76 10 13233 https://digitalcollections.lakeheadu.ca/files/original/22858aa3bc9f4b3560ee5a63f7e169ab.pdf ad8628deb5689276c27baa82945c876c PDF Text Text 51st ANNUAL MEETING Nipigon, Ontario - May 24-28, 2005 INSTITUTE ON LAKE SUPERIOR GEOLOGY e- <(j. . I Part 1 – Proceedings and Abstracts 51st ILSG Nipigon 2005 wwwIakesuperiorgeology.org �51st ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 24-28, 2005 Nipigon, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 51 Part 1 – Proceedings and Abstracts Edited by Mike Easton (Ontario Geological Survey) & Pete Hollings (Lakehead University) Cover Photos: Left - pahoehoe texture in basalts of the Osler volcanic group, Wilson Island, Middle - diabase sills on the shores of Lake Nipigon, Right - No. 1 Shaft headframe, MacLeod-Cockshutt Mine, Geraldton. �Proceedings of the 51st ILSG Annual Meeting - Part 1 51ST INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 51 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGY AND FOLD MINERALISATION OF THE BEARDMORE-GERALDTON GREENSTONE BELT TRIP 2: QUATERNARY GEOLOGY OF THE BEARDMORE – NIPIGON AREA TRIPS 3 & 6: A STRATIGRAPHIC TRANSECT ACROSS THE NORTHERN FLANK OF THE MIDCONTINENT RIFT NEAR ROSSPORT TRIP 4: GEOLOGY AND RARE ELEMENT PEGMATITES OF THE QUETICO SUBPROVINCE NEAR NIPIGON TRIP 5: GEOLOGY OF THE BLACK STURGEON AREA Reference to material in Part 1 should follow the example below: Albers, P.B., and Miller, J.D.., 2005. The Geology and Petrology of the Leveaux Porphyritic Diorite, Cook County, MN: Investigating Possible Magmatic Relationships to the Anorthositic Series of the Duluth Complex. In; Easton, M. and Hollings, P. (Eds.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 1 - Proceedings and Abstracts, v.51, part 1, 3-4. Published by the 51st Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 -i- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2005 ............................................................. iii Constitution of the Institute on Lake Superior Geology .....................................................v By-Laws of the Institute on Lake Superior Geology ....................................................... vii Goldich Medal Guidelines .............................................................................................. viii Goldich Medallists ............................................................................................................ ix Goldich Medal Committee .................................................................................................x Citation for Goldich Medal Recipient.................................................................................x Eisenbrey Student Travel Awards ..................................................................................... xi Eisenbrey Student Travel Award Application .................................................................. xii Student Paper Awards ..................................................................................................... xiii Student Paper Awards Committee................................................................................... xiii Session Chairs ................................................................................................................. xiii Membership Criteria for the Institute on Lake Superior Geology .................................. xiv Board of Directors.............................................................................................................xv Local Committee...............................................................................................................xv Banquet Speaker ...............................................................................................................xv Report of the Chair of the 50th Annual Meeting ........................................................... xvi Acknowledgements ....................................................................................................... xviii Program ........................................................................................................................... xix Abstracts .............................................................................................................................1 Author Index .....................................................................................................................70 - ii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2005 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage - iii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 # Date Place Chairs 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings - iv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Constitution of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) Article I - Name The name of the organization shall be the “Institute on Lake Superior Geology”. Article II - Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III - Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only “scientific” or “educational”, but also “non-profit”) Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV - Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V - Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI - Directors The Board of Directors shall consist of the Chair, Secretary, Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than six persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to six members. Article VII - Officers The officers of this organization shall be a Chair, a Secretary and a Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. -v- �Proceedings of the 51st ILSG Annual Meeting - Part 1 B. The Secretary shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. C. The Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. The terms of the Secretary and Treasurer shall be staggered so that there will always be a two year overlap between the two. Article VIII - Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. - vi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 By-Laws of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) I. Duties of the Officers and Directors A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the annual meeting over which he/she presides. B. It shall be the duty of the Secretary to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the Board of Directors. 3. Maintain an up-to-date mailing list 4. Act as the first point of contact for all enquires about the Institute 5. Respond to requests for back issues of the Proceedings of the Institute C. It shall be the duty of the Treasurer to: 1. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required. 2. Store the Goldich medals and each year ensure one is engraved with the name of that years winner D. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings. II. Duties and Expenses A. Regular membership dues of $5.00 or less on an annual basis shall be assessed each member as determined by the Board of Directors.. B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of students. III. Rules of Order The rules contained in Robert’s Rules of Order shall govern this organization in all cases to which they are applicable. IV. Amendments These by-laws may be amended by a majority vote (majority of those voting) of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended. - vii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. - viii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. Goldich Medalists 1979 Samuel S. Goldich 1992 William F. Cannon 1980 not awarded 1993 Donald W. Davis 1981 Carl E. Dutton, Jr. 1994 Cedric Iverson 1982 Ralph W. Marsden 1995 Gene LaBerge 1983 Burton Boyum 1996 David L. Southwick 1984 Richard W. Ojakangas 1997 Ronald P. Sage 1985 Paul K. Sims 1998 Zell Peterman 1986 G.B. Morey 1999 Tsu-Ming Han 1987 Henry H. Halls 2000 John C. Green 1988 Walter S. White 2001 John S. Klasner 1989 Jorma Kalliokoski 2002 Ernest K. Lehmann 1990 Kenneth C. Card 2003 Klaus J. Schulz 1991 William Hinze 2004 Paul Wieblen 2005 Goldich Medal Recipient Mark Smyk Ontario Geological Survey, Thunder Bay, Ontario - ix - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Goldich Medal Committee Serving through the meeting year shown in parentheses David Meineke (2005) Meriden Engineering, Hibbing, Minnesota George Hudak (2006) University of Wisconsin, Oshkosh Tom Hart (2007) Ontario Geological Survey David Meineke, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2005 meeting. Citation for Goldich Medal Recipient Mark Smyk It is a pleasure to acknowledge the many contributions of Mark C. Smyk to the understanding of the geology of the Lake Superior region at the 51st annual meeting of the Lake superior Institute on Lake Superior Geology. This is the 26th Goldich medal awarded by the Institute to individuals that have made significant contributions to Lake Superior geology. Mark was born in Dryden, Ontario, in 1961 and received his Honors Bachelor of Science degree in Geology from Lakehead University in 1984 upon completion of a thesis titled “A comparative study of silver occurrences, Island Belt Silver Region, Thunder Bay District, Ontario”. From Lakehead University Mark entered Carleton University where he received his Masters of Science degree in geology upon completion of a thesis on silver veins in the Cobalt silver area in 1987. The thesis was titled “Geology of Archean Interflow Sedimentary Rocks and their relationship to Ag-Bi-Co-Ni-As Veins, Cobalt Silver Area, Ontario”. Mark completed contract geological mapping in the Schreiber-Hemlo and Swayze greenstone belts in 1984. He has also completed contracts with David Bell Geological Services Inc. and Saarberg-Interplan Ltd. From 1998 to the present Mark has served as guest lecturer at Lakehead University, Thunder Bay. He has also lectured and prepared course curriculum in geology for the Qikiqtaaluk Corporation, Iqalut, Northern Territory. Mark has long been active in the Institute on Lake Superior Geology and co-hosted the 41st and 51st annual meetings. He is a Registered Professional Geologist in the Province of Ontario, fellow in the Geological Association of Canada, member of the Northwest Prospectors Association and guest editor for “Exploration and Mining Geology”, Canadian Institute of Mining and Metallurgy. Since 1987 Mark has worked with the Ontario Geological Survey. During this period he has authored or co-authored 7 reports, 14 abstracts and 7 guidebooks. The Ontario Geological Survey library lists 62 items as authored or co-authored by Mark. Since working with the Ontario Geological Survey Mark has advanced to become Regional Resident Geologist, Thunder Bay North. His work entails geology related publications, land use planning and interaction with the public, prospectors and industry. Submitted by Ron Sage -x- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. - xi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Award Application Student Name : __________________________________ Address: Date: ____________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ email: __________________________________________________________ Educational status: _____________________________________________________ Are you the senior author of an oral presentation or poster? Yes ____ No _____ Will any other students be traveling with you? Yes ____ No _____ If yes, then who? ___________________________________________________ ___________________________________________________ Statement of need (use additional page if necessary): __________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Signature: ____________________________________________________ Department Head: ____________________________________________________ - xii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Student Paper Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. Student Paper Awards Committee Penelope Morton, University of Minnesota Duluth Greg Stott - Ontario Geological Survey, Sudbury, Ontario Wally Rayner - Toronto, Ontario Session Chairs Charlie Blackburn, Blackburn Geological Services Terry Boerboom, Minnesota Geological Survey William Cannon, United States Geological Survey Mike Easton, Ontario Geological Survey Mary Louise Hill, Lakehead University Peter Hinz, Ontario Geological Survey Tom Lane, CAMIRO Mark Severson, Natural Resources Research Institute - xiii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Membership Criteria for the Institute on Lake Superior Geology Approved May 8, 1997. Amended by the Board—May 6, 2004 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars “Remove my name from the mailing list”; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary or attends an annual meeting. D. “Member for Life” status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary. F. The Secretary can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary to avoid unintentional lapse of membership. - xiv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Mark Smyk/Pete Hollings - General Chair 2005 meeting (2008) - Ontario Geological Survey/Lakehead University Steve Hauck (2007) - University of Minnesota, Duluth Laurel Woodruff (2006) - U.S. Geological Survey Peter Hinz (2005) - Ontario Geological Survey Peter Hollings - Secretary (2006) - Lakehead University, Thunder Bay, Ontario Mark A. Jirsa - Treasurer (2007) - Minnesota Geological Survey Local Committee Co-Chairs Mark Smyk - Ontario Geological Survey, Thunder Bay, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Program and Abstracts Editors Mike Easton - Ontario Geological Survey, Sudbury, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Field Trip Guidebook Editor Pete Hollings - Lakehead University, Thunder Bay, Ontario Organising Committee Mary Louise Hill - Lakehead University, Thunder Bay, Ontario Phil Fralick - Lakehead University, Thunder Bay, Ontario Bill Addison - Thunder Bay, Ontario Ryan Tuomi - Ontario Geological Survey, Thunder Bay, Ontario Levina Collins - Township of Nipigon Banquet Speaker Jim Franklin, Franklin Geosciences Ltd. Mineral Resources for the Future: The Resource Potential of Northern Lake Superior (See abstract on page 19) - xv - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Report of the Chair of the 50th Annual Meeting Duluth, Minnesota The Natural Resources Research Institute (NRRI) of the University of Minnesota Duluth hosted the 50th Annual Institute on Lake Superior Geology on May 4 – 9, 2004 at the Radisson Hotel Duluth-Harborview in Duluth, Minnesota. The meeting consisted of two days of technical sessions with four pre- and four posttechnical session field trips. Julie Ann Heinz, Barbara Hauck, and Jeanne Lukkarila provided excellent on-site assistance. John Heine was responsible for: the 50th ILSG website and online registration; AV expertise that kept the sessions flowing on time; and worked with Mark Severson on logistics for the field trips. Julie Oreskovich was the poster session czar. Richard Patelke was invaluable as the data base manager and the designer of the 50th ILSG T-shirt. Bill Cannon and Gene LaBerge compiled a CD of photographs of past ILSG members and field trips. Other members of the local committee included: Charlie Matsch, Jim Miller, Penny Morton, Dean Peterson, and Larry Zanko – all of whom provided much need expertise. Pre-meeting registration was 275 students and professionals, with an additional 24 on-site registrations, for a total of 299 registrants. This was the first time that on-line registration was offered, and 222 individuals registered on-line. Proceedings Volume 50 was published in two parts: Part 1 – Program and Abstracts, edited by Steven Hauck, with 69 published abstracts, for 39 oral and 30 poster presentations; and Part 2 – Field Trip Guidebook, edited by Mark Severson. The 50th meeting marked the sixth time (1958, 1963, 1971, 1979, 1989, 2004) the ILSG annual meeting was held in Duluth, Minnesota. The nine field trips (one trip with two sections) were well attended with 315 total participants attending one or two field trips. On Tuesday, May 4th, marked the beginning of two two-day field trips: George Hudak and co-leaders lead a field trip to examine the volcanic stratigraphy, hydrothermal alteration, and VMS potential of the Lower Ely Greenstone; and Terry Boerboom and co-leaders lead a trip to visit newly mapped outcrops in the Southwestern Sequence of the North Shore Volcanic Group and in the Beaver Bay Complex. On Wednesday, May 5th, there were two one-day field trips: Howard Hobbs led a trip to look at the Late Wisconsinan Superior-lobe deposits northeast of Duluth; and Richard Ojakangas, Mark Severson, and co-leaders examined the geology of the eastern Mesabi Range. On Saturday, May 10th, there were four field trips: Mark Jirsa, Terry Boerboom, and co-leaders led two sections on a two-day investigation of the classic outcrops of northeastern Minnesota; Lisa Marlow and co-leaders examined the glacial and post-glacial landscape evolution in the Glacial Lake Aitkin and Upham basin; Dean Peterson and Richard Patelke led a field trip to the Vermilion District, northeast of Soudan, MN, to look at the economic geology of Archean gold occurrences; and Mark Severson and Jim Miller visited outcrops along the western contact of the Duluth Complex to illustrate the geology and Cu-Ni mineralization. One hundred and sixty-three participants attended the annual banquet on Thursday night. This year’s banquet speaker was Bob Dott of the University of Wisconsin, Madison. Dr. Dott is professor emeritus from the Department of Geology and Geophysics. Dr. Dott’s post-banquet presentation was: The Van Hise army and other pioneers of Lake Superior geology. The other highlight of the evening was the presentation of the 2004 Goldich medal to Paul Weiblen of the Department of Geology and Geophysics at the University of Minnesota, Minneapolis, which recognized his efforts in many areas in the Lake Superior region, especially in the Duluth Complex. The technical sessions began with a special session on “The History of Geologic Investigations in the Lake Superior Region”. The “Old Prospector” (Richard Ojakangas) gave a brief historical overview of geology between 1848 and 1900 that was followed by four other presentations on the geology, mineralization, and geochronology of the Lake Superior region. As always, the student paper committee had a difficult time of picking a winner. There were six oral presentations and four poster presentations. The winners were: 1) Andy Breckenridge – Large Lakes Observatory ($250, Winner, best oral presentation) - xvi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 2) Heidi Drexler – University of Wisconsin Oshkosh ($125, Winner, best poster presentation) 3) Justin Johnson, Lakehead University ($125, Honourable mention) 4) Adam Hoffman - University of Minnesota ($125, Honourable mention, best poster presentation) In addition five Eisenbrey Student Travel awards were presented in 2004, they winners were: Paula Shafer, Indiana University ($250); Justin Johnson - Lakehead, Thunder Bay ($125); Geoff Heggie - Lakehead, Thunder Bay ($125); Adam Richardson - Lakehead, Thunder Bay ($125); and Riku Metsaranta - - Lakehead, Thunder Bay ($125) The Institute’s Board of Directors met on May 6, 2004, and brief summary of the meeting follows: 1. Accepted the Report of the Chair for the 49th ILSG from Laurel Woodruff, and the minutes of the last Board meeting, May 8, 2003 from Pete Hollings. 2. Accepted the 2003-2004 ILSG Financial Summary from Mark Jirsa. 3. Approved Steven Hauck to continue on as a Board member. 4. Approved Nipigon, Ontario as the 51st annual meeting location with Mark Smyk and Pete Hollings as co-chairs. 5. Replaced Ron Sage, who was on the Goldich Committee with Tom Hart, who had agreed to serve. 6. Discussed splitting the Secretary-Treasurer position. Pete Hollings was nominated and elected secretary by the ILSG members, and Mark Jirsa was nominated and elected treasurer. The 50th ILSG meeting was a great success, and we wish to thank all of the individuals who contributed to this success. The staffs of the Radisson Hotel Duluth-Harborview, UMD Food Service, and the Fortune Bay Casino (field trips) were always professional and responsive to our evolving needs to handle a very large group. The following organizations provided generous monetary contributions: Department of Geosciences, University of Minnesota Duluth, MN; Franconia Minerals Corporation, Spokane, WA; Idea Drilling Incorporated, Virginia, MN; Iron Mining Association, Duluth, MN; Lehmann Exploration Management, Minneapolis, MN; Meriden Engineering, LLC, Hibbing, MN; Minerals Processing Corporation, Duluth, MN; Minnesota Exploration Association (MExA), Minneapolis, MN; Minnesota Minerals Coordinating Committee, St. Paul, MN; Teck Cominco American Incorporated, Spokane, WA; and Wallbridge Mining Company, Lively, ONT. The field trips were very well attended, and our appreciation is due to the field trip leaders, van drivers, and everyone else who contributed to the success of the 50th ILSG meeting. Because of the large attendance for the meeting and field trips, the 50th ILSG meeting generated several thousand dollars to the ILSG general fund. The 50th ILSG meeting also coincided with the 50th anniversary of the Department of Geosciences at the University of Minnesota Duluth (UMD). A special session on Friday afternoon featured papers by UMD faculty, students, and alumni. A banquet celebrating this occasion was held at the Duluth Depot on Friday evening after the ILSG meeting, which was attended by many UMD alumni, faculty, and students, and ILSG members. Both of us are very happy with the results of 50th meeting, and we hope that future ILSG meetings can be as well attended as this meeting. We also hope that the attendees were as happy with the meeting as the local committee was. This meeting, like every ILSG meeting requires a lot of work and time on behalf of the cochairs and the local committee. The additional assistance of ILSG members at-large helped make the meeting a success. Respectively Submitted, Steven Hauck and Mark Severson Co-Chairs, 50th ILSG meeting - xvii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Acknowledgements The following organizations made generous contributions to the 51st Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the institute to provide one of it’s primary objectives – to promote better understanding of the geology of the Lake Superior Region. North Western Ontario Prospectors Association Lake Nipigon Region Geoscience Initiative Ontario Prospectors Association Ministry of Northern Development and Mines – Ontario Geological Survey Township of Nipigon Municipality of Greenstone Canadian Institute of Mining and Metallurgy, Thunder Bay Branch Department of Geology, Lakehead University David Malouf, Roxmark Mines Limited Chaltrek Geological Supplies Inc. - xviii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Program Tuesday May 24 8:00 a.m. Field Trip 1: Geology and gold mineralization of the Beardmore-Geraldton greenstone belt Leaders: Mark Smyk & Tom Hart (OGS) and Phil Fralick (Lakehead University) 6:00 p.m. Overnight in Beardmore Wednesday May 25 8:00 a.m. Field Trip 1: Geology and gold mineralization of the Beardmore-Geraldton greenstone belt Leaders: Mark Smyk & Tom Hart (OGS) and Phil Fralick (Lakehead University) 8:00 a.m. Field Trip 2: Quaternary Geology of the Beardmore – Nipigon area Leader: Peter Barnett (OGS) 8:00 a.m. Field Trip 3: A Stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport (I) Leaders: Peter Hollings and Phil Fralick (Lakehead University) 6:00 p.m. Return of Trips 1,2 and 3 4:00 p.m. - 8.00 p.m. Registration (Nipigon Community Centre) 7:00 p.m. - 9.00 p.m. Ice Breaker Social (Nipigon Curl-a-Drome) and Poster Setup (Nipigon Community Centre) Thursday May 26 8:00 a.m. - 4:00 p.m. Registration (Nipigon Community Centre) 9:00a.m. - 9:05 a.m. Introductory Remarks - Mark Smyk and Peter Hollings, Co-Chairs Technical Session I Session Chairs: T. Boerboom (Minnesota Geological Survey), W. Cannon (US Geological Survey) 9:05 a.m. Addison, W., Brumpton, G., Vallini, D., McNaughton, N., Davis, D., Kissin, S., Fralick, P. and Hammond, A. Discovery of distal ejecta from the 1850 Ma Sudbury impact 9:25 a.m. Schnieders, B. and Scott, J. Mining and exploration activity in the Thunder Bay South District 9:45 a.m. Hill, M.L. and Smyk, M. Penokean fold-and-thrust deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario 10:05 a.m. Middleton, R. and Heggie, G. Seagull Intrusion; A unique PGE-Ni-Cu Setting. 10:25 a.m. - 11:00 a.m. Coffee Break and Poster Session 11:00 a.m. Maric, M.*, and Fralick, P. Sedimentology of the Rove and Virginia Formations and their tectonic significance - xix - �Proceedings of the 51st ILSG Annual Meeting - Part 1 11:20 a.m. Easton, R.M. The Grenvillian Tomiko quartzites of Ontario: correlatives of the Baraboo quartzites of Wisconsin, the Mazatzal orogen of New Mexico, or unique? Implications for the tectonic architecture of Laurentia in the Great Lakes region. 11:40 a.m. Medaris, G. and Dott, B. Riddle of the sands (Proterozoic) solved by quartzites at Hamilton Mounds Wisconsin 12:00 p.m. - 1:30 p.m. Lunch Break and Poster Session (ILSG Board Meeting by invitation) Technical Session II Special Session - Lake Nipigon Regional Geoscience Initiative Session Chairs: Tom Lane (CAMIRO), Mark Severson (Natural Resources Research Institute) 1:30 p.m. Heaman, L. and Easton, R.M. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating 1:50 p.m. Smyk, M. Mineral deposits and metallogeny of the Midcontinent Rift in Ontario 2:10 p.m. Fralick, P., Metsaranta, R., and Rogala, B. Stratigraphy of the Mesoproterozoic Sibley Group and Nipigon Sills 2:30 p.m Hart, T. and MacDonald, C. Mesoproterozoic diabase sills of the Nipigon Embayment, northwest Ontario 2:30 p.m. - 3:00 p.m. Coffee Break and Poster Session 3:00 p.m. Richardson, A.* and Hollings, P. Geochemical variation within the Mesoproterozoic Nipigon diabase sills 3:20 p.m. Metsaranta, R.* and Fralick, P. Depositional setting of the Pass Lake and Rossport Formations (Sibley Group) inferred from a combined sedimentologic/geochemical approach 3:40 p.m. Laarman, J.* and Hollings, P. Petrogenesis and PGE mineralization of the Eva Kitto Intrusion, Northern Ontario 4:00 p.m. Magee, A.*, Hollings, P. and Fralick, P. Preliminary stratigraphy and geochemistry of the Mesoproterozoic Pillar Lake volcanics, Wabigoon subprovince, Superior Province, Armstrong, Ontario, Canada 6:00 p.m Annual Banquet and Award Presentation (Royal Canadian Legion) Announcement of 52nd Annual Meeting Location 2005 Goldich Award Presentation to Mark Smyk 2005 Banquet Address - Dr. J. Franklin Meeting participants not registered for the banquet are welcome to attend the address - xx - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Friday May 27 9:00 a.m. - 12:00 p.m. Registration Technical Session III Session Chairs: Mary Louise Hill (Lakehead University), Mike Easton (Ontario Geological Survey) 9:00 a.m. Holm, D., Cannon, W., Chandler, V., Schneider, D., Schulz, K., & Van Schmus, W. The incredible shrinking Penokean orogen: a new look at the accretionary history of the southern Lake Superior region. 9:20 a.m. Vallini, D.*, Cannon, W. and Schulz, K. New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions 9:40 a.m. Knudson, D.*, Saini-Eidukat, B., Miller, J., and Daniels, P. Structural state of plagioclase phenocrysts in porphyritic rocks of the Midcontinent rift, northeastern Minnesota 10:00 a.m. Albers, P.* and Miller, J. The geology and petrology of the Leveaux porphyritic diorite, Cook County, MN: Investigating possible magmatic relationships to the Anorthositic Series of the Duluth Complex 10:20 a.m. - 11:00 a.m. Coffee Break and Poster Session 11:00 a.m. Conly, A. and MacDonald, J. Origin of high-sulphate waters of the Hogarth Pit Lake, Steep Rock iron mine, Atikokan, Ontario. 11:20 a.m. Breckenridge, A. and Johnson, T. Lake Superior’s oxygen isotope record suggests overflow to Lake Ojibway between 10,000 and 9,400 CAL BP (~8.9-8.4 14C KA) 11:40 a.m. Blackburn, C. and Kor, P. Control of Quaternary erosional and depositional landforms at the Eastern outlet of glacial Lake Agassiz by Precambrian bedrock and structure, Ottertooth-Pantagreul Lakes area, northwestern Ontario 12:00 p.m. - 1:30 p.m. Lunch Break Technical Session IV Session Chairs: Peter Hinz (Ontario Geological Survey), Charlie Blackburn (Blackburn Geological Services) 1:30 p.m. Weiblen, P., Peterson, D., Vislova, T. Implications of Midcontinent Rift and oceanic ridges analogies and 3D interpretations of the subsurface structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise 1:50 p.m. Halls, H. Dyke swarms around the Lake Superior Region define Proterozoic (~2 Ga) deformation of the Archean Superior Province associated with evolution of the Kapuskasing Zone 2:10 p.m. Dahl, D. Gold grains, pathfinder elements, and till clast composition in a portion of the Vermilion Greenstone Belt, Northeastern Minnesota 2:30 p.m Miller, J. and Jirsa, M. - xxi - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology, geochemistry and PGE potential of mafic and ultramafic intrusions in Minnesota, excluding the Duluth Complex 2:50 p.m Presentation of Best Student Paper Award and Eisenbrey Awards 3:00 p.m. - 3:30 p.m. Coffee Break and Poster Session (Posters removed after the break) 7:00 p.m Mixer - Cash Bar (Nipigon Curl-a-Drome) Poster Presentations Bucholz, T., Falster, A., and Simmons, W. Mineralogy of pegmatites and spatially associated metasomatized zones, Michels Materials Quarry, Waterloo, WI Cannon, W., Schulz, K., Daniels, D., Anderson, R., Chandler, V., Holm, D., Schneider, D., Van Schmus, R. Geology of Precambrian basement rocks in Iowa and the southern parts of Wisconsin and Minnesota Jirsa, M. and Miller, J. Geologic implications of bedrock mapping in the Ely and Basswood Lake quadrangles, Northeast Minnesota Lane, C.* and Hollings P. Geochemistry and petrography of the Rabbit Islands Breccia, North Central Lake Nipigon MacTavish, A. MetalCORP Ltd. Big Lake Ni-Cu-PGE, Cu-Zn-Ag, and Mo Property Magee, A. Mining and exploration activity in northwestern Ontario Planavsky, N.* and Murphy, J.* New thoughts on old circles: A reexamination of spheroidal Gunflint taxa Rossell, D. and Coombes, S. The geology of the Eagle Lake Nickel-Copper deposit: Marquette County, Michigan Severson, M. PGE and gold potential of the Archean Deer Lake Complex, Minnesota, USA Shareef, S. and Craven, J. A view into the Nipigon Embayment: preliminary results of the largest magnetotelluric study ever in Ontario. Stott, G. and Rayner, N. Discrimination of Archean terranes in the Sachigo subprovince and relevance to volcanogenic masive sulphide exploration Trow, J. and Young, C. Correlation between self-potential and dowsing (IESG) at the Quincy Mine and at the Calumet and Hecla Mine, Michigan - xxii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Weiblen, P., Peterson, D., and Vislova, T. Implications of Midcontinent Rift and oceanic ridges analogies and 3D interpretations of the subsurface structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise Zieg, M., Forsha, C., and Habarka, J. Textural examination of Kama Point diabase sill, Nipigon, Ontario NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Saturday May 28 8:00 a.m. Field Trip 4: Geology and Rare Element-bearing pegmatites of the Quetico Subprovince Leaders: Mark Smyk (OGS) and Steve Kissin (Lakehead University) 8:00 a.m. Field Trip 5: Geology of the Black Sturgeon Area Leader: Tom Hart (OGS) 8:00 a.m. Field Trip 6: A Stratigraphic Transect Across the Northern Flank of the Midcontinent Rift near Rossport (II) Leaders: Peter Hollings and Phil Fralick (Lakehead University) 6.00 p.m. Return of Trips 4, 5 and 6 - xxiii - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Discovery of Distal Ejecta from the 1850 Ma Sudbury Impact ADDISON, W.D., R.R. 2, Kakabeka Falls, Ontario, P0T 1W0, Canada; BRUMPTON, Gregory, R., 211 Henry Street, Thunder Bay, Ontario, P7E 4Y7, Canada; VALLINI, Daniela A, McNAUGHTON, Neal, J., Centre for Global Metallogeny, School of Earth and Geographic Sciences, University of Western Australia, Nedlands, Western Australia, 6009, Australia; DAVIS, Don W., Department of Geology, Earth Sciences Centre, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1, Canada; KISSIN, Stephen A., FRALICK, Philip W., HAMMOND, Anne L., Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada A 25–70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted-grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Smectite replaced microtektites, spherules and crushed spherules, and sphere-insphere features make up the bulk of the ejecta. Accretionary lapilli appear near the middle of the ejecta column in the Ontario Gunflint Formation cores but they are not present in the Minnesota Virginia Formation cores. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between circa 1878 Ma (Fralick et al., 2002) and 1836 Ma. The Sudbury impact event, which occurred 650–875 km to the east at 1850±1 Ma (Krogh et al., 1984), is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features (French, 1998) are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features. LEGEND racks younger than Paleoproterozoic Paleoproterozoic rocks, BIF in black Archean greenstone granite terrane Figure 1. Location of drill holes in relation to the Sudbury structure. The Sudbury impact is the only known impact location close enough to have produced a craton-sourced (quartz and feldspar grains), westward-thinning ejecta layer (70 cm thinning to 25 cm) this thick, in these locations, given the time constraints established by zircon dating. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact: Geology, 33, 193-196. Fralick, P., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, 39, 1085–1091. French, B.M. 1998. Traces of catastrophe: Lunar and Planetary Institute Contribution 954, 120p. Krogh, T.E., Davis, D.W. and Corfu, F. 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area, in Pye, E.G., Naldrett, A.J., and Giblin, P.E., eds., The geology and ore deposits of the Sudbury structure: Ontario Geological Survey Special Volume 1, 431–446. -1- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Figure 2. Overview of the Gunflint and Rove Formations based on drill core 89MC-1s. The 1878 Ma tuff age is from Fralick et al (2002). Detail from column 1 showing the Gunflint-Rove boundary zone and the relationship of the ejecta to the dated Rove tuff. The Minnesota cores and other Ontario cores show similar features in this zone. Detail of the ejecta layer in drill core BP99-2, the most complete and least altered ejecta section in all the cores. The possible carbonate-replaced fireball layer fades into the overlying carbonate. Minnesota ejecta layers are thinner and lack accretionary lapilli. LEGEND C Possible fireball rayer. carbonate replaced Acoretionary lapilli Ejecta P Possible ejecla. carbonate replaced S [] Inaclastjo Grainstone — Ei Fine - grained chemical sedimentwy rooks Siliclaslic Sandstone [II — Fine-grained siliastics Recrystalljzed and silioffied carbonates -2- �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Geology and Petrology of the Leveaux Porphyritic Diorite, Cook County, MN: Investigating Possible Magmatic Relationships to the Anorthositic Series of the Duluth Complex ALBERS, Paul B., Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812, albe0167@d.umn.edu and MILLER, James D., Jr., Minnesota Geological Survey, c/o NRRI, 5013 Miller Trunk Hwy, Duluth, MN 55811, mille066@umn.edu The Leveaux porphyritic diorite (LPD) is a 72-m thick, discontinuous, hypabyssal, sheet-like intrusion located within the Mesoproterozoic (1.1 Ga) Midcontinent Rift-related Beaver Bay Complex of northeastern Minnesota (Miller and Chandler, 1997). The LPD makes up five prominent, cuesta-like ridges (Figure 1) that trend parallel to Lake Superior over a 20 km distance. In the Caribou Lake region, the LPD makes up smaller knob-like outcrops that dip in multiple directions and are segmented by the younger Beaver River diabase. The LPD is composed of an upper porphyritic zone, which contains 40-50% of 1-4 cm labradorite megacrysts in a fine-grained ferrodiorite matrix (Figure 2), and a lower aphyric zone composed of fine-grained ferrodiorite with rare megacrysts. The aphyric and porphyritic zones are separated by a 20-30 cm thick gradational transition. The orientation of this planar transition and sheet joints from the aphyric section indicate a gentle (10-20 degrees) southeast dip. The upper contact of the LPD is fault bounded. In two localities, the lower contact is observed to be chilled against a thin arkosic sandstone unit, which in turn is underlain by an ophitic basalt. Few outcrops of a sparsely porphyritic (5-20% plagioclase megacrysts) ferrodiorite off the trend of the main sheet are found and may represent feeder zones to the LPD. Figure 1. Cuesta-like ridge of LPD at Moose Mountain dipping gently toward Lake Superior (looking southeast). The ridge to the right is the Beaver River diabase. Figure 2. Nonfoliated, lath-shaped labradorite megacrysts (1-4 cm) in a dark, fine-grained ferrodiorite matrix at Oberg Mountain. Geochemical and petrographic characteristics of the aphyric zone and the matrix material of the porphyritic zone are nearly identical vertically and laterally throughout the sheet. Mineralogically, the ferrodiorite is composed of plagioclase, augite, K-feldspar, Fe-Ti oxides, and minor accessory minerals of quartz, inverted pigeonite, calcite, and apatite, which collectively display an intergranular, nonfoliated texture. Modally, the ferrodiorite varies between diorite and quartz monzonite. XRF analyses indicate a normative An (An/An+Ab+Or) matrix composition of 39, while microprobe analyses range from 19-64. Matrix augite contains a constant composition (mg # 55-61) throughout the intrusion. Microprobe traverses and Nomarski DIC microscopy of plagioclase megacrysts indicate subtle zoned cores (~An 65) and strongly zoned rims with An compositions comparable to matrix plagioclase. XRD analyses indicate that the plagioclase megacrysts have an intermediate ordered structural state (see Knudsen and others, this volume). Based on these observations, we conclude that the LPD formed by the shallow emplacement of a single plagioclase-phyric ferrodiorite magma. Density calculations indicate that the upper porphyritic section may have -3- �Proceedings of the 51st ILSG Annual Meeting - Part 1 formed by plagioclase megacryst flotation in the host ferrodiorite magma. The average composition, zonation patterns, and structural state of LPD plagioclase megacrysts are similar to cumulus plagioclase of the anorthositic series of the Duluth Complex. Also, the evolved compositions of the LPD ferrodiorite are similar to the estimated anorthositic series trapped liquid compositions (Miller and Weiblen, 1990). With these petrogenetic links, we conclude the LPD is a hypabyssal equivalent to the anorthositic series, which is thought to have formed from plagioclase crystal mushes derived from deep crustal magma chambers. References Miller, J. D., Jr., and Chandler, V. W. 1997. Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R.W., Dickas, A.B., Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, 73-96. Miller, J. D., Jr., and Wieblen, P. W. 1990. Anorthositic rocks of the Duluth Complex, Minnesota: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, 31, 295-339. -4- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Control of Quaternary Erosional and Depositional Landforms at the eastern outlet of Glacial Lake Agassiz by Precambrian Bedrock and Structure, Ottertooth-Pantafreul Lakes area, Northwestern Ontario BLACKBURN, C.E., Blackburn Geological Services, Victoria, B.C. and KOR, P.S.G., Ontario Parks, Peterborough, Ontario During late Wisconsinan/early Holocene time, glacial Lake Agassiz debouched into the Nipigon basin in catastrophic bursts at the continental divide through a number of spillway channels. Two protected areas managed by MNR Ontario Parks, Ottertooth Conservation Reserve and Pantagreul Creek Provincial Nature Reserve, encompass the eastern portion of the 15-km-wide Kaiashk Spillway corridor, the southern-most of five major channel complexes (Figure 1). The power of the bursts can be appreciated by the fact that water flowing through the outlet would have fallen from the 457 m elevation of the divide to about the 320 m level of Lake Nipigon - a drop of 137 m over a distance of 20 km. 44 1# C, C, t I. to Erosional features resulting from this outflow include plunge basins that formed at the break in slope from the Archean gneissic basement into the Nipigon lowlands. Two plunge basins are present. The more spectacular of the two, Devils Crater, has walls on the order of 100 m high down which today two water falls cascade. The second un-named plunge basin lies some 1.5 km to the southwest. Streams drain to the southeast from both plunge basins, the one from Devilʼs Canyon being through a deeply incised, narrow gorge. Similar though less prominent features suggesting plunge of water over a pre-existing bedrock “lip” can be seen: a) at the north end of Rabelais Lake; b) about 3 km to the northeast of Devils Crater, and c) near the west shore of Pantagruel Creek. Notably, all of these features lie along a NE-trending line over a distance of ~20 km. Depositional features resulting from the outflow include an alluvial fan at the mouth of the more southerly of the channel-ways exiting from the two plunge basins, which unites with the main spillway channel through which Pantagruel Creek now flows. An “island” of braided eskers forms an erosional remnant in the centre of the channel. Figure 1. Location of the Kaiashk Spillway system, and its relation to four other systems that linked Lake Agassiz with Lake Nipigon during the Nipigon Phase (from Teller and Thorleifson, GAC Special Paper 26, 1983, Fig. 7). Underlying Archean and Proterozoic bedrock geology had a profound influence on development of the above Quaternary features. To the northwest of the northeasterly-trending line joining points of plunge of Agassiz lake waters, Archean gneissic bedrock was peneplained prior to deposition of Proterozoic Sibley Group sediments, and exhumed during the time of outlet from Lake Agassiz. The result is a broad, level plain. Flat-lying Sibley sedimentary rocks were intruded by Nipigon diabase sills, the erosional remnants of which form numerous scarps and cuestas. Evidence of regional scale faulting post dating intrusion of the sills is seen at Devilʼs Crater, where Archean gneisses exposed on its northwest face are uplifted relative to Proterozoic diabase that is exposed throughout the rest of Devilʼs Crater and its gorge. Faulting is probably present at the smaller plunge basin, and at the north end of Rabelais Lake where there is also transition from peneplained Archean gneiss to the incised channel-way of Rabelais Creek. However this latter channel has eroded completely through the diabase “cap” into the gneisses below. Interpretation of -5- �Proceedings of the 51st ILSG Annual Meeting - Part 1 such a regional scale fault is supported by presence of similar NE to NNE-trending regional-scale faults that have been mapped throughout the region. Projection of the regional-scale NE-trending fault is coincident with the five identified points of plunge of Agassiz lake waters during late Wisconsinan/early Holocene time. Other such fault structures, as yet undetected, may have had considerable influence on development of landform features at the eastern exit from glacial lake Agassiz, possibly along the entire length of the spillway channels northwest of present Lake Nipigon (Figure 1). References Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz–Superior connection; in Glacial Lake Agassiz, Geological Association of Canada, Special Paper 26, p.61-290. -6- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Lake Superiorʼs Oxygen Isotope Record Suggests Overflow to Lake Ojibway between 10,000 and 9,400 CAL BP [~8.9-8.4 14C ka] BRECKENRIDGE, Andy and JOHNSON, Thomas C., Large Lakes Observatory and Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Late glacial ostracode and bivalve records from Lakes Huron and Michigan are characterized by extreme δ18O variations, ranging from values that reflect a source that is primarily glacial in origin (~–20 ‰ PDB) to much heavier values characteristic of a regional meteoric source (~–5 ‰ PDB). In contrast, a coeval record from benthic ostracodes (Candona subtriangulata) from a varve sequence in Lake Superior is consistently depleted in 18 O, ranging from –18 to –22 ‰ PDB. Between 10,000 and 9,400 cal BP [8.9-8.4 14C ka], high δ18O values in Huron strongly contrast with much lower values in Superior, which suggests Lake Superior overflow circumvented Lake Huron, and discharged through the Pic-White Otter River Valley en route to Glacial Lake Ojibway. The northern outlet may have opened when lake levels fell from an upper post-Minong water plane at around 10,000 cal BP [~8.9 14C ka]. Thereafter, lake levels probably transgressed towards the post-Minong III shoreline. Because deltaic deposits in the Pic River Valley dated around 9,200 cal BP [8.2 14C ka] record glacial meltwater flow towards Lake Superior (Bajc et al., 1997), northern drainage of Lake Superior must have been blocked by ice advance at around 9,400 cal BP [8.4 14C ka], rather than by differential uplift of a northern outlet. Negative δ18O excursions in Lakes Huron and Michigan between 9,400 and 9,000 cal BP [8.4-8.1 14C ka] record the return of Lake Superior overflow to the upper Great Lakes. In Huron the negative δ18O excursion was previously ascribed to the Late Stanley lowstand (Rea et al., 1994), and in Michigan the event was attributed to Lake Agassiz overflow and labeled ʻA2ʼ (Colman et al., 1994). During this entire period, both Lake Agassiz and glacial meltwater discharged into Lake Superior via the Nipigon inlets. Lake Agassiz and glacial meltwater fluxes into Lake Superior diminished to zero between 9,040 and 8,840 cal BP [~8.1-7.9 14C ka]. References Bajc, A.F., Morgan, A.V., and Warner, B.G. 1997. Age and paleoecological significance of an early Postglacial fossil assemblage near Marathon, Ontario, Canada. Canadian Journal of Earth Sciences. 34, 687-698. Colman, S.M., Keigwin, L.D., and Forester, R.M. 1994. Two episodes of meltwater influx from glacial Lake Agassiz and their climatic contrast. Geology 22, 547-550. Rea, D.K., Moore, T.C., Jr., Anderson, T.W., Lewis, C.F.M., Dobson, D.M., Dettman, D.L., Smith, A.J., Mayer, L.A. 1994. Great Lakes paleohydrology: complex interplay of glacial meltwater, lake levels, and sill depths. Geology 22, 10591062. -7- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineralogy of Pegmatites and Spatially Associated Metasomatized Zones, Michels Materials Quarry, Waterloo, WI BUCHOLZ, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, FALSTER, Alexander. U. and SIMMONS, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. The Michels Materials Waterloo Quarry exposes Baraboo Interval quartzites and metapelites that have been intruded by pegmatites of Wolf River age, approximately 1,440 Ma (Aldrich, 1959; cited in Brown, 1986). A pegmatite outcropping nearby in the Crawfish River has been long known, but pegmatite exposures in the quarry itself were first noted during an ILSG field trip in 2001. Samples of pegmatites and associated rocks were collected at that time and during a number of subsequent visits. The pegmatites are highly weathered and in hand specimen consist of kaolinite, quartz and muscovite; virtually all feldspars have altered to kaolinite. The dikes are small, discontinuous, apparently randomly oriented and frequently crosscut bedding in the metasediments. Despite the extensive alteration of feldspars, associated muscovite is quite fresh and a number of accessory phases persist. Small miaroles appear to be present, but are obscured by kaolinite. Most pegmatites exposed to date are in the upper level of the quarry, although recent operations (2004) have exposed one pegmatite in the lower level. Accessory minerals identified in dikes in the upper level of the quarry include dark green gahnite as small aggregates to approximately 2 mm and as well-formed octahedral crystals to approximately 500 µm: MnO content ranges from 0.4 to 1.1 wt. % and FeO content ranges from 1.5 to 2.9 wt. %. Columbite-tantalite group minerals are not uncommon but are inconspicuous due to their small grain size. They form prismatic crystals up to about 900x50 µm. Mn enrichment is pronounced and Ta-enrichment is modest, with compositions falling into the manganocolumbite and manganotantalite fields; there are two distinct composition trends present in these small dikes, one clustering around a Nb-dominant composition and the other clustered around a Ta-dominant composition. TiO2 contents range from 0.4 to 0.9 wt. %. Sn, Bi, and Sb were not detected; the absence of Bi is interesting in light of recent finds described below. Additional minerals in the upper level dikes include apatite, sprays of acicular goethite crystals, ilmenite as crude hexagonal platelets, small grains of a LREE-Ca phosphate (probably monazite-(Ce) or rhabdophane-(Ce)), zircons giving evidence of minor Hf enrichment and a grayitelike Th-phosphate associated with gahnite. Despite the intense alteration of pegmatitic feldspars, associated muscovite is well preserved and retains a substantial Li2O content of up to 0.98 wt %. In September 2004 a somewhat less weathered pegmatite was discovered in the lower level of the quarry. Similar to the upper level dikes, this pegmatite appears to cut across the bedding and terminates before intersecting the upper bench level. Remnants of partially altered feldspars are present in this pegmatite, and the overall mineralogy is somewhat more complex than in the upper level dikes. Some micas in this pegmatite have a pronounced dark pink color and contain substantial Li2O enrichment up to 2.05 wt %. Some additional minerals found include manganotantalite, goethite replacements of pyrite and chalcopyrite crystals, thin tabular crystals of hematite and sparse apatite. Noteworthy are small grains of bismutomicrolite {(Bi,Ca) (Ta,Nb)2 O6 (OH)} to approximately 0.4 mm embedded in feldspar. EMP analysis indicates Ta>Nb, and Bi>Ca+Na. Pale greenish elongated crystals forming small branching groups in vugs and voids, usually along surfaces of quartz grains, are a mixture of kettnerite {(Ca,Bi) (CO3) O, F} and other Bi secondary minerals as indicated by x-ray diffraction study. Kettnerite is a typical secondary Bi mineral probably derived from alteration of a primary Bi mineral such as bismuthinite, although the primary Bi mineral has not yet been found. The Waterloo occurrence represents the first find of gahnite from a pegmatite in the state and the overall composition of these small pegmatites indicates highly peraluminous composition, even though such common peraluminous species such as garnet and tourmaline appear to be absent in them. These are also the first finds of kettnerite and bismutomicrolite for Wisconsin. -8- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Spatially and perhaps genetically related alteration or metasomatic vein-like bodies have been found near the pegmatites. These range from quartz-muscovite±andalusite veins to massive fine-grained lithian muscovite (0.6 wt % Li2O), the two phases interfingering and grading into each other laterally. Considering the close spatial association of these bodies to pegmatites, it seems likely that the fluids responsible for the alteration may have been derived either from the pegmatites or the source igneous body, although mobilization from the metasediments cannot be excluded based on the present evidence. Schorl tourmaline is locally common in the quartz-muscovite+-andalusite veins in small vugs and adjacent pelitic schists. Colors range from black to olive green, brown and pinkish brown. Microprobe analysis indicates FeO contents of 8-9%, reflecting an overall schorl composition, though site occupancy considerations indicate a significant Li content. Hence, as the Fe-content of these tourmaline crystals is only slightly on the schorl side of the line between elbaite and schorl they might be considered elbaitic or lithian schorl. Apatite is common in small vugs near tourmaline, and ranges from dark red tabular hexagonal crystals to short visually zoned yellow-brown hexagonal prisms. Additional accessory minerals noted in vuggy portions of the veins include Ti-oxides, ilmenite-hematite, pyrite, chalcopyrite (both sulfides usually replaced by goethite), calcite and sparse REE-phosphate grains. Accessory minerals noted in the massive fine-grained muscovite-rich portions include ilmenite-hematite grains, enigmatic muscovite replacements of an undetermined mineral, and sparse zircon. The zircons may be detrital and predate metasomatism. Hence it is likely that the highly evolved pegmatites formed in conjunction with regionally widespread Wolf River age magmatism (Medaris et al., 2003). Elevated Ta and Mn levels evidenced in the pegmatites, and B and Li in the metasomatic units are anomalous for Wolf River age intrusions in Wisconsin. However, it appears likely that fluids either from these pegmatites or more directly from the parent intrusion may have had a strong, localized metasomatic effect on adjacent rocks, introducing P, B, Li and perhaps other elements to favorable areas in the host rocks. An intriguing question is what influence assimilation of peraluminous metasediments may have had on the source magma, and subsequently on pegmatite composition. Could this have influenced the mineralogy of the pegmatites and other features, or was the source magma already evolved, having originated by partial melting of different source rocks than typical Wolf River intrusions? References Brown, Bruce A. 1986. Baraboo Interval in Wisconsin, In Proterozoic Baraboo Interval in Wisconsin, Geoscience Wisconsin 10, 1-14. Medaris, L.G. Jr., Singer, B.S., Dott, R H. Jr., Naymark, A., Johnson, C.M. and Schott R.C. 2003. Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites. Journal of Geology, 111, 243-257. -9- �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology of Precambrian basement rocks in Iowa and the southern parts of Wisconsin and Minnesota CANNON, W.F., SCHULZ, Klaus J., and DANIELS, David L.,U.S. Geological Survey, Reston, VA, ANDERSON, Raymond, Iowa Geological Survey, Iowa City, IA; CHANDLER, Val, Minnesota Geological Survey, St. Paul, MN, HOLM, Daniel, Kent State University, Kent, OH, SCHNEIDER, David, Ohio University, Athens, OH, and VAN SCHMUS, W.R., Kansas University, Lareence, KS The well-known Precambrian rocks of the Lake Superior region record geologic events in the evolution of Laurentia from 3.6 Ga to 1.1 Ga. Similar rocks extend southward as the crystalline basement beneath Paleozoic and Cretaceous strata in southern Minnesota and Wisconsin and throughout Iowa. Using aeromagnetic and gravity maps of the region, and several hundred basement-penetrating drill holes, we have produced a geologic map and preliminary tectonic interpretation of the Precambrian subcrop for this region (Figure 1). Our interpretation extends the well known Superior Province (Archean) and Penokean orogenic belt (Paleoproterozoic) of the Lake Superior region southward to a prominent geophysical discontinuity, shown clearly by aeromagnetic patterns, named the Spirit Lake-Trempealeau discontinuity (SLTD). This discontinuity separates fundamentally different terranes; rocks to the south are part of the Yavapai Province and those to the north are part of the Penokean Province. Although rocks of Yavapai age, represented by extensive ultra-mature sediments, such as the Baraboo and Sioux Quartzites, and granitic plutons, extend north of the discontinuity, there are as yet no firmly identified Penokean or older rocks south of the discontinuity, although data are sparse. Thus we suggest the possibility that the discontinuity marks the southern limit of the preserved Penokean orogen and its Archean basement. South of the SLTD, rocks at the subcrop include subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. An orogenic complex of gneisses and mafic volcanic rocks, probably basement rocks on which the rhyolites were deposited and from which they formed by partial melting, are inferred from gravity and magnetic patterns to be at subcrop throughout much of this terrane, but its lithology and age are not well constrained. All of these rocks were strongly deformed at about 1.63 Ga during the Mazatzal orogeny, whose northeastern limit may underlie the extreme southeastern part of the map area. The nature of the SLTD is controversial with possibilities including a north-directed Yavapai subduction zone or a major strike-slip fault zone. Major granitic plutons were emplaced into the Yavapai terrane in the interval 1.50-1.43 Ga and some are stitching plutons of the SLTD providing an upper age limit on deformation within it. The final major Precambrian event in the region was formation of the Midcontinent Rift at 1.1 Ga. The rift transects older terranes at a high angle and consists of more than 10 km of flood basalts accumulated in and near a series of grabens, and slightly younger clastic rocks. Its location in the subsurface is clearly shown by very pronounced geophysical anomalies. A major igneous complex in NE Iowa and SE Minnesota is also interpreted to be part of the Midcontinent Rift. - 10 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 ______ _______ _______ 98L0.W _______ 94.0YW 88 OOW SBOW go•do•w EXPLANATION Ma Manson impact structure Red clastic sedimentary rocks Vo Mafic intrusive rocks Penokean Orogen Arthean basement granitic rocks Archean Granite and tonalite Pembine-Wausau Terrane Gnelss of Jeffers Block Mafic volcanic rocks LAng 1 Granite,anorthosite,norite Gneiss of Montevideo Block Mazatzal Orogen Felsic volcanic rocks Gneissof Morton Block Granitic plutons Rhyoliteandgranlte Baraga Group Orogenic complex Late- and post-tectonic granite Flood basalt, minor rtiyolite Anorogenic Plutons Foreland basin Mat Ic volcanic rocks Midcontinent Rift MRs Penokean Orogen (cont.) Yavapai Orogen (cont.) Bimodal volcanic rocks Units of undetermined age ç:: Gneiss — Yavapai Orogen Marshfleld Terrane Mafic gneiss Quartzlte (BarabooSioux, etc) Mt Yr iF Rhyolite and granite ftiOIIl]I]OI Felsic yneiss Iron-formation — Diabase dikes (mostly MCR related) Mafic and untramafic rocks 11111111 Mylonite and sheared granite Mafic volcanic rocks Volcanic rocks Figure 1. Geological Map of Precambrian basement rocks in Iowa and the southern parts of Minnesota and Wisconsin inferred from aeromagnetic and gravity data and drill holes. - 11 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Origin of High-Sulphate Waters of the Hogarth Pit Lake, Steep Rock iron Mine, Atikokan, Ontario CONLY, A.G., and MacDONALD, J.C., Department of Geology, Lakehead University, Thunder Bay, On, P7B 5E1, andrew.conly@lakeheadu.ca Hogarth Pit Lake was formed from the continual flooding of the main open pit iron mine at Steep Rock Lake near Atikokan, Ontario. Hogarth Pit Lake is located in the former middle arm of Steep Rock Lake and was mined for itʼs predominantly goethite iron ores from 1954 to 1977. During this time over 25 million tons of ore were mined [1]. Water within the pit lake is characterized by near neutral pH and extraordinarily high SO42- concentrations (1000 to 3000 ppm; [2], this study). Consequently, the aim of the research is to determine the source(s) of the high SO42- levels. Possible sources of sulphate that are evaluated include: i) oxidation and leaching of finely disseminated sulphides within hanging wall and footwall rock units by groundwater; ii) oxidation and leaching of pyritic portions of the ore zone by groundwater; and, iii) oxidation and leaching of sulphides/sulphates contained in waste rock dumps and tailings piles by surface waters Hogarth Pit Lake is approximately 160 m deep, with a surface area of approximately 100 hectares and is elongated along a north-south axis. The lake is steep sided, well-sheltered by high rock walls. Footwall rocks, east of the Jolliffe ore zone, consist of (in ascending stratigraphic order) the Marmion Geniss Complex, metasedimentary rocks of the Wagita Formation and the Mosher Carbonate [3]. To the west, hanging wall rocks include ultramafic-mafic volcanic and volcaniclastic rocks of the Dismal Ashrock Formation, which is overlain by mafic to felsic volcanic rocks of the Witch Bay Formation [3]. Since cessation of mining activities the water level within the pit continues to increase (~ 3 m/year) from surface runoff and groundwater seepage. Environmental assessment studies completed shortly after closure of the mine indicate that approximately 85% of the water entering the pit is groundwater (R. Bernachez, personal communication, 2003). The lower contribution of surface water is consistent with the limited number of tributaries that currently flow into the pit lake. Water from Hogarth Pit Lake sampled for this study has a pH of ~7.6, contains 1340 to 1381 ppm of SO42, only trace levels (0.1-0.2 ppm) of Fe and Mn, and no detectable deleterious metals (As, Cd, Pb). Pit water has δ13C and δ34S values that range from 0.8 to 0.9‰ and -2.7 to -2.9‰, respectively. Surface waters that are following into the pit are characterized by a wide range in pH (2.8 to 8.1) and SO42- content (734 and 2580 ppm), with δ13C and δ34S values ranging from -20.3 to 1.5‰ and –3.6 to -0.4‰, respectively. The composition of surface and pit waters is comparable with the data of [2], although there is a trend of decreasing SO42- between 1998/99 and 2003. The variation in the chemistry of surface water reflects differences in wall-rock interactions. Surface waters with low pH and δ13C values reflect reactions with footwall and hanging wall volcanic and genesis units and organic matter. Water with near neutral pH values and δ13C values approaching 0‰ indicate buffering by the Mosher Carbonate. Groundwater was not sample as part of this study. However, regional groundwaters in the Atikokan area are generally enriched in Cl- with substantially lower SO42- contents than observed at Hogarth Pit [4]. However, some regional groundwaters have SO42- contents that approach the lower values for surface waters [4]. Tailings Lillill Pyric waste rock Ore zone Hanging wall C Hogarlh pit waters C Hanging wall tributaries Footwall tributaries P 6'$ (permil) Figure 1. Histogram showing the distribution -of12 sulphur isotope data for Hogarth Pit Lake waters The sulphur isotope composition of the pit and tributaries waters (Figure 1) is consistent with oxidation and dissolution of pyrite from the ore zone, waste rock dumps and the hangingwall. The data for the pyritic waste rock dumps is also used to better constrain the composition of pyritic Figure 1. Histogram showing the distribution of sulphur isotope data for Hogarth Pit Lake waters and lithological units �Proceedings of the 51st ILSG Annual Meeting - Part 1 portions of the ore zone, since only one in situ ore sample was obtained for the study. The shift to slightly more depleted δ34S values for the water samples relative to pyrite-bearing rocks is within the range anticipated for pyrite oxidation [5]. However, sulphur isotopes cannot be used in isolation to discriminate potential sources of SO42-. It is necessary to consider both the total sulphur content of the potential source material and water flow pathways. Consequently, the most likely source of SO42- is the oxidation and dissolution of pyrite contained with the Jolliffe ore zone by groundwater. The high bulk sulphur content (3.5-43.8 wt% as FeS2) of pyritic ore and waste rocks indicate that there is an amply supply of pyrite. Conversely, only very minor amounts of SO42- could have been derived from the hanging wall units and tailings, since these lithogies are characterized by low sulphur contents (<2 wt% and < 0.06 wt%, respectively). Furthermore, the substantially higher proportion of groundwater to surface water filling the pit indicates that water-rock interactions with exposed lithologies contribute a subordinate amount of SO42-. References [1] Taylor, B. 1978. Steep Rock The Men and the Mines. Quetico Publishing: Atikokan, Ontario, 114 p. [2] McNaughton, K.A. 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 85p. [3] Stone, D., Kamineni, D.C. and Jackson, M.C. 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, Bulletin 405, 106p. [4] Ophori, D.U. 1996. Regional groundwater flow in the Atikokan research area: spatial variable density and viscosity. Atomic Energy of Canada Limited Report AECL-11082, COG-93-184, 44 p. [5] Ohmoto, H. and Rye, R.O. 1979. Isotopes of sulfur and carbon. in Barnes, H.L., ed., Geochemistry of Hydrothermal Ore Deposits, 2nd edition: New York, John Wiley and Sons, p. 509-567. - 13 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Gold Grains, Pathfinder Elements, and Till Clast Composition in a Portion of the Vermilion Greenstone Belt, Northeastern Minnesota DAHL, David A., Lands and Minerals Division, Minnesota Department of Natural Resources, 1525 3rd Avenue East, Hibbing, MN 55746, dave.dahl@dnr.state.mn.us. An orientation project to determine variability of glacial till in the Mud Creek area of northeastern Minnesotaʼs Vermilion Greenstone Belt demonstrates that gold grains and pathfinder elements are present in basal till, and that anomalies stand out in contrast to regional background levels. Clastic and chemical variations within the till sample set are sufficient to consider using the basal till as sampling media for gold dispersal mapping. Of the thirty-two till samples analyzed, four were highly anomalous for gold, with counts of 88-1,282 gold grains per 10 kg of –2mm table sample, and pristine gold grain proportions up to 98%; up to 8,050 ppb gold in HMC (nonmagnetic heavy mineral concentrates); and up to 1,050 ppb gold in the -63µm silt/clay fraction of till. A suit of bedrock grab samples collected as reference mineralization returned assays up to 12,247 ppb Au, and silver concentrations up to 42,500 ppb. Analytical results for the till samples support a hypothesis that clastic dispersal trains of mineralized material exist in tills in the area. Within the project area, particulate gold is more anomalous in basal till samples than in the thin drape of overlying melt-out till. The gold grain counts and gold grain morphology add a transport distance value to chemical measurements of gold in soils and till, and suggest that the gold in the samples is locally derived. Pebble counts and pebble morphology similarly suggest a local derivation for the particulate gold. Analytical results for the present study are comparable to larger, more extensive regional evaluations conducted in neighboring Ontario and further confirm anomalous soil and fine fraction gold values reported in earlier Vermilion Greenstone Belt studies. Analysis of basal till in this portion of the Vermilion Greenstone Belt, particularly for gold grains, offers a capacity to positively detect local, previously unrecognized mineralization both inside and outside of areas of detailed bedrock geologic mapping. - 14 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Grenvillian Tomiko quartzites of Ontario: correlatives of the Baraboo quartzites of Wisconsin, the Mazatzal orogen of New Mexico, or unique? Implications for the tectonic architecture of Laurentia in the Great Lakes region EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca The Baraboo and 6 other correlative red quartz arenite sequences in the southern Lake Superior region were deposited between 1710 and 1630 Ma, and locally were affected by regional deformation and metamorphism at about 1630 Ma, as well as by potassium metasomatism at 1465 Ma (Medaris et al., 2003). The Baraboo and related quartz arenites may be correlative with quartz arenites in the Athabasca and Thelon basins (Medaris et al. 2003), indicating widespread sedimentation across much of North America in the late Paleoproterozoic. Key features of the Baraboo and Athabasca basin quartz arenites, include the local presence of well-developed paleosols, a high-degree of supermaturity of the quartz arenites, and chemical index of alteration (CIA) values of 95-99. These three features have been interpreted to indicate that profound chemical weathering occurred across much of North America between 1750 and 1630 Ma (e.g., Medaris et al., 2003). A thick sequence of paragneiss, including quartz arenite and iron formation (hereafter referred to as the Tomiko supracrustal rocks), occurs within the Tomiko terrane of the northern Central Gneiss Belt of the Grenville Province in Ontario. Holmden and Dickin (1995), based on Nd-Sm model age data and regional tectonics, suggested that the Tomiko quartz arenites were possible correlatives of the Baraboo and related quartz arenite sequences. If valid, then this correlation indicates that sedimentation and weathering between 1710 and 1630 Ma occurred east of Lake Superior, and that rocks of the Penokean orogen may underlie large parts of the northern Central Gneiss Belt in Ontario. Testing this correlation has been problematic, as until recently, only limited mapping and geochronological data were available for Tomiko terrane. Mapping during the 2003 field season (Easton, 2003) provides additional constraints for regional correlation, as follows: • Discovery of B-rich (>15% tourmaline) and Mn-rich rocks in association with magnetite-chert iron formation in northern Tomiko terrane, as well as epidote-rich (>20%) rocks in association with rusty gneisses near Crocan Lake and chemically unusual (high TiO2 & P2O5) calc-silicate rocks). • The recognition of possible dacitic metavolcanic rocks and sills interlayered with the Tomiko supracrustal rocks, along with amphibolites that may represent sills or flows. • The possibility that many of the muscovitic and feldspathic units within the Tomiko supracrustal sequence may represent hydrothermally altered rocks (CIA of 60 to 64). • The recognition of possible Archean “basement” rocks infolded with the Tomiko supracrustal rocks. This basement is likely structural, not depositional. • The identification of at least 3 metamorphic events affecting the Tomiko supracrustal rocks, with the intensity of the 2 latest events increasing to the southeast. M2 forms a migmatite front between northern and southern Tomiko terrane, with lower grade rocks to the north. M3 is a regional hydrothermal event, and results in the replacement of blue kyanite and granite leucosome formed during M2 by pale-green kyanite and quartz segregations. • Stratigraphic correlation with metasedimentary rocks of the Huronian Supergroup, exposed immediately to the north, is unlikely, based on geochemistry and Nd/Sm ages. On the basis on the new data, correlation of the Tomiko rocks with the Baraboo and related quartz arenites is still compelling. The Tomiko quartz arenites appear to be mature rather than supermature (CIA of 69 to 78 for quartzites), even though they are associated with metamorphosed volcanic and plutonic rocks and minor amounts of iron formation. It can be argued that kyanite-bearing and muscovite-rich gneisses in Tomiko terrane represent a combination of metamorphosed paleosols and potassium metasomatized quartz-rich sedimentary rocks, respectively. U/Pb zircon ages on detrital zircons from quartz arenite at two localities at different metamorphic grades give a maximum depositional age of 1687 Ma (Krogh, 1989; Easton and Kamo, 2004). Minimum - 15 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 depositional age is 1250±10 Ma, based on the age of grey gneisses interlayered with the quartz arenite; identical within error to the age of several A-type granite plutons that intrude the Tomiko supracrustal rocks (1257+4/-2 and 1244+4/-3 Ma; Easton and Kamo, 2004; Lumbers et al., 1991, respectively). If the correlation with the Baraboo is valid, then the Tomiko supracrustal rocks likely have limited mineral potential. Equally compelling, however, is that the Tomiko supracrustal rocks have many lithologic, stratigraphic and geochronologic similarities to Geon 17 rocks of the Mazatzal orogen exposed in Arizona and New Mexico (Tonto Basin Supergroup and Hondo Group, respectively). For example, the Hondo Group overlies a sequence of older greenstones, and consists of a thin basal unit of quartz pebble conglomerate, over 1 km of quartz arenite, and a capping sequence of muscovite schist and arenite, metarhyolite, phyllite and calc-silicate rocks (Robertson et al., 1993). A manganese-rich horizon, possibly a paleosol, perhaps lateritic, occurs at the top of the greenstones. Furthermore, some of the muscovite schists in the Hondo Group represented hydrothermally altered felsic volcanics. Rocks of the Hondo Group were deposited between 1700 and 1644 Ma (Robertson et al., 1993), and contain detrital zircons ranging in age from 1850 to 1700 Ma. A key difference is that plutons intruded the Hondo Group between 1680 and 1650 Ma, not at circa 1250 Ma. The rock types, stratigraphy, thickness, alteration history and age of the Tonto Basin Supergroup and the Hondo Group is similar to that observed in the Tomiko supracrustal rocks, even though the Mazatzal rocks in New Mexico are now some 3500 km distant. If correlation with the Mazatzal Orogen is valid, then the iron formation in northern Tomiko terrane may be equivalent to the manganese-rich unit at the base of the Tonto Basin Supergroup and the Hondo Group, and may have served as the decollément surface along which northward-directed thrusting occurred. If the Tomiko supracrustal rocks do represent a sliver of the Mazatzal Orogen, which at one time must have stretched from Arizona to Labrador, then the eastern end of the Penokean Orogen has been truncated in Ontario. A third possibility, that the Tomiko quartz arenites were deposited close to 1270 Ma, similar in age to the grey gneiss units interlayered with them, would mean that the sequence is unique in the Lake Superior region, and further begs the question of what happens to the Penokean Orogen east of the Grenville Front? References Easton, R.M. 2003. Reconnaissance study of the geology and mineral potential of the eastern Tomiko terrane, Grenville Province; in Ontario Geological Survey Open File Report 6120, p. 16-1 to 16-25. Easton, R.M. and Kamo, S.L. 2004. The Grenvillian Tomiko quartzites of Ontario: correlative with the Baraboo quartzites of Wisconsin or the Mazatzal orogen of New Mexico? Implications for the tectonic architecture of Laurentia in the Great Lakes region; Geological Society of America, Abstracts with Program, 36, no.5, p.A-459. Holmden, C. and Dickin, A.P. 1995. Paleoproterozoic crustal history of the southwestern Grenville Province; Canadian Journal of Earth Sciences, 32, 472-485. Krogh, T.E. 1989. Provenance and metamorphic ages in the Grenville (NW); in Lithoprobe Abitibi-Grenville Project Workshop, March 1989, p. 5-7. Lumbers, S.B., Wu, T-W, Heaman, L.M., Vertolli, V.M., and MacRae, N.D. 1991. Petrology and age of the A-type Mulock granite batholith, northern Grenville Province, Ontario; Precambrian Research, 53, 199-231. Medaris, L.G., Jr., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M. and Schott, R.C. 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and proto-North America: evidence from Baraboo interval quartzites; Journal of Geology, 111, 243-257. Robertson, J.M., Grambling, J.A., Mawer, C.K., Bowring, S.A., Williams, M.L., Bauer, P.W. and Silver, L.T. 1993. Precambrian geology of New Mexico; in Precambrian Conterminous US, Geological Society of America, The Geology of North America, Volume C-2, p. 228-238. - 16 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Stratigraphy of the Mesoproterozoic Sibley Group and Nipigon Sills FRALICK, Philip, METSARANTA, Riku and ROGALA, Becky, Department of Geology, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1 This paper is designed to complement papers on the geochemistry of the Sibley Group (Metsaranta and Fralick, this volume) and Nipigon Sills (Richardson and Hollings, this volume). It will review the existing nomenclature and highlight recent advances made in deciphering the stratigraphy of the Sibley Basin during field investigations forming a portion of the Lake Nipigon Region Geoscience Initiative. Rocks correlated with the Sibley Group are present primarily to the south and west of Lake Nipigon. They are divided into five Formations (Franklin et al., 1980; Cheadle, 1986; Fralick et al., 2000; Rogala, 2003) encompassing, at their thickest, approximately nine hundred meters of stratigraphic section. The basal Pass Lake Formation lies on metamorphosed igneous and sedimentary rocks, except for a limited area in the southwest where it overlies Paleoproterozoic strata of the Rove Formation. Evidence for the existence of small hills and valleys on the paleosurface is common. Age of the Pass Lake Formation is constrained by a Rb-Sr isochron on overlying dolomitic mudstones of 1339±33 Ma (Franklin, 1978), probably representing a diagenetic reset, and detrital zircon geochronology on a Pass Lake sandstone giving a concordant youngest zircon age of approximately 1650 Ma (see Heaman and Easton, this volume). The conglomerates and sandstones of this unit represent a number of depositional environments ranging from Scott-type and South Saskatchewan-type braided streams to cobble beaches, offshore storm sand sheets and possible aeolian dunes. The subaerial environments, where present, are situated at the base of the section, overlain by a flooding event. As the water depth increased the nearshore, storm deposited, sand sheets were succeeded upwards by laminated and massive silt (Rossport Formation). Delta progradation reversed this trend in areas proximal to sediment entry points. Caliche developed in underlying fluvial, off-channel deposits attests to a semi-arid climate and this caused the water body to become saline. Cyclically banded red and cream coloured dolomitic siltstone accumulated as salinity increased. The millimeterto decimeter-scale banding probably reflects periodic movement of the redox front from the water mass to beneath the sediment surface, possibly driven by organic loading. The central portions of thicker, light coloured layers contain bladed crystals and small nodules of gypsum attesting to peak aridity during oxygen minima in the bottom sediments. As the water mass contracted sand, possibly derived from the south, formed extensive sheet deposits in most areas. The strandline is marked by the sporadic development of algal flat deposits with carbonate silt storm layers. Though most of this one to seven meter thick calcareous unit is dominated by smooth mat and breccia small panicles and more classic stromatolites are present. Tepees and gypsum filling stromatactis are also encountered, though more rarely. The upper few centimeters of the carbonates is intensely weathered and commonly overlain by either an excellently developed terra rosa or subareal debris-flows. Regional computer modeling of thickness data indicates that this interval may represent initiation of north-south orientated, half graben formation in the area. The succeeding silt-dominated mudflat assemblage contains abundant nodular and vein gypsum, attesting to the continuation of dry conditions, albeit with a high water table. The transition from the Rossport to Kama Hill Formation is marked by a flooding event and re-establishment of open water conditions, though no evaporite minerals were precipitated from this water mass. Purple shales dominate the lower portion of the succession with thin ripple laminated, fine-grained sandstones, which become more numerous, thicker and coarser grained upward. Trough and hummocky cross-stratified , medium-grained sandstones (Outan Island Formation) appear and then dominate the section, continuing the coarsening upward trend. Slump scars and large areas of chaotic slide-block deposits attest to the periodic failure of, in places, extensive portions of the outbuilding delta complex. The delta top deposits consist of fining and thinning upwards sandstone successions meters to a few tens of meters thick (channels) interbedded with fine-grained sediments (floodplains). The floodplains are mostly composed of friable mudstone with soil horizons representing subareal accumulation but parallel laminated shales deposited in floodplain ponds are also present. The upper deltaic and fluvial deposits of the Outan Island Formation are only present in a core from a hole drilled in Nipigon Bay. - 17 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Detrital zircon geochronology performed on this unit gave a maximum age of deposition of approximately 1470 Ma (see Heaman and Easton, this volume). The Outan Island Formation is erosively truncated and overlain by the Nipigon Bay Formation. Planar cross-sets up to 17 meters thick indicate that the winds which deposited this sea of aeolian sand had a predominant direction from the northeast. The medium-grained sandstones of the Nipigon Bay Formation comprise the uppermost 400 meters of the Sibley Group with conglomerates of the basal Osler Group unconformably overlying them. At approximately 1109 Ma, diabase sills were intruded into the Sibley sedimentary rocks as the Midcontinent Rift established itself in the area. The southernmost region underlying Nipigon Bay has the most complete section of Sibley strata, but only one thin sill near the base of the assemblage. Sibley on the adjacent mainland contains a thick sill in the basal Kama Hill Formation, which is traceable across the southern portion of the basin. Thus, the thick sills are confined to north of Nipigon Bay with a fundamental divide running down this topographic feature. Moving from the southern to central basin a second, lower sill appears in the stratigraphy and sill width increases (toward Muskrat Lake) where their composite thickness is approximately 300 meters. Rossport Formation strata between the two thick sills in the central basin have been extensively thermally metamorphosed reaching temperatures of 600°C locally and 300-400°C tens of meters from the sills. The sills are commonly locally discordant, up- and down-ramping through the stratigraphy. They also appear to be vertically offset by faults, in places defined by surface lineaments. Toward the northern portion of the basin the erosion level cuts downward through the stratigraphy, with only the lower sill and a thin layer of underlying, Sibley-like sediment preserved. In the northernmost region this sill directly overlies a Mesoproterozoic volcanic-intrusive complex. To the east of the basin marginal Black Sturgeon Fault a sill overlies a thin, sporadically present, baked sedimentary layer lying on basement; a scenario similar to the northern area west of the Fault. Generally sill thickness decreases to the west of the central basin and the sills may bifurcate in this direction as well. An anomalous area exists in the western region, northwest of Muskrat Lake where a drillhole encountered a thick (500 m) sequence of diabase. The significance of this occurrence is unknown. Regional sill stratigraphy implies the feeder area was probably in the vicinity of Muskrat Lake and possibly related to the Black Sturgeon Fault system. References Cheadle, B.A., 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, vol. 23, p. 527-542. Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. Institute on Lake Superior Geology, Proceedings Volume46, Part 2: Fieldtrip Guidebook, yellow section. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Franklin, J.M., Mcilwaine, W.H., Poulsen, K.H. and Wanless, R.K., 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada Canadian Journal of Earth Sciences, vol. 17, p. 633-651. Rogala, B., 2003. The Sibley Group: a lithostratigraphic, geochemical, and paleomagnetic study. Unpubl. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 254p. - 18 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineral Resources for the Future: The Resource Potential of Northern Lake Superior FRANKLIN, James M., Franklin Geosciences Ltd., 24 Commanche Drive, Nepean, Ontario, K2E 6E9 The demand per person in the industrialized nations for primary “old - economy” metals has increased tenfold since 1900. Globalization in recent years has increased competitiveness and access to highly productive regions such as South America, Africa and Oceana, leading to an overall decline in metal prices of 40%. The demand for “new - economy” metals has increased 5 to 100 times since 1985; their prices have increased significantly. In the past five years, industrial expansion in China and India has created an almost unprecedented need for new metal resources. Even with recycling, vast quantities of new resources, particularly for “green” products, must be found, mined and processed in environmentally sustainable ways. The geological attributes of the Northern Lake Superior region provide an outstanding opportunity for exploration for an exceptional range of commodities and ore deposit types. Archean volcano-sedimentary strata with much additional potential include the historically productive Abitibi – Wawa and Wabigoon belts, with world – class volcanogenic massive sulfide deposits (Manitouwadge district), orogenic gold deposits (Geraldton) and a porphyry/epithermal district (Hemlo). Significant pegmatite bodies (Georgia Lake) and Algoma iron formations (Wawa) are also present. Early Proterozoic strata are comprised of an extensional and miogeoclinal sequence that includes the giant Gunflint Superior-type iron formation. .Mid Proterozoic intrusions associated with magmatic plumes (~1500-1600Ma) and the Keweenawan plume-intracratonic rift assemblage (1130-1090Ma) have proven potential for Cu-Ni-PGE deposits, Nb-REE in carbonatite, as well as potential for IOCG (REE-Zr) mineralization. Vein deposits that are contemporaneous with Keweenawan magmatism include Pb-Zn-Ba unconformity-vein deposits, two generations of Ag veins, and unconformity-style uranium occurrences. Epithermal and breccia – pipe copper deposits occur near Batchawana Bay. Redbed copper occurrences are associated with interflow sedimentary strata in the Osler volcanic sequence. Future discoveries require innovative exploration. Quantitative estimation of key geological attributes (e.g. magma fluid contents, paleo- permeability, and fault dynamics) must be applied vigorously in the search for ore. Geophysical techniques normally used for deep imaging of the earth (seismic, magnetotelluric) are being adapted for exploring shallow crystalline terrains. Developing quantitative models of ore forming processes that can be applied at all scales will ensure the supply of metals needed for the rapidly developing nations, and for improved quality of life everywhere. - 19 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Dyke swarms around the Lake Superior Region define ~2 Ga Proterozoic deformation of the Archean Superior Province associated with evolution of the Kapuskasing Zone HALLS, H.C., Department of Geology, University of Toronto, hhalls@utm.utoronto.ca Proterozoic dyke swarms in the Archean Superior Province straddle a ~2 Ga crustal dislocation, the Kapuskasing Zone (KZ), that extends for more than 500 km from James Bay to Lake Superior and divides the Archean Superior Province into eastern and western halves. About ten swarms are present ranging in age from the Matachewan, (2446-2473 Ma) to the Keweenawan (1108-1094 Ma). Nearly all the swarms include at least one reversal of the earthʼs magnetic field. During the Matachewan igneous episode only one reversal, from R to N, occurred at 2446 Ma. Within the KZ only N dykes are found, but outside, R dykes outnumber N by more than 4 to 1. N dykes are relatively common in the focal region of the swarm north of Lake Huron, but gradually disappear to the northwest with only R dykes being found in the Lake Nipigon region. The KZ can be divided up into a number of domains based on Matachewan dyke polarity, the boundaries between R and N domains being major faults that define the crustal uplift along the KZ. N polarity dykes within the KZ represent the roots of R dykes that have been remagnetized at depth before being uplifted. Using the polarity domain concept, major faults have been mapped that define a new segment of the KZ at its southwestern end (the Pineal Lake block), which is offset about 20 km sinistrally from the main Chapleau block to the NE1. The pattern of polarity domains follows exactly that produced by variations in feldspar clouding intensity within the dykes, whereby the clouding, (caused by exsolution of magnetite and other minerals in groundmass feldspars) occurs only in N polarity domains. The secondary magnetite, thought to be caused by slow cooling of the dykes at large crustal depths, increases in concentration with increasing depth. The growth of this magnetite led to the secondary N magnetization. Only a few dykes within the KZ show vestiges of their formerly R polarity1. When Matachewan dyke populations in regions of about 100 km in lateral dimensions were compared, a positive correlation was found 2 between mean Matachewan dyke trend and magnetization declination, a correlation that was absent between dykes within the areas. This result suggested that the variation in mean dyke trend was a product of differential crustal rotation about vertical axes, and therefore that the broad Z-shaped trend of the dykes across the southwestern end of the KZ was the result of the deformation of an originally more linear dyke swarm2. Further work on Matachewan dykes3, and also on younger swarms that have equivalents on both sides of the KZ (2170 Ma Biscotasing4, 2076 Ma Fort Frances/2069 Ma Lac Esprit5), confirm that the western half of the Superior Province has rotated counterclockwise about 20° with respect to the eastern half. More recent work on 2101-2121 Ma Marathon dykes6 suggests that the western half of the Superior Province has not rotated as a single unit because the KZ as a fault zone extends for more than 100 km to the northwest of the main zone of crustal uplift. The age of the rotation is thought to be about 2 Ga old and related to the deformation along the KZ. A possible consequence of the rotation is that a rift in the Superior province opened beneath Hudson Bay and remained a region of crustal weakness to be subsequently exploited in later Phanerozoic subsidence that formed the Hudson Bay basin and associated lowlands4. References [1] Halls, H. C., Zhang, B. 2003. Crustal uplift in the southern Superior Province, Canada, revealed by paleomagnetism. Tectonophysics 362, 123-136. [2] Bates, M. P., Halls, H .C. 1991. Broad-scale Proterozoic deformation of the central Superior Province revealed by paleomagnetism of the 2.45 Ga Matachewan dyke swarm. Canadian Journal of Earth Sciences 28, 1780-1796. [3] Halls, H. C., Stott, G. 2003. Paleomagnetic studies of mafic dykes in the vicinity of Lake Nipigon, northwestern Ontario. - 20 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 in Summary of Fieldwork and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 11-1 to 117. [4] Halls, H.C., Davis, D.W. 2004. Paleomagnetism and U-Pb geochronology of the 2.17 Ga Biscotasing dyke swarm, Ontario, Canada: evidence for vertical-axis crustal rotation across the Kapuskasing Zone. Canadian Journal of Earth Sciences 41, 255-269. [5] Buchan, K.L., Goutier, J., Hamilton, M., Ernst, R E., Mathews, W. 2004. Paleomagnetism of the Lac Esprit dykes and implications for crustal rotation of the Canadian Shield. AGU-CGU Meeting, Montreal [Abstract]. [6] Halls, H.C., Davis, D.W., Stott, G. 2005. Paleomagnetism and U-Pb dating of Proterozoic dykes: a new radiating swarm and an increase in post-Archean crustal rotation westwards from the Kapuskasing Zone, Ontario. GAC-MAC Halifax, Meeting, [Abstract]. - 21 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mesoproterozoic Diabase Sills of the Nipigon Embayment, northwest Ontario HART, T.R., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 (tom.hart@ndm.gov.on.ca) and MacDONALD, C.A., Caracle Creek International Consulting Inc., Sudbury, Ontario, P3E 2V7 The Nipigon Embayment is an approximately 19,000 km2 area of Mesoproterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior. The current extent of the Embayment is defined by the sporadic occurrence of diabase sills suggesting that the Embayment was probably more extensive prior to erosion. A series of shallow dipping, 1110 - 1113 Ma, diabase sills of the Nipigon Sill Complex (Heaman et al., 2005) intrude all Archean and Proterozoic rocks in the Embayment. The Nipigon Sill Complex is composed of at least 3 major, and a series of minor, diabase sills ranging in thickness from a few tens of metres to >200 m. The diabase is commonly sub-ophitic to ophitic, massive, medium- to coarse-grained feldspar and pyroxene with trace olivine and 1 to 2% magnetite. Medium- to coarsegrained diabase forming the majority of the sills should properly be classified as gabbro, but to avoid confusion with other intrusions in the area the diabase classification has been applied to all rocks associated with the sills. The sills are generally shallow dipping occasionally forming broad, up to 6 km wide, saucers with interiors occupied by older rock types (Figure 1). Figure 1. First vertical derivative of the total field magnetic data (Ontario Geological Survey, 2004) showing an inward dipping saucer shaped diabase sill in the southern Black Sturgeon River map area (Hart, 2005). The geometry of these saucers is in part evident from contact relationships observed during bedrock mapping (e.g. Hart, 2005), and from the magnetic patterns on the first vertical derivative of the total field airborne magnetic data (e.g. Ontario Geological Survey, 2004). Modelling a saucer located in the Garden Lake greenstone belt of the Wabigoon Subprovince, west of Lake Nipigon, produced two possible models (J. Rudd, Ontario Geological Survey, personal communication, 2000). However, the model of a shallow dipping thin sill best agreed with field observations, and is the best fit between field observations and geophysical data for similar structures in the Beardmore area (Hart et al., 2000) and in the southern Black Sturgeon River area (Hart, 2005). Similar sill geometry has been suggested for other mafic sill complexes (e.g., Thomson and Hutton, 2004), although the saucers and nested saucers structures in those areas are generally smaller in diameter. One reason for the large scale of the saucers in the Nipigon Embayment may be the more competent nature of the Archean basement rocks compared to the host rocks of other sill complexes. - 22 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The majority of the sills have relatively uniform mineralogy and geochemistry complicating the correlation of the sills within the Embayment. Minor stratigraphic variations within some sills suggest that they may have formed by injection of multiple pulses of magma. However, a thickening of some sills to 250 and 400 m in thickness in the Muskrat and Gieke lakes area, in combination with geochemical trends, suggests that these thicker sills are single cooling units, and may also indicate proximity to the feeders for the sills. The exception to the uniform geochemistry of the Nipigon sills is the >1120 Ma Inspiration Sill (Heaman and Easton, 2005) in the northwest portion of the Nipigon Embayment, which has higher La/Yb and Zr/Y ratios and a positive magnetic polarity in contrast to the negative polarity of the Nipigon sills (MacDonald, Tremblay and Easton, 2005). Another significant geochemical change occurs around Thunder Bay (Hart, 2004) where the majority of the sills located to the south of the city are part of the 1115±1 Ma Logan Sill Complex (Heaman and Easton, 2005). Similar geochemical variations related to geographic position have been observed in some flood basalt provinces (e.g., Mantovani et al., 1985) suggesting that the Logan and Nipigon sills may be part of a single intrusive event. References Hart, T.R. 2004. Geochemistry of the Proterozoic intrusive rocks of the Nipigon Embayment; abstract in Institute on Lake Superior Geology, Proceedings, 50th Annual Meeting, Duluth, Mn, v.50, pt.1, p.68-69. Hart, T.R. 2005. Precambrian Geology of the South Black Sturgeon River and Seagull Lake Area, Northwestern Ontario; Ontario Geological Survey, Open File Report 6165, 63p. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian Geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, Beardmore Area, Northwest Ontario. Ontario Geological Survey, OFR 6095, 206p. Heaman, L.M. and Easton, R.M. 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; abstract in Institute on Lake Superior Geology, Proceedings, 51ts Annual Meeting, Nipigon, Ontario, v.51, pt.1. Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy Annual Meeting, Toronto 2005, Program with Abstracts. MacDonald, C.A., Tremblay, E., and Easton, R.M. 2005. Precambrian Geology of the west-central map area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6164, 48p. Ontario Geological Survey 2004. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometer data, grid and vector data, ASCII format, Lake Nipigon Embayment Area; Geophysical Data Set 1047a. Thomson K. and Hutton, D. 2004. Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough; Bulletin of Volcanology 66, 364-375. - 23 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating HEAMAN, L.M., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca and EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 The Lake Nipigon Region Geoscience Initiative, a 2-year, science-focused, Industry-Government-University collaborative geological study, collected new geoscience data in the Lake Nipigon region of Ontario between May 2003 and May 2005. As part of this study, over 40 new U-Pb baddeleyite and zircon age determinations were acquired from predominantly mafic rocks of the Lake Nipigon region. Preliminary results of this study are reported herein. William Logan first described the copper-bearing rocks of the Midcontinent Rift (MCR) in Ontario in 1863. Subsequent studies by many researchers indicate that the MCR in Ontario comprises 3 main tectonic elements: 1) the volcanic and sedimentary rocks in and along the margin of Lake Superior, 2) associated alkalic complexes, 3), the mafic and ultramafic intrusions centred on Lake Nipigon, and whether they represent a “failed arm” of the main rift. Work in Ontario has tended to focus on the Lake Nipigon area, because of accessibility and mineral potential. The best estimate for a maximum depositional age of a greywacke from the upper Rover Formation is provided by a 1777 Ma concordant detrital zircon grain, as the youngest grain in the sample, at 1731 Ma, is 5% discordant. This interpretation is supported by the abundance of grains (n=10) between 1796 to 1777 Ma, and is consistent with U-Pb ash bed ages of 1836±5 and 1832±3 Ma from the basal Rove Formation (Addison et al., 2005). Detrital zircon ages from sandstones of the lower (Pass Lake Formation) and upper (Nipigon Bay Formation) Sibley Group indicate maximum depositional ages of 1634 and 1670 Ma, respectively, with a predominance of Geon 17 and 18, not Archean, detritus. Data from the middle to upper (Outan Island Formation) Sibley Group indicates a maximum depositional age of 1450 Ma, as well as Geon 15 and 17 detritus, but no Geon 18 grains. The Geon 15 detritus may be locally derived, e.g. from the 1547±4 Ma English Bay complex volcanic rocks, the 1590±1 Ma Pillar Lake felsic intrusion, and the 1599±1 Ma Pillar Lake gabbro. In addition, sediment interbedded with flat-lying mafic volcanic rocks south of Armstrong are younger than 1514 Ma, based on detrital zircons in interbedded sediments, but older than overlying, circa 1159 Ma (minimum age of 1120±1 Ma), Inspiration diabase sills. The abundance of Geon 15 ages in the western Nipigon Embayment is impressive, as the period between 1600 and 1520 Ma in eastern North America has been recognized for some time as a period of quiescence throughout Laurentia (Gower et al., 1990). Furthermore, there are no dike swarms or reliable paleopoles for the period 1700 to 1500 Ma. The only other reliable Geon 15 age in North America is the 1576+/-13 Ma upper intercept age from the Priest River metamorphic complex in Idaho, which served as basement to the circa 1450 Ma Belt and Purcell Supergroups (Evans and Fischer, 1986). Gower (1996) suggested that, at least in the eastern Grenville Province, this quiescence might be linked with the development of a passive continental margin, however, such a setting does not explain the felsic magmatism present in the Nipigon area. The new U-Pb data obtained in this study suggest the presence of an earlier period of magnetically normal, localized alkalic (lamprophyre dikes, Queen et al., 1996) and mafic magmatism (Inspiration sills and the Pigeon River dikes (1141±20 Ma)) between 1150 and 1135 Ma in the MCR in Ontario, including the possibility of localized volcanism and sedimentation in the Pillar Lake area south of Armstrong. Baddeleyite data from the 4 ultramafic intrusions in the region shows some scatter within individual - 24 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 intrusions (e.g. from 1113 to 1124 Ma in the Seagull intrusion), which could indicate a protracted (up to 10 m.y.) crystallization history for some of these bodies. In addition, the suite of ultramafic intrusions has a range of ages from circa 1118 to 1107 Ma, however, the aforementioned scatter makes it difficult to ascertain the significance of this range – does it reflect protracted cooling or different emplacement times, or both? A related ultramafic body, the Jackfish Island sill, cuts the English Bay complex and is geochemically similar to the Kitto intrusion, although it appears to be slightly younger in age (1112±3 Ma versus 1118±2 Ma, respectively). In contrast, baddeleyite ages from 4 different Nipigon diabase sills analyzed so far cluster between 1110 and 1114 Ma. Although geochemically distinct from the Nipigon sills (Hart, 2003), a sample from the Logan sill at Mount McKay in Thunder Bay, is similar in age, at 1115±1 Ma. Thus, most mafic and ultramafic rocks in the Lake Nipigon and Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, internal between 1115 and 1100 Ma. Emplacement of alkalic intrusions, such as the Coldwell complex, as well as filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake Lake (1095±2 Ma) gabbros, and the Arrowhead dike (1093±3 Ma). References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J. Davis, D.W. Kissin, S.A. Fralick, P.W. and Hammond A.L. 2005. Discovery of detrital ejecta from the 1850 Ma Sudbury impact event; Geology, 33, 193-196. Evans, K.V. and Fischer, L.B. 1986. U-Pb geochronology of two augen gneiss terranes, Idaho – new data and tectonic implications; Canadian Journal of Earth Sciences, 23, 1919-1927. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwest Ontario; Institute on Lake Superior Geology, Proceedings, 49, pt.1, Programs and Abstracts, 21-22. Gower, C.F. 1996. The evolution of the Grenville Province in eastern Labrador Canada; in Precambrian crustal evolution in the North Atlantic Region, Geological Society of London, Special Publication 112, 197-218. Gower, C.F., Ryan, A.B. and Rivers, T. 1990. Mid-ProterozoicLaurentia-Baltica: an overview of its geological evolution and a summary of the contributions made by this volume, in Mid-Proterozoic Laurentia-Baltica, Geological Association of Canada, Special Paper 38, 1-20. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dikes from the eastern Lake Superior region: evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, 33, 958-965. - 25 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Penokean Fold-and-thrust Deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario HILL, Mary Louise, Department of Geology, Lakehead University, Thunder Bay ON P7B 5E1 CANADA and SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Flat-lying, Paleoproterozoic Gunflint Formation chemical and clastic sedimentary rocks unconformably overlie Archean basement rocks near Thunder Bay. Folds have long been recognized in the Gunflint Formation near Pass Lake, 40 km northeast of Thunder Bay, but no tectonic explanation has previously been demonstrated. The recent recognition of thrust faults in this area appears to link this folding to Penokean compression. Although Penokean orogenic events (circa 1875 to 1835 Ma) and resultant deformation have been well established south and west of Lake Superior, no structures ascribed to Penokean deformation have ever been described north of Lake Superior. Recent geochronologic data show that the Gunflint Formation (circa 1878 Ma) predates the earliest Penokean thrusting and thrust-loading caused by the collision of the Pembine-Wausau terrane (circa 1860-1850 Ma). This prompted Fralick et al. (2002) to contend that the Gunflint formed in a back-arc extensional setting, rather than in a foredeep (foreland basin) and also suggests that Penokean structures should exist in these rocks. Recent examination of the Gunflint Formation near Pass Lake has led to the recognition of structures typical of fold-and-thrust belt deformation. Discrete bedding-plane faults with locally developed gouge and breccia can be traced laterally into horizontal, hanging-wall ramps with associated fault-bend folding. Fold-and-thrust belt deformation is caused by regional compression. Previous workers had ascribed the folds to syn-sedimentary slumping and Keweenawan diabase sill emplacement and thought that they were attributable to local, rather than regional-scale, deformation. Displacement in fold-and-thrust belts tends to be localized along discrete bedding planes and not easily recognized. This may account for the perceived lack or absence of structures elsewhere in the Gunflint Formation. Penokean structures on the northern side of Lake Superior represent the northward migration of thrust faults into the foreland (passive margin Archean basement + Gunflint Formation) caused by hinterland collision to the south. Future work will focus on recognizing other deformed locales and quantifying fault displacements based on stratigraphic correlation of sedimentary units. References Fralick, P., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U–Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Science, v.39, p.1085–1091. - 26 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The incredible shrinking Penokean orogen: a new look at the accretionary history of the southern Lake Superior region HOLM, D.K., Dept. of Geology, Kent State University, Kent, OH; CANNON, W.F., USGS, Reston, VA; CHANDLER, V.W., MGS, Mnpls, MN; SCHNEIDER, D.A., Ohio University, Athens, OH; SCHULZ, K., USGS, Reston, VA; and VAN SCHMUS, W.R., University of Kansas, Lawrence, KS. The Penokean orogen in the stable continental interior of the upper Great Lakes region records the initiation of rapid late Paleoproterozoic (1.9-1.6 Ga) southward growth of Laurentia. The Penokean orogeny is generally accepted to have been a period of southward subduction from about 1900 to 1850 Ma during which juvenile Paleoproterozoic terranes were accreted to the southern edge of the Superior craton (Archean) and foreland sedimentation and deformation occurred on the craton margin. Historically, juvenile Penokean crust was interpreted to extend southward beneath Paleozoic and Cretaceous strata into southern Iowa, southern Wisconsin, and northern Illinois. However, a growing body of geochronologic data suggests that there may be no rocks older than 1.75 Ga in those areas and that the current Penokean orogen has a much more restricted geographic distribution than heretofore inferred. We have identified a fundamental ENE-trending boundary, the Spirit Lake-Trempealeau discontinuity CB: Cheyenre Belt GFTZ: Great F alIsTectoniozone (SLTD), using a combination of potential field geophysical GLTZ: Great LakesTecton Zore data and drill hole information (see Figure 1), which marks MSM: Mojave-Sonora Megashear NFZ: Niagara F ault Zone the southern limit of Archean and known Penokean rocks. SLTD: Spir Lake-Trempealeau South of this boundary rocks at the subcrop are dominated Dbonnuii 300 km by subaerial potassic rhyolite and epizonal granite, formed at about 1.75 Ga, and ultra-mature quartzite, such as the Baraboo Quartzite, which lies unconformably on them. Gneisses and mafic volcanic rocks, probably basement rocks from which the rhyolites formed by partial melting, are inferred from gravity and magnetic highs to be at subcrop in several areas. Because the SLTD transects geon 18 Penokean structures, it appears to be a post-Penokean feature perhaps marking the northern margin of a Yavapai age accreted terrane. It lies directly above an abrupt offset in Moho depth (deeper to the south) beneath Lake Michigan identified in GLIMPCE deep seismic surveys. The offset was previously interpreted as a north-dipping Penokean subduction zone, but it now seems more likely to be a Yavapai feature. We interpret the SLTD to be a fundamental Yavapai-age Proterozoic boundary, equivalent to the Cheyenne belt paleosubduction zone in N southern Wyoming. The Cheyenne belt juxtaposes geon Figure 1. Map showing major Precambrian crustal 17 Yavapai orogen crust on the south against the Archean Map showing major Precambrian crustal *je provinces age provinces and tectonic boundaries in the western Wyoming craton, and transects geon 18 (Trans-Hudson) and tectonic Ixundaries in thewestem and central and central United Stated United States. structures in southern South Dakota. The interpretation shown here suggests that the Cheyenne suture extends eastward, joins with the Spirit Lake trend in Iowa, and continues east across the Midcontinent Rift as the Trempealeau discontinuity in Wisconsin. Geon 17 plutonic rocks north of the SLTD, and a north-dipping crustal reflection across it, are consistent with intervals of north-directed subduction during Yavapai accretion. Younger, Mazatzal-age compression deformed quartzites that overlie the Yavapai, Marshfield, and Penokean terrane rocks indicating that geon 16 deformation extended far north of the still imprecisely defined Yavapai-Mazatzal terrane boundary. Barring major strike-slip motion, our revised tectonic map suggests progressive accretion of juvenile arc terranes every 100 million years or so during the late Paleoproterozoic (circa 1.85, 1.75, and 1.65 Ga). - 27 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geologic Implications of Bedrock Mapping in the Ely and Basswood Lake Quadrangles, Northeast Minnesota JIRSA, Mark A. and MILLER, James D., Jr., Minnesota Geological Survey (jirsa001@umn.edu and mille066@umn.edu) Two regional bedrock geologic compilation maps of northeast Minnesota at scale 1:100,000 have recently been completed by the Minnesota Geological Survey (Figure 1), funded in large part by the STATEMAP program of the U.S. Geological Survey. The most current—described here—covers the Ely and U.S. portion of the Basswood Lake 30ʼ x 60ʼ quadrangles (Jirsa and Miller, 2004). The compilation integrates new mapping, unpublished work contained in 7 theses, and the classic Knife Lake maps of Gruner (1941), into a lithostratigraphically consistent, digital (GIS) format. Much of the map depiction of Paleoproterozoic (Animikie Group) and Mesoproterozoic rocks (Duluth Complex and North Shore Volcanic Group) is not changed substantively from recent works by Miller and others (2001). By contrast, the Archean geology is considerably modified from earlier geologic compilations at 1:250,000 scale (Green, 1982). The area encloses the central part of the Boundary Waters Canoe Area Wilderness (Figure 2), and is the first new publication of mapping of that area in many decades. . 9 7?:. -d _______ International Falls -r—,,,•,•.-,,,,,,,•,fl,, - • VERMILION LAKE Quadrangle - . - 92 7, ELY-BASSWOOD LAKE Quadrangles MESOPROTEROZOIC Duluth Complex and North Shore Volcanic Group PALEOPROTEROZOIC Virginia-Thomson Formations Biwabik Iron Formation NEOARCHEAN Granitoid batholiths and plutons I Schist of sedimentary protolith Metavolcanic and metasedimentary rocks Duluth Figure 1. Generalized geologic map of northeastern Minnesota showing the location of recently mapped 30ʼ x 60ʼ quadrangles; Jirsa and Boerboom, 2003, and Jirsa and Miller, 2004. Exposures provide what is likely the most complete view of Archean geology anywhere in Minnesota. The Neoarchean bedrock lies within the Wawa and Quetico subprovinces of the Superior Province (Figure 3). A third, distinctive sequence of rocks—the Knife Lake Group—is interpreted to have been deposited in a complex array of successor basins developed along early-formed faults near the boundary between the two subprovinces. The Knife Lake strata include hornblende-bearing alkalic and calc-alkalic volcanic flows and unusual hornblenderich tuff, conglomerate derived from multiple sources including the 2,689 Ma Saganaga Tonalite, and feldspathic graywacke and mudstone. Much of the temporal distinction between various geologic elements of the Archean bedrock is based on fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3. All three deformation events are the result of north–south- to northwest–southeast-directed compression. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks of the Wawa subprovince at about 2,722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at about 2,689 Ma (Corfu and Stott, 1998). Folding in volcanic rocks of the Ely Greenstone attributed to D1 deformation is truncated by Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely Greenstone. As such, the Knife Lake Group is inferred to be a Timiskaming-type sequence temporally - 28 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Corfu and Stott, 1998). D2 deformation affects all of the Archean supracrustal units and is bracketed by U-Pb dates of intrusions in the Giants Range batholith that place the regional deformation and metamorphic event between about 2,674 Ma and 2,685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults in the low-grade supracrustal and intrusive rocks of the Wawa subprovince, and folding of granitic and migmatitic rocks in the Quetico subprovince. Saganta Tonallie lUSCr L9' BWCA DULUTH COMPLEX 4sorthosItIc sMn Figure 2. Schematic geologic map of the Ely and Basswood Lake 30ʼ x 60ʼ quadrangles showing major lithologic components, including subdivisions of rock units in the Archean bedrock, Paleoproterozoic rocks of the Mesabi Iron Range, and intrusions of the Mesoproterozoic Duluth Complex. f_i / / South KawlshIw- // BIdEgI C--;' jMASI IRON RANGE - - - - - - /'vr > Partndge F Greenwood , / Beaver I Comple: ,--4' / SHORE Ey-Bassswoad L Th:, SO. x 60 qJ3]. SHEBANOOWAN %. GRtENSIONE BELT _RMPLION (!It!pI@ VERMIUON SHEBANOOWAN T!k!!Q.Lypth&& I(NIFE LME SHEBANOOWN qees GROUP ASSEMBLAGE ft]'Y NEWTOn LAKE GREEN WATERJBURCHELL SobprQWEs M@ISi L—J Ok.-thd€ih-kIIi& Ia 1114 oy& {271&27M} ELYGREENSTONE M) Figure 3. Generalized geologic map showing potential stratigraphic correlation between Archean rocks of the Vermilion district in Minnesota and those of the Shebandowan greenstone belt in Ontario. Shaded areas are lakes. Abbreviated References Boerboom, T.J., and Zartman, R.E. 1993. Canadian Journal of Earth Sciences 30:2510-2522. Corfu, F., and Stott, G.M. 1998. Geological Society of America Bulletin 110:1467-1484. Green, J.C. 1982. Minnesota Geological Survey, Two Harbors Sheet; scale 1:250,000. Gruner, J.W. 1941. Geological Society of America Bulletin 52: 1577-1642. Jirsa, M.A. and Boerboom, T.J. 2003. Minnesota Geological Survey Map M-141, scale 1:100,000. Jirsa, M.A., and Miller, J.D., Jr., 2004, Minnesota Geological Survey Map M-148, scale 1:100,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W. and Peterson, D.M. 2001. MGS Map M-119, scale 1:200,000. Peterson, D.P., Gallup, C., Jirsa, M.A. and Davis, D.W. 2001. ILSG, 47th Annual Meeting, Proceedings, p. 77-78. - 29 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Structural State of Plagioclase Phenocrysts in Porphyritic rocks of the Midcontinent Rift, Northeastern Minnesota KNUDSEN, Damion, Department of Geosciences, North Dakota State University, Fargo, ND 58105, damion.knudsen@ndsu.edu; SAINI-EIDUKAT, Bernhardt, Department of Geosciences, North Dakota State University, Fargo, ND 58105, MILLER, James D., Jr., Minnesota Geological Survey, University of Minnesota, c/o Natural Resources Research Inst., 5013 Miller Trunk Hwy, Duluth MN 55811, and DANIELS, Peter, Markstrasse 123, 44803 Bochum, Germany. The structural states of plagioclase phenocrysts from porphyritic rocks of the Midcontinent Rift in northeastern Minnesota were analyzed to determine the origin of the plagioclase and to evaluate possible petrogenetic relationships between rock units. As a general rule, higher states of structural disorder in the crystal lattice of plagioclase reflect higher temperatures of equilibration. The units studied include hypabyssal sills of the Beaver Bay Complex (BBC), anorthositic rocks of the Duluth Complex, and porphyritic basalts of the North Shore Volcanic Group (NSVG). The NSVG is a 7-10 kilometer thick volcanic edifice composed of flood basalts and minor felsic volcanics, intermediate lavas, and interflow sandstones (Green, 1972). The BBC is composed of at least thirteen hypabyssal intrusions, with widely varying compositions, that were emplaced in the medial section of the NSVG (Miller and Chandler, 1997; Miller et al., 2002). The chemistry and texture of plagioclase phenocrysts in some porphyritic NSVG basaltic flows and BBC intrusives suggest they are related to plagioclase-rich magmas (mushes) that are thought to have produced the anorthositic series rocks of the stratigraphically deeper, more plutonic Duluth Complex (Miller and Weiblen, 1990). Plagioclase phenocrysts were separated for XRD using common magnetic and heavy liquid methods. Patterns were collected in 0.02° step scan mode on a Philips Xʼpert PW 3040-MPD diffractometer using 2θ from 18° to 55°, 5 sec/step, and λ =1.54178 Å. Refined unit-cell parameters were used to approximate plagioclase Al, Si distributions using Kroll and Ribbeʼs (1980) γ method, and a derivative method using t10-<t1m> vs. anorthite content (An). An contents were determined by electron microprobe at the Department of Geology and Geophysics, University of Minnesota-Twin Cities. The rock types analyzed for this study are: (a) plagioclase porphyritic, ophitic basalts from near Silver Bay and Croftville, (b) plagioclase porphyritic ferrodiorites of the Cabin Creek and the Leveaux Porphyry intrusions (early intrusions of the BBC), (c) porphyritic leucogabbros and gabbroic anorthosites of the Scott Creek and Katydid Lake intrusions that occur near the transition between the BBC and Duluth Complex (both are thought to represent offshoots of the anorthositic series; Boerboom and Miller, 1994); and (d) a porphyritic diabase dike, possibly an offshoot of the main Scott Creek intrusion. The phenocrysts in these rock types are compared with anorthosite xenoliths occurring in the Beaver River diabase (Knudsen et al., 2005) and with anorthositic series rocks of the Duluth Complex (Bandli and Saini-Eidukat, 2000). XRD results (Figure 1) show the Croftville basalt phenocrysts, along with those of the porphyritic dike, have intermediate to high disorder, plotting near the curve for disordered plagioclase. In contrast, the Silver Bay basalt phenocryst has intermediate to low structure. The Leveaux Porphyry phenocrysts have intermediate structures and plot near Duluth Complex anorthositic series samples, while the Cabin Creek phenocrysts have distinctly more disordered structures. Plagioclase phenocrysts from the Scott Creek leucogabbro and the Katydid Lake gabbroic anorthosite are intermediate to ordered and plot within the field of Duluth Complex anorthositic series samples. Anorthosite xenoliths in the BBC are intermediate to ordered. Plagioclase separated from the matrix of selected samples all have lower An contents than the phenocrysts - 30 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 they host, and show intermediate to high disorder. The relationship of the matrix t10-<t1m> values to those of the phenocrysts they host reverses in a manner similar to that noted by Hoffer (1968) for the Rock Creek porphyritic basalt flow of Idaho. Figure 1. Order distributions of measured plagioclases between the various rock types. A. γ vs. An, after Kroll and Ribbe (1980). B. derivative plot of t10-<t1m> vs. An. Abbreviated References Albers, P. and Miller, J.D., Jr.,2005. (this volume). Bandli, B. and Saini-Eidukat, B. 2000. Proc. ILSG, v. 46, part 1: 4-5. Boerboom, T.J., and Miller, J.D., Jr. 1994. MGS Misc. Map M-81. Knudsen, D., Saini-Eidukat, B., Miller, J.D., Jr., Daniels, P. 2005. GSA–N. Centr, Abstr., Mpls, MN, May 19-20. Green, J.C. 1972. in Geology of Minnesota – A centennial volume: 294-332. Kroll, H. and Ribbe, P.H. 1980. Am. Min. 65: 449-457. Hoffer, J.M. 1968. Am. Min. 53: 908-916. Miller, J.D. Jr. and Chandler, V.W. 1997. in GSA Sp. Pap. 312: 73-96. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. MGS RI 58, 207 p. - 31 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Petrogenesis and PGE mineralization of the Eva-Kitto Intrusion, northern Ontario LAARMAN, J. and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On, P7B 5E1, jlaarman@lakeheadu.ca The Eva-Kitto Intrusion is a Mesoproterozoic semicircular peridotitic ring complex situated in the Nipigon Embayment on the eastern shore of Lake Nipigon near Beardmore, Ontario. The age of the intrusion is 1117.7±1.8 Ma (L.Heaman, University of Alberta, personal communication, 2005) and has been interpreted to be emplaced as the feeder conduit to a more evolved cone sheet during initial asthenospheric upwelling of a mantle plume in the Midcontinent rift system (MCR) (Sutcliffe, 1986). The intrusion cross-cuts both the Archean Southern Sedimentary Belt (SSB) and the Southern Volcanic Belts (SVB) of the Beardmore-Geraldton Greenstone belt, dated at circa 2696-2691Ma, of the Wabigoon Subprovince (Lafrance et al., 2004). Within the SVB are sulphidized magnetite-chert iron formations that run parallel to the strike of the main structural trends of the greenstone belt and are intercalated with mafic volcanic units. In drill core, the intrusion cross-cuts calcareous sandstones and mudstones interpreted to be the Sibley metasediments and is cross-cut by Nipigon gabbro sills which are also found to crop out as horizontal circular ridges around the intrusion. The Eva-Kitto peridotite has high TiO2, K2O and P2O5 and is LREE-enriched similar to the chemistry of the Fe-Ti Osler Volcanics of the MCR (Hart et al., 2002). The sills, on the other hand, have lower TiO2, K2O and P2O5 than the ferropicritic peridotites, and are similar in chemistry to the olivine tholeiitic flood basalts of the MCR. Therefore the Nipigon sills originated from a more fractionated underplated basaltic magma later in the Keewanawan event circa 1109 Ma (Sutcliffe, 1987). j... The petrography and geochemistry of the lithologies within the Eva-Kitto Intrusion are being studied from drill core held by East West Resources Inc. Four holes were drilled by Kennecott Canada Exploration Inc. in 2002 Lake N4Igon based on airborne MegaTEM anomalies surveyed by Fugro Airborne Surveys that same year. Hole EK-2 intersected disseminated po-cpy mineralization and was the only hole to intersect magnetiferous pyritic metasediments of the Archean basement. Assays values up to 0.28%Ni, 0.13%Cu, and 563ppb Pt+Pd in a 1.22m interval have been reported (Rossell, 2003). Lithologies within EK-2 from the top to the end of hole at 345m at depth comprise lherzolite, olivine websterite, sulphidized olivine websterite, vari-textured pyroxenite, spotted pyroxenite, pyroxenite, melanogabbro, magnetiferous pyritic metasediment, and mafic volcanics. EK-2 is the only hole to intersect the more fractionated varitextured pyroxenite to orthocumulate pyroxenite lithologies that contain PGE mineralization. .::L: Figure 1: Geological map ofthe Eva Kitto Ring Complex with its Figure 1. Geological map of the Eva Kitto Ring location circled on the Lake Nipigon inset. The four ER drill holes Complex. four EK drillofthe holes are located at the are located atThe the southern extent peridotitic intrusion. southern end of the peridotitic intrusion Based on preliminary interpretation of the petrography the petrogenesis of the Eva-Kitto Intrusion can be characterized by two magmatic pulses. A first pulse of magma is suggested by the pyroxenite and melanogabbro units of EK-2. The pyroxenite has an orthocumulate texture with cumulate pyroxenes enclosed in a groundmass of lathy plagioclase. The melanogabbro has a poikilitic texture with cumulate pyroxene crystals housed in plagioclase oikocrysts. A second pulse of magma is suggested by the incipient break up of cumulate - 32 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 pyroxene and growth of clusters of secondary pyroxene in the vari-textured and spotted pyroxenites. The patchy texture within these units can be attributed to the churning up of the pyroxenite by a second influx of magma. Lherzolite is the dominant lithology in all the drill holes and may represent the primary cumulate fractionation of an enriched ferropicritic magma (Sutcliffe, 1986). Lherzolite displays a mesocumulate texture of ovoidal cumulate olivine grains and larger cumulate clinopyroxene within poikilitic orthopyroxene and plagioclase. The olivine websterite is darker due to fractionation of pockety pyroxenes with breakdown of olivine. The olivine websterite possibly formed by flow differentiation of a more fractionated magma from the central lherzolite flow zone. Both the lherzolite and olivine websterite contain large amounts of primary biotite minerals. PGE minerals are associated with disseminated sulphides in the sulphidized olivine websterite, vari-textured pyroxenite, pyroxenite and melanogabbro units of drill hole EK-2. Within the olivine websterite and pyroxenite, disseminated pyrrhotite-pentlandite-chalcopyrite sulphides are found interstitial to cumulate olivine and pyroxene minerals. Blebby pyrrhotite-chalcopyrite sulphides, possibly a result of liquid immiscibility, are locally found in the sulphidized olivine websterite/melanogabbro units at the contact with the wallrock metasediments. Ongoing studies are focusing on petrogenesis and mineralization of the intrusion. Scanning electron microscope (SEM) discs made from drill core in Holes EK-1, 3 and 4 are being analysed under the petrographic microscope to compare lithologies in these holes with the fractionated suite of EK-2. The interstitial sulphides will be examined by SEM to identify the PGE minerals. Mineral chemistries within the various lithologies will also be identified using SEM. Bulk rock chemistries, S/Se elemental ratios, trace elements, εNd and Sr patterns, and Ni abundances of olivine will be analysed to further understand the petrogenesis of source magmas, and crustal contamination involved with mineralization. References Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra townships, northwestern Ontario: Phoenix Bedrock Mapping Project. Ontario Geological Survey, Open File Report 6095, 206p. Lafrance, B., DeWolfe, J.C., Stott, G.M. 2004. A structural reappraisal of the Beardmore-Geraldton Belt at the southern boundary of the Wabigoon subprovince, Ontario, and implications for gold mineralization. Canadian Journal of Earth Sciences 41, 217-235. Rossell, D. 2003. November 2003 Report on Diamond Core Drilling on the Eva Kitto Property. Kennecott Canada Exploration Inc. Sutcliffe, R.H. 1986. Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. Sutcliffe, R.H. 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96, 201-211. - 33 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geochemistry and Petrography of the Rabbit Islands Breccia, North Central Lake Nipigon LANE, C. J. and HOLLINGS, P., Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. cjlane@lakeheadu.ca The Rabbit Islands breccia is a dyke-like intrusion, rich in calcite and dolomite, which intrudes the Nipigon diabase on the southern most island of the Rabbit Islands group in north central Lake Nipigon (Figure 1). Geological relationships indicate that the breccia must represent one of the youngest events associated with the Midcontinent rift. In the past this breccia has been referred to as a carbonatite diatreme (Sutcliffe and Greenwood, 1982), and one of the objectives of this study was to evaluate this model and to compare the breccia to the carbonatites of the Coldwell complex. Whole-rock geochemistry, petrography and scanning electron microscope (SEM) data, indicate that the groundmass material of this breccia is not of carbonatite affinity. The low abundance of trace elements such as Sr, Mn and Ba in SEM semi-quantitative scans of the carbonates and feldspars combined with the low abundances of Sr, Ba, V, and rare earth elements (REEʼs) determined by XRF and ICP-MS, do not satisfy the requirements of Samoilov (1991). Samoilov suggested that in order to be considered a carbonatite the following criteria must be met: enrichment in Sr (>700 ppm), Ba (>250 ppm), V (>20 ppm) and REE (including Y; >500 ppm). The samples analyzed from the Rabbit Islands breccia groundmass contained 191 ppm Sr, 286 ppm V, 115 ppm total REEʼs and low amounts of barium. Consequently, the breccia does not meet the limits set by Samoilov (1991) and therefore is not considered to be of carbonatitic affinity, but rather a carbonate-rich breccia. Evidence for changes in fluid composition has been observed within the groundmass material. For example, in many instances calcite has been shown to brecciate and replace dolomite crystals present in the breccia, however, quartz and feldspar brecciate and replace the dolomite. Other textures, such as calcite replacing feldspar emphasize the fact that there is an overlap of fluid phases, which may be controlled by fluid source or contamination by the (0 •0 (a C 2. a 0= = -, @02 EQ (a e. 0 = o (0 0 -o 'C 2o o = —. == OW0 ao = (a0 (a 0 'C wmz The geochemistry of the clasts and the country rock associated with the breccia has been compared with the different suites of Nipigon diabase: the Inspiration suite, the Jackfish suite, the McIntyre suite and the Main Nipigon suite (MacDonald and Tremblay, 2004; Rabbit Islands Richardson and Hollings, 2004), showing that the breccia intruded into the Main Nipigon suite (Figure 2). Clasts of diabase found within the breccia have undergone extensive alteration. Many of the clasts display consecutive zones of alteration which can be seen in both hand sample and thin section. Elements which have become mobile within the altered clasts, and show variation between each alteration zone include: potassium, calcium, magnesium, strontium, europium, yttrium, lanthanum, and neodymium. Depletion of calcium, strontium, europium, yttrium, lanthanum and neodymium can be seen from the centre of the clast outwards. Whereas, potassium, magnesium and loss on ignition (due to the presence of chlorite) increase from the centre of the clast outwards. Figure 1. Location of the Rabbit Islands (Modified from Hollings et al. 2004). - 34 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Rabbit Islands i><+><i Main Nipigon Suite RbThUNbKTaLaCePrSrNdZrHfSmEuTGdThDyYHoErTmYbALuVSc Figure 2. Primitive mantle multi-element plot to show the similarities between the Rabbit Islands Diabase and the Main Nipigon suite. country rock. References Hollings, P., Fralick, P., Kissin, S. 2004. Geochemistry and Geodynamic Implications of the Mesoproterozoic English Bay Granite-Rhyolite Complex, Northwestern Ontario. Canadian Journal of Earth Sciences, 41, 1329-1338. MacDonald, C.A. and Tremblay, E. 2004. Lake Nipigon Region Geoscience Initiative: Results of bedrock mapping in the northern part of the western Nipigon Embayment, northwestern Ontario, Canada; oral presentation abstract, Institute on Lake Superior Geology, Proceedings, v. 50, pt. 1, Programs and Abstracts p.102-103. Richardson, A.J. and Hollings, P. 2004. Lake Nipigon Region Geoscience Initiative: Geochemistry and Mineralogy of the Nipigon Diabase Sills. Summary of Field Work and Other Activities 2004. Ontario Geological Survey, Open File Report 6145, p.51-1 to 51-5. Samoilov, V.S. 1991. The Main Geochemical Features of Carbonatites. Journal of Geochemical Exploration 40, 251-262. Sutcliffe, R.H. and Greenwood, R.C. 1982. Geology of the Lake Nipigon Area. In Summary of Field Work 1982. Ontario Geological Survey, Miscellaneous Paper 106, 19-23. - 35 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 MetalCORP Ltd. Big Lake Ni-Cu-PGE, Cu-Zn-Ag, and Mo Property MACTAVISH, Allan, MetalCORP Ltd., 309 South Court Street, Thunder Bay, ON, P7B 2Y1, Canada. amactavish@metalcorp.com The Big Lake Property of MetalCORP Ltd. of Thunder Bay, Ontario comprises 33 claims (365 units totaling 5840 hectares) and is located approximately 230 km east-northeast of the City of Thunder Bay and 18 km southeast of the Town of Marathon in Northern Ontario, Canada. Work completed by MetalCORP since early 2004 includs a MegaTEM airborne survey, detailed prospecting (677 samples), linecutting, ground pulse-EM surveys, and 2 phases of diamond drilling, totaling 22 diamond holes (5672 m). This work resulted in the discovery of 4 previously unknown mineralized zones that represent 3 separate and distinct mineralization styles. The mineralized zones include: the J4 and J5 Pt-Pd reefs within the Big Lake Ultramafic Complex; the A2 Ni-Cu Zone within the Gus Creek Mafic Intrusion; and the BL14 Cu-ZnAg Zone within strongly altered mafic metavolcanic flows and associated metasedimentary rocks. The A2 and BL14 zones are not exposed on surface and are buried beneath 10 to 75 m of glacial drift. The property is also host to the historic Playter Cu-Pb-Mo-Ag Prospect which has yet to be evaluated by MetalCORP. The Big Lake Property is located near the southern margins of the eastern portion of the Archean-age Schreiber-Hemlo greenstone belt of the eastern Wawa Subprovince of the Canadian Shield. The greenstone belt is split into distinct eastern and western segments by the 1108 Ma Mesoproterozoic Coldwell Alkalic Complex. The eastern part of the belt is subdivided into the Hemlo-Black River assemblage (2.77 Ma) to the north and the Heron Bay (2.70 Ma) assemblage to the south, both of which are primarily affected by amphibolite-facies regional metamorphism. The western portions of both assemblages are lower in grade and exhibit upper greenschist facies regional metamorphism. The Big Lake Property occurs within the Heron Bay Assemblage which is intruded by the granitic to granodioritic Heron Bay Batholith, the recently recognized mafic to ultramafic Gus Creek Intrusion, the Bellʼs Lake Ultramafic Intrusion, and the Big Lake Ultramafic Complex. The J4 and J5 Pt-Pd Reefs consist of narrow, apparently stratabound intervals hosted within thick peridotite units contained within the upper and central intrusive cycles of the eastern portion of the sill-like Big Lake Ultramafic Complex. The complex is not layered. The two host intrusive cycles dip approximately 43° north and are very similar in their appearance, progression of rock units, and apparent thickness. The observed mineralization consists of low amounts (trace to ~1%) of very finely disseminated pyrrhotite and chalcopyrite within serpentinized to locally talcose, fine-grained peridotite. The J4 Reef varies between 0.58 and 2.11 m in thickness, occurs within the basal peridotite unit of the uppermost intrusive cycle of the Big Lake Complex and is usually directly adjacent to the contact with an overlying feldspathic pyroxenite unit. The J5 Reef is identical in appearance to the J4 Reef, varies between 0.75 and 3.00 m in thickness, and occurs within the basal peridotite of the central intrusive cycle of the complex near, but not adjacent to, the upper contact of the host unit with an overlying feldspathic pyroxenite. The J4 Reef has been traced for 1.60 km and contains up to 0.70 gpt Pt and 0.79 gpt Pd (1.49 gpt PGE)/1.67 m. The J5 reef has been traced for a similar distance and contains up to 0.81 gpt Pt, 0.85 gpt Pd (1.86 gpt 2PGE)/0.75 m. The A2 Ni-Cu Zone occurs near the base of the discordant Gus Creek Mafic Intrusion and consists of disseminated, blebby, and stringered, locally semi-massive pyrrhotite, chalcopyrite, and possibly pentlandite hosted within a 2 to 20 m thick host sequence. The host sequence is a complex interval of variably mineralized (1 to 30% sulphides), varitextured, inclusion-rich, gabbroic to melagabbroic intrusive rocks overlain by unmineralized, medium- to coarse-grained gabbro and quartz leucogabbro and underlain by occasionally mineralized, pyroxenephyric melagabbro and feldspathic pyroxenite. The strongest mineralization occurs near the base of the host sequence, comprises the A2 Ni-Cu Zone, and includes 1.66% Ni and 0.20% Cu/0.30 m, 1.00% Cu and 0.80% - 36 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Ni/0.40 m, 1.40% Cu and 0.27% Ni/0.77 m, and 0.98% Cu and 0.29% Ni/1.40 m. The geometry of the A2 mineralized zone remains uncertain and may be more complex that initially thought, but within the area drilled appears to strike between 125 and 150° and dip southwest at between 40 and 60°. The BL14 Cu-Zn-Ag Zone is located near the east end and stratigraphically below the sill-like Big Lake Ultramafic Complex, approximately 800 metres south of the A2 zone. The zone closely resembles high temperature Cu-rich VMS stringer mineralization and is composed of an intensely biotitized and strongly chloritized breccia containing up to 30% bands, veins, stringers and pods of chalcopyrite and pyrite with up to 5% disseminated to streaked sphalerite and minor galena. The mineralized zone occurs within a strongly to intensely K- and Mg altered, Na2O-depleted package of mafic metavolcanic rocks, minor associated interflow clastic metasedimentary rocks, and deformed bands of chert. Mineralization intersected to date includes 2.56% Cu, 1.00% Zn, 46.0 gpt Ag, 1.60 gpt Au, and 0.10% Pb/0.93 m; 0.80% Cu, 0.50% Zn, and 21.3 gpt Ag/2.62 m; and 0.74% Cu, 0.23% Zn, and 9.0 gpt Ag/0.75 m. - 37 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mining and Exploration Activity in Northwestern Ontario MAGEE, M. Angelique, Ministry of Northern Development and Mines, Resident Geologist Program, Thunder Bay, Ontario, Canada, P7B 6S7 Northwestern Ontario experienced a significant upswing in mining and mineral exploration in 2004. Six mines produced a total of 1.7 million ounces of gold in 2004, approximately 70% of the Ontario total. Gold producers included: Campbell Mine (Placer Dome Inc.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome Inc./Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 308,931 ounces of palladium and 25,128 ounces of platinum at its Lac des Iles Mine and has begun the development of an underground operation below its open pit mine. There are approximately 300 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, OnamanTashota belt and the Pickle Lake greenstone belt. Elevated mineral commodity prices are contributing to levels of exploration activity in northwestern Ontario not seen since the mid-1980s. - 38 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Preliminary stratigraphy and geochemistry of the Mesoproterozoic Pillar Lake Volcanics, Wabigoon Subprovince, Superior Province, Armstrong, Ontario, Canada MAGEE, M. Angelique, Ministry of Northern Development and Mines, Resident Geologist Program, Thunder Bay, Ontario, Canada, P7B 6S7, HOLLINGS, P. and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 The Mesoproterozoic Pillar Lake Volcanics (PLV); an approximately 50 m thick series of flat-lying, greenschist facies undeformed massive and pillowed basalt flows; are located 15 km southeast of Armstrong, Ontario. The Pillar Lake Area was originally mapped as part of a geological compilation series with the dominant lithology in the area comprising Nipigon diabase sills formed during the Keweenawan mid-continent rift system of North America (Breaks, 1980; Davies et al., 1970). Mapping completed at a scale of 1: 50,000 by MacDonald et al. in 2003 resulted in the discovery of previously unmapped lithologies which include the Pillar Lake Volcanics (MacDonald, 2004). The PLV are part of a suite of Mesoproterozoic rocks that unconformably overlie the Archean basement of the central Wabigoon Subprovince, Superior Province. The PLV have not yet been successfully dated by geochronological methods. Magnetic remanence studies of the PLV places them at 1140 Ma but there is a significant error associated with this date (C. Hercun, pers. comm., 2005). Fifty detrital zircons from an interflow lithic arenite bounded at its base and top by PLV were dated and the youngest detrital zircon was dated at 1514 Ma (Heaman et al., 2005). The dominant populations of zircons from the interflow lithic arenite fall within 1780 to 1880 Ma and 1900 to 1950 Ma (Heaman et al., 2005). The PLV unconformably overlie a 1599±1 Ma layered gabbroic intrusion but no contact has yet been found except in drill core (Heaman et al., 2005; Middleton, 2005). A nearby 1590±1 Ma syenite may also underlie the PLV (Heaman et al., 2005). The PLV are overlain by Inspiration diabase sills dated at 1120±1 Ma (Heaman et al., 2005). The Inspiration diabase sills are geochemically distinct from the Nipigon sills and hence have been given a new designation. The Inspiration diabase sills extend the timing of North American mid-continent rift-related intrusive activity. Based on these age dates, the PLV are Mesoproterozoic in age, and were erupted between 1514 Ma and 1120 Ma. The PLV have been metamorphosed to greenschist facies and are extensively chloritized with accessory epidote and sericite. The PLV are hematite and actinolite altered with intense alteration found at the top of each flow. The individual massive basalt flows are approximately 0.2 – 1 m thick and each flow is capped with a 5 cm thick flow top breccia with actinolite rims and hematite cores forming the breccia fragments (Figure 1). The pillowed basalt flows tend to be pervasively altered and exhibit autobrecciation throughout the flow layer, perhaps due to higher permeability. Near the base of the volcanic pile, the pillowed basalt flows are rarely unaltered, unbrecciated and preserve primary volcanic features such as pillow selvages, interpillow hyaloclastite, and degassing structures (Figure 2). Associated with the massive and pillowed basalt flows are numerous breccias including an undeformed volcaniclastic breccia that contains fragments of gabbro, amygdaloidal basalt with clay minerals infilling the amygdules, and possibly Figure 1. Flow top breccia capping massive basalt flow with actinolite rims and recessively weathered hematite cores - 39 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 granitoid. Field relationships between the PLV and the breccias are not always discernible due to poor outcrop control, but in several instances the breccias occur as dikes found within the PLV. Based on preliminary geochemistry, the PLV may be the result of a mantle plume and possibly predate the main North American mid-continent rift event. It is hoped that detailed mapping of the PLV in conjunction with additional geochemical analyses will determine the source and cause of the volcanism that resulted in this suite of volcanic rocks. Figure 2. Undeformed chlorite-altered pillowed basalt with preserved selvages and interpillow hyaloclastite. References Breaks, F.W. 1980. Sioux Lookout - Armstrong Sheet, Geological Compilation Series, Ontario Geological Survey, Final Map 2442. Davies, J.C., Pryslak, A.P., Pye, E.P. 1970. Sioux Lookout - Armstrong Sheet, Geological Compilation Series, Ontario Geological Survey, Final Map 2169. Heaman, L.M., Easton, R.M., Hart, T.R., MacDonald, C.A., Fralick, P., and Hollings, P. 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constrains from U-Pb zircon and baddeleyite dating; Canadian Institute of Mining and Metallurgy, 2005 Annual Meeting, April 24-27, Toronto, Ontario. Macdonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2005. Diamond Drilling on Red Granite Property, Pillar Lake Sheet, Armstrong, ON, 52I03NW, Resident Geologist Program Thunder Bay North Assessment Files, 55p. - 40 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Sedimentology of the Rove and Virginia Formations and their Tectonic Significance MARIC, Mike and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1, philip.fralick@lakeheadu.ca The Paleoproterozoic Rove and Virginia Formations form the upper portion of strata in the Animikie Basin. They are lithostratigraphically and chronostratigraphically correlative units overlying iron formation of the Biwabik and Gunflint Formations. Tuffaceous layers very near the base of the Rove and Virginia Formations gave U-Pb zircon ages of approximately 1835 Ma (Addison et al., 2005), whereas a sandstone sample collected by R.M. Easton (O.G.S.), approximately 400 m higher in the stratigraphy, had a U-Pb minimum detrital zircon age of 1780 Ma (see Heaman and Easton, this volume). These age constraints place sedimentation commencing during the final stages of Penokean igneous activity and stretching over an extended time span. The Rove and Virginia Formations would have been in the appropriate spatial and temporal setting to represent deposition in a foredeep transitional into a foreland basin. This study investigates the internal stratigraphy and sedimentology within the Rove and Virginia to ascertain what the lithofacies present and their stacking order can reveal about the dynamics of the tectonic setting. The data set consists of detailed logging of twelve continuously cored drill holes extending from south of Duluth to south of Thunder Bay. In total 3200 meters of core were examined. This approach is similar to that of Lucente and Morey (1983), though with advances in depositional models and process sedimentology, since their classic work, more information may be able to be extracted. Cl y - § . § - ..— The Rove and Virginia Formations overlie an intensely altered zone in the upper Gunflint and Biwabik Formations. Silicification, formation of neomorphic carbonate spar, pyrite growth in strataform bands and pisoliths all occur in the upper few meters of these underlying units. This is sharply overlain by the basal Rove and Virginia consisting of black, carbonaceous shale with thin interbeds of siltstone and very fine-grained sandstone. Green, friable tuffaceous layers are common in this assemblage, especially in the northern drillholes. - 41 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Approximately five meters above the base the amount of siltstone and sandstone interlayers gradually begins to decrease upward until they are relatively scarce. The next approximately one hundred to one hundred and fifty meters is almost completely dominated by fissile black shale. Microscopic examination reveals the presence of very thin laminae composed of angular silt grains and other microlayers consisting of carbon. The shale dominated succession is interrupted midway towards its top by a siltstone and very fine-grained sandstonerich interval traceable across the basin. This coarser unit is thicker near both the northern and southern basin margins. As well, the black shale dominated interval, in general, contains more coarse-grained interbeds in the southern one third of the basin. A gradational contact exists at the top of the shale assemblage with the overlying sandstone-shale unit. The gradation occurs over eighty meters in the north but thins to the south. The overlying sandstone-shale succession is up to 350 meters thick and consists of a stacked assemblage of over one hundred individually coarsening-upwards parasequences. Internally they consist of graded, commonly massive, finegrained sandstones separated by mmʼs to cmʼs of shale. The shales between parasequences are decimeters to one or two meters thick. This sandstone-shale assemblage fines appreciably to the south. The uppermost unit overlying the graded sandstone packages at approximately 500 meters above the base of the section is dominated by black shale with thin rippled sandstones. Both current and wave ripples are present. The upper Gunflint represents a subareally weathered surface exposed during upbuckling concurrent with the Penokean Orogeny. This is highlighted by the 1878 Ma age of the Upper Gunflint (immediately pre-Penokean, Fralick et al., 2002) and the 1835 Ma age of the Rove (Addison et al., 2005) immediately overlying the weathered surface. Upramped orogenic load probably resulted in resubmergence of the area and a slightly coarser unit was deposited forming the basal Rove and Virginia as the flooding surface migrated across the basin. Eruptions at this time in the core of the Penokean deformed terrain may have supplied the volcanic ash. With increased water depth a sediment starved, condensed sequence developed with anoxia probably caused by high organic loading in the bottom sediments. A slight coarsening of detritus in the southern area probably reflects a source in the Penokean deformation belt, but the general scarcity of sediment influx from this area strongly implies only minor upraising of an orogenic zone during the Penokean. The condensed sequence is further condensed by the lack of biogenic sediments other than carbon resulting in tens of millions of years being represented by only a hundred meters of black shale. Progradation of a turbiditic to shelf system from the north ends the sediment starvation with a flood of detritus from this direction. Detrital zircon geochronology and paleocurrent directions (Morey, 1973) are consistent with derivation of the sediment influx from the TransHudson Orogenic zone to the northwest. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, vol. 33, p. 193-196. Fralick, P.W., Davis, D.W. and Kissin, S.A., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, vol. 39, p. 1085-1091. Lucente, M.E. and Morey, G.B., Stratigraphy and sedimentology of the lower Proterozoic Virginia Formation, northern Minnesota. Minnesota Geological Survey, Report of Investigations 28, 28p. Morey, G.B., 1973. Stratigraphic framework of Middle Precambrian rocks in Minnesota. In, ed G.M. Young, Symposium on Huronian Sedimentation, Geological Association of Canada, Special Paper 12, p. 211-249. - 42 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Riddle of the Sands (Proterozoic) solved by Quartzites at Hamilton Mounds, Central Wisconsin MEDARIS, L.G., Jr and DOTT, R.H., Jr, Department of Geology and Geophysics, University of WisconsinMadison, Madison, WI 53706, medaris@geology.wisc.edu, rdott@geology.wisc. edu Quartzite inliers in central Wisconsin have long been enigmatic with regard to the Proterozoic evolution of the southern Lake Superior region. At Hamilton Mounds the intrusion of quartzite by 1.76 Ga granite has led some to suggest that Baraboo Interval sedimentation (1.75-1.63 Ga) was spatially and temporally associated with Geon 17 magmatism. Re-examination of the Hamilton Mounds area reveals the presence of two different quartzites, an older one intruded by granite, which we designate the Hamilton Mounds Quartzite (HMQ), and a younger one resembling the Baraboo Quartzite, which we call the Seven Sisters Quartzite (SSQ), from the present name of the quarry. Quarrying operations have largely removed the HMQ and intrusive granite, although numerous displaced blocks remain for examination. The HMQ consists of gray, laminated quartzite (Fig. 1), which commonly contains calc-silicate (qtz+ep±am) layers and ellipsoidal domains, perhaps former concretions (Fig. 2; cs, calcsilicate). The HMQ is a metamorphosed immature wacke or arenite, which contains biotite, muscovite, and substantial amounts of microcline (mc) and plagioclase (pl) (Fig. 3). The HMQ is intruded by granite (gr) (Fig. 4), which yields a U-Pb zircon age of 1763 ± 7 Ma (Van Schmus, cited in LaBerge et al., 1991). Contact metamorphic effects of the granite on the HMQ include metasomatic growth of alkali feldspar, but the coarsegrained, poikiloblastic texture of such feldspar (Fig. 5) allows easy distinction between it and fine-grained, recrystallized detrital feldspar (Fig. 3). The SSQ, which crops out in the hills surrounding the quarry, consists of pink, massive to cross-bedded supermature quartzite (Fig. 6), resembling Baraboo Interval quartzites elsewhere, except for a greater degree of recrystallization and the presence of muscovite, rather than pyrophyllite. Such features and extensive brecciation probably reflect the influence of the nearby Wolf River batholith, with which are associated wide-spread Geon - 43 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 14 hydrothermal alteration and K-metasomatism as documented in the Baraboo and Sioux quartzites (Medaris et al., 2003). p r JM1 -- S —S Geon 17 detrital zircons have recently been recognized in a single sample of the “Hamilton Mounds Quartzite” by Van Wyck and Norman (2004). However, the analyzed sample, described as supermature quartzite, was collected several hundred yards from granite and is likely to be from the SSQ, which we correlate with other supermature quartzites of the Baraboo Interval. Thus, it is not surprising that this sample should contain Geon 17 detrital zircons and display a spectrum of detrital zircon ages similar to that from the Baraboo Quartzite. Although the contact between the HMQ and SSQ is not exposed, we interpret the HMQ and intrusive granite to represent basement upon which the SSQ was deposited unconformably. Thus, like the Baraboo Quartzite, the SSQ appears to be younger than, and rests upon, Geon 17 basement. The results of this investigation are consistent with our view of the Baraboo Interval (1.75-1.63 Ga) as representing an episode of extensive weathering and widespread deposition of supermature quartz arenite on a stabilized craton in the interval following cessation of Geon 17 magmatism and prior to Geon 16 Mazatzal deformation (Medaris et al., 2003). Abbreviated References LaBerge, G. L. et al. 1991. U.S. Geol. Surv. Bull. 1904B: B1-B18. Medaris, L. G., Jr et al. 2003. J. Geol. 111: 243-257. Van Wyck, N. and Norman, M. 2004. J. Geol. 112: 305-315. - 44 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Depositional setting of the Pass Lake and Rossport Formations (Sibley Group) inferred from a combined sedimentologic/geochemical approach METSARANTA, R.T. and FRALICK, P.W., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada, rtmetsar@lakeheadu.ca The depositional setting of the lower portions of the Mesoproterozoic Sibley Group have been investigated through detailed lithofacies analysis, stable isotope and trace element geochemistry. The portion of the Sibley Group studied is comprised of a lower clastic assemblage, middle mixed carbonate-clastic assemblage and an upper clastic dominated, evaporite rich assemblage. The thickness of each unit varies but is in general about 20-80m. The lower clastic section was deposited in a variety of depositional environments including: poorly developed, coarse-grained alluvial fans; braided, sand rich fluvial systems; fluvial/wave dominated deltaic settings; and nearshore/beach environments. The middle mixed carbonate clastic succession represents lacustrine or shallow marine deposition in a semi-arid environment. The upper red siltstone, evaporite-rich section has been interpreted to represent a saline mudflat environment or sahbka-like setting. Numerous chemical sediment types are preserved within the three distinct successions. Calcrete soils are developed in coarse-grained alluvial fan conglomerates in the lower clastic dominated package as well as higher in the stratigraphy within the upper package. Dolomitic mudstones are found in a cyclic dolomite-siltstone lithofacies association of the middle assemblage. Stromatolitic carbonates are preserved in the transition from the middle to upper succession along with evidence of subaerial exposure of this strand-proximal lithofacies. Sulfate minerals occur as fine nodules within dolomites of the middle mixed carbonate-clastic assemblage and as coarser nodules in the upper evaporitic succession. Carbon and oxygen stable isotopic compositions, Sr isotopic compositions and trace element compositions were determined for various chemical sediment types to study facies (process) dependent and stratigraphic variations in isotopic compositions and to evaluate the degree of post depositional diagenetic alteration. Stable isotopic compositions for dolomitic mudstones range from –3 0/00 (PDB) to about +1 0/00 (PDB) for δ13C and span δ18O compositions from –4 to –8 0/00 (PDB). Stromatolitic carbonates have δ13C from 0 to 2 0/00 and δ18O from –3 to -14 0/00 (PDB). Soil related carbonates vary over δ13C compositions from –1.5 to 1.50/00 (PDB) and δ18O varies from -2 to -6 0/00 (PDB). Sampling through a stratigraphic thickness of about 45m of the middle carbonate clastic succession shows a good correlation between increasing stratigraphic height and heavier stable isotopic composition (Figure 1). Petrographic analysis and trace element geochemistry suggests little diagenetic effect on stable isotopic compositions. A • • • A S l S •• • U• A • • • •A.• . A A. I —a S S S dolomitic mudstone stromatolite soil carbonate • S U 0 1 .4 2 -3 -2 -1 0 81 3c (PDB) dolomitic mudstone 313C (PDB) Figure 1. Stable isotopic compostions of various Sibley Group carbonates (left), and stratigraphic variation in δ13C for the middle dolomite/siltsone lithofacies association (right) - 45 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Sr isotopic compositions were also determined for a variety of carbonate and sulfate samples. Dolomitic mudstones (n=4) display 87Sr/86Sr ratios varying from 0.70846 to 0.71249±1. Soil carbonates appear to have variable compositions from a calcrete low in the stratigraphy at 0.70478±1 (n=1) to weathered stromatolitic material higher in the stratigraphy at a value of 0.70737±1 (n=1). One unaltered stromatolite sample has a value of 0.71234±1. Sulfates (n=8) from the upper evaporitic package vary through a fairly narrow range of compositions between 0.70647±3 to 0.708836±1. A single sample of sulfate from the middle succession in a dolomitic layer yielded a fairly high value of 0.71090±1. Further analyses have been undertaken to strengthen the validity of the apparent facies related variation. Rare earth element geochemistry of carbonate samples was also undertaken using an acetic acid dissolution method to avoid clastic contamination of primary carbonate geochemical signatures. Figure 2 shows Post Archean Australian Shale normalized REE diagram for Sibley Group carbonates. Stromatolites and soil carbonates generally show a distinct negative Ce anomaly, and overall relatively flat slope with a slightly negative HREE slope. Dolomitic mudstones exhibit an enrichment in MREE, with little or no negative Ce anomaly. Positive Gd anomalies are present in most samples of all carbonate types. Stromatolite Together lithofacies analysis and geochemical data provide insight into the depositional environments of the Sibley Group. C and O isotopic compositions fall within the realm of values determined for other Mesoproterozoic carbonate rocks deposited in marine systems. Upward stratigraphic changes in carbon and oxygen isotopic compostions may have been brought about through global scale changes, with a positive shift in C values attributable to increased rates of light (organic carbon) burial. Alternatively the carbon La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tm Yb Lu isotopic shift maybe have resulted from processes Dolomitic rnudstone occurring within a restricted lacustrine basin with the upward shift corresponding to increasingly restricted depositional conditions near the top of the sampled succession. 87Sr/86Sr values also lie within the realm of possible seawater Sr values for the Proterozoic. The lowest values in calcrete from near the bottom of the lower succession suggests that the Sr source for these carbonates may have been marine. Relatively high Sr values in the middle succession may imply a lacustrine system. Diagenetic sulfate in the upper succession 0.001 also falls within “marine” values for the Proterozoic La Ce Pr Nd SmEu TbGd Dy Ho Er TmYb Lu and is generally lower than carbonates in the middle Soil Carbonate succession. REE element patterns have a signature consistent with oxygenated seawater; however, little comparative data is available for modern of ancient carbonates deposited in lacustrine or evaporitic environments. 0.001 Pr Nd SmEu Tb Gd Dy Ho Er TmYb Lu Figure 2. Post Archean average shale normalized rare earth element diagrams for various Sibley Group carbonate types. La Ce - 46 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Seagull Intrusion, Ontario: A Unique PGE-Ni-Cu Setting MIDDLETON, R.S., P.Eng. and HEGGIE, G., Thunder Bay, Ontario The Seagull ultramafic (Proterozoic) intrusion is an unique setting when compared to other mafic intrusion in the Lake Superior region and for that matter the rest of the world. A dunite layer exceeding 400 metres in thickness with pristine olivine (unaltered) having a magnesium oxide content exceeding 33% average olivine composition of Fo86 occurs with a 50-100 metre thick gabbro-pyroxenite (lherzolite) basal unit. Overlying is a peridotite 200-250 metres thick and an upper granophyric gabbro-gabbro pyroxenite-gabbro norite (50-100 metres thick). The intrusion has been age dated as 1112 Ma although older dates of 1124 Ma are appearing in other phases (see Heaman and Easton, this volume). The intrusion is adjacent to a failed arm of the Midcontinent Rift that extends north into the Nipigon Embayment and is cut by Nipigon olivine gabbro (1108 Ma) that forms extensive sills in the region. Other olivine-picrite bodies occur in the area such as Eva-Kitto and Hele, but are not known to contain the extensive ultramafic observed at Seagull. Sulphides occur in the basal lherzolite with zoned blebs containing cubanite tops and pyrrhotite-pentlandite bottoms. Assays up to 3.58g Pt +Pd have been obtained with % S and a 1 to 1.1 ratio of Pt and Pd. Nickel values up to 0.25% and Cu up to 0.35% have been observed. Horizons of fine disseminated sulphide occur in the dunite up to 5 metres in thickness containing up to 5.5g PGE, 0.68% Cu and 0.36% Ni in the 470 to 500 metre depth area. Two distinct layers have been identified so far, with highly anomalous zones in between the layers. The horizons are hosted in a medium to coarse dunite above a very coarse dunite. Fine-grained chalcopyrite with minor pyrrhotite-pentlandite occur in these layers. There is no development of feldspar or oxide mineral (magnetite or chromite) layering, however there appears to be a change in the pyroxene composition from both orthopyroxene and clinopyroxene below the mineralized horizons to just clinopyroxene above the mineralized horizon, which occurs in conjunction with a shift to slightly more primitive olivine compositions. Magnetic inversion modeling has been the most useful tool for mapping the shape of the intrusion. Down hole EM has identified conductors within the basal lherzolite which will be followed up for stringer-massive sulphide accumulations. Gravity surveying has helped identify a trough extending east and a scarn zone in the Sibley Group Metasediments. Most of the basement rocks beneath the areas containing values are Archean Quetico Metasediments. Sulphur isotope work suggests the Quetico sulphides acted as a sulphur source. Minerals identified in the basal zone are merenskyite, michnerite, sperrylite, a palladium-copper alloy, native copper, native iron-bismuth, chalcopyrite, cubanite, pyrrhotite and pentlandite. Where the horizons are dominated by bravolite, polarite, sperrylite, telluropalladinite and copper. - 47 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geology, Geochemistry and PGE Potential of Mafic and Ultramafic Intrusions in Minnesota, excluding the Duluth Complex MILLER, James D. Jr., and JIRSA, Mark A., Minnesota Geological Survey, mille066@umn.edu and jirsa001@umn.edu Exploration for polymetallic deposits, including platinum group elements (PGE), has traditionally focused on mafic intrusions of the Mesoproterozoic Duluth Complex for obvious reasons—the complex is relatively well exposed and contains occurrences of PGE associated with basal Cu-Ni deposits that were discovered decades ago. By contrast, mafic intrusions in the Archean and Paleoproterozoic terranes of the remaining four-fifths of the state have received only cursory investigations by exploration companies and the state. This stems in large part from thick glacial cover over all but the northeast and southeast parts of the state; there is Paleozoic cover in the southeast. The completion of a high-resolution aeromagnetic survey of the state between 1982 and 1990 and the increased demand for PGE has spurred more exploration activity targeting ultramafic and mafic intrusions over the past two decades. Because of the requirement that drill core and related geochemical and geophysical data be turned over to the state after mineral leases expire, a sizeable public database of geologic, geochemical, and geophysical information exists on mafic intrusions in the state. However, this database has not been consistently evaluated for what it implies about the potential for these intrusions to host PGE deposits. Beginning in 2001, a four-year, two-phase study was funded by the state for the Minnesota Geological Survey (MGS) and the Natural Resources Research Institute (NRRI) to inventory the public database and acquire new data for the purpose of evaluating the potential for PGE deposits in mafic intrusions outside the Duluth Complex. Phase 1 of this study, which was completed in July of 2003 and published by the MGS as an interim openfile report, took inventory and digitally compiled basic geologic, lithologic, and geochemical attributes of over 150 individual mafic intrusions studied from field exposures or from drill core. Information was also collected on some of the thousands of diabasic dikes and other geophysical anomalies inferred to be mafic intrusions. The project work conducted on each intrusion varied with the level of detail that existed, but included some combination of geophysical (aeromagnetic and gravity) delineation, outcrop mapping, drill core examination, petrography, and geochemical analysis. This compilation of pre-existing data was augmented with geophysical modeling, relogging of drill core, outcrop mapping and sampling in well exposed areas of north-central Minnesota, petrographic studies of over 100 thin sections from 53 intrusions, and geochemical analyses and assays of 83 samples from 58 intrusions. Also, eight new 40Ar/39Ar analyses of magmatic hornblende and biotite separates were acquired to better constrain the general temporal framework of some intrusions. Phase 2 of this study, which is currently underway, sets out to conduct a more detailed evaluation of the geologic setting, igneous stratigraphy, and geochemistry of 15 specific intrusions for which there is sufficient sampling by drill core. The petrographic and geochemical results of this study phase were not ready at the time of this writing, but will be presented at the meeting. Another component of this study, which is being conducted by the NRRI, is a detailed mapping and chemostratigraphic analysis of the well-exposed Deer Lake layered mafic complex of north-central Minnesota. Based on an evaluation of this database, mafic intrusions can be grouped into several major types according to age and type of host rock, timing of emplacement relative to tectonism and metamorphism, and general lithologic attributes. The attributes of these intrusion types and some examples are summarized in Table 1. Phase 1 of this study acquired 75 new lithogeochemical and assay analyses from more than 50 intrusions. In addition, over 400 publicly available geochemical analyses have been compiled from exploration company files and data previously acquired by various state and federal agencies. For Phase 2, an additional 85 analyses are - 48 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Table 1. Classification of mafic intrusions outside the Duluth Complex in Minnesota. Tecto-metamorphism ithologic attributes Examples (Fig. 1) Subvolcanic maficultramafic sills Intrusion type Age Ar Archean greenstone belts Host rock Pre- to syn-tectonic; low to moderate metamorphic grade Tholeiitic-komatiitic; peridotite, pyroxenite, gabbro, & diorite Deer Lake Complex, Winterfire intr, KIB intrusions, Warroad intrusion Amphibolite sills & dikes Ar Archean highgrade gneiss terrains Pre-tectonic; high metamorphic grade Schistose amphibolite (metagabbro) Small amphibolite bodies in Quetico Subprovince Lamproid dikes & plugs Ar Archean greenstone belts Syn- to post tectonic; low metamorphic grade Hbld-, oxide-, & biobearing ultramafic rocks Lamprophyre plugs in Wawa Subprovince Gabbroic to intermediate intrusions Ar Archean granitegreenstone terrains Post-tectonic; low metamorphic grade Gabbro/diorite, tonalite, monzonite w/mafic enclaves Linden pluton, Oaks intrusion, Grygla pluton Gabbroic anorthosite massifs p< Archean granitegreenstone terrains Syn- to post-tectonic; moderate metamorphic grade Coarse gabbroic anorthosite Mentor intrusive complex Layered gabbronorite intrusions p< Penokean orogen & MN River Valley Subprovince Syn- to post-tectonic; low to moderate metamorphic grade Modally & texturally layered gabbronorite, gabbro, & pyroxenite Lake Washington, LL, BKV & Providence intrs. Pyroxenite & peridotite plugs p< Penokean orogen Post-tectonic; low metamorphic grade Alkaline ultramafic rocks Small plugs in central Minnesota Hornblendic maficintermediate complex p< Penokean orogen Syn- to post-tectonic; low to moderate metamorphic grade Hornblendite, diorite, granodiorite Tibbets Brook intrusion Olivine gabbro intrusions m< Unknown Syn-continental rifting Layered oxide olivine gabbro Fillmore County intrusions currently being acquired from 15 intrusions to more completely characterize the chemostratigraphy of these select intrusions. Currently, the highest PGE values have been found in sulfidic basal contact zones of the Winterfire subvolcanic sills in north-central Minnesota with Pd, Pt, and Au values in 1-3 meter intervals in the range of 5001,000, 100-400, and 50-200 parts per billion, respectively. The discovery of anomalous PGE values is not the primary objective of this study, however. Rather, the goal is to assess the overall potential of particular intrusion types to host either contact-type or reef-type PGE deposits. Working toward this goal, geochemical data will be used to interpret the composition and PGE tenor of the parent magmas, determine the degree to which the intrusions display internal differentiation, and evaluate conditions of sulfide saturation through the intrusions. A draft report on this study will be completed in the summer of 2005 with a final report to follow. In addition to digitally compiling pertinent geologic, geophysical, petrographic, and geochemical data, this report will evaluate the data to assess the PGE potential of the various intrusion types and make recommendations as to what intrusion types merit further study and exploration. - 49 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 New thoughts on old circles: A reexamination of spheroidal Gunflint taxa PLANAVSKY, N.J. and MURPHY, J., Department of Geology, Lawrence University, Appleton WI The first systematic description of the Gunflint biota (Barghoorn and Tyler, 1965) is a seminal work in Precambrian paleontology. The Gunflint biota are represented by an assemblage of incredibly well-preserved microfossils contained within the approximately two billion year old Gunflint Iron Range of the Animikie Basin. Because the Gunflint microfossils were the first definitive and widely publicized evidence of Precambrian life, they have become one of the most famous fossil assemblages in the world. The high level of preservation and diversity of the biota has led to the microfossils becoming a benchmark against which early records of life are compared. The Gunflint and other Animikie basin microfossils are also significant because they provide the most complete record of the biosphere during a dramatic change in Earthʼs history—the initial oxygenation of the atmosphere and shallow seas and the sequestration of the iron from the oceans. The importance of the Animikie microfossils is based upon a clear understanding of the diversity and structure of the Gunflint community. Most Gunflint genera are determined based on obvious morphological characteristics such as having a filamentous or spheroid morphology. However, species boundaries within broad morphological groups such as the spheroids are more difficult to define. Diameter distribution (i.e. unimodal or polymodal) is one factor that has previously been used to determine species composition of a spheroidal population (Strother and Tobin, 1987; Knoll et al., 1978; Knoll and Simonson, 1981). Although the range of species distributions has always been noted, it has been afforded little attention in the evaluation of specific taxon (Knoll et al., 1978). We conducted a study to determine the diameter distribution within a population of the coccoid cyanobacteria Microcystis aeruginosa and a survey of previously recorded diameter ranges for 100 extant coccoid cyanobacteria taxa. Our goal is to determine the validity of Gunflint and other Animkie basin species classifications. Previous studies that have noted a polymodal diameter distribution in a microfossil population have asserted that this indicates the presence of multiple taxa. This was most notably assumed for the genus Huroniospora, one of the most abundant biota of Gunflint. Strother and Tobin (1987) recorded a skewed bimodal size-frequency distribution (modes at 4 and 7 µm) for Huroniospora microfossils found within the Schreiber Beach assemblage. Huroniospora specimens from the Sokoman Iron Formation have a size-frequency distribution with modes at 3, 5, and 10µm (Knoll and Simonson, 1981), in accordance with previous findings from the Schreiber locality. We obtained a diameter distribution for a cultured population (from Lake Winnebago, Appleton, WI) of Microcystis aeruginosa through measurements with an optical micrometer. We made wet mount preparations and recorded the diameter of all specimens within a single optical plane. The diameters were placed into .5μm groups, which is the level of accuracy of the optical micrometer used. In all, 425 individuals were measured. The population showed a distinct bimodal pattern with modes at 6 and 8.5 μm. Although previous algae populations have been shown to have a broadly unimodal diameter distribution, our results show that a monospecific cyanobacteria population can contain a biomodal diameter distribution. Polymodal distributions of cyanobacteria have also been recorded (Prescott, 1951). These results strongly suggest that polymodal diameter distributions with closely spaced modes in a microfossil population, specifically those described by Strother and Tobin (1989) and Knoll and Simonson (1981), in the definitively cyanobacteria taxon of Huroniospora are very unlikely to indicate multiple taxa. At minimum the observations show that a polymodal diameter distribution within a microfossil population cannot be used as a diagnostic indication of multiple taxa. We also conducted an evaluation of the range of diameter distributions in extant cyanobacterial species. Our goal was to see if the reported diameter ranges for fossil taxon are consistent with the ranges seen for extant species. We surveyed 100 marine and freshwater cocciod cyanobacteria species from previous reports. The average diameter distribution was 3.0, with a standard deviation of 3.1. We found a range of 0.2 to 24 μm, with only two species with a reported range greater than 10μm. This observation is relevant because many of the Gunflint taxon have very large ranges that are skewed left. A notable example is Leptoteichos golubicci. The - 50 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 taxon is broadly unimodal with a mean cell diameter of 13.5μm. However, the reported range for the species is 26μm (5-31μm), larger than any of the species we encountered and far above the mean for diameter range. Less than the five percent of Leptoteichos golubicci have a diameter above 24 μm (Knoll et al., 1978). The diameter distribution of and range reported for Leptoteichos golubicci is inconsistent with monospecfic classification of the population. We believe that the cells above 24 microns in the Leptoteichos golubicci population are most likely akinetes, which are large reproductive structures (analogous in many ways to spores) produced by filamentous cyanobacteria. Although akinetes have previously been shown to be present to present in the Gunflint (Licira and Cloud, 1968), no estimate of their abundance was made. Our reinterpretation of the Leptoteichos golubicci population could be used to determine an estimate of akinete abundance. Although the spheroidal Gunflint microfossils are exceptionally well preserved, we need to remember that we are still looking at a collection of circles, from which little morphological variation can be gathered. Therefore information on population structure from modern analogues needs to be used to help ascertain the basic ecology of the Gunflint community. Selected References Braghoorn, E.S. and Tyler, S.A. 1965. Microorganisms form the Gunflint chert. Science, 147, 563-577. Licari, G.R., and Cloud, P.E., Jr., 1968. Reproductive structures and taxonomic affinities of some nannofossils from the Gunflint Iron Formation. Proceedings of the National Academy of Sciences, 59, 1053-1060. Knoll, A.H. Barghoorn, E.S., Awramik, S.W. 1978. New microorganisms from the Aphebeian Gunflint Iron Formation, Ontario. J. Paleontology 52, 976-992. Knoll, A.H. and Simonson, B. 1981. Early Proterozoic microfossils and pencontemporaneous quartz sedimentation in the Sokoman Iron Formation, Canada. Science. 211, 478-480. Prescott, G.W. 1951. Algae of the Western Great Lakes Area. Cranbrook Institute of Science: Bloomfield Hills, Michigan. Strother, P.K. and Tobin, K. 1987. Observations on the genus Huroniospora braghoorn: implications for paleoecology of the gunflint microbiota. Precambrian Research 36, 323-333 - 51 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Geochemical Variation within the Mesoproterozoic Nipigon Diabase Sills RICHARDSON, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada, ajrichar@lakeheadu.ca The Nipigon sills are related to the Mesoproterozoic Keweenawan mid-continent (MCR) rift event. Centered on what is now the Lake Superior basin, the igneous activity is interpreted to be the result of a mantle plume interacting with the continent as far to the southwest as Kansas, USA, and to the southeast as far as the Grenville front in southern Ontario, Canada (Davis and Green, 1997). The system operated for approximately 20 Ma producing a volume of intrusive and extrusive igneous rock estimated to be in excess of 1.5 million km3 (Klewin and Shirey, 1992). The Nipigon sills are thick, relatively flat lying hypabyssal, medium-grained subophitic to ophitic olivine gabbronorite with U-Pb ages ranging from 1107 Ma to 1112 Ma (Heaman and Easton, this volume). The goals of this project are to evaluate geochemical variations within the Nipigon sills, compare them to other MCR rocks and generate an emplacement model which identifies and explains any identified intrasill relationships, magma sources. The project area (Figure 1) covers approximately 15,000 km2 of the Nipigon embayment. As part of this project a detailed sampling program was undertaken in the summer of 2003 and 2004 resulting in 250 shoreline samples being collected from Lake Nipigon (Figure 1) and 530 samples from diamond drill core in the southern Lake Nipigon basin near Black Sturgeon and Shillabeer lakes. Selected samples were analysed for major, trace and rare earth elements with a representative subset analysed for Rb-Sr, Sm-Nd and Pb-Pb isotopes. Three distinct sill suites were identified within the basin. Two additional suites have been identified by T. Hart and C.A. MacDonald (Ontario Geological Survey) as part of their mapping projects in the Nipigon Embayment (MacDonald, 2004; Hart and Magyarosi, 2004). The various suites were defined, in part, using rare earth element data (Figure 2). The most spatially abundant suite has been termed the Main Nipigon suite. This suite outcrops over much of the Nipigon embayment having La/Smcn and Gd/Ybcn ratios of 1.4-1.8 and 1.3-1.5 respectively (Figure 2). Mga s s.ys.dns El Meta,oeic • The Main Nipigon and Inspiration suites share similar Gd/Ybcn (Figure 2), TiO2 and MgO values. The Inspiration suite has a higher La/Smcn ratio, but these data lie on a possible trend from the Main Nipigon suite and the whole rock data suggest that the Inspiration and the Main Nipigon suites may be related. OS' Hole CdL3! 9Spl {U Scale 0 102O Km Figure 1. Map showing sample locations, drill holes, geology and project area. - 52 - Other suites in the Nipigon Embayment include the McIntyre and Jackfish suites. These two suites share similar Gd/Ybcn and La/Smcn ratios and lie on a fractionation trend (Figure 2) with the Jackfish �Proceedings of the 51st ILSG Annual Meeting - Part 1 possibly being the source of the McIntyre. The Shillabeer suite is geochemically similar to the Disraeli ultramafic intrusion using Gd/ Ybcn, La/Smcn and whole rock geochemistry suggesting the possibility that this suite is not part of the Nipigon sills but is related to the ultramafic intrusions of the southern Lake Nipigon embayment. 4.5 • • .1;I •a . A U Isotope data (Figure 3) reveals differences in the degree of crustal contamination of the ShiIlaber • Distoeli sill suites. The Main Nipigon and McIntyre •Jockfish suites are the least contaminated (εNd ~0) o o. 1.5 2 2.5 4 4.5 3 3.5 with the Main Nipigon data trending toward La IS men Sibley group sediments suggesting that these Figure 2. Chondrite normalized rare earth data defining sill suites in sediments may be the dominant contaminant. Shillabeer and Inspiration sill suites are the the Nipigon embayment. most contaminated (εNd -5.5 to -7) but do not 0.705 0.110 OIlS 0.720 0.850 0.900 0.950 trend towards Sibley data implying a distinct contaminant. . Moin Nipigon a £Kama Hill • McIntyre .—- 1 p £ £ References Klewin, K.K, and Shirey, S.B. 1992. The igneous petrology and magmatic evolution of the -4 0 Midcontinent rift system. Tectonophysics 213, 33-40. Davis, D.W. and Green, J.C. 1997. Geochronolgy •. •Jadcfih of the North American Midcontinent rift in AMoin Mpon A Shillthter western Lake Superior and implications for its •MCIMyT •Sibky geodynamic evolution. Canadian Journal of Earth Sciences 34, 476-488. 875r/86Sr Hart, T.R. and Magyarosi, Z. 2004. Precambrian Figure 3. Nd and Sm isotope data for the Nipigon sill suites Geology of the northern Black Sturgeon River compared with data from the Sibley sediments and Quetico basement and Disraeli Lake Area, Nipigon Embayment, rocks. northwest Ontario; Ontario Geological Survey Open File Report 6138, 56 p. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. -'.0 £ •/ - 53 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 The Geology of the Eagle Nickel-Copper Deposit: Marquette County, Michigan ROSSELL, D. M., dean.rossell@kennecott.com, Kennecott Exploration Company, 10861 N. Mavinee Dr. #141, Oro Valley, AZ. 85737 and COOMBES, S., Kennecott Canada Exploration Inc., #354-200 Granville Street, Vancouver, B.C. V6C 1S4 Kennecottʼs discovery of the Eagle nickel-copper deposit in 2002 marked the culmination of more than a decade of exploration work by Kennecott in the Paleoproterozoic in age Baraga sedimentary basin. The discovery hole, YD02-02 completed in July 2002, intersected 84.2m of massive sulfide mineralization averaging 6.3% Ni and 4.0% Cu. The resource estimate for the Eagle deposit at the end of 2003 was 5 million tonnes at 3.68% Ni, 3.06%Cu and 0.1% Co. The Eagle deposit is hosted in the westernmost of two small peridotite bodies historically referred to as the Yellow Dog Peridotite. The Yellow Dog intrusions, which lack penetrative foliations and truncate Penokean tectonic fabrics in the surrounding meta-sediments, are believed to be Keweenawan in age (Klasner et al., 1979). The intrusions are mainly comprised of coarse-grained, variably serpentinized peridotite and feldspathic peridotite. A fine-grained, olivine poor phase is found along the margins of the intrusions and as xenoliths within the peridotite. Possible amygdules in the olivine poor phase(s) suggest a shallow level of intrusion. Three principal types of sulfide mineralization are recognized in the Eagle deposit: disseminated (blebby), semi-massive (matrix) and massive. Although the nickel contents of semi-massive and massive sulfides are relatively uniform through out the deposit, copper contents vary significantly. Platinum group metals (PGM) and gold values are significantly higher in the copper rich massive sulfides. Copper rich veins and disseminations, with significant PGM and gold, in the surrounding meta-sediments may constitute a fourth type of ore. Massive and semi-massive sulfide ore types in the Eagle deposit are irregularly distributed. The contacts between different ore types are sharp and show little evidence of the gradation or layering that might be expected if gravity driven accumulation of sulfides from an overlying, sulfide saturated, silicate magma was the principle mechanism of ore formation. Sequential emplacement of various mixtures of silicate and sulfide magma and cumulus minerals, derived from a lower stratified magma chamber, may provide a better model. Reference Klasner, J.S., Snider, D.W., Cannon, W.F. and Slack, J.F. 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. Geologic Survey of Michigan DNR report of investigation 24, 31p. - 54 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mining and Exploration Activity in the Thunder Bay South District SCHNIEDERS, B.R. and SCOTT, J.F., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA The Thunder Bay South District covers an area from west of Atikokan, east to White River and north to Armstrong from the United States border. There were four producing precious metal and base metal mines, as well as 15 seasonal amethyst producers in 2004. The three Hemlo gold mines produced 655,000 ounces of gold in 2004, and more than 18 million ounces of gold has been produced from the Hemlo deposit since mining began in 1985. The Lac des Iles mine produced a record 308,931 ounces of palladium from the open pit, as well as platinum, gold, nickel and copper. Underground development was initiated in 2004 and underground mining is scheduled to begin in 2006. Exploration for gold, base metals, platinum group metals, uranium, molybdenum, diamonds and industrial minerals were all active in 2004. Highlights include a new base metal discovery by Freewest Resources Canada Inc. in early 2005. The Sungold Occurrence in the Western Shebandowan greenstone belt (Wawa Subprovince) is located 120 km west of Thunder Bay. Grab samples assayed 33.2 % zinc and 12.5 % Cu, discovered after a V-TEM (Dreamcatcher) and winter beep-mat geophysical surveys were conducted. Gold highlights include the Tower Mountain Property of Valgold Resources Ltd. who conducted more than 10,000 m of diamond drilling with results of up to 164.7 g/t gold over 1.5 m. Further drilling is ongoing. In the Hemlo area Navasota Resources Ltd. intersected 18.28 g/t gold and 47.18 g/t silver over 4 m in diamond drilling. North American Palladium Ltd. continued deep diamond drilling on the High-Grade Offset Zone, including intersecting 5.25 g/t palladium and 0.515 g/t platinum over 3.1 m. Exploration is ongoing on the mine block, and at the Shebandowan project. East West Resource Corporation and Canadian Golden Dragon Resources Ltd. entered into a $7.5 million joint venture with Platinum Group Metals Ltd. on the Seagull-Disraeli property. Several new reef horizons were discovered, assisted by the Ontario Geological Survey, the Lake Nipigon Region Geoscience Initiative and Lakehead University. Assays of up to 7.90 g/t platinum + palladium + gold over 0.44 m have been intersected with a 1:1 Pt:Pd ratio. Rampart Ventures Ltd., in joint venture with New Shoshoni Vcentures Ltd., have staked more than 2000 claims in the Sibley Basin for uranium. Airborne geophysical surveys have been flown and diamond drilling is planned. R ipple Lake Diamonds Inc. discovered three micro-diamonds in a small 3 by 3 m diatreme associated with a calc-alkalic (minette) dike, located on the Trans-Canada Highway approximately 30 km west of Marathon. More than 2200 claim units have been acquired. Exploration is active for a variety of commodities in the Thunder Bay South District. We invite you to come and explore in Northwestern Ontario! - 55 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 PGE and gold potential of the Archean Deer Lake Complex, Minnesota, USA SEVERSON, Mark J., Natural Resources Research Institute, University of Minnesota, Duluth The precious metal potential of the Deer Lake Complex (DLC) has recently been addressed by detailed geologic mapping (2004) and two detailed sampling campaigns (2002 and 2004). The DLC is Archean in age, is located in northeastern Itasca County, Minnesota, and consists of a series of multiple sills, each of which are variably differentiated and steeply inclined. Sampling results, along several traverses across the DLC, suggest that PGE contents are low with maximums of only 84 ppb Pt and 52 ppb Pd. Gold contents, on the other hand, are more encouraging and values up to 2.4 ppm are associated with a shear zone in one of the basal DLC sills. The Deer Lake Complex (DLC) consists of a series of steeply-dipping, tholeiitic, mafic-ultramafic sills that are variably differentiated and complexly interfingered. Where differentiation is complete, the sills exhibit a layering sequence consisting of a basal chilled margin grading upwards through peridotite, pyroxenite, melagabbro, melagabbro-porphyritic gabbro, gabbro, quartz gabbro (or diorite), and in some rare localities, an upper chilled margin. The sills were mostly emplaced into calc-alkaline volcanic and sedimentary rocks of the Joy Lake Volcanic Sequence (Jirsa, 1990). The bottom-most portion of the DLC was initially intruded into highMg tholeiitic basalt and basaltic komatiite; these volcanic rocks are assumed to be the early extrusive equivalent of the DLC (Jirsa, 1990; Englebert and Hauck, 1991). All of the sills are metamorphosed to greenschist facies. The DLC was explored for base metal deposits in the 1970s; however, no significant indications of Ni-Cu mineralization were found. Initial geologic mapping of the DLC (Berkley, 1972; Ripley, 1973) suggested that only 2-3 completely differentiated sills were present, and that repetitions of specific rock types were related to tight isoclinal folds. Later review (Severson, 1987; Jirsa, 1990; this study) suggests that the DLC formed from the emplacement of numerous interrupted magmatic pulses with partial to complete fractional crystallization occurring between the pulses. Thus, not all of the sills contain a peridotite base or grade completely upwards into quartz gabbro. Stratigraphic Order As can be seen on a plot of Pt+Pd versus stratigraphic position (Figure 1), most of the individual DLC sills contain extremely low Pt+Pd contents except for weakly anomalous samples in central portions of several sills. The overall patterns for two of these sills in particular consist of low Pt+Pd values that gradually increase with stratigraphic height to noticeable peaks, followed by dramatic dropoffs to virtually no PGE in the upper portions of the sills. It may be possible that this dramatic changeover in the Pt+Pd content may be related to a Sulfide Saturation??? 7 sulfide saturation event; whereby, PGE was scavenged from the melt. 6 A similar pattern has been reported for the Mesoproterozoic Sonju Lake Sulfide Saturation? intrusion, where a PGE-bearing reef 5 with up to 320 ppb Pd and 66 ppb Pt has been identified by Miller (1999). 4 3 However, detailed follow-up, close2 1 spaced, sampling at one of the “sulfide 0 50 100 150 saturation horizons” during 2004 failed Pt+Pd (ppb) to produce any significant PGE in the DLC. Figure 1. Vertical changes in Pt+Pd (ppb) contents with stratigraphic height in the various sills (numbered in boxes) of the Deer Lake Complex. Shaded areas are peridotite horizons. - 56 - Several anomalous gold values �Proceedings of the 51st ILSG Annual Meeting - Part 1 (>100 ppb) were obtained in samples collected from a drill hole (26508) near the base of the DLC. These values (up to 2,399 ppm Au) are from a well-foliated/sheared gabbro that contains 1-5% pyrite and pyrrhotite, and contains some of the highest Cu values (up to 0.07% only) in the DLC. In summary, the PGE potential of the Deer Lake Complex appears to be low but there is an outside possibility that some of the sills may contain PGE reefs that are associated with sulfide saturation events. On the other hand, Au potential appears to be better, especially in the sheared rocks in drill hole 26508. REFERENCES Berkley, J.L. 1972. The geology of the Deer Lake gabbro-peridotite complex, Itasca County, Minnesota. University of Missouri, M.S. thesis, 107p., 2 pls. Englebert, J.A. and Hauck, S.A. 1991. Bedrock geochemistry of Archean rocks in Minnesota. Natural Resources Research Institute, Technical Report NRRI/TR-91-12, 200 p. Jirsa, M.A. 1990. Bedrock Geologic Map of northeastern Itasca County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-68, scale 1:48,000. Miller, J.D. 1999. Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota. Minnesota Geological Survey Information Circular 44, 32 p. Ripley, E.M. 1973. The ore petrology and structural geology of the lower Precambrian Deer Lake mafic-ultramafic complex, Effie, Itasca County, Minnesota. Duluth, Minnesota, University of Minnesota Duluth, M.S. thesis, 143p., 2 pls. Severson, M.J. 1986. Summary report on exploration activities in the Deer Lake Area, Itasca County, Minnesota. Santa Fe Minerals Exploration report, Minnesota DNR open files. - 57 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 A view into the Nipigon Embayment: preliminary results of the largest magnetotelluric study ever in Ontario. SHAREEF, S., and CRAVEN, J.A., Geological Survey of Canada, Ottawa, ON K1A 0E9. Over 800 audiomagnetotelluric and combined audiomagnetotelluric and magnetotelluric (A/MT) sites were surveyed along five transects (Figure 1) during the period of late Aug to mid October 2004. The primary goal of the survey was to investigate the deep structures of the region to complement the studies that comprise the Lake Nipigon Regional Geoscience Initiative (LNRGI). The LNRGI was created to provide a comprehensive 4-D geoscience knowledge base in order to facilitate proper land-use decisions and to reduce the risk in mineral exploration of the region. The studies conducted as part of the Initiative are comprised of surface geological and stratigraphic investigation, surface and airborne geophysical surveys and other studies such as geochronological or paleomagnetic studies. Missing from these studies is an investigation of the deep geological structure. The A/MT survey conducted last year was designed to provide subsurface information down to mantle depths and beyond along five major transects. I\ I \. I B38AMTstes orna MT/MJT sites PR OT E H OZO IC Felsic to interrnethale metavolcanic rocks I relsic 0 Intern,edsIe fliwsive rocks —, Coarse dastic Melsedimentaly rocks ARCH EA N Felsic granitic rocks Melsedime ntaiy rocks Massive granodiorite '0 grane 20km Disraeli Lk 3-D • Larger 3-D • Figure 1. Site Locations for 2004 A/MT Nipigon Survey. Possible plan view locations for future 3-D model studies are also shown by red boxes. The magnetotelluric technique is a geophysical technique for imaging the deep structure of the Earth. It images conductors in the Earth and as such will be useful to delineate contacts between crystalline and sedimentary units at depth as they have dramatically different electrical characteristics. The novel feature of A/MT is that it uses the Earthʼs natural magnetic field as a transmitter of energy into the earth. This enables almost an infinite depth of investigation; however in practice the depth of investigation is limited to the asthenosphere. Preliminary results of the Nipigon A/MT survey will be presented with a view to providing a clue to the origin of the enigmatic sedimentary and intrusive units in this area. - 58 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mineral Deposits and Metallogeny of the Midcontinent Rift in Ontario SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA A variety of metallic and non-metallic mineral deposits are associated with Mesoproterozoic (Keweenawan) Midcontinent Rift (MCR)-related magmatism and associated hydrothermal activity in Ontario. Orthomagmatic deposits of copper, nickel and platinum group elements (PGE) are associated with early ultramafic to mafic intrusive rocks. Stratabound, “reef-style” Cu-Ni-PGE mineralization occurs in peridotite in the Seagull layered intrusion; within massive, cumulate Ti-Fe-oxide layers in the border gabbro of the Coldwell Complex; and with Cr-bearing spinel in cyclic anorthosite-gabbro units above the Great Lakes Nickel deposit in the Crystal Lake Gabbro. Varied-textured to pegmatitic gabbro at the base of the Crystal Lake Gabbro hosts the Great Lakes Nickel deposit (~ 41.4 mT @ 0.334% Cu, 0.183% Ni, 0.69 g/t Pd, 0.21 g/t Pt). Similar rocks of the Two Duck Lake gabbro in the Coldwell Complex host the Marathon deposit (24.4 mt @ 1.22 g/t Pd, 0.31 g/t Pt, 0.37% Cu). Late-stage, magmatic deposits of pyrochlore-hosted, U-Nb-mineralization have been investigated in the Prairie Lake Carbonatite Complex and in syenites of the Coldwell Complex (Nb-Zr-Th-U-Ce). Rare metal (Be-Zr-U-Th-) mineralized structures crosscut the Dead Horse Creek carbonatitic diatremes. Epigenetic deposits associated with MCR structures and hydrothermal activity are hosted in a variety Neoarchean, Paleoproterozoic and Mesoproterozoic rocks. Native copper and Cu-sulphides occur in Keweenawan basalt and interflow sedimentary rocks at Mamainse Point (e.g., Coppercorp Mine), on Michipicoten Island (e.g., Quebec Mine) and in Osler Group rocks in western Lake Superior. Hydrothermal fluids ascribed to MCR magmatism are believed to have produced Cu-Mo-Ag-Pb-mineralized breccia pipes in Archean country rocks at the Tribag Mine and Cu-Mo-mineralization at the Jogran porphyry deposit. Copper-mineralized carbonate units of the Mesoproterozoic Sibley Group are found near the contacts with Nipigon diabase sills. Silver-bearing, carbonate-quartz veins near Thunder Bay have been subdivided into two groups: the Mainland Belt (Ag + Zn-Cu-Pb-bearing veins in Paleoproterozoic Animikie Group sedimentary rocks and Logan diabase); and the Island Belt (Ag-Bi-Co-Ni-As-bearing veins in Keweenawan diabase/gabbro; e.g., Silver Islet Mine). Lead-zinc-barite veins occupy structures near the unconformity between Neoarchean basement and Mesoproterozoic Sibley Group sedimentary rocks. Some of these structures also host amethyst deposits. Silver-bearing, lead-zinc veins occur within and proximal to the Coldwell Complex. All of these veins are likely contemporaneous but were generated by different fluid sources. Uraniferous, hematite-rich veins and breccias that cut Neoarchean basement and locally contain Proterozoic xenoliths may also be related to MCR hydrothermal activity, which remobilized uranium from granitic pegmatites in the basement. - 59 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Discrimmination of Archean Terranes in the Sachigo Subprovince and relevance to Volcanogenic Massive Sulphide Exploration STOTT, G. M., Ontario Geological Survey, Sudbury, ON, P3E 6B5 (greg.stott@ndm.gov.on.ca), and RAYNER, N., Geological Survey of Canada, Ottawa, ON Geological and geophysical constraints on the mechanisms for tectonic growth of the Superior Province are progressively summarized in Williams et al., 1992; Stott, 1997; and Percival, 2005, the latter based on Lithoprobe and Natmap research activities in northwestern Ontario and Manitoba since 1997. This increased knowledge permits greater confidence in the subdivision of the Superior Province into terranes and superterranes of contrasting tectonostratigraphic history. Prior to this, for many years, the broad region of Archean crust north of Uchi Subprovince in Ontario was more commonly called the Sachigo Subprovince (e.g., Card and Ciesielski, 1986). A reconnaissance geochronological and Hf isotopic study was initiated on a transect of granitoid and felsic volcanic samples just west of the James Bay Lowlands (Figure 1) to complement the Natmap studies previously conducted near the Ontario - Manitoba border (e.g., Stone, 2005). Thus far, results from U-Pb geochronology in eastern Sachigo Subprovince further confirm that this subprovince can be subdivided into several terranes. Most notably, a broad Neoarchean, Oxford-Stull terrane contains crustal magmatic ages ranging from 2690 to 2730 Ma, and separates two major superterranes to the north and south, each of which contains evidence of > 3.0 Ga history. Since virtually all significant Cu-Zn volcanogenic massive sulphide (VMS) deposits in the Superior Province are Neoarchean in age, the presence of Oxford-Stull terrane bears some importance for exploration. A Neoarchean age of 2737±7 Ma has been determined for a felsic volcanic host of recently discovered VMS deposits in the McFaulds Lake area, inside the edge of the James Bay Lowlands (Figure 2). This argues for the potential of a chain of VMS occurrences to be explored along a Neoarchean tectonic assemblage extending from Kasabonika to the McFaulds Lake area, comparable to the VMS-rich assemblage along the discontinuous Shebandowan-Terrace Bay-Manitouwadge greenstone belt (Figure 2). Assean Lake On Lake Block Bloc SpFit Lake B!ock / / J&nes Bay Lowlands Greenstone Belt a Sedimentary Belt () Pltjtonjc Rocks Figure 1. A subdivision of NW Superior Province into terranes (modified from Stone, 2005). Uncertainty of boundaries increases eastwards reflecting current distribution of geochronological and isotopic data. - 60 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 _______ Kasabonika-MeFaulds greenslone bell I 0 100 I Olher greenstone bells A MFaulds Lake Cu-Zn * Community massive sulpriide deposits (Spider Resou rcesIKWG Resources exploration) 200 K Ia metres 100 200 — Shebandowan-Terrace Bay-Michipicoten band aigreenstone belts Other greenstone bells Kilometres a Cu-Zn massive sulphide deposits Community Figure 2. (a). The discontinuous Neoarchean Kasabonika-McFaulds greenstone belt is the focus of new Cu-Zn VMS exploration, and may resemble (b) the Shebandowan-Terrace BayManitouwadge belt, a bandlike tectonic assemblage, which contains several VMS deposits and mines along its length. References Card, K.D. and Ciesielski, A. 1986. Subdivisions of the Superior Province of the Canadian Shield. Geoscience Canada, 13, 5-13. Percival, J.A. 2005. The Ancient Earth: Development of the Canadian Shield, the oldest parts of North America – building a continental foundation from 4000 to 2500 million years ago. In LITHOPROBE Celebratory Conference Oral and Poster Presentations, R.M. Clowes and C. Li (compilers). Published by the LITHOPROBE Secretariat, University of British Columbia, E-Publication No. 5, v.1. Stone, D. 2005. Geology of the Northern Superior Area, Ontario. Ontario Geological Survey, Open File Report 6140, 94p. Stott, G.M. 1997. The Superior Province, Canada. In Greenstone Belts; Edited by M.J. de Wit and L.D. Ashwal. Oxford Monograph on Geology and Geophysics 35, Oxford Clarendon, p.480-507. Williams, H.R., Stott, G.M. and Thurston, P.C. 1992. Tectonic Evolution of Ontario: Summary and Synthesis, Part 1: Revolution in the Superior Province. In Geology of Ontario; Ontario Geological Survey, Special Volume 4, Part 2, pp.1256-1294. - 61 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Correlation Between Self-Potential and Dowsing (IESG) at the Quincy Mine and at the Calumet and Hecla Mine, Michigan TROW, Jim, Geological Sciences, Michigan State University, emeritus, 540 Lake Avenue #2, Hancock, Michigan 49930, and YOUNG, Charles, T., Geological Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931. The focus of this paper is an 11/24/04 joint traverse over and mapwise perpendicular to the strike of the native copper Calumet Conglomerate Lode at the Hecla #1 Mine, Calumet Michigan, with self-potential (Young) and dowsing (Trow) observations plotted in profile on Figure 1. According to Young (Young and Trow, 2005), “The self-potential apparatus consists of hand-made non-polarizing copper copper-sulfate electrodes, a reel containing AWG 20 stranded, tinned copper wire, patch cords and clip leads and a high-input-impedance digital voltmeter. The reference electrode is installed at a convenient sheltered position along the line where the wire can be tied to a sturdy object. The wire is clipped to the reference electrode and is unwound from the reel to the observation points. The electrodes are installed in shallow holes made with a garden trowel, which is used to remove loose and dry surface material so that the electrode is in contact with moist soil. The repeatability of these self-potential measurements is typically about five millivolts.” Horizontal distances are measured with fiberglass surveyorʼs tape. ! 9 U,. I I .4. •tlfl I 2. . .: I Ic_ — 14• 1994 99 99 —l—m ••91 •U!14944 .9*9 .11 — 191 •tt3flt 1194•t IOU . .tn. T: t 93 9 —Ic 9• 9• 4 .99919,.., ••—1199,• .9ltI. ••!T499 •tC •34 .9 fl I • ni. it/c oIl •!lM •_• 'ull •;3• •Tt •.94 l.!fl.. •4n•9& .4..•.9nulb. .9 9* 9 ' 9 ct .•9t m .mE .9 — •n..Tp •9c! .l l •jj•9] 4!—.. •.—_. •93 •lS.9 •fl9 .9!,; fl .9I •.jfl.9 1.991 •9Iul97_ •3'9n• .9—. .4,3. 9 .9 •..I—_,.9— . . *1 £9 c_fl.... ••..!p •!! 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Figure 1 shows dowsing by Trow with sensitive 3/16”-diameter 6” x 15” iron welding rods, bent into a Ushape, and with less sensitive 6” x 15” L-rods of the same diameter and material. Dowsing with U-rods reset to the standard state after each episode of turning (CCʼ) detects a broader and lower anomaly than Youngʼs –350 mV SP anomaly (AAʼ), but both appear to be distributed symmetrically about the SP negative center, as though dowsing implies an upward-continuation curve for the SP. After all, SP is a conductive method, whereas dowsing is an inductive “airborne” method, involuntarily detected by oneʼs brain (Faraday cage shielding experiments; - 62 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Trow, 1992). BBʼ shows the less sensitive L-rod dowsing results. When favorable weather returns, Trow would like to reverse by traversing SE, to see if the L-rods define the NW edge of the subcrop of the ore body as BBʼ defines its SE edge when traversing NW. DDʼ represents Trowʼs dowsing with the sensitive U-rods, when they are not reset to the standard state after turning, in conformity with most dowsersʼ procedure, which ignores important data compared to CCʼ. They have run out of lever arm (physics terminology). Figure 2 (not shown, please see poster) suggests three hypotheses for a mathematical function to reconcile SP and dowsing: The geologically most plausible is that the square root of the number (x) of Urod convergences over the total –SP anomaly is proportional to the –SP readings of its negative center. An even better fit than Figure 2 results when one considers the total SP relief on Figure 1, from +15 mV at Aʼ to the –350 mV negative center = -365 mV. Calculating for Quincy [(√7/√13)(-365mV)] yields – 268mV, 99% of Youngʼs observed – 270 mV there. Two points, Quincy and C & H, do not define a curve, but they hint at one. Figure 3 (not shown, please see poster) symbolically illustrates (some elements displaced to avoid overlap) characteristic dowsersʼ rod movements, which Trowʼs (1992) experiments with permanent magnets, low-voltage static electric charges, and Faraday cages demonstrated are related to electrostatics, not magnetostatics. REFERENCES Trow, J. 1992. Inductive electrostatic gradiometry (IESG) deciphers Keweenanwan copper plumbing system, Soc. Mining, Metall. And Expl., Phoenix, Preprint 92-32, 22p. Trow, J. 2004. Dowsing employs classical mechanics and static electricity to locate self-potential anomalies inductively and rapidly, 50th Institute on Lake Superior Geology, Duluth, Proceedings, v.50, pt 1., 158-159. Young, C.T. and Trow, J. 2005. Human-sensed fields?: Does dowsing response correlate with self-potential or conductivity anomalies?, 18th Annual Symposium on the Application of Geophysics in Engineering and Environmental Problems, Atlanta, 9p. - 63 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions VALLINI, Daniela A., School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia, CANNON, William F., and SCHULZ, Klaus J., U.S. Geological Survey, 954 National Center, Reston, VA 20192 A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior region, is attempted for the first time. Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the overlying Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317±6 Ma, in the Sturgeon Quartzite it is 2306±9 Ma and in the Sunday Quartzite it is 2647±5 Ma. The oldest hydrothermal xenotime in the Enchantment Lake Formation is 2133±11, in the Sturgeon Quartzite it is 2115±5 Ma and in the Sunday Quartzite it is 2207±5 Ma. The depositional age of the Chocolay Group is constrained to 2300-2200 Ma and the radiometric age data proves it is correlative with part of the Huronian Supergroup in the Lake Huron region, Ontario and reveals that the unconformity which separates the Chocolay Group from the overlying Menominee Group is up to 350 m.y. in duration. The detrital zircon suite in the Sunday Quartzite and part of those in the Enchantment Lake Formation and Sturgeon Quartzite were derived from the underlying Archean basement rocks. The source of the ~2300 Ma detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma as there is no known terrane of this age in the Michigan area. It is speculated that there was once a widespread volcano-sedimentary cover sequence in Michigan which was removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages are coeval with the 2219±4 Ma Nipissing Diabase in the Lake Huron region and the 2200-2100 Ma Kenora-Kabetogama mafic dyke swarm in Minnesota and adjacent Ontario, Lake Superior region. - 64 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Implications of Midcontinent Rift and Oceanic Ridges Analogies and 3DInterpretations of the Subsurface Structure of the Bald Eagle Intrusion in the Duluth Complex and the East Pacific Rise WEIBLEN, Paul, Department of Geology & Geophysics, University of Minnesota, PETERSEN, Dean, Natural Resources Research Institute, University of Minnesota Duluth, and VISLOVA, Tatiana, Department of Geology University of St. Thomas The geometry of the gravity and magnetic anomalies of the Bald Eagle intrusion and the Duluth Complex to the south, as well as the overall Midcontinent Rift, is very similar to the pattern of the seismic reflections profiles of ridge systems (Vislova, 2003). In detail, the geophysical expressions of the Bald Eagle intrusion have the same shape and dimensions as the “bulls eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. This suggests that the Bald Eagle intrusion could be a “frozen” dynamic magma chamber. In support of this analogy we note that the magmatic systems of mid-ocean ridges, extensional regimes in back-arc environments, and ophiolites have a common characteristic: the emplacement of magma in extensional environments, and the common products in all four are varieties of layered intrusions, dikes and sills, and overlying volcanic rocks. There is a long history of comparative studies of ophiolites and oceanic magmatic environments (See web sites for ophiolites and the East Pacific Rise). This suggests that it would be profitable to add the studies of the igneous systems of intracontinental rifts to the current interdisciplinary, studies of ophiolites and the oceanic environments. In the accompanying poster session we explore aspects of this approach using 3D modeling of surface mapping, geophysical, and drill hole data of the basal mineralized zone of the Duluth Complex, the Bald Eagle intrusion and seismic reflection data of the East Pacific Rise. Since its discovery in 1961, the Bald Eagle intrusion has posed unresolved questions concerning its origin and magmatic significance (Weiblen, 1965; Weiblen and Morey, 1980; Miller and others, 2002). A number of its characteristics contrast markedly with those of the other mapped intrusions in the Midcontinent Rift: (1) It has a well-defined intrusive contact in anorthositic gabbros around its perimeter except for its southern extension; (2) There is a subtle, but recognizable metamorphic contact effect on the anorthositic gabbros; (3) A primary magmatic foliation is well defined by mineral orientation and discoid segregation of plagioclase from mafic phases; (4) Over 150 measurements of the foliation define a steeply-dipping asymmetric funnel with the foliation paralleling the contact and grading from steep to horizontal inward; (5) The intrusion consists of two cumulus units, a plagioclase-olivine outer cumulate (troctolite) and an inner plagioclase-olivine-clinopyroxene (olivine gabbro) cumulate; and (6) There is only minor (< a few %) intercumulus material in the cumulates which is found as the expected lower temperature minerals in the crystallization sequence, i.e. clinopyroxene and iron oxides in the plagioclase-olivine. Recent petrologic studies include over 2000 electron microprobe analyses and interpretations using the computer-based mineral-melt equilibria routines MELTS and COMAGMAT (Vislova, 2003). These studies indicate that the following scenario could account for the characteristics of the Bald Eagle intrusion listed above. As up-welling magma streams through a dynamic (expanding) funnel-shaped feeder, a constant temperature appropriate to plagioclase-olivine crystallization is maintained by a balance between the heat content of the incoming magma plus the heat of crystallization and the heat loss through the chamber walls. Plagioclase and olivine are left behind and oriented/segregated on the walls of the expanding chamber. At some point conditions change and a temperature appropriate to plagioclase-olivine-clinopyroxene crystallization is established and maintained to form the inner olivine gabbro of the intrusion. This scenario could also produce the plagioclaseolivine-clinopyroxene-iron oxide cumulates found to the south of the mapped expression of the Baled Eagle intrusion. A dynamic, flow-through magmatic system raises questions about mineralization. We note that there are active “smokers” above the proposed magma chambers on the East Pacific Rise. In view of this and assuming - 65 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 the analogy suggested above is appropriate, comparative studies of the smokers and the sulfide mineralization in the intrusions adjacent to the Bald Eagle intrusion may be profitable. Applying this approach, some of the attributes of the Duluth Complex copper-nickel-PGE sulfide deposits resemble those of deposits at Norilʼsk, Russia and Voiseyʼs Bay, Canada that are associated with sulfide mineralization in intrusive feeder zones. The common attributes include occurrence in shallow tholeiitic intrusions associated with plateau basalt volcanism, an external sedimentary source of sulfur, and openness to repeated magma influx and expulsion. A critical attribute of the high-grade Norilʼsk-Talnakh and Voiseyʼs Bay deposits, not yet positively identified in the Duluth Complex deposits, is the location of a magma conduit. A conduit that experienced repeated influxes of magma appears to be key to the formation of high-grade copper-nickel-PGE deposits (Naldrett, 1997). One of the difficulties in evaluating the potential for feeder zone mineralization in the Duluth Complex is determining whether intrusions were fed one-by-one by local magma conduits or by master conduits that sequentially fed several intrusions. Another long-standing question is the source of sulfur. In the case of the Norilʼsk deposits, Jurassic sediments are obvious candidates as are the Virginia and Biwabik Iron formations for the mineralization at the base of the Duluth Complex. However, designation of these sources begs the question of how, when, and where the sulfur is delivered to the magmatic system. This is more straightforward with regard to smokers: circulating hydrothermal solutions deliver sulfur from sea water directly to magma chamber vents. We suggest that consideration of this mechanism in the Midcontinent Rift magmatic systems could provide new insights for exploration targets. Field mapping (Green and others, 1966; Foose and Cooper, 1978) showed the Bald Eagle and the South Kawishiwi intrusions to be in direct contact. Petrologic observations and geophysical interpretations (Chandler, 1990; Chandler and Ferderer, 1989) suggest that the Bald Eagle and South Kawishiwi intrusions were emplaced by successive overplating of magmas from a common feeder centered on the northern Bald Eagle intrusion and extending along the trace of the macrodike that links the Bald Eagle and South Kawishiwi intrusions. Another model relating the emplacement of the two intrusions to a single magma feeder has been presented by Miller and Severson (2002; their Fig. 6.15). This model further speculates that the Bald Eagle intrusion had a more complete differentiation sequence (this contrasts with the dynamic flow model of Vislova (2003) outlined above) that has been eroded away and that this magma system may have fed surface eruptions. At a more detailed level, Peterson (2001) interpreted the copper-PGE mineralization in the Maturi deposit and its extension to the east (Maturi Extension deposit) as indicative of magma input from the northwest via the arcing macrodike that connects the Bald Eagle and South Kawishiwi intrusions. Peterson (2001) envisioned a confined magma flow model that invokes a change from laminar to turbulent flow beneath a pillar of older anorthositic series rocks, increasing the R-factor of the entrained sulfides, and resulting in higher metal contents of the exited (Maturi deposit) and remaining (Maturi Extension deposit) sulfide fraction. If correct, this model predicts that a Voiseyʼs Bay-type copper-nickel-PGE massive sulfide body may exist in an area south of the Spruce Road deposit. References Chandler, V.W. 1990. Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota. Economic Geology, v. 85, no. 4, p. 816-829. Chandler, V.W. and Ferderer, R.J., 1989, Copper-nickel mineralization of the Duluth Complex, Minnesota-A gravity and magnetic perspective: Economic Geology 84, 1690-1696. Foose, M.P. and Cooper, R.W. 1978. Preliminary geologic report on the Harris Lake area, northeastern Minnesota. U.S. Geological Survey Open-File Report 78-385, 24p., 1 pl., scale 1:12,000. Green, J.C., Phinney, W.C. and Weiblen, P.W. 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:31,680. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D. and Severson, M.J. 2002. Chapter 6: Geology of the Duluth Complex, in Miller, J.D., Jr., Green, J.C., Severson, - 66 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M. and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Naldrett, A.J. 1997. Key factors in the genesis of Norilʼsk, Sudbury, Jinchuan, Voiseyʼs Bay and other world-class Ni-CuPGE deposits: Implications for exploration. Australian Journal of Earth Sciences, 44, no. 3, 283-315. Peterson, D.M. 2001. Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi intrusion, Duluth Complex, Minnesota. Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario, 3 pages. Vislova, Tatiana. 2003. Petrology of the Bald Eagle Intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift, unpublished PhD. Thesis, University of Minnesota. Weiblen, Paul W. 1965. A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota, unpublished. PhD. Thesis, University of Minnesota. Weiblen, P. W. and Morey, G. B. 1980. A summary of the stratigraphy, petrology, and structure of the Duluth Complex. American Journal of Science, 280A, Part I, 88-133. - 67 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Textural Examination of Kama Point Diabase Sill, Nipigon, Ontario ZIEG, Michael J., FORSHA, Clinton J., and HABARKA, Joseph D., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA, 16057, USA, michael. zieg@sru.edu The texture of an igneous rock provides valuable constraints on the cooling history of the magma from which it crystallized. When combined with mineralogical data, the sizes and abundances of crystals in a rock reflect the interactions between the thermal and chemical evolution of the parent magma. In order to avoid uncertainty relating to the absolute magnitude of kinetic parameters such as nucleation and growth rates, this study focuses on textural gradients. These gradients reflect differences in crystallization history and magma composition, and are related to differences in the thermal and compositional state of the magma. The lowermost 35 meters of the 1100 Ma olivine diabase sill at Kama Point, approximately 25 km east of Nipigon, Ontario, has been sampled at ~3 meter intervals from the basal contact upwards. For each sample, textures were quantified using crystal size distributions (CSDs) as well as measurement of mean crystal size. As expected, the CSDs exhibit a clear coarsening-inwards trend through the lowermost 25 meters of the sill. However, above 25 meters, the texture becomes finer-grained. Above this, the coarsening-inwards trend continues, but with a lesser gradient (Figure 1). In this study, analysis of plagioclase CSD data is shown to provide a method for quantitatively evaluating hypotheses for magmatic processes as well as important constraints on crystallization kinetics. CSD Intercept Variation CSD Slope Vailation . . . . . . . 20 . 20 . . . . . . 0 9 10 . . 11 . -14 12 Intercept, In(ni Imm41 -12 -10 -O -S -4 -2 Slope Imm11 Figure 1. Variations in CSD intercept and slope. Intercept is related to the number of crystals per unit volume; slope is related to the mean length of the crystals in the sample. With increasing height above the base of the sill, the number density decreases and the mean length increases. These trends are consistent with increasing cooling durations. These The measured textural gradients have been compared to calculated magmatic cooling durations, based on conductive cooling of an instantaneously injected 200 meter thick diabase sill. The initial conditions for the cooling model are an initial magma temperature of 1200°C and an initial country rock temperature of 100°C; far-field temperature is held at 100°C. The model is used to calculate the length of time required for the magma to cool to a temperature of 1000°C, which is the approximate solidus of the Nipigon diabase. Measured textures were compared to the calculated cooling durations (Figure 2), which yields a bulk growth rate of 1.8×10-10 mm/s (R2 = 0.925). The total number of crystals decreases with cooling duration, consistent with a nucleation rate controlled by the degree of undercooling, which is maximized during rapid cooling. The break in textural gradient at 25 meters is attributed to a recharge event in which a second pulse of - 68 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Mean Length vs Cooling Duration Total Number vs Cooling Duration 5T0 — 4T0 020 E E E 3T0 . C 0,15 a C -I . C 2T0 z 0.10 ITO . . 0 10 20 0 40 10 20 30 40 Cooling Duration yrJ Coo Iiruj Duration [yrj Figure 2. Variations in textural parameters obtained from CSD intercept and slope. Textural parameters are related to calculated cooling durations. 25-m break in texture corresponds to 22-yr cooling duration. magma is injected into the magma chamber, which had at that point crystallized a thickness of 25 meters. This event is believed to have taken place approximately 22-25 years after the initial injection. Cooling of the new batch of magma was slower than cooling in the initial injection because of the cooler, previously crystallized diabase located between the fresh magma and the cold country rock. This new cooling environment qualitatively explains the change in textural gradient above the 25-m discontinuity. Additional modeling will focus on relating this discontinuity to a specific set of recharge conditions. Further field investigation of the sills in the Nipigon area is planned. This work will extend the sampling transect higher into the sill and will include samples from higher in the sill. Goals of this research include a more thorough understanding of the injection history and original dimensions of this intrusion, as well as empirical relationships between intrusion dimensions, initial and boundary conditions, and textural profiles. - 69 - �Proceedings of the 51st ILSG Annual Meeting - Part 1 Author Index Addison, W. 1 MacDonald, C.A. 22 Albers, P. 3 MacDonald, J. 12 Anderson, R. 10 MacTavish, A. 36 Blackburn, C. 5 Magee, M.A. Breckenridge, A. 7 Maric, M. Brumpton, G. 1 McNaughton, N. Bucholz, T. 8 Medaris, L. Cannon, W. 10, 27, 64 Metsaranta, R. 17, 45 Chandler, V. 10, 27 Middleton, R. 47 38, 39 41 1 43 Conly, A. 12 Miller, J. Coombes, S. 54 Murphy, J. 50 Craven, J. 58 Petersen, D. 65 Dahl, D. 14 Planavsky, N. 50 Daniels, D 10 Rayner, N. 60 Daniels, P. 30 Richardson, A. 52 Davis, D. 1 Rogala, B. 17 43 Rossell, D. 54 Saini-Eidukat, B. 30 Dott, R. Easton, R.M. 15, 24 Falster, A. 8 Schneider, D. Forsha, C. 68 Schnieders, B. Fralick, P. 1, 17, 39, 41, 45 Schulz, K. 3, 28, 30, 48 10, 27 55 10, 27, 64 Franklin, J. 19 Scott, J. 55 Habarka, J. 68 Severson, M. 56 Halls, H. 20 Shareef, S. 58 Hammond, A. 1 Simmons, W. 8 Hart, T. 22 Smyk, M. Heaman, L. 24 Stott, G. 60 Heggie, G. 47 Trow, J. 62 Hill, M.L. 26 Vallini, D. Hollings, P. 32, 34, 39, 52 Van Schmus, W. 26, 59 1, 64 10, 27 Holm, D. 10, 27 Vislova, T. 65 Jirsa, M. 28, 48 Weiblen, P. 65 Johnson, T. 7 Young, C. 62 Kissin, S. 1 Zieg, M. 68 Knudsen, D. 30 Laarman, J. 32 Lane, C. 34 - 70 - � https://digitalcollections.lakeheadu.ca/files/original/942b8efd07100ae7b5e8a86a8ac19475.pdf 2e91b73833d712df8bd61840ebb527f9 PDF Text Text 51st ANNUAL MEETING Nipigon, Ontario - May 24-28, 2005 INSTITUTE ON LAKE SUPERIOR GEOLOGY Part 2 – Field Trip Guidebook 51st ILSG Nipigon 2005 wwwIakesuperiorgeology.org �51st ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 24-28, 2005 Nipigon, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 51 Part 2 – Field Trip Guidebook Compiled and edited by Pete Hollings, Lakehead University Cover Photos: Left - basaltic dyke cutting Osler volcanics on Wilson Island, Middle - Aerial view of the Black Sturgeon fault, Right - Ruby Lake near Nipigon, Northern Ontario. �51ST INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 51 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: GEOLOGY AND FOLD MINERALISATION OF THE BEARDMORE-GERALDTON GREENSTONE BELT TRIP 2: QUATERNARY GEOLOGY OF THE BEARDMORE – NIPIGON AREA TRIPS 3 & 6: A STRATIGRAPHIC TRANSECT ACROSS THE NORTHERN FLANK OF THE MIDCONTINENT RIFT NEAR ROSSPORT TRIP 4: GEOLOGY AND RARE ELEMENT PEGMATITES OF THE QUETICO SUBPROVINCE NEAR NIPIGON TRIP 5: GEOLOGY OF THE BLACK STURGEON AREA Reference to material in Part 2 should follow the example below: Hart, T.R., 2005. Geology of the Black Sturgeon Area. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 2-39. Published by the 51st Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table of Contents Introduction, safety considerations and acknowledgements ...............................................1 Fieldtrip 1 - Geology and gold mineralisation of the Beardmore-Geraldton greenstone belt ..............................................................................................................................3 Fieldtrip 2 – Quaternary geology of the Beardmore – Nipigon area ................................41 Fieldtrips 3 and 6 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport .....................................................................................................57 Fieldtrip 4 - Geology and rare element pegmatites of the Quetico Subprovince near Nipigon .....................................................................................................................71 Fieldtrip 5 - Geology of the Black Sturgeon Area .........................................................85 -i- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Mark Smyk Resident Geologistʼs Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada This volume is intended to serve not only as a guide for 51st ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently we have included UTM coordinates in the NAD 83 datum for stops, as well as instructions on how to reach them. As some of the stops are on private and staked land, particularly on Trip 1, please be sure to obtain the land owners’ permission before entering their land. In what is perhaps a first for the Institute this year we are offering a fieldtrip onto Lake Superior. This creates a number of unique safety issues. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. If you are planning to revisit these sites please be very careful as Lake Superior is a dangerous lake, waves can often be many metres high and even in mid summer fog can appear very quickly. major highways or busy logging roads. Please take care when crossing or parking along these roads. We would like to thank all the other authors who contributed to this field guide (Peter Barnett, Tom Hart, Phil Fralick and Steve Kissin) and also all those who provided comments and assisted with the running of the field trips themselves. We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties, particularly David Malouf, Roxmark Mines Limited. The other fieldtrips will be visiting stops along either LAKE r _____9— NIPIGON 0 ta Figure 1. Map showing the location of the five field trips. -1- �Proceedings of the 51st ILSG Annual Meeting - Part 2 -2- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 1 - Geology and gold mineralisation of the Beardmore-Geraldton greenstone belt Mark Smyk Resident Geologistʼs Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Thomas R. Hart Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction The Beardmore-Geraldton belt (BGB) is a Neoarchean metavolcanic-metasedimentary terrane at the boundary between the Quetico Subprovince and the eastern Wabigoon Subprovince of the Superior Province (Fig. 1a). The belt can be subdivided into six east-striking sub-belts, all of greenschist facies metamorphic grade (Fig. 1b; Devaney and Williams, 1989). They are: the northern metasedimentary subbelt (NMB), northern volcanic sub-belt (NVB), central metasedimentary sub-belt (CMB), central volcanic sub-belt (CVB), southern metasedimentary sub-belt (SMB) and southern volcanic sub-belt (SVB) (Fig. 2). The northern units are not as laterally continuous as the southern units. Previous studies have shown that although these belts are fault-bounded (Fig. 3), mainly along the southern boundaries of the volcanic sub-belts, they probably reflect an original sedimentary assemblage deposited on a cratonic margin in environments ranging from alluvial fan-braid plain in the NMB, through fan delta-braid delta in the CMB to a submarine fan/ramp in the SMB (Devaney and Fralick, 1985; Barrett and Fralick, 1985; Devaney, 1987; Barrett and Fralick, 1989). Original continuity of this succession prior to tectonic disruption cannot be proven, but is supported by consistent stratigraphic trends and sedimentary structures that mostly young to the north. Although isoclinal folds are present on varying scales, particularly in the southeast part of the terrane near Geraldton, the overall structure of the belt appears to be initially one of stacked, imbricate, internally northward-younging sheets which have been interpreted as the product of accretionary wedge tectonics (Williams, 1986, 1987; Devaney and Williams, 1989; Williams and Stott, V (,n Hudson I et_w. N LEGEND A Paleozoic Cover Prolerozoie Covet Grenville Provincc — •Abitpbi Churchili Province Supenor Province Uciti — Areas: E'iglish River ?ontic Winnipeg LANADA 1 U.SA -Wabigeon' rnIoAea Metasedimentary Belts Cranitic Areas Volcanic Sedimentary Belts Cneissic Terranes -'-—-Quetico kiii Wawa Figure 1a. Location of the Beardmore-Geraldton greenstone belt, Superior Province. -3- �- Proceedings of the 51st ILSG Annual Meeting - Part 2 15 /a..aaa..N?LTnsl r4 I Garuiue—gncnMone suhpnwincc BanMto,e-CcSiion Deli 8GB Aras: I RSy Ltt di*ICL 2 SticN.id.iwan. 3 Manitounfrj 4 tolhioo thNuicl Tnsuc Am" Beardmore-Geraldton Belt belts 0 parsoihests $1 -i Luio 3M IO knu NY iw :2 -r 01 lnieatd II) PROTEINOZOIC 7/4 —— ARCHEAN Da,d.a-G.nIdtoc Bell Eli couuw - kin C rn Md C a_-it ia Pset.poducloIdrrnne; ItAilch. 3 M.ctcodccxbhutt Met.moiplmaad seethtooe & s1flhit ease mdtvokiSv ivck. MaBe In • 2 BaikThdd. • -_ - Real siguiada. number • moqawI saooe. tgdlnc. polymjdk iIIoma*t ta.2à3 — Road and b#way n-ri PWd TrW IbuuaJIe In Sk Geld pefl I' L::: SmedlNemMtmeIndc.rncTocks: o P18nMIS _____ Sn arc cavok reds ___ ______________ Figure 1b. Regional geology and field trip stop locations modified after Lafrance et al. (2004); see text for sub-belt index. 86aow • + * * • * • * • * • • * * • • * • * * • . * • • * •*.,* + * • * * • * 50N • • • * * + ..:wabi9oonsubProvince:.::4:.:.:3> nCCMB t :/. * * Bea rdmore— Geraldton Belt ., 'I, SVB r?ij::::::::::Quetit •:i• _.r.Tir2_!_._&r / I . 10 krii *.] Granitoids Metnodiriiontary rooks I • Felsic 10 irlterrMdiate rrmtavolcanic rocks :E:E:E.j Quetico Subprovince sedimenlary rocks Mafic metavolcanic rocks Figure 2. Generalized geology of the Beardmore-Geraldton belt; see text for sub-belt index. -4- �Proceedings of the 51st ILSG Annual Meeting - Part 2 minor interflow chert-magnetite iron formation. The northern metavolcanic sub-belt (NVB) is subdivided into the northern Bish Bay assemblage (BBA) and the southern Poplar Point assemblage (PPA) (Hart et al., 2002; Fig. 4). The BBA is composed of eaststriking, mafic pillowed to massive flows and rare tuffs resembling the SVB. The PPA consists of northweststriking, intermediate flows, tuff-breccias and tuffs resembling the CVB, with subordinate mafic massive and pillowed flows. Qu.tIo&Wabgoon SVB OVB NVB B,! p LS Z w Figure 4. Schematic cross-section of the southern half of the eastern Wabigoon Subprovince showing imbrication of fault-bounded metavolcanic (VB’s) and metasedimentary (MB’s) sub-belts (modified after Stott and Davis, 1999); S = Southern; C = Central; N = Northern; PLSZ - Paint Lake Shear Zone; B - Blackwater Fault; P - Princess Lake Fault; W - Standingstone - Watson Lake Fault. 1991). Thus, the steeply dipping sheets probably represent the telescoped version of an original cratonic margin assemblage. South of the Beardmore-Geraldton belt, more highly metamorphosed, turbiditic sediments of the submarine margin and trench-fill are present in the Quetico Subprovince (Williams, 1986; Fralick et al., 1993). A number of rock types intrude the supracrustal rocks of the BGB, including a series of mafic to ultramafic, synvolcanic, intermediate to felsic synvolcanic, mafic post-tectonic intrusive rocks and diabase dikes (Hart et al., 2002). The synvolcanic gabbroic rocks form thin sills, subparallel to the strike of the mafic metavolcanic rocks of the SVB and the BBA. A large intrusive body located in the BBA is a composite intrusion with gabbroic and peridotitic phases in the southern and northern parts of the intrusion, respectively. A series of intermediate to felsic dikes and sills ranging from massive granodiorite to quartz-porphyritic, feldspar- N The presence of the Onaman-Tashota volcanic arc terrane (ca. 2740 Ma; Stott and Davis, 1999) to the immediate north, the imbricate thrust structure of the Quetico metasedimentary rocks to the south (Williams, 1986, 1987), and the style of sedimentary depositional systems in the region strongly suggests that the Beardmore-Geraldton belt represents a forearc assemblage or a cross section of a complete island arc system (see “Depositional Environments”) (Fig. 3). The mostly mafic volcanic rocks which form the basement to the sedimentary rocks are probably slivers of oceanic volcanic terranes which were accreted prior to development of the clastic wedge. Similar depositional and tectonic evolutionary trends in the Great Basin of California have been described by Dickinson and Seely (1979). 0 1 Dorothea Sandra 2 Km BBA NVB CSB PPA CVB Lake Nipigon Eva Summers Eva Lake Hwy 11 Hwy 580 SSB SVB Igneous rocks Mafic metavolcanic rocks of the southern metavolcanic sub-belt (SVB) consist of massive and pillowed flows with minor tuffs, lapilli tuffs, and tuff breccias and interflow chert-magnetite iron formations. The central metavolcanic sub-belt (CVB) consists of intermediate massive and pillowed flows with significant tuffs, lapilli tuffs and tuff breccias, and ELMHIRST -RICKABY Camp 72 Road 8..,dmor.-G.raldton R CN Beardmore w rail ay Mafic Intrusive rocks Ultramafic Intrusive rocks Mafic Metavolcanic rocks Intermediate Metavolcanic rocks H Kitto QUETICO SIBLEY Felsic Metavolcanic rocks Metasedimentary rocks Sibley Group sedimentary rocks Post-tectonic Intrusive rocks Synvolcanic mafic intrusive rocks Synvolcanic felsic intrusive rocks Figure 4.of theGeology ofArea,the Beardmore area (HartNVBet al., General geology Beardmore-Geraldton Kitto, Eva, Summers, Dorothea and Sandra townships. Northern Metavolcanic sub-belt, CVB-Central Metavolcanic sub-belt, SVB-Southern Metavolcanic sub-belt, SSBSouthern Sedimentary sub-belt, CSB-Central Sedimentary sub-belt, PPA-Poplar Point Assemblage (part of the NVB), 2002); BBA Bish Bay Assemblage; PPAPoplar Point BBA-Bish Bay Assemblage (part of NVB). Assemblage. -5- �Proceedings of the 51st ILSG Annual Meeting - Part 2 20 Ti/Zr 15 10 5 0 0 5 10 15 La/Yb Figure 4. Zr/Ti versus La/Yb diagram for the metavolcanic rocks of the Beardmore area. Circles - SVB, open and filled triangles - CVB, squares - NVB. Data from Hart et al. (2002) and Tomlinson (1996). porphyritic and feldspar-quartz-porphyritic phases occur within the metavolcanic rocks of the PPA. These units appear to have been emplaced along the regional foliation, although some bodies are subhorizontal in orientation. A feldspar-porphyritic granodiorite dike intrudes the mafic flows of the SVB and resembles the dikes of the PPA. Late, post-tectonic diorite sills predominantly occur within the metasedimentary and metavolcanic rocks along the contact between the SSB and CVB. Additional intrusions located along the northern and southern contacts of the PPA are generally undeformed diorite sills that have chilled contacts with the metasedimentary rocks. A swarm of narrow, generally north-striking diabase dikes intrude the supracrustal rocks and appear to be predominantly Paleoproterozoic in age. A series of Mesoproterozoic diabase sills of the Nipigon Sill Complex intrude all other supracrustal rocks of the BGB. Geochemical groupings in the metavolcanic rocks span the three belts within the western portion of the BGB. The mafic metavolcanic rocks of the SVB and NVB are predominantly basalt with high Ti/ Zr and low Zr/Y, and negative slopes on mantlenormalized, extended element diagrams characteristic of mid-ocean ridge basalt (Fig. 5; MORB; Tomlinson, 1996; Hart et al., 2002). A subset of basalt samples have a mixture of elemental abundances and ratios characteristic of subduction-related volcanic rocks which led Tomlinson (1996) to suggest that these rocks formed in a back-arc basin, similar to the Lau Basin, where there are volcanic rocks with both arc and/or MORB geochemical characteristics. The intermediate metavolcanic rocks are generally confined to the CVB and PPA of the NVB and are predominantly andesite. The andesites have low Ti/Zr, high Zr/Y, and positive slopes with prominent negative niobium and tantalum anomalies on a mantle-normalized, extended element diagram, that are generally characteristic of calcalkaline andesite formed in an island-arc environment (Fig. 5; Tomlinson, 1996; Hart et al., 2002). Two felsic metavolcanic rocks from the CVB and the PPA were analysed by Hart et al. (2002) , and have low (La/Yb)n ratios with moderately high ytterbium values characteristic of FII felsic metavolcanic rocks (Lesher et al., 1986), indicative of a potential to host volcanogenic massive sulphide mineralization. Both samples have elemental abundances and ratios characteristic of active continental margin and/or the destructive plate margin felsic volcanic rocks (e.g., Wood, 1980). A sample of the flow from the PPA has a U/Pb age from zircons of 2724.9 ± 1.1 Ma and the flow from the CVB has a U/Pb age from zircons of 2724.9±1.2 Ma (Hart et al., 2002). The geochemistry of the intrusive rocks of the western portion of the BGB was characterised by Hart et al. (2002). The mafic intrusive rocks have elemental abundances and ratios that generally reflect the composition of the metavolcanic rocks that they intrude. Unfortunately these similarities also mean that the geochemistry does not aid in differentiating coarse-grained, gabbroictextured flows from subvolcanic, gabbroic intrusions. Both the southern mafic and northern ultramafic phases of the composite intrusion in the BBA have similar tholeiitic geochemical characteristics. A few analyses that are available for the intermediate to felsic dikes and bodies indicate that these units have high Zr/Y and (La/Yb)n ratios characteristic of alkalic rocks, which may be interpreted to be the products of either an early Archean, post-tectonic intrusive event or part of a Proterozoic magmatic event related to the initial rifting during formation of the Nipigon Embayment. A number of post-tectonic, mafic intrusions located along the contact between the SSB and the CVB have Ti/Zr, Zr/Y and (La/Yb)n ratios similar to the andesite of the CVB, and generally characteristic of calc-alkaline rocks. The overlap in the geochemistry of these intrusions and the CVB andesite is interpreted to indicate that these two -6- �Proceedings of the 51st ILSG Annual Meeting - Part 2 A B north. . FlAt The turbiditic association of the SMB can be divided into a clastic association and a chemical association, with a high proportion of oxide-facies, banded iron formation (BIF) layers. In the chemical association, clastic interbeds are generally less than several centimetres thick, and range in grain-size from silt to coarse sand. Upward-thickening and -coarsening trends over several metres are locally present, as at Solomon’s Pillars and the Leitch Mine near Beardmore (Fig. 6; Barrett and Fralick, 1985). Within the overall upward trend, oscillations between silts, sands, and iron formation occur. Depending on the relations between these types of beds, four iron formation lithofacies associations (IFLA) can be defined: magn at! le, •d Iat FLAt interlaminated manstit. asp.,. ilbtL and &ate San 2mCn,, E. IFLAc sadtn; siltstone. Slate and mantit IFLAb :—t iItstoe, slot, and • IFLA-a consists of dominantly magnetite-rich sediment with millimetre- to centimetre-scale, graded or ungraded silt interbeds (Fig. 7). Figure 6. Stratigraphic sections of iron formation bearing units. A) Outcrop section measured at the Leitch Mine (for detailed location see Barrett and Fralick, 1985). B) Outcrop section measured at Solomon’s Pillars (for detailed location see Fralick 1987). From Fralick and Barrett (1995). • IFLA-b comprises centimetre-scale graded to sharply bounded silt beds, either contiguous or separated by millimetre-thick laminae of magnetiterich sediment. rock types formed by similar petrogenetic processes, but the occurrence of intrusions in both metavolcanic rocks and metasedimentary rocks indicates that some of these intrusions are younger. • IFLA-c represents sand-rich composite units up to about 1m thick, generally consisting of thin, stacked, ungraded, laminated sand beds. These composite units consist mainly of mediumto coarse-grained sand. They are separated by intervals of IFLA-b up to 15cm thick. Sedimentary rocks Lithofacies Associations The NMB, the northern (uppermost) third of the CMB and the northernmost portion of the SMB are dominated by a conglomeratic assemblage with minor amounts of sandstone. The clast-supported conglomerates are poorly to moderately sorted, and almost always non-graded with a poorly to moderately sorted sand matrix. Bedding is defined by variation in average or maximum clast size between units, but it is commonly indistinct. Scouring is locally preserved, but most other primary features such as imbrication have been destroyed by deformation. Sandstones interbedded with the conglomerates commonly appear massive, but in some outcrops planar lamination and cross-stratification are present. They form lenses in conglomeratic beds; thin, irregular sheets blanketing conglomeratic beds; wedges abutting conglomeratic beds; and thick units separating conglomerates. Clast types in the conglomerates are almost exclusively igneous, representing a suite of rocks similar to those present in the Onaman-Tashota volcanic terrane to the • IFLA-d consists of framework-supported, polymictic conglomeratic beds up to a few metres thick, interbedded with sandstone and minor iron formation, or interbedded with fairly thick iron formation and thin-bedded sands. Conglomerate contain mainly mafic to felsic volcanic and granitic clasts. Although flattened clasts indicate that IFLA-d outcrops are tectonically thinned, their associations are primary, with the conglomeratic units erosively cutting down into BIF-sandstone packages. Transitions between IFLA-a, -b and -c can be gradual or abrupt. Some silt-sand successions containing iron formation exhibit intervals of thicker and wellgraded clastic beds. They form structured sections up to several metres thick within successions that are otherwise generally disorganized. Clastic units in the lower two thirds of the CMB and the SMB are divisible into three lithofacies associations: a thin-bedded, turbidite-dominated -7- �till HUIIIILiIllP EThJliJIIi p p p 11L Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 7. Photomicrographs of siltstone associated with laminated, magnetite-rich sediment (IFLA-a). Logs to right of photos outline layer contacts and schematically illustrate grainsize variations. Dots and dashes represent siliciclastic and magnetite-rich components, respectively. Scale bars: A.) 5.0 mm; B.) 2.0 mm; C.) 3.0 mm; D.) 2.0 mm. All images are in crossed nicols. From Barrett and Fralick (1985). association (LA2); a medium-bedded, turbiditedominated association (LA3); and a thick-bedded association (LA4). LA2 consists mostly of graded, <10 cm thick siltstone and/or sandstone beds that are either unstructured or thin and fine upwards over 1 to 3 m. LA3 is divisible into two types. LA3a consists of medium- to coarse-grained sandstones with sharp bottom and top contacts. Parallel lamination is present near the tops of some of the otherwise massive beds. These successions are unstructured. LA3b is similar to LA3a except these beds are organized into either upward-thickening or upward-thinning trends. Thick, poorly graded sandstones dominate LA4. The beds usually have a coarse sand or pebbly base, grading into a thick, poorly sorted, massive central area. They are often abruptly capped by thin, fine-grained sandstone. Irregular, erosional bases and scattered rip-up clasts are common. Structured, thinning- and fining-upward sequences, metres to tens of metres thick, are present in the area south of Beardmore. The successions are topped by CDE and/or DE turbidites which are abruptly overlain by massive grain flows/high-density turbidites with internal inverse- to normal-graded, conglomeratic bands. Pebbles present in the conglomerates are mainly felsic igneous rocks (extrusive and intrusive), while rip-up clasts are not the expected mudstone or siltstone, but rather clay- and silt-rich, fine-grained sandstone. Load structures are ubiquitous throughout the area. Commonly, the base of one unit sags into the underlying beds. Locally, multiple internal loads are developed, usually in the B division. These loads sag into the A division, in places extending into the underlying beds. Depositional Environments Deposition of the clastic and chemical sediments in the Beardmore-Geraldton belt occurred within a narrow time frame. Crosscutting felsic intrusive rocks provide a minimum age for the sediments of 2691+3/-2 Ma (Anglin, 1987; Anglin et al., 1988). UPb detrital zircon geochronology on sandstone samples from the region consistently give youngest zircon ages in the range 2701±1 Ma to 2696±2 Ma (Hart et al., 2002; Don Davis, unpublished data, personal communication, 2004). These ages are very similar to -8- �Proceedings of the 51st ILSG Annual Meeting - Part 2 Subprovince on Lake Superior (Purdon, 1995). In addition to geochronology, lithofacies associations and geochemistry indicate that the depositional system probably extended from the cratonic margin (i.e., Wabigoon Subprovince), across the arc-trench gap and trench (i.e., Quetico Subprovince) to the outboard ocean floor (i.e., Wawa Subprovince) immediately prior to the commencement of collision of an island arc system to the south. The NMB is represented by sandstone and conglomerate deposited in a fluvial fan, and/or braid Figure 8a. 2700 Ma paleogeography of the region surrounding plain environment (Devaney and Fralick, 1985). The Beardmore-Geraldton. The island arc in the south represents the Schreiber-Hemlo area of Wawa Subprovince. The trench CMB displays this type of assemblage in its northern system to the north of this arc is now metaturbidites of third, with successions in the southern portion similar the Quetico Subprovince. The sediments feeding from the to those described above for the SMB. Thus, the southern flank of the Wabigoon cratonic arc in the north paleo-shoreline exposed by the present level of erosion formed the Beardmore-Geraldton succession. From Fralick lies within the CMB. All of the sediments in the SMB and Barrett (1991). are marine, except for rare conglomerates at the top ages of detrital zircons from the Quetico Subprovince of the succession and throughout the section in the and a more distal turbiditic assemblage, the McKellar highly deformed Geraldton area. The turbidites were Harbour Formation, outcropping in the northern Wawa deposited in fore-arc environments ranging from deltafront to submarine fan or ramp. Many ______________________ Onommi—Tas bota VoIcoo!c Terrc!t, unstructured sandstone sequences are -. D L ItbfdIfl Aoc!ot!on interpreted as having accumulated on LIt 24 a submarine ramp that was locally Br WI transected by channels feeding deeper fan lobes (Barrett and Fralick, 1989). / An interpretive view of the overall paleoenvironment is shown in Figures 4c' 8a and b. I r - / rTsTL I 0-- (z) c - H--H— zoooe ond — -— -I(onFormacr, 0 !1 seofloor Sediments deposited in the Quetico trench to the south are similar in many respects to the submarine ramp deposits of the adjacent fore-arc basin, but there are differences. AB and ABC Figure 8b. Suggested depositional environments for sedimentary rocks in the Beardmore-Geraldton belt. The subaqueous portion of this diagram is depicting environments mostly present in the Southern Metasedimentary Belt. The Central Metasedimentary Belt represents mainly fan-delta environments, and the Northern Metasedimentary belt a braidplain environment. The entire sedimentary assemblage in all three belts forms a large-scale progradational system. In the hinterland is the Onaman-Tashota volcanic arc terrain which supplied the detritus. This sediment was transported across braidplains to distributary mouth bars. Because of rapid accumulation rates and the tectonically active environment, gravity flows and slumps were frequently triggered. The resultant grain-flows produced the vertically unstructured, poorly graded, mud-poor, medium- to coarse-grained deposits of the ramp succession. More structured sandstone sections formed on locally developed fan lobes and thick grain-flows filled the submarine feeder channels. Iron formation was deposited immediately offshore from the channel mouths on braid deltas during periods of low sediment influx. The meter-scale coarsening-upwards cycles present in the iron formation record the effects of delta lobe outbuilding. Progradational events may result in the stacking of coarsening-upwards iron formation-bearing, subaqueous parasequences or the delta top braid system may erode through the uppermost chemical sediments during maximum outbuilding. The iron formation is limited to the shore-proximal zone indicating a precipitation model where nutrients delivered to the system by rivers caused heightened cyanobacterial growth rates in the near-shore, similar to modern systems. The oxygen produced by photosynthesis caused precipitation of iron oxides in these oxygen oases. From Barrett and Fralick (1989) -9- �Proceedings of the 51st ILSG Annual Meeting - Part 2 turbidites dominate the trench fill, whereas grainflows and A turbidites characterize the more proximal, fore-arc environment. Iron formation is present in the fore-arc basin associated with thinner-bedded, finer-grained sediments deposited in the shallow near-shore portions of the deltas. Iron formation accumulated when transgressions caused intervals of clastic sediment starvation in the proximal pro-delta and distributary bar environments. Iron formation is absent from sediments deposited further offshore in the Beardmore-Geraldton belt and the Quetico trench deposits. The limited distribution of oxide-facies iron formation (occurring only in shore-proximal areas) indicates that biological activity thrived in the nearshore waters where the commonly limiting nutrients, nitrogen and phosphorous, are delivered at higher levels due to river input. It is difficult to prove that iron compound precipitation was driven by free oxygen liberated during photosynthesis by cyanobacteria, but this would be a logical assumption. Rare successions that coarsen and thicken upward may represent limited establishment of discrete fan lobes in the distal fore-arc basin and Quetico trench CVB *CA CYB CA *•J3 OTT The discussion of the structural development is taken largely from recent work and synthesis conducted by Lafrance et al. (2004). Their treatment of the topic is the most recent structural study, outlining three phases of structural overprinting of the volcano-sedimentary assemblage and ascertaining the timing of each. CA I Sediment supplied to the fore-arc basin and the trench was derived from erosion of felsic and intermediate igneous material, with a large contribution probably coming from resedimentation of unconsolidated volcaniclastic material. Clast compositions indicate that the Onaman-Tashota arc terrane to the north was the source (Devaney, 1987; Devaney and Williams, 1989). Grain size and lithofacies trends also strongly support a source to the north. In addition, whole rock geochemistry conducted on sandstone samples from the Beardmore-Geraldton area and the Quetico Subprovince further confirm that the Onaman-Tashota terrane is the only igneous complex in the area which could have supplied the majority of the detritus (Figs. 9 and 10; Fralick and Kronberg, 1997; Fralick, 2003) The ease with which volcaniclastic material can be physically eroded from subaerially erupted debris could have led to short subaerial residency periods, and thus lowered the extent of chemical weathering and production of clays. This is probably the major factor contributing to the rarity of E division fines in the deposits. Structure * OTT 0 floor, but for the most part the trench fill was dominated by overlapping, interfering fans fed by multiple channels extending from the shallow marine portion of the fore-arc basin. One of these channels outcrops south of Beardmore (Stop 1-2). The channel is filled with thick, upward-thinning and -fining successions that grade from conglomerate and coarse sandstone grainflows or turbidites deposited from high density currents near channel bases, to CDE and DE, lowdensity current turbidites that fill abandoned channels near their tops. 2 3 Ti02/(Nb xl,000) Figure 9. Ratio plot of chemically immobile elements in sandstones showing derivation of the sands deposited in the Beardmore-Geraldton terrain and the Quetico Subprovince from the Onaman-Tashota calc-alkaline volcanic arc. From Fralick and Kronberg (1997); Fralick (2003); igneous data from Tomlinson et al. (1993) and Kresz and Zayachivsky (1989). After deposition of the clastic succession the area was tectonised by thrust faulting, regional folding and dextral shearing. Thrust faulting resulted in the regional volcanic and sedimentary packages being vertically juxtaposed in an imbricated thrust stack (Devaney and Williams, 1989). This represents D1 and may be related to uncommon, early, F1 folds. The youngest detrital - 10 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 * on NVB CVB SVB CA 160 Kresz and Zayachivsky, 1989 Tomlinson et al, 1993 mafic to intermediate volcanic Onaman - Tashota Terranc rocks Northern Volcanic Belt Central Volcanic Belt Southern Volcanic Belt Gale - Alkaline o mafic volcanic rocks A intermediate volcanic rocks • intermediate to felsic sub - volcanic intrusive rocks intermediate to felsic intrusive rocks intermediate to mafic intrusive rocks A • *CVB CA Tholeiitic volcanic CYB CA / —— a— and intrusive rocks SVBh 0 ULTRAMAFIC Onaman - Tashp calc - alkaline volcanic and intrusive rocks p I 20 40 60 p p p 00 80 20 p 40 160 p 80 p 200 hO2! Zr Figure 10. A ratio plot similar to Figure 9 using Zr. From Fralick and Kronberg (1997); Fralick (2003); igneous data from Tomlinson et al. (1993) and Kresz and Zayachivsky (1989). zircon recovered from the sedimentary units is 2696±2 Ma (Hart et al., 2002) and this places a maximum age on the thrusting event. The D2 event is recorded by the development of tight to isoclinal folds and a flattening strain fabric responsible for transposed bedding and flattening of clasts and pillows. A homoclinal, north-younging panel of regional extent developed at this time and represents the sheared-off southern limb of a larger syncline. D2 deformation affects altered and gold-mineralized porphyry dykes in the syn-tectonic Croll Lake stock which U-Pb geochronology indicates are 2691+3/-2 Ma (Anglin, 1987; Anglin et al., 1988). An age of 2699±1 Ma for a Au-mineralized feldspar porphyry dyke at the Hard Rock Mine and identical ages of 2690±1 Ma for two phases of the Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt (Corfu, 2000). The final event, D3, occurred as regional transpression developed in the compressive framework of the area. Vertical bed orientations developed during D2 did not refold but rather were overprinted by a steeply dipping, regional cleavage. Partitioning of the strain, during east-west dextral shear, between less competent argillites and more competent sandstones and porphyries resulted in cleavage refraction near lithologic contacts. The pervasive cleavage developed in the Paint Lake shear zone at this time shows a progressive rotation towards the orientation of the zone (Lafrance et al., 2004; DeWolfe et al., 2000). This is in contrast with the Barton Bay Lithotectonic Zone (BBLZ) where the S2 fabric was reactivated to accommodate the D3 shear. Some folds were generated during this interval but they tend to be smaller Z-folds, overprinting limbs of regional F2 folds. Shear zones active at this time were dextral with nearly horizontal displacements. To sum up the results of Lafrance et al. (2004): • Thrusting along the southern margin of Wabigoon Subprovince in the Beardmore-Geraldton area between 2696 Ma and 2691 Ma resulted in the largescale assemblage of what was to become the belt structure. • The rocks were then modified by a compressive event (D2), probably between 2692 Ma and 2686 Ma, which steepened the beds to a near-vertical position, caused extensive flattening and formed large-scale fold structures. • D3 is more poorly constrained in time but may be synchronous with 2670 to 2650 Ma peak metamorphic conditions in Quetico Subprovince. It is characterized by the development of dextral shear zones and small-scale Z-folding. Gold mineralisation Overview Gold was first discovered in the Beardmore-Geraldton Greenstone Belt in 1925 (Mason and MacConnell, - 11 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 1 - Gold mineralisation in the Beardmore-Geraldton area. From Mason and McConnell (1983). Mine Years of production Ore milled (tons) Gold produced (Oz) Average grade (Oz/ton) Silver produced (Oz) Bankfield 1937-42, 1944-47 231,009 66, 417 0.29 7,590 Brengold 1941, 1949 46 134 2.91 Crooked Green Creek 1980-1984 1,455 471 Hard Rock 1938-1951 1,458,375 269,081 0.18 Jellicoe 1939-1941 10,620 4,238 0.40 145 Leitch 1936-1968 920,745 847, 690 0.92 31,802 9,009 Little Long Lac 1934-54, 1956 1,780,516 605,499 0.34 52,750 MacLeod-Cockshutt 1938-1968 10,337,229 1,475,728 0.14 101,388 Magnet Consolidated 1938-43, 1946-52 359,912 152,089 0.42 16,879 Maloney Sturgeon 1937 1 73 73 16 Maylac 1946-1947 1,518 792 0.52 46 Mosher Long Lac 1962-1966 2,710,657 330,265 0.12 34,604 Northern Empire 1934-41, 1949 425,866 149, 493 0.35 19,803 Orphan (Dikdik) 1934-1935 3,525 2,460 0.70 1,558 Sand River 1937-1942 157,870 50,065 0.32 3,628 Sturgeon River 1936-1942 141,123 73,438 0.51 15. 922 Talmora-Long Lac 1942, 1948 6,634 1,417 0.21 36 Tashota-Nipigon 1935, 1938 51,200 12,356 0.24 14, 527 Theresa 1935-38, 1941-1943, 26,120 4,785 0.18 202 69, 120 0.36 1945, 1950-53, 1955 Tombill 1938-42, 1955 190, 622 Total 4,115,611 1983). The Beardmore-Geraldton greenstone belt has produced approximately 4.1 million ounces of gold and over 300,000 ounces of silver. Most of this production came from two distinct gold camps at Beardmore and Geraldton, respectively. The balance of gold production came from several small mines scattered 30 km northeast of Beardmore and one mine south of Longlac. Table 1 (Mason and McConnell, 1983) is a production summary of all past gold producers. Gold in the field trip area occurs dominantly in quartz-carbonate veins and sulphide replacement zones cutting all the major Neoarchean rock types of the area. The vein systems are most typically hosted by: 1) clastic sedimentary rocks; 2) contacts between clastic sedimentary rocks and albite porphyry; 3) interbedded iron formation and clastic sedimentary rocks (Macdonald, 1984); 4) interflow iron formation and mafic metavolcanic rocks; and 5) post-tectonic mafic intrusive rocks (Hart et al., 2002). Structures Related to Mineralization The fine-grained sedimentary rocks in the southern portion of the area are isoclinally folded with a well- 8,595 318,500 developed, sub-vertical, east-striking cleavage. The fold axes vary in trend and plunge: in the west, the fold axes plunge west at moderate angles (Hart et al., 2002; Lafrance et al., 2004); near Jellicoe fold axes are vertical; and as we move progressively to the east the fold axes return to a westward plunge (Kehlenbeck, 1983). Shear discontinuities along axial planar surfaces also become more prevalent to the east (Kehlenbeck, 1983). Earlier work also outlined a later deformational event. It formed chevron folds with cleavage co-planar to the axial surfaces (Kehlenbeck, 1983; Anglin and Franklin, 1985). The second deformational event of Kehlenbeck (1983) is probably spatially and genetically related to regional, east-trending fault zones and is thus at least partially the same event as D3 of Lafrance et al. (2004). Geraldton Camp Gold mineralization in the belt has resulted from the introduction of hydrothermal fluids in zones of high crustal permeability. Permeability was generated by prolonged, multiple periods of deformation which focused not only fluids, but magmatic activity and intrusions. In the Geraldton camp (Fig. 11), a major zone - 12 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ______ Vv ..VvV . VV - VVVVVV.vVq Vd •''VVVVv.vvvvvvvvv VVVVVVVVy vvvvvyvv i'i. VYVVVvJvvv,..' V-/VQAflqVI ./1IVJ7\#vvvvvvvvvvvvvvvvvv VVVVVVVVV 4vV.#V,..e' :.VQV.vI;--Ivvvvvv-.v., vvv1vvvyy.g Fvv-v vvvvvvv ..rnv--, vv•,v i-# iv., -VJ,V .--.r-ty,v /vVvv vVYgvvvv ,VJVV vvvv vqyyvy.g. 1'. :.v-IV.dVVvVvvvv.,vv..,vv! V vv.vvNv1vvvvv ivvvvvv . V ,.VVv VV/ VVVVVVVVVV Ivivvvv yVjVy1Vv V' .j•. I ;I.vvV;vVvvvvvvv vvvvvjvv Ra,tonliay --IIv,vvvvvv VVVVVVJ I VVVVv . -- IThH S V — Uthotectonjc Zone V 'I GERALOTQn A. - - :: - .... - . -&/ -. 4 + .. — '.IV . I— - — -,.—--._c-_ .- --———-- - . VVv'- .- — — t + 4 ...jCy. .t+ — .—-———.— --- --— —.0 I - _-—: - — - 2 + ÷ 4 4 4 + 4 + + 4 4 + .V.yvVVVVV 4 3 SMile, 6 It iti Porphyfflic lelsic intrusion Equigranular teisic intrusion Matic intruson A S Clastic rnetp$edln,ents iron tormation Gold Occurrences Major producing mines Fault ILITJ Matic to intermeiaIe metavolcamcs Figure 11. Spatial association of gold deposits to the Barton Bay lithotectonic zone north of the Bankfield-Tombill Shear Zone. Modified after Macdonald (1985). of deformation in which the gold mines are located has been alternatively termed: the Bankfield-Tombill Fault Zone (Pye, 1952; Horwood and Pye, 1955); TombillBankfield Deformation Zone (Lafrance et al., 2004); 2 — and Barton Bay Deformation Zone (Williams and Stott, 1991). In acknowledging not only the deformation within this zone, but also its lithologic variety, we will use the term Barton Bay Lithotectonic Zone (BBLZ) (Fig. 12). N Pt (/ —— N _* FFpa,tw Horwood and Pye (1955) noted that the area adjacent to and north of the BBLZ not only has S-folds related to the regional folding pattern but also has Z-folds. The Zfolds in this area only occur along the north side of and proximal to the BBLZ. This led Pye (1952), Horwood Figure 12. Structure and Stop locations within the Barton Bay Lithotectonic Zone. After Lafrance et al. (2004). 'l*ir1JS - 13 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 -- SFABRIC.. north of, and genetically linked to, the Barton Bay Deformation Zone (BBDZ) (Fig. 13). This zone of deformation varies from 1000 to 3000 m in total width (Lavigne, 1983; in press), while the crush zone of the Bankfield-Tombill Fault proper ranges from meters to hundreds of meters in width. The crush zone has been intensely silicified (Pye, 1952), carbonatized (Anglin and Franklin, 1985) and contains minor amounts of gold (Pye, 1952). - — - BANKEICLO — MAGNET MrNE Malic sIotayoicanlcs Malic Introsives _J Mastive Aikose Felsic Dike H Greywacke ore Figure 13. Fault-related fabric development in the Barton Bay Lithotectonic Zone near Geraldton. Modified after Colvine et al. (1984); from Scott (1985). and Pye (1955) and Buck and Williams (1984) to hypothesize that the second deformational event was related to the development of east-trending faults. This was confirmed by Lafrance et al. (2004) (their D3) and the faulting was attributed to transpression causing dextral shear throughout the region. Most mineralized occurrences in the Geraldton camp lie in a zone of deformation to the immediate Numerous Z-folds on various scales were formed in the deformation zone. Auriferous vein systems in the MacLeod-Cockshutt and Hard Rock mines are hosted by one of the Z-folds. This structure plunges shallowly west (Fig. 14; Horwood and Pye, 1955) and is mimicked by minor parasitic folds in the BIF. The parasitic Z-folds are generally less than 5 m in amplitude (Macdonald, 1982, 1983a) and are commonly cut by quartz-carbonate veins subparallel to axial traces. Displacement zones, also subparallel to axial traces, developed at lithologic contacts on isoclinal fold limbs, also host auriferous veins. Veins parallel to axial planes of folds are relatively straight and non-sinuous whereas those orientated at an angle to axial planes of Z-folds are highly ptygmatically folded. Beardmore Camp Gold mineralization in the Beardmore area is hosted by quartz-carbonate and sulphide-bearing quartz veins located in foliation-parallel, carbonatized, brittle-ductile shear zones in either clastic metasedimentary rocks of the SMB or in mafic metavolcanic rocks, proximal to NORTH ZONE F2 S Figure 14. Block diagram of the North Zone at the MacLeod-Cockshutt and Hard Rock mines drawn by Lafrance et al. (2004) using level mine plans published in Horwood and Pye (1955). Diagram shows the overprinting of a S F2 fold by a Z F3 fold on the north limb of the Hard Rock anticline. Ore pods are shown in black. - 14 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 T-e- '293 Figure 15. Map showing the surface trace of mineralized quartz veins on the Sand River - Leitch properties. After Ferguson (1967a). BIF of the SVB (Hart et al., 2002). Gold occurrences are also hosted by quartz veins in post-tectonic intrusive rocks and disseminated sulphides in carbonatized shears in metavolcanic and metasedimentary rocks associated with intrusive rocks. Regional mapping by Hart et al. (2002) and a composite longitudinal section of the Sand River Mine and Leitch Mine vein system based on that of Ferguson (1967a,b; Fig. 15) indicates that: 1) the Leitch Mine No. 2 vein and the Sand River Mine vein are the same vein; 2) the vein is foliation-parallel and is at a low angle to the regional fold axis at a strike of 240° and a dip of 65°; 3) the vein contains multiple ore shoots that are a series of dilatant zones controlled by the intersection of two regional fabrics such that the ore shoots plunge moderately steeply at about 320° and have good down-plunge continuity. The veins are hosted by the ductile-brittle shear system which is part of the Standingstone Lake fault (Hart et al., 2002; a.k.a. Standingstone Lake – Watson Lake fault of Bruce and Laird, 1937; Mackasey, 1975; Shanks, 1993), the western extension of the Oxaline Lake – Watson Lake fault (e.g., Williams, 1986). This fault trends about 075° northeast, and is topographically expressed by a swampy valley that includes Standingstone Lake and its associated tributaries, north of Highway 580, east of the Leitch Mine. The fault is manifested by the development of numerous small, northeast-striking, carbonatized shear zones. Lafrance et al. (2004) defined the structure hosting this Sand River vein as being parallel to the axial plane of this fold and S3. The Standingstone Fault has been interpreted to be a normal, south-side down fault (Carter, 1987) that has experienced multiphase deformation from dip-slip to dextral strike-slip (Williams, 1987). The Northern Empire Mine is hosted by a series of en échelon and composite quartz veins containing differing amounts of pyrite, arsenopyrite, pyrrhotite and chalcopyrite, hosted by carbonatized, sheared metavolcanic rocks and interflow iron formation of the SVB near the Empire fault and the SVB–SMB contact. The quartz veining occurs in brittle features of the deformed iron formation. The Empire Fault trends approximately 060° northeast, is steeply south dipping (but slightly shallower than the average dip of the overturned metavolcanic flows) and is marked by a thin fault gouge of black graphite bordered by a few feet of sheared rock (Benedict and Titcomb, 1948). Previous interpretations (Benedict and Titcomb, 1948; Shanks, 1993) placed the Empire Fault adjacent to the SVB-SMB contact and Shanks (1993) interpreted kinematic indictors to suggest a south side-up sense of motion. Hart et al. (2002) suggested that there are 2 distinct, subparallel faults: the Empire Fault to the east, and the Princess Lake Fault to the west, splaying off the SVB-SMB boundary. Both of these resulted in the formation of zones of brittle deformation in interflow iron formations in mafic metavolcanic rocks of the SVB that host auriferous quartz stringers containing arsenopyrite, pyrite, pyrrhotite and chalcopyrite (Hart et al., 2002). Co-incidence of structures related to the fault separating the BBA and the PPA northwest of - 15 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 _________ Beardmore, and the location of post-tectonic, mafic intrusion appear to have controlled the emplacement of quartz veins hosting gold, copper and/or molybdenum mineralization (Hart et al., 2002). The Tyson #1 gold occurrence (Mason and White, 1986) and the Tyson #2 molybdenum occurrence (Hart et al., 2002; Mackasey 1975) are hosted by quartz veins in the post-tectonic intrusion. Copper mineralization is hosted by sheared, carbonatized lapilli tuff within the fault (Hart et al., 2002). The fault trends approximately 075° to 080°, is marked by intense carbonatization and weak to moderate ankerite and calcite-flooding, and may have controlled the emplacement of the intrusion. Vein Systems Auriferous veins are typically 2 to 10 cm wide and primarily composed of quartz plus ankerite, dolomite and other carbonate species. Pyrite, arsenopyrite, pyrrhotite, sphalerite and chalcopyrite are locally present. The gold occurs as small blebs and fracture fills within individual quartz and sulphide crystals and in voids between crystals. It has also been observed as fine threads extending along cleavage planes in carbonate minerals (Armstrong, 1943). The auriferous veins often show evidence of brecciation and multiple generations of quartz. The brecciated zones are richer in gold that non-brecciated areas (Bruce, 1935, 1937a,b; Horwood and Pye, 1955). Crack-seal-textured ore veins, which predominate at the Leitch and Sand River mines, are indicative of successive periods of dilation and fluid influx. These veins contained minor and varied amounts of pyrite, arsenopyrite, chalcopyrite and tetrahedrite, scheelite ——— a —- - VEPN — Ii] Li .— 1: LY! JHT•::7 Figure 17. Plan of the North Zone (No. 30 Vein system), 250-foot level, Hard Rock Mine. After Horwood and Pye (1955). with septa of sericitic and carbonatized wall rocks. Alteration Halos Substantial alteration halos surround auriferous veins (Fig. 16). Carbonatization (ankeritic dolomite) dominates in host lithologies with low concentrations of iron-bearing minerals (e.g., clastic sedimentary rocks) and fracture zones associated with both major and minor faults. Siltstones have been found to contain up to 20% CO2 (Anglin and Macdonald, 1984). Carbonatization is ubiquitous in the clastic sedimentary rocks at the MacLeod-Cockshutt and Hard Rock mines. However, 300 m along strike away from this structure, carbonate alteration is rare (Macdonald, 1983a). Siderite supercedes ankeritic dolomite as the major carbonate phase within 5 cm of the vein margin. In addition to carbonatization, pyritization occurs in iron-rich rocks (e.g., BIF) adjacent to veins. These pyritic halos not only replace the magnetite/hematite laminae in BIF but also sandstone and siltstone laminae in the Geraldton area. The pyritization generally extends only centimetres to tens of centimetres away 3JN. RELATER MINERALOGY I CLI ——- IJI UAI'ON CL!AST1 4Hr / HOST BOCLM(N &A Lt Figure 16. Host rock, alteration halo and vein mineralogy for veins in iron formation-siliciclastic assemblages. From Scott (1985). - 16 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 1976; Mason and McConnell, 1983; Scott, 1985). Concentrations of gold may reach in excess of 15,000 ppb. In contrast to this, the surrounding, sulphide-free BIF consistently contains less than 20 ppb gold (Anglin and Macdonald, 1984; Fig. 19). E? TREMC CAR 0 OlAt 2 AT ION Generation of Hydrothermal Fluids and Timing of Mineralization S U LPHI OAflOti HALO Figure 18. Schematic representation of vein-related alteration, Hard Rock Mine (Scott, 1985); carbonatized zone is approximately 1 m wide. from individual veins. However, where densely packed systems of veins exist sulphide bodies meters by tens of meters in size have developed (e.g., MacLeodCockshutt-Hard Rock mines; Horwood and Pye, 1955; Figs. 17 and 18). Concentration ofOold ( ppb S r An alternative model to those outlined above is the genesis of the fluids through metamorphic de-watering. Studies conducted by Horwood and Pye (1955) indicated the fluids which deposited the auriferous veins contained Ag, As, Au, C, Ca, Co, Hg, Mg, Pb, S, Si, Te, W, Zn and possibly Fe and Ti. This assemblage is similar to the elements enriched in and near auriferous veins studied by Phillips and Groves (1983), Kerrich and Hodder (1982) and Kerrich (1981). These authors interpreted the fluids which formed those deposits to have been generated during burial and metamorphic de-watering of a volcanic pile. Cuncent,alio'i of Arsi'iic (ppm) Concentration of Arsenic ( ppm) ft Concentration ofOold ( ppb The vast majority of the gold associated with the vein systems in BIF is contained within sulphides in alteration halos. The gold generally occurs as submicroscopic particles and minute blebs within the pyrite (Pye, 1952; Horwood and Pye, 1955; Mackasey, Past studies have suggested the gold-bearing fluids in the Geraldton camp were derived either from a granodioritic body (Croll Lake Stock) lying to the immediate east of the area (Horwood and Pye, 1955; Macdonald, 1983b) or by metamorphic redistribution of gold contained in the banded iron formation (Boyle, 1976; Mackasey, 1975). Arguments can be made against gold originating in the BIF. Sulphide-free BIF in the area generally contains less than 20 ppb gold (Anglin and Macdonald, 1984). This amount is much lower than the average values of 61 ppb Au for highFe tholeiites and 78 ppb Au for volcanic-associated BIF which were obtained from a volcanic pile lying in Superior Province northwest of the BeardmoreGeraldton greenstone belt (Cowan and Crockett, 1980). Obviously, if volcanic and related chemical sedimentary rocks in the map area have similar gold contents (and sparse data indicate they do) they would provide a more adequate source of gold. In addition, alteration halos around the veins indicate the fluids were in marked disequilibrium, both in composition and Eh, with the BIF signifying that they were derived from an external source. Figure 19. Gold and arsenic distribution associated with veins at MacLeod-Cockshutt (MC) and Solomon’s Pillars (SP). From Scott (1985). The spatial association between the east-striking fracture systems and the gold mineralization has led previous authors to believe that the faults and related minor structures served as passageways along which the fluids moved (Horwood and Pye, 1955; Macdonald, 1984; Anglin and Franklin, 1985). Structural control of - 17 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 vein systems, and by inference, fluid migration paths, is common in the Superior Province (Rigg and Helmstaedt, 1981; Fryer et al., 1979; Stott and Schnieders, 1983; Poulsen, 1983; Durocher and Hugon, 1983; Hodgson, 1983). The fluids were probably generated at depth and utilized the permeability developed in deformed zones to gain access to higher crustal levels. Permeable fault zones provide conduits to the fluids allowing them to move upwards towards lower-pressure/temperature zones. Episodic and protracted development of dilatant zones and shearing is indicated by the open spacefilling nature of the veins and their brecciation. Fluids migrated towards dilation zones enveloping fault zones, driven by a pressure gradient and seismic pumping (Sibson et al., 1975). The same model has been put forward to explain structurally controlled gold veins in the western Wabigoon Subprovince (Poulsen, 1983). The development of abundant carbonate and iron sulphide in the reaction halos around veins indicates the fluid could not have been very acid (Krauskoph, 1967; Boyle, 1969) and was probably near neutral (Phillips and Groves, 1983). The formation of massive amounts of carbonate minerals also denotes a very CO2rich fluid. This conforms to data from other studies which advance H2O- CO2-dominated fluids as of major importance in the formation of Archean lode gold deposits (Kerrich, 1981; Kerrich and Hodder, 1982). Fluid inclusion studies of the MacLeod-Cockshutt deposit also indicated a CO2-rich fluid at a temperature in excess of 380°C was involved in forming at least some of the veins (e.g., Macdonald, pers. comm., 1985). Recent research has suggested that major gold deposits may be the result of prolonged mixing of two fluids: a hot, reducing, orogenic fluid that migrates along major regional fault structures; and a colder, oxidized, goldand base metal-enriched, magmatic fluid associated with late, mantle-derived plutons (c.f. Neumayr et al., 2004; Rogers et al., 2004). Gold mineralization in the Beardmore area has not been studied to the same degree as in the Geraldton area, but much of the preceding discussion is probably applicable there as well. However, there are a number of points specific to the Beardmore area. An intermediate to felsic intrusion comparable to the Croll Lake Stock has not been identified. There are a number of posttectonic mafic intrusions, and one of these intrusions along the faulted BBA-PPA boundary hosts gold and molybdenite mineralization (Tyson occurrences) and there is copper mineralization in the adjacent fault (Hart et al., 2002). Mafic intrusions of comparable composition are located to the west of the Sand River Mine, south of the interpreted trace of the Standingstone Lake Fault, but intrusions are not spatially associated with mineralization in either the Leitch or Sand River mines. The Standingstone Lake Fault may be part of the fault system separating the SMB and CVB as it, in part, parallels this contact, but it also is the approximate location of the reversal in top directions in the metasedimentary rocks of the SMB (Hart et al., 2002). Axinite, a borosilicate similar to tourmaline, is located in brittle structures in mafic metavolcanic rocks along strike with the Leitch – Sand River vein to the east, and along the BBA-PPA contact to the north (Hart et al., 2002). Although axinite is not associated with gold mineralization, as in the Chibougamau (Dubé and Guha, 1993) and Cadillac areas (Bardoux et al., 1990), it may be a late-stage mineral associated with an intrusion that may have supplied hydrothermal fluids during the gold-mineralizing event. However, the axinite may be unrelated to the gold mineralizing event and reflecting an unexposed pegmatite or the intrusion of the diabase sill (e.g., Ozaki, 1972). Gold mineralization at the Leitch and MacLeodCockshutt mines, the two largest, past-producing gold mines in the Beardmore–Geraldton Belt, is associated with D3 brittle shear zones and folds, overprinting regional F2 folds (e.g., Lafrance et al., 2004). The plunge of the ore zones is parallel to F3 fold axes and to the intersection of D3 shear zones with F2 and F3 folds. At the deposit scale, the plunge of the ore zones is similar in orientation to the plunge of F3 axes (e.g., North Zone, MacLeod-Cockshutt Mine) and to the intersection of D3 shear zones with F3 folds (e.g., Leitch Mine; Hart et al., 2002; Lafrance et al., 2004) and F2 folds (e.g., F Zone, MacLeod-Cockshutt Mine; Horwood and Pye, 1955). Lafrance et al. (2004) provided an estimate for the age of syn-D3 gold mineralization ranging from 2686 Ma (i.e., the age of the pre-D3 Ottertail granite in the Rainy Lake district) to 2640 Ma (i.e. the minimum age for the formation of D3 structures and high-temperature metamorphism in the Quetico subprovince; Zaleski et al., 1999). On a subprovince scale, regional folds that are cut by D3 dextral shear zones are promising targets for discovering the next generation of large gold deposits (ibid). - 18 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stops Geraldton gold camp Junction Hwys 11 & 584 Preface 2-1 MacLeod - Cockshutt headframe The Beardmore-Geraldton area has been the subject of numerous field trips and published and unpublished field trip guide books, including those by Mason et al. (1985), Fralick and Barrett (1991) and Williams and Stott (1991). Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Thunder Bay. Please exercise caution along highway, road right-of-ways and lake shores. 2-2 Porphyry Hill km Beardmore - South Intersection of Hwy. 11 & CNR (Beardmore) 0 1-3 Blackwater fault/pillowed volcanic rocks 0.0 2-3 Glory Hole BIF Field Trip Road Log Stop Locality 0.0 1.3 Junction Hwys 11 & 584 0.0 Ashmore/Errington boundary 0.5 Mosher Mine turn-off 1.3 2-4 Conglomerate 1.3 2-5 Gabbro 1.6 Gravel Pit turn-off 2.3 Powerline crossing 3.8 Wintering Road turn-off 4.7 Powerline crossing 5.9 McClellan Strip 6.4 Magnet Mine turn-off 8.5 Bankfield Mine turn-off 10.6 Key Lake/Malouf Vein 11.8 Wild Goose Lake Park 27.0 1-2 Quetico metasedimentary rocks 5.7 Jellicoe-Kinghorn Road 1-1 Sibley-Quetico unconformity 13.6 2-8 Southern Sedimentary belt sandstone Jellicoe Post Office Beardmore - West 2-9 Pillowed mafic flows 39.6 47 47.2 Junction of Hwy. 11 & 580 0 East Leitch turn-off 5.8 Junction Hwy 11 & Kinghorn Road 0 Longlac Superior strips 3.7 Ara Lake sign 1.7 72 Road turn-off 5.3 CNR tracks 2.9 7.1 Sturgeon River Bridge 5.3 Sand River Mine turn-off 7.4 6 km Road turn-off 6.0 1-8 Sand River 16 Vein Zone 8.3 2-6 Missing Link extension 7.2 1-7 Leitch BIF 8.4 Missing Link turn-off 7.7 1-6 Eva Creek: conglomerate/BIF 9.3 Missing Link parking spot +0.4 1-5 Eva Creek: pyroclastic rocks 9.6 2-7 Missing Link (NE corner) +0.2 1-9 Leitch mine turn-off Million Dollar Road turn-off 10.4 High Hill Harbour turn-off 12.0 1-4 Poplar Lodge pillowed flows, G & G Goodman driveway (access to lakeshore) 14.0 Stop 1-1 - Archean – Proterozoic Unconformity UTM coordinates - 0423431E 5483881N The angular unconformity between east-trending, steeply dipping Neoarchean Quetico metasedimentary rocks and flat-lying Mesoproterozoic Sibley Group - 19 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 21. Stratigraphic column of Quetico metaturbidites, Stop 1-2 (Fralick et al., 1992) t..& Figure 20. Angular unconformity between steeply dipping Quetico metaturbidites and overlying Sibley Group sandstone-carbonate units, Highway 11 (Stop 1-1). sedimentary rocks is exposed on both sides of Highway 11. The phyllitic Quetico rocks display layer-parallel slickensides (bedding-parallel fault?). This outcrop consists of metasedimentary rocks unconformably overlain by a thin layer of Sibley Group sandstone (Fig. 20). The underlying rocks were folded to a near vertical position during the Kenoran Orogeny, prior to Mesoproterozoic deposition of the Sibley sedimentary units. The Sibley here consists of an upper, 30 cm thick, orthoquartzite separated from the Archean basement by 0 to 30 cm of silicified carbonate. The carbonate occurs in fractures in the substrate up to 60 cm below the paleosurface. This gives the distinct impression that the carbonate is a well-developed, caliche zone. Caliche soils form in semi-arid settings where the evaporation rate exceeds the precipitation rate. Irregular laminations in this layer are up-buckled in places, forming domical structures, probably the result of expansion during crystallization. It is difficult to say much about the overlying orthoquartzite. It is not even known if it is wind- or water-lain. Similar, thin sandstone layers occur both to the east and west of Lake Nipigon, separating basement from Mesoproterozoic (Keweenwan) diabase sills. They are correlated with the Sibley simply on the basis of their probable Mesoproterozoic age. Stop 1-2 - Quetico Metasedimentary Rocks UTM coordinates - 0428530E 5489570N Most of the Quetico consists of an unstructured submarine ramp system (i.e., an environment where large, long-lasting channels do not exist and sediment delivery routes switch rapidly, producing a chaotic structuring to the deposits). This outcrop provides one of the few examples of structured channel outbuilding in this ramp setting. A thinningand fining-upward succession of Bouma ADE, CDE and DE turbidites is sharply overlain by a thick-bedded unit of ABCD turbidites with abundant clay rip-ups and extra-basinal pebbles (Fig. 21). The lower, 8 m-thick, fine-grained assemblage is the upper portion of a subaqueous channel feeding sediment from the Beardmore-Geraldton forearc basin to the Quetico trench. It is Orn erosively overlain by the basal section of the next channel complex. The phyllitic rocks that occur at the northern margin of the Quetico Subprovince south of Beardmore are strongly sheared, exhibiting rust-spot elongation, quartz fibre lineation, and crinkles, some of which can be attributed to non-parallel,cleavage-bedding and cleavage-vein intersections (Williams and Stott, 1991). None of these intersections appear to have reliable or consistent tectonic significance. Stop 1-3 - Blackwater Fault / Pillowed Metavolcanic Rocks UTM coordinates - 0430174E 5493162N Deformed, pillowed mafic metavolcanic rocks of the Southern Volcanic Sub-belt are exposed along the sides of Highway 11 just south of Beardmore. They form the southern margin of the Beardmore-Geraldton greenstone belt and the eastern Wabigoon Subprovince in this area, where it is in fault-bounded contact with Quetico metasedimentary rocks to the south. The Blackwater River Fault (BRF) is a regional-scale structure which separates the two subprovinces. It is likely a dextral, transcurrent fault, similar to other subbelt-bounding structures in the Beardmore-Geraldton belt. It has been traced for over 100 km and underlies the valley occupied by Camproad Creek, a few hundred - 20 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 intermediate to mafic metavolcanic rocks of the Central Volcanic Sub-belt are exposed along the shoreline of Lake Nipigon, just north of Poplar Lodge. They have been described by numerous workers, including Coleman (1907), Laird (1937), O’Brien (1985), Tomlinson (1996) and Hart et al. (2002). Figure 22. Deformed pillow basalt near Blackwater Fault, Highway 11 (Stop #1-3). metres south of this field trip stop. Immediately north of the BRF, Shanks (1993) noted a series of sub-vertical, non-penetrative planar surfaces, interpreted to be either slip surfaces or C-planes. Estimation of the finite strain ellipsoid shape by Williams and Stott (1991) using pillow shapes and selvage thickness indicated plane strain. Localized, small-scale c-s fabrics were noted by Williams and Stott (1991) in schistose selvages, indicating a north over south sense of displacement. Light grey-green weathering basalt flows are foliated at approximately 240°/80° north. They are locally phyllitic, friable and gossanous. Despite flattening and aspect ratios of 3:1 to 10:1, pillows give northward top directions. Pillows range in size from 0.5 to 1.0 m and have recessively weathered selvages (Fig. 22). They contain flattened vesicles near their rims and are locally variolitic. Isolated patches of autoclastic(?) breccia contains 0.10 to 0.25 m-sized, lenticular fragments. The flows are more massive to the south and are crosscut by rusty, “bull-white” quartz veins. Joint surfaces are coated with quartz and calcite. A rusty quartz vein occupying the sheared contact between basalt and sandstone to the north was sampled but returned no gold nor silver. A 3m wide diabase dyke trends ~155° and cuts the supracrustal rocks. Massive, vesicular/amygdaloidal and/or pillowed basalt/andesite, heterolithic breccias, as well as porphyritic and flow-banded units were described by O’Brien (1985). Individual flows in the CVB are commonly a few metres thick, with a few examples that are tens of metres thick with pillows that are generally weakly deformed to undeformed. In this outcrop, the pillows may be greater than 1 m in diameter (Fig. 23). Pillow selvages are in these flows are generally thicker than in the mafic metavolcanic rocks. Internal features include amygdaloidal and vesicular flow laminae textures and feldspar phenocrysts, with some pillows in flows to the south containing concentric flow laminae defined by the alignment of vesicles approximately 1 mm in diameter. Pillow breccias occur as a transition from the pillowed flows to the hyaloclastite-rich, pillow fragment breccias, and are exposed on the western end of the outcrop as it dips into the lake. The pillow breccia portion of the units is generally 1 m wide, with the pillows very abruptly diminishing in size away from the main body of the flow. A good example of this type of breccia is exposed in the bay to the south, ~150m from this location, but is only accessible when the water levels are low. Pillow fragment breccia forms the major part of the unit, and is generally a few metres in width adjacent to the flow front. This transition is visible near the waterline. The breccia thickens to the Srop 1-4 - Poplar Lodge Pillowed Metavolcanic Rocks Private property access to lake shore; please ask permission from cottage owners UTM coordinates - 0421034E 5499814N (northern end of section), 0421089E 5499665N (southern end of section) Weakly deformed, northward-younging, pillowed Figure 23. Pillow basalt, Poplar Lodge, Lake Nipigon (Stop 1-4). - 21 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 west, which may suggest that the direction of flow was towards the west. Fragments generally consist of sub-angular parts of pillows and rounded fragments or amoeboid fragments and small pillows in hyaloclastite matrix. Although the pillow breccias have been classified as volcaniclastic rocks, a portion of the units could be classified as hyaloclastite or rocks that are the result of fragmentation of the magma due to interaction with water (Cas and Wright, 1987; Batiza and White, 2000). There is no depth restriction to the formation of the pillow breccias, as the breccias have been observed in both shallow water and on seafloor sites. Heterolithic breccias consist of predominantly 0.5 to 10 cm sub-angular to angular clasts of intermediate metavolcanic rocks, but minor clasts of feldsparporphyritic, felsic metavolcanic rocks, cherts and mafic metavolcanic rocks overlie pillow breccias in a few locations along the lake shore. Generally, the breccias lack any internal structure. Although some areas appear to have bedding structures, they generally cannot be traced for any distance. The matrix is generally highly chloritized and some of the smaller, more porous clasts may have been totally chloritized; reaction rims along clasts margins may be developed Major element and trace element analysis indicated that the pillowed flows are tholeiitic basalt and andesite (O’Brien, 1985), characteristic of mid-ocean ridge basalt (Tomlinson, 1996; Hart et al., 2002). Stop 1-5 - Eva Creek Pyroclastic and Sedimentary Rocks UTM coordinates - 0424037E 5498780N Dacitic pyroclastic rocks along the southern margin of the CVB are exposed on a small ridge 50 m east of Highway 580 (Fig. 28). A massive, aphyric, monomictic tuff-breccia has an iron-carbonatized matrix and is cut by epidote veins (Fig. 24). It is foliated at 065°/78° south and hosts narrow shear zones with crenulated fabrics and quartz veins. There is a suggestion of flattened pillows at the northern edge of the outcrop. A north-trending diabase dyke cuts the supracrustal rocks. An outcrop on the west side of Highway 580, across from Stop #1-5, exposes typical, medium-bedded turbidites of the SMB. These consist predomintly of A, B, D and very minor E divisions. The minor E division reflects very little clay in the sediment transport system, a logical consequence of sediment feed being mostly derived from active subareal, pyroclastic volcanism to the north in the Onaman-Tashota terrain (Fralick, 2003). The interval of time between eruption, erosion and sedimentation was not sufficient to produce significant amounts of chemical weathering (i.e., clay). The relative lack of C (rippled) divisions is more difficult to account for. One possibility is that higher ocean temperatures resulted in a thinner, laminar sublayer which fine sand was capable of disrupting and, since ripples need a laminar sublayer to form, the C division was suppressed. The non-regular bed thickness variation up through this section reflects rapid changes of the position of sediment feed areas into the subaqueous portions of the basin, resulting in a somewhat chaotic interlayering of thin and thick turbidites. Stop 1-6 - Eva Creek Conglomerate / Banded Iron Formation •L. $ t , , UTM coordinates - 0423925E 5498505N Figure 24. Deformed felsic pyroclastic breccia, Highway 580 (Stop 1-5). Exploration of the local iron ranges began in earnest around 1900 when several claims in what was to become the Sand River and Leitch mines area were staked and surveyed (Laird, 1937). In 1919, the Lake Superior Ore Company was formed to option and drill a portion of the Central range. At this time, the Kokoko camps on claim A.L. 413 were built. Drilling carried on until 1920. Coleman (1907) reported that drilling returned assays in the 40 to 50% Fe range, with negligible amounts of deleterious sulphur and phosphorus. Laird (1937) characterized the iron deposits of the Central range: - 22 - “The beds have been crumpled and intricately folded, �-2. ___2 FIuviaI _________ a Proceedings of the 51st ILSG Annual Meeting - Part 2 ______ •Ly€€d. 40 Dominated .—'--.. .1 .! b SnhiIar. orMagnelite 3 -r and Sirisiclasb. ThinLayered. LI- SiIisidasbc . I 2 0 with Thin MagnetLte t.aminae 20 / No fl(ilrCp Sandstone Lense In Conglomerate 1J1/ IT Trcijh Crot Stfat,flcabnn 0m — Flooding r — Surface Figure 25. Stratigraphy, Eva Creek section (Stop 1-6). vi . ab IF. Im on tSandstV with the result that they have the appearance of great thickness in places. The formation is well-banded and consists of thin layers of red jasper alternating with steelgrey hematite, the whole being irregularly interbedded with dark slaty bands. The jasper-hematite bands are not commonly more than a few feet in width and in places they occur as mere streaks in the slaty material. Magnetite is not a prominent constituent of this band, but in places it is present in quantities sufficient to cause considerable magnetic disturbance.” This is one of the best examples of the stratigraphic position of BIF in the belt. The lower portion of the section consists of interlayered magnetite, siltstone r • • • Figure 26. Conglomerate associated with BIF, Eva Creek (Stop 1-6). and sandstone in various ratios from chemical sediment-dominated to clastic-dominated (Fig. 25). This is erosively truncated by a conglomerate. The clast-supported, polymictic conglomerate (Fig. 26) contains diffuse zones of larger and smaller clasts, lenses of medium-grained sandstone and crescent scours and sand shadows along internal surfaces where sandstone lenses are concentrated. This is overlain by a thick, medium-grained sandstone, which is, in turn, succeeded by a clastic-BIF succession. A coarsegrained sandstone-conglomerate assemblage erosively cuts down into this package, removing the upper metre in places. This outcrop highlights the juxtaposition of BIF and erosively based, fluvial channels. The fluvial system was periodically extending into a nearshore dominated by thin, graded siltstones and sandstones with iron oxide laminae. The depositional architecture is composed of a series of prograding parasequences in the littoral zone with BIF developed on flooding surfaces. Commonly iron-rich BIF directly overlies the flooding surface and is upwardly transitional into more clastic-dominated units. These may be erosively incised by the next channel assemblage. Off-delta, deeper-water turbidite units do not contain BIF. This strongly indicates that BIF deposition was the result of photosynthetic oxidation in the nearshore, producing an oxygenated oasis near nutrient-rich waters emanating from river mouths. - 23 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 1-8 - Sand River Mines Property - #16 Vein system UTM coordinates - 0424339E 5497563N The following history has been modified from a summary reports prepared by Bevan (2004) for Roxmark Mines Limited (who are the current property owners) and Mason and White (1986). Laird (1937) provided the first geological description of the property and its exploration activity. History of the Sand River and Leitch Mines Figure 27. Deformed BIF, Highway 580 (Stop 1-7). Stop 1-7 - Leitch Banded Iron Formation UTM coordinates 0424600E 5497867N This is an excellent exposure of oxide-facies BIF interlayered with siliciclastic layers. Unlike many BIF’S in this region, hematite, not magnetite, dominates this exposure. At first glance the rocks give the impression that there is intense shearing along the cleavage direction disrupting the layering (Fig. 27). Closer examination may lead to a different conclusion. The sharp-sided, clay-poor, medium-grained sandstones form lenses (averaging 1 cm thick), cutting saucershaped scours into the 1 mm-thick, stacked iron oxide laminae. That is, the sandstones are lenticular to flaserbedded. Some lenses appear to be ripple-laminated, although the cleavage cutting through at a similar angle makes identification problematic. This type of siliciclastic depositional system is typical of distal portions of distributary mouth bars in deltaic settings. The interlayering of iron formation indicates relatively rapid precipitation of the chemical sediments in a nearshore environment. The first discovery of gold in the Standingstone River area west of Beardmore was made on what became the Sand River Mine property in June, 1934 by Russell Cryderman (Laird, 1937). The Discovery or Number 1 vein averaged only 13 inches in width, but was remarkably persistent along strike, being exposed in cross-trenches and surface pits over a length of 2100 feet. The vein was traced to the northeast onto the Leitch Mine in 1935 (Fig. 28). Stripping and trenching were carried out by the newly incorporated Sand River Gold Mining Company on the Number 1 vein. Servicing was by two three-compartment shafts, 2950 feet apart in a northeasterly direction: the westerly Sand River shaft to 2656 feet and the Leitch main shaft to 3006 feet, with a winze from the 19th (2875-foot) level to the 30th (4525-foot) level. A road connecting the two properties with Beardmore was constructed in 1936. Production started with milling facilities at 75 tons per day at both properties in 1937. Between 1937 and 1942, Sand River processed 157,870 tons at a recovery grade of 0.32 ounce gold per ton, yielding 50,065 ounces gold, to their 9th (1150-foot) level. Mining was by conventional shrinkage stoping with an approximate width of 3.0 feet. The Sand River operation ceased in 1942. Leitch Mines processed 80 tons per day, milling 176,535 tons at a recovery grade of 0.71 ounce gold per ton in the same period with a high-grade resuing approach. Northern Empire Mines Limited, a subsidiary of Newmont Mining Corp., began milling operations in Beardmore in 1934 with a maximum of 186 tons per day in 1939 and ceased in 1941. They then acquired the Sand River property on a ten-year lease. They drifted out on two of the three un-mined Sand River levels above the diabase sill. Sufficient encouragement led them to collar a winze on the Sand River 11th or 1450-foot level, designed to sink through the diabase. - 24 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 The winze was stopped at 1506 feet when war broke out in Europe. Northern Empire resumed operations at Sand River in 1944 as the Undersill Gold Mining Company. Undersill de-watered and deepened the Sand River shaft through the flat-lying, 600-foot thick diabase sill to 2180 feet. The shaft was bottomed at 2656 feet. They explored the 17th or 2610-foot level with a shaft crosscut of approximately 450 feet and 2340 feet of drifting, of which 1300 feet was eastward to the Leitch boundary, and 1040 feet was westward. The 1947 Leitch Gold Mines Limited Annual Report stated that Northern Empire’s Undersill Gold Mining Company had encountered “good ore” on the Sand River 17th (2610-foot) level, an extension of the highgrade Leitch Number 2 vein. Northern Empire, unable to capitalize on its 1944 to 1947 development with gold at US$35 per ounce, closed down the Undersill Gold Mining operation in 1954 and concentrated development at its Magnet Mine, near Geraldton. Leitch acquired the Sand River property in the same year to protect the depth extension of their steeply westplunging Number 2 vein. Leitch developed the Sand River property by an internal winze at the Leitch Mine from their 19th (2875-foot) level to the 30th (4525foot) level. Leitch’s production from the Sand River - Undersill Gold Mining Company’s holdings totalled 376 283 tons at a recovery grade of 1.11 ounces gold per ton from 1955 to mine closure in 1965. This was an appreciable improvement of grade with depth from 530 112 tons at 0.79 ounce gold per ton from 1938 to 1954 above the 19th level. Environmental requirements were completed on the Leitch Mine and the Sand River property was returned to the original owners in 1968. In 1940, exploration by Halport Gold Mines outlined a parallel vein system, named the #16 vein system. The following table covers the history of this discovery. Historical Summary - #16 Vein System 1940 - Halport Gold Mines located a number of surface veins on Claim AL 415 at the eastern end to the boundary with Sand River Claim TB 12944. 1944 - Leitch acquired the Halport holdings. 1945 - 22 diamond drill holes totalling 16 510 feet were drilled. 1950 - A long crosscut on the 8th level was initiated because surface drilling had indicated ore at this horizon. 1951 - Further crosscutting and some drilling was carried out. 1952 - Further crosscutting and drifting to open up the #16 vein. Drilling of 51 underground holes totalling 11,116 feet, mainly on the 8th level. 1953 - Further drilling and stoping on the #16 vein – production and milling of 3,598 tons grading 0.56 oz. Au/ton. 1969 - Teck Corporation (together with HighlandBell Limited and F.E. Hall) purchased control of the Leitch Gold Mines property. 1971 - The entire Leitch assets were acquired by Teck Corporation. 1946 - Teck Corporation carried out exploration work until 1981 over the Leitch property, including line cutting, VLF-EM magnetometer surveys, 393 holes of overburden drill sampling and 29 short diamond drill holes totalling 3,454 feet and covering a strike length of 1,400 feet. One hole was abandoned. 1987 - Sand River-Cryderman diamond drilling program. 10 holes totalling 2,030 feet were drilled on Claim TB 12944 covering the #16 vein to the east. 1988 - Teck Corporation drilling program in joint venture with San Paulo Explorations Inc. 24 diamond drill holes totalling 22,098 feet, of which three were abandoned, one hit the open stope and one intersected the vertical diabase dyke and was terminated. Airborne and ground geophysical surveys and reverse circulation drilling were also undertaken until 1989. 2003 - Acquisition of Sand River property by Roxmark Mines Limited; Advanced Exploration project initiated. In 2003, Roxmark Mines Limited acquired the Sand River, Leitch and East Leitch properties from Rio Fortuna, Teck Cominco and Kinross, respectively (News Release, Roxmark Mines Limited, November 28, 2003) and began an Advanced Exploration program. In 2004, two vein systems were exposed by trenching, stripping, channel sampling and stockpiling at the East Leitch and Sand River #16 zones, respectively. Stripping of these areas had been recommended in preparation for taking bulk samples (ibid). Geological mapping and sampling was planned to gain a better understanding of the mode of occurrence and identify other possible veins and extensions in the #16 vein system. Further exploration in the form of diamond drilling in five areas was recommended as follow- - 25 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sand River - Lelich Mines (altar Ferguson (1967a) 0 results confirm the historical sampling data completed by Teck Corporation. Furthermore, the current assay results confirm the historical drill holes that intersected the #16-1 Vein. Recent Roxmark assays of the #16-5 West vein were 1.058 ounces Au per ton over a 26.5 foot strike length. The #16-5 East Vein returned 1.094 ounces Au per ton over a strike length of 35.0 feet. I loOm Geology of the #16 Vein Area Figure 28. Geology of the Sand River-Leitch mines area after Ferguson (1967a), showing Stop locations. up work in preparation for underground exploration and development. Bulk samples will be processed at Roxmark’s 200 ton-per-day Northern Empire Mill in Beardmore. Roxmark reported the results of samples taken from the #16-vein system on the Sand River Mine property (ibid, December 16, 2004). The Company is continuing with its plan for the surface bulk sample program and in particular, the in-fill sampling on the #16-1 and #16-5 veins. Channel samples were cut every three feet. All composite assays used a minimum width of 1.0 feet. The #16-1 West Vein returned an average assay of 1.737 ounces Au per ton over a strike length of 42 feet. The #16-1 East Vein returned 0.347 ounce Au per ton over 37 feet. The recent assay 16 Vein Zone Sand River Mine The Leitch-Sand River area (Figs. 15 and 28), most recently mapped by Hart et al. (2002), is predominantly underlain by clastic metasedimentary rocks of the SMB. Thickly bedded, southward-younging, overturned, feldspathic sandstone and siltstone have been wellexposed by recent stripping operations in the vicinity of the #16 North zone (Fig. 29). Bedding has eastnortheasterly to east-southeasterly orientations with steep, northerly dips. Sub-parallel shear zones are distinguished by well-developed, slaty S3 cleavage, as well as local sericitization and carbonatization. Oblique cleavage – bedding relationships are locally exposed. Cleavage is refracted through interbedded siltstone and sandstone beds (Fig. 30). A number of auriferous quartz veins have been exposed in the immediate area (Figs. 28 & 29). Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite in composite, “crack-seal”-textured veins. Tetrahedrite and sphalerite are also reported to occur in these fractures. Pink-beige scheelite is relatively abundant. :i The Leitch Mine No. 2 vein and the Sand River Mine vein are the same vein striking southwest parallel to a N Figure 29. Map of stripping and veins, Sand River 16 Vein zone (Stop 1-8). Figure 30. Refraction of cleavage in sandstone/siltsone, Sand River 16 Vein zone (Stop 1-8). - 26 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 subvertical to vertical tectonic fabric which is subparallel to the original bedding in the clastic metasedimentary rocks (Hart et al., 2002). The vein contains multiple oreshoots that plunge at about 320° in the plane of the 65° northwest dipping vein, and these shoots are controlled by the intersection of the first fabric with a second regional tectonic fabric. Lafrance et al. (2004) defined the strike of the vein as 240° and dip 65°, with plunges to the oreshoots being consistent with the F3 fold axes, parallel to the Standingstone - Watson Lake Fault which was likely developed in two parallel, D3, dextral shear zones. Ore zone plunges are consistent with F3 fold axes. Hart et al. (2002) identified that the potential for additional gold mineralization would be very high in structures subparallel to parallel to the Sand River Mine – Leitch Mine No. 2 vein, such as the No.16 veins that are exposed north of the Sand River Mine shaft. Stop 1-9 - Leitch Mine UTM coordinates - 0425302E 5497484N This stop allows trip participants to scour Leitch Mine dump piles for representative samples of goldbearing quartz veins and clastic sedimentary rocks. Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite in composite, “crack-seal”-textured veins. Tetrahedrite and sphalerite are also reported to occur in these fractures. Pink-beige scheelite is relatively abundant. Leitch Mine operated continuously from 1937 to 1965. Mill clean-up ensued between 1966 and 1968. One of the highest-grade gold mines in Canada during its lifetime, the mine yielded 861,981 ounces of fl• gold from 1,022,360 milled tons at a recovered grade of 0.915 ounce gold per ton. 31,802 ounces of silver were also produced at a grade of 0.035 ounce silver per ton. 64.3 tons of tungsten ore were produced at a average grade of 3.95% WO3. Figure 31 gives us a glimpse of the property at the time of its discovery. Day Two - Geraldton Area Past-producing gold mines in the Geraldton camp (~ 3.1 million ounces) are confined to the northern part of the SMB and are localized by the Barton Bay Deformation Zone (BBDZ). The BBDZ (Fig. 12) is a 1 km wide, high-strain zone that extends from the Hard Rock and MacLeod-Cockshutt mines west along Highway 11 to the Bankfield Mine (Pye, 1952; Lavigne, in press; Lafrance et al., 2004). Shear is distributed heterogeneously across the BBDZ. In contrast to the mafic metavolcanic/mafic intrusive rocks to the north, and the monotonous sequence of sandstone to the south, the rocks within the BBDZ (mafic to intermediate intrusive rocks, felsic porphyry dykes and sills, sandstone/pelite, conglomerate and BIF) are complexly intercalated and have high competency contrasts. As noted by Lavigne (in press), only a very few areas in the belt have such lithologic heterogeneity. All of the aforementioned rocks in the BBDZ, except for the conglomerates, host gold. Although Geraldton is often considered to be a BIF-hosted gold camp, it is important to note that less than 30% of the gold came from BIF-hosted ore (Macdonald, 1982). Most of the production came from clastic sedimentary rocks, particularly at or near the contact with feldspar porphyry intrusions. Anglin and Franklin (1985) cited a more consistent spatial relationship between gold mineralization, deformation zones and felsic intrusive rocks. Stop 2-1 - MacLeod-Cockshutt Mine UTM coordinates - 0504200E 5502770N 2 4 Figure 31. No. 1 vein, Leitch Mine, October 1935 (Laird 1937; Stop #1-9). The history of the MacLeod-Cockshutt Mine (Fig. 32) has been summarized by Horwood and Pye (1955) and Mason and White (1985). In 1931, following the discovery of gold by W.W. Smith on Discovery Point, Kenogamisis Lake, F. MacLeod and A. Cockshutt staked the ground adjoining the Hard Rock Gold Mines Limited property to the west. Surface exploration led to the discovery of gold-bearing quartz veins in 1931. The discovery of larger mineralized zones in 1933 - 27 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 'j ____ I Figure 32. Map of the MacLeod-Cockshutt – Hardrock mines area (modified after Ferguson 1967). Pf = albite porphyry; IF = banded iron formation; sandstone is unlabelled and unhatched. led to the organization of a new company, MacLeodCockshutt Gold Mines Limited. In 1934, shaft-sinking began here at the Number 1 shaft; the Number 2 shaft, 600 m to the southeast, was sunk in 1936. The MacLeod-Cockshutt Mine became the fifth producing gold mine in the Little Long Lac area on April 19, 1938 when a mill with a rated capacity of 600 tons per day was brought into operation. In 1967, MacLeodCockshutt Gold Mines Limited, Consolidated Mosher Mines Limited and Hard Rock Gold Mines Limited were amalgamated to form MacLeod Mosher Gold Mines Limited. Underground operations continued until July, 1970. The mine had produced 1,546,980 ounces of gold at an average grade of approximately 0.14 ounces of gold per ton. This total accounts for about half of all the gold produced by the 10 mines in the Geraldton gold camp between 1934 and 1970. In the 1980’s, Lac Minerals Ltd. (now Barrick Gold Corporation) undertook studies of existing underground reserves at the MacLeod-Cockshutt and neighbouring Hardrock mines and carried out lithogeochemical sampling (Gray, 1993). Starting in 1987, Lac conducted ground geophysical surveys, followed by 77 diamond drill holes, totaling approximately 50,000 feet. Targets, especially those with open pit potential, were investigated (e.g., Hardrock D and F; North and South Porphyry; Porphyry Hill zones). In 1992, Asarco Exploration Company of Canada Limited entered into a 5-year earn-in agreement with Lac and in 1993 carried out a program of reverse circulation overburden drilling and diamond drilling, the latter largely focusing on the near-surface portion of the F-zone and targets along the plunging nose of the albite porphyry (ibid). As a result of this work, a geological resource was calculated by Horvath (1993) for the Porphyry Hill, West and East pits: Pit Resource: 1,920,000 tons grading 0.079 ounce gold per ton (with strip ratio, including overburden, of 4.76 to 1) Ramp Resource: 1,160,000 tons grading 0.127 ounce gold per ton Asarco continued their exploration campaign into 1994, completing reverse circulation holes in overburden; sonic holes in tailings; and an additional 40,000 feet of diamond drilling, mostly on the aforementioned targets (Gray, 1994). Cyprus Canada Inc. assumed Asarco’s role in the Lac Minerals agreement in 1996 and drilled 24 holes, leading to the discovery of the B-zone (Mason and White, 1997). The agreement ended in 1997 and the properties have been dormant since then. Lac Minerals Properties Inc. began a rehabilitation program in 1996 that is still active. In the first description of the fledgling Little Long Lac (i.e. Geraldton) camp, Bruce (1935) proposed three types of gold deposits: (1) Shear zones in sedimentary rocks, along which narrow, but closely spaced quartz veins occur in parallel planes; (2) Irregular veins of quartz accompanied by pyrite, filling fractures in iron formation; and (3) Zones of pyritization and silicification in both in sedimentary and intrusive rocks. - 28 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the porphyry and the adjacent metasedimentary rocks. The second ore type is exemplified by the North, North Limb, South Limb and West zones (Fig. 33). Ore consists of masses, lenses and tongues of almost massive sulphides that replace folded wall rock, particularly laminae of incompetent, sheared sandstone and magnetite beds in banded iron formation. Sulphides, principally pyrite and arsenopyrite, also replace rocks adjacent to steeply dipping, east-trending shear fractures or shallowly west-dipping, north-trending tension fractures. Fractures are commonly healed by late(r)stage quartz. It was reported that replacement-type ore averaged approximately 0.5 ounce gold per ton, while fracture-filling ore typically returned between 1 and 3 ounces gold per ton. p West -. BodieE!\._____ ....-. ... - VS Replacement Systems Albite Porphyry = Iron Sandstone— Argillite 9 6pm Figure 33. Vertical north-south section, MacLeod-Cockshutt Mine. After Horwood and Pye (1955). The third ore type, best exemplified by the Fzone, consists of numerous quartz veins and stringers localized within a wide zone of shearing and fracturing along the northern side of the albite porphyry. The veins and stringers range in width from < 1 to 10 cm. They appear to be largely confined to sandstone and lean BIF nearer to surface and occur in altered quartz diorite at lower levels to the west. Generally, quartz stringers are bordered by buff-coloured, sericitized and carbonatized sandstone with fine- to coarse-grained pyrite and subordinate arsenopyrite. In iron-bearing wall rock, sulphides are more predominant and may Six main ore zones were exploited at the MacLeodCockshutt Mine. Horwood and Pye (1955) classified the MacLeod-Cockshutt orebodies and zones of mineralization into three distinct types: (1) Quartz veins and mineralized zones (2) Irregular, massive sulphide-quartz lenses in a folded series of sandstone and banded iron formation (3) Quartz stringer zones in sandstone and subordinate quartz diorite. The first ore type, exemplified by the South, No. 210 Quartz Vein, No.’s 516, 517 and 519 Drift zones, is manifested by numerous but small lenses of semimassive sulphides (coarse pyrite + arsenopyrite, sphalerite and chalcopyrite) scattered along the contacts of intrusions and metasedimentary rocks; by disseminated, auriferous sulphides in porphyry; and by minor quartz-carbonate veins and stringers cutting both Figure 34. No. 1 Shaft headframe, MacLeod-Cockshutt Mine (Stop 2-1). - 29 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 secondary pyrite in association with iron carbonatization and quartz veining. Transpressive/oblique compression has produced open Z-folds and localized S-folds, with evidence of re-folding and late(r) transposition (Figs. 35 and 36). Consistently shallow (15° to 25°) plunges are observed on fold hinges. Lafrance et al. (2004) noted several, small isoclinal F1folds that were folded by parasitic F2 S-folds, resulting in a strong, rod-like, coaxial lineation that plunges 20° to 40° west, subparallel to the strike of the S2 axial-planar cleavage (100°/85° southwest) of regional folds. Figure 35. Deformed magnetite BIF, MacLeod-Cockshutt Mine (Stop 2-1; coin is 2 cm in diameter). The BIF consists of a finely parallel-laminated sequence of alternating, magnetite-rich layers and silt- to very fine sand-rich layers. The sand layers rarely show grading due to deformation, although some graded examples show increasing magnetite content in their clay-rich tops, which are gradational to the overlying, pure chemical sediment layer. On the microscopic scale, the magnetite layers have small amounts of chert and detrital minerals, but are otherwise mineralogically pure. Although deformation limits the depositional interpretations that can be drawn in this area, other exposures near Beardmore (e.g., Stop 1-6) indicate that the iron-rich sediments accumulated in the shore-proximal positions near the mouths of rivers in braid-delta complexes. Stop 2-2 - Porphyry Hill, Hard Rock Gold Mines Limited 4(j UTM coordinates - 0504810E 5502850N Figure 36. West-plunging folds in BIF, MacLeod-Cockshutt Mine (Stop #2-1). form small replacement lenses. Although the quartz veinlets contain visible gold, much gold is also associated with replacement pyrite and arsenopyrite. The recently exposed outcrop at the base of the No.1 shaft headframe (Fig. 34) displays highly deformed BIF and sandstone, part of a band that extends 600 m east onto the Hard Rock Mine property and hosts the North ore zone there (Stop 2-3). The steeply dipping BIF contains primary magnetite, but displays some In 1935, Hard Rock Gold Mines Limited began surface work and diamond drilling on the “Porphyry Hill” section of the property and discovered goldmineralized zones in and along the northern tongue of porphyry (Horwood and Pye, 1955). The discovery of scattered, finely disseminated pyrite and numerous, small quartz stringers prompted underground followup, resulting in the definition of the No. 1 and No. 2 vein systems. In addition, a small, high-grade shoot outcropped near the western property boundary. The X-vein system was defined underground (3.5 feet wide, 110 feet long, with an average grade of 0.049 ounce gold per ton), within an inclusion of sandstone and lean BIF in albite porphyry (ibid). Asarco’s proposed Porphyry Hill open pit was designed to recover shallow ore from several zones, including the F-zone (sandstone-BIF-hosted; northern porphyry contact-hosted); Porphyry zone (sheared - 30 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 2-3 - Glory Hole BIF, Hard Rock Gold Mines Limited UTM coordinates - 0504862E 5503143N Deformed, gold-bearing BIF is exposed along the edges of a fenced-off, water-filled glory hole, just south of Highway 11 that represents the mined, near-surface portion of the Hard Rock North ore zone. BIF occurs with siltstone/sandstone, porphyry sills and diorite sills and dykes. This orebody consisted of numerous, irregularly shaped replacement lenses of sulphides distributed in an east-trending zone along the northern limb and in the trough of a minor synform having a plunge of approximately 30° west (Horwood and Pye, 1955). The open stope mimics the shape of a F3 Z-fold defined by folded BIF horizons (Macdonald, 1988). Individual lenses were not of great lateral and vertical extent, but had lengths of up to 300 feet and widths up to 30 feet. Collectively, these sulphide lenses formed an ore zone which persisted and plunged onto the Figure 37. Contact between albite porphyry and sandstone/ BIF, Porphyry Hill (Stop 2-2). porphyry-hosted); South Porphyry zone (southern porphyry contact-hosted) and the Southern Iron Formation zone. Horvath’s (1993) geological resource estimate for the Porphyry Hill open pit included an estimated high-grade resource of 1.02 million tons at a grade of 0.100 ounce gold per ton, and a low-grade resource of 0.48 million tons at a grade of 0.034 ounce gold per ton (for a total of 1.5 million tons grading 0.079 ounce gold per ton). Deformed porphyry dykes and sills intruding sandstone and BIF are exposed at this site (Fig. 37). Crowded albite and minor quartz phenocrysts occur in a fine-grained matrix of quartz, feldspar, sericite, calcite and chlorite. An age of 2698.6±1.3 Ma was obtained on a Au-mineralized “crowded albite” feldspar porphyry dyke at the Hard Rock Mine (Corfu, 2000). Figure 38. Oxide-facies banded iron formation (BIF) cut by massive sulphide (MS)-rich alteration envelope surrounding quatz-carbonate vein (QV), Hard Rock Mine (Stop 2-3). MacLeod-Cockshutt property to the west. This is the same BIF that is exposed below the No. 1 headframe (Stop 2-1), approximately 600 m west of this location. The so-called massive sulphide lenses contain up to 65% combined pyrite, arsenopyrite and pyrrhotite, the remainder being quartz. Minor sphalerite, chalcopyrite and galena, as well as accessory ankerite, calcite and tourmaline has also been noted. Replacement, controlled by quartz-filled fractures and bedding planes, is locally pervasive, destroying primary lithologic features (Fig. 38). Pervasive carbonatization accompanies replacement and gold mineralization. Replacement lenses are localized in zones of intense deformation - 31 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 W1iItFigure 39. Pre- and post-rehabilitation views of the MacLeodCockshutt tailings at the junction of Highways 11 and 584, showing new Heritage Interpretative Centre. along contacts between sandstone and BIF, and in the noses of small, asymmetric folds in an orientation consistent with the axial-planar cleavage in these folds (Anglin and Franklin, 1985). North zone ore mined to the mid-1940’s averaged approximately 0.25 ounce gold per ton. Stop (optional) - Geraldton Reclamation Project In the late 1990’s, Barrick Gold Corporation, in conjunction with the Town of Geraldton and government agencies embarked on a tailings reclamation project. This Tourism Development Project led to the development of the Heritage Interpretative Centre and a 9-hole expansion of the Kenogamisis Golf Course. The following description has been gleaned from the website of one of the designers (http://www.gsd. harvard.edu/people/faculty/schwartz/projects.html). In the Geraldton recalmation project, mine tailings were reshaped for both aesthetic and economic reasons (Fig. 39). 14 million tons of tailings from the mines covered 170 acres to a depth of 27 feet. In order to spur economic redevelopment, the Town made the decision to make something of the tailings by improving their appearance and adding opportunities for visitors. Design alternatives involved sculpting the flat pile into compelling sculptural landforms which serve as a dynamic roadway edge and a gateway to the town. Trails invite one to walk, bird watch, mountain bike, snow board, sled or snowmobile. Technical constraints were key to the final form of the earthwork. The different types and sizes of earth-moving equipment and their turning radii provided guidelines for the grading plan. A primary objective in the project was to balance cut and fill, and to maintain a maximum total earth moving of 150 000 cubic meters. Cut is kept to a minimum as arsenic levels are higher toward the bottom of the pile. There is a cap at the bottom of the pile of tailings, and there is a maximum of an additional 5 m that can occur on top. Standing water has been considered as a design element, but the water table has been respected by the re-grading. Storm drainage is maintained and the proposed earthwork does not impede sight lines for traffic safety. Six to twelve inches of peat topsoil were added to disturbed areas to aid in revegetation. A planting plan for the project focuses primarily on native grasses, especially those golden in color. The Geraldton project reveals the power of design to remake a wasteland into a new landscape. Even more than an earthwork, this landform is also a cultural artifact, highlighting the location and role of mining in the life of the town. Stop 2-4 - Conglomerate, Highway 11 UTM coordinates - 0502906E 5502908N Low outcrops on the south side of Highway 11 opposite the turnoff for the Consolidated Mosher Mine display deformed conglomerate, sandstone and BIF of the SMB within the BBLZ. As noted by Williams and Stott (1991), strain is exhibited by differentiated layering in pelitic rocks and elongated clasts in the conglomerates, especially in less-competent, mafic clasts (Fig. 40). Abundant, shallowly west- Figure 40. Sheared conglomerate in BBLZ, Highway 11 (Stop 2-4; Pen is 15 cm long.) - 32 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 plunging, Z-folds with attendant axial-planar fabrics, back-rotated pebbles, pressure fringes and intraclast fractures support the contention that the BBLZ is a major dextral, ductile shear zone (Williams and Stott, 1991). Lafrance et al. (2004) noted that the long axes of deformed granitic clasts in polymictic conglomerate are parallel to the axial plane of F3 folds. S2 strikes 100° in the sandstone matrix away from the clasts, but as it wraps around the clasts, it changes orientation to that of the elongated granitic clasts, resulting in a geometry similar to dextral, asymmetrical strain shadows around rigid objects (ibid). Polymictic, clast-supported, cobble-pebble conglomerate with deformed, lenticular lenses of deformed sandstone is interbedded with thicker, medium-grained sandstone sheets. They collectively represent a gravelly, braided fluvial system with sanddominated channels, separated by gravelly longitudinal bar complexes. The bars were active during flood events with small sand channels within the bars developing as chutes during falling stage. In these types of braided systems, more massive, coarser-grained gravels developed in the bar-head region of the bars. Bar-tails are often composed of interlayered sand and thin gravel layers, dissected by secondary channels. Stop 2-5 - Gabbro, Highway 11 UTM coordinates - 0502530E 5502860N Approximately 300 m west of Stop 2-4, outcrops of deformed gabbro lie on the south side of Highway 11, just east of a small pond. The amphibolitized, medium-grained gabbro is cut by a network of dark green, chloritic zones < 1 cm wide in which hornblende megacrysts are elongated. These anastomosing shear bands separate lozenges of relatively undeformed gabbro, imparting an disconcerting impression of a pillowed basalt flow. With increasing degrees of deformation, the shear bands become sub-parallel, forming a C fabric (Macdonald et al., 1990). In the most deformed rock, a third (C’) fabric develops at 45° to the C fabric. The northwestern orientation of C’ is indicative of dextral shear. Elsewhere on the outcrop, sharply bounded, metre-wide zones of fine-grained greenschist represent mylonitized gabbro (Williams and Stott, 1991). Buck and Williams (1984) considered such variations in the preservation of primary textures, versus the development of shear fabrics, to be a function of the inhomogeneity of strain and not of the protolith. Stop 2-6 - Missing Link Extension, Kinghorn Road UTM coordinates - 0466744E 5511220N The Missing Link Extension occurrence was discovered by Beardmore prospector Myron Nelson in 1986 and optioned to Freewest Resources Inc. in 1990 (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). It is situated on the west side of the Kinghorn Road, just north of the Paint Lake Deformaion Zone (PLDZ). The PLDZ is a 500 m-wide high-strain zone at or near the northern boundary of the Beardmore-Geraldton belt (Mackasey, 1976; Reilly, 1988; DeWolfe, 2002; Lafrance et al., 2004). The shear zone extends for over 40 km, from Lake Nipigon to the late, sinistral Jellicoe Fault, where it is offset by 500 m. The shear zone can be traced for ~ 9 km on the east side of the fault before it is lost under overburden. At the Missing Link property (1 km to the northeast; Stop 2-7), a northern splay/extension of the PLSZ has been exposed by mechanized stripping. At Paint Lake (DeWolfe, 2002), the PLSZ is parallel to lithological contacts between polymictic conglomerate (NMB) and volcanic rocks (NVB), and forms the boundary with the Onaman–Tashota belt to the north. Shear sense indicators are observed in both conglomerate and volcanic rocks. Several granitic clasts in conglomerate are bounded by dextral, asymmetrical strain shadows filled with quartz fibers. Steeply dipping dextral shear bands cut across S3 (Lafrance et al., 2004). The property straddles intermediate to mafic metavolcanic rocks on the north and clastic metasedimentary rocks to the south, separated by a major, east-striking structure interpreted to be the PLSZ (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). Stripping and trenching exposed two sub-parallel zones of alteration and mineralization within sheared and isoclinally folded andesite. The orientation of the major fold axis is generally subparallel to the shear fabric, which strikes between 080° and 090° and dips steeply south to vertically. The fold is believed to plunge shallowly to the east. The Main Zone was exposed over a strike length of 40 m and a width of 12 m. Sub-zones were recognized on the basis of variations in the relative amounts of chloritization, carbonatization, silicification, quartz-carbonate veining and sulphide minerals. Finely disseminated arsenopyrite, pyrite and chalcopyrite were noted. Values of up to 0.20 ounce gold per ton were returned - 33 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 variety of surveys were undertaken on the property by Beardmore prospector, Nolan Cox ca. 19861988. Older, undocumented trenches (ca. 1950’s) were discovered during the course of this work. The property straddles intermediate to mafic metavolcanic rocks on the north and clastic metasedimentary rocks to the south, separated by a major structure (at 080°) interpreted to be an extension or splay of the Paint Lake Shear Zone (PLSZ) (Lafrance et al., 2004). At the Missing Link, phyllonites (mafic metavolcanic protolith?) display intense microfolding of S3 on millimetre-scale, producing a fine L3’ crenulation lineation along S3 (ibid) (Figs. 41a,b ). Figure 41a. Crenulated and altered metavolcanic phyllonite with quartz-carbonate veins, Paint Lake Shear Zone, Missing Link property (Stop 2-7). IZ Figure 41b. Crenulated and altered metavolcanic phyllonite, Paint Lake Shear Zone, Missing Link property (Stop 2-7). in sulphide-rich grab samples. Stop 2-7 - Missing Link Property / Paint Lake Shear Zone UTM coordinates - 0467535E 5511479N The Missing Link occurrence was described by Mason et al. (1989). Stripping, prospecting and a DeWolfe et al. (2000) stated that gold mineralization and associated hydrothermal alteration were focused along second- and third-order fault splays off of the PLDZ. Second-order splays generally occur south of the PLDZ, often following lithologic contacts and refracting into an orientation sub-parallel to the PLDZ. Third-order splays are concentrated within the Northern Volcanic Sub-belt, where they are oriented both to the northeast and southeast. Gold mineralization at the Missing Link property is associated with sheared and altered mafic metavolcanic rocks. The deformation zone has been exposed over a width of 60 to 90 m by stripping. Disseminated, foliation-parallel, euhedral pyrite and arsenopyrite within zones of both shearing and quartz-carbonate veining, host gold. Pervasive carbonatization and more localized, erratically distributed silicification and hematitization have also been noted (Mason et al., 1989). Three zones (North Zone; Shaft Pit and Baseline Trench) were sampled by N. Cox. The North Zone strikes 085° and dips 61° south and has returned assay values of up to 0.256 ounce gold per ton over 1.8 m. Grab samples collected by Mason et al. (1989) from all the zones returned up to 0.14 ounce gold per ton. The property was optioned by Homestake Mineral Development Company in 1989, who conducted a number of geochemical, geological and geophysical surveys. Only low gold values (e.g. 2.06 g/t Au over 1.0 m) were returned and in 1990 the property was returned to the vendor (Resident Geologist’s Files, Thunder Bay North District, Thunder Bay). Placer Dome Inc. optioned the property in 1991 and completed just over 1000 m of drilling in four holes. Despite delineating a zone approximately 175 m wide within which intensely sheared and altered rocks occurred, no intersections of economic significance were encountered (ibid). - 34 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 2-8 - Sandstone, Southern Sedimentary Subbelt, Highway 11 UTM coordinates - 0468062E 55005080N Thickly bedded, north-younging, overturned turbidites are exposed on the south side of Highway 11 opposite the Kinghorn Road turn-off . They display a variety of well-preserved sedimentary structures, including graded bedding, ball-and-pillow structures, flames and scours (Fig. 42). Some sandstone beds display a weakly developed cleavage. @) I © 1) 04 0) 0 N) :9 Figure 43. Stratigraphic column of sandstone outcrop (Stop 2-9; scale in metres). N) ri © a 01 ® a G 01 me ICr N) a I) C N) -— 0) 04 0) e melerS 0) ____ __ it — Figure 42. Load casts at base of sandstone (arrows), Highway 11 (Stop 2-8). This section of turbidites contains graded AE and AA beds up to 8m thick (Figs. 43 and 44). Most grading is in the basal few centimeters where the amount of very coarse sand grains decreases rapidly and in a zone approximately three-quarters of the way up through the bed where there is a rapid decrease from medium- to 0 - COG® - Figure 44. Measured sections of the turbidite assemblage in the Southern Metasedimentary Belt between Jelicoe and Geraldton. Note the extreme thickness of some of the beds. These units can be divided into a thin-bedded association, a medium-bedded association and a thick-bedded association. From Barrett and Fralick (1989). - 35 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 fine-grained sand. This results in beds that are quite monotonous, with little change in their grain size through most of their thickness. These sediments were transported by typical, high-density turbidity currents and grainflows which deposit sediment in sourceproximal locations. The fine-grained sand tops on the massive beds were possibly the result of resultant turbulence after the main turbidity current swept through the area. Currents moving water in behind the tail of the turbidity current would sweep sand from the surrounding bottom onto the coarser turbidity current deposits. Stop 2-9 - Pillowed Mafic Metavolcanic Rocks, Highway 11 UTM coordinates - 0460740E 5503325N Pillowed and massive, greenschist-facies, mafic metavolcanic rocks occur on the south side of Highway 11, 200 m west of the Jellicoe post office. They occur in a small, fault-bounded enclave of metavolcanic rocks with clastic metasedimentary rocks of the Southern Sedimentary Sub-belt (Bruce, 1937; Mackasey, 1976). They display a non-penetrative schistosity, on which there is a steeply plunging mineral and stretching lineation. Parallel to the schistosity are shear zones in which a non-penetrative fabric is forming at an angle to the zone, becoming parallel to it and more intense near the axis of the zone, characteristic of S-C fabric relationships (Williams and Stott, 1991). The shear sense based on this local relationship suggests south side-down. In graphitic metapelites and BIF exposed in road cuts 200 m west, the stretching lineation is well-developed. Acknowledgements The authors would like to thank John Mason and Gerry White (Resident Geologist’s Program, OGS, Thunder Bay) for providing property information and historical data. Greg Stott (Precambrian Geoscience, OGS, Sudbury) provided useful structural information and insights on the local tectonic framework. We would also like to acknowledge the support of the Municipality of Greenstone and local exploration companies, especially David Malouf (Roxmark Mines Limited). References Anglin, C.D., 1987. Geology, structure and geochemistry of gold mineralization in the Geraldton area, northwestern Ontario. Unpublished M.Sc. thesis, Memorial University, St Johns, Nfld. Anglin, C.D. and Franklin, J.M. 1985. Gold Mineralization in the Beardmore-Geraldton area of northwestern Ontario: structural considerations and the role of iron formation; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.193-201. Anglin, C.D. and Macdonald, A.J., 1984. Gold mineralization and an iron formation-bearing lithotectonic zone, Beardmore-Geraldton, Ontario; Geological Association of Canada - Mineralogical Association of Canada, Annual meeting, London Ont., Program with Abstracts, p.42. Anglin, C.D., Franklin, J.M., Loveridge, W.D., Hunt, P.A. and Osterberg, S.A.,1988. Use of zircon U-Pb ages of felsic intrusive and extrusive rocks in eastern Wabigoon subprovince, Ontario, to place constraints on base metal and gold mineralization; in Radiogenic age and isotope studies: Report 2, Geological Survey of Canada, Paper 88-2, p.109-115. Armstrong, H.S., 1943. Gold ores of the Little Long Lac area, Ontario; Economic Geology, v.38, p.215-246. Bardoux, M., Jébrak, M., Goulet, M., Morin, D., Giguère, C. and Zadeh, H. 1990. Metallogenesis of the Cadillac shear zone in the McWatters area, Abitibi belt, Canada; abstract in Proceedings, 8th IAGOD Symposium, in conjunction with Mineral deposit modeling international conference, Ottawa, Ontario, p.A260. Barrett, T.J. and Fralick, P.W., 1985. Sediment redeposition in Archean iron formation: examples from the Beardmore-Geraldton greenstone belt, Ontario; Journal of Sedimentary Petrology, vol. 55, p. 205212. Barrett, T.J. and Fralick, P.W., 1989. Turbidites and iron formation, Beardmore-Geraldton, Ontario: application of a combined ramp/fan model to Archean clastic and chemical sedimentation; Sedimentology, v.36, p. 221-234. Batiza, R. and White, J.D.L. 2000. Submarine lavas and hyaloclastite; in Encyclopedia of volcanoes, Academic Press, San Diego, California, p.361-381. Benedict, P.C. and Titcomb, J.A. 1948. Northern Empire Mine; in Structural geology of Canadian ore deposits, Canadian Institute of Mining and Metallurgy, Jubilee Volume, p.389-399. Bevan, P. A. 2004. Qualifying report on the Sand River / Leitch Mines with specific reference to the #16 Vein Systems, Beardmore Area, Northwestern Ontario, Canada; Roxmark Mines Limited, World Wide - 36 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the Beardmore-Geraldton area of northern Ontario. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 227p. Website, http://www.roxmark.com, 35p. Boyle, R.W., 1969. Discussion of hydrothermal transport and deposition of gold;. Economic Geology, v.64, p.112-115. Boyle, R.W., 1976. Mineralization processes in Archean greenstone and sedimentary belts; Geological Survey of Canada, Paper 75-15. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geological Survey of Canada, Bulletin, v.280, 584 p. Bruce, E.L. 1935. Little Long Lac Gold area; Ontario Department of Mines, Forty-fourth Annual Report, v.XLIV, pt.III, 1935, 60p. Bruce, E.L., 1937a. New developments in the Little Long Lac area; Ontario Department of Mines, 45th Ann. Rept., v.45, pt. II, p.118-140. Bruce, E.L. 1937b. The eastern part of the Sturgeon River area (Jellicoe-Sturgeon River section); Ontario Department of Mines, Annual Report, v.45, pt.2, p.159. Bruce, E.L. and Laird, H.C. 1937. The western part of the Sturgeon River area; Ontario Department of Mines, Annual Report 1936, v.45, pt.2, p.60-117. Buck, S., and Williams, H.R., 1984. Structural studies in the Geraldton area; in Summary of Field Work 1984, Ontario Geological Survey, Miscellaneous Paper 119, p.208-211. Carter, M.W. 1987. Geology of McComber and Vincent townships, District of Thunder Bay; Ontario Geological Survey, Open File Report 5648, 144p. Cas, R.A.F. and Wright, J.V. 1987. Volcanic successions: modern and ancient; Allen and Unwin, London, United Kingdom, 528p. Coleman, A.P. 1907. Iron ranges east of Lake Nipigon; Ontario Bureau of Mines, Sixteenth Annual Report, v.XVI, pt.I, p.105-135. Colvine, A.C., Andrews, A.J., Cherry, M.E., Durocher, M.E., Fyon, A.J., Lavigne Jr., M.J., Macdonald, A.J., Marmont, S., Poulsen, K.H., Springer, J.S. and Troop, D.G. 1984. An integrated model for the origin of Archean lode gold deposits; Ontario Geological Survey, Open File Report 5524, 98p. Devaney, J.R. and Fralick, P.W. 1985. Regional sedimentology of the Namewaminikan Group, northern Ontario: Archean fluvial fans, braided rivers, deltas and an aquabasin; in Current Research, Part B, Geological Survey of Canada, Paper 85-1B, p.125-132. Devaney, J.R. and Williams, H.R., 1989. Evolution of an Archean subprovince boundary: A sedimentological and structural study of part of the Wabigoon-Quetico boundary in northern Ontario; Canadian Journal of Earth Sciences, v.26, p.1013-1026. DeWolfe, J.C., Lafrance, B. and Stott, G.M. 2000. Structurally controlled mesothermal gold mineralization in the western Beardmore-Geraldton belt; in Summary of Field Work and Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p. 17-1 to 17-6. Dickinson, W.R. and Seely, D.R., 1979. Structure and stratigraphy of forearc regions; American Association of Petroleum Geology Bulletin, v.63, p.2-31. Dubé, B. and Guha, J. 1993. Factors controlling the occurrence of ferro-axinite within Archean goldcopper-rich quartz veins: Cooke mine, Chibougamau area, Abitibi greenstone belt; The Canadian Mineralogist, v.31, p.905-916. Durocher, M.E., and Hugon, H., 1983. Structural Geology and hydro-thermal alteration in the Flat Lake - Howey Bay deformation zone, Red Lake area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.216-219. Ferguson, S.A. 1967a. Leitch Gold Mines Limited: Surface plan of eastern part of property, parts of Eva and Summers townships, District of Thunder Bay; Ontario Department of Mines, Preliminary Map P.484, scale 1:6000. Ferguson, S.A. 1967b. Leitch Gold Mines Limited: Subsurface No.1, 900-foot level / Subsurface No.2, 4525-foot level, parts of Eva and Summers townships, District of Thunder Bay; Ontario Department of Mines, Preliminary Map P.485, scale 1:6000. Corfu. F., 2000. Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon; Lithos, v.53, issue 3-4, p.279-291. Ferguson, S.A., Groen, H.A., and Haynes, R., 1971. Gold Deposits of Ontario, Part I, Districts of Algoma, Cochrane, Kenora, Rainy River, and Thunder Bay; Ontario Ministry of Natural Resources, Mineral Resources Circular, no.13, 315 p. Cowan, P. and Crocket, J.H., 1980. The gold content of interflow metasedimentary rocks from the Red Lake area, Ontario-A preliminary evaluation; Geological Survey of Canada, Paper 80-1B, p.189-133. Fralick, P.W., 2003. Geochemistry of clastic sedimentary rocks: ratio techniques; in Geochemistry of Sediments and Sedimentary Rocks, Geological Association of Canada, Geotext 4. Devaney, J.R., 1987. Sedimentology and stratigraphy of the northern and central metasedimentary belts in Fralick, P. and Barrett, T.J., 1991. Precambrian depositional systems along the southwestern edge of the - 37 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Superior Craton; Geological Association of Canada - Mineralogical Association of Canada - Society of Economic Geologists, Joint Annual Meeting, Toronto ‘91. Field Trip A3: Guidebook, 54 pp. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada; in Facies Analysis, Special Publication 22, International Association of Sedimentologists, p.137-156. Fralick, P.W. and Kronberg, B.I., 1997. Geochemical discrimination of clastic sedimentary rock sources; Sedimentary Geology, v.113, p.111-124. Fralick, P.W., Wu, J. and Williams, H.R., 1993. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield; Canadian Journal of Earth Sciences, v.29, p.2551-2557. Fryer, B.J., Kerrich, R., Hutchinson, R.W., Pierce, M.G., and Rodgers, D.S., 1979. Archean precious metal hydrothermal systems, Dome Mine, Abitibi Greenstone belt. I: Patterns of alteration and metal distribution. Canadian Journal of Earth Sciences, v.16, p.421-439. Gray, R.S. 1993. Report of work, Asarco Exploration Company of Canada Limited, Geraldton Project; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 5p. Gray, R.S. 1994. Geraldton Project, 1994 Exploration program, Asarco Exploration Company of Canada Limited; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 6p. Hart, T.R., terMeer, M. and Jolette, C. 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra townships, northwestern Ontario: Phoenix bedrock mapping project; Ontario Geological Survey, Open File Report 6095, 206p. Geraldton area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.201-203. Kerrich, R., 1981. Archean gold-bearing chemical sedimentary rocks and veins: a synthesis of stable isotope and geochemical relations; in Genesis of Archean, Volcanic-Hosted Gold Deposits; Ontario Geological Survey, Miscellaneous Paper 97, p.144167. Kerrich, R., and Allison, I., 1978. Vein geometry and hydrodynamics during Yellowknife mineralization; Canadian Journal of Earth Sciences, v.15, p.16531660. Kerrich, R., and Hodder, R.W., 1982. Archean lode gold and base metal deposits: evidence for metal separation into independent hydro-thermal systems; in Geology of Canadian Gold Deposits, Canadian Institute of Mining and Metallurgy, Special Volume 24, p.144160. Krauskoph, K.B., 1967. Introduction to Geochemistry. McGraw-Hill, New York, 721 p. Kresz, D. and Zayachivsky, B. 1989. Geology of Barbara, Meader and Pifher townships, District of Thunder Bay; Ontario Geological Survey, Report 270, 91p. Lafrance, B., DeWolfe, J.C. and Stott, G.M. 2004. A structural reappraisal of the Beardmore-Geraldton belt at the southern boundary of the Wabigoon Subprovince, Ontario, and implications for gold mineralization; Canadian Journal of Earth Sciences, v.41, p.217235. Laird, H.C. 1937. The western part of the Sturgeon River area (Sturgeon River-Beardmore section); Ontario Department of Mines, Forty-fifth annual report, v. XLV, pt.II, p.60-117. Hodgson, C.J., 1983. The structure and geological development of the Porcupine camp - a re-evalutation; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.211-225. Lavigne, M.J. (in press). Structural geology and gold mineralization in the Geraldton area, BeardmoreGeraldton greenstone belt; Ontario Geological Survey, Open File Report. Hodgson, C.J., and MacGeehan, P.J., 1982. Geological characteristics of gold deposits in the Superior Province of the Canadian Shield; in Geology of Canadian Gold Deposits, Canadian Institute of Mining and Metallurgy, Special Volume 24, p.211229. Lavigne, M.J. 1983. Gold Deposits of the Geraldton area; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.198-200. Horvath, A.S. 1993. Ore reserve estimate, Geraldton Project, 1993, Asarco Exploration Company of Canada Limited; unpublished OMIP report, Assessment Files, Thunder Bay North District, Thunder Bay, 7p. Horwood, H.C., and Pye, E.G., 1955. Geology of Ashmore Township, Little Long Lac area, Thunder Bay District; Ontario Department of Mines, Annual Report, v.60, pt.5, 105p. Kehlenbeck, M.M., 1983. Structural studies in the Beardmore- Lesher, C.M., Goodwin, A.M., Campbell, I.H. and Gorton, M.P. 1986. Trace-element geochemistry of oreassociated and barren, felsic metavolcanic rocks in the Superior Province, Canada; Canadian Journal of Earth Sciences, v.23, p.222-237. Macdonald, A.J., 1982. The MacLeod-Cockshutt and Hard Rock mines, Geraldton: examples of an iron formation-related gold deposit; in Summary of Field Work 1982, Ontario Geological Survey, Miscellaneous Paper 106, p.188-191. Macdonald, A.J., 1983a. The iron formation-gold association evidence from Geraldton area; in The Geology - 38 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.75-83. Australia; Geological Society of Australia Journal, v.30, p.25-39. Macdonald, A.J., 1983b. A re-appraisal of the Geraldton gold camp; in Summary of Field Work 1983, Ontario Geological Survey, Miscellaneous Paper 116, p.194197. Pirie, J. and Mackasey, W.O. 1978: Preliminary examination of regional metamorphism in parts of Quetico metasedimentary belt, Superior Province, Ontario; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p. 37-48. Macdonald, A.J. 1984. Gold mineralization in Ontario, I: The role of banded iron formation; Canadian Institute of Mining and Metallurgy, Special Volume 34, p.412430. MacDonald, R.D. 1940. Geology of the Kenogamisis River area; Ontario Department of Mines, Annual Report 1940, v.49, pt.7, p.1-17. Mackasey, W.O. 1975. Geology of Dorothea, Sandra and Irwin Townships, District of Thunder Bay; Ontario Division of Mines, Geological Report 122, 83p. Mackasey, W.O. 1976. Geology of Walters and Leduc Townships, District of Thunder Bay. Ontario Division of Mines, Geological Report 149. 58p. Mackasey, W.O., Edwards, G.R. and Cape, D.F. 1976. Legault Township, District of Thunder Bay; Ontario Division of Mines, Preliminary Map P.1191, scale 1:15 840. Mason, J.K. and McConnell, C.D. 1983. Gold mineralization in the Beardmore-Geraldton area; in The geology of gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.84-97. Mason, J.K. and White, G.D. 1986. Gold occurrences, prospects and deposits of the Beardmore-Geraldton area; Ontario Geological Survey, Open File Report 5630, 680p. Mason, J.K. and White, G.D. 1997. Beardmore-Geraldton Resident Geologist’s District 1996; Ontario Geological Survey, Open File Report 5958, p.1-1 to 1-22. Mason, J.K., White, G.D. and McConnell, C. 1985. Field guide to the Beardmore-Geraldton metasedimentary – metavolcanic belt; Ontario Geological Survey, Open File Report 5538, 73p Neumayr, P., Walshe, J., Connors, K., Cox, S., Morrison, R. S., and Stolz, E. 2004. Gold mineralization in the St Ives camp near Kambalda; Geological Survey of Western Australia, Record 2004/16, p.23-45. O’Brien, M.S. 1985. A detailed geological study of the mafic metavolcanic rocks north of Poplar Lodge, Ontario; unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 76p. Poulsen, K.H., 1983. Structural setting of vein-type gold mineralization in the Mine Centre-Fort Frances Area: implications for the Wabigoon Subprovince; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.174-180. Purdon, R.H. 1995. Lithostratigraphy and provenance of the Neoarchean McKellar Harbour sequence, Superior Province, Ontario, Canada; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 172p. Pye, E.G. 1952. Geology of Errington Township, Little Long Lac area; Ontario Department of Mines, Annual Report, v.60, pt.6, 140 p. Reilly, B.A. 1988. Structural analysis of the Paint Lake Deformation Zone, northern Ontario. M.Sc. thesis, Brock University, St. Catharines, Ontario. Rigg, D.M., and Helmstaedt, H., 1981. Relationships between structure and gold mineralization in Campbell Red Lake and Dickenson Mines, Red Lake Area, Ontario; in Genesis of Archean, Volcanic Hosted Gold Deposits, Ontario Geological Survey, Miscellaneous Paper 97, p.111-127. Rogers J. R., Joyce, K. A., Masterman, G. J., Halley, S. W., and Walshe, J. L. 2004. Kanowna Belle gold mine; Geological Survey of Western Australia, Record 2004/16, p.47-51. Scott, B.M., 1985. A study of the auriferous quartz veins found within the oxide facies banded iron formations of the Beardmore-Geraldton area of northern Ontario; Unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 96 p. Shanks, W.S. 1993. Geology of Eva and Summer Townships, District of Thunder Bay. Ontario Geological Survey, Open File Report 5821. Sibson, R.H., Moore, J.M., and Rankin, A.H., 1975. Seismic pumping - a hydrothermal fluid transport mechanism; Journal of the Geological Society of London, v.131, p.191-213. Ozaki, M. 1972. Chemical composition and occurrence of axinite; Kumamoto Journal of Scientific Geology, v.9, p.1-34. Stott, G.M., and Davis, D. 1999. Contributions to the tectonostratigraphic analysis of the Onaman–Tashota greenstone belt, eastern Wabigoon Subprovince; in 1999 Western Superior Transect 5th Annual Workshop. Report 70, Lithoprobe Secretariat, The University of British Columbia, Vancouver, B.C., p.122–123. Phillips, G.N., and Groves, D.I., 1983. The nature of Archean gold fluids as deduced from gold deposits in Western Stott, G.M., and Schneiders, B.R., 1983. Gold mineralization in the Shebandowan Belt and in relation to regional - 39 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 deformation patterns; in The Geology of Gold in Ontario, Ontario Geological Survey, Miscellaneous Paper 110, p.181-193. Tomlinson, K.Y. 1996. The geochemistry and tectonic setting of Early Precambrian greenstone belts, northern Ontario, Canada; unpublished Ph.D. thesis, University of Portsmouth, Portsmouth, United Kingdom, 287p. Tomlinson, K.Y., Hall, R.P, Hughes, D.J. and Thurston, P.C. 1993. Mafic metavolcanic rocks of the BeardmoreGeraldton greenstone belt: Preliminary geochemical findings; in Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 162, p.54-58. Williams, H.R., 1986. Structural studies in the BeardmoreGeraldton belt, northern Ontario; in Summary of Field Work 1986, Ontario Geological Survey, Miscellaneous Paper 130, p.138-146. Williams, H.R., 1987. Structural studies in the BeardmoreGeraldton belt and in the Quetico and Wawa subprovinces; in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p.90-92. Williams, H.R. and Stott, G.M., 1991. Subprovince accretion in the southern Superior Province. G.A.C.-M.A.C. annual Meeting, Toronto, Field trip B6, Guidebook, 26 p. Wood, D.A. 1980. The application of a Th–Hf–Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province; Earth and Planetary Science Letters, v.50, p.11-30. - 40 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 2 – Quaternary geology of the Beardmore – Nipigon area Peter J. Barnett Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction then returns to Nipigon along the same route (Fig. 1). In association with the International Quaternary Association in 1987 (INQUA’87) an excursion quidebook C-12: Quaternary features and scenery along the North Shore of Lake Superior was created (Geddes et al., 1987). Unfortunately the trip was never run. This current field trip and field trip guide utilizes much of the information presented in the INQUA’87 field trip guide particularly as it pertains to the Beardmore – Nipigon area. The route follows the east branch of the Nipigon River, initially across the Nipigon lowlands then along the Pijitawabik Canyon cut through Nipigon Sills and onto more-typical Canadian Shield terrain north of Beardmore. Although this part of Ontario is pivotal to many large-scale glacial histories and theories of the Great Lakes Region, North America and global climate change, very little if any detailed mapping of the Quaternary geology has been done. One exception is the work of Thorleifson and Kristjansson (1993) in the very northern part of the field trip area (scale 1:100,000). In addition, Mollard and Mollard (1981) undertook a terrain analysis (scale 1:100,000) of a large part of the field trip area and produced a map of some of the major landforms and material types that occur there. The field trip area has been displayed on several regional maps including Zoltai (1965a), Sado and Carswell (1987), Barnett et al. (1991) and Sado et al. (1994). In addition, regional studies on selected aspects of interest to Quaternary researchers, particularly associated with ancestral lake features and deposits (e.g., Farrand, 1960; Teller and Thorleifson, 1983, 1987; Teller and Mahnic, 1987) have been done. Zoltai (1965b) described and discussed the formation of some of the major landforms and deposits of the Nipigon area. Lake Nipigon During the trip we will visit several sites and landforms that display clues to the glacial history of the area and hopefully discussions on how they relate, support or conflict, with several of the regional histories proposed in the literature will ensue. The fieldtrip route basically follows Highway 11 north from Nipigon to Beardmore and then northwest on Highway 580 to Poplar Lodge on Lake Nipigon and Figure 1. Location, route and stops. Rectangle is for reference between the various regional figures. - 41 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Regional geological setting During the fieldtrip we will be travelling between Lake Superior and Lake Nipigon following a now abandoned drainage way used during deglaciation between these two lakes. The present drainage divide is located between McKirdy and Shamrock Lake within the Pijitawabik Canyon. Keemle Creek via a series of small lakes drains northward into Pijitawabik Bay of Lake Nipigon (approximately 260 m or 852 ft asl). An unnamed east branch of Cash Creek drains southward from McKirdy Lake into Cash Creek and onto Helen Lake and then into the Nipigon River to Lake Superior (approximately 183 m or 600 ft asl). The main branch of the Nipigon River is the present-day outlet of Lake Nipigon. The fieldtrip area, located on the Canadian Shield, is an irregular, subdued, gently rolling, bedrockdominated terrain underlain by Precambrian igneous and metamorphic rocks (Figs. 2, 3). The area immediately south of Lake Nipigon, however, is not typical shield terrain. It has been referred to as the Nipigon Plain by Bostock (1970) and “is formed on nearly flat-lying Proterozoic gabbro sills and sediments surrounding Lake Nipigon” (Bostock, 1970, p.16). Combined past tectonic activity, differential weathering and glacial/ glacial meltwater erosion of the Proterozoic sills and Sibley Group metasedimentary rock surfaces has produced a unique landscape of buttes and mesas and steep-walled valleys along the margins of the Nipigon Plain. To the east of Lake Nipigon, beyond the extent of the sills, in the area underlain by Archean igneous Nipigon diabase sills Sibley Op. j1 nepheline syenite suite III1 granodiorite to granite 2' granitic rock Metasedinientary rocks Felsic metavolcanic rocks Mafic metavolcanic rocks Figure 2. Fused digital elevation model (DEM) and hillshaded DEM. Rectangle is for reference between the various regional figures. Figure 3. Bedrock geology of the fieldtrip area (OGS 2003). - 42 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 and metamorphic rocks, more typical Shield terrain is prevalent. Farrand (1960) has referred to the southern part of the fieldtrip area as the Nipigon Lowlands. Here, the irregular surface of the Canadian Shield has been subdued and partially infilled by sand, silt and clay of glaciolacustrine origin producing a broad dissected plain. Farrand (1960) recognized at least 4 terraces or steps within this lowland at various elevations (692, 800, 907-912, 950-952 and 980 ft or about 211, 244, 276-278, 290, 299 m) that he related to ancestral lake levels in the Lake Superior basin. Relief over the entire region exceeds 350 m (about 1150 ft). Local relief in areas of Archean rocks is commonly between 15 and 50 m (50 and 165 ft). Along the eroded edges of sills that surround the Nipigon Plain it can exceed 100 m (about 328 ft). Local relief of over 100 m (about 328 ft) also exists along the walls of the Pijitawabik Canyon. Bedrock geology The study area contains rocks of the Superior Craton and the Southern Province (Fig. 3). Major geological subdivisions include the Wabigoon and Quetico subprovinces (Blackburn et al., 1991; Williams, 1991), the Proterozoic Sibley Group and Nipigon Diabase Sills (Sutcliffe, 1991). Metasedimentary and granitic rocks dominate the rocks of the Quetico Subprovince in the southern part of the fieldtrip area. Mafic metavolcanic rocks dominate the Wabigoon Subprovince with granodiorite to granite intrusions forming the inter-greenstone belt parts. Diabase/gabbo in the form of sills and dikes is the dominant rock type in the Nipigon embayment or the north-central part of the fieldtrip area (Fig. 3). Nipigon sills are massive bodies that vary in thickness from 150 to 200 m. Internal textural zoning indicates that the sills are single cooling bodies (Blackburn et al., 1991). The Nipigon sills in the area are very similar texturally and monotonous in appearance. They overlie and interfinger with Sibley Group sedimentary rocks that outcrop or subcrop within the southern part of the Nipigon embayment. The Nipigon sills formed contemporaneous with the initiation of the Lake Superior Midcontinent rift age-dated at approximately 1108 Ma (Blackburn et al., 1991). With the possible exceptions of some lateral and vertical outliers the age of the Sibley is constrained between the youngest detrital zircon in a sample from the Kama Hill Formation (1420 Ma; L. Heaman pers. comm. 2004) and a Rb-Sr isochron from authogenic minerals in the Rossport and Kama Hill Formations (1339 Ma; Franklin, 1978). The Sibley consists of unmetamorphosed red-bed sequences consisting of conglomerates, sandstones and shales. They are believed to have accumulated in a subsiding graben prior to the formation of the midcontinent rift (Sutcliffe, 1991). Greenstone belts east of Lake Nipigon are part of the Eastern Wabigoon Subprovince and have been traced beneath the diabase and sedimentary units (Blackburn, 1991; Smyk et al., 2005). Quaternary geology The surficial (Quaternary) geology of the field trip area south and east of Lake Nipigon has been investigated only at a regional scale despite its importance to the overall understanding of the glacial history of Ontario and in particular to mineral exploration using unconsolidated sediments (drift exploration). Regional maps at various scales that cover the area include Zoltai (1965a), Sado and Carswell (1987), Barnett et al. (1991) and Sado et al. (1994). Zoltai (1965b) described and discussed the formation of some of the major landforms and deposits of the QueticoNipigon area. Mollard and Mollard (1981) undertook a terrain analysis of the study area and produced maps of some of the major landforms and material types that occur there (Fig. 4). Several theses and papers have been written on significant aspects or features of Quaternary age from within the area of interest (Farrand, 1960; Teller and Thorleifson, 1983, 1987; Teller and Mahnic, 1987). Zoltai (1965a, 1965b), Elson (1967), Prest (1970), Dredge and Cowan (1989) Barnett (1991) and Sado et al. (1994) have summarized regional concepts of the glacial history surrounding Lake Nipigon. There is very little agreement as to the details of the sequence and timing of events. The deposits in the area probably represent ice movement during the Michigan Subepisode (Late Wisconsinan), however; it may reflect ice cover during the entire Wisconsin Episode or approximately the last 100,000 years. The Laurentide Ice Sheet flowed southwestward during its maximum extent about 20,000 years - 43 - �r re-advance to produce most of the major moraines in northwestern Ontario. The Marks, Mackenzie and Dog Lake moraines have been suggested to have formed at about the same time, about 10,000 years ago (Burwasser, 1977; Drexler et al., 1983) and are probably correlative to the Grand Marais Moraines that formed during the Marquette advance (Drexler et al., 1983). With further ice margin recession and a suggested major re-advance, the Lac Seul moraine formed west of the fieldtrip area (Prest 1970). Zoltai (1965b) suggested that this ice margin formed the northern part of the Kaiashk Interlobate Moraine (Fig. 5). — — a 4. ,m Proceedings of the 51st ILSG Annual Meeting - Part 2 Prest (1970) suggested that with further ice margin retreat, the Sioux Lookout and Whitewater moraines formed west of Lake Nipigon. These events were followed by a westward advance in the Lake Nipigon basin that produced the Nipigon Moraine (Stop 1) along the west and southern side of the lake (Zoltai, 1965a, b). Prest (1970) suggests that only the northern part of this moraine formed following the Whitewater Moraine and that the southern part is older. Zoltai (1965a, b) identified several other moraines east of the Nipigon Moraine in the Lake Nipigon basin that appear to be younger than the Nipigon Moraine. The Nakina Moraines that are located along the northeastern edge of the Lake Nipigon basin mark the last significant ice margin positions to directly affect the region. To complicate the history even more is the suggestion that glacial Lake Agassiz waters drained catastrophically into the Nipigon/Superior basin several Figure 4. Surficial sediments in the fieldtrip area (modified from Mollard and Mollard, 1981). ago. During deglaciation and possibly during initial ice movement into the area, ice flow direction was controlled on a regional scale by topography. During these times ice flow followed broad topographic lows, especially the Great Lakes basins and most likely Lake Nipigon, producing lobate ice margins that could have possibly behaved somewhat independently (Fig. 5). Prest (1970), following the work of Zoltai (1965b), suggested that an active ice margin in northwestern Ontario fronted glacial Lake Agassiz during ice margin retreat. They (Zoltai, 1965b; Prest, 1970) postulated several alternating stages of ice margin retreat and NbItt 2. . It k BI Ck M@! 3. IC k Mk d Lk gk!& ph d 3.4.7.8 k Kbk LIttbfl 9. k Figure 5. Ice flow direction and ice margins in the Lake Nipigon area (Sado et al., 1994). - 44 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 _________ ________ ______________ _____________ __________ AC4SSIZ EASTERN LAKE SUPERIOR OUTLETS PHASES LAKE MICHIGAN ID LAKE HURON - EARLY NIPISSING (rising): SAULT STRAIT: EARLY NIPISSING 7000 4 SAULT RAPIDS HOUGHTON — (bedrock) CHIPPEWA- STANLEY 8000 8000 I ION PROTO- (toIling) P11<1.— ST MARYS RIVER riS,rI --b (downcutting through drift) KAI. — rising MINONG POST )..::Y.cM!l!f.::: 0 8 I AU TRAIN - • ESCANADA GRAND MARAIS 10,000 PROTOST MARYS -—b RIVER MLNONQI 10,000 CIIIPPEWA / EARLY — — UPPER GROUP STANLEY SHEGUIANDAH SKORAM PAYETTE through 2 ALGONQUIN 1!000 '' 9000 PoRTAGE1jJ::RCuPINE 11,000 Tt?'"-.,, fl750 Figure 6. Chart of ice-sheet fluctuations and glacial lake phases (from Farrand and Drexler, 1985). times during the deglaciation of the Lake Nipigon and Superior basins. Farrand and Drexler (1985) produced a chart summarizing some of the ice sheet fluctuations and their relation to glacial lake phases in northwestern Ontario and the upper Great Lakes basins (Fig. 6). Agassiz drainage into the Nipigon and Superior basins occurred both before and after the Marquette maximum (Farrand and Drexler, 1985). A network of 5 groups of channels (Teller and Thorleifson, 1983, 1987) carried drainage from glacial Lake Agassiz following the Marquette advance (Fig. 7). And “as postulated by Teller and Thorleifson (1983), the outflow through the Nipigon channels must have been catastrophic” (Farrand and Drexler, 1985, p.23). These “catastrophic discharges from Lake Agassiz were responsible for cutting down the Lake Minong sill” (Farrand and Drexler, 1985, p.17). The Nipigon Moraine marks a former ice margin located along the north shore of Lake Superior and has been traced along the southern and western side of Lake Nipigon (Fig. 5). It is commonly associated with ancestral glacial Lake Minong and as such believed Figure 7. Eastern outlets of Lake Agassiz. Outlet number 26 is the Pijitawabik Canyon (from Teller and Thorleifson, 1983). - 45 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 to have formed about 9500 years ago (Farrand and Drexler, 1985). Farrand’s regional work on abandoned shoreline features along the north shore of Lake Superior (Farrand, 1960) is the most comprehensive field study on shorelines to date in the area (Figure 8). Farrand (1960) identified evidence of glacial Lake Minong about 25 km north of the Nipigon Moraine (Stop 3). And recently Slattery (2003) identified deltas that formed along the Nakina Moraines built to Minong III (?) or post-Minong levels at elevations of 360 m in the Longlac area. If catastrophic Agassiz drainage carved the outlet at St Marys River, why do shore features of Minong or fairly high Post Minong levels exist well behind the Nipigon Moraine, a feature that in part post-dates the opening or use of many of the earlier eastern outlets? The proposed eastern outlet events and features were based on several coarse-grained deposits associated with the large deep steep-walled valleys that occur throughout the region (Teller and Thorleifson, 1983, 1987). The Pijitawabik Canyon (Stop 3, Outlet number 26, Fig. 7) was proposed as one of the passageways for this meltwater (Teller and Thorleifson, 1983, 1987) and the Pijitawabik delta (Stop 2) was inferred to have formed during Agassiz drainage events (Geddes et al., 1987). An alternative explanation for the development of most, if not all, the steep-walled valleys (e.g., Pijitawabik Canyon) is that they were initially formed in part by differential erosion along fractures and faults within the diabase/gabbro sills prior to glaciation. The valleys were then subsequently carved by subglacial meltwater floods based on the similarities of this channelled landscape to that of the Channelled Scablands of eastern Washington State (Baker et al., 1987). Many of these valleys contain glacial sediments indicating that the bedrock channels themselves were not carved by postglacial Lake Agassiz drainage events, but already existed. Agassiz outflow may have reoccupied some of these steep-walled channels that were carved by the subglacial meltwater flood(s). The complex and still poorly understood deglacial history of the Nipigon basin does not make drift exploration easy. The events and features discussed above could have resulted in many areas having multiple directions of ice movement due to the shifting of ice centers through time that will require linking evidence of ice movement to specific tills (Fig. 4). Determining the history of meltwater and ancestral lakes in the study area is also very important to interpret the results of drift exploration programs. Studies of tills deposited in the BeardmoreGeraldton area (Thorleifson and Kristjansson, 1993) and in the Hemlo-Matheson-White River area (Geddes and Kristjansson, 1986; Karrow and Geddes, 1987; Hicock, 1988) have brought to light another factor that affects mineral exploration using till (drift exploration) in the fieldtrip area. The till in the Beardmore area appears for the most part to have been deposited subglacially by an actively flowing glacier (ideal for mineral exploration using till). However, where the till is thick it is commonly rich in clasts of Paleozoic carbonate and Proterozoic metasedimentary rocks derived from Hudson Bay Lowlands some 100 km or greater away (Stop 6). The content of carbonate in the till matrix is also high (up to 50%; Thorleifson and Kristjansson, 1993). This “exotic” till tends to grade into locally derived till (Stop 4) at surface in the area surrounding the community of Beardmore. Thorleifson and Kristjansson (1993) attribute the abundance of far-traveled debris to: 1) the high susceptibility of carbonate rocks to erosion 2) low erodibility of granitic rock which outcrop between the Beardmore area and the Hudson Bay Lowlands 3) the short distance of transport over the erodible greenstone belt rocks and 4) a zone of vigorous ice flow (ice streaming). Thorleifson and Kristjansson (1993) suggested that an ice stream would be required to produce and distribute the carbonate-rich or exotic till to the Beardmore-Geraldton area based in part on the regional distribution in a radiating tongue parallel to ice flow. They also suggest that the exotic till was transported beneath the glacier by pervasive shear (deformable bed). They argued that “deposition by lodgement of basal debris seem contradicted by carbonate clast preservation” (Thorleifson and Kristjansson, 1993, p.57). Hicock (1988), however, records evidence within exotic till exposures of discrete not pervasive shear that would suggest lodgement as the process of deposition, although, he also suggested ice streaming to produce the exotic till. - 46 - Geddes and Kristjansson (1986) suggested that �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stops englacial transport of the carbonate debris over long distances was required. Hicock (1986) suggested that englacial load under dynamic ice conditions can be transferred to the glacier bed in the leeside of upland settings. Stop 1 – Nipigon Moraine (J. Nichols Sand and Gravel, Nipigon) UTM coordinates – 0406748E 5430513N Regardless of the mechanism of entrainment, transport and deposition of the carbonate-rich till, it has been suggested that the “standard rules of glacial comminution as applied in mineral exploration may not be applicable” (Karrow and Geddes, 1987, p.368). And that care be taken to recognize and avoid this exotic till when sampling for mineral exploration purposes (Thorleifson and Kristjansson, 1993). Note - permission from the land owner is required for access to this stop. The Nipigon Moraine marks a former ice margin of the Laurentide Ice Sheet that fronted a large glacial lake. It is generally accepted that this lake was glacial Lake Minong and that its formation occurred about 9500 yr BP (Farrand and Drexler, 1985). In the Nipigon area it is a discontinuous ridge that rises more than 40 m above the surrounding lake plains. Elevation at the site is approximately 280 m asl. The ridge rises to the west where it becomes flat-topped at an elevation of about 295 - 296 m asl. The form here does not appear to be primary (i.e. delta) but the result of subsequent erosion and deposition (wave cut and wave built platform). A lower plain occurs to the north of the moraine here at about 290 m asl. These features raise the question of what was the actual water level in the Superior basin during the formation of the Nipigon Moraine. The field trip stops were chosen to give the participants an overview of the Quaternary geology of the area between Lake Superior and Lake Nipigon and to introduce them to some remarkable landforms and scenery that exists in this part of Ontario. For the INQUA’87 fieldtrip guide, Geddes et al. (1987) prepared a figure showing the profiles of water planes and shoreline features recognized by Farrand (1960) in the Lake Superior basin between Thunder Bay and Kama Bay. This figure appears as Figure 9 with a few modifications for reference purposes. An ice marginal feature built along the Nipigon NE SW KILOMETRES NORTH OE AGASSIZ OUTLET -J 200 taJ —300 .J C Iii -250 In 0, —200 I-ISO 400 — L.a 0 a Figure 8. Shoreline-relation diagrams and isobases of glacioisostatic rebound (modified 8. Shoreline-relation and general isobases of from Figure Farrand and Drexler, diagrams 1985). A) glacioisostatic rebound (modified from Farrand and diagram emphasizing glacial retreat (Farrand, Drexler 1985) A) general diagram emphasizing retreat 1969). B) Diagram showingglacial relationships (Farrand 1969) between Lakes Superior and Nipigon (Teller B) diagram showing lakes Superior-Nipigon and relationships Thorleifson, 1983). C) Isobases of (Teller and Thorleifson 1983) C) isobases of rebound glacioisostatic for Washburn glacioisostatic forrebound Washburn and and Minong (Fanand 1960). Minong (Farand, 1960) - 47 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 PROFILES OF FORMER WATER PLANES BETWEEN THUNDER BAY AND NIPIGON (after Parrand 1960) io o I m S 80 60 40 20 0 e I I 50 40 30 20 I I It I I a Ifl •er t 0 S . r —1 S — — — —— 5— - 0 'r-• — =i_— I * • * 0 -. -c r0 I II — • r •—• I S I I 0 •V 0 r m r r 5• 5— • C t 5• - ———... —S •lI* i 00 C I 0 -I In In , S jI 0r I 0 •j_ UGHT0N.- •p_• — 'NIPISSIN0 ALGOMA S .—4.-—--.———— I 3 •___.____1_._. ——4- — . —.—-— •—• • S ION i—-— 55 5 • —e — —;__• S 5•— a, — • -T--I I r m fli I z 0 0 — * S C S In 0 z Figure 9. Profiles of former water planes in the Lake Superior basin from Thunder Bay and Kama Bay (after Farrand, 1960; Geddes et al., 1987). Figure 9. Profiles of former water planes in the Lake Superior basin from Thuder Hay and Kama Bay (after Farrand 1960, Geddes et al. 1987) Moraine south east of Black Sturgeon Lake that has a delta form occurs at an elevation of between 300 and 304 m. This feature would fall near to the isobase passing through Nipigon, essentially making the diagram one isobar off (Fig. 8C). Shore features identified as ice-marginal deltas at Marathon and Wawa, that were reported to be built into glacial Lake Minong (Geddes et al., 1987), may not be deltas at all, but subaqueous fans later planed off by waves of a subsequent lake. This is the case at Wawa (S. Slattery, personal communications, 2003). The sedimentology of the feature at Marathon remains to be studied in detail before such a conclusion can also be made but preliminary observations are not consistent with a deltaic origin. The Jim Nichols Trucking Limited sand and gravel pit provides a look at the internal structures of the sediments making up the Nipigon Moraine. In general he ridge here is made up of a fining upward and outward sequence of stratified sand and gravel. The lower part or core is composed of steeply dipping beds of gravel and sand (Fig. 10). These avalanche or foreset beds probably formed during rapid growth by sediment gravity flow processes. The beds generally dip to the south or across the ridge form. Trough cross-bedded (dunes) sand, pebbly sand and pebble gravel overlie the steeply dipping beds. The aforementioned sediment sequence is common in subaqueous fans (Shaw, 1985). Near the top and along the ice-proximal side (northside) minor flowtill lenses are present. Faults and postdepositional slumping are common along the proximal side of the ridge likely formed following the removal of the glacier support and sediment supply. The sequence is interpreted as an ice marginal subaqueous fan. If so, it only places a minimum (280 m asl) for the lake that existed during its formation. As mentioned previously, the water here during formation could be up to 20 m deep. A shorebluff and bouldery gravel beach bar of a younger lake occurs along the roadway at the entrance of the pit at approximately 250 m asl (Figs. 10 and 11). * Alternative stop - look-out over community of Nipigon from the Nipigon Moraine (UTM coordinates – 0407343E 5430274N). - 48 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 w Profile 260 broad shallow channel /sulting shoreline a £ C' = 240 0 a fining 220 •I.. N 4flntng 260 decrease dip of beds 200 U a i6oo 500 0 C a isào Distance (ni) D N —4 — - H -- im -- -I--'_- B Figure 10. Internal structures of the Nipigon Moraine, Jim Nichols Trucking Ltd sand and gravel pit: A – longitudinal view, B – steeply dipping avalanche bedding, C – faulting and slumping along northern slope of the ridge and D – schematic crosssection of the Nipigon Moraine at this site. Stop 2 – Pijitawabik delta UTM coordinates – 041382E 545403N The route from Stop 1 to Stop 2 crosses the Nipigon lowlands along the east side of Helen Lake. Glaciolacustrine fine-grained rhythmites occur beneath the surface of the plain until just north of Mignet Creek. At the break in slope about 400 m north of the creek, Farrand identified a Dorion shore feature at 800 ft asl or about 244 m (Figs. 11 and 12). Another prominent bluff, a Post-Minong shoreline, occurs at an elevation of between 907 and 912 ft (276-278 m) asl (Farrand, 1960). Farrand (1960, p.194) suggested that the “very extensive, near level surface of sand and some rather heavy gravel” about 25 km north of Nipigon is also a — Like Profile: Lake Superior to Pijitawabik Bay /elth? ]UL}\ B shore bluff Nipigon Moraine shore bluff?—) Pijitawabik Ray V/ H Helen Lake IOD I 0 Approximate utbem limit of the Pthtawabik Canyon ::::: I Distate (m) frail LAe Stwia Figure 11. Profile from Lake Superior to Pijitawabik Bay of Lake Nipigon following the Pijitawabik Canyon. Stop 1 is on the crest of the Nipigon Moraine, Stop 2 at the Pijitawabik delta and stop 3 within the Pijitawabik Canyon. Several features controlled by ancestral water levels in the Superior basin can be seen. - 49 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 12. A fused DEM and hillshade of the Pijitawabik delta and the southern part of the Pijitawabik Canyon. product of glacial Lake Minong (Figs. 11 and 12). This feature has an elevation of 950-952 ft or about 290 m asl. At 980 ft or 299 m asl, Farrand (1960, p.194) identified another possible Minong feature within the Pijitawabik Canyon. Both these features are well inside or north of the Nipigon Moraine. The following quote from Farrand (1960) sums up some of his conclusions as to the origin and relationships that these deposits have with lakes in the Superior and Nipigon basins. “The 980-foot gravel accumulation lies nearly on the isobase of Terrace Bay where the Minong beaches lie at 950 to 1000 feet. Moreover, the abundance of sediment in the Pijitawabik Bay canyon implies the presence of the ice sheet either in Pijitawabik Bay itself or at least in the Lake Nipigon basin”. “These facts lead to the conclusion that the Nipigon basin was ice-filled (or nearly so) in Minong time. As lake levels fell in the Superior basin and the ice sheet retreated from the Lake Nipigon basin these basins apparently contained separate water bodies connected only by the ancestral courses of the Nipigon and Black Sturgeon rivers. In other words, there appears to be no validity in the assumption generally made (Stanley, 1932, p.135; Leverett, 1927, p. l.7) that the Nipigon and Superior basins were occupied at any time by one continuous lake” (Farrand, 1960, p.82). be sound, however convincing evidence that Stanley’s or Leverett’s assumptions had “no validity” was not substantiated. Down-cutting of the delta to the 290 m level may have been caused by a drop in base level as water in the Superior basin lowered in response to erosion at the outlet sill at Sault Ste Marie. The delta may have been fed from the glacier margin that remained within the Pijitawabik Canyon farther up the valley. Alternatively, during basin (Nipigon/Superior) separation, as an outlet stream from the lake Nipigon basin, or possibly built by an Agassiz drainage event through the valley. Stop 3 – Pijitawabik Canyon UTM coordinates – 0420816E 5462100N This stop may be a series of stops along the route The Pijitawabik Canyon is a deep, steep-walled valley cut into Nipigon diabase/gabbro sills. It is about 20 km long with a prominent near right-angle bend about 11 km from its south end. It can be considered to be made up of two segments; a southern segment that is about 2 km wide and a northern segment that is only about 1 km wide (Figs. 13 and 14). Local relief along the valley walls of both segments can exceed 150 m. The initial conclusions made by Farrand appear to - 50 - The valley is controlled by major structures in the �r Proceedings of the 51st ILSG Annual Meeting - Part 2 scJLT Pijitaabik Canyon ProfileA C 0— 285 m 292 m 10000 5000 UsIam(n4 Profile B Profile C 500 500 400 rTh 450 400 i 350 - 350 E30 250 10 E ooo 250 K4onielocs iS 20% 200 3000 4000 5000 20 0 1Q00 Profile D Profile E 450 350- 3000 4000 5000 Profile F - 450- Th 2000 (m) (m) 00 / E 350300 I I 500 1000 (m) 1500 I I 2000 2500 0 lOU 20(X) 310 4000 (m) 1000 2 3000 4 (m) Figure 13. Selected profiles across the Pijitawabik Canyon. bedrock. In addition, the rock characteristics of the Nipigon sills are such that weathering can be intense along fractures, producing grus-like residual soils that are easily removed by erosion. The Pijitawabik delta emerges from the southern end of the canyon and deltaic sediments occur within the canyon walls (Figs. 11, 12 & 13). Remnant terraces of sand and gravel (Figs. 12 & 13) fall from an elevation of about 305 m at the canyon bend to the delta surface at about 299 m at the mouth of the canyon. The ice margin feeding this system must have been just north of the bend as suggested by Farrand (1960), in the vicinity of this stop. The side canyon, Profile D (Fig. 13) is cut down to the level of the terraces only, so major down-cutting here, must have occurred during delta building. Stop 4 – Sand River Property, Roxmark Mines Limited UTM coordinates – 0424364E 5497579N Note - permission from the land owner is required for access to this stop. In 2003, Roxmark Mines Limited acquired the Sand River, Leitch and East Leitch properties from Rio Fortuna, Teck Cominco and Kinross, respectively (News Release, Roxmark Mines Limited, November 28, 2003) and began an advanced exploration program. In 2004, trenching and stripping at Sand River exposed vein systems filled with quartz. Visible gold occurs within fractures in quartz veins and most commonly is found along thin seams or septa of sericite and chlorite, along with arsenopyrite, pyrite and ankerite. Channel samples were cut every three feet. All composite assays used a minimum width of 1.0 feet. The #16-1 West Vein returned an average assay of 1.737 ounces Au per ton over a strike length of 42 feet. The #16-1 East Vein returned 0.347 ounce Au per ton over 37 feet. - Figure 14. View of the Pijitawabik Canyon looking north toward Lake Nipigon (photo courtesy of P. Kor, OMNR). The Sand River area was most recently mapped by Hart et al. (2002) and is underlain predominantly by clastic metasedimentary rocks. Thickly bedded, southward-younging, feldspathic wacke, sandstone, - 51 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 I i. :1 The rock surface has been polished and striated with at least two ice flow directions indicated (Fig. 15). The oldest set is oriented between 265-270°N and the youngest set at 290°N. Minor meltwater erosion forms are also present. The edges of the stripped area and trenches expose up to 2 m of glacial and postglacial sediments. II In one section, an interesting sequence of sediments is exposed. Resting directly on the bedrock surface is a thin (30 to 50 cm) layer of stony, slightly silty, sand diamicton interpreted as subglacial till (Fig. 16). Clast content is high and is composed of many locallyderived lithologies, dominated by metasedimentary rock fragments. The till is typical of areas of thin till within the Canadian Shield allowing for variations in lithology and stoniness. It has less than 2% carbonate content in the silt/clay fraction of the matrix. A sample of till from this layer contained 69% sand, 28% silt and 3% clay-sized particles (2 microns). In comparison a sample of till from Stop 6 contained 45% sand, 49% silt and 6% clay and had a matrix carbonate content of about 27%. Figure 15. Polished bedrock surface containing 2 sets of striations (photo courtesy of M. Smyk, OGS). and argillaceous rocks have been well exposed by the recent stripping operations. Bedding has eastnortheasterly to east-southeasterly orientations with steep, northerly dips. Well-developed, slatey cleavage, as well as local sericitization and carbonatization distinguish sub-parallel shear zones. Oblique cleavage - bedding relationships are locally exposed. Cleavage is refracted through interbedded siltstone and sandstone beds. Overlying the till layer is 30 to 50 cm of rhythmically bedded silt and clay. These sediments were deposited in an ancestral lake located in at least the Lake Nipigon basin during ice marginal recession. A channel sample of the rhythmites contained 7% sand, 77% silt and 16% clay and its carbonate content was about 30%. The rhythmites have been overturned toward the northwest (fold axis ~30°N) and are overlain by a thin layer of diamicton containing sorted lenses and wisps I Figure 16. “Local till” overlain by silt and clay rhythmites at the Sand River Property (photo courtesy of M. Smyk, OGS). Figure 17. Overturned rhythmites and overlying diamicton (photo courtesy of M. Smyk, OGS). - 52 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 till, overlies the bedrock surface. The bedrock surface is striated with the striations oriented at 245°N. The bedrock directly below the till is composed of fine to very fine grained sandstone, 20 to 30 cm thick, resting on Quetico metasedimentary rocks. The Archean - Proterozoic unconformity, an angular unconformity between the flat-lying Sibley Group sedimentary rocks and east-trending steeply dipping Quetico metasedimentary rocks, is exposed on both sides of Highway 11 (UTM 0423431E 5483881N). Figure 18. Transverse dune at stop 5. Inset shows internal structure (photos courtesy of M. Smyk, OGS). of sand and gravelly sand (Fig. 17). The diamicton may represent a minor fluctuation of the ice margin; overriding the glaciolacustrine rhythmites followed by ice marginal debris flow sedimentation (flowtills) or may be debris flows related to slumping or flowing of debris downslope; a post glacial event. The direction of overturning of the rhythmites, however, is similar to the direction of the younger set of striations and perpendicular to the slope direction at the site. Stop 5 – Nipigon dunes UTM coordinates – 0421102E 5497420N At this stop a road cut exposes the internal structures of a transverse dune that probably formed along the shore of an ancestral lake in the Nipigon basin (Fig. 18). Thorleifson and Kristjansson (1983) mapped out a shorebluff several metres above present-day Lake Nipigon. The till fizzes when a weak solution of hydrochloric acid (~10%) is applied. It contains about 27% carbonate (11% calcite and 16% dolomite) in the silt/ clay fraction (Chittick analysis). The till at Sand River (Stop 4) contained less than 2% carbonate. A sample of till from this site contained 45% sand, 49% silt and 6% clay-sized particles (2 microns). In comparison a sample of till from the Sand River Property (Stop 4) contained 69% sand, 28% silt and 3% clay. This exposure of thick, carbonate-rich “exotic till” is one of the furthest west exposures. This exotic till is best exposed farther to the east, between Geraldton and Jellico especially around Wildgoose Lake. It underlies a spectacular streamlined surface (Fig. 19) where “drumlins up to 1 km in length are scattered across the area, but the principal morphological feature on thick till deposits are flutes about 1 km wide and over 20 km long” (Thorleifson and Kristjannsson, 1983, p.54). Zoltai (1965a) noted that the Nipigon Moraine, west of Lake Nipigon, corresponded to the extent of carbonate-rich till and Karrow and Geddes (1986) used the extent of carbonate-rich till to suggest that the The second part of the stop is to a small field of dunes developed on an abandoned lake bottom that presently has an elevation of about 296 m asl. The route follows along the shorebluff mapped by Thorleifson and Kristjansson (1983), then crossing a subtle break in slope at about 178 m asl on our way to the dune field. Stop 6 – Carbonate till UTM coordinates – 0423418E 5483886N In the 7 m exposure on the west side of Highway 11, approximately 6 m of light brownish grey, massive to fissile, slightly silty to silty, very fine to coarse grained sand with pebbles, cobbles and boulders, interpreted as Tn 1 Figure 19. Streamlined till surface near Wildgoose Lake (photo courtesy of OMNR 74-4929-18-156). - 53 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Nipigon Moraine was the correlative of the Chapleau Moraine north of Lake Huron. Stop 7 – Nipigon Lowland Rhythmites UTM coordinates – 0407200E 5437660N This stop is to look at some of the sediments that underlie the Nipigon lowland. At this site, 0.6 to 1 m of apparently massive slightly clayey silt to silt containing freshwater molluscs overlies greater than 14 m of silt and clay rhythmites (Fig. 20). The upper 6 m of the section is well exposed in a drainage ditch just east of Highway 11. The rhythmites are slightly contorted at the top of the unit then become horizontally bedded with depth. Concretions have formed around rootlets below 4.5 m. In the woods to the east, a small abandoned sand and gravel pit occurs at the base of the bedrock-controlled hill and may possibly mark the position of the shore and level of the lake that the molluscs inhabited. Farrand (1960) identified a couple of terraces east of the highway and Helen Lake that he attributed to Nipissing/Algoma. Acknowledgements The writer would like to thank Mr James Nichols, Jim Nichols Trucking Ltd., Nipigon and Mr. Dave Malouf, Roxmark Mines Ltd., for kindly giving permission to visit their properties. Mr Phil Kor (OMNR) and Mark Smyk (OGS) provided most of the photographs used in the fieldguide and Mark kindly provided text on the bedrock geology for several of the field trip stops. Roxanne Corcoran (OGS) produced some of the line illustrations. All of their help is much appreciated. In addition, the indirect contributions of the contributors to the 1987 INQUA fieldtrip guide (Geddes et al., 1987) and Dr. W.R. Farrand (1960) for his work on ancestral lake levels in the Superior basin improved the content of this field guide. This fieldtrip guide was produced with the permission of the Director, Ontario Geological Survey. References Baker, V.R., Greely, R., Komar, P.D., Swanson, D.A., and Waitt Jr., R.B., 1987. Columbia and Snake river plains; in Geomorphic Systems of North America, Geological Society of America, Centennial Special Volume 2, p.403-468. Barnett, P.J. 1991. Quaternary geology of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p.1011-1088. Barnett, P.J., Henry, A.P. and Babuin, D. 1991. Quaternary geology of Ontario, west-central sheet; Ontario Geologic Survey, Map 2554, scale 1:1 000 000. Blackburn, C.E., Johns, G.W., Ayer, J.A. and Davis, D.W. 1991. Wabigoon Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.303-382. Bostock, H.S. 1970. Physiographic subdivisions of Canada; in Geology and Economic Minerals of Canada, Geological Survey of Canada, Economic Geology Report no.1, p.10-30. Burwasser, G.J. 1979. Quaternary geology of the city of Thunder Bay and vicinity, District of Thunder Bay; Ontario Geological Survey, Geological Report 164, 70p. Dredge, L.A. and Cowan, W.R. 1989. Quaternary geology of the southwestern Canadian Shield; in Quaternary geology of Canada and Greenland, Geological Survey of Canada, Geology of Canada, no.1, p.214-249. Dreimanis, A. 1989. Genetic classification of tills; in Genetic Classification of Glacigenic Deposits, Balkema, Rotterdam, p.17-84. Drexler, C. W., Farrand, W. R. and Hughes, J. D., 1983. Correlation of glacial lakes in the Superior Basin with eastward discharge events from Lake Agassiz; in Glacial Lake Agassiz, J. T. Teller and L. Clayton (eds), Geological Association of Canada Special Paper 26, p.261-290. Figure 20. Example of fresh water molluscs collected from Stop 7. Elson, J.A. 1967. Geology of glacial Lake Agassiz; in Life, land and water, University of Manitoba Press, - 54 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Winnipeg, Manitoba, p.36-95. Farrand, W.R., 1960. Former shorelines in western and northern Lake Superior basin; unpublished Ph.D. thesis, University of Michigan, Ann Arbor, Michigan, 226p. Farrand, W.R. and Drexler, C.W., 1985. Late Wisconsin and Holocene history of the Lake Superior basin; in Quaternary Evolution of the Great Lakes, P.F. Karrow and P.E. Calkin, eds., Geological Association of Canada, Special Paper 30, p.17-32. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34 Geddes, R.S. and Kristjansson, F.J. 1986. Quaternary geology of the Hemlo area: Constraints on mineral exploration; Canadian Geology Journal of the Canadian Institute of Mining and Metallurgy, v.1, no.1, p.5-8. Geddes, R.S., Kristjansson, F.J. and Teller, J.T., 1987. Quaternary features and scenery along the north shore of Lake Superior; International Quaternary Association, INQUA’87, XII International Congress, Excursion Guide Book C-12, 62p. Hart, T.R., terMeer, M. and Jolette, C., 2002. Precambrian geology of Kitto, Eva, Summers, Dorothea and Sandra Townships, northwestern Ontario : Phoenix Bedrock Mapping Project; Ontario Geological Survey, Open File Report 6095, 206p. Hicock, S.R. 1988. Calcareous till facies north of Lake Superior, Ontario: Implications for Laurentide ice streaming; Géographie Physique et Quaternaire, v.42, p.121-135. Karrow, P.F. and Geddes, R.S. 1987. Drift carbonate on the Canadian Shield; Canadian Journal of Earth Sciences, v.24, p.365-369. Leverett, F., 1929. Moraines and shorelines of the Lake Superior basin; United States Geological Survey, Professional Paper, 154-A, 72p. Mollard, D.G. and Mollard, J.D. 1981. Frazer Lake area (NTS 52H/SE), District of Thunder Bay; Ontario Geological Survey, Northern Ontario Engineering Geology Study 42, 32p. Accompanied by Map 5052, scale 1:100 000. Ontario Geological Survey 2003. 1:250,000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release – Data 126. Prest, V.K. 1970. Quaternary geology of Canada; in Geology and economic minerals of Canada, 5th edition Geological Survey of Canada, Economic Geology Report 1, p.676-764. Sado, E.V., Fullerton, D.S. and Farrand, W.R. 1994. Quaternary Geological Map of the Lake Nipigon 40X60 Quadrangle, United States and Canada; U.S. Geological Survey, Miscellaneous Investigations Series, Map I-1420 (NM-16), scale 1:1 000 000. Shaw, J. 1985. Subglacial and ice marginal environments; in Glacial Sedimentary Environments, Society of Economic Paleontologists and Mineralogists, Short Course no. 16, p.7-84. Slattery, S.R., 2003. Sedimentary architecture and Quaternary geology of the Longlac area, northcentral Ontario, Canada; unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 125p. Smyk, M., Fralick, P.W., and Hart, T., 2005. The geology and gold mineralisation of the Bearmore-Geraldton greenstone belt. Institute on Lake Superior Geology 51st Annual Meeting, Proceedings Volume 51, Part 2 – Field trip guide, this volume. Sutcliffe, R.H. 1991. Proterozoic Geology of the Lake Superior Area. in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.627661. Teller, J.T. and Mahnic, P. 1987. History of sedimentation in the northwestern Lake Superior basin and its relation to Lake Agassiz overflow; Canadian Journal of Earth Sciences, v.25, p1660-1673. Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz– Superior connection; in Glacial Lake Agassiz, Geological Association of Canada, Special Paper 26, p.61-290. Teller, J.T. and Thorleifson, L.H., 1987. Catastrophic flooding into the Great Lakes from Lake Agassiz; in Catastrophic flooding, Allen & Unwin, Boston, Massachusetts, p.121-138. Thorleifson, L.H. and Kristjansson, F.J., 1993. Quaternary geology and drift prospecting, Beardmore-Geraldton area, Ontario, Geological Survey of Canada, Memoir 435, 146p. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.383-405. Zoltai, S.C. 1965a. Glacial features of the Quetico–Nipigon area, Ontario; Canadian Journal of Earth Sciences, v.2, p.247-269. Zoltai, S.C. 1965b. Surficial geology, Thunder Bay; Ontario Department of Lands and Forests, Map S265, scale 1:506 880. Sado, E.V. and Carswell, B.F. 1987. Surficial geology of northern Ontario; Ontario Geological Survey, Map 2518, scale 1:1 200 000. - 55 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 - 56 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrips 3 and 6 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport Pete Hollings and Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Safety As this trip will be taking place on Lake Superior it will be weather dependant and could be cancelled or curtailed at very short notice. An alternative trip to the Winston Lake mine will be undertaken in case of bad weather. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. It will probably be very cold out on the lake so please dress warmly. Regional geology Archean granites, outcropping along the shoreline near Rossport, are unconformably overlain by strata of the Gunflint Formation (Fig. 1). These sediments were deposited on a south facing shelf at approximately 1878 Ma (Fralick et al., 2002). The Formation consists of a Lower Member composed of basal stromatolitic bioherms overlain by ankeritic, interclastic grainstones. A regressive, karstified surface caps the northern portion of this assemblage (Fralick and Barrett, 1995) and is succeeded by the Upper Member. It begins with a repetition of the underlying lithologies to which, higher in the succession, are added carbonaceous shales, tuffs and rarely mafic volcanic rocks. These chemical and fine-grained siliciclastic sediments record parasequence development on a storm-dominated shelf (Pufahl and Fralick, 2004) forming the relatively stable portion of a back-arc basin (Kissin and Fralick, 1994; Hemming et al., 1995) prior to compressive northward thrusting of the arc at approximately 1860 to 1835 Ma. As the compression of the Penokean Orogeny waned the sea again transgressed over the area depositing black, carbonaceous shales of the Rove Formation. This depositional cycle lasted from 1835 Ma (Kissin et al., 2003; Addison et al., 2005) till less than 1780 Ma ago (new detrital zircon laser ablation ICP MS data). The lower portion of the Gunflint Formation in the Rossport area is poorly exposed. The limited outcrop of the Lower Gunflint is similar to the succession comprising the thin, basal Kakabeka Conglomerate and overlying interclastic grainstones present in exposures to the west near Thunder Bay. Good exposure of the Upper Gunflint exists on Quarry Island and consists of possible basaltic flow rocks with associated stromatolites, overlain by a succession of mediumto coarse-grained, graded, sandstone beds. The geochemistry of the sandstones is similar to Archean rocks to the north indicating probable derivation from this source. Black shales, lithically correlative with the Rove Formation, outcrop on an island approximately 5 km to the west. The shales do not overly the turbidite succession on Quarry Island where arenites of the Sibley Group disconformably rest on an erosion surface at the top of the Gunflint sandstones. The basal unit of the Sibley Group is the Pass Lake Formation. It is composed of the conglomeratic Loon Lake Member and the overlying sandstones of the Fork Bay Member (Cheadle, 1986). Where the basal conglomerates are present they either represent: 1) large channel fills cutting down into sandstones to shales with abundant caliche zones, or; 2) more laterally continuous conglomerates interbedded and overlain by parallel laminated, medium-grained sandstones. The former represent channel gravels in braided fluvial systems and the latter coarse-grained strandline deposits. The overlying Fork Bay sandstones likewise record both braided fluvial deposition and subaqueous, strand proximal sand-sheet development. In addition to upward thinning and fining successions developed during transgressive systems tract formation, other sandstone assemblages thicken and coarsen upwards representing progradational, delta lobe outbuilding. It is not known whether the water body was lacustrine or marine, though preliminary isotopic analyses (C, O, S and Sr) are more in agreement with a hypersaline lacustrine setting for at least the later history of the water mass. This is consistent with Cheadle’s (1986) findings - 57 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 A B Figure 1b Osler Group 1108 Ma Nipigon diabase Stop 8 Sibley Group Lake Superior C 48°30ʼ a Animikie Group Figure 1c 1105 Ma Lake Superior 200 km Rossport N Stop 7 Quarry Island Stop 6 Simpson Island Stop 3 Vein Island Stop 2 Archean rocks a 15 km Stop 5 Stop 4 Stop 1 Channel Island Wilson Island 48°30ʼ Keweenawan intrusive rocks Osler Group volcanics Keweenawan sediments Sibley Group sediments Gunflint Formation Archean basement Copper Island 48°45ʼ 87°30ʼ 87°45ʼ 48°45ʼ 1 km Figure. 1. Map showing the location of the fieldtrip area. B) Regional geology map showing the extent of the Osler Volcanic Group. Age data from Davis and Sutcliffe (1985) and Davis and Green (1997). Modified after Sutcliffe (1986). C) Geological map of the Osler Volcanic Group showing sample locations. Modified after Giguerre (1975). based on regional paleogeography. The increasing salinity of the water resulted in dolomite, minor gypsum and rarer barite and celestite precipitation mixed with mud deposition. A red and green banding developed in this assemblage due to periodic anoxia of the bottom sediments. The final desiccation of the lacustrine basin is recorded by the development of strandline microbial bioherms (stromatolites) which are overlain by either a terra rosa (red, wind-delivered soil) or subareal, intraformational, mass-flow conglomerates. This is succeeded upwards by mudstones with abundant gypsum nodules representing mudflats formed in an arid climatic setting where hypersaline groundwaters precipitated gypsum. Together all these fine-grained sediments comprise the Rossport Formation. It is overlain by the Kama Hill Formation; a coarsening upwards deltaic succession recording flooding of the basin and development of a more humid climate. The age of the Pass Lake and Rossport Formations can be bracketed between laser ablation MS youngest ages on detrital zircons of 1600 Ma and a Rb-Sr isochron of 1339 Ma (Franklin, 1978). Youngest detrital zircons in the Kama Hill Formation are 1420 Ma. The Sibley Group is very well exposed along the shorelines of the islands off Rossport. The basal disconformity can be seen about two thirds of the way up the cliff face on the western side of Quarry Island where it overlies graded sandstone beds of the Gunflint Formation. Blocks of medium-grained sandstone were extracted from the cliff face on the island for use in the construction of buildings in Thunder Bay. These sandstones are medium- and large-scale planar crossstratified and may represent a sandflat composed of transverse bars in a braided stream or small barchan, eolian dunes. Rare pebbles indicate the former may be the case but this evidence is not conclusive. Channel and Copper Islands contain excellent exposures of the lacustrine rocks with outbuilding of channel systems along the paleolake margins. One of the best outcrops of the strandline stromatolitic carbonates occurs on Channel Island and will be visited during this fieldtrip. On Copper Island the Rossport Formation is overlain - 58 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Logan Sills (1109+4/-2 Ma, Davis and Sutcliffe, 1985), the lower portion of the North Shore Volcanics (1108±2 Ma, Davis and Green, 1997), the Swamper Lake Gabbro and Nathan’s Series intrusives (1107 Ma, Paces and Miller, 1993) and the lower portion of the Powder Mill Group (1107±2 Ma, Davis and Green, 1997). These older units outcrop on the rift flanks where erosion has removed the younger rift sequence or, in the case of the The oldest rift-related rocks on which U-Pb age Coldwell and Logan Sills, are intruded into older rocks determinations have been performed lie along the immediately north of the rift. northwestern portion of the rift. These include, from NE The Osler Volcanic Group comprise a ~3km thick to SW, the alkaline intrusives of the Coldwell Complex (1108±1 Ma, Heaman and Machado, 1992), the lower sequence (Cannon et al., 1989) lying unconformably Osler Group volcanics (1108+4/-2 to 1105±2 Ma, above Sibley Group metasediments (Fig. 2). The Davis and Sutcliffe, 1985; Davis and Green, 1997), the volcanic sequence is overlain and intruded by the disconformably by pebbly, fluvial conglomerates of the basal Osler. Thirty kilometers to the west the uppermost unit of the Sibley, the Nipigon Bay Formation, underlies the same disconformity. This highlights the fact that approximately 600 m of erosive downcutting occurred in the Rossport area before the basal Osler was deposited. V 1095 N Copper Harbour Conglmerate Portage lake Volcanics IV Portage Lake IV Volcanics Group 7 V (Group 8) III Group 6 Upper Suite R R Kallander Creek Volcanics Siemens Creek Volcanics Beaver Bay Complex Mostly basalt units Osler Group 1105 Schroeder Basalts V Duluth Complex Great Conglomerate and Group 5 Central Suite III II I Bessemer Quartzite 1110 Lower Suite I Simpson Isl Cgl Nipigon Sills Groups 3,4 Group 2 Group 1 IV IV IV 1100 NE Minnesota SW limb III II I IV North Shore Volcanic Group Copper Harbour Conglomerate 1090 Mamainse Point Michipicoten Island Michipicoten Island Formation Isle Royale 1085 Age (Ma) Isle Royale Black Bay Peninsula Lake Nipigon Upper Michigan NW Wisconsin Elyʼs Peak Basalts I, II, III Nopeming sandstone Archean Basement Figure 2. Schematic correlations of volcanic rocks of the Midcontinent Rift based on the stratigraphic position of distinctive basalt sequences, magnetic polarity and absolute age where possible. Modified after Nicholson et al. (1997). Dashed lines in Upper Michigan section separate lower and upper members of Kallander Creek and Siemens Creek volcanics. Left hand column shows magnetic polarity. Roman numerals I-IV refer to five distinctive laterally extensive basalt compositions identified on the south shore of western Lake Superior. Where equivalent basalt compositions occur in other stratigraphic successions, the appropriate Roman numeral is noted (see Nicholson et al., 1997 for data sources). Shaded regions represent intervals in which contacts are covered or obscured by plutonic rocks. - 59 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 St. Ignace Island Volcanic-Plutonic Complex an intercalated sequence of basaltic rocks and rhyolitic flows (Sutcliffe and Smith, 1988). Detailed descriptions of the Osler Group have been provided by McIlwaine and Wallace (1976) and Lightfoot et al. (1991). Generally the mafic flows of the Osler Group consist of massive to amygdaloidal flows, with locally developed ropey tops and pahoehoe textures (Sutcliffe and Smith, 1988). The flows range in thickness from 5cm to 30m (Lightfoot et al., 1991) with a regional dip of ~6-15° S (Giguerre, 1975; Lightfoot et al. 1991; Hollings et al., 2005). The majority of the exposed section is magnetically reversed with only the upper 100m displaying a normal polarity (Halls, 1974). The contact between the two units is marked by the presence of the Puff Island conglomerate and a discordance between the basalt flows above and below the contact. This has been interpreted as representing a significant break in the eruption history. A felsic porphyry near the base of the Osler Group has yielded an age of 1107.5+4/-2 Ma (Davis and Sutcliffe, 1985) whereas zircons from the Agate Point rhyolite towards the top of the reversely magnetized sequence have yielded an age of 1105±2 Ma (Fig. 1b; Davis and Green, 1997). Within the Osler Group interflow sediments are typically thin and of limited extent. Field descriptions of the sedimentary successions appear in Giguere (1975) and McIlwaine and Wallace (1976). They show that there are two main zones of sedimentary rocks within the Osler Group. One occurs near the base of the volcanic pile. The other is present approximately 2700 meters higher in the succession marking the paleomagnetic reversal. Lightfoot et al. (1991) in a study of the Osler Volcanic Group exposed along the shores of the Black Bay Peninsula to the west of the field trip location proposed that the major and trace element geochemical data could be used to subdivide the flows into an Upper, Central and Lower Suite although the boundaries between the suites were not clear cut. While the geochemical compositions of the Central (750900m) and Upper suites (1900-3000m) overlap their Lower Suite (0-750m) is distinguished by elevated Mg numbers (0.55-0.7 versus 0.3-0.6), lower Al2O3 (8-12 wt% versus 13-17wt%), lower Th/Nb ratios (0.090.70 versus 0.3-0.6) but higher Gd/Ybn ratios (3.5-4.5 versus 1.6-2.6). Nicholson et al. (1997) concluded that there were five geochemically distinct flood-basalt compositions within the Mid-continent rift that are common to most sections and appear in approximately the same stratigraphic order (Fig. 2) They recognized a lower suite in the Siemens Creek Volcanics (Basalt Type 1; Fig. 2), which they suggest is analogous to the Lower Suite of Lightfoot et al. (1991). In the United States this unit is less than 100m thick whereas the Lower Suite of Lightfoot et al. (1991) is ~750m thick. They report a narrow range of εNd(1100) values for the Siemens Creek Volcanics of -0.7 to +0.7. Nicholson et al. (1997) further suggest that there is a broadly recognizable suite of basalts above this (Basalt type II) which includes the upper Siemens Creek Volcanics and in the upper part of the Grand Portage lavas (Fig. 2). The suite is characterized by slight negative Nb anomalies and a range of εNd(1100) values of -1.4 to -6.9. They suggest that this may be analogous to the most primitive members of the Central Suite of Lightfoot et al. (1991). The volcanic flows of the Osler Group on Wilson Island are all basalts or basaltic andesites (SiO2 = 4756 wt%; MgO = 5-16 wt%; Hollings et al., 2005). The basalts are characterized by LREE enrichment (La/Smn = 1.5-3.9) in conjunction with moderately fractionated HREE (Gd/Ybn = 1.5-3.7) and slight positive to moderately negative Nb anomalies (Nb/ Nb* = 0.56-1.13; Hollings et al., 2005; Fig. 3). Major and trace element data show trends of increasing SiO2 and decreasing MgO and display strong positive correlations between La/Smn, Th/La and Th/Nb with height (Fig. 4). This correlation is most pronounced above 400m with samples from the base of the stratigraphy displaying more or less constant values of these ratios (Fig. 4). Measured 143Nd/144Nd ratios for the seven Osler basalts analysed range from 0.5118570.512286 with εNd(t=1106Ma) of +0.3 to -5.3 (Hollings et al., 2005). The high incompatible element abundances, in conjunction with LREE enrichment and strongly fractionated HREE are comparable to modern OIB, albeit at lower absolute abundances (Fig. 3). When compared to other flood basalt sequences the more primitive basalts from this study closely resemble basalts from the Parana-Etendeka flood basalt sequence (Fig. 4; Gibson et al., 2000). The εNd data from the most primitive members of the Osler Group is consistent with an enriched mantle plume rather than a contaminated depleted mantle source, given the lack of trace element evidence for contamination in these samples. Depleted mantle at 1100 Ma would have had a positive εNd perhaps as high as +6 whereas an - 60 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 by an older lithospheric component characterized by pronounced LREE enrichment, high Th abundances but generally unfractionated HREE (Hollings et al., 2005). 100 The sedimentary successions near the base of the Osler Group constitute the Simpson Island Formation and have recently been described in detail by Hollings et al. (2005). They are composed of a Lower Member dominated by trough cross-stratified, medium-grained sandstones directly overlying basement and an Upper Member with a greater variety of siliclastic units. The Lower Member sits on an irregular, erosional surface cut into the underlying quartz arenites of the Nipigon Bay Formation, Sibley Group. A massive pebble-cobble conglomerate overlies the unconformable surface and is in turn overlain by decameter-scale layers of coarse-grained and pebbly sandstone (Fig. 5, Section 1). Sandstones are parallel laminated, commonly have cross-stratified tops, more rarely contain pebbly transverse ribs and chute and pool-like structures. The central portion of the succession is composed of trough cross-stratified, medium-grained sandstone organized into a stacked assemblage of lenses. Pebble stringers and pebbly sandstones commonly occur on the deeper portions of curving set boundaries (Fig. 5, Section 2). Massive pebble-cobble conglomerates sharply overly the sandstone succession (Fig. 5, Sections 2 and 4). The conglomerates contain trough cross-stratified, mediumgrained sandstone lenses; decameter- to meter-scale wedges of planar cross-stratified sandstone and are interbedded with assemblages of trough cross-stratified sandstones up to one meter thick. Another assemblage of trough cross-stratified sandstone, similar to the one in the central portion of the succession, caps the basal sedimentary assemblage (Fig. 5, Sections 3 and 4). Clast lithologies in the pebble-cobble population are dominated by quartz, chert, various types of volcanic 10 Hawaiian OIB Deccan Traps CFB Parana-Etendeka CFB 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Rock/Primitive Mantle 100 10 Type 1 Lower Suite 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc 100 10 Type 2 1 Central Suite Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Figure 3. Comparison of primitive mantle normalized plots from the Osler Group with A) Phanerozoic OIB and Continental Flood Basalts (CFB) and B & C) the Lower and Central suites of Lightfoot et al. (1991). From Hollings et al. (2005). enriched plume source would have εNd ~0 (Nicholson and Shirey, 1990; Shirey et al., 1994). Up sequence the basalts are characterized by higher SiO2, Th and La/Smn abundances in conjunction with increasingly negative Nb and Ti anomalies and εNd(1106) values of -4 to -5. This is consistent with contamination of these basalts 900 800 SiO2 MgO Fe2O3 La/Smn Th Gd/Ybn Th/La εNd Th/Nb 700 600 500 400 300 200 100 0 40 50 60 5 10 15 10 15 1 2 3 4 1 2 3 4 1 2 3 4 0.05 0.10 0.15 0.20 0.10 0.15 0.20 -6 -4 -2 0 Figure 4. Geochemical stratigraphy of the Osler Group on Vein and Wilson Islands. The stratigraphic position of the samples has been calculated assuming a dip of 10° parallel to the section. From Hollings et al. (2005). - 61 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 V V V V V V V V V 6 No Longitudinal 24 Complex V V V V V Major 12 V V V V 6 V V V V V V 18 V 3 0 Section 5. V V No O/C V V V V V Nipigon Bay V V V Ripples Pebbles Hummocky Cross-Strat. V V V V V No O/C ? m. m V V V V V Complex No O/C ? m. Sandy Sheetfloods 6 Sheetfloods and Debris flows 3 V V V V V V V V V V Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale Sst. V V V V V V 7 15 V V V 8 m. No O/C 12 21 18 15 12 Bar V 0 Massflow 3 m. No O/C V V V V V V V V V V V V V 5 6 V V V V V V 15 V V V V V V V 10 km Older Units Section Locations Section 7. V V V V V V V V V V Sand Channels with Small Gravel Longitudinal Bars Stacked Channels 9 Sheetflood Sands with Channels 6 Small Stacked Channels Distal 6 V 4 2 1 3 VV V Sedimentary Rocks Igneous Rocks 1 27 Sandy Channels to Distributary Mouth Bar 9 V m Gravel Channels 26 V V V V V Osler Group 24 29 23 V V Section 6. 32 Longitudinal No O/C Trough Cross-Strat. Small Irregular Lenses Paleocurrent Direction V V V V V V V V V V V V V V V V V V V V V V V V V V Bar 0 0 Complex Rhyolite V V V V =318o 15 9 Longitudinal Parallel Lamination V V V V Bar Tail Sand Sheet Longitudinal Bar Nipigon Bay V Sandy Channel 3 Bar Sandy Channel V Section 4. m Basalt INTERFLOW SEDIMENTS m V Channel O/C =268o V 9 3 0 V l e g e n d 18 No 6 V V Sst. V 9 V Major Sandy Channel 0 Bar V m V 3 O/C 15 Section 1. V V No O/C V n = 38 V V 21 o Section 3. m V = 265 27 Section 2. Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale N m V SIMPSON ISLAND FORMATION ( Basal Sediments ) 3 0 Massflow Sheetflood Sands with Channels Figure 5. Sections of sedimentary rocks in the Osler Group. Sections 1 through 4 are the basal sedimentary succession of the Lower Member, Simpson Island Formation, at different locations (see inset map). Sections 5 and 6 are of the Upper Member, Simpson Island Formation, interlayered with basal basalt flows. Section 7 is the sedimentary assemblage near the top of the Osler Group on Puff Island. From Hollings et al. (2005). - 62 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 rocks, red siltstone, metamorphosed granite and at the east end of the outcrop belt, on Copper Island, a higher proportion of unmetamorphosed red granite. Current indicators show flow was to the west, averaging 265°. A sedimentary assemblage also occurs near the upper limit of outcrop of the Osler Formation, at the top of the magnetically reversed interval (Fig. 5, Section 7). These interflow sediments are located on Puff Island and overly a felsic porphyry with a U-Pb age of 1105 Ma (Davis and Green, 1997). They contain: sharp sided assemblages of laterally continuous, pebbly, coarsegrained sandstone beds with caliche horizons which are scoured into by small lenses of conglomerate; large scours filled with trough cross-sets over a meter thick; stacked assemblages of irregular lenses filled by trough cross-stratified, coarse-grained, pebbly sandstone; and, poorly sorted, disorganized, massive boulder-cobble conglomerate. Clasts are all volcanic, ranging from quartz-feldspar porphyries to mafic compositions. Paleocurrents on large-scale sedimentary structures consistently show flow to the southeast. Island. Stop 1 – Osler volcanics, Wilson Island UTM coordinates – 0462794E 5402095N Flows of the Osler Group on Wilson Island are typically >1m thick, frequently amygdaloidal towards the top and bottom of the flow with a massive core and rarely displayed a pahoehoe texture on the flow surface. The basalts are characterized by clinopyroxene and plagioclase phenocrysts in a groundmass of plagioclase, augite and Fe-Ti oxides. Rarely, basalts from the base of the sequence contained pseudomorphed olivine phenocrysts. The basalts have all been subjected to low-grade metamorphism ranging from zeolite to prehnite-pumpellyite facies (McIlwaine and Wallace, 1976). At this stop, approximately 500m above the basal conglomerates, are exposed a sequence of thin rubbly basalt flows ~50cm thick with rare massive flows ~2-4m thick and thin interflow sediments (Fig. 6). Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). Sedimentary units are predominantly quartz sandstones with thin shale partings. These are best interpreted as sands washing into small hollows on the surface of the flow units with a mud drape settling out towards the top of the layer. In places units that appear to have been deposited on the surface of basalt flows connect into sub-vertical cooling cracks in the flows. Sediment filled cooling cracks can be up to 2m deep (Fig. 7). cr1 The Simpson Island Formation is composed of a laterally continuous sedimentary succession up to 25 meters thick and discontinuous sedimentary units up to 30 meters thick interlayered with the basal basalt flows. The lowest sedimentary beds fill channelways cut into the underlying sandstones of the Nipigon Bay Formation. The channel fills and overlying sedimentary assemblage represent a braided stream system, similar to the South Saskatchawan model (Miall, 1978), where dunes composed of coarse-grained sand migrated Stop 2 – Osler Volcanics, North end of Wilson down the channels and gravelly longitudinal bars with Island chute channels and bar edge sand wedges form the UTM coordinates – 0461810E 5403438N higher relief areas (Fig. 5). The fluvial interpretation Exposed at this outcrop, are the lower flows of the is consistent with Tanton (1931) and McIlwaine and Osler Volcanic Group ~300m above the conglomerates Wallace (1976). Clast lithologies indicate debris was mainly derived from erosion of local lithologies. C I Stops The trip will depart from the public dock at Rosport and will undertake a traverse through the stratigraphy of the Midcontinent Rift, starting with the youngest rocks of the Osler Volcanic Group, proceeding through the Sibley and Gunflint Formations and finishing with a look at the granites of the Archean basement (Fig. 1). In order to make the most of the calmer weather typical of early mornings we will first travel for approximately Figure 6. Thin interflow sediments between basaltic flows, 30 minutes to the most southerly outcrop on Wilson Stop 1 on Wilson Island. - 63 - �I ') i 1 Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 8. Pahoehoe texture at Stop 2, Wilson Island. Figure 7. Approximately 2m thick mafic flow cut by sediment filled cooling crack. Stop 1, Wilson Island. of the Upper Simpson Island Formation. The basaltic flows are generally massive, ranging in thickness from 1-3m. Flow tops vary from rubbly to well-developed pahoehoe textures (Fig. 8). The basalts are vesicular and amygdaloidal and in places the vesicles are elongated giving them almost a pipe-like appearance. Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). In some areas the basalt flows incorporate xenoliths of a more vesicular material which have both sharp and diffuse contacts (Fig. 9) At the north end of the outcrop the flows are cut by a 2-3m wide mafic dyke. This dyke is geochemically distinct from the flows but comparable to the older diabase intrusions in the vicinity of Lake Nipigon. tel Figure 9. Amygdaloidal xenoliths in basalt flows at Stop 2, Wilson Island. sandstone beds (Figs. 10, 11). A covered interval separates this assemblage from overlying mediumto large-scale, trough cross-stratified, coarse-grained sandstones to conglomerates (Fig. 12). Paleocurrent indicators show flow to the west, though with a higher variance than other sections. Clast lithologies Stop 3 – Upper Simpson Island Formation, Daylight are probably locally derived from both Archean and Point, Wilson Island Proterozoic sources. The fine-grained sandstone near the base of the section represents a wave modified deltaUTM coordinates – 0461450E 5404650N front (ie., a distributary mouth bar of a small delta). A sedimentary assemblage of the Upper Member occurs on Wilson Island, overlying approximately The presence of small, dish-shaped scours suggests a 50 meters of basal basalt. This coarsening upwards shallow water environment with no large channels. The succession has oscillation rippled, very fine-grained upper part of the sequence represents badly organized sandstones at its base (Fig. 5, Section 6). These coarsen river deposits with gravelly, longitudinal bar forms and upwards by the addition of increasing amounts of channels filled with sand. The planar cross sets at the medium-grained, parallel laminated to hummocky base of the cliff were formed by transverse bars, while cross-stratified to oscillation rippled, decimeter-scale the trough cross beds represent migrating dunes. - 64 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 red siltstones outcrop on the shoreline here (Fig. 13). These are interpreted to be part of the cyclic facies (Channel Island Member) of the Rossport Formation as exposed at Kama Hill. The dolomite and minor gypsum indicate a hypersaline environment interpreted to be lacustrine because of the multiple deltas and sand sheets building in from a variety of directions. The cyclic facies is overlain by stromatolites and interbedded thin, sandy, carbonate storm layers (Fig. 14) of the Middlebrun Bay Member. This meter thick Figure 10. Fine-grained red sandstones with thin shale assemblage is similar to Recent sabkha deposits on the partings forming the base of the deltaic deposits at Stop 3. south shore of the Persian Gulf, and in particular the open water ponds on this sabkha where sandy storm layers are well preserved. The upper few centimeters of the stromatolitic unit is altered to a grey-green layer that represents a weathered horizon interpreted to ., 4r 17 ; Figure 11. Oscilation ripples with overlying hummocky, medium-grained sandstone showing the effects of wave reworking on the sediments forming the delta front at Stop 3. Figure 12. Cross-stratified sandstone and massive conglomerate forming the upper portion of the prograding deltaic succession present in the Upper Member of the Simpson Island Formation present at Stop 3. These sediments represent a longitudinal bar-channel complex of a braided stream. Stop 4 – Sibley Group, Mary Ann Bay, Channel Island UTM coordinates – 0462831E 5405801N Note - this is a very small outcrop and access to the rocks may be limited if lake levels are high. Please take care moving from the landing point to the outcrop. A succession of grey dolomites interbedded with Figure 13. Interlayered red siltstones and dolostones (lower unit underlying the more massive strandline carbonate with overlying mass-flow deposits) were deposited in a saline lake away either temporally or spatially from areas of coarse sand influx. The colour banding reflects the position of the redox boundary as the sediments accumulated. The grey layers commonly have slightly higher dolomite contents possibly reflecting higher organic productivity leading to more photosynthetically mitigated carbonate precipitation (higher dolomite content) and heavier organic loading to the sediment (redox boundary moving upward to at or above sediment water interface). Stop 4, Mary Ann Bay. - 65 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ________ U t1 ii' IIIIII!! CENTIMETRE Figure 14. Stromatolitic layering (smooth mat with small pinnacles) with interbedded coarse silt to very fine-grained sand storm layers (white). This is typical of strandline to sabkha environments and especially open water ponds on the sabkha. Some of the storm layers were remobilized in the form of clastic dykes and sills. www. ]akcu pC 1 recj Ic IgyUIr have formed as the sequence became subaerial and the stromatolites weathered in situ. This weathered zone is traceable throughout the basin with the strandline deposits below it commonly containing well developed tepee structures. The carbonates are overlain by a massflow unit with intraformational clasts of red siltstone, sandstone and dolostone up to boulder size. Although the contact between the carbonates and the mass flow units is locally obscured by the intrusion of a sill, the transition is interpreted to represent a minor time gap based on the weathered zone which expands to thick terra rosa (soil) deposits at other locations. Stop 5 – Gunflint Formation, Quarry Island UTM coordinates – 0462305E 5406730N A succession of sandstones and mafic volcanic rocks outcrop on the south shore of Quarry Island. On the northeastern end of the outcrop area a gabbro, probably related to the Midcontinental Rift, is exposed. Next to this is a small outcrop of stromatolites with a box-like appearance (Fig. 15). The rectangular to square outline of the mounds contrasts with the round to oval appearance of classic stromatolites, though their organic origin is exemplified by the high angle layering, which, when projected into the area now eroded, can be seen to form mounded structures. Areas between the stromatolites are infilled with coarser siliciclastic sandstones and cherty clasts. The next outcrop of Figure 15. Odd shaped structures of probable stromatolitic origin. within the Gunflint Formation. Stop 5, Quarry Island Gunflint volcanic rocks is problematic. Mafic volcanic flow rocks occur interbedded with Upper Gunflint lithologies southwest of Thunder Bay. These are also associated with stromatolites that developed on the firm substrate of the flow tops. Thus, the igneous rocks in the Gunflint assemblage on Quarry Island could be correlative to the other flow rocks, but it is difficult to conclusively show that these rocks are extrusive. Possible flow banding is present, as are areas of finer and coarser material in individual units. The igneous rocks are overlain by medium- to coarse-grained sandstones with bed thicknesses averaging approximately 30 cm. The sandstones are dark in appearance giving the impression they were derived from mafic detritus, but their geochemistry indicates an intermediate source similar in composition to the Archean crust to the north. The layers are excellently graded (Fig. 16) with the only sedimentary structure being sporadically developed parallel lamination. Beds such as these are commonly thought of as typical turbidites and the deposits ascribed to reasonably deep water. However, it must be remembered that graded bedding simply means a decelerating flow deposited the bed, which can occur in any water depth. These beds may be tempastites, ie. beds formed by storm events, in this case in water deeper than storm wave base but certainly not anything approaching abyssal depths. Or they may have formed - 66 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Figure 16. Well graded sandstone layers. Though these are probably turbidites they were not deposited in excessively deep water and may, in fact, represent distal tempestites. Stratigraphic position is problematic as these beds may belong to either the Upper Gunflint or Rove Formations. Stop 5, Quarry Island. from inter- or overflow sediment-water plumes off river mouths, though lack of current reworking of bed tops makes this unlikely. Alternatively they may represent prodelta deposits formed by slumping of the delta front. All of these environments are relatively shallow which agrees with the presence of stromatolites not far stratigraphically below the graded beds. Another Figure 18. A second example of unusual marking observed at Stop 6. The origin of these markings is unclear. Stop 5, Quarry Island interesting point concerning these clastic units is that although such sandstones are common in the upper Rove Formation they are not present in the Gunflint at any other location. Thus, their stratigraphic position is debatable. The third unusual attribute is the presence of difficult to interpret structures on some bedding planes. Series of enechelon small crack-fill like features cut across bedding planes (Fig. 17). In addition a jellyfishlike impression was found on a bedding plane (Fig. 18). This feature had five-fold symmetry, similar to echinoderms, but the age of the rocks and its presence in sandstone leads to the distinct possibility that it was manufactured. Stop 6 – Pass Lake Formation, Quarry Island A 50 meters inland from the last shore outcrop (Stop 5) where the iron train rail lies across the cobble-boulder beach - this stop will be time dependant. This outcrop consists of a cliff-face in sandstones, which were quarried, and the blocks produced used in the construction of buildings in Thunder Bay (Fralick et al., 2000). Here we see the basal sediments of the Sibley Group, the Pass Lake Formation. The Pass Lake forms a diverse group of basal coarse clastic deposits representing environments ranging from braided fluvial through to subaqueous sand sheets. The mediumgrained sandstones present in this cliff are organized into a series of large-scale planar cross-stratified sets with normal to low dip angles. Sorting is fairly good and only one pebble has been found in the succession. Assemblages such as this pose a dilemma in formulating Figure 17. Unusual markings on the bedding planes of the an interpretation of their depositional environment. graded sandstones. Stop 5, Quarry Island. Both aeolian sand dunes and sandflats composed of - 67 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 i —I • , • —l •d• Figure 19. Basal conglomerate of the Gunflint exposed at Gut Point. Photo courtesy of Mark Smyk. transverse bars in braided rivers are capable of creating such an organization of lithofacies. The presence of the pebble is significant as only freak wind-storms, such as tornadoes, can move material of this size, and these do not form sand dunes. So, it is more likely that these deposits are subaqueous but this rests only on a slim piece of evidence. Figure 21. Black chert with gossan within the basal conglomerate of the Gunflint Formation at Gut Point. Photo courtesy of Mark Smyk. is matrix-supported, with subangular to rounded pebbles of white, sugary quartz, lesser cherty and lithic fragments and minor jasper in a medium-grained, sandy matrix (Fig. 20). A black, pyritiferous chert breccia is marked by a conspicuous gossan (Fig. 21). Sulphide mineralization may be related to a persistent, parallel fracture set at 115°. Fractures may host quartz-calcitebarite veins ranging from < 1 to 20 cm wide as well Stop 7 – Basal conglomerate of the Gunflint, Gut as vein breccia. A conjugate fracture set at right angles to the first is locally developed. Silicification adjacent Point to the veins has preserved a thin (1 to 5 cm) veneer of UTM coordinates – 0461610E 5408587N Gunflint from being eroded (especially the carbonate The unconformity and basal Gunflint are exposed units). A 2m wide diabase dyke strikes at 115° through at Gut Point as a thin, discontinuous veneer along the the outcrop. lakeshore on top of Archean basement (Fig. 19). The basement is a medium-grained, equigranular granite, Stop 8 – Archean basement, Selim Point which has been altered (sausseritized/chloritized) UTM coordinates – 0469219E 5409146N beneath the basal Gunflint. The basal conglomerate is up to 30 cm thick and occupies depressions in the paleoFrom the dock in Rosport return to Highway 17 and erosion surface in the basement. The conglomerate head east for ~5 km. Turn right on to Lakeshore Drive just west of Whitesand Provincial Park. Follow the dirt road to a parking spot opposite a small tombola (Fig. 22). The porpyritic granite exposed here is Archean in age and part of the Wawa Subprovince. The area was mapped by Carter (1988) who described the rocks as porphyritic pink, hornblende + biotite alkali feldspar granite, a phase of the Whitesand Lake Batholith. The porphyritic “facies” is surrounded by massive pink and grey phases of alkali feldspar granite that is not exposed at this locality. The batholith is about 8 x 16 km in size and intrudes the Schreiber greenstone belt; no radiometric date has been generated. Feldspar Figure 20. Matrix supported basal conglomerate of the phenocrysts are typically 3-4 cm across (Fig. 23), Gunflint Formation containing rounded pebbles of quartz, subhedral to euhedral and in places appear to display chert and lithic fragments. Photo courtesy of Mark Smyk. localized alignment suggestive of flow banding. - 68 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Morey, G., Sutcliffe, R., and Spencer, C., 1989. The North American Midcontinent Rift beneath lake Superior from GLIMPCE seismic reflection profiling. Tectonics, 8, 305-332. Carter, M.W. 1988. Geology of the Schreiber-Terrace Bay area, District of Thunder Bay; Ontario Geological Survey, Open File Report 5692, 287p. Cheadle, B.A., 1986. alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 527-542. Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Davis, D.W. and Sutcliffe, R., H., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579.Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Figure 22. Porphyritic Archean granite, Selim Point. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada. In, ed by A.G. Plint, Sedimentary Facies Analysis, Special Publication of the International Association of Sedimentologists, v. 22, p. 137-156. Fralick, P.W., Kissin, S.A. and Davis , D.W., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Figure 23. Feldspar phenocrysts in Archean porphyritic granite at Selim Point. Acknowledgements We would like to thank Mark Smyk and John Scott for their help and advice in the preparation of this field guide. In particular Mark Smyk provided text and photographs for Stop 7. References Addison, W.D., Brumpton, G.R., Vallini, D.A., Davis, D.W., Kissin, S., Fralick, P.W., McNaughton, N.J., and Hammond, A., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, 33, 193-196. Cannon, W., Green, A., Hutchinson, D., Lee, M., Milkereit, B., Behrendt, J., Halls, H., Green, J., Dickas, A., Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. In, ed. by P. Fralick, Fieldtrip Guide Books, Forty-Sixth Annual Meeting, Institute of Lake Superior Geology. p. 7-42. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Giguere, J.F., 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay. Ontario Ministry of natural Resources, Geological Report 118, 35p. Halls, H.C., 1974. A paleomagnetic reversal in the Osler Volcanic Group, Northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200-1207/ Heaman, L.M., and Machado, N., 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contributions to Mineralogy and Petrology, 110, 289303. - 69 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evinence for the provinance of the early Proterozoic verginia Formation, Minnisota. Implications for the tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. Hollings, P., Fralick, P. and Cousens, B., 2005. Early History of the Mid-Continental Rift Inferred from Geochemistry and Sedimentology of the Proterozoic Osler Group, Northwestern Ontario. Submitted to GSA Bulletin. Kissin, S.A. and Fralick, P.W., 1994. Early Proterozoic volcanics of the Animikie Group, Ontario and Michigan, and their tectonic significance. Proceedings Institute of Lake Superior Geology, v. 40, p. 18-19. Sutcliffe, R. H., 1986. The petrology, mineral chemistry and tectonics of Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. Unpublished Ph.D. thesis, University of Western Ontario, London, 325p. Sutcliffe, R.H., and Smith, A.R., 1988. Project number 8717. Geology of the St. Ignace Island volcanic-plutonic complex. Summary of Fieldwork and Other Activities 1988. Ontario Geological Survey Miscellaneous Paper 141, 368-371. Tanton, T.L., 1931. Fort William and Port Arthur, and Thunder Cape map areas, Thunder Bay District, Ontario. Geological survey of Canada Memoir 167, 222p. Kissin, S.A., Vallina, D.A., Addison,W,D. and Brumpton, G.R., 2003. New zircon ages from the Gunflint and Rove Formations, northwestern Ontario. Proceedings Institute of lake Superior Geology, Lightfoot, P., Sutcliffe, R., and Doherty, W., 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: A trace element perspective. Journal of Geology, 99, 739-760. McIlwaine, W.H., and Wallace, H., 1976. Geology of the Black Bay Peninsula Area, District of Thunder Bay, Accompanied by Map 2304, scale 1 inch to 1 mile. Ontario Division of Mines, GR133, 54p. Miall, A.D., 1978. Lithofacies types and vertical profile models in braided river deposits: A summary. In ed. A.D. Miall, Fluvial Sedimentology, Canadian Society of Petroleum Geologists Memoir 5, 597-604. Nicholson, S.W., Shirey, S., Schulz, K., Green. J., 1997. Riftwide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences, 34, 504-520. Paces, J.B., and Miller, J.D, Jr., 1993. Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota; geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagnetic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research, B, Solid Earth and Planets, vol.98, no.8, pp.13,997-14,013. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on paleoproterozoic shallow-water iron formation accumulation, Gogebic Range, Wisconsin, U.S.A. Sedimentology, v. 54, p. 791-808. Shirey, S., Lewin, K., Berg, J., and Carlson, R., 1994. Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario, Canada. Geochimica et Cosmochimica Acta, 58, 4475-4490. - 70 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 4 - Geology and rare element pegmatites of the Quetico Subprovince near Nipigon Mark Smyk Ontario Geological Survey, Resident Geologist’s Program, Thunder Bay, Ontario, Canada Stephen Kissin Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Overview The Quetico Subprovince of the Superior Province is situated between the Wabigoon and Wawa volcanoplutonic subprovinces that bound the Quetico on its northern and southern margins, respectively. This easttrending subprovince has a fairly consistent width of 70 km and consists predominantly of metasedimentary rocks and their migmatitic and anatectic derivatives (Williams, 1991). In general, the boundaries of the Quetico, whether or not they may be primary and/or tectonic, have been mapped as steeply dipping surfaces across which there is commonly a distinct contrast in lithology. In northwestern Ontario, the Quetico has been defined to exclude mafic metavolcanic rocks and derived migmatites; these rocks are instead assigned to the bounding Wawa and Wabigoon subprovinces. In contrast, Seemayer (1992) also described an asymmetric metamorphic grade distribution that had metamorphic grade increasing from south to north across the Quetico, southwest of Lake Nipigon. The southern margin was characterized by greenschist-facies rocks, the central portions were at amphibolite facies and the northern margin displayed an abrupt decrease in grade adjacent to the Wabigoon Subprovince to the north. The Quetico Fault, which is normally situated at the northern margin of the Quetico, lies well within the Quetico southwest of Lake Nipigon (Seemayer, 1992). Seemayer (1992) determined temperatures based on garnet-biotite thermometry ranging from 526°C at the southern margin of the Quetico Subprovince, increasing asymmetrically to a maximum of 714°C, then falling sharply at the northern margin to 517°C. An overview of the lithologic, metamorphic, structural and tectonic characteristics of the Quetico Subprovince has most recently been provided by Easton (2000): “The intensity of metamorphism varies within the subprovince, such that rocks marginal to the subprovince tend to be at lower grade than in the interior. The lowest metamorphic grade is found along the northern boundary with the Wabigoon subprovince (Pirie and Mackasey, 1978). Locally, subgreenschistto greenschist-facies rocks occur along the southern boundary (Borradaile, 1982), but typically, there is a rapid rise in metamorphic grade north of the Wawa subprovince, especially north of Manitouwadge, where a belt of metasedimentary granulites occurs within the Quetico subprovince close to, and parallel with, the northern margin of the Wawa subprovince (Coates, 1968; Williams and Breaks, 1989, 1990; Pan et al., 1994). As a result, grade distribution is asymmetrical, with the maximum in temperature and pressure occurring south of the central Quetico, locally coincident with the southern margin.” 4 Figure 1. Field trip stop locations. - 71 - I �Proceedings of the 51st ILSG Annual Meeting - Part 2 _______ ________ GeneaI geology of the Quetico Subprovince near Nipigon (after Williams 1991) Key to Locations: 7. Jean lake B. Lake HSen II. Georgia Lake 50km ._— Alkalic rrptexes Y to Paleoproterozoic ver and intrusions; Pharierozoic ver Moso Trerd of regional S fabric Sttatigrapliic fadng Fault arrows iricate MusooVe-beaiing granffio raCKs (peralumir,ous). may indude slip sense, baths on upthrow &de. if mown biothe granite S yncline, Massive granadiodte to granite; unsubdivided, mainly biotfte beang antdiie Gneis&c and foliated tonalite wiles Metasedinientary rocks: wacke, paragneiss, granulite; minor mac units Wabigoon, Wawa and Abitibi subprovincos Figure 2. Generalized regional geology (after Williams, 1991). Pressure calculated at near peak temperature was 5 ± 1.5 kbar. Easton (2000) described the regional metamorphic conditions and metamorphic history: “P–T conditions increase from west to east, for example, 500ºC and 2.5 kbar at the Minnesota border west of Thunder Bay (Percival, 1989), to 700–780ºC and 5.4–6.1 kbar adjacent to the Kapuskasing structural zone (Percival and McGrath, 1986; Percival, 1989). Typical conditions in the central region are on the order of 620ºC and 3.3 kbar (Percival, 1989). Granulites north of Manitouwadge yield 680–770ºC and 4.4–6.4 kbar (Pan et al., 1994; Percival, 1989). The regional variation in P–T can be ascribed to a relatively shallow level of erosion in the west (<10 km) and a deeper level in the east (>12 km) (Percival et al., 1985). Rocks located east of the Kapuskasing structural zone are believed to be generally at upper-amphibolite-facies conditions (Williams, 1991).” “Evidence for an earlier, medium-pressure, low-temperature, pre-tectonic or early syntectonic metamorphism comes from four areas within the subprovince. In the Atikokan region, and in northern Minnesota, both at the northern margin of the Quetico subprovince, an early M1 metamorphic peak between D1 and D2 produced Ky–St–Bt assemblages (Ayres, 1978; Tabor et al., 1989). Kyanite inclusions in plagioclase within Grt–Sil–Bt–Pl–Qtz schist near Raith, north of Thunder Bay, have been reported by Percival et al. (1985). Kehlenbeck (1976) also presented textural evidence for a polymetamorphic history along the northern margin of the Quetico subprovince north of Thunder Bay. Again, along the northern margin of the subprovince, in the Beardmore–Geraldton area (Williams, 1989), amphibolite-facies conditions were attained prior to D2 deformation, with the distribution of facies being structurally controlled within thrustbounded panels (Williams, 1991).” “In contrast, the main phase of regional metamorphism (M2), which produced the observed - 72 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 map-pattern (Fig. 2), occurred late syntectonically (Sawyer, 1983; Williams, 1991). The general sequence of isograds, based on the appearance of diagnostic assemblages in pelites, is Chl–Ms–Bt, Grt+And+Sil, Grt+Crd+Sil, in situ granitic leucosome, and Opx (Pirie and Mackasey,1978; Percival and Stern, 1984). The common occurrence of Grt–And in metapelites in the western Quetico subprovince is diagnostic of bathozone 2 (<3.4 kbar; Carmichael, 1978), whereas the presence of Sil–St in the eastern Quetico is diagnostic of bathozones 3 and 4 (3.4–5.5 kbar).” “As noted by Williams (1991), tectonic thickening of the sedimentary pile and intrusion of minor Itype granitic rocks occurred prior to the thermal acme. Most of the large pre- to syntectonic granitic bodies are peraluminous and have sedimentary sources but display little evidence of thermal contact metamorphism; one exception is the South Beatty Lake pluton in the northern Quetico subprovince (Pirie & Mackasey, 1978). Steeply dipping thermal gradients, local increases in temperature around large plutons, and the general association of the highest-grade rocks with abundant generation of leucosome, indicate that the source of heat was a combination of burial, upward magmatic transport, and tectonism.” “In the northern Quetico, M1 metamorphism is estimated to have occurred between 2698 Ma, the maximum age of sedimentation (Davis et al., 1990) and 2688+4 Ma, the age of emplacement of the late syntectonic Blalock pluton (Davis et al., 1990). In the southern Quetico, M1 occurred after 2690 Ma, the maximum age of sedimentation (Zaleski et al., 1999). The timing of M2 metamorphism is less well constrained, and may have been protracted. In the Manitouwadge area, Zaleski et al. (1999) constrained regional D2 deformation to 2680–2677 Ma, and suggested that migmatization in both the northern Wawa and southern Quetico subprovinces occurred after 2679 Ma, broadly coincident with D3 deformation. This inference is consistent with observations elsewhere in the Quetico that contact aureoles around late plutons, dated at 2671+2 to 2665+2 Ma, as well as late granitic pegmatites dated at 2653+4 Ma, overprint regional metamorphic fabrics (Percival and Sullivan, 1988; Percival, 1989). North of Manitouwadge, Pan et al. (1998) reported a U–Pb zircon age of 2666+1 Ma from a granitic pegmatite concordant with respect to the D3 fabric, and suggested that the regional amphibolitefacies metamorphism occurred between 2671 and 2665 Ma, consistent with the ages cited above. The timing of peak granulite-facies metamorphism north of Manitouwadge appears to be some 15 Ma younger, on the basis of U–Pb zircon ages of 2650+ and 2651+3 Ma from a mafic granulite and a tonalitic leucosome, respectively (Pan et al., 1998). Zaleski & van Breemen (1997) reported that titanite ages young with increasing metamorphic grade, ranging from ~2686 Ma in the southern Manitouwadge greenstone belt to ~2640 Ma in the southern Quetico, suggesting that the thermal effects of regional metamorphism may have lasted over ~30 million years, from 2677 to 2640 Ma, in the higher-grade parts of the Wawa and Quetico subprovinces. On the basis of their regional geological and geochronological studies, Zaleski et al. (1999) concluded that “M2 metamorphism occurred after the tectonic juxtaposition of the Quetico and Wawa subprovinces.” This field trip will cover the southern two-thirds of the Quetico Subprovince, from south of Beardmore to Nipigon (Fig. 1). As mentioned above, there is an asymmetric distribution in metamorphic grade, with a gradual progression from greenschist-facies, clastic metasedimentary rocks near the northern contact with the Wabigoon Subprovince; to lower amphibolite-facies schists and gneisses; through to upper amphibolitefacies migmatites and derived granitic rocks near the southern contact with the Wawa Subprovince near Nipigon. Thermal and pressure maximum occurs south of the center of the Quetico. The metamorphic character is of high-temperature/low-pressure (Abukuma-type) metamorphism, associated with the abundant intrusion of granitoid rocks and the regional distribution of migmatites derived from the metasedimentary rocks (Kamineni et al., 1988; Percival and McGrath, 1986; Percival, 1989; Williams, 1989). Peak metamorphic assemblages in the field trip area suggest conditions >650ºC and 5 kbar, corresponding to bathozones 4 to 5 (14 to 15 km deep in the crust) (Easton, 2000; Percival, 1989). The nomenclature of Mehnert (1968) has been used in describing migmatites in this field guide. Overview of the Georgia Lake Pegmatite Field The Georgia Lake pegmatite field, as described by Pye (1965) and Mulligan (1965) represents the largest concentration of rare element mineralization in the - 73 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 1. Estimated reserves for the Georgia Lake pegmatite field. Deposit Estimated reserves (tons) Average grade (% Li2O) 759,475 1.65 96,000 1.50 Conway 1,830,000 0.96 Jean Lake No. 4 (Parole) 1,689,000 1.30 261,000 1.03 Nama Creek North Zone 2,784,000 1.11 Nama Creek South Zone 1,508,332 0.96 Jackpot 2,000,000 1.09 750,000 1.38 11,677,807 1.14 Aumacho No. 1 (Brink) Aumacho No. 2 Aumacho No. 3 McVittie Vegan No. 2 (Newkirk) Total Superior Province of Ontario (Breaks et al., 2003a). It comprises 38 rare element occurrences and 10 spodumene-bearing, pegmatite deposits (total resource of 11.7 million tons grading an average of 1.14% Li2O; Pye, 1965; Table 1; Fig. 3) were discovered during an exploration rush in the 1950’s (Pye, 1956). As delineated by Breaks et al. (2003a), the field covers approximately 1200 km2 (32 km by 37 km) and is hosted by upper greenschist- to lower amphibolitefacies, clastic metasedimentary country rocks. Four types of rare element pegmatites (using the classification of Cerný (1991a) have been identified locally; Breaks +:duaoeke aatholth OpSb1::*:..:::.,11rr._ + + L Diebase dikes and Rare V.] (co"pIredfoundbype5entsu'vey) Slb1o GrOtJP. xnsobdivlded ,. be beryp ,.I,wrtonaI,t and m!gn,arnic m.tas.diftl.ntary col -tent endaver GPaOer Lake eatholith Li coumbite-t.ntaILte Lithium Peraluminous S-type graniuc rocks, ssIte,1t pi-nt grantte and pota5sc pegTlaUte r1 clastic maaethn,entary rocks io and medium maçnec oxLdeminera grade metawacke and m*tap.IIte Pet spod petahte spodvnw,.e Maflc to int.rmslate metavorcan,c rocks tour tourmalint bçOOn subprwMce Figure 3. Generalized regional geology (after Breaks et al., 2003a). - 74 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 2. Subdivisions of the Georgia Lake pegmatite field, after Breaks et al. (2003a) Pegmatite group Postagoni River Lake Jean Barbara Lake Forgan Lake Type Albite-spodumene Albite-spodumene Albite-spodumene Albite-spodumene Examples McVittie; Dunning; Nama Creek Foster; Giles; Trans Vegan; Newkirk; Point; Jackpot Forgan; Lucky Lake (Lun-Echo) Cosgrave Lake Beryl±petalite sub-type Swanson; MNW Gathering Lake Albite Newly discovered, unnamed biotite-muscovite and muscovite-biotite granite and granodiorite. et al., 2003a): 1) Beryl-type 2) Albite-spodumene-type 3) Complex-type/petalite-subtype 4) Albite-type The Georgia Lake pegmatite field was initially subdivided by Milne (1962) and then Zayachivsky (1985) and Kissin and Zayachivsky (1985) into the Northern, Central and Southern groups. More recent data and mapping conducted by Breaks et al. (2003a) has led to a revised, six-fold subdivision (Table 2). The two pegmatites that will be visited on the field trip, the Dunning pegmatite (Stop 2) and the Foster pegmatite (Stop 4), are examples of the albitespodumene-type. The Glacier Lake Batholith (GLB) is a large mass of medium- to coarse-grained, S-type granite and pegmatitic granite situated immediately south of the northern metasedimentary part of the Quetico Subprovince, extending east of Highway 11 at least 50 km. It dominantly consists of grey, medium- to coarse-grained biotite and muscovite-biotite granite, which locally contains minor tonalitic and migmatitic metasedimentary enclaves. Generally massive, this rock may display a local, weak foliation developed in 1 to 2 cm-thick, shallowly lineated veins of fibrolite, muscovite and quartz that appear to have formed during aluminous hydrothermal alteration controlled by late shearing (Breaks et al., 2003a). Gradation into __ Zayachivsky (1985) divided the felsic plutonic rocks in the Georgia Lake area into three groups (Fig. 4): —— SoRe (1) Two-mica leucogranites, occurring as a large plutonic mass south of the pegmatite field (cf. Glacier Lake Batholith) and as smaller, satellite intrusions; * T—sqs — (2) Kilgour Lake Group granitoids centered around a small gabbroic unit near Kilgour Lake; and I mon I Isiopil — 2t (3) Tonalitic sills, distributed throughout the pegmatite field. / 7 .7./ - [.L Zayachivsky (1985) suggested that the two-mica leucogranites and tonalitic sills were derived from the partial melting of metapelites and metawacke, respectively (i.e. S-type granitoids). The Kilgour Lake rocks were presumed to be the products of the fractional crystallization of a mafic magma generated in the upper mantle/lower crust (i.e. I-type granitoids). Breaks et al. (2003a) described two distinct, fertile, peraluminous, S-type granite plutons (MNW and Barbara Lake stocks) that occur within metasedimentary country rocks. They consist of virtually identical, grey, medium-grained, generally homogeneous, __ __ — — —— Kikwtk. S,S. CR -— MMflOak Figure 4. Plutonic rocks in the Georgia Lake area (after Pye, 1965; Zayachivsky, 1985; Breaks et al., 2003a). - 75 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 consanguineous pods of tourmaline-muscovite potassic pegmatite up to 1 m in diameter occurs locally. There are several episodes of S-type granitic magmatism in the GLB. Undeformed, tourmaline-cordierite or tourmuscovite dikes are the youngest and most chemically evolved units (F. Breaks, Ontario Geological Survey, personal communication, 2004). can be pursued by the identification of dispersion haloes of rare alkali metals (Li, Rb, Cs) in country rocks and some other elements such as Be, Sn and Cu. Hidden pegmatite targets in promising terrain may thus be located. Pye (1965) undertook the first lithogeochemical survey for rare elements in the aforementioned felsic plutonic rocks in the Georgia Lake area. Many anomalies, ranging from 100 to 400 ppm Li, were noted. Uranium-bearing pegmatite dykes are associated with felsic plutonic rocks in this part of the Quetico Subprovince. Several occurrences have been documented by Scott (1987), including some in the vicinity of Nipigon. White, albite-rich, biotitequartz+muscovite+apatite pegmatite dykes up to 30 m wide intrude gneissic and migmatitic rocks. Yellow and yellow green stain (secondary uranophane (Ca(UO2)2Si2O7 · 6H2O?)) characterize many of the uraniferous dykes. Uraninite appears to be the primary uranium-bearing mineral (Franklin, 1978). The Lake Helen occurrence is situated on the western shore of Lake Helen, opposite the roadside rest area on Highway 11 (Stop 7). Trenching and sampling of this occurrence in 1967 by Aggressive Mining Limited yielded values up to 0.135% U3O8 (ibid). Such dykes are considered to be the primary source of uranium that occurs in structures at or near the Archean / Proterozoic unconformity in the Nipigon Basin. Two modes of source fluid derivation may have operated during the emplacement of the Georgia Lake pegmatites (Zayachivsky, 1985). Pegmatites of the Central Group were derived by igneous fractionation of a granitic parent near Barbara Lake and emplaced as low-viscosity, volatile-rich melts in a direction away from the source area. This is supported by a fractionation trend defined most clearly by the concentration of Rb and Cs in perthitic microcline and muscovite across the Central Group. The pegmatites of the Northern Group may have been the result of direct anatexis of lithium-rich Quetico metasedimentary rocks. Breaks et al. (2003a) described the MNW pegmatite (petalite sub-type) as the most evolved of the local pegmatites, based on its geochemistry, mineralogy, including the occurrence of manganotantalite. A general rule for rare element-bearing Ontario pegmatites is that tantalum mineralization is associated with lithium mineralization in the form of spodumene and/or petalite (ibid). Prediction of the source of lithium deposits in “fertile” parental granites is a somewhat uncertain art. The essential parental granite is a siliceous, peralkaline leucogranite, but the lithium content of a prospective parental granite is not highly indicative of the presence of lithium-rich pegmatites (Cerný, 1991a). Data summarized by Stewart (1978) and more recently by Cerný (1991a) indicate that Li-contents in parental granites are typically <100 ppm. Moreover, analyses of bulk compositions of spodumene- and petalitebearing pegmatites yield a closely grouped mean of 1.53 weight % Li2O (Stewart, 1978) and range between 1.5 and 2.0 weight % Li2O (Cerný, 1991b). These data indicate that differences between spodumene- and petalite-bearing pegmatites are caused by pressure and temperature differences during crystallization rather than differences in bulk composition. Cerný (1991b) suggested that exploration for rare-element pegmatites Uraniferous Dykes Stops Field Trip Road Log Stop Locality Lake Jean stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north Gorge Creek turn-off - head east Reset Postagoni River Bridge 1 Postagoni Stock 2 Dunning pegmatite Powerline crossing 3 Quetico metasedimentary rocks 4 Foster pegmatite Lake Helen stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north 5 Glacier Lake Batholith - 76 - km 0 37.4 12.4 14.4 18.4 19.1 28 28 0 17.1 �Proceedings of the 51st ILSG Annual Meeting - Part 2 6 7 8 9 10 11 Biotite leucogranite Migmatite - roadside rest area Pull-off area Pegmatitic granite Pegmatitic granite Pegmatites in migmatite Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 17 east Migmatites (optional) rocks and display no foliation nor significant internal variation. An U-Pb zircon age of 2684.8 + 2.1 Ma was determined for the Postagoni Stock (D. Davis, University of Toronto, unpublished data). 10.8 7.4 5.9 5.2 4.85 4.6 Stop 2 - Dunning Pegmatite UTM coordinates - 0427823E 5474882N 0 18.4 Stop 1 - Postagoni Stock UTM coordinates - 0425041E 5472869N Large, locally derived boulders of leucotonalite of the Postagoni Stock (a.k.a. Postagoni Lake sill) occur on the eastern side of the forest access road. Pye’s (1965) mapping identified a porphyritic granitic intrusion approximately 1.6 km long and 700 m wide in metasedimentary country rocks. The McVittie pegmatite cuts the stock north of Dive Lake; it was visited during an ILSG field trip in 1990 (Kissin, 1990). Both the McVittie and Dunning (Stop 2) pegmatites are part of the Northern Group (Zayachivsky, 1985) or Postagoni Lake Group (Breaks et al., 2003a), comprising unzoned albite-spodumene-type pegmatites. This small roadside outcrop exposes the Dunning pegmatite dyke, which has an estimated width of 2 to 3 m at this location and intrudes Quetico metasedimentary rocks. Woolverton (1956) noted that pegmatite dykes on the Dunning property were exposed intermittently in a zone extending over 600 m in length. This dyke is part of Zayachivsky’s (1985) Northern Group, which are characterized by medium- to very coarse-grained, perthitic microcline and spodumene in a fine-grained matrix of quartz, muscovite and albite, with trace amounts of apatite and garnet. They are typically unzoned to poorly zoned and commonly contain aplitic stringers, bands and pods, within and parallel to dyke contacts. This dyke is a typical albite-spodumene-type pegmatite (Fig. 5). Green spodumene crystals, up to 4.5 cm long, are aligned perpendicular to the dyke Zayachivsky (1985) described this sill as a coarsegrained tonalite with closely packed crystals of plagioclase up to 1.5 cm long, locally imparting a porphyritic texture. This texture occurs throughout the sill except near its contacts with the metasedimentary country rocks where a several metre-wide chilled margin has developed. The chilled margin texture resembles that of the porphyry dykes described by Pye (1965). A modal analysis of the Postagoni Stock was given by Zayachivsky (1985; Table 3). Table 3. Modal analysis of the Postagoni Stock. Mineral Plagioclase K-feldspar Quartz Muscovite Biotite Epidote Zircon Modal abundance(%) 66.6 5.6 25.7 0.4 1.6 0.1 Trace The Postagoni Stock and the other local tonalitic sills are invariably in sharp contact with the country Figure 5. Drusy spodumene, Dunning pegmatite (Stop 2). - 77 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 ___ ______________________ ____________ -2 _______________ Logging Camp (abandonedi - Sketch Map of the Foster Pegniatite area Amieno Resourc.s Inc. 19B7) STOP 3 nd // / •/./ /// STOP4 of the submarine margin and trench-fill are present in the Quetico (Fralick et al., 1992; Williams, 1989). 10 to 20 cm thick sandstone-siltstone beds strike 100º and likely dip steeply to the south, based on observations in the area by Pye (1965). Graded bedding suggest tops to the south (Fig. 7). Pye (1965) noted that biotitequartz-feldspar schist such as this are most commonly granoblastic; garnet porpyhryoblasts have also been noted. Some lenticular pods of quartzo-feldspathic leucosome are present. Stop 4 - Foster Pegmatite 9 a 100ut1 UTM coordinates - 0436379E 5473665N .---—----—Metasedimentary rocks Figure 6. Sketch map of Foster pegmatite area (Stops 3 & 4). contact. This phenocrystic spodumene typically contains several percent iron and is commonly altered to a dark green to black alteration product of chlorite and sericite (aka “rotten spodumene”). Some altered, brown spodumene is also present, along with quartz, albite (+cleavelandite), beige alkali feldspar, green muscovite and minor oxides. Pye (1965) reported that the lithia contents of the two dykes on the Dunning property were low and of no economic importance. Stop 3 - Quetico Metasedimentary Rocks UTM coordinates - 0436345E 5473931N Medium-grade clastic metasedimentary rocks typical of the north-central part of the Quetico Subprovince are exposed in a small outcrop north of the Foster pegmatite (Fig. 6). South of the Beardmore-Geraldton belt, more highly metamorphosed, turbiditic sediments A series of stripped and washed outcrops reveals a swarm of narrow pegmatite dykes cutting an easttrending sill of massive, biotite tonalite, termed the Parole Lake sill by Zayachivsky (1985; Fig. 8). The Foster deposit was discovered in 1956 and was tested by diamond drilling later that year by Towagmac Exploration Co. Ltd. (Walter, 1957; Pye 1965; Table 4). This work indicated that the lithium deposit was too low-grade and the property was allowed to lapse. Armeno Resources Inc. explored the property (including the neighbouring Lew deposit) from 1986 to 1989, carrying out stripping, geological, magnetometer and electromagnetic surveys, as well as diamond drilling (Assessment Files, Thunder Bay North District, Thunder Bay). The lone drill hole intersected biotite tonalite with numerous quartz-rich, pegmatitic dykes < 1m thick with up to 5% spodumene; no chemical analyses were reported (ibid). The main Foster dyke strikes approximately 085º and dips 80º to 85º south. It is exposed continuously 1 F? 'r--- r-- r - t'r Figure 7. Graded bedding in metaturbidites, Foster area (Stop 3). Figure 8. Pegmatite dykes in tonalite, Foster (Stop 4). - 78 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Table 4. Results of drilling at the Foster deposit (Walter, 1957). Drill Hole Number 7 6 9 Location (feet) 3+50 west 4+50 west 6+50 west 10 7+50 west Core length (feet) 29.0 30.0 31.3 20.0 32.7 over 75 m and averages 9 m in thickness. To the west, it splits into a number of thin, parallel dykes. In the most westerly drill hole (T-10), 7 individual dykes, 0.3 to 2.4 m wide were intersected, separated by sections of host tonalite a few centimetres to a metre wide (Pye, 1965). The deposit contains 10 to 15% spodumene, which is for the most part, unaltered. Similar to other deposits in the Lake Jean Group, crystals are prismatic and perpendicular to dyke contacts and may extend across the entire dyke (ibid). Milne (1962) listed the Foster pegmatite in the Jean Lake sub-group (with the Giles, Camp and Lew pegmatites) on the basis of welldeveloped spodumene orientation and the occurrence of narrow, branching and continuous, longitudinal aplite dykes. Lithia Content (%) 0.26 0.55 1.04 0.58 0.58 Large road cuts on both sides of Highway 11 provide excellent exposures of the Glacier Lake Batholith (GLB). At this location, it consists of white, massive to locally foliated, muscovite < biotite, medium- to coarsegrained granite. Localized pods of tourmaline-biotitemuscovite-potassium feldspar pegmatite occur within the granite. These pods may reach 1 m in diameter and locally contain tourmaline-quartz intergrowths. Numerous, curvate, fibrolite - muscovite + tourmaline veins up to 1 cm thick also occur in the host granite. Purple fluorite is exposed on a fractured outcrop face on the west side of the highway (Fig. 10). Stop 6 - Glacier Lake Batholith Leucogranite/ M igmatite UTM coordinates - 0407163E 5440408N This is another example of the southern margin of the Glacier Lake Batholith where it is in contact with metasedimentary migmatite (Fig. 11). White, biotite-muscovite leucogranite contains rare, bluegreen apatite. Foliation is developed in feldspathic segregations and biotitic seams. Leucogranite dykes and migmatite locally exhibit a lit-par-lit structure. Tight to isoclinal folds have developed in the quartzbiotite-feldspar schist (Fig. 12). All rocks display boudinage and folding. Folded leucosome suggests a Figure 9. Drusy spoduemene, Foster pegmatite (Stop 4). These are classic albite-spodumene-type pegmatites. Fresh green to dark green-black, altered spodumene crystals may reach lengths of 8 cm (Fig. 9). Other constituent minerals include K-feldspar, albite (+ cleavelandite) and quartz, minor apatite and garnet, and tiny, sparsely distributed, black tantalum-niobiumbearing oxide minerals (F. Breaks, OGS, personal communication, 2004). A locally developed border zone (< 5 cm) consists of quartz + feldspar. Stop 5 - Glacier Lake Batholith Leucogranite UTM coordinates - 0409911E 5445897N Figure 10. Fluorite on fracture surface in sericite- and fibrolite-bearing Leucogranite (Stop 5). - 79 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 r '2..—:, 4 A- I.— A Figure 11. Contact between folded metasedimentary migmatite and Leucogranite (Stop 6). Figure 13. Folded metasedimentary schist, incipient leucosome development (Stop 6). an early and protracted deformation history (Fig. 13). Metasedimentary migmatite enclaves are common in the white, S-type pegmatitic granites along the highway. Note that less evolved S-type granite may contain biotite as the only mica. whaleback outcrop behind the highway rest stop/ pull-off area. There is quite a bit of variation in the relative amounts of leucosome and restite (Fig. 14). Boudinaged and pytgmatically folded leucosome pods and veins occur in a quartzo-feldspathic with narrow, biotitic restite septa. Narrow diabase dykes cut the country rocks on the western side of the highway. Stop 8 - Pegmatitic Granite UTM coordinates - 0407272E 5435208N Stop 7 - Migmatite UTM coordinates - 0406986E 5437331N Migmatites are exposed on the shore and islands of Lake Helen and on a large, glacially streamlined, Figure 12. Folded metasedimentary migmatite (Stop 6). This outcrop shows a transition in magmatism from S-type (i.e. Glacier Lake Batholith) to I-type granitoids. Pink, coarse-grained, biotite granite here consists of pink, coarse-grained, perthitic K-feldspar, quartz, coarse-grained biotite, and brown, altered plagioclase (up to 4 cm long). These weakly peraluminous, pegmatitic, biotite granites are relatively primitive and are younger than the white, two-mica leucogranites. Such rocks are probably of I-type origin and typically are metasedimentary enclave-free. Bulk rock levels of Figure 14. Lit-par-lit migmatites with large metasedimentary inclusions (schollen) (Stop 7 area). - 80 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 • [j\ ti'a( Figure 15. Pegmatitic K-spar - quartz – biotite granite (Stop 8). rare-elements are very low: 128 ppm Rb, 2.63 ppm Cs, 1.8 ppm Nb, 0.39 ppm Ta (F. Breaks, OGS, personal communication, 2004). A K-feldspar-megacrystic, pink biotite granite (Fig. 15) with a shallowly east-dipping foliation and metasedimentary xenoliths is intruded by a more massive granite at this locality. The intrusive contact is embayed and scalloped, suggestive of co-mingling magmatic textures (Fig. 16). The megacrystic, foliated granite may represent an earlier, xenolith-bearing phase that was intruded by the massive granite while still relatively warm and plastic. Stop 9 - Pegmatitic Granite UTM coordinates - 0407458E 5434906N feldspar megacrysts may attain lengths of over 60 cm. Stop 10 - Pegmatite Dykes in Migmatite UTM coordinates - 0407587E 5434690N Approximately 200 m south of the massive pink granite, white pegmatite dykes intrude fine-grained metasedimentary schist. This dark, fine-grained, feldspathic, biotite schist displays a well-developed foliation and minor folds, indicative of a long, protracted ductile deformation history, perhaps coeval with dyke emplacement. Note the sub-horizontal mineral lineation. Flattening of feldspars, producing small-scale, augen structures, is also indicative of hightemperature (>500º C) deformation. The boudinaged and annealed, sericite- and biotitebearing dykes range from a few centimetres (lit-parlit structured) to several metres in thickness. They are mineralogically and texturally interesting, containing Lt- ! H -v A large, bare, whaleback outcrop on the east side of the highway consists of coarse-grained to pegmatitic, massive, homogeneous pink granite (i.e. the younger granitic unit at Stop 8; Fig. 17). Crystals or interstitial patches of quartz, biotite and locally sericitized, Kfeldspar average 2 to 3 cm in size (Fig. 18). Individual Figure 17. “Whaleback” outcrop of coarse-grained to pegmatitic granite; note large (>60 cm) feldspar megacryst (circled) (Stop 9). Figure 16. Scalloped contact (arrow) between foliated biotite granite (top) and pegmatitic granite (bottom) (Stop 8). Figure 18. Coarse-grained to pegmatitic granite (Stop 9). - 81 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 both cordierite and quartz-tourmaline intergrowths with alkali (locally sericitized) feldspar. Garnet is conspicuous by its absence. metamorphic rocks. John Scott (Ontario Geological Survey) generated maps for this report and provided information on local uranium mineralization. I 1 Sf C Breaks et al. (2003b) described potassic, biotite pegmatite that grades into a medium-grained, biotite granite near this stop (UTM: Easting 408339; Northing 5433016). The pegmatite contains coarse, euhedral K-spar; quartz; prismatic, medium- to coarse-grained, black tourmaline (schorl-dravite; Fig. 19); and finegrained, green and blue fluor-apatite (0.3 to 0.9 weight % MnO). A biotite-rich, metasomatic contact occurs between the biotite granite and diabase. Graphic, coarse-grained cordierite (< 2 cm) and tourmaline occur in the granite near a diabase contact. Figure 20. Schollen structure in migmatite (Stop 11). References Ayres, L.D., 1969. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield; Geological Survey of Canada, Paper 78-10, p.25-36. Ayres, L.D. 1978. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p.25-36. Figure 19. Quartz-tourmaline intergrowth in pegmatite dyke (Stop 10). Stop 11 - Migmatites (Optional) UTM coordinates - 5429614N 0425006E This optional stop displays schollen (raft)-structured migmatites in which folded and schlieric, mafic, paleosome xenoliths have been highly deformed and disrupted (Fig. 20). There is a relatively high proportion of quartzo-feldspathic neosome matrix to the xenoliths, suggesting high(er) degrees of partial melting. Patches and dykelets of coarse-grained, biotite-quartz-feldspar neosome contain garnet, cordierite and large (>10 cm), euhedral green apatite crystals. Acknowledgements Dr. Fred Breaks (Ontario Geological Survey) is thanked for the provision of unpublished data on pegmatites and host rocks. Dr. Mary Louise Hill (Lakehead University) provided useful insight into the metamorphic and structural history of the high-grade Borradaile, G.J. 1982. Comparison of Archean structural styles in two belts of the Canadian Superior Province; Precambrian Research 19, p.179-189. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003a. Fertile peraluminous granites and related rare-element pegmatite mineralization, Barbara-Gathering-Barbaro lakes area, north-central Ontario; Summary of Field Work and Other Activities, 2003, Ontario Geological Survey, Open File Report 6120, p.14-1 to 14-13. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003b. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; , Ontario Geological Survey, Open File Report 6099, 179p. Carmichael, D.M., 1978. Metamorphic bathozones and bathograds: a measure of the depth of postmetamorphic uplift and erosion on the regional scale; American Journal of Science, 278, p.769-797. Cerný, P., 1991a. Rare-element granitic pegmatites, part I. Anatomy and internal evolution of pegmatite deposits; Geoscience Canada, v.18, p.49-67. Cerný, P 1991b. Rare-element granitic pegmatites. Part II: - 82 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Regional to global environments and petrogenesis. Geoscience Canada v.18, p.68-81. north of Manitouwadge, Ontario; Canadian Journal of Earth Sciences, v.31, p.1427-1439. Coates, M.E., 1968. Stevens–Kagiano Lake area; Ontario Department of Mines, Geological Report 68, p. Pan, Y., Fleet, M.E. and Heaman, L.M., 1998. Thermotectonic evolution of an Archean accretionary complex: U–Pb geochronological constraints on granulites from the Quetico subprovince, Ontario, Canada; Precambrian Research,v.92, p.117-128. Davis, D.W., Pezzutto, F. & Ojakangas, R.W., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province; Earth and Planetary Science Letters,v.99, p.195-205. Easton, R.M. 2000. Metamorphism of the Canadian Shield, Ontario, Canada I: The Superior Province; The Canadian Mineralogist, v.38, p.287-317. Fralick, P., Wu, J., and Williams, H.R., 1992. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield. Canadian Journal of Earth Sciences, 29: 2551–2557. Franklin, J.M., 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario; Current Research, Part A, Geological Survey of Canada, Paper 78-1A, p.275-282. Kamineni, D.C., Stone, D. and Johnston, P.J. 1988. Metamorphism of Quetico sedimentary rocks near Atikokan, Ontario; in Program with abstracts, Geological Association of Canada-Mineralogical Association of Canada-Canadian Society of Petroleum Geologists Annual Meeting, v.13, p.A63. Percival, J.A and Stern, R.A., 1984. Geological synthesis in the western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 84-1A, p.397-407. Percival, J.A and Sullivan, R.W., 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 88-2, p.97-108. Percival, J.A., 1989. A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada; Canadian Journal of Earth Sciences,v.26, p.677-693. Percival, J.A. and McGrath, P.H., 1986. Deep crustal structure and tectonic history of the northern Kapuskasing uplift of Ontario: an integrated petrological-geophysical study; Tectonics,v.5, p.553-572. Percival, J.A., Stern, R.A. and Digel, M.R., 1985. Regional geological synthesis of western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.385-397. Kehlenbeck, M.M., 1976. Nature of the Quetico–Wabigoon boundary in the De Courcey – Smiley Lakes area, northwestern Ontario; Canadian Journal of Earth Sciences, v.13, p.737-748. Pirie, J. and Mackasey, W.O., 1978. Preliminary examination of regional metamorphism in parts of Quetico metasedimentary belt, Superior Province, Ontario; Geological Survey of Canada, Paper 78-10, p.37-48. Kissin, S.A., 1990. Granitoid-related mineral deposits of the western Lake Superior region; 36th annual Institute on Lake Superior Geology, Thunder Bay, Field Trip Guidebook, p.52-66. Pye, E.G., 1956. Lithium in northern Ontario; Canadian Mining Journal, v.77, p.73-75. Kissin, S.A. and Zayachivsky, B., 1985. Genesis of pegmatites in the Quetcio gneiss belt of northwestern Ontario: rare-element pegmatites and associated granitoids of the Georgia Lake pegmatite field; in Geoscience Research Grant Program, Summary of Research 1984-1985, Ontario Geological Survey, Miscellaneous Paper 127, p.186-199. Mehnert, K.R., 1968. Migmatites and the origin of granitic rocks. Elsevier. 405p. Milne, V.G., 1962. The petrography and alteration of some spodumene pegmatites near Beardmore, Ontario; unpublished Ph.D. thesis, University of Toronto, Toronto, Ontario, 242p. Mulligan, R., 1965. Geology of Canadian lithium deposits; Geological Survey of Canada; Economic Geology Report 21, 131p. Pan, Y., Fleet, M.E. and Williams, H.R., 1994. Granulitefacies metamorphism in the Quetico subprovince, Pye, E.G., 1965. Geology and lithium deposits of the Georgia Lake area, District of Thunder Bay; Ontario Department of Mines, Report 31, 113p. Sawyer, E.W., 1983. The structural history of a part of the Archean Quetico metasedimentary belt, Superior Province, Canada; Precambrian Research,v.22, p.271-294. Scott, J.F., 1987. Uranium occurrences of the Thunder Bay – Nipigon – Marathon area; Ontario Geological Survey, Open File Report 5634, 158p. Seemayer, B.E., 1992. Variations in metamorphic grade in metapelites in transects across the Quetico Subprovince north of Thunder Bay, Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, 163p. Stewart, D.B., 1978. Petrogenesis of lithium-rich pegmatites. American Mineralogist, v.63, p.970-980. Tabor, J.R., Hudleston, P.J. and Magloughlin, J., 1989. Metamorphism of the Quetico supracrustals north of - 83 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 the Vermillion granitic complex, northern Minnesota; Geological Association of Canada – Mineralogical Association of Canada, Program with Abstracts, v.14, p.A38. Walter, J.P. 1957. Towagmac Explorations Ltd., Reoprt on exploration program, 1956 season; unpublished report, Assessment Files, Thunder Bay South District, Thunder Bay, 3p. Williams, H.R., 1989. Geological studies in the Wabigoon, Quetico, and Abitibi–Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724, p. Williams, H.R., 1991. Quetico subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, v.1, p.383-403. Williams, H.R. and Breaks, F.W., 1989. Geological studies in the Manitouwadge – Hornepayne area; Summary of Field Work and Other Activities 1989, Ontario Geological Survey, Miscellaneous Paper 146, p.7991. Williams, H.R. and Breaks, F.W., 1990. Geological studies in the Manitouwadge – Hornepayne area; Summary of Field Work and Other Activities 1990, Ontario Geological Survey, Miscellaneous Paper 151, p.4751. Woolverton, R.S. 1956. Report on McVittie lithium option, Noranda Exploration Co. Ltd.; unpublished report, Assessment Files, Thunder Bay South District, Thunder Bay, 6p. Zaleski, E. and van Breemen, O., 1997. Age constraints on plutonism, metamorphism and deformation across the Wawa–Quetico subprovince boundary near the Manitouwadge greenstone belt, northeastern Ontario; Institute on Lake Superior Geology, Program with Abstracts, v.43, p.67-68. Zaleski, E., van Breemen, O. and Peterson, V.L., 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa–Quetico subprovince boundary, Superior Province, Ontario, constrained by U–Pb zircon dates of supracrustal and plutonic rocks; Canadian Journal of Earth Sciences, v.36, p.945-966. Zayachivsky, B., 1985. Granitoids and rare-element pegmatites of the Georgia Lake area, northwestern Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 234p. - 84 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Fieldtrip 5 - Geology of the Black Sturgeon Area Thomas R. Hart Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury, Ontario, P3E 6B5, Canada Introduction The Nipigon Embayment is an approximately 19,000 km2 area of Mesoproterozoic igneous and sedimentary rocks centred on Lake Nipigon, north of Lake Superior, approximately 100 km northeast of Thunder Bay, Ontario (Fig. 1). The Embayment is underlain by Archean rocks of the Wabigoon Subprovince to the north, and the Quetico Subprovince to the south. This field trip will be concentrating on the Proterozoic and underlying Archean rocks of the southern portion of the Embayment. u nI_u • nil nuN UU The first geological work in the Lake Nipigon area was conducted by Bell (1870) for the Geological Survey of Canada. Portions of the area were examined by McInnes (1896), Parks (1901), Coleman (1909), and Wilson (1910). Geological mapping by Coates (1972) covered the area south of Lake Nipigon at a scale of 1:63,360. The adjacent area to the south was mapped Figure 1. Mafic and ultramafic intrusions of the Nipigon Embayment. by McIlwaine and Tihor (1975a,b) at 1:15,840 scale. The area surrounding Lake Nipigon, and extending to the southern edge of Black Sturgeon Lake, was mapped at a scale of 1:50 000 by Sutcliffe (1982a,b; 1985a,b,c). Sutcliffe (1986) completed a doctoral thesis on the intrusive rocks of the Lake Nipigon area, including the Nipigon diabase sills and ultramafic intrusions of the Disraeli and Seagull-Leckie Lakes areas. Franklin et al. (1980), Cheadle (1986), and Rogala (2003) have carried out detailed investigations of the stratigraphy of the Sibley Group. Bedrock mapping and research studies were undertaken between 2003 and 2005 as part of the Lake Nipigon Region Geoscience Initiative (LNRGI). The LNRGI is a geoscience-based geological data acquisition and compilation program managed by the Ontario Prospectors Association (OPA) and financially supported by the Northern Ontario Heritage Fund Corporation (NOHFC). The major goals of the initiative are to conduct research and collect new geoscience data that will help to explain the geological history of the area, and identify areas of high mineral potential that will attract exploration to the region. The initiative included bedrock mapping projects (MacDonald, 2004; MacDonald and Tremblay, 2005), airborne magnetic and radiometric geophysical surveys (Ontario Geological Survey, 2004a), ground gravity surveys (Ontario Geological Survey, 2004b), and targeted surficial geochemical and lineament studies (e.g., Barnett, 2004; Dyer, 2004), a paleomagnetic study (e.g., Ernst et al., 2005) and geochronological studies (e.g., Heaman et al., 2005). Mapping of the southern portion of the Nipigon Embayment was completed by the Ontario Geological Survey as part of its commitment of in-kind support to the LNRGI (Hart and Magyarosi, 2004; Hart, 2005a). Lakehead University, as a partner in the Initiative, contributed a detailed sedimentary basin reconstruction (e.g., Metsaranta and Fralick, 2004) and petrochemical characterization of the mafic and ultramafic intrusions (e.g., Heggie and Hollings, 2004; Richardson and Hollings, 2005). - 85 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Mineral exploration and prospecting in the area has been sporadic with the first recorded activity being the exploration for iron ore in the early 1900s (Coates, 1972). Exploration in the early 1900s also resulted in discovery of lead and zinc mineralization to the southwest, and limited production (Shklanka, 1969). The discovery of sporadic copper mineralization near Disraeli Lake in 1965 resulted in a surge in copper and the other base metals in the late 1960s (Coates, 1972). Exploration for uranium has been cyclical, with the greatest activity occurring in the middle to late 1970s coinciding with the discovery of large unconformity related deposits in other locations including the Wollaston Basin of Saskatchewan (Scott, 1987). A uranium occurrence was discovered by R.H. Sutcliffe during the 1981 field season and subsequently acquired and explored by Uranerz Exploration and Mining Corporation Limited until 1985 (Scott, 1987). A number of companies are actively exploring for uranium mineralization at the time this guide was being prepared. Several companies investigated the potential of sedimentary-hosted base metal mineralization in the Sibley Group in the early 1990s, but other than an airborne geophysical survey completed by Cominco in 1993, little of the work was filed for assessment. Exploration for PGE mineralization intensified after the 1998 discovery of mineralization in the Seagull/Leckie Lake intrusion (Osmani and Rees, 1998). Initial results of diamond drilling indicated the presence of a basal pyroxenite hosting up to 1.71 ppm Pt and 1.87 ppm Pd over 2.1 m (Durham, 2000). Resampling completed in early 2004 defined two zones higher in the intrusion with up to 1.52 ppm Pt and 1.78 ppm Pd over 3.0 m and 1.56 ppm Pt and 1.87 ppm Pd over 3 m in the upper and lower zones respectively (East West Resource Corporation, 2004a). Drilling in late 2004 intersected a mineralized zone consisting of 2.38 g/t Pt, 2.65 g/t Pd, 0.19 g/t Au, 0.18 g/t Rh, 0.39 g/t Os, 0.34 g/t Ir and 0.08 g/t Ru (Pd:Pt ratio of 1.11) associated 0.19% Cu and 0.20% Ni over 3.21 metre (Platinum Group Metals Ltd., 2005). General Geology The southern portion of the Nipigon Embayment is underlain by Archean rocks of the Quetico subprovince that are unconformably overlain by Proterozoic sedimentary rocks of the Sibley Group (Fig. 2). Rocks of the Quetico Subprovince and the Sibley Group are io a Kilometres C N!p!g&,] UIt,amafi cIrtr(]iOns s'brey Group eotite Granite [3] Muscovite Gran!Ie FJ Faults Roads Outco Metaseth,]&nta Rocks Transmission Line Gas Pipel,ne Figure 2. Geology of the southern portion of the Nipigon Embayment. - 86 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 intruded by Proterozoic mafic to ultramafic intrusive rocks in the Disraeli and Seagull Lakes, and Hele and Kitto township areas (Fig. 2). Mafic sills of the Nipigon Diabase Sill Complex intrude all other rock types in the map area. Rocks of the Quetico Subprovince consist of biotite and/or andalusite schists that are gradually replaced towards the south by amphibolites (Hart, 2005a). The rocks to the north have a schistosity oriented at approximately 050° to 060°, with near vertical dips. In the south, leucogranites intrude the amphibolite or biotite gneisses and form complex mixtures that have been classified as migmatite. The airborne magnetic survey indicates a regional fabric with about an 080° to 090° orientation in the migmatitic rocks (Ontario Geological Survey, 2004). A number of areas east of the Black Sturgeon River contain a high percentage of coarse grained to pegmatitic irregular bodies and dykes of the muscovite granite suite. Irregular bodies of medium to coarse-grained rocks of the biotite granite suite intrude the amphibolites and leucocratic phases form the leucosome of the migmatitic rocks. Widely spaced, weakly deformed and metamorphosed, north trending diabase dikes intrude the Archean rocks of the Quetico Subprovince. A paleomagnetic study of similar dykes located to the west, in the area of Highway 527, by Ernst et al. (2005) has interpreted these dykes to be part of the early Proterozoic 2121 – 2101 Ma Marathon dyke swarm. The Proterozoic Sibley Group sedimentary rocks unconformably overlie the Archean rocks of the Quetico Subprovince, and are relatively flat-lying, with dips of less than 5° and strikes that are usually difficult to determine. Rocks within this area have been subdivided in to three relatively flat-lying formations, the lower Pass Lake, the middle Rossport, and the upper Kama Hill (e.g., Cheadle, 1986; Fig. 3). The Pass Lake Formation in this area is best observed in drill core, and consists of conglomerates, quartz arenites, and minor dolomites, with rare outcrops of the upper sandstone members. A sandstone of the Pass Lake Formation has a youngest detrital zircon grain with a U-Pb age of 1670 Ma (Heaman et al., 2005). The Rossport Formation consists of mudstone, dolomite and siltstones of the lower Channel Island Member, limestone and stromatolitic limestone of the Middlebrun Bay Member, and predominantly muddy variably carbonatized siltstones and mudstones of the upper Fire Hill Member (see Fig. 3). The Kama Hill Noo Diabass $11 Oonlsx, 1110le1113 S 2005) C,ta,I laed Fom,ton atleasI3silsha,ebeeelifle, d450 Ma Hea!,sa,s $1 at.. 2005) dykes may e*1ea let tholt dlanc.s from the sills .olpresenlirlmap area -1339 M -d€it!!]l dQslt tFrakls]197fl) __— Kane ll FoIrnallo,, mar!csl, one lntades lw el!lts!0 I Pi!a H!ll Mmb. —1ROSSPO,tFOlmalior. SwI. Hale. Dali t,ln,SlOflS. 1106— 1124 Ma (ReAmanetal.2005) Legend Dabs.. s!lI 4e H Mdlethn Say Membr Rosspefl Fomlaijo,, •P PsssLa&sF Ch..,! Id Mrsb., Met JCeflsls!]dMemt.r 1ROSSPOflFO!n,akn,. fl,eH!llMembe[ 0_ Ks H!II Flm!.reks Gs... I Pass Lake Fo,aleUon 2121-2101 MaMaralliorl dib dyks mscoe grane and I lO70Ma O*aman.tal.2005) I blotice andior andalusile ___-I hbls mpl]ibd. blSbs Oa salt.. Figure 3. Generalized stratigraphic column for the southern portion of the Nipigon Embayment. Formation is the uppermost formation of the Sibley Group exposed in the map area and consists of laminated mudstones and siltstones with sandstone prominent in the upper section. A sandstone of the Outan Island Formation, which overlies the Kama Hill Formation has a youngest detrital zircon with a U-Pb age of 1450 Ma (Heaman et al., 2005). A sandstone of the Nipigon Bay Formation, which overlies the Outan Island Formation, has a youngest detrital zircon grain with a U-Pb age of 1657 Ma (Heaman et al., 2005). A Rb/Sr age of 1339 +/- 33 Ma based on analyses of a combination of Rossport and Kama Hill formation samples by Franklin (1978), probably denotes a post-depositional diagenetic event. Cheadle (1986) estimated up to 200 m of topographic relief on the Archean paleosurface at the time of deposition of the Sibley Group. Although the bedding is generally flat-lying on an outcrop scale, these extremes in topographic relief probably controlled sedimentation processes on a local scale complicating stratigraphic correlations (P. Fralick and B. Rogala, Lakehead University, personal communication, 2003). Regionally, the Sibley Group thickens to the south under Lake Superior with two additional formations - 87 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 overlying the Kama Hill formation (Rogala, 2003), and thins to the north and west (e.g., Cheadle, 1986). may be difficult to distinguish from the diabase sills without petrography and lithogeochemistry. Mafic to ultramafic intrusions occur in the Disraeli and Seagull-Leckie Lakes, Hele and Kitto township areas (Fig. 1), and all four are composed of a pyroxene peridotite core with an irregular olivine gabbro zone along the margin (e.g., Hart et al., 2002b; Hart and Magyarosi, 2004). The Seagull Intrusion has been studied in the greatest detail, and petrography indicates that it consists predominately of cumulate textured dunites, lherzolites, and olivine websterite, with minor olivine- hornblende pyroxenite (Heggie and Hollings, 2004). An olivine gabbro forms an irregular border zone to the ultramafic rocks, and is the unit most commonly exposed in outcrop. All of the intrusions have a monzogabbro, or granophyre, unit occurring as irregular pods or bands in the olivine gabbro, but are distinguished from it by the presence of abundant pink feldspar and minor amphibole. An olivine gabbro from the Hele Intrusion has a U-Pb baddeleyite age of 1106+/-1.5 Ma with one fraction yielding a 1119.4 Ma age (L. Heaman, University of Alberta, personal communication, 2004). Samples from the Seagull Intrusion have U-Pb ages from zircon and baddeleyite of between 1105 and 1124 Ma (L. Heaman, personal communication, 2004). Samples from the Disraeli Intrusion have U-Pb ages from baddeleyite and zircon of 1111 Ma (L. Heaman, personal communication, 2004). Samples of an olivine gabbro from the Kitto Intrusion has a U-Pb baddeleyite age of 1117 Ma, but has baddeleyite fractions with a range of ages comparable to the other intrusions (L. Heaman, personal communication, 2004). Diabase sills have chill contacts with olivine gabbro of the Seagull, Hele and Kitto intrusions. A series of shallow-dipping Proterozoic diabase sills intrude all other rock units in the map area. The sills display a range of U-Pb zircon ages from 1110 to 1113 Ma from sills located west of Lake Nipigon (Heaman et al., 2005). The diabase sills have variable internal subdivisions, a generalized section from top to bottom would contain: 1) chill, 2) fine-grained variably amygduloidal, 3) magnetite-rich, 4) medium to coarse-grained, 5) coarse to very coarse-grained, 6) fine-grained, variable amygduloidal, and 7) chill (Hart and Magyarosi, 2004). These subdivisions resemble those proposed by Sutcliffe (1986). Massive, mediumto coarse-grained feldspar and pyroxene diabase is the most common rock type and generally lacks a sub-ophitic or diabasic texture. Although these rocks should properly be classified as gabbro the diabase classification has been applied to all rocks associated with the sills to avoid confusion with other intrusions in the area. The sills range in thickness from less than 5 m to greater than 180 m and there appears to be at least two major sills greater than 100 m in thickness. However, erosion and block faulting hinders the correlation of the sills and thus the determination of the original number of sills. A group of fine grained, massive mafic sills with a higher olivine content may intrude the Sibley Group sedimentary rocks. Some of these sills are spatially associated with the ultramafic intrusions, as in the Moraine Lake and Hele Township areas, and probably represent extensions of the intrusions (Hart, 2005). In the Shillabeer Lake and Kama Hill areas, similar sills are not associated with known ultramafic intrusions (e.g., Richardson and Hollings, 2005; Hart, 2004) and may be comparable to the Jackfish Sills described by MacDonald (2004) in the English Bay area. One of these sills intrudes the Disraeli Intrusion suggesting that some examples of this group post-date the larger ultramafic intrusions (Hart, 2005). This group of sills Quaternary ice-marginal deltaic deposits of the Nipigon Moraine form an extensive cover along the Black Sturgeon River and the area immediately to the west of the river (Barnett, 2004). The rest of the map area further from the river is covered by differing thickness of till that is reported by Barnett (2004) to be mainly locally derived. Regional metamorphism of the Archean rocks of the Quetico Subprovince varies from upper greenschist to lower amphibolite facies, with limited areas of migmatite. Hornfels metamorphism, in zones up to 10 m wide, is variably developed in the Sibley Group in contact with the peridotites, and occasionally the diabase sills. Hematite and specular hematite is associated with some late north-trending fracturing in both Archean and Proterozoic rocks, and appears to be a useful indicator of the north-trending fault systems in this area, where most structures are hidden by talus and glacial deposits (Hart and Magyarosi, 2004). Metasedimentary rocks of the Quetico Subprovince have experienced multiple phases of deformation. Schistosity is well developed along the bedding planes - 88 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 obscuring primary structures but the rocks appear to be the metamorphosed equivalents of the greywackes and siltstones present in the Beardmore-Geraldton greenstone belt (Hart et al., 2002a). North- and northwest-trending regional fault trends have displaced the Proterozoic Sibley Group, and a northeast-trend may only represent minor late movement along previous Archean structures (Hart and Magyarosi, 2004; MacDonald, 2004). The north and northwest trending faults, define the Black Sturgeon Fault Zone, which has been interpreted to be en échelon resulting in the formation of a block faulted asymmetric basin or graben (e.g., Coates, 1972). There appears to have been little lateral displacement based on the correlation of the iron formations across Black Sturgeon Lake in the area north of the current map area (Hart and Magyarosi, 2004). The north trending faults are interpreted to be regionally continuous structures that may correlate with north-trending structures in the Gull Bay area, approximately 150 km to the north. The northwesttrending faults display vertical movement with no apparent horizontal displacement, and may be traced to the west into the adjacent Wabigoon Subprovince. Accurate determination of displacements on these structures is not possible as they have been obscured by talus slopes and glacial deposits. north-trending and centred on Lake Nipigon. The paleomagnetic and geochemical study of the northtrending dykes located to the west of the Nipigon Embayment by Ernst et al. (2005) suggests that they are part of the 2101 to 2121 Ma Marathon dyke swarm, not Keeweenawan. If the identification of these dykes as part of the Marathon swarm is indeed correct, then combined with the lack of a north trending dyke swarm north or east of the Nipigon Embayment, a key element of the failed arm model appears to be absent. The timing of the different faults is not known, and it is probable that they have been reactivated. There have been several proposals made to account for the structural history of the area, with most focussing on the Sibley Group sedimentary rocks. Coates (1972) proposed a half graben with the Black Sturgeon River as the eastern boundary that also served as the main control on the deposition of the Sibley Group. Franklin et al. (1980) proposed that a failed arm of the Mesoproterozoic Keweenawan Midcontinent Rift created a basin, which included the half graben. Sutcliffe (1986) further developed the failed arm model as a major control on the emplacement of the peridotite intrusions and the diabase sills, and highlighted the existence of a subparallel fault system along the Nipigon River to the east. Alternatively, Fralick and Kissin (1995) and Hollings et al. (2004) proposed that the thermal upwelling that formed the English Bay Complex was followed by relaxation and formation of an intracratonic basin in which the Sibley Group was deposited. Timing and magnitude of displacement along faults related to the emplacement of either the ultramafic intrusions or diabase sills is also difficult to determine. The high degree of fracturing and hematite alteration in some of the diamond drill core from the 1106-1124 Ma Seagull Intrusion (Heaman et al., 2005) suggests post emplacement fault activity. Extensive hematite and the occurrence of uranium mineralization in north trending structures has been dated at 1090±20 Ma (Ruzicka and LeCheminant, 1984). Current interpretations suggest that faulting has resulted in displacement of igneous horizons within the Seagull intrusion, but it is just as possible that the intrusion was emplaced into a preexisting structure to produce its current form. Preexisting fault structures probably, in part, controlled the emplacement of the 1111-1113 Ma Nipigon sills (Heaman et al., 2005), as has occurred in other mafic sill complexes (e.g., Leaman, 1975). However, due to the lack of a distinctive marker in the area, it is difficult to determine if the apparently disjointed nature of many of the sills is the result of the sills ramping upward along pre-existing structures and subsequently being eroded away, or if it is a product of late post- The failed arm model predicts the presence of a dike swarm (e.g., Ernst and Buchan, 2003), in this case, The presence of Pass Lake Formation rocks at the base of a number of diamond-drill holes in the map area suggests that the half graben was present before, or was formed during, the deposition of the Sibley Group, sometime between 1670 and 1339 Ma (Heaman et al., 2005; Franklin, 1978). The occurrence of Pass Lake Formation at the base of Moseau Mountain, on the east side of the Black Sturgeon River, suggests that fault activity continued throughout the entire period of sedimentation, or that there has been significant postsedimentation fault re-activation. At this time, there is no definitive means of distinguishing between these models although, the location of Pass Lake Formation rocks at higher elevations to the east could mean that sedimentation originally extended across much of the Nipigon Embayment, consistent with the first model. - 89 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 emplacement fault displacement. 9 Geochemistry 8 Central suite Osler Group 7 Hele Intrusion 6 The Hele Intrusion may be subdivided into olivine gabbro, olivine melagabbro, and peridotite based on their MgO contents, with the peridotite having between 19 - 29 wt.% MgO and the olivine gabbro having MgO contents of less than 11 wt.% MgO. Elemental variations in diamond drill core samples appear to indicate that the Hele Intrusion is composed of more than one pulse of magma, resulting in an interlayering of the peridotite and melagabbro. Pink feldspar may occur as a minor mineral phase in the olivine gabbro, and is a major constituent of the monzogabbro, or granophyre. The presence of pink feldspar usually correlates with higher K2O content that may be up to 7.42 wt.%, with no appreciable change in the Fe2O3 contents compared to the other olivine gabbro. Although the high K2O is possibly related to late stage igneous fractionation, it is considered more likely that these values are a result of assimilation of Sibley Group rocks and that the pink colour is a result of potassic alteration rather than hematization. Total rare earth element (REE) and high field strength (HFSE) concentrations in the peridotite 100 I x-1r o osa S a m p le /p rim itiv e m a n tle DI 1000 10 1 0.5 Th Ta Ce Nd Hf Eu Gd Dy Ho Tm Lu Sc Nb La Pr Zr Sm Ti Tb Y Er Yb V Figure 4. Mantle-normalized extended element diagrams of the olivine gabbro, melagabbro and peridotite of the Hele Intrusion (normalizing factors from Sun and McDonough, 1989). L a /S m Mafic to Ultramafic Intrusive Rocks 5 4 Lower suite Osler Group 3 Upper suite Osler Group 2 1 1.0 1.8 - 2.7 3.5 4.3 5.2 6.0 Gd/Yb Figure 5. Gd/Yb versus La/Sm diagram of the mafic to ultramafic intrusive rocks and the Nipigon diabase sill complex with fields for the volcanic rocks of the Osler Group (after Lightfoot et al., 1991). Legend for symbols, see Figure 4. samples are generally lower than in the olivine gabbro or olivine melagabbro (Fig. 4). All of the rocks have weak negative TiO2 and Zr anomalies on the mantle normalized extended element diagram (see Fig. 4), and these anomalies are more pronounced than negative Nb-Ta anomalies for the Seagull and Disraeli intrusions (Hart and Magyarosi, 2004) although all three intrusion have flattening of the trends between Th and La. All three units have [La/Yb]mn (mantle normalized) and Gd/Yb ratios which are distinctly higher than the diabase sills, and plot toward the high end of the field defined by the Lower Suite of the Osler Group volcanic rocks (Hart, 2005; Fig. 5). Rocks of the Hele Intrusion have a wider range of Gd/Yb ratios than the rocks from the Disraeli and Seagull intrusions, but overlap with the values for the other intrusions. The peridotite and olivine gabbro have REE and HFSE ratios that are comparable to ocean island basalts and most of the rocks from the Disraeli and Seagull intrusions in contrast to the diabase sills (Fig. 6). The peridotite and gabbro are not enriched in nickel, and the peridotites follow an olivine-controlled igneous fractionation trend, which would be consistent with a parental MgO concentration of approximately 13 wt.% (Fig. 7). Both units have a flat positive slope on a primitive mantle normalized PGE diagram with enrichment in Ir and Pt (Fig. 8), compared to the trends for the diabase sills that follow a trend similar to that of - 90 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 (Fig. 9), but the [La/Yb]mn are distinct from those of the rocks of the mafic to ultramafic intrusions. Most of the diabase samples have weak negative TiO2 anomalies, but no apparent Zr anomalies, on the mantle normalized extended element diagram (see Fig. 9), and pronounced negative Nb-Ta anomalies. Some HFSE and REE ratios for the diabase show some similarities to the Central and Upper Suite volcanic rocks of the Osler Group as has been suggested by Sutcliffe (1987) (see Fig. 5). most continental flood basalts (Hart et al., 2002a). 20 Upper Suite Central Suite 10 Upper Crust SZB T h /Ta Logan sills Primitive Mantle The Nipigon diabase sills are not enriched in nickel, and follow an igneous fractionation trend (see Fig. 7). PGE contents of the diabase are similar to the trend followed by most continental flood basalts, and comparable to the PGE contents and trends noted for the diabase sills from the Beardmore area by Hart et al. (2002a; Fig. 10). OIB OPB 1 NMORB Lower Suite MORB 0.5 0.8 1 10 50 La/Yb Figure 6. La/Yb versus Th/Ta diagram of the mafic to ultramafic intrusive rocks and the Nipigon diabase sill complex with fields for the volcanic rocks of the Osler Group and the Logan diabase sills south of Thunder Bay (after Condie et al., 2002). Legend for symbols, see Figure 4. 2000 Within plate basalt Ni (ppm) 1000 Upper crust 100 Mapping and geochemical sampling by Sutcliffe (1986) and Hart et al. (2002a) suggested that the sills do not represent single cooling units. However, the results of detailed sampling of a number of continuous sill sections from diamond drill core suggest that some sills may be single cooling units. Whereas others have chemical variations suggesting a more complex history of emplacement that may include multiple pulses of magma that did not re-equilibrate before cooling. However, the variations are subtle and generally not distinctive enough to be evident in a group of outcrop samples. The diabase sills have HFSE and REE ratios with some similarities to subduction zone basalts and the Central and Upper Suites of the Osler Group (see Fig. 6). 1000 10 0 10 20 30 40 MgO (wt. %) Figure 7. MgO versus nickel diagram of the mafic to ultramafic intrusive rocks and the Nipigon sill complex (after Lightfoot et al., 2001). Legend for symbols, see Figure 4. S a m p le /M a n tle P G E 100 10 1 0.1 0.01 Nipigon Diabase Sill Complex The diabase sills plot along a line indicative of fractionation controlled mainly by plagioclase and clinopyroxene, as suggested by Sutcliffe (1987). The diabase sills display a range in REE and HFSE concentrations that will require further investigation .001 Ni Ir Ru Rh Pt Pd Au Ag Cu Figure 8. Primitive mantle-normalized platinum group element diagrams of the olivine gabbro, melagabbro and peridotite of the Hele Intrusion (normalizing factors from Barnes et al., 1987). Legend for symbols, see Figure 4. - 91 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 and low TiO2 rocks is similar to regional distributions observed in some flood basalt provinces suggesting that the Logan and Nipigon sills may be part of a single intrusive event. S a m p le /p rim itiv e m a n tle 1000 100 10 1 0.5 Th Ta Ce Nd Hf Eu Gd Dy Ho Tm Lu Sc Nb La Pr Zr Sm Ti Tb Y Er Yb V Figure 9. Mantle-normalized extended element diagrams of the Nipigon Diabase Sill Complex (normalizing factors from Sun and McDonough, 1989). Legend for symbols, see Figure 4. 1000 S a m p le /M a n tle P G E 100 10 1 The presence of variants of the sills that have specific geochemical and/or geographic distribution has resulted in the introduction of a variety of new, informal, names for the diabase sills in the past few years, such as the distinction between the Logan sills in the Thunder Bay area and the Nipigon sills around Lake Nipigon. In addition, new sill types, such as the Inspiration sills have also been identified (e.g., MacDonald, 2004; Richardson and Hollings, 2005). This does cause a problem, however, in that the historical term Logan Sills has been used exclusively in the paleomagnetic literature, and in much of the international literature, for all of the diabase sills north of Lake Superior. Until these terminology issues are resolved, it is recommended that the historical nomenclature of Logan Sills should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Logan sills in the south and Nipigon sills to the north. The Nipigon sills may be further subdivided into several chemical variants, such as the normal Nipigon, Inspiration, and Jackfish-like sills. Clarification of this nomenclature is contingent on further geochemical, petrological and geochronological work. Mineralization 0.1 Platinum Group Elements Hele Intrusion 0.01 .001 Ni Ir Ru Rh Pt Pd Au Ag Cu Figure 10. Primitive mantle-normalized platinum group element diagrams of Nipigon Diabase Sill Complex (normalizing factors from Barnes et al., 1987). Legend for symbols, see Figure 4. The Nipigon diabase sills have lower TiO2, Zr/Y, La/ Yb values than the Logan diabase sills located to the south of Thunder Bay (see Fig. 6) and have a regional distribution that extends south to at least the north side of the City of Thunder Bay (Hart, 2004). Several samples collected from the area south of Thunder Bay have geochemical characteristics comparable to the Nipigon diabase sills (see Fig. 6), but further investigation is required. This regional geographic distribution of high Mineralization has not been located in either of the two diamond drill holes or in outcrop of the Hele Intrusion. The peridotite and olivine gabbro have similar PGE contents with Pt/Pd ratios of 1.03 to 4.63 with the highest values being 75.87 ppb Pt, 50.24 ppb Pd and 24.07 ppb Au in an olivine gabbro (Hart, 2005). The peridotite and olivine gabbro also have similar base metal contents with 18 to 1320 ppm Ni, 17 to 459 ppm Cu, and 72 to 280 ppm Zn. Seagull Intrusion Mineralization in the southern portion of the Nipigon Embayment occurs as three reef style zones hosted by the Seagull Intrusion. The upper zone contains up to 1.52 ppm Pt, 1.78 ppm Pd, 0.34 ppm Os, 0.11 ppm Rh, 0.23 ppm Ir, 0.15 ppm Au, 0.58% Cu, and - 92 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 0.34% Ni over 3 m (East West Resource Corporation 2004a,b). The lower zone contains up to 1.56 ppm Pt, 1.87 ppm Pd, 0.20 ppm Os, 0.10 ppm Rh, 0.18 ppm Ir, 0.15 ppm Au, 0.55% Cu, and 0.35% Ni over 3 m (East West Resource Corporation 2004a,b). The best intersection in the basal zone, close to the intrusion Quetico contact, is 1.71 ppm Pt and 1.87 ppm Pd over 2.1 m associated with 10% disseminated to net textured sulphide minerals including pyrite, pyrrhotite and minor chalcopyrite (Durham, 2000). Diamonddrill holes completed in 2004 intersected a zone near the base of the intrusion consisting of 2.38 g/t Pt, 2.65 g/t Pd, 0.19 g/t Au, 0.18 g/t Rh, 0.39 g/t Os, 0.34 g/t Ir, 0.08 g/t Ru, 0.19% Cu and 0.20% Ni over 3.21 metre (Platinum Group Metals Ltd., 2005). Mineralization in the basal zone is present near the lower contact of the intrusion, and is interpreted to be caused by sulphur saturation of the magma during initial stages of emplacement (Heggie and Hollings, 2004). The upper and lower zones are associated with more primitive whole rock geochemistry and mineralogy possibly reflecting an influx of less evolved magma. The mineralogy has been characterized by Heggie and Hollings (2004) and the following details are from that source. Nickel and copper sulphide minerals occur as disseminated bravoite and chalcopyrite respectively, found interstitial to olivine. Copper is also present as veinlets of native copper cross cutting sulphide minerals. Platinum group minerals are located at the grain boundaries of NiFe sulphide minerals, and are dominated by sperrylite, keithconnite, stibiopalladinite, and a copper palladium alloy. Stops Field Trip Road Log Stop 1a 1b Locality Junction of Highway 585 and Highway 11/17 in the Town of Nipigon Red Rock Black Sturgeon River Rock quarry north of Big Squaw Creek Road cut Big Squaw Creek Road Cut Junction of Highway 11/17 and the Stewart Lake Road Reset Junction of the Stewart Lake Road and Fowlkes Lakes roads km 0 5.5 16 19 19.5 26.5 5.9 Stop 2 3 4 5 6 7a 7b Locality km Reset Hele Intrusion peridotite 2.9 Junction of the Fowlkes Lakes and Driftstone Lake roads 6.3 Reset Hele Intrusion olivine gabbro 3.9 Return to Stewart Lake Road Reset Junction of Stewart Lake and Black Sturgeon roads 3.8 Reset Black Sturgeon Road 3.3 Eagle Mountain 20.3 Junction of Black Sturgeon and Camp 42 roads 22.5 Reset Junction of Camp 42 and Church Road 14.1 Reset Andalusite schist 7 Reset Return to south to junction of Camp 42 and Church Road 7 Reset Road to left 0.75 Granite-diabase contact. South on trail 0.2 Pegmatite 12 Trip starts at the junction of Highway 585 and 11/17 in Nipigon. Proceed southwest on Highway 11/17. 5.5 km - Rossport Formation Oxidized units of the Rossport Formation of the Sibley Group are exposed in the hillside on the east side of the highway. These rocks unconformably overlie Archean rocks of the Quetico Subprovince, and are intruded at the top of the hill by a diabase sill of the Nipigon Sill Complex 16 km - Black Sturgeon River (Photo 1) There are two major regional fault trends, north and northwest, that define the structural zone referred to as the Black Sturgeon Fault Zone. The combination of these two trends has been interpreted to be en échelon resulting in the formation of an asymmetric basin or graben, as originally proposed by Coates (1972). A series of subparallel north-trending faults are interpreted to be regionally continuous structures that can be traced - 93 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 by geophysics and topography for over 150 km north into the area of the English Bay Complex. A series of northwest-trending faults can be traced for similar distances to the west into the Wabigoon Subprovince. Apparent flexures in the north-trending faults probably represent intersections with northwest-trending faults. Vertical displacements on these faults are varied (may be up to 350 m) and are visible along the Black Sturgeon River, interpreted by Coates (1972) to be the halfgraben hinge. There appears to have been little lateral displacement on these faults based on the correlation of the Archean iron formations that extend across Black Sturgeon Lake (Hart and Magyarosi, 2004). Corridors of north-trending faults cut, and may have controlled the emplacement of, the Seagull and Disraeli peridotite intrusions, and possibly the Hele peridotite intrusion. 19 km - turn right onto natural gas pipeline access road on the north side of the highway and immediately turn left and follow trail along the north side of the highway. Proceed 200 m to the edge of the gravel pit * r 4 • 1. 'r 1 *: and park vehicles. Walk for 115 m along trail heading north and skirting the edge of the gravel pit. Stop 1a - Rock Quarry UTM coordinates – 0396320E 5416664N This stop is located on the south side of a rock quarry in a diabase sill which produced fill for highway construction. This outcrop is the upper contact zone of the diabase sill that extends to the south into the Big Squaw Creek road cut. This contact zone is a relatively rare variant of the typical chilled contact consisting of pebble sized, rounded xenoliths of quartz and feldspar in a biotite-rich fine-grained diabase. Contact relationships observed to the south, in the next stop, indicate that the diabase is intruding sedimentary rocks of the Sibley Group. The composition of the country rock probably influences the formation of this relatively rare contact type, as the diabase sills commonly have glassy chilled contacts. This type of contact zone has been interpreted to be a result of the partial melting and assimilation of country rock xenoliths resulting in a quartz-feldspar restite and a biotite-rich diabase. Similar quartz-feldspar xenoliths in a hornfels carbonate-rich sediment are observed along some contacts in the road cut to the south suggesting that the presence of a carbonate-rich unit may be important to the formation of this contact style. However, a similar contact zone is present in a diabase sill intruding rocks of the Quetico Subprovince without any obvious evidence of Sibley sediments, about 10 km southwest of Beardmore, near the Warnesford Quarry —. ' __ 7 4- fl'_. 4. 'C Photo 1. The erosional valley of the Black Sturgeon River following a northwest fault, with the Archean to the right (east) and Hele Intrusion to the left (west). The valley is approximately 800 m wide. Photo courtesy of J. Scott. Stop 1a. Fine-grained, massive, biotitic diabase matrix containing angular to subrounded pebble-sized xenoliths of coarse-grained quartz and white to pinkish feldspar. Markings on scale card are at 1 cm intervals. - 94 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 SeE! •% "tt' Stop 1b. The east block of hornfels units of the Rossport Formation surrounded by diabase sill along the north side of Highway 11/17. on the CNR railway. In other areas, intrusion of diabase and ultramafic sills into carbonate-rich Sibley sediment has resulted in the formation of variolitic sills with a chilled margin composed in part by densely packed fine varioles and a core of widely spaced large varioles. Variolitic textures have been observed southwest of Moraine Lake, in a sill cut by diamond drill hole NI92-7 located west of Disraeli Lake, and in and drill hole HE02-01 below the Hele Intrusion. Return to the highway and turn right. Proceed west for 600 m, driving past the outcrop, located on the north side of the highway, and turn left into the entrance to a private dump to park. Stop 1b - Big Squaw Creek road cut UTM coordinates – 0396114E 5416374N A diabase sill which has intruded and metamorphosed blocks of Rossport Formation mudstone and dolostone is exposed in a rockcut along the north side of Highway 11/17, in east Stirling Township. The eastern block appears to have been lifted, resulting in a westerly dip, with diabase intruding underneath and from the east (see photo above). However, the block is abruptly truncated by a vertical contact to the west, and a less then 25 cm thick sill intrudes along the bedding eastward from this contact. The ambiguity of the injection direction of the diabase may be a result of the rock cut being oblique to the injection direction, and the block may not be totally separated from the surrounding Sibley Group. The western block is composed of sedimentary rocks comparable in composition to those in the east block but has a chaotic structure with no preservation of bedding, and is probably a detached, deformed and metamorphosed xenolith Metamorphism in the sedimentary rocks decreases away from the contact, and the sedimentary rocks close to the contact contain an assemblage of diopside, phlogopite, clinochlore, pargasite, and calcite (Hart, 2005). In some areas along the contact there are irregular zones to isolated pebble sized clasts of quartz and microcline within the calc-silicate hornfels. The quartz and microcline may represent a partial melt of the sedimentary rocks by the diabase, and may be equivalent to the xenoliths observed at the previous stop. A calc-silicate hornfels or skarn (Meinert, 1992) may be developed along the contact between the diabase sills and the Sibley Group sedimentary rocks, but is ubiquitous along the contacts between the peridotites and Sibley Group contacts (Hart, 2005). Where present, the hornfels is varied in width, extending up to 10 m into the sedimentary rocks, although an approximately 60 m thick layer of hornfels occurs in the Rossport Formation between the two diabase sills in diamonddrill hole ST02-01 (Hart and Magyarosi, 2004). The effects of metamorphism are most pronounced in the Rossport Formation, with the mudstone and siltstones beds commonly developing a light green, mauve, reddish gray to light gray banding. Dolomite-rich beds form light green, white to buff marbles dependent on the original mud content. Radial texture in 3 to 8 cm wide zones occur along some sill contacts, with radially oriented amphibole of the 0.2 to 0.5 cm in diameter. - 95 - During his regional study of the Nipigon diabas sills, �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sutcliffe (1986) identified two common assemblages of fine-grained, calc-silicate minerals, calcite-tremoliteforsterite (clinochlore) and calcite-diopside-fosterite at lower and higher metamorphic grades, respectively. The forsterite is altered to serpentine. No dolomite and only minor quartz were observed. Rogala (2001) completed a study of interbedded mudstones and dolomites of the cyclic facies of the Channel Island Member and lower Middlebrun Bay Member of the Rossport Formation, intersected by diamond drill hole NI92-5 located west of Disraeli Lake. These rocks contained a mineral assemblage of pargasite, tremolite, talc, clinochlore, calcite and barite with varying amounts of hematite and apatite, halite, sylvite, gypsum and dolomite. The halite and sylvite were interpreted to be a result of evaporite minerals present in the Sibley Group and the presence of these minerals probably controlled the formation of pargasite (Rogala, 2001). The barite was interpreted to have been introduced into the Sibley Group by later fluid associated with the lead-zinc veining. Rogala (2001) noted that one sill contact had a thin band of pyrite with chalcopyrite, magnetite, ilmenite, galena, and rare gold and silver. High gold has been noted in a sample from the McVicars Resources Muskrat Lake diamond drill hole ML01-3 at 239.8 m and also in samples from along the shore of Lake Nipigon (A. Richardson, MSc student, Lakehead University, personal communication, 2004). Past workers have suggested that the copper mineralization may be related to the intrusion of the diabase or ultramafic intrusions (e.g., Coates, 1972). However, it is not clear if the metals are igneous or sedimentary in origin. The diabase is commonly medium brown to brownish grey, massive, medium- to coarse-grained feldspar and pyroxene with trace to 3% olivine and 1 to 2% magnetite. Medium- to coarse-grained diabase forms the majority of the sills and generally lacks a sub-ophitic or diabasic texture and should properly be classified as gabbro, but to avoid confusion with other intrusions in the area the diabase classification has been applied to all rocks associated with the sills. Portions of this outcrop contain ophitic textures consisting of pyroxene oikocrysts up to 2 cm in diameter, and in diamond drill hole the coarse-grained ophitic textured diabase is often interlayered with sub-ophitic diabase. Calculations by Sutcliffe (1986) suggested that the diabase sills were emplaced at shallow crustal levels, at a lithostatic load of between 0.06 and 0.43 kilobars. The sills were estimated to have crystallized over a temperature range of 1100° to 800°C, adjacent wall rocks were estimated to have reached 665°C, and the metamorphic mineral assemblages suggest temperatures of above 540°C. 26.5 km - west along Highway 11/17, and turn right on to the Stewart Lake Road - reset. 5.9 km follow the Stewart Lake Road for 4.2 km north and then west after a right-angled turn for an additional 1.7 km. Turn right onto the Fowlkes Lakes Road - reset. 2.9 km – north on the Fowlkes Lakes Road to where a skidder trail departs to the west. Stop 2 - Hele Peridotite UTM coordinates – 0386762E 5420119N Park the trucks and proceed by foot along the skidder trail for 500 m to the outcrop. Located in the southeast corner of Hele Township, the Hele Intrusion covers approximately 40 km2 and has a maximum thickness of 130 m, as defined by diamond drilling Peridotite is very poorly exposed in the central portion of the body, east of Driftstone Lake, and is interlayered with olivine gabbro and feldspathic peridotite in diamond drill core. The peridotite is best exposed in an area of clear cut logging located on the northern edge of Stirling Township as a highly weathered and serpentinized rock with a high magnetic susceptibility overlain by a diabase sill. The peridotite is dark green, medium to coarse-grained, massive, variably ophitic, and composed of olivine and pyroxene with variable plagioclase and trace to 1% fine-grained reddish brown mica (Hart, 2005). Numerous, subparallel serpentine and chlorite-rich fractures result in a banded or layered appearance to the otherwise massive peridotite. Contacts between the peridotite, feldspathic peridotite and olivine gabbro observed in diamond drill core are generally gradual transitions in grain size and olivine and plagioclase content. There appears to be more than one band of peridotite interlayered with the melagabbro and feldspathic peridotite suggesting internal subdivisions similar to the Seagull Intrusion. The Hele Intrusion appears to be a relatively flatlying sill intruding the Sibley Group with no known feeder zone, although there are fewer diamond drill holes with which to constrain the geometry of the intrusion. Geophysical inversion modelling of the airborne magnetic data by Desmond Rainsford - 96 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 grained, reddish brown mica is common in the olivine melagabbro in the central areas of the intrusion. An absence or very low abundance of mica and a lower concentration of olivine in the olivine gabbro which forms the northern portion and margins of the intrusion makes it difficult to distinguish these rocks from the olivine-bearing diabase sills. The gabbro contains pods of monzogabbro in outcrop (see photo below) and is interbanded with monzogabbro in the upper portion of the diamond drill holes. - It Stop 2. Example of parallel fractures filled with serpentine and minor biotite in the peridotite of the Disraeli Intrusion. Pencil is approximately 15 cm long. (geophysicist, Ontario Geological Survey) supports this interpretation (Hart, 2005). Inversion modelling of the ground gravity data indicated the presence of an anomaly located along the western edge of the surface expression of the intrusion and extending to the southwest. This anomaly may represent an extension of the ultramafic intrusion, but recent diamond drill results in the area of the Seagull Intrusion intersected a magnetite rich skarn associated with a similar gravity anomaly. The Seagull Intrusion hosts PGE mineralization and differs from the Hele Intrusion in having a lopolithic shape, and intruding along or close to the contact between the Quetico Subprovince and the Sibley Group. The central portion of the intrusion appears to be saucer-shaped, but this may be a product of post-intrusion faulting, and a flat-lying sill along the periphery of the body. An underlying, or peripheral, feeder structure for the Seagull Intrusion has not yet been identified. The monzogabbro, or granophyre, is generally medium to coarse-grained, and massive with a variable amphibole content and distinctive pink feldspar or groundmass that occur in irregular shaped pods up to a metre in diameter. These pods are best exposed along the logging trails in the northeast portion of the intrusion, and are hosted by medium to fine-grained olivine gabbro containing minor pink feldspar. Less commonly, the monzogabbro is fine-grained, and massive reddish pink with minor quartz. This type of monzogabbro is present in irregular pods and dykelets with sharp contacts, with and without the coarser grained monzogabbro, in the western portion of the intrusion. Some of the best examples are in the central part of the intrusion, close to a small exposure of Sibley Group sedimentary rock that is in contact with the upper portion of the intrusion. In diamond drill core, the two types of monzogabbro are generally closely associated and in some examples the finer grained variety is rimmed by the coarser grained rock. The abundance of pink feldspar correlates with elevated K2O values, and has been interpreted to be the result of the assimilation of 6.3 km – junction with the Driftstone Road and turn right - reset. 3.9 km – east along the Driftstone Road. Stop 3 - Hele Intrusion - olivine gabbro UTM coordinates – 0390358E 5424134N The olivine gabbro to olivine melagabbro forms much of the exposed area of the Hele Intrusion. The gabbro is fine to medium-grained, medium to dark green, massive and composed of pyroxene, up to 30% plagioclase, and up to 40% olivine. Trace to 1% fine •i :• Stop 3. Contact between olivine gabbro and an irregular pod of monzogabbro / granophyre, in the western portion of the Hele Intrusion. The pen magnet is approximately 16 cm long. - 97 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Sibley Group sedimentary rocks (Hart and Magyarosi, 2004). The finer grained monzogabbro may be a poorly assimilated sediment xenolith and the coarser grained monzogabbro may be the result of assimilation of the volatile rich sedimentary rocks. Turn vehicles around and return by the same route to the corner of the Driftstone Road and the Stewart Lake Road 10.2 km to the south. Turn right onto the Stewart Lake Road. 3.8 km - Stewart Lake Road ends at the Black Sturgeon Road. Turn right onto the Black Sturgeon Road and drive 3.3 km north. contact of the sill is evident, which appears to be a result of later movement along a northwest-trending structure. Joints are rare in the outcrops forming the interior of the sills, and when present are generally linear in nature. The column-like jointing is probably a result of late stage fracturing of the cooling magma combined with later fault activity rather than a true columnar jointing. 20.3 km – this portion of the Black Sturgeon Road is underlain by units of the Sibley Group and outcrops are rare. Stop 5 - Eagle Mountain UTM coordinates – 0385387E 5433471N Stop at the 20.3 km in the open space on the east side of the road for a view of Eagle Mountain. Stop 4 - Black Sturgeon Road UTM coordinates – 0385313E 5418737N Stop at kilometer 9 of the Black Sturgeon Road and look to the east across the clear cut, a diabase sill intrudes the Rossport Formation of the Sibley Group. Jointing in the diabase sills may be either arcuate or linear. The arcuate joints formed by cooling of the magma vary in strike direction by up to 70° and are commonly observed on horizontal surfaces. Jointing is best exposed in an area to the north, of the clear cut within the 1999 burn. Jointing located close to the sill contacts forms crude polygon joint patterns with “T” or flattened “Y” junctions, and the interiors of the sills lack the “Y” junctions commonly present in polygonal jointed unit (e.g., Aydin and DeGraff, 1988). This lack of “Y” junctions suggests a slow cooling rate which would be in agreement with the 315 to 560 year cooling times for the sills estimated by Sutcliffe (1986). Looking to the northeast, an offset in the basal Eagle Mountain is an approximately 10 km long ridge located west of the Black Sturgeon River and is the first of a series of north-trending fault bounded ridges located further west. The ridges are commonly capped by a diabase sill that has intruded units of the Kama Hill Formation. The lower portions of the ridges are generally poorly exposed but appear to be composed of units of the Rossport Formation. In some areas, as in Eagle Mountain, a second diabase sill intrudes the lower portions of the ridge. There were three diamond drill holes that were completed on the lower slope of Eagle Mountain that provide a stratigraphic section though the lower portion of the Sibley Group. Two of these holes intersected a mafic dyke, which is also exposed in a ravine on the central portion of the ridge. The 5 to 10 m wide fine-grained massive dyke dips approximately 50° south. This dyke appears to be related to a series of fine grained mafic sills with a higher olivine content. Although, these sills Stop 4. Nipigon diabase sill intruding units of the Rossport Formation located along the Black Sturgeon Road. Photo is looking to the southeast. - 98 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 within the wackes (Hart and Magyarosi, 2004). The presence of andalusite, with minor cordierite, suggests a low pressure, high temperature metamorphic event possibly related to the emplacement of the felsic intrusive rocks. The andalusite pseudomorphs are partially replaced by a mica indicating some degree of retrograde metamorphism. Stop 5. Looking north from the top of Eagle Mountain, with units of the Kama Hill Formation intruded, overlain, by a diabase sill. The sill – sediment contact is approximately along the upper tree line in the cliff. are geochemically similar to the ultramafic intrusions, they do not appear to be associated with any known ultramafic intrusion (Hart, 2005; Richardson and Hollings, 2005). 22.5 km – (km 26 on the Black Sturgeon Road) turn right at the load levelers, on to the Camp 42 Road reset. 14.1 km - continue east and then north on the Camp 42 Road until the junction with the Church Road reset. These metasedimentary rocks closely resemble the clastic metasedimentary rocks of the Quetico and Wabigoon subprovinces in the Beardmore area, east of Lake Nipigon (Hart et al., 2002a). For this reason, these rocks are interpreted to have accumulated in a trench environment of an accretionary complex comparable to the depositional environment of the Beardmore-Geraldton greenstone belt (e.g., Williams, 1989). The transition from the southern Wabigoon to Quetico subprovince is located approximately 25 km to the north, where a series of iron formations appear to correlate with the iron formations of the BeardmoreGeraldton belt (e.g., Sutcliffe, 1986). However, the iron formation exposed on the east side of Black Sturgeon Lake (Hart and Magyarosi, 2004) is comparable to the chert-magnetite iron formation in the Mawn Lake area (Coates, 1972) rather than the chert poor units of the Beardmore area. Turn vehicles around 7 km - proceed back along the Church Road to the junction with the Camp 42 Road 7 km – north along the Church Road. Stop 6 - Andalusite schist 0.750 km – trail to left, proceed south up hill along the trail These rocks consist of metamorphosed feldspathic wacke, lithic wacke, and siltstone with metamorphic grade increasing progressively toward the south (Hart 2005). The metawackes and metasiltstones are light grayish brown to medium gray, with 3 to 30 cm thick beds, and a schistosity parallel to bedding resulting in the variable preservation of original textures. These rocks commonly contain 5 to 10% biotite, trace to 1% very fine-grained garnet and, a trace subhedral mineral tentatively identified as cordierite. Specific bands are composed of individual to multiple beds containing 10 to 15% coarse- to very coarse-grained, subhedral to euhedral andalusite. The andalusite occurs disseminated through some beds and occasionally concentrated along parallel bands that appear to represent the original bedding plane. This distribution probably reflects the distribution of clay minerals, with higher Al2O3 content, 0.200 km – park and walk right, to the west, for ~10 m S UTM coordinates – 0388613E 5449432N Stop 6. Coarse-grained andalusite pseudomorphs in metamorphosed feldspathic wackes and siltstones of the Quetico Subprovince. - 99 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 Stop 7a - Granite – Diabase Contact UTM coordinates – 0390536 E 5442913N The contact between the leucocratic muscovite granite of the Quetico Subprovince and the Mesoproterozoic diabase sill of the Nipigon Sill Complex is exposed in this flat-lying outcrop. The medium to coarse-grained leucocratic muscovite granite occurs are rounded xenoliths up to 20 cm in length in the chill zone of the diabase sill (see photo below). The sills is composed of a fine to very fine-grained, glomeroporphyritic diabase with polygonal jointing and some area have the glassy chill zone preserved. This outcrop is interpreted to be the upper contact of the diabase sill which is dipping shallowly to the north, and the hill to the southwest is capped to the same diabase sill. This same sill caps the hill above the outcrop at stop 7b. About 20 m to the right or east of the trail, an outcrop of the Quetico Subprovince granite is cut by a northeast-trending, approximately 8 cm wide, aphanitic diabase dyke. Dykes may occur close to the diabase sill contacts generally extending for less than a few metres, but may in some cases extend for a few tens of metres. This dyke is though to be injected from the diabase sill which is located beneath this outcrop. 1.60 km – return to the Camp 42 Road and continue back to the gravel pits. Stop 7b - Pegmatite and Nipigon Sill UTM coordinates – 0389764E 5442111N The west dipping outcrop consists of a muscovite granite dyke intruding biotite schists, and the hill above is capped by a diabase sill. Muscovite granite dykes in this area are generally a few metres in width but may be up to a few tens of metres wide, and are usually irregular in form and orientation although generally following the fabric of the surrounding country rock. This outcrop is composed of pegmatitic quartz, potassium feldspar, and plagioclase with graphic intergrowths of quartz and feldspar. Black, coarse-grained to pegmatitic tourmaline occurs as tabular grains and as graphic intergrowths with quartz. Fine grained subhedral, blue-green apatite occurs as an accessory mineral located along the contacts with the biotite schist country rock. This dyke is one of a number of dykes intruding the metamorphic rocks adjacent to the leucocratic muscovite granite. A body of leucocratic muscovite granite occurs to the north and northwest and is light gray, pinkish gray, to white, massive, and medium to very coarsegrained with occasional pegmatitic sections, composed of quartz and potassium feldspar, and plagioclase with typically less than 5% muscovite (Hart, 2005). Irregular to tabular shaped xenoliths of metasedimentary and schistose rocks up to a few metres in diameter are present through out, but appear to be more common to wards the margins of the body. Numerous muscovite granite dykes intrude the massive body and may constitute up to 30% of the outcrop. These dykes are commonly a few metres in width, but may be up to a The outcrop is located approximately 300 m east of the road, and is accessible by a trail leading from the back of the small gravel pit. Stop 7a. Xenoliths of Quetico granite in the glassy chill zone of a Nipigon diabase sill. Stop 7b. Black tourmaline in the leucocratic muscovite granite dyke. - 100 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 dykes contain intergrowths of tourmaline and quartz, and muscovite and quartz commonly observed in rocks that have been classified as fertile granites, or granites that have the potential to host rare-element mineralization (Breaks et al., 2003). Additional work is on-going to characterise these bodies. The muscovite leucogranite occurs along the regional trend extending from Georgia-Barbara lakes area, about 60 km to the northeast, to the DeCourcey Lake and Onion Lake areas, approximately 45 km to the southwest (Breaks et al., 2003), and this area may represent a similar concentration of fertile granites. Stop 7b. Northeast dipping diabase sill observed to the west of the pegmatitic dyke outcrop. few tens of metres. The granite dykes are composed of pegmatitic quartz, potassium feldspar, and plagioclase and frequently contain graphic intergrowths of quartz and feldspar, black, coarse-grained to pegmatitic, tabular tourmaline usually graphically intergrown with quartz and plumose intergrowths of muscovite-quartz. Trace to 1% fine-grained, disseminated, subhedral, reddish brown garnet, fine grained light green mica or rosy quartz was identified in a few of the dykes. Contact relationships suggest that the leucocratic muscovite granite and pegmatite dykes intruded the country rocks rather than being products of partial melting of the host country rocks. However, the initial mode of formation is not known and these rocks may have formed by partial melting of sedimentary rocks at depth. The pegmatitic, muscovite leucogranite body and Looking west across the valley, a diabase sill can be observed to dip shallowly to the northeast, whereas along the Black Sturgeon River the same sill can be observed to dip shallowly east (see Photo below). The shallow inward dipping orientation of this sill, forming a broad saucer with a width of about 5 km, is also visible in the airborne magnetic pattern (Fig. 11) (Ontario Geological Survey 2004). A number of these saucer-shaped sills, with widths of up to 6 km and interiors occupied by older rock types, are evident from airborne magnetic surveys both in this area and in sills intruding Archean rocks of the Wabigoon Subprovince to the northeast and northwest. Variation in sill orientation over short distances is common in other mafic sill complexes (e.g., Tasmania: Leaman, 1975; Karoo: Chevailler and Woodford, 1999). This change in orientation may be due to a number of factors including a change in orientation of the principal stress direction (Gretener, 1969) during sill emplacement, and the sill may follow pre-existing structures such as faults. There appear to be at least three major sills (100 to ~250 m), and possibly a number of thinner (5 to 25 m) sills in the area, based on a combination of outcrop and diamond drill hole information. However, correlation between outcrops and diamond-drill holes is complicated by the saucer shape of some sills, the presence of block faulting that may post-date sill emplacement, and the lack of distinctive geochemical differences between the sills. 12 km – back to the junction of the Camp 42 and Black Sturgeon roads 26 km – south to Highway 11/17 along the Black Sturgeon Road – bypassing the Stewart Lake Road Figure 11. First vertical derivative of the total field magnetic data the inward dipping saucer shaped diabase sill. Star marks the location of field trip Stop 7. - 101 - �Proceedings of the 51st ILSG Annual Meeting - Part 2 References Office, assessment file 52A15-2.21989. Aydin, A. and DeGraff, J.M., 1988. Evolution of polygonal fracture patterns in lava flows; Science, v. 239, p. 471-476. Barnes, S.J., Boyd, R., Korneliussen, A., Nilsson, L-P., Often, M., Pedersen, R.B. and Robins, B., 1987. 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Sutcliffe, R.H. and Greenwood, R.C., 1985b. Precambrian - 104 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2005 Description An account of the resource Institute on Lake Superior Geology. Nipigon, Ontario. May 24-28, 2005. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2005 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/12d45312aebf45dc0fad52fdffb93c7a.pdf 4b91a4fa095c82217c8aef3e3a361ccb PDF Text Text 52nd Annual Meeting Sault Ste Marie, Ontario – May 8 – 12, 2006 Institute on Lake Superior Geology Part 1 – Proceedings and Abstracts Volume 52 part 1 – Proceedings and Abstracts �Proceedings of the 52nd ILSG Annual Meeting – Part 1 52nd Annual Meeting INSTITUTE ON LAKE SUPERIOR GEOLOGY May 8 – 12, 2006 Sault Ste Marie, Ontario Hosted by R. P. Sage and A. C. Wilson Co-chairs Volume 52 Part 1 – Proceedings and Abstracts Edited by A. C. Wilson (Ontario Geological Survey) Cover Photos (clockwise from upper left) – Generalized geology of the Sault Ste Marie, Ontario area (Ontario Geological Survey Map 2543); Stone quarry in nodular anorthosite of the Agnew Lake Intrusion (photo courtesy of M. Easton OGS); Arch Rock, Mackinac Island, Michigan;Gowganda Formation, Hwy. 108 Elliot Lake area; Nicholson outcrop, Arctic Star Diamond Corp. property, Menzies Township; Chippewa Falls unconformity, Hwy.17N. i �Proceedings of the 52nd ILSG Annual Meeting – Part 1 52nd INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 52 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: GLACIAL LAKES ALGONQUIN AND NIPISSING SHORELINE BEDROCK FEATURES: MACKINAC ISLAND, MICHIGAN - FIELD TRIP GUIDEBOOK PART 3: UNUSUAL DIAMOND-BEARING BRECCIAS OF THE WAWA AREA FIELD TRIP GUIDEBOOK PART 4: THE HURONIAN SUPERGROUP BETWEEN SAULT STE MARIE AND ELLIOT LAKE - FIELD TRIP GUIDEBOOK PART 5: KEWEENAWAN ROCKS OF THE MAMAINSE POINT AREA FIELD TRIP GUIDEBOOK PART 6: GEOLOGICAL GUIDEBOOK TO THE PALEOPROTEROZOIC EAST BULL LAKE INTRUSIVE SUITE PLUTONS AT EAST BULL LAKE, AGNEW LAKE AND RIVER VALLEY, ONTARIO - FIELD TRIP GUIDEBOOK Reference to material in Part 1 should follow the example below: Brown, B. A., Czechanski, M. L., Reid, D. D. and Mudrey, M. G. Jr. 2006. New evidence for syn-depositional subsidence in the Middle Ordocivician rocks of southwest Wisconsin; in Wilson A. C. (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, v. 52 pt 1, p. 7. Published by the 52nd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology - Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2006 ............................................................. iv Constitution of the Institute on Lake Superior Geology................................................... vi By-Laws of the Institute on Lake Superior Geology ....................................................... vii Membership Criteria for the Institute on Lake Superior Geology.................................. viii Goldich Medal Guidelines ................................................................................................ ix Goldich Medallists ........................................................................................................... xi Goldich Medal Committee ............................................................................................... xi Citation for Goldich Medal Recipient.............................................................................. xii ILSG Student Research Fund ......................................................................................... xiv Eisenbrey Student Travel Awards ....................................................................................xv Eisenbrey Student Travel Award Application ................................................................ xvi Student Paper and Poster Awards .................................................................................. xvii Student Paper and Poster Awards Committee ............................................................... xvii Report of the Chairs of the 51st Annual Meeting ......................................................... xviii Board of Directors.............................................................................................................xx Session Chairs...................................................................................................................xx Local Committee...............................................................................................................xx Banquet Speaker ............................................................................................................. xxi Acknowledgements......................................................................................................... xxi Program.......................................................................................................................... xxii Abstracts .............................................................................................................................1 Author Index .....................................................................................................................72 iii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Institutes on Lake Superior Geology, 1955-2005 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn iv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 # Date Place Chairs 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste Marie, Ontario R. P. Sage & A. C. Wilson v �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Constitution of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) Article I - Name The name of the organization shall be the “Institute on Lake Superior Geology”. Article II - Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips. Article III - Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to _________ (some tax free organization). (To avoid Federal and State income taxes, the organization should be not only “scientific” or “educational”, but also “non-profit”) Minn. Stat. Anno. 290.01, subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501(c)(3) Article IV - Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors. Article V - Meetings The organization shall meet once a year. The place and exact date of each meeting will be designated by the Board of Directors. Article VI - Directors The Board of Directors shall consist of the Chair, Secretary, Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than six persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to six members. Article VII - Officers The officers of this organization shall be a Chair, a Secretary and a Treasurer. A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His/her term of office as Chair will terminate at the close of the annual meeting over which he/she presides, or when his/her successor shall have been appointed. He/she will then serve for a period of three years as a member of the Board of Directors. B. The Secretary shall be elected at the annual meeting. His/her term of office shall be four vi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 years, or until his/her successor shall have been appointed. C. The Treasurer shall be elected at the annual meeting. His/her term of office shall be four years, or until his/her successor shall have been appointed. The terms of the Secretary and Treasurer shall be staggered so that there will always be a two year overlap between the two. Article VIII - Amendments This constitution may be amended by a majority vote (majority of those voting) of the membership of the organization. By-Laws of the Institute on Lake Superior Geology (Last amended by the Board—May 6, 2004) The by-laws of the Institute on Lake Superior Geology are in revision and will be posted on the ILSG website when completed and approved. Please visit www.lakesuperiorgeology.org vii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Membership Criteria for the Institute on Lake Superior Geology Approved May 8, 1997. Amended by the Board—May 6, 2004 A. Membership in the Institute on Lake Superior Geology requires either participation in Institute activities, or an indication on a regular basis of interest in the Institute. Those individuals registering for an annual meeting will remain as members for 4 years unless: 1) they indicate no further interest in the Institute by responding negatively to the statement on meeting circulars “Remove my name from the mailing list”; or 2) two successive mailings in different years are returned by the postal service as address unknown. B. Those individuals who have not registered for an annual meeting in the past 4 years must indicate an interest in the Institute by postal, electronic, or verbal correspondence with the Secretary at least once every two years. Such individuals will be removed from the membership if they indicate no further interest in the Institute or two successive mailing in different years are returned by the postal service as address unknown. C. The Secretary will maintain a list of current members. The list will include the date of the beginning of continuous membership, dates of returned mail, dates of last contact (expression of interest), and the date membership expires, barring a change of status initiated by the member. Those individuals who have become members of ILSG by Section B will have an expiration date listed at 2 years from the upcoming meeting. For example, a member who expresses interest in September of 1997 (the next annual meeting is May, 1998) will have an expiration date of May, 2000, unless the member contacts the Secretary or attends an annual meeting. D. “Member for Life” status is granted to individuals who have been (nearly) continuous participants of the ILSG meetings for 15 years, Goldich Medal recipients, or those who have served as meeting chairs. This status will be further maintained unless the individuals indicate no further interest in the Institute, or 4 mailings in different years are returned by the postal service as address unknown, or they are deceased. E. All members will be mailed the First Circular for the Annual Meeting and the ILSG Newsletter. The Chair of the annual meeting may opt to send the first circular to additional individuals. All returned mail should be reported to the Secretary. F. The Secretary can designate any individual who is on the ILSG membership list (mailing list) as of January 1, 1997 as a member for life based on participation in ILSG activities. G. Members are strongly encouraged to send address corrections to the Secretary to avoid unintentional lapse of membership. viii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has ix �Proceedings of the 52nd ILSG Annual Meeting – Part 1 worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. x �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Goldich Medallists 1979 Samuel S. Goldich 1993 Donald W. Davis 1980 not awarded 1994 Cedric Iverson 1981 Carl E. Dutton, Jr. 1995 Gene LaBerge 1982 Ralph W. Marsden 1996 David L. Southwick 1983 Burton Boyum 1997 Ronald P. Sage 1984 Richard W. Ojakangas 1998 Zell Peterman 1985 Paul K. Sims 1999 Tsu-Ming Han 1986 G.B. Morey 2000 John C. Green 1987 Henry H. Halls 2001 John S. Klasner 1988 Walter S. White 2002 Ernest K. Lehmann 1989 Jorma Kalliokoski 2003 Klaus J. Schulz 1990 Kenneth C. Card 2004 Paul Wieblen 1991 William Hinze 2005 Mark Smyk 1992 William F. Cannon 2006 Goldich Medal Recipient Michael G. Mudrey Jr Mount Horeb, Wisconsin Goldich Medal Committee Serving through the meeting year shown in parentheses. George Hudak (2006) University of Wisconsin, Oshkosh Tom Hart (2007) Ontario Geological Survey Doug Duskin (2008) Member from Industry xi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Citation for Goldich Medal Recipient Michael G. Mudrey, Jr. 2006 Goldich Medal Recipient It is my pleasure to acknowledge the many contributions of Mike Mudrey on the occasion of his being awarded the 27th Goldich Medal for “Outstanding Contributions to the Geology of the Lake Superior Region”. Over the past 30 years, Mike has produced an extensive list of papers, maps, and abstracts that have significantly contributed to our understanding of regional geology. He has also compiled an outstanding record of service to the Institute on Lake Superior Geology. Mike Mudrey began his education at South Dakota School of Mines in 1963. In 1964 he transferred to Princeton, where he graduated with an A.B. degree in Geology and Geochemistry in 1967. At Princeton he had the opportunity to study with some of the pioneers of modern geology, and received a strong background in geology and chemistry. He began his association with economic geology working at Homestake in the summer of 1965. After working briefly with vertebrate paleontology in the summer of 1966, he began his long career in the Lake Superior Precambrian working as an assistant to Sam Goldich in the summer of 1967. Mike was a graduate student at SUNY Stony Brook in 1968, then moved to Northern Illinois University, where he completed his M.S. in 1969. His thesis topic was the petrology of the Northern Light Gneiss, completed under Sam Goldich's supervision. During the summers he assisted Sam at the Bureau of Standards and in the field in Minnesota and Ontario. In 1969 Mike moved on to the University of Minnesota, where he graduated with a Ph.D. in geology and analytical chemistry in 1973. His thesis research was a petrologic study of the Pigeon Point Sill, with Paul Weiblen as his advisor. While at Minnesota, Mike worked as a geologist for the Minnesota Geological Survey where he gained additional experience in field mapping and geochemical studies. After graduation, Mike returned to Northern Illinois University to work two years as a Scientist and Project Manager for the Dry Valley Drilling Project of the NSF Antarctic Program. In 1976 Mike joined the Wisconsin Geological and Natural History Survey, were he worked until retirement in 2005. At the WGNHS Mike's first job was to start up a Precambrian mapping program, necessitated by the discovery of volcanogenic massive sulfide deposits in the north. This work led to the first state bedrock map to subdivide the Precambrian, published in 1984. Mike was the driving force behind the long effort to complete gravity and aeromagnetic surveys of the state. Those who have been regulars at the ILSG are familiar with Mike's many contributions to the Precambrian geology of Wisconsin, but working at a small state survey requires one to wear more than one hat. Over his career at WGNHS, Mike ably served as expert on such diverse topics as earthquakes and seismicity, radioactive waste management, mineral and water resources, oil exploration, regional stratigraphy, and radon in the environment. Mike has always had a strong commitment to public service and education as well as scientific research. He was never too busy to answer a question on Wisconsin geology, whether from a legislator or a K-12 science student. In retirement he remains active, continuing to collaborate with his Survey and agency colleagues, and serving as consultant on radon to the Wisconsin Department of Health. xii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Mike has been an active contributor to the Institute on Lake Superior Geology since the early 1970s, when he first met Sam Goldich. He has served as co-chair, field trip chair, board member, member of the Goldich medal committee, Secretary-Treasurer (1990 to 1994), and field trip leader and session chair numerous times. Mike has nearly always contributed an abstract or two, and in the true spirit of Sam, he has never been at a loss for some good critical discussion. It is my pleasure to present the 2006 Goldich Medal to my friend and colleague of many years Mike Mudrey, in recognition of his many contributions to regional geology and service to the Institute. Submitted by B. A. Brown xiii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ILSG Student Research Fund The 2005 Board of Directors established the ILSG Student Research Fund with US$10,000 from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of US$500 each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • • • • • • The Board of Directors will be responsible for selecting a minimum of two awards. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31st each year. Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed), references, and a list of research expenses. The proposal should also include a proposed end date for the research. The proposal will need to be signed by the researcher’s supervisor. xiv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. The form below is optional. Successful applicants will receive their awards during the meeting. xv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Award Application Student Name : __________________________________ Address: Date: ____________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ email: __________________________________________________________ Educational status: _____________________________________________________ Are you the senior author of an oral presentation or poster? Yes ____ No _____ Will any other students be traveling with you?Yes ____ No _____ If yes, then who? ___________________________________________________ ___________________________________________________ Statement of need (use additional page if necessary): __________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Signature: ____________________________________________________ Department Head: ____________________________________________________ xvi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Student Paper and Poster Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper and Poster Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in person. 3) The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers and posters will be noted on the Program. Student Paper and Poster Awards Committee Dan England - Eveleth Fee Office Inc. John Klasner -Western Illinois University (Retired) Norm Trowell - Ontario Geological Survey xvii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Report of the Chairs of the 51st Annual Meeting REPORT OF THE 51st ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Nipigon, Ontario The Ontario Geological Survey and Lakehead University co-hosted the 51st Annual Institute on Lake Superior Geology meeting on May 24-28 in Nipigon, Ontario. The meeting consisted of two days of technical sessions with three pre-meeting and three post-meeting field trips. Ryan Tuomi provided excellent on-site AV assistance that kept the sessions running on schedule and also designed the Meeting web site. Bill Addison, Peter Hinz, Bernie Schnieders, John Scott, Mary Louise Hill and Mike Easton provided invaluable assistance with the field trips. Levina Collins of the Nipigon Economic Development Office acted as a liaison with the Town of Nipigon. Total registration for the meeting was 127 students and professionals. Proceedings Volume 51 was published in two parts: Part 1 – Program and Abstracts, edited by Mike Easton and Pete Hollings, with published abstracts for 26 oral and 14 poster presentations; and Part 2 – Field Trip Guidebook, edited by Pete Hollings. The 51st meeting marked the first time the ILSG Annual Meeting was held in Nipigon, enabling the organization of excellent field trips. On Tuesday, May 24, Tom Hart, Phil Fralick and Mark Smyk co-led a two-day trip to examine the geology and gold mineralization of the BeardmoreGeraldton greenstone belt. The following day, Peter Barnett led a small but dedicated group to view the Quaternary geology of the Beardmore-Nipigon area and, in a first for the Institute; Pete Hollings led a flotilla of small boats out on to Lake Superior to examine the Mesoproterozoic Midcontinent Rift (MCR) stratigraphy near Rossport. On May 28, three trips set out from Nipigon: Pete Hollings and Phil Fralick reprised the Rossport Trip; Tom Hart led a group to examine the geology of the Black Sturgeon area, focusing mainly on the diabase sills and ultramafic intrusions associated with the MCR; and Mark Smyk led a trip to look at the pegmatites and high-grade metamorphic rocks of the Quetico subprovince. One hundred and ten participants attended the Annual Banquet. Dr. Jim Franklin provided the after-dinner presentation, entitled “Mineral Resources for the Future: The Resource Potential of Northern Lake Superior”. Peter Hinz had the privilege of presenting the 2005 Goldich Medal to Co-Chair Mark Smyk of the Ontario Geological Survey. Mark has worked tirelessly for the Institute over the last 17 years and has also made significant contributions to the understanding of Lake Superior geology. The student paper committee (Penelope Morton, Greg Stott and Wally Rayner) were faced with the usual dilemma when it came to picking a winner from the eight talks and two posters. The winners were: 2005 Best Student Paper Awards 1) Daniela Vallini – University of Western Australia ($300, Winner, best oral presentation) 2) Noah Planavsky and Jennifer Murphy – Lawrence University ($200, Winners, best poster presentation) 3) Angelique Magee, OGS/Lakehead University ($100, Honourable mention, oral presentation) In addition, Eisenbrey travel awards in varying amounts were presented to students from: • Lawrence University (Noah Planavsky and Jennifer Murphy); xviii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 • • North Dakota State University (Damion Knudsen); Lakehead University (Adam Richardson, Riku Metsaranta, Dawn-Ann Trebilcock, Chris Lane, Mike Maric, Jordan Laarman, and Bjarne Almqvist). The Institute’s Board of Directors met on May 26, 2005, and brief summary of the meeting follows: 1. 2. 3. 4. 5. 6. 7. Accepted report of the Chairs for the 50th ILSG, Duluth, Minnesota Received, discussed, and accepted 2004-2005 ILSG Financial Summary from ILSG Treasurer Mark Jirsa. Approved Mark Smyk as on-going ILSG Board member Approved 2006 (52nd annual) meeting location—Sault Ste Marie, Ontario, and co-chairs Ron Sage (OGS - retired) and Ann Wilson (OGS). Replaced David Meineke as the “member from industry” on Goldich Committee with Doug Duskin. Amended the Institute’s by-laws in order to qualify for 501c3 status with the IRS. Established the ILSG Student Research Fund with US$10,000 from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. The 51st ILSG meeting was a great success and we would like to thank all the individuals who contributed to this success, including the people and businesses of Nipigon. The following organizations are thanked for their sponsorship of the meeting: Ontario Geological Survey, Lakehead University, Lake Nipigon Region Geoscience Initiative, Ontario Prospectors Association, Canadian Institute of Mining and Metallurgy (Thunder Bay Branch), Northwestern Ontario Prospectors Association, and Chaltrek Geological Supplies Inc. The field trips were well-attended and we would like to extend our thanks to the trip leaders and all those who found themselves with keys to rental cars thrust into their hands at short notice. The Municipality of Greenstone and Roxmark Mines Limited provided generous in-kind support for the BeardmoreGeraldton trip. We would also like to thank all those attendees who pitched in to help move poster boards, chairs and dining tables without having to be asked. The members of the Institute never cease to impress. Both of us were very pleased with the 51st meeting and thankful that it was not marred by accident, injury or inclement weather. We appreciated all the positive feedback from delegates, who enjoyed the small-town setting, meeting venues and varied field trips. Logistical arrangements, although much more daunting in a small community, did not prove to be insurmountable. It bodes well for those considering hosting the Annual Meeting in a smaller town. It was a thoroughly enjoyable and rewarding experience. Respectfully submitted Pete Hollings and Mark Smyk Co-Chairs, 51st ILSG Meeting xix �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Ron Sage/Ann Wilson - General Chair 2006 meeting (2009) - Ontario Geological Survey Mark Smyk (2008) - Ontario Geological Survey Steve Hauck (2007) - University of Minnesota, Duluth Mark A. Jirsa - Treasurer (2007) - Minnesota Geological Survey Laurel Woodruff (2006) - U.S. Geological Survey Peter Hollings - Secretary (2006) - Lakehead University, Thunder Bay, Ontario Session Chairs Theodore Bornhorst – Michigan Technological University Peter Hinz – Ontario Geological Survey George Hudak – University of Wisconsin – Oshkosh Helene Lukey – Cleveland Cliffs Inc. Joseph Mancuso – Bowling Green University James Miller – Minnesota Geological Survey - Duluth Richard Ojakangas – University of Minnesota - Duluth Laurel Woodruff – United States Geological Survey Local Committee Co-Chairs R. P. Sage - Ontario Geological Survey (retired), Sault Ste Marie, Ontario Ann Wilson - Ontario Geological Survey, Timmins, Ontario Program and Abstracts Editor Ann Wilson - Ontario Geological Survey, Timmins, Ontario Field Trip Guidebooks Editor R. P. Sage - Ontario Geological Survey (retired), Sault Ste Marie, Ontario Organizing Committee Nora Simm – Chartwells Dining Services Lisa Bagnall - Sault College of Applied Science and Technology Banquet Speaker Dr. Ed Walker, Petrologic Ltd. Exploring for Diamonds in Unconventional Rocks xx �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Acknowledgements Thank you to the following individuals, groups and organizations who contributed to making the 52nd Annual Meeting of the Institute on Lake Superior Geology a success. Ontario Prospectors Association Ministry of Northern Development and Mines – Ontario Geological Survey Minuteman Press - Timmins Volunteer Field Trip Leaders Volunteer Van Drivers xxi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Program Monday May 8 8:00 a.m. Field Trip 1: Classic Stratigraphy of the Huronian Supergroup – Elliot Lake Transect Leaders: Gerry Bennett (OGS-retired) and Mike Hailstone (OGS) 6:00 p.m. Return to Sault Ste Marie Tuesday May 9 8:00 a.m. Field Trip 1: Classic Stratigraphy of the Huronian Supergroup – Searchmount Transect Leaders: Gerry Bennett (OGS-retired) and Mike Hailstone (OGS) 8:00 a.m. Field Trip 2: Unusual Archean Diamond-bearing rocks of the Wawa Area Leader: Ann Wilson (OGS) 6:00 p.m. Conclusion of Trips 1 and 2 6:00 p.m. - 8.00 p.m. Registration (Sault College)) 6:30 p.m. - 9.00 p.m. Ice Breaker Social (Sault College Cafeteria) and Poster Setup (Sault College) Wednesday May 10 8:00 a.m. - 4:00 p.m. Registration (Sault College) 9:00a.m. - 9:10 a.m. Introductory Remarks – Ron Sage and Ann Wilson, Co-Chairs Technical Session I Session Chairs: Peter Hinz (Ontario Geological Survey), Helene Lukey (Cleveland Cliffs Inc.) 9:10 a.m. Hailstone, Mike An overview of geology of the Sault Ste Marie area 9:35 a.m. 10:00 a.m. Rainbird, Robert H. and Davis, William J. Detrital zircon geochronology of the western Huronian Basin Bennett, Gerry The “Kona Dolomite” of Ontario 10:25 a.m. – 10:45 a.m. Coffee Break and Poster Session Moran, Patrick*, Fralick, Philip and Hollings, Pete Geochemical constraints on the deposition of Mesoarchean banded iron formation at the Musselwhite Mine, North Caribou greenstone belt, Superior Province. 11:10 a.m. Fralick, Philip Iron formation in Neoarchean deltaic successions; Layering styles developed during siliciclastic and chemical sediment deposition, Superior Province, Canada. 11:35 a.m. Jirsa, Mark. A. and Chandler, Val W. Structure of the Biwabik Iron Formation, Mesabi Iron Range, Minnesota 10:45 a.m. 12:00 p.m. – 1:30 p.m. Lunch Break and Poster Session (ILSG Board Meeting by invitation) xxii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Technical Session II Session Chairs: Joseph Mancuso (Bowling Green University), Jim Miller (Minnesota Geological Survey – Duluth) 1:30 p.m. 1:55 p.m. Grabowski, Gary Sampling lamprophyre dikes for diamonds – Discover Abitibi Initiative Shute, Amy* and Hollings, Pete Geology and alteration associated with VMS mineralization in the Hamlin Lake area, Northwestern Ontario 2:20 p.m. – 2:45 p.m. Coffee Break and Poster Session 2:45 p.m. Cannon, William F., Horton, J. Wright Jr., and Kring, David A. The Sudbury impact layer in the Marquette Range Supergroup of Michigan 3:10 p.m. Hollings, Pete and Wyman, Derek Geochemistry of the ~2.7 Ga Blake River Group and Confederation Assemblages: Implications for supra-subduction zone volcanism in the Superior Province 3:35 p.m. Holm, Daniel K., Anderson, R., Boerboom, Terrence J., Cannon, William F., Chandler, Val, Jirsa, Mark, Miller, James, Schneider, D. A., Schultz, Klaus and Van Schmus, W. Randy Continental growth and evolution of the northern interior of the conterminous U. S. 3:55 p.m. Announcements 6:00 p. m. – 7:00 p.m. Cash Bar (Sault College Cafeteria) Annual Banquet and Award Presentation (Sault College Cafeteria) 7:00 p.m. Announcement of 53rd Annual Meeting Location 2006 Goldich Award Presentation to M.G. Mudrey Jr. 2006 Banquet Address - Dr. E. C. Walker Meeting participants not registered for the banquet are welcome to attend the address. Thursday May 11 9:00 a.m. – 10:30 a.m. Registration Technical Session III 9:00a.m. - 9:05 a.m. Announcements Session Chairs: George Hudak (University of Wisconsin-Oshkosh), Laurel Woodruff (United States Geological Survey) 9:05 a.m. 9:30 a.m. Planavsky, Noah*, Knudsen, Andrew and Shapiro, Russell Evidence for widespread distribution of iron dependent metabolisms in Precambrian oceans Waggoner, Thomas D. Sulphur Isotopes from pyrite in the Negaunee Iron Formation xxiii �Proceedings of the 52nd ILSG Annual Meeting – Part 1 9:55 a.m. Mudrey, Michael G. Jr. Statistical analysis of indoor radon data and relationships to geology in Wisconsin 10:20 a.m. – 10:45 a.m. Coffee Break and Poster Session 10:45 a.m. 11:10 a.m. Magee, M. Angelique*, Hollings, Pete and Fralick, Philip W. Geology and geochemistry of the Chimney Lake volcaniclastic breccia near Armstrong, Ontario Miller, James D. Jr. and Peterson, Dean M. The Precambrian Research Center – A new initiative to promote Precambrian field studies at the University of Minnesota Duluth 11:35 a.m. - 1:30 p.m. Lunch Break and Poster Session (Posters removed after lunch) Technical Session IV Session Chairs: Theodore Bornhorst (Michigan Technological University), Richard Ojakangas (University of Minnesota - Duluth) 1:30 p.m. 1:55 p.m. 2:20 p.m. Smyk, Mark C., Hollings, Pete and Heaman, Larry M. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario Halls, Henry C., Stott, Greg M., Ernst, R. E., and Davis, Donald W. A Paleoproterozoic mantle plume beneath the Lake Superior region Vallini, Daniela A., Cannon, William F., Schultz, Klaus J., and McNaughton, Neal J. The thermal history of low metamorphic grade Paleoproterozoic metasedimentary rocks of the Penokean orogen, Lake Superior Region: Recognizing a widespread 1786 Ma overprint using xenotime geochronology 2:45 p.m. Presentation of Best Student Paper and Poster Awards and Eisenbrey Awards 3:10 p.m. - 3:35 p.m. Coffee Break NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award 3:30 p.m. Field Trip 6 – Geology of the Paleoproterozoic East Bull Lake Intrusion departs Sault College, Sault Ste Marie for East Bull Lake; overnight at East Bull Lake Lodge 4:00 p. m. Field Trip 4 – Unusual Archean Diamond-bearing rocks of the Wawa Area, participants make their own way to Wawa; overnight in Wawa xxiv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Poster Presentations Bartingale, R. J.* and Shaw, C. A. Defining flow patterns: Paleomagnetic characteristics of the Wissota Dike Boerboom, T. J. Bedrock geological maps of the Split Rock Point and Two Harbors Northeast 7.5’ quadrangles, north shore of Lake Superior, Minnesota Brown, B. A., Czechanski, M. L., Reid, D. D. and Mudrey, M. G. Jr. New evidence for syn-depositional subsidence in the Middle Ordovician rocks of southwest Wisconsin Buchholz, T.W., Falster, A, U. and Simmons, Wm. B. Some accessory minerals of the Cary Mound granite/granophyre complex, Wood County, Wisconsin Cote, V. The Sault and District Prospectors Association Craddock, J. P., Patel, D., Porter, R., and Wirth, K. Anisotropy of magnetic susceptibility (AMS) analysis of Keweenaw Rift rhyolites, Minnesota Easton, R. M. Complex folding and faulting history in Huronian Supergroup rocks located north of the Murray fault zone, Southern Province, Ontario Gross, A.* and Holm, D. K. Kinematic analysis and monazite geochronology of the Eau Pleine and Niagara shear zones, Wisconsin Hudak, G. J., Hocker-Finamore, S. M. and Heine, J. Field distribution, petrography and lithogeochemistry of epidosites in the vicinities of Fivemile, Needleboy and Sixmile Lakes, Vermilion District, NE Minnesota Jirsa, M. A. New geological mapping of the Mesabi Iron Range Juda, N.*, Wirth, K., Craddock, J., Vervoort, J. and Andring, M. Petrogenesis of a granite xenolith in the 1.1 Ga Midcontinent Rift at Silver Bay, MN Kissin, S. A., Heggie, G. J., Franklin, J. M., Karimzadeh Somarin A. Sulphide saturation mechanisms in gabbroic intrusions in the Nipigon Embayment MacTavish, A. MetalCORP Ltd. Big Lake Cu-Zn-Ag-Au-Co, Ni-Cu-PGE and Mo Property Magee, A. Mining and exploration activity in northwestern Ontario Miller, J. D., Jr. and Severson, M. J. Geology of the Duluth Complex in the four Babbitt 7.5’ quadrangles, northeast Minnesota Mudrey, M. G., Jr. Statistical analysis of indoor radon data and relationships to geology in Wisconsin Peterson, D. M. 3D visualization of mafic intrusions in the Duluth Complex, northeastern Minnesota xxv �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Rousell, D. H. Unresolved problems and the evolution of Sudbury geology Stonier, P.*, Holm, D. K., Medaris, L. G., Jr. and Schneider, D. Characterizing the monazite fingerprint of Paleoproterozoic (Statherian) metasedimentary sequences in central Wisconsin Wirth, K. R., Vervoort, J., Craddock, J. P., Davison, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S. and Walker, E. Source rock ages and patterns of sedimentation in the Lake Superior region: Results of preliminary U-Pb detrital zircon studies NOTE: Asterisk * denotes a student eligible for a Best Student Poster Award Friday May 12 8:00 a.m. Field Trip 3: Keweenawan Rocks of the Point Mamainse Area Leaders: Tom Hart and Anthony Pace (OGS) 8:00 a.m. Field Trip 4: Unusual Archean Diamond-bearing rocks of the Wawa Area Leader: Ann Wilson (OGS) 8:00 a.m. Field Trip 5: Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features – Mackinac Island, Michigan Leader: Ron Sage (retired-OGS) We will be meeting at the Arnold Transit Co. boat dock in St. Ignace MI no later than 9:15 a.m. to catch the 9:30 a.m. ferry 8:00 a. m. Field Trip 6: Geology of the Paleoproterozoic East Bull Lake Intrusion Leaders: Mike Easton (OGS) and R. S. James (Laurentian University) 6.00 p.m. Return of Trips 3 and 6 to Sault Ste Marie, Ontario Field Trip 4 concludes in Wawa Field Trip 5 concludes at ferry dock on Mackinac Island, Michigan xxvi �Proceedings of the 52nd ILSG Annual Meeting – Part 1 DEFINING FLOW PATTERNS: PALEOMAGNETIC CHARACTERISTICS OF THE WISSOTA DIKE. BARTINGALE, R.J. and SHAW, C.A., Department of Geology, University of Wisconsin – Eau Claire, Eau Claire, WI 54702-4004 We analyzed a gabbro dike intruding Precambrian granite below the Lake Wissota Dam in western Wisconsin. Data consisted of alternating field demagnetization and anisotropy of magnetic susceptibility measurements (AMS). Chan (1991) interpreted previous results as consistent with a Keeweenawan age (1.1 Ga) for the dike. However, research done by Macouin et. al. (2003) show similar dikes in the upper midwest and above Lake Superior have been reinterpreted to be related to the 2.07 Ga Kenora-Kabetogama Dike swarm based on moderately SE-plunging paleomagnetic directions. This study was designed to test the age interpretation of the Wissota dike and magma flow patterns. AMS data taken with respect to the major mineral axis indicates a north-east trending, horizontal flow pattern within 4 meters of the north contact and vertical flow in the center. This suggests the concentrations of feldspar phenocrysts on the northern contact were formed near the present level, possibly being fed by the vertical flowing magma. The poles have strong correlation in the center, but weaken within 4 meters of the contact. When fit to a girdle, many samples show a strong foliation. Paleomagnetic poles in several gabbro sites have a characteristic remnant magnetization plunging between 28° and 289° in a WNW direction. Samples have an N-directed overprint we interpret as recent, and record one episode of magnetism. Plotted on an apparent polar wander path for North America, the poles plot near 24° north and 176° west, which is consistent with ages of approximately 1.1 Ga. We conclude that the Wissota dike is probably Keeweenawan in age (Figure 1). References Chan, Lung, 1991, Paleomagnetism of central Wisconsin dike swarm; constraints on thermomechanical model of Midcontinent Rift: Institute on Lake Superior Geology Proceedings and Abstracts, v.37, Part1, p.23. Macouin, M., Valet, J.P., Besse, J., Buchan, K., Ernst, R., LeGoff, M., and Scharer, U., 2003, Low paleointensities recorded in 1 to 2.4 Ga Proterozoic dykes, Superior Province, Canada: Earth and Planetary Science Letters, v. 213, p. 79-95. 1 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. Wissota dike virtual geomagnetic poles (VGPs). 2 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE "KONA DOLOMITE" OF ONTARIO BENNETT, GERALD, 123 LaSalle Court, Sault Ste Marie, ON The stratigraphic similarity between the Chocolay Group of the Marquette Range Supergroup of Michigan and the lower part of the Cobalt Group of the Huronian Supergroup of Ontario has been recognized for some time. Young (1983) and others accepted the correlation but at that time there were also doubters. Both Chocolay Group and the Cobalt Group lie (at least in part) on Archean basement rocks. The lowermost formations of both groups contain rocks generally considered to be glaciogenic, which are overlain by formations dominated by quartz-arenite. But there the lithologic similarity seemed to end. The Mesnard Formation of the Chocolay Group is overlain by the Kona Formation which contains a thick sequence of dolostone, whereas the (proposed equivalent) Gordon Lake Formation is predominantly a siltstone/sandstone sequence. There have however been reports of thin beds and nodules of dolostone within the Gordon Lake formation by Hoffman et al. (1980) and Jackson (1994). In 1986 Peter Born of the Ontario Geological Survey called the writers attention to a previously unmapped dolostone unit apparently overlying the Lorrain Formation in Fenwick Township, northwest of Sault Ste Marie, Ontario. Subsequent more detailed mapping by the writer revealed that the unit is comprised of at least 10 m of laminated dolostone and chert with clastic dolostone and oolitic dolostone. The writer correlated the dolostone of Fenwick Township with the Gordon Lake Formation of the Huronian Supergroup. Mr. Ken Hatfield, then of Lake Superior State University, pointed out the similarity to the Kona Formation of the Marquette area (Bennett et al., 1989, Born, 1988). No stromatolitic structures comparable to the "big cusp" dolomite of the Kona Formation were noted, but that some thinly laminated units are probably stromatolitic structures or algal mats (Personal communication, Dr. Hans Hoffman, 1990). The occurrence in Fenwick Township is strikingly similar to the dolostone of the Kona Formation of Michigan. Given the recent geochronological studies of Vallini et al. (2005), there is now little doubt that the Cobalt Group of Huronian Supergroup may be correlated with the Chocolay Group of the Marquette Range Supergroup. References Bennett, G., Leahy, E.J, Melisek, J. Born, P. and Hatfield, K. 1989. Sault Ste. Marie Resident Geologists District–1988; in Report of Activities 1988, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 142, p. 207-217. Born, Peter, 1987. Geology of the Havilland Bay – Goulais Bay Area District of Algoma; Ontario Geological Survey, Open File Report 5602, 114 p with map at a scale of 1:15 840 (1 inch to ¼ mile) Hoffman, H. J., Pearson, D.A.B. and Wilson, B.H., 1980. Stromatolites and fenestral fabric in Early Proterozoic Huronian Supergroup, Ontario; Canadian Journal of Earth Sciences, v.17, p.1351-1357. Jackson, S. L., 1994.Geology of the Aberdeen area; Ontario Geological Survey, Open File Report, 5903, 69p Vallini, Daniela, A., Cannon, William, F, and Schulz, Klaus J., 2005. New age data for the Chocolay Group, Marquette Range Supergroup: Implications for the Paleoproterozoic Evolution the Lake Superior and Lake Huron regions. Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part I – Proceeding and Abstracts, v.51 part 1 Young, G.M. 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir, v.160, p.15-32. 3 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 BEDROCK GEOLOGIC MAPS OF THE SPLIT ROCK POINT AND TWO HARBORS NORTHEAST 7.5-MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, TERRENCE J., Minnesota Geological Survey, boerb001@umn.edu The Minnesota Geological Survey is continuing quadrangle-scale geologic mapping of 7.5' quadrangles adjacent to Lake Superior as part of the U.S. Geological Survey STATEMAP program. This mapping effort has resulted in seven published geologic maps in an area from Duluth to Split Rock Point (Fig. 1A). Work is currently in progress on the Little Marais, Schroeder, and Tofte quadrangles, and Lutsen will be mapped in the coming year. Field mapping is at scale 1:12,000, and map compilations are at 1:24,000. The North Shore is experiencing burgeoning development, creating a growing need for understanding bedrock aquifers and for identifying construction resources. Nearly all water wells near Lake Superior are finished in bedrock aquifers, and saline brines are commonly encountered. Refining the volcanic stratigraphy is the first step in understanding where these brines originate. Identification of intrusive rocks is the first step in locating sources for crushed-rock aggregate. Also, mapping has identified potential sources of paving stone, for which new sources are being pursued by landscaping companies. Thus, the goal of this mapping is to refine the knowledge of the volcanic and intrusive rocks for societal needs, as well as to provide a geologic framework for ongoing studies of the geochemical evolution of the Keweenawan Midcontinent rift system, through collaborative studies with staff from Macalester College. TWO HARBORS NE Prior to this study, no mapping had been done in this quadrangle. Although parts of the quadrangle contain few bedrock outcrops, areas underlain by intrusive rocks are generally well exposed. The newly recognized, informally named, London intrusion is a crudely layered or composite intrusion with a basal laminated ferrogabbro, an intermediate granophyric ophitic gabbro, and a cap of ophitic diabase. Coarse-grained felsicintermediate granophyric rocks form an irregular layer between the ophitic gabbro and the upper ophitic diabase, and also lenses within the other units. Fine-grained ferromonzodiorite occurs locally at the base of the intrusion, likely as a hybridized partial melt of adjacent andesite. Other intrusions include the northward extension of the Silver Creek diabase and Lafayette Bluff diabase, and other poorly exposed units whose distributions are based largely on geophysical data. Sporadically exposed volcanic rocks include the southwestern portion of the Gooseberry River lavas (Green, 2002), here composed of olivine tholeiite in the upper part and transitional basalt to andesite in the lower part, and a newly recognized sequence termed the Gustafson Hill lavas composed of variably porphyritic ferroandesitic to basaltic rocks, separated from the Gooseberry River lavas by the Silver Creek diabase. 4 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SPLIT ROCK POINT The Split Rock Point quadrangle is dominated by volcanic rocks of the Gooseberry River lavas, with subordinate mafic to felsic intrusions that include the Silver Creek and Beaver River diabase, the Split Rock intrusion, and a narrow multilithic breccia dike (Boerboom, 2004; Boerboom and others, 2004). The upper Gooseberry River lavas include a thick porphyritic basalt flow and a faultsliced sequence of andesite sandwiched between ophitic olivine tholeiite flows. The lower Gooseberry River lavas are poorly exposed, but available outcrops indicate they are composed dominantly of andesitic rocks continuous with those mapped in the Two Harbors Northeast quadrangle. The Split Rock intrusion is a hypabyssal, north–south elongate body with the form of a south-plunging syncline that has a thin, lower mafic phase coeval with the dominant phase of pink, flow-banded, weakly porphyritic felsite that contains scattered but ubiquitous small mafic enclaves. References Boerboom, T.J., 2004, Newly recognized diatreme breccia dikes on Lake Superior near Two Harbors, Minnesota [abs.]: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, v. 50, p. 39. Boerboom, T.J., and Green, J.C., 2004, Bedrock geology of the Split Rock Point quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-147, scale 1:24,000. ———2005, Bedrock geology of the Two Harbors NE quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-155, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock geology of the French River and Lakewood quadrangles, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-128, scale 1:24,000. ———2002b, Bedrock geology of the Knife River quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-129, scale 1:24,000. Boerboom T.J., Green, J.C., and Miller, J.D., Jr., 2003a, Bedrock geologic map of the Castle Danger quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-140, scale 1:24,000. ———2003b, Bedrock geologic map of the Two Harbors quadrangle, Lake County Minnesota: Minnesota Geological Survey Miscellaneous Map M-139, scale 1:24,000. Boerboom, T.J., Miller, J.D., Jr., and Green, J.C., 2004, Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and Beaver Bay Complex: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, v. 50, pt. 2, Field trip guidebook, p. 46-85. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, chapter 5 of Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-105. 5 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. A. Index map showing the location of mapped quadrangles along the North Shore of Lake Superior. M-128–Boerboom and others (2002a); M-129–Boerboom and others (2002b); M-139– Boerboom and others (2003b); M-140–Boerboom and others (2003a); M-147–Boerboom and Green (2004); M-155– Boerboom and Green (2005). Little Marais, Schroeder, and Tofte to be published in 2006; Lutsen in 2007. 6 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 NEW EVIDENCE FOR SYN-DEPOSITIONAL SUBSIDENCE IN THE MIDDLE ORDOVICIAN ROCKS OF SOUTHWEST WISCONSIN BROWN, B.A., CZECHANSKI, M.L. Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison WI 53705 REID, DANIEL D. Wisconsin Dept. Transportation, 3502 Kinsman Blvd., Madison, WI 53704 MUDREY, M.G. JR.† 106 Ravine Road, Mount Horeb, WI 53572 Extensive new rock cuts and exposures were created during the rebuilding of U.S. Highway 151 into a modern 4-lane highway through the Driftless Area of southwest Wisconsin. These cuts, some exceeding 100 feet high, provide a unique cross section of the Middle Ordovician rock of the historic Upper Mississippi Valley Base Metal District of zinc and lead. The new cuts provide a detailed view of the stratigraphy, and expose some structures not previously described in the region. Examples of collapse structures and local block faulting have been recognized throughout the mining district for many years. These structures could be seen in older road cuts, and they were described in many early reports on the mining district. Structures of this type have traditionally been interpreted as pitch-and- flat structures, which resulted from solution and collapse related to the formation of the zinc-lead deposits. The oldest collapse features observed in the new cuts formed in Early Middle Ordovician time, during deposition of the St. Peter Sandstone. The youngest known at this time formed in Late Middle Ordovician time, during deposition of the carbonate of the Galena Formation. These features formed as much as 200 million years earlier than the zinc-lead mineralization, which has been dated as Early Permian in age. The early syn-depositional collapse structures are interpreted to be the result of local collapse of paleokarst features developed in the underlying carbonate of the Early Ordovician Prairie du Chien Group. Extensive karst is known to have formed during the interval of aerial exposure following lithification of the Prairie du Chien rock and prior to deposition of the St. Peter Sandstone. This interval contains a major regional unconformity which marks the SaukTippecanoe sequence boundary throughout the region. Collapse occurred as overlying sediment accumulated and compacted, prior to complete lithification. The role of these syn-depositional features in controlling the path of mineralizing fluids is unknown. The few examples known at this time contain no significant mineralization, although mineralized pitch and flat structures and gash vein lead deposits are known to occur nearby. It is possible that the paleokarst features were small and localized, and were not important as conduits for mineralizing fluids, which migrated along regional tectonic features. In contrast, areas of sulfide mineralization were typically associated with extensive rock alteration and deep weathering, which required modifications to the design of cuts and structures, and use of alternative slope stabilization methods during construction. 7 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SOME ACCESSORY MINERALS OF THE CARY MOUND GRANITE/GRANOPHYRE COMPLEX, WOOD COUNTY, WISCONSIN. BUCHHOLZ, THOMAS W.†1, FALSTER, ALEXANDER U.2, and SIMMONS, WM. B. 2, 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2 Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148. Early Proterozoic (1,833 ± 4 Ma) granite, granophyre and comagmatic rhyolite outcrop on Cary Mound in western Wood County, WI, and are exploited in several quarries. All phases of the complex are cut by numerous faults and fractures that served as avenues for fluid transport, resulting in widespread chloritization of the granite and granophyre and development of thin hydrothermal veins. Several studies (Sims, 1990; Bruesewitz and Cordua, 2003) postulate that the complex may be a collapsed caldera and indicate that the complex may be of anorogenic or late orogenic origin. Granophyric phases are locally miarolitic, particularly in the Haske quarry, and host a complex mineralogy ranging from simple magmatic through pegmatitic to hydrothermal mineralization, even though no pegmatites sensu stricto have yet been found on Cary Mound. Miaroles may be either simple vugs lined with crystals of quartz, microcline +biotite, probably formed as a result of local volatile saturation, or may have marginal pegmatitic facies marking the transition from granophyre to miarole, the latter primarily noted in the Haske quarry. In areas where such miaroles are in close proximity, pegmatitic margins may merge and form larger areas of pegmatitic texture. These pegmatitic facies may represent pods of pegmatitic melt generated by fractionation of the crystallizing granophyre; if so, the melt was probably enriched in volatiles and incompatible elements. Typical NYF (niobium-yttrium-fluorine = typical A-type granitic association as opposed to LCT (lithium-cesium-tantalum = typical S/I-type granitic association) pegmatite mineralization is present in these pegmatitic phases; quartz, microcline, fluorite, allanite(Ce) and zircon. Locally ferrocolumbite, samarskite-(Y) and thorite have been identified; a Ti-rich Y-Nb oxide mineral has been noted as well and may be polycrase-(Y), but confirmation is required, and a number of additional phases await further study. Pegmatite-bordered miaroles may be quartz cored or filled with quartz + late chlorite, sulfides, fluorite, siderite and calcite; these are interpreted as products of a pervasive late hydrothermal phase introduced along networks of thin fractures. The most abundant sulfides are pyrite, chalcopyrite and pyrrhotite, though sphalerite, galena, marcasite, arsenopyrite and rarely molybdenite may be present. Unusual acicular sulfide crystal morphologies are sometimes present (Buchholz et al, 1997). Small silvery-colored grains of Cu-Co-Ni sulfides have been noted; no further identification work has been done due to paucity of material. Barite is common in small amounts but is usually inconspicuous. Gypsum of secondary, weathering origin may be locally common in small amounts. Cassiterite is uncommon but has been identified from thin fissures in several sites (Cepress and County quarries), and appears to have formed early, probably as a highertemperature hydrothermal phase perhaps transitional from magmatic/pegmatitic to 8 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 hydrothermal/pneumatolytic. Xenotime-(Y) from a thin fissure in the Haske quarry may be of similar origin. Hydrothermal alteration and chloritization is widespread throughout the complex, and associated mineralized veins and fissures in general reflect the late hydrothermal mineralization noted in miaroles. Although most are rich in chlorite, one vein system in the Cary-Rock Road quarry is mineralized with fine-grained lithian muscovite (1.1 wt. % Li2O) associated with pyrite, chalcopyrite, siderite, apatite and barite. Pyrrhotite is generally absent from vein mineralization, whereas the Fe-S paramorphs pyrite and/or marcasite are usually common. Tiny grains of molybdenite are often common in metasomatized granite/granophyre in the Haske quarry. Sparse millerite (NiS) has recently been identified from a hydrothermal vein in the Haske quarry. Rutile is uncommon but has been noted from the Cepress and Haske quarries. Small late-formed crystals and grains of a LREE-phosphate (probably either monazite-(Ce) or rhabdophane(Ce) have been found on chlorite and pyrite from the Haske and Cepress quarries. The mineralization present in pegmatitic miarole margins may indicate the parent magma had locally evolved to a Nb, Y and F-enriched phase. Abundant fluorite and the existence of lithium-bearing muscovite veins suggest the possibility of pneumatolytic or greisen-type mineralization within the complex. The pervasive chloritic hydrothermal alteration and sulfide mineralization suggest that small vein-type sulfide deposits may be present. However locating these, if they exist, may be challenging due to extensive forests and remnant Cambrian sandstone cover. References Bruesewitz, Jeff and Cordua, W.S., 2003, The Cary Mound Granite: A mineralized collapsed caldera in Wood County, Wisconsin, abstract, Geological Society of America, Abstracts and Programs - North Central Section annual meeting, vol. 35 #2, St. Louis, Mo, p.45 Buchholz, Thomas W., 1997, Apatite and Lithium Bearing Muscovite from Central Wisconsin: Mineral News, June 1997, p.6. Buchholz, T. W., Falster, A. U., and Simmons, Wm. B., 1997, An Unusual Miarolitic Mineral Assemblage From Central Wisconsin, abstract, Rochester Mineralogical Symposium, Program and Abstracts Volume, p. 8. Sims, P.K., 1990, Geologic Map of Precambrian Rocks, Eau Claire and Green Bay 1º x 2º Quads, Central Wisconsin, U.S. Geological Survey Map I-1925 9 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE SUDBURY IMPACT LAYER IN THE MARQUETTE RANGE SUPERGROUP OF MICHIGAN CANNON, WILLIAM F.† and HORTON, J. WRIGHT, JR., U.S. Geological Survey, Reston VA 20192 KRING, DAVID A., Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ The 1850 Ma meteorite impact at Sudbury, Ontario created a crater estimated to be 180 km in diameter (Abramov and Kring, 2004). A layer of material formed by the Sudbury impact has been well documented in northwestern Ontario and northern Minnesota (Fig. 1) (Addison and others, 2005). In Michigan, only about 500 km from the center of impact, marine sediments of the Marquette Range Supergroup were being deposited and should record the impact. One possible record of an impact is addition to local sediments of material excavated from the crater. This material may vary from coarse fragments of the target rock from the ejecta curtain to finer particles from the impact-generated dust cloud, including accretionary lapilli and mineral grains bearing shock metamorphic features. At the time of impact at least parts of the Michigan sedimentary basin were in shallow-water suggesting the likelihood of major tsunami-related deposits. We are investigating possible impact generated rocks at five sites in northern Michigan (fig.1), which are at a comparable stratigraphic horizon to the Ontario ejecta and are similar petrographically. All localities are at or within a few hundred meters above the base of the Baraga Group and may record both airborne and tsunami deposition. Baraga Basin- layer 1-15 m thick in lower part of Michigamme Formation. Well developed accretionary lapilli (fig. 2) and planar deformation features (fig. 3). West Dead River- isolated outcrop of bedded to massive lapilli-rich material (fig. 4). Contains angular chert fragments to about 1 m diameter. Strong carbonate replacement. At least 2 m thick and probably at least 100 m above base of Michigamme Formation. East Dead River- bed of breccia about 30 m thick (fig. 5, 6). Sparse lapilli. Crudely graded with coarsest clasts at base. Underlain by banded iron-formation and overlain by black pyritic slate. Contains numerous clasts of chert. About 300 m above base of Michigamme Formation. Marenisco- bed about 2 m thick near base of Copps Formation. Coarse sand to conglomerate containing many clasts of underlying Archean granite. Also contains slabs of chert to about 2 m diameter and quartz grains with possible relict planar deformation features. Strong carbonate replacement. Republic- numerous boulders of lapilli-rich breccia in gravel pit, possibly locally derived. Our study of these localities and the search for additional sites is in its early stages. We hope that calling attention to these likely impact-related rocks will encourage additional searches and discoveries. We suspect that most new “discoveries” will result from recognizing telltale signs of impact processes within already known “unusual” breccias or “volcanic” units within this narrow stratigraphic interval. References Abramov, O, and Kring, D.A., 2004, Numerical modeling of an impact-induced hydrothermal system at the Sudbury crater: J. Geophys. Res., v. 109, p.1-16. Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact: Geology, v. 33, p. 193-19 10 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1 Location of impact layer sites Figure 4. West Dead River. Bedded accretionary lapilli unit unconformably overlain by massive breccia with angular chert. Card 8 cm long. Figure 2. Baraga Basin. Accretionary lapilli in drill core. Figure 5. East Dead River. Photomicrograph of breccia with “volcanic” shards and rounded quartz grains. Figure 3. Baraga Basin. Quartz grain with two sets of relict planar deformation features. Figure 6 East Dead River. Multi-lithic breccia of chert and a variety of “volcanic” fragments in finer breccia groundmass. 11 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE SAULT AND DISTRICT PROSPECTORS ASSOCIATION COTE, VIVIENNE, President, Sault and District Prospectors’ Association, Sault Ste Marie, Ontario The Sault and District Prospectors Association (SDPA) has been in existence since the early 1970’s. The purpose of the association is to promote mineral exploration in the area and raise awareness of the role mineral development plays in the economy of the region and the north in general. Although the group is relatively small it is quite active with speakers from various backgrounds and interests presenting a diverse array of topics at the monthly meetings. The highlight of the year is the SDPA annual field trip. The trips proved so popular that in 2005 a fall field trip was added. The latest trips have included the Archean diamond bearing rocks in the Wawa area, the Keweenawan rocks of the Mamainse Point area, Huronian stratigraphy of the Elliott Lake area as well as the Eagle River Mine in the Wawa district. The poster is a visual overview of the various fieldtrips undertaken by the group against a backdrop of the general geology of the area. A wide variety of participants have attended including prospectors, geologists, students, rock hounds and of course, our mascot, the dog “Chloe”. The SDPA is an associate member of the Ontario Prospectors Association. 12 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) ANALYSIS OF KEWEENAW RIFT RHYOLITES, MINNESOTA CRADDOCK, JOHN P. and PATEL, DHIREN, Geology Dept., Macalester College, St. Paul, MN 55105; PORTER, RYAN, Geology Dept., Whitman College, Walla Walla, WA; and WIRTH, KARL, Geology Dept., Macalester College, St. Paul, MN 55105 The North Shore Volcanic Group (NSVG) of the 1.1 Ga Midcontinent Rift System (MRS) in Minnesota is dominated by basalt, with approximately 10–25% of the bi-modal igneous suite being composed of felsic flows (rhyolites and icelandites). Several of the rhyolite flows may be rheomorphic ignimbrites due to their vast expanse and presence of tridymite paramorphs and local exposures of flow banding, laminations and lineations (Green and Fitz, 1993). In this study we identified four well-exposed rhyolite flows between Grand Portage and Duluth where the over and underlying mafic flows clearly distinguish the rhyolite flow thickness and outcrop character. Oriented samples were collected from the bottom, middle and top of each of the four flows and oriented cores (or equant cubes) were analyzed at the Institute for Rock Magnetism, University of Minnesota using the “RolyPoly”, which is an alternating current (AC) susceptibility bridge for determining anisotropy of low-field magnetic susceptibility. An alternating current in the external "drive" coils produces an alternating magnetic field in the sample space with a frequency of 680 Hz and amplitude of up to 1 mT. The induced magnetization of a sample is detected by a pair of "pickup" coils, with a sensitivity of 1.2*10-6 SI volume units. For anisotropy determination, a sample is rotated about three orthogonal axes, and susceptibility is measured at 1.8° intervals in each of the three measurement planes. The susceptibility tensor is computed by least squares from the resulting 600 directional measurements. The output is a trend and plunge for each of the principal susceptibility tensors (i.e. Kmax, Kint, Kmin), mean susceptibility, and three axial ratios L=Kmax/Kint, F=Kint/Kmin, and P=Kmax/Kmin (lineation, foliation, and degree of anisotropy respectively). Principal tensors are plotted on lower hemisphere steroenet projections. AMS is, thus, a magnetic proxy for interpreting magmatic flow in the rhyolites. From north to south, we sampled the Kimball Creek (near Hovland, n=56, 366 m thick), Devil Track (near Grand Marais, n=61; 250 m thick), Palisade Head (near Silver Bay, n=62, 100 m thick), and Lakewood (north of Duluth, n=58, 78 m thick) rhyolites. Despite an average anisotropy for the sample suite (n=237) of 7.5%, only the Devils Track (base and top) and Palisade rhyolites preserve a layer-parallel Kmax grouping that is interpretable as rift-normal, suggesting northwestward eruption from the rift axis (Fig. 1). References Green, J.C. and Fitz, T. J. III, 1992, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota; petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, 177-196. Rochette, P., Jackson, M., Aubourg, C., 1992, Rock magnetism and the interpretation of anisotropy of magnetic susceptibility: Reviews of Geophysics, v. 30, p. 209-226. 13 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 14 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 COMPLEX FOLDING AND FAULTING HISTORY IN HURONIAN SUPERGROUP ROCKS LOCATED NORTH OF THE MURRAY FAULT ZONE, SOUTHERN PROVINCE, ONTARIO EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, mike.easton@ndm.gov.on.ca Huronian Supergroup strata located north of the Murray fault zone are generally thought to record a relatively simple structural history of broad folding and faulting, related primarily to distal effects of the ~1835 Ma Penokean orogeny. Detailed mapping by the Ontario Geological Survey in Porter and Vernon townships (Easton 2005, 2006), northeast of Agnew Lake indicates that, at least in the area immediately west of Sudbury, this view is incorrect. At least 2 periods of folding are present, roughly orthogonal to one another - the resulting interference forms a dome and basin pattern (Figure 1). F1 folds Nipissing gabbro intrusions present in the lowermost part of the stratigraphy, whereas Nipissing gabbro appears to be emplaced along fractures related to F2 axial planes. This suggests either multiple periods of gabbro emplacement, or that gabbro emplacement occurred synfolding. In either case, folding cannot be significantly younger than 2210 Ma. The map pattern is also affected by at least 5 major fault sets, 4 of which are post-folding. The earliest faults are north-trending, and juxtapose Archean granitic basement against Huronian Supergroup strata. These faults appear to have been fluid conduits, as indicated by the presence of large quartz vein systems and microbrecciation in Archean basement, and hydrothermal annealing of quartz in sedimentary rocks, adjacent to the faults. Eastnortheast faults also juxtapose Huronian strata against basement rocks, but are post- F1 folding, with both vertical and lateral movement. They may be associated with a set of north to northeast, dominantly normal faults, which may have an older thrust component. Most significant in terms of map pattern, at least in the southern part of the map area closest to the Murray fault system, are east to east-northeast normal faults, across which major changes in stratigraphic level occur. There may be a thrust component to these faults, but if so, it has been obscured by subsequent vertical movement, and the fact that the east to east-northeast faults are the loci for the development of extensive zones of Sudbury breccia. The localization of Sudbury breccia along this fault set suggests that it may have developed at ~1850 Ma, due to the Sudbury impact or the peak of the Penokean orogeny, or both. Finally, significant vertical displacement, occurs along a major set of closely spaced northwest-trending faults. Some of these faults are the loci of Sudbury swarm diabase dikes (~1240 Ma). The dikes are undeformed, which suggest that this fault set formed between 1850 and 1240 Ma. The complex history of the area provides new evidence for an earlier orogenic event in the region (“Blezardian?”), and has major implications for detailed stratigraphic correlation of Huronian Supergroup strata and mineralized Nipissing gabbro intrusions. 15 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. Simplified geological map of the northeast shore of Agnew Lake, showing the distribution of fold styles within Porter and southern Vernon townships. The contact between the Mississagi and Bruce formations has been highlighted to illustrate the fold pattern, and units stratigraphically above the Bruce Formation are shown by a pattern. Between the Cameron Creek and Midport faults, the area is dominated by a dome and basin geometry, indicating the presence of two fold generations, with approximately perpendicular axial planes. North of the Midport fault, the early, north-oriented fold style (F1) dominates. Abbreviations: BB = Big Swan basin, CB = Cygnet Lake basin, HB = Hunter basin, PB = Porter basin, SB = Sutherland basin, VS = Vernon syncline. References Easton, R.M. 2005. Geology of Porter and Vernon townships, Southern Province; in Summary of Field Work and Other Activities, 2005, Ontario Geological Survey Open File Report 6172, p.13-1 to 1320. Easton, R.M. 2006a. Geology of Porter and Vernon townships; Ontario Geological Survey, Preliminary Map P.2845. Scale 1:20 000. Colour, with Marginal Notes. 16 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 IRON FORMATION IN NEOARCHEAN DELTAIC SUCCESSIONS: LAYERING STYLES DEVELOPED DURING SILICICLASTIC AND CHEMICAL SEDIMENT DEPOSITION, SUPERIOR PROVINCE, CANADA FRALICK, PHILIP, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada (philip.fralick@lakeheadu.ca) Neoarchean iron formations (IFs) developed in volcanically quiescent, shallow marine settings consist of magnetite- and/or hematite-rich chemical sediments interbedded with siltstones and slates. The mechanism responsible for depositing such successions contrasts with the two principal models for iron hydroxide or oxyhydroxide precipitation from early Precambrian seawater. The deposition of large Paleoproterozoic iron formations through the mixing of Fe+2 enriched, deep ocean waters with the oxygenated waters on shelves is generally accepted (Cloud 1973, Holland 1973, Pufahl and Fralick 2004). In contrast, many Archean IFs appear to have formed through the venting of hydrothermal fluids associated with volcanically active terrains (Fralick and Barrett 1995). The latter model is not applicable to rocks formed in shallow, volcanically inactive areas and the former has only been applied to Paleoproterozoic shelfs where precipitation was occurring during the oxygenation of the Earth’s atmosphere. The shallow water, Neoarchean iron formations form a unique class of IFs where precipitation was driven by factors other than upwelling or hydrothermal venting. This IF type was examined in the Beardmore-Geraldton area of Wabigoon Subprovince and in the Eagle Island Group of Uchi Subprovince. The shallow water Neoarchean iron formations described here were primarily deposited on flooding surfaces overlying fluvial channel and shore-proximal braid delta deposits. Magnetite and/or hematite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces on barforms to ripples. Additionally, the iron oxides are present as: disseminated detritus in the upper portion of thin graded siliciclastic layers; intervals of finely parallel laminated hematite and jasper, or magnetite and magnetite+chert, separating clastic layers; and, micro-ripple laminated jasper with hematite drapes. The chemical sediments precipitated in the water column of the nearshore deltaic environment and accumulated during periods of lower current activity and siliciclastic supply. The ferric compounds were redistributed during intervals of river plume outflow, especially accumulating in association with fine-grained detritus in event layers formed where the plume lost contact with the bottom (Fig. 1). Offshore equivalents of these assemblages do not contain IF. The model presented for IF deposition relies on an elevated nutrient flux (N, P) in the near-shore that stimulated microbially induced oxidation of Fe+2. This implies the existence of thriving microbial communities in Neoarchean, near-shore settings; communities of organisms that were able to produce their energy by photosynthesis, oxidize iron either intra- or extra-cellularly, and generate thick successions of IF (Fralick and Pufahl in press). 17 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 References Cloud, P.E., 1973. Paleoecological significance of banded iron-formation. Econ. Geol., v. 68, p. 1135-1143. Fralick, P.W. and Barrett, P.J., 1995. Depositional controls on iron formation associations in Canada; in, G. Plint (ed), Facies Analysis. Int. Ass. of Sedimen., Spec. Pub 22, p. 137-156. Fralick, P.W. and Pufahl, P.K., in press. Iron formation in Neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts. Jour. of Sed. Res. Holland, H.D., 1973. The oceans: a possible source of iron in iron formations. Econ. Geol., v. 68, p.1169-1172. Pufahl, P.K. and Fralick, P.F., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA. Sedimentology, v.51, p.791-808. Fig. 1. Schematic representation of the processes responsible for depositing iron formation in shallow Neoarchean settings. 18 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SAMPLING LAMPROPHYRE DIKES FOR DIAMONDS; DISCOVER ABITIBI INITIATIVE GRABOWSKI, GARY, District Geologist, Ontario Geological Survey, Kirkland Lake, Ontario gary.grabowski@ndm.gov.on.ca The Discover Abitibi Initiative is funded by the private sector and, the federal and provincial governments (respectively Industry Canada through FedNor and the Ontario Ministry of Northern Development and Mines through the Northern Ontario Heritage Fund). The program is designed to stimulate mineral exploration in the Ontario portion of the Abitibi greenstone belt. A project to sample lamprophyre dikes, in the Kirkland Lake – Cobalt area, was approved by the Discover Abitibi program in July, 2004. Forty-five samples, each weighing 24 kg, were submitted to SGS Lakefield Research Ltd. in Lakefield, Ontario for litho-geochemical analysis and diamond extraction, selection and description. Six of the forty-five samples submitted returned diamonds. Samples GGDA0402 and GGDA0432 each returned one microdiamond. Samples GGDA0433, GGDA0435 and GGDA0441 returned 5, 3 and 23 microdiamonds respectively. Sample GGDA0410 contained one 0.011 carat (2.214 mg) macrodiamond. The results of this project demonstrate that diamonds occur in the lamprophyric rock from the Kirkland Lake – Cobalt area. • A 25 kg sample represents about one cubic foot of rock. Although every attempt was made to collect as representative a sample as possible from each exposure, the relatively small volume sampled may have easily missed a diamond. Therefore, sample locations that did not return a diamond should not be considered to barren. • Further study is needed to determine where the diamonds are located within the dikes. Most dikes sampled that returned diamonds contained xenoliths. Spider Resources Ltd. has recently postulated that the diamonds are found in the xenoliths on their Wawa property. • A variety of rock types host lamprophyre dikes and breccia, including all types of metavolcanic and metasedimentary rocks, as well as felsic intrusive rocks including granodiorite, granite and syenite. No preference is apparent for those that contain diamonds. • There are numerous lamprophyre locations that were not sampled in this project. Published Ontario Geological Survey (and its predecessors) can be used to locate these exposures. • The Kirkland Lake – Cobalt area hosts more than 30 kimberlite pipes, over half of which are diamondiferous. There are many targets being tested for potential kimberlite. In May 2005, Tres-Or Resources Ltd. discovered a kimberlite pipe on its Temagami North property, Lapointe 1 target, located 16 km northwest of sample GGDA0402. 19 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 KINEMATIC ANALYSIS AND MONAZITE GEOCHRONOLOGY OF THE EAU PLEINE AND NIAGARA SHEAR ZONES, WISCONSIN GROSS, AMANDA, and HOLM, D.K., Geology, Kent State University, Kent OH, agross4@kent.edu; SCHNEIDER, D.A., Geological Sciences, Ohio University, Athens, OH Introduction. The southern margin of Laurentia experienced several episodes of arc accretion that account for the growth of new continental crust during the late Paleoproterozoic, 1900-1600 Ma. The Niagara fault zone and the Eau Pleine shear zone are structural remnants of an ancient arc-continent collision that occurred during the Penokean orogeny (1870-1830 Ma). As Laurentia continued to grow, these sutures likely persisted as zones of weakness; recent studies have proposed that the Niagara Fault zone may have been reactivated during gneiss dome exhumation (Schneider et al., 2004). Other structural discontinuities in the area show evidence of long-lived reactivation including the Great Lakes tectonic zone to the north. Our purpose is to evaluate the importance of tectonic heredity on the geologic history of the central Penokean orogen. Kinematic analysis. Tectonites from both shear zones contain steep penetrative foliations and dominantly down-dip stretching lineations. Oriented samples of these tectonites exhibit kinematic indicators suggestive of a multi-stage displacement history. Samples of the Niagara Fault zone from Pier’s gorge show spectacular quartz-filled strain shadows (Fig. 1a) that show south-side up relative motion, along with asymmetrical tails on feldspar grains (Fig. 1b) that show south-side down movement. Mesoscopic field indicators from a large outcrop on the north side of Highway 101 preserve definitive south-side down movement as seen in sigma structures and near isoclinal folding of late veins. Oriented samples of the Eau Pleine shear zone at March Rapids display south-side down movement in rotated feldspar grains. Sheared quartzofeldspathic gneiss at the south end of Dancy Quarry exhibits variable grain-size reduction – from coarse gneissic fabric to strongly sheared ultramylonitic fabric all of which are subparallel. The coarse gneiss samples show south-side up movement seen in rotated feldspars. In contrast, the finer-grained mylonites contain bent mica and fish structures which show south-side down relative motion. Multiple episodes of movement are evident from the kinematic work described here. Monazite geochronology. U-Pb dating of monazite has proved useful for determining the timing of formation and reactivation of large shear zones in the western U.S. (McCoy et al. 2005). Initial monazite microprobe work on a single coarse-grained sample of the Eau Pleine shear zone produced a tight cluster of monazite total-Pb ages at ~1846 Ma (Loofboro et al., 2004). This age is consistent with the timing of formation of the shear zone based on cross-cutting relations (Sims et al., 1989). Dating of monazite from the finer-grained ultramylonites is currently in progress. Medium-grained monazite spot dating on two samples of the Niagara fault zone yielded two age populations: the older age of ~1628 Ma is about the same age as the Mazatzal orogeny, whereas the younger ca. 1496 Ma date is about the time that the Wolf River batholith intruded the area (Rose, 2004). Both of these dates, although preliminary, suggest that the Niagara fault zone was influenced by younger tectonic stresses and fluid channelization events. Textural work shows that some monazite grains at the Pier’s gorge site are sheared (Fig. 1c), providing opportunity for constraining maximum age of deformation. Meso- and microscopic kinematic studies combined with total-Pb ages of metamorphic monazite hold great potential for constraining the timing and sense of relative motion along these shear zones. 20 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 21 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 AN OVERVIEW OF GEOLOGY OF THE SAULT STE MARIE AREA HAILSTONE, M. P.GEO, Ministry of Northern Development and Mines, Ontario Geological Survey, Resident Geologist Program, District Geologist, Sault Ste. Marie This presentation will provide an overview of the geology of the Sault Ste. Marie area. The variety of lithologies in the area span the ages from Archean to Paleo- and Mesoproterozoic. The Archean Batchawana Greenstone Belt is part of the Abitibi subprovince and is typical of many Archean Greenstone Belts within the Superior province dominated by metabasalts with intercalated calc alkaline to felsic metavolcanics and metasediments. This belt has been divided into an older Eastern Domain and a Western domain by Grunsky (1991) along a plate-plate collision boundary. The Batchawana Greenstone belt is surrounded on three sides by younger Archean gneisses of the Chapleau, Algoma and Ramsey Lake Gneiss domains. The belt of Paleoproterozoic Huronian rocks between Sudbury and Sault Ste. Marie are part of the Southern structural province. Standard nomenclature for the division of the sedimentary groups within the Huronian utilize a model of four glacial cycles from conglomeratic diamitictite base formations through deep marine formations to deltaic shallow marine formations. Although these environments of deposition work for the sedimentary groups, they mask the genesis of the Huronian basin which is now thought to be an early Proterozoic, active then passive rift system. (Bennett, G. 2006.) Recent studies into the earth’s early atmosphere have revealed that at approximately 2.35 Ga, oxygen made its appearance in the earth’s atmosphere. Huronian sedimentary rocks in the Elliot Lake area preserve that event. Lake Superior is the one of the longest and deepest continental rift systems on the face of the planet and is approximately 1 Ga years old. The rift is also known as the Mid Continental Rift (MCR). In the Mamainse Point Formation Keweenawan subaerial, alkaline basaltic flows, intercalated with conglomerates and intruded by felsic potassic keratophyres of the MCR are exposed on the west side of the Batchawana Greenstone Belt approximately 60 kilometers north of Sault Ste. Marie. Studies of these alkaline basalts demonstrate a geomagnetic reversal separated by a conglomerate unit with the older eastern alkaline basalt flows being reversely polarized. (Hart, T., R. and Pace, A. 2006) References Bennett, G., 2006: The Huronian Supergroup between Sault Ste. Marie and Elliot Lake-Field Trip Guidebook Institute on Lake Superior Geology, 52nd Annual Meeting, Volume 52, Part 4, 65p. Grunsky, E.,C., 1991: Geology of the Batchawana Area, District of Algoma; Ontario Geological Survey, Open File Report 5791, 214p. Hart, T., R. and Pace, A., 2006: Middle Keweenawan Rocks of the Mamainse Point Area - Field Trip Guidebook Institute on Lake Superior Geology, 52nd Annual Meeting, Volume 52, Part 5, 28p. 22 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 A PALEOPROTEROZOIC MANTLE PLUME BENEATH THE LAKE SUPERIOR REGION HALLS, H.C.†, University of Toronto at Mississauga, Mississauga, Ontario L5L IC6, hhalls@utm.utoronto.ca, STOTT , G.M., Ontario Geological Survey, Sudbury, Ontario, ERNST, R.E ., Ernst Geosciences, 43 Margrave Avenue, Ottawa, and DAVIS, D.W., Department of Geology, University of Toronto, Toronto, Ontario. New paleomagnetic and radiometric U-Pb age data on baddeleyite1 show that the 2101 to 2126 Ma Marathon dyke swarm radiates from a region, approximately in east-central Wisconsin (after closure of the 1.0 Ga Mid-Continent Rift), suggesting that these dykes are associated with a possible plume centre that lies off the southern margin of the Superior Province rather than on the northern side within the Hudson bay embayment. The period of magmatic activity includes a reversal of the magnetic field from moderately steep negatively inclined remanences (R polarity) with southeasterly declination to approximately antipodal ones (N polarity)1,2. The Marathon swarm was originally defined on the basis of a set of north-trending dykes in the general area of Marathon, but the new age data show that NE-trending dykes east of Wawa also belong to this swarm. These NE-trending dykes occur within a swarm of NE to ENE-trending “Kapuskasing” dykes that give similar paleomagnetic directions to the Marathon dykes, but with steeper positive and negative inclinations. Outside the Kapuskasing Zone (KZ) both polarities are observed for which positive baked contact tests exist1 . Inside the KZ, only Kapuskasing dykes of R polarity occur. Since feldspar clouding and negative baked contact tests are associated with Kapuskasing dykes lying in the high grade eastern part of the Chapleau Block, thereby demonstrating a secondary, magnetization3, it is possible that all Kapuskasing dykes within the southern KZ have been remagnetized, either during the uplift or as a consequence of slow cooling at depth.. By analogy with the 2.45 Ga Matachewan dykes which show the same phenomenon4, the R magnetization in Kapuskasing dykes is younger than N, which is the same age relation deduced on the basis of U-Pb ages for the Marathon swarm1. The remanence inclination of Kapuskasing dykes, whether R or N, is steeper than the average value for Marathon dykes. A few dykes with comparably steep inclinations are present in the Marathon swarm as originally defined, and one of these dykes (of N polarity) gives a U-Pb age of 2125.7 ±1.2 Ma1. Another, with a relatively shallow N inclination compared to the mean, gives an age of 2121+14/-7 Ma2, so steep inclinations may be a reflection of secular variation rather than apparent polar wander. Geochemically, Kapuskasing dykes cannot be distinguished from Marathon ones, so we provisionally place the Kapuskasing dykes with the Marathon swarm, thus defining a radiating swarm with a fan angle of about 70°, with N to ENE trends. The Fort Frances dyke swarm has R polarity5 and an age of 2076 Ma2 and trends NW, and together with the recently dated 2067± 1 Ma R polarity Franklin dyke6,7 that trends WNW in the Minnesota River valley, forms a broadly radiating swarm that converges to a focal region approximately in central-southern Wisconsin. Taken as a whole, the Marathon and Fort Frances dykes define a radiating swarm with a fan angle of about 140° and a plume centre approximately in Wisconsin. The plume had a life span of about 60 My (from 2126 to 2067 Ma), comparable to that of the older 2.45 Ga Matachewan plume to the east, which had a longevity of at least 50 million years. 23 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Geochemical data from dyke chilled margins show that the Fort Frances dykes have flatter REE patterns compared to Marathon dykes but that one or two dykes within the Marathon and Fort Frances swarms may belong to the other one, which would indicate a radial stress pattern. Alternatively, the noticeable dyke-free gap between the Marathon and Fort Frances swarms may arise if intrusion of the N-NE trending Marathon dykes changed the orientation of the maximum principal stress to favour NW-WNW intrusion of the later Fort Frances dykes. References 1 – Halls, H.C. et al., 2005. Ontario Geological Survey Open File Report 6171, 59 p; 2 - Buchan et al., 1996. Can. J. Earth Sci. 33: 1583-1795; 3 - Halls, H. C. et al., 1994. Can. J. Earth Sci. 31:1182-1196; 4 Halls, H.C. & Zhang, B., 2003. Tectonophysics 362: 123-136; 5 - Halls, H.C. 1986. Can... J. Earth Sci. 23:142-157; 6 – Schmitz, M.D. et al. 2006. GSA Bull. 118: 82-93; 7 - Cavanaugh, M.D. 1983. Unpublished Ph.D.Thesis, University of South Carolina, 79 p. 24 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOCHEMISTRY OF THE ~2.7 GA BLAKE RIVER GROUP AND CONFEDERATION ASSEMBLAGES: IMPLICATIONS FOR SUPRASUBDUCTION ZONE VOLCANISM IN THE SUPERIOR PROVINCE HOLLINGS, PETE† Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Canada, peter.hollings@lakeheadu.ca and WYMAN, DEREK School of Geosciences, University of Sydney, NSW, 2006 Australia The broadly coeval Blake River Group (BRG) of the southern Abitibi Belt and the Confederation Assemblage of the Birch-Uchi greenstone belt, have been interpreted as subduction-related volcanic assemblages generated in oceanic and continental margins respectively (Hollings and Kerrich, 2000; Péloquin et al., 1996). Both greenstone terranes contain a range of mafic rocks types (i.e., variably tholeiitic to calc alkaline) and host volcanogenic massive sulfide (VMS) deposits. The Blake River Group is host to numerous VMS deposits, ranging from the Horne Mine (55 Mt massive sulphide ore mined, total tonnage ~144 Mt), to the Quemont Mine along the southern margin of the Sequence (16 Mt), and relatively small ore bodies common in the Noranda Mine Sequence (e.g., 1-5 Mt; Gibson and Watkinson, 1990). The Confederation assemblage is host to the past-producing South Bay VMS mine which produced 1.6 million tons of ore with an average grade of 11% Zn, 2% Cu and 2.12 ounces Ag per ton (Atkinson et al., 1990). Differences in the proportions and types of rocks in the two areas suggest they represent end-members in a range of subduction settings present during the late Archean. The BRG was erupted over a short period between about 2703 - 2698 Ma and is one of the youngest pre-orogenic volcanic suites in the southern Abitibi belt. Plume-associated komatiites are inter-layered with arc-type rocks in the south-central part of the Abitibi Subprovince that contains the BRG. The ~2725-2745 Ma Confederation assemblage was not associated with plume volcanism and was situated at the margin of a proto-continent containing rocks that date back to ~3 Ga. Despite the important differences in their settings, the major element trends of tholeiitic rocks from the two areas resemble each other, and Phanerozoic arcs, more than tholeiites from continental rift settings that may be analogues for some Archean greenstone belts. Rhyolites in both areas are interpreted to be the fractionation products of mantle-derived melts. In addition to documenting variable crustal contamination, the trace elements systematics of the rhyolites provide evidence of zircon fractionation events that occurred without significant changes in major element compositions. These results are probably attributable to extraction of rhyolitic liquids from crystal mush zones that was accompanied by preferential entrainment of zircon crystals, leading to Zr fractionation. The BRG suite includes magmas generated in relict plume asthenosphere but the chemical trends also provide evidence of slab melt metasomatism (Wyman and Hollings, 2005). Primitive rocks in the Confederation assemblage define trace element trends that are analogous to typical modern arcs with no indication that melt-mobilized elements such as Zr and Nb have been introduced in significant amounts. Adakite-like rocks were formed as a result of local events such as arc rifting in the South Bay area, or as an indirect consequence of larger events such as global-scale mantle-plume episodes that 25 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 strongly influenced the southern Abitibi subprovince and the BRG (Wyman and Hollings, 2005). Niobium enriched basalts associated with crustally contaminated rhyolites in the southwest of the South Bay study area are most plausibly linked to rifting of the Uchi Subprovince proto-continent margin. The lack of evidence for HFSE metasomatism in the sources of tholeiitic and calk alkaline mafic rocks, indicates that metasomatism of the sub-arc mantle was dominated by hydrous fluids. Therefore, slab melting occurred not in response to a pervasive steep geotherm but to specific geodynamic events, which in this case were probably linked to the early phases of arc rifting along the continental margin. References Atkinson, B.T., Parker, J.R. and Storey, C.C., 1990, Red Lake Resident Geologist’s District-1990; In Report of Activities 1990, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 152, 31-66. Gibson, H.L. and Watkinson, D.H., 1990, Volcanogenic massive sulphide deposits of the Noranda shield volcano and cauldron, Quebec. In, Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J.M., Riverin, G. and Simard, A.(eds.), The northwestern Quebec polymetallic belt, The Canadian Institute of Mining and Metallurgy, Special Volume 43, 119-132. Hollings, P. and Kerrich, R., 2000. An Archean arc basalt - Nb-enriched basalt - adakite association: The 2.7 Ga Confederation assemblage of the Birch-Uchi greenstone belt, Superior Province. Contributions to Mineralogy and Petrology 139, 208-226. Peloquin, S., Potvin, R., Paradis, S., Lafleche, M., Verpaelst, P., Gibson, H., 1990. The Blake River Group, Rouyn-Noranda area, Quebec; a stratigraphic synthesis. In: Rive, M., Verpaelst, P., Gagnon, Y., Lulin, J-M., Riverin, G., Simard, A. (Eds.). The northwestern Quebec polymetallic belt; a summary of 60 years of mining exploration. Special Volume - Canadian Institute of Mining and Metallurgy, vol.43, pp.107-118. Wyman, D.A. and Hollings, P., 2005. Late Archean convergent margin volcanism in the Superior Province: A comparison of the Blake River Group and Confederation Assemblage. In: Archean Geodynamics and Environments, AGU Geophysical Monograph Series, 164, 215-237. 26 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 CONTINENTAL GROWTH AND EVOLUTION OF THE NORTHERN INTERIOR OF THE CONTERMINOUS U.S. NICE (Northern Interior Continental Evolution) Working Group HOLM, D.K.† (corresponding author), Kent State University; ANDERSON, R., IGS; BOERBOOM, T.J., MGS; CANNON, W.F., USGS; CHANDLER, V., JIRSA, M. and MILLER, J., MGS; SCHNEIDER, D.A., Ohio University; SCHULZ, K., USGS; VAN SCHMUS, W.R., University of Kansas. www.geo.umn.edu/mgs/index_wNICE.html The Penokean orogeny, long considered the dominant Paleoproterozoic event in the Lake Superior region, has been extrapolated to much of the buried basement of WI, IA, NE, and MI. In contrast, geon 17 crust (Yavapai orogen) is dominant south of the Archean Wyoming craton requiring a 100 m.y. age difference of juvenile crust along strike of the Transcontinental Proterozoic Province. We reconcile this problem with a revised history of the growth and evolution of continental crust in the northern mid-continent based on integration of modern geochronology and regional aeromagnetic data. A new aeromagnetic compilation of the region documents a complex terrane of 3.5-1.0 Ga rocks. In MN, the Archean craton is subdivided by the Great Lakes tectonic zone, a late Neoarchean suture. Paleo-proterozoic rifting of the craton created an irregular continental margin consisting of the Becker embayment and the MRV promontory. Bordering the embayment and onlapping the craton are a Paleoproterozoic fold-thrust belt and foreland basin. Within the Becker embayment are calc-alkaline volcanic and granitoid arc rocks formed by Penokean subduction and suturing of the arc with the craton. A sharp post-Penokean aeromagnetic discontinuity, the Spirit Lake tectonic zone (SLtz), extends eastward from NW Iowa, where it is defined by an abrupt southeastward decrease in the magnetic anomaly, through NC Wisconsin, where it is expressed by a sharp truncation of linear patterns in the Archean gneisses of the Marshfield terrane. The SLtz marks the northern extent of juvenile Yavapai age crust and the southern extent of Archean and Penokean crust. The nature of the SLtz is enigmatic and as yet poorly imaged in the third dimension by geophysical data. However, the preservation of Penokean juvenile crust only in an embayment along the southern rifted margin of the Superior craton suggests the SLtz formed initially as a major strike-slip fault zone responsible for margin truncation. South of the SLtz, the structural grain is subparallel to the SLtz and to axial traces of Mazatzal-age folds (Baraboo syncline). Gneisses and mafic volcanic rocks, probably basement rocks from which 1750 Ma rhyolites formed, are inferred from gravity-magnetic highs to be at subcrop in several areas. The Yavapai terrane is marked by abundant geon 14 granites identifiable by a generally smooth aeromagnetic pattern and low gravitational attraction. Additional high-relief, highintensity circular magnetic anomalies within the Yavapai terrane of Iowa delineate related granites. In southern WI and NE Iowa the aeromagnetic pattern reveals an extensive area of folded basement and Baraboo Interval quartzites beneath thin Paleozoic cover. The YavapaiMazatzal terrane boundary must be southeast of these deformed rocks. This area is bordered to the SE by large irregular shaped magnetic highs making up the Green Island 27 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 plutonic belt (GIPB) along the north edge of the Eastern Granite-Rhyolite province. We interpret the GIPB as having intruded into dominantly Mazatzal age crust. Geochronologic and thermochronologic data corroborate the new tectonic province map and provide important time constraints on the evolution of southern Laurentia during the late Paleoproterozoic. Basement crystallization ages. U-Pb zircon crystallization ages for juvenile crust exposed in WI definitively bracket the Penokean orogeny between 1880 & 1830 Ma. In contrast, U-Pb zircon ages from basement drill hole samples in NE are dominantly 1800 Ma or younger and probably represent eastward extension of the Yavapai age basement of CO. However, very few samples were available from the basement of Iowa, southern MN, SE South Dakota, and southern WI, with the result that many late-20th century interpretations for the buried basement in this region consisted of south-westward extension of the Penokean terrane. For example, the Precambrian basement of NW Iowa was commonly shown as Penokean crust abutting Archean crust, similar to the situation in northern WI along the Niagara Fault Zone. New basement ages appear representative of a growing UPb database for the region south of the SLtz. Key points relevant to this summary are as follows: (a) single-crystal TIMS U-Pb zircon data yield Yavapai-interval crystallization ages (1740 & 1760 Ma); (b) ion-probe analysis of zircons confirm these data; (c) zircons separated from gneissic xenoliths from the Manson impact structure in Iowa confirm the existence of igneous activity ca. 1760 Ma. These data yield no Penokean interval ages. Thus, all presently available U-Pb data support our interpretation that Yavapai orogenic crust extends eastward from NE into Iowa and southern WI, and that Penokean crust may be entirely absent from Iowa and SE Minnesota. Metamorphic & igneous ages. Metamorphism along the southern margin of the Superior province has been historically attributed to Penokean orogenesis. Indeed, a narrow window of amphibolite-facies rocks north of the Niagara Fault zone does record 1.831.80 Ga monazite U-Pb metamorphic ages. Peak metamorphic conditions with attendant magmatism likely mark the culmination of arc accretion. However, the dominant metamorphic and igneous imprint on the Precambrian basement is a regional Yavapai age tectonothermal event dated at ca. 1.76 Ga. Yavapai convergence led to weakening of the mid-crust and generation of the classic gneiss domes now exposed in northern MI and WI. Geon 17 metamorphism extends eastward into the Lake Huron region, where geon 18 metamorphic or magmatic activity is largely absent. Cooling/resetting ages. The results of over 100 modern Ar/Ar mineral ages from basement rocks of the NC midcontinent have allowed detailed characterization of its Proterozoic thermal history. Penokean biotite cooling ages are preserved only in lowgrade arc rocks in EC Minnesota; a few hornblende ages also record Penokean cooling in the metasedimentary rocks of the orogen. Elsewhere throughout the Superior and Huron region, hornblende and mica Ar/Ar ages are predominantly 1.76-1.75 Ga or somewhat younger, reflecting rapid, widespread cooling and orogenic collapse following the aforementioned Yavapai amphibolite-facies metamorphism and magmatism. Geon 17 rapid cooling of Archean and Paleoproterozoic metamorphic rocks of the gneiss dome corridor was caused by their exhumation and regionally was followed by a period of 28 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 tectonic quiescence, crustal stabilization, and deposition of the supermature Baraboo Interval (1730-1630 Ma) quartzites. Across much of the WI bedrock, low-temperature reheating was responsible for resetting mica cooling ages caused by geon 16 Mazatzal collision and foreland deformation. Interestingly, the northern limit of Mazatzal deformation and reheating is approximately located along the Niagara Fault zone in northern WI and upper MI. However, in MN the deformational front must bend south of the MRV promontory as those rocks are not thermally/isotopically reset and are overlain by flat-lying Baraboo Interval quartzites (Sioux quartzite). In the Huron region, Mazatzal heating is recorded only locally along the north shore of Lake Huron. Intrusion of the Wolf River batholith and associated geon 14 A-type plutons across the continental margin had a limited thermal effect on the country rock, in part reflecting their rapid emplacement at shallow levels. However, hydrothermal alteration along the Paleoproterozoic basement/cover contact occurred at considerable distances from the batholith. In summary, the continental interior straddles several terrane boundaries, including the transition from Archean tectosphere to Paleoproterozoic lithosphere. The SLtz is a fundamental Yavapai-age Proterozoic boundary, equivalent to, and possibly a direct extension of the Cheyenne belt suture zone, which also juxtaposes Yavapai orogen crust on the south against the Archean craton, and transects geon 18 (Trans-Hudson) structures in southern South Dakota. The Cheyenne-Spirit Lake structure is a fundamental feature in the evolution of the southern margin of Laurentia, the North American craton. Our new interpretation of the Paleoproterozoic continental growth and evolution of the northern interior of the North American craton suggests greater correspondence to that of the Rocky Mountains than previously thought. Although that region has structurally and magmatically modified during Cenozoic and older tectonism, relatively little tectonism has occurred in the cratonic interior in the last one billion years, providing us a uniquely unaltered perspective into Precambrian evolution of the North American continental lithosphere. 29 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 FIELD DISTRIBUTION, PETROGRAPHY, AND LITHOGEOCHEMISTRY OF EPIDOSITES IN THE VICINITIES OF FIVEMILE, NEEDLEBOY AND SIXMILE LAKES, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., HOCKER-FINAMORE, S. M., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, hudak@uwosh.edu HEINE, J., Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN 55811 The Lower Member of the Ely Greenstone Formation (LMEG) contains a well-studied, more or less east-west striking, steeply north-dipping and north-facing sequence of Neoarchean submarine volcanic, volcaniclastic, chemical sedimentary, and intrusive strata in the vicinity of Fivemile Lake, Needleboy Lake, and Sixmile Lake in the Vermilion District of northeastern Minnesota. Primary volcanological features of these rocks are generally well-preserved despite syn- to post-volcanic hydrothermal alteration (quartz, epidote, chlorite, sericite, actinolite, albite, iron carbonate, dolomite and calcite) and subsequent greenschist facies metamorphism. Based on field characteristics, the LMEG has been subdivided into an older Fivemile Lake Sequence (FLS) and a younger Central Basalt Sequence (CBS; Peterson and Patelke, 2003). Volcanogenic massive sulfide prospects have been identified in near Fivemile Lake, Skeleton Lake, Needleboy Lake and Eagles Nest Lake #4 (Giagrande, 1981; Peterson and Jirsa, 1999; Hudak and Morton, 1999; Peterson, 2001; Hovis, 2001; Hudak et al., 2002; Hudak et al., 2003). Epidosites (granular to granoblastic, high varience mineral assemblages comprising epidote + quartz ± chlorite ± actinolite) have been identified in several locations within the LMEG. South of Fivemile Lake, epidosites occurs as discrete 0.1-2m diameter round- to lens-shaped pale yellow green masses within a 400m by 500m discordant alteration zone located in an actinolite-epidote-quartz altered diabase dike – sill complex that intrudes the FLS. Petrographic and electron microprobe studies (Hocker et al., 2003) indicate the presence of both pistacite and zoisite within this alteration zone. Epidosites also occur within a 300m by 200m, northeast-trending disconformable alteration zone approximately 500-700m east- southeast of Sixmile Lake. At this location, CBS pillow lavas and lobes are intensely altered to a mineral assemblage composed of pistacite, zoisite, quartz, actinolite, Fe-chlorite, Mg-chlorite, magnetite, chalco-pyrite (locally altered to malachite) and minor sphalerite within approximately 200m of a synvolcanic gabbro sill-dike complex. Isocon analysis (Grant, 1986) has been used to evaluate metasomatism during the genesis of the epidosite alteration zones. Least-altered compositions were selected based on boxplot analysis (Large et al., 2001) and petrographic observations. The LMEG epidosites have specific gravities 5-10% greater than least-altered samples, consistent with observations from epidosites in the Josephine Ophiolite (Harper, 1999). Variation diagrams indicate that Zr and Hf are the least mobile elements during metasomatism in both the diabase intrusion and the CBS lava flows; the best-fit line for these two elements on an isocon diagram defines the isocon. Relative to least-altered diabase, epidosite masses are enriched in Ca, Al, Si, and Sr, and depleted in Fe, K, Na, Mn, Mg, Cu, Zn and Eu. These geochemical variations are consistent with alteration in a high temperature (>350°C), high water:rock ratio reaction zone deep within a synvolcanic submarine 30 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 hydrothermal system (e.g. Harper, 1999) capable of producing volcanogenic massive sulfide mineralization up-section. Variations in chemical behavior within the CBS pillowed flows with increasing distance from the gabbro sill-dike complex suggest that epidosite zones at this locality formed within localized high temperature hydrothermal zones driven by heat derived from the gabbro. Further work is needed to evaluate the extent of epidosites in the CBS, and potential VMS mineralization up-section in the Upper Member of the Ely Greenstone Formation. References Giagrande, P., 1981, Geology and sulphide mineralization of the Skeleton Lake Prospect: unpublished M. S. thesis, University of Minnesota-Duluth, 118 p. Grant, J. A., 1986, The isocon diagram – a simple solution to Gresen’s equation for metasomatic alteration: Economic Geology, v. 81, p. 1976-1982. Harper, G. D., 1999, Structural styles of hydrothermal discharge in ophiolite / sea-floor systems: Reviews in Economic Geology, v. 8, p. 53-73. Hocker, S. M., Hudak, G. J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Fivemile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion District of northeastern Minnesota: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/17, 49 p. Hovis, S. T., 2001, Physical volcanology and hydrothermal alteration of the Archean volcanic rocks at the Eagles Nest volcanogenic massive sulfide prospect, northern Minnesota: unpublished M. S. thesis, University of Minnesota – Duluth, Duluth, Minnesota, 137 p. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR2002/03, 390 pages. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, S., 2003, Needleboy Lake – Sixmile Lake Geological Mapping Progress Report: June 2003: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/18, 22 p. Hudak, G. J., and Morton, R. L., 1999, Mineral Potential Study, Minnesota Department of Natural Resources Project 326, Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion–Big Fork Greenstone Belt, Part A: Discussion of Lithology, Alteration, and Geochemistry at the Fivemile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota DNR Division of Minerals Project 326 Report, 136 p. Large, R. R., Gemmell, J. B., Paulick, H., and Huston, D. L., 2001, The alteration box plot: a simple approach to understanding the relationships between alteration mineralogy and lithogeochemistry associated with volcanic-hosted massive sulfide deposits: Economic Geology, v. 96, p. 957-971. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analogy Canadian Mining Camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, MN, 503. Peterson, D. M., and Jirsa, M. A., 1999, Bedrock geological map and mineral exploration data, western Vermilion District, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-98, scale 1:48,000. 31 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 STRUCTURE OF THE BIWABIK IRON FORMATION, MESABI IRON RANGE, MINNESOTA JIRSA, MARK A.†, and CHANDLER, VAL W., Minnesota Geological Survey (jirsa001@umn.edu; chand004@umn.edu) Six years of mapping by the Minnesota Geological Survey along the Mesabi Iron Range generated a variety of new maps, and considerable data regarding the structure of the Paleoproterozoic Biwabik Iron Formation and adjacent units (Fig. 1). Bedrock mapping utilized GIS to integrate data sources that included archived geologic and structure contour maps created by industry and government organizations, test pit and drill hole records, digital bedrock topography, and several iterations of aeromagnetic data. The aeromagnetic data delineate oxidation zones along faults, folds, and fractures in otherwise strongly magnetic iron-formation. These data and field work in more than 400 mines created a mass of structural observations that provide a context for understanding the deformation history of the range. Figure 1. Generalized geologic map of the Mesabi Iron Range showing structures in and along the subcrop of Paleoproterozoic Biwabik Iron Formation (gray). Bedrock north of the Biwabik Iron Formation is largely Archean in age; south of the Biwabik Iron Formation is Paleoproterozoic Virginia Formation bedrock; and to the southeast is the Mesoproterozoic Duluth Complex. The Biwabik Iron Formation is part of the Paleoproterozoic Animikie Group—a sequence of quartzose sandstone, iron-formation, and mudstone—that was deposited unconformably on a relatively stable shelf composed of Archean granite and greenstone. Depositional ages of the Animikie Group vary from 1,878 to 1,777 Ma (Fralick and others, 2002; Addison and others, 2005; Heaman and Easton, 2005). This broad temporal span indicates a protracted history of deposition, and probably also deformation. Much of the Biwabik Iron Formation forms a south-dipping homocline that contains little evidence of disruption, with the exception of locally well developed deformation structures. Although the precise ages of various structural elements on the Mesabi range are nearly impossible to ascertain, a relative chronology has been established from crosscutting relationships. Assigning deformation events to specific structures is probably premature; however, "D0, D1, D2…" nomenclature is applied here to refer to suites of 32 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 apparently related structures. The oldest are those presumably related to soft-sediment deformation (D0), including slumps, sedimentary breccias, and structures that appear to be the result of differential compaction. The earliest "regional" deformation (D1) is manifest in localized, small-scale rotational structures, bedding-parallel slickensides, and larger nappe and sheath folds. The structures commonly lie along boundaries between units having strong rheologic contrast, such as the contacts between thick sequences dominated by mudstone and siliceous grainstone. Nearly all of these structures display asymmetry that indicates south-over-north tectonism. This northward vergence, and the apparent timing relative to later structures, is consistent with compressional deformation—potentially related to the Penokean orogen. One of the long-standing controversies in iron-ore genesis is the question of whether the oxidation and leaching of iron-formation that formed the high-grade hematite ores occurred by supergene or hypogene processes. Although not conclusive, the observation of several early-formed, south-dipping thrust faults with folded, mineralized wall rocks, and bedding-parallel slickensides that host abundant secondary iron and silica implies that at least some of the mineralization was coincident with compressional deformation, perhaps during Penokean orogenesis. This is consistent with the hypogene model proposed by Morey (1999) that attributes oxidation and leaching to ground-water flow driven northward from uplift in the Penokean fold and thrust belt. A second regional suite of structures (D2) is largely extensional. These are monoclines and normal faults that are mutually transgressive; that is, faults that have sympathetically folded wall rocks, and folds that pass laterally to faults. These are some of the major structures along which oxidation and leaching has occurred, and the focus of most hematite ore mining. Veins, vugs, and other secondary mineralization features are abundant. D2 structures likely formed as localized responses to regional tilting. The most recent deformation effects (D3) are trough-like collapse structures, presumably related to post-leaching subsidence. The collapse, and associated oxidation and weathering, are best developed in the uppermost subcrop of iron-formation, implying supergene alteration played a significant role. Thus, the answer to the supergene vs. hypogene debate appears to be that both processes were significant, perhaps at different times. The Virginia horn—where the subcrop extent of iron-formation makes a hookshaped bend—is a complex horst, bounded by faults in the subjacent Archean rocks along which vertical movements have occurred during the entire temporal spectrum from early deposition to latest crustal accommodation. Lacking finite ages, the structures may record Penokean (Geon 18), Yavapai (Geon 17), Mazatzal (Geon 16), and/or Keweenawan (Geon 11) deformation events. The presence of diabase dikes cutting ironformation as far west as Keewatin raises the possibility that at least some of the later phases of deformation on the Mesabi range are related to development of the 1,100 Ma Midcontinent Rift. Funding for Minnesota Geological Survey mapping on the Mesabi range was provided by the Minnesota Legislature on recommendation of the Minerals Coordinating Committee, the Environmental Trust Fund administered by the Legislative Commission on Minnesota Resources, the EDMAP program of the U.S. Geological Survey, and the State Special appropriation to the Minnesota Geological Survey. Products can be digitally downloaded or 33 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 ordered on paper through the Minnesota Geological Survey website (www.geo.umn.edu/mgs). Available products include maps of bedrock topography and depth to bedrock (Miscellaneous maps M-126, M-158), historic 1899 land-surface topography, hydrology, and infrastructure (M-118, M-157), bedrock geology (M-163), and Quaternary geology (M-164). REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworded volcanic ash: Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating, in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Program and Abstracts, v. 51, pt. 1, p. 24-25. Morey, G.B., 1999, High-grade iron ore deposits of the Mesabi range, Minnesota—product of a continental-scale Proterozoic ground-water flow system: Economic Geology, v. 94, p. 133-142. 34 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 PETROGENESIS OF A GRANITE XENOLITH IN THE 1.1 GA MIDCONTINENT RIFT AT SILVER BAY, MN JUDA, NATALIE, WIRTH, KARL, CRADDOCK, JOHN, Geology Department, Macalester College, St. Paul, MN, 55105; VERVOORT, JEFF, Dept. Geological Sciences, Washington State University, Pullman, WA 99164; ANDRING, MATT, Whitman College, Walla Walla, WA This study examined a well-known granitic xenolith locality within the hypabyssal Beaver Bay Complex of the Midcontinent Rift System (MRS). The xenolith is exposed along the shore of Lake Superior at Silver Bay, MN. Our goal was to constrain the origin of the granite using U-Pb zircon geochronology, whole-rock and trace element geochemistry, and anisotropy of magnetic susceptibility. Previous researchers have interpreted the origin of granite xenoliths contained within MRS rocks as either Archean crustal fragments or MRS felsic plutons. The granite xenolith (~ 50 meters in diameter) occurs within Beaver River diabase, and is crosscut by a mafic dike. The rock consists primarily of quartz, albite, and orthoclase. Granophyric intergrowths of quartz and feldspar are common. In addition, accessory minerals including sphene, apatite, and zircon are present. At the macroscopic level, the xenolith exhibits no indications of magmatic flow or foliation, and our study of anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow confirms this (Figure 1). U-Pb analyses of zircons from the xenolith yield an age of 1094 ± 11 Ma on a concordia diagram. This is within error of the age of the youngest dated MRS granophyres. The geochemistry of the granite is similar to other MRS granophyres (e.g., Eagle Mountain, Finland Granite), except that the granite xenolith has higher concentrations of silica and sodium and very low potassium and other alkali elements (e.g., Rb, Ba; Figure 2). The apparent alkali mobility may have resulted from fluid infiltration during late-stage cooling. The compositions of several granitic dikes at Beaver Bay are similar to the Silver Bay xenolith. The Silver Bay xenolith and Beaver Bay dikes share “within plate” and A-type granite major and trace element compositions with MRS granophyres. Geochemical data from granite xenoliths at Split Rock are significantly different from those at Silver Bay and from other MRS granophyres by having volcanic arc characteristics. This suggests that the Split Rock xenoliths might have a different origin from those at Silver Bay. Figure 1. Equal area lower hemisphere projections of AMS results for samples granite xenolith (KP05-45B) and a cross-cutting mafic dike (KP05-45I). 35 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The Keweenawan age and A-type granite characteristics of the granite xenolith at Silver Bay suggest a greater distribution of MRS granophyres than previously thought. These granophyre bodies may also underlie volcanic flows in the more central portions of the rift. Further isotopic analysis of the granite xenoliths, such as with the Sm-Nd system, would help constrain the petrogenesis of the MRS granophyres. REFERENCES Kennedy, B.C., Wirth, K., Vervoort, J.D. 2000. Petrogenesis of the Midcontinent Rift Granophyric Complexes of Northern Minnesota: Proceedings and Abstracts Institute on Lake Superior Geology, vol. 46, p. 29-30. Miller, J., Chandler, V. W. 1997. Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: Geological Society of America Special Paper 312, p. 73-96. Vervoort, J.D., Wirth, K. 2004. Origin of the Rhyolites and Granophyres of the Midcontinent Rift, northeastern Minnesota: Proceedings and Abstracts Institute on Lake Superior Geology, vol.50, p. 158-159. Figure 2. 36 Harker diagrams showing the concentrations of Na2O, K2O, and Al2O3 in the granite xenolith compared to data from other MRS granophyres (Wirth and Vervoort, in prep.). Symbols: filled inverted triangles = Silver Bay granite xenolith; open inverted triangles = Split Rock granite xenoliths; other symbols are MRS granophyre bodies. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SULPHIDE SATURATION MECHANISMS IN GABBROIC INTRUSIONS IN THE NIPIGON EMBAYMENT KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 sakissin@lakeheadu.ca, HEGGIE, G.J., East West Resource Corporation, 1158 Russell Street, Thunder Bay, ON, P7B 5N2, FRANKLIN, J.M., Franklin Geosciences, 24 Comanche Drive, Nepean, ON, K2E 6E9, KARIMZADEH SOMARIN, A., Department of Geology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran Interest in the Nipigon Embayment as favourable exploration target for platinum-group element (PGE) deposits was stimulated by the suggestion by Naldrett (1992) that the area is a likely geological setting for the development of nickel-copper-PGE deposits based on criteria established in studies of the Noril’sk deposit in Siberia. Proterozoic gabbroic intrusions in the Nipigon Embayment of northwestern Ontario were studied with the aim of discerning the mechanism of sulphide saturation leading to (PGE) concentrations recently discovered. Two intrusions, the Seagull intrusion (1116.2±9.2 Ma), south of Lake Nipigon, and the Kitto intrusion (1117±1.8 Ma), on the eastern shore of Lake Nipigon, were the subject of the study, as they contain potentially economically significant PGE concentrations. Most of the study was carried out on the Seagull intrusion, as only limited samples were available from the Kitto intrusion. Neither intrusion is well exposed, and most samples were taken from drill-core. Profiles of sulfur, copper, nickel, gold, palladium and platinum as a function of depth in drillholes reveal that sulphur saturation occurred at the base of the Seagull intrusion, where a zone of sulfide mineralisation is developed. However, sulphur saturation was noted at higher levels in the intrusion, notably in the high-grade RGB zone. These observations suggest the operation of different processes in formations of the mineral occurrences – a Noril’sk-type process involving assimilation of sulphur for the basal zone and a reef-type process for the higher zones (Naldrett 1993). Olivine compositions were determined in both intrusions, and in both cases, the compositions indicate that the parental magmas were undersaturated with respect to sulphur. Thus, according to theories of PGE deposit formation, both intrusions have potential for PGE concentration. Contamination of the parental magma, either through assimilation of country rock or magma mixing, has been ascribed a crucial role in the formation of an immiscible melt (Irvine 1975; Naldrett 1989). Neodymium (Nd)-samarium(Sm) isotopes provide a means of testing for contamination. Heggie (2005) reported data on Nd/Sm isotopic studies on Seagull intrusion samples for a range of depths in several drill-holes, as well results from underlying Quetico Subprovince metasedimentary rocks and Sibley Group sedimentary rocks. The calculation of εNd for theses samples yielded values of -0.2 t0 -4.0 (±0.5) for the Seagull intrusion, but -16 to -23 for the Quetico metasediments and a mean of -5 for Sibley Group sediments. Rb/Sr isotopic studies on the same samples were used in a comparison of 143Nd/144Nd vs. 87 Sr/86Sr. Sibley Group sediments differ markedly from the Seagull intrusion in both factors, whereas the Quetico metasediments have lower 143Nd/144Nd ratios and similar 87Sr/86Sr ratios. As the 87Sr/86Sr ratios for the Seagull samples trend to somewhat higher values than those found in the Quetico metasediments, some assimilation of the high 87Sr/86Sr from Sibley material must have occurred. However, since 143Nd/144Nd in the Seagull samples decreases with depth, trending toward lower Quetico metasediment values, assimilation of Quetico material is also likely. 37 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The source of sulfur in the Seagull intrusion was investigated through the study of sulphur isotopes and selenium/sulfur ratios. Sulphur isotopic compositions of samples from the base of the Seagull intrusion were compared with those from Quetico metapelites and evaporites from the Sibley Group. Sulphides from the Seagull intrusion had ∂34S ranging from –2.3 per mil (‰) to +2.6‰ with a mean value of –0.9‰. Sulphides in Quetico Subprovince metapelites has ∂34S ranging from –2.3‰ to +1.1‰ with a mean value –0.8‰. Finally, Sibley Group evaporites had ∂34S ranging from +7.7‰ to +9.0‰. According to Franklin and Mitchell (1977), these correspond to a mean H2S composition of –4.2‰, based on prior knowledge of temperatures of formation of the sulphate minerals (barite, anhydrite and gypsum) in the Sibley Group. These data provide plausible evidence for incorporation of Quetico sulphide in the sulphide zone at the base of the Seagull intrusion. The source of sulphur in higher zones remains to be determined. Comparison of ∂34S with Se/S x 106 in Seagull sulphides and Quetico metapelites revealed that the Quetico samples lie well outside the region for mantle sulphur, whereas the Seagull samples show considerable scatter in Se/S x 106. Together with their negative ∂34S values, it is evident that assimilation of Quetico sulphide is the explanation for these data. Although the Nipigon Embayment has a number of features that seem to provide for a Noril’sk model setting, continental rifting, evaporites in the section, voluminous basaltic eruption among others, there are problems in its application to the two cases studied here. The Seagull and Kitto intrusions are among the earliest igneous events associated with the Mid-Continent Rift (Davis and Green 1997). Although some contribution from Quetico metapelite sulphide seems likely, the Sibley Group evaporites do not seem to be likely sulphur sources. Rather a reef-type of process seems to be responsible for zones of PGE enrichment at higher levels in both intrusions. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34: 476-488. Franklin, J.M., and Mitchell, R.M. 1977. Lead-zinc-barite veins of the Dorion Area, Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, 14: 1963-1979. Heggie, G.J. 2005. Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 156p. Irvine, T.N. 1975. Crystallization sequences of the Muskox intrusion and other layered intrusions-II. Origin of chromitite layers and similar deposits of magmatic ores; Geochimica et Cosmochimica Acta, 39: 991-1020. Naldrett, A.J. 1989. Magmatic sulfide deposits; Oxford University Press, New York, 186p. Naldrett, A.J. 1992. A model for the Ni-Cu-PGE ores of the Noril’sk region and its application to other areas of flood basalt; Economic Geology, 87: 1945-1962. Naldrett, A.J. 1993. Models for the formation of strata-bound concentrations of platinum-group elements in layered intrusions; in Mineral deposit modeling; Geological Association of Canada, Special Paper 40: 373-387. 38 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 MetalCORP LTD. BIG LAKE Cu-Zn-Ag-Au-Co, Ni-Cu-PGE, AND Mo PROPERTY MACTAVISH, ALLAN, MetalCORP Ltd., 309 South Court Street, Thunder Bay, ON, P7B 2Y1, Canada The Big Lake Property of MetalCORP Ltd. of Thunder Bay, Ontario comprises 33 claims (365 units totalling 5840 hectares) and is located approximately 230 km east-northeast of the city of Thunder Bay and 18 km southeast of the town of Marathon in Northern Ontario, Canada. Work completed by MetalCORP since early 2004 includes a MegaTEM airborne time-domain EM and magnetometer survey (2004), a detailed helicopter-borne ATEM III time-domain EM and magnetometer survey, detailed and reconnaissance prospecting (918 samples), linecutting, surface and down-hole pulse-EM surveys, geological mapping, and 3 phases of diamond drilling, totaling 31 diamond holes (8300 m). This work resulted in the discovery of 4 previously unknown mineralized zones that represent 3 separate and distinct mineralization styles. The mineralized zones include: the J4 and J5 Pt-Pd reefs within the Big Lake Ultramafic Complex; the A2 Ni-Cu Zone within the Gus Creek Mafic Intrusion; and the BL14 Cu-Zn-Ag-Au-Co Zone within strongly altered ultramafic metavolcanic flows and associated metasedimentary rocks. The A2 and BL14 zones are not exposed on surface and are buried beneath 10 to 75 m of glacial drift. The property is also host to the historic Playter Mo-Cu-Pb-Ag Prospect which has yet to be fully evaluated by MetalCORP. The Big Lake Property is located near the southern margins of the eastern portion of the Archeanage Schreiber-Hemlo greenstone belt of the eastern Wawa Subprovince of the Canadian Shield. The greenstone belt is split into distinct eastern and western segments by the 1108 Ma Mesoproterozoic Coldwell Alkalic Complex. The eastern part of the belt is subdivided into the Hemlo-Black River assemblage (2.77 Ma) to the north and the Heron Bay (2.70 Ma) assemblage to the south, both of which are primarily affected by amphibolite-facies regional metamorphism. The western portions of both assemblages are lower in grade and exhibit upper greenschist facies regional metamorphism. The Big Lake Property occurs within the Heron Bay Assemblage which is intruded by the granitic to granodioritic Heron Bay Batholith, the recently recognized mafic Gus Creek Intrusion, the Bell’s Lake Ultramafic Intrusion, and the >30 km long Big Lake Ultramafic Complex. The BL14 Cu-Zn-Ag Zone is located stratigraphically below the eastern end of the sill-like Big Lake Ultramafic Complex, approximately 800 metres south of the A2 zone (see below). The south-facing, high temperature, mafic dominated Cu-rich VMS zone is overturned, dips at ~25o to the north, and consists of: 1. A stringer zone of intensely biotitized and strongly chloritized and talcose breccia containing up to 30% bands, veins, stringers and pods of chalcopyrite and pyrite with up to 5% disseminated to streaked sphalerite and minor galena; 2. A semi-massive to near-massive zone of chalcopyrite, pyrrhotite, sphalerite, and minor galena underlain by a strongly to intensely biotitic, chloritic, and talcose footwall alteration zone that locally contains anthophyllite and sillimanite. Trace element lithogeochemistry shows that the footwall metavolcanic rocks contain unusually high amounts of Ni, Cr, TiO2, and Pd that suggests that the host-rocks were originally komatiitic basalts or possibly ferro-picrites. The mineralized zone is capped by relatively unaltered clastic and chemical metasedimentary rocks. The thickest BL14 Zone intersection (DDH BL06-24) to date (April 6, 2006) contained 5.57% Cu, 103.9 grams/tonne (gpt) Ag, 6.74 gpt Au, 1.66% Zn, 39 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 689 ppm Co, and 0.15% Pb over 5.38 m (17.65 ft). This interval included 7.45% Cu, 137.8 gpt Ag, 9.18 gpt Au, 2.24% Zn, 891 ppm Co, and 0.21% Pb over 3.95 m (12.96 ft). Stringer zone mineralization intersected to date contained up to 2.56% Cu, 1.00% Zn, 46.0 gpt Ag, 1.60 gpt Au, and 0.10% Pb over 0.93 m (3.05 ft). The J4 and J5 Pt-Pd Reefs consist of narrow, apparently stratabound intervals hosted within thick peridotite units contained within the upper and central intrusive cycles of the eastern portion of the north-facing, well-differentiated, unlayered, sill-like Big Lake Ultramafic Complex. The ultramafic complex is presently undated; however, it is thought to be younger than most of the supracrustal rocks observed within this portion of the Schreiber-Hemlo Greenstone Belts. Diamond drilling shows that the central portions of the complex dip to the north at ~45o, whereas the eastern portions of the complex dip to the north at ~25o. Geological mapping suggests that the western portions of the complex exhibit a steeper northerly dip. The two host intrusive cycles and are very similar in appearance, progression of rock units, and apparent thickness. The observed mineralization consists of trace amounts of very finely disseminated pyrrhotite and chalcopyrite within serpentinized to locally talcose, fine-grained, pyroxene-oikocrystic peridotite. The J4 Reef varies between 0.58 and 2.11 m in thickness, occurs within the basal peridotite unit of the uppermost (J4) intrusive cycle of the Big Lake Complex and is usually directly adjacent to the contact with an overlying olivine-bearing pyroxenite unit. The J5 Reef is identical in appearance to the J4 Reef, varies between 0.75 and 3.00 m in thickness, and occurs within the basal peridotite of the central (J5) intrusive cycle of the complex near, but not adjacent to, the upper contact of the host unit with an overlying olivine-bearing pyroxenite. The J4 Reef has been traced for 2.20 km (1.37 mi) and contains up to 0.70 gpt Pt and 0.79 gpt Pd (1.49 gpt 2PGE)/1.67 m. The J5 Reef has been traced for a similar distance and contains up to 0.81 gpt Pt, 0.85 gpt Pd (1.86 gpt 2PGE)/0.75 m. It is interesting to note that both reefs were intersected while drill testing the BL14 Zone described above. The A2 Ni-Cu Zone occurs near the base of the discordant Gus Creek Mafic Intrusion (2669.3 ± 1.8 Ma., Jack Satterly Geochronology Laboratory, University of Toronto, 2005) and consists of disseminated, blebby, and stringered, locally semi-massive pyrrhotite, chalcopyrite, and pentlandite hosted within the 2 to 20 m thick, conduit-like, A2 host intrusion breccia sequence. The A2 intrusion breccia is a complex interval of variably mineralized (1 to 30% sulphides), varitextured, inclusion-rich, gabbroic to melagabbroic intrusive rocks overlain by unmineralized, medium- to coarse-grained gabbro and quartz leucogabbro and underlain by occasionally mineralized, pyroxene-phyric melagabbro and feldspathic pyroxenite. Inclusion/fragment types include a variety of gabbros, ultramafic intrusive rocks, and clastic and chemical metasediments. The strongest mineralization occurs near the base of the intrusion breccia, comprises the A2 NiCu Zone, and includes 1.66% Ni and 0.20% Cu/0.30 m, 1.00% Cu and 0.80% Ni/0.40 m, 1.40% Cu and 0.27% Ni/0.77 m, and 0.98% Cu and 0.29% Ni/1.40 m. The geometry of the A2 mineralized zone remains uncertain and may be more complex that initially thought, but within the area drilled appears to strike ~140o and dip southwest between 40 and 60o. It is presently thought that much of the observed sulphide mineralization within the A2 Zone consists of fragments from a high R-factor, massive, Ni-Cu sulphide body located somewhere at depth. 40 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 MINING AND EXPLORATION ACTIVITY IN NORTHWESTERN ONTARIO MAGEE, ANGELIQUE, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA Northwestern Ontario continued to see a significant increase in mining and mineral exploration in 2005. Six mines produced a total of 1.5 million ounces of gold in 2005, approximately 70% of Ontario’s total. Gold producers included: Campbell Mine (Placer Dome Inc.); David Bell Mine (Teck Cominco Limited and Barrick Gold Corporation); Golden Giant Mine (Newmont Canada Limited); Musselwhite Mine (Placer Dome Inc./Kinross Gold Corporation); Red Lake Mine (Goldcorp Inc.); and Williams Mine (Teck Cominco Limited and Barrick Gold Corporation). North American Palladium Ltd. produced 177 167 ounces of palladium and 18 833 ounces of platinum at its Lac des Iles Mine and development of the underground operation below its open pit mine continues. The Golden Giant Mine closed its mining operation in December of 2005 and will be decommissioning the mine site in the first half of 2006. There are approximately 400 active exploration projects in the northwest, the vast majority of which are focused on gold. Areas receiving the most interest from exploration companies were the Red Lake greenstone belt, Shoal Lake area, Dogpaw Lake area, Shebandowan greenstone belt, Fort Hope greenstone belt, Onaman-Tashota belt and the Pickle Lake greenstone belt. Elevated mineral commodity prices are contributing to levels of exploration activity in northwestern Ontario not seen since the mid-1980’s. Exploration in northwestern Ontario continues for the following mineral deposit types: diamonds, uranium, copper-nickel-platinum group elements, volcanic hosted massive sulphides, rare earth elements in pegmatites, iron-oxide-copper-gold, coppermolybdenum-gold porphyry, and lode gold. 41 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY AND GEOCHEMISTRY OF THE CHIMNEY LAKE VOLCANICLASTIC BRECCIA NEAR ARMSTRONG, ONTARIO MAGEE, M. ANGELIQUE† 1, 2, HOLLINGS, PETE 2, FRALICK, PHILIP W. 2 Ontario Geological Survey, Resident Geologist Program, Suite B002, 435 James St. S., Thunder Bay, Ontario, Canada, P7E 6S7, 2 Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 1 The Chimney Lake volcaniclastic breccia (CLVB) is part of a group of Mesoproterozoic rocks that unconformably overlie the Archean basement of the central Wabigoon Subprovince near the northwestern end of Lake Nipigon. Mapping in 2003 resulted in the discovery of a number of previously unmapped, Mesoproterozoic units, including the Badwater layered gabbro intrusion (Middleton 2005), the Pillar Lake volcanic assemblage (PLV), and an undeformed volcaniclastic breccia located on the north shore of Chimney Lake (MacDonald 2004). The Badwater layered gabbro (1599 Ma; Heaman and Easton 2006) intrusion has anorthositic to gabbroic layers. The PLV are a series of flat-lying, greenschist-facies, undeformed, massive and pillowed basalt flows. A robust age date of the PLV has not yet been obtained, but dates obtained by geochronological methods indicate that the PLV were erupted between 1514 Ma and 1120 Ma (Heaman and Easton 2006). The CLVB was originally mapped as a conglomerate within the Pass Lake Formation of the Sibley Group and has not been dated by geochronological methods (MacDonald 2004). Field relationships between the CLVB and surrounding lithologic units, such as the PLV, are not discernible due to poor outcrop control. CLVB contains fragments of gabbro, basalt, amygdaloidal basalt, and fragments tentatively described as granitoid and sedimentary rocks. Fragments range in size from 0.2 cm to 30 cm in diameter. Gabbroic fragments are angular with sharp edges, displaying no evidence of assimilation. Basalt fragments sometimes exhibit assimilation features and have angular to subrounded and ameboidal shapes. Amygdules within basalt fragments are infilled with chlorite and locally clay minerals. Alteration envelopes of chlorite, hematite and clay minerals surround the amygdules. The breccia matrix consists of very fine- to coarse-grained, angular to sub-rounded, hematite- and actinolite-altered volcanic and igneous fragments. Preliminary geochemical results indicate that the CLVB contains fragments of basalt that are similar in composition to the alkaline PLV basalt. The gabbroic clasts vary in composition but it appears that they are similar to the Badwater layered gabbroic intrusion. The Badwater layered gabbroic intrusion and the PLV are geochemically dissimilar. The similarity between the fragment composition in the CLVB and nearby lithologic units suggests a local source. The CLVB could be a reworked autoclastic breccia related to Pillar Lake volcanism, or alternately it may represent a diatreme breccia dike that was emplaced after Pillar Lake volcanism. Ongoing detailed mapping of the volcaniclastic breccia in conjunction with additional geochemical analyses will determine the source and cause of the volcanism that resulted in this breccia unit, as well as other local volcanic rocks. 42 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 REFERENCES Macdonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. Middleton, R.S. 2005. Diamond Drilling on Red Granite Property, Pillar Lake Sheet, Armstrong, ON, 52I03NW, Resident Geologist Program Thunder Bay North Assessment Files, 55p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Miscellaneous Release of Data 191, 86p. 43 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE PRECAMBRIAN RESEARCH CENTER—A NEW INITIATIVE TO PROMOTE PRECAMBRIAN FIELD STUDIES AT THE UNIVERSITY OF MINNESOTA DULUTH MILLER, JAMES D., JR.†, Minnesota Geological Survey (mille066@umn.edu) and PETERSON, D.M., Natural Resources Research Institute (dpeters1@nrri.umn.edu) As the minerals industry enters an anticipated period of protracted expansion, a major impediment to this growth is a scarcity of new geoscientists adequately trained in basic field methods. This is especially true in field studies of Precambrian terrains, which host much of the world's ore deposits. The Precambrian Research Center (PRC) is being created at the University of Minnesota Duluth (UMD) with the primary goal of satisfying this new demand for field geologists by providing training and support to upper-level undergraduate students, graduate students, and professional geologists in modern methods of geologic mapping in glaciated Precambrian terrains. A secondary goal of this center is to attract exceptional students, who have an interest in conducting field-oriented thesis research, to the University of Minnesota Duluth's graduate program. Ultimately, it is our hope that the PRC will sustain and enhance the reputation the geology department at UMD has developed over the past 50 years for producing well-trained field geologists. The PRC will have five programmatic components: 1. Summer geology field camp in northeastern Minnesota Beginning in 2007, the PRC will offer a summer field camp focused on the unique aspects of field mapping in Precambrian terrains of the southern Canadian Shield. The field camp will be a course accredited through the College of Science and Engineering at UMD. It will be open to undergraduate and graduate students from throughout the U.S., Canada, and abroad. It will be staffed by 4 to 6 professional field geologists contracted with the PRC. Field camp highlights • Introduction to basic field methods in glaciated Precambrian shield terrains. • Overview of the Precambrian geology of the southern Canadian Shield. • Week-long capstone field mapping projects with small field excursions supervised by professional field geologists; Boundary Waters Canoe Area Wilderness option. • GIS compilation of field data and digital geologic map-making. • Integrating structure, drill core, and geophysics into 3-D geologic interpretations. 2. Graduate assistantships and grants The PRC will offer several yearly research assistantships for students accepted into the UMD graduate program who wish to pursue field-based research projects focused on the Precambrian geology of the Lake Superior region. Small research grants will also be available to undergraduate and graduate students to assist in various aspects of fieldbased studies of Precambrian geology. Undergraduate or graduate students may apply for these grants, but preference will be given to students from UMD and those who have attended the PRC field camp. 3. Professional workshops/field experiences The PRC's goal of providing advanced training to professional geologists, as well as students, will be achieved by sponsoring a regular series of workshops and field 44 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 experiences (at least two per year) on various topics related to field mapping of Precambrian shield geology. In addition, customized geologic mapping experiences for groups of industry geologists and/or the geologic staff of individual companies can be arranged. The PRC will work with UMD to ensure that these programs meet the continuing professional development requirements of geologic licensing boards. Industry members of the PRC will receive registration waivers to these advanced training sessions depending on their level of support. 4. Advanced geology courses at UMD Three new field-based courses will be offered for upper level undergraduates and graduate students within the Department of Geological Sciences. These courses include Advanced Field Methods and Geological Maps; Geology in Three-Dimensions; and Geologic Problem Solving Using Digital Methods. 5. Outreach, field trip offerings, and career planning for students The PRC will offer outreach education to K-12 students and the general public on the geology and mineral resources of the Lake Superior region. It will also offer to lead field trips on the Precambrian geology of the area for UMD students and students visiting the area from other colleges and universities. Finally, the PRC will serve as a clearinghouse for students to find job opportunities in the public or private sector that require field mapping skills. PRC activities and finances will be overseen by the heads/directors of three principal organizing institutions within the University of Minnesota: the Natural Resources Research Institute (NRRI) at UMD, the Department of Geological Sciences at UMD, and the Minnesota Geological Survey (MGS) at the University of Minnesota Twin Cities campus. The NRRI will oversee the business activities of the PRC; the geology department will oversee the PRC's educational programs; and MGS will provide guidance on geologic mapping projects. The PRC will employ 3 to 4 permanent staff and will contract with many field-experienced academic and professional geologists from throughout the Lake Superior region for its various programs and activities. A Board of Advisors consisting of preeminent geologists from industry and academia will be established to provide advice and oversight of PRC activities and programs. Funding for the PRC will come from many sources. Base funding will be sought from the State of Minnesota to support administrative expenses, from the University of Minnesota in the form of tuition deferments, and from students and professional geologists by tuition and fees paid for summer field camp, academic courses, and workshops/field experiences. Funding for particular research projects will be sought from the U.S. Geological Survey through their EDMAP program and from the minerals industry for sponsorship of industry-generated thesis projects. The minerals industry, which stands to be a major beneficiary of the PRC by its professional workshops and by having access to well-trained students, will also be requested to serve as a key benefactor of the PRC. 45 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY OF THE DULUTH COMPLEX IN THE FOUR BABBITT 7.5' QUADRANGLES, NORTHEAST MINNESOTA MILLER, JAMES D., JR., Minnesota Geological Survey (mille066@umn.edu) and SEVERSON, MARK J., Natural Resources Research Institute (mseverso@nrri.umn.edu) The Minnesota Geological Survey published 1:24,000-scale digital geologic maps of the Babbitt, Babbitt Northeast, Babbitt Southwest, and Babbitt Southeast 7.5' quadrangles in 2005 (Severson and Miller, 2005; Miller and others, 2005; Miller and Severson, 2005; and Miller, 2005, respectively). The area was originally reconnaissance mapped by Bonnichsen (1970a-d). The Precambrian rocks in the four Babbitt quadrangles are best known for hosting the easternmost Mesabi Iron Range taconite mines and some of the Cu-Ni-PGE deposits that occur along the northwestern margin of the Duluth Complex. The maps, which include three cross sections, will be on display as a poster presentation. Trending northeasterly through the map area, the Duluth Complex is in intrusive contact with Paleoproterozoic rocks of the Animikie Group and Archean granitic rocks of the Giants Range batholith. The Animikie Group units include the basal Pokegama Quartzite, the overlying Biwabik Iron Formation, and the Virginia Formation. The Peter Mitchell (Northshore Mining) and Dunka Pit (closed) taconite mines occur in the map area. The northwestern margin of the Duluth Complex is exposed in the Babbitt quadrangles. The Duluth Complex is the largest exposed intrusive component of the Mesoproterozoic (1.1 Ga) Midcontinent Rift. It was emplaced as multiple intrusions into the lower section of comagmatic volcanic rocks of the North Shore Volcanic Group, which is evident from field relationships in the map area. The oldest Mesoproterozoic rock units in the Babbitt quadrangles are several types of mafic hornfels inclusions, which represent thermally metamorphosed remnants of the North Shore Volcanic Group. The most common type is basaltic hornfels, which by the common occurrence of meta-amygdaloidal textures, are clearly thermally metamorphosed mafic volcanic lava flows. Magnetic, nonmagnetic, and plagioclase porphyritic varieties of basalt hornfels are recognized, with the magnetic types usually occupying a stratigraphically lower position. An interesting hornfels type is a well crossbedded mafic hornfels that is interpreted to be a strongly metamorphosed volcanogenic interflow sandstone unit (Patelke, 1996). An enigmatic hornfels type is a medium- to fine-grained, equigranular oxide olivine gabbro that displays a domainal distribution of granular mafic phases possibly representing granoblastic recrystallization of an originally ophitic texture. The thickness, homogeneity, and average medium grain size imply that the unit may be a metamorphosed subvolcanic sill or a large lava flow. The oldest intrusive rocks of the Duluth Complex in the map area are gabbroic to anorthositic rocks of the early anorthositic series. The anorthositic series consists of a structurally complex suite of plagioclase-rich gabbroic rocks that cover large expanses of the upper reaches of the Duluth Complex (Miller and others, 2002). Over most of the map area, anorthositic rock types, with plagioclase ranging from 75 to 95 percent, typically occur as meter- to decameter-sized inclusions in troctolitic rocks. However, in 46 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 the Babbitt Southwest quadrangle, several varieties of anorthositic-series rocks occur over large areas. In addition to standard plagioclase-rich anorthositic-series lithologies, typically with poikilitic to intergranular olivine, these large areas of anorthositic-series rocks also contain a distinct olivine oxide gabbro lithology that only locally is leucocratic. This rock type was referred to as the Powerline gabbro by Bonnichsen (1972), who suggested it was an upper differentiate of the Partridge River intrusion. Paces and Miller (1993) acquired a U-Pb age of 1,098.6 ± 0.5 Ma for this unit, also considering it part of the Partridge River intrusion. However, this mapping has clearly shown this gabbro to be in gradational contact with other anorthositic-series rocks and to be crosscut by troctolitic rocks of the Partridge River intrusion. The main Duluth Complex units in the Babbitt quadrangles are various types of troctolitic (Ol + Pl) cumulates of the layered series. The layered series is the youngest component of the Duluth Complex and is composed of a suite of discrete layered mafic intrusions that show variable degrees of internal differentiation (Miller and others, 2002). The Babbitt quadrangles include parts of four major layered series intrusions: the Partridge River intrusion, the South Kawishiwi intrusion, the Greenwood Lake intrusion, and the Bald Eagle intrusion. Only the Partridge River and the South Kawishiwi intrusions are sufficiently exposed to subdivide their igneous stratigraphies into map units; both intrusions are composed mostly of olivine-plagioclase cumulates, and different map units are distinguished on the basis of subtle differences in the amount of interstitial augite and Fe-Ti oxide, the occurrence of melatroctolitic intervals, and on variable concentrations of anorthositic-series inclusions. The igneous layering in the lower 500 meters of both intrusions is better known because of the high density of exploration drill core. Several of the major Cu-Ni-PGE sulfide deposits that occur along the base of the Duluth Complex in this area were first discovered in the early 1960s. From southwest to northeast, these include the Northmet (formerly Dunka Road), the Mesaba (formerly Babbitt), the Serpentine, the Dunka Pit, and the Birch Lake deposits. Polymet is in the final stages of permitting the Northmet deposit, and if successful, is scheduled to begin development in 2008. Assessment activity on the other deposits has increased as well, particularly on the Birch Lake deposit. The geologic picture portrayed in these maps and cross sections provide new insights and important constraints on models for the emplacement, crystallization, and mineralization histories of the Partridge River and South Kawishiwi intrusions. The potential development of Cu-Ni-PGE deposits will provide further insights into the detailed geology and mineralization of this economically important part of the Duluth Complex. REFERENCES Bonnichsen, B., 1970a, Reconnaissance geologic map of Babbitt quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970b, Reconnaissance geologic map of Babbitt NE quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970c, Reconnaissance geologic map of Babbitt SE quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1970d, Reconnaissance geologic map of Babbitt SW quadrangle: Minnesota Geological Survey Open-File Map, scale 1:24,000. ———1972, Southern part of the Duluth Complex, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 361-388. 47 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Miller, J.D., Jr., 2005, Bedrock geology of the Babbitt Southeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-162, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J.D., Jr., and Severson, M.J., 2005, Bedrock geology of the Babbitt Southwest quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-161, scale 1:24,000. Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2005, Bedrock geology of the Babbitt Northeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-160, scale 1:24,000. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14,013. Patelke, R.L., 1996, The Colvin Creek body, a metavolcanic and metasedimentary mafic inclusion in the Keweenawan Duluth Complex, northeastern Minnesota: Duluth, Minn., University of Minnesota Duluth, M.S. thesis, 232 p. Severson, M.J., and Miller, J.D., Jr., 2005, Bedrock geology of the Babbitt quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-159, scale 1:24,000. 48 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOCHEMICAL CONSTRAINTS ON THE DEPOSITION OF MESOARCHEAN BANDED IRON FORMATION AT THE MUSSELWHITE MINE, NORTH CARIBOU GREENSTONE BELT, SUPERIOR PROVINCE MORAN, PATRICK†, FRALICK, PHILIP and HOLLINGS, PETE, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, pcmoran@lakeheadu.ca Iron formations (IFs), chemical sedimentary rocks containing greater than 15% iron (James 1954), can be broadly divided into Superior- and Algoma-type. Superior-type IF are laterally extensive, associated with sedimentary rocks deposited in shallow water settings, and are generally Paleoproterozoic in age. They formed from the upwelling of oxygen-deficient, Fe+2bearing ocean water onto shallow shelves where oxygen was present (Cloud 1973). In contrast, Algoma-type IFs are laterally and vertically limited, associated with volcanic and sedimentary rocks deposited in deep-water, and mostly Archean in age (Gross 1996). They are commonly considered to have formed by precipitation from venting hydrothermal fluids, although shallowwater deposits of Algoma-type are present and probably represent microbially induced precipitation (Fralick, this conference). This study utilized banded chert-magnetite iron formation present in surface exposures at the Musselwhite Mine. The site sampled is an unmineralized area of the gold-bearing horizon. Sixteen samples were collected, from which monomineralic layers were separated and analyzed using XRF and ICP-MS. This is an amphibolite facies Algoma-type IF that overlies a thick mafic metavolcanic succession with apparent conformity. Millimeter to approximately one centimeter thick layers of chert and magnetite alternate in the lower half of the IF. These layers contain very small amounts of siliciclastic material. Higher in the IF there is a gradational increase in the amount of siliciclastic debris intercalated with the chemical sediment layers, until the succession is dominated by siliciclastics. All samples came from the lower, relatively siliciclastic free zone. Concentrations of most major and trace elements are relatively low in the magnetite and chert samples. Exceptions to this are Si in the cherts and Fe, Mn and P in the magnetite layers. The Si, Fe and Mn are self-explanatory. The slightly elevated phosphorous values indicate possible scavenging of PO4-2 from seawater by iron hydroxides or oxyhydroxides during precipitation or microbial activity in the sediment. Trace element abundances normalized to chondrite (Fig.1a,b) indicate the fluid that precipitated the IF was depleted in Ni, Cr, Zn, Co, Cu, Ti and K; and enriched in Sc, Y, W and Cs, relative to chondrites. Figure 1C shows the chert layers have less admixed siliciclastic material and also less Ni, Cr, Zn, Co and Cu, possibly denoting the iron compounds precipitated from higher temperature fluids. Rare earth element plots portray similar patterns to REE plots of recent metalliferous sediment and vent water from the TAG field (Atlantic) and the Atlantis II Deep (Red Sea) (Peter 2003). The geochemical data indicate both the Si- and Fe-rich layers precipitated from vented fluids in an environment where there was sufficient oxygen to form iron hydroxides or oxyhydroxides. References Peter, J.M., 2003, Ancient iron formations: their genesis and use in the exploration for stratiform base metal sulfide deposits, with examples from the Bathurst Mining Camp, in Lentz, D.R., Ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments: Geological Association of Canada, GeoText 4, p. 145-176 James, H.L., 1954, Sedimentary Facies of iron-formation: Economic Geology, v. 49, p. 29-44 Cloud, P.E., 1973, Paleoecological significance of banded iron-formation: Economic Geology, v. 68, p. 1135-1143 Gross, G.A., 1996, Stratiform Iron: Lake Superior-type iron-formation, Algoma-type iron-formation, Ironstone, in Eckstrand, O.E. Sinclair, W. D., And Thorpe, R.I., Eds., Geology of Canadian Mineral Deposit Types, The Geology of North America, n. 8: Geological Survey of Canada, p. 41-80 49 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Figure 1. (a) Logarithmic plot of trace element values from magnetite layers taken from the BIF at Musselwhite mine. Trace element values normalized to CI Carbonaceous chondrite values of McDonough and Sun (1994). (b) Logarithmic plot of trace element values from chert layers taken from the BIF at Musselwhite mine. Trace element values normalized to CI Carbonaceous chondrite values of McDonough and Sun (1994). (c) Chert values normalized to magnetite values from the present study. (d) REE plot normalized against CI chondrite. 50 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 STATISTICAL ANALYSIS OF INDOOR RADON DATA AND RELATIONSHIPS TO GEOLOGY IN WISCONSIN MUDREY, M.G. JR., 106 Ravine Road, Mount Horeb, WI 53572 (mgmudrey@mhtc.net) In the 1986 with the discovery of exceeding high radon values in Pennsylvania, the US Environmental Protection Agency initiated a nation-wide study to determine the population and geographical risk associated with radon. Because Wisconsin has a strong state radiation protection program, and known occurrences of uranium and other radionuclides, it was one of 10 initial states to be analyzed. Since then, 84,262 residential indoor radon in air covering all areas has been collected. These data are not randomly distributed and reflect collection by interested by home owners. Nearly 50% of the analyses have been accurately located and digitized permitting geologic analysis; the remainder has only zipcode locational information. This study compares the EPA radon survey with the more extensive Wisconsin data base and evaluates the data with respect to various geologic attributes in order to define those areas and geologic units in Wisconsin where radon may pose a higher risk. The original EPA survey of 1194 homes is considered the only statistically useful survey for evaluating average values of indoor radon in Wisconsin: the mean of 3.4, with a Q1 of 1.2 and a Q3 of 4.1. The data are highly skewed. The highest value found in Wisconsin is 938 near Hudson. Elevated indoor radon is found in all areas of Wisconsin as is predicted by geological analysis. Soil derived from granite and carbonate rock that is the principal geological factor leading to elevated indoor radon in Wisconsin. Because radon can migrate only a few meters over the 3.8 day half-life, soil chemistry and soil texture principally influence elevated indoor radon. Because elevated radon is found in all areas of Wisconsin, and because do-ityourself radon testing is inexpensive, it is highly recommended that all houses in Wisconsin should be tested for radon. This study was funded by the U.S. Environmental Protection Agency to the Radiation Protection Section, Wisconsin Division of Public Health, Wisconsin Department of Health and Family Services, and to the Wisconsin Geological and 51 Natural History Survey, University of Wisconsin Extension, while Mudrey was with the WGNHS. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 3D VISUALIZATIONS OF MAFIC INTRUSIONS IN THE DULUTH COMPLEX, NORTHEASTERN MINNESOTA PETERSON, DEAN M., Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu) One of the main hallmarks of science is that it allows one to imagine reality. In the geosciences, one of the main realities that geologists try imagine is the geometry and structural juxtaposition of geological units and/or mineralized zones in the subsurface. This is especially true for geologists that work in the mineral exploration and mining industries, earthquake monitoring and hazard assessment agencies, and at contaminated groundwater sites. The advent of 3D geological computer programs has brought about a revolution in the understanding of the Earth in three dimensions. This digital poster presentation using the computer program gOcad (by Earth Decision Science) will display geological features of numerous troctolitic intrusions within the Duluth Complex, and will highlight how such visualizations advance our understanding of geological processes that ultimately led to the formation of billions of tonnes of Cu-Ni-PGE mineralized rocks. An image of the basal surface of a portion of the Partridge River Intrusion is presented in Figure 1. The bowl-shaped depression hosts the Babbitt deposit, currently held by Teck-Cominco. Figure 1. 3D view of the basal surface of a portion of the Partridge River Intrusion. View looking due west and down 10º. Grid lines are UTM coordinates, in meters. 52 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 EVIDENCE FOR WIDESPREAD DISTRIBUTION OF IRON DEPENDENT METABOLISMS IN PRECAMBRIAN OCEANS PLANAVSKY, NOAH †, KNUDSEN, ANDREW, Lawrence University Appleton, WI SHAPIRO, RUSSELL, Gustavus Adolphus College Saint Peter, MN noah.j.planavsky@lawrence.edu Starting with the first detailed descriptions of Precambrian microfossils (Barghorn and Tyler, 1965), the dominant view has been that cyanobacteria were the primary producers in Paleoproterozoic, and more generally in all early Precambrian, ecosystems (Awramik, 1992). An increased understanding about the chemical evolution of the ocean atmosphere system lead some to question the theoretical and observational foundation of this dogma (Blank, 2004) Currently, there are little constraints on many of the basic attributes of most pre1.8ga ecosystems. For instance, the diversity of bacterial metabolism in early environments in still debated and distribution of bacterial metabolisms in early earth’s history is poorly constrained. Although the atmosphere became oxygenated approximately 2.5Ga widespread marine anoxia and sulfate limitation resulted in pre1.8Ga oceans with significant quantities of dissolved iron. There is strong theoretical support from ecological modeling that these iron rich oceans supported an abundant iron dependent microbial community (Konhauser et al., 2002). In our analysis of the Paleoproterozoic Animikie basin we found empirical evidence for the widespread distribution of a microbial community with iron dependent metabolisms that thrived at a characterizeable oxygenic chemocline. There are two distinct stromatolites Animikie Basin; large, hemispheroidal, calcitic, peritidal stromatolites, and iron rich, subtidal, digitate stromatolites. Based on the morphology of the stromatolites, the inferred primary silica composition (Barghoorn and Tyler, 1965), and the microstructure the iron rich stromatolites were proposed as having formed as either sinter deposits or in the hot spring apron (Walter, 1972). The stromatolites proposed to be sinter deposits are defined by the presence of thin laminations (thinner in width than the majority of Gunflint microfossils) with distinct boundaries between the laminations (Hoffman, 1969; Walter, 1972). The spring origin of the stromatolites has been widely accepted, (Sommers and Awramik, 2002; Siminson, 1987) and even popularized to general audiences (Knoll, 2003). Field and microscopy work revealed that the ‘abiogenic’ stromatolites formed under a strong microbial influence. Field observation demonstrated that the stromatolites represent one facies in a well preserved sedimentary package. The iron rich stromatolites also display mesostructural features that cannot be readily explained by abiotic processes. For instance, the columnar sections of the stromatolites display crestal thickening, often with tuffs similar those seen in modern microbial mats and laminations that are far above angle of repose. The presence of alternating residual organic rich and residual organic poor lamination couplets and total organic carbon values of up to 2.5% also strongly suggests microbial mediation. In the least metamorphically altered stromatolites the thinner organic rich laminations are composed predominately of hematite in siliceous cement. The organic poor laminations and the surrounding siliceous material contain very low concentrations of iron. The iron distribution cannot be explained by secondary 53 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 oxidation and therefore necessities a means for iron disproportionation within the organic rich laminae. The rare earth element (REE) pattern and sedimentological characterization suggest that the iron rich stromatolites formed at an oxygenic chemocline. Ce is the only REE that can be oxidized in surface aqueous solutions, which results redox reactions controlling the element’s cycling and abundance. The stromatolites display a negative cerium anomaly, which indicates stromatolite formation in aerobic conditions or at an oxygenic mixing zone. The stromatolites occur at a transition out a zone with regular authogenic iron deposition, which implies formation at an oxygenic chemocline. In modern environments, iron oxidizing β proteobacterium dominate at oxygen mixing zones or in microaerophilic conditions where there are similar or even significantly lower ferric iron concentrations than the predicted (Holland, 1984,) values for the Paleoproterozoic oceans (Emerson and Revsesbach, 1994). Iron oxidizing β proteobacterium induce iron precipitation on average 60 times faster than abiotic reactions providing a means for the observed iron disproportionatation in the organic rich stromatolite laminae. The stromatolites also display a positive Gd anomaly, which serves as independent biogenicity proxy. Abiogenic iron precipitates, because of the lanthanide tetrade effect, display a negative Gd anomaly (Bau, 1999). Microbial communities slightly preferential or proportionately adsorb Gd compared to Tb, and Dy (Anderson and Pedersen, 2003). Modern ocean water has positive Gd anomalies that are mirrored by negative Gd anomalies in the largely abiogenic ferromanganese pavements. The stromatolites have similar or even more pronounced Gd anomalies than modern oceans. The stromatolites positive Gd anomaly necessitates biogenic precipitation of iron oxides within the stromatolites. References: Anderson, C. R. & Pedersen, K. 2003. In situ growth of Gallionella biofilms and partitioning of lanthanides and actinides between biological material and ferric oxyhydroxides. Geobiology 1 (2), 169-178. Barghoorn, E.S. and Tyler S.A., 1965. Microorganisms from the Gunflint Chert. Science 147 (3658), 563577. Bau M., and Dulski P. (1999) Comparing yttrium and rare earths inhydrothermal fluids from the MidAtlantic Ridge: Implications for Yand REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater. Chem. Geol. 155, 77–79. Emerson, D., and N. P. Revsbech. 1994. Investigation of an iron-oxidizing microbial mat community located near Aarhus, Denmark: field studies. Applied Environmental Microbiology 60:4022-4031 Holland, H. D., 1984. The Chemistry of the Atmosphere and Oceans. Wiley, New York. Konhauser, K.O., Hamade, T., Morris, R.C., Ferris, F.G., Southam, G., Raiswell, R., and Canfield, D., 2002. Could bacteria have formed the Precambrian banded iron formations? Geology, 30:1079-1082. Simonson B. M. n, 1985, Sedimentological constraints on the origins of Precambrian iron-formations, GSA Bulletin, 96: 244-252. Walter, M.R. 1972. A hot spring analog for the depositional environment of Precambrian iron formations of the Lake Superior region. Economic Geology, 67: 965-972. 54 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 DETRITAL ZIRCON GEOCHRONOLOGY OF THE WESTERN HURONIAN BASIN RAINBIRD, ROBERT H.† and DAVIS, WILLIAM J. Geological Survey of Canada, 601 Booth St, Ottawa, Ontario, K1A 0E8 rrainbir@nrcan.gc.ca The Paleoproterozoic Huronian basin hosts an up to 12 km-thick succession of mainly siliciclastic sedimentary rocks deposited along the southern margin of the Superior Province (Huronian Supergroup). Paleocurrent data from crossbedding in fluvial sandstones throughout the succession suggest provenance from the west and northwest. U-Pb SHRIMP analysis of detrital zircon from six sandstone samples from the western part of the Huronian basin indicates provenance mainly from Neoarchean sources with prominent modes ca. 2.67 and 2.72 Ga. A sample from the stratigraphically lowest unit (Livingstone Creek Formation-Elliot Lake Group) contains zircon ranging in age from 2.90 - 2.65 Ga, with one ca. 2.50 Ga grain (weighted mean 207Pb/206Pb age of 2497 ± 10 Ma). This grain probably was derived from co-eval volcanic rocks erupted during rifting and initiation of the Huronian basin and provides a maximum age for deposition of the Huronian. A sample from the overlying Matinenda Formation has a unimodal zircon age population at ca. 2.67 Ga. The overlying Mississagi Formation (Hough Lake Group) has a polymodal zircon population varying in age from 3.62 - 2.45 Ga. Given the easterly paleocurrent indicators at the sampling locality, the pre-3.0 Ga grains could have derived from gneisses of the Minnesota River Valley terrane, southwest of the Huronian basin. The two youngest grains from the Mississagi Formation (weighted mean 207Pb/206Pb ages-2445± 9 Ma and 2451 ± 6 Ma), likely were eroded from volcanic rocks (or their intrusive equivalents) in the unconformably underlying Elliot Lake Group. Samples from succeeding thick fluvial quartz arenites of the Serpent (Quirke Lake Group) and Lorrain (Cobalt Group) formations show similar detrital zircon age profiles with a range of ages from 2.88-2.68 Ga, and a significant ca. 2.72 Ga population. A marine quartzarenite from the uppermost unit of the Huronian (Bar River Formation-Cobalt Group) has a generally similar population to that of the Serpent and Lorrain formations but with a broader range of ages, including 3 grains at ca. 2.53 Ga of unknown provenance, and 4 grains at ca. 3.00 Ga. Overall, detrital zircon geochronology and sedimentology of the western Huronian basin is compatible with provenance from the Wawa Subprovince of the western Superior craton with contributions from adjacent older gneissic terranes. 55 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 56 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 UNRESOLVED PROBLEMS AND THE EVOLUTION OF SUDBURY GEOLOGY ROUSELL, D. H. Department of Earth Sciences, Laurentian University, Sudbury, ON, P3E 2S4, drousell@laurenian.ca. Geological events, which shaped the area now occupied by the Sudbury Basin and surrounding footwall, span at least 1470 Ma from 2711 Ma, the minimum age of the Levack Gneiss Complex, to 1238 Ma, the age of olivine diabase dikes. The area has undergone several tectonic, magmatic, metamorphic and mineralization events which have been largely overshadowed by the Sudbury Event at 1850 Ma. Ascribing the Sudbury Event to meteorite impact is an entrenched paradigm; several unresolved problems are largely ignored. The aim of this abstract is to outline the geological evolution of the area and to identify certain outstanding problems. The events which have affected the area may be grouped as follows: doming, Sudbury Event and post Sudbury Event. Early workers recognized that the Paleoproterozoic rocks of the Sudbury Basin were superimposed on an Archean dome, with the NW boundary extending to the Huronian outliers and the other boundaries obscured by later deformation. Evidence for the dome is as follows: 1) in the South and East Ranges the footwall rocks become progressively younger away from the dome; 2) rocks of the Levack Gneiss Complex, metamorphosed to the granulite facies at depths of up to 28 km and uplifted to depths of 5 to 11 km, possibly during emplacement of the Cartier batholith, suggests that the complex cored a paleodome; 3) mafic dikes in the footwall of the North and East Ranges, located between 10 and 15 km from the outer margin of the Sudbury Igneous Complex ( SIC) , are oriented normal to the adjacent margin of the SIC which is consistent with dike emplacement during local magmatic doming; 4) the Nipissing gabbro, which has an affinity for rocks of the Huronian Supergroup, is absent between the NW edge of the SIC and the Huronian outliers, which suggests little or no deposition of Huronian sediments or their complete erosion, implying that the site of the basin was a topographic high; and 5) three felsic plutons , viz., Murray, Creighton and Skead intrude the area of the inferred paleodome. Prior to the Sudbury Event, rocks of the Southern Province and the Huronian outliers were folded about EW- to NE-trending axes and locally about NNE-to NNW-trending axes. The Sudbury Event is ascribed, without question, to meteorite impact by virtually all investigators. The bolide coincidentally struck a local dome which was presumably pregnant with sufficient Ni-Cu-PGE- Zn-Pb-Cu-Ag-Au mineralization to form one of the world’s largest mining camps. The notion that the ores are of cosmic origin has received little support. Features formed by the event include: Footwall Breccia and Sudbury Breccia (SB); shatter cones; planar deformation features; Onaping Formation; and SIC. SB in the granitic rocks of the North Range footwall consists of pseudotachylite, a rock considered to have formed by frictional melting in dry rocks during high-speed slip along large faults concentric with the outer margin of the North Range SIC. Field work has led others to question the existence of the faults. In contrast to the North Range SB, clastic SB occurs in the footwall of the South and East Ranges. It apparently formed by explosive dilation, fluidization and flowage into extension fractures. Clastic SB occurs at Lake Temagami, 80 km NE of the outer margin of the SIC and the most distant locality reported to date. Clasts in breccias surrounding impact craters tend to increase in size with distance from impact. Clasts at Lake Temagami are smaller than those in some bodies near the SIC. Curved and striated fracture surfaces known as shatter cones are supposedly unique to impact craters. At Sudbury, they are scarce in the North Range footwall but are common in the South and East Range footwall where they tend to occur in clusters. Most fractures are in the shape of curved surfaces rather than cones. In attempts to define the size of the Sudbury crater, numerous authors have subscribed to the thesis, first proposed in 1970, that the Huronian outliers 57 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 were preserved in down-faulted blocks formed by the simultaneous collapse of the central uplift and the emplacement of the SIC. Despite convincing evidence that the sediments were deposited in a rim syncline around a domal structure long before the Sudbury Event, relating the preservation of the outliers to impact still persists The origin of the Onaping Formation has been interpreted as follows: volcanic, fall-back breccia from meteorite impact, impact-induced volcanism or hyaloclasis. A discontinuous breccia at the base of the formation contains fragments as much as 79 x 23 km which reflect the lithology of the adjacent country rocks. The clasts were possibly emplaced by sliding down a submarine slope (crater wall?). Above the basal unit the formation consists of a series of breccias. Possibly melt, continuously fed from below, underwent passive to explosive fragmentation and rapid cooling on interaction with water. The “Onaping melt” may represent an impact melt, a hypabyssal intrusion or some combination. Diamonds in the Onaping Formation are referred to, by their discoverers, as “impact diamonds” without even considering alternatives such as a diatreme origin. Based largely on geochemical data, most investigators interpret the SIC to be an impact melt sheet. However, the granophyre, the upper unit of the SIC, intrudes the Onaping Formation. This implies that the SIC crystallized from a magma, possibly impact-triggered. An alternate scenario is that the granophyre represents a later intrusion and that the bulk of the SIC was emplaced before the Onaping, perhaps as a melt sheet. This means that the Onaping “melt” was injected through the SIC before fragmenting into a hyaloclastite. Structural data suggests that the SIC was emplaced in approximately the present disposition of the North Range i.e., dipping inward at 420. Evidence is as follows. Igneous layering in the norite dips less than the dip of the base of the SIC. Folding of a horizontal melt sheet would produce a foliation with a steeper dip than the basal contact and a strain in possible hinge zones such as the lobes located at both ends of the East Range. A plagioclase lineation in the north lobe is orthogonal to the base of the SIC. The lineation is attributed to crystal growth in a magma chamber. Apparently, even minor deformation will destroy the orthogonality. Thus the mineral fabric and the low overall strain in the North Lobe preclude a fold origin for the present shape of the SIC. The Sudbury Event was followed by differentiation of the units of the SIC (1850 Ma), formation of the Ni-Cu-PGE deposits, deposition of the Vermilion, Onwatin and Chelmsford Formations, hydrothermal alteration and formation of Zn-Pb-Cu deposits inside the basin. NW-directed compression and a weaker SW-directed compression (Penokean Orogeny, 1900 to 1700 Ma) folded the rocks of the basin about NW-trending, doubly-plunging fold axes and developed a prominent cleavage in them. Deformation died out to the NW as the North Range Onaping Formation and SIC have undergone only local and mild ductile deformation. In the South Range, the Onaping Formation and the SIC are steepened to the NW and displaced by a SE-dipping zone of reverse shear. On the outcrop-scale, the SIC displays anastomosing conjugate shear zones. A revised model is required which embraces all aspects of Sudbury Geology. Many features are either force-fitted into an impact model or ignored. Some, but not all, elements are better explained by endogenic models such as diatremes and plumes. Because of the time span of Sudbury geology, perhaps too many pieces of the puzzle have been lost, thus precluding an unequivocal history of Sudbury Geology. 58 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 GEOLOGY AND ALTERATION ASSOCIATED WITH VMS MINERALIZATION IN THE HAMLIN LAKE AREA, NORTHWESTERN ONTARIO SHUTE, AMY† and HOLLINGS, PETE, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada; alshute@lakeheadu.ca, The Shebandowan Greenstone Belt is located within the Wawa Subprovince of the Superior Province and has been the target of numerous exploration efforts over the last century. The belt is host to many different precious and base metal mineral deposits. Past producers include the Shebandowan Ni-Cu PGE mine, the North Coldstream Cu-AuAg mine, and the Ardeen Au mine, Northern Ontario’s first gold producing mine. With a renewed interest in the belt by many exploration companies, the potential for new discoveries is growing. The Hamlin Lake area is within the Shebandowan Greenstone belt and located approximately 120 km west of Thunder Bay, Ontario. This project will be looking at the alteration and tectonic setting that is associated with the Hamlin Lake volcanogenic massive sulfide (VMS) system. The VMS system was first recognized when massive sulfides were found while surface sampling during the 2005 field season. The mineralization includes pyrite, chalcopyrite and pyrrhotite all at the surface with stringer mineralization found in some areas as well. Trenching and drilling followed during the fall and winter of 2005/06 and revealed significant additional mineralization. The copper values as high as 1.49% have been found in surface samples with 4.88g of gold, and the Ray Smith showing is part of the Hamlin Lake property with copper values as high as 6% and 6.0g of gold. A B Figure 1. (a) Rhyolite showing round amygdules; (b) Felsic fragmental showing partial alignment of the clasts (magnet for scale is approximately 12.0cm long) The Hamlin Lake area consists of four major units; felsic volcanic rocks; mafic volcanic rocks; pink brecciated rock; and banded iron formation. The smallest unit is the banded iron formation, found in one larger outcrop, but also found sporadically throughout the Hamlin Lake area in small (1-2m) lense-like showings. The mineralized pink breccia is the second least abundant unit and is the focus of a currently ongoing drilling project because of its higher grade mineralization. 59 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The second major unit is the mafic volcanic rocks. This unit consists of a fragmental unit and can vary in groundmass from being chlorite rich to magnetite rich in the field. Geochemically the mafic volcanic rocks have SiO2 values that range from 51.1 to 64.4 wt.%, TiO2 from 0.08 to 0.61 wt.%, and Fe2O3 from 7.0 to 34.3 wt.%. The most abundant rock type and the focus of this study are the felsic volcanic rocks. In the field, the felsic volcanics contain quartz-eyes, amygdules, and fragments with a fine-grained groundmass and overall light grey colour (Fig.1a and b). These rhyolites vary in colour, texture and in geochemistry with preliminary work showing at least two distinct felsic suites when the REE’s are plotted (Fig.2b). The major elements in the felsic volcanic rocks show SiO2 values that range from 76 to 89 wt.%, TiO2 from 0.2 to 0.8 wt.%, and Fe2O3 from 0.9 to 8.1 wt.%. Although the groundmass is different between the felsic and mafic fragmental volcanic rocks, their fragments are both lenticular-shaped and chert-like in appearance. These fragments have yet to be studied to distinguish their origin, but they are likely either pumaceous clasts or fragmented chert layers. 7000 100.00 6000 10.00 4000 REE/PM Ti (ppm) 5000 3000 AS-05-012 AS-05-015 1.00 AS-05-029 AS-05-057 2000 0.10 1000 0 0.01 0 50 100 150 200 250 300 350 400 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al Zr (ppm) Figure 2. (a) Preliminary Zr vs. Ti (ppm) plot showing two distinct groupings of samples. (b) PM normalized REE plot showing two distinctly different rhyolites suites. Felsic volcanic rocks associated with VMS systems have been the subject of considerable study over the past two decades. Several classifications been created in an attempt to characterize felsic volcanic rocks that are associated with VMS deposits, and that are barren of VMS deposits. Lesher et al. (1986) classified felsic volcanic rocks as being FI, FII and FIII’s with distinctions between their REE patterns, Zr/Y ratios and abundances in high field strength elements. Preliminary work has shown that there are at least two distinct felsic volcanic suites, suite I having flat REE patterns and positive Zr and Hf anomalies and suite II having more fractionated REE and lacking positive anomalies. When comparing Lesher’s classification with the felsic volcanic rocks in the Hamlin Lake area, suite I most closely resemble FII whereas suite II is similar to the FI group. References Lesher, C.M., Goodwin, A.M., Campbell, I.H., and Gorton, M.P., 1986, Trace-element geochemistry of ore associated and barren, felsic metavolcanic rocks in the Superior Province, Canada: Canadian Journal of Earth Sciences, v. 23, p. 222-237. 60 V Sc �Proceedings of the 52nd ILSG Annual Meeting – Part 1 PRELIMINARY INVESTIGATIONS OF THE PETROLOGY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE ST. IGNACE ISLAND COMPLEX, MIDCONTINENT RIFT, NORTHERN LAKE SUPERIOR, ONTARIO SMYK, MARK C.†, Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 CANADA, HOLLINGS, PETER, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada and HEAMAN, LARRY M., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3. As part of the Lake Nipigon Region Geoscience Initiative, a helicopter traverse was undertaken in 2005 in northern Lake Superior, in order to sample igneous rocks associated with the Mesoproterozoic Midcontinent Rift (MCR), including the St. Ignace Island Complex (SIC; Fig. 1). The SIC intruded the upper portions of MCR-related, Osler Group volcanic rocks (ca.1008 Ma; Davis and Sutcliffe 1985). It consists of a gabbroic to anorthositic ring dyke, which encloses quartz-feldspar porphyritic volcanic rocks (Sutcliffe and Smith 1988; Giguere 1975). Sutcliffe and Smith (1988) described the volcanic component of the SIC as intercalated plagioclase-glomeroporphyritic basaltic rocks, quartz-feldspar-phyric rhyolite flows and fragmental rocks. The pink to grey, rhyolitic rocks in the core of the SIC are dominantly quartz-phyric, with rare pyroxene and feldspar phenocrysts set in a fine-grained to glassy groundmass. They commonly contain wispy to amoeboid, mafic (basaltic?) inclusions, which are typically plagioclasephyric. Geochemically, the quartz-feldspar-phyric rocks from the core of the SIC are dacites and rhyolites (62 to 74 wt% SiO2) with elevated K2O contents (2.3 to 4.8 wt%). The lower silica contents within the core of the complex are apparently associated with small mafic inclusions within the more felsic units. The sampled mafic intrusive rocks from the ring dyke are plagioclase- and pyroxene-phyric, coarse- to fine-grained gabbros to monzogabbros (53 to 58 wt% SiO2). Figure. 1. A) Map of upper Great Lakes showing the location of the study area. B) Regional geology map showing the extent of the exposed portion of the Osler Group and the location of the St. Ignace Island Complex. The SIC samples yielded both zircon and baddeleyite. Subhedral baddeleyite grains from a rhyolite in the core of the SIC yielded a 207Pb/206Pb age of 1107.2±2.4 Ma, whereas zircons recovered from the rhyolite yielded a 207Pb/206Pb age of 1124 Ma. The zircons are large and show signs of resorption; they are also characterized by high Th/U contents that are typical of zircons derived from a mafic source. The fact that this latter age is much older than that of the Osler Group basalts that the SIC has intruded, combined with a lack 61 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 of euhedral zircon and baddeleyite grains, suggests that these grains may be of xenocrystic origin. The presence of baddeleyite in a rhyolite is also somewhat unusual as these are more commonly associated with syenites found in alkalic complexes (e.g. Coldwell Complex). Gabbro from the margin of the SIC yielded a small number of zircon grains with baddeleyite cores. These grains yielded a 207Pb/206Pb age of 1089.2±3.2 Ma. The growth of euhedral zircon on baddeleyite cores is occasionally observed in mafic rocks and is interpreted to indicate increasing silica activity conditions during magma crystallization. This is consistent with field relationships, which suggest that rhyolitic and gabbroic magmas may have intermingled during emplacement. Consequently, the 1089 Ma age may represent the emplacement age of both the rhyolite and the gabbro and suggests that all dates obtained from the rhyolite are xenocrystic. This is consistent with textures observed by Sutcliffe and Smith (1988) who also reported evidence for localized magma mixing and the presence of vesicular basalt fragments in felsic, welded tuffs. This age is similar to that of other MCR-related intrusions in the area (e.g. Crystal Lake, Blake and Moss Lake gabbros; Arrow River dyke) that have intruded Paleoproterozoic rocks, older MCR intrusions and/or Osler Group volcanic rocks during the late stages of MCR magmatism (Heaman and Easton 2006). Further field, petrographic and geochemical studies will seek to better determine the relationships between the various volcanic and intrusive rocks in order to understand the development of the SIC within the Midcontinent Rift. References Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Davis, D.W. and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, 34, p.476-488. Giguere, J.F. 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay; Ontario Division of Mines, Geological Report 118, 35p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Miscellaneous Release of Data 191, 86p. Sutcliffe, R.H. and Smith, A.R. 1988. Geology of the St. Ignace Island volcanic-plutonic complex; Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 141, p.368-371. 62 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 CHARACTERIZING THE MONAZITE FINGERPRINT OF PALEOPROTEROZOIC (STATHERIAN) METASEDIMENTARY SEQUENCES IN CENTRAL WISCONSIN STONIER, PEGGY, and HOLM, D.K., Kent State University, Kent, OH, peshelma@kent.edu; MEDARIS, L.G., JR, Univ. of Wisconsin-Madison, Madison, WI; SCHNEIDER, D., Ohio University, Athens, OH. Introduction: Supermature siliciclastic rocks of the 1750-1630 Ma Baraboo Interval are widespread in the southern Lake Superior region and signify a period of crustal stability following the Penokean Orogeny and subsequent geon 17 magmatism (Medaris et al., 2003). Although the chronology and tectonic significance of Baraboo Interval sedimentation has been firmly established, certain quartzite inliers in central Wisconsin remain enigmatic. At Hamilton Mounds Baraboo Interval quartzite is reported to be intruded by geon 17 granite (Greenberg, 1986; Van Wyck and Norman, 2004). This proposed cross-cutting relation conflicts with recent detrital zircon age data showing all Baraboo Interval quartzites to be younger than 1750 Ma (Holm et al., 1998; Medaris et al., 2003). Van Wyck and Norman (2004) propose that early onset of Baraboo Interval quartzite deposition was synchronous with magmatism, an interpretation that is unusual for this rock type and in disagreement with deposition on a recently stabilized craton (Dott, 1983). Instead, Medaris et al. (in review) demonstrate that the metasedimentary rocks at Hamilton Mounds consist of two Paleoproterozoic sedimentary sequences: an older meta-arkose intruded by geon 17 granite, and a younger, overlying supermature quartzite, which is likely correlative with Baraboo Interval quartzite elsewhere. Our purpose here is to characterize and date monazite grains in these two units. Recently, studies of detrital zircons in Baraboo Interval rocks have been invaluable for establishing their maximum age and identifying their source terrane. Assessing the monazite "fingerprint" in these rocks may allow us to differentiate between the two depositional interpretations and to better establish sedimentalogical aspects of post-Penokean crustal stabilization. Monazite Textures: The quartzite unit contains only tiny (10-40 micron diameter), rounded to subrounded monazite grains that show simple chemical zonation (rims/cores). Many have cores that are high in both Yttrium and Uranium (Fig. 1a). In contrast, the meta-arkose contains a larger monazite grain-size variance (10-90 micron diameters) and more variable morphology. A few grains are rounded and chemically simple, but many have embayed grain boundaries and are complexly zoned (Fig. 1b). Some grains are irregular and some have a bladed elongate morphology that are concordant to pre-existing textural features (Fig. 1c, d). Monazite Geochronology: Electron microprobe total-Pb analyses of the fine-grained detrital monazite from the quartzite unit yield spot ages ranging from ~2050 Ma to ~1750 Ma, with dominant peaks at ~1800 Ma and ~1860 Ma (Fig. 2a; composite mean age of 1837 ±23 Ma). Similar analyses on the meta-arkose unit yields ages ranging from ~1900 Ma to ~1730 Ma, with a single dominant age peak at 1850 Ma (Fig. 2b; composite mean age of 1849 ±7 Ma). Interpretation: Both rock units at Hamilton Mounds contain abundant Penokean age 63 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 detrital grains, consistent with their having been deposited after the Penokean orogeny. Both rock units also contain some geon 17 ages. The geon 17 ages within the quartzite unit are clearly from detrital grains. However, the geon 17 ages in the meta-arkose are unlikely to be detrital considering that this unit is cut by a ca. 1761 Ma granite dike. We interpret these ages to instead reflect metamorphism associated with dike intrusion. This interpretation is most consistent with the varied monazite morphology in this unit. Our results show that monazite geochronology of metasedimentary units is a powerful tool when combined with detailed in situ textural analysis aided by a comprehensive understanding of the area's geologic context. Figure 1a: Figure 1b: Figure 1c & 1d: In-situ BSE image of meta-Meta-arkose monazite In-situ BSE images of meta-arkose monazite grains quartzite monazite grain grain mapped for Y References Dott, R.H., Jr., 1983. The Proterozoic red quartzite enigma in the north central United States: resolved by plate collision?; Geological Society of America Memoir, v. 160, p. 129-141. Greenberg, J.K., 1986. Magmatism and the Baraboo Interval: breccia metasomatism and intrusion; Geoscience Wisconsin 10, 96-112. Holm, D.K., Schneider, D., and Coath, C., 1998b. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia; Geology, v. 26, p. 907-910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C., 2003. Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and protoNorth America: Evidence from Baraboo interval quartzites; The Journal of Geology, v. 111, p. 243-247. Van Wyck, N., and Norman, M., 2004. Detrital zircon ages from Early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites; The Journal of Geology, v. 112, p. 305315. 64 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 THE THERMAL HISTORY OF LOW METAMORPHIC GRADE PALEOPROTEROZOIC METASEDIMENTARY ROCKS OF THE PENOKEAN OROGEN, LAKE SUPERIOR REGION: RECOGNIZING A WIDESPREAD 1786 MA OVERPRINT USING XENOTIME GEOCHRONOLOGY VALLINI, DANIELA A. University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 CANNON, WILLIAM F.†, SCHULZ, KLAUS J. U.S. Geological Survey, MS 954, Reston, VA 20192 MCNAUGHTON, NEAL J. University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the Penokean foreland. These strata were metamorphosed by multipleevents, all previously attributed to the Penokean orogeny (1875-1830 Ma). We sampled 10 localities (Fig. 1) in the Marquette Range Supergroup in Michigan and the Animikie, Mille Lacs, and North Range Groups in Minnesota that contain xenotime suitable for in situ SHRIMP U-Pb geochronology and where the metamorphic grade is greenschist to sub-greenschist. The units sampled are Enchantment Lake Formation (sample 1), Sunday Quartzite (sample 5), Ajibik Quartzite (sample 7), and Michigamme Formation (samples 8, 9, 10) in Michigan, and the Mille Lacs Group (sample 2, 3), the Mahnomen Formation (sample 4), and Pokegema Quartzite (sample 6) in Minnesota. Thirty-two U-Pb ages of xenotime in these samples give a population at 1786 ± 4 Ma and 9 ages give a population at 1861 ± 10 Ma. Both populations are contained in samples from the Chocolay Group in Michigan and the Mille Lacs and North Range Groups in Minnesota (Fig. 1) and thus record a region-wide 1860 Ma low-temperature thermal event that is slightly older than the basal units of the Baraga Group in Michigan and the Rove Formation in Minnesota and Ontario. This event coincides with regional uplift that led to the unconformity between the Baraga and Menominee Groups in Michigan, hence xenotime growth must have occurred at shallow depths. Younger units, including the Animikie Group in Minnesota and the Baraga Group in Michigan, record only the 1786 Ma event. Amphibolite-granulite facies rocks within a gneiss dome corridor in the southern part of the foreland, south of our sample sites, show an 1800-1790 Ma monazite population that overprints 1830 Ma Penokean metamorphism (Schneider and others, 2004). These high grade rocks are adjacent to gneiss domes and early geon 17 post-Penokean granite plutons. Our samples are 50 to 150 km away from these features so the 1786 Ma xenotime ages do not appear to reflect local thermal imprints from plutonism and gneiss dome formation. Several sample sites in Michigan are within the low temperature zones of the Republic metamorphic node where metamorphic monazite has been dated at 1760 ± 5 Ma (Rose and others, 2003). Thus, most of our xenotime ages are significantly older than the Republic metamorphism, which does not appear to have been a significant xenotime-forming event at our sample sites. The geographic extent of the 1786 Ma xenotime growth event suggests it was a basin-wide subtle thermal pulse. We suggest two possible causes for this event. First, all of our age localities lie north of a corridor of gneiss domes and granitic plutons that formed in the interval 1800-1765 Ma, during a period of gravitational collapse of overthickened crust of the Penokean orogen (Schneider and others, 2004). This period of 65 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 gravity-driven tectonism and coincident heating may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in growth of xenotime. Alternatively the xenotime ages may record very distal effects of events within the Yavapai orogen, which truncated the southern part of the Penokean orogen on the south in central Wisconsin and southeastern Minnesota, about 200 km south of our sample sites. This early geon 17 crust-forming event occurred across the central and southwestern U.S. and may, in some as yet poorly understood manner, have caused widespread subtle heating across a broad foreland on its north. References Rose, S., Schneider, D.A., Loofboro, J., and Holm, D.K., 2003, Results and implications of monazite geochronology from the central Penokean orogen, WI & MI (abs): Geological Society of America Abstracts with Programs, v, 35, no.6, p. 505. Schneider, D.A., Holm, D.K., O’Boyle, C.O., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: in Whitney, D.L., Teyssier, C., and Siddoway, C.S., eds., Gneiss domes in orogeny: Geological Society of America Special Paper 380, p. 339-357. Figure 1. Map of the western Lake Superior region showing sample locations in relation to major tectonic and stratigraphic units. Inset shows density functions of xenotime ages divided into older and younger stratigraphic groups. 66 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SULFUR ISOTOPES FROM PYRITE IN THE NEGAUNEE IRON FORMATION WAGGONER, T.D., Negaunee, MI, USA 49866-1007 In 2003 evidence was presented at the ILSG on hydrothermal venting systems preserved in the sediments older than the Negaunee iron formation. Rare earth elements patterns for the hard ore and vent hematite suggested a commonality for the iron source. During the study it was noted that within the hard iron oxide deposits on the Marquette Range there are numerous occurrences of veins, disseminated and massive sulfides. Pyrite is the common sulfide but both chalcopyrite and bornite can be present. Nine pyrite samples associated with hard ores from geographically diverse locations on the Marquette Range were submitted (Geochron Labs) for sulfur isotope analysis. The isotope ratios were determined by using the Canon Diablo troilite (CDT) standard. The physical relation of the oxides to sulfides indicated either a syndeposition or post replacement of the chert and iron oxides by pyrite. Fig. 1 Location of sulfur isotopes from the Marquette Iron Range Sulfur associated with sedimentary processes reflect the composition of biogenic sulfide produced by bacterial reduction of marine sulfate and is likely to result in δ34S values. Sulfur associated with igneous rocks is isotopically similar to that of meteorites and have δ34S values close to 0%o. Further variations are due to complex and interactive chemistry of the fluids and host. 67 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 The values obtained range from + 1%o to +6.8%o with the mean value of 1.9%o The narrow low positive values would suggest that the sulfur is attributable to a hydrothermal source thus supporting the earlier conclusion based on the REE data. Since a hydrothermal component has been shown to exist for portions of the Brockman (Hagemann et al, 1999) Caue (Rosiere et al, 2004) and Carajas (Guedes et al, 2002), it is logical to project an igneous-hydrothermal source be considered for the formation of BIFs in general. It is further suggested that water deposited BIFs could be a natural end product of hydrothermal IOCG type mineralization. Many features (e.g. age, extensional cratonic or continental margin setting, not easily related to igneous activity, mineral assemblage and alteration patterns) common to Iron Oxide deposits (IOCG) are also common to banded iron formations. The existence of end member BIFs in IOCG deposits (e.g. Pilot Knob, MO and Olympic Dam, South Australia) supports the hypothesis. References Guedes, S.C. et al, 2002, Carbonate Alteration Associated with the Carajas High- Grade Hematite Deposits, Brazil. Proceedings: AusIMM Iron Ore 2002, p. 63-66. Hagemann, S.G. et al, 1999, A Hydrothermal Origin for the Giant BIF-Hosted Tom Price Iron Ore Deposit. In: Stanley et al. (eds), Mineral Deposits: Processes to Processing, Balkema, Rotterdam, p. 41-44. Rosiere, C.A. et al, 2004, The Origin of Hematite in High-Grade Iron Ores Based on Infrared Microscopy and Fluid Inclusion Studies: The Example of the Conceicao Mine, Quadrilatero Ferrifero, Economic Geology v. 90 p. 611-624. 68 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 SOURCE ROCK AGES AND PATTERNS OF SEDIMENTATION IN THE LAKE SUPERIOR REGION: RESULTS OF PRELIMINARY U-PB DETRITAL ZIRCON STUDIES WIRTH, K.R.1, VERVOORT, J.2, CRADDOCK, J.P.1, DAVIDSON, C.3, FINLEY-BLASI, L.3, KERBER, L.4, LUNDQUIST, R.3, VORHIES, S.5, WALKER, E.6 1 Geology Department, Macalester College, St. Paul, MN 55105 (wirth@macalester.edu) 2 Department of Geological Sciences, Washington State University, Pullman, WA 99164 3 Department of Geology, Carleton College, Northfield, MN 55057 4 Geology Department, Pomona College, Claremont, CA 91711 5 Department of Geology, Smith College, Northampton, MA 01063 6 Department of Geology, Allegheny College, Meadville, PA 16335 U-Pb age analysis of detrital zircons provides information about source region ages and patterns of sedimentation. Although most commonly applied to orogenic belts and accreted terranes, this technique also has great potential for illuminating the evolution of cratonic regions. Here we report preliminary results of U-Pb analyses of detrital zircons from Paleoproterozoic (Denham Formation, Pokegama Quartzite, Palms Formation, Rove Formation, Thomson Formation), Neoproterozoic (Puckwunge Sandstone, Nopeming Sandstone, Rift Interflow sediments, Fond du Lac Sandstone, and Hinckley Sandstone), and early Paleozoic (St. Peter Sandstone) rocks from Minnesota and Wisconsin. U-Pb analyses of detrital zircons were conducted using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Washington State University. Approximately 120 grains were analyzed from each sample. All reported ages are 207 Pb/206Pb ages. Only those grains that are <10% discordant, based on comparison of 206 Pb/238U and 207Pb/206Pb ages, are presented. Arkosic conglomerate and quartz arenite of the Denham Formation are the oldest rocks that we examined in this study. The Denham samples contain zircons with ages between 3.6 and 2.1 Ga, however most grains fall into two age ranges: 3.5 – 3.4 Ga and 2.8 – 2.5 Ga. The youngest grains observed in the Denham Formation are 2.07 Ga. Basal Sandstones of the Animikie (Pokegama) and Marquette (Palms) Supergroups in Minnesota and Wisconsin, respectively, contain mostly Neoarchean zircons with similar age distributions (2.9 to 2.6 Ga). Both formations contain scattered grains of Mesoarchean (Pokegama) and Paleoarchean age (Palms), but neither contains grains with ages < 2.6 Ga. Fine-grained sandstones from upper Rove Formation (NE Minnesota), Thomson Formation (E. Central Minnesota), and Tyler Formation (NW WI) were deposited in a migrating foredeep north of the Penokean orogen. Most zircon grains from these three formations have ages between 2.05 and 1.80 Ga (Fig. 1). All three formations also contain some Paleoproterozoic to Paleoarchean grains, but these ages are relatively few in number. The zircon age histograms also lack the major peak at 2.7 Ga that occurs in 69 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 the basal Pokegama and Palms Formations. Zircons from sandstones immediately below basal volcanics of the Keweenawan Midcontinent Rift (1.1 Ga) of east-central (Nopeming Sandstone) and NE Minnesota (Puckwunge Sandstone) have age distributions that are strikingly different. Zircon ages from the Nopeming Sandstone form three groups: 2.8 – 2.5 Ga, 2.1 - 1.8 Ga, and 1.2 - 1.1 Ga (Fig. 1). A few grains also have ages from 3.3 to 2.8 Ga and 2.4 to 2.2 Ga. Puckwunge zircons have a similar age distribution except that no Mesoproterozoic ages (1.2 – 1.1 Ga) are present. Zircons from interflow sediments of the SW limb of the North Shore Volcanic Group have a dominant age peak at 1.15 to 1.0 Ga, and scattered ages in the range of 2.7 to 2.3 Ga. Neoproterozoic Fond du Lac and Hinckley Sandstones were deposited after the main pulse of rift-related magmatism. Fond du Lac zircons have ages that range from 1.5 to 1.0 Ga; a few grains have older ages at 2.9, 2.5, and 1.9 to 1.6 Ga. Hinckley zircons have similar age distributions, but with many more ages from 3.1 – 2.7 Ga and 2.1 – 1. 5 Ga (Fig. 1). Zircons from the Middle Ordovician St. Peter Sandstone have two age populations: 2.8-2.6 Ga and 1.5-1.0 Ga (Fig. 1). Only three grains have ages between 2.5 and 1.5 Ga. Most of the observed zircon ages can be correlated with known source rock ages in the Lake Superior region (Fig. 1). Some ages, however, have no obvious local sources (e.g., 2.5 – 2.1 Ga, 1.6 – 1.5 Ga, and 1.4 – 1.1 Ga) and must have been derived from more distal sources (Van Wyck and Norman, 2004) or from regional sources with unrecognized multicyclic components. In particular, all Neoproterozoic and Paleozoic sediments that we studied have abundant ages between 1.5 and 1.1 Ga that might have been derived from Grenville sources (e.g., Rainbird et al., 1992; Johnson and Winter, 1999). Figure 1. 70 Histograms of 207Pb/206Pb ages from detrital zircons in Thomson Formation, Nopeming Sandstone, Hinckley Sandstone, and St. Peter Sandstone. Shaded bands indicate possible source region ages in Lake Superior Region. �Proceedings of the 52nd ILSG Annual Meeting – Part 1 References Johnson, C.M. and Winter, B.L, 1999, Provenance analysis of Lower Paleozoic cratonic quartz arenites of the northern Midcontinent region: U-Pb and Sm-Nd isotope geochemistry: Geological Society of America Bulletin, v. 111, 1723-1738. Rainbird, R., Heaman, L., and Young, G., 1992, Sampling Laurentia: Detrital zircon geo-chronology offers evidence for an extensive Neoproterozoic river system originating from the Grenville orogen: Geology, v. 20, p.351-354. Van Wyck, N. and Norman, M., 2004, Detrital zircon ages from Early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites: Journal of Geology, v. 112, p. 305315. 71 �Proceedings of the 52nd ILSG Annual Meeting – Part 1 Author Index Magee, M.A. McNaugton, N.J. Medaris, L.G. Jr. Miller, J.D. Jr.27, Moran, P. Mudrey, M.G. Jr. Patel, D. Peterson, D.M. Planavsky, N. Porter, R. Rainbird, R.H. Reid, D.D. Rousell, D.H. Schneider, D.A. Schultz, K.J. Severson, M.J. Shapiro, R. Shaw, C.A. Shute, A. Simmons, W.B. Smyk, M.C. Stonier, P. Stott, G. M. Vallini, D. A. Van Schmus, W.R. Vervoort, J. Vorhies, S. Wagonner, T.D. Walker, E. Wirth, K.R. Wyman, D. Anderson, D.K. 27 Andring, M. 35 Bartingale, R J. 1 Bennett, G. 3 Boerboom, T.J. 4, 27 Brown, B.A. 7 Buchholz, T.W. 8 Cannon, W.F. 10, 27, 65 Chandler, V.W. 27, 32 Cote, V. 12 Craddock, J.P. 13, 35, 69 Czechanski, M.L. 7 Davidson, C. 69 Davis, D.W. 23 Davis, W. J. 55 Easton, R.M. 15 Ernst, R.E. 23 Falster, A.U. 8 Finley-Blasi, L. 69 Fralick, P. 17, 42, 49 Franklin, J.M. 37 Grabowski, G. 19 Gross, A. 20 Hailstone, M. 22 Halls, H.C. 23 Heaman, L.M. 61 Heine, J. 30 Heggie, G.J. 37 Hocker-Finamore, S.M. 30 Hollings, P. 25, 42, 49, 59, 61 Holm, D.K. 20, 27, 63 Horton, J.W. Jr. 10 Hudak, G.J. 30 Jirsa, M.A. 27, 32 Juda, N. 35 Karimzadeh Somarin, A. 37 Kerber, L. 69 Kissin, S.A. 37 Knudsen, A. 53 Kring, D.A. 10 Lunquist, R. 69 MacTavish, A. 39 72 41, 42 65 63 44, 46 49 7, 51 13 44, 52 53 13 55 7 57 20, 27 27, 65 46 53 1 59 8 61 63 23 65 27 35, 69 69 67 69 13, 35, 69 25 � https://digitalcollections.lakeheadu.ca/files/original/2494bf3ae2892ac712fe458477742e86.PDF 843d0608ade7fa8cb4884c9ec370ae36 PDF Text Text Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan i Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan t ) ). By: U Ronald P. Sage, PhD Victoria L. Sage, BSc 2006 -- Field Trip Guidebook, Volume 52, Part 2 Institute on Lake Superior Geology ti. Sault Ste. Marie, Ontario PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ii Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan iii Glacial Lakes Algonquin and Glacial Lakes Algonquin and Nipissing Nipissing Shoreline Bedrock Features: Shoreline Bedrock Features: Mackinac Island, Michigan Mackinac Island, Michigan By: Ronald P. Sage Victoria L. Sage 2006 52nd Field Trip Guidebook for the Institute on Lake Superior Geology Sault Ste. Marie, Ontario By: Ronald P. Sage, PhD Victoria L. Sage, BSc 2006 Field Trip Guidebook, Volume 52, Part 2 Institute on Lake Superior Geology Sault Ste. Marie, Ontario On the cover: Lithograph of Arch Rock on east shore of Mackinac Island. See page 16-17. Colorization: V.Sage. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �iv Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan v Acknowledgements This one-day trip is designed to visit all the prominent rock exposures carved by glacial Lakes Algonquin and Nipissing into the limestones exposed on Mackinac Island. The senior author has more than 30 years of geological field experience working in the Canadian Shield. The bedrock exposures on Mackinac Island are of interest to the author as they relate to the island economy. Victoria L. Sage, BSc, has worked in scientific and technical communication for the medical field. She has provided the computer programming skills required to put this guidebook together and has provided editorial assistance with text and guide format. Both authors have worked on Mackinac Island as Guest Service Representatives for Mackinac State Historic Parks. During this period of employment Mr. Greg Hokans, Marketing; Mr. Phil Porter, Museum Programs and Dr. David Armour, Deputy Director offered encouragement and support to the staff working on the island and assisted in locating various published articles describing the features of Mackinac Island. Mr. Steve Brisson, Curator of Collections, provided copies of most of the historical lithographs of rock formations used in this guide that have made the Island famous. Carol R. Sage, MS, has provided editorial assistance in preparing the Guidebook. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �vi Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan vii Table of Contents Introduction.....................................................................................................................................................1 Silurian–Devonian............................................................................................................................................2 Point aux Chenes Formation.................................................................................................................2 Bois Blanc Formation............................................................................................................................2 Detroit River Formation.......................................................................................................................5 Mackinac Breccia.............................................................................................................................................5 Breccia Origin......................................................................................................................................7 Age of Brecciation.................................................................................................................................7 I. Stop Descriptions................................................................................ 13 Stop 1: Robinson’s Folly............................................................ 14 Stop 2: Arch Rock..................................................................... 17 Stop 3: Eagle Point Cave........................................................... 18 Optional.................................................................................... 18 Stop 4: Pulpit Rock................................................................... 19 Stop 5: Chimney Rock.............................................................. 20 Stop 6: Devil’s Kitchen and Lover’s Leap.................................. 21 Stop 7: Sugar Loaf..................................................................... 22 Stop 8: Skull Cave..................................................................... 24 Stop 9: Crack-in-the-Island and Cave-of-the-Woods................... 24 Additional Suggestions.............................................................. 25 iii r jirA iil!!Y..:. In Recent...............................................................................................................................................................8 Upper Algonquin.................................................................................................................................9 Lower Algonquin..................................................................................................................................9 Submerged (Buried) Stream Valley of the Straits of Mackinac.............................................................. 10 Nipissing Shorelines.............................................................................................................................. 11 Post-Nipissing......................................................................................................................................... 11 Lithograph of ramp into Fort Mackinac and “Gibraltar Rock,” which forms the foundation of the Fort above Lake Huron (Allen 1891, p.185). Gibraltar Rock is part of a sea cliff formed by glacial Lake Nipissing. Bibliography.............................................27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �viii Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 1 M ackinac Island occurs between Lakes Huron and Michigan and just south of Lake Superior. It lies on the northeast rim of the Michigan Basin, a basin structure that underlies the state of Michigan and portions of the states of Wisconsin, Illinois, Indiana, Ohio and the province of Ontario, Canada (Figures 1 and 2). It is a continental scale basin that is generally poorly exposed except in scattered locations along its rim where the older rocks forming the basin are sometimes exposed. Within central Michigan and in the center of the basin, the rocks exposed along the rim are buried beneath many thousands of feet of younger rocks. Mackinac Island represents one of the best exposures of rocks of Silurian (440-395 million years) and Devonian (395-345 million years) age along the basin perimeter, and these Silurian and Devonian rocks project above the highest water level of the older glacial Lake Algonquin. The effects of coastline erosion are well recorded in the wave cut cliffs and abandoned beaches found on the island. The island offers an excellent opportunity to examine shoreline features related to the glacial Lakes Algonquin and Nipissing formed at the edge of the receding continental ice sheets of 10,000 to 12,000 years ago. Since shoreline features are commonly in unconsolidated Riyer material only the latest events N are recorded in these materi5Mb Boypal Slim Michigan als, the earlier features being SMncn •Mo obliterated by later events. It is SMbb Bno.Bedford SMC ElbwocTh-&a,im anticipated that the actual Mü.ü1t. events are much more complex SMrYc Mtthu than presented here. The SD? Traven. City present island contains a surSüd Thridss SOdr DMrcitR$vw face area of 2,221 acres SDbb Bali Blanc — — (Russell, 1905, p. 56). 0-Sn. MOdSuIc -S — Sand Pdrt St S. — — Oi.ia Erqra&. ajif Sme 0. RId.susid Oc 0? TrSan Cnn £j CrnnbIIOn MILES Figure 1: Bedrock geology of the state of Michigan (Dorr and Eschman, 1970). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan FNWQOTU — Figure 2: Outline map of the distribution of Silurian age rocks around the Michigan basin (Dorr and Eschman, 1970, p. 104). The line of section passes very close to Mackinac Island near point A. Silurian – Devonian Mackinac Island is composed of rocks of Silurian and Devonian age. The lower Silurian stratigraphy consists of the Point aux Chenes or Salina formation and the upper Devonian consists of the Bois Blanc and Detroit River formations (Figures 3, 4 and 5). Point aux Chenes formation The Silurian Point aux Chenes formation consists of upper beds of variegated shale and thin brown dolomites, which overlie a lowersalt series that can contain 1600 feet of salt in a number of beds (Landes et al., 1945, p. 159). The salt beds are thin along the northeast and northwest flanks of the Michigan Basin and the upper salt beds have been leached from the formation in upper Michigan ( Landes et al., 1945, p. 159160). Landes et al., (1945, p. 161) report that shale beds commonly separate salt beds in the upper half of the formation and dolomite in the lower half. A basal dolomite occurs everywhere below the lowest salt and, where unleached, the Point aux Chenes formation in northeastern Michigan has a thickness of 1175 to 2886 feet (Landes et al, 1945, p.161). Bois Blanc formation Landes et al., (1945, p. 163) report that the Bois Blanc formation contains Onondaga fossils of lower middle Devonian age. The formation consists of limestone and dolomite and ranges from 165 to 1000 feet in thickness in upper Michigan (Landes et al., 1945, p. 165-166). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 3 IEGENO SAEI$?011 lvii. r—I .vp._ L____J —NtS*flI fl SALT SItar Figure 3: Generalized columnar section of the region around the Mackinac Straits (Landes et al., 1945, p. 154). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �4 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan CLASSIFICATION CF ROCKS IN STRAITS THE MACKINAC STRAITS AREA CITY '0. 4 TO. ii:; ROflRC CITY LIMESTONE CflA N OVIA Iii (it______ DUNDEE LIMESTONE T Figure 4: Generalized stratigraphic column at the Straits of Mackinac showing the distribution of Mackinac breccia across the Silurian and Devonian time periods (Sheldon, 1959, p. 12). It C Cr 0 I- 4 a V Eu U I- C, to -J 0015 BLANC FM. PARI( 400' IS. F GARDEN 250•. ST. IGNACE DOLOMITE C, C z C z 'C U 600 —IC POINTE AUX CHENES 511. S IN INDICATE STRATIC ENGADINE DOLOMITE AR MX I V t et. 7 ci 4- ROGERS CITY ORMAIION GARDEN IS. FORMATION DUNDEE FORMATION St IGNACE DOLOMITE DETROIT RIVER GROUP POINTE AUX CHENES C BLANC FORMATION SHALE MACKINAC BRECCIA Figure 5: Generalized surface geologic map of the Mackinac Straits area that displays the distribution of the Mackinac breccia (Sheldon, 1959, p. 3). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 5 Detroit River Formation This formation is not exposed in the area of Mackinac Island. The lower part of the formation consists of limestone and dolomite that is difficult to separate from the Bois Blanc formation (Landes et al., 1945, p. 174). Above the basal limestone of the Detroit River formation, there is an evaporate series consisting of dolomite, anhydrite and salt that in the center of the Michigan Basin may be 1145 feet in thickness (Landes et. al., 1945, p. 174). The evaporate thickness ranges up to 600 feet along the periphery of the basin, but the evaporate series is absent along the rim in the vicinity of Mackinac Island (Landes et al., 1945, p. 174175). Mackinac Breccia Mackinac breccia is the most prominent rock type found on Mackinac Island when one examines the rock formations created by the interaction of glacial Lakes Algonquin and Nipissing. All the rock formations that have received special attention on Mackinac Island are composed of this rock unit. The term “Mackinac breccia” was originally applied by Douglas Houghton, first state geologist for the state of Michigan (Shelden, 1959, p. 19). The Mackinac breccia is an indurated breccia although, in the region of Mackinac Island, non-indurated breccia is the dominate lithology (Landes et al.; 1945, p. 135). Mackinac Island and the St. Ignace peninsula are composed of a non-indurated megabreccia and a small amount of transformational breccia (Landes et al., 1945, p. 134-135). Landes et al. (1945, p. 133) report that the cement to the indurated breccias is carbonate. The indurated Mackinac breccia is easy to recognize in an outcrop by its pock-marked surface that is caused by differential weathering of the breccia fragments. The more soluble fragments are removed leaving large cavities in the breccia units giving rise to the well-developed pock-marked surface. The fragments are angular and up to 10 feet or more in maximum length and consist of a mixture of rock fragments from units higher in the stratigraphy. The resistance to weathering of the indurated breccia suggests that some silica also serves as a cementing agent for the breccia fragments. If all the cement was carbonate, it would be dissolved upon exposure to weathering or solution and the Mackinac breccia would collapse and become indistinguishable from the regional brecciation. Erosion of the regional limestones of Silurian and Devonian age by the glacial Lakes Algonquin and Nipissing has removed the softer, less indurated material enclosing the indurated breccia masses. This has resulted in the prominent rock formations now preserved along the former coastlines of the glacial lakes. Mackinac Island occurs in a broad zone of brecciation found at the northeast corner of the Michigan Basin (Figure 6). Salt decreases in thickness from 1200 feet to 0 feet northwest of Alpena County and north of Cheboygan County, Michigan, and the zone of brecciation corresponds with the disappearance of salt in the stratigraphy. This salt has a blunt edge suggesting this boundary may be a leached rather than a natural, depositional edge (Figure 7) (Landes et. al., 1945, p. 146). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �6 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan c SR*NO I £ L CO 14* KALKAflA TN*YflSE SUROCE LYIDOIG! OF COLLAPSI OF' .COLLAPSt WITH 0 (VIQCHCC OF Figure 6: Map of the Mackinac Straits area showing the region of solution collapse and Mackinac breccia distribution (Landes et al., 1945, p. 175). Figure 7: Isopach map showing combined thickness of Salina salt (Landes et al., 1945, p. 146). The salt disappears abruptly just southeast of Mackinac Island and outcroppings of Mackinac breccia. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 7 Breccia Origin The origin of the Mackinac breccia has been discussed by Stanley (1945), Landes et al. (1945), and Shelden (1959). The most complete discussion of the origin is that of Landes et al. (1945), who completed a detailed summary of all previous work in determining breccia development. Landes et al., (1945, p.142) attribute the development of a modern model for the formation of the Mackinac breccias to Mr. Henry H. Hindshaw, former assistant to the state geologist for New York. A brief summary of the model for the formation of the Mackinac breccia is as follows (Landes et al., 1945, p. 143-145) and has been previously summarized by Shelden (1959). The land surface is floored with rocks of Niagara age (Silurian) and was submerged beneath the Pointe aux Chenes Sea. During this time, several hundred feet of shale, dolomite, salt and gypsum were deposited. Post-deposition, emergence followed along the rim of the Michigan Basin and percolating ground waters leached salt from the rim of the salt-bearing rocks. Caves were produced when the salt in the Pointe aux Chenes was removed from rocks lying above the ground water table. The solution of the salt created caverns that became unstable and collapsed. There were two types of collapse: regional and local. The collapse was probably sudden and the overlying rocks broke into angular fragments of all sizes. This probably created localized sink holes above the collapse, and the larger areas of regionalized collapse created tilted stratigraphy in the region. The period of collapse took place during emergence following Detroit River deposition and preceding Dundee deposition. This collapse took place over a time interval rather than at a specific time. Shelden (1959, p. 23) reports slickensides and normal faults within larger blocks of collapsed rock. The Mackinac breccias occur in columns, and some clasts may represent down drop of 600 to 750 feet (Landes et al., 1945, p. 129). After collapse, erosion of the surface developed a peneplane and surface irregularities (such as sinkholes) were filled. The limestones of the Dundee formation were deposited on this peneplane surface. Percolating ground waters gradually cemented the collapse breccias. Recent emergence has brought the rocks to surface where differential erosion has completed sculpting the breccias into the forms observed on the Island. Age of brecciation Landes et al., (1945, p.137) interpret the age brecciation as likely post-Detroit and pre-Dundee. The brecciation likely took place over an extended period of time. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Recent Mackinac Island has been affected by recent glaciation of the Wisconsin period and glacial Lakes Algonquin and Nipissing following retreat of the glaciers. Glacial Lake Algonquin represents an age of 11,000 to 12,000 years ago, and glacial Lake Nipissing represents an age of approximately 4,000 years ago. Viewing of the island from St. Ignace and Mackinaw City presents a profile of an island with a hump in the middle with two relatively flat planes separated by a steep slope (Figures 19 and 20). The present mean level of Lake Huron is 580.37 feet above sea level and the highest point on the island is at Fort Holmes, 900.5 to 904.1 feet above sea level (Stanley, 1945, p. 65, 72-73). The highest level reached by glacial Lake Algonquin is 809 feet above sea level (Taylor, 1915, p. 69), which is the highest peneplane observed in the profile of the island. This former high water level is approximately 229 feet above the present lake level (Taylor, 1915, p. 69). That portion of Mackinac Island lying above this high water mark is referred to as the “Ancient Island”. The “Ancient Island” part of Mackinac Island was glaciated during the Wisconsin period and a few glaciated boulders, cobbles and pebbles displaying glacial striations can be found in the area (Stanley, 1945, p. 13). The ice moved in a southwest direction and left a thin covering of glacial till on the “Ancient Island” (Stanley, 1945, p. 12-13). Glacial ice retreated northeastward in a series of retreats and advances with the retreats exceeding the advances (Landes et al., 1945, p. 10). The process of retreat-and-advance by glacial ice will destroy evidence left in unconsolidated materials from earlier (older) retreats and advances and only the last event is preserved in the geological record. _____________________ U sot SOUTH URE HURON ISO ALSn#01 LIWL soJJlHt4sr NAUlNO LEVEL LAKE NUAON 550 Figure 8: Profile across Mackinac Island showing former shore levels looking east from St. Ignace (Stanley, 1945, p. 22). Glacial LoRe Nipissiog water plane Ancient Island" I Fort Holmes atop GIOCICI Luk. àiqooQuir. wave cut cliff w°ve Lake Nipássing wove duff Glacial UII4 AIUOMU1O watt, PWM Glacial LaKe Nipissing woterplone I I I MdernIoke$evht I Figure 9: Profile of Mackinac Island looking eastward from St. Igance with the various glacial lake levels noted (Dorr and Eschman, 1970, p. 177). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Upper Algonquin The Lake Algonquin shoreline is a well-developed major feature in the region of the Straits of Mackinac (Landes et al., 1945, p. 11) (Figures 10 and 11). Landes et al. (1945, p. 11-12) indicate that drainage from this glacial lake was eastward towards North Bay and Mattawa, Ontario. But, as the ice sheet withdrew, isostatic rebound caused uplift to the northeast until the waters overflowed to the south at Port Huron and Chicago. This is a three-stage outlet for Lake Algonquin. As isostatic uplift continued in response to the retreating ice sheets, the lake level was stabilized by the outflow through Port Huron and Chicago and the eastward drainage abandoned (two stage outlet) (Landes et al., 1945, p. 12). As uplift continued, the lake level continued to recede and gradually erosion along the Port Huron drainage caused the abandonment of the Chicago outlet (Landes et al., 1945, p. 12). Beach lines associated with the Upper Algonquin of glacial Lake Algonquin lie between 799 and 759 feet above sea level and the type section is the Short Rifle Range lying between Fort Mackinac and Fort Holmes (Stanley, 1945, p. 31-32). Taylor (1915, p. 69) reports that the highest beach level is 809 feet above sea level or approximately 229 feet above the present level of Lake Huron. Glacial straie have not been observed on glacial boulders below the level of 205 feet above lake level (Taylor, 1892, p. 212-213). 9 1' Figure 10: High water level for glacial Lake Algonquin approximately 11,000 years ago. This illustration indicates most of the region was under water. This view is oriented north looking south (Porter and Nelhiebel, 1984, p. 11). Lower Algonquin Below the Upper Algonquin is a zone relatively free of beach development, which is thought to represent a urrLa Iauw SNORE period of relatively rapid falling of the lake level 10.000 Years Ago 116 Abow tflc flunn todsy (Stanley, 1945, p.32). This zone lies between approximately 762 and 635 feet above sea level (Stanley, 1945, Figure 11: Upper glacial Lake Algonquin shoreline approximately 10,000 years ago (Porter and Nelhiebel, 1984, p. 47). p. 32-36). The best-described beach line in the Lower Algonquin is the “Battlefield Beach” at an elevation of 718 feet above present sea level located in the north central part of the island (Stanley, 1945, p. 33-34; Landes et al., 1945, p. 13). Stanley (1945, p. 35-36) and Landes et al. (1945, p. 13-14) describe other locations as examples of beaches within the Lower Algonquin. The Lower Algonquin beaches are characteristically less well developed than the Upper Algonquin. The quick lowering of the lake level is in response to the opening of discharge channels to the east as the ice sheet retreats (Landes et al., 1945, p. 13). The lowering probably extended to much lower levels than present lake levels, and a buried river valley between PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Lake Huron and Lake Michigan was probably reactivated (Landes et al., 1945, p. 14). At this low water stage the present islands within the Straits did not exist but were part of what is now the mainland. Landes et al. (1945, p. 14) indicate that the lake levels ceased falling when the lowest outlet from the Huron Basin, the Mattawa Valley (east of North Bay), was freed of ice and unobstructed flow eastward was established. The removal of glacial obstruction left only isostatic rebound as a mechanism of changing lake water flow. The Mattawa Valley is located to the east and northeast of Mackinac Island where this continental uplift would be most effective (Landes et al., 1945, p.1 4). Continental uplift to the east generated continually rising lake levels until southern flow was reestablished, and then eastward flow through the Mattawa Valley was terminated (Landes et al., 1945, p. 14). As the lake waters rose, topographic features created in the unconsolidated glacial deposits as the lake level dropped were obliterated or buried beneath the rising waters. Submerged (buried) stream valley of the Straits of Mackinac The presence of a submerged valley in the Straits of Mackinac was disclosed by soundings between 1918 and 1924 (Stanley, 1938, p. 966) (Figure 12). This valley loops around Mackinac Island to the north and is likely the result of Pleistocene or earlier erosion (Stanley, 1938, p. 966, 974). This valley was likely filled with glacial drift during the Wisconsin ice advance and cleaned out during the low water level between the glacial Lakes Algonquin and Nipissing (Stanley, 1938, p. 974). Stanley (1938, p. 966) and Sheldon (1959 p. 59) suggest flow along this valley was towards to the east. This valley lies 150 to 250 feet below the present level of Lake Huron with the greatest depths being recorded through the Straits near Mackinaw City (Stanley, 1938, p. 968). Stanley (1938, p. 968) reports the greatest depth to be 289 feet below present lake level. This valley exceeds 70 miles in length (Stanley, 1938, p. 966). LAKE M'GHIGAN 7 Figure 12: Submerged valley through the Straits of Mackinac (Stanley, 1938, p. 967). At this time, the water level of Lake Huron was approximately 120 feet lower than it is today. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 11 Nipissing Shorelines As isostatic rebound continued, the level of Lake Huron returned to a Scott's Cavg level 50 feet above its present level Croglian Water (Stanley, 1945, p. 39) (Figure 13). Stanley (1945, p. 39) suggests the process of uplift and higher water levels took place slowly over a period of several thousand years and the previous geological features in unconsolidated glacial material were de- Devil's stroyed. Nipissing beaches reached elevations of 629 to 635 feet above sea level (Stanley, 1945, p. 42). At Robinson's rally abmt Laict Huron the time of Nipissing beach developlakE Huron rreseifl ShorelIne ment, 85% of the isostatic rebound had been completed (Shelden, 1959, Figure 13: Modified oblique view of Mackinac Island showing the shoreline of glacial Lake Nipissing with scenic rock formations noted (Porter and Nelhiebel, 1984, p. 13). Friendship p. 59; Landes et al., 1945, p. 14). Nipissing beaches do not completely Altar and Pulpit Rock are the same feature. This shoreline is approximately 4,000 years old. enclose Mackinac Island but obliquely transect the lower Algonquin beaches (Stanley, 1945, p.39, 41). Nipissing beaches are preserved on the northwest and southeast portions of Mackinac Island; the City of Mackinac is built on Nipissing beaches (Landes et al., 1945, p. 15; Stanley, 1945, p. 44; Leverett and Taylor, 1915, p. 452). Post-Nipissing During post-Nipissing time, Lake Huron drains south past Port Huron and is approximately 55 feet lower than the highest level of glacial Lake Nipissing (Sheldon, 1959, p. 63). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �12 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 13 Stop Descriptions T he Mackinac Island field trip is a one-day trip that is designed to visit all the major bedrock outcrops that contribute to the scenery of the island. These outcrops have been described in previous technical and non-technical literature for more than 150 years. These scenic bedrock exposures have contributed much to the island economy over the years. Figure 14 is a simple sketch of Mackinac Island showing the location of the sites on this tour and those geographic features mentioned in this text. The island’s major bedrock outcrops are the direct product of the interaction between glacial lakes and bedrock during the retreat of the Wisconsin ice sheets. Upon arrival, one disembarks on the south side of Mackinac Island where the glacial Lake Nipissing beach deposits are best developed. Fort Mackinac is built on the Lake Nipissing sea cliffs. Below this cliff lie the deposits of glacial Lake Nipissing. These beach deposits are largely obscured in most areas by subsequent construction. The sea cliff below Fort Mackinac has a prominent rock formation called Gibraltar Rock that projects out from the cliff but is not detached. Continued erosion around this prominent rock could have caused its detachment from the sea cliff to create a sea stack (see the Table of Contents for a lithograph of Gibraltar Rock). The cliff face on which Fort Mackinac is built is fenced off from the public and, if time permits, a closer examination can be made after completion of the field trip. The field trip consists of two parts. The first part is a trip around the island on Shoreline Road to examine the erosive activities of glacial Lake Nipissing. The second part is an interior tour of the island to examine the erosive activities of glacial Lake Algonquin. Cave Cake For Fairy Arch hore Former ScoWs Cave Foot N CQve cc the Woods N A 1000 Feet & trn]k HerLot Kitchen Rock Figure 14: Sketch map indicating the sites of rock outcroppings to visit and those geographic features needed to locate them. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �14 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop1: Robinson’s Folly Moving eastward along Shoreline Road (through the settlement) approximately one mile from the landing on Mackinac Island, the first outcrop one encounters is a prominent cliff face of Mackinac breccia known as Robinson’s Folly (Figure 15). This prominent headland is reportedly named after Captain Daniel Robertson of the British 84th Regiment who supposedly built a summer house on the promontory, which subsequently collapsed into Lake Huron (Wood, 1918, p. 584-585). The name “Robinson” is a corruption from the French addressing him as “Robinçon” (Wood, 1918, p. 584-585). Meade (1986 [1897], p. 165-170) presents five stories on the naming of Robinson’s Folly. The reader should refer to Meade’s lengthy discussion of the name if interested in the origin. Robinson’s Folly is 127 feet (Van Fleet, 1970, p. 147; 1882, p. 24) or 128 feet (Winchell, 1861, p. 210) above the present level of Lake Huron. It represents a prominent portion of the sea cliffs formed along the eastern side of Mackinac Island through the erosive action of glacial Lake Nipissing. A short distance north of Robinson’s Folly is a second outcropping of breccia that was the former site of a sea arch known as Fairy Arch (Figure 16). This arch was destroyed during road construction around the island, but a photograph was published by an anonymous source (1899, p. 5). Ak Figure 15: Lithograph of Robinson’s Folly, a glacial Lake Nipissing sea cliff (Woolson, 1894, p. 289). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 15 fr Figure 16: Lithograph of Fairy Arch, a glacial Lake Nipissing shoreline feature (Woolson, 1894, p. 285). This arch was destroyed during road building along the east side of Mackinac Island. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan .1 16 Figure 17: Lithograph of Arch Rock from the shore of Lake Huron (Disturnell, 1875). This is a glacial Lake Nipissing feature. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 17 Stop 2: Arch Rock Arch Rock is located on the east side of Mackinac Island and is the best known rock formation on the island (Figures 17 and 18). The limestone arch is butted against seacliffs of glacial Lake Nipissing to the north and a promontory of Mackinac breccia that projects out from the cliff face but remains attached. The top of the arch is approximately 140 feet above Lake Huron and the top of the buttress that projects out from the cliff is 105 feet above present-day Lake Huron (Winchell, 1861, p. 210; Van Fleet, 1970, p. 147; 1882, p. 24). Arch Rock has a span of 40 to 50 feet (Van Fleet, 1882, p. 20). Figure 19: Lithograph looking down through Arch Rock toward Lake Huron. This view is from the top of the glacial Lake Nipissing sea cliffs (Woolson, 1894, p. 281). Arch Rock is composed of highly fractured limestone. Many of the fractures have been sealed with man-made cement. The northern abutment has been reinforced with man-made cement and rock. In the spring, loose fragments that have fallen from the arch commonly lie below the structure. The highly fractured appearance of the limestone prompted McKenny (1959 [1827], p. 390) and Foster and Whitney (1851, p. 163) to predict a very short life for the arch. In terms of geologic time, this will likely be true. I At the base of the promontory which forms the south abutment to Arch Rock, there is a small arch known as the Sannillac Arch. This arch is named after an Indian warrior named Sannillac, who is the subject of a poem by Henry Whiting that was published in 1831 (Wood, 1918, p. 588-589). In front of Arch Rock on the lake side of the shoreline road there is a large boulder of Mackinac breccia that has broken free of the promontory. This boulder has been referred to as Gitchie Manitou (Stanley, 1945, p. 51). Figure 18: Lithograph of Arch Rock in the moonlight looking up from Lake Huron through the arch that forms part of the glacial Lake Nipissing sea cliffs (Woolson, 1894, p. 279). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �18 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 3: Eagle Point Cave Eagle Point Cave is located approximately two miles northwest beyond Arch Rock. Take Lake Shore Road to Scott’s Shore Road; turn inland approximately 500 feet to Scott’s Road. The cave is situated approximately 1000 feet south of the junction of Scott’s Shore Road and Scott’s Road on the west side of the Scott’s Road. The site of this cave was once used for some unknown purpose. The remains of concrete steps are found in front of the cave and the floor of the cave has been leveled using logs and dirt fill. Eagle Point Cave occurs in Mackinac breccia and is represented by a large amphitheater-type opening in the breccia. Eagle Point Cave is typical of most of the shoreline features described on the island as caves. These caves are commonly amphitheater-type openings of very limited depth that have resulted from lakeshore weathering-erosion processes. Eagle Point Cave is a glacial Lake Nipissing shoreline feature. Optional After the visit to Eagle Point Cave one can return to the junction of Scott’s Shore Road and Scott’s Road and continue north for 200-300 feet where approximately 100-150 feet west of the road is a small promontory of bedrock in which the former Scott’s Cave was displayed. This cave has either caved-in or become filled-in and relatively little remains to view. The site warrants some restoration effort. Stanley (1945, p. 47) published a good photograph of the cave when it was exposed for viewing. This cave occurred in the same glacial Lake Nipissing shoreline bluff as Eagle Point Cave. One can walk a trail along the top of the bluff from one cave to the other. Scott’s Cave was named after Captain Thomas Scott of the British 53rd Regiment who was stationed at Fort Mackinac in 1787 (Wood, 1918, p.591). Scott’s Road occurs along a flat land surface representing the action of glacial Lake Nipissing. Scott’s Road continues around the northern portion of Mackinac Island but remains relatively unscenic. Return to the Lake Shore Road and continue north around the island to British Landing. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 19 Stop 4: Pulpit Rock Figure 20: Pulpit Rock, a glacial Lake Nipissing shoreline feature on the northwest corner of Mackinac Island. This sea stack occurs 50 to 75 feet west of the wave-cut bench, representing beach erosion of unconsolidated material of glacial Lake Nipissing (Allen, 1891). Pulpit Rock (Figure 7), at present better known as Friendship Altar, is located a short distance northeast of British Landing along the western end of Scott’s Road. The vertical standing rock formation consists of Mackinac breccia standing approximately 10 feet in front of the bluff formed by glacial Lake Nipissing. While the most commonly used name for this rock unit is Friendship Altar, the outcrop is vertical standing like a pulpit and not horizontal lying as an altar would be. The author prefers the term “Pulpit Rock” because it best describes the outcrop shape or form. Wood (1918, p.536-537) mentions that both names, Pulpit Rock and Friendship Altar, have been applied to this outcropping near British Landing. But, he also suggests the term “Pulpit Rock” may have been applied to another exposure known as Vista Rock in the area of Sugar Loaf. Vista Rock, as indicated on the map of Wood (1918), is a poorly exposed outcrop that does not resemble a pulpit and, perhaps, is an outcrop of no particular note. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �20 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 5: Chimney Rock Continuing along the shoreline road for approximately two miles one passes numerous raised beaches of former glacial Lake Nipissing and of present-day Lake Huron (Figure 21). Bluffs and sea cliffs of the former glacial Lake Nipissing begin to appear south of Heriot Point (Wood, 1918). A prominent vertical rock formation, sometimes referred to as Sunset Rock, occurs approximately ¼ mile from where these bluffs begin to appear along the shoreline road. The name “Chimney Rock” is the original name of this shoreline exposure and the name “Sunset Rock” appears to be of recent origin, perhaps generated by whomever built the platform on the top of the formation. The name “Sunset Rock” appears on this platform outlined by dark pebbles set in concrete used in building the structure. The author uses the term “Chimney Rock” since it is of historical usage and is a very good description of the appearance of the exposure. This feature has only recently become part of the Mackinac State Historical Park holdings and accessible to the public. This was largely due to the efforts of David Armour, PhD, former deputy director of Mackinac State Historic Parks. Van Fleet (1870, p. 144; 1882, p. 24) and Wood (1918, p. 521) have cited Alexander Winchell, professor of geology at the University of MichiFigure 21: Lithograph of Chimney Rock, a glacial Lake Nipissing shoreline gan (Winchell, 1870) and a former state geologist feature. The viewing platform that has been constructed on top of this formation may have damaged the original profile as shown in this lithograph for the State of Michigan (Winchell, 1861), as de- (Woolson, 1894, p. 283). scribing this rock exposure as “one of the most remarkable masses of rock in this or any other state”. The author has failed to identify the original source for this comment. Chimney Rock is a promontory of Mackinac breccia developed by selective erosion and removal of less resistant limestone enveloping the more indurated breccia forming the vertical column of rock making up Chimney Rock by glacial Lake Nipissing. The breccia column remains attached to the headlands, so Chimney Rock is not a sea stack. The top of Chimney Rock was originally 131 feet above the level of Lake Huron (Van Fleet, 1870, p. 147, 1882, p. 24; Winchell, 1861, p. 210). It is unknown whether those that built the platform on Chimney Rock vandalized the upper portion of the chimney-like rock outcrop thereby altering the original profile. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 21 Stop 6: Devil’s Kitchen and Lover’s Leap 1 Continuing along the shoreline road for ½ to ¾ mile, one comes to the Devil’s Kitchen. Devil’s Kitchen represents an amphitheaterlike opening in a large outcrop of Mackinac breccia. This feature is the product of recent interaction of the shoreline of Mackinac Island and the waters of Lake Huron. Earlier erosion by glacial Lake Nipissing may have also had an influence. 4 3: I IL a w Lover’s Leap occurs on the wave-cut bluff behind and slightly west of Devil’s Kitchen. It remains private property and, thus, is unavailable for examination (Figure 22). Lover’s Leap is composed of Mackinac breccia and is comparable to Chimney Rock in appearance and origin. The top of Lover’s Leap is 145 feet above the present level of Lake Huron (Van Fleet, 1870, p. 147; 1882, p. 24; Winchell, 1861, p. 210). - Figure 22: Lithograph of Lover’s Leap, a glacial Lake Nipissing shoreline feature (Woolson, 1894, p. 288). I nner Island Tour Upon completion of the visit at Devil’s Kitchen, one continues back towards the village on Mackinac Island. As we approach the village, turn left onto Mahoney Ave. and continue to Cadotte Ave. Turn left on Cadotte Ave. and travel to Huron Road passed the Grand Hotel. Turn right on Huron Road and continue to the eastern side of Mackinac Island to continue an inner island tour dominated by features related to glacial Lake Algonguin. Huron Road passes along the top of east bluff that represents the effects of glacial Lake Nipissing. Along Huron Road, the first stop will be Robinson’s Folly (Stop 1) where participants can view Lake Huron from the top of the rock formation. From Robinson’s Folly, the group will continue to Arch Rock (Stop 2). The revisit of Stop 2 allows participants to view Arch Rock from above. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �22 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 7: Sugar Loaf From the revisit of Arch Rock, one continues into the interior of Mackinac Island along Rifle Range Road to Sugar Loaf Road (Figures 23 and 24). Continue along Sugar Loaf Road to Sugar Loaf. Sugar Loaf is a sea stack that is the product of glacial Lake Algonquin. This sea stack is composed of Mackinac breccia and is separated from the headlands of the former glacial Lake Algonquin by approximately 300 feet. The sea cliffs to the west are composed of breccia. The wooden stairs from the former terrace of glacial Lake Algonquin allow great access to the cliffs where the details of the breccia can be closely examined. The stairs continue to the top of the sea cliff known as Point Lookout where an excellent overall view of Sugar Loaf and Lake Huron is possible. The Lake Algonquin terrace around the Sugar Loaf is approximately 140-150 feet above the present Lake Huron. The top of the Sugar Loaf is 855.81 feet above sea level or 274.94 feet above the present level of Lake Huron (Stanley, 1945, p. 65, 72). The base of Sugar Loaf is approximately 134 feet above Lake Huron (Van Fleet, 1870, p. 20-21; 1882, p. 140). Wood (1918, p. 594-595) indicates the Sugar Loaf is 79 feet high using the road as a base. The conical shaped sea stack reportedly received its name from honey bees that constructed a hive in the formation (Wood, 1918, p. 595). t Figure 23: Lithograph of Sugar Loaf, a glacial Lake Algonquin shoreline feature. View is of the west side (Woolson, 1894, p. 287). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 23 Figure 24: Lithograph of Sugar Loaf, a glacial Lake Algonquin shoreline feature. View is of the east side (Woolson, 1894, p. 286). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �24 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan Stop 7: Skull Cave From Sugar Loaf, we continue along Sugar Loaf Road to Rifle Range Road and east to the junction of Rifle Range Road and Garrison Road. Skull Cave is a crumbling sea stack that has a large amphitheater-like opening. This feature is composed of Mackinac breccia and is the result of glacial Lake Algonquin. The sea stack has a summit of 831 feet above sea level, is between 15 and 25 feet high, and is separated from the headland of the former Lake Algonquin sea cliff and bluff by 50 feet (Stanley, 1945, p.26-27). Skull Cave is named after an experience by Alexander Henry, a fur trader who survived the massacre in 1763 of the British garrison at Fort Michilmackinac in Mackinaw City. Henry was sequestered in this cave by Wawatam, a Chippewa Indian, thus saving him from massacre at the Fort. Henry found the cave full of human bones and skulls and was very happy to leave his place of refuge at his earliest opportunity. It is unknown whether the bones are the result of burial or ceremonial sacrifice. Wood (1918, p. 592) stated that Alexander Henry was of the opinion that the bones were from prisoners devoured at war feasts, but there is no evidence as to which interpretation is most probable. Slight variations in this story occur in almost all discussions of Skull Cave. Stop 8: Crack-in-the-Island and Cave-of-the-Woods From Skull Cave, continue north along Garrison Road to State Road and turn left. Continue along this State Road until you reach Island Trail that leads from the road to the west. This trail is accessible with a trail bike but is very rough; caution is recommended. Crack-in-the-Island and Cave-of-the-Woods lie approximately ¼ mile west of State Road and at the edge of Mackinac Island Airport. They are separated by only a few tens of feet. Crack-in-the-Island is a solution crack in the limestone that, at this location, is on the order of one foot wide and 1-2 feet deep. Wood (1918, p. 523) described the crack as a deep fissure several feet wide, but the crack as seen today does not fit that description. Wood (1918) indicates these solution cracks occur in several other places on the island. Cave-in-the Woods is an amphitheater-shaped opening or sea cave in Mackinac breccia formed by glacial Lake Algonquin that sits approximately 140 feet above the level of present-day Lake Huron (Porter and Nelhiebel, 1984, p. 47). Upon completion of the examination of this site, return along the trail to State Road. Even though it is longer in distance, the easiest way back to town and the ferry docks is to continue north along State Road, which rejoins with Garrison Road and continues on towards British Landing. This route is down hill through the center of the island and, then, relatively flat along the perimeter of the island, Lake Shore Road. At British Landing, turn left on to Lake Shore Road and return to the ferry docks, a distance of several miles that passes previous stops illustrating glacial Lake Nipissing shore features. If time permits, it is suggested to tour Fort Mackinac, a Revolutionary War fort. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 25 Additional Suggestions The tour of Mackinac Island is designed to visit all prominent glacial-lake modified bedrock outcroppings on the island in a single day. At the visitor’s discretion and convenience, some outcroppings in and near the city of St. Ignace on the mainland of the Upper Peninsula are worth visiting. St. Anthony’s Rock occurs in a parkette behind the business establishments lining the main street of the city. This is a sea stack of Mackinac breccia formed by glacial Lake Nipissing. It sits several yards in front of a Lake Nipissing cut headland. Castle Rock located just north of St. Ignace is a commercial property that permits tourists of the area to obtain a view from its high vantage point. Castle Rock is a prominent promontory that remains attached to the glacial Lake Nipissing headlands and is, thus, not a true sea stack. Landes et. al. (1945, p. 136) classify Castle Rock as being an indurated transformational breccia, which lies immediately east of megabreccia that is regional in extent. Landes et al. (1945, p. 125) report that the best place to see the various styles of regional brecciation is from the road cut leading to the north entrance of the Mackinac Bridge. Landes et. al. (1945) provides the most complete description of the geology of Mackinac Island and the surrounding area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �26 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan 27 Bibliography Allen, E.A.; 1891. A Jolly trip; or, where we went and what we saw last summer; Central Publishing House, Atlanta, Georgia, 266p. Anonymous, 1899. The Standard Guide, Mackinac Island and Northern Lake Resorts; Foster and Reynolds, 88 p. Disturnell, J., 1875. Island of Mackinac; Philadelphia, 96 p. Dorr Jr., John A. and Eschman, Donald F., 1970. p. 176-217; in Geology of Michigan; University of Michigan Press, Ann Arbor, Michigan; 475 p. Foster, J. W. and Whitney, J.D., 1851, p. 163-166; in Report on the Geology of Lake Superior Land district; A. Boyd Hamilton, Washington D.C., 456p. with illustrations Landes, K. K., Ehlers, G. M. and Stanley, G. M., 1945. Geology of the Mackinac Straits Region and subsurface geology of Northern southern Peninsula; State of Michigan Department of Conservation, Geological Survey Division; pub. 44, Geological Serial 37, 204 p. Leverett, F. and Taylor, F.B., 1915. p. 452-453; in The Pleistocene of Indiana and Michigan and the History of the Great Lakes; United States Geological Survey Monograph, v. 53 McKenny, Thomas L., 1827. Sketches of a tour to the Lakes; Ross and Haines Inc., Minneapolis, Minnesota, 1959, 493 p. Meade, William C., 1897[1986]. Early Mackinac. A sketch Historical and Descriptive with Introductory essay by Larry Massie; Republished 1986 by Avery Color Studios, Au Train, Michigan, 184 p. NOTE: Early Mackinac was first published in 1897 and then revised and published in 1901 and 1912 Porter, Phil and Nelhiebel, Victor, R., 1984. The Wonder of Mackinac, Mackinac Island State Park Commission, Pendall Printing Inc., 52 p. Russell, Israel C., 1905, p. 44-45, 55-57, 102-104; in A geological reconnaissance along the north shore of Lakes Huron and Michigan; Report of the State Board of Geological Survey of Michigan for the year 1904, Wynkoop Hallenbeck Crawford Co., Lansing, Michigan Sheldon, Frances D., 1959. Geology of Mackinac Island and lower and middle Devonian south of the Straits of Mackinac, Michigan; Michigan Basin Geological Society Guide Book; 63 p. Stanley, G.M., 1938. The submerged Valley through Mackinac Straits; Journal of Geology, v. 46, n. 7, p. 966-974 Stanley, George M., 1945. Pre-Historic Mackinac Island; State of Michigan Department of Conservation, Geological Survey Division, Publication 43, Geological Series 36, 74p. Taylor, F.B., 1892. The highest old shoreline on Mackinac Island; American Journal of Science, v. 43, p. 210-218 Taylor, Frank B., 1915. Old shorelines of Mackinac Island and their relations to the Lake History; Geological Society America Bulletin (abst), v. 26, p. 68-70 Van Fleet, J. A., 1870. Old and New Mackinac; Courier Steam Printing-house, Ann Arbor; 176 p. Van Fleet, J. A.; 1882. Mackinaw Region and Adjacent Localities; Lever Print, Detroit, Michigan, 49 p. Winchell, A., 1861. First Biennial Report of the Progress of the Geological Survey of Michigan Geology, Zoology and Botany of the lower Peninsula; Geological survey of Michigan, 339 p. Winchell, Alexander, 1870. p. 247-245; in Sketches of Creation; Harper and Brothers, New York, 459 p. Wood, Edwin O., 1918. Historic Mackinac; The Historical, Picturesque and Legendary Features of the Mackinac Country; The MacMillan Company, New York, 540 p. Woolson, Constance F., 1894. Mackinac, p. 279-291; in Picturesque America, No. 5, March 17, 1894, (previously published 1872), Artist F. T. Woodward; D. Appleton, Publishers PDF compression, OCR, web-optimization with CVISION's PdfCompressor �28 Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan References not cited Schoolcraft, Henry R. 1832 [1953]. Narrative Journal of travels through the northwestern regions of the United States extending from Detroit through the great chain of American lakes to the sources of the Mississippi River in the year 1820; Republished Michigan State College Press, 1953, edited by Mentor L. Williams, 520 p. Strickland, W.P. 1860. Old Mackinac or the Fortress of the Lakes; James Challen and Son, Philadelphia, 404 p. PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/0c4e5c4f4cf1d00d709139b4dc0e22f7.pdf 28c0b52caa344bdb859324c05ee86abc PDF Text Text Unusual -bearing Rocks of Unusual Diamond Diamond-bearing the Wawa Area - - . : - £- a — L . Institute on Lake Superior Geology 52nd Annual Meeting Sault Ste Marie, Ontario Volume Part 3 – Field Trip Guidebook PDF compression, OCR,52web-optimization with CVISION's PdfCompressor �Unusual Archean Diamond-bearing rocks of the Wawa Area by A. C. Wilson Ministry of Northern Development and Mines, Resident Geologist’s Program, Ontario Geological Survey, Timmins, Ontario On the cover (clockwise from top): Giant lower crustal to upper mantle xenoliths, Enigma Property, Menzies Township, Oasis Diamond Corporation Inc.; Diamonds from the Festival Property (photo courtesy of Pele Mountain Resources Inc.); Sandor Diamond Occurrence, Highway 17, Spider Resources Inc. & KWG Resources Inc.; Heterolithic diamond-bearing breccia, Engagement Zone, Northern Sierra Minerals Corporation PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table of Contents Introduction .................................................................................................................................................... 3 Geological Overview of the Michipicoten Greenstone Belt........................................................................... 3 Quaternary Geology ....................................................................................................................................... 6 Structural Geology of the Diamond-bearing Rocks ....................................................................................... 6 Description of the Diamond-bearing Rocks ................................................................................................... 7 Relationship between diamond content and lithology .................................................................................. 10 Geochronology ............................................................................................................................................. 11 Geochemistry of the Diamond-bearing Rocks ............................................................................................. 11 Diamond characteristics ............................................................................................................................... 11 Origin of the Diamond Deposits................................................................................................................... 12 Field Trip Road Log ..................................................................................................................................... 14 STOP 1 - GQ Diamond Discovery Site ................................................................................................... 14 STOP 2 - Northern Sierra Minerals Corporation Area B......................................................................... 17 STOP 3 - Northern Sierra Minerals Corporation Engagement Zone ....................................................... 21 STOP 4: - Moet Occurrence, Festival Property ....................................................................................... 23 STOP 5: - Sandor Diamond Occurrence.................................................................................................. 25 STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre .................................................................... 27 STOP 7: - Monchiquite Dike ................................................................................................................... 28 STOP 8: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel ................................. 28 Bibliography................................................................................................................................................. 29 Figures 1. Generalized geological map of the Michpicoten greenstone belt ............................................................. 4 2. Composite structural section through the central part of the Michipicoten greenstone belt. .................... 7 3. Detailed geology of the southwestern corner of the Festival Property ..................................................... 8 4. Geological compilation of the GQ Property ........................................................................................... 15 5. Occurrences of diamondiferous bedrock on the GQ Property................................................................ 16 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property.................................................. 18 7. Northern Sierra Minerals Corporation Area B ........................................................................................ 20 8. Geology and sample locations at the Engagement Zone ......................................................................... 22 9. Simplified cross section through the Engagement Zone looking northwest........................................... 23 10. Detailed geology of the Moet Occurrence.............................................................................................. 24 10. Detailed geology of the Sandor Occurrence ........................................................................................... 26 11. Generalized geology of the Wawa Project ............................................................................................. 26 12. Lower crustal to upper mantle xenolith, Dubreuilville dike ................................................................... 27 Tables 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program ................. 17 2. Diamond results from Engagement Zone bulk samples (2001). ............................................................. 21 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence......................... 25 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Unusual Diamond-bearing rocks of the Wawa Area Introduction In 1991, local prospector C. “Mickey” Clement recovered at least three alluvial diamonds from the Michipicoten River. Two of the stones were sent to the Department of Mineralogy, Royal Ontario Museum, where they were identified as industrial-grade diamonds with weights of 1.05 and 1.13 carats. Both stones were described as frosted and graphite-inclusion riddled. In 1995, prospector Sandor Surmacz and geologist Marcelle Hauseux recovered diamonds from a bedrock occurrence on the Trans Canada Highway, approximately 20 km north of the town of Wawa. An 18.1-kg bulk sample of a xenolith-rich lamprophyre yielded 1 macrodiamond and 5 microdiamonds. All but one was gem quality. Since then, more than 50 occurrences of diamondiferous bedrock have been reported in an area of approximately 30 km2 in size, centred approximately 20 km north of the town of Wawa. The occurrences are hosted within a sequence of unusual, Archean-aged, heterolithic breccias. Historically, this sequence of rocks has received little exploration interest and was considered to have little economic significance. This field trip will focus on exposures of a series of foliated lamprophyre dikes and associated heterolithic breccias outcropping in Lalibert, Leclaire, Menzies and Musquash townships. This field trip guide represents a summary of information available at the time of writing and should not be considered the final analysis of these rock types. Much more research is required on these rocks. Active exploration and research is still underway on the properties included in this field guide. Given the limitations of time, the field trip will visit only some of the more accessible properties. Bear in mind that when visiting active exploration or mine properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office in Timmins, or the District Geologist’s Office in Sault Ste Marie, Ontario. Geological Overview of the Michipicoten Greenstone Belt The Wawa region lies within the Wawa subprovince of the Canadian Shield. The Michipicoten greenstone belt extends inland for approximately 150 km from the Lake Superior shore and has an average width of 38 km. The greenstone belt consists of supracrustal rocks of Archean age. Younger Archean granitic rocks surround the greenstone belt. Figure 1 shows a generalized geological map of the Michipicoten greenstone belt. The oldest volcanic cycle is approximately 2900 Ma and is of limited distribution. This cycle is best developed in Esquega Township along the southern flank of the supracrustal assemblage. Portions of this metavolcanic cycle extend into eastern McMurray and western Lastheels townships. The base of this volcanic cycle consists of massive to pillowed komatiitic flows intruded by mafic sills. The ultramafic rocks are overlain by intermediate to felsic tuffs capped by thinly bedded chert-magnetite-sulphide iron formation (Judith-Kathleen Iron Formation). Intermediate to felsic metavolcanic tuffs below the Judith iron formation have been dated at 2889 ± 9 Ma (Turek et al. 1992). Overlying the 2900 Ma cycle is a 2750 Ma volcanic cycle. This volcanic cycle is predominately composed of intermediate to felsic tuffs, breccias and flows. Porphyritic and spherulitic flows are not common and most of the intermediate to felsic metavolcanic rock is fragmental. The base of this cycle consists of high magnesium and high iron tholeiitic massive and pillowed flows. It lies conformably atop the JudithKathleen iron formation at the east end of Wawa Lake, but the basal unit is poorly exposed elsewhere. Overlying the mafic metavolcanic rocks is a sequence of heterolithic, intermediate to mafic breccia that has been traced for a distance of over 13 kilometers. This unit is in turn, overlain by a thick section of intermediate to felsic tuffs, breccias and massive flows that reaches a maximum thickness of approximately 2000 m below the Helen Iron Formation. The upper part of the intermediate to felsic metavolcanic rocks has been dated at 2749 ± 2 Ma (Turek et al. 1992). The Michipicoten (Helen) iron formation caps this 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �— 10 I —— Lake Superioi Creek Fault an I I I I . \ Diamond Occurr ences in Archean Heterol ithic Breccia Faults Mafic to fic Intrusive Rocks Rocks I Chemical Clastic Metasedi mentary Rocks I Felsic Metavolca nic Rocks Mafic Metavolcai riic Rocks I Carbonatite Intr usive Rocks Alkalic Intrusion S Sanukitoid Suite Intrusions I Granitoids Figure 1. Generalized geological map of the Michipicoten greenstone belt showing some of the diamond occurrences (modified after Stott et al. 2002). N ickenson Lake Stock 1J Lalibert Lec / Discussed in 1 Enigma Prop erty 2 Sandor Occu rrence 3 Cristal Occur rence 4 Engagement Zone 5 GQ Occurren ce 6 Leadbetter P roperty Diamond Pro pe1 rties \\\\ PDF compression, OCR, web-optimization with CVISION's PdfCompressor �this volcanic cycle. It commonly exceeds 100 m in thickness and, in the vicinity of the past-producing Helen Mine; it has been tectonically thickened to over 300 m. This iron formation was the source of virtually all commercial iron ore production in the Wawa area from 1898 to 1998. The youngest metavolcanic rocks in the area are those of the 2700 Ma volcanic cycle. These rocks underlie approximately fifty-percent of the Michipicoten greenstone belt and are found in the central and northern parts of the belt. The basal unit is composed of massive and pillowed mafic amygdaloidal flows that attain a maximum thickness of 5.5 km in the Goudreau area. This unit is overlain by intermediate to felsic metavolcanic rocks, or their stratigraphic equivalent, the Doré metasedimentary rocks. These metavolcanic rocks are typically composed of tuffs and monolithic and heterolithic breccias. Quartz + feldspar crystal tuff and an intermediate tuff from this volcanic cycle returned a U-Pb zircon age of 2701 ± 8 Ma (Turek et al. 1992) and 2701.4 ± 2.1 Ma (Ayer et al. 2003). The intermediate to felsic tuffs interdigitate with clastic metasedimentary sequences that include cross-bedded sandstone and a tonalite cobble conglomerate (Doré conglomerate). Corfu and Sage (1987, 1992) reported an age of 2698 ± 2 Ma for a tonalite clast in the Doré conglomerate and maximum ages of 2680 ± 3 and 2682 ± 3 Ma for sedimentary sequences in northern and central parts of the Michipicoten greenstone belt. Geochronological and structural evidence indicates that sedimentation continued after cycle 3 volcanism and predated a major folding and faulting event. Arias (1996) noted that the rocks comprising cycle 3 in the central part of the Michipicoten greenstone belt are upside down and represent the overturned limb of a belt-scale recumbent nappe fold. This inverted limb has been refolded and imbricated by subsequent southverging thrust faults, which caused local repetition of the stratigraphic sequence (Wilson 2004). Felsic plutonism occurred synchronous with all of the major volcanic cycles and continued after volcanism ceased at Wawa. Plutonic rocks associated with cycle 1 volcanism include the Murray-Algoma porphyry (2881± 6 Ma) and the Regnery biotite granite of the Hawk Lake granitic complex (2888 ± 2 Ma). Both intrusions are situated in Esquega Township. The Jubilee granitic stock, located in McMurray Township, was dated at 2745 ± 3 Ma and is coeval with cycle 2 volcanism. Plutons associated with cycle 3 volcanism range in composition from tonalite through granodiorite and granite and have ages ranging from 2698 to 2693 Ma. These plutons are located south and west of the Michipicoten greenstone belt (Stone and Semenyna 2004). The Kapuskasing Structural Zone extends east from the shore of Lake Superior, northeast through Kapuskasing and into the Hudson Bay Lowland. Local features interpreted to be associated with it include northeast-striking Proterozoic lamprophyre dikes (Sage 1994; Morris 1999). Lamprophyre dikes of middle Proterozoic age are common in the region south of the Wawa-HawkManitowik Lake Fault and rare to non-existent north of the fault. They are carbonate, biotite, and sometimes olivine-rich and usually less than 1.0 m in width. The dikes generally strike northeast. Dikes exposed in McMurray Township commonly have blue to blue-green sodic amphibole developed in their wall rocks. This mineral has been interpreted to be a product of fenitization. These dikes are likely spatially and temporally related to the emplacement of the Keewenawan-age Firesand River Carbonatite (Sage 1994). North of the Wawa-Hawk-Manitowik Lake Fault, in the area extending west from the former Magpie Mine (Leclaire Township), to the east side of the Dickenson Lake Stock (Lalibert Township), there are a series of what appear to be narrow, biotite-amphibole-rich dikes that have been interpreted as Archean lamprophyres. These dikes commonly have large, rounded inclusions (lower crust to upper mantle-derived xenoliths) up to 3.0 m in size, the centres of which are often completely altered to radiating clusters of actinolite crystals. Titanite from the matrix of one of these dikes returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the same dike returned an age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Several narrow (<10 m) kimberlite dikes also intrude the Michipicoten greenstone belt. These dikes are largely restricted to the eastern part of the greenstone belt and are spatially related to the northeast-trending Kapuskasing Structural Zone. Several of these dikes have been dated. Intrusions K115 and K121, located in Isaac Township, returned an average 87Rb/86Sr age of 1097 ± 7 Ma (Kaminsky et al. 2002). A kimberlite dike intersected in Pele Mountain Resources Inc.’s drill hole 97-34, drilled in Riggs Township, returned a 207 Pb/206Pb age of 1197 ± 24 Ma (R. P. Sage, Ontario Geological Survey, unpublished, 2000; Wilson 2004). Quaternary Geology All Quaternary deposits within the Wawa area were deposited during the Late Wisconsin by the Labrador sector of the Laurentide Ice Sheet. Peat, recovered from a bog located within the surface of a terrace associated with the highest glacial lake in the Lake Superior basin, was radiocarbon dated at 9759 ± 170 BP. A caribou antler, recovered from near the surface of the delta of glacial lake Minong III, yielded a radiocarbon date of 8820 ± 80 BP (Morris 2001). Bedrock striae indicate that there were two prominent ice flow directions. The oldest and most pervasive ice flow was south to southwest (159° - 240°). A later, weaker ice flow was to the southwest and west (220° - 290°). The younger set of striae was formed during the latter stages of glaciation as the ice sheet began to thin and bedrock topography began to influence the direction of ice flow. Much of the overburden consists of a thin (< 1 m), discontinuous till veneer draped over the bedrock. Most of the area's thicker surficial deposits are located within bedrock controlled valleys where glaciolacustrine waters from the Lake Superior basin covered the area. In several of these valleys, the glaciofluvial material reaches thicknesses up to 32 m (Morris 2001). Structural Geology of the Diamond-bearing Rocks The greatest concentration of diamond-bearing rocks in the Michipicoten greenstone belt is constrained to a roughly 30 km2 block of land coinciding with a D1 recumbent nappe identified by Arias (1996). The nappe lies immediately north and adjacent to the Kapuskasing Structural Zone and has affected the two youngest rock sequences in the greenstone belt. In cross section, a generalized structural section across the greenstone belt would be upside down and represents the overturned limb of a belt-scale recumbent fold. This inverted limb of the nappe fold has been refolded and imbricated by subsequent south-verging thrust faults. The result is that the geometry of the rocks hosting the diamondiferous bedrock is that of an inverted anticline (Arias and Helmsteadt 1990). The effect is to create a tectonic repetition of the diamond-bearing sequence of rocks. In simplified terms, the rocks hosting the diamonds in the Wawa area can be considered as a single, overturned fold limb that has been faulted along a series of thrust zones. As a result, the diamond-bearing breccias, the lamprophyres associated with them and the surface onto which these metavolcanic rocks were deposited has been repeated at least four times (Walker 2002). The western limit of the nappe is likely the Dickenson Lake fault that passes along the west side of the Dickenson Lake stock. Extrusive lithologies similar to those hosting diamonds were recognized to the east of the stock by Sage (1993) during reconnaissance mapping. To date, no diamondiferous occurrences have been discovered to the west of the Dickenson lake fault. The eastern limit of the nappe is the Marsden Lake fault. Prior to 2002, rocks favourable to hosting diamonds had not been observed east of this structure (Wilson 2004). However, in 2002, Oasis Diamond Exploration Inc. made a discovery of diamonds on the east shore of the Magpie River and in 2004, diamondiferous occurrences of bedrock were found on the west shore of the Magpie River in Chabanel Township. These discoveries suggest that the potential for these host rocks extends farther eastward than previously believed. Figure 2 shows a graphic representation of the nappe structure. 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �500 F oslon Figure 2. Composite structural section through the central part of the Michipicoten greenstone belt. Section X-Y from south-central Corbiere Tp. (Josephine Iron Range) to Andre Lake central Corbiere Tp. Section Y-Z is a schematic section from west central Lalibert Tp. The sketch in the lower left explains the present configuration of the belt as a regional nappe fold (F1) refolded about F2. Imbricate thrusts are considered related to Fs (Arias 1996). Description of the Diamond-bearing Rocks The diamond-bearing breccia and associated lamprophyre is broadly distributed throughout Lalibert, Leclaire, Menzies and Musquash townships. On-going mapping by Pele Mountain Resources Inc. on the Festival property and by Nathalie Lefebvre on the GQ Property has helped to refine the classification of these rocks. Systematic exploration and sampling suggest that the individual diamond occurrences are part of a much larger suite of rocks and that diamonds occur primarily within discrete layers at the base of diamond-bearing zones. North-northwest-trending diamond-bearing zones of breccia and lamprophyre are up to 1500 m in length and up to 800 m in width (Pele Mountain Resources Inc., press release, January 18, 2005). The breccia forms thick units (maximum true thickness is approximately 110 m) dipping to the northeast 30°. The lateral extent and thickness of the breccia unit is not well constrained, owing to the large-scale regional folding and thrusting (Lefebvre 2004). Figure 3 provides a detailed map of the southwest corner of the Festival Property showing a recent interpretation of these diamond-bearing zones. The diamond-bearing rocks can be visually subdivided into two classes, lamprophyre (dikes and bodies of indeterminate morphology) and heterolithic or polymict breccias. It is often difficult to differentiate between the two classes since the lamprophyre dikes frequently contain an assortment of inclusions that 7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Genesis West II I Geri Mumm ---: I I Rosigtj Bollinger Lake Perth Per Dom .Perlgnon Musquash Tp. Leclaire Tp. it East Perch Salon Salon Cristal Taft • Other • iyric Suite I km • South Zor • North Zor Leger * Diamond I Occurrences F' Figure 3. Detailed geology of the southwestern corner of the Festival Property (modified from Pele Mountain Resources, Inc. press release, January 18, 2005). The units shown in green consist of mafic to intermediate massive and pillowed flows, breccias and tuffs (Vaillancourt 2005b, Sage et al. 1982) Veuv e iot Menzies Tp. —— — — — I_ �give them the appearance of breccia. The lamprophyre dikes cut the breccia units. Both lithologies have been metamorphosed to upper greenschist facies. The breccia primarily consists of angular, pebble-sized, lithic fragments, mainly of volcanic composition, contained within a green to grey fine-grained matrix. The matrix grain size ranges from < 2 mm to 1 mm. At least eleven distinctive types of lithic fragments have been observed in the breccia and the fragments are irregularly distributed throughout the breccia. The clast population is primarily derived from rocks with which the breccia is intercalated. Most typically these clasts are mafic and felsic metavolcanic rocks and intermediate to mafic intrusive rocks. Other clast types include fragments of clast-supported breccia within matrix supported breccia, fragments of earlier matrix-supported breccia with fewer than 5% fragments and coated lithic fragments (Lefebvre 2004). The breccia is characteristically massive, unstratified and poorly sorted with clast size ranging from sand to boulders up to 9 m. Primary sedimentary structures such as bedding and crude grading are rare (Lefebvre 2004). Petrographic work by Lefebvre (2004) on the breccias identified a typical fragmental texture within a mineralogically variable matrix. Typically the groundmass is dominated by actinolite, but chlorite and biotite dominated groundmass are locally predominant. Juvenile magmatic material also was observed as discrete fragments and rims on other clasts in the breccia. Its petrography is distinct from the breccia groundmass. The juvenile magmatic material contains more abundant actinolite grains; fewer epidote and fine-grained plagioclase grains and more fine-grained oligoclase and muscovite; and more oscillatoryzoned hornblende. Most exploration efforts over the past few years have concentrated on the breccias because they are considered to have the best potential for hosting commercial diamond deposits. Over the course of the last few years, explorationists have subdivided the diamond-bearing breccias into three separate facies. These facies are volcanic (pyroclastic), subvolcanic/intrusive breccia and hypabyssal facies. A variable, but distinctive proportion and composition of fragments and/or xenoliths characterizes each facies (Wilson 2004). The volcanic facies contains breccia, lapilli- and ash sized fragments and consists of medium to thickly bedded pyroclastic air-fall deposits. They are characterized by angular to sub-angular Archean supracrustal fragments, some hypabyssal fragments and rare lower crustal to upper mantle xenoliths. The subvolcanic/intrusive breccia facies are the most variable in texture and appearance. The rock is characterized by observed intrusive relationships, a high proportion of fragments and a close proximity to the volcanic facies. The fragment characterization is variable. The facies can contain all or some of the following fragment types: supracrustal fragments, crustal fragments and lower crustal to upper mantle xenoliths. This facies was once included with the lamprophyres but is now interpreted by various exploration companies as a debris flow. The hypabyssal facies hosts variable proportions (<25%), of sub-rounded to rounded mantle xenoliths, as well as minor proportions of gneiss and/or trondhjemite fragments. This facies also was originally classified as lamprophyre but is now considered to be part of the debris flows (Lefebvre et al. 2003). The brecciated unit(s) could also be a single or multiple diatreme(s) localized within Lalibert, Leclaire, Menzies and Musquash townships. A tectonic repetition of the diamond-bearing diatreme(s) has been achieved through the regional deformation events described previously. This regional deformation may help to explain the layered appearance observed at some of the diamond occurrences (J. Ayer, Ontario Geological Survey, personal communication 2006). Lamprophyre occurs as narrow dike-like intrusions or in bodies of indeterminate morphology cross-cutting or intercalated with local country rocks. Dikes range in width from 50 cm to 2 m and may display 2-3 cm offshoots in some locations. Contacts with the host rocks vary from sharp and straight to highly irregular. No variation in grain size within the dikes has been observed and the no variation in colour or mineralogy 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �has been observed within country rock adjacent to the dikes. The lamprophyre is fine-grained, grey in colour and contains approximately 5-10% subrounded to subangular fragments. The fragment population is dominated by actinolite-rich monomineralic rocks, or by biotite-rich greenstone and hornblende-rich ultramafic rocks (Lefebvre 2004). Lamprophyre is differentiated from the breccia by: a lower clast content; a predominance of highly altered, coarse-grained actinolite fragments; scarcity of wall rock fragments; the rounded shape of the xenoliths; and the presence of a weaker fabric (Lefebvre 2004). The lamprophyre dikes post-date all other lithologies. The lamprophyre with indeterminate morphology predates most of the host lithologies since fragments of each are found within these lamprophyres. However, in some locations, lamprophyre fragments have been observed within intermediate and mafic intrusive rocks near the contact with lamprophyre (Lefebvre 2004). Fragments within lamprophyre dikes commonly have a biotite-rich rim enclosing the xenoliths and the fragments found in the lamprophyres of indeterminate morphology do not. The lamprophyres of indeterminate morphology also show positive relief of less severely weathered xenoliths more so than the lamprophyre dikes. Lastly, the lamprophyre dikes have a less variable fragment lithology (Lefebvre 2004). The lamprophyre is petrographically distinct from the breccia. It contains a lower abundance of clasts and fewer clast types. Unlike the breccia, the lamprophyre contains no juvenile magmatic material and oscillatory-zoned hornblende grains are rare. Detailed petrographic descriptions of the lamprophyre can be found in Lefebvre (2004). Relationship between diamond content and lithology Microdiamonds have been recovered from a wide array of breccias and lamprophyres in the Wawa area. Between 2003 and 2004, the Ontario Geological Survey (OGS) investigated a number of diamond occurrences during a mapping program conducted in Menzies and Musquash townships. Closer look at locations where bulk samples had been collected suggested that the bulk samples probably included more than one rock type. Diamonds were recovered from each of these bulk sample sites, but it would be difficult to establish from which of the rock types the diamonds were recovered. The OGS undertook a limited sampling program to further investigate the diamond content of specific lithologies (Vaillancourt et al. 2005a). The OGS collected three small samples (95.21 kg total weight) from three different lithological units at the Cristal and Genesis diamond occurrences. These occurrences are on the Festival Diamond Property owned by Pele Mountain Resources Inc. Results from this limited sampling program reinforce the observation that microdiamonds are not restricted to a single unit. Microdiamonds were recovered from heterolithic breccia, both with and without ultramafic magma pockets, and from a fragment-free ultramafic dike. There is the possibility, however, that the diamonds recovered from the ultramafic dike are xenocrysts derived from diamond-bearing host breccia (Vaillancourt et al. 2005a). The OGS concluded that the results from only three samples are not sufficient to draw irrefutable conclusions regarding the location of the microdiamonds. Collection and analysis of more, well constrained samples is necessary to further refine the diamond potential of specific host rocks. In 2005, Spider Resources Inc. and KWG Resources Inc. conducted a similar bedrock sampling program to investigate the diamond-bearing potential of specific lithologies. Separate representative samples (approximate weight 16 kg each) of the matrix, xenoliths and run of mill (ROM) portions of the bedrock were collected from the Wawa Diamond Project and were sent for caustic dissolution. The results are as follows: matrix sample returned 67 diamonds (0.008 total ct), xenolith sample returned 244 diamonds (0.051 total ct) and the ROM sample returned 86 diamonds (0.006 total ct) (Spider Resources Inc., press release, February 20, 2006). 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochronology In conjunction with recent geological mapping in the area, the Ontario Geological Survey has been conducting geochronological work to help understand the nature and timing of the diamondiferous units and their host rocks within the Michipicoten greenstone belt. A felsic volcanic horizon hosting diamondiferous units returned a 207Pb/206Pb age of 2701.4 ± 2.1 Ma. Maximum 207Pb/206Pb ages of 2685.1 ± 1.0 Ma and 2684.9 ± 1.4 Ma have been returned for diamondiferous lamprophyre dikes cutting the Catfish assemblage (2.7 Ga) intermediate to felsic metavolcanic rocks in Lalibert and Menzies townships (Ayer et al. 2003). A second sample of felsic lapilli tuff, part of the Catfish assemblage, adjacent to the Moet Occurrence contains zircons that returned a 207Pb/206Pb age of 2698.7 ± 1.1 Ma (Vaillancourt et al. 2004). A sample was collected from the diamondiferous breccia at the Moet Occurrence in order to determine the age of brecciation. Five zircons were analyzed. The three oldest ages are 2687 ± 2 Ma, 2683 ± 2 Ma and 2681 ± 2 Ma. The two youngest zircons cave results that precisely overlapped one another at 2679.2 ± 2.1 Ma (Vaillancourt et al. 2004). Since this is the youngest zircon age obtained from the breccia, it represents either the time of cystallization or emplacement of the body if the zircons are magmatic, or a maximum time of emplacement if the zircons are xenocrystic (Vaillancourt et al. 2005a). If the zircons are xenocrystic, this age must still be close to the time of breccia emplacement since a lamprophyre dike from the GQ property returned a date of 2673 ± 8 Ma from titanite (R. P. Sage, Ontario Geological Survey, unpublished 2000). The data indicate that the felsic metavolcanic rocks hosting the diamondiferous breccias are part of the Catfish assemblage. The maximum age for the diamondiferous breccias and the associated dikes is less than 2680 Ma. These absolute age constraints indicate that the breccias are not volcaniclastic units belonging to the Catfish assemblage (Vaillancourt et al. 2005a). Zircons from a sample of felsic lapilli tuff from northwestern Menzies township returned an age of 2736.0 ± 0.8 Ma which is taken to represent the age of eruption and crystallization of the tuff. This age clearly indicates that the volcanic package underlying the iron formation in the western part of Menzies Township is part of the Wawa assemblage (2.75 Ga) and brackets the uppermost part of the assemblage at 2736 Ma (Vaillancourt et al. 2005a). Geochemistry of the Diamond-bearing Rocks Both Williams (2002) and Lefebvre (2004) conclude that the whole rock major element geochemistry is consistent with a calc-alkaline classification for both the lamprophyres and the associated breccia. Both authors also noted that the compositions of chromite in the Wawa metavolcanic rocks are in the range typical for lamprophyres and dissimilar to those in kimberlites and lamproites. Whole rock geochemistry for the diamond-bearing rocks is tabulated in Sage (2000), Williams (2002), Lefebvre (2004), Stone and Semenyna (2004) and Vaillancourt et al. (2005c). Sage and Williams’ work is specific to the diamond-bearing and non diamond-bearing lamprophyres. Work by the other authors relates to both the breccia and the lamprophyres. Whole rock geochemistry for kimberlites of the Wawa area can be found in Kaminsky et al. (2002). Diamond characteristics Lefebvre (2004) undertook a study examining a parcel of 80 macrodiamonds recovered from the volcaniclastic breccia on the GQ Property. Results from this work are summarized below and in De Stefano et al. (2006). Additional work on the morphology of the Wawa diamonds can be found in Stone and Semenyna (2004). Stachel et al. (2004) summarize results of analysis conducted on diamonds from the Genesis and Cristal diamond occurrences presently held by Pele Mountain Resources Inc. 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Shape: The diamonds display a highly variable primary growth form. Most of the diamonds are either octahedral aggregates (44% of the population) or single octahedral crystals (26%). Single cubic and cubicoctahedral crystals and their aggregates, as well as macles form the remainder of the population. Fortyeight percent of the diamond population is single crystals and only 28 diamonds could be evaluated for crystal regularity. The majority of the diamonds also display some degree of distortion. Colour and transparency: The diamonds included in this study are classified into colourless, brown, grey, black, yellow and white. No pink, green, violet or blue diamonds were observed. The colour distribution within the population is: colourless (48%), heterogeneous (24%), yellow (11%), black (3%), brown (10%) and grey (3%). The heterogeneity in colour is observed only in aggregates. The diamond population consists of 48% transparent crystals, 25% translucent crystals, 14% opaque crystals and 14% is a combination of opaque and translucent crystals. Transparent crystals are typically colourless and also comprise a few yellow octahedral single crystal and coarse aggregates as well as macles. Translucent crystals comprise all possible primary crystal forms and colours. Opaque crystals are mostly fine-grained aggregates which have black body colouring. Resorption: Generally speaking, the diamonds have experienced low degrees of resorption. Only 21% of the diamond population displays extensive resorption. Some crystals (14%) exhibit non-uniform resorption where one part of the crystal is more strongly resorbed than another. Inclusions: Mineral inclusions were identified in 58% of the diamonds. Both primary and secondary inclusions were observed. The mineralogy of the recovered primary inclusions is listed in descending order of abundance: olivine (Fo92 and Fo89), clinopyroxene (omphacite), plagioclase (albite and An-rich), orthopyroxene (En93) and Fe-Ni sulphide (pentlandite). Cathodoluminescence: The relative abundances of cathodoluminscence (CL) colours for the Wawa diamonds are: orange-red (46%), yellow (28%), orange-green (10%), green (6%), and other non-uniform colours (10%). None of the 69 diamond examined displayed the more common blue CL. Impurities: Fournier Transform Infrared (FTR) spectrometry was used to investigate the nitrogen and aggregation states for 41 diamonds. The majority of the diamonds have low nitrogen contents, < 300 ppm. The diamonds show two modes of nitrogen aggregation suggesting mantle storage at 1100 - 1170° C. Diamonds from the Genesis occurrence are almost exclusively cubes including some fragmented, twinned and moderately resorbed cubes. Most of the crystals contain clouds. Fully transparent stones are dominantly brown although colourless stones also are common; one diamond was yellow in colour. Nitrogen concentrations range from below detection (<10 ppm) to 600 atomic ppm. Nitrogen aggregation is very low (Stachel et al. 2004). Diamonds from the Cristal occurrence range from un-resorbed octahedra to highly resorbed dodecahedra. Octahedral and weakly resorbed octahedral stones dominate the population. About 25% of the population are irregular crystals, macles are common (15%) and about 5% of the diamonds show cubo-octahedral growth. The stones fall into two dominant colour classifications, colourless and a range of brown colouration. Nitrogen contents range from <10 – 560 ppm, but with only one exception nitrogen is ≤170 ppm. Nitrogen aggregation varies between 0 and 97% B-centre. Olivine is the most common mineral inclusion, followed by pyrope garnet and Mg-chromite (Stachel et al. 2004). Origin of the Diamond Deposits Based on published data on the diamond-bearing rocks at Wawa and Cobalt, Wyman et al. (in press) suggest that the tectonic setting of the deposits and nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 – 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of lowtemperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �diamonds in most Phanerozoic scenarios may be too deep to be accessed by the lamprophyric magmas. Shallow subduction, as proposed for these occurrences of adakitic-type rocks in the Wawa subprovince, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas. Wyman et al. (in press) states that any tectonic model for these Archean diamond occurrences must address several requirements. These requirements include 1. a deep source for oxidized metasomatic fluids that is activated prior to lamprophyre emplacement 2. a mechanism to isolate this isotopically aged and depleted source for tens or hundreds of millions of years until it is heated in the mantle during orogeny 3. a hybridized mantle source for primitive, hydrous, shoshonitic lamprophyres 4. sustained cold finer effect in the mantle to establish a shallow-mantle diamond stability window Two theories of diamond origin are postulated by De Stefano et al. (2006). Both a cratonic and orogenic model of diamond formation are discussed in an effort to rationalize the observed diamond characteristics. The authors conclude that neither model fully explains all of the observed characteristics. 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Field Trip Stops Field Trip Road Log Stop 1 2 3 4 5 6 7 8 Locality Intersection of Hwy 101 and Hwy 17 Take Highway 17 north km 0 Catfish Road forestry road – turn east 19.2 GQ Diamond Discovery Northern Sierra Minerals Corporation Area B Northern Sierra Minerals Corporation Engagement Zone 23.3 23.9 29.1 Return to Highway 17 reset odometer Drive north to access road – turn east Park Walk eastward along trail Moet Occurrence, Festival Property 0 6.2 6.6 7.6 7.6 Return to Highway 17 reset odometer Drive north, park on shoulder of highway Sandor Diamond Occurrence Continue north on Highway 17 to intersection of Highway 519, turn right 0 4.3 4.3 12.6 Safely turn in parking area and return to Intersection of Hwy 17 and 519 reset odometer Drive south on Highway 17, park on shoulder of road Dubreuilville Dike Continue south on Highway 17 Turn left into access road to gravel pit, park Walk south approximately 150 m Monchiquite Dike Drive north on Hwy 17 to Wawa Wawa Motor Hotel 0 3.1 3.1 44.9 45 53.2 STOP 1 - GQ Diamond Discovery Site Northern Sierra Minerals Corporation Area A UTM co-ordinates – 0665570E 5333291N NAD83 Several outcrops of diamondiferous breccia outcrop on the west side of a forestry road. This exposure is an example of the hypabyssal facies of the three identified diamond-bearing units. The rock cut displays the apparently conformable nature of these “lamprophyre” dikes. The most notable features of these outcrops are the actinolite-rich nature of the matrix and the presence of biotite-rich reaction rims around the xenoliths. It is frequently difficult to distinguish these dikes from the mafic to intermediate agglomeratic and tuffaceous host rocks. In the vicinity of the discovery area, located at the south end of Area A, the diamondiferous breccias are arrayed linearly along the logging road where the topography indicates a 5 – 10 m thick, northwest-trending dike (Cavey 2002). A compilation of the geology of the GQ Property is shown in figure 4. Figure 5 provides a compilation of the diamond occurrences on the GQ Property. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �GQ PROPERTY I / I AR! ' I GEOLOGY REVISED FROM CANADA EXPLORATION INC. GABBRO LII METASEDIMENTS GO PROPERlY INTERMEDIATE TO FELSIC METAVOLCANICS L CALCALKALINE IIVB AND LAMPROPIIYRE LOCATIONS ARE APPROXIMATE PROPERTY GEOLOGY MAP Sault Ste. Mane Mining Division Ontario Atter K. KM (2003), Kennecott Figure 4. Geological compilation map of the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �_____ ____________ iT ___________ a § Drill Hde Occurrences ci In&usive and/or Exinisive flreccias KEY • drilled in 2002 I 2003 KEY D;scovemdprlorto Opthn during a GO PROPERTY COMPILATION MAP drilled in 2000 logging road. 2004 MusquashTownship It Ste. Marie Mirlfng Division Ontario Figure 5. Occurrences of diamondiferous bedrock on the GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2004). 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �A relatively precise 207Pb/206Pb age of 2674 ± 8 Ma was returned from a sample collected in 2000 by R. P. Sage (R. P. Sage, Ontario Geological Survey, unpublished 2000). The sample analysed was titanite. The titanite grains are considered to be a primary mineral and not xenocrystic. In thin section, the rock is characterized by a green, medium-grained, granoblastic to decussate groundmass of actinolite, biotite, chlorite, plagioclase and accessory minerals. Amphibole or biotite macrocrysts up to 1 mm are common (Stone and Semenyna 2004). Local prospectors T. Nicholson, J. Robert and M. Tremblay made the discovery in the fall of 1999. The first two bedrock samples (63.4 kg and 70.5 kg) collected were processed by Kennecott Canada Exploration Inc. in their Thunder Bay laboratory. According to Kennecott’s report, the 63.4 kg sample yielded 45 diamonds, of which 10 were macro diamonds and 35 were microdiamonds. One of the macro diamonds measured 1.01 mm in one dimension. The 70.5 kg sample yielded 9 microdiamonds. All stones were white in colour and transparent in clarity. Duplicate samples were collected by Band-Ore Resources Ltd. in early 2000 and were processed at SGS Lakefield Research Limited. A 54.6 kg sample yielded 98 microdiamonds. A confirmation sample from the same area yielded 98 microdiamonds from a 54.6 kg sample. In 2000, Band-Ore Resources drilled 3 short holes (75 m total) at the discovery site. Table 1 details the diamond recovery results from the drill program. Only partial intervals from drill hole DDH GQ-00-3 were submitted for microdiamond recovery since portions of the core were used for thin sections, microprobe analysis and display purposes. In total 5 diamonds were recovered from DDH GQ-00-3, including one champagne coloured macro diamond and one white microdiamond from a sample weighing 7.5 kg. Drill sections for DD GQ-00-01 through 03 are shown on figure 6. Table 1. Diamond recovery results from 2000 Band-Ore Resources Ltd. diamond-drilling program Drill Hole DDH GQ-00-1 DDH GQ-00-2 Sample No. Sample Size (kg) No. Macro Diamonds No. Micro Diamonds Sample 1A 63.35 1 Sample 1B 30.17 10 Sample 1C 28.51 6 Sample 2A 37.52 1 30 Sample 2B 30.97 434 Sample 2C 28.51 30 results compiled from Band-Ore Resources Ltd. press releases 2000 To date, the discovery site has yielded 746 diamonds, including 15 macro diamonds, from sample material weighing 785 kg. The largest diamond recovered exceeds 1.0 mm in size and the majority of the stones are gem quality, white, clear and transparent. STOP 2 - Northern Sierra Minerals Corporation Area B UTM co-ordinates – 0665425E 5334748N NAD83 The exposure on the east side of the forestry road provides an excellent exposure of the subvolcanic/intrusive breccia facies. Subrounded to rounded xenoliths dominate the vertical exposure (Figure 7). Field relationships between the intrusive breccia and other heterolithic breccias can be observed in several outcrops along the road. Texturally, the subvolcanic/intrusive facies may resemble both the hypabyssal facies and the intrusive heterolithic breccias. The facies consists of mica and amphibole phenocrysts (<2mm) in a groundmass of mica, actinolitic amphibole and lesser albite, carbonate, sphene and oxides. Alteration includes variably chloritized mica while the other phenocrysts have been extensively altered to varying proportions of mica, albite and actinolitic amphibole. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �z Q 000 / MAFIC VOLCAXICS A S\\IPI I lB -, / MARC VOLCANICS Band-Ore Resources Ltd. GQ PROPERTY Drill Section 3Dm Musquash Township Ste. Marie Mining Division Ontario Drill Hole GQ-OO-O1 Section Looking West Figure 6. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �SAM PLI- XENOLITHIC microdinmonds A (33.81kg) 3 macrodiamonds DIATREME 00) 2A (3 7.52kg) / XYN(YLITIIIC DI.\I I 2B 7 GQ-OO-03 S\\IPLI 2€ 4, Band-Ore Resources Ltd. GQ-OO-02 27m GQ PROPERTY Drill Section Musquash Township Sault Ste. Marie Mining Division Ontario Drill Holes G-OO-2 and GQ-OO-03 Section Looking West Figure 6 cont’d. Drill hole sections GQ-00-01, GQ-00-02 and GQ-00-03, GQ Property, Musquash Township, Northern Sierra Minerals Corporation (Cavey 2002). 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �I- - . —I —.-..' - ½ 4 - ( "- Ip .t •q Figure 7. Stop 2 - Northern Sierra Minerals Corporation Area B – rounded lower crustal to upper mantle xenoliths Stone and Semenyna (2004) completed a petrographic examination of one of the ultramafic xenoliths from this site. The sample was dominated by a coarse, radiating to decussate, clear to pale green amphibole of tremolitic to magnesium-rich actinolite composition. Carbonate occurs locally and biotite is concentrated at the rims of the xenolith. Band-Ore Resources Ltd. discovered area B in 2000. Thirty-three (33) reconnaissance samples were collected from this area and a total of 273 diamonds was recovered from 352 kg of material (Cavey 2002). One 24 kg sample returned 126 microdiamonds (1.37 mg total weight). No macro diamonds were recovered from Area B. Band-Ore Resources Ltd. completed only a reconnaissance sampling program, minor stripping and trenching in this area. The Barnett Zone lies approximately 1.6 km to the northwest. It was discovered in the fall of 2001 by Kennecott Canada Exploration Inc. who completed a limited program of mechanical stripping, channel sampling and washing of outcrops over the area. A total of 27 outcrop channel samples (270 kg) were collected. The channel samples returned 330 microdiamonds and 3 macro diamonds. A single 24 kg sample of heterolithic breccia from the Barnett Zone returned 3 macrodiamonds and 123 microdiamonds. A total of 273 diamonds (261 microdiamonds and 12 macrodiamonds) were recovered from 34 samples (352 kg) collected between September 2000 and July 2001 by Kennecott Canada Exploration Inc. in Area B (including the Barnett Zone). 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 3 - Northern Sierra Minerals Corporation Engagement Zone Bulk Sample Site UTM co-ordinates – 0667760E 5336073N NAD83 The main outcrop exposure consists of medium to thickly bedded pyroclastic air-fall deposits and debris flows described as heterolithic tuff-breccias that grade upwards to lapilli-tuff and tuff. The groundmass of the extrusive phase is fine-grained with variable proportions of relatively small (<2mm) altered phenocrysts, including chloritized mica. The groundmass consists primarily of actinolitic amphibole with rare to minor mica or granular albite. The pyroclastic rocks are mineralogically and compositionally similar to intrusive varieties, but have a significantly higher proportion of mica phenocrysts. Both diamond-bearing intrusive and extrusive rocks host significant proportions of fragments derived from the local country rock. Rare to minor, deep crustal and upper mantle xenoliths, such as banded gneiss and extensively altered talcose ultramafic xenoliths are present. Fresh mantle rocks, such as lhertzolite, harzburgite and eclogite have not been identified. The matrix material is typically fine-grained, green, weakly foliated actinolite schist. Albite is present, although it is less abundant (<10%) than in the lamprophyre dikes and implies a more ultramafic composition for the breccia matrix than for the lamprophyre dikes. Titanite is fairly abundant and calcite, epidote, apatite and sulphide minerals occur locally. Macrocrysts of actinolite are commonly observed. Rare macrocrysts of amphibole also are observed and are frequently altered to actinolite. The actinolite macocrysts are probably metamorphic in origin, whereas the amphibole macrocrysts may represent accidental or cognate crystals (Stone and Semenyna 2004). The Engagement Zone has a minimum strike length of 335 m and a horizontal width in excess of 75m. The zone strikes northwesterly and has a shallow dip to the northeast. This zone may represent the southeast extension of Pele Mountain Resources Inc.’s Cristal diamond occurrence located approximately 2 km to the northwest. In 2003, a 0.72 carat macrodiamond was recovered from a bulk sample collected from the Cristal. The geology of the Engagement Zone and sample locations are shown in Figure 8. A simplified cross section of the Engagement Zone is shown in Figure 9. Band-Ore Resources Ltd. discovered this zone in January 2001. A 16 kg sample from a single angular boulder of diatreme breccia returned 128 microdiamonds. Four subsequent samples (96 kg total weight) returned 5045 microdiamonds and 65 macrodiamonds. In 2001 a mini-bulk sample weighing 12.5 tonnes was collected under the supervision of Kennecott Canada Exploration Inc. and shipped to the Saskatchewan Research Council. The largest diamond recovered from this sample was a 0.254 carat, broken, white octahedral stone. Two additional bulk samples were collected in 2003. A 22 tonne sample tested an area where 6 channel samples (63 kg total) recovered 1752 stones. A 20 tonne sample tested an area where 5 channel samples weighing 41.8 kg returned 552 stones. The results from these bulk samples are shown in Table 2. Table 2. Diamond results from Engagement Zone bulk samples (2001) Sample Weight Sieve +1mm Sieve +2mm Sieve +3mm Sieve +5mm Sieve +6mm Total Diamonds Total Carat Weight Engagement Zone East 22.1 tonnes 1 4 4 2 1 12 0.375 Engagement Zone West 20.4 tonnes 2 3 3 8 0.155 Occurrence results compiled from Band-Ore Resources Ltd. press releases 2004 To date, exploration on the Engagement Zone has included an orientation geochemical survey, geological mapping, channel sampling, trenching, bulk sampling and a 9-hole (1775 m) diamond drilling program designed to test the strike continuation of the zone. The drilling program demonstrated that thick diamondiferous breccia deposits can feather out and thin to a few centimeters thickness (Cavey 2003). 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Engagement Zone // N 1/ II / APPARENT OUTLINE OF // ii 0 / // MICROS 73 MACROS FROM 481.83 I - excludes the resuj from the Kenneec channel sampling / II V C \ I Mt.51262 12 tonne Exploration Sample hz' - -- I y '.APPROX. DII' - -. O.2o4CaratWhrtsOctahedral / LOG A1FONØ ARE APP ROXIM so icc Engagement Zone • OCCURRENCES OF BRECCIA large angular slabs and/or Duicrop) * SAMPLE PROCESSED FOR MICRODIAMOND RECOVERY (caustic fusion) Figure 8. Stop 5 – Geology and sample locations at the Engagement Zone, GQ Property, Northern Sierra Minerals Corporation (Cavey 2002) 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2001 MINI-DULK SAMPLE A SIRIFFLNG N N VOLCAN N4AFLC VOLCANICS AND INIERELOW N HETEKOLtIHIC MEGALITH VOLCAN[CLAST:C I CC IA — VOLGAN[C •I "NN 105, N I HETEWLFIHIC N N N LOCATIONS SKECCIA ArFROX!MATE "N Figure 9. Simplified cross section through the Engagement Zone looking northwest (Cavey 2004). STOP 4: - Moet Occurrence, Festival Property Pele Mountain Resources Inc. UTM co-ordinates – 0662709E 5338009N NAD83 The Moet occurrence is a large stripped outcrop that extends in a north-south orientation across the forestry road. The outcrop displays all three facies of diamond-bearing bedrock exposed over an area 500 m by 300 m. It is hosted within fine- to medium-grained mafic metavolcanic rocks with closely associated intermediate to felsic metavolcanic rocks and metasediments. The volcanic facies was found concentrated within a series of outcrop exposures along the west side of a 5-8 m north-trending ridge and the subvolcanic/intrusive breccias and hypabyssal rocks are present in several outcrops east of the volcanic facies. The subvolcanic/intrusive breccias are hosted in the metavolcanics and the volcanic facies appears to overlie these rocks, and are in turn overlain by intermediate to felsic metavolcanic rocks. The fragments 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �within the volcanic facies consist of country rock fragments with lesser crustal fragments and lower crust to upper mantle xenoliths. The hypabssyal facies displays primarily mantle xenoliths (Walker 2003). A detailed geological map of the Moet locality is shown in Figure 10. ' Stop4 t Moot 5—-/ — I I 1YPE \">" — — — M.lic rock. + an&'o reck, tomainjin mamle xpnnMha cpsTs 0 Figure 10. Detailed geological map of the Moet Occurrence, Festival Property, Pele Mountain Resources Inc. (Kjarsgaard et al. 2003). The access trail to the outcrop passes through a sequence of intermediate to felsic tuffs and tuffaceous breccias. The composition of the fragments and that of the matrix are highly variable from felsic to intermediate and a combination of felsic matrix with intermediate fragments and vice versa is not uncommon. At a distance of approximately 1km, the trail passes through a sequence of mafic to intermediate tuffs and lapilli tuffs (Vaillancourt et al. 2005b). An age date of 2698 ±1 Ma was returned from sample of the felsic metavolcanic rocks at the occurrence. A sample of the breccia returned an age date of 2680 Ma (J. Ayer, Ontario Geological Survey, personal communication, 2004). Discovered by Pele Mountain Resources Inc. in 2001, the occurrence initially gained interest because the breccia has a size distribution of diamonds that includes coarser sized diamonds from relatively small samples. For example, an 8 kg sample collected in 2001 recovered a total of 9 diamonds, 4 of which were in the +600 mesh fraction. Diamonds are consistently recovered from both the volcanic and subvolcanic facies at this showing. A summary of the results of the 2001 and 2002 sampling of the occurrence by Pele Mountain Resources Inc. is found in Table 3. 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 3. Summary of the diamond results from the 2001-02 sampling of the Moet Occurrence Facies Weight (kg) <425 mesh >425 mesh volcanic volcanic hypabyssal hypabyssal volcanic volcanic hypabyssal subvolcanic breccia 117 16 24 8.6 9.3 14.1 9.3 32 48 2 0 0 7 12 0 30 0 1 0 0 0 5 0 1 total diamonds 48 3 0 0 7 17 0 31 results compiled from Pele Mountain Resources Inc. press releases 2002 Further exploration was conducted at the occurrence in 2003 when DeBeers Canada Exploration Inc. completed a detailed airborne geophysical survey, stripping, mapping and sampling program. A 47.8tonne bulk sample from the site returned 5 diamonds with a total carat weight of 0.13. All diamonds were recovered from the +1 to +3 sieve class screens (Pele Mountain Resources Inc., press release, March 17, 2004). STOP 5: - Sandor Diamond Occurrence Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0659805E 5342191N NAD83 The Sandor occurrence the first confirmed occurrence of diamonds in bedrock in the Wawa area The occurrence is located in a 4 m high road cut on the east side of the Trans Canada Highway (Highway 17). The dike is approximately 5 m wide, steeply dipping and strikes roughly parallel to the regional schistosity at 120°. The dark, greenish-grey rock weathers olive grey, is highly fractured, moderately carbonatized and is non-magnetic. It is composed of up to 40% actinolite replaced mantle xenoliths and supracrustal xenoliths. Towards the margins of the dike xenoliths are less common and the rock grades into an adjacent micaceous dike. Only remnants of the dike remain in situ. The dike is hosted by gabbros and intermediate to felsic crystal tuffs. A (308.6 kg) sample of the dike, collected by Spider Resources Inc. in 1997, returned a total of 97 diamonds comprising 1 commercial stone, 13 macrodiamonds and 83 microdiamonds. A short walk into the forest from the top of the outcrop leads to a larger stripped area where field relationships between the host gabbro and the dike can be observed. A second, xenolith-bearing dike (occurrence LAL-3) is located at the north end of the outcrop. This dike is 2 m wide and closely resembles the Sandor occurrence. A 34.6 kg sample of this dike contained 1 microdiamond. A detailed geological map of the Sandor occurrence is shown in Figure 11. A compilation map of the geology of the Spider Resources Inc. and KWG Resources Inc. property is shown in Figure 12. Using normative mineralogy, Sage (2000) concluded that this dike should be classified as a spessartite. A spessartite is defined as a lamprophyre composed of phenocrysts of green hornblende or clinopyroxene in a groundmass of sodic plagioclase with accessory olivine, biotite, apatite and opaque oxides. Titanite and rutile from the matrix of the Sandor dike returned an age of 2703 ± 42 Ma (Sage 2000). The date is interpreted to be a minimum age of intrusion. Subsequent dating of a zircon from a gneissic xenolith from the Sandor dike returned a 207Pb/206Pb age of 2684.9 ± 1.4 Ma (Ketchum, Kamo and Davis 2003). Spider Resources Inc. and joint venture partner KWG Resources Inc. have taken 3 mini-bulk samples along the 1 km strike extent of the dike since the fall of 2001. The three bulk samples had a combined weight of 7.61 tonnes and returned 11 commercial stones and 9 macro diamonds. These samples were tested only for macro diamond and commercial diamond content (Spider Resources Inc., press releases, February and March 2002). 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �0 ' - — M*VSc.iic — tbcoveryOflop ——-Opel— 2__21' S Stops Figure 11. Detailed geology of the Sandor Occurrence, Spider Resources Inc. & KWG Resources Inc. (Kjarsgaard et al. 2003). Figure 12. Generalized geology of Wawa Project, Spider Resources Inc. and KWG Resources Inc. (Spider Resources Inc. and KWG Resources Inc., CD-ROM presentation, update February 24, 2004) 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 6: - Dubreuilville Dike - Xenolith-rich lamprophyre Spider Resources Inc. & KWG Resources Inc. UTM co-ordinates – 0657251E 5347023N NAD83 The dike outcrops in a 1.5 m high exposure on the west side of the Trans-Canada Highway (Highway 17). In the area between the former Magpie Iron Mine in Leclaire Township to approximately the eastern contact of the Dickenson Lake Stock, there are a number of exposures of unusual dike rocks. These rocks are characterized by prominent round, to elliptical inclusions of actinolite or actinolite plus talc. Xenoliths are altered to fine-grained actinolite with or without talc, and some display zoning from talc core to an actinolite rim. The actinolite inclusions may consist of prismatic green crystals as large as 8 cm in length, which may be randomly oriented or radiating inward towards the core. The inclusions containing talc consist of a talc core with the prismatic to acicular actinolite projecting radially inward towards the core. The xenoliths are believed to represent at least two original mafic compositions which are likely to be originally of lower crust origin or deeper. A weakly developed regional schistosity crosses the dike implying an Archean age (Sage 1993, 2000). An example of one of the lower crustal to upper mantle xenoliths is shown in Figure 13. Figure 13. Lower crustal to upper mantle xenolith, Dubreuilville Dike Stop 6, Highway 17 North. This dike may be the one described by Higgins (1986). He reported that the dike consists of 60% euhedral amphibole, 20% biotite replacing amphibole and 15%plagioclase. Minor sphene and opaque minerals are present and chromite is reported from the core of the talc-bearing clasts. The bulk composition of the nodules is reportedly pyroxenite, but Higgins did not indicate whether their source was mantle or crust. The rounded outline of the clasts may reflect magmatic erosion during transport and the present mineralogy is the product of regional metamorphism (Higgins 1986). 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 7: - Monchiquite Dike Michipicoten Post Provincial Park UTM co-ordinates – 0663600E 5309223N NAD83 On the west side of Trans Canada Highway (Highway 17) two, 1-metre wide and one, 3-metre wide monchiquite dikes outcrop at the base of a 3 m high outcrop. The dikes are parallel, strike 060° and crosscut a portion of the Mission Stock. A 4 m wide, east-trending microsyenite dike also can be observed cutting the granodiorite and the monchiquite at this location. The north-northwest-trending Trembley Fault cuts across the north end of the outcrop (Massey 1985). These lamprophyres are most commonly seen in road cuts along the highway and along the shore of Lake Superior. Typically, they are black when fresh and weather to an orange-brown colour. The dikes are usually narrow and are typically less than 1 m wide. Biotite-phyric and olivine-phyric varieties are most common, but pyroxene and feldspar phenocrysts also are observed. Often the dikes show evidence of multiple and composite intrusion, sometimes with tin screens of country rock trapped within the dikes. Lamprophyre dikes south of the Michipicoten River typically have a northeast trend (Massey 1985). Lamprophyre dikes are commonly found cross cutting all lithologies south of the Wawa – Hawk – Manitowik Lakes Fault. Because of their ease of weathering they are infrequently seen in outcrop. However, the dikes are frequently found in underground mine workings. These dikes have a number of macrocrystic resemblances to the dikes seen here. As early as 1927, geologists working in the area had noted a similarity between these lamprophyre dikes and kimberlite (Gledhill 1927). Recent petrographic and mineralogical studies on several dikes in both McMurray and Lendrum townships have suggested that some of the geochemistry falls within the classification of type II kimberlites (orangeites) (Barnett 2001). To date, no diamonds have been recovered from these dikes. These lamprophyre dikes are probably Proterozoic in age and are interpreted to represent Proterozoic alkalic magma emplacement into structures related to the Kapuskasing Structural Zone, perhaps consanguineous with the nearby Firesand Creek carbonatite complex. Rocks from the Firesand Creek carbonatite complex have a U-Pb date of 1078 ± 2.4 Ma (Sage 2000). STOP 8: - Contemplation of the rocks on the fireplace at the Wawa Motor Hotel Acknowledgments The author would like to thank Wayne O’Connor, Northern Sierra Minerals Corporation, Al Shefsky, President, Pele Mountain Resources Inc. and Neil Novak, Vice-President of Exploration, Spider Resources Inc. for permission to access properties described in the field guide. Editorial comments by R. P. Sage also were appreciated during the preparation of the field guide. My thanks also go to Glenn Seim for his technical assistance in the preparation of the guidebook. 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Bibliography Arias, Z. G. 1996. Structural evolution of the central Michipicoten greenstone belt, Superior Province, Wawa, Ontario, Canada; unpublished MSc thesis, Queen’s University, Kingston, Ontario, 140p. Arias, Z. G. and Helmstaedt, H. 1990. Structural evolution of the Michipicoten (Wawa) greenstone belt, Superior Province: Evidence for an Archean fold and thrust belt; in Geoscience Research Grant Program, Summary of Research 1989-1990, Ontario Geological Survey, Miscellaneous Paper 150, p. 107-114. Ayer, J. A., Conceição, R. V., Ketchum, J. W. F., Sage, R. P., Semenyna, L. and Wyman, D. A. 2003. The timing and petrogenesis of diamondiferous lamprophyres in the Michipicoten and Abitibi greenstone belts; in Summary of Field Work and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p. 10-1 to 10-9. Barnett, R. L. 2001. Fletch Kimberlite correspondence, Report of Work, Matchinameigus and Fletch Properties, Dolson & Echum Townships, Sault Ste Marie Mining Division, Northern Ontario; Timmins Resident Geologist’s Office, Dolson Township, assessment file WP–Dolson–4, unpaginated. Cavey, G. 2002. Geological report on the Wawa properties for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 50p. Cavey, G. 2003. Updated geological report on the Wawa property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 35p. Cavey, G. 2004. Summary geological report on the GQ Property for Band-Ore Resources Ltd. Sault Ste Marie Mining Division, Ontario; unpublished NI 43-101 report, 39p. Corfu, F. and Sage R. 1987. A precise U-Pb zircon age for a trondhjemite clast in Doré conglomerate, Wawa, Ontario; in Proceedings and Abstracts, Institute on Lake Superior Geology Annual Meeting, v. 33, p. 18. Corfu, F. and Sage, R. 1992. U-Pb age constraints for deposition of clastic metasedimentary rocks and late-tectonic plutonism, Michipicoten belt, Superior Province; Canadian Journal of Earth Sciences, v. 29, p. 1640-1651. De Stefano, A., Lefebvre, N. and Kopylova, M. 2006. Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada; Contributions to Mineralogy and Petrology, on-line edition, January 5, 2006. Gledhill, T. L. 1927. Michipicoten Gold Area, District of Algoma; Ontario Department of Mines, Annual Report, 1927, v. 36, pt. 2, p. 1-49. Higgins, M. D. 1986. Nodule-bearing spessartite (lamprophyre) dike near Wawa, Northern Ontario; in Program with Abstracts GACMAC-GCU-AGC-AMC-UCG, vol. 11, p. 81. Kaminsky, F. V., Sablukov, S. M., Sablukova, L. I, Shchukin, V. S. and Canil, D. 2002. Kimberlites from the Wawa area; Canadian Journal of Earth Sciences, v. 39, p. 1819-1838. Ketchum, J., Kamo, S. and Davis, D. 2003. U-Pb ages from the Superior and Grenville Provinces of Ontario; unpublished report of the Jack Satterly Geochronology Laboratory, Toronto. Kjarsgaard, B. A., McClenaghan, M. B., Boucher, D. R. and Kivi, K. 2003. Kimberlites and ultrabasic rocks of the Wawa, Chapleau, Kirkland Lake and Lake Timiskaming Areas; in VIIIth International Kimberlite Conference, Northern Ontario Field Trip Guidebook, B. A. Kjarsgaard (ed) p. 1-37. Lefebvre, N. S. 2004. Petrology, volcanology, and diamonds of Archean calc-alkaline lamprophyres, Wawa, Ontario, Canada; unpublished MSc thesis, The University of British Columbia, Vancouver, British Columbia, 265 p. Lefebvre, N., Kopylova, M., Kivi, K. and Barnett, R. 2003. Diamondiferous volcaniclastic debris flows of Wawa, Ontario Canada; long abstract prepared for VIIIth International Kimberlite Conference, Victoria, British Columbia, Canada. Massey, N. W. D.1985. Geology of the Mishewawa Lake Area, District of Algoma; Ontario Geological Survey, Open File Report 5532, 167p. Morris, T. F. 1999. Overburden as a media for kimberlite, base metal and gold exploration, Wawa region, northeastern Ontario; Geological Association of Canada – Mineralogical Association of Canada, Joint Annual Meeting, GAC-MAC Sudbury 1999, Field Trip B6 guidebook, 67p. Morris, T. F. 2001. Quaternary geology of the Wawa Area, Northeastern Ontario; Ontario Geological Survey, Open File Report 6055, 67p. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Sage, R. P. 1993. Geology of Killins, Knicely and Lalibert townships, District of Algoma; Ontario Geological Survey, Open File Report 5589, 141p. Sage, R. P. 1994. Geology of the Michipicoten greenstone belt; Ontario Geological Survey, Open File Report 5888, 592p. Sage, R. P. 2000. The “Sandor” diamond occurrence, Michipicoten greenstone belt, Wawa, Ontario: A preliminary study; Ontario Geological Survey, Open File Report 6016, 49p. Sage, R. P., England, D., Calvert, T., Oudkerk, G., Worona, R. and Kusciusko, R. 1982. Precambrian geology of Musquash Township, Algoma District; Ontario Geological Survey, Map P.2556, scale 1:15 840. Stachel, T., Blackburn, L., Kurszlaukis, S., Barton, E. and Walker, E. C. 2004. Diamonds from the Cristal and Genesis volcanics, Wawa area, Ontario; in Abstracts of Talks & Posters, 32nd Annual Yellowknife Geoscience Forum, 16-18 November 2004, p. 74-75. Stone, D. and Semenyna, L. 2004. Petrography, chemistry and diamond characteristics of heterolithic breccia and lamprophyre dikes at Wawa, Ontario; Ontario Geological Survey, Open File Report 6134, 39p. Stott, G. M., Ayer, J. A., Wilson A. C. and Grabowski, G. P. B. 2002. Are Neoarchean diamond-bearing breccias in the Wawa area related to late-orogenic alkalic and “sanukitoid” intrusions?; in Summary of Field Work and Other Activities 2002, Ontario Geological Survey, Open File Report 6100, p. 9-1 to 9-10. Turek, A., Sage, R. P. and Van Schmus, W. R. 1992. Advances in the U-Pb ziron geochronology of the Michipicoten greenstone belt near Wawa, Ontario; Canadian Journal of Earth Sciences, v. 27, p. 649-656. Vaillancourt, C., Ayer, J. A., Zubowski, S. M. and Kamo, S. L. 2004. Synthesis and timing of Archean geology and diamond-bearing rocks in the Michipicoten Greenstone Belt: Menzies and Musquash townships; in Summary of Field Work and Other Activities 2004, Ontario Geological Survey, Open File Report 6145, p. 6-1 to 6-9. Vaillancourt, C., Ayer, J. A. and Hamilton, M. A. 2005a. Synthesis of Archean geology and diamond-bearing rocks in the Michipicoten greenstone belt: Results from microdiamond extraction and geochronological analyses; in Summary of Field Work and Other Activities 2005, Ontario Geological Survey, Open File Report 6172, p. 8-1 to 8-11. Vaillancourt, C., Dessureau, G. R. and Zubowski, S. M. 2005b. Precambrian geology of Menzies Township; Ontario Geological Survey, Preliminary Map P.3366, scale 1:20 000 Vaillancourt, C., Zubowski, S. M. and Dessureau, G. R. 2005c. Lithogeochemical data and field photographs for the Wawa area: Menzies and Musquash Townships; Ontario Geological Survey, Miscellaneous Release – Data 151. Walker, E. C. 2002. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 41p. Walker, E. C. 2003. Diamond deposits of the Festival Property, Wawa, Ontario; Report prepared for Pele Mountain Resources Inc. under National Instrument 43-101, 39p. Williams, F. 2002. Diamonds in late Archean calc-alkaline lamprophyres Ontario, Canada: Origins and implications; unpublished BSc thesis, University of Sydney, Sydney, Australia, 82 p. Wilson, A. C. 2004. Diamond exploration targets, Michipicoten greenstone belt; Canadian Institute of Mining Bulletin, v. 97, no. 1077, p. 41-46. Wyman, D. A., Ayer, J. A., Conceição, R. V., and Sage, R. P. in press. Mantle processes in an Archean orogen: evidence from 2.67 Ga diamond-bearing lamprophyres and xenoliths, Lithos. 30 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/97cacb27af6036606c9fbddb715df7e4.PDF 878291d60fa2702c5fc3825c929f6be8 PDF Text Text tFu3 iitiitiuitui blflv..a sri .aridi 7 — - IL - ' r4' -4 V 1. - I -4 'I / Gerald Bennett 2006 Field Trip Guidebook, Volume 52, Part 4 Institute on Lake Superior Geology Sault Ste Marie, Ontario — PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Acknowledgements I would like to acknowledge the rocks of the Huronian Supergroup for providing me with so many years stimulating and rewarding work and for teaching me so much, only to eventually show me how much more there was to learn. I would like to thank my wife and children for their love and support during my long absences while a field geologist for the Ontario Geological Survey. Dr. Ron Sage edited an early version of this manuscript and made many useful suggestions. Cover and page layout by Victoria L. Sage, BSc. Cover Photo: Ripple marks in the lower red quartzite member of the Lorrain Formation at Stop S2.2, Highway 17, west of the Town of Desbarats. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents IlSG Field Trip No 1- Day 1 ........................................................................................ 1 Part 1 - Overview of the Huronian Supergroup ....................................................... 1 Introduction ................................................................................................................. 1 The Elliot Lake Group ................................................................................................ 4 The Livingstone Creek Formation ................................................................................ 4 Huronian Magmatism ................................................................................................. 7 Introduction ................................................................................................................... 7 Basement Dikes ............................................................................................................. 7 Layered Gabbro/Anorthosite Complexes ...................................................................... 8 Huronian Volcanic Rocks of the Sudbury Area ............................................................ 8 The Huronian Volcanic Rocks of the Sault Ste. Marie-Elliot Lake area ...................... 8 Sedimentary Rocks Associated with the Thessalon Formation .................................. 14 The Matinenda Formation ........................................................................................... 15 The McKim Formation ................................................................................................ 16 Stratigraphic Relationships within the Elliot Lake Group of the Sault Ste. MarieElliot Lake Area. .................................................................................................... 17 Hough Lake Group .................................................................................................... 17 Introduction ................................................................................................................. 17 Ramsay Lake Formation.............................................................................................. 17 Pecors Formation ......................................................................................................... 18 Mississagi Formation .................................................................................................. 18 Aweres Formation........................................................................................................ 19 Quirke Lake Group ................................................................................................... 19 Bruce Formation .......................................................................................................... 19 Espanola Formation ..................................................................................................... 20 Serpent Formation ....................................................................................................... 20 Cobalt Group ............................................................................................................. 20 Gowganda Formation .................................................................................................. 20 Lorrain Formation ....................................................................................................... 21 Gordon Lake Formation .............................................................................................. 22 Bar River Formation .................................................................................................... 22 Nipissing Intrusions ................................................................................................... 21 Huronian Paleosols and Evidence for the Accumulation of Oxygen in the Huronian Atmospher.............................................................................................23 Regional Tectonic Patterns and Metamorphism .................................................... 25 Tectonic Models for the Development of the Huronian Basin .............................. 29 ILSG Field Trip No. 1 - Day 1 .................................................................................. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �The Elliot Lake Transect ........................................................................................... 29 Stop Descriptions and Road Log .............................................................................. 30 STOP 1.1: McKim Formation and Nipissing Diabase ............................................... 30 STOP 1.2: Radioactive quartz-pebble conglomerate of the Matinenda Formation. .. 30 STOP 1.3: Mississagi Formation, Hough Lake Group. ............................................. 35 STOP 1.4: Nipissing Diabase (Gabbro) altered Mississagi Formation, Bruce Formation. ......................................................................................................................... 35 STOP 1.5: Espanola Formation and Nipissing diabase sills. ..................................... 36 STOP 1.6: Stratified Gowganda Formation. .............................................................. 36 Stop 1.7: Laminated varvite? ..................................................................................... 37 Stop 1.8: Gowganda Formation Serpent disconformity............................................. 38 Stop 1.9: Espanola Limestone Member of the Espanola Formation........................... 38 Stop 1.10: Ramsay Lake Formation overlain by Pecors Formation. .......................... 38 STOP 1.11: Huronian volcanic rocks of the Thessalon Formation (Dollyberry Volcanics). ....................................................................................................................... 38 STOP 1.12. Archean metavolanics with pillow structures. ......................................... 39 STOP 1.13: Bar River Formation. ............................................................................... 40 STOP 1.14: Red beds of the Gordon Lake Formation ................................................ 40 STOP 1.15: Gordon Lake Formation. ......................................................................... 40 STOP 1.16: Lorrain Formation.................................................................................... 40 Highway 556 Transect ............................................................................................... 41 Road Log and stop Descriptions .....................................................................................................40 ILSG Field Trip No. 1 -......................................................................................................................40 Day 2 - Part 1............................................................................................... ...................................40 Huronian Stratigraphy along Highway 556 and correlation with the Chocolay Group of the Marquette area. ............................................................................. 40 STOP 2.1: Island Lake Fault Zone ............................................................................41 STOP 2.2: Gowganda Formation unconformably overlying Conglomerate of the Aweres Formation. ................................................................................................. 45 STOP 2.3: Drop-stones of the Gowganda Formation ............................................... 45 STOP 2.4: Contact between Gowganda Formation and Huronian metabasalt of the Thessalon Formation .............................................................................................. 46 STOP 2.5: Archean granitic rocks and Gowganda Formation. ................................. 46 STOP 2: 6: Jasper Pebble Conglomerate Member of the Lorrain Formation. .......... 46 STOP 2.7: Outcrop of Archean metavolcanic rocks. ................................................ 46 STOP 2.8: Outcrops of Dolostone at the Dolostone Knob. ...................................... 48 STOP 2.9: This stop will be visited as time allows. ................................................... 48 Geological Features and Correlation of Dolostone Unit in Fenwick Township Northeast of Sault Ste Marie ............................................................................... 49 Correlation with the Gordon Lake Formation............................................................. 50 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Correlation with the Chocolay Group of Michigan .................................................... 50 Road Log and Outcrop Descriptions. ...................................................................... 51 STOP S1-1: Lowermost Gordon Lake Formatiom......................................................................51 Supplimentary Road Log for Highway 638 49 STOP 2.9: Dolostone nodules and discontinuous dolostone beds in the Gordon Lake Formation. .............................................................................................................. 52 STOP S1-2: Mudstone overlying matrix-supported conglomerate (diamictite) of the Gowganda Formation. ............................................................................................ 52 STOP S1-3: Dropstones in laminated siltstone and mudstone of the Gowganda Formation. .............................................................................................................. 53 STOP S1-4: Diamictite of the Gowganda Formation. ............................................... 53 STOP S1-5: Jasper pebble conglomerate of the Lorrain Formation (“Pudding Stone”). .............................................................................. 53 Supplimentary Road Log for the area between Sault Ste Marie and Highway 108 ....................................................................................................... 54 STOP S2.1: Sub-Jacobsville unconformity ................................................................ 54 STOP S2.2: Lower red quartzite member of the Lorrain Formation. ......................... 54 STOP S2.3: Purple siltstone member of the Lorrain Formation. ................................ 54 STOP S2.4: Basal Arkose Member of Lorrain Formation. ......................................... 54 STOP S2.5: Bruce Mines Copper Vein. ...................................................................... 54 STOP S2.8: Outcrop of Matinenda Formation on east side of Highway 129. ........... 55 STOP S2.9: Laminated siltstone of the Gowganda Formation with dropstones. ....... 55 STOP S2.10: Rhyolite of the Thessalon Formation. .................................................. 55 STOP S3.1: Mineralized quartz vein breccia. ............................................................ 56 STOP S3.3: McKim Formation - Staurolite (pseudomorph) schist. ........................... 58 STOP S3.2: Pronto Mine Location............................................................................. 58 Publications Cited or Consulted .............................................................................. 59 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �List of Figures Figure 1.1: The Distribution of the Huronian Supergroup in Ontario…………… 1 Figure 1.2: Rock- Time chart for southeast Lake Superior region……………… 2 Figure 1.3: Stratigraphic column in the Elliot Lake area…………………….... 3 Figure 1.4: The cyclicity of Huronian Formations………………………….…… 4 Figure 1.5: Paleocurrent directions in the Huronian Supergroup…………......... 5 Figure 1.6: The distribution of some formations of the Elliot Lake Group…… 6 Figure 1.7: Stratigraphic relationships within the Elliot Lake Group…………… 9 Figure 1.8: Photo of anorthosite of the East Bull Lake Suite, Foul Bight ………… 10 Figure 1.9: Thessalon basalt overlying Livingstone Creek Fm ……………….… 10 Figure 1.10: Internal stratigraphy of the Thessalon Formation………………… 11 . Figure 1.11: Discrimination diagrams for volcanic rocks of the Thessalon Fm…… 13 Figure 1.12: Photo of radioactive quartz-pebble conglomerate and grit overlying the Livingstone Creek Formation……………………………………………… 14 Figure 1.13: Quartz pebble size vs. pyrite size in Matinenda ore beds……..... 15 Figure 1.14: Diamictite of the Bruce Formation, Highway 108…………… 19 Figure 1.15: Concentration ratios for sub-Thessalon, sub-Matinenda and sub-Lorrain Fm. paleosols………………………………………………… 24 Figure 1.16: Geological cross-section of the Blind River and Sudbury-Manitoulin areas. ……………………………………………… 27 Figure 1.17: Metamorphism of the Huronian Supergroup…………………… 28 Figure 1.18: Index maps for geological maps and some areas referred to in the text………………………………………………………… 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 1.19: Geological Map of Quirke Lake Syncline ……………………... 33 Figure 1.20: Legend for Figure1.19 and 1.21……………….......................... 32 Figure 1.21: Geological maps of the Flack Lake area………………………... 34 Figure 1.22: Trough cross-bedding in the Mississagi Fm……………………... 33 Figure 1.23: Metamorphosed Espanola Fm………………………………….... 36 Figure 1.24: Stratified conglomerate of the Gowganda Fm. ……………............ 37 Figure 1.25: Dropstone in laminated siltstone of the Pecors Fm………….......... 39 Figure1.26: Pillow Structure in Archean metavolcanics ……………….......... 40 Figure 2.1: Geological map of the Highway 556 transect. ………………....... 42 Figure 2.2: Stratigraphic column for the Highway 556 transect. ………..…..... 43 Figure 2.3: A diagrammatic cross-section along the Highway 556 transect...... 44 Figure 2.4: Gowganda Formation unconformably overlying Aweres Formation. 44 Figure 2.5: Photo of a “drop-pebble” in laminated Gowganda argillite. ……. 44 Figure 2.6: Polymictic conglomerate of the Aweres Formation……………… 45 Figure 2.7: Dolostone of the Kona Formation intruded by a mafic dike. …… 47 Figure 2.8: Dolostone and chert, Fenwick Township………………………..... 47 Figure 2.9: Dolostone nodules in the Gordon Lake Formation ………………… 47 Figure 2.10: Jasper-pebble conglomerate of the Lorrain Fm…………………… 47 Figure 2.11: Map of Dolostone are in Fenwick Township …………………… 49 Figure 2.12: Correlation of the dolostone occurrences of Ontario and the Marquette area, Michigan ……………………………………………... 50 Figure 2.13: Geology of the Pronto Mine……………………………................. 57 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �1 ILSG Field Trip Number 1 - Day 1 Part 1 - Overview of the Huronian Supergroup These first pages are in tended to give participants new to the Huronian Supergroup of Ontario a summary of what we think we knowof these ancient rocks. Much of the material is borrowed from an earlier ILSG guidebook (Bennett et al , 1997). The important ontributions to that guidebook by K. D. Card and Kirsty Tomlinson are acknowledged with thanks. The Huronian Supergroup is one of the Earth’s most studied sequences of rocks. Since the turn of the century the results of hundreds of studies of Huronian rocks have been published in scientific journals and government publications. These studies have led geoscientists, to present evidence for the Earth’s earliest glacial periods, the development of free oxygen in the atmosphere of the early Earth, the deposition of paleoplacer deposits of uranium, and evidence for plate tectonic activity during the Paleoproterozoic. Much of the evidence presented is based on rock exposures, which will be visited during this field trip. The 2219 Ma radiometric age of the Nipissing intrusions places an upper limit on the age of the Huronian Supergroup, while the Copper Cliff Ryolite (2450 Ma) is probably close to the date of initial Huronian deposition in the Sudbury area. The Huronian Supergroup consists of four groups, which in ascending stratigraphic order are: The Elliot Lake Group, Hough Lake Group, PROTEROZOIC Mid-Late Early Archean Hudson Bay Pr ov inc e Introduction nv i lle e North The Huronian Supergroup, of the James Bay Su Southern Tectonic Province of the peri c or Provin Canadian Shield, is a sequence of Range Paleoproterozoic sedimentary and minor Marquette Supergroup re G volcanic rocks lying unconformably 250 km e c Huronian n i v upon Archean rocks of the Superior o Southern Pr Supergroup Province of the Canadian Shield. The Huronian rocks extend eastward from Lake Superior, along the north shore of Figure 1.1: Distribution of Early Proterozoic Rocks Lake Huron to Sudbury and then in the Great Lakes region. northward to the Noranda area of Quebec, a distance of about 450 km Quirke Lake Group and Cobalt Group (Figure (270 miles) (Figures 1.1, 1.2). The Huronian 1.3). Formations of the three upper groups, with Supergroup attains its greatest thickness of the exception of the Serpent Formation of the 12,000 meters (40,000 feet) southeast of Quirke Lake Group, show stratigraphic Sudbury. The sequence thins northward due to continuity, and display a remarkable cyclicity of the wedging out of basal units, thinning of lithological units. Each cycle begins with clastic units, and erosion within the sequence matrix-supported conglomerate (Roscoe, 1969; Frarey and Roscoe, 1970). (diamictite/mixtite), followed by mudstone, gb 2006 (after Jackson, 2001) PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 G Erosion Erosion LEGEND 200 500 Michigan 600 700 800 900 Erosion 1000 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 Continental glaciation Radiometric age determination Penokean Orogeny Chocolay Group Folding, faulting, metamorphism Erosion Nipissing Intrusions G G Huronian volcanics G Layered gabbro Dike Swarms Kenoran Orogeny Elliot Lake, Ont. Sault Ste. Marie, Ont. Erosion Free oxygen accumulating in the atmosphere 1800 G G Huronian Supergroup Rifting Erosion folding faulting metamorphism methane-rich atmosphere (no oxygen) 1700 Mainly granite Erosion East Lake Superior, Ont. 1600 Michigan 1500 Marquette Range Supergroup 1400 Thousands of years before the present Proterozoic 1300 Mainly gabbro Keweenawan Mid-continent Rifting Supergroup 1100 1200 Volcanic Rocks Elliot Lake, Ont. 400 Sedimentary Rocks Sault Ste Marie, Ont. 300 East Lake Superior, Ont. Michigan Basin Phanerozoic 100 Archean Pleistocene Glaciation 2900 3000 G. Bennett, 1994-2005 Figure 1.2: A rock-time chart for the southeast Lake Superior region. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �3 Nipissing Diabase (2.22Ga) Gabbro, diabase, granophyre Stop Number Quartz arenite, subarkose minor mudstone, siltstone. 300 m 1.14, 1.15 Gordon Lake Fm. 500 m 1.16 Siltstone, mudstone chert, sandstone Cobalt Group Bar River Fm. 1.13 Quartz arenite, quartzpebble conglomerate Arkose, subarkose Lorrain Fm. 2500 m Mudstone, siltstone 1.6, 1.7 Gowganda Fm. Arkose, siltstone 600 m (top missing) Diamictite, polymictic conglomerate Subarkose, wacke 250m 1.5, 1.9 Espanola Fm. 200 m Diamictite, sandstone siltstone Bruce Fm. 1.4 Dolostone , siltstone, wacke limestone 50 m Subarkose, arkose Mississagi Fm. quartz-pebble 1.3 1.10 450m conglomerate Pecors Fm. Mudstone, siltstone 127 m Ramsay Lake Fm. Diamictite Disconformity Hough Lake Group Serpent Fm. Quirke Lake Group Unconformity 1.8 20 m McKim Fm 1.2 Matinenda Fm 0 - 80 m U, Th 1.11 East Bull Lake Suite 2.48 Ga Gabbro anorthosite Arkose, subarkose quartz-pebble conglomerate Disconformity 180 m Thessalon Fm. Disconformity 0 - 100 m? Livingstone Creek Fm. 0 - 10 m? Archean Basement 1.12 Metavolcanics, metasediments, granitic rocks Stop Numbers Mafic volcanics Minor qtz. peb. cong. Elliot Lake Group U, Th U, Th Mudstone, siltstone wacke 1.1 Subarkose, conglomerate Unconformity The thickness of formations was determined from three drill holes drilled near the transect. gb 1996, 2006 Figure 1.3: A Stratigraphic column for the Elliot Lake transect PDF compression, OCR, web-optimization with CVISION's PdfCompressor �4 siltstone or limestone and capped by a thick sequence of coarse, cross-bedded sandstone( Figure 1.5). Many paleocurrent studies have shown paleocurrent flowed south to southeast, with southeast being the predominant direction (Figure 1.5). The Elliot Lake Group The Elliot Lake Group differs from the overlying Huronian groups in that: (1) Its internal stratigraphy is generally discontinuous and less extensive. (2) It does not show the diamictite-mudstone-sandstone sequence of the overlying groups. (3) It contains the only important uranium deposits and the only volcanic rocks of the supergroup. (4) Most formations have disconformable surfaces. The Livingstone Creek Formation Clast-supported, polymictic conglomerate is the prominent rock type in the lower sections of the Livingstone Creek Formation of most areas. Cobble- to boulder-sized clasts generally of grey granitic rocks and minor mafic plutonic and metamorphic rock clasts are set in a sparse matrix of coarse, grey arkose or arkosic grit. The writer has not observed clasts of Huronian volcanic rocks in these conglomerates. Thin units of cross-bedded, grey arkose are locally interbedded with the conglomerate (Frarey, 1977, Bennett et al., 1991). The granitic mega-clasts of the conglomerate member are predominately pale grey in contrast with the predominantly reddish hues of the underlying Archean basement rocks. A few of the granitic Main environment of deposition COBALT GROUP Main rock types Arkose, subarkose, quartz arenite quartz-pebble conglomerate Disconformity Cycle 2 QUIRKE LAKE GROUP Cycle 3 Bar River Formation Gordon Lake Formation Lorrain Formation Gowganda Formation Serpent Formation Espanola Formation Bruce Formation Fluvial Mudstone, siltstone, carbonate (Espanola Fm.) Marine HOUGH LAKE GROUP Diamictite, mudstone, siltstone clast supported conglomerate Mississagi Formation Pecors Formation Ramsay Lake Formation Elliot Lake Group Cycle 1 Reducing Atmosphere inferred from grey-hued sandstones. Oxidizing atmosphere inferred from presence of red sandstones The conglomerates and sandstones of the Livingstone Creek Formation (Frarey, 1967, 1977) form the lowermost Huronian formation. The Livingstone Creek Formation is at least 1200 feet (400 m) thick in the Sault Ste Marie area and between 300 feet (100 m) and 1000 feet (300 m) thick in the Thessalon area. It consists of two distinctive rock types: an upper, well-sorted, grey sandstone and clast-supported polymictic conglomerate (Bennett et al, 1991), (Figures 1.3, 1.6, 1.7). Glaciogenic Volcanic rocks, marine and fluvial deposits. Paleoplacer uranium deposits Note: Where present, the upper part of the Gowganda Formation is predominantly mudstone. Figure 1.4 The cyclicity of Huronian rocks PDF compression, OCR, web-optimization with CVISION's PdfCompressor �km Elliot Lake Paleocurrents - Matinenta Fm. Lake Huron 50 Huronian Supergroup Figure 1.15: Paleocurrents in the Mississagi and Matinenda Formations N Sault Ste Marie 0 Post Huronian rocks LEGEND Paleocurrents - Mississagi Fm. Archean rocks Some modifications made for display purposes. Data from Long, 1977 and McDowell, 1957. Sudbury Igneous Complex Sudbury 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �No subsurface information No subsurface information Murray Fault City of Pecors Lake Elliot Lake Area East Bull Lake Complex No Huronian volcanic rocks (from Drilling Dolleyberry lake area Archean rocks Fault Gabbro, anorthosite East Bull Lake Suite Sandstone , polymictic conglomerate (from outcrop data). Livingstone Creek Fm. The areal extent of some units are exaggerated Crazy Lake Area No Huronian volcanic rocks (from drill hole data) Lake Huron Foul Bight No Huronian volcanic rocks (from drill hole data)) Mafic dikes of Thessalon Fm. intrude Livingstone Creek Fm. East Bull Lake Suite present (From drill hole data) No subsurface information elt eB ak rL No subsurface information Quartz pebble conglomerate at base of Thessalon flows. (locally radioactive) Haughton Tp. area pe Co o Thessalon ) ke ke La La ss en Ba rde rea e a b (A Figure 1.6: The distribution of some formations in the Elliot Lake Group Sault Ste. Marie No subsurface information Duncan Volcanic belt Basalt, andesite, rhyolite etc. (From outcrop and drill hole data) Thessalon Formation Upper Huronian rocks Meso-Proterozoic and Phanerozoic rocks LEGEND 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �7 megaclasts in the predominately grey conglomerate near the south end of Pine Ridge Road show the distinct texture of the typical Archean, massive, pink, k-spar megacrystic granite – but with only a hint of the pink color in the phenocrysts. The grey colour of the Livingstone Creek Formation conglomerates appear to be due to the reduction of ferric iron in the feldspars of the granitic clasts, and not a result of differing provenance as some authors have suggested. This conclusion is supported by the writer’s observation that granitic rocks in a “paleosol zone” a few meters to a few tens of meters below the base of the Livingstone Creek Formation commonly display a grey colour as well. conglomerate member occurs as far east as Pecors Lake (Figure 1.6) on the south limb of the Quirke Lake Syncline (Jensen, 1990) and below Huronian volcanic rocks south of Stinson Lake in the Elliot Lake area. The Livingstone Creek Formation has not been recognized east of the Quirke Lake Syncline. The clast size, local source and low stratigraphic position of the Livingstone Creek conglomerates are consistent with deposition on an alluvial fan. The uniform, fine- to medium-grained sand of the trough cross-bedded sandstone member suggests a different, although probably related, more distal depositional environment than that of the conglomerate. The sandstone member may represent deposition by median streams flowing in a fault-bounded valley with walls of Archean rocks partly covered by alluvial fans (Bennett et al., 1991). The well-sorted nature of the sandstone suggests an aeolian component or even aeolian deposition as proposed by Meyer (1983). The grey, sandstone member of the Livingstone Creek Formation may be distinguished from most Huronian sandstones by the uniform grain size (fine- to medium-sand) of the former. Mudstone and pebbly units are lacking within the sandstone member of the Livingstone Creek Formation. Another characteristic feature of the sandstone units is the presence of carbonate along the foreset beds of the commonly Huronian Magmatism well-developed trough cross-beds. Introduction At Maud Lake in Duncan Township near Sault Ste Marie fine to medium-grained, pale-grey quartz-arenite forms the upper few meters of the Livingstone Creek Formation, suggesting a more prolonged period of weathering in that area. Four distinct more-or-less coeval (ca. 2450 Ma), post-Kenoran igneous rock sequences are associated with the Huronian Supergroup: (1) Mafic dike swarms in the basement rocks but not found cutting the Huronian Supergroup. In addition to the well-known exposures in the (2) Igneous complexes of layered Thessalon and Sault Ste Marie areas the gabbro/anorthosite. (3) Mafic to felsic volcanic Livingstone Creek Formation has been flows. (4) 2.48 Ga. felsic plutons in the Sudbury correlated with grey sandstone and conglomerate area (Krogh et al., 1996). near Crazy Lake in Nicholas Township (Bennett, The widespread intrusion of post-Huronian (Ca 1978; Bottrill, 1971) (Figure 1.6). The writer 2200 Ma) Nipissing gabbro/diabase dikes and correlated a basal grey sandstone unit directly sills have yet to be placed in a plate tectonic underlying the Matinenda Formation in context. Haughton Township (Figure 1.6) with the Livingstone Creek Formation (Bennett et Basement Dikes al.1991). An area of clast-supported, grey granite-cobble conglomerate near Samried Lake The north to northwest trending Matachewan-Hearst dike swarm is the second reported by Jackson (2001) is probably an largest dike swarm of the Canadian Shield. The erosional remnant of the Livingstone Creek 2.45 Ga age of the swarm (Heaman, 1988) is Formation. Other probable remnants of the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �8 essentially the same as that of the East Bull Lake thick), and Stobie Formations (1500 m thick) and the felsic, Copper Cliff Formation (760 m Suite and the Huronian volcanic rocks of the thick) . Krogh et al. (1984) obtained and age of Copper Cliff Formation. 2450+25-10 Ma (U-Pb zircon) age for rhyolite of the Layered Gabbro/Anorthosite Complexes Huronian Copper Cliff Formation; the only Huronian supracrustal rock for which there is an Several petrographically distinctive, mafic to absolute age. The volcanic rocks of the Sudbury ultramafic sills and cone-shaped bodies have been recognized between the eastern end of the area show evidence of submarine eruption from fault controlled vents along the edge of a Sudbury Igneous Complex and the nose of the depositional basin into which arkosic sandstones Quirke Lake Syncline. The intrusions are were being transported from the Archean characterized by the presence of anorthositic phases, and locally by a well-developed, primary granitic terrain to the north, while turbidites rhythmic layering of alternating anorthositic and were deposited from sediment from both the volcanic and rejuvenated basement marginal to gabbroic layers. Segregations, dikes and the basin (Card, 1978). sheet-like bodies of granophyre are locally present (Card and Palonen, 1976; Peck et al., The volcanic rocks of the Sudbury differ in 1995). Major intrusions of gabbro-anorthosite terms of internal stratigraphy, overall thickness are found at Agnew Lake (Card and Palonen, and depositional environment from the 1976) and at East Bull Lake (Born and James, Huronian volcanic rocks in the Sault Ste 1978; Kamineni et al., 1984) Marie-Elliot Lake area. Krogh et al. (1984) reported a U-Pb age The Huronian Volcanic Rocks of the Sault 2491+5-5 Ma for zircons from the Ste Marie-Elliot Lake area gabbro-anorthosite intrusion at Agnew Lake and 2480+10-5 for the East Bull Lake Frarey (1967) named the Huronian volcanic gabbro-anorthosite body. The similarity of these rocks in the Thessalon and Sault Ste Marie ages to the 2450+25-10 Ma (U-Pb zircon) age areas that overlie the Livingstone Creek of rhyolite of the Huronian Copper Cliff Formation as the “Thessalon Formation” Formation (Krogh et al., 1984) suggests that (Figures 1.3, 1.6). these gabbro-anorthosite intrusions form part of This writer has examined all known exposures of the early Huronian volcanic events. Bennett et Huronian volcanic rocks as well as all available al. (1991) report two additional bodies between drill-core and drill-hole logs reporting volcanic Blind River and Thessalon. Layered anorthosite rocks in the Elliot Lake–Sault Ste Marie area. It and associated coarse-grained gabbroic rocks was concluded that there is no credible evidence form a sill-like body about 5 km long at Foul for more than one period of Huronian volcanism Bight on the North Shore of Lake Huron in the area of the study and that all the (Figure 1.8). A drill-hole drilled from the ice Huronian volcanic rocks west of the nose of the south of the Town of Thessalon intersected Quirke Lake Syncline are stratigraphically layered gabbroic rocks (Figure 1.6). All correlative with the Thessalon Formation gabbro-anorthosite bodies found to date have (Figure 1.6)(Bennett 1978, Bennett et al 1991). been emplaced in the Archean basement at, or just below, the Archean-Huronian boundary. Unfortunately, none of the many attempts to obtain an absolute age determination from rocks Huronian Volcanic Rocks of the Sudbury of the Thessalon Formation have been Area successful. However, the spatial association of The Huronian bimodal volcanic sequence in the the East Bull Lake intrusions with the Huronian volcanic rocks of the Quirke Lake Syncline Sudbury area has been subdivided in the suggests Huronian volcanism in that area predominately mafic Elsie Mountain (1000 m PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Th,U,py THESSALON AREA py HAUGHTON-OTTER TWPS Feeder dike of Thessalon Fm. Th, U, py Fault U,Th, py U,Th,py ELLIOT LAKE AREA Unconformity or disconformity Quartz-pebble conglomerate Granite, greenstone ARCHEAN BASEMENT Grey arkose, subarkose Polymictic conglomerate LIVINGSTONE CREEK FM. Basalt, andesite, rhyolite flows basalt dike THESSALON FM. CRAZY LAKE AREA NICHOLAS TWP. Paleosol on Livingstone Cr. Fm and basaltic dike (Thessalon Fm.) The thickness of some units is exaggerated purposes of visualization Figure 1.7: Stratigraphic relationships in the Elliot Lake Group. METERS SAULT STE MARIE AREA 1000 LEGEND Arkose, grit quartz-pebble conglomerate MATINENDA FM. ELLIOT LAKE GROUP HOUGH LAKE GROUP 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10 - -4 JjJ622910LJ EW Figure 1.8: Layered anorthositic gabbro. Foul Bight, Lake Huron. Figure 1.9: Stratigraphic relationships at Cullis Lake, Thessalon area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �11 Thessalon Basalt Upper Island Lake Bass Lake 500 m 1200 m 650-850 m Unit6 Unit 1 Unit 6 Unit 6 Rhyolite Unit 5 Basalt to andesite cycles Mg-lava Radioactive, Quartz-pebble conglomerate Livingstone Creek Fm. Unit 4 Unit 3 Unit 3 Unit 2 Unit 1 Unit 1 Livingstone Creek Fm. I'll Unit 7 ? Livingstone Creek Fm. From Tomllinson, 1996 Figure 1.10: Internal Stratigraphy of the Thessalon Formation in the Sault Ste Marie Area occurred about the same time as the emplacement of the East Bull Lake intrusions (2480 Ma). Given the potential error for the relevant age dates (Krogh et al., 1984), the age of the Thessalon Formation may not differ significantly from that of the Copper Cliff Formation of the Sudbury area (2450 Ma). The maximum thickness of the Thessalon Formation in the Sault Ste Marie area is approximately 650 m (2100 feet) and 820 m (2700 feet) (Frarey, 1977). Diamond drilling has indicated at least 670 m (2200 ft) of Thessalon volcanics under Lake Huron south of the town of Thessalon. The Thessalon Formation may be as much as 1080 m (3500 feet) thick north of Bass Lake in Aberdeen Township. in the Dollyberry Lake, Pecors Lake and Thessalon areas. The lower flow sequences show much lower concentrations of Ni, Cr, and contain higher amounts of Ti and P than the upper flows. In the Dollyberry Lake area the upper basalts appear to be missing. This is assumed to be due to erosion (unpublished analyses in the writers files). Tomlinson (1996), completed a comprehensive analysis the geochemistry of the Huronian volcanic rocks between Sault Ste Marie and Thessalon, concluded that the lavas of the Thessalon Formation are divisible into 7 distinct units based on mapping, petrography and major and trace element geochemistry (Figures 1.10, 1.11). The 7 units were grouped into two “lava In the Sault Ste Marie, Thessalon and Aberdeen series”. The upper lava series (units 6) of Lake areas the Huronian volcanics rocks can be Tomlinson, 1996) is equivalent to the upper tholeiitic basalt sequence of Bennett et al. subdivided into upper tholeiitic basalt unit and lower complex or “mixed member”(Bennett et al (1991). The lower lava series (units 1-5, of Tomlinson, 1996) consists mainly of basaltic 1991) which includes more fractionated rocks andesite with subordinate, local rhyolite, including basaltic andesite, tholeiitic andesite, mugearite, andesite and high magnesium basalt mugearite, hawaiite, rhyolite, basalt. flows; and corresponds to the “diverse member” Magnesium-rich basalts with some of the chemical characteristics of komatiites are present of Bennett et al. (1991). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �12 Bennett et al. (1991) proposed that the upper, basaltic flows of the Thessalon Formation (upper lava series) probably represent part of a continental flood basalt sequence, while the diverse member (lower lava series) appears to have erupted from central vents. The Huronian volcanic rocks of the Quirke Lake Syncline display lithological and geochemical similarities to the lower lava series of the Thessalon Formation west of the Quirke Lake Syncline (G. Bennett, unpublished data). The upper, tholeiitic basalt flows of the Thessalon Formation (upper lava series of Tomlinson, 1996) are almost uniformly greenish-grey fine- to medium-grained tholeiitic metabasalt. The essential minerals are albite, actinolite, chlorite, clinozoisite, epidote and Fe-Ti oxides. Primary clinopyroxene is present in only a few samples of basalt from the Sault Ste Marie area. The andesitic rocks of the lower lava series of are generally darker coloured and commonly contain stilpnomelane and biotite with green pleochroism (Fe+3 rich?) in addition to albite and actinolite (Bennett et al. 1991). Tomlinson (1996) reported the presence of tremolite in some flows. Quartz is a minor component of basaltic and andesitic types. In most areas the metamorphic grade of the Thessalon Formation flows is lower greenschist facies although the presence of albite and primary pyroxene along with elevated soda indicates sub-greenschist grades at the northern end of the Duncan volcanic belt near Sault Ste Marie. “The field and geochemical data indicate that the lavas are comparable to modern day continental basalt and andesites but that their source was similar to that of island arc basalt. Figure 1.11 shows the lavas plotted on a variety of discrimination diagrams where they are most comparable to modern within-plate basalt. Their arc-basalt geochemical characteristics result from their derivation from an upper mantle that had been metasomatically enriched during subduction in the Kenoran orogeny. Units within the sequence were generated from different pulses of magma, each of which stemmed from a different region of the relatively homogenous Huronian mantle. Only the unit 2 lavas require generation from part of the mantle with slightly different characteristics to a source similar to that required to generate island arc basalt ID16 (a primitive basalt used by Tomlinson (1996) in making calculations. gb). Different degrees of partial melting of this source that contains garnet in the residue (at less than 29.8% partial melting) are responsible for the REE characteristics of each unit. In the generation of the lower lava series, different batches of magma underwent crustal contamination to a greater or lesser extent within the lower or upper crust (or both) prior to eruption. The upper lavas series rocks are not contaminated. In all but the unit 2 lavas crystal fractionation occurred both early, as magma ascended to shallower depths, and later, in shallow level sills or magma chambers. Early fractional crystallization was responsible for the low Mg, Ni and Cr values in most units, while later stage crystal fractionation was responsible for the major element and compatible element trends within single units, as shown on bi-element plots”(Tomlinson, 1996). Amygdules of epidote, chlorite, calcite, quartz and stilpnomelane, in complex zonal arrangements, are common. Flattened chlorite-filled amygdules a centimeter or less across is a distinctive feature of most mafic flows of the Thessalon Formation. Pillow structures are rare but were observed in most areas. Scoriaceous flow-tops and crosscutting breccias On the geochemistry and tectonic setting of the are commonly filled with a fine-grained mixture Thessalon volcanic rocks Tomlinson (1996) states: of quartz and grey to red secondary albite. Tomlinson (1996) states: PDF compression, OCR, web-optimization with CVISION's PdfCompressor �13 V - M!1PIU B- pA .100 JO kiP S .1 - MbV : VI VII 3- E 2- Mbk: VII C ki AVB: CD - B D A Figure 1.11: Discrimination diagrams for the Thessalon Formation in the Sault Ste Marie area. From Tomlinson, 1996. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �14 “Various lines of evidence from the Elliot Lake Group as a whole can be used to characterize the Huronian rifting event. Active rifting involves deep mantle upwelling or plume magmatism, which splits the continental crust, whereas passive rifting involves mantle diapirism and adiabatic upwelling induced by differential stresses and stretching in the lithosphere....…..The geochemical evidence from the Thessalon Volcanic Formation indicates that the source of the lavas was metasomatised upper mantle rather than a deep mantle or plume component. Structural subsidence patterns in the Archean basement (Zolnai et al., 1984) are thought to be responsible for lithospheric stretching, in-turn causing mantle upwelling, episodic partial melting and volcanism. This is supported by the presence of syndepositional faults in up to Archean basement. These features indicate that initially volcanism was a consequence of rifting. In active rifts just one uplift and melting event occurs as a plume impacts the lithosphere, but in passive rifts uplift and melting are episodic ........... Multiple erosional surfaces within the Elliot Lake Group indicate that numerous episodes of uplift occurred. Based on these lines of evidence the Huronian rifting event can best be characterized as a typical passive rifting event” (Tomlinson, 1996). Jolly (1987) concluded the Thessalon Formation is a continental flood basalt sequence associated with continental rifting. Sedimentary Rocks Associated with the Thessalon Formation Some early reports refer to the presence quartz-pebble conglomerate within the Livingstone Creek Formation. The writer has examined most of the known occurrences of the Livingstone Creek Formation between Sault Ste Marie and the Quirke Lake Syncline and found no interbedded quartz –pebble conglomerate. However at many locations a thin unit (< 1 m) of radioactive, pyritic, quartz-pebble conglomerate overlain by a few meters of coarse, arkose was found to lie upon the Archean Figure 1.12: Sandstone of the Livingstone Creek Fm. Overlain by basement or directly upon radioactive quartz-pebble conglomerate and grit of the Thessalon the Livingstone Creek Fm. Thessalon Township. Formation where the latter is present. In the absence of the Livingstone Creek Formation the 400 meters of Huronian sedimentary rocks quartz-pebble conglomerate lies directly upon the below the volcanics. G. Bennett (personal basement rocks. In Duncan Township of the communication, 1996) has also described Sault Ste Marie and Thessalon areas this syndepositional features (at 2 locations near quartz-pebble conglomerate-arkose sequence is Elliot Lake and Sault Ste Marie) where found within the lower flows of the Thessalon sedimentary rocks of the Livingstone Creek Formation. (Hay, 1963; Bennett et al., 1978; Formation infill fractures in the underlying Meyer, 1983) (Figures 1.6, 1.7, 1.9, 1.12). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �15 The Matinenda Formation Size of pyrite grains (mm) Bennett et al. (1991) proposed that the conglomerate-arkose units indicate the presence of a disconformity between the Huronian volcanics of the Thessalon Formation and the Livingstone Creek Formation. The wide distribution of these units (Figure 1.6) suggests that they reflect early Huronian erosional period of regional extent. The resistate nature of the mineral assemblage in the quartz-pebble conglomerate (assuming an oxygen deficient atmosphere) points to a period of extreme weathering. Some of this 2.4 quartz-rich regolith may still be visible as a quartz breccia upon the granitic basement west of 1.8 Highway 639 (Stop 1-11of this guide). mudstones of the McKim Formation (Card, 1978). The most abundant rock type of the Matinenda Formation in the Elliot Lake area is generally described as medium- to coarse-grained subarkoses, arkoses and grits consisting of poorly sorted quartz and feldspar grains set in a matrix of sericite and comminuted rock and mineral Correlation Coefficient = 0.93 1.2 The Matinenda Formation of 0.6 the Elliot Lake Group is a sequence of arenites and From Theis, 1976 intercalated quartz-pebble conglomerates which host the 7 14 21 28 35 42 49 56 63 once important uranium Pebble size (mm) deposits of the Elliot Lake Figure 1.13: Quartz pebble size vs. pyrite grain size area. The Matinenda in the ore beds of the Matinenda Formation. Formation is up to 180 m (600 feet) in the Elliot Lake area where it lies on Huronian volcanic rocks and fragments. The ratio of K-spar to sodic feldspar Archean basement (Roscoe, 1969; Robertson, is about 8:1. Minor constituents are pyrite, 1968,1976). In the Thessalon and Sault Ste calcite, chlorite, zircon and rarely, leucoxene Marie areas the Matinenda Formation consists coated iron oxides and monazite. Variable predominantly of fine-to medium-grained, amounts of sericite produce what has various subarkose to subwacke and is probably less than been described as a green, apple green or 50 m (150 ft) thick (Bennett et al., 1978). In greenish-yellow colour. Well-sorted Haughton Township the Matinenda Formation quartz-pebble conglomerate beds, containing lies upon grey sandstones that the writer equates well-rounded pebbles and cobbles of quartz and with the Livingstone Creek Formation (Figure chert, and pebbly subarkose units are scattered 1.7). throughout the coarse subarkose units of the In the Sudbury area clastic units correlated with Matinenda Formation, but are more common the Matinenda Formation are as much as 600 m near the base. (Robertson, 1968; Pienaar, 1963). (2000 ft) thick but they thin rapidly eastward and is intercalated with the mainly metavolcanic Trough cross-bedding, scour and fill structures rocks of the Stobie Formation and the are common in arkosic units (Robertson, 1968; Roscoe, 1969). Paleocurrent studies by PDF compression, OCR, web-optimization with CVISION's PdfCompressor �16 McDowell (1957) and Long (1977) have established a northwest source area for the sediment of the Matinenda Formation (Figure 1.5). Fralick and Miall (1989) suggest the Matinenda Formation was deposited from shallow braided streams flowing down a south dipping paleoslope which underwent tilting to the southeast during deposition. Kimberly et al. (1980) reported that the uraniferous conglomerates contained almost no magnetite-ilmenite and had very high potash/soda ratios. These are also features of Huronian paleosols, and suggest the sediment of the Matinenda Formation was formed by the intense weathering of a granitic source terrain as proposed by Roscoe (1969). Two southeast trending ore zones were recognized since the early days of uranium mining in the Elliot Lake camp. The Nordic zone, east of the City of Elliot Lake is about 1 mile (1.6 km) wide and 3.6 miles (5.6 km) long. The Quirke Zone, in the Quirke Lake area, is about two miles (3.2 km) wide and six miles (9 km) long. Paleotopographic features of the basement are thought to have had a determining influence on the position and orientation of the zones. Ore grade (circa 0.1 % U3O8) conglomerate occurs as persistent lenses with individual units up to 15 feet (4.5 m) thick. The uraniferous quartz-pebble conglomerates are commonly well developed at the base of the Matinenda Formation but also occur within the arkose up to 150 feet (45 m) above the base (Roscoe, 1969, Robertson, 1968). modified paleoplacer origin of the ores as outlined by Roscoe (1969). Advocates of this hypothesis propose that prior to the accumulation of significant free oxygen in the Earth’s atmosphere, southeastward flowing streams carried quartz, pyrite and uraniferous minerals released by the extensive weathering of the Archean granitic terrain, and deposited them as southeast trending units determined by basement topography. Evidence supporting the paleoplacer hypothesis was provided by Theis (1979) who demonstrated that a direct relationship exists between the size of quartz pebbles and pyrite grains (Figure 1.13), and the concentration of many other components of the ore zones. The McKim Formation The McKim Formation is the uppermost formation of the Elliot Lake Group. It is mainly a turbidite sequence of generally dark grey, subarkosic wackes, mudstones, subarkoses, lithwacke and litharenite that extends from the Blind River area to the Grenville Front. Graded beds, parallel laminations, ripple marks, ripple-drift cross-laminations and Bouma cycles, indicative of deposition by submarine turbidity currents are reported (Parvianen, 1973; Card et al., 1977). Robertson (1968) gives a thickness of 0-380 feet (0-100 m) for the McKim Formation on the south limb of the Quirke Lake syncline. It is missing on the north limb. The McKim Formation is thickest in the Sudbury area, where The quartz-pebble conglomerate consists mainly it is up to 2400 meters (8000 feet) thick. of well rounded pale-grey to dark-grey quartz The Murray Fault appears to have exerted an and chert pebbles set in matrix of pyrite with important influence on the deposition of the some quartz/feldspar grit and sericite. Pyrite McKim sediments. North of the Murray Fault commonly forms from about 15% of minable the McKim rarely exceeds a few hundred meters units. Radioactive minerals include uraninite, in thickness, whereas south of the fault the brannerite, and uranothorite (Roscoe, 1968). thickness of the McKim Formations is at least Monazite and zircon are characteristic heavy 2400 meters. Card (1978) noted the change minerals. from laminated siltstone in the west to more The sedimentological and mineralogical features wacke in the east indicated a change from more of the uranium bearing zones of the Elliot Lake distal to proximal facies, in turn suggesting more camp are now generally believed to support a tectonic activity and possibly a source for the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �17 McKim sediments in the east. Fralick and Miall (1989) concluded that the McKim Formation of the Elliot Lake area represents a marine transgression that gradually drowned the Matinenda fluvial plain. extended to the Quirke Lake Syncline as well. The outcrop pattern of the Huronian volcanic rocks on geological maps also suggests that the volcanic rocks are erosional remnants preserved in basement depressions. Stratigraphic Relationships within the Elliot The Thessalon Formation may once have extended beyond the limit suggested from its Lake Group of the Sault Ste Marie-Elliot present outcrop distribution. Its present Lake Area. The stratigraphic relationship between the Matinenda Formation, Thessalon Formation and Livingstone Creek Formation is revealed on a rock face near the northern boundary of Haughton Township about 30 km (18 miles) north of the town of Thessalon (Figures 1.6, 1.7). At this location pyritic quartz-pebble conglomerate of the Matinenda Formation (Chandler, 1976) directly overlies an apple-green paleosol on grey, fine-grained sandstone, which the writer correlated with the Livingstone Creek Formation (Bennett et al.1990). About 600 meters (380 feet) northwest of the above occurrence arkose and quartz-pebble conglomerate of the Matinenda Formation disconformably overlies a steeply dipping, east-striking, mafic dike; the upper few meters of which is a sericite-leucoxene paleosol. The dike intrudes grey sandstone and apple-green paleosol of the Livingstone Creek Formation (Bennett et al. 1990, Bennett et al, 1991; Sutton and Maynard, 1993). Less than 2 km south of the above location Chandler (1976) identified a fault-bounded block of Thessalon Formation volcanic rocks with a minimum thickness of approximately 500 m. The mafic dike referred to above was a feeder for Thessalon flows, since the Thessalon Formation is the only known igneous activity at this stratigraphic position The above observations show clearly and unequivocally that there was a period of volcanic activity, as well as a significant period of erosion, separating the Matinenda Formation and the Livingstone Creek Formation. Since paleosols are well known upon Huronian flows in the Elliot Lake area, the sub-Matinenda unconformity seen in Haughton Township distribution may be but erosional remnants of a once extensive continental flood basalt sequence. This is to be expected on consideration of Macdougall’s (1988) statement “Since many CFB provinces have been uplifted and occur as elevated plateaus, dissection and removal is rapid”. He notes as well that the general scarcity of pre-Cretaceous Continental flood basalt provinces is probably due to their erosion. Hough Lake Group Introduction The Hough Lake Group (Robertson, et al., 1969; Roscoe, 1969) is lowest of the three Huronian groups that display the cyclic deposition of diamictite followed by a mudstone, siltstone, and turbidite or carbonate sequence; and overlain by a cross-bedded arenite unit. Each cycle is generally thought to represent a sequence of glaciogenic - marine - fluvial and/or shallow marine deposition (Figure 1.4). Ramsay Lake Formation The Ramsay Lake Formation is the lowermost formation of the Hough Lake Group and is the oldest of three such conglomerate units that define the base of Hough Lake, Quirke Lake and Cobalt Groups (Roscoe, 1969; Pienaar, 1963) (Figures 1.3, 1.4). The Ramsay Lake Formation is a widespread, relatively thin unit, which is up to 41 m thick in the Bass Lake area (Chandler, 1973). In the Elliot Lake area the thickness of the Ramsay Lake Formation is from 0 to just over 30 m. (Diamond drill logs Assessment files, Sault Ste Marie District Geologist’s Office). The Ramsay PDF compression, OCR, web-optimization with CVISION's PdfCompressor �18 Lake Formation is from 70 to 170 m thick in the common (Card, 1978; Robertson, 1976). The Pecors Formation is the result of transgressive Sudbury-Manitoulin area (Card 1978). units formed in deep water by turbidity currents Matrix supported polymictic conglomerate (Card, 1978). The presence of drop stones is (diamictite) is the most abundant rock type in evidence of a cold paleoclimate, and provides the Ramsay Lake Formation, especially near the supporting evidence for the glaciogenic origin of base. The lowermost few meters of the the underlying Ramsay Lake Formation. formation usually reflects the underlying rock type (Robertson, 1968; Parviainen, 1973). Mississagi Formation Minor amounts of mudstone, wacke and arenite The Mississagi Formation is a thick sequence of are locally present. predominantly grey, arenitic rocks extending Subrounded to well rounded pebbles and throughout most of the length of the Huronian cobbles of grey granitic rocks and angular to outcrop belt. Within the Quirke Lake syncline rounded clasts of very dark green to black the Mississagi Formation is from 344 to 704 m volcanic rocks generally form less than 30 thick. South of the Murray Fault the formation percent of the total volume of the diamictite. is notably thicker, being more than 3000 m in The dark matrix consists of quartz, feldspars, the Sudbury area (Card, 1978; Long, 1978). By chlorite, muscovite-sericite-illite and pyrite far the dominant rock type in the Mississagi (Parvianen, 1973). Formation is moderately well sorted, medium to coarse-grained subarkose and arkose. Small to Although some writers have argued for a debris medium quartz/chert pebble conglomerate is a flow origin, most writers now accept the Ramsay minor component of the formation, but is more Lake Formation as having a significant common in the western and northeastern parts glaciogenic component (Roscoe, 1969; of the Huronian belt. Fine-grained pyrite along Robertson, 1976). Fralick and Maill (1989) forsets commonly results in rusty staining of identified an ice proximal association of pebbly outcrops. Greenish, sericitic units form sandstone and diamictite; subaqueous gravity relatively thin planar-bedded units between flows and ice rainout deposits; and ice-proximal, cross-bedded sandstones. Palonen (1973) fluvial outwash deposits. provided evidence supporting a marine origin for the Mississagi Formation; however, Long (1978) Pecors Formation argued that the abundance of mud-grade matrix The Ramsay Lake Formation is conformably in the immature arenites, and the predominance overlain by a sequence of generally dark, bedded of unimodal paleocurrent directions, and the and laminated wackes, mudstone, siltstones and lack of quartz arenites argued against a marine sandstones of the Pecors Formation (Roscoe, environment for the Mississagi Formation. He 1969). The Pecors Formation is 30 m thick at concluded that the Mississagi Formation was Quirke Lake (Robertson, 1968) but is as much deposited from braided streams with low to as 900 m thick south of the Murray Fault in the intermediate sinuosity and high width to depth Sudbury area (Card, 1978). It was not identified ratios. in the area between Thessalon and Sault Ste Bedding units are commonly about a meter thick Marie (Frarey, 1977). Ripple marks, graded bedding, cross-laminations parallel laminations, but range from a few centimeters to over four meters. Trough cross-stratification and ripple ball and pillow structures, clastic dikes and cross-stratification are common sedimentary slumpage features have been reported in the structures (Long, 1978). Individual formation. The basal part of the formation is commonly laminated resembling varves and, in cross-stratified beds may show grain size gradation (McDowell, 1957). places, has dropstones (Robertson, 1968; Parvianen, 1973). Partial Bouma sequences are PDF compression, OCR, web-optimization with CVISION's PdfCompressor �19 Long (1978) measured over 2500 cross-stratified units in the Mississagi Formation (Figure 1.5). Two major stream systems were recognized: a stream system flowing southeast to easterly from the Sault Ste Marie area joined a stream system flowing southwestward from the Cobalt Plain to form southward flowing system southwest of the Sudbury area. These observations suggest that the area now occupied by the Sudbury Igneous Complex was elevated during the period of Mississagi deposition(Long, 1978). Aweres Formation fan system that extended in a more or less northeast direction beyond the present northern limit of the groups below the Cobalt Group. The Mississagi Formation may represent a more distal depositional environment than that of the Aweres Formation. Quirke Lake Group Bruce Formation The Bruce Formation extends from the Garden River Indian Reserve near Sault Ste Marie to about 70 km northeast of Sudbury. It consists mainly of matrix supported and minor clast supported conglomerate. Pebbly wacke, arkose, wacke and siltstone are locally present. Robertson (1968) reports the Bruce Formation is from 79 m (260 feet) to 12 m (40 feet) thick in the Elliot Lake area. It is 26 m (85 feet) to 37 m (120 feet) thick under the greater part of the Quirke Lake Figure 1.14: Diamictite of the Bruce Fm. Syncline. Highway 108, Elliot Lake area. In the Sault Ste Marie area, the 1700 m thick sequence of conglomerate and sandstone of the Aweres Formation (McConnell ,1927) unconformably overlies the mafic volcanics of the Thessalon Formation (Figures 2.1, 2.2). The internal stratigraphy and lithologies of the Aweres Formation is consistent with deposition in an alluvial fan environment. The base of the formation is almost entirely of mafic volcanic clasts while higher levels show a progressive increase in granitic components. The uppermost rocks of the Aweres Formation south of Aweres Lake are mainly arkose with thin pebble conglomerate beds. The lithological variation with stratigraphic height indicates the continual erosion of an uplifted fault-bounded plateau of Huronian volcanic rocks. Pebble- to boulder-sized, angular to subrounded clasts generally consist of pale-grey granitic rocks, Archean supracrustal rocks and fine-grained mafic clasts. The upper parts of the formation may contain up to 5% carbonate (Robertson, 1968). The Bruce Formation is generally interpreted as a tillite with minor beds and lenses of glacially derived sandstone. Dropstones have been observed in laminated units (Robertson, 1968). Casshyap (1969) concluded the formation was The distinct lithology of the Aweres Formation deposited from terrestrial wet-base glaciers. prevents its direct correlation with other However Sims et al. (1981) proposed that the Huronian rocks. The upper surface is partly fault Bruce Formation is an accumulation of debris bounded but is unconformably overlain by the flows released as a result of normal faulting, a Gowganda Formation on Highway 556 (Figure sudden increase in paleoslope and a sudden 2.3). It is possible that the Aweres Formation is increase in water depth. an erosional remnant of a once extensive alluvial PDF compression, OCR, web-optimization with CVISION's PdfCompressor �20 Espanola Formation The Espanola Formation is the only widespread carbonate unit of the Huronian Supergroup. It is a present from the Sault Ste Marie area to the Maple Mountain area about 70 km northeast of Sudbury. Its widespread distribution and distinctive lithology make it the most useful stratigraphic marker unit of the Huronian Supergroup. In the Elliot Lake area the Espanola Formation can be subdivided into three parts: a lower limestone member (the Bruce Limestone), a middle siltstone, wake, arenite member (the Espanola Greywacke) and an upper dolomite member (The Espanola Limestone) (Robertson, 1968). The latter generally contains 3 - 4% total iron which gives a distinctly brownish colour to weathered surfaces. All 3 members are generally thinly bedded to laminated. The threefold subdivision is not so well developed south of the Murray Fault (Young, 1982). Contacts between members tend to be gradational. Intraformational breccias, mud cracks, ripple-marks, flame structures and ball-and-pillow structures are common sedimentary features. Hofmann et al. (1980) described stromatolites in the Espanola Formation on Quirke Lake. These features suggest deposition in a quiet shallow water environment with carbonate deposition being interrupted by influx of fine-grained sediment. Young (1973) proposed that the relatively sharp change from the diamictites of the Bruce Formation suggests warm climatic conditions following a glacial advance. However, the recognition of carbonate deposition at high latitudes (Williams, 1975) and the association of detrital uranium with intense chemical weathering (Maynard et al, 1991) adds uncertainty to attempts to model Huronian paleoclimate. activity preceding deposition of the Gowganda Formation of the Cobalt Group. Reported thickness estimates of the Serpent Formation range from 150 up to 1500 m (Bennett et al., 1991). Robertson (1968) states that nowhere in the Blind River-Elliot Lake area is there any evidence that the total thickness of the Serpent Formation has been preserved. The Serpent Formation is mainly fine to medium-grained, quartz arenite and arkose. Conglomeratic units have been noted, especially near the base of the Formation. Carbonate is a significant component near the base of the formation in the Elliot Lake area (Robertson, 1968). Planar cross-bedding, festoon cross-bedding, rip-up clasts, fine-laminations and mud cracks have been reported. Long (1976) proposed that the Serpent Formation was deposited in a distal braided stream environment with calcareous units representing a sabkha environment. Young (1982) noted that the presence of very large cross-beds and a bi-modal size distribution suggest aeolian processes may have been active at least locally. Cobalt Group Gowganda Formation The Gowganda Formation is a complex sequence of conglomerates, sandstones, siltstones and mudstones, which comprise the lowermost formation of the Cobalt Group. Its thickness ranges from 1070 m in the Sault Ste Marie area; from 970 to 1150 m around Whitefish Falls, on the North Shore of Lake Huron; and from 950 to 2700 m near Sudbury. Near Dunlop Lake, in the Elliot Lake area, the Gowganda Formation is about 600 m thick. Matrix-supported conglomerates are common, especially in the lower parts of the formation. However, these are commonly intercalated with clast-supported conglomerates, sandstone, Serpent Formation wacke units. Laminated mudstones and siltstone are especially prominent in the upper parts of The Serpent Formation is found throughout the Gowganda Formation. Many occurrences of much of the Huronian belt; however, it is locally ice-rafted drop-stones have been reported in removed by erosion during a period of tectonic laminated mudstone/siltstone units. Individual PDF compression, OCR, web-optimization with CVISION's PdfCompressor �21 units are generally relatively thin and discontinuous making subdivision of the Gowganda formation difficult except in well-exposed areas. Most granitic clasts of Gowganda conglomerates have a distinctly pinkish or reddish hue, in comparison to the grey, granitic clasts in the matrix-supported conglomerates of the stratigraphically inferior Ramsay Lake and Bruce Formations. Also, pink and red hued sandstones first make their appearance in the Gowganda Formation. Roscoe (1969) pointed out the appearance of red coloration (i.e. ferric iron) just above the basal units if the Gowganda Formation, and argued that it represents the appearance of free oxygen in the Earth’s atmosphere, and a change from the previously reducing atmospheric conditions that allowed the accumulation of readily oxidized minerals such as pyrite and uraninite in a fluvial environment. Lorrain Formation The Lorrain Formation is generally well exposed throughout most of the Huronian belt, where it commonly forms a background to some of the most scenic views in northern Ontario. It is overwhelmingly an arenitic sequence, with local siltstone units present in lower parts of the section. The formation is up to 2500 m thick near Sault Ste Marie and in the LaCloche Syncline, southwest of Sudbury. It is up to 2300 m thick in the Cobalt Embayment. In general, the lower part of the Lorrain Formation is dominated by pink, arkosic sandstones; the middle by hematite-rich subarkose and quartz-arenite; and the upper portion by pale grey to white mature, quartz-arenite. A distinctive jasper-pebble conglomerate found in the Sault Ste Marie area is a popular decorative stone, known locally as “pudding stone”. The presence of aluminous minerals is a characteristic feature of the upper, quartz-arenites of the Lorrain Formation. Diaspore and kaolinite The depositional environment of the are common in the quartz-arenite of the Sault Ste diamictites in the Gowganda Formation have Marie area and north of Elliot Lake (Wood, 1973) been the subject of discussion since Coleman while kyanite, andalusite and kaolinite are present (1905) proposed a glacial origin for the matrix as metamorphosed equivalents in the LaCloche supported conglomerates. Many later writers Lake-Kilarney area (Card, 1978). Young (1973) including Ovenshine (1965); Casshyap (1969); and Wood (1973) interpret the presence of Lindsay (1971); Young and Nesbitt (1985), diaspore and kaolinite as the result of and others have also supported a glacial, post-depositional in-situ alteration of feldspar glacial-marine or glaciolacustrine origin for the under hot and humid climatic conditions. Gowganda Formation. The presence of abundant detrital hematite in the Card (1968) concluded that, although Lorrain Formation and the occurrence of glaciation may have supplied coarse detritus to thorium-rich, monazite-bearing, quartz-pebble the basin initially, debris flows and turbidity conglomerate north of Elliot Lake has been currents, released by vertical tectonic interpreted by Frarey and Roscoe (1970) as movement, may better explain the thickness indicating an oxidizing environment. variations, rock associations and distribution he observed it the Sudbury Manitoulin area. Planar and trough cross-bedding are common, as Roscoe (1969) also emphasized that glaciation are ripple marks and other primary depositional is only one of the several processes responsible structures. There is no consensus as to the for the deposition of Gowganda sediments. depositional environment of the Lorrain PDF compression, OCR, web-optimization with CVISION's PdfCompressor �22 Formation. Most of the sedimentary structures of the Lorrain Formation can be found in either shallow marine or fluvial environments. Wood (1973), Young (1973) and Frarey (1977) favored a fluviatile model while Pettijohn (1970) supported a marine environment for the Lorrain Formation. Card (1976) proposed that the Lorrain Formation represents near-shore coastal shelf deposition during episodic marine transgression and regression. Gordon Lake Formation The Gordon Lake Formation displays a gradational contact with the underlying Lorrain Formation. It is made up predominantly of variegated mudstone, siltstone, chert and minor fine-grained sandstone. Robertson (1986) subdivided the Gordon Lake Formation of the Flack Lake area into a lower member of reddish arenite, siltstone, and chert with anhydrite and gypsum nodules; a middle member of green siltstone and mudstone; and an upper reddish mudstone, siltstone and chert. Abundant sedimentary features include small-scale cross-bedding, ripple marks and desiccation cracks. Some of the features of the Gordon Lake Formation are unique for the Huronian Supergroup. Wood (1973) noted on the abundance of feldspar in the formation, a marked contrast to rocks of the immediately underlying Lorrain Formation. He also described hematite ooliths and the abundance of grains in the 0.02 to 0.05 mm range, a relatively uncommon grain size in sedimentary rocks. Since this size is found in loess deposits, Wood (1973) proposed that the quartz silt of the Gordon Lake Formation was formed by glacial action and carried by the wind and deposited in a tidal flat environment. Bar River Formation The Bar River Formation is the uppermost formation of the Huronian Supergroup. It is characterized by quartz-arenite with minor ferruginous arenite and siltstone. It is approximately 300 m thick in the Flack Lake area, north of Elliot Lake. Wright and Rust (1985) concluded that the Bar River Formation was deposited in a tidal environment. Nipissing Intrusions Dikes, sills, and cone sheets of gabbro, diabase and granophyre, commonly referred to, as “Nipissing diabase” are the most widespread igneous rocks associated with the Huronian Supergroup. Baddelyite from Nipissing gabbro sills in the Gowganda area has been dated at 2219 Ma; the minimum age for the Huronian Supergroup (Corfu and Andrews, 1986). Buchan and Card (1985) report that paleomagnetic data suggests at least two periods of Nipissing intrusive activity. Olivine-bearing hypersthene gabbro, gabbro, feldspathic pyroxenite, two-pyroxene quartz gabbro, hornblende gabbro, granophyric gabbro and granophyre have been identified in Nipissing intrusions. Many of the Nipissing sills are characterized by chilled margins 50 cm to 5 m wide, overlain by 10-20 m of quartz gabbro, then 100-500 m of hypersthene-poor gabbro-norite and vari-textured diabase (Lightfoot and Naldredt, 1996) Nipissing intrusions are widely and evenly distributed throughout the Huronian belt but, with few exceptions, are not recognized within the Archean terrain. Individual intrusions may be up to several hundred meters thick and extend over an area of several hundred square kilometers. The form and orientation of Nipissing intrusions indicate that their emplacement may be controlled by older faults, folds and competency of the enclosing rocks (Card and Pattison, 1973). Lightfoot and Naldredt (1996) discuss the geochemical characteristics of the Nipissing magmas and the potential for platinum group metal deposits. They conclude that the Nipissing magmas were emplaced into the Huronian sedimentary sequence over a period of less than 10 million years. Parental magmas of remarkably uniform composition underwent PDF compression, OCR, web-optimization with CVISION's PdfCompressor �23 in-situ contamination and differentiation within the intrusions Lightfoot and Naldredt (1996). A spatial association between Nipissing intrusions and both magmatic and vein-type mineralization have long been recognized. Huronian Paleosols and Evidence for the Accumulation of Oxygen in the Huronian Atmosphere It has long been recognized that the study of Huronian paleosols (ancient soil profiles) could provide information pertaining to the development the Earth’s atmosphere and climate during the Proterozoic. Since iron is much less soluble in the ferric state than when in the ferrous state, the behavior of iron in paleosols should provide some indication of the oxygen partial pressure of the environment. Many of the best descriptions of Precambrian paleosols have been from those associated Huronian unconformities (Gall, 1992). Grandstaff et al. (1986) identified 8 features of paleosols; most of which have been described in Huronian paleosols. These features are: 1. Stratiform 2. Relatively thin (<20 m) Roscoe, 1970; Gay and Grandstaff, 1980; Kimberly et al, 1984; G-Farrow and Mossman, 1988; Prasad and Roscoe, 1991, Sutton and Maynard, 1992, 1993). Bennett et al. (1991) have proposed that there are three disconformities or unconformities within the Elliot Lake Group, which have, potential for paleosol development (Figure 1.7). These are in descending stratigraphic order the sub-Matinenda disconformity, the sub-Thessalon Formation disconformity, and sub-Livingstone Creek Formation unconformity. The sub-Livingstone Creek Formation unconformity is the basal unconformity of the Huronian Supergroup and is the only entirely sub-Huronian unconformity (Figure 1.3, 1.7). This unconformity is exposed in the Thessalon area, where the upper few meters of the Archean granitic rocks can be seen to progress from angular, slightly rotated blocks, separated by grey grit and fine-grained sandstone, upward, to more rounded boulders with a higher proportion of finer clastic material (Collins, 1925). This zone may be termed a “paleo-regolith”, since there is little or no obvious development of the yellow, sericitic paleosol commonly found in the younger, sub-Matinenda paleosols. 3. Transitional lower boundary-sharp upper boundary Prasad and Roscoe (1996) described two paleosols in the same drill core from the Denison Mine at Elliot Lake. A paleosol was found 4. Colour variations above Huronian volcanic rocks and another, less 5. Destruction of primary rock textures well-developed paleosol, was found upon accompanied by the development of soil textures Archean tonalite below a short section of quartz-pebble conglomerate and grit (Prasad, 6. Destruction of primary minerals with personal communication, 1997) below the 9 m formation of clay minerals or metamorphic thick volcanic unit (Figure 1.15). equivalents 7. Dikes of material from overlying sediment washed down into desiccation cracks in the soil 8. Rip-up clasts of overlying sediments Well-preserved paleosols below the Matinenda Formation in the Elliot Lake area have been described by many workers (Roscoe, 1969; Pienaar, 1963; Robertson, 1968; Frarey and The best developed, and most studied, Huronian paleosols have been found directly below the Matinenda Formation. On mafic rocks, the sub-Matinenda paleosols can generally be recognized by the presence of an upper, distinctly apple-green to yellowish, sericitic zone which grades downward, over few centimeters to several meters, to an underlying, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �24 granitic rocks is generally lacking or relatively thin. black, fine-grained, chlorite-rich eluvial zone up to several meters thick. Abundant pseudomorphs of titanium oxides after ilmenite are a feature of paleosols on mafic igneous rocks. Rip-up clast of sericitic paleosol (the “argillite scraps” of mine geologists) are commonly found in the lower few meters of the overlying Matinenda Formation. Prasad and Roscoe (1996) report significant amounts of carbonate and pyrite in sub-Matinenda paleosols in the Elliot Lake area. Paleosol Denison Drill hole S-62 Sub-Matinenda Formation Paleosol K K 0 C CO2 Sub-Matinenda Paleosol -178 Sub-Lorrain Formation Paleosol Massive Basalt -1336 1850 -1950 Fe2 Fe3 Ca Mg Fe2 K Fe3 Paleosol [( Na liii Ville Marie granite Transition zone -899 500 Metabasalt-increasingly weathered upward Fet Mg Ca Sub-Thessalon Fm. Paleosol Chlorite, sericite rich bands in paleosol "Argillite" (paleosol) NaCa Mg Fe2 Fe3 Coglomerate Grit 0 Tonalite - increasingly weathered upward cm Hematitc breccia Lorrain Formation Sub-Matinenda paleosols commonly show the pronounced loss of soda found in most paleosols. Lime, and magnesia are also depleted, but there is generally a large increase in potash content (Gay and Grandstaff, 1980) (Figure 1.15). In most cases, iron and manganese are depleted in upper parts of the paleosol. This is held to provide good evidence of weathering in a reducing environment. However, Gay and Grandstaff (1980) noted an upward increase in The uppermost sections of sub-Matinenda Formation paleosols on Archean granitic rocks total iron in paleosol from the Pronto Mine area. After Prasad and Roscoe(1996), Rainbird et al. (1990). Figure 1.15: A comparison of sub-Matinenda Fm. and sub-Lorrain Fm. Paleosols and arkosic sedimentary rocks, is generally apple-green to yellowish rock composed mainly of quartz and sericite (Robertson, 1968; Gay and Grandstaff (1980); Sutton and Maynard (1992). Where the texture of the protolith is well preserved, but the original mineralogy is replaced, the paleosol may be termed a saprolith (Rainbird et al, 1990). The chlorite-rich eluvial zone of paleosols on They concluded this upward increase in iron content indicated the presence of free oxygen in early Huronian atmosphere although at approximately 1% of the present level (Gay and Grandstaff, 1980). They suggested that the loss of iron shown in most Huronian paleosols could be due to local reducing environments. Some writers have concluded that the increase in potash (as sericite) in Huronian paleosols is largely due largely to diagenetic and PDF compression, OCR, web-optimization with CVISION's PdfCompressor �25 metamorphic processes which may mask the environmental and hydrologic conditions operative during paleosol development (Gay and Grandstaff (1980); G-Farrow and Mossman (1988). purple siltstone member of the Lorrain Formation near Sault Ste Marie (Frarey, 1977). Not all workers accept the above explanation for the preservation of uraninite and pyrite, and the observed change of colour with stratigraphy. The mineralogy and geochemistry of Sub-Lorrain Ohmoto (1996) has stated “ the loss of total iron in paleosols of all ages is not due to a Formation paleosols have been described by Rainbird et al. (1990) and Sutton and Maynard reducing atmosphere but to the reductive (1992). In contrast to the older, sub-Matinenda dissolution of ferric hydroxides under an oxic paleosols. Sub-Lorrain paleosols commonly show atmosphere”. an enrichment of Fe+3 at the expense of Fe+2 Regional Tectonic Patterns and without a significant loss of total iron. Hematite Metamorphism is a common mineral in the upper parts of sub-Lorrain paleosols, in contrast to the presence Major structures of the Huronian belt follow two of pyrite in sub-Matinenda paleosols. In this main trends: 1) west-northwest trending folds respect the sub-Lorrain paleosols resemble many and faults of the Sault Ste Marie-Elliot Lake post-Huronian paleosols and are consistent with area; and 2) east to northeast striking folds and weathering in an oxidizing atmosphere (Prasad faults of the Sudbury-Manitoulin area (Figure and Roscoe, 1996; Rainbird et al., 1990). 1.16). These two structural orientations are associated with differing fold styles, Since pyrite and uraninite are unstable under metamorphic grade and metamorphic fabric. oxidizing conditions, the abundance of detrital Changes are most notable across the east-west pyrite and uraninite in the paleoplacer uranium faults of the Murray fault system. This northeast ore zones in the Matinenda Formation provide striking fault zone (Figure 1.16) is the most good evidence for an oxygen deficient significant structural feature of the Huronian atmosphere during weathering, transport and belt. Since many formations show a significant deposition of early Huronian sediments. increase in thickness south of the fault, it is In marked contrast to the common red beds of generally interpreted as an inverted growth fault. younger clastic sequences, sandstones (and most (i.e. an early listric normal fault, active during granitic clasts in conglomerates) below the sedimention; which, during a later compressive Cobalt Group are almost all drab coloured in regime, was converted to a thrust or reverse spite of the abundance of red and pink granitic fault (Card, 1978; Jackson (2001). rocks in the continental source area (Roscoe The rocks of the Huronian Supergroup have 1969, 1973). Frarey and Roscoe (1970) noted been subjected to several deformational events. the above and proposed that the drab colour of This is particularly evident south of the Murray lower Huronian clastic rocks is due to the lack of Fault zone. In the Whitefish Falls area (south of free oxygen in the atmosphere during the period the Murray Fault) Young and Nesbitt (1985) of deposition of lower Huronian rocks. conclude that some large-scale folding is related Red hued, hematite bearing rocks, which Roscoe to syn-depositional and/or post-depositional (1969) proposed mark the presence of an deformation of unconsolidated sediment. Early oxidizing atmosphere, make an appearance with syndepositional deformation is also indicated by the Gowganda Formation of the Cobalt Group, an important unconformity below the and are important in parts of the Lorrain and Gowganda Formation of the Cobalt Group; and Gordon Lake Formations of that group. What the presence of ragged, slumped contacts and may be the first true red-beds are found in the large slump blocks along major faults (Card, 1978; Young, 1983). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �26 Convincing evidence of at least one important pre-Nipissing (2.2 Ga) deformational event, assigned to the Blezardian Orogeny (Stockwell, 1982), is provided by the observation that Nipissing bodies in the Sudbury-Whitefish Falls area transects axial surfaces of major folds (Card, 1978). Jackson (2001) did not observe such relationships in the areas north of the Murray Fault. However, Jackson (2001) points out, as earlier noted by Robertson (1964), that the tendency of Nipissing dikes to occupy structures parallel to the axial plane of the Chiblow Anticline, suggests pre-Nipissing folding. Jackson (2001) also noted evidence of pre-Nipissing faults north of the Murray Fault zone. Following emplacement of the Nipissing intrusions but prior to the emplacement of the Sudbury Igneous Complex there was further deformation and regional metamorphism. Rb-Sr isotope studies of Huronian metasediments indicate that prograde metamorphism occurred at about 1.90-1.85 Ga (Fairbairn et al., 1969). This event is probably correlative with the Penokean Orogeny of Michigan-Minnesota (Sims et al., 1989). After the emplacement of the SIC and the deposition of the Whitewater Group, there is evidence of further deformation and low-grade metamorphism, followed by intrusion of granite plutons at about 1.75 and 1.5 Ga. The post-SIC deformation and retrograde metamorphism was found mainly south of the Murray Fault, especially in the area between the SIC and the Grenville Front. Shanks and Schwerdtner(1991) report that this deformation is characterized by south-dipping thrust faults or deformation zones with northeast-trending foliation and southeast-plunging lineation. The last recognized deformational feature affecting Huronian rocks is the movement along faults, which post-date the diabase dikes of the 1.25 Ga Sudbury swarm (Jackson, 2001). A study of magnetic fabrics, strain patterns, and microstructures in granitoid rocks of the Creighton and Murray plutons and their Huronian host rocks by Riller (1996) have provided information on the pre-2.2 Ga “Blezardian Orogeny” (Stockwell, 1982). Riller concluded that major folding and amphibolite facies regional metamorphism in the Sudbury area was coeval with the emplacement of the Creighton Pluton and Murray Plutons, which were dated at 2.3 Ga by Frarey et al (1982) and Krogh et al. (1984) and more recently at 2477+/- 9 Ma by Krogh et al. (1996). Large-scale dome-and-basin structures in Archean basement and Huronian cover rocks represent major south-vergent nappes. The Sudbury Structure, a deformed relic of an astrobleme, is superimposed on a major antiformal dome cored by Archean granulite and granitoid rocks and flanked by overturned Huronian strata on the south and several remnant rim synclines of Huronian rocks on the north (Riller, 1996; Bennett et al, 1997). In the Sault Ste Marie-Elliot Lake area fault and fold structures trend in general west-northwest to northwest direction. Folds are generally upright, open concentric structures with gentle, variably plunging hinges (Figure 1.16). There is only a weak development of minor tectonic structures. The metamorphic grade is subgreenschist (Card, 1978b). The major structural features of the Elliot Lake area include a gently south-dipping homocline south of the Flack Lake Fault, the open fold of the Quirke Lake Syncline and the Chiblow Anticline between the Quirke Lake Syncline and the Murray Fault. Jackson (2001) found no evidence of detachment at or near the basement-cover interface in the Elliot Lake area. He also proposed that the “inverted growth-fault” model as applied by Zolnai et al. (1984) to structural-stratigraphic relationships in the Huronian may, in some cases at least, be interpreted as thrust faults with flats following depositional boundaries, and ramps that cut up through the stratigraphic section. Given the data available, neither model could be rejected for major northwest trending faults in the Sault Ste Marie area (Jackson, 2001). Jackson (1997) points out that the curvature of the Flack Lake PDF compression, OCR, web-optimization with CVISION's PdfCompressor �27 M q V ±PLfl2I 4011f4' Mall uqicc1G blOCK roK6 gOflil ?b04 fl CCLGUA!IIG bLOAIUCG) QOMU4JLOMU ?!qG c°p°ii CLOnb L0CK2 EQflI4 (Op?GLAGq — bk6-COPOU 43LOflb LOCK2 qsLIAGq 410w VLCIJ6OU pO2GWGU4 I2lauq ELOU4 IGC4OU!C LOCI(2 (3flbGLiOL bLOAIUCa) E0fl14 wE2 VU4!Cl!LJ& bIG—CopoK Eafl14 (4LOC6OpJ6 OU 2flL4CC6 4L0C60P16 22 wee IU E r000iIou 241nc4n16 El r1uuaw6q 5 U0141J 04 Figure 1.16: Main structural features of the Huronian Supergroup. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �28 Fault is in the opposite direction to that expected if it is a thrust fault as proposed by Zolnai et al. (1984). In the Sault Ste Marie area the presence of trust faults, as suggested by out-of-sequence units, has been confirmed, in at least one location, by diamond drilling (Assessment files, Sault Ste Marie Resident Geologist’s Office). The Murray fault zone separates the moderately developed. More than one age of major and minor structures can be discerned south of the Murray Fault (Jackson, 2001). Metamorphism south of the Murray fault ranges from low greenschist to low amphibolite facies (Figure 1.17). Rocks of higher metamorphic grade occur in two zones or nodes, one along the Murray Fault zone and another northwest of the Grenville Front. Both zones coincide with major r.ø.uq 4dCIIU LJ rUM 1 w!qqç dLG6L)UCIJ!24 WQ OLGUAIIIG LLOU4 EIUIC IU4LUUIOUU bLOA!UCG Figure J1.17: Metamorphism of the Huronian Supergroup deformed, low metamorphic grade rocks to the north from the multi-deformed, higher-grade rocks of the Sudbury-Manitoulin area to the south. In the Sudbury-Manitoulin area is characterized by open to sub-isoclinal, flattened buckle folds with upright to northward overturned axial surfaces. Elongate domes and basin are formed by reversals in plunge. Penetrative axial place cleavage and steeply plunging rodding or mineral lineations are well anticlinoria, although in detail, metamorphic isograds transect fold axes (Jackson, 2001). Higher-grade metamorphic nodes do not coincide with the few granitic intrusions that intrude the Huronian rocks south of the Murray fault. The 1.850-1.900 Ga age of metamorphism is much younger than the Creighton-Murray granite (2.45 Ga) and older than the PDF compression, OCR, web-optimization with CVISION's PdfCompressor �29 1.700-1.750 Ga of the Chief Lake and Cutler granites. Jackson (2001) considers the origin of the high-grade staurolite-biotite assemblages of the McKim Formation in the hanging wall of the Murray Fault as one of the most enigmatic aspects of the tectonic history of the Southern Province. He concludes that geobarometry indicates a relatively low- pressure metamorphism (2-3 kbar) at high temperature. These conditions differ significantly from the 6-7 kbar pressures estimated for the Penokean metamorphism in Minnesota as determined by Holm and Silverstone (1990). He concluded that the high-temperature metamorphism was at or below pressure corresponding to the thickness of the Huronian rock column, thereby precluding crustal thickening as the origin of the metamorphism. Jackson (2001) concluded that a high heat flow regime as developed in areas of crustal extension and related mantle upwelling Jackson (2001). Such a model is compatible with Card’s (1964) view that the high-grade metamorphism may be the result of rapid, focused heat flow. Tectonic Models for the Development of the Huronian Basin There have been various tectonic models proposed for the early development and later deformation of the Huronian basin. Many reconstructions are essentially modifications of the model put forth by Deitz and Holden(1966) which stated that the Huronian Supergroup represents a rift and passive margin sequence that was compressed, partly tectonically buried and metamorphosed during a collision with the Superior craton and another mass which overrode its southern edge. Zolnai et al. (1984), Bennett et al. (1991) accept the essential aspects of the Dietz and Holden (1966) model. They propose that rifting and continental break-up was coeval with Huronian volcanism (2.45 Ga) and that the much later deformation was equivalent to the Penokean Orogeny (1.860-1.835 Ga). This model does not attempt to account for the multiple deformation events affecting Huronian rocks and the origin of the Nipissing magmatic event. Young (1982) proposed that the Huronian Supergroup was deposited in an aulocogen, an easterly trending fault bounded trough, which opened towards an ocean in the area now occupied by the Grenville Province. Sims et al. (1980, 1981) concluded that the Huronian Supergroup, the Marquette Range Supergroup and Animikie rocks were deposited as intra-continental, fault controlled basins developed along a major, late Archean structure, the Great Lakes Tectonic Zone. More recently, Roscoe and Card (1992), noting the close stratigraphic correlation between the Early Proterozoic sequences of the Wyoming craton and the Huronian Supergroup, proposed that the Superior and Wyoming cratons are rifted portions of what was once a single continental land mass. They suggest the direction of the Matachewan-Hearst dike swarm (2.45 Ga) indicates an east-west tensional regime, which resulted in, a Huronian basin elongated in a north-south direction. On this larger craton the Huronian sediment was deposited in a southward deepening intracratonic basin. Roscoe and Card (1992) propose that it was during the Nipissing igneous event (2.2 Ga) that successful rifting of the Superior Province took place with the eventual drifting of part of the missing Superior Province to its present location as Wyoming craton. They attribute pre-Nipissing folding to the Blezardian Orogeny of Stockwell (1982) and the later more important deformation to be coeval with the Penokean Orogeny of Michigan, Wisconsin and Minnesota (Roscoe and Card, 1992). Jackson (2001) supports the model of Roscoe and Card (1992) since the high heat flow, which he considers necessary to give the observed features of the high-grade metamorphic rocks, would be a necessary effect of mantle upwelling during continental break-up. He also interprets some of the early high-strain deformation as being consistent with a Nipissing age break-up of the Superior craton. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �30 ILSG Field Trip No. 1 - Day 1 The Elliot Lake Transect On the first day of this field trip we will examine many of the excellent rock exposures of one of the Earth’s most well-known and most completely studied stratigraphic sequences. The rocks of the Elliot Lake transect display clearly visible evidence of the nature of the climate and atmosphere of the Earth more than 2 billion years – of course not all Earth scientists agree as to what this evidence tells us. Stop Descriptions and Road Log From Sault Ste Marie, Ontario proceed east on Highway 17 for 162 km (101 mi) to the intersection of Highway 17 and Highway 108, then 29 km (18 mi) north on Highway 108 to Elliot Lake. At Elliot Lake continue north past Hillside Drive South and the first few stoplights to Hillside Drive North. Turn left (west) onto Hillside Drive North. Continue west on Hillside Drive North for about 1 km (0.6 mi) to Spruce Street. Turn right (North) on Spruce Street to Valley Crescent continue on Valley Crescent to Balsam Place and stop at the cull-de-sac. When collecting samples please exercise care to avoid damaging any vehicles parked nearby. cul-de-sac. Note the deflection of the axial plane cleavage in the mudstone units. The movement of adjacent beds inferred from the deflection of the cleavage indicates the south limb of a syncline. A gabbroic dike is separated from the sedimentary rocks by a zone of sheared and fractured rocks. The McKim Formation is missing on the north limb of the syncline. Return to Hillside Drive North and continue west for about 1 km to Spine Road. Turn right (west) onto Spine Road to Lawerence Avenue at the west end of Spine Road. 14.9 km = 9.3 mi. 17-370462E, 5137991N STOP 1.2: Radioactive quartz-pebble conglomerate of the Matinenda Formation. The low outcrops on north side of Spine Road are grey, buff and dark-grey arkose and radioactive, pyritic, quartz-pebble conglomerate of the Matinenda Formation. Pebble units are about 20-30 cm thick and dip about 10 degrees to the north. Pebbles in this outcrop are about 1-2 cm across and are generally much smaller than the typical pebbles in the ore zones of the Elliot Lake mines. Ruzicka and LeCheminant, (1984) report the radioactive conglomerate contains “rare-earth-element-bearing uranothorite (?), large zircons, a Ti-U-Si-Fe phase (brannerite?), 17 – 372751E, 5138695N (All coordinates in chalcopyrite and chromite. The distribution of the Elliot Lake are within UTM zone 17; NAD radioactive minerals in the conglomerate 83 which is essentially equivalent to NAD displays layering thus indicating a detrital WGS84). origin of these grains” STOP 1.1: McKim Formation and Nipissing Diabase The dark-grey areas in the radioactive beds are due to the presence of minor amounts Outcrops are argillite/slate and grey sandstone radioactive carbon generally known in the of the Mckim Formation on the east side of the Elliot Lake area as “thucolite” but also referred PDF compression, OCR, web-optimization with CVISION's PdfCompressor �North 2.1 Hy. 17 2.7 Hy. 556 Sault Ste. Marie Area of Figure 2.1 2.8 2.9 1.12 Hy. 17 from drill hole Hy. 128 LAKE HURON Thessalon Bruce Mines Hy. 638 Foul Bight Highway 556 transect Elliot Lake transect Field trip stop Faults Cutler Granite Haughton Twp. area. Gordon Lake area Hy. 17 Hy. 556 Fenwick Township Hy. 546 Area of Figure 1.21 Hy. 108 50 km City of Elliot Lake cline Hy. 108 Area of Figure 1.19 e syn e Lak Quirk Hy. 639 Flack Lake area Archean rocks East Bull Lake Suite Gabbro anorthosite Elliot Lake, Hough Lake and Quirke Lake Groups Cobalt Group Gordon Lake Fm. Huronian Supergroup Murray Fault LEGEND Figure 1.18: Index map for included geological maps and some areas mentioned in the text. Lake Superior Paleozoic and Keweenawan rocks 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �32 LEGEND POST-HURONIAN MAFIC INTRUSIVE ROCKS NIPISSING DIABASE HOUGH LAKE GROUP Subarkose, arkose, conglomerate Diabase, gabbro, metagabbro, granophyre 16 HURONIAN SUPERGROUP COBALT GROUP Bar River Formation 15 Pecors Formation (4) Argillite siltstone 8 Ramsay Lake Formation Quartz arenite Gordon Lake Formation 14 Siltstone, argillite, subarkose Lorrain Formation 13 Quartz arenite, subarkose, arkose, conglomerate, thorium-bearing conglomerate Gowganda Formation 12 Conglomerate, argillite, wacke, subarkose, siltstone DISCONFORMITY QUIRKE LAKE GROUP Serpent Formation 11 Subarkose, conglomerate Espanola Formation 10 Limestone, dolomite, calcareous siltstone Bruce Formation Diamictite Not Shown Mississagi Formation Fault 3 9 7 Diamictite ELLIOT LAKE GROUP McKim Formation 6 Siltstone, argillite, wacke Matinenda Formation 5 Subarkose, arkose, conglomerate, uranium-bearing conglomerate DISCONFORMITY Thessalon Formation 4 Basalt, andesite, mugearite-hawaiite, minor basal quartz-pebble conglomerate 4 DISCONFORMITY Livingstone Creek Formation 3 Sandstone, polymictic conglomerate UNCONFORMITY ARCHEAN 2 ARCHEAN Plutonic granitic rocks, gneisses, INTRUSIVE CONTACT Mine (Past Producer) 1.2 Field trip stop 1 Felsic to mafic metavolcanic rocks, metasedimentary rocks Sources of Information Giblin and Leahy(1979) Figure 1.20: Legend for figures 1.19 and 1.21. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �7 8 9 10 2 12 12 7 8 12 9 5 8 7 10 12 14 2 13 9 16 9 16 5 2 5 Elliot Lake 1.2 10 12 13 11 4 6 1.1 1.3 2 2 16 12 City of Elliot Lake 1.4 1.5 1.6 Hy 108 1.11 Hy 639 9 16 9 12 1.7 1.8 1.9 9 10 2 16 5 8 16 6 11 5 12 11 Qirke Lake 12 7 6 1.10 16 10 2 Figure 1.19: Geological map of the Quirke Lake Syncline. 5 9 10 11 Dunlop Lake 4 9 2 7 4 1 11 4 9 7 10 2 2 14 1 11 10 3 rs 1 co Pe 16 12 ke 6 12 1.1 La 9 16 2 16 5 10 2 1.1 9 16 0 1 9 1 9 2 2 9 16 3 2 1 6 1 N 1 4 miles km 6 2 Whiskey Lake 2 33 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2 12 13 2 13 12 12 12 3 14 Hy 546 13 12 16 12 9 13 1.14 Ten Mile Lake 4 9 4 3 Flack Lake 1.15 Hy 639 16 11 15 10 14 1.16 12 2 14 2 15 1.13 14 9 15 Hy 639 1.12 13 Mount Lake Figure 1.21: A geological map of the Flack Lake area. 10 14 16 16 12 2 14 1.11 16 1 16 151 16 2 14 1 16 Rawhide Lake 4 10 Semiwite L Lake 16 13 12 2 2 0 16 1 3 2 2 13 16 12 2 1 2 North 4 miles km 6 2 34 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �35 to a hydrocarbon kerogen. Ruzicka and LeCheminant, (1983) note that several generations of carbon occur in the conglomerates of the Matinenda Formation. The earliest generation occurs as layers concordant with the bedding or a component of the matrix and appears to have been deposited in areas of quiescent sedimentation during the last phase of an upward fining sedimentary cycle. Diamond drilling has indicated that there are no ore-grade units in this area. Grab samples collected by the author in 1982 returned up to 0.80 lbs U3O8 /ton and 0.78 lbs ThO2/ton. A continuous chip sample returned 0.31 lbs U3O8/ton and 0.53 lbs ThO2/ton. Return to Highway 108 and proceed north on Highway 108. Reset odometer at Stanleigh Road. About 1.5 km north of Hillside Drive North). 1.0 km = 0.63 mi. 17 - 371717E, 5140426N STOP 1.3: Mississagi Formation, Hough Lake Group. One-meter thick beds of grey subarkose of the Mississagi Formation on the west side of the highway display the rusty staining on the face of the outcrop reflecting the minor pyrite content along the foreset beds of trough cross-beds (Figure 1.22). The paleocurrent direction (from the west) can be best observed on the upper surface of the outcrop. – Please Figure 1.22: Trough cross-bedding in the Mississagi Fm. exercise caution when walking on smooth, wet rock surfaces - The grey Stop 1.3. colour of these sandstones and the presence of apparently detrital pyrite Later generations are probably remobilized is held by most geoscientists to indicate of the phases of the first generation of carbon. The very low partial pressure of free oxygen of the carbonaceous matter in the Elliot Lake ores is atmosphere during the deposition of the comparable in occurrence and composition to Mississagi Formation. similar hydrocarbon in the Witwatersrand gold 1.4 km - 0.9 mi. 17-371652E, 5140948N reefs; interestingly Ruzicka and LeCheminant (1984) report elevated gold content (1000 STOP 1.4: Nipissing Diabase (Gabbro) 2000 ppb) in the carbonaceous matter of the altered Mississagi Formation, Bruce Elliot Lake ore beds. The radioactive carbon at Formation. this site is reported to be auriferous, although A sill-like body of Nipissing gabbro/diabase is the gold content is not available. exposed on the east side of Highway 108. In 1955 Rio Algom Mines Limited completed a Rhythmic, compositional layering is visible on diamond drill hole about 30 m south of this the vertical face of the road-cut. Near the north location. The logs of this hole indicate that the end of the Nipissing outcrop face note the radioactive beds exposed here are about 35 relatively planar, striated, surface is truncated by meters above the Archean basement rocks. more irregular surface that has been interpreted PDF compression, OCR, web-optimization with CVISION's PdfCompressor �36 as the result of erosion by high-pressure, waterborne sediment, presumably as a result of melting of an adjacent Pleistocene ice sheet. North of gabbro sill, the upper portion of the Mississagi Formation is exposed along the east side of the highway. The distinctly pinkish hue of the sandstone is probably due to the formation of albite by hydrothermal fluids activated by the adjacent intrusion. Further evidence of hydrothermal activity is seen by dark-green to black chlorite deposited along fractures. below the north-dipping diabase sill near the north end of the exposure, where it occurs as sub-parallel groups of pale-grey to white prismatic crystals about one mm wide and up to a centimeter long. X-ray diffraction analysis has determined that the pink coating on joint surface is apophyllite (KFCa4[Si8O20]8H20) an uncommon mineral, sometimes found in amygdules in basalts, but is also associated with calc-silicates (Figure 1.23). Young (1991) states Continue northward on foot for a few tens of meters. Here (17-371668, 5141245) the Mississagi Formation is overlain by diamictite of the Bruce Formation at the base of the Quirke Lake Group. The dispersed megaclasts of the Bruce Formation are predominantly grey granitic rocks with smaller mafic clasts of predominantly Huronian volcanic rocks of the Thessalon Formation. The abundant matrix of the conglomerate is dark-grey to black. Sand-sized quartz grains have a glassy, black appearance, a reflection of the dark matrix behind the Figure 1.23: Espanola Formation with calc-silicate minerals. Stop 1.5 clear quartz. There is no evidence of significant disconformity at the base of the Bruce Formation. that the small scale thrust faults and folds in the limestone on the west side of the highway is Proceed by vehicle to near the top of the hill. probably the result of slumping during early 1.8 km = 1.4 mi. 17-371621E, 5142684N tectonic activity. STOP 1.5: Espanola Formation and Nipissing diabase sills. The upper, ferruginous dolostone-bearing member of the Espanola Formation and the The base of the Espanola Formation is a green, overlying Serpent Formation are not present at this location but are well represented on the laminated unit about a meter or so thick. Laminated silty limestones and minor thin, chert north limb of the syncline. Since the Serpent beds of the Bruce Limestone member, Espanola Formation is missing we can infer that the ferruginous dolostone was removed during a Formation, overlie this unit. At this location the proximity of Nipissing gabbro intrusions has period of pre-Gowganda erosion. led to the development of calc-silicate minerals Continuing north the basal units of the including: grossularite garnet, idocrase Gowganda Formation are visible in road-cuts (vesuvianite), diopside, and wollastonite typical along the highway. of a skarn (Robertson, 1968). Wollastonite 2.0-2.5 km = 1.3-1.6 mile. Gowganda (identified by X-ray diffraction) is found just Formation diamictite. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �37 Megaclasts of pink granite, grey granite and granitic gneiss and mafic rocks are widely distributed in a dark green matrix. Most geologists now consider at least some of the diamictites in the Gowganda Formation to be tillites, although a debris-flow origin, either glaciogenic or as submarine debris flows is a more reasonable interpretation at specific localities. Roscoe, (1969) places the appearance of free oxygen in the atmosphere (“oxyatmoversion”) as coinciding with the appearance of the reddish hue of hematite just above the base of the Gowganda Formation a short distance north of STOP 1-5. (Figure 1.24). The mineral assemblage and metamorphic grade of the few of the mafic clasts examined by the writer many years ago indicated that the clasts were probably from Huronian basaltic flows. 4.7 km. = 2.9 mi. Pink sandstones and diamictite of the Gowganda Formation. 6.7 km = 4.2 mi. Diamictite with large boulder, Gowganda Formation. t —-.1 3.8 km. = 2.4 mi. 17- 371841E, 5143180N STOP 1.6: Stratified Gowganda Formation. Park just south of the rock-cut on east side of the highway. This is an impressive exposure though a stratified sequence of diamictites, Figure 1.24: Crudely stratified conglomerate of the clast-supported conglomerates and sandstones of the Gowganda Formation. Gowganda Fm. with mafic volcanic clasts. Stop 6. The base of the sequence at the extreme southern end of the exposure on the west 8.2 km. = 5.0 mi. Stanrock Mine Road. Reset side of Highway 108 consists of massive odometer. Turn east onto Stanrock Road diamictite overlain by a thin unit of laminated Optional Stop mudstone-siltstone with a few small dropstones. 2.2 km = 1.4 mi. 17-374911E, 5147007N About a meter of stratified Gowganda overlies the siltstone. Distinct beds of diamictite, Stop 1.7: Laminated varvite? sandstone, pebbly sandstone and clast supported Laminated siltstone/mudstone of the Gowganda polymictic conglomerate are present. Some Formation on the north side of the road. This conglomerate units display normal and reverse unit has been interpreted as varves found as grading suggestive of debris flows. The rocks deposits from Pleistocene glacial lakes. Return to displayed here may be interpreted to represent a Highway 108. depositional environment proximal to retreating glacial margin. Set odometer to 0 at Stanleigh Road and Highway 108. Note the predominance of red and pink granitic clasts, in marked contrast to the pale grey clasts 3 km = 1.9 mi. Denison Mine Road - Turn east. of the Bruce and Livingstone Creek Formations Reset odometer to 000 km. seen earlier. There is also a significant 1.5 km. = mi. 17 -375102E, 5150499N proportion of black pebble to cobble-sized clasts PDF compression, OCR, web-optimization with CVISION's PdfCompressor �38 Stop 1.8: Gowganda Formation Serpent disconformity Stop 1.10: Ramsay Lake Formation overlain by Pecors Formation. The Serpent Formation is not present in the south limb of the Quirke Lake Syncline and the Blind River Sault Ste Marie area where it was probably removed during a period of pre-Gowganda erosion. At this location, on the south side of the road, well-sorted sandstone of the Serpent Formation is overlain by polymictic conglomerate of the Gowganda Formation. The contact is sharp but irregular. Evidence of a sub-Gowganda disconformity at this location is given by the presence of pebble and cobbles of the Serpent Formation near the base of the Gowganda Formation. Diamictites of the Ramsay Lake Formation contain cobbles of grey granitic rocks, mafic clasts of Huronian volcanic rocks and Archean felsic volcanic clasts in an abundant dark-grey to black sandy matrix. The Ramsay Lake Formation is overlain by dark laminated siltstone and mudstone (argillite) of the Pecors Formation. The latter contains a few dropstones (Figure 1.25). Note: the Matinenda Formation of the Elliot Lake Group, expected between the basement and the Ramsay Lake Formation, is truncated by the Ramsay Lake Formation in this area. The Matinenda Formation does occur in the mine workings down-dip from this location. Return to Highway 108. Reset odometer at Highway 108 and Denison Mine Road. Continue east to the end of the Panel Mine road. This is the rehabilitated area of the former 0.5 km = 0.3 mi. Disseminated carbonate in Panel Mine. There is little evidence of the sandstone of the Serpent Formation on east side uranium mine and mill complex that was on this of Highway 108. site until 1993. 1.1 km = 0.68 mi. Road to Quirke Lake and Return to Highway 108. Reset Odometer. former Panel Mine. Reset odometer to 0. Turn Continue north on Highway 108. Tailings dam east onto Panel Mine road. of the Quirke Mine is visible west of the 1.5 km = 0.9 mi. 17-375258E, 51511331N Highway. Stop 1.9: Espanola Limestone Member of the Espanola Formation. Rock-cut in ferruginous dolomite and siltstone of the Espanola Limestone Member of the Espanola Formation. The Espanola Limestone Member is the uppermost of the thee members of the Espanola Formation recognized in the Elliot Lake area (Robertson, 1968). It is characterized by intercalated siltstone and reddish-brown weathering, ferruginous dolostone beds containing 3-4% FeO. Intraformational breccia, ripple marks, small-scale cross-bedding, and various soft sediment features are present but faintly visible on the south outcrop. Near the east end of the outcrop a grey clastic dike crosses the stratification at a high angle. 4.1 km = 2.5 mi. 17 – 377379E, 5152019N Highway 108 ends and Highway 639 begins. 1.0 km = 0.6 mi. Diamictite of the Bruce Formation is exposed west of the Highway. 1.5 km = 0.9 mi. Outcrops of Mississagi Formation are exposed along Highway 639. Note the yellowish colour characteristic of the Mississagi Formation where it lies directly on the Archean granitic basement (Robertson, 1968). 2.5 km = 1.6 mi. 17 – 371508E, 5152302N STOP 1.11: Huronian volcanic rocks of the Thessalon Formation (Dollyberry belt). A gated road leads west from Highway 639 leads to one of the Quirke Mine tailings dams. Park near the gate and walk a short distance along a rough road from the gate to the base of the tailings dam. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �39 Note the very dark-green to black, flattened, chlorite amygdules characteristic of the Huronian mafic volcanic rocks between Sault Ste. Marie and the Elliot Lake area. Cross the stream and proceeding northward a short distance along a rough road to the crest to the low hill. The Huronian volcanics at this location (17 – 371428E, 5152342N) include transitional observed identical scattered, quartz pebbles were found along the contact of the Huronian volcanic rocks where they overlie Archean mafic volcanic rocks in the haulage drift of the Stanleigh Mine on the south limb of the Quirke Lake Syncline. The writer proposes that these quartz cobbles are lag deposits left behind while the finer sediment was washed off the surface. A few kilometers west of this location, occurrences of this conglomerate unit contain more rounded quartz, and are commonly are overlain by a thin arkosic unit. Quartz-pebble conglomerate, which locally contain significant pyrite as well as uranium, occur sporadically at the base of the Huronian volcanic sequence of the Quirke Lake Syncline and westward to the Sault Ste. Marie area. The conglomerate at this location occupies the same stratigraphic position as the radioactive, quartz-pebble conglomerate. Figure 1.25: Dropstone in laminated argillite of the Pecors Formation. Stop 10. alkalic types, hawaiite and mugearite (unpublished analyses in the authors files). The eastward trending, south dipping unconformity between the Archean granitic basement rocks and Huronian volcanic is visible near the crest of the hill. There appears to be no visible paleosol development at this location. Near the west end of the outcrop, a thin, quartz-pebble conglomerate/breccia unit, comprised mainly of angular, quartz-clasts, overlies the granitic rocks at the base of the volcanic unit. Scattered, isolated, mainly cobble-sized clasts of quartz can also be found along the unconformity. Some visitors to this site have proposed that the contact between the Archean and Huronian volcanics is not an unconformity, but a fault contact. However, during a visit to the Stanleigh Mine in 1990, the writer along with Rob Henderson, Mine Geologist and Dr. Larry Jensen of the Ontario Geological Survey Return to vehicle, set odometer to 0 and continue north on Highway 639. 0.2 km = 0.1 mi. Archean outcrops cut by diabase outcrops along the highway. 6.8 km = 4.3 mi. 17 – 368530E, 515773N STOP 1.12. Archean metavolanics with pillow structures. Archean mafic metavolcanics displaying well-developed pillow structures are exposed, on north sloping outcrop, the east side of highway (Figure 1.25). Pillows are deformed, but facing directions can easily be determined. Small amygdules are concentrated near the upper surface of many pillows. 7.7 km =4.9 mi. Flack Lake fault occupies a valley near this point. 8.2 km = 5.2 mi. Outcrops of hematite-stained sandstone of the Bar River Formation. 8.8 km = 5.6 mi. Entrance to Mississagi Provincial Park. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �40 9.1 km = 5.7 mi. Pale grey sandstone of the Bar River Formation with herringbone cross-bedding. to suggest that these features are the result of the transportation of consolidated desiccation fracture fillings. 9.8 km = 6.2 mi. Christman Lake. 13.6 km = 8.6 mi. 17 – 364395E, 5162758N 17 – 366863E, 5160228N STOP 1.14: Red beds of the Gordon Lake Formation 10.2 km - 6.5 mi. Laminated, maroon buff siltstone and chert with reduction spots in the upper part of the Gordon Lake Formation. This is another red-bed occurrence within the Cobalt Group. 17 –366742E, 5160652N STOP 1.13: Bar River Formation. 14.9 km = 9.4 mi. 17 – 363062E, 5163194N STOP 1.15: Gordon Lake Formation. — - 'U: __t - - -. Siltstones and sandstones of the Gordon Lake Formation display ripple marks, desiccation cracks, cross bedding and a late cleavage . Note: the presence of pyrite in contrast to hematitic nature of the Gordon Lake Formation near the top of the formation. 19.3 km = 12.20 mi. 17 – 361771E, 5166967N STOP 1.16: Lorrain Formation White to pale pink quartz arenite of the upper portion of the Lorrain Formation is exposed on the east side of the highway. Figure 1.26: Pillow structures in mafic metavolanics. Stop 1.12. 21.0 km = 13.3 mi. At Little White River Road (Highway 546) – End of Field Trip Return to Sault Ste Marie. Sandstones of the Bar River Formation display ripple marks, mud cracks and sinuous structures, 22.8 km 17.8 mi. Junction with Little White which have been described as possible worm River road (Highway 546.) casts. Comparison with desiccation structures in End of Day one field trip. the Gordon Lake Formation led Young (1969) PDF compression, OCR, web-optimization with CVISION's PdfCompressor �41 ILSG Field Trip No. 1 Day 2 - Part 1 Huronian Stratigraphy along Highway 556 and correlation with the Chocolay Group of the Marquette area. On Day 2 we will examine some of the Huronian rocks of the area north of Sault Ste Marie where we will see important differences in the stratigraphy and structural relationships when compared to the sequence in the Quirke Lake Syncline of Day 1. The transect along Highway 556 will show that all three formations of the Quirke Lake Group are missing and correlation with formations of the Hough Lake Group is problematical. Also, there is a significant unconformity at the base of the Gowganda Formation. We will spend part of Day 2 examining a dolostone unit in Fenwick Township near the north shore of Goulais Bay, and a dolostone-bearing unit in the Gordon Lake Formation, southeast of Sault Ste Marie. Those familiar with the Kona Formation of the Chocolay Group in the Marquette Range Supergoup will notice the correspondence in lithology and stratigraphy. Road Log and Stop Descriptions (Refer to Figures 2.1 and 2.2) Island Lake fault in this area are locally chloritized and albitized micro-breccia. The lack of such alteration in the Huronian rocks south of the fault suggests that the alteration pre-dates Huronian sedimentation (Delio Tortosa personal communication, 2005). IMPORTANT: When observing roadside rock-cuts, be especially careful to park your vehicle well away from the pavement and exercise the necessary caution when crossing STOP 2.1: Island Lake Fault Zone the road. Where stops are located on or near blind turns, park in a safe place and walk to the Rocks of the Island Lake Fault Zone separating rock exposure. the Archean rocks on the north from Huronian rocks on the south are exposed along Highway GPS datum is NAD 83 =WGS 84. UTM 638 near Lower Island Lake. Rocks within the Zone17. fault zone are highly sheared and brecciated. Pink rock in the shear zone is a Keweenawan 000 km- Second Line and Highway 17 N. felsic intrusion (felsite). Continuing north on Highway 17. 13.3 km. = 8.2 mi. Intersection of Highway 17 6.4 km. = 4.9 mi. - Railroad crossing and Highway 566. Reset odometer to 0. 8.1 km. = 5.0 mi. 16-709788E, 5175568N Proceed east on Highway 556. 5.6 km. =3.5mi. Fractured altered Archean rocks. The Archean granitic rocks north of the Park on shoulder just west the outcrops. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �42 1a 6 1a 1a Hy 556 2.7 9 7 9 9 2.4 2.3 6 2.6 7 1a 2.2 2b 6 7 2.5 ACR 2b Hy 552 2.1 4 2a 2a o -50 25 Jarvis Lake 2b 20 o 4 5 2a Reserve Lake o -45 5 15 Aweres Lake N 3 -45 Hy 5565 Island Lake Upper Island Lake LEGEND 0 2 1 Keweenawan Supergroup 9 1 Jacobsville Fm. Red Sandstone, conglomerate 2 Gabbro, diabase Huronian Supergroup Mafic flows Livingstone Creek Fm 3 Sandstone, conglomerate Archean 2b. Granitic gneiss 2a. Massive granitic rocks 2a, 2b 7 Lorrain Fm. Quartz arenite, quartz-pebble conglomerate. 6 1 Mafic metavolcanics Gowganda Fm Mudstone, siltstone, diamictite 5 km Thessalon Fm 4 Nipissing 8 3 miles Aweres Fm Sandstone, polymictic conglomerate 2.4 Field trip stop. (Day2) Fault (inferred or assumed) Figure 2.1: Geology of part of Highway 566. (Day 2) From Bennett et al, 1975 Frarey, 1977, unpublished maps and notes of P.E. Giblin and E. J. Leahy. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Jacobsville Fm. Red sandstone, conglomerate Gordon Lake Fm. Lorrain Fm. Keweenawan Mudstone, sandstone, minor dolostone in the Gordon Lake area and Fenwick Twp. only. Quartz arenite, quartzpebble conglomerate Cobalt Group (2.21 Ga) Gabbro, diabase, granophyre 43 Mudstone, siltstone Nipissing Diaabse Gowganda Fm Arkose, siltstone Diamictite, polymictic conglomerate Unconformity Hough Lake Group ? Mainly arkose, subarkose & pebble conglomerate Aweres Fm. ( = Ramsay Lake / Mississagi Fm. ? ) Mainly subarkose, granite & volc. cobble conglomerate Thessalon Fm. U, Th Disconformity Metabasalt, basaltic andesite Minor qtz. peb. cong. near base Disconformity Livingstone Creek Fm. Archean Subarkose, conglomerate Unconformity Elliot Lake Group Mainly volcanic cobble conglomerate Metavolcanics, metasediments, granitic rocks Figure 2.2: A stratigraphic column for the Highway 556 transect PDF compression, OCR, web-optimization with CVISION's PdfCompressor �44 Symbols Fault Unconformity Gowganda Formation Stop 2.7 Archean metabasalt Belleview Fault Stop 2.5 Stop 2.6 Gowganda Fm. Stop 2.2 Stop 2.4 Stop 2.3 Stop 6 Lorrain Formation Archean Archean Metabasalt syenite Nipissing Gabbro Island Lake Fault Aweres Formation Huronian Metabasalt Stop 2.1 Hydrothermally altered Archean granitic rocks Livingstone Creek Fm. (Not exposed on Highway 556) Figure 2.3: A diagramatic cross-section along the Highway 556 transect. I' i1'jq G. Bennett, 2005 Figure 2.4: Gowganda Formation. (Behind figure) unconformably overlying conglomerate of the Aweres Formation. Stop 2.2. Figure 2.5: Small dropstone in laminated siltstone of the Gowganda Formation. Stop 2.3. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �45 STOP 2.2: Gowganda Formation unconformably overlying Conglomerate of the Aweres Formation. On south side of highway an almost black, volcanic pebble to cobble, clast-supported conglomerate of the Aweres Formation is overlain by massive, grey, pebbly wacke of the Gowganda Formation. The contact is abrupt and visible in outcrops along both sides of the The Aweres Formation seems to have been deposited as alluvial fans along the valley walls of a down-dropping fault block (rift valley?). The surrounding plateau of Huronian volcanic rocks were eventually eroded to the underlying Archean basement. The Aweres Formation is probably stratigraphically equivalent to the Mississagi Formation. At the north boundary of the Garden River Indian Reserve, about 12 km southeast of this area, the writer noted a southward transition from Aweres-type conglomerate to the sandstones of the Mississagi Formation (Bennett and Sawiuk, 1979). 9.2 km = 5.6 mi. 16 - 710458E; 5176230N STOP 2.3: Drop-stones of the Gowganda Formation The vertical face of an outcrop of laminated argillite of the Gowganda Formation on the Figure 2.6: Polymictic conglomerate of the Aweres south side of the highway contains a few Formation, Aweres Twp. pebble-size dropstones. (Figure 2.5). The juxtaposition of pebble to boulder-sized clasts, normally indicative of a high-energy highway at this location (Figure 2.4). The entire environment, with laminated mudstone of a low Quirke Lake Group is missing in this area energy environment is unusual. Dropstones are (Figures 1.3, 2.2). It is possible that tectonic the important indicators of paloclimate because, activity north of this area, such as that which in Precambrian rocks, dropstones can only be produced the Kapuskasing structure may be responsible for the uplift and erosion of some of explained as a rain of coarse sediment from floating ice onto silt and mud of a relatively still the sub-Gowganda rocks in this area. lake bottom or sea floor. It is the presence of dropstones at within stratified, fine-grained The lithology of the Aweres Formation seen metasediment of Gowganda Formation (also the here is identical to the lowermost part of the Aweres Formation as seen directly overlying the Bruce and Ramsay Lake Formations) which points to the glaciogenic origin of Huronian Thessalon Formation in the Jarvis Township a matrix supported conglomerates. A very few kilometres southeast of this point. The proportion of volcanic clasts within the Aweres impressive occurrence of dropstones is found in formation decreases as the proportion of granitic a cliff of Gowganda Formation about 50 meters clasts and sandstone increases with stratigraphic west of Highway 129 about 30 km north of the intersection of Highway 129 and Highway17. height (Figure 1.14) (Bennett et al., 1975). PDF compression, OCR, web-optimization with CVISION's PdfCompressor �46 9.5 km. = 6.9 mi. 16 - 710962 E, 5176196 N granitic rocks. About 15 meters of overburden separates the granitic rocks from an outcrop of STOP 2.4: Contact between Gowganda grey pebbly wacke assigned to the Gowganda Formation and Huronian metabasalt of the Formation. The east end of the outcrop (nearest the syenitic rocks) contains angular Thessalon Formation blocks of reddish the syenite/syenodiorite suggesting an unconformable relationship Park on south shoulder just west of outcrop. Pebbly wacke and matrix supported, polymictic rather than a fault. The unconformity crosses pre-Gowganda rocks that dip from 10o to 30o conglomerate with abundant granite clasts west. (Gowganda Formation) show an abrupt contact with the dark-grey Huronian basalt 12.6 km. - 7.9 mi - Quartzite of the Lorrain flows to the east. The contact between the Formation. Fractured reddish quartzite volcanics and the Gowganda conglomerate strikes 135 dips 65 W. Faint bedding (?) in Gowganda Formation is essentially parallel to 14.9 km. = 9.2 mi. 16- 714951E, 5177887N the contact with the volcanic rocks. There is minor fracturing parallel to the contact in the STOP 2: 6: Jasper Pebble Conglomerate Member of the Lorrain Formation. volcanics but no significant deformation is noticeable in the conglomerate near the This striking rock is a pebbly sandstone and contact. The writer (eventually) concluded quartz pebble conglomerate known locally as that the contact is an unconformity (Figure “puddingstone”. The abundant pebbles of 2.3). variously coloured jasper presumably had a provenance in Archean iron formation, which The Huronian volcanics (Thessalon Formation) contains clots of altered plagioclase is well represented in the Superior Province to the north. This member is in about the crystals (glomeroporphyritic texture). Whole rock chemical analyses of the basaltic flows in stratigraphic middle of the Lorrain Formation this area show the elevated soda similar to that (Frarey 1977). At this point the dip of the bedding here is vertical or nearly so. A possible of spilite. Albite, clinozoizite, actinolite and some (original) clinopyroxene are the essential axial plane cleavage is visible, suggesting the rocks lie on the south limb of an anticlinal minerals. structure. 10.8 km. - Railroad trestle. 16.4 km.= 10.25 mi - Northland Lake Road. 11.3 km. = 7.2 mi. 16- 712648E, 5176124N. Sharp Corner – watch for traffic. You may park 17.6 km. = 10.9 mi (Optional) on the level area south of the Highway if your vehicle has a relatively high road clearance. STOP 2.7: Outcrop of Archean metavolcanic rocks. STOP 2.5: Archean granitic rocks and Gowganda Formation. The easternmost, high rock-cut is of pink, coarse-grained Archean syenite to syenodiorite. Two mafic dikes intrude the Fine to medium-grained amphibolite is part of the hornfels zone near the granitic plutons. End of road log for Highway 556. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 2.7: Dolostone of the Kona Formation. Highway 63, Marquette area, MI. Figure 2.8: Dolostone and chert. Fenwick Township, ON. 47 0 .:Y3 Figure 2.10: Jasper-pebble conglomerate member of the Lorrain Formation. Stop 2.6. Figure 2.9: Dolostone nodules and lenses (dark areas) in the Gordon Lake Formation. Near Highway 638, Gordon Lake area, ON. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �48 Return to Highway 17. At intersection of Highway 17 and Highway 556 reset odometer to 0. Continue north on Highway 17 to Highway 552 (about 13 km). Turn left (west) onto Highway 552. 5.5 km = 3.4 mi. Goulais Mission Road. Turn right onto Goulais Mission Road. 11.4 km = 7.1 mi. Private road of Case Construction Company. Permission must be obtained from Case Construction before continuing. We will continue to the end of the road as indicated by a locked gate. A short walk (> 100m) in a southwest direction will bring us to a small hill we refer to as the “dolostone knob”. STOP 2.8: Outcrops of Dolostone at the Dolostone Knob. Thinly bedded and laminated dolostone and chert are found along the steep sides of an outcrop knob. Some laminated silty sections may be stromatolitic bedding. STOP 2.9: This stop will be visited as time allows. This stop is located near Highway 638. Directions and a stop description are included in the supplementary road log for Highway 638. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �49 Geological Features and Correlation of a Dolostone Unit in Fenwick Township, Northeast of Sault Ste Marie, Ontario millimeters to at least 1 m thick. The metachert beds have a similar thickness range but form less than 25 percent of the visible section (Figure 2.8). No clearly identifiable stromatolitic structures were observed but irregular and wavy, closely spaced laminae of brownish chert, suggestive of algal mats, were noted. Pale grey, clastic dolostone and oolitic dolostone appear to be the main rock types in the upper part of the carbonate unit. A few lenses, up to 50 cm wide, of coarse, pink barite are found within dolostone on the south side of the knob. Born (1987) described, a previously unrecognized occurrence of dolostone and associated clastic rocks in Fenwick Township, approximately 20 miles (30 km) northwest of Sault Ste. In the spring and fall of 1988 the writer mapped the known extent of the dolostone, and some of the surrounding rocks, was mapped by at a scale of about 1:8000 (Bennett et al, 1990). Mr. Paul Morra of Sault Ste Marie provided a stadia survey of selected areas. 1 1 1 1 1 1 1 1 1 3 Gravel Pit 1 4c 1 4c 70 4c,b 3 3 4c,b 1 1 1 3 3 1 1 75 4b 3 4b 75 4b 3 3 Dolostone knob 4b 3 4b 4a 35 65 4b 3 No outcrop 4c 2 3 Sand and gravel 2 Mi ss North 0 100 200 Meters 300 Legend Gordon Lake Fm 4a. Red Sandstone, siltstone 2 2 3 4c 4b 4b 4b 70 io n 4a Ro ad 4b. Dolostone, chert 4c. Siltstone, mudstone Lorrain Fm 3. Quartz arenite Outcrop with code Fault Gowganda Fm 2. Mudstone, sandstone Archean Figure 2.11: Geology of dolostone occurrence in Fenwick Township. 1. Metavolcanics Geology by G. Bennett, 1988 The general geological setting of the dolostone units in Fenwick Township is shown on Figure 2.11. The best exposures of the dolostone unit are found in the area indicated on the figure as the “dolostone knob”. In the area of the dolostone knob, the lowermost third of the dolostone sequence consists mainly of very fine-grained, pale pink to reddish pink dolostone with intercalated grey to pink metachert. Individual dolostone beds are from a few At the western side of the knob, the clastic dolostone is directly overlain by deep-red sandstone, and red to pink laminated siltstone. The dolostone unit is underlain by green and maroon siltstone and argillite. The southern contact between the Lorrain quartz arenites and the dolostone is not exposed but lies within an east-trending, narrow, linear, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �50 recessive zone about 10 m wide, and is inferred to be a fault. lithological and stratigraphic similarities of the Chocolay Group (and equivalent groups further west) and the Cobalt Group have been pointed Correlation with the Gordon Lake out by many authors (Figure 2.12). At the time, Formation this correlation was not widely accepted because previous radiometric dating of rocks considered Red siltstone and sandstone, purple and green to underlie the Chocolay Group was seen as siltstones, chert and dolostone of Fenwick providing evidence that the Chocolay Group is Township have been assigned to the Gordon significantly younger than the Huronian Lake Formation of the Huronian Supergroup Supergroup of Ontario (Young, 1983). (Bennett et al, 1990). This correlation is However, Vallini et al (2005) have successfully indicated by the juxtaposition of the dated hydrothermal xenotime in the Kona dolostone and associated rocks with the Formation and detrital zircons in the underlying Lorrain Formation (Although this is likely a Mesnard Formation thereby constraining the fault contact). The reddish hues of the associated sandstones Gordon Lake area Flack Lake Area Marquette Area Fenwick Twp indicate they are Sault Ste Marie Bar River Fm. Wewe Slate Ontario stratigraphically superior Kona Gordon Lake Fm. Cu Cu Fault Fm. to the Serpent Formation Mesnard Fm. Lorrain Fm Lorrain Fm (Roscoe (1969). Most Gowganda Enchantment L. Fm. Fm importantly, nodules and Gowganda Fm. Archean Archean continuous beds of 250 km dolostone have been Lower Huronian Groups Dolostone identified in the Gordon Lake Formation of the Diamictite 130 km Gordon Lake area SIltstone, sandstone (Jackson, 2001; Hofmann Slate, argillite et al 1980). Quartz arenite Figure 2.12: Proposed stratigraphic relationships between the During a discussion (in Lower Proterozoic rocks of Ontario and Michigan. 1988) of the problematic correlation of the timing of deposition of the Chocolay Group dolostone units with Mr. Ken Hatfield of between 2300-2200 Ma thus proving a Lake Superior State University, the latter correlation with the Huronian Supergroup of suggested a correlation with the Kona Ontario. Dolomite of the Marquette, Michigan area (Figure 2.7). There seems litte doubt now that the Cobalt Correlation with the Chocolay Group of Group rocks of the Huronian Supergroup are Michigan stratigraphically equivalent to the Chocolay Group of the Marquette Range Supergroup of There has been a long-standing debate as to Michigan and to correlative units further west. stratigraphic and temporal equivalence of the Huronian rocks of Ontario and the Middle Precambrian rocks of the Lake Superior region of Michigan (Young, 1983) The PDF compression, OCR, web-optimization with CVISION's PdfCompressor �51 Supplimentary Road Log for Highway 638 Highway 638 extends north from Highway 17 at A few small, ground-level rock exposures on the Echo Bay and continues in a broad loop to rejoin north side of the road near the western limit of Highway 17 at Bruce Mines (Figure 1.18). the northern rock-cut exhibit ancient mud-cracks as polygonal patterns of apple-green Only STOP 2-9 of this road log is part of the mudstone and pink sandstone. Day 2 sequence. Stops preceded by “S” are supplementary or optional stops. A few of these Ripple-marks and laminated, red siltstone can be stops may be made if time allows. The seen at about midpoint on the north outcrop. remaining outcrop descriptions are abstracted Small-scale (less than 20 cm) cross-bedding is from a previous guidebook (Bennett et al, 1997) well developed in pale grey and pink sandstone for the general interest of the reader. on the eastern portion of the southern road-cut but can be observed in most of the sandstone of the road-cut. Road Log and Outcrop Descriptions. 0 km = 0 mi. The Husky Service Station on Highway 17 near the eastern limits of the City of Sault Ste. Marie. 17.8 km =11.1 mi. Junction of Highway 638 and Highway 17 in the Town of Echo Bay. Reset odometer. Continue along Highway 638. 5.4 km = 3.4 mi. Outcrops of pink weathering sandstone of the Lorrain Formation. 10.6 km = 6.6 mi. Large road cuts on both sides of highway. The west limit of the outcrop area is at NAD 83 (WGS 84) 17-273278E, 5015292N. STOP S1-1: Lowermost Gordon Lake Formation The sandstones and minor mudstone exposed in the rock cuts are either the lower units of the Gordon Lake Formation or the uppermost rocks of the underlying Lorrain Formation. A large portion the sandstone is “bleached” almost white in contrast to the pink colour dominant in much of the outcrop. Note that the boundary between the pink and pale-grey sandstone clearly crosses the stratification in the rocks. That the bleaching (reduction) is the later event can be seen near the eastern limit of the road-cut. There the pale-grey sandstone is cut by dark red, hematite staining for several meters on both sides of steeply dipping, northeast striking fractures in the sandstone. A wide zone of reduction can be seen on both sides of the oxidized fractures. A hypothesis consistent with these observations is that chemically active fluids passing upward though the fractures first reduced the red hematite in the pink feldspar of the sandstone over a broad area along the fracture and in doing so became enriched in ferrous iron. Some time later, the system became oxidizing resulting in the precipitation of the much less soluble ferric iron as hematite adjacent to the fracture zone. These later fluids may have been oxygenated groundwater percolating downward from the surface encountering the ascending Fe+2 bearing reducing fluids that had scavenged iron from the underlying rocks. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �52 A mafic dike about 10 cm wide intrudes the sandstone near the fracture zone. The dike occupies a fracture with a strike (direction) and dip similar to that of the hematite-filled (reddened) fractures. This suggests that hot magma was in the vicinity when the fractures existed and thus may have provided the thermal energy to drive the fluids through the fracture system. 10.9 km = 6.7 mi. East end of outcrop at 17-273416E, 5150243N. 11.6 km = 7.2 mi 17 – 274165E, 5150594N STOP 2.9: Dolostone nodules and discontinuous dolostone beds in the Gordon Lake Formation. NOTE: The outcrop at this location is on private property. To protect the rights of the property owner the exact location of the outcrop will not be provided here. Outcrops of buff coloured siltstone and sandstone contain deeply recessed, discontinuous beds and scattered dark-brown to black dolostone. Jackson (1994) states: “Buff brown to black weathering siliceous dolostone nodules are common throughout the lowermost Gordon Lake Formation. The nodules are commonly amoeboid and concentrically zoned with carbonate-rich core and dark hematitic-rich rinds. A large number of the amoeboid forms are, however, asymmetrically zoned with the hematitic rind being concentrated, or better developed, towards the stratigraphic top of the amoeboid forms suggesting at least local upward flow of fluids. In red to deep maroon siltstones, the carbonate-rich nodules are very pale pink to white, reflecting the widespread reduction and/or removal of iron from these volumes of the rock” Return to Highway 638. 23.2 km = 14.4 MI. Village of Leeburn Corner and general store. 25.0 km = 15.3 mi. East side of road. Dark grey-green mudstone and siltstone of the Gowganda Formation. Thin-bedded to very thick bedded with some thin beds of pink siltstone. 25.3 km = 15.8 mi. Dark grey siltstone with smooth, striated outcrop surface due to most recent (Pleistocene) glaciation. 26.4 km = 16.4 mi. Nipissing granophyre and diabase, fractured and cut by calcite and iron carbonate veinlets. 30.7 km = 19.1 mi. Stop Sign at the village of Ophir. 32.7 km = 19.7 mi. West side of highway. STOP S1-2: Mudstone overlying matrix-supported conglomerate (diamictite) of the Gowganda Formation. Matrix supported conglomerate consist of pebbles and bounders dispersed in a fine matrix so that the pebble and boulder sized clasts are supported by the matrix, not each other. Such conglomerates cannot be formed by the transport and deposition of clay to boulder-sized clasts by water or wind. Matrix supported conglomerates usually imply deposition from mudflows or directly from ice as glacial till. 32.9 km = 20.4 mi. West side of Highway. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �53 STOP S1-3: Dropstones in laminated siltstone and mudstone e of the Gowganda Formation. STOP S1-5: Jasper pebble conglomerate of the Lorrain Formation (“Pudding Stone”). A few granitic pebbles lie within dark grey laminated siltstone. It is generally agreed that the only process that can allow the juxtaposition of pebbles or boulder-sized clasts in stratified, fine-grained sediment of is the release of stones by melting icebergs onto the sea bed or lake bottom. Outcrops of pale-grey quartz arenite and jasper-quartz pebble conglomerate are found at east side of the highway. It is assumed that the jasper pebbles were derived from Archean banded iron formation, which is well represented in the Batchawana greenstone belt about 130 km northwest of these outcrops. The jasper-pebble conglomerate member (Frarey, 1977) of the Lorrain Formation is a minor but persistent unit. It has been found near the middle of the Lorrain Formation from the Sault Ste Marie area in the west to the Quebec border in the east. Although the high concentration of red, maroon and pink jasper presents a striking appearance it has a limited commercial use as a decorative stone because of its extreme hardness. 33.6 km = 20.9 mi. Large gravel pit with high rock face on east side of the highway. A short unsurfaced road leads to the gravel pit. STOP S1-4: Diamictite of the Gowganda Formation. The high rock face is worn smooth by rushing water charged with abrasive sand etc. Vertical channels in the rock face suggest the erosive effect of sand-charged water under the extreme pressure of an overlying ice sheet. The northern section of the outcrop is matrix-supported conglomerate of the Gowganda Formation. A diabase dike (probably of Nipissing age) intrudes the southern part of the outcrop. The dike is about 20 meters wide (estimated). 42.8 km =26.6 Outcrops of Lorrain Formation sandstone on west side of highway. At least two faults show hematization in the fracture zone. A pebble unit is seen to be displaced along one of the fault. 44.1 km = 27.4 mi. Bridge at Rydal Bank. 36.2 km = 22.5 miles Rock Lake Road. 52.7 km = 32.7 mi Junction of Highway 628 and Highway 17 at Bruce Mines. 39.8 km = 24.7 mi. End of road log for Highway 638. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �54 Supplimentary Road Log for parts of Highway 17 and Highway 129 between Sault Ste Marie and Highway 108 Most of the outcrops included in this section are not included in the ILSG 2006 field trip. Stops indicated as supplementary are preceded by the letter “S”. They are included to allow those interested to visit them if traveling in the area at some future date. UTM coordinates have been provided for some locations. Note: the differing ripple directions and local interference ripples. This impressive outcrop is known locally as the ”Ripple Rock”. 26.8 km = 16.6 mi. STOP S2.3: Purple siltstone member of the Lorrain Formation. Dark red to purple siltstone and sandstone Proceed east along Highway 17 from Sault Ste. constitute true red-beds in the Huronian (Frarey, 1977). These hematite-rich rocks Marie. display the reduction along fractures typical of The road log starts at the intersection of red beds, and a good indicator of deposition in Highway 638 and Highway 17 East in the an oxidizing environment. The bedding is Town of Echo Bay faint, but locally distinct, striking 140o and dipping 10o south. 0.0 km = 0.0 miles - Highway 638 at Echo Bay - continue east on Highway 17. 27.7 km = 17.2 mi. Lake Huron Drive in the town of Desbarats. 8.8 km = 5.5 miles - At sign for Calabogie Road UTM 16- 725322E,5144204 N,. 28.0 km = 17.4 mi. 17- 5136434N, 275510E, STOP S2.1: Sub-Jacobsville unconformity STOP S2.4: Basal Arkose Member of Lorrain Formation. Outcrop on west side of Highway 17. The most southerly outcrop is white and grey quartzite (quartz arenite) of the white orthoquartzite member (Frarey, 1977) of the Lorrain Formation quartzite is overlain by a sedimentary breccia of the Keweenawan, Jacobsville Formation. The conglomerate is comprised of angular clasts of Lorrain quartzite up to 60 cm across in a red siltstone matrix. The contact is irregular. No stratification was observed. Large outcrops of pink, medium-grained, rather massive arkose beds with hematite-rich spots about one centimeter across. Pale laminations in outcrop on south side of the road indicate shallow dips of about 10o west. The unit is about 1700 feet thick in this area (Frarey, 1977). 40.5 km = 25.0 mi. STOP S2.5: Bruce Mines Copper Vein. 25.0 km = 15.5 mi. In a rock-cut on the north side of Highway 17, in the town of Bruce Mines, is a 3 m wide STOP S2.2: Lower red quartzite member quartz vein in Nipissing diabase. The vein contains small amounts of chalcopyrite, of the Lorrain Formation. chalcocite, bornite and malachite. It is one of On the north side of Highway 17 steeply the few remaining remnants of the copper dipping beds of the lower red siltstone member mineralization that once made this town of the Lorrain Formation reveal well preserved known throughout the mining world. Mining oscillation ripples an a large south facing began here in 1846 and continued to 1875, surface. making Bruce Mines the first mining town in Canada. Please do not take samples from this outcrop. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �55 About 75 meters north of this outcrop there is a reconstruction of a copper mining operation of that period built on one of the original mine workings, known as the “Simpson Shaft”. 42.6 km = 26.5 mi. Entrance to the quarry of Ontario Trap Rock (R.W. Tomlinson Limited). The quarrying of trap rock (Nipissing diabase) at Bruce Mines resumed in 1990. A predecessor near Lake Huron operated during the First World War. chlorite. The relatively small, monomineralic chlorite amygdules (“chlorite buttons”) are characteristic of the mafic volcanics of the Thessalon Formation between Sault Ste. Marie and Elliot Lake. Glacial striae and chatter marks are well developed on the outcrop surface. Proceeding north on Highway 129. Reset odometer to 00 at Intersection of Highway 17 and Highway 129. 45.3 km = 28.1 mi. 17-289512E, 5129999N 1.0 km = 0.62 mi. STOP S2.6: Low outcrops of Mississagi Formation. STOP S2.8: Outcrop of Matinenda Formation on east side of Highway 129. Medium-sand to grit-sized, grey, subarkose with thin quartz-pebble beds and prominent planar cross-bedding are well exposed along the north side of the highway. Rusty staining on the outcrop reflects minor detrital (?) pyrite along the foreset beds. Black chert pebbles in pebbly beds, minor pyrite and meter scale bedding are common features of the Mississagi Formation. Roscoe, (1969) concluded that the drab colour and presence of detrital pyrite in Huronian sandstones and conglomerates is evidence of the reducing nature of the early Huronian atmosphere. 45.4 km = 28.2 mi. Waltonen Road near east end of the outcrop area. This fine-grained, pale pink to greenish grey, subarkose which directly overlie the Thessalon Formation in the Thessalon area has been correlated with the Matinenda Formation of the Elliot Lake Group (Bennett et al., 1991). Faint outlines of trough cross-bedding, visible on the upper surface of the outcrop, indicate paleocurrent directions from the northwest. The greenish (sericitic) units and the trough cross-beds are typical of the Matinenda Formation, but the fine grain size is not. At Thessalon Point about 4 km southwest of this location these rocks contain thin quartz pebble conglomerate units, but the pyrite and radioactivity, characteristic of the Matinenda Formation of the Elliot Lake area, is lacking. 60.6 km = 37.6 mi. 17 - 303650E, 5126650N 23.0 km = 14.9 miles The village of Warncliffe Intersection of Highway 17 and Highway 129. 29.1 km = 18.4 miles north end of Appleby Lake. Stop S2.7: Thessalon Formation tholeiitic basalt. Dark green, fine to medium grained basalt is exposed on the east side of Highway 129 just north of the intersection with Highway 17. This tholeiitic basalt forms the top of the Thessalon Formation in the Thessalon area and is typical of the thicker tholiitic sequences which make up the upper parts of the Thessalon Formation in the Sault Ste. Marie and Aberdeen Township areas. The rocks consist of albite, chlorite, epidote, clinozoisite, leucoxene and minor quartz and oxides. The metabasalt contains amygdules of albite, quartz, epidote, calcite and 30.3 km = 19.4 miles UTM 17, 5 145 009N, 320 904E STOP S2.9: Laminated siltstone of the Gowganda Formation with dropstones. West of Highway 129. Proceed up steep slope to cliff face located a few tens of meters into the bush. Vertical joint faces in laminated siltstones-mudstone of the Gowganda Formation form an impressive cliff just west of Highway 129. The bedding comprises regular and remarkably continuous mudstone-siltstone PDF compression, OCR, web-optimization with CVISION's PdfCompressor �56 couplets from 0.5 to 2 cm thick. The grey siltstone unit at the base of each couplet grades up to a darker mudstone which is in sharp contact with the siltstone base of the overlying couplet. The sequence closely resembles varves of Pleistocene glacial lakes. A few pink, discontinuous, fine, sandstone beds are also present. Widely scattered trough the sequence are pebble to boulder-sized “drop stones”, predominantly of Archean granitic rocks. In the absence of volcanic activity within the Gowganda Formation, the only plausible source of the drop stones is the presence of floating ice which released its rock load during melting. This outcrop, and many others of a less spectacular nature, provide some of the best evidence for a cool climate during Gowganda deposition more than 2 Ga.. Return to Highway 17 Reset Odometer. 3 km = 1.9 Pine Ridge Road Proceeding east on Highway 17 from Pine Ridge Road Reset odometer to 0. 13.5 km = 8.4 mi. Melwell Road 14.1 km = 8.7 mi. 17-321874E, 5129338N – Outcrop with prominent white quartz veins near crest of hill . STOP S3.1: Mineralized quartz vein breccia. Quartz-carbonate veins and vein breccia cut pebbly siltstone of the Gowganda Formation Church and Young, (1972) interpret the on the south side of Highway 17. The vein laminated unit as a varved sequence. They and mineral assemblage also note the presence of small sedimentary (quartz-carbonate-pyriteclasts, lithologically similar to the enclosing rocks. They interpret the clasts of sedimentary chalcopyrite-hematite) are typical of many in rocks as having been derived from the ablation such occurrences the area between Sault Ste. of sediment on the upper surface of the glacier. Marie and Sudbury. The extensive brecciation suggests surface venting with resulting 30.9 km = 19.5 miles. Intersection with hydrothermal boiling, adiabatic cooling with Highway 554. Return to Thessalon along coincident brecciation and mineral deposition. Highway 129. Some sections of the vein system show well-developed mineral zoning and pre-vein 30.9 km = 19.5 miles. Intersection of alteration. Minerals identified include: quartz, Highway 17 and Highway 129 at Thessalon. iron-carbonate (ankerite?), pink calcite, Reset odometer to 0.0 chalcopyrite and specular hematite. The lack 7.3 km - 4.6 miles. 17 - 306085E, 5126967N. of pyrite and the late deposition of hematite, indicates a sulfur deficient hydrothermal fluid. Green Lane. Cross the railway tracks (with care) and immediately turn left then south to short section of unpaved road between private 47.6 km = 29.6 mi. Town of Blind River properties. Outcrops of Thessalon Formation 66.7 km = 41.4 mi. Pronto Road. 17 – felsic metavoclanics lie at the end of the road. 367425E, 5117509N STOP S2.10: Rhyolite of the Thessalon Formation. Access to granitic paleosol and a remnant of radioactive quartz-pebble conglomerate of the former Pronto Mine. Volcanics are fine grained, grey to pink and maroon rhyolite consisting of a fine grained mosaic of albite, k-feldspar, and quartz with At the time of writing (2006) the Pronto Mine minor green pleochroic biotite. Amygdules are site is currently not open to the public. Those filled with quartz, biotite and stilpnomelane. planning to visit this site should contact the Collapsed vesicules are also present. Local iron staining and darker chloritic areas indicate the presence of iron sulfides. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �57 A S3.2 Former mine site B Pr on to Th ru st LEGEND Fa ul Diabase t HURONIAN Mine workings Higher Formations Matinenda Fm. Ore bed Paleosol ARCHEAN After Robertson (1970). Granitic rocks N A S3.2 Former mine site Since rehabilitated rust Fault nto Th Pro Hy dro El ec tri c Line Pond Pond B nto Pro d Roa l Fau r r ay Mu t ay ghw 17 Hi Lake Huron 1 km After Robertson (1970). HURONIAN SYMBOLS Formations above the Matinenda Matinenda Formatioin Pater Formation ARCHEAN Granitic rocks S3.2 Field trip stop Fault Projection of original ore body Figure 2.13. Plan and section of the geology of the (former) Pronto Mine area. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �58 displayed as dikes and irregular bodies of aplite and pegmatite, can be recognized in the paleosol (saprolith). Continuing up the southward sloping outcrop the yellowish colour of the paleosol can be seen to take on a pinkish and reddish hue normal to the Archean granitic rocks of the area. Continuing north Turn north onto the Pronto road and continue and west to near the highest point on the until prevented by a gravel barrier from outcrop area one can discern the faint outline proceeding further. Continue on foot past the of core-stones in the original granitic rocks. barrier to the west end f a beaver pond just north of the road. Proceed along a trail across 11.6 km = 7.2 mi Highway 108 to Elliot Lake the west outlet of the pond, then around the Return to Highway 17. Continue east along north end of the pond and up a steep hill. Turn left (to the north) on a gravel road at the Highway 17 to Highway 108 to Elliot Lake. top of the hill. Continue to an area of outcrop Continue eastward for a few hundred meters and gravel which marks the former open stope past Highway 108 to an outcrop just south of of the Pronto mine at 17-367804, 5119019. If Highway 17. not accompanied by a guide, please do not Continue for about 100 m past the intersection attempt to access this site without a GPS of Highway 17 and Highway 108. receiver or compass as well as suitable footwear and clothing. Low outcrops a few tens of meters south of Highway 17. (Refer to Figure XXX). District Geologist office of the Ministry of Northern Development and Mines at Sault Ste Marie, Ontario (705 - 945-6931) to learn of the site status and the phone number of a contact person with the authority to grant permission to access the property. STOP S3.2: Pronto Mine Location This property is currently (2006) undergoing restoration to a condition as closely as possible to it’s original state. STOP S3.3: McKim Formation Staurolite (pseudomorph) schist. This outcrop, which lies a few hundred meters south of the Murray Fault Zone, displays the The Pronto Mine of Rio Algom Limited, the higher metamorphic grade and deformation first operating uranium mine of the Algoma common to rocks south of the fault. At this District, produced 2.1 million tones of ore location argillaceous units of the McKim between 1955 and 1960. (Figure 2.13. Formation contain pale prismatic crystals of staurolite that have undergone retrograde This is one of the few locations where metamorphism to sericite-quartz aggregates. A exposures of uraniferous quartz pebble conglomerate of the Matinenda Formation can few pseudomorphs show the characteristic cruciform twinning of staurolite. The growth of be found on surface in the Elliot Lake-Blind large staurolite crystals in the originally more River area. In the depression south of the argillaceous upper portions of the beds has road, a few remnants of rusty-weathering resulted in “reverse graded bedding” in a few outcrops of radioactive, pyritic, quartz-pebble places. conglomerate of the Matinenda Formation mark the base of the ore bed of the Pronto Return to the intersection of Highway 17 and Mine. The conglomerate lies directly upon Highway 108. Proceed north along Highway yellowish, sericitic paleosol developed on 108 to the Town of Elliot Lake. Archean granitic basement. Visitors are requested not to take samples of the Continue north on Highway 108 to the City of conglomerate. Elliot Lake. Proceeding in a general northerly direction, some features of the original granitic rocks, PDF compression, OCR, web-optimization with CVISION's PdfCompressor �59 Publications Cited or Consulted Bennett, G. 1978. Huronian volcanism, districts of Algoma and Sudbury; in Summary of Field Work, 1978; Ontario Geological Survey, Miscellaneous Paper 82, p.105-111. Bennett, G. 1982. Geology of the Two Horse Lake Area, District of Algoma; Ontario Geological Survey, Geological Report 210, 63p. Bennett, G., Hillier, R. D., Nentwich, F., Dupuis, C. P. and Pucovsky, M.., 1975. Jarvis Lake-Garden River area, District of Algoma, Ontario Division of Mines; Preliminary Geological map, P.1064, 1 inch to ¼ mile or 1:15,840. Bennett, G and Sawiuk, Myron. 1979, Jarvis Lake-Garden River area (Southern Part), District of Algoma, Ontario Division of Mines (Now Ontario Geological Survey); Preliminary Geological map, P.2241, 1 inch to ¼ mile or 1:15,840. Born, Peter, 1987. Geology of the Havilland Bay – Goulais Bay Area District of Algoma; Ontario Geological Survey, Open File Report 5602, 114 p with map at a scale of 1:15 840 (1 inch to ¼ mile) Born, P. and James, R.S. 1978. Geology of the East Bull Lake anorthosite intrusion, District of Algoma, Ontario; in Proceedings and Abstracts, Geological Association of CanadaMineralogical Association of Canada, Joint Annual Meeting, v.3, p.369. Born, P., Worona, R., and Stephenson C., 1986. Precambrian Geology of the Haviland-Goulais Area. District of Algoma; Ontario Geological Survey, Map P2959. Geological Series-Preliminary Map. Scale 1:15 840 or 1 inch to ¼ mile. Geology 1985. Bottrill, T. J., 1971. Uraniferous conglomerates of the Canadian Shield; in Report of Activities, Part A: April to October, 1970, Geological Bennett, G., Leahy, E. J, Melisek, J. Born, P. and Survey of Canada, Paper 71-7, p.77-83. Hatfield, K. 1989. Sault Ste. Marie Resident Buchan, K. L. and Card, K.D., 1985. Preliminary Geologists District—1988; in Report of comparison of petrographic and paleomagnetic Activities 1988, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 142, p. characteristics of Nipissing diabase intrusions in northern Ontario; in current Research, Part A, 207-217. Geological Survey of Canada, Paper 85-1A, Bennett, G., Leahy, E. J. and Walmsley, J. 1990. p.131-140. The Sault Ste. Marie Resident Geologist’s District-1989; in Report of Activities, Regional Cannon,W. F., 1962. Plutonic evolution of the and Resident Geologists, Ontario Geological Cutler area, Ontario; unpublished PhD thesis, Syracuse University, Syracuse, New York, 105p. Survey, Miscellaneous Paper 147, p.205-215. Bennett, G., Dressler, B.O. and Robertson, J.A. Card, K. D., 1964. Metamorphism in the Agnew 1991. The Huronian Supergroup and associated Lake area, District of Sudbury, Ontario, Canada; intrusive rocks; in Geology of Ontario, Ontario Geological Society of America Bulletin, v.75, no. Geological Survey, Special Volume 4, Part 1, p. 10, p.1011-1030. 549-592. Bennett, G., Card K. D. and Tomlinson, K. Y. 1997. The Huronian Supergroup between Sault Ste Marie and Elliot Lake; Institute on Lake Superior Geology, 43rd Annual Meeting, May 6, 11, 1997, Sudbury, Ontario. Card, K. D., 1976. Geology of the MacGregor Bay – Bay of Island Area, Districts of Sudbury and Manitoulin, Ontario Division of Mines Geoscience Report, 131, 63p. Card, K. D., 1978. Geology of the Sudbury-Manitoulin area, districts of Sudbury PDF compression, OCR, web-optimization with CVISION's PdfCompressor �60 and Manitoulin; Ontario Geological Survey, Report 166, 238p. Card, K. D., Church, W.R., Franklin, J.M., Frarey, M.J., Robertson, J.A., West, G.F. and Young, G.M. 1972. The Southern Province; in Variations in Tectonic Styles in Canada, Geological Association of Canada, Special Paper 11, p.335-380. Card, K. D., Innes, D.G. and Debicki, R.L., 1977. Stratigraphy, sedimentology, and petrology of the Huronian Supergroup in the Sudbury-Espanola Area; Ontario Division of Mines, Geoscience Study 16, 99 p. Chandler, F.W., Young, G.M. and Wood, J., 1969. Diaspore in early Proterozoic quartzite (Lorrain Formation) of Ontario; Canadian Journal of Earth Sciences, v.6, p.337-340. Church, W. R. and Young, G. M., 1972. Precambrian geology of the southern Canadian Shield with emphasis on the lower Proterozoic (Huronian) of the North Shore of Lake Huron; International Geological Congress 24th, Montreal, Guidebook to field excursion A36-C36, 65 p. Coleman, A. P., 1905. The Lower Huronian ice age; Journal of Geology, v.16, no. 2, p.149-158. Card, K. D. and Pattison, E. F., 1973. Nipissing diabase of the Southern Province; in Huronian Collins, W. H., 1925. North shore of Lake Huron; Geological Survey of Canada, Memoir Stratigraphy and Sedimentation, Geological 143, 160p. Association of Canada, Special Paper 12, p.7-30. Corfu, F., and Andrews, A., 1986. A U-Pb age Card, K. D. and Palonen, P.A., 1976. Geology for mineralized Nipissing diabase, Gowganda, Ontario; Canadian Journal of Earth Sciences, of the Dunlop-Shakespeare area, District of v.23, p.107-112. Sudbury; Ontario Division of Mines, Geoscience Report 139, 52p. Dietz, R. S. and Holden, J.C, 1966. Card, K. D. and Jackson, S.L., 1995. Tectonics Miogeosynclines in space and time; Journal of Geology, 74, p. 566-583. and metallogeny of the Early Proterozoic Huronian fold belt and the Sudbury Structure Dyer, B. D., Krumbein, W.E. and Mossman, of the Canadian Shield; Field Trip Guidebook, D.J., 1988. Nature and origin of stratiform Precambrian 95, Geological Survey of Canada kerogen seams in lower Proterozoic Open File 3139. Witwatersrand-type placers-the case for Casshyap, S. M., 1969. Petrology of the Bruce biogenicity; Geomicrobiology Journal, v.6, p.33-47. and Gowganda formations and its bearing on Huronian Sedimentation in the Eisbacher, G. H., and Bielenstein, H.U.,1969. Espanola-Willisville area, Ontario, Canada; The Flack Lake Depression, Elliot Lake area, Paeleogeography, Paleoclimatology and Ontario (41J/10); in Report of Activities, Part Paleoecology, v.6, p5-36. B, Geological Survey of Canada, paper 69-1B, Chandler, F. W., 1973. Geology of McMahon p.58-60. and Morin Townships, District of Algoma, Fahrig, W. F., 1987. The tectonic setting of Ontario Division of Mines, Geoscience report continental mafic dike swarms: failed arms and 112, 77p. early passive margins; in Mafic Dike Swarms, Geological Association of Canada, Special Chandler, F. W., 1976. Geology of the Paper 34, p.331-348. Saunders Lake area, District of Algoma, Ontario Division of Mines, Geoscience report Fralick, P. W. and Miall, A.D., 1989. 155, 46p. Sedimentology of the lower Huronian Supergroup (early Proterozoic), Elliot Lake PDF compression, OCR, web-optimization with CVISION's PdfCompressor �61 area, Ontario, Canada; Sedimentary Geology, v.63, p.127-153. Frarey, M J., 1967. Three new Huronian names; Geological Survey of Canada, Paper 67-6, 3p. Frarey, M. J., 1977. Geology of the Huronian belt between Sault Ste. Marie and Blind River, Ontario; Geological Survey of Canada, Memoir 383, 87p. Frarey, M. J. and Roscoe, S.M., 1970. The Huronian Supergroup north of Lake Huron; in Symposium on Basins and Geosynclines of the Canadian Shield; Geological Survey of Canada, 70-40, p. 143-158. Frarey, M. J., Loveridge, W.D. and Sullivan, R.W., 1982. A U-Pb zircon age for the Creighton granite, Ontario; in Rb-Sr and U-Pb Isotopic Age Studies, Report 5, Current Research, Part C, Geological Survey of Canada, Paper 82-1C, p.129-132. Frarey, M. J. and Krogh, T.E., 1986. 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Lithophile elements in Huronian low-Ti continental tholeiites from Canada and evolution of the Precambrian mantle; Earth and Planetary Science Letters, v.85, p.401-415. Junnila, W. T., 1987. A bibliography of the Huronian Supergroup; 1921-1987; Ontario Geological Survey, Open File Report 5651, 71p. igneous province; Ontario Geological Survey Study 58, 80p. Lindsay, D.A., 1971. Glacial marine sediments in Precambrian Gowganda Formation at Whitefish Falls, Ontario; Paeleogeography, Paleoclimatology and Paleoecology, v.9, p7-25. Long, D. G. F., 1976. Stratigraphy and sedimentology of the Huronian (Lower Aphebian) Mississagi and Serpent Formations; Kamineni, D. C., McCrank, G.F.D., Stone, D., unpublished PhD thesis, University of Western Ejeckman, R.B., Flindall, R. and Sikorsky, R., Ontario, London, Ontario, 291 p. 1984. Geology of the central plateau of the Long, D. G. F., 1978. 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The sedimentology of the Huronian Ramsay Lake and Bruce formations, north shore of Lake Huron; unpublished PhD thesis, University of Western Ontario, London, Ontario. Peck, D. C., James, R. S., and Chubb, P. T. , Previc, S. A. and Keays, R. R., 1995. Geology, metallogeny and petrogenesis of the East Bull Lake Intrusion, Ontario; Ontario Geological Survey, Open File Report 5923, 117p. Pettijohn, F. J., 1970. The Canadian Shield: a status report; in Symposium on Basins and Geosynclines of the Canadian Shield, Geological Survey of Canada, Paper 70-40, p.329355. Pettijohn, F. J., 1975, Sedimentary Rocks, Harper and Rowe, 628p. Pienaar, P. J., 1963. Stratigraphy, petrography and genesis of the Elliot Group, Blind River, Ontario, including the uraniferous conglomerate; Geological Survey of Canada, Bulletin 83, 140p. Prasad, N. and Roscoe, S M. ,1991. Profiles of altered zones at ca 2.45 Ga unconformaties beneath Huronian strata, Elliot Lake Ontario: evidence for early Aphebian weathering under anoxic conditions; in Current Pretorius, D.A. 1981. Gold and uranium in quartz-pebble conglomerate; Economic Geology, 75th Anniversary Volume, p.117-138. Rainbird, R. H., Nesbit, H. W. and Donaldson, J. A., 1990. Formation and diagenesis of sub-Huronian saprolith: comparison with a modern weathering profile; Journal of Geology, 98, p. 801-822. Rice, R. J., 1991. Regional sedimentology and paleoplacer gold potential of the Lorrain Formation, Huronian Supergroup, in the Cobalt plain; Ontario Geological Survey, Open File Report 5761. Riller, U. P. ,1996. Tectonometamorphic episodes affecting the southern footwall of the Sudbury Basin and their significance for the origin of the Sudbury Igneous Complex, Central Ontario, Canada; unpublished Ph.D. thesis, University of Toronto, 135 p. Robertson, J. A., 1961. Geology of townships 143 and 144; Ontario Department of Mines, Geological Report 4, 66p. Robertson, J. A., 1962. Geology of Townships 137 and 138; Ontario Department of Mines, Geological Report 10, 94p. Robertson, J. A.,1968. Geology of Township 149 and Township 150, District of Algoma; Ontario Department of Mines, Geological Report 57, 162p. Robertson, J. A., 1970. Geology of the Spragge area, District of Algoma; Ontario Department of Mines, Geological Report Number 76, 109p. Robertson, J. A., 1976. The Blind River uranium deposits; the ores and their setting, Ontario Division of Mines, Miscellaneous Paper 65, 45p. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �64 Robertson, J. A., 1986. Huronian Geology and the Blind River (Elliot Lake) uranium deposits, 7-31, the Pronto Mine, in Uranium Deposits of Canada, Canadian Institute of Mining and Metallurgy, Special Paper 33, p. 46-43. Sutton, S.J. and Maynard, J.B. 1992. Multiple alteration events in the history of a sub-Huronian regolith at Lauzon Bay, Ontario, Canada; Canadian Journal of Earth Sciences, vol. 29, p. 432-445. Robertson, J. A., Frarey, M .J. and Card, K. D., 1969. The Federal-Provincial Committee on Huronian Stratigraphy: Progress Report; Canadian Journal of Earth Sciences, v.6, p.335-336. Sutton, S.J. and Maynard, J.B. 1993. Sediment and basalt hosted regoliths in the Huronian supergroup: role of parent lithology in middle Precambrian weathering profiles; Canadian Journal of Earth Sciences, vol. 30, p. 60-76. Roscoe, S. M., 1969. Huronian rocks and uraniferous conglomerates; Geological Survey of Canada, Paper 68-40, 205p. Theis, N.J. 1979. Uranium-bearing and associated minerals and their geochemical and sedimentological context, Elliot Lake, Ontario; Geological Survey of Canada, Bulletin 304, 50p. Roscoe, S. M., 1981. Temporal and other factors affecting deposition of uraniferous conglomerates; in Genesis of Uranium- and Gold-Bearing Precambrian Quartz-Pebble Conglomerates; United States Geological Survey, Professional Paper 1161-A-BB, p.W1W7. Tomlinson, K.Y. (1996). The geochemistry and tectonic setting of early Precambrian greenstone belts, Northern Ontario, Canada; unpublished Ph.D. Thesis, University of Portsmouth, 278p. Vallini, Daniela, A., Connon, William, F, and Schulz, Kalus J., 2005. New age data for the Chocolay Group, Marquette Range Supergroup: Implications for the Paleoproterozoic Evolution the Lake Superior Ruzicka, V. and LeCheminant, G. M., 1984. and Lake Huron regions. Institute on Lake Uranium deposit research 1983; in Geological Superior Geology Proceedings, 51st Annual Survey of Canada paper 84-1A, p. 39-44. Meeting, Nipigon, Ontatio, Part I – Proceeding Shanks, W. S. and Schwerdtner, W. M., 1991. and Abstracts, v.51 part 1 Structural analysis of the central and Williams, G. E. 1975. Late Precambrian glacial southwestern Sudbury Structure, Southern climate and the Earth’s obliquity; Geological Province, Canadian Shield; Canadian Journal Magazine, v112, p.441-465. of Earth Sciences, vol. 28, p. 411-430. Williamson, M. and Keen, C.E., 1995. How Sims. P. K., Card, K. D. and Lumbers, S. active are passive margins? Modern analogues B.,1981. Evolution of early Proterozoic basins of magmatism in rifts; the Canadian of the Great Lakes Region; in Proterozoic Mineralogist, vol. 33, p. 943-944. Basins of Canada, Geological Survey of Wright, D. J. and Rust, B. R., 1985. Canada, paper 81-10, p. 379-397. Preliminary report on the stratigraphy and Stockwell, C. H.,1982. Proposals for time sedimentology of the Bar River Formation; in classification and correlation of Precambrian Geoscience Research Grant Program, rocks in Canada and adjacent areas of the Summary of Research 1994-1995, Ontario Canadian Shield: part 1- a time classification Geological Survey, Miscellaneous Paper, 127, of Precambrian rocks and events; Geological p. 119-123. Survey of Canada Paper 80-19, p690-698. Roscoe, S.M. and Card, K.D., 1992. Early Proterozoic tectonics and metallogeny of the Lake Huron region of the Canadian Shield; Precambrian Research, V. 58, 9. 99-119. PDF compression, OCR, web-optimization with CVISION's PdfCompressor �65 Wood, J., 1973.Stratigraphy and sedimentation in Upper Huronian rocks of the Rawhide Lake-Flack Lake area; in Huronian Stratigraphy and Sedimention, Geological Association of Canada Special Paper 12, p73-95. Young, G. M., 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America Memoir, v.160, p.15-32. Young, G. M., 1973. Tillites and aluminous quartzites as possible time markers for Middle Precambrian (Aphebian) rocks of North America; in Huronian Stratigraphy and Sedimentation, edited by G. M. Young; Geological Association of Canada Special Paper 12, p. 97-127. Young, G. M. and Nesbitt, H. W., 1985. The Gowganda Formation in the southern part of the Huronian outcrop belt, Ontario, Canada; Precambrian Research, v.29, p.265-301. Young, G. M., 1991. Stratigraphy, sedimentology and tectonic setting of the Huronian Supergroup; Field Trip B5:Guidebook; Young, G. M., 1982. Field excursion guide book; Geological Association of Canada and excursion 13B: Depositional environments and Mineralogical Association of Canada, Society of Economic Geologists Joint Annual Meeting, tectonic setting of the early Proterozoic Huronian Supergroup; International Association Toronto 1991., 34 p. of Sedimentologists, Eleventh International Zolnai, A. I., Price, R. A. and Helmstaedt, H., Congress on Sedimentology, McMaster 1984. Regional cross-section of the Southern University, Hamilton, Ontario, Canada. Province adjacent to Lake Huron, Ontario: implications for the tectonic significance of the Murray fault zone; Canadian Journal of Earth Sciences, v.21, p.447-456. PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/7f3aee4f6a39e32e2c43f4b4a0ddd107.pdf 0babd4791a930a720e5fac1812b24e76 PDF Text Text Keweenawan Rocks of the Mamainse Point Area Field Guide for the 52nd Annual Institute on Lake Superior Geology Vol. 52, Part 5 U . -J • S U By: Thomas R. Hart, Ontario Geological Survey, Ministry of Northern Development and Mines Antonio Pace, Resident Geologist Program, Sault Ste. Marie District, Ministry of Northern Development and Mines PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents Page INTRODUCTION ............................................................................................................ 1 REGIONAL GEOLOGY.................................................................................................. 1 GENERAL GEOLOGY.................................................................................................... 2 Mafic Volcanic Rocks........................................................................................... 3 Felsic Rocks .......................................................................................................... 4 Clastic Sedimentary Rocks ................................................................................... 5 Younger Clastic Sedimentary Rocks .................................................................... 6 ALTERATION ................................................................................................................. 7 STRUCTURAL GEOLOGY ............................................................................................ 7 LITHOGEOCHEMISTRY ............................................................................................... 9 ECONOMIC GEOLOGY................................................................................................. 10 Mamainse Mine .................................................................................................... 10 Coppercorp Mine .................................................................................................. 11 GEOCHRONOLOGY ...................................................................................................... 15 PALEOMAGNETISM ..................................................................................................... 15 REFERENCES ................................................................................................................. 15 FIELD TRIP GUIDE ........................................................................................................ 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 1989; Nicholson et al. 1997). Although there is only very limited geochronology in the MPF, recent work in the Lake Nipigon area indicates that rift related magmatic activity began at ~1114 Ma (Heaman and Easton 2005) which extends the period of magmatic activity from the ~22 Ma by Davis and Green (1997) to ~28 Ma. This extended period of time is much longer than the 1 to 5 Ma of volcanic activity proposed for more Phanerozoic continental flood basalt provinces by Jerram and Widdowson (2005), but is comparable to the time periods proposed for Archean age magmatic events possibly related to mantle plumes. This trip will hipiccten provide an opportunity to reIs Ia rid examine many of the same Ontado stops described by Giblin (1974) and Annells (1973). MIDDLE KEWEENAWAN ROCKS OF THE MAMAINSE POINT AREA INTRODUCTION Mid-Keweenawan mafic volcanic flows, felsic intrusive to extrusive rocks, and clastic sedimentary rocks of the Mamainse Point Formation (MPF) are located about 64 km north of Sault Ste. Marie, along the east shore of Lake Superior (Fig. 1). This area was the subject of a Nipigon Emb ayment—.5. M anitoba Say -Jr to z Minnesota ir. - REGIONAL GEOLOGY The ~1.1 Ga Midcontinent Rift extends for over 2000 km and is interpreted to be an aborted Wisconsin continental rift (Fig. 1) (e.g. Van Schmus and Hinze, 'C 1985). Seismic profiling of Michigan Lake Superior indicates that -J the rift consists of a series of a series of asymmetric grabens 'S.. separated by accommodation zones filled with up to 30 km Inferred extent of Keweenawan and sediments of volcanic and sedimentary Iowa rocks (e.g. Cannon et al., Upper sediments 1989). Initiation of rifting has KeweenaA'an igneous been related to a mantle KeweenaNan extrusive rocks plume, or hot spot (e.g. sediments Kansas Hutchinson et al. 1990; Figure 1. The Midcontinent Rift System and some of the Nicholson and Shirey 1990), major regional geological features from Lightfoot et al. which resulted in the production of < 1 500 000 (1999). km3 of volcanic and intrusive rocks (Klewin and Shirey 1992). Rift related volcanic and field trip by Giblin (1974) during the Annual sedimentary rocks of the Keweenawan Meeting of the Institute of Lake Superior Supergroup are generally exposed along the Geology. Since that time, there has been various margins of the rift, with the greatest volumes studies completed on the MPF (e.g. Massey 1980; exposed west of Lake Superior (Fig. 2) Klewin and Berg 1991; Shirey et al. 1994; (Nicholson et al. 1997). Limited exposures are Lightfoot et al. 1999; Walker et al. 2002) and on present along the east shore of Lake Superior at the rocks in other parts of the Keweenawan Mamainse Point, Cape Gargantua south of Wawa, Midcontinent Rift (e.g. Green 1983; Cannon et al. and Michipicoten Island with a more extensive Penr Ia \VVVV. S v'.vv 1 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 1114 r 1 Seagull intrusion 1117 Ma Kltto Intrusion 1114 r 1 Disraeli intrusion Ca. 1,1 Ga Pigeon River Arr: call 1540 Ma English eay Grante ca. :.i GaKenora-Fortrrances GENERAL GEOLOGY The Keweenawan rocks are underlain by Archean rocks of the Batchawana Greenstone Belt that consists of mafic to intermediate metavolcanic and minor felsic metavolcanic rocks, and Algoman-type iron formation (Giblin 1974). The Archean rocks strike east, have been metamorphosed to amphibolite facies, and deformed resulting in northeast 01.12 TtCtOflICZOflt trending isoclinal North Bhoro folds and penetrative Pr fabric with steep dips. MW. MichipicoMn Mand Faue The Mamainse Point Formation (MPF) unconformably Archean overlies the Archean rocks, and consists of a sequence * alkalic complexes — International boundary estimated to be Faults between 4200 and ..A.._A.. Thrustfauks 6000 m thick Dikes consisting of subaerial mafic flows, intercalated clastic sedimentary and felsic igneous rocks (Fig. 4 and 5). The mafic flows can be subdivided into lower olivine- and plagioclasebearing basalts and upper plagioclase-bearing basalts (Annells 1973), which corresponds to a major change in the chemistry of the basalts (e.g. Klewin and Berg 1991; Lightfoot et al. 1999). Clastic sedimentary rocks consist of predominately conglomerates, and the 550m thick Great Conglomerate horizon marks the break between the lower and upper basalts. Minor felsites, quartz porphyry and flowNCF KF OF Paleozoic Illinois and Michigan basins lesoproterozoic Midcontinent Rift CIa stic sedimentary rocks AnorogenicfPost-orogenic suites Sibley Group-sandstone, - >134 Ga) Granitoid rocks (1.48.1.77 Ga) Paleoproterozoic Penokean Orogen and Related Rocks Granitoid and volcanic arc rocks (1 .89 - 1.84 G a) Intrusive rocks (1 11-1 Group Marquette Sup ergroup (ca, 2.1 1,85 Ga) Figure 2. Geological map of the Lake Superior basin after Lightfoot et al. (1999). and modified based on the recent mapping in the Nipigon Embayment (Hart and MacDonald in press). sequence of volcanic rocks present on Simpson and St. Ignace islands and Black Peninsula near Nipigon (Fig. 2). A rift-wide correlation of the volcanic rocks was presented by Nicholson et al. (1997) (Fig. 3), but the position of the MPF could only be estimated using paleomagnetic data due to a lack of detailed geochronology. C FauC —— 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide — (b) (a) a (d) (c) n Upper Michigan NWWisocnsin I 085 —0.- NE MInnesota OWlimb NE Minnesota NElirtib (e) Isle Royale Black Bay Peninsula a LakeNipigon o Lake Silo,. Trspsk*, 3 Man,ainse Point Michipicolen Island echipicoten and Forniator logo 1095 1100—s1105 1110 —0APPROX. THICKNESS OF VOLC. SECTION: Isle Royele: Oaler Group: Figure 3. Schematic correlation of MRS volcanic rocks in western and eastern Lake Superior from Nicholson et al. 1997. 3.5 lust 3 lust Midi. Island: Manieinse Pt: Along the north side, the Mamainse Point Formation is unconformably overlain by the Mica Bay Formation which is considered to be the equivalent of the Freda Formation south of Lake Superior (Fig. 4) (e.g. Annells 1973; Giblin and Armsburst 1969). To the south, the Mamainse Point Formation is in fault contact with a red sandstone interpreted to be part of the Jacobsville Formation. Paleomagnetic age estimates by Halls and Pesonen (1982) suggest that both of these units are late Keweenawan. 4 lan 5 km Mafic Volcanic Rocks The MPF consists of 300 to 350 individual mafic flows that commonly range in thickness from 1.5 to 9.0 m, with some flows being up to 30m thick and other as thin as 0.15 m (Annells 1973). Many of the flows have upper and lower vesicular zones, with the lower zone vesicles often pipe-like and bent in the direction of flow. Most of the flows have ropy pahoehoe surfaces, but some have clinkery scoriaceous flow tops, and Annells (1973) noted that the olivinerich flows commonly have scoriaceous rather than pahoehoe flow tops. Prismatic jointing is also common in the olivine-rich flows, but also observed in the finer grained flows. The mafic flows were subdivided into a Lower and Upper Division by Annells (1973) with the polymicitic conglomerates of the Great Conglomerate forming the break between the divisions (Fig.5). The Lower Division, located along the north side of the MPF, is about 1700 m thick and consists of about 20% flows with olivine phenocrysts and 80% flows with plagioclase and minor olivine and pyroxene phenocrysts. Two volcanic conglomerates and two thin felsic horizons are intercalated with the flows. The Upper Division, located to the south, consists of banded rhyolites occur as intrusive and possibly extrusive units within the MPF. Felsic dykes, porphyries, and breccias also intrude the Archean rocks to the east, and are considered to be related to the felsic volcanic and intrusive rocks occurring within the MPF. Some of the intrusions host mineralization, with the best examples being the ~1055 Ma breccia pipe hosting Cu-Au-Ag at the past-producing Tribag Mine (Wanless et al. 1968) and several Cu-Mo prospects at the Jogran Porphyry (Tortosa and Moss 2004). The volcanic rocks generally strike to north to northwest with homoclinal dips of 15o-60o to the southwest and are cut by northwest- and northeast-trending faults. The mafic flows of the Alona Bay area form an about 1300 m thick sequence of basaltic flows with less lithological variation than the MPF, and lacking intercalated sedimentary and felsic igneous rocks (Annells 1973). Located approximately 4 km to the north of the MPF, the Alona Bay flows consist of olivine- and plagioclase-bearing basalts that have been proposed to be equivalent to the lower division of the MPF (Walker et al. 2002; Annells 1973). 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide 2 ki Iometree KINCAID + RYAN about 65 km to the north. These flows also resemble a plagioclase-rich ponded flow in the Osler Volcanic Group volcanic rocks which is exposed on the southeast corner of St. Ignace Island, in the Thunder Bay area (Sutcliffe and Smith 1988). The top of that flow has a texture resembling the glomeroporphyritic “Daisey Stone” while the core of the flow consists of plates of feldspar up to 5 cm in width. Felsic Rocks Generally rhyolitic in composition, the felsic igneous rocks in the MPF have been classified as felsite, quartz porphyry and flowbanded rhyolite. Many of these rocks occur as plugs, Cong dykes, and sheets intruding Basalt the mafic flows commonly Baserrent with auto-brecciated zones Ro Ia centimeters to tens of centimeters in width containing clasts of basalt and Dip orb felsite and occasional agglomeratic zones suggesting there may be eroded rhyolitic domes (e.g. Annells 1973; Giblin 1974). The depth of intrusion is not known, or whether some of these units may have formed high level cryptodomes that may have breached surface. These rocks are commonly reddish brown to pink to grey-white, and vary from fine-grained and massive to quartz and/or feldspar porphyritic, with the groundmass and massive portions composed of quartz, altered feldspar, mica, chlorite, and epidote. Some units may be flow banded with the best exposed examples located along the margins of one body exposed at Cottrell Bay (Fig, 3). This flow banding shows tight asymmetric isoclinal folds with bands that are generally parallel to the contact. The quartz porphyries commonly form small plugs or thin intrusive sheets with paramorphed b-quartz Figure 4. Geological map of the Mamainse Point Formation. Based on OGS Map 2251 (Giblin and Armburst 1969) and modified after Lightfoot et al. (1999). about 3000m of fine-grained to aphanitic, subophitic to ophitic flows. There are a number of clastic sedimentary and felsic horizons intercalated with the Upper Division flows. The plagioclase glomeroporphyritic “daisy stone” flow is a distinctive unit towards the base of the Lower Division and contains radiating calcic plagioclase laths, up to 5 cm across (Annells 1973). Annells (1973) also described a flow in the Mamainse Point area, near the base of the Upper Division, as containing a 3.25 m zone of large euhedral plagioclase up to 10 cm long. Annells (1973) also notes that there is a flow with similar textures in the Cape Gargantua sequence 4 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide it P Mafic Intrusive Rocks Mafic intrusive rocks are relatively rare in the MPF and typically consist of narrow diabase and fine-grained dykes cutting the flows. Annells (1973) describes the dykes as typically less than 3 m, east or north striking with steep dips, and varying degrees of alteration. No connection was observed between the dykes and flows. I rhyolite magmaltsm Diab ass V rhyolite magmaltsm to ophitic tholeiite rhyolite magmatism Diab ass to fine-grained aphyric tholeiite — VI Cong 10 m crate A — rhyolite magmatism Ophftio basalt III — rhyolite magmatism Oltvine-phyric basalt basalt Fbgioclase-oliv basalt basalt spherulite daisystone 1 Oltvine-phyric pioritic basalt Figure 5. Petrographic and geochemical variations through the Mamainse Point Formation with the stratigraphic subdivisions of Annells (1973), Massey (1983) and Klewin and Berg (1991) modified by Lightfoot et al. (1999). Magnetic polarity is based on Halls and Pesonen (1982). phenocrysts up to 8 mm in size and altered euhedral feldspar (Annells 1973). In one location Annells (1973) described a composite sheet of felsic and mafic material which was interpreted to indicate that the felsic and mafic lavas were erupted simultaneously. Felsite dykes cut, and are cut by, structures hosting the copper mineralization at the Coppercorp and Mamainse mines, and are considered to have been emplaced contemporaneously with fault movement, brecciation and sulphide deposition (Heslop 1970). Keweenawan quartz-feldspar porphyries and breccia pipes hosting copper and molybdenum mineralization intrude the Archean rocks to the east of the MPF (e.g. Tribag Mine), and may be related to the felsite dykes. Clastic Sedimentary Rocks Clastic sedimentary rocks occur at a number of levels within the stratigraphy of the MPF and consist of volcanic conglomerates in the Lower Division and polymicitic conglomerates in the Upper Division (Fig. 5). The volcanic conglomerate was described by Annells (1973) as being composed of poorly sorted, angular to subangular clasts of fine- to coarse-grained Keweenawan basalt in a silty basaltic matrix. Some of the clasts are irregular, rounded, amoeboid shaped which was interpreted to be a result of deposition while still hot. Rare red siltstone, but no basement rocks or felsic volcanic, clasts were observed. It was noted that the horizons become finer grained and well laminated upwards. There are a number of horizons of clastic sedimentary rocks intercalated with the mafic flows of the Upper Division, but the largest volume occurs in the Great Conglomerate which is located approximately in the middle of the Formation between the Lower and Upper Divisions. The Great Conglomerate is a sequence of predominantly polymictic boulder conglomerates containing thin lenses of red to grey sandstone, siltstone, shale and granite, and is interbedded with two basalt flows of the Upper Division. The base of the conglomerate is described by Annells (1973) as a thin layer of red sandstone, overlying a scoriaceous flow top. The conglomerates are poorly sorted, with indistinct bedding containing well rounded to sub-rounded, pebble to boulder sized clasts of granite and basalt, with minor amounts of diabase, felsite, and vein quartz. Conglomerates exposed along strike to the south of the highway occur as 10 to 20 cm thick, graded beds with some beds displaying weakly developed cross bedding suggesting that the trough in which the Great Conglomerate was deposited is deeper to the north-northwest. Some of the well laminated sandstone lenses in the 5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide with fresh feldspar. Plagioclase is variably zeolitised and some have patches of albite and epidote. Augite is partially altered to clinoamphibole and chlorite. Opaque minerals are commonly hematized, with titanium-rich phases altered to leucoxene and sphene. In the felsic rocks, the feldspar is kaolinized and quartz fills cavities. The felsic rocks at Cotrell Cove are mottled light reddish brown to beige, and also contain irregular patches or funnel-shaped features. The mottling resembles the variations observed in the oxidized Sibley Group sedimentary rocks of the Nipigon area, suggesting that this variation may be in part a result of possibly deuteric reduction of an originally oxidized rock. However, the funnel-shaped features may be a result of alteration during degassing of the flows. All vesicles within the mafic flows are always filled, and the composition of the amygdules is variable. Annells (1973) describes some fillings as an outer zone of chlorite with a chalcedony rim, and a core of colourless quartz and zeolite but some have yellow epidote, colourless prehnite and calcite. Similar mineralogy may also occur as fracture fillings. Lightfoot et al. (1999) suggest that there was a stratigraphic variation with zeolites, including heulandite and stilbite, common in the Upper Division flows and prehnite and pumpellyite common in the Lower Division flows. Epidote alteration is also more common in the Lower Division flows (e.g. Annells 1973) and, along with hematite and specular hematite, in major crosscutting veins and fissures associated with mineralization at the Coppercorp and Mamainse Mines (e.g. Richards and Spooner, 1989). Calcite is also present in fractures cutting the clastic sedimentary units interbedded with the Lower Division and in the Great Conglomerate. highway section are cross-bedded. Annells (1973) suggested that the conglomerates were deposited in an alluvial fan environment. The presence of granite clasts suggests a reduction in the rate of, or possible hiatus in, volcanism so that material could be transported from the adjacent Archean terrains or a more extended period of time than previous interpreted. Regionally, the Great Conglomerate resembles the conglomerate exposed on Puff Island, near the top of the exposed portion of the Osler Group which is located near the top of the ~3 km of exposed volcanic rocks. Younger Clastic Sedimentary Rocks The Mica Bay Formation is an approximately 61 m thick sequence of clastic sedimentary rocks unconformably overlying the volcanic rocks of the MPF along the north side of the formation (Giblin 1974). An up to 30 cm thick, polymictic, matrix supported conglomerate forms the basal member of the formation. Most of the formation is composed of grey-brown siltstones, arkoses, and minor immature quartz pebble conglomerates with siltstones forming approximately 70% of the section. Giblin (1974) interpreted these rocks to have been deposited in a shallow water environment based on the presence of ripple marks, graded bedding, cross-lamination, flame structures, and ball-and-pillow structures with flute casts and clastic dikes indicating current flow towards the north. The Mica Bay Formation is considered to be the equivalent of the Freda Formation (e.g. Annells 1973; Giblin and Armburst 1969). To the south, the Mamainse Point Formation is in fault contact with sandstones displaying the typical red and white mottling of the Jacobsville Formation. The Jacobsville Formation is interpreted to form the floor of much of the east part of Lake Superior (e.g. Giblin 1974). Paleomagnetic age estimates by Halls and Pesonen (1982) suggest that both of these younger sedimentary units are late Keweenawan. STRUCTURAL GEOLOGY The flows of the MPF vary in strike from south to north from northwest to north with dips decreasing from east to west from 55o to 15o west defining a broad antiform that gently plunges to the west (Annells 1973). In the area of Pancake Point and Cottrell Cove, the strike of the flows is highly variable and some flows appear to be overturned which has been interpreted to be a ALTERATION All of the mafic flows appear to have undergone low-grade hydrothermal metamorphism, and some alteration appears to be deuteric. Annells (1973) noted that olivine is replaced by saponite and hematite even in basalts 6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Major faults (upthrown - side indicated) I - L 0 100km Cross-structure faults Extent of MCR rocks the flows. These fractures are up to 30 cm wide and commonly filled with silt and other material similar to the matrix of the conglomerates. Annells (1973) speculated that these fracture fillings were clastic dykes formed as a result of minor adjustment of the flows during deposition. There are 2 major regional faults, Mamainse Point and Montreal River faults, that bound the MPF to the north and south (Fig. 4 and 6) (Manson and Halls 1997). To the south, the northeast-trending Mamainse Point Fault juxtaposes the flows of the MPF against red sandstones of the Jacobsville Formation. This fault is a re-activated ductile shear zone that forms the southern margin of the Batchawana greenstone belt to the east and may correlate with faults in northern Michigan and Wisconsin. To the north, the Montreal River Fault forms the northern boundary of the MPF and is the western boundary of the Abitibi and Wawa subprovinces, and the southwest extension of the Ivanhoe Lake fault which is the eastern boundary of the Kapuskasing Structural Zone. This fault may correspond to the Keweenaw Fault to the west (Fig. 6). Both of these faults have been interpreted to be major reverse faults associated with late compression related to the Grenville Orogen in the late Keweenawan. Two major faults, Mamainse Lake and Hibbard Bay faults, that transect the MPF and offset or truncate stratigraphy (Fig. 4 and 6). The Mamainse Lake Fault trends northeast, has a variable sinistral displacement, and appears to Figure 6. Major faults of the Lake Superior portion of the Midcontinent Rift. From Manson and Halls (1997) result of the intrusion of a number of felsic bodies. Rocks of the Mica Bay Formation, that overlie the flows of the MPF to the north, strike 065o, are gently folded, and dip 15o to 30o north or south (Giblin 1974). The Jacobsville sandstones, which overlie the flow to the south, strike 335o to 320o and dip 15o to 25o west. The Alona Bay flows strike north and dip 45o to 49o west with the basal unit unconformably overlying Archean felsic plutonic rocks (Annells 1973). This section terminates against a northwest-trending fault filled by a diabase dyke. The orientation of these flows is comparable to the northern flows of the MPF. A large number of northeast- and northwest-trending normal faults with limited vertical displacements cut the flows of the MPF (Annells 1973). However, Annells (1973) considered the ~6000 m of stratigraphy within the MPF to be in part a result of possible fault repetition. A series of apparently radially distributed faults, with a focal point roughly near the Coppercorp Mine, offset stratigraphy (Tortosa and Moss 2004). The focal point corresponds to a magnetic high and an adjacent large felsite body located about 4 km east of the Coppercorp Mine. A large number of fractures of variable orientation and no discernable displacement cut 7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide converge with the Mamainse Point Fault under Pancake Bay. The Hibbard Bay Fault is northwest-trending, subparallel to the rift axis located to the west under Lake Superior, and truncates stratigraphy at an acute angle. Cu in some flows; other flows have high La/Sm and SiO2 suggesting crustal contamination. Height in section (m) LITHOGEOCHEMISTRY Geochemical studies by Massey (1980) and Klewin and Berg (1991) have characterised the flows of Mamainse Point Formation, and have identified geochemical variations correlating with stratigraphic position within in the MPF. Massey (1983) identified five major groups of flows with the break between the Lower and Upper Division occurring stratigraphically at the break between Groups III and IV, across the Great Conglomerate (Fig. 5). Klewin and Berg (1991) subdivided the MPF into seven groups, with Groups 1 to 5 corresponding to Groups I to III of Massey (1983) (Fig. 5). Lightfoot et al. (1999) proposed a subdivision of the groups of Klewin and Berg (1991) (Fig. 5). The Lower Division contains olivine-rich flows (Annells 1973), and some of these flows were identified by Klewin and Berg (1991) to be picritic in composition. Based on the presence of skeletal olivine and the fact that the flow composition did not fall along an olivine control line, Berg and Klewin (1990) concluded that these flows were not olivine cumulates but rather formed as a result of melting at deeper levels in the mantle. Overall the Lower Division flows, Groups 1 to 5, consist of basal picrites to picritic basalts overlain by the “daisy stone”, and then a series of tholeiitic to high Mg-tholeiitic basalts. Some of the main characteristics of the different groups were summarized by Lightfoot et al. (1999) (Fig. 5, 7, and 8a, b): Figure 7. Variations in selected elements with stratigraphic position after Klewin and Berg (1991) and arrows indicating the general direction of geochemical evolution. GC - Great conglomerate; BCC - basalt clast conglomerate. Sample position in metres above the base (from Lightfoot et al. 1999). Most of the variation in the Upper Division flows is a difference in the absolute trace element abundances rather than incompatible element ratios. Some of the main characteristics of the different groups were summarized by Lightfoot et al. (1999) (Fig. 5, 7, and 8a, b): Group 6 - high Yb (2.3-5.7 ppm), highest TiO2 (1.8-3.4 wt.%) and Mg-number (0.30-0.48); subgroups 6a, 6b, and 6c defined by varying Ce, Yb, and Zr contents and may reflect interdigitate throughout the stratigraphy, possibly in part due to local fault repetition Group 7 – more primitive than Group 6 with elevated Ni (>75 ppm), low Cu (dominantly less than 100ppm), and TiO2 (~0.9 wt.%); subgroup 7a has elevated Mg-number (0.62-0.70), low TiO2 (0.9-1.2 wt.%), and low incompatible elements abundances (e.g. Ce=15-20 ppm); subgroup 7b has lower Mg-number (dominantly 0.52-0.60), Group 1 and 2a - high Mg-numbers (0.640.70), TiO2 (1.1-2.0 wt.%) and Gd/Yb, with low CaO and Al2O3. Groups 2b and 3 - lower MgO, TiO2, and Gd/Yb compared with Groups 1 and 2a, but high La/Sm, Al2O3, and CaO. Group 4 – higher MgO (9.0-10.5 wt.%) but low P, Zr and Hf compared to other groups Group 5 - quite low TiO2 (<1.5 wt.%), a wide range of MgO and La/Sm with low Gd/Yb, with very low Ni and moderate 8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide but higher TiO2 (1.3-1.9 wt.%) and incompatible element abundances (Ce=20-25 ppm). Group 8 – are interspersed with Groups 6 and 7; elevated incompatible elements and LREE (e.g. Ce: 120-130ppm) and high TiO2 (3.6-3.7 wt.%) Group 9 - elevated TiO2, Ce, Yb, Zr, La/Sm, LILE, and low Mg-number, Al2O3, and Ni 1973; Lightfoot et al. 1999). These rocks have Zr/Y ratios of 1.9 to 7.3, 10 to 53 ppm Y, (La/Yb)cn ratios of 2.67 to 8.92 with (Yb)cn values of 4.55 to 25. They also have moderately positive Nb+Ta, and negative P, Ti, and Eu on a chrondrite normalized extended element diagram (Lightfoot et al. 1999). Lightfoot et al. (1999) compared these rocks to sialic volcanic rocks found in other CFB such as the Salsette Island suite north of Bombay, India, in the Deccan Trap, and based on this similarity suggested that these rocks formed by a similar mechanism of partial melting of basaltic crust. These rocks could also be classified as FII type felsic volcanic rocks which suggests that they may have the potential to host volcancogenic massive sulphides mineralization, and that they formed by partial melting of a basaltic source at depths >10 km (e.g. Hart et al. 2004). The model proposed by Klewin and Berg (1991) to explain the upward geochemical variations in the MPF was partial melting of different sources at decreasing depths followed by fractional crystallization replaced by periodic replenishment, tapping, and fractionation in magma chambers combined with assimilation and fractional crystallization, and finally simple fractional crystallization at two broad levels in the crust. This model was modified by Shirey et al. (1994) based on isotopic data to reflect interaction with old subcontinental lithospheric mantle in the lower flows, crustal assimilation in the middle flows, and a mixture of plume and depleted mantle comparable to Phanerozoic mid-ocean ridge basalt (MORB). A study of the picritic basalts by Shirey (1997) suggested a mixed source of enriched mantle plume and unradiogenic continental lithosphere, and possible involvement of recycled slab from late Archean subduction. Lightfoot et al. (1999) compared the MPF with the Osler Group volcanic rocks and observed that Groups 1 to 3c, except for the possible lithospheric interaction, resemble but are not identical to the Lower Formation of the Osler Group. Picritic basalts are also present in the Lower Formation but are not the lowermost flows and are underlain by tholeiites. The Osler Group lacks flows comparable to Group 5. Groups 6 and 7 broadly resemble the least contaminated basalts of the Central formation of the Osler Group. 4.6 Gd/Yb I I Osler Group 'Upper Formation 'Central Form A Lower Fom,ation Mamainse Point Group 09 • 7a-d • Ga-c 0 5ac ft 2o A l,2a,2c, 1000 La/Lu Figure 8a. Variation in La/Sm versus Gd/Yb with symbols representing samples from different groups in the MPF and Osler Group volcanic rocks; Figure 8b. Variation in Ce/Nb versus La/Lu in MPF and Osler Group flows. (from Lightfoot et al. 1999) The felsic rocks have SiO2 of 72-81 wt.% with moderate Al2O3 (11-15 wt.%) (e.g. Annells, 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide There are only a couple of picritic basalt flows in the Osler Group, and although they are located near the base of the Group but are not the lowest flows (Hart 2002). ECONOMIC GEOLOGY Copper deposits of Keweenawan age are of economic importance in the area and consist of 3 types: (i) fissure filling carbonate vein breccias which carry chalcocite, bornite, chalcopyrite, specular hematite, and very minor amounts of native copper (ii) porphyry copper type deposits, in which chalcopyrite, chalcocite, molybdenite and pyrite are disseminated in quartzfeldspar porphyry (iii) breccia pipe deposits in which wallrock fragments are set in a matrix of quartz, carbonate and minor fluorite. Chalcopyrite and pyrite, with minor molybdenite, sphalerite, galena, tetrahedrite, stibnite and scheelite occur in the matrix of the breccia. Photo a: Mamainse copper mill (Coleman 1899) The Tribag Mine is the best example of the breccia pipe deposits but will not be visited during this field trip. The deposit is located 16.1 km north of Batchawana Bay, and past production included copper and minor gold and silver from a breccia pipe during the period 1967 to 1974. Milling rate at these mines averaged 400 to 500 tons per day of ore grading approximate 2% Cu with minor values in gold and silver. Photo b: Shaft and engine house at Mamainse (Coleman 1899) of 1890, the deepest workings were on the 300foot horizon at the Copper Creek shaft. There is no record that any significant amount of copper was produced until recent years when two mines, the Coppercorp and the Tribag came into production in the Batchawana Bay area. There are three shafts at the Mamainse Mine with 4 levels established. Work began by the Lake Superior Native Copper Company in 1882. The shafts are about 300 m apart on the vein. Amongst the companies engaged in early exploration were the Montreal Mining Company in 1856-57, the Ontario Mineral Lands Company in 1871, the Silver Islet Consolidated Mining and Lands Company in 1882-84, the Canada Lands Purchase Company in 1890, and the Nipigon Mining Company, probably about 1892. From 1906 to 1908, the property was optioned by the Mamainse Mine The existence of copper at Mamainse Point was known in very remote times and the socalled “Indian diggings” near Hibbard Bay attest to the mining activities of the natives in this region. The similarity of the geology at Mamainse Point to that of the Michigan copper district attracted the attention of mining interests when the Michigan camp was being developed. There are sketchy records of intermittent mining activity from 1842 until 1894 (Photo a and b). During this period shafts were put down at several places in the area: the best known developments are those at the Mamainse mine and the Copper Creek and Silver Creek shafts. According to the report of the Royal Commission 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide mineralization consists of disseminated native copper in amygdules, and veins and vein hosted copper sulphides that occur in fault-related breccia zones that transect flood basalts, conglomerates, and felsic intrusive and volcanic rocks of the Keweenawan-age Mamainse Point Formation (Fig. 11). Mineralization consists of chalcocite with minor malachite and chalcopyrite associated with pyrite and hematite. Regional westward folding of the Mamainse Point Formation combined with possible contemporaneous radial faulting may have provided the structural conduits for the mineralizing fluids in the Coppercorp Mine and elsewhere on the property (Fig. 12). The presence of a high area of magnetic intensity in the focal area of the radial fault system combined with associated felsic intrusive and extrusive activity in the lower volcanic sequence suggests the presence of a volcanic or intrusive centre in the area. Calumet and Hecla Mining Company of Michigan. Mineralization at the Mamainse Mine is hosted by a quartz-carbonate vein / fracture fill, striking 334o dipping 50o east and varying from 0.46 to 4.0 m wide. Tracable for 457 m, the vein and an associated felsite intrusion pinch and swell along strike in the northwest-trending fracture which cross-cuts the north striking mafic volcanic flows at an oblique angle. Mineralization in these veins consists of hematite and chalcocite in veins with minor chalcopyrite and native copper along with malachite and azurite as weathering products. The structure associated with the mineralization at the Mamainse Mine is similar to the main NNW trending, east dipping structures seen at the Coppercorp (Tortosa and Moss 2004). The main vein can be examined along the shoreline (see Stop 6), and it was along this vein where three shafts were sunk to 18.3, 85.3 and 97.6 m with four levels established. The vein cuts flows of the Upper Division and hosts minor native copper and finely disseminated chalcocite in cross fractures. The basalts are thin flows of diabasic and ophitic olivine tholeiite with amygdules containing quartz, agate, calcite, copper sulphides and native copper. Annells (1973) reported that specks of native copper are common in the vesicles and flow tops of the Upper Division flows and that a 67 kg piece of native copper was discovered during highway construction in the area south of the Mamainse Mine. History Exploration on the property probably began at the same time as the work on the nearby Mamainse Mine, and much of the early work appears to have concentrated on the native copper mineralization in amygdules and fractures (Table 1). Between 1948 and 1952, work by Macassa Mines, and C.C. Houston and Associates examined the old copper showings and drilled 10,180 m outlining several mineralized zones, including the C Zone, D Zone, SB Zone and Silver Creek Zone (Fig. 11 and 13). Table 1: History of Ownership of Montreal Mining Sand Bay Location Years Ownership Coppercorp Mine The Coppercorp Mine is a past producer located in the Mamainse Point area, about 80 km north of Sault Ste. Marie, Ontario. The mine produced 1.021 million tons grading 1.16% Cu, along with 237,603 oz Ag and 1,964 oz Au from a number of veins between 1965 and 1972 (Tortosa and Moss 2004). Nikos Explorations Ltd. currently has an agreement to acquire Amerigo Resources Ltd. interest in the property. The following information has been extracted from the technical report of Nikos Explorations Ltd. completed by Tortosa and Moss (2004). The Coppercorp deposit is hosted by volcanic and intrusive rocks situated on the eastern edge of the Midproterozoic Midcontinental Rift system. Copper 1856-1857 1871 1882-1884 1890 1892 1906-1908 1948 1951 1955 1964 2002 Montreal Mining Co. Ontario Mineral Lands Co. Silver Islet Consolidated Mining and Lands Co. Canada Lands Purchase Synd. Nipigon Mining Co. Calumet and Hecla Co. Macassa Mines Ltd. C.C. Huston and Associates Coppercorp Ltd. leased by Vauze Mines Ltd North Canadian Enterprises Ltd. Terry Nicholson & William Gibbs from Tortosa and Moss (2004) 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Figure 10: Detailed Geology around the Coppercorp Mine area showing the location of some of the surface and projected mineralized zones (Giblin, 1973). Blue outline shows the C Zone, L Zone, Lutz Vein and Mamainse Vein. J — — — Oldest Figure 11: Distribution of faults and simplified regional geology of the Mamainse area with the outline of the Coppercorp Property shown in yellow (after Giblin, 1973; Richards, 1995) from Tortosa and Moss (2004). Figure 12: Mineralized structures in the Coppercorp deposit (Heslop, 1970) from Tortosa and Moss (2004) 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Coppercorp Ltd. was a new company created in 1954 to sink a shaft, to 168 m with levels at 76, 114, and 152 m (Tortosa and Moss 2004). During the underground development, 4,267 m of lateral development were completed and 60,000 tons of ore were stockpiled. Operations ceased in 1957 due to falling copper prices. Vauze Mines Ltd. (controlled by Sheridan Geophysics Limited) leased the mine in 1963 and completed additional drilling along with a surface exploration program which included geophysical surveys and geological and geochemical examinations. In 1965, a decision was made to bring the Coppercorp deposit into production and the original shaft was dewatered and deepened to 192 m. The operation produced 500 tons per day processed into a 50% copper concentrate at a recovery rate of <90%. The Coppercorp Mine produced 1, 021,358 tons grading 1.16% Cu plus approximately 237,603 ounces of silver and 1,964 ounces of gold from such veins between 1965 and 1972. Some of the available historical statistics on underground development, drilling, pre-production ore reserve estimates and production figures are provided in Table 2. Recent exploration on portions of the property was completed by J.F. Paquette and Cominco Ltd.. J.F. Paquette held a property covering the Lutz vein and L zone in 1991-92, and a self-potential survey along with prospecting and sampling programs were conducted (Fig. 11). Cominco Ltd. optioned this property in 1993 and completed geological mapping, surficial geochemistry, electromagnetic (UTEM) and magnetic surveys. Mineralization Copper mineralization occurs as disseminated native copper in amygdule and veins, and vein-hosted copper sulphide deposits. Production on the property has concentrated on the copper sulphides mineralization. The copper sulphide veins occur in faultrelated breccia zones that generally display gradual transitions from high grade sulphide veins to barren oxide cement. The copper sulphides are dominantly chalcocite, with lesser chalcopyrite and bornite, rarely native copper and are usually accompanied by specular hematite. Massive chalcocite veins, 20 to 25 cm wide, were found at numerous localities within the deposit. Richards (1985) recognized four stages of mineralization, which were: 1) pyrite-chalcopyrite, 2) chalcopyrite-bornite, 3). chalcocite-hematite, 4). native copper, native silver, copper arsenides, malachite and hematite. The third stage was the most important source of copper producing rich veins of chalcocite and replacing earlier sulphides. The veins and breccias consist of quartz and carbonate with subordinate laumontite and fluorite. Large vugs of varying size are lined with quartz, calcite, and sulphides and were commonly found throughout the deposit, suggesting a shallow ‘open space filling’ type of mineralizing process (Heslop, 1970). The wallrock is commonly chloritized and sericitized and may contain epidote. The faults hosting mineralization cut Keweenawan basalt flows and conglomerates. The width of the fault zones varies along strike from shears less than 1 m to disrupted lenses up to 12 m across (Richards, 1985). The faults have two orientations, northeast and northwest. Northnortheast trending faults dip 60o to 65o east and host the Copper Creek Zone, Silver Creek Zone and `G`, `H`, and `F` Zones. North-northwest Table 2: Historical statistics on underground development and drilling at the Coppercorp Mine (Tortosa and Moss 2004). ______________________________ _______________________ ________ ltI,naitlo. Sourer FsplunIIun Activist I Indcrgruund flnckçrnenl DnAuig. 34.8*2 [ccl SMS1R (1150X57 Cronuculs 3,62* (cci L)nllang StutTacc. 16,000 [cci ShOOK 000152 20,000 eel j • SMDR 000152 - Iable 4 H,stcrral Pre'Production Ore Reserve Eshma4cC at the Coppertoip Muir Mi•erslIrS Zone Ij,toemitiss Source Tore Maceve Rationale C Zone red C7.apa Soisth" 400000 ions® 23% Cu ShOOK 000852; Cuppntcoqi L Silta Creek ScoUt Zone 490,000 ices a I 9%Cu SB and Silsti Creek North 630,000 Ions Tolal Ore Runurce 1,540000 loot I I I%Cu I 965 SMOR '100852, Coppeccoep SMOR 000852, Coppccccep Report lee 1983 • Cu SMOR 000*52, Cnppeeroqi Repoil (or 1965, Northern MIner 1961 Iv,, Year •J9570 1965 1966 1967 Au(Os) US$2 2*519 3*6 115,848 1146,601 149691 Mills 1971 tT.thr" Ag(ck) ('uØI*) 411000 - 1 I 146,441 142,9*6 — 1969 1970 (Source 5M014000*52L toot Hoisted 3,716.325 290 I - 268 33,622 3,175,730 231 1,785 33,570 3,109,73* t(pj,1181 141,955 111,811 83.5/') 156111 84.492 , ni on 440 '1 I 7 23,782,128 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide trending faults that dip 50o to 70o east and almost parallel the volcanic and sedimentary rocks (Fig. 12 and 13). These faults are the most productive structures, hosting the C Zone, SB Zone, D Zone and B Zone. Where northwest faults intersect the Great Conglomerate, the fault narrows and the sulphide mineral content decreases possibly due to the lower competency of the conglomerate compared to the mafic volcanic rocks (Heslop 1970). Mineralized structures also cut, and are cut by, felsite and diabase dykes. Both the diabase and felsite intrusions are considered to have been emplaced contemporaneously with fault movement, brecciation and sulphide deposition. Heslop (1970) defined four major stages of fault development based on the crosscutting relationships of the faults (Table 3) with an apparent younging in the faults from south to north, but mineralogical changes in the ore or other characteristics associated with this relative structural timing have not been documented. Some zones, including the C, SB, and L zones, Lutz and Mamainse veins, display an apparent stratigraphic control. Mineralization in these zones occurs primarily where the structure is hosted by basalts of the upper section of the Mamainse Point Formation, 75 to 150 m above the Great Conglomerate (Fig. 11). Table 4: Copper deposits in the Mamainse PointBatchawana Area (Tortosa and Moss 2004) Strike Dip SB Zone Copper Creek Zone C Zone Silver Creek Zone D Zone B Zone F Zone G Zone H Zone 342-335 020 East 55-60 E 1 2 345 010 55-68 E 50-65 E 3 4 300 345 030 020 020 45 NE East Southeast East East 4 4 4 4 4 Production Rarnts 1965to in 102 M cons 3 4 Copperguatlz YCLO iBSZco 1884 3 3 2 Ratio I%7ro1973 2% Co 4CM tons oboes 300ns 125M tons (410.13% I Deposit Typo Coppacorp CoppoT. Mancciose Thus8 qon coin Sours . a • I 16% Co ii I6S%Cu Pipes Jlrrson lien cIa &sss8rrccso 3 CootcdoO4%MoS2 F Wnsltrrrcaa F 0 RI tons a 0.6 to I IO%WO, Jogs00 porphyry I8Mtomta#0.19% N6 SoorcesiRoport, 1%?; 2 Moore 1426, 3 I Cuond080%Mo5 1 M&R. t989, 4 SMDR 000852 Deposit Model Tortosa and Moss (2004) have proposed that the Coppercorp deposit is an iron-oxide copper-gold (IOCG) type deposit similar to Olympic Dam based on: 1. A continental rift-related tectonic setting on the eastern margin of the Midcontinent Rift system. 2. The Keweenawan basalts represent a significant volume of potential copper source rocks with an estimated thickness of 4,300 to 6,000 m. 3. The presence of a massive magnetite vein grading 3.9% Cu over 1.05 m at the Jogran porphyry and fluorite associated with the Breton Breccia at Tribag and with Coppercorp ore. 4. The presence of numerous faults some of which are splays off major crustal faults including the Mamainse Point Fault to the south of the property. 5. The apparent high level emplacement of the felsic intrusives 6. The presence of dilational sites along active structures. 7. The presence of a high temperature saline brine (350o to 450oC) 15-20 eq. wt.% CaCl2 believed to be magmatic in origin, and a lower temperature fluid (<100o to 350oC, 0 to 15 eq. wt. %) believed to be a mixture of magmatic and meteoric fluid (Richards, 1985). 8. The occurrence of widespread Cu mineralization in the area as both low tonnage medium grade deposits (e.g. Coppercorp) and high tonnage low grade Table 3: Relative age of fault zones based on crosscutting relationships with 1= oldest and 4=youngest (Heslop, 1970) from Tortosa and Moss (2004). Mineralized (Fault) Zone Production Yonn Deposit Relative Age There are several other deposits in the Batchawana area and these are summarized in Table 4. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide PALEOMAGNETISM Paleomagnetic studies of Keweenawan rocks of the Midcontinent Rift (e.g. Portage Lake volcanic rocks, Michigan; North Shore volcanic and intrusive rocks, Minnesota; various dykes south of Thunder Bay; Logan sills, Thunder Bay; Osler Group volcanic rocks, Nipigon; Cape Gargantua volcanic rocks, Wawa) indicate that the older rocks have reverse magnetic polarity and the younger rocks have normal polarity (e.g. Halls and Pesonen 1982; Nicholson et al. 1997). The volcanic rocks of the MPF have more complex pattern with 2 zones of reversed polarity overlain by zones of normal polarity with the polarity changes coinciding with the base of clastic sedimentary units (Fig. 5) (Palmer 1970). It was originally proposed that this repetition was the result of a strike fault located at the boundary between the lower normal and upper reverse zones, but the lithogeochemical stratigraphy indicates another explanation is required. deposits (e.g. East Breccia zone of Tribag mines). 9. The presence of a broad, regional aeromagnetic anomaly over the property and the presence of several gravity anomalies. 10. The production of limited amounts of gold and silver along with the copper at the Coppercorp Mine and the anomalous concentrations of gold and silver found in the outlying copper occurrences. GEOCHRONOLOGY There is a limited amount of geochronology completed in the Mamainse Point area and correlations with other areas in the Midcontinent Rift are often based on paleomagnetic data (e.g. Nicholson et al, 1997). A number of K/Ar ages have been determined but are regarded as minimum ages due to the probability of Ar loss (Wanless et al, 1966, 1967, 1968). These ages include the basalt exposed at Chippewa Falls (Stop 11), south of Pancake Bay and the main portion of the Mamainse Point Formation, which have an age of 915 +/-140 Ma, and samples of the Tribag Mine breccia pipe which returned ages of 785 +/- 103 Ma to 1055 +/- 35 Ma. A more accurate age of volcanism is the 1070 +/- 50 Ma Rb/Sr age for a felsites in the Mamainse Point area (Van Schmus, 1971). A U/Pb age of 1096.2 +/- 1.9 Ma for a felsic unit in the area of the volcanic conglomerate of the Lower Division is very similar to the Rb/Sr age but younger than the 1105 +/-4 Ma felsic volcanism in the Osler Group (Davis et al. 1995). A Re-Os age of 1128 +/- 54 Ma for the basal picrite volcanic rocks (Shirey 1997) probably represents a maximum age for the Mamainse Point Formation, and is comparable to the 1124 to 1114 Ma age for the ultramafic Seagull Intrusion of the Lake Nipigon area (Heaman and Easton 2005). However, if the picritic flows of Mamainse Point are equivalent to the Seagull Intrusion, the basal portion of the MPF is much older previous interpretations, and 1096 Ma felsic age of Davis et al. (1995) means that the sediments of the Great Conglomerate may represent a longer hiatus in volcanic activity. REFERENCES Annells, R. N. 1973. Proterozoic Flood Basalts of Eastern Lake Superior: the Keweenawan Volcanic Rocks of the Mamainse Point Area, Ontario; Geological Survey of Canada, Paper 72-10. 51 p. Cannon, W.F., Green, A.G., Hutchinson, D.R., 1989. The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling. Tectonics, v. 8, p. 305-322. Coleman, P.A., 1899. Copper Regions of the Upper Lakes, Report of the Bureau of Mines Vol. VIII Part 2, p.152 Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.33, p.476-488. Davis, D.W., Green, J.C., and Manson, M., 1995. Geochronology of the 1.1. Ga North American Mid-continent Rift; Institute of Lake Superior Geology, 41st Annual Meeting Proceedings Volume 41, Part 1: Abstracts, p. 9-10. Giblin, P. E. 1974. Middle Keweenawan Rocks of the Batachawana-Mamainse Point Area. Institute of Lake Superior Geology 20th Annual Meeting, Nipigon, Ontario, Program, Abstracts, and Field Guides, v. 20, p. 39-67. Giblin, P. E., and Leahy, E. J. 1967. Sault Ste. Marie - Elliot Lake Sheet, Districts of Algoma, Manitoulin, and Sudbury. Ontario Department of Mines, Geological Compilation Series, Map 2108. Scale 1 inch to 4 miles. Giblin, P.E. and Armsburst, G.A. 1969. Batchawana, Ontario Department of Mines. Map 2251, scale 1:63 360. 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Gmitro, T.T., 1990. Chemostratigraphy and petrogenesis of Keweenawan lavas at Alona Bay, Ontario. unpub. M.S. thesis, Northern Illinois University, DeKalb, Ill. Green, J.C., 1983. Geological and geochemical evidence for the nature and development of the Middle Proterozoic (Keweenawan) midcontinent rift of North America: Tectonophysics, v. 94, p.413-437. Halls, H.C. and Pesonen, L.J., 1982. Paleomagnetism of Keweenawan rocks; in Geology and Tectonics of the Lake Superior Basin; ed. R.J. Wold and W.J. Hinze; Geological Society of America, Memoir 156. p.173-201. Hart, T.R. 2002. Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data Associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario; Ontario Geological Survey, Miscellaneous Release Data 114. Hart, T.R. and MacDonald, C.A., submitted, Proterozoic and Archean Geology of the Nipigon Embayment: Implications for Emplacement of the diabase sills and PGE-enriched mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, Hart, T.R., Gibson, H.L., and Lesher, C.M., 2004. Trace Element Geochemistry and Petrogenesis of Felsic Volcanic Rocks Associated with Volcanogenic Massive Cu-Zn-Pb Sulfide Deposits; Economic Geology, v. 99, p. 1003-1013. Heaman, L.M., and Easton, R.M., 2005. Proterozoic history of the Lake Nipigon area, Ontario: Constraints from U-Pb zircon and baddeleyite dating. ; Institute of Lake Superior Geology, 51st Annual Meeting, Nipigon, Ontario, Part 1 – Proceeding and Abstracts, v. 51, part 1, 24-25. Heslop, J.B., 1970, Geology, Mineralogy and textural relationships of the Coppercorp Deposit, Mamainse Point area, Ontario. unpublished M.Sc. Thesis, Department of Geology, Carleton University, Ottawa, 95 p. Hutchinson, D.R., White, R.W., Cannon, W.F., and Schulz, K.J., 1990. Keweenaw hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system in western Lake Superior; Journal of Geophysical Research, v. 95, p. 10 86910 884. Jerram, D.A. and Widdowson, M. 2005. The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism; Lithos, v. 79, p. 385– 405 Klewin, K.W. and Berg, J.H. 1990. Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record; Canadian Journal of Earth Sciences, v. 27, p. 1194-1199. Klewin, K.W., and Berg, J.H. 1991. Petrology of the Keweenawan Mamainse Point Lavas, Ontario: Petrogenesis and continental rift evolution. Journal of Geophysical Research, v. 96, p. 457-474. Klewin, K.W. and Shirey, S.B. 1992. The igneous petrology and magmatic evolution of the Midcontinent rift system; Tectonophysics, v. 213, p.33-40. Lightfoot, P.C., Sage R.P., Doherty W., Naldrett A.J., and Sutcliffe, R.H. 1999. Mineral potential of Proterozoic Keweenawan intrusions: implications of major and trace element geochemical data from bimodal felsic and volcanic sequences of Mamainse Point and the Black Bay Peninsula, Ontario; Ontario Geological Survey OFR 5998, 91p. Manson, M.L. and Halls, H.C. 1997. Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent Rift. Canadian Journal of Earth Sciences, v. 34, p. 562-575. Massey, N.W.D. (1980). The geochemistry of some Keweenawan metabasites from Mamainse Point, Ontario. Ph.D. thesis. McMaster University, Hamilton, Ontario. 352p. Massey, N.W.D. 1983. Magma genesis in a late Proterozoic proto-oceanic rift: REE and other trace-element data from the Keweenawan Mamainse Point Formation, Ontario, Canada. Precambrian Research, v. 21, p. 81-100. Nicholson, S.W., and Shirey, S.B., 1990. Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic study of the Midcontinent Rift System in the Lake Superior region; Journal of Geophysical Research, v. 95, p. 10 851-10 868. Nicholson, S.W., Shirey, S.B., Schulz, K. and Green, J.C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences, v. 34, p. 504-520. Palmer, H. C.1970. Paleomagnetism and Correlation of some Middle Keweenawan Rocks, Lake Superior. Canadian Journal of Earth Sciences, v. 7, p. 14101436. Ojakangas, R.W., and Morey, G.B. 1982. Keweenawan sedimentary rocks of the Lake Superior region: a summary. In Geology and tectonics of the Lake Superior basin. Edited by R.J. Wold and W.J. Hinze. Geological Society of America, Memoir 156, pp. 157–164. Richards, J.P., 1985, A fluid inclusion and stable isotope study of Keweenawan fissure-vein hosted copper sulphide mineralization, Mamainse Point, Ontario. Unpublished M.Sc. thesis, Department of Geology, University of Toronto, 290p. Richards, J.P. and Spooner, E.T.C. 1989. Evidence for Cu(Ag) Mineralization by Magmatic-Meteoric Mixing in Keweenawan Fissure Veins, Mamainse Point, Ontario. Economic Geology, v. 84, pp. 360385. Shirey, S.B., 1997. Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources. Canadian Journal of Earth Sciences, v. 34, p. 489-503. Shirey, S.B., Klewin, K.W., Berg, J.H. and Carlson, R.W., 1994.Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keewenawan Mamainse Point Formation, Ontario, Canada; Geochemica et Cosmochimica Acta, v. 58, pp. 4475-4490. 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Sutcliffe, R.H. and Smith, A.R. 1988. Project Number 87-17. Geology of the St. Ignace Island Volcanic-Plutonic Complex; in Summary of Field Work and Other Activities, Ontario Geological Survey Miscellaneous Paper 141, p. 368-371. Tortosa, D. and Moss, R. 2004. Geology and Exploration of the Coppercorp Property, Sault Ste. Marie Mining Division, Ontario; March 23, 2004, prepared for: Nikos Explorations Ltd., 158p.; filed with SEDAR May 11, 2004; available from http://nikosexplorations.com/_resources/Coppercor p2004Report.PDF Van Schmus, W. R. 1971. Rb-Sr Age of Middle Keweenawan Rocks, Mamainse Point and Vicinity, Ontario, Canada. Geological Society of America, Bulletin 82, p. 3221-3226. Van Schmus, W.R. and Hinze, W.J. 1985. The mid-continent rift system. Annual Review of Earth and Planetary Sciences, v. 13, p.345-384. Walker, J.A., Gmitro T.T. and Berg J.H. 2002. Chemostratigraphy of the Neoproterozoic Alona Bay lavas, Ontario; Canadian Journal of Earth Sciences, v. 39, p.1127-1142 Wanless, R. K. , Stevens, R.D., Lachance, G.R., and Rimsaite, J. Y. H. 1966. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 6. Geological Survey of Canada, Paper 65-17, p. 58. Wanless, R. K., Stevens, R. D., Lachance, G. R. and Edmonds, C. M. 1967. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 7. Geological Survey of Canada, Paper 66-17, p. 8485. Wanless, R. K., Stevens, R. D., Lachance, G. R. and Edmonds, C. M.1968. Age Determinations and Geological Studies, K-Ar Isotopic Ages, Report 8. Geological Survey of Canada, Paper 67-2, Part A, p. 95-96. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide ITINERARY Stop Locality Km (south) Km (north) 1 Alona Bay 0 105.7 2 Unconformity: Mica Bay Formation Mamainse Point Formation 8.0 97.7 3 Glomeroporphyritic Basalt "Daisy Stone" 8.5 97.2 4 Ropy Flow Top 9.2 96.5 5 Volcanic Conglomerate 10.7 or 11.3 10.7 95.0 6 Mamainse Mine 14.2 91.5 entrance to road 15.0 90.7 7 Ubetuwantit (Coppercorp Property) 2.3 8 Coppercorp Mine 4.6 9 Interbedded Conglomerate and Basalt 21.8 83.9 10a Domeykite Occurrence 25.4 80.3 10b Pseudotechylite 25.9 79.8 10c Cottrell Cove Felsic Intrusion 26.1 79.6 Pancake Provincial Park 32.9 72.8 Pancake River 34.6 71.1 Mamainse Point Fault (approx.) 35.5 70.2 Road to the Tribag Mine Site 40.1 65.6 Batchawana River 45.3 60.4 Chippewa Falls 54 51.7 Keweenawan basalt flows 65.2 40.5 Jacobsville Formation sandstone 69.0 36.7 Intersection at Highway 17 and 556 Water Tower Inn 105.7 0 11 the 1840's. A sample of pitchblende was described by Le Conte in the American Journal of Science in 1847 with the location given as a vein 5.1 cm wide located about 112.6 km north of Sault Ste. Marie, but subsequent papers gave the location as between 64.4 to 144.8 km. In 1948, Robert Campbell of Toronto found pitchblende at the northwest end of Theano Point, which can be seen north across the bay. Subsequently prospectors found several other pitchblende deposits inland, to the north and east. Point Aux Mines on the south side of Alona Bay, was the site of the earliest organized mining venture in Ontario. In 1772-73, a mining company formed by Alexander Henry, the noted fur-trader, worked a copper deposit and shipped a small amount of ore to England. Theano Point to the north and Point Aux Mines to the west southwest consists of Archean felsic plutonic rocks cut by diabase dikes. The Alona Bay volcanic rocks are an approximately 1200 m thick, southward younging sequence of Keweenawan basalt flows unconformably overlying the Archean rocks (Annells 1973; Gmitro 1990). These flows are unconformably overlain by siltstones and sandstones similar to the Mica Bay Formation, and possibly comparable to the post-magmatic Freda Sandstone (e.g. Annells 1973; Ojakangas and Morey 1982). The Alona Bay flows have been correlated with the Lower Division flows of the Mamainse Point Formation (MPF) (Walker et al. 2002; Annells 1973). The Alona Bay flows are described by Annells (1973) and Gmitro (1990) as being composed of about 107 mafic flows averaging 6.8 m in thickness. Most of the flows have pahoehoe surfaces and vesicular-amygdular flow tops, and pipe vesicles are relatively common just above flow bases. Gmitro (1990) subdivided these flows into olivine phyric, plagioclase phyric, olivineplagioclase phyric, and aphyric groups. Two basic dikes cut the section, one about 500 m from the base of the section and one at the top of the section in fault contact with Archean rocks. Clastic dikes are common throughout the Alona Bay section. All of the Alona Bay lava flows have undergone some low-grade burial metamorphism to prehnite-pumpellyite facies and deuteric alteration, This field guide is an amalgamation of the field trip guides of Annells (1973) and Giblin (1974). All coordinates are in UTM (Universal Transverse Mercator), Zone 16, with a NAD83 Canada datum. STOP 1 Alona Bay Scenic Lookout UTM coordinates - 673636E 5219651N The following historical description is from Giblin (1974). An historical plaque commemorates the first discovery of uranium in Canada, made nearby in 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide _______ General Geology and Field Trip Stop Locations Map-Batchawana-Mamainse Point Area N 'b in ,C ii Stop Stop Stop Stop Stop lOb Stop I 200 0 200 400 Meters Legend Jacobsville Formation Archean Rocks Granitic Sandstone, Siltstone Mets Mafic Intrusive ROCkS 1 Feisk Diabase, Gabbro Keweenawan Felsic Intrusive and Volcanic ROCkS Felsite, quartz-feldspar porphyry Cong Iomer2te Mat ic r Basalt Figure 14. Field trip stops geology from Giblin and Leahy (1967) 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide shallowly west dipping outcrop located along the edge of the water. similar to that observed in the MPF (e.g. Massey 1980). Walker et al. (2002) identified 4 geochemical groups based on decreasing TiO2 Ni and MgO contents that reflect an upwards progression through the volcanic sequence. Although proposed to be equivalents to the flows of the Lower Division of the MPF, the majority of the Alona Bay flows have MgO concentrations between 6 and 10 wt.%, and none have MgO > 15.0 wt.%. BE CAUTIOUS AS THE OUTCROP WILL BE SLIPPERY IF WET The underlying Keweenawan basalts of the MPF strike N 30° W and dip 45° to 50° W. The basalts are thin olivine tholeiitic flows near the base of the MPF (Annells 1973), with upper vesicular zones exposed in this outcrop. Unconformably overlying the basalts is an up to 30 cm thick polymictic, matrix supported conglomerate of the Mica Bay Formation (Photo 1a). Very little of the basal conglomerate remains, please do not collect samples or deface the exposure The Mica Bay Formation has a total thickness of approximately 61 m, but only the lower 18.3 m are exposed along the beach. This section includes the basal conglomerate, which is overlain by grey-brown siltstones, arkoses, and minor immature quartz pebble conglomerates. Siltstones constitute approximately 70% of the section. Sedimentary structures include ripple marks, graded bedding, cross-lamination, flame structures, and ball-and-pillow structures indicative of a shallow water environment (Photo 1b and 1c). In other exposures, north of the section examined, flute casts and clastic dikes indicate a northerly direction of current flow. The sedimentary rocks strike N 65° E, and in the section examined, dip 15° N. Steeply-dipping faults cut the sedimentary rocks with offsets of up to 1.8 m. These rocks lie on the south limb of a regional syncline, with the north limb exposed between the creek and the small granite headland at the north end of the beach. The north limb is in fault contact with the underlying Archean granite, and a thin basal breccia occurs on the granite north of the fault. Similar basal breccias are found overlying the granite, at several points along the shore of Mica Bay. The age and correlation of these rocks are uncertain. The unconformity observed at this stop indicates that they are post-Middle Keweenawan, but they are lithologically and structurally different than the rocks of the Jacobsville Formation. The Mica Bay Formation appears to underlie the Jacobsville, which forms nearby islands in Lake Superior and much of the floor of Proceed for ~8 km to the south 0 – 2 km: Alona Bay flows 2 – 6.6 km: Archean felsic plutonic rocks and diabase dykes unconformably underlying the Alona Bay and MPF 6.6 – 8 km: Mica Bay Formation STOP 2 Unconformity: Mica Bay Formation and Mamainse Point Formation UTM coordinates – 673691E 521891N Park in the east side of the highway on a portion of the old highway roadbed. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Cross the highway and proceed north to the culvert where the stream can be followed down to the beach. BE CAUTIOUS AS THE MUDDY SILTSTONES CAN BE SLIPPERY, ESPECIALLY IF WET Fine-grained siltstone and graphitic siltstone with trace sulphides are exposed in the section cut by the stream. Proceed about 200 m to the south along the cobble beach, passing exposures of the Mica Bay Formation. An angular unconformity between the clastic sedimentary rocks of the Mica Bay Formation and the underlying Keweenawan basalt of the Mamainse Point Volcanic Group is exposed in a 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide the eastern part of the lake. The age of the Jacobsville Formation is uncertain and has been variously assigned to the Lower and Middle Cambrian, and to the Upper Keweenawan. Thus, the Mica Bay Formation may be either Upper Keweenawan or Lower Cambrian. A thinner section of similar clastic sedimentary rocks occurs in Alona Bay, 6.4 km north, and may also be part of the Mica Bay Formation. Proceed south for ~1.0 km STOP 3 BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Glomeroporphyritic "Daisy Stone" flows of the Lower Division of the MPF STOP 4 Ropy Flow Top UTM coordinates – 673127E 5217816N Park in the drive about 200 m south of the rock cut Basalt UTM coordinates – 673437E 5218459N Cross the highway and walk north along the shoulder of the highway to the south side of the rockcut at the top of the hill. Proceed south along the highway for about 400 m from the same parking site as used for Stop 2. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS A ropy flow top is exposed on the south face of the outcrop located on the east side of the highway (Photo 3). The pahoehoe marks the top of a 6.5 m thick olivine tholeiite flow (Annells, 1973). Proceeding to the north through the rock cut, there are several flows with thin basal pipe vesicle zones, thicker amygdaloidal zones in the upper parts of the flows, and ropy flow tops. The vesicles and amygdules are filled with calcite, celadonite, and a green mica. About 150 m further north, on the east side of the highway, a northwest trending, 26o E dipping diabase dyke crosscuts the flows located near the top of the olivine phyric group (Annells, 1973). Scoriaceous flowtops probably on olivine-rich flows, are reported to occur in the west side of the rock cut in the area of the dyke. The distinctive glomeroporphyritic “daisy stone flows” are located interbedded with olivine tholeiite flows of the Lower Division of Annells (1973), towards the north side of the MPF. The flow is exposed in the rock cut located on the east side of the highway, and can also be observed along the shoreline to the northwest. This flow strikes north to northwest and dips about 50° W which is steeper than the dips present in subsequent stops higher in the sequence. Giblin (1974) reported that the flow can be traced intermittently along strike for 11.3 km, and a new exposure located about 4.3 km south of this location provides an excellent exposure of these flows. This flow varies from massive, fine-grained basalt to glomeroporphyritic, with radiating calcic plagioclase laths, up to 5 cm across (Photo 2). Annells (1973) reported that the massive portions of the flows are non-porphyritic containing abundant pseudomorphs after small olivine crystals. The plagioclase is variably epidotized or hematitized in different portions of the flow. A plagioclase-rich ponded flow in the Osler Volcanic Group volcanic rocks is exposed on the southeast corner of St. Ignace Island, in the Thunder Bay area (Sutcliffe and Smith 1988). The top of that flow has a texture resembling the glomeroporphyritic “daisy stone” texture. Proceed south for 1.5 km 0 – 1.0 km flows of the Lower Division of the MPF 1.0 – 1.5 km clastic sediments (volcanic conglomerate horizon) STOP 5 Volcanic Conglomerate UTM coordinates – 670790E 5214845N Parking is along the shoulder of the road. 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Photo 1a: Polymictic conglomerate of the Mica Bay Formation unconformably overlying a MPF flow. Photo 2: Glomeroporphyritic Basalt (daisy stone) with a distinctive radiating calcic-plagioclase in a fine grained to aphanitic basaltic matrix (new location to the south of the highway). Photo 1b: Ball and Pillow Structures within the siltstone of the Mica Bay Formation. Photo 3: Ropy flow top on the south face of the outcrop, east of the highway. Photo 1c: Cross-laminations in the siltstones and arkoses of the Mica Bay Formation. Photo 4: Mamainse Vein looking east. Fracture fill carbonate-quartz vein hosted by the basaltic flows of the Upper Division. 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS mining operation, containing abundant specular hematite with lesser chalcocite. The area is underlain by thin, dark green, amygduloidal olivine tholeiite flows (Annells 1973), that strike north and dip 30o to 40o west. Amygdules filled with quartz, agate, calcite, chlorite and / or epidote, and carbonate and quartz-carbonate fracture fillings are common in the flows. The Mamainse Vein is described as striking o 335 and dipping 50o east, similar to the C Zone on the Coppercorp property (Tortosa and Moss 2004). The vein was traced for 457 m, pinching and swelling along strike varying in width from 0.46 to 4.0 m. This vein appears to be exposed along the shore to the south, and there are a number of smaller fractures containing mineralization including one north of the parking site. Interbedded with the basalt flows in the Mamainse Point Group are volcanic conglomerates composed entirely of basalt clasts. The conglomerate was described by Annells (1973) as poorly sorted, angular to subrounded clasts in a matrix of fine basaltic debris with some silty material. The clasts are massive to vesicular, up to 0.30 m in diameter, and some clasts are irregular shaped which Annells (1973) interpreted to be a result of emplacement while still semisolidified. Annells (1973) also reported that this unit has a fine-grained, well laminated upper zone resembling a hyaloclastite or peperite. Although there are clasts and lenses of occasional crossbedded red-brown silt there are no clasts of Archean basement rocks. The conglomerate is cut by carbonate veins which carry minor amounts of quartz and laumontite. BE CAUTIOUS AS THE OUTCROP WILL BE SLIPPERY IF WET Proceed south for 3.7 km Approximately 60 m south of the parking area on the lake shore, a west northwest trending fracture hosts an about 10 cm wide felsite dyke and the quartz-carbonate Mamainse vein fracture filling (Photo 4). Very fine fractures, filled with native copper and calcite, are oriented at right angles to the main northwest-trending fracture. The carbonate-quartz fracture filling and veining contains disseminated chalcocite, chalcopyrite, lesser native copper, with malachite and azurite weathering products, and associated specular hematite. North of the parking area, a subparallel 5 to 10 cm wide fracture filled with carbonate and quartz also hosts chalcocite and chalcopyrite, with malachite and azurite. Along the shore to the north, and in the rock cut on the east side of the highway, a polymictic conglomerate is interbedded with the flows. This conglomerate is located stratigraphically above the “Great Conglomerate” (Giblin and Armburst 1969). 0 – 1.8 km: Flows of the upper portion of the Lower Division 600 m: distinctive hematite alteration along fractures in the flows - east side of the highway 1.8 – 2.6 km: clastic sediment, mainly polymicitic conglomerates, of the “Great Conglomerate” 2.6 – STOP 6 3.5 km: mafic flows and intercalated conglomerates of the Upper Division of the MPF Mamainse Mine UTM coordinates – 669939E 5214040N Exit to the west onto the bush road, proceed about 100 m to the second opening and park. The Mamainse Mine consists of 3 shafts that were put into production between 1842 and 1894, along with a stamp mill, service facilities and a village were constructed by the Lake Superior Native Copper Company. The area to the south of the parking site consists of rubble from the Proceed south for ~875 m Turn east on the bush road Proceed south for 2.3 km 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide STOP 7 Ubetuwantit Property) Vein (Coppercorp Proceed south for ~5.8 km UTM coordinates – 670432E 5211228N 0 – 5.8 km: flows and intercalated clastic sediments and felsic rocks of the lower portion of the Upper Division, MPF Park the vehicles along the bush road and proceed about 300 m to the east along the trail to the pits. This vein is exposed in two shallow pits on the west side of the powerline. Sampling by Nikos Explorations (Tortosa and Moss 2004) indicates that the 5 to 10 centimetre wide vein is composed of quartz and carbonate hosted by fracturing and minor brecciation within a basalt flow. The veins contain chalcocite and chalcopyrite with abundant specular hematite. The flow is amygduloidal with calcite veinlets and trace malachite staining. Some portions of the flow are pervasively hematitized with up to 1% specular hematite. 4.3 km: south entrance to the Coppercorp Mine. STOP 9 UTM coordinates – 66836E7 5207904N Park in the area near the top of the rise on the west side of the highway BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Continue south for ~ 2.3 km Walk north along the highway to the rock cut and cross with caution. Passing through the area of the main shaft and waste pile for the Coppercorp Mine. STOP 8 Interbedded Conglomerate and basalt At this location, two polymictic conglomerates are separated by a basalt flow. There are a number of conglomerate horizons interbedded with the flows the MPF, with thicker concentration occurring in the “Great Conglomerate” located to the north and stratigraphically below this stop, and this conglomerate is typical of the polymicitic conglomerates of the MPF (e.g. Annells 1973; Giblin 1974). The conglomerates generally contain subrounded, up to 0.61 m clasts of predominantly Archean granitic rocks, with minor amounts of Archean mafic metavolcanic rocks, iron formation, and Keweenawan basalt (Fig. 5). In some locations, well laminated to cross-bedded sandstone is interbedded with the conglomerates (Annells 1973). The basalt flow separating the conglomerate beds was described by Giblin (1974) as having a narrow amygdaloidal zone, with a few pipe vesicles, at the base, rapidly coarsening upwards through the flow, and a thicker amygdaloidal zone near the flow top. Minor faulting, ranging from a few to 30 cm, occurred at the base of the flow, and has been filled by carbonate. Carbonate occurs in fractures cutting the conglomerate and also in the matrix. Coppercorp Mine UTM coordinates – 671103E 5209365N In the open cut, the C Zone can be observed in a north-northwest trending fault. This is one of a set of north-trending, generally 50o-70o east dipping, faults that are almost parallel to the strike of the flows. The veins and breccia fillings consist of quartz and carbonate with subordinate laumontite and fluorite. The width of the fault zones varies along strike from shears less than 1 metre to disrupted lenses up to 12 metres across (Richards, 1985). The wallrock is commonly chloritized and sericitized and may contain epidote. Mineralization consists of predominantly copper sulphides, chalcocite with lesser chalcopyrite and bornite, usually accompanied by specular hematite. Large vugs of varying size are lined with quartz, calcite and sulphides and were commonly found throughout the deposit. Samples may be obtained from the blocks piled to the south of the open cut. The mine may be exited either by returning north for ~4.6 km along the bush road to the highway, or if the roads are passable proceed south and west for ~2.3 km to the highway Proceed south for ~3.6 km. 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide felsic magma, perhaps concentrated in late stage magmatic fluids migrating along zones of shearing and deformation during the last stages of crystallization of the magma. 0 – 3.6 km: flows and intercalated clastic sediments and felsic rocks of the Upper Division ~2.6 km: hyaloclastite intercalated with the massive flows – west side of highway STOP 10a Return to the vehicles Domeykite Occurrence Proceed south for ~800 m UTM coordinates – 669783E 5204844N STOP 10b Park in the lane on the west side of the highway, south of the outcrop Cottrell Cove Felsic Intrusion UTM coordinates – 670390E 5204502N Park in the bush road on the east side of the highway and proceed about 60 m north to the outcrop on the west side. BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS BE CAUTIOUS AS THE HIGHWAY IS BUSY AND HEAVILY TRAVELLED BY TRACTOR TRAILERS Walk north to the large sloping outcrop located on the east side of the highway and cross the highway with caution. The rock cut consists of a rock of rhyolitic composition (Annells 1973; Lightfoot et al. 1999), and is part of an approximately 250 m wide body that includes the pseudotachylite stop to the north (Stop 10c). The domeykite outcrop has been interpreted to be the northwest branch of this unit (Giblin and Armburst, 1969). This unit is also exposed along the shore of Cottrell Cove to the west. The felsic unit is fine-grained, reddish brown, and varies from massive in the core to quartz porphyritic and flow banded towards the margins. The flow bands forms tight asymmetric isoclinal folds that are generally parallel to the contacts (Photo 6a). The contact consists of a breccia ranging from centimetres to tens of centimetres, and containing subangular to subrounded, pebble to cobble sized fragments of this unit, basalt, and granite in a felsic matrix (Fig. 6b). Dykes of felsite breccia reportedly intrude the adjacent basalts (Giblin 1974). The massive core contains light brown to beige lenses and irregular patches or funnels Domeykite (Cu3As) is isometric, hextetrahedral, gray to yellow-brown to white, with a brittle fracture and habits that include botryoidal, reniform, and massive uniformly indistinguishable crystals. It has a hardness of 3 to 3.5, metallic luster, and a black gray streak (http://webmineral.com /data/Domeykite.shtml). Domeykite mineralization is restricted to a pink to red, fine-grained felsic rock of rhyolitic composition that dips from 30o to 40o northwest with an overall thickness from 10 to 15m. The lowermost 3 to 4 m of the unit is strongly flow banded with tight asymmetric isoclinal folds. The lower contact with the basalt is sharp and has an aphanitic, 2 to 10 cm chilled zone that contains subangular to subrounded fragments of basalt. Spherical calcite-filled amygdules occur within the felsic unit for up to a few centimetres from the basalt. The basalt is a medium-grained, diabasic, dark green amygduloidal olivine tholeiite. This felsic unit is interpreted to be an intrusion into the older basaltic flows (e.g. Annells 1973). Domeykite is a copper arsenide concentrated in green to grey patches best developed in the flow banded portions of the lower and central parts of the unit. The domeykite has not been observed in the basalts stratigraphically above or below. The restriction of the copper arsenide mineralization to the felsic unit suggests that the mineralization may be genetically related to the 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide Photo 5: Polymicitic conglomerate containing clasts of Archean felsic plutonic rocks intercalated with basaltic flows. Photo 6c: Alteration along vertical fractures in rhyolite possibly related to magmatic degassing. Photo 6a: Flow-banded rhyolite near the contact, with pink, grey to white alteration. Photo 7: Pseudotachylite consisting of an aphanitic, matrix containing fragments of quartz, felsite, and basalt — -— -k.- i Photo 6b: Autobrecciated felsite in contact with diabasic basalt. Photo 8: Keweenawan basalt unconformity overlying Archean felsic plutonic rocks. (http:// www.start.ca /users/ mharris/ waterfalls/ chippewa-falls.html) 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide are set in a matrix of finely comminuted fragments of the above-noted materials and larger, irregular grains of carbonate and chlorite. An overall pinkish colour was interpreted to be a result of finely disseminated hematite. Giblin (1974) tentatively interpreted the pseudotachylite to be a result of gas escaping from the underlying felsic magma streamed upwards, under high pressure, through fractures in the overlying rocks. Fragments of the wallrocks, detached by erosive power of the gas and structural dislocation, were transported in a fluidized gas-solid system, which further eroded the wallrocks and the entrained particles. The fluidized material filled the fractures, and upon the eventual decrease in gas pressure, remained as veins of very fine-grained rock powder. (Photo 6c). Giblin (1974) reported that this change was a result of alteration and extensive development of kaolin. The orientation of the funnel suggests that this alteration may be a result of post-depositional degassing and possibly related to the pseudotachylite of the previous stop. This irregular colour variation also resembles variations observed in the oxidized Sibley Group sedimentary rocks of the Nipigon area, suggesting that some of this variation may be in part a result of reduction of an originally oxidized rock. These felsic units or felsites have been interpreted to be intrusive in nature (e.g. Annells 1973; Giblin 1974) based primarily on the brecciated nature of the exposed contacts. The depth of intrusion is not known, or whether some of these units may have formed high level cryptodomes that may have breached surface. The only known geochronology for the Mamainse Point area is a U/Pb zircon age of 1096+/-2.1 Ma (D. Davis, personal communication, 2005) for a unit with similar characteristics located north of the Mamainse Mine. However, the contact relationships at that location are ambiguous. Proceed south for ~27.7 km 6.6 km: Pancake Bay Provincial Park UTM coordinates – 675727E 5204458N 8.3 km: Pancake River UTM coordinates – 678422E 5203459N Proceed for ~200 m north ~8.8 km: Mamainse Point Fault separates the MPF to the north from the Jacobsville Formation sediments to the south. outcrop located on the west side STOP 10c Pseudotachylite 8.8 – 21.8 km: Jacobsville Formation UTM coordinates-670190 5204595 ~13.8 km: the road to the Tribag Mine exits on the east side of the highway Please do not collect samples from the outcrop, as there are abundant fragments in the rubble used as fill for the road bed. ~19.0 km: Batchawana River UTM coordinates – 688231E 5200663N The pseudotachylite was originally described by Giblin (1974) and occurs as narrow branching veins, ranging in thickness from a few centimetres to thin films on fracture surfaces, cutting both the massive fine-grained felsic rock and the wallrock basalts (Photo 7). Veins are composed of a dark grey, black, to dark brown, aphanitic, matrix, that often breaks with a sub-conchoidal fracture, containing fragments of quartz, felsite, and basalt. Thin-sections material show the veins to be a microbreccia consisting of angular fragments of quartz (free of strain shadows), feldspar, devitrified glass, quartz-feldspar porphyry with a matrix of devitrified glass, opaque minerals, felsite, and basalt (Giblin 1974). These fragments ~21.8 – 27.7 km: Archean felsic plutonic rocks The highway crosses over the Harmony River The entrance to the parking lot is to the south side of the river on the east side of the highway. STOP 11 Chippewa Falls UTM coordinates – 695982E 5200414N 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �nd 52 ILSG – Mamainse Point Field Trip Guide A monument in the small roadside park marks the approximate mid-point of the Trans-Canada Highway. Follow the trails for approximately 90 m to the top of the lower falls, where there is an excellent exposure of Mamainse Point flows unconformably overlying Archean trondhjemite and diabase dykes. The best exposure is on the small island which is accessible only during periods of low water (Fig. 8). The contact on the island and the east side of the river can be viewed from the trail. Giblin (1974) described the unconformity as generally 10° - 30° southwest dipping, but is nearly vertical on the east bank of the river. The basalt flow has a chilled lower contact with rare pipe vesicles, and near the east side of the island, a few pillows ranging in length from 0.3 to 1.2 m occur in the lower 3 m of the flow. These are the only pillows found to date in the Keweenawan lavas of the east shore of Lake Superior. 25.0 – 32..1 km: Jacobsville Formation in the Goulais River valley. 32.1 – 45.9 km: Archean granitic gneisses and migmatites cut by diabase dikes with Huronian sediments to the east along Highway 550. 45.9 - 51.7 km: Jacobsville Formation to the Water Tower Inn Proceed 51.7 km south to Sault Ste. Marie 0 – 9.8 km: Archean felsic plutonic rocks 9.8 - 13.1km: Keweenawan mafic flows 11.2 km: Optional Stop, Keweenawan Basalt Optional Stop – Keweenawan Basalt UTM coordinates –700261E 5192354N Good cross-sections of the basaltic flows are exposed in the rock cut on the eats and southeast side of the highway, and include thin basal amygdaloidal zones, thicker upper amygdaloidal zones, and common reddish, ropy flow tops. 13.1 – 15.2 km: Jacobsville Formation 15.2 – 17.4 km: Archean felsic plutonic rocks 17.4 – 19.9 km Archean felsic to intermediate metavolcanic rocks with the hill top to the west being a Nipissing sill. 19.9 – 25.0 km: sedimentary rocks of the Gowganda Formation of the Southern Province, intruded by gabbro sills of the Nipissing diabase to the northeast. 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � https://digitalcollections.lakeheadu.ca/files/original/22ea3b680000e76ba99ce28da77c8e43.PDF 384ef9ee0a2ad25d4df809f0ebf0283f PDF Text Text Ontario Ontario Geological Survey Open File Report 6135 Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario 2004 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ONTARIO GEOLOGICAL SURVEY Open File Report 6135 Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario by R.M. Easton, L.S. Jobin--Bevans and R.S. James 2004 Parts of this publication may be quoted if credit is given. It is recommended that reference to this publication be made in the following form: Easton, R.M., Jobin--Bevans, L.S. and James, R.S. 2004. Geological guidebook to the Paleoproterozoic East Bull Lake intrusive suite plutons at East Bull Lake, Agnew Lake and River Valley, Ontario; Ontario Geological Survey, Open File Report 6135, 84p. e Queen’s Printer for Ontario, 2004 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �e Queen’s Printer for Ontario, 2004. Open File Reports of the Ontario Geological Survey are available for viewing at the Mines Library in Sudbury, at the Mines and Minerals Information Centre in Toronto, and at the regional Mines and Minerals office whose district includes the area covered by the report (see below). Copies can be purchased at Publication Sales and the office whose district includes the area covered by the report. Although a particular report may not be in stock at locations other than the Publication Sales office in Sudbury, they can generally be obtained within 3 working days. All telephone, fax, mail and e-mail orders should be directed to the Publication Sales office in Sudbury. Use of VISA or MasterCard ensures the fastest possible service. Cheques or money orders should be made payable to the Minister of Finance. Mines and Minerals Information Centre (MMIC) Macdonald Block, Room M2-17 900 Bay St. Toronto, Ontario M7A 1C3 Tel: (416) 314-3800 Mines Library 933 Ramsey Lake Road, Level A3 Sudbury, Ontario P3E 6B5 Tel: (705) 670-5615 Publication Sales 933 Ramsey Lake Rd., Level A3 Sudbury, Ontario P3E 6B5 Tel: (705) 670-5691(local) 1-888-415-9845(toll-free) (705) 670-5770 pubsales@ndm.gov.on.ca Fax: E-mail: Regional Mines and Minerals Offices: Kenora - Suite 104, 810 Robertson St., Kenora P9N 4J2 Kirkland Lake - 10 Government Rd. E., Kirkland Lake P2N 1A8 Red Lake - Box 324, Ontario Government Building, Red Lake P0V 2M0 Sault Ste. Marie - 70 Foster Dr., Ste. 200, Sault Ste. Marie P6A 6V8 Southern Ontario - P.O. Bag Service 43, 126 Old Troy Rd., Tweed K0K 3J0 Sudbury - Level B3, 933 Ramsey Lake Rd., Sudbury P3E 6B5 Thunder Bay - Suite B002, 435 James St. S., Thunder Bay P7E 6S7 Timmins - Ontario Government Complex, P.O. Bag 3060, Hwy. 101 East, South Porcupine P0N 1H0 Toronto - MMIC, Macdonald Block, Room M2-17, 900 Bay St., Toronto M7A 1C3 This report has not received a technical edit. Discrepancies may occur for which the Ontario Ministry of Northern Development and Mines does not assume any liability. Source references are included in the report and users are urged to verify critical information. Recommendations and statements of opinions expressed are those of the author or authors and are not to be construed as statements of government policy. If you wish to reproduce any of the text, tables or illustrations in this report, please write for permission to the Team Leader, Publication Services, Ministry of Northern Development and Mines, 933 Ramsey Lake Road, Level B4, Sudbury, Ontario P3E 6B5. Cette publication est disponible en anglais seulement. Parts of this report may be quoted if credit is given. It is recommended that reference be made in the following form: Easton, R.M., Jobin-- Bevans, L.S. and James, R.S. 2004. Geological guidebook to the Paleoproterozoic East Bull Lake intrusive suite plutons at East Bull Lake, Agnew Lake and River Valley, Ontario; Ontario Geological Survey, Open File Report 6135, 84p. iii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Contents Abstract ...........................................................................................................................................................xiii Introduction........................................................................................................................................................ 1 Safety......................................................................................................................................................... 1 Acknowledgements ................................................................................................................................... 2 East Bull Lake Intrusive Suite............................................................................................................................ 2 Nomenclature ............................................................................................................................................ 2 Regional Geological Setting...................................................................................................................... 3 Overview of East Bull Lake Intrusive Suite .............................................................................................. 9 Depth and Mechanism of Emplacement.................................................................................................. 10 Magma Composition and its Relationship to Mineralization .................................................................. 10 Orthopyroxene Hornblendite Bodies....................................................................................................... 12 Geochemistry .................................................................................................................................. 14 Field Trip Guidebook....................................................................................................................................... 16 East Bull Lake Intrusion—Overview ...................................................................................................... 16 Country Rocks, Structure and Geometry of the Intrusion............................................................... 16 Stratigraphy and Petrography.......................................................................................................... 17 Marginal Series ...................................................................................................................... 18 Lower Series........................................................................................................................... 18 Main Series............................................................................................................................. 18 Upper Series ........................................................................................................................... 20 Geochemistry .................................................................................................................................. 21 Mineralization................................................................................................................................. 26 Road Log, Day 1, East Bull Lake Intrusion............................................................................................. 27 Stop 1. Paleoproterozoic Metagabbro............................................................................................. 28 Stop 2. Neoarchean Parisien Lake Syenite ..................................................................................... 28 Stop 3. Southern Margin of the East Bull Lake Intrusion ............................................................... 28 Stop 4. Country Rocks .................................................................................................................... 29 Stop 5. Rhythmically Layered Zone ............................................................................................... 29 Stop 6. Folson Lake Deformation Zone.......................................................................................... 30 Stop 7. Dendritic Texture in the Varitextured Gabbronorite Zone ................................................. 30 Stop 8. Layered Gabbronorite Zone................................................................................................ 30 Stop 9. Olivine Gabbronorite Zone................................................................................................. 31 Stop 10. Rhythmically Layered Zone, East Bull Lake Intrusion .................................................... 31 Stop 11. Moon Lake Traverse......................................................................................................... 31 Stop 12A. Parisien Lake Traverse................................................................................................... 33 Stop 12B. Parisien Lake Traverse................................................................................................... 34 Agnew Lake Intrusion—Overview.......................................................................................................... 35 Country Rocks, Structure, and Geometry of the Intrusion.............................................................. 35 Stratigraphy and Petrography.......................................................................................................... 35 Marginal Series ...................................................................................................................... 35 Lower Series........................................................................................................................... 37 Upper Series ........................................................................................................................... 39 Geochemistry .................................................................................................................................. 39 v PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Mineralization................................................................................................................................. 40 Road Log, Day 2, Agnew Lake Intrusion................................................................................................ 42 Stop 1. Inclusion-Bearing Unit ....................................................................................................... 43 Stop 2. Nodular Anorthosite Unit, Marginal Leucogabbronorite Zone .......................................... 44 Stop 3. Inclusion-Bearing Unit, Lower Gabbronorite Zone............................................................ 44 Stop 4. Inclusion-Bearing Unit, Lower Gabbronorite Zone............................................................ 46 Stop 5. Sudbury-Type Breccia........................................................................................................ 47 Stop 6. Varitextured Unit, Marginal Leucogabbronorite Zone ....................................................... 47 Stop 7. Dendrite Unit, Lower Gabbronorite Zone .......................................................................... 48 River Valley Intrusion, Overview............................................................................................................ 49 Country Rocks, Structure and Geometry ........................................................................................ 49 Structural State and Degree of Preservation........................................................................... 50 Contacts.................................................................................................................................. 52 Stratigraphy and Petrography.......................................................................................................... 55 North of the Sturgeon River ................................................................................................... 55 Marginal Zone ............................................................................................................... 55 Inclusion and/or Fragment-Bearing Zone ...................................................................... 56 Olivine Gabbronorite Zone............................................................................................ 57 Gabbronorite Zone......................................................................................................... 57 Leucogabbronorite Zone................................................................................................ 57 South of the Sturgeon River ................................................................................................... 57 Stratigraphic Comparison with other East Bull Lake Suite Intrusions ................................... 59 Geochemistry .................................................................................................................................. 60 Road Log, Day 3, River Valley Intrusion................................................................................................ 64 Stop 1. Shear-Zone Hosted Orthopyroxene Hornblendite Body..................................................... 64 Stop 2. Rocks of the Crerar Gneiss Association ............................................................................. 65 Optional Stop. Crerar Gneiss Association ...................................................................................... 65 Optional Stop. Red Cedar Lake Gneiss........................................................................................... 65 Stop 3. Varied Degrees of Preservation in Rocks of the River Valley Intrusion ............................ 66 Stop 4. Autolith Fragments and Layering, Layered Gabbronorite Zone......................................... 66 Stop 5. Layered Olivine Gabbronorite............................................................................................ 67 Stop 6. Footwall Alkali Feldspar Granite and Sudbury Diabase Dike............................................ 67 Stop 7. Azen Creek Copper-Nickel-PGE Occurrence .................................................................... 68 Optional Stop. Huronian Metavolcanic Rocks................................................................................ 69 Optional Stop. Variably Preserved Nipissing Gabbro .................................................................... 69 Stop 8. Mylonitized Anorthosite of the River Valley Intrusion ...................................................... 70 Optional Stop. Mississagi Formation and Mylonitic Contact with the River Valley Intrusion....... 70 Stop 9. Dana Lake Copper-Nickel-PGE Occurrence ...................................................................... 71 Stop 9A. L6+00N, Contact environment................................................................................ 74 Stop 9B. L7+00N, Contact environment................................................................................ 74 Stop 9C. Road Zone ............................................................................................................... 74 Stop 9D. Central Zone............................................................................................................ 75 Stop 9E. Trench Zone............................................................................................................. 76 Stop 9F. South Zone............................................................................................................... 76 References........................................................................................................................................................ 77 vii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �FIGURES 1. Distribution of U-Pb zircon ages from Neoarchean and Paleoproterozoic rocks in the Sudbury area, as well as the distribution of East Bull Lake intrusive suite bodies................................................................. 4 2. Geologic map of East Bull Lake Intrusion .................................................................................................. 16 3. Stratigraphic sections for the East Bull Lake and Agnew Lake intrusions.................................................. 17 4. Pearce-element ratio diagrams for the East Bull Lake, Agnew Lake and River Valley intrusions ............. 22 5. Chondrite-normalized rare earth element plots for the East Bull Lake and Agnew Lake intrusions........... 23 6. Geological map of the Moon Lake area, East Bull Lake Intrusion, showing the route of the traverse undertaken during Stop 11........................................................................................................................... 32 7. Geology of the Agnew Lake Intrusion ........................................................................................................ 36 8. Map showing the degree of primary textural preservation within rocks of Crerar and Dana townships..... 51 9. Interpreted stratigraphic sequence and distribution of primary mineral phases and compositions within the River Valley intrusion north of the Sturgeon River..................................................................................... 56 10. Generalized cross-sections for the Street metagabbro and the River Valley, East Bull Lake and Agnew Lake intrusions of the East Bull Lake intrusive suite .................................................................................. 59 11. Chondrite-normalized rare earth element plots for the River Valley intrusion for rocks north of the Sturgeon River............................................................................................................................................. 61 12. Map of the Dana North property of Pacific North West Capital Corporation, showing the location of stops 9A through 9F, as well as the extent of the drilling program undertaken between 1999 and 2002............. 73 PHOTOS Frontispiece. Typical copper-nickel-PGE mineralization from the River Valley intrusion ................................. xv 1. Orthopyroxene phenocrysts in matrix-rich orthopyroxene hornblendite in Henry Township..................... 13 2. Igneous layering in the East Bull Lake Intrusion ........................................................................................ 19 3. Dendritic texture within the Upper Series of the East Bull Lake Intrusion ................................................. 20 4. Centimetre- to decametre-scale igneous layering within rocks of the Agnew Lake Intrusion .................... 38 5. Dendrite texture developed with the Lower Gabbronorite Zone of the Agnew Lake Intrusion .................. 38 6. Stop 1, Day 2. Large pyroxenite pod hosted by rocks of the Inclusion-Bearing unit, which at this locality are hosted in a predominantly leucogabbronoritic matrix ........................................................................... 43 Stop 2, Day 2. a) Stone quarry in Nodular Anorthosite unit. b) Close-up of nodular anorthosite, showing closely packed plagioclase glomerophenocrysts with minor interstitial, altered, pyroxene ........................ 45 Nodular anorthosite from the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion ...................................................................................................................................................... 46 Diabase dike containing large, rounded plagioclase crystals, cutting across rocks of the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion................................................................ 47 7. 8. 9. ix PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PHOTOS Cont'd 10. Progressive textural changes. a) Igneous-texture in medium-grained norite of the River Valley intrusion, Crerar Township. b) Partly recrystallized rock of the River Valley intrusion, preserving igneous-texture. c) Recrystallized but massive leucogabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. d) Typical undeformed but recrystallized leucogabbronorite from the Positano Quarry, Loughrin Township ......................................................................................................... 52 11. Layering in the River Valley intrusion. a) Igneous layering within the norite zone. b) Igneous layering near the top of the anorthosite zone and the base of the olivine gabbronorite zone. c) Layering within leucogabbronoritic rocks immediately above the heterogeneous basal zone. d) Large-scale igneous layering within leucogabbronoritic rocks exposed in the Dana Quarry....................................................... 53 12. a) Typical leucogabbroic gneiss of an East Bull Lake intrusive suite intrusion in Henry and Loughrin townships. b) Protomylonitic to mylonitic textures present in a high-strain zone in the River Valley intrusion. c) Strain gradient within rocks of the River Valley intrusion, northern Crerar Township. d) Epidote-clot developed in leucogabbro gneiss of the River Valley intrusion, Crerar Township................. 54 13. Photographs illustrating the textural variation observed in the Inclusion-Bearing unit in the Dana North area .............................................................................................................................................................. 75 TABLES 1. Summary of stratigraphic, mineralogical and geochemical data for the East Bull Lake intrusive suite...... 5 2. Summary of geochronology on rocks of the East Bull Lake intrusive suite................................................ 7 3. Timing of major geological events and summary of age constraints on the main rock units present in the study area..................................................................................................................................................... 8 Representative and average anhydrous whole-rock geochemical analyses from the East Bull Lake and Agnew Lake intrusions................................................................................................................................ 24 Representative chemical analyses from the River Valley intrusion ............................................................ 62 4. 5. xi PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Abstract Intrusions of the ~2480 million-year-old East Bull Lake intrusive suite occur as an east-northeast-trending belt along the boundary of the Archean Superior and the Proterozoic Southern provinces in Ontario, Canada. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic, bimodal magmatic event resulting from a mantle-plume driven, intracontinental rifting event that formed a major basin, filled by sedimentary and igneous rocks of the Huronian Supergroup. Intrusions of similar age and composition in Finland and Wyoming were once contiguous with the East Bull Lake intrusive suite prior to tectonic dispersion during the Proterozoic. Field and geochemical evidence indicates that the 3 largest intrusions (East Bull, Agnew, River Valley) crystallized from similar, low-Ti, high-Al, PGE-rich, tholeiitic parent magmas that originated in deeper, more mafic chambers. The primary magmas were likely second-stage melts derived from a sublithospheric depleted mantle source modified by Neoarchean subduction and accretion events. Petrologically, the intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe-enrichment trend in the residual magmas. Olivine and orthopyroxene occur as cumulus phases in only the interpreted lower parts of the stratigraphy. Rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle part of the stratigraphy. Ferrogabbro and ferrosyenite occur only in the uppermost Agnew Lake Intrusion. A marginal facies, comprising brecciated and, locally, thermally recrystallized and/or partially melted footwall rocks, is common and is testimony to a high-energy flow regime during initial emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite with erratically distributed leucocratic to melanocratic autoliths and footwall xenoliths and eventually into a thick interval of undifferentiated, plagioclase-rich cumulates, locally with spectacular glomerocrystic textures. Disseminated, bleb and interstitial magmatic chalcopyrite-pyrrhotite-pentlandite mineralization (1 to 5% sulphide minerals, up to 10 g/t Pt+Pd) occurs in autolith-rich gabbronorite breccia within 5 to 50 m of the contact of the intrusions. The mineralization is commonly spatially associated with pyroxenite cumulates and autoliths that are otherwise poorly represented in the stratigraphy. Vigorous convection and explosive breccia-producing emplacement of sulphide-saturated magma formed PGE-rich zones at the margins (sidewall or floor?). The magmatic sulphide zones are overprinted and enclosed by a broader envelope of metamorphic and/or hydrothermal sulphide mineralization of similar mineralogy (± pyrite) that extends into adjacent leucogabbronoritic to anorthositic units, and less so into the country rocks. The hydrothermally enriched zones contain 2 to 10% sulphide minerals with 1 to 10 g/t Pt+Pd, and represent the main exploration target. The high-grade zones are enclosed by broader, lower grade mineralization with Pt+Pd levels in the background range of 20 to 50 ppb. Day long field excursions to each of the East Bull Lake, Agnew Lake, and River Valley intrusions are also included in this report. The combination of road accessible and walk-in stops is designed to illustrate important aspects of the geology, stratigraphy and mineralization found in each of the intrusions. xiii PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �S Frontispiece. Typical copper-nickel-PGE mineralization from the River Valley intrusion. a) Typical outcrop exposure of mineralized olivine gabbronorite from the River Valley intrusion in the vicinity of the Jackson’s Flats occurrence. Note sulphide “burns” to the right of the 8.5 cm long scale card. b) Drill core from drill hole DL-14 in the Dana North area (Stop 9C, Day 3), showing the typical low tenor of sulphide and the presence of blue-grey quartz. A sample like this typically contains greater than 2 g/t of Pt and Pd mineralization. c) Close-up of mineralization from the Inclusion-Bearing unit at Dana South (Stop 9F, Day 3). All photographs courtesy of S. Jobin-Bevans. xv PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario R.M. Easton1, L.S. Jobin-Bevans2 and R.S. James3 Ontario Geological Survey Open File Report 6135 2004 1 Precambrian Geoscience Section, Ontario Geological Survey Ministry of Northern Development and Mines, Sudbury, Ontario, Canada P3E 6B5 2 Caracle Creek International Consulting Inc., Sudbury, Ontario, Canada P3E 2V7 3 Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Introduction Since the summer of 1998, mafic intrusions of the East Bull Lake intrusive suite, specifically the East Bull Lake, Agnew Lake and the River Valley intrusions, have been the subject of ongoing copper-nickelplatinum group element (PGE) mineral exploration. This increased economic interest in these intrusions has led to renewed interest in the geology, stratigraphy and geochemistry of the East Bull Lake intrusive suite, as summarized by James et al. (2002a, 2002b) and Easton (2003). After some 3 decades of study there are only a very few field trip guides to any of these intrusions. Peck et al. (2000) and Brisbin et al. (2001) produced guidebooks for the East Bull Lake Intrusion for PGE Exploration Workshops at Laurentian University in 2000 and 2001. A field guide for the Agnew Lake Intrusion has never been prepared. Hrominchuk and Jobin-Bevans (2000) prepared a guidebook for the River Valley intrusion to compliment the first PGE Exploration Workshop at Laurentian University in 2000. Earlier guides by Lumbers (1978) and Davidson (1986) included a few stops in the River Valley intrusion, but were prepared prior to detailed mapping of the intrusion. The geology of the East Bull Lake suite intrusions is a significant topic in understanding the earliest evolution of the Southern Province in central Ontario. There are now numerous professional publications that describe and discuss the geology, geochemistry and mineral deposit potential of these intrusions (Table 1, and references in James et al. 2002a, 2002b). This integrated field guidebook to the East Bull Lake, Agnew Lake and River Valley intrusions is intended to compliment those studies. The field trip stops have been selected to show typical stratigraphic units in each intusion, as well as the geology of mineralized zones near the contacts of all 3 intrusions. The stops are also designed to compare and contrast the geological characteristics of the 3 major intrusions. The field trip uses a combination of road accessible outcrops, as well as some stops that require some short traverses through the bush, generally utilizing trails of varied condition. Although most of the stops can be accessed using a two-wheel drive vehicle, use of a vehicle with high ground clearance and fourwheel drive capability is recommended for both the Agnew Lake and River Valley intrusion stops, especially in the spring and fall. The structure of the guidebook is as follows: following the introduction, a brief overview of the East Bull Lake intrusive suite is provided. This is followed by descriptions of each one-day field trip to the East Bull Lake, Agnew Lake and River Valley intrusions. These descriptions include an overview of the geology, stratigraphy, geochemistry and mineralization in each intrusion, as well as a detailed road log and stop description. Day 1, the East Bull Lake Intrusion, was prepared mainly by R.S. James. Day 2, the Agnew Lake Intrusion, was prepared mainly by S. Jobin-Bevans and R.S. James. Day 3, the River Valley intrusion, was prepared mainly by R.M. Easton and S. Jobin-Bevans. The guidebook was compiled and edited by R.M. Easton. SAFETY For users of this guidebook, please bear in mind that some of the stops listed in this guidebook involve hiking in the bush. Therefore, standard bush safety practices should be followed by users of this guidebook. Such practices include travelling in pairs; advising others of your starting time and location and your expected return time; carrying sufficient water for the trip; being prepared for sudden changes in the weather; and carrying the appropriate emergency and safety gear. Most of the trip routes are on Crown land, but access is on or near private property for some routes. As in all such situations, please respect the property rights of others, so that future access for other geologists is not adversely affected. 1 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ACKNOWLEDGEMENTS The authors acknowledge the contributions of information kindly made available to them by Mustang Minerals Corporation with regard to access and information on the geology and mineralization of the East Bull Lake Intrusion. Pacific North West Capital Corporation and its partner Anglo American Platinum Corporation Limited are also thanked for similar information on the Agnew Lake and River Valley intrusions. Some of the material on the River Valley intrusion is related to work conducted as part of Ontario Geological Survey bedrock mapping projects PU99-007, 99-019, 01-009 and 02-008, as described in Easton (2003). This guidebook was prepared initially for use with a field trip held in conjunction with the Geological Association of Canada–Mineralogical Association of Canada joint annual meeting in St. Catharines, May 12 to 14, 2004. East Bull Lake Intrusive Suite NOMENCLATURE The term East Bull Lake intrusive suite was only recently introduced by Easton (1999), and is similar to the term, East Bull Lake suite, used by Vogel et al. (1999). Prior to 1991, rocks of this suite were generally only described as individual intrusions, or as gabbro-anorthosite bodies that were intrusive into the lower part of the Elliot Lake Group (e.g., Bennett, Dressler and Robertson 1991). Prevec et al. (1995) referred to rocks of the East Bull Lake intrusive suite as “trans-Huronian” intrusions. Keays et al. (1995) and Peck et al. (1995) grouped them as part of the Huronian–Nipissing Magmatic Province, Domain or Belt. Nomenclature of individual intrusions within the East Bull Lake intrusive suite has been inconsistent over the years; however, James et al. (2002a) attempted to standardize the nomenclature for the intrusions within the suite, and to make this terminology consistent with the North American Stratigraphic Code (NACSN 1983). For example, the intrusion located west of Agnew Lake has been termed the Shakespeare–Dunlop intrusion, the Shakespeare–Dunlop gabbro-anorthosite intrusion, the Agnew Lake Intrusion and the Agnew intrusion, which was standardized in James et al. (2002a) as the Agnew Lake Intrusion. The standardization of nomenclature recommended by James et al. (2002a) is used herein, and is summarized in Table 1. Table 1 also includes the location of reference sections for the better-studied intrusions, details on contact relationships, and brief descriptions of the various intrusions. Recent studies on the East Bull Lake intrusive suite have used a three-tiered classification scheme to describe the rocks of the suite (e.g., Chubb 1994; Vogel 1996; James et al. 2002a), which has been used in this report. The first-tier differentiates rock types based on colour index and variation in modal abundance of mafic minerals and plagioclase. The second-tier groups individual rock layers into mappable series, zones, subzones and units (Irvine 1982). It should be noted that units outlined by this second-tier classification are not formal stratigraphic units as defined by the North American Stratigraphic Code (NACSN 1983), although some units, in particular the zones, are roughly equivalent to lithodemic units. The third-tier is based on petrography or distinctive textures, but also uses the IUGS classification scheme for plutonic rocks (Le Maitre 1989) where primary mineralogy or whole-rock geochemistry is present. Most primary igneous mafic minerals are replaced by amphibole, chlorite and epidote group minerals (e.g., Kamineni et al. 1985; Kamineni 1986), which reflects regional metamorphism in East Bull Lake intrusive suite bodies in the Southern Province. Plagioclase may be unaltered or partially to completely 2 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �saussuritized. Rocks of the East Bull Lake intrusive suite within the Grenville Province can range from almost wholly unaltered to completely recrystallized to an upper amphibolite facies, amphiboleplagioclase assemblage (e.g., Easton and Hrominchuk 1999; Tettelaar 2000; Easton 2002). In either geological province where metamorphic alteration is pervasive, rock nomenclature is based on geochemically determined CIPW-normative mineralogy combined with petrographic data. Unless otherwise stated, all UTM co-ordinates are in Zone 17, datum NAD 83. Unless otherwise noted, all analytical work in this report was completed by the Geoscience Laboratories, Geoservices Centre, Sudbury. REGIONAL GEOLOGICAL SETTING The Paleoproterozoic East Bull Lake intrusive suite (Easton 1999; James et al. 2002a) consists of several bodies of dominantly gabbronorite to gabbroic anorthosite that occur in both the Southern and Grenville provinces between Elliot Lake and the Ottawa River (Figure 1). The 3 largest bodies are the East Bull Lake and Agnew Lake intrusions (Southern Province) and the River Valley intrusion (Grenville Province) (see Table 1). All were emplaced between ~2491 and 2475 Ma (Table 2). Smaller bodies include the intrusions in Drury, Falconbridge, May, Street and Wisner townships (see Table 1, see Figure 1). Most bodies of the East Bull Lake intrusive suite were emplaced into Archean rocks present in either the Superior or Grenville provinces. Table 3 summarizes the main geological events that have occurred in this part of the Canadian Shield during the Precambrian. The distribution of the bodies of the East Bull Lake intrusive suite approximates the base of the Huronian Supergroup in the Southern Province (see Figure 1). Contact relationships between rocks of the East Bull Lake intrusive suite and the Huronian Supergroup are either faulted or equivocal. Consequently, it is not known for certain if the East Bull Lake intrusive suite intruded the Huronian Supergroup, or was unconformably overlain by it, or both (e.g., Card 1978; McCrank et al. 1989; Peck, James and Chubb 1993). Emplacement of the bodies of the East Bull Lake intrusive suite, subsequent eruption of volcanic rocks of the Huronian Supergroup, and formation of the depositional basin filled by Huronian Supergroup sediments is attributed by most authors (e.g., Card et al. 1972; Young 1983; Fahrig 1987; Hoffman 1989; Bennett, Dressler and Robertson 1991; Heaman 1997; Card and Poulsen 2000; Ernst and Buchan 2001) to an intracontinental rifting event resulting from a mantle-plume centred near Sudbury. A suite of igneous rocks (hereafter called the “rifting suite”) records the initial trace of this plume-induced rifting event; from oldest to youngest, they include: 1. The East Bull Lake intrusive suite (~2490 to 2470 Ma). 2. Elliot Lake Group metavolcanic and minor plutonic rocks, for example, the Elsie Mountain, Stobie and Copper Cliff formations, May Township metavolcanic rocks, and minor synvolcanic mafic intrusions (~2490 to 2450 Ma). Metavolcanic rocks of the Elliot Lake Group constitute the lowermost of 4 stratigraphic groups in the Huronian Supergroup. 3. Matachewan (2473 Ma) and Hearst (2446 Ma) mafic dike swarms (Heaman 1997). 4. Granitic intrusions within the Elliot Lake Group and the Superior Province (~2385 to 2460 Ma), including the Creighton and Murray plutons and the Street Township granitic plutons that may be coeval with Elliot Lake Group felsic volcanism. 3 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x < x 2473 >Matachewan x x x x x x Hearst x x x x -x Province x x X X x X x x x x x X x x x /x x )< / x x xI x A x,'x x I x X )< x X x x X x X x X x x 2681 ±13 x x X / x + )( + )< x + < > x + + ++ Province i- + > + + )< ) x X x +25 + -r —In. - + + + + + + + + + + + + + + - + + - + + + + + + +++ + + + + + + + + + + + + + + + + + + 2475±2+ + • •+ + + - + + - Lake Nipissing + 1+ East Bull Lake intrusion tonalite gneiss Agnew Lake intrusion Archean basement Falconbridge Twp. intrusion ® MayTwp. River Valley intrusion ® Levackgneiss Sudbury Igneous Complex intrusion plutonism + metamorphism Copper Cliff rhyolite 8 (fl\ Drury Twp. '—' intrusion 1 Temagami Island Gabbro + + Flett Twp. intrusions Grenville Province rocks Wisnerlwp. Huronian metasedimentary rocks intrusion Huronian volcanic rocks southern Wanapitei complex Street Twp. intrusion EBLI suite rocks x Superior Province rocks Figure 1. Distribution of U-Pb zircon ages from Neoarchean and Paleoproterozoic rocks in the Sudbury area, as well as the distribution of East Bull Lake intrusive suite bodies. Dashed circle indicates 125 km radius from a point central to the Sudbury Structure; most intrusions lie within this radius. Sources used: 1, 2, 8, 9) Krogh, Davis and Corfu 1984; 4) Wodicka and Card 1995; Krogh, Davis and Corfu 1984; 5) Chen, Krogh and Lumbers 1995; 6 and 10) Prevec 1993; 7) Heaman in Easton, Davidson and Murphy 1999; below and above circle 16, Corfu and Easton (2000). Age of Hearst and Matachewan dikes from Heaman (1997). Abbreviations: GFTZ = Grenville Front tectonic zone; EBLI = East Bull Lake intrusive suite. Current geochronological data suggests 2 main pulses in magmatism. 1. A predominantly mafic pulse at ~2475 Ma, represented by the East Bull Lake intrusive suite, Matachewan dikes, and the mafic part of the Elliot Lake Group. 2. A felsic-dominated pulse at ~2450 Ma, represented by felsic metavolcanic rocks of the Elliot Lake Group, related granitic intrusions, and Hearst dikes. The rifting event produced an east-trending, southward-deepening basin that likely resulted in formation of oceanic crust (e.g., Card et al. 1972; Young 1983; Fahrig 1987; Hoffman 1989; Bennett, Dressler and Robertson 1991). The geometry of the Matachewan and Hearst dikes and their distribution with respect to the metavolcanic rocks of the Elliot Lake Group is consistent with the north-northwest trend of the dikes reflecting the orientation of a failed arm of the rift (e.g., Fahrig 1987; Ernst and Buchan 2001). If this geometry is correct, then the east-trending East Bull Lake intrusive suite bodies not only occur along the continent-side of the rift basin, but also within a 125 km radius of a plume centered on Sudbury (see Figure 1). At present, it is uncertain if the rocks of East Bull Lake intrusive suite occur directly along a cryptic structure, or structures, related to this rift basin, or if the present distribution of the bodies reflects some other emplacement mechanism (James et al. 2002a). 4 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �5 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Size (km2) Dominant Rock Types 15 ~5 May Twp. or Salmay Lake Wisner Twp. 43 East Bull Lake, includes outliers at Splake and Tee Lakes 1-2 8-10 Drury Twp. or Worthington Falconbridge Twp. or Norduna 50 Agnew Lake or Shakespeare –Dunlop Leucogabbronorite Leucogabbro, gabbro, melanogabbro. Leucogabbronorite gabbronorite. Leucogabbronorite gabbronorite, olivine gabbronorite. Leucogabbro, gabbro. Leucogabbronorite gabbronorite, olivine gabbronorite. Southern Province hosted intrusions Intrusion Shock metamorphosed, upper greenschist? Lower to upper greenschist Shock metamorphosed, upper greenschist Lower to upper greenschist Shock metamorphosed, locally upper greenschist Upper greenschist to lower amphibolite Metamorphic Grade n = 8 major and trace element analyses, REE and isotope data also available n = ~15 major and trace element analyses, REE and isotope data also available n = ~5 major and trace element analyses, REE and isotope data also available n = ~200 major and trace element analyses, REE and isotope data also available n = ~100 major and trace element analyses, REE and isotope data also available Geochemisty 5 No data No data Plag = An60-80 Rhythmically layered zone: ol = Fo59-65, opx = En64-66Fs30-32Wo4, cpx = En41-45Fs15-16 Wo39-44 Olivine gabbronorite zone: ol = Fo65-74, opx = En67-706Fs26-29Wo4, cpx = En41-43Fs14-15 Wo43-44 Plag = An60-76 No data Plag = An2-79 Primary Mineral Chemistry Unknown Contacts typically zones rich in Sudbury breccia. Intrusion in contact aureole of the Sudbury Igneous Complex. Melanogabbro better preserved than leucocratic rocks. Ramsey-Algoma granitoid complex rocks in footwall, diking and brecciation in footwall at contacts, to east, possible unconformable contact with Huronian Sgp. metasedimentary rocks. Contacts typically zones rich in Sudbury breccia. Intrusion in contact aureole of the Sudbury Igneous Complex. Parisien Lake syenite, Whiskey Lake greenstone belt, RamseyAlgoma granitoid complex rock, diking and brecciation in footwall at contacts, possible unconformable contact with Huronian Supergroup metasediments to the north. Contact relationships Unknown Unknown Not defined Not defined Not defined Not defined ~850 m Unknown Not defined Not defined Location of Reference Sections, UTM Zone 17, NAD 83 Unknown at least 1000m, as much as 2100 m in some interpretations Stratigraphic Thickness Table 1. Summary of stratigraphic, mineralogical and geochemical data for the East Bull Lake intrusive suite (modified from James et al. 2002a; Easton 2003). Vogel, James and Keays 1998 Cape 1973, Robertson 1976, Prevec 1993 DeGagne 1982, Prevec 1993 Born 1979; James and Born 1985; McCrank et al. 1989; Peck, James and Chubb 1993; Peck et al. 1995; Prevec 1993, Vogel , James and Keays 1998 Brons 1984; Prevec 1993 Vogel 1996; Vogel, James and Keays 1998; Vogel et al. 1999 References �6 PDF compression, OCR, web-optimization with CVISION's PdfCompressor Size (km2) Dominant Rock Types ~25 ~5 ~5 Contact Third Concession ~100 Loughrin Lake River Valley Gneissic leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Gneissic, leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Gneissic leucogabbronorite, gabbronorite, associated orthopyroxene hornblendite. Leucogabbronorite gabbronorite, olivine gabbronorite. Grenville Province hosted intrusions Intrusion Table 1. continued Upper amphibolite Upper amphibolite Upper amphibolite Greenschist to upper amphibolite, with some areas of primary mineralogy preserved, mainly in Dana Township. Metamorphic Grade n = 1 major and trace element and REE analyses No data n = 12 major and trace element analyses, REE data also available n = ~70 major and trace element analyses, REE data also available Geochemisty 6 No data No data No data Marginal zone: Plag = An79 (core); An68, (intercumulate) Olivine gabbronorite zone: ol = Fo74 plag = An70-75 opx = En60-80 Primary Mineral Chemistry Primary Contacts: Dana Twp.: Sharp contact with paragneiss or alkali feldspar granite, no dikes in footwall, marginal zone contains footwall xenoliths. Fault contacts with Huronian Sgp. metasedimentary and metavolcanic rocks, equivocal contact with Nipissing gabbro. Crerar Twp: Sharp contact with alkali feldspar granite, alteration of granite in footwall, fine-grained phase at contact, no dikes in footwall, marginal zone contains footwall xenoliths. Hosted by Crerar gneiss association. North contact mainly tectonic. South contact sharp, with scattered orthopyroxene hornblendite bodies. Hosted by Street gneiss association. North contact locally cut by Geon 24 to 23 felsic intrusions. South contact sharp, with scattered orthopyroxene hornblendite bodies. Hosted by Street and Crerar gneiss association. Same as Contact intrusion. Both contacts locally intrude migmatitic garnet amphibolite. Hosted by Street gneiss association. Contact relationships ~500 m less than 500 m ~500 to 1000m ~1000 m Stratigraphic Thickness Not defined Not defined Marginal zone: 555310E, 5172625N; 556610E, 5174225N to 556510E, 5171225N; 558760E, 5167825N to 558510E, 5167475N. Olivine Gabbronorite zone: 559260E, 516825N; 563260E, 5166225N; 560410E, 5167825N. Gabbronorite zone: 558510E, 5165575N Not defined Location of Reference Sections, UTM Zone 17, NAD 83 Easton 2003 Easton 2003 Easton 2003 Easton and Hrominchuk 1999; Hrominchuk 1999, 2000; Easton 2003 References �7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor ~5 ~10 ~5 Street metagabbro Red Deer Lake southern Wanapitei complex Gneissic leucogabbro, gabbro, norite. Gneissic leucogabbronorite, gabbronorite. Leucogabbronorite gabbronorite, norite, associated orthopyroxene hornblendite. Dominant Rock Types Upper amphibolite Upper amphibolite Upper amphibolite Metamorphic Grade n = 8 major element analyses n = 1 major element analysis n = 25 major, trace element and REE analyses Geochemisty Coronitic olivine present No data Coronitic orthopyroxene present Primary Mineral Chemistry Unknown Relative Ages River Valley Street Twp. River Valley Drury Twp. Falconbridge Twp. Intrusion Absolute Ages Agnew Lake East Bull Lake Rb/Sr whole-rock >2446±3 >2460±20, 2475+25/-10 Ashwal and Wooden (1989) Pb/Pb whole-rock Nd/Sm whole-rock and mineral 2562±165 2377±68 2165±130 2185±105, 1960±100 2468±5 U/Pb baddeleyite and zircon U/Pb zircon 7 Heaman (1997) Corfu and Easton (2000), Easton and Hrominchuk (1999) Corfu and Easton (2000) Krogh, Davis and Corfu (1984) Krogh, Davis and Corfu (1984) McCrank et al. (1989) Kamineni (1986) McCrank et al. (1989) McCrank et al. (1989) Prevec (1993) Prevec (1993) Prevec (1993) Heaman, unpublished in Easton, Davidson and Murphy (1999) Ashwal and Wooden (1989) Ashwal and Wooden (1989) U/Pb zircon U/Pb baddeleyite and zircon Nd/Sm whole-rock and mineral Ar/Ar hornblende K/Ar whole rock K/Ar whole rock U/Pb zircon U/Pb zircon U/Pb zircon U/Pb baddeleyite and zircon 2491±5 2480+10/-5 2472±76 1859±5 1725±18 1155±14 1855±10 2441±3 ~1850 2475+2/-1 U/Pb zircon Reference Method Age (in Ma) Unknown Unknown less than 500 m Stratigraphic Thickness Not defined Not defined Location of Reference Sections, UTM Zone 17, NAD 83 530510E, 5155225 to 5155725N Rousell and Trevisiol 1988 Lumbers 1975 Easton 1999, 2003; Easton and Murphy 2002 References Dikes of the Matachewan dike swarm cut the River Valley intrusion. Felsic dikes likely related to felsic intrusions coeval with Huronian Stobie Formation cut the River Valley intrusion. age from orthopyroxene hornblendite body thought to be genetically related to East Bull Lake intrusive suite. n=4, second age is resetting event. lower intercept, dates Sudbury event. from layered gabbronorite zone, either in Erana or Dana quarries, north of Sturgeon River. n=6, igneous textured samples only. n=4, igneous textured samples only, second age is resetting event. from alkali feldspar granite that may not be part of the intrusion. from olivine gabbronorite zone, few grain, hence high error. from olivine gabbronorite zone. no details on location. from olivine gabbronorite zone. from olivine gabbronorite zone. shocked zircons, dates Sudbury event. Comment Basal contact intrudes migmatitic garnet amphibolite, ~2475 Ma granites contain fragments of intrusion. Hosted by Street gneiss association. Unknown Contact relationships Table 2. Summary of geochronology on rocks of the East Bull Lake intrusive suite (from Easton 2003). Size (km2) Intrusion Table 1. continued �Table 3. Timing of major geological events and summary of age constraints on the main rock units present in the study area (from Easton 2003). Event and/or Map Unit Age Constraint (in Ma) Comment and/or Source Grenville dike swarm 586±4 Kamo, Krogh and Kumarapeli (1995) Pegmatite vein emplacement 989±2 Corfu and Easton (2000) Age of peak metamorphism in the hanging wall of the Grenville Front tectonic zone 1000 to 990 Corfu and Easton (2000) Age of peak Grenvillian metamorphism in Crerar and Dana townships 1040 and 1030 Sudbury dike swarm 1238±4 emplaced in or along northwest-trending faults in the Southern Province. Deformed and metamorphosed within the Grenville Province. Krogh et al. (1987). Killarney magmatic belt second-stage magmatism, coincident with magmatism in the Eastern Granite Rhyolite Province and in the Central Gneiss Belt 1471±3 van Breemen and Davidson (1988) Regional albitization metasomatic event 1701±4 U/Pb monazite, Schandl, Gorton and Davis (1994); fluid focussed along northwesttrending faults Killarney magmatic belt volcanism and high-level plutonism 1740, 1747±3, 1749±12 van Breemen and Davidson (1988); Sullivan and Davidson (1993); Davidson and van Breemen (1994) Northwest-trending regional faults Pre-1700, post-1850 Penokean Orogeny (folding and metamorphism of Huronian Supergroup rocks) ~1835 Peak metamorphism. Holm et al. (2001). Impact event and formation of Sudbury breccia 1850±1 Krogh, Davis and Corfu (1984) Thrust faulting post-F2 pre-regional faulting Sudbury breccia localized along these faults, suggesting they are pre-Sudbury Structure F2 folding post-2200, pre-1700, pre 1850? Pre-regional faulting, Nipissing sills folded, relationship to Sudbury Structure uncertain F1 folding Pre-2200 Emplacement of Nipissing gabbro sills 2219±4 to 2210±4 Huronian sedimentation >2220 but <2460 Huronian felsic volcanism and related plutonic rocks ~2477 to 2375 (2450±25, 2460±20, 2477±9, 2415±5, 2376±2) Emplacement of East Bull Lake intrusive suite rocks 2475±2 see also Table 2 Emplacement of orthopyroxene hornblendite bodies 2468±5 Emplacement of alkali feldspar granite bodies in Crerar and Dana townships 2640 High-grade Archean metamorphism and migmatization 2647±4 Emplacement of megacrystic granodiorite bodies in Crerar gneiss association 2660 Emplacement ages of Archean units in the Sudbury area 2711±7 to 2642±1 this study; see also Carr et al. (2000) Faults cut Sudbury Structure Nipissing sills in Street Township appear unaffected by this folding Corfu and Andrews (1986); Noble and Lightfoot (1992) Krogh, Davis and Corfu (1984), Corfu and Easton (2000), Krogh, Kamo and Bohor (1996), Smith (2002) Heaman (geochronologist, University of Alberta, personal communication, 1999) Corfu and Easton (2000) Bodies intrude Crerar and Pardo gneiss, this study Krogh, Davis and Corfu (1984); Wodicka and Card (1995) this study Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Chen, Krogh and Lumbers (1995); Meldrum et al. (1997) 8 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Within the Southern Province, rocks of the East Bull Lake intrusive suite are affected by folding and upper greenschist to lower amphibolite facies metamorphism associated with the Penokean Orogeny, likely around 1835 Ma (Holm et al. 2001; see also Table 3). Within the Grenville Province, rocks of the East Bull Lake intrusive suite may have been locally affected by metamorphism and deformation at ~1700 Ma and ~1450 Ma, in addition to the main pulse of Grenville orogenesis at ~1070 to 1040 Ma (Corfu and Easton 2000). OVERVIEW OF EAST BULL LAKE INTRUSIVE SUITE Table 1 lists all known major members of the East Bull Lake intrusive suite along with notable features common to all and a listing of all studies performed to date. The distribution of the bodies is illustrated in Figure 1. Table 2 summarizes the geochronology of the intrusive suite. The 3 largest intrusions are the East Bull Lake, Agnew Lake, and River Valley intrusions, all of which have been mapped in the last 25 years at either 1:10 000 or 1:20 000 scale in whole or in part (Born 1979; Kamineni et al. 1984; Chubb 1994; Peck et al. 1995; Vogel 1996; Easton and Hrominchuk 2001a, 2001b), with accompanying petrological, geochemical or economic studies (James and Born 1985; McCrank et al. 1989; Chubb 1994; Peck, James and Chubb 1993; Peck et al.1995; Vogel 1996; Vogel, James and Keays 1998; Vogel et al. 1998, 1999; Easton and Hrominchuk 1999; Hrominchuk 1999, 2000; Peck et al. 2001; James et al. 2002a, 2002b). The preserved size of the intrusions varies from 1 to ~100 km2 (Table 1). The most completely preserved body is the Agnew Lake Intrusion with approximately 2 km of stratigraphic section; both the East Bull Lake Intrusion and River Valley intrusion contain roughly 1 km of section. The East Bull Lake, Agnew Lake and River Valley intrusions are sporadically layered at centimetre- to decametre-scale throughout their stratigraphy. Leucogabbronorite and gabbronorite are the dominant rock types in the lower and middle parts of most bodies of the East Bull Lake intrusive suite. Olivine gabbronorites and leucogabbronorites are common in the Main Series of the East Bull Lake intrusion and the Lower Series of the Agnew Lake Intrusion. Melagabbronorites and troctolites are common in the lower parts of the River Valley intrusion. Ferrogabbros, ferrosyenites and alkali granites form the top 150 m of the Agnew Lake Intrusion. The significant volume of melanocratic norites and troctolites recognized in the River Valley Intrusion is not present in those intrusions west of the Grenville Front, and may indicate that the former represent a deeper part of the stratigraphy of this intrusive suite. The crystallization order of primocryst phases in most of the intrusions is plagioclase, olivine, orthopyroxene, clinopyroxene and titanomagnetite. The attitude of metre-scale phase layering and stratigraphic units in the East Bull Lake and Agnew Lake intrusions suggest that they represent lopoliths joined by dike-like units (James et al. 2002a). Original geometry of the bodies present in the Grenville Province cannot be reliably ascertained, although the northeast part of the River Valley intrusion appears to be a shallowly-dipping (10 to 30°) sheet. Without exception, breccias containing footwall and mafic cognate xenoliths in a gabbronorite matrix occur at the base of all intrusions, where the footwall contact is preserved. The thickness and abundance of these breccia units varies considerably. Also near the contact, footwall breccias and zones of extensive footwall dikes may be present. Disseminated chalcopyrite and pyrrhotite, typically in modal amounts from 0.5 to 2.0%, occur in the matrix of the marginal and brecciated rocks. This sulphide mineralization commonly contains between 1 and 5 g/t of combined Pt+Pd+Au, and is the focus for current mineral exploration (James et al. 2002a, 2002b). 9 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �DEPTH AND MECHANISM OF EMPLACEMENT Researchers studying East Bull Lake suite intrusions in the Southern Province have suggested shallow depths of emplacement (<8 km), possibly at the contact between the Archean basement and overlying supracrustal rocks of the Huronian Supergroup and its precursors (e.g., Card 1978; Peck et al. 1995; Vogel et al. 1998, 1999). In contrast, researchers studying the River Valley intrusion have favoured moderate to deeper depths of emplacement (8 to 12 km), either at the boundary between Archean plutonic rocks and greenstone belts or at the boundary between gneisses and a plutonic rock-dominated midcrustal layer (e.g., Easton 2000; J.L. Hrominchuk, personal communication, 2000). Easton (2002) went further, suggesting that several depths of emplacement are recorded within the East Bull Lake intrusive suite, based on the fact that the intrusive bodies display subtly different features depending on which country rock gneiss association they are in contact with. Two models have been proposed with respect to the emplacement mechanism of the East Bull Lake intrusive suite; both are discussed in James et al. (2002a), and are summarized here. The first model has been proposed specifically for the area extending from Elliot Lake to Sudbury. Based on geologic reconstruction and “unfolding” of the effects of Penokean deformation on the East Bull Lake and the Agnew Lake intrusions, and their similar stratigraphy, it has been proposed that the middle and upper parts of both these intrusions were part of an extensive, subhorizontal sheet. This sheet may have been approximately 2 km thick, representing the upper 75% of a lopolith that had horizontal dimensions of 30 to 50 km in a northerly direction and greater than 100 km in an easterly direction (James et al. 2002a; Vogel et al. 1999). In this model the lower and mineralized sections of the intrusions occupied rift faultrelated structures or embayments that controlled magma intrusion, replenishment, and mineralization, and thus, may not have been as widespread as the main mass of the lopolith. Furthermore, the mineralized zones would not have been connected. This model has been expanded to include the possibility that the East Bull Lake, Agnew Lake, and River Valley intrusions, as well as the intervening smaller intrusions (see Figure 1, Table 1), are the remnants of one large interconnected Great Dike-like lopolith that can be traced for at least 300 km across central Ontario (see Figure 1) (James et al. 2002a). If so, then small size is not a concern in terms of modelling the potential size of mineralized sections, and the area of potential interest for exploration is much larger. The second model proposes that the East Bull Lake intrusive suite was emplaced as a group of smaller, separate bodies, rather than as one or two large, now dismembered, sill complexes (James et al. 2002a). One consequence of this model is that the various bodies could have been emplaced at different crustal levels. This model would also provide an explanation for the differences in stratigraphy between the various intrusions, as not every intrusion would differentiate to the same degree. Finally, an important implication of this model is that mineralization style and grade will vary between intrusions. MAGMA COMPOSITION AND ITS RELATIONSHIP TO MINERALIZATION Detailed studies (Peck et al. 1995, 2001; Vogel et al., 1999; James et al. 2002a, 2002b) demonstrate that the magma(s) that formed the East Bull Lake and the Agnew Lake intrusions were high-Al, low-Ti (Al2O3 ~17.5 wt %; TiO2 ~0.5 wt %) tholeiites. In these 2 intrusions, the main fractionation trend formed by leucogabbronorite and norites is interrupted by emplacement of more primitive (less fractionated), olivine-rich rocks (the Olivine Gabbronorite zones) which form a distinctive geochemical pattern not easily attributed to the same magma source. 10 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �For the River Valley intrusion, at least north of the Sturgeon River, its stratigraphy, including the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones, could be attributed to a magnesian variant (8 to 10 % MgO) of the high-Al, low-Ti, tholeiite (James et al. 2002a). Such magma would have olivine followed by plagioclase on the liquidus, which satisfies the petrographic data for these rocks. The magma giving rise to the sporadically mineralized orthopyroxene-phyric norite and gabbronorite that is the main component of the Marginal Zone of the River Valley intrusion is part of an earlier magmatic sequence: its pyroxene-phyric, olivine-absent character and disseminated PGE mineralization distinguish it from norites and gabbronorites higher in the stratigraphy (James et al. 2002a). The magmatic evolution of River Valley intrusion, however, is not as well constrained as it is for the East Bull Lake and Agnew Lake intrusions. As noted by Hrominchuk in James et al. (2002a), a unit of massive, medium-grained, orthopyroxene-phyric norite with a boninitic composition (MgO ~11.5 wt %, Al2O3 ~13.5 wt. %, SiO2 ~51.5 wt %), located near the southeast margin of the River Valley intrusion, could represent the parent magma of the intrusion. Furthermore, Hrominchuk argued that the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones were derived from this boninitic parental magma by a combination of fractional crystallization and mixing of variably fractionated magma batches within the magma chamber. With the available analytical data, it is equally plausible, however, that the Olivine Gabbronorite, Gabbronorite and Leucogabbronorite zones of the River Valley intrusion formed from a high-Al, low-Ti magma similar to that which produced the East Bull Lake and the Agnew Lake intrusions. The boninitic magma may have only been injected near the base of the intrusion to form the Marginal and Inclusion and/or Fragment-bearing zones (James et al. 2002a). An argument against a boninitic parental magma is that such magmas are currently found only in ocean island arc settings (e.g., Bonin islands, Japan, Papau New Guinea), unlike the inferred continentalrift plume-setting for the East Bull Lake intrusive suite. Nonetheless, boninites are thought to form from a relatively high degree of partial melting of previously melted metasomatized lithospheric mantle (Hamlyn et al. 1985). Metasomatism is generally thought to occur from fluids derived from dehydration of a subducted slab of oceanic crust (Hickey and Frey 1982) and can produce light rare earth element (LREE) enrichment in the resulting melt. Alternatively, crustal contamination of a second-stage mantle melt can produce LREE enrichment in an otherwise primitive melt (Vogel et al. 1999). Vogel et al. (1999) found crustal contamination to be an unlikely mechanism for the uniformly elevated LREE in the Agnew Lake Intrusion, and suggested that the parent magma must have come from a partial mantle melt enriched by LREE-rich fluids. In the case of the River Valley intrusion, the LREE and trace element patterns are much less uniform and contamination is an important process, especially in the Marginal and Inclusion and/or Fragment-bearing zones (Hrominchuk 2000), which is one reason to favour a boninitic parental magma (James et al. 2002a). Regardless, both the proposed high-Al, low-Ti and boninitic parental magmas are second-stage, mantlederived magmas as described by (Hamlyn and Keays 1986). Hamlyn and Keays (1986) argued that second-stage magmas are critical to the formation of PGE deposits, a view also favoured in James et al. (2002a, 2002b). These fertile magmas arise from previously depleted mantle, typically in areas underlying Precambrian cratons or island arcs. Both the interpreted high-Al and boninitic parental magmas of the East Bull Lake intrusive suite are PGE fertile (Pt+Pd ~20 to 50 ppb) and sulphur-poor (~100 to 200 ppm), and together with their continental rift-margin tectonic setting, are good examples of second-stage magmas. Their low sulphur-content likely reflects prior extraction of MORB-type basalt liquids from the mantle, which removed most of the mantle sulphide phase, but left a small amount of a PGE-rich residual sulphide phase (Keays 1995). Pd and Se data from barren cumulates and possible feeder dikes from the East Bull Lake Intrusion (Peck et al. 2001) illustrates the PGE-rich nature of the parental magmas compared to first stage (MORB-type) magmas derived from fertile mantle, which have significantly higher S(Se) and lower PGE/S(Se) ratios. 11 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �In terms of crust-mantle scale processes, James et al. (2002a) suggested that fractional crystallization of a mantle-derived, second-stage magma in the lower to middle crust generated PGE-enriched, sulphurundersaturated leuconorite or norite magmas in the upper part of a zoned ultramafic to mafic magma chamber. During ascent and intrusion into the upper crust, this buoyant residual magma (parent to much of the marginal parts of the East Bull Lake and Agnew Lake intrusions) became sulphur-saturated, and apparently escaped from the more primitive subchamber without the loss of the sulphide phase. In this model, rocks of the more primitive Olivine Gabbronorite zone in the East Bull Lake and Agnew Lake intrusions represent influxes of magma from the more mafic parts of the same magmatic system. They should be relatively barren in PGE, except where mixing with fractionated liquids from the high-Al magmas may have generated reef-type deposits. Similar layered olivine-rich rocks in the River Valley intrusion appear to be part of a fractionating system that forms norites and leucogabbros in the upper part of this body (James et al. 2002a). Detailed studies on PGE mineralization have been done only on the East Bull Lake Intrusion. Many fine-grained gabbros in the Marginal Series and feeder-type plagioclase-phyric dikes in the footwall, have 20 to 100 ppb Pt+Pd (Pd/Pt = 2:1 to 1:1) and contain small (<1%) amounts of fine-grained disseminated sulphide minerals (pyrite ± pyrrhotite ± chalcopyrite). These PGE and sulphide contents are interpreted as evidence that early magma pulses were sulphur-saturated when they entered the magma chamber (James et al. 2002a). The PGE values in these rocks far exceed those expected for basaltic liquids, even those derived from depleted mantle sources (Hamlyn and Keays 1986; Keays 1995), which is further evidence for sulphur saturation. Vigorous convection within the magma chamber, as evidenced by the Inclusion-bearing and Anorthosite zones at the East Bull Lake Intrusion, produced high silicate/sulphide liquid mass ratios that allowed growing sulphide liquid droplets to attain the observed high PGE contents (1 to 10 g/t) (James et al. 2002a). ORTHOPYROXENE HORNBLENDITE BODIES Bodies of orthopyroxene hornblendite consist of orthopyroxene (bronzite) phenocrysts, ranging from 0.5 to 5 cm in length, hosted in a fine-grained amphibole-plagioclase matrix (Photo 1). These bodies occur throughout Awrey, Dryden, Henry, Loughrin and Street townships (Easton 1998, 1999, 2002, 2003; Easton and Murphy 2002; Davidson 1998). Large glacial erratics of orthopyroxene hornblendite, some up to 6 m across, are common in Henry, Loughrin and Street townships. The erratics are generally found within several hundred metres of observed bodies in outcrop, or in areas where bodies may be hidden beneath overburden. In most of these aforementioned townships, the orthopyroxene hornblendite bodies are found in close proximity to mafic intrusive rocks of the East Bull Lake intrusive suite. Zircon from an orthopyroxene hornblendite body in Street Township has been dated at 2468±5 Ma (Corfu and Easton 2000), similar in age to other intrusive rocks of the East Bull Lake intrusive suite (James et al. 2002a). Even though some orthopyroxene hornblendite bodies occur within 100 m of the Grenville Front (Easton and Murphy 2002; Hubbard 1998), equivalent rocks to these bodies have not been reported in conjunction with East Bull Lake intrusive suite bodies located within the Southern Province. No orthopyroxene hornblendite bodies have been found in association with the River Valley intrusion, although some ultramafic layers present within the River Valley intrusion in Crerar Township are chemically similar to rocks of the orthopyroxene hornblendite suite, even though they are texturally and mineralogically distinct. Easton (2002) suggested that the orthopyroxene hornblendite bodies are restricted to country rocks of the Street gneiss association, although some bodies do occur in association with other East Bull Lake intrusive suite rocks along the boundary between the Street and Crerar gneiss associations. This would explain the absence of these rocks in the Southern Province and in association with the River Valley intrusion, as well as their abundance in Awrey, Dryden, Henry, Loughrin and Street townships, all of which are underlain by rocks of the Street gneiss association. 12 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �—I-', Photo 1. Orthopyroxene phenocrysts in matrix-rich orthopyroxene hornblendite in Henry Township. Hammer handle is 30 cm long. UTM 547314E 5158815N. Easton and Murphy (2002) suggested that these bodies exhibited only 2 types of contact relationships; however, work by Easton (2003) suggests that there might be 4 types of contact relationships. These contact types are summarized below: 1. The first type occur near sharp, possibly modified igneous contacts between metagabbroic and metaleucogabbroic rocks of the East Bull Lake intrusive suite and their host gneisses (Street gneiss association). Bodies of this type appear to have been emplaced into pre-existing embayments along the metagabbro-gneiss contact. All of the Street Township bodies are of this type. 2. The second type occurs within deformation zones characterized by flattened gneissic rocks (e.g., the body exposed on the south side of Highway 17 in Awrey Township, Stop 1, Day 3). 3. Some bodies in Loughrin Township, show a linear distribution and may represent boudinaged dikes. It is possible that types 2 and 3 are related, with type 2 representing a more deformed version of type 3. 4. Many of the larger bodies found in Henry and Loughrin townships, are not clearly associated with the contact of any of the East Bull Lake intrusive suite bodies. Some are located over 1 km from the contact. The field term for these rocks, orthopyroxene hornblendite, is based on the dominant mineral phases observed in hand sample. Some of the better-preserved bodies contain relict olivine grains. Mesonorm calculations suggest a metamorphic mineralogy dominated by pyroxene and amphibole consistent with the observed mineralogy in the field. CIPW normative calculations for these rocks suggest a primary mineral assemblage dominated by olivine (4 to 22%), hypersthene (32 to 62%), diopside (3 to 22%) and calcic plagioclase (13 to 21%) (Easton and Murphy 2002). The CIPW normative mineralogy suggests 13 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �that these rocks were originally olivine gabbronorites or olivine websterites, depending on whether they contained moderate or only minor amounts of plagioclase, respectively. Orthopyroxene hornblendite bodies located near the Grenville Front contain euhedral bronzite grains up to 5 cm long. The bronzite grains become subrounded to rounded and overall grain size decreases with increasing distance from the Grenville Front. Similar observations can be observed from the margin to the core in some of the smaller hornblendite bodies. This change in bronzite form with increasing distance into the Grenville Province can be attributed to greater degrees of metamorphic recrystallization toward the interior of the Grenville Province. Mineralogically, the orthopyroxene crystals in the bodies are complex and represent primary phenocrysts that recrystallized during regional metamorphism. Evidence that they were primary phenocrysts comes from the presence of zircon and chromite inclusions within the bronzite grains. Both types of inclusions contain well-developed cores and rims. In the case of the zircon grains, subrounded cores are surrounded by euhedral fractured rims. The cores yielded the aforementioned U/Pb age of 2468±5 Ma, whereas the rims yielded an age of 1471±10 Ma (Corfu and Easton 2000). Chromite cores are rimmed by iron chromite that typically forms only during regional metamorphism (Deer, Howie and Zussman 1966). Small euhedral zircons are also present in the fine-grained amphibole matrix and yielded an age of 1052±19Ma (Corfu and Easton 2000). Mineral chemistry data on the matrix amphiboles indicates that they are magnesium hornblende, tremolite, cummingtonite and magnesium cummingtonite. The amphiboles occur both as individual grains and complexly exsolved and intergrown grains. Olivine grains from a body located near the Grenville Front have compositions of Fo73. Orthopyroxene compositions lie in the bronzite field and are magnesium-rich (En75-80) (Easton and Murphy 2002; Hubbard 1998). Geochemistry Analyses from 11 separate bodies show only a limited compositional range in major element chemistry (Easton 2003). The SiO2 content of the unaltered orthopyroxene hornblendites ranges from 46.0 to 50.5 weight percent which is unusually high given the corresponding MgO content of 21.0 to 27.0 weight percent. On a Jensen diagram the data from the orthopyroxene hornblendites plot as a cluster in the peridotitic komatiite field although there is no evidence to suggest that these rocks were extrusive. Plots of Ti versus Cr and Cr-Ni are consistent with these rocks being crystallized from layered mafic sills. Trace element data from orthopyroxene hornblendite samples plot in 2 distinct groups on chondritenormalized rare earth element (REE) diagrams (Easton 2003). One group has a relatively flat, 10 times chondrite pattern with a negative Eu anomaly and total REE less than 35 ppm, whereas the second group has moderate light REE enrichment (to 60 times chondrite), no Eu anomaly, and total REE greater than 80 ppm. The REE content and high silica content of the orthopyroxene hornblendites suggests that they do not represent the most primitive rocks of the East Bull Lake intrusive suite. The first group, characterized by low total REE contents and magnesium contents greater than 20 weight percent, occur as isolated bodies, near, but not intimately associated with other East Bull Lake intrusive suite rocks. In contrast, the second group with the higher REE contents and lower magnesium contents (<19.0 wt. %) occur at primary intrusive contacts, or as layers within East Bull Lake intrusive suite bodies. For example, hornblendite samples from the Street metagabbro plot in the LREE enriched group, as do olivine gabbronorites and pyroxenites from the River Valley intrusion. It can be argued that the two groups are related to one another, with the lower magnesium group being a more fractionated differentiate of the magma that produces these unusual rocks. 14 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �All unaltered orthopyroxene hornblendite samples contain from 2900 to 4700 ppm Cr and 750 to 1000 ppm Ni. Copper content is generally low (Easton 2003). Chromium is present as chromium-spinel, iron-chromite and chromite and is concentrated within olivine and orthopyroxene crystals. Platinum group element content in all of the analyzed orthopyroxene hornblendite samples is low (generally at or below detection limits), regardless of degree of alteration. The orthopyroxene hornblendites differ from other rocks of the East Bull Lake intrusive suite in their high chromium and nickel contents, as listed above. One explanation for this difference is that precipitation and removal of pyroxene from the primary magma (possibly by crystal settling) increases the silica content of the melt. The increase in silica would move the composition of the melt away from the olivine-chromite cotectic into the field of chromite and lead to the co-precipitation of bronzite and chromite. This process, in conjunction with olivine precipitation, would effectively strip the primary magma of much of its chromium and nickel, leaving rocks of the East Bull Lake intrusive suite with low chromium and nickel contents. Chromium numbers of the orthopyroxene hornblendites are 20 to 38 and magnesium numbers are 78 to 82. The bodies have chemical affinities to high-aluminium chromitite from ophiolitic complexes, most notably, chromium number less than 50, TiO2 greater than 0.2 weight percent, CaO greater than 1.5 weight percent, and Al2O3 greater than 1.2 weight percent. High aluminium chromitites are found in back-arc spreading and rifting tectonic environments that are consistent with previous suggestions of a rift-setting for the lower Huronian Supergroup (e.g., Bennett, Dressler and Robertson 1991). 15 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Field Trip Guidebook EAST BULL LAKE INTRUSION—OVERVIEW Country Rocks, Structure and Geometry of the Intrusion The East Bull Lake Intrusion, located roughly 80 km west of Sudbury (see Figure 1), is the type example of the East Bull Lake intrusive suite. The intrusion is hosted by a Neoarchean granite-greenstone belt to the southwest, but principally by tonalitic gneiss and syenite (James and Born 1985). The geometry of the intrusion (Figure 2) has been interpreted as primary (Peck, James and Chubb 1993; Born 1979) or as a result of polyphase open folding related to Penokean deformation (Vogel, James and Keays 1998). The East Bull Lake Intrusion consists of 2 interconnected magma chambers that locally exceed 1 km in thickness (see Figure 2). The eastern end of the East Bull Lake Intrusion is connected to the western end of the Agnew Lake Intrusion by the Streich dike (see Figure 2). The dike-like conduit that connects the eastern and western lobes of the East Bull Lake Intrusion (see Figure 2) consists of the lowermost stratigraphic units of the intrusion, and may represent part of the feeder dike to the magma chambers. These stratigraphically lowest parts of the intrusion are significant, as they are proximal to mineralized zones and feeder dikes that are largely obscured by the main mass of the lopolith. An unknown thickness of the upper part of the intrusion apparently has been eroded away. The East Bull Lake Intrusion was metamorphosed at the greenschist to lower amphibolite facies (Card 1978; Kamineni 1986; Chubb 1994). Igneous textures are typically well preserved throughout the intrusion, although primary pyroxene and olivine are commonly pseudomorphed by secondary amphiboles. Primary mineralogy has been deduced mainly from detailed textural and geochemical studies (e.g., James and Born 1985; Chubb 1994). EAST BULL LAKE SCALE 1 20000 0 05 1 2 25 Kflometres Figure 2. Geologic map of East Bull Lake Intrusion. Numbered units within the intrusion correspond to stratigraphic zones shown in Figure 3. Units 1 to 3 are Archean strata; units 14 to 17 are post-intrusion Paleoproterozoic units; for details see Peck et al. (1995). 16 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �w C a >< 12 Agnew Intrusion .2 (U 5: Transition unit ö o 0 a E .2 ö 1= _2000 _1 800 _1 600 (0 w w a 0) An<3 Ferrosyenite unit Fe-Ti Oxide Zone Leucogabbro unit G) a Porphyritic unit w 0. 0. Upper .1400 Gabbronorite ( Zone _1200 Pod-bearing unit Porphyritic unit East Bull Lake Intrusion Ce w Mixed unit Dendrite unit Layered unit Massive Gabbronorite Zone _1 000 Olivine Gabbronorite•- _800 Layered Gabbronorite Zone Olivine Gabbronorite Zone — Dendrite unit Rhythmically layered Zone Lower Gabbronorlte Zone Van-textured Gabbrononite Massive unit Leucogabbronorite Zone 0 Anorthosite Zone ow .E 0)01 Inclusion-bearing Zone Gabbronorite Zone Border Zone _600 Inclusion-bearing Gabbronorite Zone Marginal Leucogabbronorite Zone 5 Marginal Gabbronorite Zone Figure 3. Stratigraphic sections for the East Bull Lake and Agnew Lake intrusions. Mineral distribution and plagioclase compositions are from Vogel et al. (1999). Unit numbers correlate with geologic maps shown in Figures 2 and 6a. Stratigraphy and Petrography The type stratigraphic section for the East Bull Lake Intrusion is shown in Figure 3. Three mappable subdivisions are recognized: the Lower, Main and Upper Series. 17 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �MARGINAL SERIES The contact between the East Bull Lake Intrusion and Archean basement is represented by the Border and Gabbronorite zones of the Marginal Series. The Border Zone is commonly several tens of metres thick, and comprises varied proportions of gabbroic and anorthositic veins and locally derived basement xenoliths. The relative proportion of footwall xenoliths and gabbroic veins and the size of the basement xenoliths increase away from the intrusion and into the undisturbed basement. The basement xenoliths are commonly thermally recrystallized to hornfels and locally display evidence of in situ melting. Adjacent gabbroic veins commonly contain several percent quartz and/or patchy, granophyric-textured leucotonalite and leucogranite. The Border Zone is locally separated from the main mass of the intrusion by the Gabbronorite Zone, a 1 to 50 m thick texturally and modally banded gabbronorite. Chubb (1994) suggested that the Gabbronorite Zone, as exposed in the conduit linking the western and eastern lobes of the East Bull Lake Intrusion, probably developed from late injections of mafic magma that were unable to penetrate the overlying Lower Series rocks. Elsewhere in the East Bull Lake Intrusion, the Gabbronorite Zone is clearly gradational into the overlying Inclusion-Bearing Zone and underlying Border Zone, and likely represents an irregular chilled margin. LOWER SERIES The Lower Series hosts most of the known PGE mineralization in the East Bull Lake Intrusion, and comprises a lower inclusion-rich unit (Inclusion-Bearing Zone) and an upper anorthositic unit (Anorthosite Zone). The Inclusion-Bearing Zone consists of a matrix of medium- to coarse-grained gabbronorite and/or leucogabbronorite and a heterolithic inclusion suite. The inclusions typically represent only 1 to 10% of the unit and include basement xenoliths, anorthosite and leucogabbro autoliths and discontinuous, podiform pyroxenite and melagabbro bands and pods. The Anorthosite Zone consists of monotonous, coarse-grained anorthosite and leucogabbronorite. Locally, these anorthositic rocks are brecciated by crosscutting gabbroic vein networks. Spectacular “nodular” (glomerocrystic) textures occur in anorthositic rocks in the lower parts of the Anorthosite Zone. These occurrences are largely restricted to the conduit linking the west and east lobes and to the easternmost part of the east lobe. The nodular textures consist of tightly packed centimetre-size spheroidal aggregates of coarse-grained plagioclase in a matrix of coarse-grained pyroxenite and/or finer grained gabbronorite. The textures are believed to have formed due to enhanced cooling rates in the narrow parts of the intrusion (Chubb et al. 1995). MAIN SERIES The Main Series incorporates the Leucogabbronorite Zone, the Rhythmically Layered Zone and the Olivine Gabbronorite Zone (see Figure 3). Metre-scale, isomodal, phase layering becomes increasingly better developed upward through the Main Series (Photo 2). Igneous layering becomes better developed upward through the Main Series. In the Leucogabbronorite Zone, most of the layering reflects modal variations of plagioclase and pyroxene. Phase layering in the Rhythmically Layered Zone is typically caused by variations in olivine and plagioclase and like the underlying zone, overall, these rocks are leucocratic. The Olivine Gabbronorite Zone is distinct in having a high proportion of olivine-rich layers throughout its 50 to 70 m stratigraphy, relative to the underlying layered zones. Throughout the Main Series, modally graded layers typically become more plagioclase-rich upward. They are believed to have formed by separation of high-temperature, buoyant, cumulus plagioclase from dense, Fe- and Mg-rich residual magma ± olivine. 18 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �—. .' 4 •! .,. Photo 2. Igneous layering in the East Bull Lake Intrusion. a) Well-developed layering between plagioclase-rich and plagioclasepoor units. Hammer handle is approximately 30 cm long. b) More typical, subtle layering within gabbronoritic rocks of the Rhythmically Layered Zone, Main Series. 19 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �UPPER SERIES The most chemically evolved rock types in the East Bull Lake Intrusion occur in the Upper Series, but they are less evolved than rocks present in the Upper Series of the Agnew Lake Intrusion (see Figure 3). Accordingly, it is believed that the uppermost parts of the East Bull Lake Intrusion have been eroded away. The Upper Series comprises the Layered Gabbronorite Zone and the Massive Gabbronorite Zone. The Layered Gabbronorite Zone consists of planar or disrupted, irregular, texturally and modally layered gabbronorite. The Massive Gabbronorite Zone comprises planar, fine- to medium-grained gabbronorite and ferrogabbronorite (Peck et al. 1995). Varitextured gabbronorite occurs throughout the East Bull Lake Intrusion, forming irregular, metresized veins, pods and discontinuous layers that commonly display dendritic textures and have highly varied grain sizes (Photo 3). The thickest band occurs in the Layered Gabbronorite Zone to the west of Moon Lake (see Figure 2). Born (1979) mapped these rocks as a stratigraphic unit in the central plateau area of the East Bull Lake Intrusion. In the eastern part of the East Bull Lake Intrusion, these same rocks are not apparently stratigraphically controlled (Chubb 1994) leading to the schematic interpretation in Figure 3 of these rocks transecting stratigraphy. At the Agnew Lake Intrusion these same rocks form a distinctive stratigraphic unit as indicated in the stratigraphic section in Figure 3 (Vogel 1996). The crystallization order throughout the East Bull Lake Intrusion is plagioclase ± olivine → orthopyroxene → clinopyroxene + magnetite → sodic myrmekite + apatite. Core plagioclase compositions vary from An70-80 in the Anorthosite Zone to near An60 at the top of the stratigraphy. Analyses of preserved igneous plagioclase, olivine and 2 pyroxenes in small areas of the Rhythmically Layered and the Olivine Gabbronorite zones (see Table 1) show Fo60-74 compositions for primocryst olivine. This suggests that the Olivine Gabbronorite Zone formed from a more primitive magma composition than the underlying Rhythmically Layered Zone. Where present, cumulus olivine appears to have co-precipitated with plagioclase and is commonly mantled and partly resorbed by orthopyroxene. Orthopyroxene includes hypersthene and iron-rich bronzite, whereas the dominant clinopyroxene is augite. Disseminated, chromium-rich spinel and clotty, coarse-grained magnetite locally occur within the Gabbronorite Zone (Marginal Series) in the southwestern part of the East Bull Lake Intrusion. '. Photo 3. Dendritic texture within the Upper Series of the East Bull Lake Intrusion. a) Frond-like pyroxene dendrites. Hammer handle is approximately 30 cm long. b) Large, pyroxene-dendrites present just to the right of the 5.5 cm lens cap. 20 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochemistry A comprehensive geochemical database for the East Bull Lake Intrusion can be extracted from Peck et al. (1995) and Chubb (1994), which includes several hundred, whole-rock major and minor element analyses and selected trace and rare-earth element analyses. Additional, relevant geochemical data for the East Bull Lake Intrusion is contained in several Atomic Energy of Canada Ltd. publications (e.g., Ejeckam et al. 1990) and previous academic studies (Born 1979; James and Born 1985). Selected representative and average geochemical compositions for the major stratigraphic units in the East Bull Lake Intrusion are presented in Table 4 and illustrated in Figures 4 and 5. Included in Table 4 are 2 estimates of the parent liquid composition for the majority of the rocks in the East Bull Lake Intrusion. These are represented by a probable feeder dike (sample 91DCP291; Chubb 1994) from the eastern lobe of the East Bull Lake Intrusion, and a weighted bulk composition for the entire intrusion (Average WEBLI). The bulk composition was determined using the calculated volumes of all of the major stratigraphic units and their average chemical compositions. Both estimates give similar, Al-rich, low-Ti leucogabbroic magma compositions. The East Bull Lake Intrusion is dominated by rocks having leucogabbronorite compositions that are olivine-normative in parts of the Lower Series and much of the Main Series. Detailed geochemical studies of surface and diamond-drill core samples from both the eastern and western lobes of the intrusion (e.g., James and Born 1985; Ejeckam et al. 1990; Chubb 1994; Peck et al. 1995) have shown that welldefined chemical fractionation trends are absent through the East Bull Lake Intrusion stratigraphy. This likely reflects the fact that 1. the early crystallization of the parent magmas was dominated by plagioclase with intermediate to calcic compositions that, as calculated by James et al. (2002a), were near neutrally buoyant in their Al-rich parent magmas. 2. the chamber was fed by multiple injections of chemically similar magma that became mixed with the resident magma, thereby precluding the development of chemical fractionation trends. Nonetheless, some geochemical trends are present in the intrusion, a few of which are illustrated on Pearce-element ratio diagrams (see Figure 4) and chondrite-normalized rare-earth element plots (see Figure 5). In Figure 4a, Olivine Gabbronorite Zone rocks from the East Bull Lake and Agnew Lake intrusions (olivine-plagioclase cumulates) clearly form a more olivine-controlled group than do the rest of the samples from these intrusions. These data support the interpretation that intrusion of the Olivine Gabbronorite Zone magma represents a separate and more primitive magma pulse into the chamber. Figure 4b shows that plagioclase is the only significant calcium-bearing phase involved in the fractionation of the East Bull Lake Intrusion magma. This is consistent with petrography and rules out clinopyroxene as a mineral phase influencing fractional crystallization for this intrusion. In both Figures 4a and 4b, Upper Series rocks plot closer to the origin of the diagram reflecting their more fractionated compositions. Many of the mafic veins in the Border Zone are quartz normative and display field and geochemical evidence of significant assimilation of basement-derived, siliceous partial melts. Nevertheless, there is no evidence for significant contamination of the main body of magma in the East Bull Lake Intrusion through partial or total assimilation of country rock inclusions. The majority of the rocks in the lower parts of the East Bull Lake Intrusion are enriched in Al and depleted in incompatible trace elements in comparison to the proposed parent magma estimates (see Table 4, Figures 4a and 4b). The Main Series contains, on average, the most magnesian rocks in the East Bull Lake Intrusion, with olivine-plagioclase cumulates occurring throughout the Olivine Gabbronorite Zone and in parts of the underlying Rhythmically Layered Zone. The Upper Series contains the most evolved rocks in the East Bull Lake Intrusion, and on most chemical fractionation plots, fall at the evolved end of the fractionation trends (e.g., see Figure 4b). 21 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �a) 300 0I X Opx Cpx 250 • Lower and Main Series A Upper Series, EBLI X OGNZ, EBLI • Marginal and Lower Series, AGI 0 Upper Series, AGI X OGNZ,AGI • Feeder Dyke, EBLI • Streich Dyke, AGI ——Linear (OGNZ, EBLI) Linear (Lower and Main Series, EBLI( Plag .X 200 • . •U 0, XX. U- U U • 150 y=O.3085x-O.1328 R2= 0.7853 • 100 • U U 0 0 0 100 200 400 300 500 600 S11T1 140 120 80 60 20 200 100 300 Al/Ti Figure 4. Pearce-element ratio diagrams for the East Bull Lake, Agnew Lake and River Valley intrusions. a) Fe+Mg/Ti versus Si/Ti plot for East Bull Lake and Agnew Lake intrusions. b) Ca/Ti versus Al/Ti plot for East Bull Lake and Agnew Lake intrusions. c) Ca/Ti versus Al/Ti plot for the River Valley intrusion. In figures 4a and 4b, solid symbols are for lower and middle portions of the stratigraphy, with open symbols for the upper part. The Olivine Gabbronorite Zone is shown separately as are the estimated parent magma compositions for the 2 intrusions. Solid lines are fitted curves for the East Bull Lake Intrusion, as examples. Mineral vectors indicate how the data should vary if trends are due to that phase only. The data in figures 4a and 4b show that the 2 intrusions exhibit identical chemical variations. In Figure 4a, Olivine Gabbronorite Zone rocks are clearly more mafic (primitive) in composition than the remainder of each intrusion which fall on a single trend with a much shallower (plagioclase-enriched) slope. The positions of the parent magma composition are consistent with the 2 groups being derived from cogenetic magmas. In Figure 4b, data for all samples fall on a linear trend with a slope in the range 0.40 to 0.45 indicating that calcic plagioclase feldspar mainly controls this trend, and that clinopyroxene is not a major modal phase in these rocks. In the lower left of 4a, samples of Upper Series rocks deviate from the trend near the origin of the diagram because they have primocryst ilmenomagnetite and thereby, Ti is no longer a conserved element. In figure 4c, rocks from the River Valley intrusion display a similar plagioclase-controlled trend, but with a greater proportion of samples lying closer to the origin, reflecting a slightly more mafic bulk composition of the samples. 22 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �1000 a) C C 0 C.) 0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu —4-- Ferrosyenite Subzone (7b) b) —0— Leucogabbro Subzone (7a) —U— Porphyritic unit (6c) —X— Olivine Gabbronorite Zone (5) unit (4b) —0—Inclusion-bearing and Massive units (3+4a) —4— Marginal Leucogabbronorite Zone (2) 100 ci) •0 C 0 0 C.) 0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 5. Chondrite-normalized rare earth element plots for the East Bull Lake and Agnew Lake intrusions. a) East Bull Lake Intrusion. b) Agnew Lake Intrusion. Data are from Peck et al. (1995) and Vogel (1996). Normalizing values are from Sun and McDonough (1989). In both diagrams, all data are the average for 2 or more samples. “Feeder dike” in Figure 5a and “Streich dike” in 5b are estimates of parent magma compositions to the East Bull Lake Intrusion and the Agnew Lake Intrusion, respectively. In 5a, note the U-shaped boninite-like pattern for pyroxenite layers from the Inclusion-Bearing Zone. Figure 5a shows that all major rock types in the intrusion are enriched in light REE; this is typical for the East Bull Lake intrusive suite as a whole. The plagioclase-rich and cumulate character of rocks from the Lower and the Main Series in the East Bull Lake Intrusion is illustrated by their depleted total REE relative to the parent magma composition (“feeder dike”), and strong to moderate Eu anomaly. 23 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 4. Representative and average anhydrous whole-rock geochemical analyses from the East Bull Lake and Agnew Lake intrusions (from James et al. 2002a). Analysis Series Zone Rock Type N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Analysis Series Zone Rock Type 1 Basement Footwall Archean tonalite 11 69.2 0.09 17.5 1.70 0.02 0.44 2.89 6.94 1.21 0.04 33.75 17 61.8 0.54 17.8 4.19 0.09 1.16 2.48 5.36 6.22 0.27 35.46 1 50.9 0.09 25.7 2.74 0.04 2.24 13.3 4.28 0.56 0.08 61.82 1 52.9 0.09 22.2 4.34 0.08 4.88 11.1 3.80 0.52 0.08 69.02 2 51.8 0.21 13.8 9.72 0.20 11.9 10.4 1.71 0.22 0.08 70.86 6 Marginal Border Quartz-rich gabbronorite vein 8 56.1 0.50 17.1 8.17 0.13 5.33 9.13 2.72 0.74 0.09 56.38 <10 <5 20 1.8 16 12 31 23 784 378 8.8 74.6 13.6 <10 8 7 4.0 53 17 77 201 1252 723 32.4 550.6 41.6 38 51 28 <1.0 32 377 31 27 225 607 4.9 37.8 7.6 113 107 41 <1.0 39 88 50 24 195 675 4.1 37.5 7.3 682 392 61 <1.0 131 922 112 20 103 259 2.6 24.4 8.6 138 192 42 15.2 153 214 70 27 370 367 7.0 73.7 15.9 12 Lower Anorthosite Anorthosite 14 Main LGN Leucogabbro norite 42 50.5 0.34 20.3 8.09 0.13 6.46 11.1 2.44 0.60 0.05 61.28 72 49.6 0.42 21.0 8.28 0.12 6.16 11.1 2.58 0.59 0.06 59.55 25 46.9 0.30 14.6 13.4 0.18 14.5 8.19 1.56 0.27 0.04 68.22 155 194 41 17.2 100 95 71 26 166 271 6.2 46.9 11.1 81 195 40 17.2 127 80 72 24 201 295 6.0 48.6 12.1 230 505 90 13.1 104 91 91 15 118 183 6.1 41.5 14.4 9 Lower IBZ Pyroxenite pods, EEBLI 2 Basement Footwall Parisien Lake syenite 10 Lower IBZ Pyroxenite layers, WEBLI 3 Marginal Border Anorthosite vein 11 Lower IBZ Chlorite schist, WEBLI 4 Marginal Border Leucogabbro vein 5 Marginal Border Melagabbro norite vein N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # 10 53.8 0.38 6.28 13.5 0.23 17.0 8.12 0.52 0.11 0.05 71.37 2 52.71 0.17 4.59 14.15 0.21 23.76 4.29 0.05 0.06 0.01 76.88 1 38.55 0.12 16.96 20.74 0.26 17.99 4.77 0.56 0.03 0.02 63.21 88 49.7 0.36 21.9 7.67 0.12 5.50 11.6 2.52 0.56 0.05 58.71 13 Lower Anorthosite Matrix to nodular anorthosite 10 49.2 0.84 13.9 13.1 0.19 10.0 10.8 1.49 0.31 0.12 60.19 Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y 1360 383 87 <1.0 196 80 116 15 51 31 6.7 57.7 17.3 87 403 59 47 152 1010 95 <2 <3 4 <1.0 11 2 998 2670 152 12 100 2000 365 <2 8 59 <1.0 21 3 116 289 40 14.1 120 338 69 21 177 300 6.9 40.5 11.2 289 332 96 <1.0 296 72 97 19 76 117 9.9 70.4 24.3 7 Marginal GN Gabbronorite EEBLI 8 Lower IBZ Matrix, EEBLI 4 50.7 0.56 15.0 13.6 0.19 7.55 9.63 2.21 0.52 0.06 52.38 9 51.5 0.57 15.3 12.2 0.19 7.67 9.79 2.24 0.53 0.06 55.41 264 201 79 39.1 340 188 99 19 113 333 4.1 62.7 16.7 210 244 43 19.4 127 209 67 16 140 253 <1.0 34.9 7.8 15 Main RL Leucogabbro norite 16 Main OGN Olivine gabbronorite 24 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 4. continued. Analysis Series Zone Rock Type N SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Mg # Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y 17 Main LGN Gabbronorite, Folson Lake 18 Main LGN Gabbronorite, Bull Lake 191 Upper GN Varitextured gabbronorite 20 Upper GN Massive gabbronorite 21 Dikes 22 Dikes Plagioclasephyric dikes, WEBLI 8 51.3 0.80 15.0 12.7 0.20 6.36 10.7 2.04 0.73 0.10 49.74 89 85 47 37.3 250 119 97 35 224 230 <1.0 67.9 18.8 24 51.2 0.52 18.1 10.0 0.16 6.24 10.6 2.37 0.76 0.07 55.25 30 52.2 0.69 18.1 10.5 0.16 4.79 9.73 2.67 1.07 0.09 47.48 29 51.7 0.65 15.6 11.5 0.18 6.81 10.4 2.32 0.71 0.08 53.90 32 52.9 0.90 15.3 13.3 0.22 5.59 7.79 2.80 1.13 0.12 45.46 Gabbronorite dike, Folson Lake 6 51.0 0.80 15.8 12.7 0.19 6.30 10.2 2.17 0.74 0.11 49.60 138 121 43 25.3 167 90 85 27 258 261 6.3 60.2 14.2 72 92 37 25.6 200 182 85 42 328 255 <1.0 66.8 15.9 184 113 51 30.6 220 121 90 27 245 223 6.4 68.3 16.6 63 67 43 32.1 264 138 125 46 392 193 7.4 100.8 20.5 136 116 45 30.8 235 138 97 34 260 210 <1.0 80.5 18.5 23 Dikes Aphyric diabase dikes, WEBLI 24 Parent Average WEBLI 25 Parent 11 41.0 1.22 14.1 15.2 0.22 5.37 9.01 2.86 0.89 0.16 41.13 342 50.2 0.45 19.5 9.31 0.14 6.60 10.6 2.44 0.65 0.06 58.41 Feeder 91DCP291, Streich dike 1 50.8 0.54 19.6 9.25 0.16 5.47 10.8 2.96 0.30 0.07 53.94 69 65 44 33.7 310 153 120 44 312 239 <1.0 116.6 32.6 125 224 45 18.9 142 179 80 26 211 267 6.5 52.5 13.1 153 140 33 <1.0 171 66 70 8 93 261 2.5 65.0 14.6 Notes: Major element oxides are in wt. %, trace element data are in ppm; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron. All samples from Peck et al. (1995), except analyses 7, 8, and 13, which are from Chubb (1994). Abbreviations: EEBLI = eastern East Bull Lake Intrusion; GN = Gabbronorite Zone; IBZ = Inclusion-Bearing Zone; LGN = Layered Gabbronorite Zone; OGN = Olivine Gabbronorite Zone; RL = Rhythmically Layered Zone; WEBLI = western East Bull Lake Intrusion. Thermodynamic modelling of the crystallization of the East Bull Lake Intrusion based on the “feeder dike” parent magma (see Table 4), using the program COMAGMAT (Ariskan et al. 1993), suggests that plagioclase was the sole liquidus mineral over the temperature range 1300 to 1200°C, followed by olivine and then orthopyroxene (1176°C) (James et al. 2002a). The most robust model is in agreement with the predictions made by James and Born (1985), using Pearce-element ratio diagrams, who indicated that plagioclase accounts for nearly 85% of the phase volume of the fractionating assemblage in the Lower Series and 77% in the Upper Series. Orthopyroxene and clinopyroxene represent only about 20% of the phase volume throughout most of the East Bull Lake Intrusion, and olivine control is only evident in the middle to upper parts of the Main Series. The average Mg-number for the parent magmas is about 73, and the average, theoretical, plagioclase composition is about An73. Our calculations also suggest that most of the pyroxenite pods in the Inclusion-Bearing Zone represent dense, fractionated, Mg- and Feenriched residual magma that originated in enclosing or overlying strata. Their REE-enriched character (see Figure 5a), relative to the main mass of Anorthosite Zone rocks, is consistent with this interpretation. There are, however, discontinuous, metre-scale, cumulate layers in the PGE-rich, basal part of the Inclusion-Bearing Zone along the southwest margin of East Bull Lake Intrusion (Bullfrog zone, Mustang Minerals Corporation) that have U-shaped REE profiles (see Figure 5a) indicating that they formed from boninitic magma. The origin and distribution of these atypical rocks requires further study, as they suggest that 2 magma types were present in the early (mineralized) stages of formation of this intrusion. 25 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Mineralization Mineral exploration in the East Bull Lake Intrusion began with prospectors who gained access along newly completed logging roads in the early 20th century. They focused their attention on gold and sulphide mineral prospects. Several companies, including Noranda Mines Ltd., El Pen-Rey Oil-Gas and Mines Ltd., Sylvanite Gold Mines Ltd. and the Mining Corporation of Canada Ltd., investigated the base metal potential of previously discovered sulphide mineral occurrences in the 1950s and 1960s. The PGE potential of the East Bull Lake Intrusion was first recognized by independent prospectors E. Gallo, M. Hauseux and S. Surmacz, as well as by BP Minerals. Their observations were corroborated by an extensive study of the mineral potential of the East Bull Lake Intrusion carried out by researchers from Laurentian University and the Ontario Geological Survey in the early 1990s (Chubb 1994; Peck, James and Chubb 1993; Peck et al. 1995). Both Inco and WMC International undertook small exploration programs on the East Bull Lake Intrusion, but sustained exploration did not take place due to low palladium prices and the unfamiliar mineralization style. Renewed exploration of the East Bull Lake Intrusion began in 1998, led by Mustang Minerals Corporation, whose property covers about 90% of the intrusion. The current exploration target is a bulk, near surface, low grade (e.g., 2.5 g/t Pd+Pt) PGEcopper-nickel resource suitable for open pit mining (Brisbin et al. 2001). The geological environments for PGE-copper-nickel mineralization in the East Bull Lake Intrusion were first described in detail by Peck, James and Chubb (1993). They recommended the non-genetic term “contact sulphide mineralization” (now “contact-type”) be used to describe all of the PGE-rich disseminated sulphide mineralization occurring in the Marginal and Lower Series of the intrusion. Sulphide mineralization is present throughout the Lower and underlying Marginal Series but is best developed within the Inclusion-Bearing Zone, within a few tens of metres of the footwall or sidewall contact. Recent diamond drilling by Mustang Minerals Corporation and Freewest Resources Canada Inc. has proven that this zone is typically 20 to 50 m thick and, despite the nugget-type distribution of the sulphide minerals, consistently displays an average sulphide mineral content of 1 to 2%. The sulphide minerals typically consist of approximately equal amounts of coarse chalcopyrite and pyrrhotite, and subordinate, finer-grained pentlandite. Where their original textures are preserved, the sulphide minerals in the Inclusion-Bearing Zone form coarse-grained blebs up to 3 cm in diameter. Sulphide minerals (<1 to 10%) also commonly occur in the Border Zone and the Gabbronorite Zone of the Marginal Series, but these occurrences are enriched in pyrrhotite and/or pyrite relative to the chalcopyrite-rich mineralization in the Inclusion-Bearing Zone. Sulphide minerals rarely occur in the Main and Upper Series and, where present, are principally composed of pyrrhotite and appear to have formed as late-crystallizing grains from trapped intercumulus liquids. In Olivine Gabbronorite Zone samples, however, disseminated finegrained chalcopyrite and magnetite have been reported in the most altered rocks (James et al. 2002a) and may account for Pd+Pt+Au assays in the 0.5 to 1.0 g/t range from this unit (Peck et al. 1995, 2001). Disseminated to massive sulphide minerals occur within the Parisien Lake deformation zone (PLDZ on Figure 2), a major east-trending structure. Most of the known mineralization is exposed in trenches located about 1 km east of East Bull Lake (see Figure 2). The sulphide minerals occur in metre-size, ovoid zones that are elongate parallel to the strike of the primary shear fabric in the Parisien Lake deformation zone. The sulphide minerals consist of pyrrhotite and lesser chalcopyrite, pyrite and pentlandite and are intergrown with quartz, amphibole, chlorite and magnetite. Field and geochemical data suggest that hydrous fluids permeating the Parisien Lake deformation zone leached chalcophile metals from pre-existing, magmatic sulphide mineralization in the deformed Lower Series rocks and redeposited them within dilatant zones. The geochemistry and genesis of the contact-type PGE-copper-nickel mineralization in the East Bull Lake Intrusion is discussed in detail by Peck et al. (2001). Based on all available data, the mineralization 26 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �is characterized by highly varied whole-rock PGE grades that reflect the erratic distribution of the sulphide minerals on a hand sample scale. Diamond drilling confirms, however, that the average PGE grade of the Inclusion-Bearing Zone in the southwestern part of the East Bull Lake Intrusion is roughly 0.5 to 2.5 ppm combined Pd+Pt+Au. Analyses from grab samples range from typical values of 1 to 2 ppm to 43 ppm combined Pd+Pt+Au (sample collected by Freewest Resources Canada Inc. from a trench on their Folson Lake property). Data presented by Peck et al. (1995) show that 20% of all of the reconnaissance samples collected from the Lower Series in the western lobe of the intrusion contain greater than 1 ppm combined Pd+Pt+Au, with an average Pd content of approximately 200 ppb. These values represent highly anomalous background PGE contents in comparison to most other mafic or ultramafic intrusions. For comparison, barren norites, pyroxenites and harzburgites from the Lower, Critical and Main Zones of the Bushveld Complex have values ranging from 13 to 478 ppb Pt+Pd (Barnes and Maier 1999). Average Pd/Pt ratios are highest in the Anorthosite Zone (3.2 to 4.3) and lowest in the pyroxenitic pods of the Inclusion-Bearing Zone (1.56) and the Border Zone (2.0). Average Cu:Ni ratios range from 0.6 in samples from the matrix in Inclusion-Bearing Zone to 4.8 in the pyroxenitic pods; however, most samples have values between 2 and 3. Border Zone sulphide occurrences have the lowest average PGE grades (at similar total sulphide mineral abundance), but show a considerable range in abundance. Average S/Se ratios for the PGE-rich sulphide mineralization range from 1240 to 2840, which are within the empirically-determined range for mantle-derived sulphide mineralization (Eckstrand et al. 1989), and are indicative of a magmatic origin for the contact-type PGE mineralization. Copper displays a strong positive correlation with both S and Se. Pd and Pt display significant, positive, correlation with each other, with Cu and, to a lesser degree, with S. As is typical of most mafic-hosted magmatic sulphide mineralization, the contact-type sulphide mineralization in the East Bull Lake Intrusion shows strong enrichment of the low-melting point PGE (e.g., Pd, Pt) relative to the refractory PGE (Ir, Rh, Ru) on a chondrite-normalized chalcophile metal plot (James et al. 2002a). The structurally-controlled sulphide mineralization from the Parisien Lake deformation zone show lower PGE contents per unit sulphide in comparison to the contact-type mineralization. The former sulphide mineralization type also have much higher Pd:Ir ratios, a feature consistent with the known differences in degree of mobility of Pd versus Ir during hydrothermal alteration of mafic and ultramafic rocks (Keays et al. 1982). ROAD LOG, DAY 1, EAST BULL LAKE INTRUSION Note: Highway 533 is used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder and when examining outcrops along Highway 533. Geological map reference: Chubb, Hannila and Peck (1994). 0.0 km Start at the junction of Highway 69S and the Highway 17W bypass in Sudbury. Set odometer to zero. Drive west on Highway 17 to Massey. 0.4 km Junction of Highway 17W bypass and Regional Road 80 (Long Lake Road), continue west on Highway 17 bypass. 70.9 km Junction of Highway 17 and 6, just north of Espanola. Continue west on Highway 17. 97.0 km Junction of Highway 17 and 553 in Massey. Turn right (north) onto Highway 553 toward East Bull Lake. 119.9 km Pull over on shoulder of Highway 533 at the top of a high ridge. Examine cleaned outcrops on both sides of the highway. 27 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 1. Paleoproterozoic Metagabbro The high ridge consists of a linear(?) body of massive, medium-grained, metagabbro which may represent part of a feeder system to other East Bull Lake intrusive suite sills that have been eroded from the surrounding terrain. Prevec (1993) suggested that the rocks from this stop have a suitable parent composition for the East Bull Lake Intrusion. Alternatively, these rocks could be part of a large Matachewan diabase dike, but unlike most of the Matachewan dikes, it is not plagioclase-phyric. Return to vehicles and continue north on Highway 533. UTM 408177E, 5133860N. 122.4 km Power line crosses the Highway, continue north. 124.8 km Approximate southern margin of the Neoarchean Parisien Lake syenite (2665±2; Krogh, Davis and Corfu 1984). 125.3 km Junction of Highway 533 and logging road to the east. Park vehicles and walk a short distance east along the logging road to examine outcrops on the south and north sides of the logging road. Stop 2. Neoarchean Parisien Lake Syenite At this locality we see small amounts of coarse-grained syenite showing a well-developed igneous flow fabric on a local scale, as well as more abundant exposures of massive, medium-grained syenite. Regional mapping indicates that several types of medium-grained syenite intrude the coarser-grained variety; the latter is the more abundant rock type in the body. The Neoarchean age of the syenite is distinct from the 2470 to 2490 Ma age range of the East Bull Lake intrusive suite (see Table 2). These rocks form the footwall to the East Bull Lake Intrusion at Stop 12b. Return to vehicles and continue north on Highway 533. 126.0 km Low outcrops on the east side of Highway 533 consist of a plagioclase-phyric diabase dike typical of the Matachewan dike swarm. The dike contains greenish altered plagioclase phenocrysts in a fine-grained metadiabase matrix. Mafic diabase dikes, some plagioclasephyric, but many not, intrude both the Archean basement and the Parisien Lake syenite. Many of the same dikes also intrude the East Bull Lake Intrusion. A few very large dikes do not cut the intrusion, and are interpreted to be potential feeders to the sills to the East Bull Lake intrusive suite bodies in this area and elsewhere in the Southern Province. Typically, the dikes strike to the northwest, but they also strike west to west-northwest when they are proximal to the Southern–Superior province boundary. 126.5 km Outcrop of coarse-grained syenite, in the central part of the Parsien Lake syenite. The outcrop is intruded on its south side by a 10 to 20 m wide, northwest-trending, Matachewan diabase dike. 127.6 km Pull off to shoulder just past the curve in road where the highway passes between 2 large outcrop ridges. Leave vehicles and work your way up to the top of the large ridge on the west side of the road. Stop 3. Southern Margin of the East Bull Lake Intrusion This area of the East Bull Lake Intrusion forms a high ridge of very coarse-grained, massive varitextured leucogabbronorite. At its southern extremity, it is inclusion-bearing and sparsely mineralized. The outcrop area represents both the Inclusion-Bearing and Anorthosite zones, of the Lower Series, inward of 28 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �the intrusion contact. At this locality, the contact with the Parisien Lake syenite is sharp and most likely is a fault. Return to vehicles and continue north on Highway 533. 128.0 km Folson Lake deformation zone. At this location Highway 553 is “offset” to the east due to this major shear zone that strikes northwest across the entire intrusion (see Figure 2). 128.7 km Pull off to the shoulder and examine low outcrops on both sides of the road exposed on a curve as it passes over a small rise. Stop 4. Country Rocks These outcrops consist of country rocks of the Neoarchean Ramsay–Algoma gneiss complex. The Ramsay–Algoma gneiss complex (Jackson and Fyon 1991) is intruded by both the Parisien Lake syenite and the East Bull Lake Intrusion. Typically these gneisses are mixtures of strongly foliated granitic gneiss and metatextite to diatextite that enclose strongly deformed enclaves of mafic gneiss that likely represent remnants of greenstone belt metavolcanic rocks and/or diabase dikes. UTM 408300E, 5141425N. 128.8 km Parisien Lake road leads to mineralized zones along southern contact of the East Bull Lake Intrusion. 129.3 km Junction to the west between Highway 533 and the AECL (Atomic Energy of Canada Limited) road. The AECL road leads to Moon Lake, Central Plateau and the Folson Lake deformation zone. 129.8 km Outcrops present in this sandpit, 30 m east of Highway 533, expose metre-scale isomodal phase layering in the Rhythmically Layered Zone, Main Series, East Bull Lake Intrusion. The layering consists of thin leucogabbronorite layers (with scarce olivine primocrysts) that pinch and swell in thickness and continuity, and metre-thick isomodal layeres of olivine melagabbronorite. Boundaries between the layers are sharp (similar to Photo 2a). Outcrops at the road at this locality show graded layers, as well as layers with more gradational boundaries (similar to Photo 2b). UTM 409022E, 5142098N. 130.5 km East Bull Lake Lodge just to the west of Highway 533. Lodge is owned by Gerry Vauteur. 130.7 km Pull over and park where space is available. Examine vertical faces in outcrop on the west side of the road. Stop 5. Rhythmically Layered Zone This stop is located within the Rhythmically Layered Zone of the Main Series of the East Bull Lake Intrusion. On the west side of the road, vertical faces in outcrop exhibit several 0.3 to 1 m thick layers that dip west into the outcrop at 20 to 30°. The layers contain primocrysts of olivine and plagioclase enclosed in a matrix of orthopyroxene and clinopyroxene. Typically within the East Bull Lake Intrusion, all of the mafic minerals are replaced by secondary amphiboles, but plagioclase is commonly preserved. In this outcrop, however, well-formed outlines of olivine are easily observed; their proportion changes abruptly across layer boundaries. Layer compositions vary from olivine leucogabbronorite to olivine melagabbonorite; in all cases the layer boundaries are typically sharp and evidence of grading is very sparse. In this outcrop, evidence of a gravity-driven convection system is negligible. Return to vehicles, turn around and retrace route south to the junction of Highway 533 and the AECL road. Turn on to the AECL route and follow it to the end. Park vehicles in the pullout area (UTM 409405E, 5142725N), and walk south along the trail to a series of stripped outcrops. 29 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 6. Folson Lake Deformation Zone Be careful of your footing on these smooth outcrops if the surfaces are wet. At this locality the Folson Lake deformation zone intersects the Layered Gabbronorite Zone of the Upper Series of the East Bull Lake Intrusion (see Figure 2). Kamineni et al. (1984) refer to this altered gabbronorite that extends as a narrow zone along the northwest-trending Folson Lake fault as the “Stockwork Gabbro”. It contains a complex network of at least 2 generations of quartz veins as well as feldspar veins (both plagioclase and adularia); that together comprise up to 30 volume per cent of the rock. Diabase dikes transect the altered gabbro stockwork both parallel and perpendicular to the northwest-striking zone of deformation and faulting. Petrological studies of the gabbros in this and related areas by Kamineni et al. (1985) indicate that the regional greenschist facies to epidote-amphibolite facies mineralogy is altered on a local scale to minerals characteristic of prehnite-pumpellyite facies and zeolite facies (e.g., laumontite, analcite, calcite). UTM location at parking area is 406330E, 5142108N. Return to vehicles. 0.0 km Reset odometer to zero and head east toward Highway 533. 0.8 km Park at edge of road by large outcrop. Stop 7. Dendritic Texture in the Varitextured Gabbronorite Zone This outcrop is located in the Varitextured Gabbronorite Zone, Upper Series, East Bull Lake Intrusion. Pods and lenses, 0.5 to 1.0 m in maximum dimension, of coarse-grained, pegmatitic and dendritic gabbronorite occur sporadically in the massive, medium-grained gabbronorite host rock. The rapid textural variation is diagnostic of this zone, as is the appearance of pyroxene dendrites forming frond-like feather patterns in a plagioclase matrix (see Photos 3a, 3b). Rarely, the feldspar also exhibits this growth pattern. At this locality the dendritic pegmatites occur as pods; elsewhere (e.g., Agnew Lake Intrusion) they have been observed in layer-like zones, dendrites pointing upward, reminiscent of crescumulate textures. In the East Bull Lake Intrusion, rocks hosting these textural varieties of gabbronorite have been mapped by Born (1979) as a conformable stratigraphic zone of the Upper Series of the intrusion. More recently, D Peck and P. Chubb have interpreted these rocks as late diapiric melts that were intrusive into the pre-existing stratigraphy. UTM 406755E, 5142610N. Return to vehicles, continue east. 1.1 km Park at edge of road by large outcrop. Stop 8. Layered Gabbronorite Zone This outcrop exposes delicate small-scale, centimetre- to decimetre-scale layering in the Layered Gabbronorite Zone, Upper Series. Unlike the layering observed so far today, there is the suggestion here of current action and erosion of earlier formed cumulates suggestive of either active convection or slumping and/or soft sediment deformation of units. Rock compositions in the lower part of the outcrop are medium-grained gabbronorite, whereas at the top of the outcrop it is a coarse-grained leucogabbronorite. UTM 406900E, 5142790N. Return to vehicles, continue east. 2.2 km Park at edge of road in pullout area. To reach this stop, head downhill toward the north end of Moon Lake. Cross the beaver dam at the end of the lake and follow the slope roughly ~200 m to the south to an area where scattered low-lying outcrops occur on the west side of Moon Lake at the base of the central plateau. This traverse should not be made alone and should be only conducted using appropriate footwear and field equipment. 30 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9. Olivine Gabbronorite Zone This stop is located in the Olivine Gabbronorite Zone, within the uppermost part of the Main Series of the East Bull Lake Intrusion. Rocks of this zone occur as metre-scale layered sequences with the more mafic olivine-rich cumulates forming the dominant rock types interspersed with thin, leucocratic plagioclasedominant layers. In these rocks olivine, plagioclase and occasionally orthopyroxene are primocryst phases. Olivine and plagioclase compositions are more primitive than in zones above and below this sequence. The Olivine Gabbronorite Zone is interpreted to represent an influx of more primitive magma into the magma chamber which interrupted the fractional crystallization process that links the Rhythmically Layered Zone of the Main Series with the Layered Gabbronorite Zone of the Upper Series. UTM 407370E, 5142942N. Return to vehicles, continue east. 2.6 km Park alongside the road where possible. Stop 10 is located on an outcrop about 20 m west of the AECL road. Stop 10. Rhythmically Layered Zone, East Bull Lake Intrusion This outcrop shows metre-scale layering in rocks of the Rhythmically Layered Zone, Main Series, that have a typical shallow dip to the west. At the road, these rocks are cut by a 5 to 10 m wide, northwesttrending diabase dike. As previously noted, such dikes are abundant throughout the intrusion. UTM 407585E, 5142708N. Return to vehicles, continue east. 3.3 km Park on the west side of the road in small pullout area. This stop is a kilometre-long walk through the units that form the margin (bottom or sidewall) of the East Bull Lake Intrusion in the Moon Lake area. The trace of the traverse is shown in the accompanying map of this area (Figure 6) that was prepared by D. Peck and P. Chubb. This traverse should not be made alone and should be only conducted using appropriate footwear and field equipment. Stop 11. Moon Lake Traverse Stop 11 is a traverse through the Lower Series and the Border Zone of the intrusion, which are both areas where disseminated copper-PGE mineralization is common. The Border Zone (see Figure 6, units 4a to 4e) represents footwall and/or sidewall lithologies to the main intrusion. It consists of Archean gneissic and metaplutonic rocks that are intruded by, or which contain, irregular to linear zones of anorthosite, leucogabbnorite and gabbronorite that appear to be related to the East Bull Lake Intrusion. The Border Zone is in abrupt contact with the Lower Series, which at this locality consists of the Inclusion-Bearing Zone (see Figure 6, units 6c and 6d) and the Anorthosite Zone (see Figure 6, unit 7a and 7b). Sulphide mineralization occurs in units 6c and 6d (Inclusion-Bearing Zone) proximal to the margin of the intrusion; leucogabbronorites in these units are typically very coarse grained, commonly varitextured, variably inclusion bearing, and show no evidence of layering. Xenoliths and autoliths vary from abundant to scarce to absent; they are typically 1 to 10 cm in maximum dimension. Autoliths vary in composition from pyroxenite to anorthosite. 31 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �LEGEND Aphyric and PlagioclasePhyric Diabase Dikes GABBROIC ANORTHOSITE SUBZONE 1 rn-Scale Layered Unit Leacogabbronorite ANORTHOSITE SUBZONE C Leacogabbrononte to Pyroxenite (pods) BORDER ZONE to Mediurn-Grained Gabbronorite and Anorthosite Camulaten Gabbroic and Anorthositic Veins and Anorthositic Veins and Granitoids and Anorthositic Veins, Sabordinate Granitoids ARCH EAN GRANITOIDS Tonalite, Tonalitic Gneiss and Granite SYMBOLS Geological Boandary Observed Geological Boundary Approximate Fault Traverse Route C:.) Outcrop Main Zone ot PGE-Cu-Ni Mineralization Figure 6. Geological map of the Moon Lake area, East Bull Lake Intrusion, showing the route of the traverse (heavy line) undertaken during Stop 11. Shaded areas indicate areas of disseminated sulphide mineralization. D.C. Peck and P.C. Chubb originally prepared the map in 1991 for the Department of Earth Sciences, Laurentian University. Inset map in the upper right indicates in detail the route of the traverse, as well as outcrop areas crossed during the traverse. Numerical rock codes in the legend correspond to those used in Figures 2 and 3, and on the map of Chubb, Hannila and Peck (1994). Note that UTM coordinates in this figure are in Zone 17, datum NAD 27. Trace to 3% disseminated chalcopyrite, pyrrhotite ± pentlandite and pyrite ocurs in these units. Typically the sulphide minerals are medium- to coarse-grained aggregates in an epidote (saussurite)-rich zone of alteration in plagioclase, and also as much finer grained xenoblasts in the same silicate minerals. The sulphide mineral assemblage appears to have both a magmatic and metamorphic and/or hydrothermal paragenesis. In these rocks Cu+Ni = 0.1 to 0.5 wt %, with Cu>Ni, and Pd+Pt = 0.5 to 10g/t and Pd/Pt = 1 to 5. The Anorthosite Zone (see Figure 6, units 7a and 7b) consists of coarse- to very coarse-grained massive anorthosite and leucogabbronorite. Altered pyroxenes locally occur as coarse-grained interstitial anhedral to subhedral crystals and also as 1 to 10 cm zones (aggregates of crystals) that are interpreted to have formed from fractionated interstitial liquids due to prolonged early crystallization of plagioclase feldspar. As illustrated in the inset map in Figure 6, the traverse route encounters the following sequence 32 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �of rock types: units 7a and 7b (Anorthosite Zone); unit 6c (Inclusion-Bearing Zone), unit 4a (Border Zone), units 6d and 6c (Inclusion-Bearing Zone). The shaded area in Figure 6 shows the general distribution of the PGE mineralization, which may be associated with disseminated sulphide minerals. Some more specific data on the mineralization in the Moon Lake area is given below, summarized from detailed information presented in Peck et al. (1995, Appendix 5, Part A). Border Zone (unit 4) rocks related to the East Bull Lake Intrusion include gabbro, quartz gabbro and leucogabbro. Unlike the sulphide mineral assemblage in the Lower Series of the intrusion, pyrite is particularly abundant. In this zone Pd+Pt normally does not exceed 700 ppb with Pd/Pt ≈ 2. Cu+Ni is in the range 80 to 1300 ppm and typically Cu ≈ Ni. Mineralization in the Inclusion-Bearing Zone shows a range in Pd+Pt = 150 to 3000 ppb and Pd/Pt = 2 to 6; with Cu+Ni = 700 to 7000 ppm and Cu/Ni = 2 to 4. The data that is reported by Peck et al. (1995) indicates that the range and ratios for the PGE, Cu and Ni for the Anorthosite Zone are very similar to those for the Inclusion-Bearing Zone. Unpublished semiquantitative data collected by D. Peck and B. Jago on the platinum group minerals in 2 samples from the Inclusion-Bearing Zone in the Moon Lake area indicate that the PGE in the samples comes from 1 to 10 micron-size grains of sperrylite, moncheite, Bi-kotulskite and platarsite/hollingsworthite. These phases are normally found at grain boundaries of one or more of chalcopyrite, actinolite, epidote, albite or quartz, or within the silicate minerals in fractures or cleavage planes and much less commonly in chalcopyrite. Return to vehicles, continue east on AECL road to Highway 533. 4.0 km Junction of the AECL road and Highway 533. Turn south (right) onto Highway 553. 4.5 km Junction, Highway 533 and Parisien Lake trail on the east side of the highway. The trail is poorly marked and partly overgrown by saplings. To reach the key outcrop areas of Stops 12A and 12B, walk east on the trail for 20 to 30 minutes to a stripped north-trending ouctrop area (Stop 12A). Following examination of these stripped outcrops, continue east to a large east-trending stripped area (Stop 12B). The 2 stripped areas expose rocks of the Marginal and Lower Series, and associated mineralization along the south side of the East Bull Lake Intrusion, respectively. This traverse should not be made alone, and should be only conducted using appropriate footwear and field equipment. Stop 12A. Parisien Lake Traverse This series of north-trending, stripped outcrops lies perpendicular to the contact of the East Bull Lake Intrusion, and consists of rocks of the Border Zone (footwall lithologies) and mineralized leucocratic rocks of the Lower Series. Walking north from the southernmost outcrops in the Border Zone, one observes layered Archean metasedimentary gneisses intruded by Parisien Lake syenite. Both rock types are in sharp contact on the north side of the outcrop with a Matachewan plagioclase-phyric diabase dike; an alternative interpretation is that the diabase represents a unit of the Gabbronorite Zone in the Marginal Series. The north side of the diabase is in sharp fault (?) contact with a 3 to 5 m thick, lens-shaped zone of sulphide-rich metapyroxenite to peridotite. The silicate minerals in this zone consist of pyroxene and/or olivine replaced by medium-grained calcic amphibole (tremolite?) enclosed and intergrown with fine- to medium-grained magnetite-chalcopyrite-pentlandite. The opaque-rich material in the zone is typically a foliated, medium- to coarse-grained, calcic amphibole-magnetite-pyrite-pyrrhotite assemblage. To the north of this mafic autolithic zone is a 20 m thick unit of varitextured leucogabbronorite which has abundant disseminated interstitial magmatic pyrrhotite-chalcopyrite-pentlandite and minor pyrite mineralization. Locally the silicate mineral assemblage has been altered to rusty zones of fine- to medium-grained biotite-actinolite-chlorite-feldspar and up to 5% idioblastic tourmaline (no sulphide 33 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �minerals). Platinum group element content in grab samples range from 1.4 to 2.4 g/t; channel samples have analysed 0.84 g/t over 21 m, including 1.36g/t over 6 m. This rock becomes “flinty” in the northernmost part of the stripped outcrop, probably reflecting mylonite development proximal to the Parisien Lake deformation zone. Continue to walk east along trail. Roughly 3 minutes to the east, you will encounter a long series of stripped outcrop that are oriented subparallel to the trail. Stop 12B. Parisien Lake Traverse The dominant rock type is coarse-grained leucogabbronorite, with lesser amounts of melagabbronorite, at the west and north ends of this large stripped outcrop. Some of the melagabbronorite could be interpreted as disrupted dikes. In a few places discontinuous, sinuous, steeply dipping, centimetre- to metre-scale layering is observed in the leucogabbronorites. The orientation of the layering is subparallel to the margin of the intrusion. Both rock types host disseminated sulphide minerals. Convoluted boundaries with reaction zones indicate interaction between the leucogabbronorite and the melagabbronorite, suggesting that magma mixing occurred between these phases. At the east end of the stripped outcrop area, leucogabbronorite is in sharp contact with Parisien Lake syenite. This contact represents the boundary between the Lower and Marginal Series of the intrusion. UTM co-ordinates at the east end of the stripped area are 409788E, 5141490N. This concludes Day 1. Retrace route back to Highway 533. 34 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �AGNEW LAKE INTRUSION—OVERVIEW Country Rocks, Structure, and Geometry of the Intrusion This section is largely based on data reported in Vogel (1996), Vogel, James and Keays (1998) and Vogel et al. (1999). The main mass of the Agnew Lake Intrusion is located on the west side of Agnew Lake, roughly 65 km southwest of Sudbury (see Figure 1). It is similar in age and size to the East Bull Lake Intrusion (2491 versus 2480 Ma and 50 km2 versus 43 km2, respectively, see Table 2). It is linked to the East Bull Lake Intrusion, on its northwest side, by the Streich dike, a 200 to 300 m wide composite body with a strike length of approximately 10 km. Pink to white, coarse-grained granites and orthogneisses of the Neoarchean Ramsay–Algoma gneiss complex (Jackson and Fyon 1991) form the footwall rocks to the intrusion around its north, west, and south sides (Figure 7a). There are only rare occurrences of footwall breccia zones at the intrusion's contact with the granitic rocks equivalent to the Border Zone of the East Bull Lake Intrusion. More commonly there is an abundance of felsic dikes (thermal aureole partial melts) intruding older granitic basement rock, particularly where mafic (feeder) dikes are abundant near the intrusion contact. The east margin of the intrusion is in fault contact with, or is apparently unconformably overlain by, McKim Formation metasedimentary rocks of the Huronian Supergroup (see Figure 7a). The longitudinal axis of the Agnew Lake Intrusion is 110 to 120° (see Figures 7a and 7b), which is parallel to the axis of the East Bull Lake Intrusion, the Streich dike and Matachewan diabase dikes in the southernmost Superior Province (Halls and Bates 1990). This trend is thought to reflect the orientation of the rift structure that permitted magma intrusion. The attitude of igneous layering in the intrusion is illustrated in Figure 7b. It shows that the body has an asymmetric (synclinal) shape with the axis of the structure plunging gently northeast. Its axial plane is parallel to that of Penokean folds in younger Huronian Supergroup strata. This fact and its high angle of intersection with the longitudinal axis of the intrusion and Streich dike, suggests that the present geometry of the Agnew Lake Intrusion is due to Penokean deformation of a lopolith. Stratigraphy and Petrography Figure 3 presents a composite stratigraphic section for the Agnew Lake Intrusion. It consists of a Marginal, Lower and Upper Series which correlate, as illustrated, with the Lower, Main and Upper Series of the East Bull Lake Intrusion. Mafic mineralogy is replaced by calcic-amphiboles formed during lower amphibolite facies, Penokean-age metamorphism. Compositions of plagioclase cores commonly appear to be near original magmatic values. Primary igneous textures are preserved, but less clearly than in the East Bull Lake Intrusion. MARGINAL SERIES The Marginal Gabbronorite Zone (unit 1, see Figures 3 and 7a) is characterized by a massive, mediumgrained, commonly olivine-normative, gabbronorite. It occurs as units up to 200 m thick with chilled outer contacts against the footwall and encloses large in situ remnants of granitic footwall. Sills or dikes, 1 to 20 m thick with chilled margins, commonly occur at the intrusion contact, and may be related to young dike sequences that transect the entire intrusion. 35 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Figure 7. Geology of the Agnew Lake Intrusion. a) Geologic map showing main stratigraphic units; unit numbers correspond to those in Figure 3 and in the text. b) Structural map showing attitude of centimetre- and metre-scale layering. The strike and plunge of the axial plane of the synclinal, lopolithic structure is the same as that in the Huronian Supergroup supracrustal rocks on the east side of Agnew Lake. Both figures modified from Vogel (1996). 36 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �The Marginal Leucogabbronorite Zone (unit 2, see Figures 3 and 7a) averages 200 m thick and occurs along most margins of the intrusion, where it either overlies the younger, Marginal Gabbronorite Zone or is in direct contact with the footwall. It consists of 3 distinctive units, namely, the Varitextured, Mottled and Nodular units. The Varitextured Unit occurs in the west and south margins of the intrusion, where it is dominated by leucocratic gabbronorites; mafic layers or lenses are common as are felsic differentiates. Grain size is highly varied on a centimetre-scale, from medium-grained to pegmatitic. These rocks are very similar to varitextured gabbronorite pods in the Anorthosite Zone in the East Bull Lake Intrusion. The equivalent, or somewhat younger, unit in the north-central margin of the Agnew Lake Intrusion is the Mottled Unit, which is characterized by large amphibole oikocrysts (after pyroxene) in a plagioclase-rich matrix. As with the Varitextured Unit, smaller but significant gabbronorite, melanogabbronorite and anorthosite also occur. Layering in the Mottled Unit is irregular and discontinuous. Of great significance in both units is the presence of centimetre- to metre-scale inclusions and pods of melanogabbronorite, footwall granite and, rarely, massive quartz where the unit is adjacent to the footwall. Disseminated PGE-rich chalcopyrite and pyrrhotite are common in the inclusion-bearing parts. The Nodular Unit forms large lens-like areas in the northwest part of the intrusion. It is distinguished by the dominance (70 to 90 modal percent) of 6 cm diameter, well-sorted and closely packed plagioclase nodules in a coarse-grained amphibolitized pyroxenitic matrix. Disseminated PGEbearing sulphide minerals sporadically occur in the matrix. Whole-rock Mg-number indicates this unit is the most fractionated of the Marginal Series. All of the units in this series have significant proportions of rock with normative olivine in the 1 to 10% range and supporting textural evidence for the presence of modal olivine. Plagioclase core compositions range from An77 to An32. LOWER SERIES The base of this series is the Inclusion-Bearing Gabbronorite Zone (unit 3, see Figures 3 and 7a). The zone is about 180 m thick and hosts footwall and cognate xenoliths; associated copper-nickel-PGE mineralization; and is limited in outcrop to the northwest margin of the Agnew Lake Intrusion where it leads to the Streich dike. Inclusions of Marginal Series rocks are common—especially of the Nodular Unit—in a gabbronorite matrix (An79-52). Similar rocks form the Inclusion-Bearing Zone at the base of the Lower Series in the eastern lobe of the East Bull Lake Intrusion (Chubb 1994). The overlying 350 to 400 m of the Lower Series is represented by the Lower Gabbronorite Zone, consisting of a Massive (4a) and a Lower Layered Unit (4b) (Photo 4), that are separated by a younger conformable phase of the Dendrite Unit (8) (Photo 5). The Massive Unit consists of massive, medium- to coarse-grained gabbronorite and leucogabbronorite (An79-61) and rare melanocratic inclusions. Unit 4b (see Figures 3 and 7a) is a layered (centimetre- to metre-scale), medium- to coarse-grained gabbronorite or leucogabbronorite (An65-53). Rocks in both units are commonly olivine-normative. These 2 units are similar to rocks in the Anorthosite and Rhythmically Layered zones, respectively, in the East Bull Lake Intrusion. Dendrite Unit rocks are coarse-grained to pegmatitic gabbronorites, commonly weakly layered, and typically varitextured, with a diagnostic feature being the common occurrence of delicate, branching, pyroxene crystals several centimetres long with interstitial plagioclase, titanomagnetite and granophyre. This unit also occurs higher in the stratigraphy and formed from a more evolved magma than its host rocks. 37 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 4. Centimetre- to decametre-scale igneous layering within rocks of the Agnew Lake Intrusion. V Photo 5. Dendrite texture developed with the Lower Gabbronorite Zone of the Agnew Lake Intrusion. Compare with Photo 3a and 3b. Similar rocks will be observed at Stop 7, Day 2. 38 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �UPPER SERIES Most of the rocks in this series have not been recognized in other intrusions of the East Bull Lake intrusive suite; either they were never deposited or they were eroded away. The Olivine Gabbronorite Zone (unit 5, see Figures 3 and 7a) is notably more mafic than other units in the Agnew Lake Intrusion. It consists of centimetre- to metre-scale, layered olivine gabbronorite and leucogabbronorite. Disseminated sulphide minerals are common and PGE contents can be as high as those present in the inclusion-bearing rocks in the Marginal and Lower Series. Similar strata, 50 to 60 m thick, occur at the same stratigraphic level in the East Bull Lake Intrusion. The Upper Gabbronorite Zone (units 6a to 6e, see Figure 3) forms about 1000 m of the stratigraphy of the Agnew Lake Intrusion. Vogel, James and Keays (1998) and Vogel et al. (1999) divided it into 5 units. Olivine is absent from rocks in the Upper Gabbronorite Zone. Plagioclase and orthopyroxene are the primocryst (cumulate) phases, with plagioclase compositions being mostly in the range An65-40. The upper Layered (6a) and Mixed units (6b) underlie the Porphyritic Unit (6c), which forms the main mass of this part of the stratigraphy (see Figures 3 and 7a). The Porphyritic Unit is a dominantly quartz-normative rock unit characterized by varied amounts of plagioclase phenocrysts and glomerophenocrysts set in a gabbronorite matrix. Diffuse, macrorhythmic, decametre-scale layering of plagioclase-phyric leucogabbronorite and nonporphyritic gabbronorite is typical of this unit. The Fe-Ti Oxide Zone (units 7a and 7b, see Figure 3) forms the upper 200 to 400 m of the Agnew Lake Intrusion; its base is marked by the first appearance of titanomagnetite primocrysts. Primary igneous plagioclase is totally altered in this zone and magmatic textures are very poorly preserved. The Leucogabbro Unit (unit 7a, see Figure 3) forms the lower part of this zone. It consists of massive, coarsegrained leucogabbro (normative clinopyroxene greater than orthopyroxene) that has a distinctive clotty texture as a result of metamorphic biotite growth around altered primocryst titanomagnetite. Metamorphic garnet occurs sporadically in these iron-rich rocks. Of particular interest is the presence of disseminated sulphide minerals (with anomalous PGE), most commonly at the top of the unit. The basal contact of the overlying Ferrosyenite Unit (unit 7b, see Figure 3) is not exposed, but has been drilled by New Millenium Minerals Corporation (which became Platinum Group Metals Limited in October 2001 as the result of a merger). In this unit, ferrosyenite grades upward into alkali feldspar granite. The ferrosyenite consists of relatively scarce phenocrysts of albite (An3) and as much as 25 modal percent primocryst magnetite in a recrystallized feldspar-quartz matrix. The magnetite-rich phase of this unit is steel-grey, massive and fine grained. The overlying granite is a white, recrystallized quartz-feldspar assemblage with less than 5% magnetite. Normative calculations suggest that the igneous assemblage had as much as 30% potassium feldspar. Geochemistry A comprehensive geochemical database for the Agnew Lake Intrusion can be extracted from Vogel (1996) and a representative suite is presented in Vogel et al. (1999). Representative analyses for units from the Upper Series (not represented in the East Bull Lake Intrusion) and the Streich dike are presented in Table 4. The latter is an estimate of the parent magma composition for all units in the intrusion except for the Olivine Gabbronorite Zone (see Figures 3 and 7a). As with the East Bull Lake Intrusion, it is a high-Al, low-Ti, tholeiitic composition. Rocks in the Marginal and Lower Series of the Agnew Lake Intrusion (except for unit 8, the Dendrite Unit) are olivine-normative. Olivine leucogabbronorite compositions dominate in the Marginal Series (Mg-number 65-76), whereas olivine gabbronorite is dominant in the Lower Series (Mg-number 39 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �55-74). In the lower two-thirds of the Upper Series, gabbronorite and their leucocratic equivalents (Mgnumber 63-31) dominate. Ferrogabbro, ferrosyenite, and alkali granite compositions (Mg-number 46-12) characterize the Fe-Ti- Oxide Zone. Detailed stratigraphic studies in the Marginal and Lower Series show that these rocks formed in an open system where frequent magma pulses produced a rather monotonous sequence of mafic rocks with thin irregular intervals of fractionated material (Vogel 1996). Pearceelement ratio plots in Figures 4a and 4b show that the main chemical variation is due to plagioclase and olivine for the Marginal Series, and plagioclase, olivine and orthopyroxene for the Lower Series. All units in the Agnew Lake Intrusion show flat heavy rare earth element (HREE) patterns (10 to 40 times chondrite) and light REE enrichment (6 to 100 times chondrite for La) (see Figure 5b). REE element concentrations for the Marginal (unit 2) and Lower Series (units 3, 4a, 4b) are the most primitive of the suite (see Figure 5b). Both show positive Eu anomalies and both are more primitive in composition than the Streich dike parent magma composition. Vogel et al. (1999) illustrated, using incompatible elements such as Y, Zr, and Nb, that rocks from the Upper Series of the Agnew Lake Intrusion ultimately fractionate to form iron-rich differentiates, namely, the Fe-Ti Oxide Zone of associated ferrogabbros and oxide-rich ferrosyenites. There is no physical or geochemical evidence that these differentiates are due to magmatic assimilation of country rock. Rather, this part of the intrusion, at least in its upper portion, has apparently become a closed system, i.e., Skaergaard-like in terms of magmatic process (McBirney 1996). Pearce-element ratio diagrams (see Figure 4) and petrography indicate that plagioclase and orthopyroxene are the major phases that account for the chemical variation of rocks in the Upper Gabbronorite Zone. REE data for this zone (unit 6c) are consistent with it being less primitive than the underlying Marginal and Lower Series rocks (see Figure 5b). Rocks in the Leucogabbro subzone (unit 7a) near the top of the stratigraphy are classified as iron-rich gabbronorites by Vogel (1996). Here titanomagnetite ± clinopyroxene join plagioclase and replace orthopyroxene as the dominant mafic phases. Titanium is not a conserved element in rocks of the Fe-Ti Oxide Zone; consequently, as seen in Figure 4b, rocks from this zone are distinct from all previously described units, and their trend can not be ascribed to fractionation by a specific phase or phases. The REE pattern for this subzone (see Figure 5b) mimics the previous patterns but has substantially higher total REEs, consistent with these rocks being comagmatic with the underlying part of the Upper Series. In the Ferrosyenite subzone (unit 7b) at the roof of the intrusion, titanium is not conserved in the Pearce-element ratio plots. The trend of the data for these rocks extends from near the origin parallel to the Al/Ti axis, reflecting the varied proportion of alkali feldspar in these calcium-poor rocks. The slope of the data in Figure 5b for this subzone, and their increased total REE abundance is consistent with it being a fractionated part of the intrusive sequence. The negative Eu anomaly is due to the absence of plagioclase. Although rocks of the Olivine Gabbronorite Zone occur in the central part of the Agnew Lake Intrusion stratigraphy (see Figure 3), the trend they define in the Pearce-element ratio plot (see Figure 4a) shows that they are, on average, more mafic than strata above and below. Their trend intersects that defined by the rest of the rocks in the Agnew Lake Intrusion near the estimated parent magma composition (Streich dike) indicating that the 2 groups are comagmatic. These data are consistent with previously discussed mineralogical data. In Figure 5b, the REE pattern for rocks from the Olivine Gabbronorite Zone has a slope that is similar to other Agnew Lake Intrusion rocks, supporting the comagmatic nature of the 2 groups. The low total REE abundance is indicative of the relatively primitive and mafic character of this zone. Mineralization Finely disseminated and erratically distributed, blebby (up to 3 cm in diameter) chalcopyrite and pyrrhotite, with minor to trace pentlandite, in modal amounts from 0.5 to 2.0%, typically characterize 40 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �mineralized localities in the Agnew Lake Intrusion. Typically Cu is greater than Ni and combined Cu+Ni is approximately 1 to 1.5 wt %. In a high proportion of sulphide-rich samples, values of Pt+Pd = 0.5 to 3.0 g/t are not uncommon and values as high as 10 g/t do occur, with Pd/Pt typically 1 to 3 (data from Assessment File Reports filed by BP Resources Canada Limited, Resident Geologist’s office, Sudbury). Although there is often a positive correlation between PGE, Cu and total sulphide contents, some of the highest PGE samples have less than 100 ppm S. As with the East Bull Lake Intrusion, the greatest sulphide mineralization and highest PGE values have been found in inclusion-bearing units, with the sulphide minerals normally occurring in the matrix. Mineralization occurs in the Marginal Leucogabbronorite Zone at the stratigraphic base of the Agnew Lake Intrusion, specifically along its west margin. The varitextured Leucogabbronorite unit (unit 2a, see Figure 7a) is highly prospective within 50 to 100 m of the contact, as is the Mottled Leucogabbronorite (unit 2b) along the north margin of the body. The rocks that host this mineralization are very similar in mineralogy, texture and stratigraphic position to those that host sulphide mineralization in the Anorthosite Zone in the western East Bull Lake Intrusion (i.e., Moon Lake area). Mineralized, fragment-rich rocks also occur in the Inclusion-Bearing Gabbronorite Zone (unit 3, see Figures 3 and 7a), which forms the lowest subdivision of the Lower Series. The mineralogy and textural distribution of the silicate and sulphide mineral phases is the same as in the Marginal Series. These inclusion-bearing rocks intrude units of the Marginal Series, in particular the Nodular Unit, and clearly represent a major phase of magma injection into the magma chamber. The similarity in rock types, mineralization and field relationships with those at the east and west ends of the eastern lobe of the East Bull Lake Intrusion, suggest that the Lower Series, Inclusion-Bearing Zone, in the Agnew Lake Intrusion is correlative with the Lower Series, Inclusion-Beaing zone, in the East Bull Lake Intrusion (see Figure 3). A third significant mineralized zone occurs near the top of the intrusion in the uppermost part of the Leucogabbro subzone and immediately below the Ferrosyenite subzone (see Figure 3). The mineralization was predicted to be present by Vogel (1996), forming as a result of closed-system fractionation of a sulphur-undersaturated, PGE-rich magma. This type of occurrence is characterized by disseminated chalcopyrite and pyrrhotite, in amounts of 1 to 2 modal percent, that increase in abundance toward the contact with the overlying Ferrosyenite subzone. Mapping and trenching by New Millenium Minerals Corporation has identified several occurrences of this type of mineralization near this contact along the eastern side of Agnew Lake. A fourth environment merits mention although mineralization has yet to be recognized. The Olivine Gabbronorite Zone at the base of the Upper Series is believed to represent an influx of primitive magma (Mg-number ~70) into a chamber dominated by fractionated liquids (Mg-number ~55). Thus, the opportunity may have existed for the formation of reef-type mineralization due to magma mixing. This idea is supported by the presence of Pt+Pd values as high as 0.5 to 1.0 g/t in rocks of the same type and stratigraphic level in the East Bull Lake Intrusion (Peck et al. 2001). Vogel (1996) analyzed 22 barren samples from the Agnew Lake Intrusion, sampling all stratigraphic units shown in Figure 3. Since the PGEs partition strongly into sulphide liquid, their abundance in a silicate rock is an excellent indicator of the status of sulphur-saturation of the magma from which the sample crystallized (Hamlyn and Keays 1986; Vogel and Keays 1997). In the barren samples, Pt varies from 0.7 to 62 ppb, with most samples in the 10 to 30 ppb range; and Pd varies from 0.2 to 58 ppb, with most samples ranging from 10 to 40 ppb. These data mimic those for the East Bull Lake Intrusion (Peck et al. 1995) and are very similar to most, non-reef, silicate mineral assemblages from the Lower, Critical and lower Main Zones of the Bushveld Complex (Barnes and Maier 1999). When plotted against stratigraphic position, Pt and Pd data show peaks: 1. at the base of the Marginal Series 2. at the base of the Inclusion-Bearing Zone, Lower Series 41 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �3. near the estimated position of the Olivine Gabbronorite Zone, and 4. at the base and the top of the Leucogabbro subzone near the roof of the intrusion. All correspond to areas of known or suspected mineralization. Mantle-normalized PGE distribution for barren rocks from the Agnew Lake Intrusion is similar to that for rocks of the East Bull Lake Intrusion. The Pt-Pd-Au-Cu part of the figure is elevated (1 to 10 times mantle abundance except for Au) relative to the Ni-Ir-Ru part (<0.01 to 0.5 times mantle values) (James et al. 2002a). The anomalous PGE concentrations, together with typically very low sulphur contents (≤150 ppm), of barren samples from the Agnew Lake Intrusion indicate that the main mass of Agnew Lake Intrusion magma did not experience sulphide saturation prior to emplacement into the chamber. All samples plot in the sulphur-undersaturated area in the Cu-Pd diagram of Vogel and Keays (1997). Peach, Mathez and Keays (1990) estimated that saturation for mantle-derived magmas is about 800 ppm, well above what is observed in these rocks. Peck et al. (2001) argue that contact-type mineralization in the Marginal and Lower Series of the East Bull Lake Intrusion formed from magmas that were sulphur-saturated prior to intrusion. Mineralization was further upgraded in PGE content by prolonged high temperature mixing of very small volumes of sulphide phases with high volumes of silicate liquid upon entering the magma chamber. The complex, chaotic, varitextured nature of the rocks that host mineralization in the Marginal and Lower Series of the Agnew Lake Intrusion, and the evidence that the magma system was “open” during formation of this part of the intrusion is consistent with the model of Peck et al. (2001). The mechanism to explain mineralization near the roof of the intrusion, i.e., sulphide saturation due to fractionation in a closed system, is also consistent with a PGE-rich, sulphur-undersaturated, parent magma. It also allows for the possibility of reef-type mineralization at the base of the Upper Series due to magma mixing between the sulphur-undersaturated, parent magma and the sulphur-saturated, fractionated magma present within the evolving magma chamber (e.g., the Olivine Gabbronorite Zone, unit 5, see Figure 3.) ROAD LOG, DAY 2, AGNEW LAKE INTRUSION Note: Most of the roads used during day 2 are used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder and when examining outcrops during day 2. Geological map reference: Vogel (1996), Card and Palonen (1976). 0.0 km Start at the junction of Highway 69S and the Highway 17W bypass. Set odometer to zero. Head west on Highway 17 to Massey. 0.4 km Junction of Highway 17W bypass and Regional Road 80 (Long Lake Road). Continue west on Highway 17 bypass. 70.9 km Junction of Highway 17 and 6, just north of Espanola. Continue west on Highway 17. 76.4 km Junction of Highway 17 and Agnew Lake Lodge Road on the west side of the hamlet of Webbwood. Turn right (north), and follow Agnew Lake Lodge Road north to the intersection with West Branch Road. 82.4 km Turn left onto West Branch Road and drive north for 17.0 km to the intersection with Power Line Road. 42 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �99.4 km Junction between West Branch Road and Power Line Road, turn left and drive west on Power Line Road. 100.8 km Junction between Power Line Road and Quarry Road. Park vehicles and walk south and west from junction onto a large outcrop. Stop 1. Inclusion-Bearing Unit UTM 4226691E, 5137144N. This outcrop is within the Inclusion-Bearing unit, which occurs at the base of the Lower Gabbronorite Zone (Unit 3, Vogel (1996) map, outcrop #153). On the west side of this outcrop the Inclusion-Bearing unit is in contact with the rocks of the Neoarchean Ramsay–Algoma gneiss complex. This stop shows the following features. 3 1. Overall the unit consists of metre-scale, pod-like autoliths of nodular anorthosite and pyroxenite in a medium grained, massive leucogabbronorite to melagabbronorite matrix (Photo 6). The outcrop is cut by plagioclase-phyric Matachewan diabase dikes with chilled margins. The dikes strike 340°, and exhibit fractures and veins that trend 250° and which show sinstral displacement. Photo 6. Stop 1, Day 2. Large pyroxenite pod in centre of photograph is hosted by rocks of the Inclusion-Bearing unit, shown in the foreground, which at this locality are hosted in a predominantly leucogabbronoritic matrix. Scale card near contact between the pod and the leucogabbronorite is 8.5 cm long. 43 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �2. PGE concentration for disseminated chalcopyrite-pyrrhotite-pentlandite mineralization averages 107 ppb Pd+Pt based on 723 analyses; the range is 1 to 1873 ppb, Pd/Pt = 2.3 and Cu/Ni = 2.1. 3. In part of the outcrop, pyroxenite (assemblage blue-green hornblende+quartz+magnetite in one sample) occurs as discontinuous lenses, or pods, that are 30 to 40 m in length and 5 m in width, strike 290° (see Photo 6), and which contain trace disseminated copper-nickel-PGE mineralization. 4. Assay data for the pyroxenite pod shown in Photo 6. Sample 154547, at the edge of pyroxenite pod has Pd+Pt = 264 ppb, Pd/Pt = 4.6, Ni+Cu = 969ppm, Cu/Ni = 4.2, S = 1800 ppm. Sample 154533 has Pd+Pt = 37 ppb, Pd/Pt = 1.5; Cu+Ni = 220 ppm, Cu/Ni = 1.3, S = 200ppm. Samples 154607 and 154739, middle of pyroxenite pod, have PGE at or below detection limits. 5. Granitic xenoliths are common in the Inclusion-Bearing unit at the contact between the intrusion and the older granitic gneisses at the west margin of the outcrop. Return to the Power Line Road and walk north for about 200 m along the Quarry Road to Stop 2. Stop 2. Nodular Anorthosite Unit, Marginal Leucogabbronorite Zone The quarry outcrop (Photo 7a) is an excellent example of the Nodular unit, the youngest unit of the Marginal Leucogabbronorite Zone, Agnew Lake Intrusion (Unit 2c, Vogel (1996) map, outcrop #126). This is the only area of the intrusion where this unit is observed and its age relationship relative to the previous outcrop area is clear as it occurs as autoliths in the Inclusion-Bearing unit. It occurs in a similar stratigraphic and marginal position at the east margin and the central narrow part of the East Bull Lake Intrusion (Nodular Anorthosite unit, Anorthosite Zone; Chubb, Hannila and Peck (1994)). At this locality the principal rock type is a very coarse-grained, nodular, glomeroporphyritic anorthosite (Photo 7b). It consists of glomerophenocrysts of plagioclase (1 to 14 cm in diameter) that form 80 to 90% of the rock and interstitial altered pyroxene. One to 3% disseminated chalcopyrite and pyrrhotite occur in the matrix to the nodules. Peck et al. (1995, Appendix 5, Part A) reported Pd+Pt in the range 35 to 470 ppb, Pd>Pt, Cu+Ni = 244 to 2138 ppm and Cu>Ni from similar sulphide mineral-bearing rocks from the East Bull Lake Intrusion. Chubb et al. (1995) suggested that the nodules formed due to elevated degrees of undercooling within dike-like constrictions at the margin of the intrusion. Nodule formation may have resulted from feldspar growth on pre-existing plagioclase phenocrysts to form primary nodules, followed by agglomeration processes forming larger secondary and tertiary nodules. Northwest-trending plagioclase-phyric Matachewan diabase dikes intrude the Nodular unit in this outcrop (see Photo 7a). UTM 426718E, 5137364N. Return along the Quarry Road for about 30 m and walk onto the outcrop immediately east of the road. Stop 3. Inclusion-Bearing Unit, Lower Gabbronorite Zone In walking from Stop 2 to Stop 3 you have traversed into the Inclusion-Bearing unit of the Lower Gabbronorite Zone (Vogel (1996) map, outcrops #192 to 195). The main phase of the Inclusion-Bearing unit consists of metre-scale zones or autoliths of nodular anorthosite enclosed in a coarse-grained matrix 44 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 7. Stop 2, Day 2. a) Stone quarry in Nodular Anorthosite unit. Author R.S. James is standing on nodular anorthosite; dark rocks in foreground belong to a Matachewan swarm diabase dike that intrudes the Nodular Anorthosite unit. b) Close-up of nodular anorthosite, showing closely packed plagioclase glomerophenocrysts with minor interstitial, altered, pyroxene. Scale card is 8.5 cm long. 45 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �of gabbronorite to leucogabbronorite. Two several metre-wide Matachewan diabase dikes, striking 290°, intrude the unit. A younger, Mesoproterozoic, 10 to 20 cm wide Sudbury swarm diabase dike (~1240 Ma) strikes 300° and intrudes all rock units. The Sudbury dike has narrow chilled margins, cooling fractures perpendicular to its margin, and contains trace disseminated sulphide minerals. It also contains small pyroxenitic and granitic xenoliths, and weathers a rusty brown colour, in contrast to the older Matachewan swarm dikes. At this locality Pd+Pt are as high as 2155 ppb and average 178 ppb (based on 154 analyses), with Pd/Pt = 3.1 and Cu/Ni = 1.4. UTM 426870E, 5137333N. Return to vehicles, and head east from the Quarry Road–Power Line Road junction. Reset odometer to zero. 0.2 km Park by side of road and walk to outcrop beside the road. (Vogel (1996) map, outcrop #215.) Stop 4. Inclusion-Bearing Unit, Lower Gabbronorite Zone This is an additional stripped outcrop area illustrating the Inclusion-Bearing unit. It contains autoliths of pyroxenite and nodular anorthosite in a matrix of leuco- to melagabbronorite (Photo 8). Disseminated sulphide minerals occur in the matrix assemblage and not in the autoliths and range from 1 to 5 modal percent. Assay values for this mineralization are Pd+Pt average 180 ppb (based on 451 analyses), ranging from 2 to 2012 ppb, with Pd/Pt = 3.1 and Cu/Ni = 1.5. Note the 2 to 5 cm wide plagioclase-phyric dike that is locally choked with euhedral plagioclase phenocrysts where the dike width narrows (Photo 9). UTM 427074E, 5137348N. Return to vehicles and head east on Power Line Road. 0.6 km Park by side of road and examine outcrop on the north side of the road (Vogel (1996) map, outcrops #216 and 217). Photo 8. Nodular anorthosite from the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion, roughly 200 m south of the nodular anorthosite quarry, in the vicinity of Stop 4, Day 2. Note the pyroxenite fragment in the centre of the photograph. 46 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 9. Diabase dike containing large, rounded plagioclase crystals, similar to the nodules of the Nodular Anorthosite unit, cutting across rocks of the Inclusion-Bearing Gabbronorite Zone, Lower Series, of the Agnew Lake Intrusion. Stop 5. Sudbury-Type Breccia In this outcrop, Sudbury-type breccia cuts through a Matachewan diabase dike that strikes 230°. The dike intruded rocks of the Ramsay–Algoma gneiss complex. The breccia consists of xenoliths of plagioclase, diabase, and granitic material, ranging from millimetre- to centimetre-sized, in a foliated, fine-grained matrix that, in part, has a distinctive flow-like fabric. Chubb et al. (1995) provide detailed descriptions of several similar dikes and argue that they represent pseudotachlyte that formed as a result of the Sudbury impact event at 1850 Ma. UTM 427504E, 5137368N. Return to vehicles and head east on Power Line Road. 1.4 km Junction with West Branch Road, turn right and head south on West Branch Road. 7.1 km Pull over and park vehicles, examine outcrops on both sides of the road (Unit 2a, Vogel (1996) map, outcrops #326 (east) and #329 (west side of road)). Stop 6. Varitextured Unit, Marginal Leucogabbronorite Zone This outcrop lies within the Varitextured Unit of the Marginal Leucogabbronorite Zone, Marginal Series. Both outcrop areas (AZ 1 – east side of road, and AZ 2 – west side of road) consist dominantly of gabbronorite to melagabbronorite and minor leucogabbronorite. Inclusions of pyroxenite and scarce nodular anorthosite are scattered throughout the matrix assemblage, indicating that at least part of the 47 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �outcrop area may belong to the Inclusion-Bearing unit. Disseminated sulphide mineralization occurs mainly in the gabbronorite matrix. Over 200 analyses from the 2 outcrop areas average 232 ppb Pd+Pt with individual analyses as high as 2375 ppb; Pd/Pt is about 2 as is Cu/Ni. Later quartz veins strike at 220° and are offset by fractures and/or faults that strike 290°. UTM 425257E, 5133932N. Return to vehicles and continue south on West Branch Road. 8.0 km Junction with logging road. Turn left (east) and drive east on the logging road for ~200 m to red flagging on road side, located at UTM 425917E, 5133294N. Follow the flagged line through bush, walking north at about 334° for approximately 440 m. On the route, you will cross Gridline 3-5, 275E of Pacific North West Capital Corporation. This traverse should not be made alone, and should be only conducted using appropriate footwear and field equipment. Stop 7. Dendrite Unit, Lower Gabbronorite Zone UTM 425744E, 5133665N. The outcrop is the bottom of a cliff face that shows extensive excellent exposure of the dendritic unit of the Lower Gabbronorite Zone. This unit is recognized as a distinct stratigraphic unit in the Agnew Lake Intrusion where it is well-exposed along the west and southwest margin of the plateau area that forms the central part of the intrusion. The textures and mineralogy are the same as observed the previous day at East Bull Lake Intrusion (Stop 7, Day 1), but here the dendritic texture is much more extensively developed. The texture occurs either as large irregular masses, sometimes almost layer-like, or as pod-like zones, particularly where this unit is gradational into the overlying layered gabbronorites. At this locality the dendritic habit of the altered pyroxenes can be rapidly replaced by coarse-grained gabbroic pegmatite assemblages, so that the rocks become quite varitextured. Examine the outcrop at the base of the cliff face and then work your way up the ridge, all the time observing the variation in rock types or the lack of variation. Having climbed the cliff face we will traverse east through the Dendritic Unit and into the overlying Layered unit of the Lower Gabbronorite Zone. In doing so we are walking through outcrop #333 into #334 (Vogel (1996) map). At the top of the cliff face (in outcrop #333) the dendritic gabbronorite is more typically restricted to local pod-like areas and enclosed in a coarse-grained to almost pegmatitic gabbroic rock. In the western part of outcrop #334 metre-scale isomodal phase layering is present, identical in general features to that observed in the East Bull Lake Intrusion. At this locality it dips approximately 30° to the east. Locally, pods of dendritic gabbronorite are observed in the Layered unit. The stratigraphic units that overlie the Layered unit to the east exhibit decametre or thicker scale layering. As a result, shallow dipping surfaces consisting of individual layers form extensive outcrops of apparently only one rock type. Return to the vehicles by walking southward (~160°) across the top of the plateau to intersect the logging road, then follow the road southwest to the parked vehicles at the base of the plateau. This ends Day 2. Retrace route to West Branch Road, Agnew Lake Lodge Road, and Highway 17. 48 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �RIVER VALLEY INTRUSION, OVERVIEW Country Rocks, Structure and Geometry The River Valley intrusion is located about 65 km east-northeast of Sudbury (see Figure 1) near the community of River Valley. The intrusion underlies parts of Crerar, Dana, Henry, Janes, McWilliams and Pardo townships, but is best exposed in Dana Township, where it locally contains well preserved primary mineralogy and textures (Hrominchuk 1999, 2000; Easton and Hrominchuk 1999, 2001a, 2001b; Easton 2001, 2003). Although recently mapped (Easton and Hrominchuk 1999, 2001a, 2001b; Easton 2003; Easton and terMeer 2004; Hrominchuk 1999, 2000), studies of the stratigraphy, geochemistry and mineralization present within the River Valley intrusion are limited in comparison to similar studies on the East Bull Lake and Agnew Lake intrusions. Of all the East Bull Lake intrusive suite bodies studied to date, the River Valley intrusion exhibits the clearest relationships with other members of the Paleoproterozoic rifting suite described earlier. It is cut by mafic dikes geochemically correlated with the Matachewan and Hearst dike swarms, as well as by felsic intrusive rocks coeval with the Huronian Supergroup metavolcanic rocks (Easton and Hrominchuk 1999, 2001b). Furthermore, in Street Township, granitic rocks dated at 2460±20 Ma, contain inclusions of East Bull Lake intrusive suite leucogabbroic and anorthositic rocks (Easton and Murphy 2002). Easton (2003) has reported similar inclusions in granitic plutons in Henry and Loughrin townships. Unfortunately, the relationship between the River Valley intrusion and the Huronian Supergroup is obscured by tectonism along the Grenville Front. Along the Grenville Front, the River Valley intrusion is either in thrust contact with quartzite of the Mississagi Formation (Davidson 1986) or is in an unknown contact relationship with mafic and felsic metavolcanic rocks of the lower Huronian Supergroup (Easton and Hrominchuk 1999). On the basis of geochemical correlation, however, these metavolcanic rocks are likely ~2460 Ma in age (Easton and Hrominchuk 1999), and consequently, most likely were deposited after emplacement of the River Valley intrusion. The River Valley intrusion is also cut by olivine diabase dikes of the 1238 Ma Sudbury dike swarm (Easton 2000) and the Grenville dike swarm (590 Ma). Intrusive contacts between the River Valley intrusion and the Archean basement are exposed throughout the eastern half of the intrusion, and are generally sharp. Zones of intense diking, equivalent to the Border Zone in the East Bull Lake Intrusion, have not been observed. In fact, in both Dana and Crerar townships, Archean para- and orthogneisses adjacent to the intrusion are devoid of any mafic dikes, apart from a few Matachewan–Hearst swarm diabase dikes. In some parts of Dana Township, a fine- to medium-grained gabbro is present at the contact, and may represent a chilled facies of the intrusion. In many areas, however, an inclusion and/or fragment-bearing facies occurs proximal to the contact, making it difficult to ascertain the true extent and significance of this finer grained facies. As in the case of the East Bull Lake Intrusion, a “syenite” (actually an alkali feldspar granite) body in Dana Township was originally interpreted as a felsic differentiate of the River Valley intrusion (e.g., Card and Lumbers 1975). On the basis of contact relationships and geochemistry, Easton (2000) interpreted it as Archean in age, an interpretation that has been recently confirmed by U/Pb zircon dating, which yielded a preliminary age of ~2660 Ma (Easton and Kamo 2003). 49 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Previous maps of the River Valley intrusion (e.g., Lumbers 1973; Card and Lumbers 1975) indicated that the intrusion extended west into Henry and Loughrin townships as a series of “tails” surrounded by gneissic country rocks. This map pattern was interpreted to be the result of either 1. a pattern of complex interference folding during Penokean and Grenvillian orogenesis, or 2. a system of feeders to the main body of the intrusion located in Crerar and Dana townships. Furthermore, it was also speculated that other isolated bodies of East Bull Lake intrusive suite rocks in the Grenville Province, west of the River Valley intrusion (e.g., Red Deer Lake, Street Twp., southern Wanapitei complex, Table 1), represented parts of the River Valley intrusion now isolated from the main mass through erosion and tectonism. Work conducted by Easton (2003) suggests otherwise. East Bull Lake intrusive suite rocks west of Crerar Township are different in form, stratigraphy, and composition, and are intruded into distinctively different country rocks than those found in the River Valley intrusion. Consequently, East Bull Lake intrusive suite rocks located west of Crerar Township likely represent part of one or more intrusions that were emplaced separately from the River Valley intrusion. STRUCTURAL STATE AND DEGREE OF PRESERVATION Figure 8 outlines the general structural state of the River Valley intrusion and its country rocks in Dana and Crerar townships, as well as the degree of textural preservation within the intrusion and the country rocks. Six main preservation regimes can be recognized, which are described by Easton (2003), these form the basis of the divisions shown in Figure 8. The Sturgeon River, and the Neoproterozoic to early Paleozoic fault that extends along it, is an important structural feature within the River Valley intrusion (see Figure 8). North of the Sturgeon River, the River Valley intrusion contains large areas of preserved or partly preserved primary mineralogy. Deformation is concentrated along discrete shear zones, both vertical and subhorizontal, that cut the intrusion (Easton and Hrominchuk 1999, 2001b; Hrominchuk 2000). Grenvillian deformation is largely concentrated near the Grenville Front, along the west side of the intrusion (see Figure 8). North of the Sturgeon River, Grenvillian metamorphism results in the incipient development of corona textures in rocks of the River Valley intrusion and the Sudbury diabase dike swarm, with metamorphic conditions estimated at 5 to 7 kb and 625°C (Tettelaar 2000; Easton and Hrominchuk 1999). An empirical observation is that rocks of leucogabbroic composition are most commonly recrystallized, whereas orthopyroxene-bearing rocks are generally more resistant to metamorphic recrystallization. The geometry of the intrusion north of the Sturgeon River appears to be largely sheet-like, with igneous layering generally dipping shallowly (20 to 30°) to the south and southeast. This has resulted in exposure of the basal contact (margin or sidewall?) of the River Valley intrusion for 8 to 10 km along its north and east margins, providing an extensive target area for mineral exploration. In contrast, south of the Sturgeon River, pockets of preserved mineralogy and texture (Photo 10) are less abundant, occurring mainly in eastern Crerar Township (see Figure 8). Igneous layering at several scales is preserved within this area of the intrusion (Photo 11) (Easton 2003). Where present, igneous layering dips moderately (40° to 60°), and stratigraphy is apparently folded (Easton and Hrominchuk 2001a). This folding results in a variety of trends for the basal, mineralized contact of the intrusion, making mineral exploration more difficult than in the north. Penetrative fabrics of Grenvillian-age are common (Photo 12), with much of the River Valley intrusion consisting of gneissic rocks of gabbroic and leucogabbroic composition. Grenvillian metamorphic effects result in more complete development of corona textures in rocks of both the River Valley intrusion and the Sudbury diabase dike swarm in Crerar Township, with metamorphic conditions estimated at 7 to 9 kb and 650°C (Tettelaar 2000; Easton and Hrominchuk 1999). 50 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �5174225 N C) drift covered Structural and Preservation Regimes Regime I Regime2a Regime 2b Regime2c Regime3a Regime 3b Regime4a Regime 4b Regime 5 Regime 6 • Regime 7 ,,' geological contact road ,)( railway (abandoned) ,V township boundary 5153225 N Figure 8. Map showing the degree of primary textural preservation within rocks of Crerar and Dana townships (from Easton 2003). 51 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 10. Progressive textural changes. a) Igneous-texture in medium-grained norite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. Note dark colour of feldspars. UTM 556115E 5171466N. b) Partly recrystallized rock of the River Valley intrusion, preserving igneous-texture in medium-grained gabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. Mafic minerals have thick, green-weathering amphibole coronas rimming dark orthopyroxene grains. UTM 556886E 5161009N. c) Recrystallized but massive leucogabbronorite from leucogabbronorite zone of the River Valley intrusion, Crerar Township. This rock no longer preserves primary mineralogy, but is relatively undeformed. Note white colour of feldspars, and lack of well defined crystal shapes. Mafic mineral clots are aggregates of fine-grained amphibole. UTM 557400E 5160225N. d) Typical undeformed but recrystallized leucogabbronorite from the Positano Quarry, East Bull Lake intrusive suite, Loughrin Township. UTM 547000E 5160600N. Hammer handle in all photos is 30 cm long. CONTACTS Copper, nickel and platinum group element exploration in the River Valley intrusion has focussed on its contacts, as outlined by Lumbers (1973). This is only an effective exploration tool if the contacts of the intrusion are primary and not tectonic. Easton (2003) noted 3 main contact types that are present between the River Valley intrusion and other rock units in the map area: 1. preserved intrusive contacts, 2. deformed and disrupted intrusive contacts, and 3. wholly tectonic (fault) contacts. Near the northwest edge of Dana Township, the western contact of the intrusion is tectonic, as originally described by Davidson (1986). The northern and eastern contacts of the intrusion are primary, although they are locally cut, with minimal lateral offset, by mylonite of the Southern–Grenville province boundary zone. Mineralization in northwestern Dana Township (Dana North and South zones of Pacific North West Capital Corporation) is spatially associated with preserved intrusive contacts. Much of the 52 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Photo 11. a) Igneous layering within the norite zone, River Valley intrusion, Crerar Township. UTM 556769E 5159980N. b) Igneous layering within the River Valley intrusion in Crerar Township, near the top of the anorthosite zone and the base of the olivine gabbronorite zone. Dark layers are melanorites, some of which are olivine-bearing. UTM 557390E 5160360N. c) Layering within leucogabbronoritic rocks immediately above the heterogeneous basal zone of River Valley intrusion, Crerar Township. UTM 557585E 5159590N. d) Large-scale igneous layering within leucogabbronoritic rocks of the River Valley intrusion exposed in the Dana Quarry, Dana Township. UTM 558630E 5165385N. Hammer handle in all photos is 30 cm long. northern contact of the intrusion in Dana Township is likely primary, and both the Lismer’s Ridge and Azen Creek zones occur near this contact. It should be noted that based on surface exposure, the Azen Creek zone is apparently located near, but not at the contact. Recent drilling by Pacific North West Capital Corporation has shown that the contact in the Azen Creek zone dips generally shallowly to the south and in fact the Azen Creek zone is proximal to the contact – the difference is that the mineralization is hosted in “inclusion-bearing” or fragment-bearing units rather than the breccias observed in the Dana North area. Also, recent drilling at Jackson’s Flats shows the contact is oriented in a northwest direction and is steeply dipping, with mineralization occurring in fragment-bearing units rather than “breccia”, much like the situation at the Azen Creek zone. In Crerar Township, a primary intrusive contact is preserved in the area that includes the Tomrose occurrence, and Crerar 2 to 4. To date, most of the mineralization identified in Crerar Township lies near this contact. 53 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �a. rN Photo 12. a) Typical leucogabbroic gneiss of an East Bull Lake intrusive suite intrusion in Henry and Loughrin townships. Note the lack of a prominent leucosome phase. UTM 548443E 5159789N. b) Protomylonitic to mylonitic textures present in a highstrain zone in the River Valley intrusion, Crerar Township. UTM 559751E 5163890N. c) Strain gradient within rocks of the River Valley intrusion, northern Crerar Township. Foliated to gneissic leucogabbro on top, straight and irregularly layered gneiss on bottom. UTM 557225E 5162512N. d) Epidote-clot developed just right of the hammer handle in leucogabbro gneiss of the River Valley intrusion, Crerar Township. Such epidote clots are common in gneissic leucogabbro throughout the study area. UTM 555495E 5163580N. Hammer handle in all photos is 30 cm long. In contrast, most of the southern contact of the intrusion in Crerar Township, although poorly exposed, is a major shear zone, which is discordant to internal contacts in the intrusion. It is likely that the eastern contact of the River Valley intrusion north of the Sturgeon River, is also a tectonic contact. Examples of tectonized intrusive contacts may be present in 3 areas. 1. The western contact of the intrusion along Highway 805, particularly in the southern half of Dana Township. However, it is possible that this contact represents an upper, rather than a basal, contact of the intrusion. 2. The other 2 examples occur in Crerar Township, but represent areas where zones of mafic straight gneisses are in contact with other highly deformed gneisses or different units within the intrusion. Similar rocks occur just south of Glen Afton, south of the Sturgeon River; however, to date no mineralization has been reported from the latter area. 54 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Hrominchuk (1999, 2000) recognized 2 types of mafic rock associations near areas of primary contacts. These are a marginal zone and an inclusion-bearing zone. There is considerable variation in thickness of both these zones along the primary contacts of the intrusion, ranging from 50 m thick to almost 500 m thick in the Dana North area. To date, mineralized zones along the contact occur where these units are relatively thick (e.g., Dana North, Varley, Azen, Tomrose). Stratigraphy and Petrography Although historically referred to as an anorthosite to anorthositic gabbro intrusion, the River Valley intrusion is dominated (~60% of surface area) by gabbro, norite, gabbronorite, leucogabbronorite and leuconorite, with some units containing modal olivine. True anorthosite makes up less than 10% of the surface area of the intrusion, but even in these areas, the distribution of mafic minerals is irregular, so that the areas of anorthositic rock are closer to noritic anorthosite or gabbroic anorthosite in composition. Noritic and gabbroic anorthosite probably underlie about 30% of the surface area of the intrusion. Observed stratigraphy in the intrusion differs north and south of the Sturgeon River. NORTH OF THE STURGEON RIVER Low regional dip and poor exposure in key areas in Dana Township has hampered stratigraphic analysis, despite the textural and mineralogical preservation of the rocks. Despite these difficulties, Hrominchuk (2000), as presented in Figure 9, developed an interpretive stratigraphic section for the River Valley intrusion located north of the Sturgeon River. The intrusion north of the Sturgeon River has been subdivided into 5 zones (Hrominchuk 2000; James et al. 2002a), as follows: 1. Marginal zone 2. Inclusion and/or Fragment-Bearing zone 3. layered Olivine Gabbronorite zone 4. Layered Gabbronorite zone, and 5. massive, locally varitextured, Leucogabbronorite zone. Marginal Zone The Marginal zone is characterized by a fine- to medium-grained, black plagioclase-phyric, quartzphenocryst-bearing, gabbronorite that is partly chilled and is typically strongly mineralized at the contact with granitic plutonic and gneissic rocks. This contact is commonly sheared and normally consists of a complex chaotic mixture of footwall blocks and fine- to medium-grained gabbronoritic rocks. Because rocks immediately adjacent to the contact usually contain large blocks and screens of footwall material, exact definition of the contact in some areas is problematic. The Marginal zone consists mostly of medium-grained norite to gabbronorite and is typically well preserved. In thin section, the rock consists of dark-brown to tan cumulate (An79 core; An74 rim) to intercumulate (An68) plagioclase, woody-brown cumulate orthopyroxene (En44-76), interstitial clinopyroxene, with accessory magnetite, quartz, biotite, sulphide minerals, and calcic myrmekite. Fragments are commonly quartzofeldspathic gneiss, alkalifeldspar granite and amphibolite gneiss. Granitic fragments contain quartz with a small amount of alkali feldspar and biotite or amphibole. Quartzofeldspathic gneiss fragments commonly have a preserved gneissic texture and are mostly quartz with a small amount of biotite. Amphibolite gneiss fragments are a fine-grained, commonly foliated, matte of blue-green amphibole and epidote group minerals. 55 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �______ 0) tic. U C.0.2 a -s 900— vv vvv vv vvv vv a a u 9 a ._a I En% Fo°k An°/o 800— v v v a I I I I Leucogebbronorite . zone 56-63 700— v"v"v vvv vv vvv 56-69 I I - - - - - - t - 600— : I 500— 57-66 5875 Gebbronorite zone : I ' • . : 300— . olivine Gebbronorite zone 200— -. . es;-:->: Inclusion and/or Fragment Bearing -zone Ivierginel zone 67-78 72-76 ' . 170-76 ' I • • 68-73 1 68 155-69 I • 39-81 I :44-76 1 1 : 1 • • I 1 :I 1 1 1 I I (meters) Primocryst mineral Interstitial mineral Figure 9. Interpreted stratigraphic sequence and distribution of primary mineral phases and compositions within the River Valley intrusion north of the Sturgeon River. From Easton (2003). Inclusion and/or Fragment-Bearing Zone This zone is commonly in gradational contact with the underlying Marginal zone, but its appearance is marked by a sharp increase in the size and amount of felsic fragments and the appearance of compositionally and texturally varied mafic to ultramafic inclusions or xenoliths. Inclusions consist of 3 main types: alkali feldspar granite and other granitic rocks, amphibolite, and olivine-rich rocks (troctolites, peridotites). Granitic, gneissic and amphibolite fragments are similar to those described above for the Marginal zone, but they are commonly surrounded by pods or lenses of a very coarsegrained to pegmatitic assemblage of biotite, muscovite, amphibole, plagioclase, quartz and alkali feldspar (Hrominchuk 2000). It is important to note, however, that in all of the mineralized areas explored by Pacifc North West Capital Corporation to date, there have been less than 1% exotic fragments in the Breccia and Inclusion-Bearing units, which is their equivalent to this zone. Autoliths are abundant (typically up to 90%) and locally (e.g., outcrop scale) may constitute up to 100% of the rock. Troctolite or peridotite autoliths are similar in all respects to the olivine norite to melatroctolite and peridotite from the Olivine Gabbronorite zone described below; most are olivine-bearing gabbronorite to peridotite in composition, with subordinate leucogabbronorite. They are usually well rounded and weakly to very strongly mineralized. The host matrix to the autoliths and fragments is typically a fine- to mediumgrained, green to black gabbro or olivine-bearing gabbro to gabbronorite, often with disseminated to blebby sulphide mineralization. 56 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Olivine Gabbronorite Zone The Olivine Gabbronorite zone is composed of 0.3 to 1.5 m thick layers that are olivine-rich at the base, and erosive into underlying layers, and which grade upward into olivine gabbronorite or troctolite to slightly more plagioclase-rich end members. Layers are deeply weathered due to the presence of serpentine minerals and fine-grained magnetite after olivine. This zone is well exposed in only a few places, but is normally 50 to 150 m thick, and grades into the overlying, olivine-poor layered Gabbronorite zone. The Olivine Gabbronorite zone contains a wide range of rock types; however, olivine is present in most, with troctolite and olivine norite predominant. Olivine is generally medium- to coarsegrained, Mg-rich (Fo72-76), coronitic, and variably serpentinized. Plagioclase is calcic (An72-76), darkcolored, and inclusion rich. Orthopyroxene (En70-76) occurs in 2 forms, as euhedral woody prisms, and as large (up to 10 cm) oikocrysts enclosing olivine and plagioclase. Clinopyroxene is found only as interstitial anhedral crystals. Gabbronorite Zone The Gabbronorite zone is composed of metre-scale, modally layered rocks with a maximum continuous thickness of about 200 m. This represents a minimum stratigraphic thickness, since faults or zones of intense deformation always truncate the top of the section. The unit typically consists of alternating metre-thick layers of norite and gabbronorite (±olivine) and leucogabbronorite. Locally, this unit is very coarse-grained, with zones rich in felsic fragments surrounded by pegmatitic pods containing quartz, epidote, magnetite, alkali feldspar, plagioclase, amphibole, pyroxene, biotite and garnet. The Gabbronorite zone consists of layers that begin as olivine-bearing norite and fractionate to gabbronorite. Coarse-grained black norite dominates; it consists of large (2 cm) orthopyroxene (En57-66) and brown plagioclase (up to 3 cm)(An58-75) primocrysts, interstitial clinopyroxene and inverted pigeonite, and rare olivine. Metamorphosed equivalents contain altered plagioclase, amphiboles (fine-grained actinolite and coarse-grained hornblende), small garnets and quartz. Leucogabbronorite Zone This zone consists of non-layered, massive, medium- to coarse-grained rocks ranging in composition from leucogabbronorite to anorthosite. It forms the highest stratigraphic levels of the intrusion (generally the tops of topographic highs) in Dana Township. They are commonly highly deformed or faulted; a complete section of these rocks has not yet been found in Dana Township; however, a more complete section may be present in Crerar Township (Easton and Hrominchuk 1999). These rocks consist mainly of dark plagioclase (An56-63) with subordinate interstitial and primocryst clinopyroxene and inverted pigeonite (En56-69). Where metamorphosed, the more leucocratic rocks form very white-weathering rocks with small pods of amphibole-rich material dispersed throughout. SOUTH OF THE STURGEON RIVER As previously noted, establishing a stratigraphic column for the River Valley intrusion has been difficult, despite the state of preservation of the rocks. South of the Sturgeon River, steeper regional dips have made the task somewhat easier; however, the section there may contain more layer-parallel faulting, and units are only well preserved in the southwestern part of the intrusion. Easton and Hrominchuk (1999) originally suggested that a section up to 3.5 km thick could be identified; however, this interpretation assumed that the units were dipping uniformly to the west at roughly 50°, that the section was not folded, 57 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �and that there were no layer-parallel faults present. Further analysis indicated that the section was indeed folded (Easton and Hrominchuk 2001a). Unlike the area north of the Sturgeon River, this section has not been formally divided into stratigraphic zones; for reference, however, informal zone names are given in italics to facilitate comparison with the Dana Township section. The Crerar Township section has a maximum thickness of 1.4 km, and from base upward, consists of: • 150 to 200 m of gabbro and melanogabbro, forming a heterogeneous Marginal zone that contains fragment-rich zones, disrupted layering, and considerable diking. Sulphide minerals are common in this zone. This zone grades upward into leucogabbro and anorthositic gabbro. This zone corresponds roughly to the Marginal and Inclusion and/or Fragment-Bearing zones present in the intrusion north of the Sturgeon River. • 400 to 500 m of gabbroic anorthosite to anorthosite (Anorthosite zone and thin Olivine Gabbronorite zone). Melanogabbro and olivine melanogabbronorite layers up to 2 m thick occur near the top of this zone. This zone apparently does not have an equivalent unit north of he Sturgeon River, but a lowermost gabbroic anorthosite to anorthosite is present in most other East Bull Lake suite intrusions (Figure 10). It is possible that the melanocratic layers in the upper part of this zone correspond to the Olivine Gabbronorite zone present north of the Sturgeon River. • 350 m of norite and gabbronorite (Gabbronorite zone), commonly layered on the scale of centimetres to decametres, with layers having pyroxenite bases and gabbroic anorthosite tops (Photo 10b). Layering on the scale of metres to decametres is also inferred to be present in this zone. This zone may be equivalent to the Gabbronorite Zone present north of the Sturgeon River. • 200 m of leucogabbro to gabbroic anorthosite (Leucogabbronorite zone). This zone may be equivalent to the Leucogabbronorite Zone present north of the Sturgeon River. • 200 m or more of oxide-bearing leucogabbro to gabbroic anorthosite (Oxide-Bearing Leucogabbronorite zone). A regional magnetic anomaly is associated with this unit. Rocks within this zone show a marked decrease in Mg-number (less than 50) compared to other stratigraphic units in the River Valley intrusion (typically greater than 57). • The top of the section is not observed due to intense deformation in the rocks above this stratigraphic level. The most significant difference between the Dana and Crerar township sections is the presence of a thick unit of leucogabbro to anorthosite between the Marginal zone and the Olivine Gabbronorite and Gabbronorite zones in the Crerar Township section. In one locality, a few outcrops of nodular anorthosite are present near the base of the leucogabbro to anorthosite unit. At another locality, this unit contains a decametre-thick layer of megacrystic anorthosite, with 1 to 5 cm equant, blocky plagioclase crystals partly separated by clotty, coarse-grained and weakly poikilitic, amphibolitized pyroxene. In addition, the varitextured to massive uppermost Oxide-Bearing Leucogabbronorite zone is not observed in Dana Township, and is the only unit that contains both Mg-number and normative An less than 55. 58 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �____________ ______________ ___________ __________ , +. __________ __________ well mineralized zones Fe-Ti oxide zone e layered zones C leucogabbronorite C gabbronorite, norite Agnew Lake Intrusion EliEl gabbroic anorthosite a ] melatroctoiite, 0 livine nonte and 2250 gabbronorite 2000 olivine gabbronorite inciusion - bearing zone 1750 border zone Upper Series 1500 River Valley Intrusion (south) East Bull Lake Intrusion / V VVVV — 1250 River Valley Intrusion 1 (noah) Norite-gebbro1 norite zone Upper Series Street Intrusion 1000 750 — norite zone1 Main Olivine 500 Series zone £ s SASS a a aS 1 Inclusion! Fragment Bearing zone tLeU000abbro Contact 250 H ii —in— ni Lower -S-if. 0 Marginal Series (metres) Figure 10. Generalized cross-sections for the Street metagabbro and the River Valley, East Bull Lake and Agnew Lake intrusions of the East Bull Lake intrusive suite. East Bull Lake section modified from Peck et al. (1995), Agnew Lake Intrusion section modified from Vogel (1996), River Valley data from Easton (2003). STRATIGRAPHIC COMPARISON WITH OTHER EAST BULL LAKE SUITE INTRUSIONS As shown in Figure 10, the composite stratigraphic section from the best preserved enclave of the River Valley intrusion in Crerar Township contains elements common to both the northern River Valley, the East Bull Lake, and the Agnew Lake intrusions. If the Crerar Township stratigraphy is a true section through the intrusion, then it is similar in thickness to other East Bull Lake intrusions (see Table 1). The exception is the Agnew Lake Intrusion (∼2.3 km thick), which unlike the other intrusions, includes felsic differentiates near the top. The Crerar Township section also suggests that the River Valley intrusion locally includes an anorthositic zone near the base of the body, similar to most other East Bull Lake intrusions (see Figure 10). 59 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Geochemistry Geochemical data from the intrusion can be found in Easton and Hrominchuk (2002) and Easton (2003). Overall, the chemistry of the rocks are similar to that of other intrusions of the East Bull Lake suite. Silica is generally less than 50 weight percent. Vogel et al. (1999) noted that in the case of the Agnew Lake Intrusion, SiO2 less than 50 weight percent is characteristic of the stratigraphically lower part of the intrusion. The same effect may be present in the River Valley intrusion, as most of the samples reported to date are from the lower part of the intrusion (below the Oxide-Bearing Leucogabbronorite zone). A few samples, located stratigraphically higher in the River Valley intrusion in Crerar Township show a marked decrease in Mg-number (from typically greater than 57 to less than 43). These rocks are generally of gabbroic anorthosite composition and oxide mineral-bearing. Representative analyses of each stratigraphic unit present in the River Valley intrusion north of the Sturgeon River are listed in Table 5, as well as in Hrominchuk (2000) and James et al. (2002a). Rocks of the Marginal zone are CIPW quartz-normative; Olivine Gabbronorite zone rocks are olivine-normative, whereas most rocks in the upper units are only silica saturated. Samples from the Marginal zone (see Table 5, analyses 1 and 2) show much higher Ti, Fe, K, Na, P, Ba, Zr and Y contents than do most other River Valley intrusion samples. Hrominchuk (2000) interpreted these elemental variations as a possible contamination signature. The high concentration of Ti and Fe in some footwall phases (see Table 5, analysis 12), relative to typical River Valley intrusion rocks, is consistent with some of the Ti and Fe contents in sample CZ7-1000 resulting from wall rock assimilation (James et al. 2002a). The matrix of the Inclusion and/or Fragment-Bearing zone is high in Mg, Ca, Al, and Fe, consistent with its plagioclase, orthopyroxene and olivine-dominated mineralogy. The fragments, inclusions and xenoliths of this zone are extremely varied in composition, partially resorbed, and commonly mineralized. The felsic fragments show similarities to the footwall alkali feldspar granite (James et al. 2002a). Mafic inclusions are similar to the rocks in the lowest part of the layered Olivine Gabbronorite zone. The Olivine Gabbronorite zone is characterized by rocks high in MgO (15 to 24 wt.%) and low in SiO2 (38 to 45 wt.%), with anomalous Ni contents attributed to the predominance of primocryst olivine (see Table 5, analyses 5 and 6). Rocks from the layered Gabbronorite zone show gradual fractionation to more Si-, Al-, Ca-, Na-, K-rich and Mg-poor compositions (see Table 5, analysis 9), a trend also observed in the uppermost stratigraphic unit. A sample of magnetite gabbro from the uppermost part of the stratigraphy in Crerar Township (Easton 2003) may be similar to the lowermost ferrogabbros from the Agnew Lake Intrusion, and is the most evolved sample found to date in the River Valley intrusion. Pearce element ratio diagrams for the River Valley intrusion (James et al. 2002) and detailed petrography indicate that major and trace element trends are controlled by plagioclase, olivine and orthopyroxene, with very little contribution from clinopyroxene (James et al. 2002a). Samples from the Olivine Gabbronorite zone show a variation from olivine + plagioclase near the base to plagioclase + orthopyroxene controlled geochemistry near the top of the zone (James et al. 2002a). Plagioclase is commonly a phenocryst phase within marginal and fine-grained rocks indicating very early crystallization, and most likely quickly joined by olivine as the first cumulate phases. Fractionation and cooling of the magma caused orthopyroxene to replace olivine as a cumulate phase during crystallization of the marginal rocks and the Olivine Gabbronorite zone. This is indicated by orthopyroxene rims on olivine and large orthopyroxene oikocrysts enclosing olivine and plagioclase near the top of this zone (James et al. 2002a). Rocks above this point show trends that indicate fractionation of magma due to plagioclase and orthopyroxene toward more Fe- and alkali-rich compositions. Chondrite-normalized rare earth element patterns for samples north and south of the Sturgeon River are shown in Figure 11. Most samples show total REE contents and patterns similar to those found in the 60 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �East Bull Lake and the Agnew Lake intrusions. In Figure 11, REE patterns for the proposed Dana Township stratigraphy show a slight fractionation trend with stratigraphic height and the influence of cumulate plagioclase (reflected in the magnitude of the negative Eu anomaly). The contaminated contact sample shows significant light REE enrichment over other marginal samples and rocks of all other zones. There are 2 groups of melanocratic rocks (Easton 2003). One with patterns and REE contents similar to other River Valley intrusion rocks; the other with high total REE and La/Yb. This second group has higher normative-orthopyroxene and lower-normative olivine contents than the first. A similar two-fold grouping occurs in the orthopyroxene hornblendite samples, perhaps suggesting a genetic linkage between the River Valley intrusion and these bodies. A sample of a primitive dike or layer from the Marginal zone exhibits a flat (~10 times chondrite) to slightly U-shaped pattern (see Figure 11), and is also characterized by relatively high silica and MgO, and low Ti and alkalis. It may represent a boninite-like parent magma composition for the River Valley intrusion (James et al. 2002a). The significance of such a parental magma has already been discussed in the section “Magma Composition and its Relationship to Mineralization”. —4— Contaminated Contact —0—Marginal Zone —A—Inclusion and\or Fragment Bearing Zone —X—Olivine Gabbronorite Zone —— Gabbronorite Zone —0-— Leucogabbronorite Zone —0-— Primitive dike/layer La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 11. Chondrite-normalized rare earth element plots for the River Valley intrusion for rocks north of the Sturgeon River. Sample from the “primitive dike or layer” may represent the parent magma composition to the mineralized parts of the intrusion in Dana Township. 61 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 5. Representative chemical analyses from the River Valley intrusion (data from Easton and Hrominchuk 2002). Analysis Sample No. 1 CZ7-500 2 CZ7-1000 3 99RME-2242 4 99RME-2243 Zone Rock Type MZ fg gabbrodiabase MZ fg gabbrodiabase IFBZ Stop 7, cg troctolite inclusion 559150 5167678 IFBZ Stop 7, mg olivine gabbronorite 559149 5167677 Easting Northing wt. % SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Mg # Ppm Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Zr/Y Ppb Au Pt Pd Pt/Pd Total (Au+Pt+Pd) 558431 5168104 558431 5168604 5 99RME2371A OLGN cg troctolite 559597 5166341 6 JLHS1-S3 7 JLHS2-S1 8 JLHS2-S4 OLGN cg melatroctolite GN cg gabbronorite GN cg norite 559657 5166341 558643 5165245 558644 5165393 52.51 0.93 14.17 13.40 N/A 0.17 5.16 8.79 2.16 0.85 0.09 0.26 0.024 1.36 99.59 40.9 51.85 0.91 14.36 13.09 N/A 0.18 5.48 8.96 2.25 0.94 0.10 0.22 <0.005 1.60 99.72 43.0 44.14 0.16 12.79 2.57 13.21 0.20 15.67 7.18 1.18 0.20 0.01 <0.03 0.67 0.01 97.32 62.1 48.42 0.21 18.81 1.51 8.38 0.14 10.16 9.82 1.87 0.30 0.03 00.07 0.18 0.25 99.90 62.9 43.32 0.10 18.30 9.34 N/A 0.12 14.06 8.37 1.34 0.28 0.03 0..22 <0.005 4.51 99.77 73.0 38.30 0.14 7.85 15.10 N/A 0.20 23.72 3.70 0.70 0.12 0.04 0..22 <0.005 10.12 99.99 73.9 50.21 0.13 19.95 8.23 N/A 0.13 6.90 10.32 2.09 0.21 0.03 0..18 <0.005 1.99 100.19 60.2 50.81 0.18 18.43 8.92 N/A 0.14 7.46 11.19 1.87 0.20 0.03 0.15 <0.005 0.43 99.66 60.1 90 57 50 29 284 171 109 37 341 255 2.9 86 16 5.5 115 75 39 30 287 401 108 46 251 193 0.9 80 18 4.3 203 1840 119 15 93 4512 122 3 81 151 0.4 12 4 3.0 250 623 65 14 91 1320 84 5 110 219 0.7 21 6 3.5 136 595 78 11 43 16 92 4 93 235 0.3 7 4 1.7 159 1360 155 17 48 162 111 1 79 94 0.9 17 8 2.2 73 97 41 20 71 18 56 16 139 238 0.2 3 0.7 4.7 90 92 38 28 109 94 66 10 121 220 0.2 4 2 2.4 <5 18 19 0.95 36 6 18 25 0.72 49 145 576 1980 0.30 2701 59 106 293 0.36 458 <5 <8 <8 <5 10 11 0.95 21 <5 <8 19 <5 11 29 0.38 40 <21 19 Abbreviations: cg = coarse-grained, fg = fine-grained; mg = medium-grained; N/A = not analyzed; IBZ = inclusion-bearing zone, GN = gabbronorite zone, LGN = leucogabbronorite zone, MZ = marginal zone, OLGN = olivine gabbronorite zone. Easting and Northings are in UTM Zone 17, datum NAD 83. 62 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Table 5. continued. Analysis Sample No. Zone Rock Type Easting Northing wt. % SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Mg # Ppm Cr Ni Co Sc V Cu Zn Rb Ba Sr Nb Zr Y Zr/Y Ppb Au Pt Pd Pt/Pd Total (Au+Pt+Pd) 9 99RME-2116 LGN cg gabbronoritic anorthosite 556346 5166811 10 99RME-2244 LGN cg gabbronoritic anorthosite 558522 5165429 11 99RME-2139 Footwall Stop 6, alkalifeldspar granite 559785 5167228 12 99RME-0375 Footwall biotite diorite gneiss 49.59 0.16 25.52 1.99 2.55 0.07 3.17 13.65 2.41 0.30 0.01 0.03 <0.01 0.98 100.40 54.2 51.07 0.10 22.84 1.08 4.47 0.11 5.38 10.91 3.03 0.22 0.01 0.08 <0.01 0.72 99.94 61.6 72.26 0.10 15.21 0.16 0.44 0.01 0.28 1.14 4.02 5.52 0.03 0.20 <0.01 0.61 99.78 46.1 55.29 1.57 13.8 3.37 10.02 0.17 3.35 6.73 2.58 1.94 0.19 0.11 0.04 0.76 99.77 31.4 63.76 0.58 15.62 2.09 3.11 0.06 3.08 3.23 3.63 2.97 0.13 0.12 0.09 1.18 99.44 52.4 50.36 1.15 14.07 3.09 11.11 0.22 5.89 10.02 1.97 0.68 0.12 <0.03 0.12 0.68 99.36 43.0 45.50 2.97 15.60 2.16 13.60 0.23 5.21 8.21 3.26 1.45 0.75 0.12 0.12 0.09 99.03 37.4 138 44 13 13 75 46 36 4 85 258 0.4 9 5 1.8 86 83 28 11 55 18 51 3 89 246 0.1 3 2 1.5 20 <5 <5 <1 11 <5 15 106 1748 950 1.2 93 1 93 30 57 41 26 330 92 142 75 593 157 12.5 166 47 3.5 183 64 19 9 17 10 94 139 670 464 6.8 117 8 14.6 90 81 57 37 319 278 67 13 91 145 4.8 92 31 3.0 37 68 57 26 204 71 190 41 673 250 21.6 348 59 5.9 5 15 19 0.79 39 <5 <8 <8 <5 <8 <8 <5 <8 <8 <21 7 38 72 0.53 117 <5 <8 <8 <21 9 6 14 0.43 29 558780 5159433 13 99RME-0025 Footwall Migmatitic metwacke, Pardo gneiss 560005 5173855 <21 14 99RME-0031 Dike Matachewan diabase dike 15 99RME-0137 Dike Sudbury diabase dike 559913 5174428 561255 5169258 <21 Abbreviations: cg = coarse-grained, fg = fine-grained; mg = medium-grained; N/A = not analyzed; IBZ = inclusion-bearing zone, GN = gabbronorite zone, LGN = leucogabbronorite zone, MZ = marginal zone, OLGN = olivine gabbronorite zone. Easting and Northings are in UTM Zone 17, datum NAD 83. 63 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ROAD LOG, DAY 3, RIVER VALLEY INTRUSION Note: Highway 805 is used regularly by logging trucks. Consequently, extreme caution should be taken when parking vehicles on the shoulder of the highway and when examining outcrops located along Highway 805. Geological map reference: Lumbers (1973); Card and Lumbers (1975); Easton and Hrominchuk (2001a, 2001b), Easton (2001). 0.0 km Start at the junction of Highway 69S and the Highway 17 southeast bypass. Get on the bypass heading east toward Coniston and North Bay. 11.8 km Junction Highway 17 and southeast bypass, turn right onto Highway 17 and head east toward North Bay. 20.0 km Bridge over the Wanapitei River. The river lies along the trace of the Grenville Front. 30.0 km Pull off onto a pullout located on the south side of Highway 17. Stop 1. Shear-Zone Hosted Orthopyroxene Hornblendite Body Examine the outcrop and large blasted boulders present on the west side of the pullout. They belong to an orthopyroxene hornblendite body that is present within a high-strain zone that extends subparallel to the highway. Examples of these highly strained felsic gneisses can be examined in outcrops at the base of the hill east of the pullout. The top of the ridge south of the road and above the stop consists of layered leucogabbronorite of the East Bull Lake intrusive suite. Thus, although proximal to rocks of the suite here, the orthopyroxenite body is not directly in contact with the suite, although that relationship has been observed elsewhere in Street and Awrey townships (Easton and Murphy 2002). Note the large, equant, orthopyroxene crystals, and the fine-grained amphibole matrix. Return to vehicles and head east on Highway 17. 60.6 km Junction of Highway 17 and 539 in Warren. Turn north onto Highway 539, reset odometer to zero. 0.0 km Junction, Highway 17 and 539 in Warren, turn north onto Highway 539 and proceed toward River Valley. After heading 200 m north on Highway 539, turn right, then after going 100 m east, turn left and continue north on Highway 539. 4.8 km Coloured Aggregates Limited/Crea-Mac quarry operation is located east of the highway. This operation is one of several quarries in Ratter Township which produces pink aggregate from syenite pegmatite of probable Archean age that occur within this part of the Grenville Province. 8.3 km Junction Highway 539 and Kipling Road West, continue north on Highway 539. 12.5 km Outcrops on both sides of the road expose typical rocks of the Crerar gneiss association, which consists of granodiorite, tonalite, and amphibolite rocks (2680 to 2660 Ma), commonly migmatized. Tight to isoclinal folds are common in the outcrop on the east side of the road. 14.5 km Pull over onto right shoulder and park by outcrops at crest of hill. 64 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 2. Rocks of the Crerar Gneiss Association This outcrop illustrates typical mafic and intermediate composition gneisses of the Crerar gneiss association (Easton 2003), including tonalite, 2 phases of granodiorite, amphibolite layers and pods, and minor calc-silicate rock. Rocks of the Crerar gneiss association form the country rocks to the southern margin of the River Valley intrusion. A sample from this outcrop yielded a Nd/Sm model age of 2710 Ma (Dickin 1998). Return to vehicles and continue northward on Highway 539. 21.6 Park on shoulder of road by large, isolated outcrop knob on east side of the highway. Optional Stop. Crerar Gneiss Association UTM 561947E, 5157087N. The outcrop on the east side of the road exposes a megacrystic granodiorite, which is a late intrusion into the Crerar gneiss association. This rock cuts earlier migmatitic fabrics, but is itself locally strongly deformed. A sample from this outcrop has yielded a U/Pb zircon age of 2663±4 Ma, with titanite dating the waning of Grenville metamorphism at 974±12 Ma (Easton and Kamo 2003). Return to vehicles and continue northward on Highway 539. 23.1 km Bridge over the Sturgeon River. 23.6 km Junction Highway 539, which heads east toward Field, and Highway 539A, which heads west-northwest toward River Valley. Turn left. Immediately pull over and park for optional stop. Optional Stop. Red Cedar Lake Gneiss This is Stop 2.3 of Lumbers (1978). Outcrops north of the highway at and just west of the junction expose typical granular gneiss of the Red Cedar Lake gneiss. The Red Cedar Lake gneiss is a widespread unit that extends from its western terminus here, at the Sturgeon River, east to the Ottawa River, a distance of over 80 km. It is thought to be a diatextite derived from Archean metasedimentary and intermediate composition intrusive rocks (Easton 2003), and forms the country rock along the eastern side of the River Valley intrusion north of the Sturgeon River. Much of the texture in the gneiss is probably Archean, rather than Grenvillian, in age. Return to vehicles and continue northward on Highway 539. 24.9 km Dupras Store in "downtown" River Valley. 26.2 km Bridge across the Temagami River. 27.6 km Junction Highway 539A and Erana Mines Road. The Erana Mines Road leads to Erana building stone quarry and Crea-Mac aggregate quarry, both are hosted in well-preserved rocks of the Gabbronorite Zone of the River Valley intrusion. 29.5 km End Highway 539A, start Highway 805. End of continuously paved road. 29.55 to 29.9 Pull over and park by the shoulder at roughly 29.55 km. The 350 m long section of outcrop on the north side of the road constitutes Stop 3. 65 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 3. Varied Degrees of Preservation in Rocks of the River Valley Intrusion This is Stop 2.4 of Lumbers (1978). UTM at the west end of the outcrop is 559691E, 5163308N. This 350 m long section along the north side of the highway exposes rocks of the River Valley intrusion exhibiting a wide variety of textures and deformation. The east end of the outcrop consists of dark-grey to grey weathering gabbronorite and leucogabbronorite, with well-preserved plagioclase laths and only incipient corona development around mafic minerals. As one proceeds east, the rocks become paler, largely due to whitening of the plagioclase grains during recrystallization. Note the presence of many small, thin, anastomosing shear zones. Heading west, the outcrop becomes a black and white leucogabbronorite, then a foliated equivalent of the same, finally becoming protomylonitic to gneissic at the west end of the outcrop where the road curves slightly. The variation seen in this outcrop is typical of the variation in the degree of preservation and deformation observed throughout the River Valley intrusion north of the Sturgeon River. In general, well-preserved, little-deformed igneous rocks form the core of topographic highs, whereas valleys and low areas are generally underlain by gneissic rocks similar to those observed in the west end of this outcrop. South of the Sturgeon River, recrystallized to gneissic varieties of gabbronorite and leucogabbronorite predominate. 30.2 km Junction with Giroux road. Turn right (north) and proceed north on gravel road. Note that the road is gated. Seek permission (and key) for entry from Mr. Giroux, who lives in the house just east of the gate. 31.3 km Road crosses Azen Creek, a major north-northeast-trending deformation zone within the River Valley intrusion. Most of the copper-nickel-PGE mineralization in the River Valley intrusion lies to the west of the Azen Creek zone, even though similar rock units occur along the contact of the intrusion to the east of the Azen Creek shear zone. 31.8 km Junction, stay to the left and follow side road roughly 600 m to the first of several quarry pits present in the Dana quarry. Park at base of hill by first large cliff face, just past the large quarry building. Proceed uphill along the sideroad to the west (right) to large open area of outcrop. Stop 4. Autolith Fragments and Layering, Layered Gabbronorite Zone This is the southernmost quarry face of a large building stone quarry, which has been exploited at various times over the last 30 years. Stone from this quarry face was used as floor tiles in the Willet Green Miller Centre, Ministry of Northern Developments and Mines, at Laurentian University in Sudbury. The rocks exposed here span nearly the entire Layered Gabbronorite zone, ranging from olivine-bearing norite in the south part of the exposure to a pigeonite gabbro in the northwest part. The dominant rock type here is a very coarse-grained, black norite to gabbronorite with varied amounts of oxide minerals in addition to metamorphic amphibole and garnet. Layering is almost horizontal in this area (~15 to 25°). Layering is subtle, and commonly disrupted or convoluted. Pods, dikes and irregular layers of pegmatitic and granophyric material are found in the more central region of this outcrop area. Pegmatite and granophyre commonly occur together and in association with mafic cognate inclusions and felsic fragments. The development of very coarse grains of pyroxene, plagioclase, amphibole and magnetite around these inclusions led Hrominchuk (2000) to conclude that these pods and dikes were not formed simply by differentiating magma. He suggested that they may have formed by contamination of a relatively unfractionated magma (olivine norite) by cooler, compositionally different, footwall and cognate xenoliths. These pegmatitic patches are very similar to those developed in the Inclusion-Bearing zone, which we will see subsequently at Stop 7. 66 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �If one continues to follow the road north, it passes by an abandoned quarry pit at 400 m and ends above another 20 m high quarry face at 600 m. In this quarry, subtle, shallow dipping (~15 to 25°) layering is present (see Photo 11d) as are pegmatitic veins and pods. Unlike the main stop, mafic cognate inclusions and disrupted layering are not observed. Turn vehicles around and retrace road back to main gravel road. 32.5 km Junction, continue north on main road. 34.4 km Pull off onto small road to the left (west) of the road and park. Follow old skidder trail through the bush west for approximately 200 m to the top of a cliff face, then work your way down safely along the side of the cliff. Watch for loose rocks on the slope and falling rocks. Stop 5. Layered Olivine Gabbronorite This stop displays olivine-rich rocks of the Olivine Gabbronorite zone, which may be stratigraphically equivalent to the Olivine Gabbronorite zones in both the East Bull Lake and Agnew Lake intrusions (see Table 5, analyses 5 and 6). The sequence of olivine-bearing rocks here is roughly 30 m thick. The section grades upward from melatroctolite to orthopyroxene oikocrystic melatroctolite near the base to a clinopyroxene-bearing orthopyroxene melatroctolite near the top. Layering is well developed in the lower portion of the cliff face and contains erosive scours that grade from plagioclase-poor olivine cumulates to more plagioclase-rich troctolites. All olivine in the section is partially to extensively serpentinized, creating a fractured and rubbly appearance. Plagioclase in this zone is generally interstitial with some euhedral primocrysts. Mineral compositions obtained by Hrominchuk (2000) are plagioclase, An70-76; olivine, Fo72; orthopyroxene forms oikocrystic or euhedral primocrysts at En70 and clinopyroxene is either absent or interstitial. Low-temperature alteration of the olivine-rich rocks produces serpentine, carbonate and magnetite. Return to vehicles, continue north on gravel road. 35.5 km Junction with side road to Azen Creek occurrence to the left (west), continue on main road. 36.3 km Park vehicles and walk south to clean outcrop exposures. Stop 6. Footwall Alkali Feldspar Granite and Sudbury Diabase Dike This quick stop shows a few outcrops of a weakly foliated alkali granite, which is the main footwall rock type to the River Valley intrusion north of the Sturgeon River in Dana Township. This granite cuts the migmatitic, metasedimentary-derived Pardo gneiss. Lumbers (1973) classified these granitoid rocks as monzonite to quartz syenite; however, geochemistry (Easton 2003; see also Table 5, analysis 11) suggests that these rocks are more appropriately termed alkali feldspar granite using the normative classification of Streckeisen (1976). The alkali feldspar granites are peraluminous, with low total REE (less than 50 ppm), positive Eu anomalies (Eu/Eu* 7.9), and are corundum-normative, suggesting an S-type (sedimentaryderived) affinity. Breaks and Moore (1992) reported similar REE patterns from their Type 2 phase of the Neoarchean, peraluminous, S-type Ghost Lake batholith in northwest Ontario, and suggested that the unusual chemistry of these rocks resulted from feldspar accumulation. U/Pb zircon geochronology from this outcrop yielded an age of 2660 Ma (Easton and Kamo 2003). Although we are at least 300 m north of the contact with the River Valley intrusion at this point, lead loss related to emplacement of the intrusion at 2475 Ma has affected this granite, and prevents determination of a more precise age. Titanite from this outcrop gives an age of 966±6Ma (Easton and Kamo 2003), confirming that these rocks, as well as the River Valley intrusion, have been affected by Grenvillian metamorphism. As was the case for the Parisien Lake syenite adjacent to the East Bull Lake Intrusion, this granite was originally thought to be a felsic differentiate that was coeval with the River Valley intrusion (e.g., Card and Lumbers 1975). 67 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Also present in the outcrop is a Sudbury swarm diabase dike. In the Southern Province, the Sudbury swarm dikes trend northwesterly, and consist of titaniferous (2.5 to 3.5 wt. % TiO2; see also Table 5, analysis 15) olivine-biotite-magnetite-gabbros. In Dana Township, Sudbury swarm dikes generally show northwest or northerly trends, and can be traced for several hundreds of metres along strike. Dikes range in size from a few centimetres in width to greater than 100 m wide. Olivine in the Sudbury swarm dikes north of the Sturgeon River commonly show incipient corona formation, similar to that observed in the River Valley intrusion, suggesting that coronas in both rock types developed in response to Grenvillian metamorphism. Sudbury swarm dikes south of the Sturgeon River are more extensively metamorphosed, exhibiting nearly complete replacement of olivine by orthopyroxene, spinel and garnet, and dike segments are shorter and more randomly orientated. UTM 559785E, 5167228N. Return to vehicles, turn around and retrace route. 37.1 km Sideroad to Azen Creek occurrence, turn right (west) and proceed roughly 500 metres to stripped outcrops. Stop 7. Azen Creek Copper-Nickel-PGE Occurrence The Azen Creek zone was one of the initial discovery sites for PGE mineralization in the River Valley intrusion in the fall of 1998. Its location along the north contact of the intrusion, some 6 km from Dana Lake, was the first indication that PGE mineralization might occur discontinuously to continuously over a considerable strike length. The stripped area south of the road contains inclusions of medium-grained gabbronorite, troctolite, amphibolite, alkali feldspar granite and other felsic material within a mediumgrained, grey-green olivine gabbronorite. Inclusions are more abundant on the west side of the stripped area, and vary from centimetre- to metre-scale. Mineralization consists of finely disseminated chalcopyrite and pyrrhotite in the matrix, as well as some sulphide-rich inclusions. Hrominchuk and Jobin-Bevans (2000) suggested that the inclusion-bearing zone might be intrusive into the contact zone of the intrusion, roughly coincident with the boundary between the fine-grained contact zone rocks and overlying olivine-rich cumulates of the Olivine Gabbronorite zone (e.g., Stop 5). The stripped area north of the road contains less mineralization, but shows a variety of pegmatitic veins and pods. The pegmatitic pods are commonly cored by inclusions of felsic material, likely incorporated from the footwall of the intrusion. Return to vehicles, turn around, and retrace route to Highway 805. 0.0 km Junction with Highway 805, reset odometer to zero. Proceed west on Highway 805. 0.3 km Highway bends south, side road heads west to Upper Canada Stone Company and Giroux gravel pit. Note piles of crushed, black aggregate derived from the well-preserved gabbronorite of the River Valley intrusion. From this point westward, the road parallels the Sturgeon River on the south, as well as winding its way back and forth across an abandoned Canadian National Railways right-of-way. 1.4 to 1.1 km High ridges to the north of the road are well preserved gabbronorite of the River Valley intrusion. 5.4 km Outcrop ridge north of the road and the right-of-way consists of gabbronorite of the River Valley intrusion which contains 2 fabrics, the later fabric being parallel to the Sturgeon River. Rocks similar to these were previously mapped as being deformed equivalents of Nipissing gabbro (Dressler 1979), however, chemically and petrographically they belong to the River Valley intrusion. 68 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �6.8 km Road turns north at Glen Afton. Glen Afton lies near the eastern margin of the 1 to 2 km wide boundary zone that marks the Grenville–Southern Province boundary in this area. For the next 20 km, Highway 805 weaves back and forth across this boundary zone, exposing zones of intense mylonization separated by lenses of relatively well-preserved rocks of the Huronian Supergroup, the River Valley intrusion, and the Nipissing intrusive suite. Note that Highway 805 was re-aligned between 1988 and 1989; consequently, users of Davidson's (1986) guidebook for the Grenville Front in this area are cautioned that the distances and stop locations listed in that guide are not always easily determined. Where possible, this guide notes the location of some of Davidson's stops. 7.0 km Park vehicles just past old railroad right-of-way and examine outcrops on west side of Highway. Optional Stop. Huronian Metavolcanic Rocks This is Stop 2.6 of Lumbers (1978). UTM for the south outcrop 554000E, 5166220N; for the north outcrop 554005E 5166267N. The outcrop just north and west of the abandoned right-of-way consists of medium-layered dacite, which may be part of the Stobie Formation of the Huronian Supergroup. The low outcrop beside the road (100 m to the north) consists of felsic to intermediate pyroclastic rocks assigned to the Huronian Supergroup. These outcrops represent the easternmost extent of Huronian felsic metavolcanic rocks that have been recognized to date. It is difficult to differentiate fine-grained metavolcanic rocks from mylonitic rocks along the Grenville Front, due to the intensity of the deformation. Return to vehicles and continue north on Highway 805. 8.0-8.2 km Outcrops west of the highway consist of fine-grained amphibolite, possibly representing Huronian Supergroup metavolcanic rocks, whereas outcrops east of the highway, generally covered by vegetation, consist of deformed leucogabbronorite of the River Valley intrusion. 9.5 km Low outcrops of gneissic leucogabbronorite of the River Valley intrusion occur on both sides of the road. 9.8 km ATV trail to Lismer's Ridge area on property of Pacific North West Capital Corporation on the right (east). 10.1 to 10.6 km Outcrops in this stretch of road, particularly those on the north side of the road, represent an optional stop. Optional Stop. Variably Preserved Nipissing Gabbro This location approximates Stop 2.7 of Lumbers (1978). The highway transects a body of Nipissing gabbro that has intruded the River Valley intrusion. Outcrop exposures on the north side of the road, in the vicinity of UTM 554777E 5168511N, exhibit varied degrees of preservation of the Nipissing rocks, similar to that observed in rocks of the River Valley intrusion. Dark green to almost black-weathering rocks are Nipissing gabbro retaining primary orthopyroxene and minor olivine grains, whereas greenweathering outcrops consist of typical, greenschist-grade, amphibolitized Nipissing gabbro, typical of what most of these rocks look like in the Southern Province. Note that plagioclase is much less abundant in the Nipissing rocks than in the River Valley intrusion. Return to vehicles and continue north on Highway 805. 10.7 km Outcrops north of road consist of protomylonitic gabbronorite of the River Valley intrusion and ultramylonite of unknown protolith. 69 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �10.9 to 11.1 km Outcrops east of the road consist of foliated to protomylonitic leucogabbronorite of the River Valley intrusion, intruded by deformed, pink felsite veins that are chemically similar to Huronian Supergroup felsic metavolcanic rocks (Easton 2003). UTM 554415E, 5168855N. 11.2 km Outcrop east of the road consists of varitextured gabbro of the River Valley intrusion. UTM 554500E, 5169105N. 11.8 km Park vehicles and examine outcrops on east side of road. Stop 8. Mylonitized Anorthosite of the River Valley Intrusion This is Stop R6a of Davidson (1986) and Stop 2.8 of Lumbers (1978). Outcrops west of the road consist of ultramylonite derived from rocks of the River Valley intrusion. Please note a few large blocks in the area, in which mylonitic texture is developed in rocks that compositionally are anorthosite. Return to vehicles and continue north on Highway 805. 12.0-12.2 km Road extends uphill and around a curve, passing through outcrops of greenish weathering Nipissing gabbro. This curve is the location of stop R5 of Davidson (1986). 12.3 km Outcrops on both sides of the road consist of black, flinty, ultramylonite of unknown protolith, but which may be derived, in part, from Huronian felsic metavolcanic rocks. This outcrop approximates stop R4 of Davidson (1986). 12.5 km Outcrop west of road consists of boulder conglomerate, likely belonging to the Gowganda Formation, in contact with weathered granite of unknown age. Outcrops east of the road are a black, flinty ultramylonite (UTM 554057E, 5170254N). The section of road between kilometre 11.8 and 12.5 illustrates the rapid transition between weakly deformed and intensely deformed rocks within the Southern–Grenville province boundary. Metasedimentary and metavolcanic rocks of the Huronian Supergroup occur in close proximity to rocks of the River Valley intrusion in this area, but deformation has obscured primary contacts. 13.3 km Outcrops on curve consist of felsite and black, flinty ultramylonite. 15.1 km Pull over and park, examine outcrops on the east side of Highway 805. Optional Stop. Mississagi Formation and Mylonitic Contact with the River Valley Intrusion This is Stop R3a and R3c of Davidson (1986). Outcrops on the east side of Highway 805 consist of deformed quartz arenite of the Mississagi Formation. UTM of road outcrop is 554413E, 5172600N. Proceed east through the bush for approximately 100 m to a cliff face. Black, flinty ultramylonite occurs at the base of the cliff, above quartz arenite of the Mississagi Formation. The mylonite is graditional upward, over about 1 to 3 m, into foliated leucogabbronorite of the River Valley intrusion. The Dana North zone, at the next field trip stop, occurs about 50 m stratigraphically above this location, and about 400 m east of this stop. Return to vehicles and continue north on Highway 805. 15.5 km Turnoff on right (east) leads to Pacific North West Capital Corporation Dana North exploration area. 16.3 km Junction, continue south roughly 500 m on road and park in flat area opposite large stripped outcrop. 70 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9. Dana Lake Copper-Nickel-PGE Occurrence Prospectors L. Luhta, R. Bailey and R. Orchard first reported promising platinum-group element assay values from rocks of the River Valley intrusion near Dana Lake and Azen Creek in November 1998. Subsequently, Pacific North West Capital Corporation optioned and staked most of the north contact of the River Valley intrusion in Dana Township (inset map in Figure 12). Mustang Minerals Corporation optioned and staked most of the River Valley intrusion in Crerar Township, as well as part of the north contact near the Dana–McWilliams Township boundary. As of this writing, Pacific North West Capital Corporation has identified a dozen exploration targets along this northern contact. These are, from northwest to southeast, the Pardo, Dana North, Dana South, Banshee, Lismer's Ridge (Lismer's North and Lismer's South), Macdonald, Varley, Azen, Jackson's Flats, and Razor zones. The Thomson zone is located more centrally in the intrusion. Between 2000 and spring 2003, Pacific North West Capital Corporation, in conjunction with their partner, Anglo American Platinum Corporation Limited, had completed 5 diamond-drill programs. Approximately 288 diamond-drill holes totalling 58 000 m have been completed on their River Valley intrusion property in Dana Township. Diamond drilling was focussed almost entirely on the Dana North (see Figure 12) and Lismer's Ridge targets. A 6th diamond-drill program, aiming for a potential total of another 40 000 m of diamond-drill core, is underway, also focussing mainly on the Dana North and Lismer's Ridge zones (Pacific North West Capital Corporation, Press Release, April 10, 2003; http://www.pfncapital.com/s/NewsReleases). The scope of these diamond drilling programs is too large to effectively summarize herein; consequently, the reader is referred to the comprehensive assessment file reports (Resident Geologist’s office, Sudbury) related to these programs. Pacific North West Capital Corporation has released 2 Mineral Resource estimates for the Dana North zone, based on the Phase 1 to 5 drilling programs. In both cases, Derry, Michener, Booth and Wahl Consultants Limited conducted the resource studies. The first in situ resource estimate (Pacific North West Capital Corporation, Press Release, October 16, 2001) was a total measured, identified and inferred resource of 593 000 ounces palladium, platinum and gold at Dana North, Dana South and Lismer's Ridge. This estimate used a 0.7 g/t Pt+Pd cut-off grade, and can also be expressed as 12.7 million tonnes at 1.46 g/t Pt+Pd+Au. This can be broken down into 7.74 million tonnes at 1.60 g/t Pt+Pd+Au at Dana North and South, and 4.97 million tonnes at 1.24 g/t Pt+Pd+Au at Lismer's Ridge. The second Mineral Resource estimate (Pacific North West Capital Corporation, Press Release, October 17, 2002) was a total measured and indicated resource of 825 900 ounces palladium, platinum and gold. There were inferred resources of 200 600 ounces palladium, platinum and gold, yielding a total of 1 026 500 ounces at Dana North and Lismer's Ridge. This estimate used a 0.7 g/t Pt+Pd cut-off grade, and can also be expressed as 18.1 million tonnes of measured and indicated at 1.36 g/t Pt+Pd and 5.4 million tonnes inferred at 1.11 g/t Pt+Pd. Roughly 60% of this resource is in the Dana North area. A description of the Dana North zone has been published by S. Jobin-Bevans, Projects Manager for Pacific North West Capital Corporation, in James et al. (2002b). The description below, in a slightly smaller font, is derived from that published report. The distribution of the marginal Inclusion-bearing/Breccia zone (roughly equivalent to the Inclusion- and/or Fragment-bearing zone of Hrominchuk (2000) and James et al. (2002a)), the loci of mineralization, is in abrupt, intrusive contact to the east with the Neoarchean-age Pardo gneiss, and to the west these zones are replaced by a weakly layered to massive leucogabbro-gabbro-melagabbro sequence. Small pod-like, alkalic intrusions of unknown age are observed at two places along the intrusive contact where they displace the Inclusionbearing/Breccia Zone. As well, faults related to the Grenville Front and a 1238 Ma Sudbury swarm olivinemagnetite gabbro dyke cut or displace this zone. Silicate assemblages that host the mineralization in this part of the 71 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �River Valley intrusion exhibit upper greenschist facies mineralogy in contrast to the upper amphibolite facies and preserved magmatic assemblages that dominate the remainder of the intrusion. The stratigraphy of the Inclusionbearing/Breccia Zone is normally ~100 m wide in plan view. From the footwall Pardo gneiss, westward into the intrusion, the sequence and character of the distinguishable units are: 1. Footwall Breccia Unit: typically 5 to 15 m wide, but may be absent. It consists of partly rounded to angular, centimetre- to decimetre-size fragments of country rock (~75%; Pardo gneiss, Archean gabbro, diabase, diorite, minor Huronian Supergroup metasedimentary rocks) and River Valley intrusion material (~25%; chilled gabbro and medium-grained melagabbro) in a matrix of finer grained rock of similar composition and (or) an aplitic to granitic matrix. A narrow zone of migmatite at the contact of the intrusion is probably due to contact metamorphism, and granitic veins can be traced from this unit into the footwall at Lismer’s Ridge. Sulphide minerals are dominantly pyrite and pyrrhotite with local areas of trace to 1 volume percent chalcopyrite + pyrrhotite. Platinum group element concentrations are normally less than 25 ppb. 2. Boundary Unit: 5 to 20 m wide, but may be absent. It contains partly rounded to subangular, centimetre- and decimetre-size fragments of country rock (typically 10-25%), and cognate xenoliths of melagabbro, gabbro and less commonly leucogabbro to anorthosite in a matrix of gabbro to melagabbro ± aplite/granite, as in the Footwall Breccia. Sulphide minerals are mainly pyrite and pyrrhotite, locally up to 3 volume percent chalcopyrite + pyrrhotite occur; platinum group element contents are typically <75 ppb with local concentrations greater than 1000 ppb. 3. Breccia Unit: 20 m wide to greater than 100 m. It contains as much as 95% dominantly cognate xenoliths of gabbro to melagabbro and subordinate leucogabbro in a medium-grained matrix of similar composition; fragments are partly rounded to round probably due to partial assimilation, and centimetre to decimetre in size. Those greater than a metre are mainly footwall compositions (including Huronian Supergroup metasedimentary rocks) and tend to be larger with increasing proximity to the intrusive contact. Sulphide minerals (1 to 5 volume percent pyrrhotite + chalcopyrite) occur as both bleb and disseminated types; platinum group element contents are highly varied, but most values range from 500-6000 ppb with local concentrations greater than 10,000 ppb. 4. Inclusion-bearing Unit: 10 to 50 m wide. It contains >90% autoliths of leucogabbro, subordinate gabbro and less melagabbro in a matrix of either medium-grained leucogabbro or gabbro; the leucogabbro xenoliths are subangular to partly rounded, dominantly decimetre to metre in scale, and appear to be stoped inclusions from the adjacent (overlying) Leucogabbronorite zone. Sulphide minerals include trace to 3 volume percent pyrrhotite + chalcopyrite; platinum group element contents range from 100-500 ppb with local concentrations greater than 1000 ppb. Interestingly, the Breccia Unit, which shows the highest and most persistent sulphide-associated platinum group element mineralization, has the smallest proportion of footwall inclusions (<1%); perhaps an indication that chemical contamination from footwall lithologies is not a major controlling factor on mineralization. Fine-grained gabbro and diabase dikes cut all of the above units as well as the Leucogabbronorite zone in the main part of the intrusion. These dikes are metamorphosed at a grade similar to the intrusion in the Dana North area and are distinct from younger dikes of the Sudbury swarm. Drill hole data suggest that the dip of the contact of the Inclusion-bearing/Breccia zone with the footwall is at least 45° to 65° and toward the intrusion, whereas the apparent dip of the surface between the Inclusion-bearing Unit and Breccia Unit is steeper (~70° ) and toward the intrusive contact. The attitude of metre-scale layering in the Leucogabbronorite zone adjacent to the mineralized Inclusion-bearing/Breccia zone is poorly constrained but is estimated to be near-vertical (~70° to 90° west and east) and possibly shallowing (i.e. less than 70° west dip) westward into the intrusion. S/Se ratios for 13 of 15 samples (mineralized and barren) from the Dana North and Lismer’s Ridge areas have values from 500 to 2120, which are well within the magmatic range indicted by the Merensky and J-M reefs, and Konttijarvi-Portimo contact-type mineralization (i.e., 700-800 S/Se). Footwall rocks have low Pt, Pd and Au, and Pd/Pt and Cu/Ni are less than 1; these metal values indicate that the Archean footwall rocks are not genetically related to the platinum group element mineralizing event(s). Estimates of the metal values for barren magma that forms large parts of the intrusion or which are feeders, have less than 100 ppm Cu and anomalous Pt + Pd 72 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �concentrations averaging 32 to 35 ppb, but their metal ratios, Pd/Pt and Cu/Ni, are both less than 1, which is unlike mineralized samples (see below). Analytical data for sulphide-bearing felsic and mafic dikes and the Boundary, Breccia and Inclusion-bearing Units all show moderate to high average Pt+Pd (peaking in the Breccia Unit, as expected), and all show Pd/Pt and Cu/Ni ratios based on metal averages in the range 1.1-4.8, distinct from the low sulphide, Cu-poor assemblages. The mineralized zones at the Bull Frog Zone of the East Bull Lake Intrusion show the same geochemical features. River Valley Project -2004 • Dr,II Hole Locetono EHilIholeTrecen — — - lntrno,oe Content — — - Faults — Roads Trenches Lnke Figure 12. Map of the Dana North property of Pacific North West Capital Corporation, showing the location of stops 9A through 9F, as well as the extent of the drilling program undertaken between 1999 and 2002. 73 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Stop 9 is located on the Pacific North West Capital Corporation property, and consists of an examination of several outcrop areas that were exposed between 1999 and 2001 (see Figure 12). These outcrops provide an excellent opportunity to examine the contact of the River Valley intrusion, the mineralized zones and the character of the inclusion-bearing unit (Photo 13). Stop 9, Day 3, makes for an interesting comparison with Stop 11, Day 1, the traverse across the East Bull Lake mineralized contact in the Moon Lake area. STOP 9A. L6+00N, CONTACT ENVIRONMENT The line 6+00N clearing to the west of the road was completed in May 2000 in order to expose and sample the up-dip geology and mineralization intersected in diamond-drill hole RV00-08. This clearing provides excellent exposure of the stratigraphy in the contact environment. Walk to the western edge of the cleared area, and then walk back past diamond-drill hole collar RV00-08 toward the road. This will bring you from massive gabbronorite through the inclusion-bearing zone at the west end of the clearing through to the footwall Archean migmatite, paragneiss and gabbroic rocks at the east end of the clearing. The highest assay value from samples collected in this area is 10.1 g/t Au+Pd+Pt (sample L6N-09). Note that the footwall gneisses are pale green and washed out, reflecting a regional chloritic alteration zone, first noted by Lumbers (1973), which affects all rock units in northwest Dana Township and southwest Pardo Township. Walk north and across the access road for about 100 m to the next clearing to the east that intersects the road, this is grid line 7+00N. STOP 9B. L7+00N, CONTACT ENVIRONMENT The line 7+00N clearing was completed in May to June 2000 in order to expose and sample an extensive induced polarization chargeability anomaly outlined in this area. This clearing offers excellent exposure of the stratigraphy in the contact environment including numerous complex structural features that crosscut both country rocks and intrusive rocks. As on line 6+00N, walk to the western edge of the cleared area, then walk back toward the road. Diamond-drill hole collars RV00-23 and 24 are located at the north edge of the clearing, on your left. Pay special attention to the extensive east-northeast trending mylonite zone that cuts across the exposure. The highest assay value from samples collected in this area is 8.6 g/t Au+Pd+Pt (sample L7N-164). Walk south along the road for about 150 m to north end of large stripped outcrop ridge. Start by the rusty weathering part of outcrop closest to the road. STOP 9C. ROAD ZONE The Road zone exposes several different rock types of the River Valley intrusion, including massive gabbroic anorthosite and gabbro that “overlie” the inclusion-bearing and breccia units. A large, faulted felsic dike cuts the breccia zone and appears to originate from the footwall contact that is exposed toward the north end of the clearing. This area is one of the original discovery zones from the fall of 1998, with the remnants of the original exposed pit located at the extreme northeast corner of the clearing, alongside the road. Another cleared area, referred to as the Road zone east, is located immediately east of the road. The highest assay value from samples collected in this area is 13.4 g/t Au+Pd+Pt (sample RZ-361). 74 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Follow the ATV trail situated on the east side of the road by the Road zone and walk southeast. On the way you will pass by North zones 1, 2 and 3 on your left. After about 200 m (about grid line 3+00N), follow a drill road north (toward the left) for about 100 m (near diamond-drill hole collar RV00-05). The Central zone is located on a north-trending outcrop ridge. STOP 9D. CENTRAL ZONE The Central zone exposes the inclusion-bearing unit (northwest part of clearing) (Photo 13b, 13d) and the mafic breccia unit (southeast part of clearing). Note the large fragments in the outcrop at the northeast edge of the clearing and note the leucocratic nature of the matrix (see Photo 13b) for comparison to the melanocratic matrix you will see in the South zone (see Photo 13c). The highest assay value for samples collected from this zone is 9.1 g/t Au+Pd+Pt+Rh (sample CZ-54). r Retrace your route back to the ATV trail and continue south for roughly 250 m. • .-.. ..•, • Photo 13. Photographs illustrating the textural variation observed in the Inclusion-Bearing unit in the Dana North area (Day 3, Stop 9). a) Large, leucogabbronorite fragment hosted in a gabbronorite to pyroxenite matrix. Scale card is approximately 8.5 cm long. b) Gabbronorite to pyroxenite fragments hosted in a leucogabbronorite matrix in stripped outcrop at the central zone (stop 9D, Day 3). c) Mafic fragment and matrix-dominated portion of the Inclusion-Bearing unit at Dana South (stop 9F, Day 3). Hammer handle is approximately 35 cm long. d) Gabbronorite to pyroxenite fragments hosted in a leucogabbronorite matrix in stripped outcrop at the Central zone (stop 9D, Day 3). Marker is approximately 14 cm long. 75 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �STOP 9E. TRENCH ZONE The Trench zone is one of the original discovery areas from the fall of 1998, with one of the original trenches (circa 1969) still visible along the west edge of the clearing. This zone exposes the mafic breccia unit, which is best developed along the western edge of the clearing (Photo 13c). Note the “shotgunpattern” of sulphide mineralization “bleeding” in outcrop. A plagioclase-phyric dike (Matachewan swarm?) occurs in the sheared contact with footwall migmatitic paragneiss (altered Pardo gneiss) and is in sharp contact with rocks of the intrusion along the southern edge of the clearing. Samples from the dike have analyzed up to 818 ppb Au+Pd+Pt+Rh and 448 ppm Cu (sample TZ-03); finely disseminated sulphide mineralization is visible. Take time to look at the “fault breccia” located at the east edge of the clearing, just north of the dike. What is this breccia reminiscent of? The highest assay value from samples collected from this zone is 13.2 g/t Au+Pd+Pt+Rh (sample TZ-07). Keep walking south along the trail down the hill from the Trench zone and cross the stream (beaver dam) that connects Platadium Pond on the right with Dana Lake on the left. A worn bush trail will take you to the South zone clearing. Total distance from the Road zone to the South zone is about 650 m. STOP 9F. SOUTH ZONE The South zone is one of the original discovery areas from the fall of 1998. Initially, several small pits along the eastern and northwestern edges of the clearing were the only exposed areas of mineralized outcrop. It is in this area that one grab sample assayed greater than 12 g/t Au+Pd+Pt. This clearing exposes the mafic mineralized breccia unit along the east half of the outcrop and the Inclusion-Bearing zone along the western half. Note the presence of several northeast-trending diabase dikes that cut across stratigraphy. The highest assay value to date from samples collected in this area is 16.4 g/t Au+Pd+Pt+Rh (sample SZ-219). Note the presence of blue-grey quartz grains in the mafic rocks and its general association with sulphide mineralization, particularly in the area of the large blast pit. This ends Day 3. Return to vehicles and retrace route to Highway 805, head south on Highway 805 to River Valley, Warren and Highway 17. 76 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �References Ariskin, A.A., Frankel, M.Ya., Barmina, G.S., and Nielsen, R. 1993. COMAGMAT: A Fortran program to model magma differentiation processes; Computers and Geoscience, v.19, p.1155-1170. Ashwal, L.D. and Wooden, J.L. 1989. River Valley pluton, Ontario: A late-Archean/early Proterozoic anorthositic intrusion in the Grenville Province; Geochimica et Cosmochimica Acta, v.53, p.633- 641. Barnes, S.-J. and Maier, W.D. 1999. 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Kamineni, D.C., McCrank, G.F.D., Stone, D., Ejeckam, R.B., Flindall, R. and Sikorsky, R. 1984. Geology of the central plateau of the East Bull Lake pluton, northeastern Ontario; in Current Research, Part B, Geological Survey of Canada, Paper 84-1B, p.75-83. 80 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Kamineni, D.C., McCrank, G.F.D., Stone, D., Ejeckam, R.B. and Sikorsky, R. 1985. A preliminary report of alteration and fracture-filling mineralogy in the East Bull Lake pluton, District of Algoma, Ontario; in Current Research, Part B, Geological Survey of Canada, Paper 85-1B, p.81-88. Kamineni, D.C. 1986. A petrochemical study of calcic amphiboles from the East Bull Lake anorthosite-gabbro layered complex, District of Algoma, Ontario; Contributions to Mineralogy and Petrology, v.93, p.471-481. Kamo, S.L., Krogh, T.E. and Kumarapeli, P.S. 1995. Age of the Grenville dyke swarm, Ontario-Quebec: Implications for the timing of Iapetan Rifting; Canadian Journal of Earth Sciences, v.32, p.273-280. Keays, R.R. 1995. The role of komatiitic and picritic magmatism and S-saturation in the formation of ore deposits; Lithos, v.34, p.1-18. Keays, R.R., Nickel, E.H., Groves, D.I. and McGoldrick, P.J. 1982. Iridium and palladium as discriminants of volcanic-exhalative, hydrothermal and magmatic nickel sulfide mineralization; Economic Geology, v.77, p.1535-1547. Keays, R.R., Vogel, D.C., James, R.S., Peck, D.C., Lightfoot, P.C. and Prevec, S.A. 1995. Metallogenic potential of the Huronian-Nipissing magmatic province; The Canadian Mineralogist, v.33, p.932-933. Krogh, T.E., Davis, D.W. and Corfu, F. 1984. Precise U-Pb zircon and baddelyite ages for the Sudbury Structure; in Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p.431-446. Krogh, T.E., Corfu, F., Davis, D.W., Dunning, G.R., Heaman, L.M., Kamo, S.L., Machado, N., Greenough, J.D. and Nakamura, E. 1987. Precise U-Pb isotopic ages of diabase dykes and mafic to ultramafic rocks using trace amounts of baddeleyite and zircon; in Mafic Dyke Swarms, Geological Association of Canada, Special Paper 34, p.147-152. Krogh, T.E., Kamo, S.L., and Bohor, B.F. 1996. Shock metamorphosed zircons with correlated U-Pb discordance and melt rocks with concordant protolith ages indicate an impact origin for the Sudbury structure; in Earth Processes: Reading the Isotopic Code, American Geophysical Union, Geophysical Monograph 95, p.343-353. Le Maitre, R.W. 1989. A classification of igenous rocks and glossary of terms; Blackwell Scientific Publications, Oxford, United Kingdom, 193p. Lumbers, S.B. 1973. Geological series, River Valley area, districts of Nipissing and Sudbury; Ontario Division of Mines, Preliminary Map, P.844, scale 1:63 360. Lumbers, S.B. 1975. Geology of the Burwash area, Districts of Nipissing, Parry Sound, and Sudbury; Ontario Division of Mines, Geological Report 116, 158p. Lumbers, S.B. 1978. Geology of the Grenville Front Tectonic Zone in Ontario; in Toronto ’78, Field Trips Guidebook, Geological Society of America–Geological Association of Canada–Mineralogical Association of Canada, p.347-361. McBirney, A.R. 1996. The Skeargaard Intrusion; in Layered Intrusions, Elsevier, New York, Developments in Petrology, v.15, p.147-180. McCrank, G.F.D., Kamineni, D.C., Ejeckam, R.B. and Sikorsky, R. 1989. Geology of the East Bull Lake gabbroanorthosite pluton, Algoma District, Ontario; Canadian Journal of Earth Sciences, v.26, p.357-375. Meldrum, A., Abdel-Rahman, A.-F.M., Martin, R.F. and Wodicka, N. 1997. The nature, age and petrogenesis of the Cartier Batholith, northern flank of the Sudbury Structure, Ontario, Canada; Precambrian Research, v.82, p.265-285. 81 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �NACSN. 1983. North American Stratigraphic Code, 1983; Association of Petroleum Geologists, Bulletin, v.67, p.841-875. Noble, S.R. and Lightfoot, P.C. 1992. U-Pb baddeleyite ages of the Kerns and Triangle Mountain intrusions, Nipissing diabase, Ontario; Canadian Journal of Earth Sciences, v.29, p.1424-1429. Peach, C.L., Mathez, E.A. and Keays, R.R. 1990. Sulfide melt-silicate melt distribution coefficients for noble metals and other chalcophile elements as deduced from MORB: Implications for partial melting; Geochimica et Cosmochimica Acta, v.54, p.3379-3389. Peck, D.C., James, R. and Chubb, P. 1993. Geological environments for PGE-Cu-Ni mineralization in the East Bull Lake Gabbro-Anorthosite Intrusion, Ontario; Exploration and Mining Geology, v.2, p.85-104. Peck, D.C., James, R.S. and Chubb, P.T. and Keays, R.R. 1995. Geology, metallogeny and petrogenensis of the East Bull Lake intrusion, Ontario; Ontario Geological Survey, Open File Report 5923, 117p. Peck, D.C., Keays, R.R., James, R.S., Chubb, P.T. and Reeves, S.J. 2001. Controls on the formation of contact-type PGE mineralization in the East Bull Lake Intrusion; Economic Geology, v.96, p.559-581. Peck, D.C., Barrie, T., Turmel, R., James, R., Wood, P., Larrivierre, J. and Lapierre, K. 2000. Geology of the East Bull Lake intrusion and its contact-type PGE-Cu-Ni mineralization; Laurentian University SEG Student Chapter PGM Exploration Short Course Field Trip Guidebook, November 3, 2000, 38p. Prevec, S.A. 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanics near Sudbury, Ontario; unpublished PhD thesis, University of Alberta, Edmonton, Alberta, 223p. Prevec, S.A., James, R.S., Keays, R.R. and Vogel, D.C. 1995. Constraints on the genesis of Huronian magmatism in the Sudbury area from radiogenic isotopic and geochemical evidence; The Canadian Mineralogist, v.33, p.930932. Robertson, J.A. 1976. Geology of the Massey area, districts of Algoma, Manitoulin and Sudbury; Ontario Department of Mines, Geological Report 136, 130p. Rousell, D.H. and Trevisiol, D.D. 1988. Geology of the mineralized zone of the Wanapitei complex, Grenville Front, Ontario; Mineralium Deposita, v.23, p.138-149. Schandl, E.S., Gorton, M.P. and Davis, D.W. 1994. Albitization at 1700+/-2 Ma in the Sudbury–Wanapitei Lake area, Ontario; Canadian Journal of Earth Sciences, v.31, p.597-607. Smith, M.D. 2002. The timing and petrogenesis of the Creighton pluton, Ontario: an example of felsic magmatism associated with Matachewan igneous events; unpublished MSc thesis, University of Alberta, Edmonton, Alberta, 123p. Streckeisen, A. 1976. To each plutonic rock its proper name; Earth-Science Reviews, v.12, p. 1-33. Sullivan, R.W. and Davidson, A. 1993. Monazite age of 1747 Ma confirms post-Penokean age of the Eden Lake Complex, Southern Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 7, Geological Survey of Canada Paper 93-2, p.45-48. Sun, S.S, and McDonough, W F. 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle compositions and processes; in Magmatism in ocean basins, Geological Society of London, Special Publication 42, p.313-345. 82 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Tettelaar, T. 2000. Petrography and geothermobarometry of variably metamorphosed and deformed leucogabbronorites of the River Valley intrusion, Grenville Province, Ontario; unpublished BSc thesis, Carleton University, Ottawa, Ontario, 59p. van Breemen, O. and Davidson, A. 1988. Northeast extension of Proterozoic terranes of mid-continental North America; Geological Society of America Bulletin, v.100, p.630–638. Vogel, D.C. 1996. The geology and geochemistry of the Agnew intrusion: implications for the petrogenesis of early Huronian mafic igneous rocks in central Ontario, Canada; unpublished PhD thesis, University of Melbourne, v.1, 292p. Vogel, D.C. and Keays, R.R. 1997. Stratigraphic correlation and structure of the East Bull Lake intrusions: implications for the evolution of the Paleoproterozoic Huronian rift zone in Ontario; Geological Association of Canada, Program with Abstracts, v.22, p.A-154. Vogel, D.C., James, R.S. and Keays, R.R. 1998. The early tectono-magmatic evolution of the Southern Province: implications from the Agnew Intrusion, central Ontario, Canada; Canadian Journal of Earth Sciences, v.35, p.854-870. Vogel, D.C., Vuollo, J.I., Alapieti, T.T. and James, R.S. 1998. Tectonic, stratigraphic, and geochemical comparisons between ca. 2600-2440 Ma mafic igeneous events in the Canadian and Fennoscandian Shields; Precambrian Research, v.92, p.89-116. Vogel, D.C., Keays, R.R., James, R.S. and Reeves, S.J. 1999. The geochemistry and petrogenesis of the Agnew Intrusion, Canada: a product of S-undersaturated, high-Al and low-Ti tholeiitic magmas; Journal of Petrology, v.40, p.423-450. Wodicka, N. and Card, K.D. 1995. Late Archean history of the Levack gneiss complex, southern Superior Province, Sudbury, Ontario: New evidence from U-Pb geochronology; Precambrian ’95, Program with Abstracts, p.191. Young, G.M. 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes area; in Early Proterozoic Geology of the Great Lakes Region, Geological Society of America, Memoir 160, p.15-32. 83 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �Metric Conversion Table Conversion from SI to Imperial Conversion from Imperial to SI SI Unit Multiplied by Gives Imperial Unit 1 mm 1 cm 1m 1m 1 km 0.039 37 0.393 70 3.280 84 0.049 709 0.621 371 LENGTH inches 1 inch 25.4 inches 1 inch 2.54 feet 1 foot 0.304 8 chains 1 chain 20.116 8 miles (statute) 1 mile (statute) 1.609 344 mm cm m m km 1 cm@ 1 m@ 1 km@ 1 ha 0.155 0 10.763 9 0.386 10 2.471 054 AREA square inches 1 square inch square feet 1 square foot square miles 1 square mile acres 1 acre 6.451 6 0.092 903 04 2.589 988 0.404 685 6 cm@ m@ km@ ha 1 cm# 1 m# 1 m# 0.061 023 35.314 7 1.307 951 VOLUME cubic inches 1 cubic inch cubic feet 1 cubic foot cubic yards 1 cubic yard 16.387 064 0.028 316 85 0.764 554 86 cm# m# m# CAPACITY 1 pint 1 quart 1 gallon Multiplied by 1L 1L 1L 1.759 755 0.879 877 0.219 969 pints quarts gallons 1g 1g 1 kg 1 kg 1t 1 kg 1t 0.035 273 962 0.032 150 747 2.204 622 6 0.001 102 3 1.102 311 3 0.000 984 21 0.984 206 5 MASS ounces (avdp) 1 ounce (avdp) 28.349 523 ounces (troy) 1 ounce (troy) 31.103 476 8 pounds (avdp) 1 pound (avdp) 0.453 592 37 tons (short) 1 ton (short) 907.184 74 tons (short) 1 ton (short) 0.907 184 74 tons (long) 1 ton (long) 1016.046 908 8 tons (long) 1 ton (long) 1.016 046 90 1 g/t 0.029 166 6 1 g/t 0.583 333 33 CONCENTRATION ounce (troy)/ 1 ounce (troy)/ ton (short) ton (short) pennyweights/ 1 pennyweight/ ton (short) ton (short) Gives 0.568 261 1.136 522 4.546 090 L L L g g kg kg t kg t 34.285 714 2 g/t 1.714 285 7 g/t OTHER USEFUL CONVERSION FACTORS 1 ounce (troy) per ton (short) 1 gram per ton (short) 1 ounce (troy) per ton (short) 1 pennyweight per ton (short) Multiplied by 31.103 477 grams per ton (short) 0.032 151 ounces (troy) per ton (short) 20.0 pennyweights per ton (short) 0.05 ounces (troy) per ton (short) Note: Conversion factors which are in bold type are exact. The conversion factors have been taken from or have been derived from factors given in the Metric Practice Guide for the Canadian Mining and Metallurgical Industries, published by the Mining Association of Canada in co-operation with the Coal Association of Canada. 84 PDF compression, OCR, web-optimization with CVISION's PdfCompressor �PDF compression, OCR, web-optimization with CVISION's PdfCompressor �ISSN 0826--9580 ISBN 0--7794--5906--7 PDF compression, OCR, web-optimization with CVISION's PdfCompressor � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2006 Description An account of the resource Institute on Lake Superior Geology. Sault Ste Marie, Ontario. May 8-12, 2006. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2006 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/30b3fdc2335802234c13370066ca4fc8.pdf d8abacfe9bcc9e8d2baa6532033dc6cb PDF Text Text 53 ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 — 13,2007 PROCEEDINGS VOLUME 53 PART 1- PROGRAM AND ABSTRACTS �INSTITUTE ON LAKE SUPERIOR GEOLOGY 53RD ANNUAL MEETING MAY 8-13, 2007 LUTSEN, MINNESOTA HOSTED BY: LAUREL G. WOODRUFF AND JAMES D. MILLER, JR. Co-Chairs U.S. Geological Survey Minnesota Geological Survey Volume 53 Part 1 – Proceedings and Abstracts Edited by Laurel Woodruff, U.S. Geological Survey Cover Photos: Top - Lake Superior shoreline at Lutsen Resort; Right - Grand Portage National Monument stockade and Hay Bay; Left - High Falls on Pigeon River; Bottom - Suzie Islands (photographs courtesy of Bill Cannon) �53RD INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 53 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (FORMERLY NATHAN’S LAYERED SERIES) TRIP 2: GEOLOGIC AND CULTURAL HISTORY OF GRAND PORTAGE NATIONAL MONUMENT TRIP 3: MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER TRIP 4: GEOLOGY AND CU-NI-PGES MINERALIZATION – NICKEL LAKE MACRODIKE, SOUTH KAWISHIWI INTRUSION TRIP 5: GEOLOGY OF THE NORTH SHORE FROM LITTLE MARAIS TO GRAND MARAIS TRIP 6: GEOLOGY OF THE GUNFLINT TRAIL Reference to material in Part 1 should follow the example below: Hart, T.R. and MacDonald, C.A., 2007, Emplacement of the Nipigon Sill Complex and mafic to ultramafic intrusions of the Nipigon Embayment [abstract]; Institute on Lake Superior Geology Proceedings, 53rd Annual Meeting, Lutsen, MN, v. 53, part 1, p. 36-37. Published by the 53rd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 53 PART 1—PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2007.............................................................. iv Goldich Medal Committee................................................................................................. vi Past Goldich Medalists ...................................................................................................... vi Citation for 2007 Goldich Medal Recipient ..................................................................... vii Sam Goldich and the Goldich Medal............................................................................... viii ILSG Student Research Fund ..............................................................................................x Student Paper Awards........................................................................................................ xi Eisenbrey Student Travel Awards .................................................................................... xii Report of the Chair of the 52nd Annual Meeting ............................................................ xiii 2007 Board of Directors ....................................................................................................xv 2007 Session Chairs...........................................................................................................xv 2007 Student Paper Awards Committee ............................................................................xv 2007 Local Committees .....................................................................................................xv 2007 Banquet Speaker ..................................................................................................... xvi Program........................................................................................................................... xvii Poster Presentations ....................................................................................................... xxiii Abstracts ..........................................................................................................................xxv iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst iv �30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. v �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1993 Donald W. Davis 1980 not awarded 1994 Cedric Iverson 1981 Carl E. Dutton, Jr. 1995 Gene La Berge 1982 Ralph W. Marsden 1996 David L. Southwick 1983 Burton Boyum 1997 Ronald P. Sage 1984 Richard W. Ojakangas 1998 Zell Peterman 1985 Paul K. Sims 1999 Tsu-Ming Han 1986 G.B. Morey 2000 John C. Green 1987 Henry H. Halls 2001 John S. Klasner 1988 Walter S. White 2002 Ernest K. Lehmann 1989 Jorma Kalliokoski 2003 Klaus J. Schulz 1990 Kenneth C. Card 2004 Paul Weiblen 1991 William Hinze 2005 Mark Smyk 1992 William F. Cannon 2006 Michael G. Mudrey 2007 GOLDICH MEDAL RECIPIENT Joseph Mancuso Bowling Green State University Bowling Green, Ohio GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Tom Hart (2004-2007) Doug Duskin (2005-2008) Richard Ojakangas (2006-2009) vi Government representative Industry representative Academic representative �CITATION FOR GOLDICH MEDAL RECIPIENT Joseph Mancuso, 2007 Goldich Medal Recipient Membership in the Institute of Lake Superior Geology is composed of government personnel who produce maps and other essential services; exploration geologists who walk the bush looking for ore deposits, and university professor who teach geology to future government personnel, exploration geologists, and professor. Membership also includes students who are learning the dimensions of the profession. In addition to teaching, Joe was an active exploration geologist during the 40 years he taught at Bowling Green State University. Joe has been an active member of ILSG for over 50 years. He attended the first meeting when he was an undergraduate student at Carleton College. During the past 50 years he has contributed more than 20 oral presentations and posters to ILSG meetings, plus 42 published papers and abstracts on the geology of the Lake Superior region. He also direct 39 MS theses on the geology of the Lake Superior region, as well as conducting annual field trips for his students to examine Precambrian geology in the field and in mines in Michigan, Wisconsin, and Ontario. Joe has produced some outstanding students. One brought two gold mines into production; another induced Freeport to drill Grasberg Mountain (4100 meters elevation) in Irian Jaya, Indonesia. Grasberg is now the largest single gold producer in the world in terms of ounces and revenue per year. It is also a high profitable porphyry copper mine (225,000 tons per day milled) that has produced the cash to Freeport to purchase PhelpsDodge. Another student recognized the alteration associated with a blind VMS deposit in Manitoba and directed a successful drilling program. He was recognized for this achievement by being named "Prospector of the Year" by the Prospectors and Developers Association of Canada. Another student collected the grab samples on the J-M platinum/palladium layer in the Stillwater layered Igneous Complex in Montana. This layer now supports two mines: Stillwater and Boulder. Finally, two of his students did the grab and development sampling at the Jarrett Canyon gold mine prior to being brought into production. Joe, you earned the Goldich award. Submitted by Ron Seavoy vii �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received viii �the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL ix �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Funs with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2006 the ILSG Board of Governors awarded the first three $500 awards from the Student Research Fund. This year awards were made to: Cole Edwards (University of Wisconsin - Oshkosh) - Controls on the formation of the earliest marine phosphate deposits, Marquette Supergroup, Michigan Noah Planavsky (Rosenstiel School of Marine and Atmospheric Sciences Marine Geology and Geophysics, Miami) - Iron isotopes as oceanographic tracers in Animikie Basin Iron Formations Michael Taylor (University of Minnesota - Duluth) - Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin x �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2006 the ILSG Student Paper Committee presented four awards from the ILSG Student Paper Fund. The awards were made to: Patrick Moran -- Lakehead University, Thunder Bay, ON (best oral presentation) -- $250 Noah Planavsky -- Lawrence University, Appleton, WI (oral presentation, honorable mention) -- $50 Amanda Gross -- Kent State University, Kent, OH (best poster) -- $250 Ryan Bartingale -- University of Wisconsin - Eau Claire, Eau Claire, WI (poster, honorable mention) -- $50 xi �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2006 the ILSG awarded six travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Peggy Stonier -- Kent State University, Kent, Ohio Amanda Gross -- Kent State University, Kent, Ohio Jenny Murphy -- Lawrence University, Appleton WI Noah Planavsky -- Lawrence University, Appleton WI Clare Stielstra -- Lawrence University, Appleton WI Davis Voights -- Lawrence University, Appleton WI xii �REPORT OF THE CHAIRS OF THE 52ND ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY SAULT STE MARIE, ONTARIO The Ontario Geological Survey hosted the 52nd Annual Institute on Lake Superior Geology meeting on May 10-11 2006 in Sault Ste Marie, Ontario. The meeting consisted of two days of technical sessions with two pre-meeting and four post-meeting field trips. Pete Hollings and Mike Mudrey provided invaluable technical assistance with the web site and Anthony Pace provided on site assistance with the audio-visual component of the program. Gerry Bennett, Mike Easton, Mike Hailstone and Anthony Pace provided assistance with field trip preparation. Vivienne Coté and Roger Poulin (Ontario Prospectors Association) and Marc Gaudreau and Anthony Pace (Ontario Geological Survey) provided aid with the loan, retrieval, delivery and set-up of the poster boards for the poster session. Lisa Bagnall, Scheduling Office, was the on site co-ordinator with the Sault College of Applied Arts and Technology. Total registration for the meeting was 110 students and professionals. Proceedings Volume 52 was published in six parts: Part 1 – Proceedings and Abstracts, edited by A. C. Wilson contained abstracts for 19 oral presentations and 21 posters; Part 2 – Glacial Lakes Algonquin and Nipissing Shoreline Bedrock Features: Mackinac Island, Michigan – Field Trip Guidebook; Part 3 – Unusual Diamond-bearing breccias of the Wawa Area – Field Trip Guidebook; Part 4 – The Huronian Supergroup between Sault Ste Marie and Elliot Lake – Field Trip Guidebook; Part 5 – Keweenawan Rocks of the Mamainse Point Area – Field Trip Guidebook and Part 6 – Geological Guidebook to the Paleoproterozoic East Bull Lake Intrusive Suite Plutons at East Bull Lake, Agnew Lake and River Valley, Ontario – Field Trip Guidebook. All guidebook volumes were edited by R. P. Sage. Last held in Sault Ste Marie, Ontario in 1974, a return visit to the area allowed participants to take part in a smorgasbord of new field trips. On Monday, May 8, Gerry Bennett, assisted by Mike Hailstone, led a two-day field trip to examine the classic stratigraphy of the Huronian Supergroup in the Elliot Lake and Searchmont areas. The next day, Ann Wilson introduced a rain-soaked but enthusiastic crowd of participants to the unusual diamond-bearing breccias of the Wawa area. Following the conference four field trips were held: Ann Wilson reprised the Wawa field trip; Tom Hart and Anthony Pace’s trip examined the Keweenawan rocks along the Lake Superior shoreline; Ron Sage led a small group of bicyclists to Mackinac Island to examine bedrock exposures there; and Mike Easton and Dick James (Laurentian University) took a group east of the Sault to investigate a suite of intrusive rocks along the boundary between the Archean Superior and the Proterozoic Southern provinces. Eighty-five participants attended the Annual Banquet. Dr. Ed Walker gave the after dinner key note address providing his insight into the “Emplacement and Geochemistry of Archean-aged diamondiferous rocks of the Wawa Area”. Jim Miller presented the 2006 Goldich Medal to Mike Mudrey for his 30 years of contributions to the understanding of the regional geology and for his service to the Institute. The student paper committee consisted of Dan England, John Klasner and Norm Trowell all of whom conscientiously deliberated over the four oral presentations and four poster presentations. The winners were: xiii �2006 Best Student Paper Awards 1) Patrick Moran – Lakehead University ($250, winner best oral presentation) 2) Amanda Gross – Kent State University ($250, winner best poster presentation) 3) Noah Planavsky – Lawrence University ($50, honourable mention oral presentation) 4) Ryan Bartingale – University of Wisconsin ($50, honourable mention poster presentation) In addition, Eisenbrey travel awards were presented to: 1) 2) 3) 4) 5) 6) Noah Planavsky – Lawrence University ($150) Jenny Murphy – Lawrence University ($150) Clare Stielstra – Lawrence University ($150) Davis Voights – Lawrence University ($150) Penny Stonier – Kent State University ($200) Amanda Gross – Kent State University ($200) The Institute’s Board of Directors met on May 10, 2006 and a brief summary of the meeting follows. 1. Accepted the Report of the Chairs for the 51st ILSG from Mark Smyk and Pete Hollings, and the minutes of the last Board meeting, May 26, 2006 from Pete Hollings. 2. Received, discussed and accepted the 2005-06 ILSG Financial Summary from ILSG Treasurer, Mark Jirsa. 3. Approved Ann Wilson as the on-going board member. 4. Approved 2007 (53rd annual) meeting location as Lutsen, Minnesota with Laurel Woodruff as chair. 5. Replaced George Hudak, academic member, on the Goldich Committee with Dick Ojakangas. The 52nd meeting was not without its challenges, including the last minute resolution of a strike at Ontario’s Colleges. However, no challenges were insurmountable and the meeting and field trips went off without a hitch. We thank all individuals who contributed to the success of the meeting, including the staff of Sault College, the bus drivers from AJ Bus Lines Limited and Reid Bus Line, volunteer field trip leaders and drivers and the Ontario Geological Survey. The field trips were astonishingly well attended and we are grateful for the kind feedback that we received from the field trip participants. Both of us were extremely satisfied with the 52nd meeting. We appreciated all of the positive feedback from the delegates who enjoyed a return visit to the Sault. As always, we wouldn’t have succeeded without the continuing support of the ILSG members. Respectfully submitted Ron Sage and Ann Wilson Co-Chairs, 52nd ILSG Meeting xiv �2007 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Laurel Woodruff and Jim Miller, Co-Chairs 53rd meeting (2010) U.S. Geological Survey, St. Paul, MN Minnesota Geological Survey, Duluth, MN Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Mark Smyk (2008) Ministry of Northern Development and Mines, Thunder Bay, ON Steve Hauck (2007) Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN Peter Hollings – Secretary (2007) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2009) Minnesota Geological Survey, St. Paul, MN 2007 SESSION CHAIRS Ted Bornhorst, Michigan Technological Survey, Houghton, MI Mary Louise Hill, Lakehead University, Thunder Bay, ON Gordon Medaris, Jr., University of Wisconsin, Madison, WI Klaus Schulz, U.S. Geological Survey, Reston, VA Thomas Waggoner, retired, Cliffs Mining Services Co., Ishpeming, MI Ann Wilson, Ontario Geological Survey, South Porcupine, ON 2007 STUDENT PAPER COMMITTEE Marcia Bjornerud (Chair), Lawrence University, Appleton, WI Daniela Vallini, Woodside Energy, Ltd., Perth, Western Australia Graham Wilson, Magma Metals, Campbellford, ON 2007 LOCAL COMMITTEES General Co-Chairs Laurel G. Woodruff – U.S. Geological Survey, St. Paul, MN James D. Miller, Jr. – Minnesota Geological Survey, St. Paul, MN Program and Abstracts Editor Laurel G. Woodruff -- U.S. Geological Survey, St. Paul, MN Field Trip Guidebook Editor James D. Miller, Jr. – Minnesota Geological Survey, St. Paul, MN Acting Local Committee, Lutsen, MN Gretchen Klasner – Marquette, MI xv �2007 BANQUET SPEAKER Don Hunter PolyMet Mining, Inc. Hoyt Lakes, MN PolyMet’s NorthMet Project – the long road from exploration to production xvi �PROGRAM xvii �TUESDAY MAY 8, 2007 8:00 a.m. FIELD TRIP 1: IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (FORMERLY NATHAN’S LAYERED SERIES) Jim Miller, Minnesota Geological Survey Eric Jerde, Morehead State University WEDNESDAY MAY 9, 2007 8:00 a.m. FIELD TRIP 2: GEOLOGIC AND CULTURAL HISTORY OF GRAND PORTAGE NATIONAL MONUMENT Bill Cannon, U.S. Geological Survey Brian Phillips, Lakehead University David Cooper, National Park Service 8:00 a.m. FIELD TRIP 3: MIDCONTINENT RIFT-RELATED INTRUSIONS NORTH OF THE INTERNATIONAL BORDER Mark Smyk, Ministry of Northern Development and Mines Peter Hollings, Lakehead University 6:00 p.m. Return of Trips 1, 2 and 3 4:00 p.m. - 8:00 p.m. Registration 7:00 p.m. - 9:00 p.m. Ice Breaker Social and Poster Setup THURSDAY MAY 10, 2007 Note: Asterisk * denotes a student eligible for Best Student Paper Award Presenter underlined 8:00 a.m. - 9:00 a.m. REGISTRATION 8:25 a.m. INTRODUCTORY REMARKS Laurel Woodruff and Jim Miller, Co-Chairs, 2007 ILSG TECHNICAL SESSION I Session Chair: Klaus Schulz, U.S. Geological Survey 8:30 a.m. Addison, William D., Cannon, William F. and Brumpton, Gregory R. How to identify Sudbury impact ejecta in the Lake Superior Region 8:50 a.m. Cannon, William F. and Addison, William D. The Sudbury impact layer in the Lake Superior iron ranges: A time-line from the heavens 9:10 a.m. Burton, Justin and Fralick, Philip Deposition and cementation of Paleoproterozoic Gunflint Formation carbonate: Implications for early hydrosphere chemistry xviii �9:30 a.m. Planavsky, Noah* and Murphy, Jennifer Rare earth element patterns in Steep Rock Carbonates 9:50 a.m. - 12:00 p.m. COFFEE BREAK AND EXTENDED POSTER SESSION 12:00 p.m. Lunch Break – 2007 ILSG Board Meeting (by invitation) TECHNICAL SESSION II Session Chairs: Mary Louise Hill, Lakehead University Gordon Medaris, Jr., University of Wisconsin, Madison 1:30 p.m. Green, John C. and Slade, Andrew A State Scientific and Natural Area and a North Shore non-profit association 1:50 p.m. Boerboom, Terry J. Newly recognized thick interflow sandstones in the upper northeast limb of the North Shore Volcanic Group, Minnesota 2:10 p.m. Mudrey, Michael G., Jr. and Wooden, Joseph L. A chemical and Sr isotopic study of the Pigeon Point Sill, Cook County, Minnesota 2:30 p.m. Bjornerud, Marcia Evidence for paleoseismic events during closure of the Midcontinent Rift, Atkins Lake-Marenisco Fault, N. Wisconsin 2:50 p.m. Hudak, George J., Hoffman, Adam T., Peterson, Dean M. and Heine, John Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota 3:10 p.m. COFFEE BREAK AND POSTER SESSION 3:30 p.m. Puumala, Mark A. New insights into the metallogeny of the eastern portion of the Archean Uchi Domain, Superior Province, Ontario 3:50 p.m. Alexander, Malcolm* and Mitchell, Roger H. Rare metal mineralization in pegmatites of the Coldwell Alkaline Complex 4:10 p.m. Good, David and Walford, Phillip PGE-rich mineralization at the Marathon Deposit, Coldwell Alkaline complex, Ontario xix �4:30 p.m. Brown, Alex C. Deposition of native copper lodes on the Keweenaw Peninsula, northern Michigan, from a gravity-driven evolved meteoric brine 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 54th Annual Meeting Location • 2007 Goldich Award Presentation to Joseph Mancuso • 2007 Banquet Address Meeting participants who are not registered for the banquet are welcome to the banquet address FRIDAY MAY 11, 2007 8:25 a.m. INTRODUCTORY REMARKS Laurel Woodruff and Jim Miller, Co-Chairs TECHNICAL SESSION III Session Chairs: Thomas Waggoner, retired, Cliffs Mining Services Co. Ann Wilson, Ontario Geological Survey 8:30 p.m. Student, James J., Wark, David A., Mutchler, Scott R. and Bodnar, Robert J. Thermal evolution of Proterozoic (>1Ga) rhyolite magma based on analysis of melt inclusions and trace elements in quartz from the Keweenaw Peninsula of Michigan 8:50 p.m. Hart, Tom R. and MacDonald, Carol Anne Emplacement of the Nipigon Sill Complex and mafic to ultramafic intrusions of the Nipigon Embayment 9:10 p.m. Hollings, Pete, Smyk, Mark C. and Hart, Tom Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: new insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes 9:30 p.m. Forsha, Clint J.* and Zieg, Michael J. Textural stratigraphy of Nipigon diabase sills: A tool for correlation and petrologic interpretation 9:50 p.m. Zieg, Michael J. and Forsha, Clint J. A comparison of textural profiles in diabase sills from the Midcontinent and Transantactic rift 10:10 a.m. COFFEE BREAK AND POSTER SESSION xx �10:30 p.m. Cockerton, Sarah, Conly, Andrew G. and Lee, Peter An experimental water-rock interaction study into the origin of high-sulfate waters associated with the Steep Rock Mines, Atikokan, Ontario 10:50 p.m. Stevens, Larissa B.* and Fralick, Philip Investigation of ferromanganese nodule precipitation and arsenic uptake in modern lacustrine biochemical sediments 11:10 a.m. Rodengen, Tommy*, Theissen, Kevin and Sugita, Sugita Elemental and isotopic shallow lake proxies of landscape changes in the Prairie Pothole region of Minnesota 11:30 a.m. Diedrich, Tamara* and Sharp, Thomas G. The effect of H2O on olivine to ringwoodite transformation: Implications for subduction zone dynamics and the deep earth water cycle 11:50 a.m. Thorliefson, Harvey Potential use of the Midcontinent Rift for CO2 sequestration 12:10 p.m. LUNCH BREAK TECHNICAL SESSION IV Session Chair: Ted Bornhorst, Michigan Technological University 1:40 a.m. McSwiggen, Peter L. Trace element analyses: Avoiding data distortion 2:00 p.m. Waggoner, Tom D. Definition of the Proterozoic terrain under the Paleozoic – Central U.P., Michigan 2:00 p.m. Oreskovich, Julie Documenting underground mine workings on the Mesabi Iron range in GIS format 2:40 p.m. Metsaranta, Riku T., Fralick, Philip W. and Bowdidge, Colin Metamorphosed halite-dominated evaporates of the Lower Sibley Group 3:00 p.m. Presentation of Student Paper Awards Marcia Bjornerud, Lawrence University: Student Paper Committee 3:10 a.m. FINAL COFFEE BREAK AND POSTER SESSION – POSTERS TO BE REMOVED AFTER THE BREAK xxi �SATURDAY MAY 12, 2007 8:00 a.m. FIELD TRIP 4: GEOLOGY AND CU-NI-PGES MINERALIZATION – NICKEL LAKE MACRODIKE, SOUTH KAWISHIWI INTRUSION Dean Peterson, Natural Resources Research Institute Paul Albers, Duluth Metals 8:00 a.m. FIELD TRIP 5: GEOLOGY OF THE NORTH SHORE FROM LITTLE MARAIS TO GRAND MARAIS Terry Boerboom, Minnesota Geological Survey John Green, University of Minnesota - Duluth Jim Miller, Minnesota Geological Survey 8:00 a.m. FIELD TRIP 6: GEOLOGY ALONG THE GUNFLINT TRAIL Mark Jirsa, Minnesota Geological Survey Paul Weiblen, University of Minnesota – Twin Cities 6:00 p.m. Return of Trips 5 & 6 SUNDAY MAY 13, 2007 6:00 p.m. Return of Trip 4 xxii �POSTER PRESENTATIONS Behling, Stuart J. Mercury in fish: a geologist’s perspective Boerboom, Terry J., Green, John C., Albers, Paul and Miller, James D., Jr. Bedrock geologic map of the Little Marais, Schroeder, and Tofte 7.5 minute quadrangles, North Shore of Lake Superior, Minnesota Boisjoli, Troy* and Flood, Timothy P. Petrographic differentiation of the five phases of the Lower Cretaceous Star Kimberlite, Saskatchewan, Canada Buchholz, Thomas W., Falster, Alexander U. and Simmons, William B. A novel locality for pseudobrookite – the Nine Mile-Pluton, Marathon County, Wisconsin Costello, Daniel E.*, Flood, Timothy P. and Thole, Jeffrey T. Origin of a mafic pegmatite within the Duluth Complex, Northern Minnesota Dean, J. Frederick and Phillips, Brian A.M. Distribution of certain ichthyofauna in relation to eastern outlets of Lake Agassiz, with emphasis on the Gunflint-Arrow Lakes corridor and the Keating Complex Elsenheimer, Don, Frey, Barry A. and Hudak, Joe N. Gold mineralization in the Virginia Horn Greenstone Terrain, St. Louis County, Minnesota: A prospect re-visited Hogan, Amanda*, Jacobs, Travis* and Theissen, Kevin Lacustrine sedimentary organic matter proxies of recent lake state changes and climatic conditions on Christina and Morrison Lakes of western Minnesota Hudak, Joe N. and Frey, Barry A. Evaluation of mineral exploration drill cuttings in the Rice River area, east central Minnesota Jasinevicius, Renata R.* and Gordon, Elizabeth A. Paleoenvironmental interpretation of a Lower Paleozoic stromatolite reef, northeastern Wisconsin Jirsa, Mark A., Boerboom, Terry J., Chandler, Val W., Lively, Richard S., Miller, James D., Jr., Mossler, John H., Runkel, Anthony C., Setterholm, Dale R. and Wahl, Timothy E. Proposed new bedrock geologic map of Minnesota Medaris, L. Gordon, Jr., Fournelle, John H. and Guggenheim, Stephen J. An occurrence of agrellite in the Wausau Alkaline Igneous Complex, Marathon County, Wisconsin xxiii �Nicholas, Sarah L.*, Wirth, Karl R., Engstrom, Jennifer and Lapakko, Kim A. Investigations of sulfide minerals leached in the presence of alkaline solids Quigley, Patrick O.* Michigan kimberlites revisited: New mineral, chemical and petrographic analyses Rymaszewski, Jody A.*, Friedrich, Jason L. and Czeck, Dyanna M. Bedrock fractures in southeastern Wisconsin: Paleostress estimates and relationships to the Waukesha Fault Saxton, Samantha* and Cordu, William Precambrian geology of the Opelt Quarry, Neillsville, Wisconsin Severson, Mark J. and Heine, John Revised stratigraphy of the Biwabik Iron Formation, Mesabi Range, Minnesota – developing the “Rosetta Stone” Stott, Greg, Corkery, Tim, LeClair, Alain, Boily, Michel and Percival, John A revised terrane map for the Superior Province as interpreted from aeromagnetic data Taylor, Michael L.* and Swenson, John B. Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin Theriault, Stephanie A.* and Hickson, Thomas A. Possible influence of a buried fault on elevated indoor radon levels, south Washington County, Minnesota Thorliefson, L. Harvey, Harris, Kenneth L., Hobbs, Howard C., Jennings, Carrie E., Knaeble, Alan R., Lively, Richard S., Lusardi, Barbara A. and Meyer, Gary N. Till geochemical and indicator mineral reconnaissance of Minnesota xxiv �ABSTRACTS xxv ��HOW TO IDENTIFY SUDBURY IMPACT EJECTA IN THE LAKE SUPERIOR REGION Addison, William D., R.R. 2, Kakabeka Falls, Ontario, POT 1W0, Canada Cannon, William F., U.S. Geological Survey, MS 954, Reston VA 20192, USA Brumpton, Gregory R., 211 Henry St., Thunder Bay, Ontario, P7E 4Y7, Canada For at least 15 years geologists looked for ejecta from the 1850 Ma Sudbury impact in sedimentary rocks of the Lake Superior region. Addison and others (2005) first reported its occurrence but more recent work has shown that “volcaniclastic breccia” or “submarine slump deposits” described and mapped as early as the 1940’s are, in fact, ejecta deposits and other impact-related rocks (Cannon and Addison, 2007). To date, Sudbury ejecta deposits have been identified in six areas in the western Lake Superior region and provide a precise regional stratigraphic marker (Cannon and Addison, 2007). It seems nearly certain that other sites will be discovered. Toward that end we pass on some of our observations and experience in the commonly frustrating search for the impact layer in hopes that others will make additional discoveries of this important chrono-stratigraphic marker bed. Ejecta, the material thrown upward and outward from the Sudbury impact, arrived in the Lake Superior region in two ways. First, was coarse to fine material, primarily melted and solid target rock, that followed a ballistic trajectory to about five times the final crater radius (5r), most recently estimated at 130 km (Spray and others, 2004). Thus, ballistic ejecta should not be expected beyond 650 km from the impact center. Five of the six known occurrences lie within the 5r distance. At those sites ballistic ejecta arrived with horizontal velocities as high as a few km/sec, and were capable of causing extreme disruption and erosion of surface sediments. The result was breccia deposits of ejecta and local country rock. Ejecta inside the 5r ballistic zone include devitrified glass in vesicular blobs (both streamlined and irregular shapes) and microtektites and spherules (frozen melt droplets), both singly and in clusters. Other features include accretionary lapilli, and angular grains of quartz, a small percentage of which contain relict planar deformation features, the only proof that the ejecta are of impact origin (French, 1998). Second, a huge super-heated expanding cloud of vapor, sub-mm melt droplets, and sub-mm rock shards reached hundreds of km above Earth. Condensed and solidified melt droplets and tiny rock shards settled over a period of days to more than a year covering all of Earth. These ejecta do not substantially alter the surface on which they are deposited and do not form the breccias typical of ballistic ejecta. Microtektites and spherules and quartz and feldspar grains, some showing shock metamorphic features, from the cloud have been found at sites near Hibbing, Minnesota, beyond the range of ballistic ejecta. Finally, most of the ballistic deposits and perhaps some of the earlier vapor cloud deposits have been reworked by impact-induced tsunamis resulting in polymict breccias of local rocks and ejecta (Cannon and Addison, 2007). 1 �So, how and where should you look for a Sudbury ejecta deposit and how will you recognize it? Known sites are near the top of the principal iron-bearing units (Gunflint, Biwabik, Ironwood, etc.) in the region to about 300 m farther up section and new discoveries will most likely be within or not far from this interval. Within the 5r zone, easily seen macroscopic features include breccia, especially black chert breccia, mm-cm size blobs of devitrified vesicular glass, and clusters or beds of accretionary lapilli. Smaller-scale features include 0.5-2 mm gray ovoid, round, or irregular microtektites and spherules that may also be replaced by green clay minerals. Similar sized rounded quartz and feldspar grains occur along with sub-mm angular quartz and feldspar grains, a few of which contain one or more sets of thin parallel lamellae, the diagnostic shock deformation features. Note that these lamellae are very rare and commonly require hours or days of careful petrographic examination under high magnification to find. Beyond the 650 km ballistic zone, represented by only the sites near Hibbing, MN, there are no characteristic macroscopic features. Ejecta at the top of the Biwabik Iron-formation are within a siliceous, light gray coarsely recrystallized carbonate layer. Within the light carbonate there is a darker gray reticulated pattern giving a vague appearance of 2-4 mm circles. Thin sections from the reticulated zone reveal microtektites, spherules and vesicular glass. Rounded quartz grains are present as are angular quartz and feldspar. Shocked quartz and feldspar grains with the relict lamellae are exceedingly rare. All known localities are in pristine to weakly metamorphosed rocks. Unfortunately, the considerable regions of higher grade metamorphism in the Lake Superior region may never yield any of the features described here, particularly the diagnostic shock lamellae in quartz, because of thorough recrystallization. Certainly this is true at Peter Mitchell Mine, Babbitt, MN. Searches in weakly metamorphosed sequences are most likely to succeed. References: Addison, W. D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Cannon, W. F. and Addison, W.D., 2007, The Sudbury impact layer in the Lake Superior iron ranges: a time-line from the heavens: Institute on Lake Superior Geology Proceedings, Vol. 53, Part 1. French, B.M., 1998, Traces of catastrophe: Lunar Planetary Inst. Contrib. 954, 120 p. Spray, J. G., Butler, H. R., and Thompson, L. M., 2004, Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling: Meteoritics & Planetary Sci., V. 39, p. 287-301. 2 �Rare Metal Mineralization in Pegmatites of the Coldwell Alkaline Complex Malcolm Alexander and Roger H. Mitchell Department of Geology, Lakehead University, Thunder bay, Ontario ABSTRACT The Coldwell Complex (northwestern Ontario), is a multiphase alkaline intrusion that is host to rare earth element, actinide and other high field strength element mineralization. Preliminary studies have shown that these minerals are concentrated in pegmatites associated with Center 1 ferrorichterite-ferroaugite syenites. These syenites exhibit the geochemical characteristics of A-type granitoid magmatism. The focus of this study is the distribution, nature and petrochemical controls of rare metal mineralization within these pegmatitic bodies. Within the Center 1 formation there are two subunits; the border gabbro, a massive to layered intrusion ranging in composition from wehrlite to diorite and locally to anorthosite; and oversaturated massive to locally layered ferroaugite syenites. Both subunits host pegmatitic quartz bearing residua characterized by cumulus perthitic alkali feldspar, hedenbergite-aegirine pyroxenes, with interstitial quartz, calcite, and calcic-tosodic-calcic-to-sodic amphiboles (Fig. 1). The pegmatitic residua are of the niobiumyttrium-fluorine (NYF) type, and characterized by the enrichment of Nb-Ti-Ta oxides, with niobium predominant over tantalum. Back-scattered electron petrography and quantitative X-ray spectrometry have been used to characterize the mineral paragenesis. Pegmatitic syenitic residua emplaced in, but not derived from, the border gabbro contain a wide range of rare element minerals which include: chevkinite; fergusonite; monazite; allanite; kainosite; xenotime; REE fluorocarbonates: bastnaesite, synchysite and parisite. Other rare element enriched minerals include fersmite, apatite, zirconolite, U-Th-Sipyrochlore, Nb-rutile and Nb-bearing ilmenite. Early-formed rare element minerals such as chevkinite and pyrochlore are commonly replaced by complex aggregates of laterforming phases. REE-fluorocarbonates commonly exhibit syntaxial intergrowths. Other ferroactinolite-quartz bearing pegmatitic residua, occurring in the upper series of a differentiated aenigmatite-ferroaugite syenite unit, contain a more limited range of rare element minerals, including zircon, xenotime, monazite and the fluorocarbonates together with REE bearing apatite and pyrochlore. The differences in rare element mineralization between the two pegmatites examined to date suggest that they were derived from different batches of ferroaugite syenite magma. Initial data indicate that different pegmatitic residua in the Center 1 subunits also evolved differently. Intensive parameters have been estimated using amphibole mineralogy and Fe-Ti-oxide compositions, as seen in Figure 2. These parameters indicate the pegmatites intruding the border gabbro and syenite residua had equilibration temperatures of 450 ºC at oxygen fugacities of +0.5-1.0 and +2.0-2.2, relative to QFM, respectively. 3 �Calcic Amphiboles 1.0 Tremolite 0.8 Actinolite Magnesiohornblende Magnesiotschmakite 0.6 0.4 0.2 FerroFerrohornblende Ferrotschermakite actinolite Mg/(Mg + Fe2+) Mg/(Mg + Fe2+) 1.0 Sodic-Calcic Amphiboles 0.0 0.8 Richerite Magnesiokatotophorite Magnesiotaramite 0.6 0.4 Ferro0.2 richterite Katophorite Taramite 0.0 8.0 7.5 7.0 6.5 6.0 5.5 8.0 Si in Formula 7.5 7.0 6.5 6.0 5.5 Si in Formula A B Figure 1: Stability fields of calcic (A) and sodic-calcic (B) amphiboles. Each point represents a mean value of 5 to 9 point analyses. Circles represent the residua injected into the border gabbro, while triangles represent residua injected into the saturated ferroaugite syenite. A B Figure 2: Oxygen fugacity diagrams, using the quartz-fayalite-magnetite (QFM) oxygen buffer. Diagram A contains two points, the right hand value is representative of the ferroaugite syenite and the left hand value is representative of the pegmatitic residua (Mitchell and Platt, 1977). Diagram B is a Fe – Ti oxide geothermometer and oxygen barometer. ILM is given in mol% ilmenite, USP is mol% ulvöspinel. Triangles are saturated ferroaugite syenite residua, circles are border gabbro residua. Adapted from Ghiorso and Sack (1991). REFERENCES Ghiorso, M.S. and Sack, O.S. 1991, Fe – Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas: Contributions to Mineralogy and Petrology, v.108, p. 485. Mitchell, R.H. and Platt, R.G. 1977, Mafic mineralogy of ferroaugite syenite from the Coldwell Alkaline Complex, Ontario, Canada: Journal of Petrology, v. 19, p. 627. 4 �MERCURY IN FISH: A GEOLOGIST’S PERSPECTIVE BEHLING, Stuart James, Retired, Superior National Forest Geologist 4712 Anderson Rd., Duluth, MN 55811 USA stu-ball@mchsi.com Mercury contamination of fish has been a health concern in Northeastern Minnesota since testing began over 30 years ago. In spite of this concern, less than 15% of the lakes in this area have been tested. The data show that fish mercury contamination varies considerably between lakes but no pattern of distribution has been recognized. The data also show that fish mercury contamination varies between species and increases with length of fish. For example, walleyes are more contaminated than northern pike of the same size, from the same lake, and longer fish have more mercury than shorter ones. However, there is not enough data available from most lakes to plot these contamination trends. A model that could predict fish mercury contamination levels by species and length of fish would be a great asset in understanding the extent and distribution of fish mercury in this area. In this study, Northeastern Minnesota was divided into groups of geologically related lakes termed lake provinces. Ten lake provinces were identified in two distinct geomorphic areas; seven are in an area of bedrock controlled terrain and three are in an area of glacial deposition. In the bedrock controlled terrain, the size shape and orientation of the lakes are directly related to structural or compositional weakness in the underlying bedrock. Weathering and glacial erosion exploited these weaknesses to produce distinct patterns of lakes that were used to identify the provinces. In the areas of glacial deposition, the lake provinces were associated with the deposits left by three distinct glacial advances. When fish mercury was plotted against length of fish by province, the correlation coefficients increased significantly. The R squared values for the provinces ranged from .58 to .81 as compared to an R squared value of .32 for all samples from the study area plotted collectively. A map showing the distribution of mercury contamination in walleyes and northern pike was produced using the results from this ongoing study. 5 �EVIDENCE FOR PALEOSEISMIC EVENTS DURING CLOSURE OF THE MIDCONTINENT RIFT, ATKINS LAKE-MARENISCO FAULT, N. WISCONSIN Bjornerud, Marcia, Geology Department, Lawrence University, 115 S. Drew St. Appleton, Wisconsin 54911 USA. bjornerm@lawrence.edu The closure, or inversion, of the Midcontinent Rift at ca. 1.0 Ga remains one of the least well understood chapters in the Precambrian tectonic history of the Lake Superior region. Rift-bounding normal faults were reactivated as major reverse faults, some with estimated net reverse displacements of several kilometers. One such structure is the north-dipping Atkins Lake-Marenisco fault in Bayfield, Ashland and Iron Counties of northern Wisconsin. According to the interpretation of Cannon et al. (1993), the Atkins Lake-Marenisco fault is a crustal-scale listric fault zone along which lower Proterozoic metasedimentary rocks and middle-Proterozoic rift-related volcanic rocks have been thrust southward over Archean gneisses (the Proterozoic sequences had originally occupied a structurally lower position in the axial graben of the rift). The Atkins Lake-Marenisco Fault is well exposed near the falls of the Marengo River in eastern Bayfield County. There the fault zone is ca. 100 m wide and cuts through the Archean (ca. 2.7 Ga) Puritan Batholith, a granitic body with a weak gneissic fabric. In a downstream transect beginning just below the falls, the rocks record extreme strain gradients. Over a distance of < 20 m, pristine gneiss gives way first to cataclasite and then to dark, extremely fine-grained phyllonites, all heavily veined with K-feldspar, chlorite and zeolite. In thin section, cataclastic textures can be seen to overprint quasiductilely deformed feldspars, indicating that displacement along the fault zone may have begun at temperatures close to 450°C and continued as the rocks cooled. Most of the cataclasites have a crudely fractal (log-linear) clast size distribution, with the average grain size decreasing progressively with strain intensity, as expected when cataclasis is the dominant deformation mechanism. Samples from one outcrop near the base of Marengo Falls, however, have textures suggestive of devitrified pseudotachylyte. Dark-colored veins and lenses, now chloritized, contain relatively large (mm-sized) clasts of feldspar, but finer-grained clasts are notably absent. Many of the 6 �larger clasts, moreover, show unusual microstructural features. Some have embayments and concave edges – shapes not typical of ordinary cataclasites, in which grains are commonly equant and polygonal to rounded. Some of the clasts also display mosaicism, a texture that can form as a result of rapid volumetric changes. The bimodal grain size distribution and embayed and mosaicized grains could be explained by the presence of a seismically generated frictional melt, which could have preferentially assimilated the finest grains, partly resorbed larger ones, and created a short-lived pressure pulse as a result of expansion upon melting. If this interpretation is correct, we should begin to consider the role seismicity may have played in facilitating transient fluid flow and consequent mineralization during the closure of the Midcontinent Rift. Reference cited: Cannon, W., Z. Peterman, and P.K. Sims, 1993. Crustal-scale thrusting and origin of the Montreal River monocline: A 35-km thick cross section of the Midcontinent Rift in Northern Michigan and Wisconsin. Tectonics v. 12, p. 728-744. 7 �NEWLY RECOGNIZED THICK INTERFLOW SANDSTONES IN THE UPPER NORTHEAST LIMB OF THE NORTH SHORE VOLCANIC GROUP, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu New mapping, funded in part by the USGS STATEMAP program, has identified several major units of interflow sandstone between lava flows in the upper Northeast Sequence of the North Shore Volcanic Group (NSVG), which are as much as 200-300 feet (60-90 meters) thick. The recognition of these sandstones approximately triples the total thickness of known interflow sandstones in that area. These sandstones were intersected in several water well cutting sets collected by McKeever Well Drilling of Schroeder, Minnesota. Although the wells intersect only partial sections of the sandstone units, these intersections project into prominent northeast-trending topographic lows which correspond with linear low aeromagnetic anomalies. These lie between and parallel to ridges held up by basalt that correspond to linear positive magnetic anomalies. The eastern extent of these sandstones has yet to be determined, but will be examined in the coming field season. The sandstones identified in water well cuttings are pinkish-tan, medium-grained, and similar in composition and appearance to the well-known Cutface Creek sandstone (Fig. 1). In the northern-most sandstone, a well just south of the Monker Lake diabase (Fig. 1) intersected 80 feet (25 meters) of sandstone above the Devil’s Track rhyolite. However, a well located 8 miles along strike to the southwest intersected only 10 feet of sandstone, below basalt and above the Devil’s Track rhyolite, indicating either that it pinches out to the southwest or that the thickness varies due to deposition on an irregular surface. The middle sandstone unit shown on Fig. 1 was intersected in only one well, which penetrated 15 feet (5 meters) of sandstone beneath basalt. The color, composition, and grain size is consistent with it being the uppermost part of a thick interflow sediment, quite distinct from the typical thin interflow sediments that are found throughout lava flows in the NSVG. The southern-most of the three sandstone units is inferred entirely on the basis of topographic and aeromagnetic features that mimic those of the middle unit. Sandstone of substantial thickness was also intersected in water wells near the top of Eagle Mountain at the Lutsen Ski Resort (Fig. 1), which penetrated 15–60 feet (4-20 meters) of tan, medium-grained sandstone. This sandstone lies below the Leveaux ferrodiorite and above the Eagle Mountain basalt, and likely acted as an anisotropic plane of weakness that controlled emplacement of the Leveaux ferrodiorite sill. Its irregular thickness may be the product of having been variably cut out by the intrusion. Two thick interflow sandstone units – the Cut Face and Indian Camp Creek sandstones, had long been recognized in the upper NE limb of the NSVG from outcrops near Lake Superior. Both of these are soft and easily eroded relative to the surrounding volcanic rocks, hence are exposed only near the shore in actively eroding stream channels. Based on inferences from paleocurrent measurements, the interflow sandstones in the NSVG were deposited in two separate depositional basins that correspond to the SW and NE limbs of the NSVG (Jirsa, 1984). Sedimentary rocks between the SW and NE limbs, in the area from Illgen City to Lutsen (Fig. 1), are composed mainly of conglomerate and fragmental volcanic rocks, in contrast to sandstone that dominates on either side. This area coincides with a negative gravity anomaly known as “White’s Ridge” (White, 1966), that represents a block of uplifted crust (Boerboom, 1994). To the southwest of White’s ridge (SW Limb NSVG), the stratigraphically lower lavas dip more steeply towards the center of the rift than higher lavas, implying greater subsidence towards the rift axis during volcanism. To the NE of White’s ridge (NE Limb NSVG), the volcanic rocks throughout the stratigraphy show no appreciable change in dip, implying that subsidence during volcanism was uniform, and that tilting towards the rift axis occurred after volcanism. (Green, 1972). The 250 foot (76 meter) thick Cut Face Creek sandstone, exposed at Terrace Point (Fig. 1), is dominantly a reddish-brown, medium-grained, well-sorted, lithic arkose, but nearly 30% of the section is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay, with lithologic and 8 �sedimentary features indicative of deposition into a fluvial-lacustrine environment, such as streams flowing into ponded water (Jirsa, 1984). The lacustrine aspect is speculated to be caused by blockage of flowing water against an obstruction, perhaps related to deformation and magmatism along White’s ridge. The recognition of several thick interflow sandstones indicates that clastic sedimentation in the NE limb of the NSVG played a more significant role than in the SW limb, but whether this is a result of a higher rate of clastic input, more efficient trapping of sediment against a topographic impediment such as White’s ridge, a higher rate of subsidence, or longer periods of volcanic quiescence is not clear. The inferred fluvial-lacustrine depositional environment of the Cut Face Creek sandstone implies that ponding and sediment-trapping processes were more operative than in the SW limb, where dominantly fluvial sedimentary features indicate sediment flow-through, and deposition closer to the axis of the rift. From a practical standpoint, these sandstones may be important aquifers, however, little is presently known about their water-yielding characteristics. Boerboom, T. J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on Lake Superior Geology 40th Annual Meeting, Houghton, MI: Proceedings v. 40, pt. 1 Abstracts, p. 5-6. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, p. 294-332 Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p. White, W.S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region: U.S. Geological Survey Professional Paper 524-E, 23 p. 9 �BEDROCK GEOLOGIC MAP OF THE LITTLE MARAIS, SCHROEDER, AND TOFTE 7.5MINUTE QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu ALBERS, Paul, Duluth Metals, palbers@duluthmetals.com MILLER, James D., Jr., Minnesota Geological Survey, mille066@umn.edu The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in ten published 1:24,000 scale maps from Duluth to Tofte, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Lutsen quadrangle will be published in July 2007; the Deer Yard Lake and Good Harbor Bay quadrangles will be mapped in the 2007 and published in 2008 (Fig. 1A). All the maps in this series are available as printed maps, or as PDF and ArcView export files at the MGS website (http://www.geo.umn.edu/mgs/). Field trip 5 of this meeting will begin in the Little Marais quadrangle at the southwest, and end in the Good Harbor Bay quadrangle at the northeast. With the exception of a strip near the shore of Lake Superior (Green, 1992), parts of the Little Marais quadrangle that were mapped but not published, and thesis work by Albers (UMD M.S. 2007), much of the area covered by these maps had never been mapped. The combined area of these three maps straddles the transition across the uppermost Southwest to the uppermost Northeast sequences of the North Shore Volcanic Group (NSVG), as well as the SchroederLutsen sequence, which unconformably overlies the Southwest and Northeast sequences (Fig. 1B). Together, these maps cover the stratigraphically highest portion of the NSVG in Minnesota. In addition, components of the Beaver Bay Complex, including multiple phases of the Beaver River diabase, the Blesner Lake diorite, and the Leveaux ferrodiorite are present in the area of these maps. The new mapping has delineated a series of dominantly felsic to intermediate composition volcanic rocks (rhyolite, icelandite, and andesite) beneath the Schroeder-Lutsen basalts, informally termed the Onion River lavas, which include some tholeiitic basalt flows. The strike of these flows at a high angle to that of the overlying Schroeder-Lutsen basalts, which are nearly parallel to the Lake Superior shoreline (Fig 1B). The basal contact of the Schroeder-Lutsen sequence is repeated by several shore-parallel reverse faults. Poorly exposed units of rhyolite, volcanic breccia, and conglomeratic to sandy sedimentary rocks, which are likely components of older volcanic and interflow sedimentary rocks such as the Onion River lavas, are exposed within the Schroeder-Lutsen sequence by normal and reverse faults. The Carlton quarry lavas (Fig. 1B) form a northwest-dipping group of lavas composed of rhyolite at the base, andesite, and ophitic basalt at the top. The position of these lavas relative to the surrounding lavas is poorly constrained due to lack of outcrop, but they are assumed to be in fault contact, given the anomalous northwest dip. The rhyolite here has been dated at 1094.3±2.0, the youngest age obtained to date from the NSVG in Minnesota, but the surrounding volcanic rocks have not been dated. Intrusive rocks of the Beaver Bay Complex are most abundant and diverse in the Little Marais quadrangle, where they are composed of multiple phases of the Beaver River diabase and the slightly older, multi-phase Blesner Lake diorite. Elsewhere, the Beaver River diabase is restricted to erosional remnants of a larger, generally southeast-dipping sheet that commonly contains large anorthosite inclusions, such as at Carlton Peak. The porphyritic Leveaux ferrodiorite occurs mainly in the Tofte quadrangle as a subvolcanic sill emplaced into volcanic rocks, and as inclusions in the Beaver River diabase. 10 �REFERENCES Green, J.C., 1992, Geologic map of the North Shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marais to Tofte: Minnesota Geological Survey Miscellaneous Map M-71, scale 1:24,000. Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. Visit the MGS website for references to individual maps. Lutsen will be published in 2007; Deer Yard Lake and Good Harbor Bay in 2008. B. Index map showing the locations of the major units mentioned in the abstract. 11 �PETROGRAPHIC DIFFERENTIATION OF THE FIVE PHASES OF THE LOWER CRETACEOUS STAR KIMBERLITE, SASKATCHEWAN, CANADA BOISJOLI, Troy and FLOOD, Timothy P., Department of Geology, Saint Norbert College, DePere, WI 54115; troy.boisjoli@snc.edu Geologic Setting The Star Kimberlite in the Fort a la Corne region, east-central Saskatchewan, is a large lithologically complex structure that preserves both intra-crater and extra-crater kimberlite (Zonneveld et al. 2004). The Star Kimberlite occurs on the eastern rim of the North American Platform. The oldest rocks in the area are believed to be part of the Archean Sask craton, which are overlain by other Precambrian rocks similar to those exposed to the north in the Glennie Domain. These Precambrian rocks are successively overlain by 300-500m of mixed siliciclasticcarbonate sediments that range in age from Cambrian to Devonian; these sediments are overlain by approximately 150m of Lower Cretaceous sediments; which are overlain by 85 to 115m of glacial deposits. The Star Kimberlite itself is intermittent and interstratified with Lower Cretaceous rocks that include the Cantuar Formation inferred as coastal plain facies, the Pense Formation inferred as shoreface facies, and the Joli Fou and Viking Formations inferred to be proximal offshore to shoreface facies. Geology of the Study Area Five different phases of the Star Kimberlite have been recognized (Zonneveld et al. 2004). In stratigraphic order from oldest to youngest they are; Cantuar Kimberlite (CK), Pense Kimberlite (PK), Early Joli Fou Kimberlite (EJFK), Middle Joli Fou Kimberlite (MJFK) and Late Joli Fou Kimberlite (LJFK). The purpose of this study was to determine if petrography and geochemistry could be used as tools to differentiate these different phases of the Star Kimberlite. The CK is the oldest kimberlite in the Star body. In drill core, kimberlite intersections in this unit range in thicknesses from 0.3 to 11.8m and are bedded at the 0.2 to 9.0m scale. The beds within the kimberlites have sharp contacts and are usually massive or fining upwards. The CK is interpreted as medial to distal primary pyroclastic airfall material, with some reworked sedimentary equivalents. The location of the feeder vent for this event has not been determined. The PK overlies CK and drill core intersections range in thickness from approximately 2 to 20m. The individual beds are highly variable, with massive, normal graded and reverse graded beds. Sharp contacts are typical and in some beds the upper 0.1 to 1m show evidence of marine reworking. PK is interpreted as proximal to distal, primary volcaniclastic material. The EJFK overlies the PK and is volumetrically the most important phase of the Star Kimberlite. The EJFK deposits are 1 to 25 m thick, with bedding ranging from 1 to 10 m. Individual beds have sharp to diffuse contacts and are typically massive or normally bedded. Reverse bedding is rare. The EJFK is interpreted as primary subaerial and distal marine fall material. The MJFK overlies the EJFK and until recently was lumped with the upper portion of the EJFK (Kjarsgaard 2007). 12 �The LJFK overlies the MJFK The thickness of this unit is unknown but bedding within the unit occurs on a scale of 0.5 to 30m, with beds of 10 to 20m the most common. The LJFK is interpreted primarily as pyroclastic subaerial fall material to pyroclastic flow material (Zonneveld et al. 2004). Analysis Petrographic analysis was performed on 23 thins sections using an average of 600 points per thin section. For all units, altered olivine comprised from 40-60% of the sections. Juvenile clasts comprised from 5-30% of the sections and opaques ranged from 1-4%. Late alteration calcite and serpentine accounted for between 0.5-6% of the sections. Quantitative petrographic analysis was not useful for distinguishing the different phases of the Star Kimberlite. An alternate petrographic method based on textural and mineralogical criteria of the juvenile clasts (Webb 2006) was performed. In this analysis, ten criteria are the basis for distinguishing various mechanisms of emplacement for kimberlites. These criteria include; sphericity, roundness, irregularity, internal structure, vesicularity, clast-host relationship, primary phenocrysts, groundmass crystallinity, mineralogy and size range/modal average. A preliminary compilation of these criteria suggest that some units are distinguishable from other units relative to emplacement mechanisms. For example, vesicularity in the CK is high relative to other phases and consistent with the original interpretation as distal primary pyroclastic airfall. Geochemical analysis was preformed on the primary olivine clasts using an SEM-EDS. The initial results of this analysis were inconclusive due the highly altered nature of the grains but work is on going. References: Kjarsgaard, B.A., 2007, personal communication. Webb, K.J., 2006, Juvenile clasts in kimberlites: Standardized comprehensive description towards unraveling emplacement mechanisms: in Kimberlite Emplacement Workshop, DeBeers Canada Inc, p. 1-5. Zonneveld, J.P., Kjarsgaard, B.A., Harvey. S.E., Heaman, L.M., McNeil. D.H., and Marcia. K.Y., 2004, Sedimentologic and stratigraphic constraints on emplacement of the Star Kimberlite, east-central Saskatchewan: Lithose 76 115-138. 1: Juvenile clast with primary olivine and vesicles infilled with calcite. Field of view 10mm. 13 �DEPOSITION OF NATIVE COPPER LODES ON THE KEWEENAW PENINSULA, NORTHERN MICHIGAN, FROM A GRAVITY-DRIVEN EVOLVED METEORIC BRINE BROWN, Alex C., Dept. of Civil, Geol. and Min. Eng., École Polytechnique de Montréal P.O. Box 6079, Sta. Centre-Ville, Montreal, QC, Canada H3C 3A7, acbrown@polymtl.ca The Keweenaw Peninsula of northern Michigan is the site of famous native copper lodes hosted by flow-top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the Portage Lake Volcanics (PLV). This mineralization is generally attributed to an up-dip flow of a cupriferous metamorphogenic fluid generated during deep burial metamorphism of the host strata in axial portions of the Midcontinent Rift (Stoiber & Davidson 1959; White 1968; Jolly 1974; Livnat et al. 1983). In an alternative model proposed here, copper is leached from immature rift sediments and porous portions of the PLV by deeply circulating, gravity-driven meteoric water. The meteoric water is initially cool, fresh, slightly acid and oxygen-rich. At depth, it becomes warmer, highly saline by the assimilation of evaporites or evaporitic brine, near-neutral in pH by equilibration with silicates and carbonates, and oxygen-deficient by reddening of its aquifers (hematitization of ferrous iron in abundant labile mafic mineral constituents of the rift sediments and mafic constituents of the PLV basalts). At first, the Eh of the brine evolves progressively toward moderately oxidizing conditions at which it becomes capable of leaching and transporting trace amounts of copper from its aquifers. Metamorphogenic water and copper may have been incorporated during circulation at deep burial levels. Highland recharge eventually drives the hybrid cupriferous brine up-dip along PLV aquifers. With continued hematitization of its aquifers, the brine becomes highly reducing by oxygen depletion, and native copper is deposited (in the absence of sulfide). Coincidentally, the same highly reducing conditions favor the solution of hematite, a feature readily visible as locally bleached rock (hematitic pigment removed) where native copper mineralization occurs on the Keweenaw Peninsula. This conceptual model provides 1) an explanation for the highly saline brine needed to mobilize copper, and 2) a means to drive dense brines up-dip from axial portions of the rift basin. It also explains the pervasive hematitic reddening of the PLV, and ties the timing of copper leaching to that diagenetic reddening event. The overall timing of native copper mineralization may be linked to the Grenvillian compressional event which tilted PLV strata to deep levels along the rift axis and probably raised highlands around the rift basin. References: Brown, A.C. (2006) Genesis of native copper lodes in the Keweenaw district, northern Michigan: a hybrid evolved meteoric and metamorphogenic model: Economic Geology, v. 101, p. 1437-1444. Jolly, W.T. (1974) Behavior of Cu, Zn and Ni during prehnite-pumpellyite rank metamorphism of the Keweenawan basalts, northern Michigan: Economic Geology, v. 69, p. 1118-1125. Livnat, A., Kelly, W.C., Essene, E.J., and Rye, R.O. (1983) P-T-X conditions of sub-greenschist burial metamorphism and copper mineralization, Keweenaw Peninsula, northern Michigan: Geological Society of America Abstracts, v. 15, p. 629. Stoiber, R.E. and Davidson, E.S. (1959) Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district; Part I, Part II: Economic Geology, v. 54, p. 1250-1460, 1444-1460. White, W.S. (1968) The native-copper deposits of northern Michigan, in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967: Amer. Inst. of Mining, Metallurgical and Petroleum Eng., GratonSales Volume, p. 303-326. 14 �p. 2 Fig. 1 Schematic illustration of the geologic setting of native copper lodes (from Brown 2006) NW tion a r d Hy / ll y it e al t Ba s l Cg 0 Cu S an H2 O H o 2+ Cu Conditions favorable for significant copper solubilities in redbeds Cu S Cu 2O an H2 O H o Cu H 2 Meteoric Water 2CuCl 3 + CuCl 2 CuO Ground Water o Cu Conditions favorable for significant copper solubilities in redbeds CuO Cu 2S dj O2 O Cu 2O Ground Water 2 Cu 2S 5 6 ppm O H2 2 O Cu 2S -0.4 25 C Cl= 0.5 M Contours of copper solubility 64 ppm Cu4 (OH)6Cl 2 -4 10 (6 ppm) -3 10 mg/l (64 ppm) Cl=0.5 M Meteoric Water Water dj ult o s 2 3 oC 25 25 -4 C a =10 mg/l 2CuCl 3 + CuCl 2 Eh (V) lt sa Ba Cu4 (OH)6Cl 2 o 25 C +0.4 t B) System Cu-O-H-Cl A) System Cu-O-H-S-Cl 2+ Cu l sa Ba lt sa Ba n w Fa ratio d y h De een a pe Pum ns it ion Tr a e t o id Ke w Fig. 2 Eh-pH stability diagrams (below) for A) the transport of copper, and B) the deposition of native copper (from Brown 2006) +0.8 In In Flow-Top Breccias Conglomerates and Amygdaloids Pr esent-day Er osion Sur face Ep Contours of copper solubility SE Native Copper Lodes Cu 2S 7 pH 9 7 5 3 11 pH 9 11 I ON IPITAT PREC Fig. 3 Schematic illustration of the deep flow of meteoric and metamorphogenic waters, to form native copper lodes (from Brown 2006) Hi g h la nd s Evapo rites Meteoric water o +Cl - Cu 2- - CuCl 3 + CuCl2 Progressive reddening of aquifers and leaching of copper Assimilation of metamorphogenic water and copper Mantle heat 15 Deposition of native copper �A NOVEL LOCALITY FOR PSEUDOBROOKITE - THE NINE MILE PLUTON, MARATHON COUNTY, WISCONSIN T.W. Buchholz1, A.U. Falster2, Wm. B. Simmons2. 11140 12th St. N., Wisconsin Rapids, WI 54494; 2Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148. Pseudobrookite, (Fe3+, Fe2+)2(Ti,Fe3+)O5, is a typical pneumatolytic mineral found in lithophysae in extrusive Ti-rich rocks such as andesite and rhyolites (Anthony et al., 1997). Recent work in the Ladick Trucking & Excavating weathered granite quarry has exposed a novel occurrence of pseudobrookite within the Proterozoic Nine Mile Pluton, the youngest and most silicic of four plutons comprising the Wausau Complex. The Ladick quarry is located approximately ¼ mile east of STH 107 in the southwest portion of the Nine Mile Pluton. Black to dark grey elongated crystals of pseudobrookite up to approximately 1.5 mm in length and clustered in radiating sheaves and as single crystals were found in an alteration assemblage associated with a thin quartz-pyritefluorite vein. The material was recovered from a pile of shot rock derived from blasting large boulders removed from the grus in the northern portion of the operation. The vein was emplaced in a fracture cutting the granite, and it appears fluids circulating along the fracture preferentially removed quartz and perhaps other minerals from adjacent granite to a distance of up to about 12 cm from the fracture. Subsequently K-feldspar, biotite, minor quartz and a series of associated minerals described below were deposited. EMP analysis of clean grains of pseudobrookite shows no significant components other than Ti and Fe and minor amounts of Mn and Nb, and XRD analysis yielded excellent structural agreement. Morphology is that of ideal, short-prismatic pseudobrookite. Associated minerals other than K-feldspar, biotite and quartz include ilmenite, rutile, anatase, sphalerite, chalcopyrite, pyrite, arsenopyrite or marcasite (now represented by goethite pseudomorphs), molybdenite, cassiterite, fluorapatite, fluorite, monazite, zircon and a LREE-carbonate mineral, probably bastnaesite-(Ce). Secondary minerals include small but attractive sprays of gypsum crystals; the Ca was probably derived from weathering fluorite, and the S from weathering sulfides. Interestingly, the cassiterite crystals often contain inclusions of fluorapatite and monazite; all monazite identified so far has been as inclusions in cassiterite. The monazite and fluorapatite inclusions may represent an earlier, otherwise transient paragenesis that was preserved by inclusion in cassiterite; the LREE released by breakdown of early-formed monazite may have contributed to the later formation of the LREE-carbonate mineral. Although pseudobrookite typically is found in lithophysae in certain eruptive rocks and this would seem to be a very different environment, the Nine Mile pseudobrookite probably formed in an environment that was not inconsistent with its usual environment of formation. As indicated by the association with cassiterite, molybdenite (this is the first report of molybdenite for the Nine Mile granite), fluorite and other Ti-bearing oxides the conditions were probably high-temperature, volatile- and Ti-rich. Conditions were likely close to a pneumatolytic environment – consistent in these respects with typical 16 �pseudobrookite occurrences. The principal variant may have been ambient pressure, although it is believed that the Nine Mile granite was intruded at a shallow, albeit as yet undefined level in the crust. Hence the environment of formation probably had at least some features in common with the usual volatile-rich, high temperature andesites and rhyolites that are the normal hosts for pseudobrookite. Reference cited: Anthony, J.W., Bideaux, R.A., Bladh, K.W. and Nichols, Monte C., 1997, Handbook of Mineralogy, V. III, p. 453. 17 �Deposition and Cementation of Paleoproterozoic Gunflint Formation Carbonate: Implications for Early Hydrosphere Chemistry Justin Burton and Philip Fralick, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, philip.fralick@lakeheadu.ca The Gunflint Formation extends 190 km towards the northeast from an intrusive contact with the Duluth Complex. Surface exposures form a narrow band of outcrops with basal siliciclastics and underlying Neoarchean rocks to the west and overlying black shales of the conformable Rove Formation to the east. Erosional remnants 98 km northeast of Thunder Bay at Schreiber, Ontario, suggest a pre-erosional continuation of the chemical sedimentary rocks to the east. Rock comprising the Gunflint Formation primarily consists of an assemblage of chemical sediments including chert, iron oxides and carbonates. Volcanogenic shales and similar debris within the chemical sediments is also present. In 1956 the Formation was divided by Goodwin into six major sedimentary facies representing two depositional cycles and four members: 1) Basal Conglomerate, 2) Lower Gunflint, 3) Upper Gunflint, and 4) Upper Limestone. These units were deposited on a wave and tide influenced broad shelf deepening to the south (Fralick and Barrett 1995, Pufahl and Fralick 2004). Storm events eroded the fine-grained chemical sediments abrading them into sand-sized fragments that accumulated as cross-stratified layers and lenses (Fralick 1988). In this study we examine the limestone unit at the top of the Gunflint Formation and compare its petrology and geochemistry to ferronian dolomitic (ankeritic) grainstones common in the shoreproximal exposures near Thunder Bay. The stratigraphy of the limestones is consistent in the outcrops studied, even though some are tens of kilometers apart. The unit is conformable with underlying silicified ankeritic grainstones. The boundary between the two units forms a replacement front in places, with microscopic examination clearly revealing replacement of calcite by ankerite. The limestone consists of a very coarse sand to granule grainstone layer overlain by cabbage sized and shaped stromatolites. Layers of both fine-grained and very coarsegrained calcareous sand are banked up against and overlie the stromatolites. This assemblage is less than one meter thick. It is sharply overlain by a pebble to boulder conglomerate with some tabular clasts up to four meters long. Fragments of the underlying ankerite and limestone layers form most of the clasts. Ankerites from lower in the succession are composed of neomorphic spar with none of the original texture preserved. Geochemically they have high concentrations of Ca, Fe, Mg and Mn with relatively low abundances of most other elements. Their REE curves are LREE enriched with positive La and, to a lesser extent, Ce anomalies. Eu is flat to positively enriched. The limestones have clasts composed of Fe-rich chlorite, consistently with V contents of 1-2 percent. They are surrounded by blocky calcite cement that in places overgrows the grains. Samples from higher in the grainstone assemblage have more pronounced dissolution pathways along calcite crystal boundaries. These dissolution channels are filled with either calcite or quartz. The stromatolites have alternating layers of Fe-rich chlorite and blocky calcite cement similar in composition to the adjacent grainstones. The grains in some grainstones are pieces of stromatolite. The limestones are very geochemically different from the ankerites. The limestones have higher values of most elements, but especially U (5 to 10 times more abundant), REE’s (10 to 100 times more abundant) and V (100 times more abundant). Their REE curves are LREE enriched and show distinct Ce and Eu depletion. 18 �New U-Pb geochronological data (Don Davis, Pers. Comm) indicates that the pebble to boulder conglomerate is the tsunami deposit that resulted from the Sudbury impact. This reinforces previous interpretations that a significant nondepositonal interval with subaerial exposure existed between deposition of the Gunflint and Rove Formations in the study area (Addison et al. 2005). The blocky calcite cements and Fe chlorites would have formed in the phreatic zone during this interval. The extreme V-U enrichments indicate oxidized fluids leaching V and U from the subaerial environment and precipitating these redox sensitive elements upon encountering the organic-rich sediments. The negative Ce anomalies of the REE curves for the calcite cements agree with this interpretation. The ankeritic sediments deposited in the marine environment have REE abundances typical of precipitation from oxygen deficient water with abundant dissolved Fe and Mn. This data points to an imbalance in oxygen levels at this time with a relatively oxic atmosphere and anoxic oceans. This agrees with findings from other geochemical investigations of the Gunflint and Rove Formations (Poulton et al. 2004). Addison, W. et al., 2005. Geology, vol. 33, p. 193-196. Fralick,P.F. 1988. Memoir 13, Can. Soc. of Petroleum Geologists, p. 24-29. Fralick, P.W. and Barrett, T.J., 1995. Spec. Publ.22, Inter. Ass. of Sedimentologists, p. 137-156. Poulton, S.W., Fralick, P.W. and Canfield, D.E., 2005. Nature, vol. 431, p. 173-177. Pufahl, P.K. and Fralick, P.W., 2004. Sedimentology, vol. 54, p. 791-808. Figure 1. Archean mudstone normalized REE curves for ankeritic samples (lower 5 curves) and limestone samples (upper 3 curves). 19 �THE SUDBURY IMPACT LAYER IN THE LAKE SUPERIOR IRON RANGES: A TIME-LINE FROM THE HEAVENS Cannon, William F., U.S. Geological Survey, MS 954, Reston VA 20192 Addison, William D., R.R. 2, Kakabeka Falls, Ontario, POT 1W0, Canada A large meteorite impact near Sudbury, Ontario at 1,850 Ma produced a geologically instantaneous regional, and probably global, catastrophe leaving a unique imprint within the classic iron ranges of the Lake Superior region 500 to 900 kilometers from the impact site. We have now identified the impact layer in or near five of the iron ranges. The layer is a unique and ultra-precise time line, having formed over a time span of days or less across the region, and allows an unequivocal temporal correlation of the stratigraphic units with which it is interbedded. The impact layer consists of various rock types and may have formed by processes including direct deposition of airborne ejecta either from a high energy ejecta curtain traveling at ballistic velocities, or from a turbulent cloud of finer ejecta. More commonly the layer consists of polymict marine breccias of reworked ejecta and local sediments possibly formed by 1) “ballistic erosion” and mixing of sediment caused by high energy ejecta deposition, 2) erosion and redeposition caused by mega-tsunamis generated by the impact, 3) resurge of ocean water toward the “hole in the bottom of the sea”, or possibly 4) submarine slumping triggered by impact generated earthquakes. Regardless of diverse lithology and depositional processes, occurrences of the impact layer are linked by the presence of quartz grains exhibiting relict shock metamorphic features, which form only under the extreme pressures produced by hypervelocity impacts. 20 �The following are brief summaries of the characteristics each site: Gunflint- At top of Gunflint Iron-formation; nearly pure ejecta deposits of devitrified vesicular glass, microtectites, and spherules; accretionary lapilli and shocked quartz; apparently not reworked by tsunamis. Thunder Bay- Tsunami debrisite at top of Gunflint Iron-formation containing reworked, locally derived breccia fragments plus ejecta similar to the Gunflint ejecta; shocked quartz known from one locality. Mesabi- Poorly known unit at top of Biwabik Iron-formation; contains devitrified spherules, microtectites, and vesicular glass; shock metamorphosed quartz. Puritan- Known from a few thin sections of drill core from 1960’s study. “Volcaniclastic breccia” in Tyler Formation about 100 m above Ironwood Ironformation; rare shock metamorphosed quartz. Marenisco- Possible tsunami deposit at base of Copps Formation lying on Archean granite. Shock metamorphic features not well documented at this time. Huron River- Breccia and fine “volcaniclastic rocks”; near base of Michigamme Formation a few meters above contact with Archean granite. Abundant shock metamorphosed quartz grains. Connors Creek- Breccia, greywacke, and accretionary lapilli beds; erosional contact on slightly ferruginous chert about 150 m above base of Michigamme Formation; shock metamorphosed quartz. McClure- Breccia and greywacke about 300 m above base of Michigamme Formation; erosional contact on carbonate iron-formation; shock metamorphosed quartz; grades up to black slate. Iron River- Breccia and greywacke forming lower part of Hiawatha Graywacke; shock metamorphosed quartz; known best from thin section collection from 1940’s; erosional contact on Riverton Iron-formation. The diagram below shows the precise temporal correlation of stratigraphic units in the five areas where we have identified the Sudbury impact layer. . 21 �AN EXPERIMENTAL WATER-ROCK INTERACTION STUDY INTO THE ORIGIN OF HIGH-SULFATE WATERS ASSOCIATED WITH THE STEEP ROCK IRON MINES, ATIKOKAN, ONTARIO. COCKERTON, S., and CONLY, A.G., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 andrew.conly@lakeheadu.ca LEE, P., Department of Biology, Lakehead University, Thunder Bay, ON, P7B 5E1 Continual flooding of the two primary open pits of the Steep Rock iron mines near Atikokan, Ontario, has lead to the formation of Hogarth and Caland pit lakes, which are located in the former middle and eastern arms of Steep Rock Lake, respectively. At present both lakes are ~200 m deep and display distinct differences in water chemistry [1]. Hogarth pit lake is characterized by non-stratified to weakly stratified and oxygenated water column (4-12 mg/L O2), near neutral pH (6.4-8.0), extraordinarily high SO42- concentrations (1200-2000 mg/L), lower alkalinity (50-125 mg/L), increased hardness (1200-1800 mg CaCO3/L) and is chronically toxic to aquatic fauna. On the other hand, Caland pit lake is well stratified with an upper oxygenated fresh water lens (~20 m in depth and 4-12 mg/L O2) that is not toxic, which overlies an anoxic (<1 mg/L O2) and moderately saline (200-500 mg/L SO42-) water column. The pH of Caland pit water is similar to that of Hogarth, but is more alkaline (100-200 mg/L) and has a lower hardness (400-1000 mgCaCO3/L). The concentration of metals in both lakes is negligible. With the pits being filled at a rate of ~3 m/year [2] and ground water presumed to be the largest input of water, the differences in water chemistry between the two pit lakes likely reflect differences in the nature of water-rock interactions between ground water and bedrock. Previous stable sulfur isotope studies have concluded that the dissolved sulfate of the pit waters is derived from oxidation and leaching of pyrite-bearing units of the Jolliffe ore zone by either ground water or due to pit water-wall rock interaction [1,3]. The geology of two pits is largely similar and includes [4,5]: 1) footwall rocks that consist of the Marmion Geniss Complex, Wagita Formation and Mosher Carbonate; 2) the Joliffe ore zone, which is comprised of the Manganiferous Paint, Goethite and Pyrite members; and, 3) hanging wall rocks that include the Dismal Ashrock Formation and the Witch Bay Formation. In order to better determine the nature of ground water-rock interactions a series of batchreaction column leaching experiments were performed. A series of four PVC columns (4 inches in diameter and 24 inches in length) were loaded with crushed (1-2 mm) pyrite and goethitehematite waste rock, representing the Goethite and Pyrite members. Two of the columns also contained 2 inches of crushed Mosher Carbonate at the base. Columns were filled with Atikokan ground water ensuring that the crush rock was completely submerged. In addition, another three columns were loaded with crushed goethite-hematite tailings (jit reject), goethite-hematite waste rock and pyritic waste rock, and filled with rain water in order to assess the contribution provided from surface water. Approximately 30 mL of water was extracted and filtered at 3 hr, 24 hr, 3 days, 5 days, 7 days and then weekly for a total of 14 weeks. Cations and metals were determined by ICP-AES, anions (SO42-, Cl-) were determined by ion chromatography, and pH was measured using an Accumet AB15 pH meter. Results from the column leaching experiments indicate that the high SO42-(aq) concentration of the pit lakes could only be produced from ground water oxidation of pyritic portions of the Joliffe ore zone (Fig. 1a). 22 �Figure 1. Normalized plots, to the average composition of Hogarth pit lake, showing the (A) relative concentration of pH, anions and cations from goethite-hematite and pyrite waste rock experiments, and (B) results of mixing between leachates derived from goethite-hematite and pyrite waste rock. To generate the near neutral pH of either pit lake requires subsequent interaction with Mosher Carbonate (Fig. 1a). Figure 1b shows the results of mixing the leachates derived from goethitehematite and pyrite waste rocks. Consequently, the pit waters are likely produced from the mixing of ground waters derived from both iron oxide and pyritic portions of the ore zone that were subsequently buffered by carbonate wall rock.. However, discrepancies between experimental waters and measured Hogarth pit lake water (e.g., Mg) likely reflect interaction of pit lake water with other wall rock lithologies and/or other ground water contributions. Experiments are currently underway to assess both the interaction of ground water with different lithologies and the effect of varying water-rock ratios. Initial results from these experiments will be presented at the meeting. Notwithstanding these discrepancies, the results from batchreaction column experiments supports stable isotope evidence [1,3] that the sulfate in the two lakes is derived from ground water leaching of pyritic portions of the Joliffe ore zone. In addition, the differences in water chemistry between Hogarth and Caland pit lakes primarily reflects differences in the relative pyrite content of the Joliffe ore zone, with the ore zone beneath Hogarth pit lake containing a higher abundance of pyrite. References: [1] Goold, A., Conly, A.G., and Lee, P., 2006. Integrated uses of water chemistry, stable isotope geochemistry and tie studies in identification of metal toxicity in pit lakes: A case study at the Steep Rock iron mine, Atikokan, Ontario: GAC-MAC Program with Abstracts, v. 31, p. 57-58. [2] McNaughton, K.A., 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 85 p. [3] Conly, A.G. and MacDonald, J.C., 2005. An Investigation into Potential Sulphate Sources of the Hogarth Pit Lake, Steep Rock Iron Mine, Atikokan. ILSG 51st Annual Meeting, Proceedings Volume 51. [4] Joliffe, A.W. 1966. Stratigraphy of the Steeprock Group, Steep Rock Lake, Ontario. GAC Special Paper No. 3, p. 75-96. [5] Stone, D., Kamineni, D.C., and Jackson, M.C., 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, v. 405, 106 p. 23 �ORIGIN OF A MAFIC PEGMATITE WITHIN THE DULUTH COMPLEX, NORTHERN MINNESOTA COSTELLO, Daniel E.1, FLOOD, Timothy P.1, and THOLE, Jeffrey T.2 Department of Geology, Saint Norbert College, De Pere, WI 54115, dan.costello@snc.edu, 2 Department of Geology, Macalester College, St Paul, MN 55105 1 Geologic Setting The Duluth Complex of Northern Minnesota is a layered igneous intrusion that formed due to rifting of continental North America approximately 1.1 billion years ago. The Complex is composed of numerous layered plutonic bodies, which formed through multiple intrusions of mafic to felsic tholeiitic magmas. These layered series are typically thick and exhibit foliation and layering that dips to the southeast, towards the axis of the Keweenawan Rift (Miller et. al. 2002). This study focuses on units within the South Kawishiwi intrusion (SKI). Near the base of this intrusion, two similar troctolitic units are separated by a pegmatitic unit. The petrogenetic relationship between the overlying and underlying troctolitic units is uncertain, as is the origin of the pegmatite. This project aims to determine the origin of and relationship between these three units using petrographic and geochemical data. Igneous Stratigraphy The pegmatitic unit, hereafter referred to as the PEG, was first noted by Foose and Weiblen (1984). This unit is used as a marker bed to differentiate the surrounding troctolites: The rock below the PEG is classified as the ultramafic three unit (U3), and the rock above the PEG is classified as part of the ultramafic two unit (U2). In drillholes where there is no PEG present, it is impossible to differentiate between these two troctolitic units. The U3 is the lowermost ultramafic unit within the SKI. The thickness of this unit averages approximately 100 feet, and ranges from 3 to 410 feet. (Severson 1994). The U3 is characterized by massive oxide horizons as well as mineralization of sulfides and platinum group elements. The PEG overlies the U3 unit and has an average thickness of approximately 95 feet. The pegmatite is used as a marker boundary between a sulfide-rich zone below and a sulfidepoor zone above (Foose and Weiblen 1986). The pegmatite is not always present in drill core. The U2 overlies the PEG and is petrologically similar to the U3 unit, with the exception that the U2 lacks massive oxides and sulfide-rich horizons (Severson 1994). This unit averages about 90 feet in thickness. Analysis Petrographic study revealed visible variations among the three units. The U2 and U3 appear to be similar; however, multiple differences occur within the pegmatite. The presence of minerals such as amphibole and biotite in the PEG suggests that this unit formed at a later stage of crystallization and/or within higher P(H20) conditions than its surrounding units. The PEG also contains significant amounts of a plagioclase symplectite texture (Figure 1). This texture may indicate the presence of a late stage magmatic fluid which would promote the growth of a pegmatitic unit. This symplectitic texture is rare below the pegmatitic unit, but is present in small amounts in all units above the PEG (Severson 1994). 24 �Geochemical studies focused on the changing concentrations of elements, notably the different behaviors of compatible and incompatible trace elements. From the U3 to the U2, not including the PEG, the concentrations of both Ni and Co decrease, suggesting olivine fractionation. In a similar fashion, the concentration of Ba increases from the U3 to the U2, suggesting plagioclase fractionation. This is consistent with a model of Ba as an incompatible element during early stages of crystallization. Preliminary mineral chemistry was obtained on plagioclase and olivine grains from all units. The two troctolitic units contain olivine of Fo63-65 and plagioclase of composition An75-77. In comparison, the pegmatite contains olivine with Fo47-60 and plagioclase with An59-63. These results are consistent with a model of fractional crystallization between the U3 and the U2 and with elevated P(H20) conditions within the pegmatite. Using the preliminary results outlined above, we infer that the three units are syngenetic and the U2 and U3 units formed through fractional crystallization. The pegmatite formed as a water rich phase concentrated within the magma chamber, with some water being derived from the footwall. As normal fractionation occurred from the U3 to the U2, a concentration of water resulted in the formation of a pegmatitic horizon between these two units. References: Foose, M.P. and Weiblen, P.W., 1986, The physical and chemical setting and textural and compositional characteristics of sulfide ores from the South Kawishiwi Intrusion, Duluth Complex, Minnesota, USA: in 27th Int. Geol. Congress (Moscow), Special Copper Symposium: Springer-Verlag, New York, p. 8-24. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. Figure 1: Plagioclase symplectite texture found in the pegmatitic unit. Field of view 1.5mm 25 �Distribution of Certain Ichthyofauna in relation to Eastern Outlets of Lake Agassiz, with emphasis on the Gunflint-Arrow Lakes Corridor and the Keating Complex J. Frederick Dean and Brian A. M. Phillips, Lakehead University, Thunder Bay, ON Canada. Based on a retrodeformed elevation model (Phillips, 1997) of the Upper Herman stage of Lake Agassiz, Phillips and Hill (2004) proposed that the lake discharged via the Gunflint-Arrow Lakes Corridor into Lake Superior down the Arrow valley. Fisheries work in Minnesota and Ontario reveals a distribution of fish species that corroborates a flow of Agassiz water into the Arrow, Whitefish and lower Kaministiquia watersheds. There are two sub-species of the temperate Johnny Darter (Etheostoma nigrum eulepis and Etheostoma nigrum nigrum), a small benthic fish unable to migrate up rapids or falls. The former, the earlier of the two, is found along the trend of the Herman shoreline and into the Gunflint-Arrow Lakes Corridor (Figure 1). The later sub-species relates to the Campbell shoreline, appearing in the Matawin and Shebandowan rivers. Intergrades developed where the two sub-species mixed (Underhill, 1963; Schmidt, 1993; Momot and Stephenson, 2007). An Arctic marine glacial relict, Deepwater Sculpin also inhabits Gunflint-Arrow Corridor lakes. Dean (2006, pers. comm.) found Pontoporeia affinis (Lindstrom), another Arctic marine relict amphipod, in bottom waters of Arrow Lake. Both migrated along the northern, icebound margin of Lake Agassiz, as flow was funneled eastwards through the corridor. The topographically rugged west-east trend of the borderlands, the “Keating Complex”, likely caused ice margin retreat to be intricate in detail (Figure 2). During and after the Steep Rock phase of Rainy Lobe retreat (circa 11,000 B.P.), Dean suggests successive spillways directed Agassiz waters towards Lake Superior. While Superior Lobe ice blocked the ArrowPigeon valley, water ran through the Stump and Swamp valleys and along the northern edge of coastal ice, first exiting via the Brule and Kimball valleys. Underscored is the potential for people to have migrated eastwards into the corridor at this time. Further ice retreat later deflected flow down the Whitefish-Kaministiquia valleys, before the Matawin exit opened. Momot, W.T. and Stephenson S.A., 2007, Atlas of the Distribution of Fishes within the Canadian Tributaries of Western Lake Superior: Lakehead University, Thunder Bay, 383p. Phillips, B.A.M., 1997, Retro-deformation of the D.E.M. of Northern Minnesota as a Tool for Reconstruction of Former Glacial Lakes, in Proceedings of the 9th Minnesota GIS/LIS Consortium Conference, St. Cloud, Mn. Phillips, B.A.M. and Hill C.L., 2004, Deglaciation History and Geomorphological Character of the Region Between the Agassiz and Superior Basins, Associated with the ‘Interlakes Composite' of Minnesota and Ontario, in L.J. Jackson and A. Hinshelwood, Eds., The Late Palaeo-Indian Great Lakes - Geological and Archaeological Investigations of Late Pleistocene and early Holocene Environments, Mercury Series, Archaeology Paper 165, Canadian Museum of Civilization, Chapter 10, p. 275-301. Schmidt, K.P., 1993, Minnesota Parks Fish Species Lists: http://www.nativefish.org, 67p. Underhill, J.L., 1963, Distribution in Minnesota of the Sub-species of Percid Fish Etheostoma nigrum, and their Intergrades: American Midland Naturalist, v.70, p. 470-478. 26 2 �Figure 1 – Distribution of Johnny Darters and Flow from Lake Agassiz to Lake Superior. Figure 2 – Flow from Lake Agassiz to Lake Superior and the Keating Complex, MN-ON. 27 3 �The effect of H2O on olivine to ringwoodite transformation: Implications for subduction zone geodynamics and the deep earth water cycle Tamara Diedrich * † Thomas G. Sharp School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287 As the most abundant minerals in the earth’s upper mantle and transition zone, phase transformations between olivine and its high-pressure polymorphs, wadsleyite and ringwoodite, have major consequences for mantle geodynamics. Based on the thermal profile of subducting lithospheric slabs, it has been proposed that metastable olivine could persist into the mantle transition zone. Its presence would decrease negative buoyancy of the slab and slow subduction. In addition, the rapid transformation of metastable olivine has been identified as a potential initiating mechanism for deep-focus earthquakes. Several lines of evidence suggest that mantle olivine may contain significant H2O in the form of hydroxyl point defects. While previous studies have shown that H2O enhances olivine to wadsleyite transformation rates, under sufficiently cold conditions, olivine would transform directly to ringwoodite. Our research measures the effect of H2O on olivine to ringwoodite transformation rates and shows that even modest hydration of olivine results in near equilibrium transformation of olivine under subduction zone conditions and no metastable wedge of olivine below 400 km. For this study, a novel experimental approach was developed that consisted of two stages: hydration of mantle derived olivine in a piston-cylinder apparatus, and partial transformation of * Present address: Economic Geology Group, Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Hwy, Duluth, MN 55811 † Corresponding author e-mail: tdiedric@nrri.umn.edu 28 �these spheroids in a multi-anvil device. After hydration, the olivine contained 298 (±29) ppm by weight H2O. The multi-anvil experiments were conducted for varying run durations at 18 GPa and 700°C, 900°C, and 1100°C. The partially transformed samples were thin sectioned and the ringwoodite rim thicknesses were plotted as a function of run duration. Both growth rates and activation enthalpy for growth were determined. Water appears to promote olivine to ringwoodite transformation by enhancing growth rates, decreasing activation enthalpy for growth, and facilitating the dissipation of transformation stress. Growth rates for the hydrated olivine are almost two orders of magnitude faster than for anhydrous olivine at 1100 °C and about four orders faster at 700°C. At 700°C, growth is fast enough to be observed on an experimental timescale; this is significant because kinetic parameters from previous studies suggest that, at 500°C, anhydrous olivine does not transform on a geologic timescale. The addition of water also enables ringwoodite growth rates to remain constant throughout transformation. The large volume decrease associated with olivine to ringwoodite transformation, if not accompanied by sufficient plastic deformation, causes transformation stress. As transformation progresses, this stress results in the build-up of strain energy, which slows growth by counteracting the driving force for transformation. Hydrolytic weakening of the olivine and/or ringwoodite facilitates the dissipation of transformation stress so that growth rates can remain constant throughout transformation. Using our kinetic parameters for hydrated olivine transformation, the addition of as little as 289 ppm by weight H2O effectively eliminates metastable olivine from the transition zone. Alternatively, if the presence of metastable olivine is confirmed, it must contain significantly less than 298 ppm H2O, placing a constraint on models of the deep earth water cycle. 29 �GOLD MINERALIZATION IN THE VIRGINIA HORN GREENSTONE TERRAIN, ST. LOUIS COUNTY, MINNESOTA: A PROSPECT RE-VISITED Elsenheimer, D.1, Frey, B. 2, and Hudak, J. 2, 1Minnesota Department of Natural Resources, 500 Lafayette Rd., St. Paul, MN 55155, 2Minnesota Department of Natural Resources, 1525 Third Ave. E, Hibbing, MN, 55746, donald.elsenheimer@dnr.state.mn.us Petrologic, geochemical and fluid inclusion data from archived drill core have been combined with a digital compilation of historic exploration data and new maps to reassess the gold mineralization potential of the Virginia Horn Greenstone Terrain, a fifty square kilometer exposure of Wawa subprovince rocks located in St. Louis County, Minnesota. The Virginia Horn Greenstone Terrain consists of a folded and faulted metamorphosed assemblage of Archean volcanic and sedimentary strata, intruded by a quartzofeldspathic porphyry (QFP) and overlain by a conglomerate sequence that was interpreted by Jirsa and Boerboom (2003) as a Timiskiming-type package correlative with parts of the Knife Lake and Shebadowan Groups (Figure 1). Metamorphic grade varies from prehnite-pumpellyite to middle greenschist facies. Visible gold within QFP outcrops were first identified in the 1930’s and associated with quartz veining, carbonate-sericite alteration and major shear zones. Historic mineral leasing and gold exploration activity involved the completion of more than fifty diamond drill core borings, mostly within or near the highly-altered and sheared QFP (Figure 2). Assay results within portions of drill core were significant, but too inconsistent to be considered economic. There has been no drilling within the Virginia Horn since 1991, and all mineral leases held on state-owned or managed lands (which cover more than 50% of the terrain) have been allowed to expire. Completion of subsequent Shebandowan 25km Moss Lake geologic mapping and analog modeling, Ardeen combined with a resurgence of priceSHEBANDOWAN Thunder sensitive gold exploration activity GREENSTONE BELTBay prompted the Minnesota Department of Mud Creek Shear Zone Natural Resources (MnDNR) to reassess Prospect Knife Lake the potential for economic gold deposits Timiskaming sequences Ely within the Virginia Horn. A digital Plutonic rocks Virginia Horn compilation was created of historic Greenstone Greenstone Belt mineral lease records and subsequent Gold Deposit After Jirsa and Boerboom (2003) Gold Occurrence petrologic and geochemical analyses on drill core housed in the MnDNR’s drill core repository. This compilation includes the conversion of Manitoba hard-copy sample location reports and laboratory reports into GIS-friendly maps, shape files and data tables. Drill core Ontario samples were reviewed to fill identified data gaps and better correlate drill core logs prepared by different companies, and new petrologic, geochemical and fluid inclusion samples were Minnesota collected to better constrain the conditions and timing of gold mineralization. Figure 1 30 �Historic exploration efforts focused on a lode gold mineralization model for the QFP. The depth and density of drill cores completed using this model were not sufficient to rule out the possibility of economic lode gold deposits within the QFP. That said, reinterpretation of the Virginia Horn as a Timiskiming-type greenstone terrain would support new exploration efforts beyond the QFP, and into both the overlying basal conglomerate and associated country rock, particularly where major shearing and alteration is found. Figure 2 # Virginia ### # # # # # # ## # ### ### # # # # # ## # #### ###### # ## Paleoproterozoic Animiki Group # # # ## Eveleth # Exploratory Drill Hole Virginia Fm Biwabik Iron Formation Pokegama Quartzite Neo-Archean Giants Range # Batholith Monzodiorite Tonalite Conglomerate Archean Greenstone Quartz Feldspar Porphyry Argillite/Greywacke Mafic intrusives Mafic volcanics 0 miles 5 Support for new drilling within relatively unexplored portions of the Virginia Horn comes from a new visible gold occurrence (Figure 3), discovered during re-logging and examination of drill core Figure 3 DML-3. This bore hole intersected a major fault and was completed within a meta-argillite unit located approximately 8,000 feet from the QFP. The visible gold occurred in a quartz-calcite vein with local minor pyrite. The quartz vein, at 207.5 to 209 feet, had a broken ribbon texture parallel to the vein margins. The vein was oriented about 5 degrees to the core axis. Other quartz-calcite (and Field of view = 1mm minor pyrite) veins were more numerous, but the internal texture was one of crackled quartz with infilling calcite. Prior orientation of the vein during this stage of deformation may have created the different styles. Gold grain counts and pathfinder element concentrations in glacial till collected from locations down ice from both the QFP and this new visible gold occurrence will be used to further constrain competing gold mineralization models. 31 �Textural Stratigraphy of Nipigon Diabase Sills: A Tool for Correlation and Petrologic Interpretations FORSHA, Clinton J. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA 16057, USA, cjf8856@sru.edu The relationship between texture and position in an igneous intrusion can be used to interpret the injection dynamics of a sill. In addition, because texture is a strong function of cooling history, textures can be used to determine position in the sill, when the upper and lower contacts are not exposed. The primary purpose of this study is to examine the textures variations in sections through a diabase sill at multiple locations in the Nipigon area, in an attempt to correlate the profiles and construct a composite section. Previous investigations of the Nipigon sills, 1.1-Ga olivine tholeiites, have demonstrated, on the basis of mineralogical variations (e.g., Sutcliffe, 1989), and textural anomalies (Zieg and Forsha, this volume), that the sills were filled via a series of injection pulses rather than in a single, instantaneous injection. This episodic filling history impacted the modal mineralogy and textural variations in the lower contact zone, and is clearly discernible using crystal size distribution (CSD) variations. In order to test the use of textural variations for stratigraphic correlation in igneous intrusions, three sample profiles, from different heights within the sill, were investigated. The Kama Point section includes the lowermost 35 m of the sill, and exhibits strong internal discontinuities related to reinjection events. The Moseau section, which has a total thickness of ~45 m, but does not include an upper or lower contact, is significantly coarser-grained than the Kama Point section and contains no significant textural anomalies. The Palisades section, which has a thickness of ~40 m but does not include an upper or lower contact, does not follow same coarsening-upwards trend as the Kama Point and Moseau sections. The rocks in this section display a slight coarsening toward the center of the section, and appear to become slightly finer-grained in the upper part. With the coarse nature of the Palisades section and its gradual change from coarsening to fining, it is believed that this section is located in the center of the sill. As an application of this method, we combine the different sections into a composite profile through the lower half of the sill. This technique is useful in areas such as Nipigon, where total relief is less than the thickness of individual sills. Used together with mineralogical and chemical constraints, textural stratigraphy is a powerful tool for comparing and interpreting the histories of igneous intrusions. References Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. Canadian Mineralogist, 27: 67-79. 32 �PGE-rich mineralization at the Marathon Deposit, Coldwell Alkaline Complex, Ontario David Good and Phillip Walford, Marathon PGM Inc. Mineralization at the Marathon deposit occurs within the Two Duck Lake gabbro, a late-stage gabbroic subunit of the Eastern Gabbro located along the eastern margin of the Coldwell alkaline complex. To date, 70 million tonnes of in-pit resource including 750,000 tonnes of PGE-rich ore have been outlined. PGE-rich ore occurs within a mineralized envelope that is stratigraphically above relatively Curich ore in the Malachite and Southern Resource areas and has been intersected in 64 drill holes over a strike length of 1 km. The highest grade intersection is 106 ppm Pt+Pd+Au and 0.02% Cu over 2m and the highest value intersection is 55 ppm Pt+Pd+Au and 0.59% Cu over 8m. The 64 drill hole intersections were subdivided into a subset of 28 intersections based on a cut-off value of 3ppm Pt+Pd and a minimum thickness of 4 m (2 samples). The average grade and thickness of this subset is 9.38 ppm Pt+Pd+Au and 0.21% Cu over 7.56m. The Pd/Pt ratio is 3.25+/-0.81. The host Two Duck Lake gabbro consists of coarse grained to pegmatitic gabbro with subhedral plagioclase and olivine with ophitic-textured clinopyroxene and magnetite, and minor apatite and biotite. Xenoliths of fine-to medium-grained gabbro are common. Alteration or secondary minerals such as chlorite, sericite, serpentine and calcite occur locally and typically make up less than 5% of the rock. Sulphide mineralization occurs as disseminated chalcopyrite and minor pyrrhotite within coarse grained to pegmatitic olivine gabbro. A detailed study of platinum-group minerals in samples from the Malachite zone determined that approximately 70% of the observed individual grains were closely associated with chalcopyrite or bornite and that less than 7% were associated with secondary minerals such as chlorite or serpentine (Liferovich, 2007). Previous examination of metal-metal distribution diagrams for the Main zone showed strong inter-element correlations between Pd-Pt, Pd-Rh, Pd-Cu and Pd-Au (Good and Crocket, 1994). A comparison of PGE-rich ore to that of the Main zone shows two interesting relationships. First, the trend for Pd-Pt and Pd-Rh shown in the Main zone is continuous with that for the high grade ore. Second, there is no correlation between Pd and Cu in the high grade ore. The Au data are less decisive and somewhat scattered but seem to show behaviour similar to that for Pd. A magmatic model for partitioning of PGE and Cu by sulphide segregation and variations in the R-factor as described for mineralization in the Partridge River Intrusion (Theriault et al, 2000) is somewhat consistent with the data. A simple calculation using a concentration factor of 1000 for the mineralized zones suggests a magma column that is at least 10,000 m thick is required to produce a 10 m intersection with 10 ppm Pd, but the host Two Duck Lake gabbro is only 200 to 400 m thick. 33 �Thériault, R. D., Barnes, S.-J. and Severson, M. J., 2000, Origin of Cu-Ni-PGE Sulfide Mineralization in the Partridge River Intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-943 Liferovich, R., 2007, Mineralogy of PGE, Gold and Silver in the Malachite ore zone, Two Duck Lake deposit, Marathon, NW Ontario, Internal Company report. Good, D.J., and Crocket, J.H., 1994, Genesis of the Marathon Cu-platinum-group element deposit, Port Coldwell alkalic complex, Ontario: A mid-continent rift-related magmatic sulfide deposit: Economic Geology, v. 89, p.131 –149 34 �Sugarloaf – A State Scientific and Natural Area and a North Shore Non-Profit Association John C. Green Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Andrew Slade Sugarloaf: The North Shore Stewardship Association, 6008 London Road, Duluth, MN 55802 The 7-acre Sugarloaf Point Scientific and Natural Area (SNA) in southwesternmost Cook County, Minnesota, was designated in 1992 to preserve and protect the geological and ecological features of this bit of the North Shore, especially the basalt lava-flows and the dynamic beaches. Despite its history as a logging-era brownfield, it preserves excellent and accessible evidence for the character of both the 1.1 Ga Midcontinent Rift volcanism and Holocene postglacial processes, still active today. The bedrock consists of olivine tholeiite flows, part of the Schroeder basalts, near the top of the North Shore Volcanic Group (Miller et al., 2001; Miller et al., 2006). Features such as ophitic and amygdaloidal textures, columnar jointing, pipe amygdules, segregation veins, ropy surfaces, vesicle cylinders, and clastic dikes are well exposed (Green, 1989). Surficial features include an abandoned strandline and the overall tombolo form, with modern beaches linking the rocky point to the mainland. Beach clasts imply derivation from Rainy Lobe as well as Superior Lobe glacial deposits, along with local bedrock. The SNA, as well as 27 surrounding acres, is managed by a nonprofit group known as Sugarloaf, the North Shore Stewardship Association. The Association has built a log interpretive center, and has accomplished considerable environmental restoration on this heavily impacted site. Major restoration projects include excavation and replanting, with locally-grown native vegetation, of a rare coastal wetland between the berms of the tombolo. Sugarloaf hosts many popular interpretive programs, including day-long workshops on various aspects of North Shore natural history. It works with landowners and public land managers throughout the North Shore on both landscape restoration and interpretation. A major current initiative is the opening of the North Shore Interpretive Center at the University of Minnesota Duluth’s historic “Limno Lab” building at the Lester River in northeastern Duluth. Exhibits there will introduce visitors to North Shore natural and settlement history and UMD based research on the North Shore and Lake Superior. References Cited Green, J. C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., and Jerde, E.A., 2006, Bedrock geologic map of the Little Marais quadrangle, Lake and Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-172, scale 1:24,000. 35 �Emplacement of the Nipigon Sill Complex and Mafic to Ultramafic Intrusions of the Nipigon Embayment T.R. Hart and C.A. MacDonald Precambrian Geoscience Section, Ontario Geological Survey (tom.hart@ontario.ca) (caroleanne.macdonald@ontario.ca) The Nipigon Embayment consists of Proterozoic sedimentary and intrusive rocks underlain by Archean rocks of the English River, Wabigoon, and Quetico subprovinces. Recent geological mapping suggests that the Nipigon Embayment is underlain by a series of north-, northwest- and northeast-trending faults (e.g. Hart 2005; MacDonald et al. 2005). The north-trending faults can be traced for over 150 km, and commonly have apparent sinuous traces resulting from an intersection with north- and northwest-trending faults. A number of the northwest-trending faults control the roughly circular-shape of diabase sills as in the Kopka River (Sutcliffe 1986). The east- to northeast-trending faults include deformation zones exposed in surrounding greenstone belts, as well as greenstone belt and subprovince boundaries. Some faults, and particularly the intersections of a number of faults, appear to have been the locus for magmatic events active at a number of geological periods. A number of 2697-2686 Ma late- to post-tectonic mafic to ultramafic intrusions (Heaman and Easton 2006) occur across the central Wabigoon Subprovince from Lac des Iles north to Awkward Lake in the Obonga Lake greenstone belt near major east- to northeast-trending structures. The ~1540 Ma English Bay Complex and ~1595 Ma Badwater Intrusion (Heaman and Easton 2006) are located near the junction of the east-trending Obonga Lake mylonite zone / Humboldt Bay high-strain zone and a number of north-trending faults traceable south to Black Sturgeon Lake. These intrusions may represent magmatism related to formation of the Nipigon Embayment. In the same area, the 1159±33 Ma Inspiration diabase sill and 1112 Ma ultramafic Jackfish sill (Heaman and Easton 2006) may represent early stages of the MRS. To the south, the 1109 Ma Disraeli, 1113 Ma Seagull, and 1107 Ma Hele ultramafic intrusions are located near the intersection of northand northwest-trending faults with the Quetico-Wabigoon subprovince boundary (QWSB), also represent early MRS magmatic events. The 1117 Ma mafic to ultramafic Kitto Intrusion (Heaman and Easton 2006) is located near the intersection of the QWSB, and the north-trending Nipigon River fault. Many of the diabase sills have morphologies controlled in part by these three faults directions, particularly the roughly circular sills exposed in the Kopka and Garden PROTEROZOIC ARCHEAN Volcanic rocks Osler Group (1109 Ma) Lake areas. Granitic rocks Nipigon Sills (1112 Ma) Franklin et al. (1980) suggested that Mafic and ultramafic Ultramafic intrusions Ultramafic intrusions intrusive rocks the Embayment represented a failed Sibley Group Metasedimentary and arm of the Midcontinent Rift System Metavolcanic rocks English Bay Complex Sanukitoid intrusions 36 �(MRS). Alternatively, the Embayment could represent a structurally controlled basin formed by subsidence following an anorogenic thermal upwelling event represented by the ~1540 Ma English Bay Complex (Hollings et al. 2004). Studies completed by Rogala et al. (2005) indicate that deposition of the lower portions of the Sibley Group sedimentary rocks (>1339 Ma; Franklin 1978) pre-dated the formation of the ~1100 Ma MRS by ~200 Ma. The presence of sills rather than dykes and apparent lack of a dyke swarm common in many failed rift arms (Ernst et al. 2006) suggest that the Embayment was not extensional during the MRS event. A uniform lithogeochemistry for the diabase sills suggests a single magma source (e.g. Hollings et al. in press). Limited anisotropy of magnetic susceptibility data (Middleton et al. 2004) indicates westerly flow directions in the western Embayment suggesting a source, or sources, near the north-trending faults in the Black Sturgeon Lake area. Manson and Halls (1997) proposed that the geometry of the MRS could be the result of the interaction between the MRS and a series of faults within the bounding Archean rocks. The faults in Nipigon Embayment could be part of this series of faults and partially explain the lack of a classic failed arm. Some of these faults could have been deep structures providing pathways for the emplacement of a variety of magmas at different periods. The timing and magnitude of displacement along these faults is difficult to determine, but a high degree of fracturing and hematite alteration in diamond drill core from the 1112 Ma Seagull Intrusion (Heaman and Easton 2006) suggests multiple periods of fault activity. Late stage activity may be related to a 1090+/-20 Ma uranium mineralization (Ruzicka and LeCheminant 1984) located along a north-trending structure east of Black Sturgeon Lake. This younger age is closer to many of the events in the late Keweenawan, including the 1091+/-4.5 Ma Blake Gabbro (Heaman and Easton 2006), and suggests fault activity in the Nipigon Embayment for a period of up to 20 Ma. REFERENCES Ernst, R.E., Buchan, K.L., Heaman, L.M., Hart, T.R., and Morgan, J. 2006. Multidisciplinary study of N to NNE trending dykes in the region west of the Nipigon Embayment: Lake Nipigon Region Geoscience Initiative; Ont. Geol. Sur., Misc. Release Data, MRD 194. Franklin, J.M. 1978. The Sibley Group, Ontario; in Rubidium-strontium isochron age studies, report 2; ed. R.K. Wanless and W.D. Loveridge; Geol. Sur. Canada, Paper 77-14: 31-34. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H. and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada; Can. J. Earth Sci., 17: 633-651. Hart, T.R. 2005. Precambrian geology of the southern Black Sturgeon River and Seagull Lake area, Nipigon Embayment, northwestern Ontario; Ont. Geol. Sur., Open File Report 6165, 63p. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb Geochronology Results: Lake Nipigon Region Geoscience Inititative; Ont.Geol. Sur., Misc.Release—Data, MRD191 Hollings, P., Hart, T.R., Richardson, A., and MacDonald, C.A. in press. Geochemistry of the Mesoproterozoic Intrusive Rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the earliest phases of rift development; Can. J. Earth Sci. Hollings, P., Fralick, P. and Kissin, S. 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay granite-rhyolite complex, northwestern Ontario; Can. J. Earth Sci., 41: 1329-1338. MacDonald, C.A., Tremblay, E. and Easton, R.M. 2005. Precambrian geology of the west-central map area, Nipigon Embayment, northwestern Ontario: L.N.R.G.I.; Ont. Geol. Sur., Open File Report 6164, 49p. Manson, M.L. and Halls, H.C. 1997. Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent Rift. Can. J. of Earth Sci., 34: 562-575. Middleton, R. S., G. J. Borradaile, D. Baker, and K. Lucas, 2004. Proterozoic diabase sills of northern Ontario: Magnetic properties and history; J. of Geop. Res., 109, B02103, doi:10.1029/2003JB002581. Rogala, B., Fralick, P.W. and Metsaranta, R. 2005. Stratigraphy and Sedimentology of the Mesoproterozoic Sibley Group and Related Igneous Intrusions, Northwestern Ontario: Lake Nipigon Region Geoscience Initiative; Ont. Geol. Sur., Open File Report 6174, 87p. Ruzicka, V. and LeCheminant, G.M., 1984. Uranium deposit research, 1983; in Current Research, Part A, Geol. Sur. Canada, Paper 84-1a: 39-51. Sutcliffe, R.H. 1986. Proterozoic rift related igneous rocks at Lake Nipigon, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 325p. 37 �Lacustrine Sedimentary Organic Matter Proxies of Recent Lake State Changes and Climatic Conditions in Christina and Morrison Lakes of Western Minnesota Amanda Hogan, Travis Jacobs, and Kevin Theissen Department of Geology, University of St. Thomas, 2115 Summit Ave., St. Paul, MN 55105 Organic matter in shallow lake sediments reveal past states of the lake, indicating dryer or wetter periods as well as periods which were dominated by algal or vascular plants. We have analyzed lake sediment cores dating approximately back to the year 1400 and have correlated stable isotopic and elemental data to support recent lake transitions. In the summer of 2006, we collected sediment cores from Lake Christina and Morrison Lake, shallow lakes (< 4 m max depth) in western Minnesota’s Prairie Pothole Region. We took samples from the cores at 1 cm intervals and prepared them to be sent to the Stanford University Stable Isotope laboratory where they were analyzed for stable isotopic (δ13C and δ15N) and elemental (C/N, TOC, TN) values. Finally, over the last two months, the data have been analyzed and correlated to a sedimentation-rate based time scale to determine changes in ecological status of the lakes as well as past changes of precipitation in the areas surrounding them. Here we report preliminary results for Lake Christina. C/N ratios (8 to10) and δ13C values (-24 to-19 per mil) show a history where the lake has been dominated by lacustrine algae (Figure 1). The range of these values is small, and while they fluctuate, the levels never reach those of a lake dominated by vascular plants (1). These small changes indicate slight changes in organic matter production and sourcing. Based on our preliminary age estimates for Lake Christina, the values for δ15N show a steady increase from the year 1680 to 1900 where they begin to decline sharply over the next 100 years (Figure 2). The low values in the years 1400-1700 might indicate a wetter climatic period where the lake level was higher and algal productivity in the lake was greater (1). Precipitation slightly decreased over the next 300 years causing upward shifts in the value of δ15N as well as a decrease in %CaCO3. This drier period was possibly windier, causing more sediment particles to be blown into the lake leading to more turbulent waters with large amounts of mixing (3). It is likely that land use changes had an impact on the overall carbon isotopic values of the sediments. Values of δ13C show a sharp decline around 1900, which corresponds to a time when most of this area was farmland rather than grassland. This might signal a change to a more algal dominated lake at this time. However, around the year 1940, δ15N values begin to decline again indicating a wetter period. In the years since 1938, more wet years are on record than in the years prior. The decline in δ15N could be attributed to the wetter climatic conditions in the area over the last 60 years (1). 38 �Lake Christina d13C vs. C:N Ratio -19.50 -20.00 -20.50 -21.00 -21.50 -22.00 -22.50 -23.00 -23.50 -24.00 7.50 8.00 8.50 9.00 9.50 10.00 C:N Ratio Figure 1. Lake Christina d15N vs. Time 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 1400 1500 1600 1700 1800 1900 2000 2100 Time (years) Figure 2. References: (1) Meyers, P.A., and Lallier-Verges, E., 1999, Lacustrine sedimentary organic matter records of Late Quaternary Paleoclimates. Journal of Paleolimnology, 21, 345-372. (2) Pratt, L.M., Comer, J.B., and Brassell, S.C., 1992, Geochemistry of Organic Matter in Sediments and Sedimentary Rocks, 29-72. (3) Xu, H., Ai, L., Tan, L., An, Z., 2006, Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications. Chemical Geology, 235, 262-275. 39 �Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada, HART, Thomas, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON, P3E 6B5, Canada Recent geochronological studies (Heaman et al., 2007) have shown that the diabase sills and mafic/ultramafic intrusions in the vicinity of Lake Nipigon are among the oldest expression of igneous activity associated with the Midcontinent Rift (~1106 to 1115 Ma). Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern expression of the Midcontinent Rift (MCR), and have previously been collectively referred to as the Logan sills (Stockwell et al. 1972). However recent work suggests a geochemical difference between the sills to the north and south of Thunder Bay (Hart 2003; Hart et al. 2005). It has been proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite (Miller et al. 2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into more informal categories (i.e. Nipigon sills for the sills north of Thunder Bay, and Logan sills for those to the south) (Hollings et al., 2007). In 2006 a sampling traverse was undertaken through Thunder Bay in order to help determine the boundary between these two suites. The large geochemical data set for the Nipigon sills obtained as part of the Lake Nipigon Region Geoscience Initiative (Hollings et al., 2007) and a number of recently analyzed, east-northeast- to northeast-trending dykes south of Thunder Bay provide a means of comparing the sills near Thunder Bay. The sills and dykes sampled for this study are all characterized by LREE and enrichment and negative Nb anomalies typical of the majority of the local intrusive rocks associated with the MCR (Hollings et al., 2007). The majority of these sills fall are geochemically similar to the Nipigon sills. However, two ultramafic sills are geochemically similar to the Logan sill suite (i.e. with higher TiO2 and Gd/Ybn; Fig. 1). This suggests that the boundary between the Nipigon and Logan sills lies southwest of Thunder Bay. Data from other dykes south of Thunder Bay (L. Hulbert and R. Ernst, Geological Survey of Canada, pers. comm., 2006) show that the geochemistry of the Pigeon River dykes (Arrow River and Rita Bolduc occurrences) is similar to the range of data from other dykes of the Pigeon River swarm sampled on Lake Superior (Cloud Bay and Jarvis Point; Fig. 1). The Pigeon River dyke swarm geochemistry more closely resembles that of the sills of the Nipigon suite (Hollings et al., 2007) than that of the ultramafic intrusions or the Logan sills (Fig. 1). In contrast, the Mt. Mollie dyke appears to be transitional between Nipigon sills and Inspiration sills. Geochemical trends in individual dykes suggest fractionation or assimilation/contamination but data is currently limited. A sample taken from a Pigeon River dyke near its chilled contact with Rove Formation on Jarvis Point (JP-3) shows elevated Th and LREE compared to the dyke interior (JP-1, -2, -4; Fig. 2). A granophyric phase of the Mt. Mollie dyke on the southern end of Victoria Island (VI-9) displays similar, but elevated, REE compared to the main gabbroic part of the dyke. The granophyre also shows a marked depletion in Al and also in Ti and V, perhaps due to the relative lack of oxides in this late-stage differentiate. Additional isotopic and 40 �geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Figure 1: Major and trace element variation diagrams illustrating the geochemical affinities of the dykes and sills in around Thunder Bay. Data from Logan sills are from Hart (2002), Nipigon data are from Hollings et al. (2007). Date for dykes south of Thunder Bay are from and L. Hulbert and R. Ernst (Geological Survey of Canada, pers. comm.. 2006) from samples collected by M. Smyk and J. Scott (Ontario Geological Survey). Figure 2: Primitive mantle normalised diagram comparing geochemistry of dykes south of Thunder Bay. References Heaman, L.M., Easton, R.M., and Hart, T.R, 2006. Further Refinement to the Timing of Mesoproterozoic Magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, in press. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; ILSG, Proceedings Volume 49, Part 1-Programs and Abstracts: 21-22. Hart, T.R. 2005. South Black Sturgeon River–Seagull Lake Area, Nipigon Embayment, Northwest Ontario: Lithogeochemical, Assay and Compilation Data. Ontario Geological Survey, Miscellaneous Release of Data 147. Hollings, P., Hart, T.,Richardson, A. and MacDonald, C.A., 2007. Geochemistry of the Mesoproterozoic Intrusive Rocks of the Nipigon Embayment, Northwestern Ontario: Evaluating the Earliest Phases of Rift Development. Canadian Journal of Earth Sciences, in press. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E. 2002. Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58. Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie, K.L. 1972. Geology of the Canadian Shield. In Geology and economic minerals of Canada. Edited by R. Douglas. Geological Survey of Canada Economic Geology Report 1: 838. 41 �RECENT DEVELOPMENTS UNDERSTANDING THE VOLCANIC, MAGMATIC, TECTONIC AND METALLOGENIC EVOLUTION OF THE ELY GREENSTONE FORMATION, VERMILION DISTRICT, NE MINNESOTA HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, hudak@uwosh.edu HOFFMAN, A. T., Department of Geology, University of Minnesota – Duluth, Duluth, MN 55812 PETERSON, D.M., HEINE, J., Natural Resources Research Institute, University of Minnesota – Duluth, Duluth, MN 55811 The Ely Greenstone Formation comprises a steeply north- to southwest-dipping sequence of Neoarchean supracrustal and associated intrusive rocks associated with the Tower-Soudan anticline in the Vermilion District of northeastern Minnesota. Three distinctive units comprise this formation. The Lower Member of the Ely Greenstone (LMEG) is composed of calc-alkalic and tholeiitic basalt and basalt- andesite lava flows and tuffs with subordinate felsic lava flows, tuffs, epiclastic rocks and iron formations (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002, Hoffman, in press). The LMEG has been subdivided into the older Fivemile Lake Sequence (FLS) and the younger Central Basalt Sequence (CBS; Peterson and Patelke, 2003). The Soudan Iron Formation Member (SMEG) comprises Algoma-type interlayered cherty iron formation, basalt lava flows, epiclastic rocks and felsic tuffs (Peterson and Patelke, 2003; Hoffman, in press). The Upper Member of the Ely Greenstone Formation (UMEG) is composed of a monotonous sequence of poorly vesiculated tholeiitic basalt lava flows and localized Algoma-type iron formation lenses (Schulz, 1980; Southwick et al., 1998). The UMEG is commonly interstratified with the Lake Vermilion Formation (LVF:Schulz, 1980; Southwick et al., 1998), which is composed of greywacke, slate, conglomerate, and dacite tuff, as well as subaerial to submarine dacite to trachyandesite lava flows, tuffs, and associated intrusions (the informally named Gafvert Lake Sequence (GLS)); locally, however, LVF unconformably overlies LMEG and SMEG strata (Southwick et al., 1998; Peterson and Patelke, 2003). Schulz (1980) interpreted volcanological and sedimentary textures to indicate a change from a subaerial / shallow subaqueous setting to a deeper subaqueous environment during the temporal genesis of the Ely Greenstone Formation. Southwick et al. indicate a sharp transition from arc-associated volcanism (LMEG) to MORB-like volcanism (UMEG) occurs abruptly at the top of the SMEG. Recent volcanological reconstructions between the Soudan Mine and Armstrong Lake provide further evidence for the interpretations of Schulz (1980). The initial deepening of the submarine volcanic setting occurred during the development of the LMEG (Peterson et al., 2005; Hoffman, in press). Abundant primary mafic and felsic volcaniclastic strata, highly vesicular basalt to basalt-andesite pillow lavas and sheet flows, multiple selvege pillows, and epithermallike zinc stringer mineralization that characterize the FLS indicate a depositional setting in a shallow subaqueous environment (Hudak et al., 2002; Hoffman, in press). The overlying CBS comprises exceptionally well-preserved, single-selvege pillow lavas and associated sheet flows with sparse (generally <5%) vesicularity which are interstratified with subordinate banded iron formation and mafic tuff (resedimented basalt hyaloclastite) interpreted to have formed in a deep subaqueous setting (Peterson and Patelke, 2003; Peterson et al., 2005). The SMEG contains 42 �finely laminated chemical sediments and resedimented tuffs, as well as sparsely vesicular mafic lava flows suggestive of deposition within a deep subaqueous setting (Peterson and Patelke, 2003; Hoffman, in press). This deeper water setting appears to have persisted through the development of the UMEG, and at least locally during the development of the LVF. New major- and trace element data indicate the lithogeochemistry of volcanic rocks in the LMEG is more complicated than previously recognized. Arc-related basalts and basaltic andesites and FI- and FII-type rhyodacites and rhyolites characterize the FLS. In the CBS, arcassociated basalts and basaltic andesites transition up-section into E-MORB, OIB and back-arc basin-like basalts which are associated with FIII-type felsic volcanic rocks (Hudak et al., in prep. Hoffman, in press). The UMEG is characterized by MORB compositions that may also be the product of back-arc spreading (Southwick et al., 1998). A model encompassing initial arc development followed by back-arc development and rifting during the CBS, with subsequent development of the SMEG, appears to be most consistent with the observed volcanological and lithogeochemical characteristics in this part of the Vermilion District. Iron formations within the SMEG occur immediately up-section from the arc – back-arc transition, a stratigraphic position shown in many studies to have high prospectivity for hosting volcanogenic massive sulfide orebodies. References Hoffman, A. T., in press, Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: Unpublished M. S. thesis, University of Minnesota – Duluth. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Heine, J., Newkirk, T. T., Hocker, S. M., and Hauck, S., in prep., Comparative Geology, Stratigraphy, and Lithogeochemistry of the Needleboy Lake – Six Mile Lake Area, Vermilion District, NE Minnesota: Natural Resources Research Institute Geological Report of Investigation. Peterson, D. M., Jirsa, M. A., and Hudak, G. J., 2005, Architecture of an Archean greenstone belt: stratigraphy, structure, and mineralization; in Robinson, L, ed., 2005, Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21, p. 154-180. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL). Geological site investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003, 88p. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 43 �EVALUATION OF MINERAL EXPLORATION DRILL CUTTINGS IN THE RICE RIVER AREA, EAST CENTERAL MINNESOTA Hudak, J. N. and Frey, B. A., Minnesota Department of Natural Resources, Lands and Minerals Division, 1525 3rd Ave. East, Hibbing, MN 55746 The Rice River area is located at the east end of the Cuyuna South Range, near the town of Aitkin. From 1949 to 1952, M.A. Hanna drilled 334 reverse circulation drill holes along approximately 12 km strike length of an iron formation related magnetic anomaly, to search for naturally enriched iron ore. Current DNR work in the Rice River area involves relogging of core and cuttings, the analysis of 325 samples for indications of base and precious metals, and the recovery of more accurate drill hole locations. The mid-Proterozoic basement rock does not crop out in the area, and drill cuttings and core are currently the only available bedrock samples. Recent replotting of geochemical lake sediment survey data revealed clustered metal anomalies in the Cuyuna South Range area. The Rice River area drill holes have been predominantly analyzed for iron and manganese and not fully evaluated for mineral potential. The main goal of the current evaluation is to see if there is any evidence of those anomalies in the Rice River area. Plausible ore deposit models for the area include SEDEX and iron-oxide-copper-gold. The magnetic iron anomaly is linear except for the large s-shaped fold, which is spatially associated with stratified sulfide occurrences and large quartz veins. Pyrite is often remobilized within shear zones of the graphitic argillite associated with this area. Veinlets with small amounts of bornite also occur scattered within the Rice River area. Currently, 60 of the 325 samples collected have been analyzed yielding anomalous levels of copper (2200 ppm) and antimony (6.75 ppm). A report of the work and findings are scheduled for release on June 30, 2007. 44 �PALEOENVIRONMENTAL INTERPRETATION OF A LOWER PALEOZOIC STROMATOLITE REEF, NORTHEASTERN WISCONSIN JASINEVICIUS, Renata R., GORDON, Elizabeth A., St. Norbert College Geology Department, De Pere, WI 54115, elizabeth.gordon@snc.edu INTRODUCTION Strata recently exposed in Schaal Quarry near Gillett, Wisconsin, provide an exceptional opportunity to examine the facies architecture of a lower Paleozoic stromatolite reef. The threedimensional nature of the reef, and its stratigraphy, are the focus of this study. The dolomitic rocks yield conodonts of early Ordovician age (Miller, 2006) and these strata are therefore assigned to the Oneota Formation. The Oneota dolomite overlies an unconformity, considered by many to be of regional extent. At this quarry, sandstones of uncertain Cambrian(?) age lie beneath the unconformity. Bedrock exposures in northeastern Wisconsin are extremely rare, due to widespread glacial deposits. This site therefore provides an important new opportunity to study Cambro-Ordovician boundary strata along the eastern margin of the Wisconsin Arch. RESULTS The Oneota Formation contains diverse stromatolite morphologies. At this locality, four stromatolite types occur in regular stratigraphic order: laterally linked hemispheroids, cabbageshapes, isolated small domes, and larger elliptical compound domes. In cross section, the larger stromatolites appear to form a mound- or ridge-shaped structure. The following paragraphs offer a summary and interpretation of the major sedimentary features associated with these stromatolites. The base of the Oneota in Schaal Quarry is dominated by an intraclastic conglomerate of variable thickness which overlies an unconformity. Desiccation cracks on the base of the conglomerate record subaerial exposure of the underlying units. Clast types include red shale, dolostone, sandy to silty dolomite pebbles and cobble to boulder-sized silcrete. The matrix is a dolomitic sandstone, and the unit grades upwards to sandy and silty dolomites. The silty dolomites contain discontinuous layers with microbial-like structures and laminated mats. These layers are interbedded with cross- and planar laminated, flaser- to wavy-bedded grainstones, and rare desiccation cracks. Grains types are primarily coated quartz sand and minor dolomite intraclasts. Upsection, discontinuous horizons of laterally linked hemispheroids (LLHS; Logan, 1964) mark the first visibly distinctive stromatolites, which predominate several horizons. In places they are vertically stacked, forming beds up to 20 cm thick. Individual hemispheres are round to elliptical with variable dimensions. LLHS are buried by ripple cross- and planar-laminated grainstones, grading to wackestones. Clay and glauconite locally contribute a greenish color to these rocks. 45 �Farther upsection, a fine-grained silty dolomite contains chert-lined vugs interpreted to be molds of anhydrite. These structures suggest hypersaline conditions. The overlying grainstones contain abundant crescent-shaped fossils of unknown origin, similar to “chitons” reported by Raasch (1952). Gastropod molds are present but rare. Conodonts include “Teridontus sp., Acanthodus uncinatus, Aloxoconus sp., Cordylodus lindstromi, and are assigned to the interval from the uppermost Cordylodus lindstromi Zone, the Iapetognathus Zone, and into the lower part of the Cordylodus angulatus Zone” (Miller, 2006), indicating an early (but not earliest) Ordovician age. Continuing upsection, there is a general upwards increase in bed thickness, accompanied by a decrease in coated quartz sand grains, and an increase in peloids and sparry cored ooids. Local grapestone intraclasts are also present. Within this interval cabbage-shaped stromatolites (CS) form discontinuous patches along one horizon. They are widely associated with chert. One wall reveals a curious relationship: adjacent stromatolite heads successively increase in size, suggesting a mound-like geometry of this cluster. CS are overlain by cross-bedded, oolitic peloidal grainstones, indicating burial by subaqueous dunes. Near the top of the quarry, thicker cross-bedded grainstones enclose nearly circular, isolated stromatolite domes. The domes appear to be concentrated in a mound or ridge. Higher still, the top surface of the quarry exposes large elliptical domes. A visible change in stromatolite synoptic relief indicates increasing water depth with time (Gerdes and Krumbein, 1994). The elliptical domes are clustered locally with subparallel long axes. The axes are aligned with paleocurrent azimuths measured from adjacent crossbeds and parting lineations. This association suggests stromatolite morphology was controlled in part by subaqueous currents. The predominant paleocurrent direction is approximately perpendicular to the inferred paleoshoreline. This implies subaqueous currents were tidal in origin and that stromatolite domes developed in subtidal channels. Collectively, these sedimentary features record evolution of a reef during marine transgressions. Interpreted paleoenvironments reflect tidal flat to subtidal environments under extreme conditions. REFERENCES Gerdes, G. and Krumbein, W., 1994, Peritidal stromatolites; in Phanerozoic Stromatolites II, ed. Bertrand-Sarfati, J. and Monty, C.: 114-17. Logan, B.W., Rezak, R., and Ginsburg, R.N.; 1964, Classification & environmental significance of stromatolites; Journal of Geology 72: 68-83. Miller, James F., 2006, Southwest Missouri State University; personal communication. Raasch, G. O., 1952, Oneota Formation, Stoddard Quadrangle, Wisconsin; Illinois Academy of Science Transactions, V. 15 p. 85-95. 46 �PROPOSED NEW BEDROCK GEOLOGIC MAP OF MINNESOTA JIRSA, Mark A. (jirsa001@umn.edu), BOERBOOM, T.J., CHANDLER, V.W., LIVELY, R.S., MILLER, J.D., Jr., MOSSLER, J.H., RUNKEL, A.C., SETTERHOLM, D.R., and WAHL, T.E, Minnesota Geological Survey (www.geo.umn.edu/mgs) Having conducted detailed geologic mapping in many areas of the state, and reprocessed state-wide geophysical data, the Minnesota Geological Survey (MGS) is well positioned to produce a new interpretation of bedrock geology. We are preparing to construct a state-wide bedrock geologic map— one that will be comprehensive, up to date, digital, and multi-layered. In the process, we also intend to upgrade digital bedrock topography and depth to bedrock maps and make them consistent with the new geologic framework (for preliminary imagery, see MGS website, Open-File 06-02). Together, these maps will provide the context necessary for various minerals and water-related applications in progress and under consideration. They will also establish the understanding of bedrock geology and crustal structure needed to address a number of impending, high-profile regional and national science initiatives, including EARTHSCOPE (www.earthscope.org). At this writing, the work of compiling hundreds of archival maps is underway. The existing state-wide bedrock map (MGS map S-20), completed in 2000, was reasonably detailed and accurate for its time; however, more than 50 new bedrock geologic maps have been created in the intervening 7 years since its production. Furthermore, the earlier map was compiled at a generalized 1:1,000,000 scale. As such, it portrays contacts that locally are inconsistent with high-resolution geophysical imagery, and it is 2-dimensional—in the sense that it depicts only the uppermost bedrock units. By contrast, the new map will be compiled at more detailed scales overall (compilation scale 1:100,000; publication scales 1:500,000 and 1:1,000,000), and geologic contacts will be reconciled with reprocessed geophysical data. It will also contain thematic layers, including trajectories of diabasic dikes, generalized metamorphic grades, and themes depicting Mesozoic, Paleozoic, Mesoproterozoic, Paleoproterozoic, and Archean geology. The Quaternary layer will be represented at this stage by revised depth to bedrock imagery. This layering will give users the opportunity to consider the third dimension by permitting removal of geologically younger layers to reveal older ones, to the extent that this can be done with some certainty. Perhaps more important than the printed maps and printable pdfs of archived maps, is the aim of producing a digital platform that will allow web-based user navigation from compilations to the most recent, accurate, and detailed mapping. Associated digital files will be searchable on attributes of lithology, age, and other themes. Mapping will be integrated with broader digital efforts using compilation formats consistent with those developed nationally (e.g., USGS) and globally (e.g., OneGeology.org). The project requires compilation of the best available maps—both digital and analog—together with new work designed to fill voids, augment existing map imagery, and evaluate the nature of buried geologic terranes. In addition, a small program of high-precision geochronology will acquire ages for a hand-full of critical samples. The results will be a seamless state-wide geologic interpretation, a webbased project that links this image to original mapping, and a digital cache of associated files. The poster presented here for general discussion depicts an initial effort to bring together the various digital map coverages. 47 �TRACE ELEMENT ANALYSES: AVOIDING DATA DISTORTION McSWIGGEN, Peter L., McSwiggen and Associates, P.A., 2855 Anthony Lane South, Suite B1, St. Anthony, MN (PMcS@McSwiggen.com) Electron microprobes have been used for trace element analyses for decades. This is because they can analyze minerals in situ, the size of the analytical area is very small, and the cost per analysis is very low. None-the-less, there is an ongoing push to measure trace elements in ever-lower concentrations. Over the years, improvements in both the electronics and the spectrometers of microprobes have made this possible. However, how the data is collected and how it is subsequently processed can mean the difference between meaningless numbers and real insights into the geologic question being investigated. Typically, people collecting trace element data want to know the minimum detection limit (MDL) of the method being used. Secondly, they want to know what should they do with the data that fall below the MDL. Should those values be set to zero? Should those values be set to the MDL? Should all of the data that fall below the MDL be discarded? If the set of data is going to be used to determine an average concentration for a rock body or a mineral type, all three options are wrong. In most cases involving trace element analyses, the calculated minimum detection limit for a single analysis should be ignored. Often, analysts will attempt to adjust their analytical conditions to get the lowest calculated minimum detection limit. However this can result in erroneous data. The MDL can be lowered using a number of strategies – for example, by counting for a longer period of time or by using a higher beam current to produce a higher count rate, both will lower the MDL. However both of these strategies can work against the overall objective. A higher beam current can damage the mineral being analyzed and therefore completely throw-off the results. Similarly, extremely long counting times can mask electronic instability in the instrument, thereby producing erroneous results. A far better strategy would be to collect a series of analyses, using more reasonable counting times and beam currents, and determine an average value from that set of data. This would minimize the effect of beam damage and would both monitor and correct for any electronic drift in the instrument; thus yielding a true detection limit that is much lower than that from the individual analyses. If one is using a set of analyses to determine an average composition, the calculated MDL becomes a meaningless value. Figure 1 shows a set of analyses run on a glass standard with a reported Ni content of 0.046 wt% using only a two second counting time per analysis. The average for this set of data was 0.053 wt%, a difference of only 0.007 wt% from the reported value. However, the calculated MDL for a single analysis was 0.25 wt%. Therefore, if all of the data that fell below the MDL was discarded, most of the data would have been thrown out, and those remaining values would have produced a completely erroneous composition of 0.275 wt% (Fig. 1). 48 �Not only should the data that fall below the MDL not be discarded, results that are below zero should also NOT be discarded. These negative values are just as important in determining the true average, as are the positive values. Each analysis has a certain error component associated with it. That error may be positive or negative. For trace element analyses, a negative error added to the real value, may result in a measured value less than zero. To get the true value, you need to average all of the data. By discarding only values on the negative side of the histogram, the data set becomes biased to a higher average value (Fig. 2). This is a big problem, because most commercial analytical instruments handle their data in this manner. Figure 1. Histogram showing multiple analyses of the NIST standard SRM-610. Two second counting times were used, resulting in a calculated MDL of 0.25 wt%, for a single analysis. The calculated average composition is 0.053 wt%, which agrees closely with the reported composition of 0.046 wt%. Figure 2. Shown is the same data set used in Figure 1, however all of the analyses that reported a negative concentration were set to zero. This is the typical procedure used by commercial analytical equipment. However, this method produces a distortion of the results. The average has been shifted from 0.053 wt% to 0.064 wt%, and this will distort any other statistics generated from the data. 49 �AN OCCURRENCE OF AGRELLITE IN THE WAUSAU ALKALINE IGNEOUS COMPLEX, MARATHON COUNTY, WISCONSIN MEDARIS, L. G. Jr and FOURNELLE, J. H., Dept. of Geology and Geophysics, Univ. of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu, johnf@geology.wisc.edu; GUGGENHEIM, S., Dept. of Earth and Environmental Sciences, Univ. of Illinois at Chicago, Chicago, IL 60607, xtal@uic.edu Agrellite is a rare, triclinic sodium-calcium fluorosilicate that occurs in alkaline (agpaitic) igneous complexes. Agrellite was first described from metamorphosed nepheline syenite in the Kipawa Complex, Quebec (Gittins et al., 1976), and its crystal structure was determined by Ghose and Wan (1979). To date, the only other reported occurrence is in the Khibina alkaline complex, Russia (Khomyakov, 1995). During construction of Interstate Highway 39 in northcentral Wisconsin, an agrellite-bearing dike was exposed in a roadcut, which was subsequently covered. The agrellite-bearing dike is located in the northern margin of Center 1 in the Wausau Complex (Fig. 1), where it cuts syenite and monzodiorite. White to light gray, tabular crystals of agrellite, ranging up to 15 mm in length, are concentrated along the center of the dike, where they are associated with quartz, perthitic alkali feldspar, plagioclase, aegirine-augite, miserite, apatite, fluorite, and locally eudialyte (Fig. 2). Fig. 1 Simplified map of igneous complexes near Wausau and the agrellite locality (modified from LaBerge and Myers, 1983) Fig. 2 Photomicrograph under crossed polarizers; a, agrellite; p, pyroxene (aegirine-augite); mf, finegrained miserite and feldspar 50 �Agrellite is biaxial, with 2VX = 55º, nX = 1.570(1), nY = 1.581(1), nZ = 1.584(1), and nZ - nX = 0.014. Cleavage is excellent on {110} and {110} and poor on {010}. The optic directions, Z and Y, are nearly parallel to the crystallographic axes, c and b, respectively, and the angle between the {110}and {110} cleavages is 40º (Fig. 3). Agrellite is triclinic, space group P1, and has the following cell parameters: a = 7.757(7) Å, b = 18.90(3) Å, c = 6.975(6) Å, α = 90.00(8)º, β = 116.81(6)º, γ = 94.4(1)º, V = 909.5(1) Å3, and Z = 4. Fig. 3 Sterographic projection of optical and crystallographic elements of agrellite The theoretical end-member formula for agrellite is NaCa2Si4O10F. Naturally occurring agrellite lies close to this composition, with the main deviations arising from the substitution of minor amounts of REE, Mn, and Sr for Ca (Table 1). Agrellite from the Wausau Complex is similar compositionally to that from the Kipawa Complex, except for having lower contents of REE and higher contents of MnO and SrO. The phase equilibrium conditions for agrellite are presently unknown, but the crystallization of agrellite, rather than wollastonite, in alkaline igneous complexes is likely promoted by high alkaline contents, i.e., (Na + K) / Al > 1, and high fluorine activity. The coexistence of agrellite with quartz in the Wausau Complex and with nepheline in the Kipawa Complex demonstrates that agrellite is stable in both silicasaturated and silica-undersaturated environments. Table 1 Composition of Agrellite SiO2 ZrO2 REE* MnO CaO SrO BaO Na2O K2O F -O=F Total 1 59.7 na 1.18 0.79 24.8 2.91 0.15 7.39 0.07 4.55 101.5 1.9 99.6 2 58.8 0.18 3.84 0.25 25.7 0.16 0.06 7.90 0.22 4.45 101.6 1.9 99.7 3 60.9 28.4 7.86 4.82 102.03 2.03 100.00 na, not analyzed 1 Wausau Complex, EMP analysis, UW-Madison 2 Kipawa Complex, Gittins et al., 1976 3 Theoretical end-member composition * Total of REE oxides References Ghose, S.G. and Wan, C (1979) Amer. Mineral. 64, 563-572. Gittins, J. et al. (1976) Canadian Mineral. 14, 120-126. Khomyakov, A.P. (1995) Mineralogy of Hyperagpaitic Alkaline Rocks: Oxford, 223 pp. LaBerge, G.L. and Myers, P.E. (1983) Wisc. Geol. Nat. Hist. Survey, Inf. Circ. 45, 88 pp. 51 �Metamorphosed, halite-dominated evaporites of the Lower Sibley Group Metsaranta, R.T.1, Fralick, P.W.2 and Bowdidge, C.1 1 RPT Uranium Corp. 537 Hilldale Rd., Thunder Bay, Ontario, P7B5N1, Canada 2 Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, Ontario, P7B 5E1, Canada. This paper describes newly discovered sedimentary halite deposits of Mesoproterozoic age from the Lower Sibley Group. The deposits were intersected in two closely spaced diamond drill holes, BSW-06-1a and BSW-06-4, drilled by RPT Uranium Corporation during the summer of 2006. The drill holes are located in the central portion of the preserved Sibley Basin, immediately west of the Black Sturgeon Fault, a major structure controlling the outcrop pattern of the Sibley Group. In the area, the Sibley Group is intruded by two thick Nipigon diabase sills that have produced significant contact metamophic effects. The evaporites occur in an approximately 50m thick section of Sibley Group rocks preserved between the lower diabase sill and Archean basement rocks. The basic stratigraphy beneath the lower diabase sill consists of from, bottom to top: a lower, well-cemented, sandstone dominated unit 14m thick, an interbedded halitemudstone unit 5m thick, an upper almost totally unconsolidated, partially halite cemented sandstone unit approximately 10-15m thick and an upper unit of interbedded calcareous mudstones and siltstones 20m thick. Lithostratigraphically, the lower sandstone unit can be correlated with the Pass Lake Formation while calcareous mudstones and siltstones in the upper portion of the section likely correlate with the lower Rossport Formation. Halite occurs in several forms within the section. First, it occurs as relatively large angular nodules within sandstones in the top few metres of the lower sandstone dominated interval (Fig. 1a). Second, it occurs as massive relatively pure beds 1-25cm in thickness within the interbedded halite-mudstone interval (Fig. 1b and c). Third, it occurs in irregular mixtures with clastic material, where clastic material is found as inclusions within halite and/or halite occurs in nodular form within the clastic sediment (Fig. 1.d). Finally, it occurs as cement and isolated nodular material in clastic sediments of the upper unit (Fig. 1e and f). Mineralogically, the composition of the evaporites is dominated by halite. Based on xray diffraction and reflectance spectroscopy gypsum is entirely absent from salt samples analyzed but there may be trace sylvite in some cases. Interbedded clastic materials commonly consist of mineral assemblages containing tremolite-actinolite, calcite, phlogopite, and serpentine group minerals consistent with contact metamorphism of carbonate-rich lithologies. Some samples contain clay minerals vermiculite, saponite and allophane which likely resulted from post metamorphic alteration of the contact metamorphic mineral assemblage. Sedimentologic and geochemical analysis of these unique deposits is currently underway. 52 �A B C D E F Figure 1. Various halite facies in the BSW-06-01a and BSW-06-4. A) Angular halite nodules within quartz-rich sandstone. B) Interbedded halite and mudstone, showing cyclically interbedded nature of halite beds (dark) with mudstone (light). C) Detail of a relatively pure, massive halite bed. D) Irregularly interbedded clastic material and halite. E) Very poorly consolidated halite-cemented sandstone. F) Photomicrograph of a halite cemented quartz sandstone. 53 �A CHEMICAL AND SR ISOTOPIC STUDY OF THE PIGEON POINT SILL, COOK COUNTY MINNESOTA M.G. Mudrey, Jr., 106 Ravine Road, Mount Horeb, WI 53572, mgmudrey@mhtc.net Isotopic and trace element by J.L. Wooden, then of LOCKHEED, NASA-Johnson Space Center, Houston, TX) In 1980, Wooden and Mudrey presented some isotopic and trace element data for the Keweenawan troctolite sill on Pigeon Point, Minnesota, the type locality of the mineral pigeonite. Wooden’s data were collected at the Johnson Space Center, Houston Texas, under exceedingly controlled laboratory conditions, comparable to lunar specimens at that time. The data have never been published, and interpretation has been limited because of other obligations of Mudrey and Wooden. Major elements were analyzed by a combination of colormetric, gravimetric and other classical techniques by S.S. Goldich and colleagues at Northern Illinois University and Pennsylvania State University. The Pigeon Point sill is an approximately 120 meter thick. differentiated troctolite to olivine gabbro intrusion of Keweenawan age situated on the Pigeon River between Minnesota and Ontario. The sill is of petrologic interest because of the association of three rock types troctonite/olivine gabbro, a ferro granodiorite, and a granophyre, whose genetic relationships have been debated for many years. Processes that have been invoked to explain the basaltic magma composition and significant abundance of granophyre (30 percent of sill thickness) on Pigeon Point include: Crystal Fractionation (Grout,1928); Magma Mixing Assimilation(Daly, 1917); Partial Melting (Bayley, 1893); Hydrothermal Alteration (Bastin, 1938); and Assimilation-Differentiation (Mudrey, 1973). The basal, chilled troctolite (PP-219-3) is a high alumina olivine tholeiite with a light enriched REE pattern (normalized La=19, Yb=6.8) having a positive Eu anomaly (Eu/*Eu=1.22), and for many years was viewed as a possibility as a parental magma for some parts of the Keweenawan. While this sample may approximate a parental liquid composition, an origin involving plagioclase and olivine accumulation is also possible. Rb-Sr isotopic data for 5 olivine gabbro and ferrogranodiorite samples form an approximate 1.429 Ga (initial 87Sr/86Sr = 0.70417). isochron. This "age" is similar to Rb-Sr ages determined for the RoveA Formation that the Pigeon Point sill intrudes. Compositional data for the sill suggests that selective contamination by alkalies and radiogenic Sr from the Rove Formation has occurred. The Sr isotopic data of the granophyre does not follow the pattern of the other samples, and its Rb/Sr ratio of 1.64 allow model ages of 1.11 to 1.15 Ga (probably representative of the crystal1ization age) to be calculated when the other sill samples are used for initial ratio control (I=0.7076 - 0.7046). (Best model age 1.132 Ga, 87Sr/86Sr = 0.70549). Petrologic, and trace element data argue for magma ascent from the upper mantle through Archean basement with low degrees of partial melting of potassic/alkaline constituents adjacent to the conduit followed by differentiation at an intermediate reservoir with ascent and emplacement at Pigeon Point at shallow depths (vaiolitic cavities, 1000 m). Local partial melting 54 �of overlying Rove Formation and stoping of quartzite within the Rove Formation account for the significant volume of granophyric rock. Bastin, E.S., 1938, Hydrothermal alteration in rocks of Pigeon Point, Minnesota, Journal of Geology, v. 46, p. 1058-1072. Bayley, W.S. 1893, The eruptive and s3edimentary rocks on Pigeon Point, Minnesota and their contact phenomena: U.S. Geological Survey Bulletin 109, 121 p. Daly, R.A., 1917, The geology of Pigeon Point, Minnesota: American Journal of Science, Series 4, v. 43, p. 423-448. Grout, F.F., 1928, Anorthosites and granite as differentials of a diabase sill on Pigeon Point, Minnesota, Geological Society of America Bulletin v. 39, p. 555-577. Mudrey, M.G., Jr., 1973, Structure and petrology of the sill on Pigeon Point, Cook County, Minnesota: University of Minnesota PhD dissertation, 310 p. Wooden, J.L., and Mudrey, M.G., 1980, A Chemical and Sr Isotopic Study of the Pigeon Point Sill, Cook County Minnesota (abs): EOS (Transactions AGU) v. 61, no 48 p. 1193. 55 �INVESTIGATIONS OF SULFIDE MINERALS LEACHED IN THE PRESENCE OF ALKALINE SOLIDS Nicholas, S.L. and Wirth, K.R., Geology Dept., Macalester College, St. Paul, MN 55105 Engstrom, J. and Lapakko, K.A., MN-DNR, Div. of Lands and Minerals, St. Paul, MN 55155 The oxidation of sulfide minerals present in mine waste exposed to the atmosphere can lead to the generation of acidic drainage. We used scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) to examine unreacted rock and eight remnant solids. The solids were leached for periods of 117 to 899 weeks in laboratory experiments conducted by the Minnesota Department of Natural Resources (MN-DNR) on finely-ground (<0.149 mm) Duluth Complex rock (75 g) (Lapakko et al. 1997; 2000). This rock, which contains approximately four percent pyrrhotite (Fe1-xS) and lesser amounts of chalcopyrite-cubanite, pyrite, and pentlandite, was mixed with different amounts of rotary kiln (RK) fines or limestone. The rock samples were rinsed weekly with 200 mL of distilled deionized water, and the resultant drainage was analyzed regularly to assess acid-producing and acid-neutralizing reactions of each mixture and the controls. The effects of chemical reactions on water quality were documented and evaluated in detail, but little was known about the effects on the solidphase sulfide minerals. A working hypothesis was that oxidation of sulfide minerals occurred from the outside of the grain inward, leaving a sulfur-depleted rind around the grain. This oxidation rind might retard diffusion of sulfur from the center of the grain into solution. We identified the presence or absence of residual alkaline solids using a binocular microscope. SEM was used to describe the degree of sulfide oxidation, using the relative proportions of sulfur and oxygen determined by EDS. The extent of oxidation was described as the fraction of total analyzed sulfide area that was oxidized to a specific degree. Limestone grains persist in the limestone-loaded reactors, whereas no alkaline grains were detected in the reactors loaded with RK fines. Oxidation of sulfide grains begins along the edges, cracks, and partings of grains, eventually forming irregular rinds of sulfur-bearing iron oxide (Figures 1 and 2). Less of the pyrrhotite area was oxidized in samples with an alkaline addition than in the control (Figure 3), and reactors with limestone additions had smaller extents of oxidized areas than did the reactors loaded with RK fines. Comparison of the extent of pyrrhotite oxidation in two reactors with the same loading that were leached for different lengths of time (Figure 3, reactors loaded with 4.7g RK fines) indicates that the rate of pyrrhotite oxidation is greater under acidic conditions, a result that is consistent with leachate water chemistry. The four treated reactors that generated no acidic drainage were among the five least oxidized. References Lapakko, K.A., D.A. Antonson, and J. Wagner 1997. Mixing of limestone with finely-crushed acid-producing rock. Proc. 4th Int. Conf. on Acid Rock Drainage, Vol. 3, Vancouver, B.C., p. 1345-1360. Lapakko, K.A., D.A. Antonson, and J. Wagner 2000. Mixing of rotary-kiln fines with finegrained acid-producing rock. Proc. 5th Int. Conf. on Acid Rock Drainage. SME, Littleton, CO. p. 901-910. 56 �Figure 1: Backscattered electron (BSE) image of pyrrhotite from reactor (0.8 g RK fines addition) showing complex oxidation (darker regions) along grain edge and fractures (scale = 50 µm). Figure 2: BSE image of pyrrhotite with a rind of sulfur-bearing iron oxide from reactor with 7.9 g limestone addition. Shattering of grain occurred during sample preparation (scale = 50 µm). Figure 3: Extent of pyrrhotite oxidation as a function of reactor conditions and water quality data. The degree of oxidation increases from unoxidized pyrrhotite (unleached original) to sulfur-bearing iron oxide (>95% of control reactor). Weeks pH<6 is the number of weeks of leaching under acidic (pH<6) conditions. Alkaline Addition (g) is the total amount of Rotary Kiln Fines or limestone added to a reactor. Total S Leached (%) is the total sulfur measured in the reactor leachate expressed as a percentage of the initial sulfur present in the solid sample prior to leaching. 57 �DOCUMENTING UNDERGROUND MINE WORKINGS ON THE MESABI IRON RANGE IN GIS FORMAT ORESKOVICH, JULIE A., Natural Resources Research Institute, Duluth, MN 55811 (joreskov@nrri.umn.edu) Seventy years of mining natural iron ore by underground methods on the Mesabi Iron Range of northeastern Minnesota has resulted in an extensive network of subsurface voids along much of the Range. Subsidence resulting from these voids is manifested today as sinkhole depressions, pools, ponds, lakes and, most notably, road and structure damage. These voids impact the area hydrology and pose unknown risks to existing and potential surface developments, as well as to present-day mining operations. Because towns grew up adjacent to the early mines, today’s Range cities are often located, at least in part, on top of the Biwabik Iron Formation (BIF). Underground workings faded from the collective memory as generations passed and open pit mining became the norm, with an occasional jolt to the public consciousness caused by a dropped highway lane, a shifted building, or a newly opened hole in the ground. However, with prospects for several major developments on the Range, along with an expected population increase, planning groups such as the Central Iron Range Initiative (CIRI) have recognized the need to factor the underground mine workings into planning and design work in the region. The Central Iron Range Sanitary Sewer District (CIRSSD) has secured funding for the Minnesota Department of Natural Resources (MN DNR) to research the underground mines of the central Mesabi Range. CIRSSD’s study area extends from the Itasca county line on the west (T 57 N, R 21 W) to midway between the cities of Buhl and Mountain Iron on the east (T 58 N, R 19 W), following the trend of the Biwabik Iron Formation. Nearly 150 natural ore mines (underground, open pit, or both) operated in this area. Mining of natural iron ore on the Mesabi Range began with underground workings in the early 1890’s. The last of the underground mines, the Godfrey Mine, closed in 1963. The history of each mine in the study area can be traced through mining directories dating back to 1922. Data, including land parcels, operators, dates of operation, type of operations, fee interests, and lessees, is captured and entered into an Access database. Mine maps are collected from numerous sources, including mining companies, fee representatives, the MN Department of Revenue, the MN DNR map collection, and the archives of the Iron Range Resource Center (IRRC) located at Ironworld in Chisholm, MN. These maps are digitally scanned, and then rectified and digitized in ArcGIS. A unique USX aperture card collection housed at the IRRC contains annual level maps for Oliver Iron Mining Company operations dating back to 1903, enabling 3-D renderings of these mines. The end product of this research will be a GIS database that will be made accessible to counties, local communities, development organizations and mines for planning purposes. 58 �Rare Earth Element Patterns in Steep Rock Carbonates Noah Planavsky: Rosenstiel School of Marine and Atmospheric Sciences, 4600 Rickenbacker Causeway Miami FL 33149. nplanavsky@rsmas.miami.edu. Jennifer Murphy: Eastern Mineral Resources, USGS. Reston, VA 20194. jmurphy@usgs.gov One of the most dramatic changes in Earth’s history is the shift from a reducing to an oxidizing atmosphere. Multiple lines of evidence suggest the atmosphere first became oxygenated around 2.4 billion years ago. The evolution of oxygenic photosynthesis is ultimately responsible for the Earth’s oxygenation. The timing of the emergence of the earliest oxygenic photosynthetic organisms, likely cyanobacteria, is typically thought to predate the rise the atmospheric oxygen. Thus there exists the possibility for redox disequilibrium between ocean atmospheric systems; local areas of high photosynthetic productivity could induce local oxidizing conditions under an overall reducing atmosphere. Rare earth element (REE) trends can be used to explore paleoredox conditions. Modern oxygenated oceans display a strong negative cerium anomaly when normalized to shale composites, reprehensive of bulk crustal values, (Byrne and Sholkovitz, 1996) while anoxic or microaerophilic conditions display a positive or no cerium anomaly (shale normalized). The negative cerium anomaly is largely due to the oxidation of Ce3+ to Ce4+ and its preferential removal from the water column onto Mn-Fe oxide particles or as precipitate. In suboxic and anoxic waters Ce anomalies are smaller due to reductive dissolution and redox cycling of settling Mn- and Fe-rich particles within the suboxic and anoxic zones, respectively. Microbial induced carbonate precipitates and carbonate cements have been shown to faithfully record seawater REE values in modern reefs and can be used as a paleoredox proxy (Kamber and Webb, 2001). REE provide a unique tool at basin scale redox cycling in ancient environments. The approximately 2.9 Ga Steep Rock Group is one of the oldest extensive carbonate successions with a low metamorphic grade and a high state of preservation. The Steep Rock Group, therefore, is an ideal place to explore the redox state of mid-Archean marine environments. Stromatolites and abiogenic carbonate cements from various depositional settings do not contain negative cerium anomalies. The trace element systematics, traditional stable isotopes, and carbonate textures indicate exceptional preservation and rule out the possibility of a diagenetic overprint. Since Ce is preferentially released during anoxic diagenesis (Byrne and Sholkovitz, 1996), the cerium anomalies observed in the siliceous stromatolites could not have formed during early anoxic diagenesis. Thus despite apparently high biological productivity, noted by the abundance of stromatolites and organic matter, the Steep basin was anoxic or suboxic. The lack of negative cerium anomalies in previously investigated early Archean stromatolites has been used to argue for absence of cyanobacteria (e.g., Tice and Lowe, 2005). Basic modeling of reductant and oxidant fluxes at the basin scale, rather than the more commonly modeled global 59 �scale, provides atmospheric scenarios in which oxygenic photosynthesizing communities could exist in reducing conditions. References: Byrne R. H. and Sholkovitz E. R. 1996. Marine chemistry and geochemistry of the lanthanides. In Handbook on the Physics and Chemistry of Rare Earths (eds. K. A. Gschneidner, Jr. and L. Eyring), Vol. 23, pp. 497–593. Elsevier, Amsterdam. Kamber, B.S. and Webb, G.E. 2001. The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history. Geochimica et Cosmochimica Acta. 65: 2509-2525. Tice, M.M. and Lowe, D.R. 2006. Hydrogen-based carbon fixation in the earliest known photosynthetic organisms: Geology, 34: 37-40. 60 �New Insights into the Metallogeny of the Eastern Portion of the Archean Uchi Domain, Superior Province, Ontario PUUMALA, Mark A., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 Canada In 2005, the Ontario Geological Survey began the Far North Geological Mapping Initiative (FNGMI). The goal of this program is to obtain a better understanding of the geological history and mineral resource potential of the portions of Ontario that are located north of 51ºN latitude. The Far North is a remote area that has a much smaller historical geological database than the southern portions of Ontario. Nevertheless, this region does have a long history of mineral exploration that has resulted in the collection of much valuable information about the nature of economic mineralization. These data are currently being compiled to evaluate metallogenic patterns that can in turn be used to better understand the regional tectonic framework and economic mineral distribution. Figure 1. Map illustrating Far North Mineral Deposit Compilation study area. To date, geological data have been compiled for mineral occurrences located within the eastern portions of the Archean Uchi Domain of the Sachigo Superterrane (Stott and Rainsford, 2006), of the Superior Province (Figure 1). The majority of the mineral deposits in this area are located within or immediately adjacent to supracrustal rocks of the Miminiska-Fort Hope, Pickle Lake, Lake St. Joseph, Meen-Dempster and Lang Lake greenstone belts of Stott and Corfu (1991). A wide variety of mineral deposit types have been identified in the study area, including: vein and replacement gold; polymetallic vein; mafic to ultramafic intrusion-hosted coppernickel-platinum group elements (PGE); rare-element-bearing pegmatite; volcanogenic massive sulphide (VMS) copper-zinc; intrusive porphyry-related copper-molybdenum-gold; and Algomatype banded iron formation. Although the individual greenstone belts listed above are useful geographic subdivisions, it is perhaps more useful to ascribe styles of mineralization to individual assemblages that were deposited during a discrete interval of time in a common depositional or tectonic setting (Stott 61 �and Rainsford 2006). The relationships between mineral occurrences and the individual tectonic assemblages have been used to obtain information regarding the possible locations and timing of significant metallogenic events in the study area. Preliminary observations suggest the following sequence of metallogenic events: 1. Mafic to ultramafic intrusion-hosted Cu-Ni-PGE mineralization in rocks of the >2860 Ma Pickle Crow assemblage in the Pickle Lake and Miminiska-Fort Hope belts; 2. VMS-type mineralization associated with rocks of the 2825-2842 Ma Kaminiskag (former Woman) Assemblage in the Pickle Lake and Meen-Dempster belts; 3. Later VMS-type mineralization associated with rocks of the <2744 Ma Confederation Assemblage in the Lang Lake, Meen-Dempster, and Pickle Lake greenstone belts, and with rocks of the <2723 Ma St. Joseph Assemblage in the Miminiska-Fort Hope Greenstone Belt; 4. Copper-nickel-PGE mineralization associated with late-tectonic mafic (commonly anorthositic) intrusions located near the southern boundary of the Miminiska-Fort Hope Greenstone Belt, and in the Lang Lake Greenstone Belt near the Bear Head Fault Zone; and 5. Structurally controlled gold mineralization that was likely to have been associated with the 2.72-2.70 Ma collision between the Uchi Domain and the Winnipeg River Terrane (Percival et al. 2006). In the Pickle Lake Greenstone Belt, this event post-dates Confederation Assemblage volcanism (>2739 Ma) and pre-dates the emplacement of the post-tectonic 2697-2716 Ma Hooker-Burkoski stock (Young et al. 2006). The metallogeny of other deposit types in the study area is currently more difficult to interpret due to the small number of known occurrences and/or a lack of geological data. However, as additional information is collected through FNGMI mapping projects, it is anticipated that the tectonic events associated with porphyry copper mineralization observed in the Lang Lake Belt and polymetallic veins found in the North Bamaji Lake area, for example, can be determined. References Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe Studies; Canadian Journal of Earth Sciences, Vol. 43, p. 1085-1117. Stott, G.M. and Corfu, F. 1991. Uchi Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 145-236. Stott, G.M. and Rainsford, D.R.B. 2006. The Precambrian geology underlying the James Bay and Hudson Bay lowlands as interpreted from aeromagnetic data and a revised terrane map for Northwestern Ontario; in Summary of Field Work and Other Activities 2006, Ontario Geological Survey, open File Report 6192, p. 13-1 to 13-10. Young, M.D., McNicoll, V., Helmstaedt, H., Skulski, T., and Percival, J.A. 2006. Pickle Lake revisited: New structural, geochronological and geochemical constraints on greenstone belt assembly, western Superior Province, Canada; Canadian Journal of Earth Sciences, Vol. 43, p. 821-857. 62 �MICHIGAN KIMBERLITES REVISITED: NEW MINERAL, CHEMICAL AND PETROGRAPHIC ANALYSES Quigley, P. O., Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55414, quig0026@umn.edu Over twenty kimberlite pipes have been reported in the Western Upper Peninsula of Michigan (Jarvis, 1993). Most of the known activity in assessing these pipes took place during the 1980’s, and therefore did not include classification according to currently available mineral chemical and petrographic methods. Drill core samples held by the Michigan DNR from several pipes across the Michigan kimberlite field therefore are presently being analyzed in order to provide a further assessment of diamond grade prospects based on mineral chemistry, as well as an updated petrographic classification. Data collection consisting of electron microprobe analyses of garnet, ilmenite, and pyroxene from heavy mineral concentrates have been completed for three of the kimberlites, and have shown a high degree of variability. Two of the intrusions appear to have sampled a portion of the mantle within the diamond stability field, due to the presence of harzburgitic garnets, and other favorable compositions, according to the Grütter et al. (2004) classification scheme. Overall diamond grade is, however, inferred to likely be economically insignificant as the proportion of diamond indicator garnets to overall garnets is moderate to low. Analyses currently in progress will further examine degree of variability in mineral chemical analyses among these kimberlite intrusions, as well as examination of polished thin sections to permit use of textural features to categorize the intrusions as crater, diatreme, or hypabyssal facies kimberlite. REFERENCES Grütter, S. H., Gurney, J.J., Menzies, H.A., and Winter, F., 2004. An updated classification scheme for mantle-derived garnet, for use by diamond explorers. J. Barry Hawthorne Volume Proc. 8th Int. Kimb. Conf., pp 841-857. Jarvis, William, 1993. Michigan Kimberlites: An Update. Abstract at Prospectors and Developers Association of Canada 61st Annual Meeting (Paper M-10). 63 �Elemental and Isotopic Shallow Lake Proxies of Landscape Changes in the Prairie Pothole Region of Minnesota Tommy Rodengen1, Kevin Theissen1, and Shinya Sugita2 1 Department of Geology, University of St. Thomas, 2115 Summit Ave, St. Paul, MN 55105 2 Department of Ecology, Evolution, and Behavior, University of Minnesota, 1987 Upper Buford Circle, St. Paul, MN 55108 Investigations of the manipulation of the landscape around lakes have shown to have profound impact on their sedimentary records (1). We have explored lake sediment cores with records spanning the last two centuries and correlated changes in proxy data to large shifts in landscapes around lakes in Western Minnesota. In February 2006 we collected sediment cores from Lee, Bore, 8-Mile and Sweet lakes in the Prairie Pothole region of Minnesota. Cores were sampled for organic matter (SOM) at 1 cm intervals, dried, ground, weighed and treated with sulfurous acid (2). These samples were sent to the Stanford University Stable Isotope laboratory and analyzed for elemental (TOC, TN, C/N) and stable isotopic (δ13C and δ15N) values. Lastly, pollen was extracted from sediments using standard palynological methods (3). Over the past 6 months I have analyzed a set of aerial photographs spanning 1938-present for the region of interest. δ13C (-29 to -22 per mil) and C/N (5-17) values reflect a dominant algal source in the sediments but also show times of mixing and terrestrial sourcing across the lake core record (4). These values suggest changes in productivity and/or changes in sourcing organic matter. We used sharp increases in the abundance of ragweed (ambrosia) as a marker of the cultural horizon (~1850 A.D.) (5). We believe the sharp changes in N isotopes indicate land use change by humans and the addition of fertilizers. This is illustrated by a large δ15N spike between 39cm and 21cm depth (Figure 1), which if you assume a linear rate of sedimentation (.224 cm/yr) is between ~1832 A.D. and ~1912 A.D. According to our earliest records of aerial photography these years were a time of extreme drought and farming of fertile lake soil was common, directly introducing high levels of nitrogen to the sedimentological record. The importance of human manipulation of the landscape, specifically synthetic fertilizer, appears to have a clear effect on the sedimentological record found in these lakes. References: (1) Lamb, H.F., Damblon, R.W., Maxted, R.W., 1991, Human Impact on the Vegetation of the Middle Atlas, Morocco, During the Last 5000 Years. J. Biogeography, 18, 1-14. (2) Verardo, D.J., Froelich, P.N., McIntyre, A., 1990, Determination of organic carbon and nitrogen in marine sediments using Carlo-Erba NA1500. Deep-Sea Res., 37, 157165. (3) Faegri, K. Iversen, J., 1989. Textbook of Pollen Analysis. 4th ed. J. Wiley & Sons, Chichester. 328pp. 64 �(4) Meyers, P.A., and Lallier-Verges, E., 1999, Lacustrine sedimentary or Late Quaternanry paleoclimates. J. Paleolimnology, 21, 345-372. (5) Jacobson, G.L. and Grimm, E.C., 1986. A numerical analysis of Holocene forest and prairie vegetation in central Minnesota. Ecology, 67, 958-966. Figure 1: 8-Mile lake δ15N values 8-Mile Lake 0 10 Depth (cm) 20 30 40 50 0.00 60 1.00 2.00 3.00 4.00 5.00 6.00 δ N 15 65 7.00 8.00 9.00 10.00 �BEDROCK FRACTURES IN SOUTHEASTERN WISCONSIN: PALEOSTRESS ESTIMATES AND RELATIONSHIPS TO THE WAUKESHA FAULT RYMASZEWSKI, Jody A., FRIEDRICH, Jason L., and CZECK, Dyanna M. Department of Geosciences, University of Wisconsin – Milwaukee, P.O. Box 413, Milwaukee, WI 53201 jody@uwm.edu INTRODUCTION The Waukesha Fault is an enigmatic structure cutting through Silurian dolomite in southeastern Wisconsin. It is a normal fault, oriented ~N40E, 60SE with an apparent offset of 10 m (Mikulic and Mikulic, 1977; Sverdrup et al., 1997). It is only known to outcrop at the Waukesha Lime and Stone Co. quarry in Waukesha, Wisconsin. Detailed gravity surveys in the region reveal the lateral and vertical extent of the fault (Sverdrup et al., 1997; Skalbeck et al., 2006). The fault trace extends NE to the town of Port Washington, Wisconsin on the shore of Lake Michigan (Sverdrup et al., 1997) and can be traced to depths greater than 600m (Skalbeck et al., 2006), showing that it is a major feature of an otherwise undeformed region. The goals of our study are to 1) estimate the paleostress orientations that formed the Waukesha Fault, 2) measure the orientations of nearby small-scale fractures, 3) estimate the paleostress orientations that formed the small-scale fractures, and 4) compare the paleostress orientations for all the structures. METHODOLOGY The one known exposure of the Waukesha Fault is currently not available for active study. Therefore, we conducted paleostress orientation estimates on the Waukesha Fault, assuming its geometry matched that predicted by Sverdrup et al., 1997. We measured the orientations of 158 fractures in Silurian bedrock at the Lannon Stone Products quarry (located in Lannon, Wisconsin: ~11km NE of the known Waukesha Fault outcrop and ~3 km NW of the Waukesha Fault trace) during field seasons from 2004-2006. We also measured the orientations of 38 fractures in Devonian bedrock at the Harrington Beach State Park abandoned quarry and beach outcrops (located ~ 10km NE of Port Washington, and ~2.5 km NW of the Waukesha Fault trace). We used standard stereonet procedures to calculate the orientations of the paleostresses likely to have formed the Waukesha fault and the fractures at each of the field locations. RESULTS Based on the assumed geometry of the Waukesha Fault, σ1, the maximum principal stress was oriented vertically, σ2, the intermediate principal stress, was oriented (plunge/trend) 0°/040 σ3, the minimum principal stress, was oriented 0°/130. The Lannon quarry contains fractures with no discernable offset, and faults with small-scale (maximum ~4 cm, mostly normal sense) offsets. Some rare fractures have preserved ridge and groove lineations or plumose features, allowing classification as shear fractures or extensional fractures, respectively. The fractures at the top of the quarry have a random distribution. The small faults found at the low-mid levels of the quarry exhibit an approximate N32E/56SE 66 �orientation. Most of the fractures with no apparent offset at mid and lower levels have the approximate orientation N30E/65S; a second group has approximate orientation of N45W/ steepsubvertical. Cross-cutting relationships between the two sets are inconclusive. Two prominent fracture sets formed at Harrington Beach State Park. The approximate orientations of these two sets are 1) N74E/subvertical and 2) N25W/ subvertical. Unfortunately, the textures of most fracture surfaces are too weathered to determine whether the fractures are extensional or shear fractures. Thus, the paleostress estimates must be partly based on the relative geometries. The small faults at the Lannon quarry have estimated paleostress orientations: σ1 = vertical; σ2 = 0°/040, σ3 = 0°/130. Most of the fractures at the Lannon quarry have a similar orientation (but often with slightly steeper dips) to the small faults. Therefore, we interpret that most of these fractures are small extensional or shear fractures with the same paleostress orientations as the small faults. The second group of fractures at the Lannon quarry could either be extensional or shear fractures, but lack of any offset supports the likelihood that they are extensional fractures. If so, the paleostress orientations are σ1 = vertical; σ2 = 0°/315, σ3 = 0°/225. The fractures at Harrington Beach could either be two distinct sets of extensional fractures or a conjugate set of shear fractures. If they are extensional fractures, the paleostress orientations are 1) σ1 = vertical; σ2 = 0°/074, σ3 = 0°/164 and 2) σ1 = vertical; σ2 = 0°/335, σ3 = 0°/245. If they are shear fractures, the paleostress orientations are σ1 = 0°/114 σ2 = vertical, σ3 = 0°/024. CONCLUSIONS While the paleostresses on all fractures could not be conclusively determined, the paleostress orientations inferred from most of the geometries of the small-scale features in the Silurian bedrock at Lannon are consistent with those estimated for the Waukesha Fault. However, the paleostress orientations of the Devonian rocks at Harrington Beach are inconsistent with those estimated for the Waukesha Fault. Therefore, it seems likely that 1) the timing of the Waukesha Fault may be bracketed by the deposition of the Silurian rocks at Lannon and the Devonian rocks at Harrington Beach, or 2) the deformation associated with the Waukesha Fault encompassed a broader region in the south. Further study of fractures in the region is required to test these hypotheses. REFERENCES Mikulic, D.G., Mikulic, J.L., 1977. History of geologic work in the Silurian and Devonian of southeastern Wisconsin: Guidebook 41st annual tri-state field conference, A19-A27. Skalbeck, J.D., Couch, J.N., Helgesen, R.S., Swosinski, D.S., 2006. Coupled modeling of gravity and aeromagnetic data to estimate subsurface basement topography in southeastern Wisconsin. Geoscience Wisconsin 17, 53-64. Sverdrup, K.A., Kean, W.F., Herb, S., Brukardt, S.A., Friedel, R.J., 1997. Gravity signature of the Waukesha Fault, southeastern Wisconsin. Geoscience Wisconsin 16, 47-54. 67 �Precambrian Geology of the Opelt Quarry, Neillsville, Wisconsin SAXTON, Samantha and CORDUA, William, Department of Plant and Earth Science, University of Wisconsin- River Falls, River Falls, WI 54022, samantha.saxton@uwrf.edu, william.s.cordua@uwrf.edu Near Neillsville, Clark County, Wisconsin, a quarry owned and operated by Opelt Sand and Gravel exposes a complex group of rocks dominated by amphibolites but including biotite gneisses, granitic to dioritic plutons, intrusive breccias and chlorite-rich shear zones. Given the quarry's location south of the Eau Plaine shear zone, it gives an important fresh exposure of the rocks of the Marshfield Terrain in central Wisconsin. We did field reconnaissance at the quarry combined with thin section, geochemical and structural analyses to study the metamorphic history. Published research for the surrounding area suggests the Opelt rocks are associated with an accreting micro-continent, which formed the foundation for later deformation during the Penokean orogeny (1880 - 1830 mya) and perhaps younger metamorphic events (Holm and Schneider, 2007). Previous work performed by Maass (1983) on rocks within 10 miles of the quarry using U-Pb zircon dating gives ages in a range of 1875 to 1820 million years old. A study by Sims and Peterman (1980) using a Rb-Sr whole-rock technique places them at roughly 1885 +/- 65 million years, correlating with the Penokean Orogeny. Thin section analysis showed the common mineral assemblage in the gneisses and amphibolites to be a combination of plagioclase, biotite, hornblende, and clinozoisite, with several minor minerals such as calcite, chlorite, and epidote among others. Within the granitic samples, quartz, plagioclase, and clinozoisite become the dominant minerals, with the same range of minor minerals. Veins of quartz and calcite are found cross-cutting the granites. Replacement textures were evident from the thin sections. Amphibolite samples display replacement rims or coronas of titanite surrounding the opaque ilmenite (Figure 1). We also see replacement relationships between several sets of minerals: primarily clinozoisite, muscovite, epidote and/or calcite replacing plagioclase feldspar, and chlorite replacing biotite. These two suites of minerals most likely represent a retrograde metamorphic event. Figure 1: Amphibolite showing titanite replacing the opaque mineral, ilmenite, seen in the middle of the rings. Geochemical analysis using discrimination diagrams derived from Pearce et al. (1982) and Gomez-Pugnaire et al. (2003) proved to be helpful in defining the protolith of the assorted rocks. A discrimination diagram for granitic rocks, based on the ratio of Rb versus Y + Nb, allocates samples to three origins- anorogenic, oceanic ridge, and island arc. Our data points, two granites and a diorite, indicate within a reasonable margin of error, a calc-alkaline island arc origin for the plutonic rocks found in the quarry. In addition, 68 �several diagrams from Pearce (1982) show the igneous rocks that were protoliths for the amphibolites to be tholeiitic volcanic or island arc basalts. Maass (1983) describes several folding events within in nearby rocks: F-1- isoclinal folds, to F-3 - very open to tight folds, as well as a mineral lineation associated with the F-3 folding. He also proposes that the plutonic activity of the area seems to have started at the latter part of the F-1 folding and continued past the end of the F-3 event. (Maass, 1983) Many of these same features can be seen in the Opelt Quarry. Field, thin section, and geochemical data suggest the following sequence of events in the Opelt rocks: 1. Amphibolite protolith = tholeiitic island arc basalts 2. Penokean Orogeny ~ Metamorphic Event #1 - Amphibolite Facies (Hb, plag) Occurrences F-1 Foliation F-2 Veins and Granitic Intrusions ~ Metamorphic Event #2 - Greenschist Facies (Chl, epi) Occurrences F-3 Foliation ~ Formation of Shear Zones ~ Veins- quartz, calcite, epidote ~ Multiple series of faulting 3. Deposition of Younger Sediments/ Later Glacial Sediments The later folding and retrograde metamorphism events could be late stage Penokean orogeny, or could be related to the Yavapai (1800-1750 mya) and Mazatzal (1650 mya) events that took place post-Penokean Orogeny (Holm and Schneider, 2007). Many structures and features originally attributed to the Penokean Orogeny now may actually be an overprint from later Yavapai and Mazatzal events (Cannon and Schulz, 2007). Radiometric work and further geochemical analysis will help resolve these questions. Cannon, W. and Schulz, K., 2007, The Penokean Orogeny in the Lake Superior Region (abst.): The Geological Society of America Abstracts with Programs, 41st Annual Meeting, Lawrence, KS, v. 39, p. 76. Gomez-Pugnaire, M.T., Azor, A., Fernandez-Soler, J.M. and V. Lopez Sanchez- Vizcaino, 2003, The amphibolites from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif): geochemistry and tectonic interpretation: Lithos, v. 68, p. 23-42. Holm, D. and Schneider, D., 2007, Metamorphic Record of the Yavapai and Mazatzal accretion in the upper Great Lakes region, U.S. and Canada (abst.): The Geological Society of America Abstracts with Programs, 41st Annual Meeting, Lawrence, KS, v. 39, p. 76-77. Maass, R.S., 1983, Early Proterozoic tectonic style in central Wisconsin: Geological Society of America Memoir, v. 160, p. 85-95. Pearce, J.A., 1982, Trace element characteristics of lavas from destructive plate boundaries, In: Thorpe, R.S. (ed.) Andesite, p. 525-548. Sims, P.K. and Peterman, Z.E., 1983, Evolution of Penokean foldbelt, Lake Superior region, and its tectonic environment: Geological Society of America Memoir, v. 160, p. 3-14. 69 �Revised Stratigraphy of the Biwabik Iron Formation, Mesabi Range, Minnesota – Developing the “Rosetta Stone” SEVERSON, Mark, J. and HEINE, John, J., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 The NRRI is currently evaluating the possibility of using specific waste rock products from taconite mining as aggregate materials. Paramount to defining specific horizons with good aggregate potential, a better understanding of the stratigraphy of the iron-formation has been accomplished through detailed logging of drill holes and detailed in-pit mapping (where core are unavailable). To date, 130 holes have been logged from seven areas that include: CliffsErie site (old Erie/LTV mine), Biwabik area (Mittal Steel’s planned “east reserve”), Laurentian Mine, United Taconite, Minntac, Hibtac, and the oxidized taconite of the Coleraine area. In each of these areas, the stratigraphy, as determined by bedding types in each of the holes, is compared to the stratigraphy as defined by each of the mining companies. Through this approach, a new picture of the iron-formation layering is evolving and for the first time it has become possible to correlate the mining units of one mine to those in adjacent mines - even though each of the mines uses a different stratigraphic terminology. In essence, this work has set forth the beginnings of a “Rosetta Stone” whereby ore and waste horizons can be better understood. Hopefully, the “Rosetta Stone” can eventually be used to aid in determining the depositional environments of internal units within the iron-formation. All descriptions of the Proterozoic iron-formations consistently attribute thin-bedded ironformations as reflecting deposition in a deep water environment, and granular, variably-bedded iron-formations as reflecting deposition in shallower water. Overall, the iron-formation has been divided into four members consisting of Lower and Upper Cherty members (regressive sequences), and Lower and Upper Slaty members (transgressive sequences). Using these simple concepts, coupled with other bedding types as seen in drill core, several unique relationships have been defined along the length of the Mesabi Range that include: • The Lower Cherty is the most consistent member of the iron-formation and consists of a single shallowing-upward parasequence consisting of thin-bedded iron-formation at the base grading upwards through sequences of regular-bedded, wavy-bedded, irregular-bedded, and thick-bedded granular iron-formation units. • The top of the Lower Cherty consists of an iron-poor, variably-bedded, granular chert that is currently being used as road aggregate, e.g., the “Mesabi Select” material from United Taconite’s Thunderbird North mine. This unit is inferred to have been deposited in shallow water near the edge of the shelf. • The base of the Lower Slaty is the most persistent marker bed in the iron-formation and at most locales consists of carbonaceous mudstone referred to as the “Intermediate Slate.” Tuffaceous units are reported to be present at the base of the Lower Slaty but have yet to be encountered in the drill holes looked at in this study. • For the most part, the Lower Slaty consists of a thick sequence of thin-bedded, ironcarbonate and iron-silicate iron-formation that was deposited in deep water. However, in the “east reserve” area, lenses of granular iron-formation (deposited in shallow water and similar to the “Mesabi Select” unit of the Lower Cherty) are commonly interbedded with the thin-bedded rocks of the Lower Slaty. Internal structures within these lenses 70 �• • suggest that they formed from slumpage of shelf material into the basin; possibly related to large storm events or earthquakes during periods of tectonic instability. The top of the Lower Slaty member is perhaps the most poorly defined as it is transitional into the overlying Upper Cherty member. This is most evident in the Virginia Horn area where both slaty and cherty iron-formation types are common and alternate at all scales. This has lead to stratigraphic inconsistencies wherein the actual division between the Upper Cherty and Lower Slaty members varies drastically from mine to mine. Recent stratigraphic correlations suggest that the upper portion of the Lower Slaty contains several laterally-restricted tongues/channels of Upper Cherty-type beds that have locally been referred to as “interbedded cherts” or IBCs. These IBCs appear to be related to several regressive/transgressive events and represent several small parasequences at the top of the Lower Slaty. The Upper Cherty member is profoundly different from the Lower Cherty in that it does not display a consistent stratigraphy nor is a shallowing-upward sequence evident. Furthermore, the Upper Chery is commonly “oolitic.” These relationships suggest that the Upper and Lower Cherty members were deposited in distinctly different depositional environments. While development of the “Rosetta Stone” adequately displays the various differences of the Biwabik Iron Formation along the entire length of the Mesabi Range, only continued logging of holes will aid in exhibiting the 3D nature of the iron-formation. When that point is reached, a more complete analysis of the actual depositional environments can be ascertained. 71 �Investigation of Ferromanganese Nodule Precipitation and Arsenic Uptake in Modern Lacustrine Biochemical Sediments Stevens, Larissa B. and Fralick, Philip, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, (larissa.stevens@lakeheadu.ca) This study was conducted to evaluate the environmental setting, geochemistry and formational processes involved in the precipitation of lacustrine ferromanganese nodules, and to investigate the mechanisms involved in arsenic uptake in these deposits. The research was conducted on precipitates in Lake Charlotte, Nova Scotia. However, the findings are applicable to the Lake Superior region as ferromanganese deposits have been found in a number of temperate North American lakes and may be present in many more. Ferromanganese deposits in the form of nodules, coated sand and layers in the sediment are present in all the Great Lakes (Sly and Thomas, 1974; Callender, 1970). Mothersill and Shegelski (1973) studied iron- and manganese-rich layers in Lake Superior near Thunder Bay. Ferromanganese concretions have also been found in inland lakes of the Lake Superior Region including: Lake Schebandowan, Ontario (Carpenter et al., 1972), Trout Lake, Wisconsin (Twenhofel et al., 1945) and the Minnesota Lakes (Zumberge, 1952). The layering styles of ferromanganese nodules can be used to infer the approximate position of the redox boundary where precipitation is occurring. The concretions found in Lake Charlotte, NS, developed on the bottom in a shallow lacustrine setting. These deposits consist of 1 cm thick crusts divisible into 1) a lower zone of iron precipitates cementing siliciclastic sands; 2) thin laterally continuous laminae of manganiferous and ferric precipitates; 3) micro-stromatolites composed of manganiferous and ferric precipitates and 4) grumulous and clotted textured manganiferous and aluminous precipitates. These deposits formed by the interaction of oxygenated lake water with diffuse reduced groundwater seeps on the sandy bottom. The development of microbial mats and presence of stromatolites and clotted precipitates indicates biochemical processes played a role in the formation of the crusts. The ferromanganese nodules are accumulating arsenic, with concentrations ranging from ~ 670 to 1400 ppm. Elevated arsenic concentrations were also found in groundwater samples suggesting that the arsenic is entering the lake from the groundwater seeps. Microbiological experiments investigating bacterial involvement in arsenic coprecipitation have found that no arsenic oxidizers were present. However, iron oxidizers are. This suggests a possible mechanism whereby: the iron oxyhydroxide precipitates were formed, at least in part, by the iron oxidizers in the microbial mats on the bottom. As the iron oxyhydroxide precipitated from the diffuse groundwater flow entering the oxygenated lake it scavenged the arsenic that was also being transported in solution. 72 �References Sly, P.G. and Thomas, R.L., 1974. Review of geological research as it relates to an understanding of Great Lakes limnology. J. Fish. Res. Bd Can., 31: 795-825. Callender E., 1970. The economic potential of ferromanganese nodules in the Great Lakes. Proc. 6th Forum Geol. Ind. Miner. Michigan Geol. Surv. Misc., 1: 55-65. Mothersill, J.S. and Shegelski, R.J., 1973. The formation of iron and manganese rich layers in the Holocene sediments of Thunder Bay, Lake Superior. Can. J. Earth Sci., 10: 571-576. Carpenter, R., Johnson, H.P. and Twiss, E.S., 1972. Thermomagnetic behavior of manganese nodules. J. Geophys. Res., 77: 7163-7174. Twenhofel, W.H., McKelvey, V.E., and Feray, D.E., 1945. Sediments of Trout Lake, Wisconsin. Bull. Geol. Soc. Am., 56: 1099-1142. Zumberg, J.H., 1952, The lakes of Minnesota, their origin and classification. Bull. Minn. Geol. Surv., 35: 1-90. A C B D A) Picture of a typical ferromanganese nodule overgrowing a cobble. The small domes are stromatolites; B) Cross section of a nodule with a stromatolitic zone at the top; C) Reflected light photomicrograph of stromatolitic layering in a nodule. The round white areas are sand grains; D) Reflected light photomicrograph of the internal layering of a nodule. 73 �A REVISED TERRANE MAP FOR THE SUPERIOR PROVINCE AS INTERPRETED FROM AEROMAGNETIC DATA STOTT, Greg, Ontario Geological Survey, Sudbury, ON, P3E 6B5, greg.stott@ontario.ca CORKERY, Tim, Manitoba Geological Survey, Winnipeg, MB LECLAIR, Alain, Géologie Québec, Québec, QC BOILY, Michel, GÉON, Montréal, QC PERCIVAL, John, Geological Survey of Canada, Ottawa, ON The Superior Province has in the past been subdivided into “Subprovinces” characterized largely by contrasting lithology (e.g., Card and Ciesielski 1986). With an increasingly refined understanding of the tectonic assembly of the Superior Province through a progression of orogenies (Stott and Corfu 1988; Percival et al. 2006), supported by geochronological and SmNd isotopic data (e.g., Tomlinson et al. 2004; Rayner and Stott 2005), we can identify individual terranes and associated domains. Recent aeromagnetic interpretation of the Precambrian crust underlying the James Bay and Hudson Bay lowlands (Stott and Rainsford 2006), permits more confident terrane correlations across James Bay. Figure 1 illustrates a new map of the Superior Province in Canada with its principal tectonic subdivisions as currently used in Manitoba, Ontario and Quebec. Archean terranes are crustal blocks with geological histories substantially different from adjacent blocks, although parts of the late (Neoarchean) history of adjacent terranes typically overlap. They include one or more tectonic assemblages and plutonic suites, and at least parts of the terrane boundaries are clearly marked by major faults. Those regions with more complex Meso- to Neoarchean orogenic histories are superterranes thought to comprise at least two terranes. Domains are marginal parts of terranes or superterranes that are thought to have been added autochthonously or for which sparse evidence exists (e.g., Marmion domain) to treat them as separate terranes. Card, K.D. and Ciesielski, A. 1986. DNAG #1. Subdivisions of the Superior Province of the Canadian Shield; Geoscience Canada, 13, 5-13. Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H. and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies; Can. J. Earth Sci., 43, 10851117. Rayner, N. and Stott, G.M. 2005. Discrimination of Archean domains in the Sachigo Subprovince: a progress report on the geochronology; Ontario Geological Survey, Open File Report 6172, 10-1 to 10-21. Stott, G.M. and Corfu, F. 1988. Whither the Kenoran Orogeny?; Geological Association of Canada/Mineralogical Association of Canada Annual Meeting, Program with Abstracts, 13, A120. Stott, G.M. and Rainsford, D.R.B. 2006. The Precambrian geology underlying the James Bay and Hudson Bay lowlands as interpreted from aeromagnetic data and a revised terrane map for northwestern Ontario; Ontario Geological Survey, Open File Report 6192, 13-1 to 13-10. Tomlinson, K.Y., Stott, G.M., Percival, J.A. and Stone, D. 2004. Basement terrane correlations and crustal recycling in the western Superior Province: Nd isotopic character of granitoid and felsic volcanic rocks in the Wabigoon subprovince, N. Ontario, Canada; Precambrian Research, 132, 245-274. 74 �Figure 1 (next page). This map displays a composite of an aeromagnetic base and bedrock geology of the northern portion of the Archean Superior Province across Manitoba, Ontario and Quebec with terrane and domain boundaries. Names of superterranes, terranes and domains are as currently used in each of the provinces. Some features to note: - The older, Mesoarchean North Caribou terrane forms a core flanked to the north and south by outward additions of Meso- to Neoarchean crust across Uchi and Island Lake domains, which merge eastward where the North Caribou terrane pinches out. Uchi domain and Oxford-Stull domain merge under the James Bay Lowland; - A large indentor-like feature underlies the James Bay Lowland, flanked by faults; - English River and Quetico metasedimentary domains are separated east of the Wabigoon Subprovince by a ridge of felsic plutonic rocks, with high magnetic field intensity, extending westward from the Opatica gneisses of Québec. The Opatica terrane might extend to the Winnipeg River terrane where the latter is interpreted to underlie the northern part of the eastern Wabigoon Subprovince; - The Trans-Hudson Orogen underlies the northern half of the Hudson Bay Lowland; - Apparent reworked Archean Northern Superior superterrane crust under Hudson Bay Lowland within the Trans-Hudson Orogen; - Broad areas of pronounced aeromagnetic field intensity characterize large parts of the Northern Superior superterrane under the Hudson Bay Lowland in Ontario and correspondingly resemble the Pikwitonei gneisses, including the Assean and Split Lake blocks on strike in Manitoba. - Potential correlation of Northern Superior superterrane with the 3.8 Ga Nuvvuagittuq greenstone belt (David et al. 2002) in Tikkerutuk domain and the >3.5 Ga Assean gneisses (Böhm et al. 2000). - Areas interpreted to be part of the Paleoproterozoic Sutton Inlier are outlined where exposed. The Sutton “Inlier”, of shallow water platform sedimentary units and overlying gabbro sill, does not appear to be one large area but a set of inliers and should in future be referred to as the Sutton Inliers. The eastern Sutton Inliers with subsurface extensions are shallowly dipping, northward concave and crescent-shaped, based on aeromagnetic patterns, which correspond closely with the distribution of outcrops observed by Bostock (1970). They resemble “klippe” that might have been transported a short distance southwards. Folded strata related to the Sutton Inliers are shown aeromagnetically to extend discontinuously northwards towards the Hudson Bay coast upon apparently reworked Archean crust within the Trans-Hudson Orogen. The TransHudson Orogen, including areas of reworked Archean crust, appears to underlie the northern half of the Hudson Bay Lowland, based on interpretation of aeromagnetic images. 75 �76 Figure 1 Ibis map tJisiavs tha tectonic subdivision o!Lbe Archean Superior Province across Ntarutotm. Ontario and Quebec on a composite aeromaurtetic and hcdrocL aeolog\ hase Names arc as currently used or proposed In each of the pio inces- The northerriniost edge ol the Superior Ioeince in Maiinotxs and Ontario is locaIk o erprintcd hs the Paleoprocerozoic Trans-I [udson Orogeir �THERMAL EVOLUTION OF PROTEROZOIC (>1Ga) RHYOLITE MAGMA BASED ON ANALYSIS OF MELT INCLUSIONS AND TRACE ELEMENTS IN QUARTZ FROM THE KEWEENAW PENINSULA OF MICHIGAN STUDENT, James J., Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, stude1jj@cmich.edu, WARK, D.A., Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, MUTCHLER, S.R., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, and Bodnar, R.J., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061. Melt inclusions (MI) form when small droplets (less than a few hundred microns in diameter) of magma become trapped inside growing crystals in magma. MI provide the best source of information concerning the chemical and physical evolution of a magma chamber. Glassbearing MI are common in quartz and zircon from porphyritic rhyolite occurrences in the Midcontinent Rift System, including rhyolite flows in the North Shore Volcanic Group, the Porcupine Volcanics, and the Portage Lake Volcanics on the Keweenaw Peninsula of Michigan. Porphyritic rhyolite cobbles are abundant in the Allouez conglomerate within the Portage Lake Volcanics, and the characteristics and geochemistry of MI and quartz phenocrysts contained in this conglomerate are described below. The MI have been categorized based on their phase assemblages and preservation style. Type 1 MI contain clear glass and a shrinkage bubble, Type 2 contain clear glass, a shrinkage bubble and 1 or more, coarser grained (> 3 um) crystals, and Type 3 MI are totally devitrified or otherwise breached. The MI range in size from 1 to over 200 um in diameter, and they typically have a negative hexagonal bi-pyramidal morphology. Cathodoluminescence (CL) observations of the quartz phenocrysts (n=42) reveal several periods of new quartz growth and dissolution on what we tentatively interpret to be nonmagmatic cores (Fig. 1A). In eight quartz phenocrysts from four texturally different porphyritic rhyolite cobbles, Ti (measured by EPMA) increases from about 50 to 220 ppm towards the rim in the magmatic growth zones, corresponding to a temperature increase from 675 to 850 °C, estimated using the TitaniQ method of Wark and Watson (2006) with a fixed TiO2 activity equal to 1. Zircon microphenocrysts and MI are distributed throughout the magmatic portions of the quartz phenocrysts. Type 1 and Type 2 MI were analyzed by EPMA and LA-ICPMS for major elements and 13 trace elements, including Zr. Zircon saturation temperatures calculated using the model described by Watson and Harrison (1983) based on Zr and major oxide concentrations in MI (n=11) from one cobble range from 670 to 866 °C and agree well with the Ti in quartz geothermometry from the same cobble (Fig. 1B). Petrographic observations and chemical analyses indicate that many (if not most) of the type 1 and 2 MI have remained chemically closed systems for over 1 Ga. Zoned calcite (imaged by CL) associated with native copper that is related to later non-magmatic mineralization has selectively replaced some of the feldspar in rhyolite cobbles from the Allouez conglomerate, and random fractures that are partially filled with calcite and secondary planes of fluid inclusions cut across the quartz phenocrysts and some Type 3 MI. The well-preserved growth zoning recorded by CL imaging of quartz is consistent with the preservation of other 77 �chemical characteristics in the MI. Based on both textural and chemical evidence, zircon and quartz remained saturated in the magma while temperatures increased by more than 175 °C sometime prior to eruption. Blundy et al., (2006) and Wark et al., (2007) recently presented evidence that the magma temperature increased during eruption at Mount Saint Helens WA and prior to the eruption of the Bishop Tuff at Long Valley CA. Quartz-bearing porphyritic rhyolite from the Keweenaw shows a similar thermal trend. The thermal and chemical evolution of the Midcontinent Rift System rhyolite magmas may be better constrained utilizing the chemical record preserved in MI, quartz, and zircon. Figure 1. A) CL image of a typical quartz phenocryst from porphyritic rhyolite cobbles of the Allouez conglomerate. Light and dark regions in the quartz correspond to high and low Ti concentrations, respectively. The dark region in the core has very low Ti concentrations (15 ppm) and may be “non-magmatic” based on TitaniQ temperature estimates (567 °C). B) Calculated zircon saturation temperatures from MI (left) and TitaniQ temperature estimates for quartz growth (right) for one Allouez conglomerate rhyolite cobble. References Blundy, J., Cashman, K., and Humphreys, M. (2006): Magma heating by decompressiondriven crystallization beneath andesite volcanoes. Nature, 442, 76-80. Wark, D.A, and Watson, E.B, (2006): TitaniQ: A Titanium-in-Quartz geothermometer. Contributions to Mineralogy and Petrology, 152, 743-754. Wark, D.A., Hildreth, W., Spear, F.S., Cherniak, D.J., And Watson, E.B. (2007): Preeruption recharge of the Bishop magma system. Geology, 34, 235-238. Watson, E.B, and Harrison, T.M., (1983): Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth Planet. Sci. Lett., 64, 295-304. 78 �Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin MICHAEL L. TAYLOR and JOHN B. SWENSON Department of Geological Sciences, University of MinnesotaDuluth, Duluth, Minnesota Along the shores of Lake Superior, in northeast Minnesota and northern Wisconsin, highly saline groundwater exists at anomalously shallow depths of 30 meters or less, contaminating domestic wells and community water supplies. These saline waters, which produce significant hydrogeologic problems for determining locations for domestic water sources, appear geochemically similar to high salinity groundwater (Na-Ca-Cl brines) commonly found at depths greater than one kilometer on the Canadian Shield (Frape and Fritz, 1982). Currently there is little research that attempts to explain this phenomenon of anomalously shallow, highly saline groundwater from a hydrodynamic perspective. Most work completed thus far has been from a geochemical perspective. One recent hydrodynamic model suggests that glaciation acted as a mechanism for flushing deep brines closer to the surface: During the Pleistocene, the Superior Lobe of the Laurentide ice sheet repeatedly occupied the Lake Superior basin. The temperate (wet-based) lobe created a strong hydraulic-head gradient across the sedimentary and volcanic rock package beneath the ice. High hydraulic head existed down the axis of the lobe and decreased towards the margins. In response to this head gradient, meltwater at the base of the glacier may have been driven deep into the underlying rock package beneath the lobe axis, where it could mix with and flush brines to the basin margins. As the Superior Lobe retreated, the head gradient was removed and the high-salinity groundwater displaced to the lobe margins began to migrate (back-flush) to its original position deep within the basin. This study develops further this model and attempts to test it via a combination of mathematical modeling and geochemical fingerprinting. Mathematical modeling comprises the development of a relatively simple model of groundwater flow and solute transport to test basic physical plausibility of the ‘flushing’ model. The flushing model predicts that samples of groundwater should represent a combination of parent brine formed deep within the rock package underlying the Lake Superior basin, isotopically light, subglacial Pleistocene recharge, and modern meteoric recharge. Geochemically fingerprinting saline groundwater samples will test the model predictions which involved: (1) field work to sample wells in the Lake Superior basin, with emphasis on the south shore, where data are sparse, (2) stable isotope and major element analyses of the fluids, and (3) stable isotope and major-element analyses of the sedimentary rocks that comprise the Mid-continent Rift System. Reference: Frappe, S.K. and Fritz, P., 1982. The Chemistry and Isotopic Composition of Saline Groundwaters from the Sudbury Basin, Ontario. Canadian Journal of Earth Sciences, v. 19, p. 645-661. 79 �Possible influence of a buried fault on elevated indoor radon levels, south Washington County, Minnesota Stephanie A. Theriault and Dr. Thomas A. Hickson Geology Department, University of St. Thomas, St. Paul, Minnesota 55105 Correlation between high indoor radon levels and a local fault in south Washington County were found in a 2004-2005 study done by the Geology Department at the University of St. Thomas and the Washington County Public Health and Environment Department. The location of the fault was important to this study because radon characteristically can travel farther along leaky faults, which could explain the unusually high levels of radon in this specific area. However, the Minnesota Geological Survey revised their mapping shortly after the 2004-2005 study and did not include the specific fault on their new map. This research focuses on determining if there is evidence for the proposed fault. Stratigraphic offset, as seen in well logs, aid in determining the possible presence of the fault. Three cross sections were created across the proposed fault from well log data. Offset was found both in the Prairie du Chien and Jordan units across the proposed fault. Structure contour maps of the top of the Prairie du Chien and Jordan units also support the presence of this fault. In conclusion, evidence provided by the well logs from the study area suggests the possible presence of a fault and that it could be related to the unusually high indoor radon levels encountered in this area. 80 �POTENTIAL USE OF THE MIDCONTINENT RIFT FOR CO2 SEQUESTRATION Thorleifson, L. H., Minnesota Geological Survey, 2642 University Ave West Saint Paul, Minnesota 55114-1057 Increasing concern about climate change and reliance on fossil fuels necessitate the exploration of carbon capture and geologic CO2 sequestration as a technology to reduce CO2 emissions. Characterization of the Midcontinent Rift, a southwestward extension of the Lake Superior basin that presents the best sequestration opportunity in the immediate region, is needed to permit a full feasibility assessment of this technology in the region. Unlike better known rocks in oil or coal producing regions, we have little information on the Rift, yet geologic factors suggest that it may be a potential target. A recent meeting therefore was held to develop a shared understanding of current knowledge on the potential opportunities and tradeoffs for deep geologic sequestration of CO2 in Minnesota, Iowa, and western Wisconsin, and to facilitate collaboration between stakeholders. At the meeting, held on March 21, 2007, over forty representatives from upper Midwest governmental, community, academic, and industry groups met to discuss prospects for geologic CO2 sequestration in the region. At the meeting, Brad Crabtree, of the Great Plains Institute (GPI), described work done by the Coal Gasification Workgroup to build consensus on a long-term vision for low-carbon utilization of coal. In May 2007, GPI will release a 50-year regional energy roadmap including a chapter on the role of advanced coal technologies with geologic carbon capture and sequestration (CCS). Bill Grant of the Izaak Walton League outlined a bill currently before the Minnesota Legislature (H.F. 1666) that would provide funding for assessment of both geologic and terrestrial carbon sequestration in Minnesota. If enacted, this bill would provide funds to the Minnesota Geologic Survey and other agencies to begin geologic assessment of the Midcontinent Rift along with steps toward analysis of required policy and technology. Elizabeth Wilson of the University of Minnesota Hubert Humphrey Institute provided an overview of policy considerations for a carbon-managed energy system. Carbon Capture and Sequestration (CCS) is currently in commercial operation at several sites around the world, and is the focus of a major research effort by the U.S. Department of Energy (DOE). Technology alone, however, will not be enough, as deployment will need to be embedded in a wider societal dialogue. States will play a key role in property rights and liability issues, as well as in managing potential risks to public safety, health and the environment. Wilson drew the attention of meeting participants to the Intergovernmental Panel on Climate Change report on Carbon dioxide Capture and Storage as an important reference. Ed Steadman of the Energy & Environmental Research Center (EERC) at the University of North Dakota described the work of the Plains CO2 Reduction Partnership (PCOR), one of seven regional partnerships under the U.S. Department of Energy’s Regional Carbon Sequestration Partnership Program. PCORP has completed Phase I and Phase II research, including assessment and ranking of sequestration opportunities, outreach materials, and a PCOR Partnership Regional Atlas. PCOR is now preparing to conduct a number of CCS pilot projects 81 �in the Williston Basin under Phase III. Ed discussed the Midcontinent rift as part of his broad presentation, both in terms of its potential and the lack of available information on the topic. Ray Anderson of the Iowa Geologic Survey gave a presentation on the geology of the Midcontinent Rift. He described how the billion-year-old rift can be clearly seen across the region as a gravity and magnetic anomaly, while rocks of the rift are only exposed in the Lake Superior Area. The limited additional information available about rift formations comes from deep test borings done for petroleum exploration in Iowa and Wisconsin in the mid 1980s, along with a few shallower drillholes in Minnesota. He indicated that available data indicate that the rift is composed of a central volcanic block, flanked by sedimentary basins up to 5 miles in thickness. Smaller sedimentary basins sit atop the volcanic block in several areas. Geologic sequestration requires porous rock formations at a depth of at least 2,500 feet or roughly a km, overlain by impermeable rock formations to trap the carbon dioxide. Preliminary indications are that the Midcontinent Rift formations may have the necessary characteristics, but much more geologic assessment is required. A breakout on policy and technology recommended that we pursue coordinated policy and regulatory development among neighboring states to pursue early development of a carbon management infrastructure, linking jurisdictions, power plants, coal resources, enhanced oil recovery (EOR) options, along with other sequestration options. This is presently emerging among Illinois Basin states, but could be expanded to other neighbors and even beyond the Illinois Basin. Once initial geological results are available, we will need to conduct comprehensive analysis of different energy futures. This analysis will examine the policy, economic and environmental implications of a) importing electricity from out of state, b) producing electricity in-state but exporting produced CO2 through pipelines, and c) producing electricity and sequestering CO2 within the state. Such an analysis will allow for the advantages and tradeoffs of different energy futures to be considered. The geology breakout recommended that geologic assessment be conducted in phases. Phase One, at a cost of about $0.1M, would assemble existing information, and could include new modeling based on existing seismic data, in order to lay the groundwork for a more rigorous geologic assessment of the CCS potential in the Midcontinent Rift. Bill H.F. 1666, currently before the Minnesota Legislature, could potentially fund Phase One. An important purpose of a Phase I study would be to inform a decision on whether it is warranted to move on to a phase II study. A Phase Two would be a geologic assessment based on new geophysical surveys and about 3 to 6 boreholes, at a cost of perhaps $5M to $10M, perhaps through public funding or a public-private partnership. The objective of Phase Two would be to determine whether Geologic Sequestration potential exists in the Midcontinent Rift. If the results of Phase Two are positive, Phase Three would expand the geological assessment to estimate the geologic storage capacity of the Rift, and begin characterizing the most promising sites. References: • • http://www.geo.umn.edu/mgs/co2_seq.htm http://arch.rivm.nl/env/int/ipcc/pages_media/SRCCS-final/IPCCSpecialReportonCarbondioxideCaptureandStorage.htm 82 �TILL GEOCHEMICAL AND INDICATOR MINERAL RECONNAISSANCE OF MINNESOTA Thorleifson, L. H., K. L. Harris, H. C. Hobbs, C. E. Jennings, A. R. Knaeble, R. S. Lively, B. A. Lusardi, G. N. Meyer, Minnesota Geological Survey, 2642 University Ave West Saint Paul, Minnesota 55114-1057 As a cooperative project of the Minnesota Geological Survey and industry, the entire State of Minnesota and adjacent regions was sampled for till geochemistry and indicator minerals at a 30km spacing during summer 2004. Within target cells, each a quarter-degree latitude by a halfdegree longitude, till from between about 1 and 2 m depth was sampled by filling a 15 liter plastic pail. At a few sites, vertical profiles were collected. In addition, three transects to the north were sampled, to help identify sediments derived by long-distance glacial transport, to obtain reference samples from the Thompson nickel belt, and also to extend sampling to the limit of Hudson Bay-derived carbonate-bearing sediments, to permit comparison to Minnesota carbonate-bearing sediments. Three control samples anomalous in kimberlite indicator minerals from Kirkland Lake, Ontario, were also obtained. The resulting batch consisted of 250 samples covering Minnesota and adjacent areas, 20 samples from Canada, and the three standards. Upon completion of the sampling, the samples were randomized, given numeric laboratory identifications, and shipped to a processing lab, where four quarter-liter splits, two for fine fraction geochemistry, one for texture, and one for an archive were removed. The remaining 14 liters were disaggregated, screened at 2 mm, and the gravel was retained for lithological analysis. The <2 mm fraction was then processed for gold grains, a ferromagnetic heavy mineral concentrate, and a nonferromagnetic heavy mineral concentrate that supported subsequent analysis for precious metal, base metal, and gemstone indicator mineral counts, indicator mineral chemistry, bulk mineralogy counts, and heavy mineral geochemistry. The resulting data are now a significant new information resource with respect to environmental geochemistry topics such as understanding the distribution of deleterious elements in food and water, while providing insights into transport history and composition of the sediments that make up our soil parent materials. Many variables provide insights into regional geology, and reflect known mineral deposits. Some of the data seem to provide faint insights into what may be mineralization that was not previously recognized, such as various base-metal and precious-metal-related elements that show patterns of varying clarity over portions of the state. None of these patterns, however, are obvious discoveries of something that was previously unknown, at the current stage of interpretation. With respect to kimberlite indicator minerals, however, there are two noteworthy patterns, including a few Cr-pyrope garnets in an area from the Twin Cities to southwestern Minnesota (Figure 1), as well as Mg-ilmenites and high-chrome Cr-diopsides in the far northcentral part of the State. Sample spacing in the thin sediments of northeastern Minnesota was not adequate to fully test for the presence of sources such as potential single kimberlite pipes, although samples at a closer spacing are presently being processed by Natural Resources Research Institute to address this point. The current results are, however, faint but clear indications of kimberlite indicator minerals sources that are not unlike several of the patterns that have been found, for example, in Canada, where some of such patterns have eventually resulted in kimberlite discoveries. In the case of the Minnesota results, the data may indicate sources 83 �within the state, or quite possibly could be manifestations of long distance glacial sediment transport, possibly from known or unknown sources in neighboring states or in Canada. In summary, the results are a highly significant step forward in mapping our geochemical landscape, in clarifying mineral potential, in provision of reference data useful to environmental protection, public health, and exploration, and in supporting follow-up with respect to potential mineralization. Figure 1: Example of results from the statewide survey Reference: Thorleifson, L. H., K. L. Harris, H. C. Hobbs, C. E. Jennings, A. R. Knaeble, R. S. Lively, B. A. Lusardi, G. N. Meyer, 2007, Till geochemical and indicator mineral reconnaissance of Minnesota. Minnesota Geological Survey Open File Report OFR-07-01, 512 p., 15 pdf digital files, 5 digital images 84 �DEFINITION OF THE PROTEROZOIC TERRAIN UNDER THE PALEOZOIC-CENTRAL U.P., MICHIGAN T.D. WAGGONER 141 CHIPPEWA, NEGAUNEE, MI 49866 E-mail: thomaswaggonergeo@hotmail.com As soon as it was recognized in the 1880’s that the eastern Menominee range was covered by Paleozoic sediments east of Waucedah, exploration has sought to identify iron mineralization under cover in the central Upper Peninsula. For over 100 years work has progressed to identify the sources of the magnetic anomalies that are found west of the NW-SE rift hinge line and the western limit of the Paleozoic cover. Fig. 1 Magnetic highs under the Paleozoic cover of the central U.P. The amplitude of the airborne total intensity magnetic anomalies range from 3,000 to 23,000 gammas. The intensity is generally dependent on both the amount of magnetite and depth of burial. Airborne and ground magnetics along with ground gravity defined target areas that have since been drilled in the search for iron ore through the 1970’s. Most of the major anomalies are caused by steeply dipping banded iron formations (bifs) similar to the Proterozoic equivalents that outcrop west of the covered area. The primary iron oxide minerals are magnetite, specularite. These are accompanied by iron carbonate silicates, carbonate and chert. Supergene oxidation and enrichment is present in about half of these deposits. Paleozoic 85 �cover ranges from inches to at lease 1800 feet. Associated rocks include slates, quartzites and schists and in one case gneiss. Some of these rocks have been intruded by diabase and granite dikes. The east-west structural attitude of the anomalies and bifs are consistent with outcrop areas to the west. Metamorphic rank from chlorite through garnet is present as noted by the gangue mineralogy associated with the bifs. There have been geophysical data bases generated for diamond exploration that would be useful in further defining the buried Proterozoic terrain. Although economic deposits of bif are probably not present, there is a distinct possibility that IOCG type deposits, feeders for the water deposited bifs are present. Further geophysical surveys like airborne gravity could help provide better definition. 86 �Detrital Zircon Provenance and Structural Geology of the Hamilton Mounds Inlier, Central Wisconsin Jakob Wartman, John P. Craddock, Karl R. Wirth, Geology Department, Macalester College, St. Paul, MN 55105; e-mail: craddock@macalester.edu Gordon Medaris, Jr., Dept. of Geol. & Geophysics, UW-Madison, Madison, WI 53706 Jeff D. Vervoort, Dept. of Geology, Washington State Univ., Pullman, WA 99164 Cam Davidson, Geology Department, Carleton College, Northfield, MN 55057 Hamilton Mounds is one of a series of small inliers in Adams County, WI that are composed of Proterozoic quartzites (Oslander, 1931). Exposures in the Seven Sisters quarry at Hamilton Mounds reveal the presence of a lower, folded metasedimentary sequence, the Hamilton Mounds meta-arkose (HMM), that is intruded by granite (1762 ± 7 Ma [2σ], TIMS-zircons; Medaris et al., 2007). The overlying Seven Sisters orthoquartzite (SSQ) is also folded and contains detrital zircon and monazite that suggest correlation with Baraboo-equivalent quartzites (see Table 1). Finite strains in the Seven Sisters quartzite are also consistent with the regional, NW-SE shortening Mazatzal orogen strain pattern (Craddock and McKiernan, 2007). Both sedimentary units at Hamilton Mounds are crosscut by breccia zones associated with Wolf River batholith intrusion at 1470 Ma. The HMM and SSQ are overlain unconformably by Cambrian sandstones (see Brown and Greenberg, 1986). Detrital zircons are abundant in the Hamilton Mounds meta-arkose. We analyzed 120 grains of which 68 yielded nearly concordant (<10% discordant) results (Figure 1). The youngest zircon age we observed is 1750 ± 16 Ma (2σ), the approximate age of the crosscutting granite dike. The zircon population includes large numbers of post-Penokean (1775 Ma) to Penokean (1800-1875 Ma) grains, and lesser amounts of Archean (25003500 Ma) grains. These data suggest that the Hamilton Mound sediments were sourced Table 1: Summary of Geochronology for Proterozoic Quartzites n= Quartzite Barron 6* Sioux 9* 9* Flambeau McCaslin 2 Hamilton Mounds HMQ1 (SSQ) 69 HM-SSQ (m) 11* HM-Basal (m) 10* HM-Basal (z) 68 Baraboo Basal 7 1250 m up 7 BARQ2 47 BARQ1 100 Method SHRIMP SHRIMP SHRIMP TIMS Youngest Age 1751 Ma 1730 1714 1773 Range 1751-1180 Ma 1730-1850 1714-1880 1773-1775 Reference Holm et al., 1998 Holm et al., 1998 Holm et al., 1998 Van Wyck, 1995 LA-ICPMS Electron Probe Electron Probe LA-ICPMS 1772 1750 1720 1783 1772-3566 1750-2165 1720-2120 1783-3456 Van Wyck & Norman Medaris et al., 2007 Medaris et al., 2007 This Study TIMS TIMS LA-ICPMS LA-ICPMS 1691 1844 1716 1724 1691-1865 1844-2588 1716-3200 1724-3100 Medaris et al., 2003 Van Wyck, 1995 Van Wyck & Norman Van Wyck & Norman [Key: All ages are 207Pb/206 Pb. HM is Hamilton Mound, with upper Seven Sisters Quartzite (SSQ) and older, basal arkose (HM-Basal). Detrital monazite (m) and zircons (z) are indicated. Monazite ages (*) are from multiple analyses: Barron (6 grains, 28 spots); Sioux (9/19); Flambeau (9/16); HM-SSQ (11/83) and HM-Basal (10/154)]. Younger HMM monazite ages (<1760 Ma) are from non-detrital grains. 87 �from the north, including the Penokee Mountains, Archean granite-greenstone terranes, and the Marshfield (or Watersmeet) Archean gneiss terranes. This is broadly consistent with detrital zircon results for other contemporaneous units (e.g., Virginia, Rove and Michigamme Fms.; see Wirth et al., 2006) along the Penokean margin. The Hamilton Mound arkose was deformed in the Yavapai orogeny, then eroded and buried by quartz arenites of the Baraboo interval (1750-1630 Ma). 12 10 8 6 4 2 0 1500 2000 2500 3000 3500 Age (Ma) Figure 1. Histogram of detrital zircon ages from basal Hamilton Mound arkosic quartzite. References Cited Brown, B.A. and Greenberg, J.K., 1987, Proterozoic quartzite at Hamilton Mound, central Wisconsin: Geological Society America Centennial Field Guide—North Central Section (DNAG), p. 195-98. Craddock, J. P. and McKiernan, A.W., 2007, Finite strain gradient in Baraboo-interval quartzites, Wisconsin and Minnesota, USA: Prec. Res. Sp. Vol. Holm, D., Schneider, D., and Coath, C.D., 1998, Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology, v. 26, p. 907-910. Medaris, L.G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., Schott, R.C., 2003. Late Paleoproterozoic climate and tectonics in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites: J. Geol., v. 111, p. 243-257. Medaris, L.G., Van Schmus, W.R., Loofboro, J., Stonier, P.J., Zhang, X., Holm, D.K., Singer, B.S., Dott, R.J., Jr., in press, Two Paleoproterozoic (Statherian) siliciclastic metasedimentary sequences in central Wisconsin: the end of the Penokean orogeny and cratonic stabilization of the southern Lake Superior region: Precambrian Research. Ostrander, A.R., 1931, Geology and structure of Hamilton Mounds, Adams County, Wisconsin: unpublished M.S. thesis, Univ. of Wisconsin, Madison, 27 p. Van Wyck, N., 1995, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of pre- and early Proterozoic rocks in Wisconsin. unpublished Ph.D. thesis, Univ. of Wisconsin, Madison, 47-280. Van Wyck, N. and Norman, M., 2004, Detrital zircon ages from early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites: J. Geol. 112, p. 305-315. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior region: results of preliminary U-Pb detrital zircon studies: 52nd Institute on Lake Superior Geology, p. 69-71. 88 �A Comparison of Textural Profiles in Diabase Sills from the Midcontinent and Transantarctic Rifts ZIEG, Michael J. and FORSHA, Clinton J., Department of Geography, Geology, and the Environment, Slippery Rock University, 1 Morrow Way, Slippery Rock, PA 16057, USA, michael.zieg@sru.edu The 176-Ma Ferrar Dolerite of the Transantarctic Rift (TAR) and the 1.1-Ga Nipigon Diabase of the Midcontinent Rift (MCR) represent magmatic activity during the early stages of continental rifting. Both of these systems include large sills intruded at or near the unconformity between basement rock and overlying sedimentary sequences (the Beacon Group and Sibley Group). Understanding the emplacement history of these sill complexes is critical to understanding the thermal and mechanical properties of the developing rifts. As in the Nipigon area, there are very few feeder dikes exposed in the Ferrar system. Thus, it has been suggested that a significant proportion of the magma transport in the TAR occurred within the sills themselves. Hart and Macdonald (2005) and Sutcliffe (1987, 1989) have suggested, and this work provides additional evidence, that the Nipigon sills experienced multiple injections of magma, which may be an indication that the sills acted as conduits for the distribution of magma through the system. Focused magma flow results in a very different thermal environment than episodic, discrete injection events, and therefore it is very important to understand the nature of the injection history when evaluating the tectonic development of a magmatically active region. In this study, we focus on the application of textural stratigraphy within mafic sills to the understanding of sill emplacement history. In particular, textural profiles from Nipigon sills are compared to profiles from Ferrar sills. These comparisons illustrate the textural signatures of singular, pulsed, and flow-differentiated injections. While the Nipigon sill from Kama Point exhibits a strong reinjection signature, the reinjection signatures in the Antarctic sills are less clear. The significance of different injection histories is that, while a network of sills and dikes emplaced via single injection events can efficiently distribute heat through a large crustal volume, reinjection and/or extended flow through a sill or dike can produce higher temperatures, though in a more localized volume. References Hart, T.R., and McDonald, C.A. 2005. Mesoproterozoic diabase sills of the Nipigon Embayment, northwest Ontario. In Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Vol. 51, Part 1, pp. 22-23. Sutcliffe, R.H. 1987. Petrology of middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96: 201-211. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. Canadian Mineralogist, 27: 67-79. 89 � https://digitalcollections.lakeheadu.ca/files/original/22fb6182a89be392ecbc6d2c2bc1ab99.pdf 29c15d555e9c2109dc1f30c0cb447f7b PDF Text Text 53RD ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 – 13, 2007 PROCEEDINGS VOLUME 53 PART 2 – FIELD TRIP GUIDEBOOK �INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA MAY 8 – 13, 2007 HOSTED BY: LAUREL G. WOODRUFF, U.S. GEOLOGICAL SURVEY MEETING CHAIRPERSON JAMES D. MILLER, JR., MINNESOTA GEOLOGICAL SURVEY FIELD TRIP COORDINATOR PROCEEDINGS VOLUME 53 Part 2 Field Guidebook Compiled by James D. Miller, Jr. (MGS) and Dean M. Peterson (NRRI) -i- �- ii - �Table of Contents Proceedings Volume 53 Part 2 – Field Trips 1) IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION 1 (FORMERLY NATHAN’S LAYERED SERIES) Leaders: J. Miller & E. Jerde 2) GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT 23 Leaders: B. Cannon, D. Cooper, & B. Phillips 3) MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER 53 Leaders: M. Smyk & P. Hollings 4) GEOLOGY OF THE NICKEL LAKE MACRODIKE AND ITS ASSOCIATION WITH CU-NI-PGE MINERALIZATION IN THE NORTHERN SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA 81 Leaders: D. Peterson & P. Albers 5) GEOLOGIC HIGHLIGHTS OF NEW MAPPING IN THE UPPER SOUTHWESTERN TO NORTHEASTERN SEQUENCE OF THE NORTH SHORE VOLCANIC GROUP AND BEAVER BAY COMPLEX 109 Leaders: T. Boerboom, J. Miller, & J. Green 6) GEOLOGY ALONG THE GUNFLINT TRAIL 143 Leaders: M. Jirsa & P. Weiblen Cover Illustrations (clockwise from upper left) • Canoeing on Poplar Lake (photo by Jim Miller) • Beach at Grand Portage (photo by Bill Cannon) • 3-D model of the South Kawishiwi and Bald Eagle intrusions (from Dean Peterson) • Stromatolitic Gunflint Iron-formation, Magnetic Rock Trail (photo by Jim Miller) • North shore view from Sugar Loaf Point (photo by Jim Miller) • Logan Sills in Rove Formation (photo by Mark Smyk) - iii - �- iv - �-v- �53rd Annual Institute on Lake Superior Geology FIELD TRIP 1 IGNEOUS STRATIGRAPHY OF THE POPLAR LAKE INTRUSION (formerly Nathan’s Layered Series) James D. Miller, Jr. Minnesota Geological Survey & Department of Geological Sciences University of Minnesota Duluth Duluth, Minnesota and Eric A. Jerde Department of Physical Sciences Morehead State University Morehead, Kentucky ILSG07 1 Trip 1 �Introduction The Duluth Complex is the largest exposed intrusive component of the Mesoproterozoic Midcontinent Rift, covering an area of over 5,000 km2 in northeastern Minnesota (Fig. 1-1). The Complex comprises a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage. These rocks were emplaced as sheet-like intrusions into the lower section of a comagmatic volcanic edifice (the North Shore Volcanic Group), which itself was erupted onto a peneplained surface composed of Paleoproterozoic sedimentary rocks and Neoarchean granite-greenstone assemblages. The Duluth Complex is generally subdivide into four major series on the basis of intrusive age, dominant lithology, internal structure, and structural position within the complex (Miller and others, 2002). In order of younging, these are: Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semicontinous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occurs along the northeastern contact of the Duluth Complex and also was emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabboic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Poplar Lake Intrusion, which is the focus of this field trip, and the Crocodile Lake intrusion to the east comprise the only two known intrusions in the Early Gabbro Series. The early emplacement of these intrusions was first implied by cross-cutting field relations mapped by Nathan (1969), who recognized that the Poplar Lake intrusion units are cut on their west end by rocks now recognized as belonging to the Layered and Anorthositic Series. An early intrusive age was also implied by their reversed magnetic polarity (Beck, 1970), which is similar to the lowermost lavas of the North Shore Volcanic Group (Green, 1972). U-Pb ages of zircons from the base of the Poplar Lake intrusion confirm its early emplacement by yielding an age of 1106.9±0.6 Ma (Paces and Miller, 1993). This age is similar to lower reversed-polarity lavas of the Midcontinent Rift (1109-1107 Ma, Davis and Green, 1997) and to the reversed polarity Logan Sills (1108(+4/-2) Ma; Davis and Sutcliffe, 1985), which occur in the footwall of the Poplar Lake Intrusion (Fig. 1-1). ILSG07 2 Trip 1 �Figure 1-1. Generalized geology of northeastern Minnesota showing the locations of the Poplar Lake and Crocodile Lake intrusions, which comprise the Early Gabbro Series of the Duluth Complex. Previous Studies The geological survey of Minnesota (1872-1901), directed by N.H. Winchell and assisted by U.S. Grant, H.V. Winchell, and A. H. Elftman, visited the Gunflint Trail area many times over the latter years of the survey. A geologic map of Cook County (Fig. 1-2), as well as more detailed (1”= 2mi) plates of the Gunflint Lake and Rove Lake areas were published in volume 4 of the Final Report (Winchell, 1899). These maps do not distinguish the various types of gabbro, but they do accurately portray the basal contact of the gabbro with the Rove formation and gabbro-granophyre contact just south of Winchell Lake. Frank Grout and coworkers conducted reconnaissance field mapping in Cook County in a period between 1913 and 1953. This work was compiled soon after Grout’s death in 1958 in MGS Bulletin 39 (Grout, Sharp, and Schwartz, 1959). In township maps of the Poplar Lake area (Fig. 1-3), Grout distinguished gabbro, oxide-rich gabbro, anorthositic gabbro, intermediate ILSG07 3 Trip 1 �N Figure 1-2. Geologic map of Cook County by U.S. Grant from the Minnesota Geological Survey Final Report vol. 4 (Winchell, 1899). EWLAMA lION F.' IF I I I I..I! .i!! I!. I IF - if K U *S I •'I(iI,IIr.X'.- I,..XIV — (..,!III'Ii!nIK,F,l'I',iI.1,,r,r INIFrIlI. RIIIIKFIK'FFI II.PI.Iii.u,I.p*lt,\IIlIil,OtNI!rII,.IIInug,, I Figure 1-3. Reconnaissance township geologic maps of the Poplar Lake area from Grout, Sharp, and Schwartz (1959) ILSG07 4 Trip 1 �intrusive rocks and granophyre and noted the moderate southernly dip of layering and foliation. The oxide-rich intervals of the Poplar Lake intrusion were previously cited by Broderick (1917) and by Grout (1950) as potential titanium and iron ore resource. By far the most complete study of the Poplar Lake intrusion comes from the PhD dissertation work of Harold Nathan (1969). Nathan mapped the bedrock geology of the Duluth Complex in the Gunflint Lake, South Lake, and Hungry Jack Lake and followed this up with a very thorough petrographic study. Reconnaissance geologic maps of these quadrangles were published nearly a decade later (Morey and Nathan, 1977; Mathez, Nathan, and Morey, 1977; and Morey and Nathan, 1978). Phinney (1972) briefly summarized Nathan’s work for the MGS Centennial Volume in a section describing the northern prong of the Duluth Complex (Figure 1-4). In that same volume, Davidson (1972) first adopts the name “Nathan’s layered series”. Later, Weiblen and Morey (1980) referred to the intrusion as the “Layered Series of Nathan” and Weiblen (1982) used the term “Nathan’s Layered Series”. In the new geologic map compilation of the Duluth Complex (Miller and others, 2001) and the companion report (Miller and others, 2002), the informal name of Nathan’s Layered Series was replaced by the Poplar Lake intrusion to acknowledge the location where most of the lithologies recognized by Nathan (1969) are exposed and accessible. 9P00' 9045 9045 9O3O' C) SO!09y 07 fljfl7flfl7 LOIW, oorn Lose. EXPLANATION Hugry Jack Lake quadrangles r0m Nalhon. (969) Remainder (ran, 6.-our 010 orhrs (see ak I 2 3M!I,) ljt7IQlfljn aIte,otinn ci 71 by 4J J IA (mone ,hcei—liIc. nçI.niyn or Al 2 0 _5 IlifT fl Mac IJ F,GlkicIusicns ai P LIfT] °(::?rr iL±J cn4 HI Figure 1-4. Generalized geology of the Poplar Lake intrusion as portrayed by Phinney (1972) based on mapping by Nathan (1969) and Davidson (1972). Except for limited reconnaissance mapping and geochemical sampling by Jerde (2001), little new mapping or petrologic research has been devoted to this very interesting early gabbroic intrusion of the Duluth Complex. Therefore, the igneous stratigraphy and unit descriptions given for this field trip through the lower half of the intrusion will largely follow Nathan’s (1969) conventions. One notable exception is that whereas Nathan (1969) recommended naming rocks by listing the minerals present ILSG07 5 Trip 1 �in the rock in decreasing order of abundance (e.g., a plagioclase-augite-olivine rock), we have adopted more conventional modal rock name nomenclature (e.g., an olivine gabbro; see Fig. 1-5 for the modal classification scheme used here). Also, we will point out where our petrographic analysis of samples from certain field stop locations differ from Nathan’s descriptions. Mela - Pl < 60-50% Leuco - Pl > 75% Gabbroic Rocks Plagioclase = 30-80/85% OLIVINE TROCTOLITE 3:1 AUGITE TROCTOLITE OLIVINE GABBRONORITE OXIDE TROCTOLITE OXIDE-OLIVINE GABBRONORITE OXIDE-OLIVINE NORITE 1:1 OLIVINE GABBRO OLIVINE NORITE OXIDE-OLIVINE GABBRO 1:3 NORITE OXIDE GABBRONORITE FE-TI OXIDE OXIDE-RICH ROCKS 1:3 1:1 OXIDE GABBRO 3:1 3:1 GABBRONORITE LOW-CA PYROXENE GABBRO HIGH-CA PYROXENE Figure 1-5. Modal classification for gabbroic rocks (Pl 30-80/85%) used in this guide. Prefixes melaor leuco- (e.g. melatroctolite, olivine leucogabbro) are used when plagioclase abundance is <60 to 65% or >75%, respectively. Taken from Miller and others, 2002. Igneous Stratigraphy and Petrologic Interpretations of the Poplar Lake Intrusion Nathan (1969) distinguished 27 different units in his mapping of the three quadrangles that include the Poplar Lake intrusion (Fig. 1-4). He labeled these alphabetically from A through AA in order of decreasing age. Nathan interpreted the relative ages of the units on the basis of fine-grained margins, cross-cutting intrusive relationships, inclusions, and/or thermal effects. The various map units Nathan (1969) indentified within the Poplar Lake intrusion can be generally grouped into 6 major lithologies: 1) oxide-rich, gabbro to olivine gabbro: F, G, H, T, U, V 2) olivine gabbro to troctolite: P, Q 3) gabbronorite to olivine gabbronorite: A, B, M 4) anorthositic rock types: J, some M, some Q, S 5) intermediate to felsic rock types: K, O, AA, Maa 6) fine-grained mafic rocks (inclusions?): C, D, E, N, X, Y ILSG07 6 Trip 1 �Davidson conducted reconnaissance mapping in quadrangles to the south of the three that Nathan (1969) mapped and had some different interpretations of the upper units of the Poplar Lake Intrusion (Davidson, 1972, 1977a, 1977b; Davidson and Burnell, 1977). Davidson (1972) interpreted the extensive mass of granophyre at the top of the Poplar Lake intrusion, which Nathan (1969) designated as unit AA (Fig. 1-4), to be a separate intrusion that belongs to the felsic series of the Duluth Complex. Like Nathan (1969), he interpreted the massive granophyre to be one of the youngest intrusion of the Duluth Complex based on observations that granophyre dikes and irregular masses commonly cut most other rock types. Nathan’s unit Maa, which underlies the granophyre mass and is composed of a highly altered assemblage of intermediate rock types, is interpreted to be an altered and contaminated phase of unit M caused by the late intrusion of granophyre (unit AA) (Fig. 1-4). Davidson (1972, 1977a, 1977b; Davidson and Burnell, 1977) interpreted these intermediate rocks (unit grd) to be marginal phases of the granophyre. Another reinterpretation of Nathan’s (1969) mapping by Davidson (1972) and others (Morey and Nathan, 1978; Morey and others, 1981) is that Nathan’s unit R is actually part of the Tuscarora Intrusion and thus belongs to the layered series (or the troctolite-olivine gabbro series as Davidson (1972) called it). In the most recent map compilation of the Duluth Complex (M-119; Miller and others, 2001), several reinterpretations were made of both Nathan’s (1969) and Davidson’s (1972) map interpretations. One change is the suggestion that Nathan’s unit S, a gabbroic anorthosite that cross cuts lower units of the intrusion, is actually part of the younger anorthositic series. More significantly, whereas both workers considered the granophyre mass, now termed the Misquah Hills granophyre, to be one of the youngest intrusions is the Duluth Complex, it is now considered to be one of the oldest. Major evidence for this comes from U-Pb age dating, which indicates a crystallization age of 1106 ± 6 Ma (Vervoort and Wirth, 2004). Consequently, the intermediate rocks of Nathan’s unit Maa and Davidson’s unit grd, which underlie the granophyre, are now seen to represent hybridization between partially melted granophyre and underlying mafic magmas of the Poplar Lake intrusion. Miller and others (2002) suggested that the granophyre acted as a low-density trap, which caused the Poplar Lake intrusion to underplate and partially melt the felsic mass. Figure 1-6. Diagrammatic cross-section through the central part of the Poplar Lake intrusion. Figure from Phinney (1972, Fig. V-33) based on data from Nathan (1969). ILSG07 7 Trip 1 �Examining the stacking of Nathan’s (1969) units, it is immediately clear that he interpreted the emplacement of the Poplar Lake intrusion to be a complex multistage process (Figs. 1-6 and 1.7). He envisioned various sill-like intrusions to have been multiply emplaced at seemingly random levels in the intrusion. Nathan (1969) interpreted most intrusive units to have crystallized in place with little evidence of fractional crystallization. This rather unconventional interpretation of the igneous stratigraphy of a mafic layered intrusion compels some skepticism as to Nathan’s model. For example, the sulfide-mineralized heterogeneous basal unit (F), commonly the earliest formed rock type in other Duluth Complex layered intrusions, is thought to have been emplaced after considerable volumes of A and B were emplaced. Many of the fine-grained gabbro units have limited areal extent and appear similar to hornfels basalt inclusions commonly found throughout other Duluth Complex intrusions. Moreover, the anorthositic units are mostly seen as late, however, anorthositic rock are typically some of the earliest intrusive phases throughout most of the Duluth Complex. Nevertheless, until a rigorous follow-up field and petrologic of this intrusion is conducted, Nathan’s multiple emplacement hypothesis stands as the current model. F [11 kHIll Mesonroterozoic POPLAR LAKE INTRUSION '' (Units based on Nathan (1969)) prismatic 01; gradational into dp 01 gabbro- fine?, foliated, dp intergran, 0l:subpoik-subprism — dm dj dg F2 df dc db Gabbronorite- coarse,oph. intergran, commonly leucocratic Gabbronoritic Anorthosite - coarse, ophitic-subophitic Olivine oxide gabbro- crs, foliated, intergran,0l:spoik-sprism; dg'- oxide-rich (>10%) zone Olivine oxide gabbrovan-textured, locally sulfidic Mafic hornfels- fine, granoblastic; basalt inclusions? Gabbronorite- medium, fOliated, Opx:poikilitic-subpoikilitic rc1:i: Gabbronoritic Troctolite- — -.- 1-' Troctolite- medium, foliated, medium, fOliated, intergranular, Opx:poikilitic — —— LOGAN INTRUSIONS Diabase- fine-coarse, suboph- VJTLT.. + HI—I LrIrIai+t — 500 Ii iitrrm- i— -+ r-n.r4 - Fr4tI--I P 0 IHi-iH4:frHiLrT Hji500 1000 Meters H - d rri dt intergranular, locally PI-phyric Paleoproterozoic ROVE FORMATION 1 Argillite/Graywacke- thin-to thick-bedded; locally hornflsic Figure 1-7. Geologic map of the field trip area based on Morey and Nathan (1977). Unit labels after Nathan (1969), but standard modal rock names (see Fig. 1-5) interpreted from Morey and Nathan’s (1977) descriptions. Locations of more detailed maps of stop locations (Figs. 1-8, 1-9 and 1-14) are shown by inset boxes. ILSG07 8 Trip 1 �FIELD TRIP 1- DAY 1 Field Stop Descriptions Upon arriving at Rockwood Lodge and unpacking gear, we will set out to investigate roadcuts along a one-mile stretch of the Gunflint Trail for the rest of the morning. STOP 1-1. BASAL MARGIN UNITS – F, C, G Location: Roadcuts along Gunflint Trail between entrance and exit of Cook County Road 82. South Lake 7.5' quadrangle (T64N, R2W, Sec. 1). UTMs: start – 683540E 5326180N; end – 684950E 5325380N Description: Roadcuts along this section of the Gunflint Trail expose some of the rock types composing the marginal zone of Nathan's layered series (Fig. 1-8). Beginning from the junction of the Gunflint Trail with County Road 92, we will proceed east along the north side of the Trail back toward the Rockwood Lodge entrance. Mesoproterozoic POPLAR LAKE INTRUSION (Units based on Nathan (1969)) dp dg 01 gabbro- fine?, foliated, intergran, Ol:subpoik-subprism Olivine oxide gabbro- cr5, foliated, intergran,Ol:spoik-sprism; —-- df dc dg'- oxide-rich (>10%) zone Olivine oxide gabbrovan-textured, locally sulfidic Mafic hornfels- fine, granoblastic; basalt inclusions? LOGAN INTRUSIONS Diabase- fine-coarse, subophIi intergranular, locally PI-phynic Paleoproterozoic ROVE FORMATION 1 Argillite/Graywacke- thin- to thick-bedded; locally hornflsic Figure 1-8. Geology of the Gunflint Trail area northwest of the Rockwood Lodge (based on Morey and Nathan, 1977). Roadcuts A – E are described in the text for Stop 1-1. Roadcuts A – These exposures show a more homogeneous and sulfide poor phase of Unit F. The rock is a coarse-grained ophitic oxide olivine gabbro. Similar exposures occur in the roadcuts on the south side of the Trail, west Co Rd 92 junction. This is the location of geochronology sample NLS-5,which yielded a U-Pb zircon age of 1106.9±0.8 Ma (Paces and Miller, 1993). Roadcuts B – This series of a low, deeply weathered roadcuts and low pavements are more typical of unit F. About 150 m from the junction is of coarse-grained, decussate, ophitic, biotitic oxideolivine gabbro. This noncumulate, vari-textured (taxitic) gabbro forms the lowermost unit of ILSG07 9 Trip 1 �Nathan's (1969) layered series (his unit F). Locally, Cu-Fe sulfide is present and gives rise to the very rusty appearance of the outcrop. Sulfide mineralization in the lower contact zone is characteristic of many parts of the Duluth Complex (Bonnichsen, 1972; Miller and others, 2002). As is interpreted from the Cu-Ni-PGE sulfide deposits of the NW Duluth Complex, the probable source of the sulfur are the shale and graywacke of the Paleoproterozoic Rove Formation, which forms the footwall of the gabbro. The lack of significant sulfide mineralization here compared to the NW area may indicate a lower sulfur content to the sediments or depletion of sulfur by earlier intrusion of the Logan Sills. Roadcut C – At this prominent roadcut, a very fine grained gabbro is exposed. It has irregular areas of coarser gabbro scattered throughout and some pockets of sulfide. This is Nathan's unit C, which he suggested could be recrystallized inclusions of Keweenawan lavas, Logan sills, or more likely, the chilled contact of a precursor intrusion (his units A and B). In thin section, the rock is a strongly recrystallized, poikiloblastic gabbronorite with small (<5 mm) high-density oikocrysts of augite and hypersthene enclosing granular plagioclase. Because the texture of Logan sills remains unaffected as the contact with the layered series approached (Nathan, 1969), it is unlikely that this body is such an inclusion. Rather, the zones rich in ovoid clots of coarse feldspar are similar in appearance to what have been interpreted elsewhere in the complex as recrystallized amygdules (Bonnichsen, 1972), and this inclusion is most likely a basalt flow. The abundance of these types of inclusions and the relative lack of quartz- or cordierite-bearing inclusions of Rove Formation suggest that the focus of emplacement of Nathan's layered series was at the horizontal to shallow-dipping discontinuity between Early Proterozoic sedimentary rocks and Keweenawan lava flows. Roadcuts D and E About 700 m farther east past a bend in the road, another deeply weathered roadcut exposes a medium- to coarse-grained, moderately laminated oxide troctolite with ophitic augite. This belongs to Nathan's unit G which he characterized as "coarse-grained, olivine-plagioclase and augite-plagioclase rocks with strongly foliated plagioclase and abundant tironals (Fe-Ti oxides) . . . bioite is conspicuous . . . density graded layering is prominent" (Nathan, 1969, p. 68). Although they form structurally lowest cumulates in the layered series, Nathan interpreted this unit to be younger than structurally higher troctolitic to gabbronoritic units on the basis of a complex sequence of discordant relationships (Units A and B, Fig. 3.21). The validity of Nathan's intrusive stratigraphy clearly needs to be tested. --Return to Rockwood Lodge for lunch and prepare for afternoon canoe trip.-- WARNING: The ice just went out (we hope) on the lake and so the water is just above freezing. We will take some time over the lunch hour to instruct everyone on basic canoe safety and what to do in the event that someone dumps in the water. We have BWCAexperienced assistants on the trip and we will bring special equipment to guard against hypothermia. Despite these precautions, the best preventative is using extreme care and common sense. ILSG07 10 Trip 1 �it-' '} POPLAR LAKE INTRUSION Units based on Nathan (1969) dq d Troctolite medium, foliated, prismatic 01; gradational into dp 01 gabbro- fine?, foliated, intergran, 0l:subpoik-subprism Gabbronoritic Anorthosite - coarse, ophitic-subophitic dg Olivine oxide gabbro- crs, fOliated, intergran,0l:spoik-sprism; dgt oxide-rich (>10%) zone df dc —db - da Olivine oxide gabbro van-textured, locally sulfidic Mafic hornfels - fine, granoblastic; basalt inclusions? Gabbronorite- medium, fOliated, Opx:poikilitic-subpoikilitic Gabbronoritic Troctolite medium, fOliated, intengranular, Opx:poikilitic Figure 1-9. Geologic map of the central Poplar Lake area showing field stop locations for the afternoon of Day 1. Dashed line shows canoe route. Geology after Morey and Nathan (1977). STOP 1-2. UNIT G’ – OLIVINE OXIDE GABBRONORITE Location: Float by shoreline ledges on island about 300m due south of Rockwood Lodge. UTMs: 684800E 5325000N Description: These lichen-covered, grussy outcrop ledges are composed of medium-grained, wellfoliated, oxide gabbro typical of Unit G’, an oxide-rich (locally >20%) interval within Unit G. Paddling to the east, a prominent outcrop can be seen at the western point of a peninsula. The well-developed foliation gives rise to a sheet jointing that dips about 20° to the south. --Canoe around the east end of this island then south into a narrow channel. Head WSW to eastern point of another island which has a sizeable dock for disembarking. -- STOP 1-3. UNIT G – OXIDE OLIVINE GABBRONORITE Location: Private residence and dock at east point of island; please request permission from owners to view exposure. UTMs: 684905E 5325355N Description: The main rock type exposed over this outcrop point is a medium coarse-grained, foliated, intergranular, oxide olivine gabbronorite. This exposure is typical of Unit G. One curious feature is the occurrence of a 20- to 30-cm-thick, fine grained sill of a similar mineralogy ILSG07 11 Trip 1 �and texture exposed in the cliff face back from the boat dock. The lower contact is rather sharp but the upper contact is gradational (Fig. 1-10A). Throughout the sill and especially near the upper contact, coarse crystals and crystal clusters of pyroxene and plagioclase occur (Fig. 1-10B). Petrographic observations across this sill show that it differs from the coarser host in several subtle ways. Olivine in the fine-grained gabbro is commonly subpoikilitic compared to granular olivine in the host gabbro. Low-Ca pyroxene is less abundant in the fine-grained gabbro (5% compared to 20%) and is dominantly hypersthene rather than inverted pigeonite. Perhaps this represents a new pulse of magma into the chamber or it marks venting event from the magma chamber causing decompression quenching of a water-saturated evolved magma. The presence of up to 2% biotite and the generally similar mineralogy and texture of the fine-grained and medium-grained gabbros suggest the latter explanation is more likely. A. B. Figure 1-10. A) Fine-grained gabbro sill in medium-grained gabbro host; B) Close-up of piece of float from the sill show dispersed coarse plagioclase and pyroxenes in the fine matrix. --Canoe about 400m due south to northernmost point of eastern end of peninsula. Limited landing spots, so this stop is optional,-- STOP 1-4. UNIT G OR P?- LAYERED OLIVINE GABBRO (Optional) Location: Point of land in southwestern Poplar Lake near the far north central part of Sec. 12. UTMs: 684785E 5324469N Description: Nathan implies by his map and unit designation that this as a thin inclusion? of unit G enclosed between younger units P and J (Fig. 1-9). The exposure here is a medium- to medium fine-grained, moderately layered, well-foliated, intergranular oxide olivine gabbro. A thin section sample from here shows a medium fine-grained oxide olivine gabbro that has a very granular (granoblastic?) texture and is lacking in low-Ca pyroxene. This rock has similarities to Nathan’s Unit P by being fine, olivine-rich, and oxide poor. The lack of Opx makes it distinct from either Unit P (commonly contains some poikilitic hypersthene) and especially Unit G (typically contains significant amounts of inverted pigeonite). --Canoe about 550m to the east-southeast to island. Land canoes against outcrops on the southeast and southwest sides of the island.,-- ILSG07 12 Trip 1 �STOP 1-5. UNIT J – OLIVINE OXIDE LEUCOGABBRO Location: Island in southern part of Poplar Lake. UTMs: 685210E 5324200N Description: Exposures on the south side of the island are composed of coarse-grained, foliated, ophitic to intergranular olivine oxide leucogabbro, which Nathan classifies as Unit J. Plagioclase ranges from 70 to 85% of the rock and is the only consistently cumulus phase. Augite and Fe-Ti oxide are the dominant mafic minerals and vary from anhedral granular to poikilitic. Minor olivine (3-5%) is also variable in texture from subprismatic (Fig. 1-11A) to subpoikilitic. Layers and pods rich in granular Fe-Ti oxides and pyroxene are evident on the southwestern corner of the island (Fig. 1-11B). In poor exposures on the north side of the island (downsection), granular (cumulus) augite, olivine, and oxide are present and the rock seems less leucocratic. A. B. B Figure 1-11. A) Common texture of leucogabbro with granular mafic phases (Ol, Aug, FeOx). B) Massive Fe-Ti oxide layer in leucogabbro. -- Paddle ~175m to the southeast to small island with campsite. Land canoes against outcrops on the south side or in cove on east side of the island. Welcome to Happy Island!-- STOP 1-6. UNIT P – OLIVINE GABBRO Location: Island in southern part of Poplar Lake. UTMs: 685310E 5324050N Description: “Happy Island” is composed of two east-west oriented outcrop areas. Exposures on the south side of the island are composed of medium-grained, well-foliated, intergranular olivine gabbro with variable abundances of Fe-Ti oxide (1-7%). Olivine occurs as small (<1cm), subpoikilitic clots or as prisms 0.5 to 3 cm long (~1:10 aspect ratios; Fig. 1-12) that tend to be concentrated in thin layers. This rock type is consistent with Nathan's description of unit P. Morey and Nathan’s (1977) map shows a contact between unit P to the south and unit Q, a troctolite, to the north running through the middle of the island. However, outcrop exposures and petrographic observations of two samples from the northern end show the rock to be mostly an oxide olivine gabbro like Unit P. The only significant differences are a slightly coarser grain size, subtle textural and modal layering, and a slightly higher concentration in inverted pigeonite. Nothing on the north half looks like an augite-poor troctolite, which characterizes Unit Q (See Stop 1-9). ILSG07 13 Trip 1 �A. B. Figure 1-12. Textures of olivine found in Unit P: A) 5mm subpoikilitic clots (Stop 1-7); B) 1cm long prismatic olivine (Stop 1-6) -- Paddle ~200m to the east-southeast to the south side of an E-W island. Land canoes against outcrops on the south-central of the island. -- STOP 1-7. UNIT P – OLIVINE GABBRO Location: Island in southern part of Poplar Lake. UTMs: 685532E 5323990N Description: Island is composed of two outcrop areas on the south side of the island, between which access is difficult. Both exposure areas are composed of medium-grained, well-laminated, intergranular olivine gabbro (PAOf cumulate) as observed on the south side of the island at Stop 3-3. Locally, 2-5-cm clots of coarse pyroxene and oxide are present and elsewhere are pods (inclusions?) of leucogabbro (Unit J?). Again olivine occurs as small subpoikilitic clots or as prisms up to 3 cm long (Fig. 1-12). Because prismatic olivine tends to be concentrated in thin layers along which joints tend to develop, this texture is preferentially displayed on the outcrop face. Note that on the plane of layering, prismatic olivine is commonly oriented parallel to the strike of layering. This observation and the delicate prismatic habit of the olivine argues against crystal settling by density currents. -- Canoe ~250m to the southwest to steep cliffs lining the bay leading the portage to Lizz Lake. Float along the cliffs and then land canoes at the portage. -- STOP 1-8. UNIT J –LEUCOGABBRONORITE-GABBROIC ANORTHOSITE Location: Bay leading to portage (to Lizz Lake) at south shore of Poplar Lake. UTMs: 685460E 5323700N Description: Exposed in sheer cliffs along the west side of the bay leading to the portage to Lizz Lake is leucogabbroic rock typical of Unit J. Lichen and moss cover make it difficult to see much of the rock. A thin section of a sample collected here shows the rock to be a medium coarse-grained ophitic leucogabbronorite. In some places , pockmarked surfaces are evident and indicate the presence of large oikocrysts of poikilitic olivine. This texture is very well ILSG07 14 Trip 1 �displayed in the exposure on the west side of the portage trail (Fig. 1-13). Here the olivine oikocrysts are up to 5 cm across. Ophitic augite is also evident here in oikocrysts up to 10 cm across. Interestingly, this rock type is very similar to the dominant lithology of the Anorthositic Series of the Duluth Complex. It would worthwhile determining the magnetic polarity of this rock type to see if it has a typical Poplar Lake reversed polarity, or is if is normal like all known Anorthositic Series rocks. Figure 1-13. Mottled surface of ophitic olivine gabbroic anorthosite at Stop 1-8. Clots are formed from both olivine and augite oikocrysts. -- Canoe ~600m to the northeast past the eastern point of island where Stop 1-7 is located. Continue northeast to middle point of large island. Not sure how many canoes can land here so this is an optional stop.. -- STOP 1-9. UNIT Q –TROCTOLITE (OPTIONAL) Location: Middle point of west side of large island in southern Portage Lake. . UTMs: 685850E 5324150N Description: According to a thin section sample of this outcrop that shows a medium-grained, well foliated troctolite with small olivine grains, this site is more typical of Nathan’s Unit Q. -- Canoe northwest back to Rockwood Lodge (~1.6 kilometers). End of Day 1.— ILSG07 15 Trip 1 �FIELD TRIP 1- DAY 2 Depart from Rockwood Lodge and return to portage to Lizz Lake (Stop 1-8). Portage canoes to Lizz Lake landing then double back on portage trail to view first stop of the day. Mesoproterozoic POPLAR LAKE INTRUSION (Units based on Nathan (1969)) 01 gabbro- fine?, foliated, dp intergran, oI:subpoik-subprism F • Gabbronorite- coars%oph- - dni - - intergan, cornnoniy eucocratic Gabbronoritic Anorthosite dj — - db cia - coarse, ophitic-subophitic Gabbronorite- medium, bliated, Opx:pcikilitic-subpoikilitic Gabbroncritic Troctolitemedium, Ibliated, intergranular, Opx:poikiiitic Figure 1-14. Geologic map of the Lizz and Caribou Lakes area showing field stop locations for Day 2. Dashed line shows canoe route. Geology after Morey and Nathan (1977). STOP 1-10. UNITS J AND B AND THE LIZZ LAKE FAULT Location: Lizz Lake portage trail just 50m south of junction with Banadad trail. UTMs: 685500E 5323600N Description: Exposed on the east side of the trail is a medium coarse-grained ophitic oxide gabbronoritic anorthosite typical of leucocratic Unit J. About 10m to the west of the trail is a poorly exposed ledge of medium-grained, foliated, gabbronorite with poikilitic hypersthene. The rock contains no olivine or Fe-Ti oxide. This rock type is typical of Nathan’s Unit B. The juxtaposition of these two distinct rock types across this gap is the best evidence of right lateral offset across a fault that Morey and Nathan (1977) inferred through here and down the elongate trend of Lizz Lake (Fig. 1-14). The Lizz Lake fault is the only fault that Nathan (1969) identified in the interior of the Poplar Lake Intrusion. -- Canoe a short distance to the outcrop island about 100 m to the southeast of the portage-- ILSG07 16 Trip 1 �STOP 1-11. UNIT J? – GABBRONORITIC ANORTHOSITE Location: Island with abundant outcrop on north end of Lizz Lake. UTMs: 685685E 5323490N Description: Good exposures over most of this island show the rock to be a medium coarsegrained, poorly foliated, ophitic, gabbronoritic anorthosite similar to the Unit J outcrop at Stop 1-10. However, Nathan has this exposure mapped within Unit B. This may indicate that the Unit J-Unit A/B contact is farther south on the east side of the Lizz Lake fault. Alternatively, perhaps this is an inclusion in Unit B though this would violate Nathan’s interpretation of Unit B being an early phase of the Poplar Lake intrusion. More mapping in this area is needed to resolve this. -- Float along a series of cliff faces on the western shore of Lizz Lake to view Stop 12 exposures-- STOP 1-12. UNIT B – GABBRONORITE Location: Series of outcrop ledges along the northwestern shoreline of Lizz Lake. UTMs: 685715E 5323345N Description: Although largely obscured by lichen, the ledges along this shoreline display a homogeneous, medium-grained, well-foliated gabbronorite with Opx > Cpx. Orthopyroxene (hypersthene and inverted pigeonite) is typically subpoikilitic to poikilitic, whereas clinopyroxene (augite) is typically anhedral granular (Fig, 1-15A). In one well exposed location beneath a tree tip-up, a subtle modal layering was recognized (Fig. 1-15B). A. B. Figure 1-15. A) Texture of the gabbronorite typical of Unit B. B) Subtle modal layering common observed under a tip-up at Stop 1-12. -- Canoe to the south end of Lizz Lake to portage to Caribou Lake. From the portage landing, head west about 8m to a rock cliff -- ILSG07 17 Trip 1 �STOP 1-13. UNITS B AND X? – GABBRONORITE AND FINE GABBRO Location: Outcrop ledge southwestern corner of Lizz Lake near portage to Caribou Lake. UTMs: 686195E 5322605N Description: Most of this exposure is a very fine-grained mafic rock in sharp contact with a medium-grained gabbro. Thin sections of the fine-grained rock shows it to be a feltytextured, oxide gabbronorite and the medium-grained gabbro is a well foliated, intergranular gabbronorite with poikilitic olivine. Nathan mapped this area as largely unit B with an occurrence of Unit X, which he defines as a fine-grained Pl-porphyritic oxide gabbro. By its unit letter designation, Nathan interpreted this as young intrusive unit. The medium gabbronorite is similar to typical unit B, except for the poikilitic olivine. Perhaps it is a hybrid between Units B and A. The fine-grained rock being an intrusion into B would be consistent with its felty texture. One would expect the rock to display a granoblastic texture if it was a hornfels inclusion. We will investigate the exposure to ascertain whether the fine-grained rock is an intrusion or an inclusion. -- Take the portage to Caribou Lake. Canoe 180m south to a campsite fronted by a large outcrop pavement-- STOP 1-14. UNIT A – OLIVINE GABBRONORITE Location: Outcrop pavement at campsite on south shore of northeast arm of Caribou Lake. UTMs: 685960E 5322155N Description: Very homogeneous exposure of medium-grained, foliated, olivine gabbronorite with poikilitic inverted pigeonite. Some of the inverted pigeonite oikiocrysts can be up to 30 cm across. This exposure is identified as Unit A by Morey and Nathan (1977), which they describe as being a fine-grained troctolite with up to 9% inverted pigeonite and 3% augite. The thin section observed from this outcrop contains 8% augite and 12% inverted pigeonite as well as 20% olivine. Given that unit A occurs at many statigraphic levels in the upper part of the Poplar Lake intrusion, it is perhaps not surprising that it is variable in mode, -- Canoe ~1200 m to the southeast arm Caribou Lake to outcrop-fronted campsite on northern shore-- STOP 1-15. UNIT M – AUGITE NORITE Location: Outcrop pavement at campsite on north shore of southeastern arm of Caribou Lake. UTMs: 686347E 5321875N Description: Homogeneous outcrop of medium-grained, well-foliated, intergranular augite norite. Thin section show that 25% of rock is anhedral to subprismatic granular inverted pigeonite with anhedral granular augite comprising about 10% of the rock. The prismatic inverted pigeonite weathers brown which makes it appear similar to olivine in outcrop (Fig. 1-16). The rock is devoid of olivine and contains only trace Fe-Ti oxide. ILSG07 18 Trip 1 �Nathan and Morey (1977) describe unit M as a coarse-grained, poorly foliated, gabbronorite with average mafic mineral concentrations of up to 2% oxide, 13% inverted pigeonite, and 24% augite. Figure 1-16. Texture of the prismatic augite norite at Stop 1-15. -- Canoe ~400 m south to the portage to Horseshoe Lake -- STOP 1-16. UNIT M – GABBRONORITIC ANORTHOSITE Location: Small outcrop at portage landing to Horseshoe Lake in southeastern Caribou Lake. UTMs: 686375E 5321490N Description: A small outcrop at portage landing appears to be an example of what Nathan cites as the “plagioclase-rich facies” of Unit M, which typically contains coarsely poikilitic pyroxene (Morey and Nathan, 1977). A thin section of this rock shows it to be medium coarse-grained, poorly foliated, ophitic gabbronoritic anorthosite. -- Canoe ~1500 m to a campsite on a peninsular point in western Caribou Lake -- ILSG07 19 Trip 1 �STOP 1-17. UNIT M OR A? – OLIVINE GABBRONORITE Location: Outcrop-fronted campground on peninsula in western Caribou Lake. UTMs: 685030E 5322070N Description: Nathan includes this exposure within Unit M, but the rock is clearly olivine-bearing and is more similar to Unit A lithologies. A thin section from this location reveals a mediumgrained, foliated, ophitic olivine gabbronorite with about 17% granular olivine, 13% poikilitic inverted pigeonite, and 4% subophitic augite. Basically, it is similar to the olivine gabbronorite observed at Stop 1-14, which is mapped as Unit A. Morey and Nathan (1977) place the Unit M – Unit A contact just north of this peninsula (Fig. 1-14), suggesting that it should be farther south. -- Canoe north around island and then east; continue past small peninsula on north shore to campsite; total distance approximately 800m. -- STOP 1-18. UNIT M OR B – LAYERED GABBRONORITE Location: Outcrop pavement at BWCA campsite on the north shore of Caribou Lake. UTMs: 685575E 5322400N Description: Generally a medium coarse-grained, subophitic to intergranular leucogabbronorite (Fig. 1-17A) locally with cm-scale mesocratic to melanocratic layers (Fig. 1-17B). This exposure is situated right at a contact between Unit B and an irregular body of Unit M. Given its coarser grain size and generally leucocratic composition, it seems closer to Unit M. A. B. Figure 1-17. A) Texture of the leucogabbronorite at Stop 1-18. B) Cm-scale wavy modal layering displayed at Stop 1-18. -- Continue along north shore to Lizz Lake portage. Return to Rockwood Lodge. – END OF FIELD TRIP 1 ILSG07 20 Trip 1 �References Beck, M.E., 1970, Paleomagnetism of Keweenawan intrusive rocks, Minnesota. Journal of Geophysical Research, v. 75, p. 4985-4996. Bonnichsen, B., 1972, Southern part of the Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A centennial volume. Minnesota Geological Survey, p. 361-388 Broderick, T.M., 1917, The relationship of the titaniferous magnetites of northeastern Minnesota to the Duluth Gabbro. Economic Geology, v. 12, p. 663-696 Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M, Jr., 1977a, Reconnaissance geologic map of the Eagle Mountain quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-28, scale 1:24,000 Davidson, D.M, Jr., 1977b, Reconnaissance geologic map of the Lima Mountain quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-32, scale 1:24,000 Davidson, D.M, Jr., and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-29, scale 1:24,000 Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488 Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geological Society of America Bulletin 96, p. 1572-1579 Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume. Minnesota Geological Survey, p. 294-332. Grout, F.F., 1950, The titaniferous magnetites of Minnesota: Department of Iron Range Resources and Rehabilitation, 117 p. Grout, F.F., Sharp, R.P., and Schwartz, G.M, 1959, The geology of Cook County Minnesota. Minnesota Geological Survey Bulletin, v. 39, 163 p. Jerde, E.A. , 2001, The early gabbroic series of the Midcontinent Rift system: Continued assessment magmatic origins. 47th Annual Institute on Lake Superior Geology, Madison, WI, p. 36-37. Mathez, E.A., Nathan, H.D., Morey, G.B., 1977, Geologic map of the Hungry Jack Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-39, scale 1:24,000 Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Morey, G.B., and Nathan, H.D., 1977, Geologic map of the South Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-38, scale 1:24,000 ILSG07 21 Trip 1 �Morey, G.B. and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-42, scale 1:24,000 Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Phinney, W.C., 1972, Northern prong. In Sims, P.K., and Morey, G.B., eds., Geology of Minnesota— A centennial volume. Minnesota Geological Survey, p. 346-353 Vervoort, J.D. and Wirth, K.R., 2004, Origins of the rhyolites and granophyres of the Midcontinent Rift, northeast Minnesota. 50th Annual Institute on Lake Superior Geology, Duluth, MN, p. 158-159. Weiblen, P.W. and Morey, G.B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex. American Journal of Science, v. 280-A, 88-133. Weiblen, P.W., 1982. Keweenawan intrusive igneous rocks. In: Wold, R.J. & Hinze, W.J. (eds.) Geology and tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, 57-82. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/ 100 plates. ILSG07 22 Trip 1 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 2 GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT William F. Cannon U.S. Geological Survey David J. Cooper U.S. National Park Service, Grand Portage National Monument Brian A.M. Phillips Lakehead University, emeritus Engraving of Grand Portage area made from field sketch done in 1849 by survey party from U.S. General Land Office. Illustration is included in report by David Dale Owen in 1852. Engraving shows physiography of the area as well as geologic cross sections. ILSG07 23 Trip 2 �GEOLOGIC AND CULTURAL HISTORY OF THE GRAND PORTAGE NATIONAL MONUMENT William F. Cannon, U.S. Geological Survey Brian A.M. Phillips, Lakehead University, emeritus Including a section on cultural history excerpted from a report by Douglas Birk, Senior Archeologist emeritus, Institute for Minnesota Archeology Introduction The Grand Portage National Monument, maintained by the U.S. National Park Service, consists of a restoration of an 18th century fort and trading post on the shore of Lake Superior and the 8.5 mile portage trail extending northwestward from the lake shore to the Pigeon River. The unrestored Fort Charlotte lies at the northwest end of the trail, on the south bank of the Pigeon River, at the head of a long series of rapids and waterfalls that made this long portage necessary in order to continue westward canoe travel from Lake Superior. The portage trail was the principal highway to the west for 18th century explorers, settlers, and fur traders who crossed the portage from Lake Superior to the Pigeon River, then into the westward draining Rainy River system, and eventually deep into the northern Great Plains and Rocky Mountains. It was over this trail that the wealth of furs was brought on their way to Montreal and eventually to Europe. Those who crossed the trail were largely responsible for the exploration and development of the Canadian west and northern U. S. Great Plains. The early fur traders, the Voyageurs, made their annual trek over the portage in late summer and then began their long canoe journey to the west to trade with native peoples for furs over the ensuing winter. They returned over the portage the following summer to the trading post to deliver their bounty of furs to representatives of the North West Company (NWC) who annually traveled from Montreal to the Grand Portage trading post. The geology of the Monument and surrounding countryside consists of: 1) the Rove Formation, a Paleoproterozic clastic sequence of shale and sandstone, 2) mafic dikes and flows formed in the Mesoproterozoic Midcontinent Rift, and 3) glacial and post-glacial deposits produced by the last tongue of Pleistocene glaciers that occupied the western Lake Superior basin and a variety of lake shoreline features formed by a series of post-glacial lakes that inundated the area shortly after deglaciation. During this trip we will tour the restored fort and trading post, discuss the European and Native American history of the site, and examine bedrock and glacial to post-glacial geologic features of the area, particularly as they affected the human history through the unique set of physiographic features that they produced. Figure 2.1 is a map of the Monument showing the location of restored buildings on Grand Portage Bay, and three geologic field trip stops. ILSG07 24 Trip 2 �Figure 2.1. Map of the Grand Portage region showing the location of the National Monument and portage trail. Geologic field trip stops are also shown. Cultural history (This account of cultural history is excerpted and slightly modified from Douglas Birk, 2005, National Register of Historic Places, Grand Portage) Grand Portage derives its name and much of its historical significance from a formidable portage, more than eight miles long, which links the Great Lakes with an interconnected series of inland waterways to the northwest. Key to the development of the North American fur trade, the Grand Portage gave Canadian explorers access to the northern Plains and to travel routes extending to the Arctic and Pacific Oceans. In the late eighteenth century the portage became a hub of seasonal coalescence and transshipment activities that linked suppliers in far-off places like England, the West Indies, and Brazil with a network of trading posts set in mosaic Native hunting territories throughout western Canada. The setting of Grand Portage, on the west side of Lake Superior and near the mid-point of a 3,000-mile commercial canoe transportation system, enhanced its role in channeling, shaping, and controlling the northwestern Montreal fur trade. Inspired by these qualities, one historian has declared that Grand Portage was once “probably the single most important fur trade location in the world.” North American Indian peoples knew the use of canoes and portaging thousands of years before the arrival of Europeans, and they initially opened the Grand Portage route. French colonials traveling west from Montreal first arrived at Lake Superior early in the seventeenth century. Having learned of the Grand Portage from Indian informants by at least 1722, the French began crossing it in 1731 to advance their affairs and alliances in more remote areas to the Northwest. From then through the mid-1800s, the portage attracted a colorful succession of French, British, and American fur traders. Between 1780 and 1802, at the height of the commercial traffic, Grand Portage was a western headquarters and regional trade center of the North West Company. It was also the scene of an annual summer gathering that made it one of the busiest nodes of human activity in the midcontinent. By promoting the mercantile interests of France and Great Britain, the Grand Portage facilitated the spread of European colonialism and the development of the nations of North America. In the nineteenth century Grand Portage also played an important part in settling the disputed boundary between Minnesota and Ontario, which is to say between Canada and the contiguous United States in the region west of Lake Superior. ILSG07 25 Trip 2 �The most enduring human presence at Grand Portage is that of aboriginal North Americans, including, most recently, the Ojibwe, Algonkian-speaking peoples with Woodland cultural traditions. Details surrounding the emergence of the Ojibwe people or the Ojibwe identity are often debated. Some traditional accounts indicate that, in the seventeenth and eighteenth centuries, the Saulteur (Ojibwe) of the eastern Lake Superior region extended their territorial prerogatives westward with the advancing fur trade. A group since known as the Grand Portage Ojibwe established ties to Grand Portage by the 1730s. They were active in that region throughout the fur trade era, and they remain a vital community today. All landscapes, resources, sites, properties, themes, and contexts that are now part of the Grand Portage National Monument story are in some way linked to the Grand Portage Ojibwe. The National Monument was formed through their invitation and cooperation, and efforts to maintain and interpret historic properties and traditions at Grand Portage have long been assisted by that local band or by individual members of the band. French and later British traders commonly entered the Great Lakes-Northwest trade by traveling west from Montreal. The use of Indian guides, birchbark canoes, and a vast network of established Indian canoe routes and portages assisted their movements (Morse, 1969; Birk, 1994). From the beginning, the fur trade built on entrenched Indian exchange practices while catering to Indian preferences and needs (e.g., White, 1982, 1987). Over time, as the business of accruing and transporting goods grew more complex and exchange frontiers expanded westward, the systems of trade were institutionalized. Certain individuals, like voyageurs, became specialists in the trade hierarchy just as certain places gained wider distinction as nodes or corridors of commerce. The most influential settlements were those that played strategic roles in the flow of workers, provisions, merchandise, and information (e.g., Hirth, 1976). Grand Portage became such a place in the eighteenth century. Through regular use, the Grand Portage emerged as a hub for local trade and as a transshipment center--a gateway community that linked markets and linear transportation lanes in the east with branching trade routes and mosaic trade districts to the northwest (Birk, 1984). Between 1731 and 1804 tons of supplies and furs were shuttled over the portage; some, in and out of the warehouses eventually built at either end of the trail. After 1760 the portage became a general rendezvous. It was a veritable beehive of activity during summers, but in winters the outposts there were comparatively quiet and staffed only by small crews engaged in local trading and facilities management (Gilman, 1992). At the height of the trade, around 1800, Grand Portage was the western headquarters of the North West Company (NWC) and the rival XY Company (XYC), two of the largest commercial establishments in North America. When the NWC and XYC moved their operations north to Kaministikwia (later Fort William, Ontario) at the start of the nineteenth century, Grand Portage lost its identity and its status as trade center, abruptly becoming remote to the main channels of trade and communication and less important to the outside world. Grand Portage is a place of majestic scale and natural beauty with many prominent landmarks. The lakefront area faces Grand Portage Bay, a horseshoe-shaped inlet that forms the deepest natural indentation on the Minnesota coast of Lake Superior (Schwartz, 1928). Flanking the northeast side of the bay is the rolling ridge of Hat Point (Pointe au Chapeaux), backed farther inland by the promontory of Mount Josephine, the top of which towers nearly 750-feet above the surface of the lake. The opposite, southwest edge of the bay is framed by the sloping headland of Raspberry Point (Pointe à la Framboise). About a mile offshore, in the center of the bay and partly shielding the bay from the big lake, is Grand Portage Island (also known as Pete’s Island, Sheep Island, Isle aux Mouton, etc.) (Gates, 1965; Grout and others, 1959). Near the lakefront within the Grand Portage National Monument is an imposing hill known as Mount Rose. The rounded and rocky summit of the hill rises several hundred feet above the bay and overlooks all of the historic properties clustered along the shoreline there (Winchell, 1899; Thompson, 1969; Gilman, 1992), as well as the mouth of Grand Portage Creek, a stream channel that drains from the interior highlands and skirts the base of Mount Rose before entering Lake Superior. Mount Rose once ILSG07 26 Trip 2 �served as a vantage to watch for approaching watercraft, the earliest such documented use perhaps occurring in 1767 (Carver, 1956). The Grand Portage is the first leg of a remarkable inland canoe route that passes from Lake Superior through a chain of lakes and rivers along the present international boundary between Ontario and Minnesota. That mainline route, sometimes called the “Voyageur’s Highway,” links with a vast network of other branching canoe trails in the hinterlands beyond (Morse, 1969). Few viable inland water routes emanate from the west side of Lake Superior, and the Grand Portage-Pigeon River route early proved to be the most direct and efficient gateway to the “border lakes” region and the far Northwest (Gilman, 1992). The Pigeon River is a relatively short stream whose headwaters at the rim of the Superior basin are only about 50 miles west of Grand Portage Bay and enters Lake Superior about five miles to the northeast of Grand Portage. The river is navigable by canoe for much of its length above Fort Charlotte. For 20 miles below Fort Charlotte, however, the Pigeon River is impassable. That stretch follows an eastward course through a sinuated series of rapids, canyons, and falls before reaching the big lake. The terrain on the Canadian side of the lower Pigeon River is too rugged and the distance between the lake and navigable parts of the river too great for portaging to be practical there (Schwartz, 1928; Buck, 1931; Burpee, 1931; Morse, 1969, Birk, 1998). The ascent from Lake Superior through the Grand Portage and up the Pigeon River to the height-of-land separating the Laurentian and Hudsonian basins is the steepest part of the entire historic canoe route that once ran between Montreal and Athabasca (Morse, 1969; Birk, 1998). Prior to the time the NWC made improvements to the trail, heavily outfitted canoe brigades could take up to ten days to move from Lake Superior to the Pigeon River (Wallace, 1934). During the heyday of the fur trade, with the NWC installed at Grand Portage, the same carry might take from five to seven days (Gilman, 1992). Some men loaded with packs or kegs weighing 180 pounds or more were able to make a round trip on the portage within six hours (Buck, 1931). Because porters often moved at a trot and took short rests en route, their actual progress was about as expedient as that of an unencumbered walker, who, under optimal conditions and without rests, might cross the portage in 2.5 hours (Thompson, 1969). Early fur traders tried using horses and oxen on the portage but under normal conditions in warmer seasons of the year, the trail was best suited for human porters (Burpee, 1931; Thompson, 1969; Gilman, 1992). Animal-drawn carts or sleds were more commonly used during the “off season” (other than summers) or during the nineteenth century, when traffic on the portage was greatly reduced. Diminished forest cover along some parts of the portage in the 1800s opened extensive views of the surrounding terrain and perhaps changed the surface environment of the trail (e.g., Winchell, 1899). In 1858, for example, the trail was said to be dry and in good condition, making it passable for oxen teams (Hind in Dawson, 1968). The Fort Charlotte site complex involves the remains of fur trade facilities that once stood along the Pigeon River at the head of the portage on either side of the mouth of Snow Creek. Evidence suggests that the NWC conducted operations along the river north of the creek (between the creek and the whitewater rapids), while the XYC and perhaps other earlier firms occupied an area along the river south of the creek. To reach these various facilities the portage trail probably forked somewhere east of the Pigeon River so that one branch of the trail led to the south side of the creek and the other continued along the north side. The main trail, the one most used and over the greatest length of time, was that on the north side. Underwater archeological investigations conducted at the main Pigeon River landing suggest that, through about 1780 or so, the main portage intersected the river very close to the mouth of Snow Creek. With the formation and growth of the NWC, the portage landing migrated downstream, first to the place where the NWC built a canoe landing and quay or “dock” (Winchell, 1899), and later to where the trail meets the river today, at a point about midway between Snow Creek and the head of the whitewater rapids (Birk, 1975; Birk and Wheeler, 1976; Wheeler and others, 1975; Birk, 1979). Modern recreational use of the landing, along with floating ice, fluctuating water levels, river currents, and log drives on the ILSG07 27 Trip 2 �Pigeon River, may have damaged early landing structures or associated offshore archeological deposits at Fort Charlotte (Birk, 1975). French traders frequented the portage between 1731 and 1760 (Nute; 1944; Woolworth and Wooworth, 1982) and perhaps earlier (e.g., Burpee, 1931), yet suspected French-era deposits or structures at Grand Portage Bay have eluded archeological discovery. Surprisingly little information about French activities at Grand Portage is found in available written records either. Nevertheless, some scholars suggest that French traders erected buildings at Grand Portage before 1760 (e.g., Warren, 1957; Buck, 1931; Babcock, 1940; Nute, 1944; Woolworth, 1967; Woolworth and Woolworth, 1982), that they had facilities at both ends of the portage as early as 1732 (Woolworth and Woolworth, 1982), or that they maintained structures at each end of the portage from ca. 1732 through 1760 (Woolworth and Woolworth, 1982). An unsubstantiated “French post” at the Pigeon River landing even has an assigned cultural resource inventory number (CR-105) (Woolworth and Woolworth, 1982). Other researchers are skeptical of such varied interpretations (e.g., Burpee, 1931; Thompson, 1969; Gilman, 1992). The apparent lack of Frenchcolonial materials on the lakefront at Grand Portage Bay west of Grand Portage Creek hints that the greatest potential for such materials there may lie in areas east of the creek (Brown, 1937). To date, the only documented French colonial archeological loci are those identified during field investigations at the Pigeon River landing (Birk and Wheeler, 1976) and at the suspected Parting Trees pose on the Grand Portage trail (Douglas A. Birk, personal observation). Barring future archival discoveries, most questions regarding French presence at Grand Portage may only be answered through further archeological inquiry. Figure 2.2. Artist’s conception of North West Company Depot at Grand Portage The earliest known trading houses at Grand Portage Bay were those established by British traders in about 1768. The houses were built west of Grand Portage Creek at the place later developed for the NWC depot (Nute, 1940; 1944). Other trade facilities were later constructed along the lakefront and, by 1793, the NWC alone was said to have sixteen buildings within its fort. The NWC also claimed dominion over adjoining parcels at the bay and may have opened facilities at the Pigeon River at the time of the its formation (1783). Rival traders soon followed the NWC’s example by building a “hangard or store” on the Pigeon River in 1785 (Gates, 1965; Thompson, 1969). Being more abundant and pervasive, material ILSG07 28 Trip 2 �evidence of British presence at Grand Portage has proved easier to find, identify, and interpret than that relating to earlier French colonial operations. Some features noted or implied in early historical records have yet to be identified through archeology. For example, the NWC had horses, cattle, hogs, and sheep at Grand Portage (Thompson, 1969; Gilman, 1992), which likely necessitated the use of barns, stables, pens, or corrals. The central clearing at Grand Portage Bay could have been used for pasturage, and grazing livestock there may actually have given the NWC a reason and excuse to fence the perimeters of that opening. To overwinter livestock required labor-intensive preparations. Hay was said to be abundant at Grand Portage in the late 1700s, but it had to be cut and stacked by hand. At the same time, cold damp fog, along with high levels of ground moisture and lingering winters, often caused harvested hay to rot (e.g., Thompson, 1969). Gardening was similarly impaired by weather and soil conditions. Potatoes were the only food crop typically grown with any success (e.g., Thompson, 1969; Woolworth and Woolworth, 1982). The size, location, and arrangement of the NWC’s barns, corrals, pastures, and gardens are presently unknown. Grand Portage National Monument Grand Portage was designated a National Historic Site in 1951. In 1958 Grand Portage National Monument was established to commemorate and preserve a premier site and route of the 18th century fur trade that led to pioneering international commerce and exploration in North America as well as cultural contact between Ojibwe and other Native societies and the North West Company partners, clerks, and canoe-men. The Monument was also established to work with the Grand Portage Band in preserving and interpreting the heritage and lifeways of the Ojibwe people. Grand Portage National Monument is a homeplace of tribal and family history and cultural persistence. Grand Portage National Monument contains reconstructed buildings and well-preserved archeological remains of several fur trading posts instrumental in the exploration of the West and in the economic history of the United States and Canada. The national monument contains the entire length of the portage that marked the entrance into the interior of western Canada. The national monument is significant because of the fundamental interrelationship of Ojibwe heritage and fur trade history. Grand Portage National Monument protects, commemorates, and interprets a reconstructed fur depot of the North West Company, a rendezvous site for international commerce and canoe route for transcontinental exploration, Native heritage, natural scene, and history of cross cultural contact and accommodation between traders, Ojibwe, and other participants in the fur trade. Today nearly 90,000 visitors pass through the reconstructed stockade and related buildings each year. Three reconstructed historical buildings currently serve these visitors. Based on archeological excavations and research from 1938 through the present, the reconstructed buildings have been furnished in the period of the late 1700’s. Technology from the time can be viewed in the Ojibwe Village and Voyageur Encampment staffed by National Park Rangers. The Great Hall and kitchen buildings contain reproduction furnishings similar to those used by late 18th century North West Company business partners, clerks, voyageur guides and Ojibwe families. In a room of the Great Hall from June until September, Ojibwe artisans create and sell beaded designs and birchbark basketry popular to Indian people of the Lake Superior region. From late May until early September interpretive programs are presented around the historical site. Subjects include historic cooking and baking, Ojibwe craft demonstrations, historic black powder musket firings, lever fur press operation, historic gardens and conducted walking tours. An annual highlight is Rendezvous Days, held the second week of August, during which the annual meeting of company representatives from Montreal, fur traders and local native people is recreated. Geologic history The geology of Grand Portage is discussed in regard to Precambrian bedrock features, a relatively uncomplicated story based on both Paleoproterozoic and Mesoproterozoic rock units, and Pleistocene and ILSG07 29 Trip 2 �Holocene history representing a dynamic story of geologically rapid landscape change caused by postglacial changes in the levels of ancestral Lake Superior. These latter changes overlapped with early human occupation of the area and undoubtedly had a major influence on human activity. The most recent geologic investigations of the monument were done in 2001 and funded by the National Park Service. Results are in administrative reports to NPS: (1) the bedrock geology was studied by W.F. Cannon and Laurel G. Woodruff of the USGS; (2) glacial and post-glacial geology was surveyed by Brian A.M. Phillips of Lakehead University. Much of the following descriptions are taken from those reports. Bedrock geology The bedrock geology of extreme northeastern Minnesota, including the Grand Portage National Monument, consists of Precambrian rocks of two separate ages. The bedrock geology of the Grand Portage region is shown in Figure 2.3. Most of the area is underlain by sedimentary rocks of the Rove Formation, formed during Paleoproterozoic time, approximately 1,850 million years ago. These rocks have been slightly tilted toward the southeast but otherwise are remarkably little altered for rocks of such antiquity. A second period of rock formation occurred during Mesoproterozoic time, approximately 1,100 million years ago. At that time diabase dikes were emplaced into the Rove Formation. These dikes are of two distinct generations separated by a short time interval and exhibit contrasting magnetic polarity. The oldest are known as the Grand Portage dike swarm and are generally east-trending features no more than a few tens of feet thick. Slightly younger dikes of the Pigeon River swarm are much more voluminous and support the higher ridges of the region such as Mt. Rose and Mt. Josephine. The dikes form a roughly orthogonal array of NE-trending and NW-trending bodies. Figure 2.3. Bedrock geologic map of the Grand Portage area in northeastern Minnesota. Geology extracted from digital files by Miller and others, 2001 The essentials of the geologic relationships were established by the earliest geological reconnaissance of the region conducted by J.G Norwood in 1849 (Owen, 1852). An etching from Owen’s report (see title ILSG07 30 Trip 2 �page) illustrates in a schematic manner the geology of the Grand Portage area. It shows large diabase bodies that form both sills, as on Pigeon Point, and dikes, as on Hat Point. “Slates” of the Rove Formation in Grand Portage Bay are cut by thin diabase dikes. There are three principal references on the geology of the region. First, Grout and Schwartz (1933) studied the Rove Formation and diabase intrusions and presented the first detailed maps of the Grand Portage area. In addition to a compilation of the regional geology, they presented maps of individual townships showing the geology and the location of bedrock exposures observed during their studies. Later, Grout, Sharp, and Schwartz (1959) published a report on the geology of Cook County, which provided some updated information on the Grand Portage area. Finally, in 1969, Morey published a detailed examination of the Rove Formation in northeastern Minnesota and adjacent parts of Ontario. This publication provides the best description and interpretation of the geologic history of the Rove and includes some specific observations within the Monument, particularly at Mt. Rose. Rove Formation The report by Morey (1969) provides an excellent description of the Rove Formation. The abstract of that report is presented below as a summary of the regional character of the Rove and some of our specific observations within the Monument follow. According to Morey (1969, p. 1-2): “The Middle Precambrian Rove Formation, the upper part of the Animikie Group, is estimated to be at least 3,200 feet thick and is exposed between northwestern Cook County, Minnesota and the Thunder Bay district, Ontario. It is a sequence of graywacke, argillite, locally abundant intraformational conglomerate, quartzite, and carbonate rocks. The Formation was deposited sometime between 2.0 and 1.7 b.y. ago in a northeast-trending basin, the configuration of which was controlled by a pre-existing structural grain. Detailed mapping in the 7 ½ minute South Lake quadrangle combined with a field and laboratory study of approximately 150 other scattered stratigraphic sections provide a basis for the recognition of three informal lithic units. From oldest to youngest these are: (1) lower argillite, 400 feet thick; (2) transitional beds of interbedded argillite and greywacke, 70 to 100 feet thick: and (3) thin-bedded greywacke, as much as 2,700 feet thick. It is concluded that the argillite and associated greywacke-sandstone and graywacke-siltstone units were deposited in moderately deep, quiet water. Repeated greywacke sedimentation units indicate sediment transport and deposition by turbidity currents. A sedimentation unit reconstructed from composite sections consists of (1) a basal conglomeratic graywacke, (2) a structureless unit which grades indistinctly into (3) a graded graywacke that is overlain by (4) a laminated graywacke, which may be modified by (5) small-scale cross-bedding, or (6) contorted bedding. Any one or several of these may be absent, but the units are always overlain by (7) an argillite. Post-depositional soft-sediment structures such as load casts, flame structures, clastic dikes, bed pullaparts, overfolds, and micro-faults indicate rapid deposition of Rove sediments, active bottom currents, and post-depositional deformation, implying a significant paleoslope. A detailed analysis of paleocurrent directional indicators such as groove casts, flute casts, dendritic ridges, and cross-bedding shows that the turbidity currents had a southerly trend about perpendicular to the axis of the Rove basin. However, ripple marks, winnowed lag deposits at the tops of many greywacke beds, and possibly some festoon-type cross- bedding show that the turbidites were later modified by bottom currents that trended southwesterly or parallel to the axis of the basin. ILSG07 31 Trip 2 �The heavy minerals of the Rove are characterized by epidote-group minerals, apatite, sphene, and tourmaline, and are typical of older Precambrian igneous rocks now exposed north of the present Rove outcrop area. Thin-section and x-ray analyses of 200 samples show that the graywackes consist of angular, poorly sorted grains of clastic quartz and plagioclase (An10-An20) embedded in an argillaceous matrix that now consists of quartz, chlorite, and muscovite. The fine-grained fissile argillite and mudstone have the same mineralogy and microtextures as the greywacke. Erosion subsequent to pre-Keweenawan tilting removed an unknown amount of the formation prior to deposition of Lower Keweenawan sedimentary rocks. The intrusion of Middle Keweenawan mafic igneous rocks caused local metamorphism of the Rove Formation to a variety of mineral assemblages now assigned to the pyroxene- and hornblende-hornfels facies, but the remainder of the formation is essentially unmetamorphosed.” An additional study of the Rove has been reported recently (Maric and Fralick, 2005) and presents a somewhat different picture from Morey’s 1969 description. Based on examination of a series of drill holes near Thunder Bay, they defined an internal stratigraphy consisting of: 1) a 150 m-thick basal unit of carbonaceous shale with lesser siltstone and fine-grained sandstone beds, which grades upward into, 2) a unit about 350 m thick containing more than 100 individually coarsening upward beds of sandstone with interlayered shale. This sequence is overlain by the uppermost preserved unit of 3) black shale with thin rippled sandstone beds. They interpreted the lower carbon-rich sediments to have been deposited in a sediment-starved anoxic basin in which very slow sedimentation rates resulted in accumulation of only 100 m of black shale during tens of millions of years of sedimentation. The upper coarser-grained units were deposited by a southward prograding turbiditic to shelf system. Most, if not all, of the Rove Formation exposed in the Grand Portage Monument is part of the upper thinbedded greywacke unit described by Morey (1969). Most exposures consist of flaggy, fine- to mediumgrained greywacke, with minor interbeds of argillite. The unit tends to split into slabs 1-2 inches thick parallel to indistinct bedding. The extensive outcrops and roadcuts on the north side of Highway 61 where the portage trail crosses the highway (Fig. 2.1, Stop 2.3) are typical of many of the other exposures along the trail to the north. The exposures on Mt. Rose (Stop 2) differ in that bedding of the greywacke is somewhat thinner and more distinct. Features such as cross-bedding and contorted bedding are present, although rare. An additional significant difference is the abundance of units of fine-grained fissile argillite as much as several tens of feet thick. The Rove Formation immediately adjacent to the Mt. Rose diabase dike is strongly metamorphosed. Original sedimentary textures are largely obliterated and metamorphic minerals, including biotite and clots of a mineral preliminarily identified as cordierite, have formed in the original greywacke matrix. Over the past few years, the age of the Rove Formation has been clarified by radiometric dating and the results provide a more precise age estimate than the 300 m.y. span indicated by Morey’s study in the 1960’s. Thin volcanic layers near the base of the Rove Formation near Thunder Bay, Ontario have been dated by the U-Pb zircon technique and indicate that deposition began somewhat before 1835 Ma. The dated volcanic layer is about 6m above the base of the formation and has an age of 1836+/- 8 Ma (Addison and others, 2005). The base of the Rove in that same area lies on a layer of ejecta believed to have originated from the major 1850 Ma meteorite impact at Sudbury, Ontario (Addison and others, 2005), further helping to bracket the age of the basal beds of the formation between 1850 and 1836 Ma. An additional age constraint comes from radiometric dates on detrital zircons from a bed about 400 m above the base and near the top of the preserved part of the Rove about 15 km north of Grand Portage. Concordant ages of individual grains are as young as 1777 Ma, providing a maximum age for deposition of the middle part of the Rove Formation (figure 2.4) (Heamon and Easton, 2006). A minimum age is not well constrained at this time. ILSG07 32 Trip 2 �Figure 2.4. Age spectrum for detrital zircons in the Rove Formation presented by Heamon and Easton (2006, figure 33). Spectrum indicates that a majority of detrital zircons were derived from a source younger than rocks of the Penokean orogeny to the south, and have come from a terrane with an abundance of zircons with ages of 1850 to 1750 Ma. These new age data require a rethinking of traditional interpretations of the Rove, both in terms of stratigraphic correlations and tectonic setting of deposition. The Rove Formation has been long considered to be the stratigraphic equivalent of major turbidite sequences to the south, such as the Thomson Formation in east-central Minnesota and the Michigamme Formation in northern Michigan. Recent discovery of the Sudbury ejecta layer in Michigan near the base of the Michigamme Formation confirms that the onset of sedimentation was nearly synchronous across the region at 1850 Ma. However, the duration of sedimentation between Michigan and northern Minnestoa/Ontario appears to be drastically different. The 1777 Ma concordant detrital zircons from the Rove are from a bed about 400 m above the base of the formation. Although this is near the top of the preserved formation in the sample area, a total thickness nearly 1000 m was estimated for the Rove (Morey, 1969). Therefore, sedimentation may have lasted well beyond the 1777 Ma maximum age. In Michigan, it is well established that the Michigamme Formation was deposited, deeply buried by tectonic overthrusting, metamorphosed, and intruded by posttectonic granite by 1835 Ma (Schneider and others, 2002; Schneider and others, 2004), which is the approximate age of the volcanic layers near the base of the Rove. Thus, much of the thick clastic sequence of the Rove appears to be entirely younger than the Michigamme Formation. The Rove has commonly been considered to have been deposited in the foreland sedimentary basin north of the 18901835 Ma Penokean orogen, but it now appears that sedimentation was almost entirely after the Penokean deformation ceased in Michigan and Wisconsin. The detrital zircon suite in the Rove, as shown in figure 4, consists dominantly of grains in the 1850-1750 Ma age range rather than Archean zircons, indicating that a source area with rocks of that age was well established and contributing the majority of detritus to the basin during at least the younger part of Rove deposition. Detrital zircon analyses of the Thomson and Rove Formations in Minnesota yielded somewhat similar results (Kerber, 2006). Although no zircons younger than about 1800 Ma were found in either formation based on analyses of a single sample from ILSG07 33 Trip 2 �each, the great preponderance of zircons are in the range 2000-1800 Ma and Archean zircons are scarce). Thus, based on presently available data, it appears that the preponderance of detritus contained in the vast turbidite deposits of the Rove, Virginia, Thomson, Tyler, and Michigamme Formations across the western Lake Superior region was derived from a source terrane(s) with ages in the approximate range 2000-1750 Ma. A widely accepted model for this turbidite deposition has been a tectonic foreland in which subsidence was being driven by northward overthrusting of arc rocks of the Wisconsin magmatic terranes, now exposed in northern Wisconsin, and that these rocks provided much of the detritus of which the turbidites are composed. This model is still acceptable for the southern parts of the basin, but complications arise in applying it to our current understanding of the Rove Formation. Relationships in Michigan and Wisconsin indicate that tectonic loading by overthrusting onto the craton margin was completed by 1835 Ma (Schneider and others, 2002). However, deposition of the basal units of the Rove was just beginning at that time. Deposition of the Rove continued for at least 60 million years after that time, long after the end of the Penokean orogeny, and basin subsidence of about 1 km occurred during that period. What was the driving mechanism for that subsidence? An additional enigma is provided by a wealth of paleocurrent data from the Rove which indicates a consistent southerly sediment transport (Morey, 1969). This direction is inconsistent with a sediment source from the Wisconsin magmatic terranes. The preponderance of Paleoproterozoic detrital zircons suggests that much of the Rove detritus was derived from a Paleoproterozoic orogen north of the Lake Superior region, perhaps the Trans-Hudson orogen in part, which contains rock units as young as 1775 Ma. Thus the Rove Formation no longer fits well into the model of Penokean foreland sedimentation because the time of sedimentation and sediment source directions are inconsistent with that model. The nature and cause of the Rove basin is a newly emerging topic for further study. Because so much of the Rove sedimentation is significantly younger than the Penokean orogeny and paleocurrent data indicate that the Penokean orogen is not the source for the Paleoproterozoic-aged detritus that seems to make up most of the Rove, it is necessary to divorce the Rove, and perhaps at least part of the correlative Virginia Formation, from the suite of features ascribed to the Penokean orogeny, and seek a cause in younger orogenic events. It appears, based on the most recent data, that the basal parts of the Rove Formation composed of black shale deposited in a sediment-starved basin may be a product of Penokean foreland basin development. However, the remaining coarser clastic deposits of the upper part of the Rove reflect a reactivation of the basin during a time spanning the 1775 Ma maximum age of the middle part of the formation. A recent reinterpretation of the tectonic assembly of the north-central part of the US (NICE Working Group, in press) indicates that the mid-geon 17 Yavapai orogeny extends eastward from its originally defined extent in the southwestern US into the southern part of the Lake Superior region. A belt of gneiss domes, granite plutons, and metamorphic nodes also developed across central Minnesota and northern Michigan at that time (Schneider and others, 2004; Holm and others, 2005). Thus, it has been established recently that major mid-geon 17 tectonic activity occurred only about 200 km south of the Rove basin. Is this tectonic episode responsible for reactivation of the older Penokean foreland basin and deposition of the Rove Formation? Although these Yavapai-age events might in some poorly understood manner have caused basin subsidence and also provide a source of detritus of the same age as that established for the Rove, the enigma of southerly-directed sediment transport in the Rove of material of that age remains unresolved. Grand Portage diabase dikes A set of thin diabase dikes was described by Green and others (1987) that they named the Grand Portage dike swarm. These dikes are present throughout the Grand Portage region and intrude the Rove Formation as well as the Middle Proterozoic volcanic rocks exposed south of Grand Portage. More than 50 of these dikes have been identified, some of which are shown on maps in Green and others (1987) and ILSG07 34 Trip 2 �in the digital compilation by Miller and others (2001). The dikes have a relatively uniform trend from N65o – 90o E. The Grand Portage dikes tend to be quite strongly magnetized, but acquired their magnetic properties as they cooled during a period when the earth’s magnetic field was inverted. This reversed magnetic polarity interferes with the present earth’s magnetic field resulting in distinctive linear negative anomalies on magnetic maps. This characteristic makes them quite easy to identify, even in areas where they are not exposed. A combination of surface exposures and magnetic expression has allowed us to identify 11 new dikes of the Grand Portage swarm that cross the monument. The most prominent of these are a set of five dikes that are exposed along the east side of Mt Rose (one is just south of the Monument boundary, the other four are on U.S. Park Service property). Each dike stands as a low to prominent ridge as much as 10 m wide as a result of differential erosion between the relatively hard diabase and much softer Rove Formation. One of these dikes shown in Figure 2.5a is probably the dike that inspired the somewhat fanciful etching that appeared in Owen’s 1852 report (Figure 2.5b). Figure 2.5. a- Diabase dike of Grand Portage swarm at south edge of Monument property. b- Etching from Owen (1852) based on field sketch, probably of the dike in a. ILSG07 35 Trip 2 �The magnetic profile shown in Figure 2.6 illustrates the sharp negative magnetic anomalies caused by these dikes. Three other similar anomalies are shown on a magnetic profile along the Grand Portage trail. Although dikes are not exposed at the surface in these areas they are inferred with confidence to occur beneath the surficial cover material. Two other dikes were identified in outcrop just north of Highway 61 but their magnetic expression is less distinct. Figure 2.6. Magnetic profile measured at 100 foot spacing along roadway northward from south boundary of Monument. The four sharp negative anomalies are caused by the four Grand Portage dikes exposed along the roadway. Pigeon River diabase dikes The Pigeon River diabase dike swarm (Green and others, 1987) is compositionally similar to the Grand Portage dikes but is slightly younger and volumetrically much more prominent. The Pigeon River dikes trend mostly about N65oE and less commonly about N40oW. Because of their hard, massive nature they form the prominent, steep-sided ridges of the region, including Mt. Rose. Because they have normal magnetic polarity, these dikes tend to produce positive anomalies on magnetic maps, but they are apparently somewhat variably magnetized so cannot be recognized in magnetic data as readily as the Grand Portage dikes. Absolute ages of Grand Portage and Pigeon River dikes The Grand Portage and Pigeon River diabase dikes have not been dated directly, but their age can be closely approximated because of their relationships with other nearby units that have been dated. A regional correlation of rock units related to the Midcontinent Rift and a summary of radiometric ages was published by Nicholson and others (1997). The Grand Portage dikes are known to cut the Hovland and Grand Portage basalts south of the Monument. Those basalts were erupted about 1,110 to 1,105 million years ago. The shift of the earth’s magnetic field from reversed to normal polarity occurred about 1,100 years ago. The intrusion of the Grand Portage dikes, therefore, must have occurred after eruption of the Grand Portage basalts, but before the magnetic field reversal in the interval 1,105 to 1,100 million years ILSG07 36 Trip 2 �ago. The Pigeon River dikes, which have normal magnetic polarity, are younger than 1,100 million years. They may be related to the Lutsen basalts that were erupted about 1095 to 1090 million years ago. Glacial and post-glacial geology The effect of ice Because ice advances reached as far south as Illinois, Iowa, Nebraska, and Kansas numerous times during the Pleistocene it is safe to assume that the Grand Portage area has been affected by the passage of several ice lobes over the last 500,000 years. The general direction of ice movement was from the north and northeast, and present-day landscapes in the borderlands first were sculpted of sub-glacial processes of plucking, abrasion and the high pressure flow of sub-glacial meltwater. Next, the processes of deglaciation resulted in deposition of ablation tills and a variety of fluvioglacial landforms (eskers, kames, ice-contact deltas) and sediments (sorted and unsorted silts, sands and gravels). Meltwater, temporarily ponded between the waning ice margin and enclosing topography, formed short-lived pro-glacial lakes that changed in area and depth as the configuration of the ice margin changed. Many of these lakes existed long enough to deposit substantial layers of lacustrine clays, and many ended their existence by draining catastrophically down spillways exhumed by the melting ice. Inundation of the land recently revealed by ice melt was common as various lake bodies occupied the Lake Superior basin at different times and water levels, depending upon the rate of water supply and the loss through such outlets as were available. In the Grand Portage area, the present topography is the product of the effects of glacial ice. Rock types more resistant to glacial processes remain upstanding and less resistant rock materials have been more deeply eroded and removed. However, ice must be considered as a very viscous liquid, not only exerting its influence on the underlying topography, but also being influenced by that topography. Though ice crossed the area from generally north to south, the already upstanding diabase ridges that characterize the area will have deflected more recent ice events. As shown in R.P. Sharp’s map (Fig. 2.7), glacial striae, the grooves ground into solid rock by the scraping action of sub-glacial debris, are found sub-parallel to the ridges rather than orthogonal to them (as in R5E) (Sharp, 1953). The most effective ice erosion is performed early in ice advance. Later, under increasing thickness of ice, erosion of the underlying ground surface all but ceases. Shearing takes place between upper layers of ice sliding over lower layers of ice that are trapped within the topographic depressions. This trapped ice becomes largely stagnant and therefore ineffective at erosion. The topography of the Grand Portage area suggests that this scenario is quite likely to have taken place. ILSG07 37 Trip 2 �Figure 2.7. Part of R.P. Sharp’s map of Cook County, MN (Sharp, 1953) showing glacial striae directions (arrows). Sub-glacial water flow It is likely that the topographic differentiation between the hard intrusive rocks and the less resistant sedimentary rocks began in pre-glacial time by the time-honored agents of runoff, wind and chemical weathering. At this time too, the conspicuous breaks in the ridge features are likely to have first been formed. A number of major faults, along which detectable vertical movement has taken place, cut across the area (Fig. 2.3), but many unmapped smaller faults and shatter zones also are present. These linear weaknesses in the hard rock materials are the first to be exploited by the agents of denudation, and once initiated, processes such as running water become focused in these locations, exacerbating the erosional effects. The role of sub-glacial water flow was particularly important. Unrecognized by earlier workers studying glacial geomorphology, water flowing beneath and within ice is often under considerable hydraulic pressure. Unaffected by gravity, such flow can run counter to the slope or topographic grain of the underlying surface. The force of water, carrying subglacial debris and running at the interface of the ice and ground surface, is a highly effective erosive tool. Scouring of rock surfaces and the erosion of steep sided channels into the rock surface (called tunnel valleys) can take place. In post-glacial landscape glacial meltwater flow in these valleys might be seen to run uphill, cross divides or run contrary to the drainage pattern. Some are used by present drainage, but many remain dry or hold lakes and bogs. Associated with these sub-glacial features are eskers, which represent a final stage of deposition within the tunnels. These eskers might run along the valleys in places or climb over the margins onto ground alongside the main channel. ILSG07 38 Trip 2 �Isostatic uplift Although the mechanism for post-glacial isostatic uplift or rebound remains inadequately explained by geophysics, the effect of this mechanism can readily be observed and measured. The process began as ice margins began to wane and withdraw, and continues to the present day. Greater uplift occurs where greater depression was made by a greater weight of ice. As a result, models for post-glacial rebound see greater and longer continuing uplift towards the center of the former ice mass. Figure 2.8. Shoreline diagram for the Grand Portage area showing present elevations of former lake shores. Along the Minnesota north shore the axis of uplift is sub-parallel to the shore, and currently, uplift at the Pigeon River is about 250 feet greater than at Duluth. Consequently, an initially horizontal line, such as a lake shoreline, is now tilted such that its elevation rises over this range between Duluth and the Pigeon River. The sequence of lakes that occupied the Superior Basin as ice withdrew and isostatic uplift occurred resulted in a staircase of shorelines found along the north shore, diverging to the northeast (Fig. 2.8). Thus in Cook County, the vertical distance between these former shoreline features is greatest at Grand Portage. A more detailed account of pre-historic water levels can be found in Phillips (2001, pp.828). Evidence of former lake margins Typically, the presence of a lake margin, sustained for a period of time sufficient for waves to do work, is represented either by an erosional form such as a bluff, or by a depositional form such as a beach. In addition, rivers entering the lake body tend to deposit fluvial sediments in the form of deltas, or at least a series of river mouth bars that characteristically curve out of the river mouth and along the shore. Both features mark the position at which rivers formerly met with the shore of the lake. ILSG07 39 Trip 2 �Streams along the Minnesota north shore have had to respond to steady isostatic uplift and falling water levels. Typically, this has resulted in the lengthening of stream courses and the deep incision of their beds as they struggle to meet the lakeshore at grade. Not surprisingly, the north shore abounds in gorges, waterfalls and rapids as outcrops of more resistant rock inhibit equal downcutting along the stream course. Another characteristic feature that results from isostatic rebound is the building of deltas at successively lower elevations as the stream course extends, each delta in part built from the material eroded from an earlier one as a result of incision into the former delta surface. The Grand Portage Trail crosses a number of these features, each denoting a period of lake level stability lengthy enough for a defined landform to be formed (Figs. 2.9, 2.10). The features have significance for finding Paleo-Indian artifacts, since it is established that early people tended to favor shoreline and river mouth locations for their temporary habitation and activities. The succession of shorelines on the Grand Portage Trail As Marquette ice, the youngest glacial lobe in the western Superior basin, melted back and exhumed the basin successively from southwest to northeast, the highest post-Marquette shorelines are those of Lake Duluth, the earliest phase of which was restricted to the Duluth area. When ice still covered the Thunder Bay region, Lake Duluth first extended across the international border into the southern part of the Nor’wester mountains. At Grand Portage, the high Duluth level was at 1340 feet and although Mt. Maud was now part of the mainland, no part of the Grand Portage Trail was above water. Figure 2.9. Idealized former water levels at Grand Portage. The gradual withdrawal of ice uncovers recently glaciated ground. If, as in this case, the ground is lower than the ice marginal (pro-glacial) lake, it is immediately inundated and protected from exposure to weathering and erosion, even obtaining a modest cover of pro-glacial lake (glaciolacustrine) clays or silts if under water long enough for offshore sediment to accumulate. ILSG07 40 Trip 2 �Portions of the Grand Portage Trail first appeared above water during the later Low Lake Duluth phase (1290 feet). A good portion of the Trail north of Old Hwy 61 lay above water, including the uppermost part of the narrow ridge along which the Trail runs for a mile or more. This part of the Trail, having been inundated only briefly, is not overlain by lacustrine sediments. The thin till is wave washed, and bare rock is exposed in many places. A pattern can be seen emerging here. High areas, subject to inundation for a lesser period, are less likely to be covered by lacustrine sediments. Lower areas, subject to a long period under water may accumulate lacustrine sediments, though only if a source of such sediment is available. To a large extent, what happens is related to the degree of shelter or exposure to wave action. This will vary locally as the water level lowers. Sheltered areas may show little modification of the existing sediments, but exposed areas are likely to show winnowing of the glacial sediments, removal of finer particles and a lag of larger clasts left on the surface. Sufficient wave action may have time to sort and transport sediment, forming recognizable beach or bar features where longshore transport is consistent. Extreme wave action may have the energy to erode and form soft-sediment bluffs, or even undercut and fashion solid rock into wave-abraded forms. In summary, as the already complicated topography of the Grand Portage Trail area slowly emerged, an ever-changing pattern of wave refraction around the shoals, islands and peninsulas led to a localized sculpting of the topography. At the Lake Highbridge level (1200 feet), the present Grand Portage Trail was divided into two clearly different portions. The Trail above Old Hwy 61 was above water, while that below the highway was still inundated. This dual character is still very visible today. The character also occurs into the High Lake Washburn phase (1110 feet), with the steep section of the Trail immediately above Old Hwy 61 forming a fairly stable promontory, down which the water level fell in stages (Figure 2.10). Meanwhile, Mt. Josephine emerged, as did higher parts of the narrow diabase ridges, the links gradually joining as water level fell. During the High Lake Washburn phase (Fig. 2.10A), two ‘gaps’ were noticeably still inundated. The eastern one marks the line of the Grand Portage fault, and the Grand Portage Trail passes through the western one. At this time, the Poplar Creek basin had become significantly protected from wave action, though, as in present Lake Superior, water flow through the two gaps was likely intensified by seiche and meteorological events, leading to possible scouring and some transfer of sediment to one or both sides of the ‘channel’. The lakeward side of the emerging ridges probably experienced the full force of Lake Superior wave action, so the transfer was likely to have been mostly towards the Poplar Creek basin. Within the relatively calm waters of the basin, lacustrine sediment would have accumulated liberally. It is during the High Lake Manitou level (1020 feet) (Fig. 2.10B) that the ‘gaps’ became dry. On both sides of the gaps, some crossing feature such as a beach or a bar was likely, though if earlier scoured to bedrock, little sediment might have been available to form these. If soft sediments formed a shoreline feature in such an emerged gap, it would be named a ‘tombolo’, in that it joined mainland to a former island. This is a feature only occasionally seen on the shore of present Lake Superior. During the High Lake Manitou level, the basin of the Grand Portage Creek also became a more sheltered reentrant, and the upper part of the creek course probably flowed. At the subsequent High Lake Beaver Bay level (920 feet), a distinct Grand Portage Bay emerged and Grand Portage Creek was a sheltered river mouth extending far up valley. This character was less pronounced by the time of Mid Lake Beaver Bay (845 feet) (Fig. 2.10C), when the summit of Mt. Rose appeared as a small island, but the potential for river mouth habitation by Paleo-Indian people is good during the five Beaver Bay lake stages. As Mt. Rose became a larger island during the Lower Lake Beaver Bay stage (775 feet), it probably protected the river mouth and the potential for habitation was sustained. At this time, wave refraction around Mt. Rose would have been quite complex and the area immediately behind the steep island would have been very protected. Lake Minong, dated as about 9,500 BP, is notable as the first of the succession of lakes to occupy the whole of the ice-free Superior basin, and its shoreline can be traced around the present lake. Lake ILSG07 41 Trip 2 �Minong time is also notable in the Minnesota-Ontario borderlands as a period in which evidence of PaleoIndian activity became more pronounced than before. In the Grand Portage area, the Minong shoreline occurs at 715 feet. A defined bluff forms a deep ‘V’ shaped reentrant into the valley of the Grand Portage Creek, which the Grand Portage Trail now surmounts. At this time, Mt. Rose ceased to be an island and further contributed to the sheltered nature of Grand Portage Bay (Fig. 2.10D). The potential for PaleoIndian occupation around this sheltered river mouth is high. Figure 2.10. Reconstruction of former shorelines at five levels of post-glacial lakes in the Grand Portage region. ILSG07 42 Trip 2 �The Post-Minong period was a time when pro-glacial Lake Agassiz flowed through a series of eastern outlets into the Nipigon basin, and thence into Lake Superior and the lower Great Lakes. At least five episodes of catastrophic discharge have been recognized (Teller and Thorliefson, 1983). Each had the potential to briefly raise the level of the Post-Minong lakes, and erode the glacial sediments that swathed the St. Mary’s river outlet to Lake Huron. Ultimately, the outlet was eroded to bedrock and the Lake Houghton phase was established as the lowest of the succession of post-glacial lakes in the Superior basin. Water level declined to a low position of 582 feet in Grand Portage Bay, placing the shoreline adjacent to Grand Portage Island, which separated two shallow bays. Little if anything remains of this portion of history, because there followed a steady rise of water level in the Superior basin as the St. Mary’s outlet was isostatically raised. This transgression drowned the recently exposed Post-Minong surface and culminated in the Nipissing shoreline, which in places truncates and reoccupies older shoreline features. In the Grand Portage area the Nipissing rose to 636 feet, preserving the Minong and the upper PostMinong shorelines (Fig. 2.10E). Near Grand Marais, the Nipissing inherited the older Minong bluff, refreshing it, and further south the Minong was destroyed and drowned by the Nipissing transgression. Subsequently, water level fell to the Lake Algoma level (621 feet) and the Sault level (610 feet) before declining to present lake level (602 feet). Features formed by these more recent water levels run through the area of the Stockade, though more recent shore erosion has removed the lowest. In summary, the present Grand Portage Trail traverses a landscape that has been fashioned first by ancient tectonic activity that emplaced the pattern of dikes, which are responsible for the basic topographic character of the area. Secondly, multiple glaciations have ground across the ridges and basins, steepening and smoothing rock surfaces, and depositing varying depths of sandy, coarse tills, particularly in lower areas. Thirdly, a succession of post-glacial lakes has fully submerged the course of the Trail. As the water level gradually declined, the Trail topography progressively emerged. These lakes have left some evidence of their brief periods of stillstand in the form of wave-washed sediments, erosional bluffs and constructional beaches and bars. They have also deposited a variable thickness of lacustrine clays and silts, particularly within the basins and valleys of the bedrock topography. The geological underpinnings of human history of the Grand Portage region Early human activity in any region is strongly influenced by physical barriers to transit and by the easiest solutions to the impediments that those barriers represent. Geological events that occurred from as little as ten thousand years ago to as much as 1.1 billion years ago largely predetermined the course of European history in the Grand Portage region because they controlled the physiography and the unique physiographic characteristics of Grand Portage that made it the obvious location for the “highway to the west”. These events not only dictated that the portage was necessary, but determined the best route for the portage, and, on a broader scale, were responsible for the enormous advantage to westward travel inherent in making the portage. European exploration and settlement of the continental interior was strongly guided by the relatively easy water access provided by the Great Lakes as far west as western Lake Superior. Access farther west into the deep interior of the northern Great Plains, Canadian prairies, and beyond was facilitated by additional relatively easy water routes up the Pigeon River, over the continental divide, and through the Rainy River system westward to major north-flowing drainages and their tributaries. The shortest distance from Lake Superior to waters navigable westward by canoe is over the Grand Portage. This short route to the west was known and used by Native Americans well before the first Europeans arrived in the early 1600’s. ILSG07 43 Trip 2 �But the stretch of high ground from the lakeshore to the upper reaches of the Pigeon River was the most prominent physical barrier along the entire westward route from Montreal to the Rocky Mountains. In fact, the length and difficulty of the portage lead to a mutiny among the first westward-bound French exploration party that arrived at the site in 1731. The portage was necessary because of a long series of rapids and falls along the lower reaches of the Pigeon River which falls about 660 feet from the upper end of the portage down to the level of Lake Superior. But a remarkable feature of this area is that once past the 8.5 mile portage, the way west was through the relatively flat water of the upper Pigeon River and a series of interconnected lakes. Within only about 40 miles from the west end of the portage one leaves the Lake Superior drainage, crosses the continental divide, and enters the Rainy Lake watershed which drains ultimately to Hudson Bay. Thus, three regional features: 1) Lake Superior and the easy route that it provides for water-borne commerce from the east, particularly Montreal, 2) the steep gradient of the Pigeon River in its lower reaches that made canoe transit impossible, and 3) the very small size of the Lake Superior watershed which, once exited, provided essentially unlimited canoe access to the interior of North America, all played fundamental roles in making the Grand Portage the most favored and easiest route for westward travel. The present basin of Lake Superior is the exhumed sediment-filled central valley of an ancient rift that formed 1.1 billion years ago. At that time a great rift valley, known as the Midcontinent rift, began to split the North American continent. The rift was filled first with tens of kilometers of volcanic rocks, mostly basalt such as the Grand Portage basalts exposed just south of the Monument, and finally by many kilometers of sediments, now mostly red sandstones. Figure 11 shows the trace of the axis of the Midcontinent rift which faithfully follows the southward concave shape of the present lake. The sandstones in the central part of the rift presented an easily erodible substrate for glacial scouring in contrast to the much more durable igneous rocks along the rift flanks, which today stand as the prominent ridges and uplands of the region. As a result, repeated glaciations during the Pleistocene gouged the deep valley now occupied by Lake Superior. Thus, Lake Superior and its favorable transportation route owe their existence to these two ancient events of rifting and glaciation. But both of these events combined to create the unfavorable situation of the extensive series of rapids and waterfalls along the lower reaches of the Pigeon River, which required the arduous “grand portage” to circumvent them. The Pigeon, along with all other major rivers along the northwest shore of Lake Superior, reach the lake over a series of rapids and falls that extend nearly to the lake shore. This is in contrast to the more typical morphology of rivers which commonly have extended reaches of slack water upstream from their terminus and thus provide waterborne access well inland. It is again the repeated glacial scouring along the ancient rift axis that created this situation. Numerous lobes of ice flowed southwestward through the western Lake Superior basin and oversteepened the topographic gradient near the present lake shore. This steep gradient extends well below the present lake level into a bedrock trough whose bottom is more than 1500 feet below the lake and about 1000 feet below sea level. In the relatively short period since the last glaciation, about 10,000 years, the rivers have not had adequate time to modify this glacially induced gradient and thus plunge down this steep topography until they intersect the modern level of Lake Superior making upstream canoe travel from the river mouths impossible. A still more fundamental geologic phenomenon is responsible for the highly unusual situation of a remarkably small watershed surrounding Lake Superior thus allowing a short distance of transit from the lake to outward flowing drainages. Modern interpretations of the geodynamic causes of the Midcontinent rift suggest that the region was underlain by a newly developing plume of upwelling mantle which caused both intense basaltic volcanism along the rift and the active splitting of the continent (Hutchinson and others, 1990; Cannon and Hinze, 1992). As the new plume waned in intensity, welding of cooling mantle material to the base of the crust, a so called rift pillow, resulted in an unusually thick crust in the Lake Superior region. Interpretation of seismic refraction surveys shows that present day crust beneath the lake is as much as 55 km thick in contrast to a typical thickness of about 40 km in surrounding regions (Halls, ILSG07 44 Trip 2 �1982). The isostatic effect of this thick crust is to maintain the Lake Superior region as a subtle topographic high (Allen and others, 1992). Examination of the continental scale drainage patterns (Fig. 11) shows that the Lake Superior watershed is anomalously small in relation to the size of the lake, and that major river systems drain radially away from the lake as a result of this isostatic bulge. Thus Lake Superior is, in a sense, a hub from which numerous possible outward draining water routes are available once the Superior divide is crossed. The Grand Portage is the shortest of these available routes. On a smaller scale, geologic features also played a significant role in determining the course of the portage and both its upper and lower terminus. Grand Portage Bay is the most sheltered harbor anywhere along the northwest shore of Lake Superior and thus was a naturally favorable location for developing the trading post at the western terminus of the Great Lakes transportation route. The bay is underlain by relatively non-resistant sedimentary rocks of the Rove Formation which make up most of the low country in the region. The sheltering headlands are formed from resistant igneous rocks. To the northeast, Hat Point and the high ridge extending inland is a reflection of the Mt. Josephine dike of the Pigeon River swarm. To the southwest, basalt flows of the Grand Portage Volcanics support the topographic high that forms the southern shore of the harbor. These same volcanic rocks also underlie Grand Portage Island that lies across the mouth of the bay and provides additional shelter. In order to traverse westward from Grand Portage Bay a way needed to found through the imposing steep-sided northeast-trending ridges that are supported by the resistant diabase of the Pigeon River dikes. A gap through these ridges is now followed by Grand Portage Creek, which flows along a fault trace (see Fig. 2.3). Although this fault has only minor offset, it apparently created weakened rocks that were either eroded by pre-glacial streams or were plucked by glacial ice or subglacial streams. In any case, this topographic gap is readily seen from Lake Superior and likely attracted the earliest humans in the area as a relatively easy course to the west. At the western terminus of the portage, local geology also played a role in determining the most favorable location at which canoes could be easily embarked and disembarked. Fort Charlotte was established at a point where a diabase dike, probably of the Pigeon River swarm, intrudes the Rove Formation and trends across the Pigeon River. This resistant dike forms a small topographic rib which creates a small rapid on the river that could not be crossed by canoes and, perhaps more significantly, impounded an upstream reach of still and unusually deep water creating a small natural harbor. These features illustrate the importance of the geologic history of the region and its influence on present physiography. It was both local and regional physiography that dictated the course of human occupation and exploitation of the Grand Portage region and its enormous significance in the 18th century history of the northern part of the continent. This “geological predetermination” of human events serves as a prime example of the interaction between the physical challenges encountered by early human exploration and settlement of a region and the ingenuity and industry with which they adapted to those challenges and opportunities. ILSG07 45 Trip 2 �Figure 2.11. Map showing the Lake Superior watershed (stippled), the axis of the Midcontinent Rift (short dashed line), and regional drainage patterns, simplified from Allen and others (1992). Arrows indicate direction of drainage and illustrate the central position of Lake Superior relative to the outward radial continental drainage pattern. Reference circles of 250 km and 500 km radius from western Lake Superior, the presumed central point of the mantle plume responsible for the rift. ILSG07 46 Trip 2 �GEOLOGICAL STOPS The following are brief descriptions of specific geological features that can be observed in several different areas of the Grand Portage National Monument. Locations of stops are shown in Figure 2.1. Stop 2.1. Stockade to Visitor Center: Grand Portage diabase dikes, Rove Formation, and glacial lake features. Shoreline features of some of the lower post-glacial lakes are visible near the Stockade. A distinct feature that runs along the shore zone and through the Stockade is the gentle bluff of the Sault stage (610 feet). To the east of the Grand Portage Creek, the bluff curves lakeward on Premiers Point, and runs northeast through the picnic area, parallel to the shore until rising erosional bluffs of Lake Superior truncate and eliminate it. Offshore, a lake floor of large boulders that fringes the east side of the sandy creek channel indicates the former extension of Premiers Point. Just above the Sault shoreline lies another curving bluff, the Algoma (621 feet). To the west of Grand Portage Creek Creek, the Sault bluff runs through the Stockade enclosure. The Great Hall building is constructed on the bluff, such that the lakeward side lies at the foot and the inland side lies on the top. The bluff forms a marked step through the Ojibwe Village exhibit to the west of the Stockade and is then truncated again by rising erosional bluffs near the Visitor Center. Behind the Canoe Warehouse in the Ojibwe Village exhibit, a small section of Algoma bluff has survived the disturbance of road construction. As you walk the roadway between the stockade and visitor center (under construction in 2007) you will cross four diabase dikes of the Grand Portage swarm (Green and others, 1987). A fifth dike crops out just south of the Monument boundary and is especially prominent as a free-standing wall of diabase about 20 feet tall, a classic geomorphic dike form. Figure 5 compares a recent photograph of this dike with an etching from Owen’s 1852 General Land Office report. All of the dikes form linear ridges up to a few tens of feet wide as a result of being much more resistant to erosion than the Rove shale into which they are intruded. The topography created by the dikes was probably emphasized by wave erosion along the glacial Lake Nipissing shore when water level was a few tens of feet higher than present Lake Superior. Pocket beaches were probably formed on the Rove Formation between the resistant dikes although talus from the steep south flank of Mt. Rose now covers these areas. Individual dikes here are 10-20 feet thick, fine grained, massive, and commonly show chilled contacts against the Rove Formation. The dikes were formed during early stages of development of the Midcontinent rift. In addition to the Rove, they also intrude volcanic flows of the Grand Portage Volcanics to the southwest of the Monument. The dikes are rather stongly magnetized and have a reversed remanent polarity. This makes them easy to find and identify by magnetic surveying because they appear as linear negative anomalies. A magnetic profile measured along the road with a proton precession magnetometer in 2001 shows four distinct magnetic lows that correspond to the four dikes that cross the road (Fig. 2.6). A profile along the portage trail, also done in 2001, found several additional dikes that had not been know previously. The dikes can be dated rather precisely by a combination of geologic relationships and magnetic polarity. They cut the Grand Portage Volcanics that were extruded early in the history of the Midcontinent rift and acquired their remanent magnetization before the magnetic reversal, which is recognized widely in igneous rocks of the Midcontiinent rift, at about 1,100 Ma. Stop 2.2. Rove Formation and Pigeon River diabase on Mt. Rose ILSG07 47 Trip 2 �The Mt. Rose Trail provides a well exposed section of the Rove Formation. The summit is underlain by a diabase dike of the Pigeon River swarm and a contact metamorphic halo in the Rove near the diabase contact. The following descriptions are keyed to numbered placards along the trail. Much of the eastfacing flank of Mt. Rose, over which the trail climbs, consists of steep bluffs of bedrock exposures of the Rove Formation. The steepness is probably due to wave erosion during higher level stands of post-glacial lakes. The Nipissing transgression reoccupied the base of Mt. Rose around 636 feet, with wave action vigorously cutting a bluff seen from Stop 2.1 on the Mt. Rose Nature Trail. The base of the bluff is now mostly covered by more recent talus. Cut in shale, no abrasion notch is expected, but an angular undercut can be seen in places to the south of the Trail. The Trail follows a bench at approximately 678 feet between lookout Stops 5 and 6 that faces Lake Superior and which would have experienced strong wave action at the post-Minong water levels. Large accumulations of blocky talus below a sharp bluff between Mt. Rose Nature Trail Stops 7 and 10 represent the work of waves during the period of Lake Minong (715 feet). In several places, a convincing undercut is seen, though the base of the rock bluff is almost everywhere covered by more recent talus accumulation. At 775 feet, at the time of Lake Low Beaver Bay, Mt Rose was an island separated from the mainland by a shallow strait. On the Mt. Rose Nature Trail, there are one or two rock benches that could have been wave modified by this level of Beaver Bay, but the strata outcrops in nearly horizontal form and it is not possible to distinguish between wave formed features and structural control. The Rove Formation along the trail is mostly flaggy sandstone in which prominent bedding partings emphasize bedding at inch-scale. Cross bedding is seen rarely and a few beds show soft-sediment slump structures. The sandy units are separated by thinner units of finer-grained fissile argillite. Beds are nearly flat lying with dips of only a few degrees toward the southwest. Within 10 or 20 feet of the contact with the diabase, the Rove is distinctly baked in the contact aureole. Closest to the contact, bedding is largely obliterated and the sediments are recrystallized to a hornfels containing biotite and clots of a secondary mineral in the matrix that is tentatively identified in thin section as cordierite. The diabase is mostly medium-grained and massive. The nearly vertical southeastern contact can be seen near the top of steep bluff as well as in pavement outcrops near the summit. The rock seen here is typical of the Pigeon River diabase dikes in the region in that it forms dikes hundreds of feet thick, has normal magnetic polarity and forms the prominent high, steep-sided ridges that are the most distinctive elements of the local topography. Stop 2.3. Rove Formation and Grand Portage dikes along the Beaver Bay shoreline. At this stop, both bedrock and post-glacial lake features are displayed. On the north side of Highway 61 at the trail crossing there are extensive exposures of the Rove Formation and two diabase dikes of the Grand Portage swarm. The unusual amount of bedrock exposure is probably a result of wave washing along the coastline of Beaver Bay stage lakes. This shoreline erosion stripped the cover of glacial till or lake sediment that probably originally mantled the bedrock, as it does over most of the surrounding area. The Rove in this area is fine-grained gray sandstone that breaks into approximately inch-thick flaggy slabs. Bedding dips about 5 degrees to the southeast, typical of the Rove over most of the region. Vertical diabase dikes of the Grand Portage swarm can be traced across the outcrop area and into the roadcut exposures along Highway 61. Just north of the bedrock outcrop, the Grand Portage Trail follows a terrace along the 816-foot contour, the inner edge of which is bounded by an indistinct bluff at 828 feet. The surface of the Trail is ILSG07 48 Trip 2 �noticeably roughened with large boulders, suggesting scour and winnowing of the finer sediments. It seems likely that this remnant terrace represents the floor immediately within the river mouth as it was in Mid Beaver Bay time (Fig. 2.12). As it is traced up valley, the bluff rises to 831 feet and 834 feet as would be expected within the river mouth. The Trail drops down a bluff from the 816 foot level into a small boggy embayment. This represents the shore of the 4th level of Beaver Bay (810 feet, between Mid and Low) and is a strong feature. The Trail rises briefly again to the former terrace (now 822 feet) and then descends into a broad embayment, backed by the continuation of the same 4th level of Beaver Bay before rising again to the terrace, (now 834 feet). A tributary coming in from the east breaks the bluff. Figure 2.12. Sketch map of the area along Grand Portage Trail north of Highway 61 showing outcrop of the Rove Formation and the trace of the bluff of the Beaver Bay stage shoreline. References Addison, W.D., Brumpton, G.R., Vallini, D.A, McNaughton, N.J., Davis, D.W., Kissin, S. A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, V., 33, p. 193-196. Allen, D.J., Hinze, W.J., and Cannon, W.F., 1992, Drainage, topographic, and gravity anomalies in the Lake Superior region: evidence for a 1110 Ma mantle plume: Geophysical Research Letters, v. 19, p 2199-2122. Babcock, Willoughby M. Jr. 1940, Re-building the Grand Portage Stockade: Some Problems in Historical Reconstruction: The Museum News, December 15, 1940, pages 6-8. Birk, Douglas A.,1975, Recent Underwater Recoveries at Fort Charlotte, Grand Portage National Monument, Minnesota: The International Journal of Nautical Archeology and Underwater Exploration 4(1):73-84. Birk, Douglas A., 1979, Whitewater Archeology. The Minnesota Volunteer 42(244):12-19 ILSG07 49 Trip 2 �Birk, Douglas A., 1984, John Sayer and the Fond du Lac Trade: The North West Company in Minnesota and Wisconsin Rendezvous: in Thomas C. Buckley, editor, Selected Papers of the Fourth North American Fur Trade Conference, 1981, pp. 51-61. St. Paul, North American Fur Trade Conference. Birk, Douglas A., 1994, When Rivers Were Roads. Deciphering the Role of Canoe Portages in the Western Lake Superior Fur Trade: The Fur Trade Revisited. Selected Papers of the Sixth North American Fur Trade Conference, Mackinac Island, Michigan, 1991, 359-76. Jennifer Brown, W. Eccles, and Donald Heldman, eds., East Lansing, Michigan State University Press. Birk, Douglas A.1998,The Hudson Bay Trail: A Study of Nineteenth Century Travel Routes Between Grand Portage, Minnesota, and Fort William, Ontario: Institute for Minnesota Archaeology Reports of Investigations, Number 466. Minneapolis. Birk, Douglas, 2005, National Register of Historic Places, Grand Portage. Birk, Douglas A., and Robert C. Wheeler, 1976, Fort Charlotte Underwater Archeology Project: Geographic Research Reports 1975, 791-99. Washington: National Geographic Society. National Brown, Ralph D., 1937, Recent Excavations at Grand Portage: Minnesota History, v, 18, p.456-458. Buck, Solon J., 1931, The Story of the Grand Portage: Minneapolis: Private printing. Burpee, Lawrence J., 1931, Grand Portage: Minnesota History, v. 12, p. 359-377. Cannon, W.F. and Hinze, W.J, 1992, Speculations on the origin of the North American Midcontinent rift: Teconophysics, v. 213, p. 49-55. Carver, Jonathan, 1956, Travels Through the Interior Parts of North America, in the Years 1766, 1767, and 1768: Reprint. Minneapolis: Ross and Haines. Dawson, Samuel J., 1968, Report on the Exploration of the Country Between Lake Superior and the Red River Settlement, and Between the Latter Place and the Assiniboine and Saskatchewan: Appendix No. 36. Reprint of 1859 edition. New York: Greenwood Press. Gates, Charles M. (editor), 1965, Five Fur Traders of the Northwest: St. Paul, Minnesota Historical Society. Gilman, Carolyn, 1992, The Grand Portage Story: St. Paul, Minnesota Historical Society Press. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Jr., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterozoic Midcontinent rift of North America: Geologic Association of Canada Special Paper 34, p. 289-302. Grout, F.F., and Schwartz, G.M., 1933, The geology of the Rove Formation and associated intrusives in northeastern Minnesota: Minnesota Geological Survey Bulletin 24, 103 p. Grout, Frank F., Robert P. Sharp, and George M. Schwartz, 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey, Bulletin 39. Halls, H. C, 1982, Crustal thickness in the Lake Superior region: in Wold, R. J, and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin, Geological Society of America Memoir 156, p. 239-244. Heamon, L.M., and Easton, R.M., 2006, Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative: Ontario Geological Survey Miscellaneous Release-data 191, 78 p. Hirth, Kenneth G., 1976, Interregional Trade and the Formation of Prehistoric Gateway Communities: American Antiquity, v. 43, p. 35-45. Holm, D.K., VanSchmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D, and Schneider, D, 2005, U-Pb zircon geochronology of the Paleoproterozoic plutons from the northern mid-continent, USA: evidence for subduction flip and continued convergence after the geon 18 Penokean orogeny: Geological Society of America Bulletin, v. 117, p. 259-275. Hutchinson, D.R., White, R.S., Cannon, W.F., and Schulz, K.J., 1990, Keweenaw hotsopot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent rift system: Journal of Geophysical Research, v. 95, p. 10869-10884. ILSG07 50 Trip 2 �Kerber, L., 2006, Minimum age and provenance of the correlated Thomson and Rove Formations of eastern Minnesota: 19th Annual Keck Symposium, p. 142-146. Maric, M., and Fralick, P., 2005, Sedimentology of the Rove and Virginia Formations and their tectonic significance: Proceedings of the 51st Institute on Lake Superior Geology, v. 51, part 1, Proceedings with Abstracts, p. 41-42. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, digital or print on demand, nominal scale 1:200,000. Morey, G.B., 1969, The geology of the Middle Precambrian Rove Formation in northeastern Minnesota: Minnesota Geological Survey, Special Publication Series, SP-7, 62 P. Morse, Eric W., 1969, Fur Trade Canoe Routes of Canada/ Then and Now: Queen's Printer, Ottawa. NICE working group, in press, Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation: Precambrian Research. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, John C., 1997, Rift-wide correlation of the 1.1 Ga Midcontinent rift sywtem basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Science, v. 34, p. 504-520. Nute, Grace Lee (editor), 1940, A British Legal Case and Old Grand Portage: Minnesota History, v. 21, p. 117-148. Nute, Grace, 1944, Lake Superior: The American Lakes Series, Milo M. Quaife, editor, Indianapolis, Bobbs-Merrill Company Owen, D.D., 1852, Report of a geological survey of Wisconsin, Iowa, and Minnesota and incidentally of a portion of Nebraska Territory: Report to U.S. General Land Office, 623 p. Phillips, B.A.M., 2001, Water Level History and Shoreline Change – Grand Portage National Monument, MN: A Report for the U.S. National Park Service, Grand Portage National Monument, 88 pages. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schneider, D.A., Holm, D.K., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: in Whitney, D.L., and Siddoway, C.S., eds., Gneiss domes in orogeny, Geological Society of America Special Paper 380, p. 339-357. Schwartz, George M., 1928, The Topography and Geology of the Grand Portage: Minnesota History, v. 9, p.26-30. Sharp, R.P. 1953, Glacial features of Cook County, Minnesota: American Journal of Science, v. 251, p. 855-883. Teller, J.T., and Thorleifson, L.H., 1983, The Lake Agassiz-Lake Superior connection: in. Teller. J.T, and Clayton, L., eds., Glacial Lake Agassiz: Geological Association of Canada Special Paper 26. pp. 261-290. Thompson, Erwin N., 1969, Grand Portage. A History of the Sites, People, and Fur Trade: U. S. Department of the Interior. Washington: National Park Service. Wallace, W. Stewart (editor), 1934, Documents Relating to the North West Company: Reprint edition (1968). New York: Greenwood Press. Warren, William W.,1957, History of the Ojibway Nation: Minneapolis: Ross and Haines. Wheeler, Robert C., Kenyon, Walter A., Woolworth, Alan R., and. Birk, Douglas A.,1975, Voices From the Rapids. An Underwater Search for Fur Trade Artifacts, 1960-73: Minnesota Historical Archeology Series No. 3, St. Paul, Minnesota Historical Society. White, Bruce M., 1982, Give Us a Little Milk. The Social and Cultural Meanings of Gift Giving in the Lake Superior Fur Trade: Minnesota History, v. 48, p. 60-71. ILSG07 51 Trip 2 �White, Bruce M., 1987, A Skilled Game of Exchange. Ojibway Fur Trade Protocol: Minnesota History, v. 50, p. 229-240. Winchell, Newton H. (editor), 1899, The Geology of Minnesota: Geological and Natural History Survey of Minnesota, Volume 4 of the Final Report, St. Paul: Pioneer Press Company. Woolworth, Alan R., 1967, Archeological Excavations at Grand Portage: An 18th Century Fur Trade Metropolis: The Minnesota Archaeologist, v. 29, p.3-17. Woolworth, Alan R. and Nancy L. Woolworth, 1982, Grand Portage National Monument. An Historical Overview and An Inventory of Its Cultural Resources: Two Volumes. Unpublished, typewritten report prepared for the National Park Service. St. Paul: Minnesota Historical Society. ILSG07 52 Trip 2 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 3 MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS NORTH OF THE INTERNATIONAL BORDER Mark Smyk Ontario Geological Survey, Ministry of Northern Development and Mines Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada Peter Hollings Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Frontispiece: View looking southwest from the top of ridge (Crystal Lake Gabbro) at the Great Lakes Nickel property. Mesas are underlain by Logan diabase sills which have intruded Rove Formation sedimentary rocks. ILSG07 53 Trip 3 �INTRODUCTION This field trip covers an area that has been the focus of much research and economic interest. The guidebook has benefited from recent geochronologic and geochemical studies conducted as part of the Lake Nipigon Region Geoscience Initiative. These studies have elucidated the nature of magmatism in the northern part of the Midcontinent Rift (MCR) and have been used to augment and refine our previous understanding of these magmatic events. This trip focuses on a variety of mafic intrusions associated with the MCR north of the International Border near Pigeon River. These intrusions represent changes in the nature of early to mid-stage MCR magmatism over a span of ~20 million years. They include Logan sills, Pigeon River and other dykes and the layered Crystal Lake Gabbro, host to the Great Lakes Nickel coppernickel deposit. Contacts with Paleoproterozoic Rove Formation sedimentary rocks are well-exposed in this area and illustrate some of the mechanisms of dyke/sill emplacement, as well as magma-wallrock interactions. These interactions may play an important role in local metallogenesis. This guide book builds upon those previously written and compiled by Franklin and Kustra (1972), Miller and Smyk (1995), Parker et al. (2001) and Miller et al. (2002). Bear in mind that when visiting exploration or private properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office, Ontario Geological Survey, in Thunder Bay. Please exercise caution along highway right-of-ways, cliffs and mine workings. REGIONAL GEOLOGY Situated within the Southern Province of the Canadian Shield, the field trip area is dominantly underlain by Paleoproterozoic Rove Formation clastic sedimentary rocks (Animikie Group) which have been intruded by MCR mafic intrusions (Figure 3-1). Previous mapping has been conducted by Tanton (1936a,b), Geul (1970, 1973) and Smith and Sutcliffe (1989). Detailed mapping, geophysical surveys and diamond drilling undertaken by exploration companies have provided additional detail and much-needed information about sub-surface geology. This area is a rugged, upland area of diabase-capped mesas and ridges that occupies a 70 km by 30 km, northeast-trending topographic feature between Thunder Bay and the Minnesota border, termed the “Logan Basin” by North (2000). Logan Sills underlie mesas that commonly rise 150 m above valleys underlain by deeply eroded, flat-lying, Rove Formation sedimentary rocks (see Frontispiece). Northwest of the Logan Basin, Archean granitoid rocks of the Superior Province form low, rolling hills. Southeast of the Logan Basin the topography is dominated by northeast-trending, linear ridges underlain by Pigeon River dykes. Animikie Group At approximately 2.45 Ga a rift to passive margin developed along the southern edge of the Superior Province when a land-mass to the south separated (Fralick and Miall 1989). Later closure of the resultant ocean led to the deposition of the Animikie and North Range Groups as a backarc basin developed (Pufahl and Fralick 1995, 2004: Hemming et al. 1995; Van Wyck and Johnson 1997; Pufahl et al. 2000), which collision later transformed into a foreland setting (Hoffman 1987; Southwick and Morey 1991; Hemming et al. 1995; Ojakangas et al. 2001; Maric and Fralick 2005). ILSG07 54 Trip 3 �Figure 3-1: General geology of the field trip area (after Smith and Sutcliffe 1987) ILSG07 55 Trip 3 �The Animikie Group in the study area is represented by the Rove Formation, which overlies the Gunflint formation and has an approximate thickness of 500 to 600 m south and east of Thunder Bay, thickening to the south. Rocks of the Rove Formation are flat-lying or dip gently to the southeast. Recent work by Amurawaiye (2001) and Maric and Fralick (2005) described a submarine ramp system in which the movement of coarse sediments into the deeper parts of the basin was mainly through the action of lowand high-density turbidity currents. Fair-weather and storm-generated currents dominated depositional activity at the edge of the basin. Mud and ash particles accumulated during tranquil periods (ibid). Amurawaiye (2001) has stated that approximately 70% of the Rove Formation locally consists of organic shale whose hydrocarbon contents have now been degraded. The lower 100 to 150 m of the Rove Formation and the correlative Virginia Formation in Minnesota consist of alternating shale-siltstone and black, pyritiferous shale successions, probably reflecting fluctuations in sea level (Maric and Fralick 2005). These successions, and especially the upper black shale, likely represent a major condensed interval deposited in water ~100 to 200 m deep. Lucente and Morey (1983) ascribed sedimentation of this interval to pelagic rainout of fine-grained sediment from dilute suspension or hemipelagic processes involving diffuse turbidity currents. The presence of abundant, sub-millimeter rip-up intraclasts also denotes the operation of sporadic bottom currents (Maric and Fralick 2005). Tidal deposits present in correlative rocks to the south of Lake Superior (Ojakangas et al. 2001) confirm open connection to the ocean. Above the upper, pure black shale interval, graded finegrained sandstones are organized into a coarsening-upward succession approximately 100 m thick that is transitional into 400 m of medium-grained, sandstone-dominated, stacked parasequences (Maric and Fralick 2005). This is overlain by lenticular to wavy bedded sandstones and shales with both wave and current ripples. The coarsening-upward to sandstone-dominated portion of the Virginia and Rove Formations has been interpreted as a submarine fan (Lucente and Morey 1983, Maric and Fralick 2005) with the uppermost ripple laminated succession representing progradation of distal distributary mouth bars of a delta (Maric and Fralick 2005). A sandstone sample from the submarine fan portion of the succession yielded a youngest U-Pb detrital zircon age of approximately 1780 Ma (Heaman and Easton 2006). The predominantly Paleoproterozoic zircon population and paleocurrents indicating sediment derivation from the north (Morey 1973), strongly suggest the Trans-Hudson Orogen was the source of the detritus. Keweenawan Supergroup Mesoproterozoic intrusive, volcanic and minor sedimentary rocks associated with the MCR collectively constitute the Keweenawan Supergroup. On the northern margin of the MCR, Keweenawan rocks include a variety of intrusive rocks and Osler Group volcanic rocks which represent some of the earliest magmatism in the MCR. As shown in Table 3-1, ages range from ca. 1140 Ma (Heaman and Easton 2006) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green 1997). A tabulated synopsis is provided below; bolded units occur within the field trip area. Most mafic and ultramafic rocks in the Lake Nipigon and northern Lake Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, interval between ca. 1115 and 1100 Ma (Heaman and Easton 2006). Emplacement of alkalic intrusions, such as the 1108 Ma Coldwell Complex (Heaman and Machado 1992), and filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake (1095±2 Ma) gabbros, and a Pigeon River dyke near Arrow River (1093±3 Ma; Heaman and Easton 2006). ILSG07 56 Trip 3 �Table 3-1: Geochronology data of MCR-related rocks in Northwestern Ontario. Lithologic Unit St. Ignace Island Complex gabbro Arrow River Dyke Pigeon River Dyke Blake Gabbro Moss Lake Gabbro Crystal Lake Gabbro Pine Point – Mt. Mollie dyke Osler Group rhyolite (central suite) Osler Group rhyolite (lower suite) St. Ignace Island Complex Rhyolite Coldwell Complex Locality / Age (Ma) St. Ignace Island / 1089.2 +3.2 Arrow River / 1078 + 3 Rita Bolduc / 1141±20 Blake Township / 1091.0 + 4.5 Black Bay Peninsula / 1094.7 ±3.1 Great Lakes Nickel / 1099.6 + 1.2 n/a Agate Point / 1105±2 Reference(s) Smyk et al. (2006) Heaman and Easton (2006) Heaman and Easton (2007) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Black Bay Peninsula / 1107.4 +4/-2 St. Ignace Island / 1107.2 ± 2.4 Davis and Sutcliffe (1985) Smyk et al. (2006) Coldwell Complex / 1108 + 1 Logan Sills Nipigon Sills Ultramafic Intrusions Pigeon River Inspiration Sill Marathon lamprophyre dykes Mt. McKay / 1115 + 1 Nipigon Embayment / 1114-1110 Nipigon Embayment / 1124-1113 Crooks Twp. / 1141 + 20 Lake Nipigon / 1141 + 20 McKellar Harbour / 1145 +15/-10 Heaman and Machado (1987) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Heaman and Easton (2006) Queen et al. (1996) Davis and Green (1997) Hypabyssal Mafic Rocks Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern remnants of the Midcontinent Rift, and have previously been referred to as the Logan sills (Stockwell et al. 1972), however recent work suggests a geochemical difference between the sills to the north and south of the City of Thunder Bay (Hart 2003; Hart et al. 2005b). Hollings et al. (2007) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite of Miller et al. (2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Logan sills generally consist of fine- to coarse-grained, ophitic to intergranular, quartz tholeiitic diabase/gabbro (Smith and Sutcliffe 1987; Geul 1970, 1973). Coarse-grained, intergranular gabbro, locally rich in granophyric mesostasis, is common in the interior of the thicker sills. The upper sections of the diabase sills are commonly plagioclase-porphyritic, containing as much as 60% phenocrysts. Chilled margin and bulk compositions are iron-rich, quartz-tholeiitic basalt. Compositional and textural variation in sills has been noted by North (2000) and Beskar (2001) in Blake Township, where varitextured, "taxitic" gabbro has been described. Logan sills are recognized by their reversed magnetic polarity and generally take the form of columnarjointed, thick sheets and sills whose geometry is strongly controlled by the subhorizontal bedding of the country rock. They form conspicuous erosional remnants that create mesa and cuesta topography. From the international boundary area to Thunder Bay as many as six diabase sheets were emplaced nearly conformably into Animikie sedimentary rocks (Weiblen et al., 1972; Smith and Sutcliffe 1987, 1989). Diamond drilling has also shown that stacked sills exist in the subsurface. For example, Dumont Nickel Inc. reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). North of the border, ILSG07 57 Trip 3 �Smith and Sutcliffe (1989) reported sills up to 50 m thick, whereas in Minnesota, Jones (1984) studied four sills ranging from 50 to 160 m in maximum thickness. Rare exposures of feeder dykes to sills and preserved sill terminations have been noted (e.g., Stop 3-3). The textural stratigraphy, which varies from a lower, ophitic zone to an upper pegmatitic zone, indicates that in most cases, the sills cooled as single units, probably over a period of 200 to 500 years (Smith and Sutcliffe 1989). Chilled contact zones are developed against sedimentary country rocks; sedimentary xenoliths are rare. Pigeon River dykes trend east-northeast to northeast and dip steeply to the southeast (Geul 1970, 1973; Smith and Sutcliffe 1989). Displacement and warping of the Rove Formation is evident along many of the dikes. Composite intrusions are noted in several dykes (add reference). Dyke widths average between 50 and 70 m, but may be as much as 150 m across in Ontario (Smith and Sutcliffe 1987) and 500 m in Minnesota (Green et al. 1987). Forming northeast-trending, linear ridges, dykes can be traced semicontinuously for 15 km along strike. As noted by many workers, some dykes clearly crosscut Logan sills. However, Geul (1973) and Smith and Sutcliffe (1987) noted that others display somewhat ambiguous crosscutting relationships. In these latter cases, dykes may appear to merge with sills, suggesting that they were contemporaneous or that sills impeded the upward migration of the dykes. The presence of multiple sets of horizontal columnar jointing suggests the development of multiple or composite dykes. The dykes typically consist of ophitic diabase that may be plagioclase-porphyritic. A typical, nonporphyritic olivine diabase consists of approximately 60% plagioclase (zoned labradorite; An 55-70), 20% augite + hypersthene, up to 15% olivine and up to 5% magnetite, ilmeno-magnetite and sulphides (Geul 1970, 1973). Average whole rock compositions of Pigeon River dikes are moderately evolved (mg# = 52) olivine tholeiitic basalt. A 15-km long, northwest-trending diabase dike, termed the Arrow River dyke by Smith and Sutcliffe (1989) crosscuts Pigeon River dikes in Ontario. This dike and two shorter, similarly oriented dikes are composed of intergranular, quartz diabase that is commonly plagioclase-phyric. They are collectively known as the Arrow River dykes, despite the fact that they occur well north of the Arrow River. The authors recommend that they be named Cloud River dykes.) Confusion has arisen because the term “Arrow River dyke” has also been used colloquially to denote a prominent Pigeon River dyke that occurs between the Arrow River and Pigeon River in southern Devon Township (i.e. Stop 3-1). The youngest intrusions in the area tend to be more irregularly shaped and internally zoned. One of these is the Crystal Lake gabbro, which is Y-shaped in plan view, with a west-northwest-striking limb 5km long and an east-northeast-striking, southern limb 2.75 km long (Figure 3-2). Internal layering and foliation suggest that the surface geometry of the northern limb may result from the tilting of a canoeshaped body, open on its western end (Smith and Sutcliffe 1987, 1989). The intrusion was subdivided into three major zones by Reeve (1969) and Geul (1970) and has been further subdivided by Smith and Sutcliffe (1987, 1989) and Cogulu (1990) into four major, roughly equivalent, lithologic zones: (1) an upper zone (60 to 80 m) of sulfide-barren troctolite, olivine gabbro and anorthositic gabbro; (2) a middle zone (30 to 42 m) of cyclic, layered anorthositic and olivine gabbro, Cr-spinel- bearing anorthosite, and olivine gabbro (Figures 3-3 and 3-4); (3) a lower, unlayered zone (50 m) of vari-textured gabbro and leucotroctolite, which hosts the bulk of the Cu-Ni-sulfide deposit; and (4) a basal zone (1 to 7 m) of fine-grained, chilled melagabbro and hornfelsed country-rock xenoliths. ILSG07 58 Trip 3 �Table 3-2: Geochemical analyses of select intrusive rocks from the Border Area Intrusive Unit: Source: Logan Sills Grand Portage dikes Mt Josephine Pigeon River dikes A B C D C Chilled Margin Bulk Comp Avg. of 17 dikes Composite dike Average of 14 dikes SiO2 49 50.1 52.5 47.9 48.9 TiO2 3.4 3.6 2.48 3.66 1.65 A12O3 13.1 13.1 13.4 11.8 16.3 FeOt 15.6 14.3 13.3 16.2 11.5 MnO 0.22 0.16 0.19 0.24 0.18 MgO 5.6 3.9 3.68 4.04 6.46 CaO 7.45 7.2 6.72 8.9 10.21 Na2O 2.52 3.4 3.17 2.63 2.42 K2O 1.16 1.5 1.79 1.29 0.52 P2O5 0.38 0.43 0.48 0.53 0.18 Volatiles 0.92 2.5 ----- ----- ----- Total 99.35 100.19 99.11 99.6 99.8 mg# 38.4 32.2 34.1 33 50.1 Cr 50 --- 44 50 100 Ni 80 --- 61 22 136 Rb --- --- 55 28 15.7 Sr 700 --- 420 359 280 Zr 250 --- 281 280 --- Hf --- --- 9 --- 2.94 Description: Trace Elements (ppm) Table from Miller and Smyk (1995); Sources: (A) Geul (1970), Table 3, Sample 2; (B) Jones (1984), Table A-I, Sill A; (C) Green et al. (1987), Table II; (D) Green (1986), Appendix E, Sample GP-60. ILSG07 59 Trip 3 �Figure 3-2: General geology of the Crystal Lake Gabbro after Smith and Sutcliffe (1987) ILSG07 60 Trip 3 �Figure 3-3: Polished slab of massive Crspinel layers in contact with medium-grained gabbro, Great Lakes Nickel property (Stop 3-4). White bar is 4 cm long. Figure 3-4: Igneous layering in the Crystal Lake Gabbro, exposed in the upper cliff section on the Great Lakes Nickel property (Stop 3-4). This is part of the middle zone of the intrusion, characterized by cyclic, layered anorthositic and olivine gabbro, Crspinel-bearing anorthosite, and olivine gabbro. ILSG07 61 Trip 3 �Projecting east of the Crystal Lake gabbro and arcing to the northeast to form a string of islands in Lake Superior is a major composite dike called the Pine River-Mount Mollie dyke (Geul 1973). Smith and Sutcliffe (1989) described this 35 km-long body as consisting of gabbroic, dioritic and granophyric rocks. Gabbroic rocks in the margins of the dike commonly have modal layering and sulphide mineralization similar to that in the Crystal Lake gabbro. Inward from the gabbro, quartz-bearing diorite cores much of the dike. In the central and western portions, granophyre occupies the core of the dike and shows contact relationships with diorite that indicates mixing of felsic and mafic liquids (Smith and Sutcliffe 1987, 1989). As described by Geul (1973), the Pine River-Mount Mollie Gabbro comprises two segments of an eaststriking intrusion which appears to postdate the Pigeon River dykes. The poorly exposed Pine River segment apparently connects to the eastern end of the Crystal Lake Gabbro. Magnetometer survey data and drilling indicate that the Pine River and Mount Mollie segments are connected beneath deep overburden. Field relationships and diamond drilling show that the dip of the gabbro contacts varies from near vertical to 35º north, indicating that the body has the shape of a tilted, truncated cone in crosssection. The exposed width ranges from approximately 60 m (true thickness) to 300 m (apparent thickness), depending upon the attitudes of the contacts. The gabbro has a distinct mottled appearance and is further distinguished from most other diabases in the area by larger grain size, leucogabbroic composition, and degree of mineralization. Diamond drilling logs summarized by Geul (1973) defined a crude zonation in the gabbro, based on variations in grain size, feldspar (i.e. anorthosite) and sulphide content. Thin section petrography by Geul (1973) indicated that the gabbro ranges in composition from anorthositic gabbro to olivine gabbro and quartz gabbro. Deuteric alteration is slight to intense and consists of secondary amphibole, biotite, saussurite, sericite, and hematite and appears to be closely related to the presence of an intergranular granophyric phase. Bodies of granophyre cut the gabbro in several locations and appear to represent the differentiated end product of the gabbroic magma. The gabbro is composed essentially of pyroxene and zoned labradorite. DISCUSSION OF GEOCHEMISTRY As part of the Lake Nipigon Region Geoscience Initiative whole rock analyses were performed on a number of the intrusions south of Thunder Bay; these data can be combined with published (Cogulu 1990), unpublished data for some of the dykes in the region (Larry Hulbert, Geological Survey of Canada, personal communication, 2006) and data from exploration projects (e.g., Rosatelli 2002), in order to assess variations in geochemistry. On a primitive mantle-normalized diagram, the samples of Crystal Lake gabbro and Pigeon River dykes show similar patterns (Figure 3-5). Pigeon River dykes from the Arrow River (Stop 3-1) and Rita Bolduc occurrence (Stop 3-2) are similar to the range of data from other dykes of the Pigeon River swarm sampled in Lake Superior (Victoria Island, Cloud Bay and Jarvis Point (Figure 3-6; Larry Hulbert, Geological Survey of Canada, personal communication, 2006). However, samples of the Pigeon River dyke swarm from Lake Superior are characterized by higher Th abundances than those from the study area, resulting in more pronounced negative Nb anomalies. The only published analysis for the Arrow Dyke swarm (Rosatelli 2002) is indistinguishable from the Pigeon River swarm. By contrast, samples from the Mount Mollie dyke are characterized by elevated La/Smn ratios, higher Th contents and more pronounced Nb anomalies (Figure 3-6). This may be a reflection of greater crustal contamination during emplacement or possibly a distinct mantle source. Regardless, this geochemistry provides another means for distinguishing these dykes. ILSG07 62 Trip 3 �When the data from the dyke swarms are compared to the regional data set generated for the sills and intrusions of the Lake Nipigon embayment (Hollings et al. 2007) it can be seen that Pigeon River dyke swarm geochemistry more closely resembles that of the sills of the Nipigon suite than that of the ultramafic intrusions or the Logan sills (Figure 3-6). By contrast, the Mt Mollie dyke geochemistry appears to be transitional between that of the Nipigon sills and Inspiration sills. Additional isotopic and geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Figure 3-5: Primitive mantle-normalized plots for mafic intrusive rocks in the field trip area. Fields from data contributed by L. Hulbert, Geological Survey of Canada, personal communication (2006). Normalizing values from Sun and McDonough (1989). CONTACT METAMORPHISM There is a remarkable range in the reported intensity and nature of contact metamorphic effects in Rove sedimentary rocks at diabase dyke and sill contacts, owing mainly to the subjectivity of the mapper and the exposures in question. Geul (1973) noted that sedimentary hornfelsic rocks are restricted to a narrow zone of baking between 2 to 10 cm wide at diabase dike contacts. Metamorphosed siltstone displays two stages: first: slight recrystallization of biotite aggregates in an incipient hornfelsic texture; and second, a more complete recrystallization of biotite, surrounded by pale sericitic aggregates, set in a quartzofeldspathic matrix. Franklin (1970) suggested that contact effects existed up to 8 m from sill contacts and possibly up to 23 m. They were manifested as microporphyroblasts of mica and chlorite (“spotted alteration”), graphite destruction and the conversion of pyrite to pyrrhotite. Geul (1973) noted that minute particles (< 0.01 mm) of oxide and sulphide minerals are locally abundant in the contact zone. Rove Formation sedimentary rocks may be deformed along dyke contacts. As noted by Geul (1973) beds appear to dip toward the dykes or are “up-dragged” along dyke contacts (e.g. Stop 3-2). Up-dragging of Rove Formation rocks was also noted along the southern contact of an east-striking Pigeon River dyke at Arrow Rapids by Geul (1970). Deformed and fractured sedimentary rocks have been noted near sill terminations (e.g. Stop 3-3). Narrow, parallel, tension gashes filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstones at Stop 3-2 (Figure 3-8). ILSG07 63 Trip 3 �Figure 3-6: Discrimination diagrams for mafic and ultramafic intrusions near Thunder Bay. Data are from Hollings et al. (2007) and Hart (2002). ILSG07 64 Trip 3 �MINERAL EXPLORATION Active prospecting was first carried out during the latter part of the nineteenth century, following the discovery of silver vein deposits along the shoreline and islands of Lake Superior (Island Silver Belt) and to the northwest (Mainland Silver Belt; Oja 1967). Small silver mining operations took place on Spar and Jarvis Islands and at the Prince Mine. Barite was produced from veins on McKellar Island and Jarvis Island. Silver-bearing veins are in or near crustal-scale, extensional listric faults that formed during MCR extension and are spatially associated with MCR-related mafic intrusions (Franklin et al. 1986). Copper-nickel-mineralized boulders were found in 1903 by J. A. McCuaig in Pardee Township (Geul 1970). This discovery led to a concerted exploration effort to find their source (Geul 1970, 1973). Local copper-nickel occurrences were soon described by Tanton (1935, 1937). United States Smelting, Refining and Mining Company conducted exploration in 1936, and were followed by Frobisher Exploration Company in 1942. In the summer of 1952, J.S. Brodie and T.W. Page examined a large outcrop of gabbroic rocks, 6 km northeast of the original float discovery on the property now held by the Great Lakes Nickel Corporation Limited. Following this discovery, several blocks of ground were staked in Crooks Township by Whitegate Mining Company Limited, J. S. Strickland, and others, in an effort to cover the interpreted eastward extension of the favourable host rock. In 1967, Canadian Exploration Limited investigated the Strickland-Whitegate claims in southwestern Crooks Township and Anaconda American Brass Limited prospected the Pine River-Mount Mollie area as part of a regional reconnaissance program. In 1968, Phelps Dodge Corporation of Canada Limited explored and drilled claims in the Pine RiverMount Mollie area of Crooks Township. In 1989, International Platinum Corp. drilled a 3592 m hole in central Crooks Township. A renewed interest in platinum group elements in the late 1990s led to a reevaluation of the area’s potential and a flurry of exploration and drilling by companies like BHP Billiton World Expl. Inc., Falconbridge Ltd., McVicar Minerals Ltd., North Atlantic Nickel Corp. and Dumont Nickel Corp. A Noril’sk-style model was employed, as had been earlier suggested by Lightfoot and Lavigne (1995) and others in promoting the area’s mineral potential. ILSG07 65 Trip 3 �Figure 3-7: Columnar joints in the Crystal Lake Gabbro, exposed above the upper adit on the Great Lakes Nickel property (Stop 3-4). The adit opening (lower left) is approximately 3m wide. Figure 3-8: Narrow, parallel tension gashed filled with quartzo-feldspathic leucosome/neosome in metatectic, deformed siltstone near contact with Pigeon River dyke, Stop 3-2 ILSG07 66 Trip 3 �FIELD TRIP STOPS: ROAD LOG AND DESCRIPTIONS STOP NAME Middle Falls Dyke Arrow River Dyke STOP NUMBER LANDMARK (0.0 km) DISTANCE (km) Reset Trip Odometer Visitor Centre @ Border 0.0 / travel north Junction with Hwy. 593 2.3 / turn left, travel west Middle Falls 4.5 Junction with Old Border Road 10.9 / turn left, travel west 500 m Junction with Logging Road 11.4 / turn right, travel northwest 3.8 km Arrow River bridge 14.4 Optional Reset Trip Odometer McCuaig Float Occurrence ILSG07 Junction of Hwy. 593 and Old Border Road 0.0/ left turn, travel north Junction with Hwy. 597 (Pardee Road) 1.7 (on right) 3.8 / 150 m north of hwy. (flagged trail) Optional Reset Trip Odometer Rita Bolduc Occurrence / Pigeon River Dyke 15.2 3-1 Junction of Hwy's. 593 and 61 NORTHING EASTING (NAD 83) 5321108 304872 5324134 296694 5325546 299009 0.0 / turn left, travel north 4.4 3-2 67 5324701 310563 Trip 3 �Mount Mollie Lookout Great Lakes Nickel Property 6.6 (gated, on left) Pine River Bridge 6.75 Junction with Memory Road 6.9 / turn right, travel east 3.1 km Trail to Mount Mollie lookout 10.0 / trail on east side of road Follow trail ~300 east to top of ridge Optional Reset Odometer Rove Formation Quarry Junction with Great Lakes Nickel Road Return to Highway 61; turn left, travel south Travel south 300 m to Great Lakes Nickel Road (gated, on right) Junction of Hwy. 61 with Great Lakes Nickel Road 0.0 / travel west 5325444 313843 3-3 4.7 (south side of road) 5328687 306401 3-4 7.5 (western end of ridge; cliff face) 5328392 303612 STOP (OPTIONAL): Middle Falls Location: South side of Highway 593, 4.5 km west of Visitor Centre at Border on Highway 61 (UTM 304872E / 5321108 N; NAD83) (N.B. On older maps, this location is referred to as Little Falls.) Description: This roadside pull-off offers a picturesque view of Middle Falls, an 18 m high waterfall underlain by a resistant, west-northwest-trending, diabase dyke cutting Rove Formation clastic sedimentary rocks. The dyke is exposed in a roadcut on the highway opposite the pull-off area. It is cut by a number of parallel, rusty carbonate veins. STOP 3-1: Pigeon River Dyke (a.k.a. the “Arrow River Dyke”) Location: Road cut, 15.2 km west of Visitor Centre at Border on Highway 61, via Old Border Road (@ 10.9 km) and logging road (@ 11.4 km) (UTM 296694E / 5324134N; NAD83) ILSG07 68 Trip 3 �Description: This stop is situated on top of a prominent northeast-trending ridge which extends for over 20 km and rises up to 150 m above the surrounding countryside. The ridge is underlain by a Pigeon River dyke which, owing to its location, has been colloquially referred to as “the Arrow River dyke”, although it is not part of the northwest-trending Arrow River dyke swarm of Smith and Sutcliffe (1989). The exposed dyke consists generally of massive fine- to medium-grained diabase. Conspicuous, pyroxene-phyric patches have diffuse to indistinct contacts with the finer-grained diabase, suggesting that they may represent fluid-rich pockets or cognate xenoliths. (Similar textures are noted at Stop 3-2 as well.) A very small amount (~50 grains) of tiny baddeleyite fragments and blades were recovered from a sample of diabase at this location (Heaman and Easton 2006). A larger number of 40–100 μm colourless resorbed zircons were also recovered, but these were interpreted to be of xenocrystic origin. The U/Pb results are from one small multi-grain baddeleyite fraction. This baddeleyite fraction has a low uranium content (188 ppm U) and a low Th/U ratio (0.022); the latter is typical for magmatic baddeleyite. This baddeleyite analysis displays slight reverse discordance (–1.4%) and has a 206Pb / 238U age of 1092.7±2.6 Ma, which is interpreted as a good estimate for the minimum age of the Arrow River diabase dike (ibid). STOP (OPTIONAL): McCuaig Float Occurrence Location: North side of Highway 593, 3.8 km west of junction of Highway 593 and Old Border Road / 1.7 km west of junction of Highways 593 and 597 (Pardee Road) (UTM 299009E / 5325546 N; NAD83) Description: This stop comprises a number of pits and trenches developed in a hummocky, boulderstrewn hillside. Copper-nickel-mineralized boulders were found in 1903 by J. A. McCuaig (Geul 1970). In 1936, a magnetometer survey was conducted by United States Refining and Mining Company. In 1942, Frobisher Exploration Company Limited completed a second magnetometer survey. In 1949-1950, J. A. McCuaig (original discoverer’s son) carried out some minor trenching and sampling and drilled a 200 foot subvertical hole which reportedly showed the mineralized rock was in the form of float boulders. A small bulk sample was tested by Falconbridge Nickel Mines Limited. In 1967, Hollinger Consolidated Gold Mines Limited carried out geophysical surveys. Hanna Gold Mines Limited completed a vertical hole nearby. In 1990, Fleck Resources Ltd. carried out magnetometer and geochemical surveys along with limited prospecting, trenching and sampling just north of this occurrence (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Tanton (1935, 1937), McCuaig (1950) and Geul (1970) have described mineralized, vari-textured diabase, gabbroic anorthosite and norite. Interstitial to blebby pyrite, pyrrhotite, chalcopyrite and pentlandite characterize the sulphide mineralization. Assay results are shown below: Reference Cu (%) Ni (%) Co (ppm) Pd (ppb) Pt (ppb) Geul (1970) 0.23 .018 n/a Trace n/a Resident Geologist Files, OGS 1.76 0.72 190 n/a 610 Tanton (1935) 0.62 0.34 n/a n/a 5140 ILSG07 69 Trip 3 �STOP 3-2: Rita Bolduc Occurrence (Pigeon River Dyke) Location: Highway 61, 4.4 km north of the junction between Highways 593 and 61; (UTM 310563E / 5324701N; NAD83) Description: This highway roadcut (Figure 3-9) exposes the southern contact and an almost complete cross-section of a sulphide-mineralized Pigeon River dyke. The property was explored with a self potential survey and trenching in 1957 (Geul 1973). It was part of a property that was acquired and explored by Falconbridge Limited and McVicar Minerals. The main sulphide showing is situated along the south edge of a Pigeon River olivine diabase dike, carrying disseminated to massive coppernickel-sulphide mineralization in a fracture zone, up to 35 feet in width and extending for a distance of at least 100 feet along the dike (Geul 1973). A grab sample of massive to disseminated pentlandite, pyrrhotite with minor chalcopyrite returned 0.70% Cu and 0.35% Ni and trace amounts of Pd (ibid). Net-textured pyrrhotite and chalcopyrite blebs occur locally. Grab samples collected from sulphide-mineralized sections of the dyke on the west side of the highway yielded the following results: Sample 01-BRB-01 01-BRB-02 01-BRB-03 Au (ppb) Cu (ppm) Ni (ppm) Pd (ppb) Pt (ppb) 1442 707 28.03 21.84 14.29 880 505 17.55 13.90 25.02 6517 2664 23.16 n.d. (Resident Geologist's Files, Thunder Bay South District, Thunder Bay) Hornfelsed, rusty weathering, Rove Formation sedimentary rocks are exposed along the southern dyke margin. They are fissile and display an orthogonal joint pattern. Beds have been up-folded along the contact, suggesting either dip-slip fault motion and/or high magma flow rates. Similar “up-drag” features were noted on the southern contact of the Arrow River dyke at Arrow Rapids by Geul (1970), also in association with a copper-nickel sulphide-mineralized fault/fracture zone. Narrow, parallel tension gashed filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstone; infilled fractures may produce leucosome dykes (Figure 3-8). They are oriented parallel to the dyke contact and occur several metres away from the contact. Rare, small (< 1 cm diameter), round chloritic patches may represent incipient “spotted” alteration, similar to that described by Franklin (1970). Geochronologic sampling by Heaman and Easton (2006) at this location showed that the least magnetic fraction contained a large amount of colourless euhedral apatite, pyrite and a small amount of baddeleyite and zircon. A very small amount of tiny tan baddeleyite blades and fragments was recovered and a number of composite baddeleyite/zircon grains were identified. Although not that common, primary igneous baddeleyite overgrown by magmatic zircon has been reported previously and is consistent with the phase relationship for crystallization of these two minerals in a mafic magma where the silica activity is progressively increasing throughout crystallization. The four zircon analyses have varied uranium and thorium contents (163–1104 ppm and 34–237 ppm, respectively), but have high Th/U (1.17–1.81), consistent with primary magmatic zircon crystallizing from a mafic magma.The four zircon analyses are discordant (8.7–11.7%) and have 207Pb/206Pb ages that vary between 1050 and 1090 Ma. The baddeleyite fraction (#5) is least discordant (3.8%) and contains relatively low uranium (213 ppm) for baddeleyite. The 1133.8 Ma 207Pb/206Pb age obtained for this baddeleyite fraction is interpreted as a minimum estimate for the age of this dike. A Model 1 regression treatment of three zircon (#1, #2, #3) and the baddeleyite (#5) yields and upper intercept age of 1141±20 Ma, which is interpreted as the best estimate for the emplacement of the Pigeon River dike (Heaman and Easton 2006). ILSG07 70 Trip 3 �Figure 3-9: Sketch map of the road cut exposing a Pigeon River dyke on the eastern side of Highway 61 at Stop 3-2. Geology modified after Peck and Rosatelli (personal communication, 2001) in Parker et al.(2001). Geochemical sample locations are shown as numbered black dots. STOP (OPTIONAL): Mount Mollie Lookout Location: Approximately 300 m north along old road bed, 3.1 km east of Highway 61 on Memory Road. The junction of Memory Road and Highway 61 is 6.9 km north of the junction of Highways 593 and 61, just north of the Pine River bridge (UTM 313843E / 5325444N; NAD83) Please be advised that this stop is located on private land. Permission must be granted by the land owner. Contact the Resident Geologist’s Office in Thunder Bay for further information. Description: This stop, atop a prominent northeast-trending ridge, affords a panoramic view of Pine Bay, Lake Superior and the surrounding, ridge-dominated topography. This and other nearby ridges are underlain by ophitic Pigeon River diabase dykes. Pigeon Point, Minnesota (type locality for pigeonite) can be seen on the horizon to the south. Northwest-trending ridges, underlain by Arrow River dykes, extend inland from south of Big Trout Bay, just north of this location. A long (>1 km) and challenging trail extends north and east from this vantage point to copper-nickel-mineralized occurrences in the Pine River-Mount Mollie dyke (ca. UTM 314748E / 5325640N; NAD27), which is approximately 60 m wide at that location. Heslop (1968) noted blebs of pyrrhotite-pentlanditechalcopyrite and magnetite-ilmenite intergrowths at this occurrence. Supergene(?) native copper has also been noted there. ILSG07 71 Trip 3 �STOP 3-3: Rove Formation Quarry, Great Lakes Nickel Limited Road Location: 4.7 km west of Highway 61 on the unmarked Great Lakes Nickel Road (private access). The GLNR extends west from Highway 61 at a point just south of the Pine River bridge, 6.6 km north of the junction of Highways 61 and 593 (UTM 306401E, 5328687N; NAD83). Description: A dormant shale quarry on the south side of the Great Lakes Nickel Limited access road provides a representative section of nearly flat-lying Rove Formation turbidites, diabase dykes and a sill. The quarry face is capped by a 1.1 m thick wacke bed that is underlain by a 1.1 m thick, conformable diabase sill in the main part of the quarry. Approximately 12 m of thinly to thickly bedded wacke and shale are exposed below the sill. The wackes are typically massive, and display sharp, planar contacts with the recessively weathered shale units. Scours of underlying shale are locally developed. Some of the shale interbeds display sediment loading features and distinctive sole features and are locally flaser-bedded. Another, 22 cm wide diabase dyke intrudes the sedimentary rocks east of the sill/dyke exposure, strikes ~060º and dips ~80º east. The columnar-jointed diabase sill is fine- to medium-grained and has conspicuous, grey chilled margins. It pinches out on the eastern end of the quarry face where it apparently “steps down” through the sedimentary rocks and merges with a near-vertically dipping diabase (feeder?) dyke. This dyke is weakly magnetic, strikes 030º and has an exposed thickness of approximately 4m. A sample collected near the chilled margin returned the following analysis: Sample Al2O3 CaO Fe2O3 K2O LOI MgO MnO Na2O P2O5 SiO2 TiO2 TOTAL 01-BGL-01 14.51 9.15 13.92 0.78 1.08 5.31 0.2 2.69 0.15 51.78 1.52 101.1 (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay) In contrast a sample of the upper sill was considerably more silica rich (GL-1; SiO2 = 72 wt%) and is characterized by elevated La/Smn ratios (3.2) and Th contents (7 ppm vs. <1.3 in other dykes and sills). This likely reflects localized assimilation of Rove wacke by the sill during emplacement. A thin veneer of the hornfelsed sedimentary host rocks is locally preserved against the chilled dyke margin. The diabase is plagioclase-phyric and contains small sub-rounded shale xenoliths. A tendril of diabase extends west from the dyke (below the sill) and forms a bulbous, pillow-like termination with radial cooling fractures. The adjacent shale is chaotically folded and crenulated, suggesting that shearing accompanied intrusion. Two metres further west, an isolated ovoid (45 cm by 75 cm) “blob” of diabase intrudes the shale. Similar fingering termination features were described at the periphery of sills by Pollard et al. (1975). At finger terminations, the host rock is wedged aside and compacted and along the edges of the fingers, buckling and shearing of strata are common. Pollard et al. (1975) ascribed finger formation to the instability of the advancing interface between a viscous magma and a more viscous host rock. Antonellini and Cambray (1992) also studied a possible structural control on local sill emplacement. A block of wacke was selected at this location for a detrital zircon study (Heaman and Easton 2006). The detrital zircon ages form two distinct age clusters, with the majority of grains having 207 Pb/206Pb ages in the 1780–1880 Ma range. A second population consists of Archean grains, most of which are in the 2600–2700 Ma range. The oldest grain has a 207Pb/206Pb age of 2914 Ma. The youngest grain, with a 206Pb/238U age of 1731 Ma places a maximum age on the depositional age of this wacke. This grain is 5% discordant, consequently, a better estimate for a maximum depositional age is provided by a 1777 Ma concordant detrital zircon grain. This interpretation is supported by the abundance of grains (n=10) between 1796 and 1777 Ma, and is consistent with U/Pb ash bed ages of 1836±5 and 1832±3 Ma from the basal Rove Formation (Addison et al. 2005). The section exposed in the quarry walls represents a portion of the middle of the almost kilometer-thick Rove Formation (P. Fralick, Lakehead University, personal communication, 2007). The lower, underlying Rove is dominated by approximately 120 m of carbonaceous shale. This gradationally coarsens and thickens upwards through 100 m by the addition of thin sandstone ILSG07 72 Trip 3 �turbidites, culminating in beds similar to the ones exposed in the quarry. Proceeding upwards, above the rocks exposed in this section, ripples become more abundant in the sandstones until the assemblage is dominated by lenticular to flaser-bedded, wave- and current-rippled units. The Rove Formation represents a period of sediment starvation followed by progradation of a prodeltaic turbidite apron and distal delta top distributary mouth bar complex. Sections in cored drillholes, equivalent to the exposed strata here, are composed of parasequences that coarsen and thicken upwards over meters to a few tens of meters by the addition of thicker sandstone beds. These represent prograding turbidite lobes that are rapidly abandoned as the subaqueous feeder channel switches position. A period of sediment starvation follows with mud accumulation. This is ended by the gradational outbuilding of another lobe as the feeder channel moves closer to this area again. The thicker shales overlying the sand-rich tops of the parasequences are carbonaceous and pyritic, compared to the thinner shales forming the tops of individual turbidites. This denotes that the clays forming the thicker shales separating lobe outbuilding periods were deposited slowly allowing the remains of microbes to become common in the sediment. During diagenesis the microbes degraded, forming strongly reducing conditions that led to formation of diagenetic pyrite in the sediment. In contrast, the more rapid deposition of clay between turbidity current events during lobe outbuilding produced lower concentrations of carbon and much less pyrite. The difference between the two types of shale can easily be seen due to staining of the pyrite-rich rock during oxidation. Complete Bouma Sequences are rarely exhibited by the turbidites as the rippled C-division is commonly not present. However, flute marks produced by scouring of the viscous turbidity current into the muddy bottom and subsequent filling with sand, are ubiquitous (Figure 3-12). Commonly, in other turbiditic formations, flute marks are randomly developed on the bedding surface, but here they are in places arranged geometrically in diagonally offset rows. They also take on unusual elongate shapes, more akin to groove marks produced by something dragging across the bottom. The cause of the abnormal patterns and shapes is not known. Figure 3-10: View, looking south, of the eastern end of the dormant quarry in Rove Formation sedimentary rocks on the Great Lakes Nickel road, Stop 3-3. A diabase feeder dyke (left-center db) branches out into two sills: a main one, near the top of the quarry face (top-center db) and midway up the face, where it fingers and terminates (white box corresponds to Figure 3-11). The white dashed line separates oxidized, pyritic shale below and relatively pyrite-free shale above. ILSG07 73 Trip 3 �Figure 3-11: Diabase (db) “finger and blob” in Rove Formation wacke, Stop 33. The diabase was injected from a feeder dyke to the left of the photograph. A propagation fracture, with attendant folding and shearing, is denoted by the dashed line. Figure 3-12: Casts of flute marks on the bottom of a sandstone layer, Stop 3-3. Some have the classic “V” shape with their deepest area at the apex of the V. The V opens in the down-current direction. Others are drawn out and appear to have migrated down-current as they cut, producing a groove rather than a V. ILSG07 74 Trip 3 �STOP 3-4: Crystal Lake Gabbro / Great Lakes Nickel Property Location: 7.5 km west of Highway 61 on the unmarked Great Lakes Nickel Road (private access). The GLNR extends west from Highway 61 at a point just south of the Pine River bridge, 6.6 km north of the junction of Highways 61 and 593 (UTM 303612E, 5328392N; NAD83). Description: A switch-back road extends from the base of a 140 m high ridge to the lower adit, excavated at the contact between Rove Formation country rocks and the base of the Crystal Lake gabbro. An overgrown trail continues upward to the upper adit in “taxitic”, varied-texture gabbro and talus boulders. The upper portions of the intrusion, including layered rocks, are visible from this vantage point but are inaccessible for safety reasons. A stockpile of mineralized gabbro is available for sampling on the road at the base of the ridge. A second dump, further up the road, consists mainly of Rove Formation shales and argillites with abundant zeolite and carbonate veins. The discovery of copper- and nickel-mineralized float boulders in this area early in this century led to a concerted exploration effort to find their source. United States Smelting, Refining and Mining Company conducted exploration in 1936, and were followed by Frobisher Exploration Company in 1942. In the summer of 1952, J.S. Brodie and T.W. Page examined a large outcrop of gabbroic rocks, 6 km northeast of the original float discovery. Prospecting soon indicated a Cu-NiPt-mineralized zone and the ground was staked for Mattawin Gold Mines. Due to lack of operating capital, the Mattawin property was optioned to Falconbridge Nickel Mines in late 1952. A trench was subsequently excavated to test the lower gabbro contact. Although trench samples returned anomalous copper and nickel values, they did not warrant further investigation and the option was allowed to lapse. Additional work by J.S. Brodie drew attention to the area north of the trench. The property was optioned to R. Barker and W. Dawidowich in 1954. Six diamond drill holes, totalling 1058 m, were completed. One 55-foot section returned 0.54% Cu and 0.18% Ni with some precious metal values. Mogul Mining Corporation Limited held the property from 1954 to 1957 and drilled seven holes, totalling 1693 m. Intersections of the mineralized zone averaged 9 to 12m and assayed 0.9% combined Cu and Ni. Late in 1964, Great Lakes Nickel Corporation Limited acquired the property and initiated their exploration program in June, 1965. From 1965 to 1970, 47 803 m of surface drilling was completed; 19 underground holes, totalling 392 m, were also drilled. Underground drilling was conducted from a newly constructed, 37 m long adit, driven into the base of the hillside. In addition, Thunder Bay Nickel Mining Corporation Limited drilled 16 holes, totalling 13 579 m, on the down-plunge extension of the deposit, 2.5 km east of the main workings. In 1972, access and development work was undertaken by Great Lakes Nickel to further test the deposit. This work included the excavation and driving of a 522 m development portal and drift and over 12 000 m of surface and underground drilling. Plant site surveys, bulk sampling, metallurgical and feasibility tests were also conducted, financed largely by the Swedish company, Boliden Aktiebolag. By 1974, plans were made to mine the deposit at an initial rate of 1.8 million tons per year (subsequently increased to 2.5 million tons per year). Up to that point, about $10 M (Canadian) had been spent on the property on 58 689 m of surface drilling, 26 182 m of underground drilling and the driving of the adit, which eventually reached a length of 1041 m. This work had outlined a deposit of 32.8 million tons grading 0.36% Cu and 0.20% Ni, with a further potential reserve of 40 million tons of about the same grade. However, cost escalations, high interest rates and uncertain metal prices forced suspension of mine development in October, 1974. Rising interest in platinum-group elements in the mid-1980’s prompted Fleck Resources Ltd. to re-evaluate the deposit whose reserves then stood at 45.6 million tons at a grade of 0.334% Cu and 0.183% Ni. Between September, 1986 and February, 1987, Fleck completed geological mapping and sampling, as well as the re-logging and assaying of more than 9144 m of drill core. Six holes were drilled to test the deposit for its PGE potential. Sampling by Fleck returned the following assays on a 3.7 million ton portion of the deposit: 0.006 oz./ton Pt, ILSG07 75 Trip 3 �0.57% Cu, 0.027 oz./ton Pd, 0.264% Ni, 0.003oz./ton Au, 0.016% Co, 0.04 oz./ton Ag. Based on February, 1987 metal prices, this material was valued at $34.29 (Can.) per ton (Fleck Resources Ltd., Annual Report, 1987). Great Lakes Nickel has undertaken a comprehensive, due-diligence review of the property (Hubacheck 2001) in order to generate a valuation opinion and summary for potential investors or joint-venture partners. The geology of the Crystal Lake gabbro has been described by Geul (1970), Reeve (1969), MacRae and Reeve (1968), Whittaker (1986) and Smith and Sutcliffe (1987, 1989). It intrudes Rove Formation argillaceous and arenaceous rocks. The intrusion was subdivided into three major zones by Reeve (1969) and Geul (1970) and has been further subdivided by Smith and Sutcliffe (1987, 1989) and Cogulu (1990) into four major, roughly equivalent, lithologic zones: (1) an upper zone (60 to 80 m) of sulphide-barren, troctolite, olivine gabbro and anorthositic gabbro; (2) a middle zone (30 to 42 m) of cyclic, layered anorthositic and olivine gabbro, Cr-spinel- bearing anorthosite and olivine gabbro (Figure 3-4); (3) a lower, unlayered zone (50 m) of vari-textured gabbro and leucotroctolite, which hosts the bulk of the Cu-Ni-sulphide deposit; and (4) a basal zone (1 to 7 m) of fine-grained, chilled melagabbro and hornfelsed country rock xenoliths. Orthocumulate rocks predominate in the Crystal Lake intrusion; adcumulates occur only in the cyclic, layered rocks, in association with Cr-spinel mineralization (Cogulu 1990). Plagioclase, with lesser olivine and Cr-spinel (Figure 3-3), is the main cumulus mineral, while clinopyroxene, magnetite and Cu-Ni-sulphides are intercumulus (Cogulu 1990; Smith and Sutcliffe 1987, 1989). Total REE, LREE/HREE ratios, and sulphide content decrease from the basal zone to the cyclic sequence, while the modal proportion of olivine increases (ibid). Smith and Sutcliffe (1987, 1989) have suggested the following crystallization sequence: Cr-spinel → olivine → plagioclase → clinopyroxene → magnetite → apatite. Biotite is epitaxial to olivine and augite, and it may also rim intercumulus sulfides and oxides (Cogulu 1990). Sulphides are disseminated, interstitial and included grains and droplets. Pyrrhotite, chalcopyrite, cubanite and pentlandite are the main sulphides. Accessory minerals include violarite, troilite, niccolite, maucherite, native bismuth, mackinawite, bornite, millerite, nickeloan pyrite, sphalerite and marcasite (Cogulu 1993a). Adcumulate and orthocumulate Cr-spinels have been subdivided by Cogulu (1993b) into compositionally and texturally distinct groups with complex re-equilibration histories. Heterogeneity between Cr-spinels was also noted by Whittaker (1986). Local sulphur sources exist in local sedimentary rocks, as well as in the Archean basement. Data from the Duluth Complex indicated similar sulphur-isotopic signatures exist between basement sedimentary rocks and Ni-Cu-sulphides (Mainwaring and Naldrett 1974). Similar S-isotopic signatures also exist between Crystal Lake gabbro and Rove Formation shales (J. Franklin, personal communication, 1999.) Franklin (1970; personal communication, 2001) has also noted contact effects in Rove shales below diabase sills which include the conversion of pyrite to pyrrhotite, the loss of carbon in the spotted contact aureole and the rearrangement of clay mineral structures. Eckstrand and Cogulu (1986) presented Se/S data which supported the contention that sulphur in the Ni-Cu-sulphide minerals at Crystal Lake was derived from the barren, Se-poor, sedimentary sulphides of the Rove Formation. Sulphide concentrates from the Rove Formation and the lower parts of the intrusion (which host the sulphide deposit) yielded lower Se/S ratios than those gleaned from the Crspinel-bearing, cyclic layered zone whose Se/S ratios correspond closely to chondritic values (~200 to 600 x 10-6), implying that the sulphur was likely mantle-derived. ILSG07 76 Trip 3 �Stratigraphic Unit Cyclic Layered Zone Lower Unlayered Zone Basal Contact Zone Rove Formation (Footwall) (Se/S) x 10-6 213 - 687 70 - 151 28 - 114 2 – 118 (Eckstrand and Cogulu 1986) A grab sample of non-magnetic, massive subhedral pyrite with minor, interstitial, fine-grained silicates taken from the lower adit area, near the basal contact of the Crystal Lake Gabbro, returned 116 ppb Au, 688 ppb Pd, 14 ppb Pt, 13494 ppm Cu, 4756 ppm Ni and 255 ppm Zn (Resident Geologist's Files, Thunder Bay). ACKNOWLEDGEMENTS The trip leaders wish to thank the following individuals and companies for their assistance in the preparation of this field trip guide book and the field trip. Phil Fralick (Lakehead University) contributed sections on the Animikie Group and wrote the Rove Formation description for Stop 3-3. John Scott (Resident Geologist’s Program, Ontario Geological Survey) provided logistical support. The authors have benefited from discussions with Tom Hart and Mike Easton (Precambrian Geoscience Section, OGS), Larry Heaman (University of Alberta) and Jim Miller (Minnesota Geological Survey / University of Minnesota Duluth). The ongoing cooperation of Great Lakes Nickel Limited and other exploration companies is also appreciated. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology, v.33, p.193-196. Amurawaiye, O. 2001. The Paleoproterozoic Rove Formation of northwestern Ontario: A turbidite-dominated shelf sequence; unpublished H.B.Sc thesis, Lakehead University, Thunder Bay, Ontario, 44p. Antonellini, M.A. and Cambray, F.W. 1992. Relations between sill intrusions and bedding-parallel extensional shear zones in the Mid-continent Rift of the Lake Superior region: Tectonophysics, v. 212, p. 331-349. Beskar, S. 2001. The Blake gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario; 47th Institute on Lake Superior Geology, Annual Meeting, Madison, Wisconsin, May 9-12, 2001, Proceedings Volume 47, Part 1, p.1 Cogulu, E.H. 1990. Mineralogical and petrological studies of the Crystal Lake intrusion, Thunder Bay, Ontario: Geological Survey of Canada, Open File 2277, 15 p. plus figures and tables. -----. 1993a. Mineralogy and chemical variations of sulphides from the Crystal Lake intrusion, Thunder Bay, Ontario; Geological Survey of Canada, Open File 2749, 21 p. plus figures and tables. -----. 1993b. Factors controlling postcumulus compositional changes of chrome-spinels in the Crystal Lake intrusion, Thunder Bay, Ontario; Geological Survey of Canada, Open File 2748, 28 p. plus figures and tables. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 1572-1579. ILSG07 77 Trip 3 �Eckstrand, O.R. and Cogulu, E. 1986. Se/S evidence relating to genesis of sulphides in the Crystal Lake gabbro, Thunder Bay Ontario; Geological Association of Canada-Mineralogical Association of Canada-Canadian Geophysical Union, Joint Annual Meeting, Ottawa, Ontario, Program with Abstracts, p.66. Fralick, P.W. and Miall, A.D., 1989. Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake, Ontario, Canada. Sedimentary Geology, v. 63, p. 127-153. Franklin, J.M. 1970. Metallogeny of the Proterozoic rocks of the Thunder Bay District, Ontario; unpublished Ph.D. thesis, University of Western Ontario, London, 317p. Franklin, J.M. and Kustra, C.R. 1972. The Proterozoic rocks of the Lake Superior area, northwestern Ontario; in Field Excursion C34: The Precambrian rocks of the Atikokan-Thunder Bay-Marathon area, 24th International Geological Congress, Guidebook, p.20-46. Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D. 1986. Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, v.23, p.1576-1591. Geul, J.J.C. 1970. Geology of Devon and Pardee Townships and the Stuart Location; Ontario Department of Mines, Geological Report 87, 52 p. -----. 1973. Geology of Crooks Township, Jarvis and Prince Locations, and Offshore Islands, District of Thunder Bay; Ontario Department of Mines, Geological Report 102, 46 p. Green, J.C. 1986. Lithogeochemistry of Keweenawan igneous rocks; Minnesota Department of Natural Resources, Division of Minerals, St. Paul, Project 241-4, 94p. Green, J.C., Bornhorst, T.J., Chandler, V.W. et al. 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America; in Mafic dike swarms, H.C. halls and W.F. Fahrig, eds., Geological Association of Canada, Special Paper 34, p.289-302. Hart, T.R. 2002. Proterozoic volcanic and intrusive whole rock geochemical data associated with the Keweenawan Midcontinent Rift, Lake Superior area, Ontario; Ontario Geological Survey, Miscellaneous Release of Data, MRD 114. -----. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; 49th Institute on Lake Superior Geology, Proceedings volume 49, Part 1, Programs and abstracts, p.21-22. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. -----. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, in press. Heaman, L.M. and Machado, N. 1987. Isotope geochemistry of the Coldwell alkaline complex: 1. U-Pb studies on accessory minerals; Geological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Saskatoon, Saskatchewan, Program with abstracts, p.54. Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evidence for the provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. Heslop, J.B. 1968. Mineralogy and textural relationships of the Mount Mollie sulphides, Pine Bay area, Thunder Bay District, Ontario. Unpublished B.Sc. thesis, University of Western Ontario, London, Ontario. Hoffman, P.F., 1987. Early Proterozoic foredeeps, foredeep magmatism and Superior-type iron-formations of the Canadian shield. In, ed. Kroner, A., Proterozoic Lithospheric Evolution, American Geophysical Union Series, v. 17, p. 85-98. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment. Canadian Journal of Earth Sciences, in press. ILSG07 78 Trip 3 �Hubacheck, P. 2001. Great Lakes Nickel copper-nickel-PGE project: Executive summary; unpublished report, Resident Geologist's Files, Thunder Bay, 7p. Jones, N.W. 1984. Petrology of some Logan diabase sills, Cook County, Minnesota; Minnesota Geological Survey, Report of Investigations 29, 40p. Lightfoot, P.C. and Lavigne, Jr., M.J. 1995. Nickel, copper, and platinum group element mineralization in Keweenawan intrusive rocks: new targets in the Keweenawan of the Thunder Bay region, northwestern Ontario; Ontario Geological Survey, Open File Report 5928, 32p. Lucente, M.E. and Morey, G.B., 1983. Stratigraphy and sedimentology of the lower Proterozoic Verginia Formation, northern Minnesota. Minnesota Geological Survey Report of Investigations 28, 28 p. MacRae, N.D. and Reeve, E.J. 1968. Differentiation sequence of the Great Lakes Nickel intrusion: 14th annual Institute on Lake Superior Geology, Superior, Wisconsin, Program with Abstracts, p.28. Mainwaring, P.R. and Naldrett, A.J. 1974. Genesis of Cu-Ni sulfides in the Duluth Complex; in Society of Economic Geologists, Annual Meeting, Economic Geology, v.69, no.7, p.1183-1184. Maric, M. and Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, v. 51, p. 41-42. McCuaig, J.A. 1950. A copper-nickel occurrence in Pardee Township, Thunder Bay District; unpublished M.Sc. thesis, McGill University, Montreal. Quebec, Miller, J.D. and Smyk, M.C. 1995. Gabbroic intrusions of the International Boundary area; in Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior region; International Geological Correlation Program Project 336, Minnesota Geological Survey, Guidebook 20, p.171-181. Miller, J.D., Smyk, M.C., Severson, M.J., Lavigne, M.J. and Middleton, R.S. 2002. PGE occurrences in mafic intrusions around western Lake Superior, USA and Canada; 9th International Platinum Symposium, Field Trip Guidebook, 135p. Morey, G.B. 1973. Stratigraphic framework of middle Proterozoic rocks in Minnesota. In, ed. G.M. Young, Huronian Stratigraphy and Sedimentation. Geological Association of Canada Special Paper 12, p. 211-249. North, J. 2000. Nature and distribution of Logan diabase sills and gabbro channels in the Keweenawan rift near Thunder Bay, Ontario: Brief comparison to Noril'sk; Abstract, 46th Institute on Lake Superior Geology, Annual Meeting, Thunder Bay, Ontario, Proceedings Volume 46, Part 1 (2000). Oja, R. 1967. Geochemical investigations of the Thunder Bay silver area; in Proceedings, Symposium on Geochemical Prospecting, Ottawa, 1966, Geological Survey of Canada, Paper 66-54, p.211-221. Ojakangas, R.W., Morey, G.B. and Southwick, D.L., 2001. Palaeoproterozoic basin development and sedimentation in the Lake Superior region, North America; Sedimentary Geology, v. 141, p. 319-341. Parker, D.P. (ed.), Middleton, B., Schnieders, B.R., Smyk, M.C. and Scott, J.F. 2001. Intrusions of the Nipigon Basin; Superior PGE 2001, Canadian Institute of Mining and Metallurgy, Geological Society Field Conference, Thunder Bay, September 16-19, 2001, Field Trip Guidebook, 43p. Pollard, D.D., Muller, O.H. and Dockstader, D.R. 1975. The form and growth of fingered sheet intrusions; Geological Society of America Bulletin, v.86, p.351-363. Pufahl, P.K. and Fralick, P.W., 1995. Paleogeographic reconstruction of the Gunflint-Mesabi-Cuyuna depositional system: a basin analysis approach; 41st Institute on Lake Superior Geology, v. 41, Proceedings with abstracts, p.59-60. Pufahl, P. and Fralick, P. 2000. Depositional environments of the Paleoproterozoic Gunflint Formation; 46th Institute on Lake Superior Geology, v.46, pt.2, Proceedings with abstracts. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA.. Sedimentology, v. 51, p. 791-808. Pufahl, P.K., Fralick, P.W. and Scott, J., 2000. Depositional environments of the Palaeoproterozoic Gunflint Formation; 46th Institute on Lake Superior Geology, v.46, pt.2, Field Trip Guide, 46 p. ILSG07 79 Trip 3 �Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: Evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, v.33, p.958-965. Reeve, E.J. 1969. Petrology and mineralogy of a gabbroic intrusion in Pardee Township near Port Arthur, Ontario: unpublished M.Sc. thesis, University of Wisconsin, Milwaukee, 79 p. Rosatelli, M.P., 2002. Assessment report on the 2002 lithogeochemical rock sampling program, Pigeon River block. McVicar Minerals Ltd., BHP Billiton World Exploration Inc.. and Falconbridge Limited; Assessment Files, Thunder Bay South District, Thunder Bay, FN 2.24485, 36p. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks of the Thunder Bay area: in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248-255. -----. 1989. Precambrian geology of Keweenawan intrusive rocks in the Crystal Lake-Pigeon River area: Ontario Geological Survey, Map P.3139, scale 1:50 000. Smyk, M.C., Hollings P. and Heaman, L.M. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, p.6162. Southwick, D.L. and Morey, G.B., 1991. Tectonic imbrication and foredeep development in the Penokean Orogen, east-central Minnesota - an interpretation based on regional geophysics and the results of test-drilling. United States Geological Survey Bulletin 1904-C, 17 p. Stockwell, C.H., McGlynn, J.C., Emslie, R F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F. and Currie K L. 1972. Geology of the Canadian Shield, in Geology and Economic Minerals of Canada, edited by R.J.W. Douglas, Geological Survey of Canada, Economic Geology Report 1, 838 p. Sun, S.S. and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes; in Saunders, A.D. and Norry, M.J. (Eds.), Magmatism in ocean basins, Geological Society of London Special Publication 42, p.313-345. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton, T.L. 1935. Copper-nickel mineral occurrences in Pigeon River area, Ontario; Geological Survey of Canada, Paper 35-1, 11p. -----. 1936a. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63 360. -----. 1936b. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 2, Map 355A, scale 1:63 360. -----. 1937. Copper-nickel occurrences in Pigeon River area, Ontario. The Precambrian, May, 1937, p.18-19. Van Wyck, N. and Johnson, C.M. 1997. Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture and tectonic setting of the Penokean Orogen (Paleoproterozoic) in Wisconsin, U.S.A. Geological Society of America Bulletin, v. 109, p. 799-808. Weiblen, P.W., Mathez, E.A. and Morey, G.B. 1972. Logan intrusions; in Sims, P.K. and Morey, G.B. eds., Geology of Minnesota: A centennial volume; Minnesota Geological Survey, p.394-410. Whittaker, P.J. 1986. Chromite deposits in Ontario: Ontario Geological Survey, Study 55, 97 p. ILSG07 80 Trip 3 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 4 GEOLOGY OF THE NICKEL LAKE MACRODIKE AND ITS ASSOCIATION WITH CU-NI-PGE MINERALIZATION IN THE NORTHERN SOUTH KAWISHIWI INTRUSION, DULUTH COMPLEX, NORTHEASTERN MINNESOTA Dean M. Peterson University of Minnesota Duluth, Natural Resources Research Institute Paul B. Albers Duluth Metals Limited, Ely, Minnesota “Imagination is more important than knowledge” Albert Einstein INTRODUCTION Mineral deposits are concentrations of specific ore-minerals that society utilizes in immeasurable ways. They have formed from Archean times up to the present, and vary greatly in commodity, mineralogy, alteration, trace element signature, geophysical properties, and grade-tonnage. Individual ore deposits are always unique, and this uniqueness arises from two main sources: 1) fundamental differences in geologic processes and environments; and 2) local, site-specific, geologic variations and associated bounding geometries. The seemingly limitless number of permutations of these (and other) features of mineral deposits defies imagination. Geologists employed in the search for ore deposits have, over the last century, developed the intellectual concept of Ore Deposit Models, which seek to organize many of ILSG07 81 Trip 4 �these variables, lump individual mineral deposits into classes, and establish criteria to aid in mineral exploration (Peterson, 2001a). Such models may be strictly empirical – a collection of observable facts associated with the occurrence of certain metals in economic proportions – or genetic – which attempts to describe the physical and chemical processes responsible for the development of an ore deposit and its related empirical features. Advances in our collective understanding of ore deposit types is nonlinear, rather it occurs in fits and starts. Take, for example, the great advance in knowledge – and thus mineral exploration models and exploration success – for porphyry copper deposits in the 1960s and 1970s (Lowell and Guilbert, 1970), volcanogenic massive sulfide deposits in the 1970s and 1980s (Franklin et al., 1981), and mesothermal lode gold deposits in the 1980s and 1990s (Hodgson, 1993). These advances were the product of many factors, which conceivably the most important was collaborative ore deposit research between the mineral industry, academia, government agencies, and research organizations. We are currently in the midst of a similar great advance in knowledge of magmatic Ni-Cu-PGE deposits (Arndt et al., 2006; Barnes and Lightfoot, 2006; Eckstrand and Hulbert, in prep.). Perhaps the most important impetus for this current revolution was the fall of the Iron Curtain, which opened the door in the early 1990s to some of the world’s greatest magmatic Ni-Cu-PGE deposits, i.e., the Noril’sk-Talnakh deposits in Russia and Jinchuan deposit in China, to economic geologists trained in open western societies (especially Canada). In addition, the 1993 discovery of the magmatic Ni-Cu deposit at Voisey’s Bay, Labrador, Canada, and its inferred origin as a magmatic feeder dike, has brought about a revolution in our collective understanding of the origin of magmatic Ni-Cu±PGE deposits. This revolution can simple be stated as “find the magmatic feeder dike and/or channelized magma flow zones” to the orebearing mafic/ultramafic intrusion(s). A magmatic conduit that experienced repeated influxes of magma appears to be the key to the formation of high-grade, world-class, Ni-Cu-PGE deposits (Naldrett, 1997). This field trip is truly the result of over two decades of dedicated Duluth Complex research, almost entirely funded by the State of Minnesota and the University of Minnesota, by geologists of the University of Minnesota Duluth’s (UMD), Natural Resources Research Institute (NRRI). Steve Hauck, director of the NRRI’s Economic Geology Group (EGG), is here thanked for his decades of dedicated research and seeing through that the EGG remains financially solvent through a never-ending series of budgetary crises. As well, the NRRI’s Mark Severson – who has logged well over 900,000 feet of Duluth Complex drill core – has without a doubt the more knowledge than anyone on the geology of the mineralized zones within the deposits of the Duluth Complex. The authors of this guidebook have simply built upon this intellectual capital in new ways, including geochemical and 3-D modeling of drill core data, and especially the most basic geologic endeavor, detailed field mapping. FIELD TRIP TENETS Many basic attributes of the Duluth Complex Cu-Ni-PGE sulfide deposits resemble those of deposits at Noril’sk, Russia, Jinchuan, China, and Voisey’s Bay, Canada that are associated with sulfide mineralization in intrusive feeder zones. Such attributes include shallow tholeiitic intrusions associated with plateau basalt volcanism, external sedimentary sources of sulfur, and openness to repeated magma influx and expulsion. However, the biggest difference between these world-class magmatic ore systems and the deposits of the Duluth Complex is the lack of significant nickel-rich (Ni>Cu) massive sulfide orebodies at Duluth. A critical attribute of the high-grade Noril’sk–Talnakh and Voisey’s Bay deposits, not previously positively identified in the Duluth Complex, is the location of a magmatic conduit, i.e., the feeder zone. Several geologists have previously identified possible conduits that may have fed the Partridge River (PRI) and South Kawishiwi (SKI) intrusions of the Duluth Complex. Severson and Zanko (unpub. data) suggest that the Grano fault might mark a possible feeder zone for the Local Boy ore zone at the northeast ILSG07 82 Trip 4 �end of the PRI. Thériault et al. (2000) postulated that a PRI conduit was present somewhere between the Wetlegs and Dunka Road deposits. Another possible PRI feeder zone may have been along the prolongation of the Siphon fault, which is a Paleoproterozoic growth fault (Graber, 1993) that may have been reactivated during emplacement of the Duluth Complex (Severson and Hauck, 1997). Several authors have suggested the presence of a sub-vertical magmatic feeder beneath the Bald Eagle intrusion (BEI) based on field relations (Weiblen and Morey, 1980) and geophysical attributes (Chandler, 1990). Peterson (2001b) interpreted the systematic variation in Cu-Ni-PGE mineralization in the Maturi deposit and its extension east to Maturi Extension, as indicative of magma input from the east-northeast via an arcing macrodike (herein first termed the Nickel Lake Macrodike (NLM)) that connects the deep-seated source of the BEI and the SKI. Subsequent mapping and research (Peterson, 2002a-f, 2006a, Peterson et al., 2004, Peterson and Hauck, 2005) has built on this model, which recently culminated with the publication of a new detailed bedrock geology map of the area (Peterson et al., 2006). If correct, this model predicts that a Voisey’s Bay-type Ni-Cu-PGE massive sulfide body may exist at depth in the area where the NLM meets the SKI, south-southeast of the Spruce Road deposit (see Figs. 4.2 and 4.3). This field trip will investigate evidence that the NLM is a feeder to the SKI, and thus is one of the principal conduits (only?) that brought Cu-Ni-PGE from the Earth’s mantle and/or lower crustal magmatic staging chambers into the Earth’s upper crust via the NLM into the SKI. This will be accomplished through scientific discussions (hopefully heated) on numerous outcrops (Day 1) as well as visualization of the geology in 3-D and displays of recently drilled cores by Duluth Metals Limited (Day 2). The fundamental tenet of this field trip is to convey to the participants the notion that science gives one the ability to imagine reality. Herein, science is geologic research of the Duluth Complex (geologic mapping, drill core logging, geochemical studies, and exploration drilling), and reality is new understanding how this magmatic system concentrated and enriched known and potential concentrations of Ni-Cu-PGE at the base of the NLM in the field trip locale as well as adjacent areas of the SKI. The field trip leaders ask the participants (and others who may use the guidebook in the future) to use their imaginations throughout the field trip (or subsequent field excursions) to think about a few basic known and possible realities of the Duluth Complex: 1) The Duluth Complex is perhaps the world’s largest untapped resource of Ni-Cu-PGE, with multibillion tons of geologic resources estimated to be worth >1 trillion dollars, (Peterson, 2006c); 2) The general geologic setting of the deposits in the Duluth Complex is similar to other world class Ni-Cu-PGE mining camps hosted by rocks in rift settings (Noril’sk-Talnakh, Jinchuan); 3) Overall, the ratio of Ni to Cu in Duluth Complex deposits average 1:3; 4) Worldwide, the ratio of Ni to Cu in similar deposits averages between 1:1 to 2:1; 5) By analogy, there seems to be an enormous mass of missing Ni-rich mineralization; 6) At depth, the Nickel Lake Macrodike may host a large percentage of this missing Ni in the SKI. MAGMATIC NI-CU-PGE ORE DEPOSIT MODEL The basic starting point to begin our quest to understand the significance (both geologic and economic) of the Nickel Lake Macrodike is a quick review of the magmatic Ni-Cu-PGE ore deposit model. There are literally hundreds of recent publications in the geological literature that deal with this important class of ore deposit, and readers interested in this topic may find that Barnes and Lightfoot (2006), Arndt et al. (2006), Eckstrand and Hulbert (in prep.), Naldrett, (1989, 1997, 1999), Naldrett et al. (2000), Li and Naldrett (1999), Lightfoot et al. (1994) and references therein are excellent reviews that describe magmatic Ni-Cu-PGE deposits in general, and major deposits in particular. ILSG07 83 Trip 4 �Magmatic Ni-Cu-PGE sulfide deposit occur as sulfide concentrations associated with a variety of mafic to ultramafic rocks in four major geological settings: 1) rifts and continental flood basalt settings (Noril’skTalnakh, Russia; Duluth Complex, Minnesota; Jinchuan, China); 2) meteorite impacts (Sudbury, Ontario, the only mining camp in this class); 3) komatiite lava flows and related intrusions (Thompson, Manitoba; Raglan, Québec; Kambalda, Australia; Pechenga, Russia); and 4) a variety of miscellaneous tholeiitic intrusions (Voisey’s Bay, Labrador; Lynn Lake, Manitoba). The ores are enriched in sulfur, iron, nickel, copper, cobalt, and the platinum group elements (Pt, Pd, Rh, Ru, Ir, and Os) and may contain minor Ag, As, Au, Bi, Hg, Pb, Sb, Se, Te, and Zn. Grade-tonnage diagrams for magmatic Ni-Cu-PGE sulfide deposits/camps are presented in Figure 4.1 which highlights the major camps/deposits listed above. Figure 4.1. Tonnage and Ni grades of magmatic Ni-Cu sulfide deposits; B. Tonnages and Cu grades of magmatic Ni-Cu sulfide deposits. Inclined contours show quantities of contained metals (tonnes) in each figure. Figure modified from Eckstrand and Hulbert, in prep. ILSG07 84 Trip 4 �The basic intrinsic geological features characteristic of a vast majority of magmatic Ni-Cu-PGE sulfide deposits include: (1) olivine-rich magmas; (2) proximity to a major crustal fault; (3) sulfide-bearing country rocks; (4) chalcophile element depletion in related intrusive or extrusive rocks; (5) field and/or geochemical evidence of interaction between the magma and the country rocks; and (6) presence of, or proximity to, a magma conduit (Naldrett, 1999). Fundamental geologic processes and constraints that together leads to the formation of these deposits include: 1) Deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magmas; 2) Chalcophile elements from the silicate melt partition into the droplets as a result of turbulent magma flow; 3) An appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals; 4) Sulfides must cluster in a restricted locality, generally due to the influence of gravity, so that the resulting metal concentration is of ore grade; 5) Massive sulfide concentrations form a high-temperature monosulfide solid solution (MSS); 6) As the MSS cools, it exsolves minerals and fractionates; a. Forms a solid cumulate mass of pyrrhotite-rich massive sulfide (enriched in Fe, Ni, Co, Ir, Ru, Rh). b. Forms a liquid residuum that is enriched in Cu, Pd, Pt, Au, and other minor elements, including As, Bi, Te, Sb (which will crystallize later into minerals as the system cools). 7) For some time, the Cu-Pd-Pt residual liquid can move and form high grade ore shoots/deposits a. i.e., footwall veins in Sudbury can be 30 wt. % Cu and multi-ounce/ton Pt + Pd. Possibly the greatest recent advance in understanding Ni-Cu-PGE sulfide deposits has been the appreciation of coherent and compelling scientific arguments that have shown that magma dynamics play a key role in the concentration and metal enrichment of sulfide minerals in these deposits (Naldrett, 1997). These arguments build on the long held notion that sulfide-rich orebodies achieve their concentrations mainly through the settling of sulfide droplets in magmas due to the effects of gravity. The new fundamental tenet has been the realization that decreases in the flow rate of magmas, principally due to the geometry and obstructions in magmatic conduits, is the major factor in settling entrained sulfide droplets and forming sulfide-rich orebodies (Eckstrand and Hulbert, in prep.). These geometries include, for example, widened parts of magmatic feeder dikes, the lowest zones of undulating basal zones of conduits, and the location where such dikes enter larger magma chambers. Geologists that are engaged in Ni-Cu-PGE exploration projects should use their imaginations to envision magma dynamics like a person fly-fishing along a stream, and search for deposits behind the large boulders and in the slow-moving pools below rapids where the big rainbow and brown trout are most likely to be found. REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 5,700 square kilometer arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, that imply intrusive roots more than 13 kilometers deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying most of northeastern Minnesota were emplaced during the development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 2,000 kilometer-long, segmented, arcuate path to ILSG07 85 Trip 4 �Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >275 kilometers in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 4.2). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Fig. 4.2). In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice between 1108 and 1098 Ma. Figure 4.2. Generalized geologic map of northeastern Minnesota. Highlighted intrusions include the Bald Eagle (BEI) and South Kawishiwi (SKI) intrusions, as well as the linking Nickel Lake Macrodike (NLM) (modified from Miller et al., 2002). The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al, 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. ILSG07 86 Trip 4 �Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semicontinuous mass of intrusions strung along the eastern and central roof zone of the complex, emplaced during an early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—A structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—A suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). This field trip will investigate rocks of the Layered Series – the SKI, NLM, and by implication the BEI – and Anorthositic series rocks in outcrops within and along the margins of the NLM. It is hoped that discussions on the outcrop (Day 1), coupled with examinations of selected sections of mineralized Duluth Metals Limited drill core from the SKI, and visualization of the geology in a 3-D presentation (Day 2) will bring new insight to the geology and mineral potential of the field trip area to the participants. Prior to these investigations, the quick descriptions of the local geology, basal contact-associated styles of CuNi-PGE mineralization, and calculated Cu-Ni grade-tonnage geologic resources within this area that follow will give the field trip participants a better appreciation of the significance of the NLM and its inferred potential for hosting great quantities of Ni-rich sulfide mineralization at depth. LOCAL GEOLOGIC SETTING Robust field and geophysical data suggest that the emplacements of the BEI and SKI may be closely linked (Weiblen and Morey, 1980). At the northern margin of the BEI, a macrodike of well-foliated troctolite (herein termed NLM) arcs northwest to southwest and merges with the middle of the northern SKI (Fig. 4.3). Green et al., (1966) mapped the macrodike as part of the SKI, but its composition is very similar to the troctolitic phase of the BEI. Peterson (2001b) proposed a model for the mineralization in the SKI whereby the initial emplacement of the intrusion was formed by sulfide-contaminated magmas that emerged from the NLM and flowed southwest between a footwall of Archean granite and a hanging wall of Anorthositic series rocks. Miller et al. (2002) present an important interpretive model for the emplacement of the BEI and SKI via a common feeder system, as well as depict the origin of sulfur saturation of basal SKI magmas (Fig. 4.4) due to contamination from Paleoproterozoic sedimentary rocks (Ripley, 1986). Peterson et al. (2006) completed detailed geological mapping along the western end of the NLM and adjacent SKI, and have defined distinct mappable units in both of these troctolitic bodies. BALD EAGLE INTRUSION The BEI is a large (4.5 to 16.5 km x 31 km) troctolitic to gabbroic body that was emplaced partially within Anorthositic series rocks, the SKI, and the Greenwood Lake Intrusion (Fig. 4.2). Weiblen (1965) mapped the well-exposed northern portion of the intrusion and showed that it consists of an outer zone of troctolite and an inner zone of olivine gabbro. In the poorly exposed southwestern portions of the intrusion, field mapping by Green et al., (1966) and Foose and Cooper (1978) showed the BEI and SKI in direct conformable contact. Steep foliation and modal layering (Weiblen, 1965; Green et al., 1966) integrated with a distinct gravity anomaly over the northern BEI imply that the northern part of this intrusion is funnel shaped and necks down to a steep feeder dike. Weiblen and Morey (1980) interpreted the limited cryptic variation (Weiblen, 1965), the steep dip of lamination and layering, and adcumulate nature of the BEI as indicative of its being an open conduit to higher intrusions and perhaps volcanic flows. ILSG07 87 Trip 4 �Figure 4.3. Simplified views of an integrated 3-D model of the BEI, NLM, and SKI of the Duluth Complex. A. plan view and B. view to the southwest. Model surfaces built from drill hole piercing points, detailed geological mapping, and interpretation of gravity and aeromagnetic data. ILSG07 88 Trip 4 �Petrologic observations and geophysical interpretations (Chandler, 1990; Chandler and Ferderer, 1989) suggest that the BEI and SKI were emplaced by successive overplating of magmas from a common feeder centered on the northern BEI and extending along the trace of the NLM that links the BEI and SKI. A model that depicts the origin of the BEI and SKI via a common dynamic feeder by Miller et al. (2002) is presented in Figure 4.4. In a related analogy, Cartwright and Møller-Hansen (2006) have shown that interconnected sill complexes transect the middle to upper crust over a vertical distance of 8-12 km offshore of Norway. The geometry of the gravity and magnetic anomalies of the BEI, as well as the overall Midcontinent Rift is very similar to the pattern of the seismic reflections profiles of active ridge systems (Vislova, 2003). In detail, the geophysical expressions of the BEI have the same shape and dimensions as the “bulls’ eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. These data suggest that the BEI could be a “frozen” dynamic magma chamber (Weiblen et al., 2005; Peterson and Hauck, 2005). The BEI has posed unresolved questions concerning its origin and magmatic significance since its discovery in 1961 (Weiblen, 1965, Weiblen and Morey, 1980, Miller et al., 2002). A number of its characteristics contrast markedly with those of the other mapped intrusions in the Midcontinent Rift: 1) it has a well-defined intrusive contact in Anorthositic series rocks around its northern perimeter; 2) there is a subtle, but recognizable metamorphic contact effect on these anorthositic gabbros; 3) a primary magmatic foliation is well defined by mineral orientation and discoid segregation of plagioclase from mafic phases; 4) foliation measurements define a steeply-dipping asymmetric funnel with the foliation paralleling the contact and grading from steep to horizontal inward; 5) the intrusion consists of two cumulus units, an outer troctolite and inner olivine gabbro; and 6) there is only minor (< a few %) intercumulus material in the cumulates, i.e., clinopyroxene and iron oxides (Weiblen et al., 2005). SOUTH KAWISHIWI INTRUSION The South Kawishiwi intrusion (SKI), together with the similar sized Partridge River intrusion (PRI) immediately to the south, are most renown for hosting the largest tonnage of Cu-Ni sulfide mineralization in the world (Naldrett, 1997). The realization that the SKI hosts vast quantities of Cu-Ni mineralization over 50 years ago has lead to the publication of numerous geologic maps, (Green et al., 1966; Bonnichsen, 1974; Foose and Cooper, 1974; Miller et al., 2001; Peterson, 2002e, f; Peterson et al., 2004; Peterson, 2006b; Peterson et al., 2006), articles (Bonnichsen et al., 1980; Weiblen and Morey, 1980; Ripley, 1986; Chandler and Ferderer, 1989; Lee and Ripley, 1996; Hauck et al., 1997; Peterson, 2001b) theses (Weiblen, 1965; Vislova, 2003; Marma, 2003), and reports (Phinney, 1969; Phinney, 1972; Listerude and Meineke, 1977, Morey and Cooper, 1977; Foose, 1984; Dahlberg, 1987; Dahlberg et al., 1989; Kuhns et al., 1990; Severson, 1994; Zanko et al., 1994; Hauck et al., 1997; Peterson, 1997; Peterson, 2001c; Miller et al., 2002; Peterson, 2002d; Patelke, 2003; Severson and Hauck, 2003). The SKI is shallow dipping (~20º to the east-southeast) sill-like intrusion dominantly composed of troctolitic cumulates that are exposed in an 8- x 32-kilometer arcuate band along the northwestern margin of the Duluth Complex (Fig. 4.2). Footwall rocks include the Paleoproterozoic Virginia Formation in the Serpentine and Dunka Pit deposits, the Paleoproterozoic Biwabik Iron Formation in the Dunka Pit and Birch Lake deposits, and the Archean Giants Range batholith from the northern Birch Lake deposit north to the Spruce Road deposit (see Fig. 4.3 for deposit locations). The presence of shallow-dipping Biwabik Iron Formation inclusions as far north as the Spruce Road deposit indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the SKI, leaving the Giants Range batholith as the dominant footwall rock type. Alternately, the Virginia and Biwabik Iron Formations may simply have been largely eroded prior to the development of the Mid Continent Rift. Also present as inclusions in the SKI are mafic volcanic hornfels (North Shore Volcanic Group), quartz sandstone hornfels (either the Puckwunge or Nopeming sandstones), and anorthosite (of ILSG07 89 Trip 4 �the Anorthosite series). Anorthositic series rocks abut the SKI on the northeast – and enclose an interpreted SKI feeder dike (the NLM) that extends farther northeast – the PRI forms the southern sidewall of the SKI, and the BEI and Anorthositic series rocks overlie the SKI to the east (Fig. 4.2). Figure 4.4. Interpretive model which depicts the emplacement and mineralization mechanisms of the South Kawishiwi and Bald Eagle intrusions via a common feeder system. A) Intrusion of plagioclase crystal mushes into volcanic rocks to create Anorthositic series rocks; B) Intrusion of the SKI below Anorthositic series rocks, with the early basal units reaching sulfur saturation via contamination from Paleoproterozoic sedimentary rocks; and C) Intrusion of the BEI above Anorthositic series rocks. Modified from figure 6.15 of Miller et al. (2002). ILSG07 90 Trip 4 �On the regional Duluth Complex map of Miller et al. (2001), the SKI is subdivided into five major map units. These are, from the base upward, 1. Heterogeneous sulfide-bearing troctolite, gabbro, and norite with localized hornfels inclusions; 2. A thick unit of subophitic to ophitic augite troctolite; 3. Discontinuous and localized layers of poikilitic leucotroctolite; 4. A thick homogeneous sequence of ophitic troctolite; and 5. A thick uppermost sequence of homogeneous troctolite that contains numerous anorthositic layers. Severson (1994) and Zanko et al. (1994) further subdivided the SKI into 17 different lithostratigraphic units that are present in over 180 drill holes over a strike length of 31 kilometers. Sulfide mineralization is confined to the BH, BAN, UW, and U3 units near the base of the intrusion, and to a lesser extent the U1, U2, and PEG units. Major marker horizons that are correlated in drill holes include three horizons with abundant cyclic ultramafic layers (U1, U2, and U3 units) and a pegmatite-bearing unit (PEG unit) that was initially recognized by Foose (1984). The understanding of the significance of a large anorthositic inclusion (Fig. 4.3), originally intersected in six deep drill holes east of the Maturi deposit, and its role in magma dynamics of the SKI has been a key feature in the development of an exploration model for Duluth Metals Limited’s Maturi Extension deposit (Peterson, 2001c). Basal SKI Mineralization Basal mineralization within the South Kawishiwi intrusion has traditionally been divided into five distinct deposits: 1) Serpentine, 2) Dunka Pit, 3) Birch Lake, 4) Maturi, and 5) Spruce Road. Recent drilling by Duluth Metals Limited has confirmed the existence on an additional economically significant deposit east of Maturi first envisioned by Peterson (2001c), named the Maturi Extension deposit (Fig. 4.3). Although the style of mineralization in all of the deposits is dominated by disseminated Cu-Ni sulfides, differences occur between the deposits in igneous stratigraphy, sulfide mineralogy, Cu-Ni and PGE grade, mineralization thickness, and contained tonnes. In addition to mineralization spatially associated with the base of the intrusion, a distinct zone of Cu and PGE-enriched mineralization occurs thousands of feet above the base of the intrusion within linear zones in the South Filson Creek deposit (Kuhns et al., 1990). Compilation and analysis of drill hole assay data by Peterson (2001, 2002a-d) has led to new understanding of two distinct styles of mineralization associated with the base of the SKI. These distinctive styles of mineralization are spatially coherent, i.e., the boundaries between them are linear (Fig. 4.5), and Peterson (2001b, c) informally termed them open and confined, which are described in more detail below. "Open" - vertically extensive (can be > 450 meters) mineralization with low - high Cu-Ni grade and low Au+PGE grades. Cu-Ni grades commonly increase towards the basal contact although mineralized zones are typically erratic in their spatial extent and grade. Restricted zones of massive sulfide locally occur at, and/or immediately below, the basal contact. The erratic pattern of mineralization in part mirrors the lithologic heterogeneity of the basal units and may reflect repeated input of small pulses of barren and sulfur-contaminated magma. Examples of this "Open" style include the Spruce Road, Serpentine, and Dunka Pit deposits. The Serpentine deposit is unique within this group as it contains significant tonnage of pyrrhotite-rich massive sulfide at the basal contact that is associated with an immediate footwall sulfide source (Zanko et al., 1994). "Confined" - vertically restricted (< 150 meters) mineralization with moderate - high Cu-Ni grades and moderate to very high (locally) Au+PGE grades. Cu-Ni grades typically are the highest near the top of the mineralized zone (upper BH into U3) and gradually decrease with depth toward the basal contact. Only limited zones of massive sulfide occurring at, and/or immediately below, the basal contact have been identified. For example, the upper portion of the mineralized zone within the Maturi deposit (which averages ~150 feet thick) commonly exhibits copper values nearing 1.0% ILSG07 91 Trip 4 �that decrease to ~0.25% at the basal contact. The spatial continuity of both the igneous stratigraphy and Cu-Ni-PGE grades of this style of mineralization point toward larger sustained inputs of magma (and/or more turbulent input of magma, thus higher fractionation of base- and preciousmetals into the sulfide fraction) than the "Open" style. Examples of the "Confined" style include the Maturi, Maturi Extension, and the Birch Lake deposits. Figure 4.5. Location map of Open and Confined styles of mineralization in the South Kawishiwi Intrusion. Recently completed (Peterson, 2002a, 2006a, b, c) and ongoing research on the distribution of Cu-NiPGE in the SKI integrates detailed geological mapping, drill hole logging, assay compilation, gradetonnage calculation, recalculation of assay data to 100% sulfide, and 3-D visualization in order to imagine the coupled magmatic dynamics and mineralization history of the area. The most profound difference in the Open and Confined styles of Cu-Ni-(PGE) mineralization is perhaps best revealed in plots of assay data that have been recalculated to 100% sulfide compositions (Kerr, 2001, Naldrett et al., 2000). Such plots give one clues to the complicated history of immiscible sulfide droplets within SKI magmas, and could be used to calculate R-factors (herein the mass transfer of chalcophile elements between immiscible sulfide and silicate liquids in magmas) of the mineralization systems. As well, discrimination of different styles of mineralization and their internal metal budgets lets one come to grips with item 3 of the previously described Ni-Cu-PGE ore deposit model, that states, “An appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals”. Geochemical plots of drill core assay data recalculated to 100% sulfide for the Open and Confined styles of mineralization are presented in Figure 4.6, and reveal profound differences in the metal budgets of the sulfide mineralization. Any genetic ore deposit model and/or mineral exploration model used to search for additional mineralization in the SKI must attempt to explain these differences in context with the overall NLM-SKI magmatic system, i.e. its timing, geometry, and magmatic plumbing. ILSG07 92 Trip 4 �Figure 4.6. Geochemical plots utilizing recalculation of assay data to 100% sulfide (Kerr, 2001) to discriminate variations of metals in sulfide minerals in the two major styles of basal contact associated mineralization in the SKI. A) Open-style PGE+Au vs. Cu plot, B) Open Style Cu/Pd ratio vs. Pd plot, C) Confined-style PGE+Au vs. Cu plot, D) Confined-style Cu/Pd ratio vs. Pd plot. Recent research in developing Cu and Ni grade maps (Peterson, 2002d) for deposits in the SKI, coupled with the realization that there are two distinct styles of basal-contact associated mineralization, has lead to the publication of inferred grade-tonnage estimates of Open and Confined styles of mineralization (Peterson, 2002c). Grade and tonnage data categorized into the Open and Confined styles of Cu-Ni mineralization within the South Kawishiwi intrusion are given in Table 4.1, which used all publicly available assay data in the year 2001. These mineral resource estimates have been calculated by the senior author (Peterson) following as much as possible the definitions and guidelines adopted by the Canadian Institute of Mining, Metallurgy, Petroleum (CIM "Standards on Mineral Resources and Reserves") in August 2000 (Postle et al.). Due to the fact that they rely on historic data from a variety of different sources, the level of data verification expected by the guidelines was not possible in all cases. Inherent uncertainties in the estimation and accuracy of these mineral resource estimates are a function of the quantity and quality of the available drill hole assay data and the quality of the methods used to determine them, which for the Spruce Road deposit are outlined in Peterson (2002d). It is hoped that the data presented so far in this field guide, once integrated together in one’s mind, leads to the conclusion that understanding the geologic history of the NLM may lead to profound advances in our understanding of Cu-Ni-PGE mineralization in the SKI. Such understanding may ultimately lead to the discovery of the “Missing Nickel” in the Duluth Complex Cu-Ni-PGE deposits, thus redefining the district as the Duluth Complex Ni-Cu-PGE deposits. ILSG07 93 Trip 4 �Table 4.1. Cumulative inferred mineral resource estimates for the Open and Confined styles of mineralization in the South Kawishiwi intrusion. Open style modeled to 1500 Ft. (457m.) and Confined style modeled to 500 Ft. (152m.) above base. Copper Cutoff 1.05 1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.01 Open Style Mineralization Cu % 1.043 1.029 0.969 0.933 0.924 0.844 0.818 0.758 0.685 0.644 0.597 0.547 0.502 0.461 0.421 0.374 0.329 0.286 0.242 0.201 0.156 0.107 Ni % 0.559 0.415 0.388 0.324 0.327 0.313 0.304 0.290 0.246 0.227 0.210 0.194 0.184 0.171 0.158 0.143 0.129 0.114 0.099 0.083 0.065 0.045 Tonnes 114,000 229,000 800,000 1,715,000 1,943,000 4,572,000 6,058,000 11,773,000 33,262,000 61,608,000 122,874,000 245,862,000 448,519,000 747,645,000 1,154,558,000 1,862,655,000 2,894,340,000 4,343,339,000 6,371,617,000 8,950,828,000 12,606,986,000 19,087,646,000 Confined Style Mineralization Cu % 1.035 1.010 0.989 0.962 0.936 0.893 0.852 0.802 0.748 0.693 0.641 0.590 0.553 0.517 0.482 0.446 0.408 0.369 0.329 0.283 0.219 0.131 Ni % 0.233 0.222 0.227 0.233 0.238 0.244 0.243 0.239 0.232 0.222 0.211 0.197 0.187 0.176 0.165 0.153 0.141 0.128 0.115 0.099 0.078 0.047 Tonnes 3,200,000 24,232,000 37,034,000 52,579,000 69,038,000 100,585,000 139,905,000 206,200,000 316,386,000 505,669,000 785,936,000 1,230,797,000 1,667,886,000 2,181,785,000 2,754,206,000 3,428,584,000 4,254,298,000 5,230,432,000 6,406,364,000 7,998,811,000 10,943,215,000 18,987,747,000 Note: 1) Cutoff grade intervals calculated from summation of modeled data (see Peterson, 2002d). For example, the 0.55 Cu cutoff includes all gridded data that falls between 0.575 and 0.525 wt. % Cu. 2) Cu%, Ni%, and tonnes represent cumulative addition of data from the Cu cutoff value and all data of higher grade. 3) Low Cu cutoff data dominantly represents barren rock within the mineralized zone of the Open style and barren rock above the mineralized zone within the Confined style. 4) Tonnes rounded to the nearest 1,000. 5) All intervals not assayed were assigned Cu and Ni values of 0.00 wt% and integrated into the model. NICKEL LAKE MACRODIKE Detailed geological mapping at a scale of 1:5,000 by the authors and Chris White, a Masters Candidate in the Department of Geological Sciences at the University of Minnesota Duluth, was completed in the late summer and fall of 2006, and published at a scale of 1:10,000 by the NRRI (Peterson et al., 2006). This map, available online at http://www.nrri.umn.edu/egg/REPORTS/MAP200604/MAP200604.html is the foundation upon which the field component of this trip will be based. During the course of this mapping, approximately 1,000 outcrops along nearly 100 kilometers of field traverses were examined to identify and confirm the internal lithologic variability, contact relationships, and structure of the western extent of the NLM, the adjacent SKI, and bounding rocks of the Anorthositic series. The authors wish to acknowledge Dr. Paul Weiblen (emeritus professor of geology at the University of Minnesota) for his keen insight on the geology of the area and Dr. George Hudak and undergraduate student Jeremiah ILSG07 94 Trip 4 �Gowey of the University of Wisconsin Oshkosh for assistance in mapping outcrops around and south of Omaday Lake. As well, a one day field excursion to Nickel Lake with Dave Peck (Anglo American), Harry Noyes (Encampment Resources), and Theodore DeMatties (consulting geologist) prior to the mapping campaign developed new insight on identifying dynamic magmatic systems in the field to the senior author (Peterson). Additional reconnaissance mapping in early November by Dean Peterson was completed to field check compiled outcrop locations depicted on the 1957 INCO map of the Spruce Road Deposit and the 1968 Hanna Mining map of the South Filson Creek deposit (both of which are publicly available in the DNR archive at Hibbing, Minnesota). The reconnaissance mapping confirmed the location of gossanous Cu-Ni bearing INCO outcrops and reconfirmed the outstanding field mapping of all types of Duluth Complex rocks by Hanna Mining Company geologists of the late 1960s. The NLM is a northwest to southwest-trending (Fig. 4.3), steeply dipping, asymmetric troctolitic and gabbroic intrusion interpreted to be a feeder dike for the northern portions of the SKI. The macrodike is interpreted to be located within a major rift-parallel normal fault (down to the southeast) now obscured by intrusion of NLM igneous rocks. Regional southward tilting (based on the deep level of erosion of the northern Bald Eagle Intrusion directly east of this area) leads to the interpretation that the southwest end of the NLM (near Omaday Lake) is structurally higher than the northeastern portion of the dike, and represents the location where magma flow changed from dike-like to sill-like, as it exited the dike – thus the magma velocity slowed – and entered the growing SKI magma chamber. Excellent potential exists for Ni-Cu rich massive sulfide at the basal contact where the dike enters the SKI (Section 31, T62N, R10W). The dike is composed of three main units: 1) inclusion-rich, locally sulfide-bearing, heterogeneous troctolite (unit N-Th); 2) layered troctolite, melatroctolite, and dunite (unit N-Tl); and 3) a late, crosscutting, coarse-grained to pegmatitic oxide-rich, olivine-gabbro to melagabbro (unit N-xG). Description of NLM Map Units The basis for all of the field aspects of this field trip is the aforementioned recently published bedrock geologic map of the NLM (Peterson et al., 2006). As well, the description of “Field Trip Stops” to follow differ from most geology field trip guidebooks in that we are simply going to take some walks in the bush, mostly along logging roads and snowmobile trails, look at numerous outcrops of the NML and adjacent rocks, and discuss the geology. We are NOT GOING TO SPECIFIC OUTCROPS to try to make a case for our interpretations; instead we urge you to use your imagination while we look at outcrops and add to the conversation. Instead of writing detailed descriptions of specific outcrops we’ll visit during the “stops” in the field trip, the authors have instead decided to simply copy verbatim the Description of Map Units from the map NRRI/MAP-2006-04 (Peterson et al., 2006) below, and use this as a reference during the field trip. The rocks of the NLM include, generally from youngest to oldest: Oxide Gabbro (N-xG) - Dark-grey, coarse-grained to pegmatitic, recessive weathered, oxide-rich (magnetite and ilmenite), olivine-gabbro to melagabbro. Contains small inclusions of anorthosite (unit N-Ai), basalt (unit N-Bi), and troctolite (unit N-Th). Interpreted to be the youngest phase of the dike based on inclusion types and cross-cutting relationships. Layered Troctolite to Dunite (N-Tl) - Grey to black, medium-grained, well-layered troctolite, melatroctolite, and dunite. Lamination of plagioclase and olivine parallel to modal layering is commonly observed as well as igneous scours and crossbedding. Small inclusions of anorthosite (unit N-Ai) and basalt (unit N-Bi) rare. Layering possibly developed as the up-welling magma streams through the dynamic (expanding) feeder dike. A constant temperature appropriate to plagioclase-olivine crystallization is maintained by a balance between the heat content of the incoming magma plus the heat of crystallization and the heat loss through the chamber walls. ILSG07 95 Trip 4 �Plagioclase and olivine are left behind and oriented/segregated on the walls of the expanding chamber (Weiblen, pers. comm.). Sulfide noted locally in outcrop in the SE corner of Section 31. Heterogeneous Troctolite (N-Th) - Light to dark grey, medium- to coarse-grained, inclusion-rich, heterogeneous troctolitic rocks with local igneous scour structures. Unit composed of intermixed troctolite, anorthositic-troctolite, melatroctolite, and gabbroic phases surrounding numerous local country rock (unit N-Ai) and exotic (units N-IF and N-Bi) inclusions that are elongate parallel to the macrodike, as well as hosts the sulfide-bearing unit N-Ts. Interpreted to be the initial highly dynamic magmatic phase of the dike that carried exotic inclusions from deep in the crust to their present level. Sulfide-Bearing Troctolite (N-Ts) - Rusty weathered, medium- to coarse-grained, sulfide-bearing, heterogeneous troctolitic and gabbroic rocks. Generally forms recessive weathering, Fe-stained, gossanous outcrops near, but not at, the northern margin of the macrodike. The current extent of this unit on the map is confined to those areas with outcrop, which in reality may be much more extensive as these outcrops generally end along linear swampy areas. Anorthosite Inclusion (N-Ai) - Light-grey, medium- to coarse-grained troctolitic-anorthosite, commonly with 1-2 cm poikilitic olivine pits. The large anorthosite inclusions at the southwest end of the dike (around Omaday Lake) are interpreted to represent a "logjam" of blocks that quit moving due to the decreased speed of the macrodike magmas as they entered the South Kawishiwi Intrusion magma chamber. Includes blocks within the SKI adjacent to the Nickel Lake macrodike. Basaltic Hornfels Inclusion (N-Bi) - Grey, fine-grained, steeply-dipping to vertical, granoblastic, locally magnetic, massive to amygdaloidal basaltic hornfels. Includes a highly magnetic block within the SKI adjacent to the Nickel Lake macrodike. Biwabik Iron Formation Inclusion (N-IF) - Well-bedded, steeply-dipping, recrystallized (layered magnetite and pyroxenite) iron-formation commonly with disseminated Cu-Ni sulfides. Forms an intense localized positive magnetic anomaly. ------------------------------------------------------------------------ Anorthositic Series - Subsuite of the Duluth Complex composed predominantly of plagioclase cumulates displaying complex internal structure and lacking obvious signs of in situ differentiation. Occurs throughout the Duluth Complex as anorthosite, troctolitic-anorthosite, and gabbroic-anorthosite, commonly poikilitic. Anorthositic Rocks Undivided (A-tA) - Mixed group of anorthositic cumulates occurring as large sill-like masses and as inclusions within troctolitic cumulates. Common rock types include troctoliticanorthosite, leucotroctolite, anorthosite, and olivine-bearing gabbroic-anorthosite. Olivine ranges from 2 to 15 percent in mode and from granular to poikilitic in texture, with oikocrysts ranging from 1 to 3 centimeters in diameter. Plagioclase mode ranges from 75 to 95 percent and varies from being non-foliated to well-foliated. Inclusions range in size from a few centimeters to elongate bodies hundreds of meters long that are parallel to foliation in the enclosing troctolite. DAY 1, FIELD TRIP STOPS Traverse #1 The location of the first field trip traverse (#1) is given in Figure 4.7. This walk in the bush begins within numerous outcrops of the bounding Anorthositic series rocks (map unit A-tA) on the northern margin of the NLM. The trail will take us southwest into inclusion-rich heterogeneous troctolitic rocks (map unit N-Th) and into southwestern most zone of known Cu-Ni-(PGE) mineralization in the NLM (map unit NTs), which in this location is associated with a large, sub-vertical inclusion of Biwabik Iron Formation (map unit N-IF). One must try to imagine from where such an inclusion came from (see Figure 4.4), how ILSG07 96 Trip 4 �it relates to the model that the NML is a feeder dike to the SKI, why it and other large mapped inclusions in the NML (Fig. 4.7) are concentrated at the southwestern end of the NML (magma velocity). Think back to the previously described fundamental tenet of Ni-Cu-PGE magmatic sulfide deposits, “… realization that decreases in the flow rate of magmas, principally due to the geometry of the conduit, is a major factor in settling entrained sulfide droplets and forming sulfide-rich ore bodies…”. Figure 4.7. Bedrock geology map of portions of the southwestern end of the Nickel Lake Macrodike in Sections 29 and 30, T62N, R10W. Dark lines are superimposed locations of field trip traverse #1, and traverse #2 along logging roads and snowmobile trails. Dashed lines represent short traverses through the bush to additional known outcrops. Traverse #2 The location of the second field trip traverse (#2) is presented in Figure 4.7. This walk to the southeast along a logging road/snowmobile trail begins in coarse-grained, oxide-rich olivine gabbro to melagabbro of map unit N-xG. Contact relationships between the N-xG and layered troctolitic to dunitic rocks (map unit N-Tl) can be complex, and we’ll investigate these relationships early on the traverse. The bulk of this traverse will be through the widest section (>650 m) of unit N-Tl that has been mapped in the NLM. Careful attention should be directed towards igneous textures in the N-Tl, and what they imply to magmatic processes (expanding dike with time, modal layering, pasting plagioclase and olivine phenocrysts on dike walls, solidification fronts, etc…). Optional traverses shown include a walk to ILSG07 97 Trip 4 �outcrops of the very large (~ 1.5 km long) basalt inclusion (map unit N-Bi), and to exceptional exposures of layered troctolite and dunite around a small beaver pond. Traverse #3 The location of the third field trip traverse (#3) is presented in Figure 4.8. This walk to the northnortheast of Nickel Lake will traverse through most of the main units identified in the NLM, including map units N-Th, N-xG, N-Tl, N-Ts, N-Bi, and N-Ai. An important walk through the bush (optional traverse on Figure 4.8) will visit several Cu-Ni-(PGE) mineralized outcrops near the northwestern margin of the NLM, and allow us to view some spectacular exposures, around the margin of a drained beaver pond, of the Anorthositic series rocks immediately northwest of the NLM. The authors cannot speak to strongly on the importance of finding these types of exposures (totally free of lichen, moss, trees, etc…), early in a mapping program, as they provide proxies for subsequent mapping of outcrops deep in the bush, where trees, shrubs, shade, forest litter, dirt, and black flies partially obscure exposures and/or ones willingness to observe them. Figure 4.8. Bedrock geology map of portions of the Nickel Lake Macrodike in the vicinity of Nickel Lake, Section 29, T62N, R10W. Dark line in the center of the image is the superimposed location of field trip traverse #3 along a logging road. Dashed line in the northwest quadrant represents a short traverse through the bush into the Cu-Ni-PGE mineralized northern zone of the NML. Note that beaver dams along the main traverse may cause very wet and/or impassable conditions. ILSG07 98 Trip 4 �Field relationships that offer evidence that a dike-like mafic to ultramafic intrusion is a conduit through which magma ascended upwards in the Earth’s crust all lead back to the fundamental tenets of the Ni-CuPGE ore deposit model. Such relationships provide evidence that the rocks formed in a dynamic, sulfidebearing magmatic system (once again, think like a person fly fishing a trout stream) that include: 1) early phases should be inclusion-rich (some of which should be from a deeper crustal level) and form as the igneous conduit breeches upwards into the Earth’s crust; 2) imbrication and/or elongation of entrained country rock inclusions parallel to igneous foliation; 3) igneous scour structures; 4) prominent steeplydipping igneous foliation and localized disruption due to magmatic injection; 5) cross-bedding of modal layering; 6) evidence of sulfide mineralization; and 7) evidence that that magma velocity varies. One of the authors’ goals of the field trip is to expand this list based on conversations on the outcrop with the participants. Outcrop photographs given in Figure 4.9 show a few of these lines of evidence for the dynamic nature of the NLM. Figure 4.9. Photographs of selected outcrops that give us clues to the dynamic nature of the magmatic processes which ultimately led to the formation of the NLM, and by interpretation, the northern portion of the SKI and its associated Cu-Ni-PGE mineralization. A) Scour structure in map unit N-Th. B) Disrupted igneous foliation along the southern margin of map unit N-Tl. C) Cross-bedding of troctolitic rocks in map unit NTl. D) Cu-Ni sulfide mineralization in map unit N-Ts. ILSG07 99 Trip 4 �DAY 2, 3-D VISUALIZATION AND DRILL CORE DISPLAYS Day two of this field trip will be spent at Duluth Metals Limited’s field office and drill core logging facility in Ely, Minnesota. The day will be split up into two principal activities: 1) 3-D visualization of subsurface geological features of the Nickel Lake macrodike and South Kawishiwi intrusion utilizing the computer program gOcad (geologic object computer aided design) and a Geowall (bring your camera because we’ll all be wearing those funny looking 3D glasses), and 2) examination of selected core intervals from a number of holes drilled by Duluth Metals Limited over the last year. 3-D GEOLOGICAL MODELING AND VISUALIZATION The science of geology uses a variety of tools to study the earth. However, the basis for every type of geologic study is fundamentally rooted in observations made of rocks in their natural habitat – “in the field”. Geologists that do not have an intimate appreciation of the power and fundamental nature of field geology cannot, in turn, appreciate coherent and compelling field-based scientific arguments from which all other geologic interpretations grow. This basic tenet may never be truer than for geologists engaged in mineral exploration. The advance in computer technology over the last twenty years has revolutionized all aspects of our lives. One such advance in geology has been the development of sophisticated 3-D geological software that, if used correctly, i.e., created and maintained by geologists who understand the rocks, can be an outstanding tool for letting geologists interpret data (field observations, drill hole data, etc...) into the subsurface where direct observation is impossible. The first author of this guidebook (Peterson) integrates many types of geological data into the computer program gOcad®, which is perhaps the most sophisticated 3-D geological modeling software available (see websites http://www.gocad.org/www/ and http://www.earthdecision.com/). 3-D models of active mines and/or advanced exploration projects benefit from continuous validation and upgrading of the underlying database, as well as the production of regional geological syntheses, integrating new geological, geophysical, geochemical, geotechnical, and geohydrological models into a single platform. The natural outgrowth of 3-D geological models is their extrapolation away from areas with large amounts of data to more remote areas with less and/or no data, and can assist in the definition of new exploration targets. For geologic teams working in a mineral exploration setting, the main advantages of using integrated 3-D geological models are to (from Fallara et al., 2006): 1) Share the information a. Conveys the data and their interpretation in an immersive format b. Avoids the loss of knowledge and interpretation which may be, i. Distributed within various locations within the company ii. Filed in disorganized ways iii. Never filed and existing only in a geologists memory 2) Be a catalyst in the development of geologic knowledge a. Easily integrates data in a common format b. Preserves data that is easily shared, seen, and analyzed 3) Hastens problem solving throughout the company a. Adaptable to team-driven resolution b. Easy access to geologists and management c. Allows for the shared comprehension of the data ILSG07 100 Trip 4 �4) Accelerates the process of data integration and interpretation a. Define potential exploration targets with reduced uncertainty b. Focuses work on interpretation c. Optimizes data subsets 5) Direct access to data and manipulation within the gOcad® software a. Integrated 3-D querying within the geological model b. Saves time and money, rapid validation of data, and uncertainty resolution A 3-D display of the first author’s gOcad modeling of geologic data from the Duluth Complex in general, and the SKI, NLM, and BEI in particular, will be presented in Duluth Metals Limited’s drill core logging facility at the start of Day 2 of this field trip. A simple screen dump out of gOcad of some of this data is presented in Figure 4.10. Figure 4.10. Print screen image of the SKI and NLM gOcad model (Peterson, unpublished data). DULUTH METALS LIMITED DRILL CORE DISPLAYS Recent drilling (Fig. 4.11) and core logging by Duluth Metals Limited east of the Maturi deposit in 2006 and 2007 has confirmed numerous stratigraphic units described by Severson (1994), especially the sulfide-bearing PEG, U3, BH, BAN, and upper GRB units. ILSG07 101 Trip 4 �Figure 11. Property map of Duluth Metals Limited. Included are historic (black) and recent (red) drill holes. The numerous correlative units of the SKI from historic and recent drill holes include the following: Main AGT – Thick zone of homogenous augite troctolite to olivine gabbro. AT&T – Thick zone of homogeneous medium-grained anorthositic troctolite with troctolites. AT(T) – Homogeneous anorthositic troctolite grading to troctolite and lesser amounts augite troctolite. T-AGT – Homogeneous medium-grained troctolite and augite troctolite. Pic – Thin horizon of medium-grained picrite that is rarely developed, and thus usually uncorrelative. Upper Gabbro – Heterogeneous, medium- to coarse-grained, oxide-rich gabbro and olivine gabbro. AN Group – Combination of coarse-grained homogenous anorthosite, troctolitic anorthosite, anorthositic troctolite, and medium-grained homogeneous gabbro, and olivine gabbro of the Anorthositic series of the Duluth Complex. Basalt Inclusion – Fine-grained, typically magnetic, basalt hornfels with sharp external contacts and locally preserved stretched amygdules. U2 – Ultramafic horizon of locally serpentinized, medium-grained picrite and troctolite with sharp contacts. PEG – Heterogeneous pegmatoidal and pegmatitic troctolite and anorthositic troctolite with minor amounts of augite troctolite, olivine gabbro, and troctolitic anorthosite. The PEG unit typically is weakly sulfide-bearing with sulfide-barren rocks above. ILSG07 102 Trip 4 �U3 – Ultramafic zone of locally serpentinized, sulfide-bearing, medium-grained dunite, picrite, and troctolite with sharp contacts. This unit is the PGE-dominate horizon at the Birch Lake deposit. BH – Basal heterogeneous zone that is the main sulfide-bearing unit of the SKI. Consists of fine- to medium- to coarse-grained and pegmatitic troctolite and to a lesser extent anorthositic troctolite and augite troctolite. BAN – Basal augite troctolite and norite which is the contaminated zone between the BH and underlying GRB. Consists of fine- to medium-grained, sulfide-bearing augite troctolite and norite. GRB – A variety of rocks from the footwall Archean Giants Range Batholith. Rock types include medium- to coarse-grained hornblendite, diorite, and porphyritic or nonporphyritic monzonite to quartz monzodiorite. Sulfide mineralization is usually restricted to the upper 20 feet of the GRB, but can exist in small concentrations over a hundred feet into the granitic footwall. Simplified drill hole logs portrayed as stratigraphic columns from two holes recently drilled in the eastern (MEX-07) and western (MEX-08) portions of the Maturi Extension deposit are presented in Figure 4.12. In addition, reported Cu-Ni-PGE grades from Duluth Metals Limited for these holes are given in Table 4.2. The stratigraphy observed in MEX-07 is typical of the eastern exploration area and the deep holes in this region (>3,000 feet) that are situated below a very large pillar/xenolith of Anorthosite series rocks (see Fig. 4.3). Channelized magma flow out of the NLM and under this Anorthosite block (think of eddies due to overlying friction as a fisherman would) has been interpreted by Peterson (2001b, c) as one of the driving mechanisms for enhanced turbulent flow of sulfide-bearing magmas, thus forming one of the elements of the Ni-Cu-PGE ore deposit model, “an appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals”. The stratigraphy observed in MEX-08 correlates with many of the drill holes in the western exploration area (as well as Franconia Minerals’, Maturi deposit) immediately to the west of the Anorthosite series pillar/xenolith. Mineralization in both of these holes consist of chalcopyrite-dominated disseminated sulfides (typically 2-5% total sulfide) restricted to a confined zone directly above the footwall and contain moderate to high Cu, Ni, and PGE concentrations. If one would very simply (unlike the data depicted in Figure 4.6, which used the strict rules of Kerr, 2001) recalculate these reported grades (Table 4.2) to 100% sulfide, i.e. as a specific gravity concentrator would do as a first step at an active mine, then one would have to multiply the assay data by a factor between 20 to 50 (5% to 2% total sulfide minerals in the assayed drill hole intervals) to imagine the true reality that is the enrichment of Cu-Ni-PGE in the sulfide minerals of the Maturi Extension deposit. Such recalculations lead to grades (at 100% sulfide) of ~33% Cu, ~9% Ni, and ~11 – >82 g/t PGE+Au for the sulfide fraction of the Maturi Extension deposit, which point to these preliminary conclusions: 1) The extensive sulfide mineralization at the Maturi Extension deposit (as well as the Maturi, and Birch Lake deposits, i.e., the Confined mineralization of Peterson 2001, 2002a-d) is similar at a recalculated 100% sulfide grade to the metal budget of the fractionated residuum of a MSS; 2) One can imagine such mineralization (the Confined-style of Peterson 2002a) initially forming from incorporation of the Cu-PGE residuum of a fractionating Ni-rich MSS at the junction of the NLM and SKI into magmas streaming out of the NLM and into the SKI. 3) A seemingly robust geologic model of a dynamic magmatic system that incorporates the BEI, NLM, and the SKI exists herein that vector towards an exploration target for a Ni-rich massive sulfide body (the “missing Ni-rich MSS”) at the junction of the NLM and the SKI. ILSG07 103 Trip 4 �Table 2. Reported mineralized intervals for holes MEX-07 and MEX-08 drilled by Duluth Metals Limited on the Maturi Extension property in 2006. MEX-08 Overall including including From (Ft) 2032 2052 2202 To (Ft) 2236.5 2142 2236.5 Total (Ft) 204.5 90 34.5 Cu % 0.69 0.81 0.89 Ni % 0.22 0.24 0.28 Pd (g/t) 0.313 0.365 0.352 Pt (g/t) 0.121 0.147 0.12 Au (g/t) 0.108 0.056 0.096 MEX-07 Overall including including From (Ft) 2543 2608 2628 To (Ft) 2798 2673 2663 Total (Ft) 255 65 35 Cu % 0.429 0.824 0.898 Ni % 0.129 0.275 0.310 Pd (g/t) 0.370 0.906 1.004 Pt (g/t) 0.183 0.444 0.472 Au (g/t) 0.077 0.173 0.177 Figure 4.12. Stratigraphic sections of holes MEX-07 and MEX-08 drilled by Duluth Metals Limited on the Maturi Extension property in 2006. 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Lightfoot, P.C., Naldrett, A.J., Gorbachev, N.S., Fedorenko, V.A., Hawkesworth, C.J., Hergt, J., and Doherty, W., 1994, Chemostratigraphy of Siberian trap lavas, Noril’sk District: Implications for the source of flood basalt magmas and their associated Ni-Cu mineralization, in Lightfoot, P.C., and Naldrett, A.J., eds., Proceedings of the Sudbury–Noril’sk symposium: Ontario Geological Survey, Special Volume 5, p. 283312. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Hibbing, Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74 p. Lowell, J.D. and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, p. 373-408. Marma, J.C., 2003, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit in the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Unpublished M.S. thesis, University of Wisconsin: condensed version, University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/39, 112 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey, Report of Investigations 58, 207 p. 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Postle, J., Bernie-Haystead, B., Clow, G., Hora, D., Vallée, M., and Jensen, M., 2000, CIM standards on mineral resources and reserves, definitions and guidelines: Canadian Institute of Mining, Metallurgy and Petroleum, 26 p. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones—a dilemma: Economic Geology, v. 81, no. 4, p. 974-978. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-93/34, 210 p., 15 pls. Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen quadrangle, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-97/19, 102 p., 4 pls. Severson, M.J., and Hauck, S.A., 2003, Platinum-group elements (PGEs) and platinum-group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/37, 296 p., 1 CD. Thériault, R.D., Barnes, S.J., and Severson, M.J., 2000, Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-944. Vislova, T., 2003, Petrology of the Bald Eagle intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift: Unpublished Ph.D. Thesis, University of Minnesota. Weiblen, P. W., Morey, G. B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex: American Journal of Science, vol. 280A, Part I, p 88-133. Weiblen, P., Peterson, D.M., and Vislova, T., 2005, Implications of Midcontinent Rift and oceanic ridges analogies and 3-D interpretations of the subsurface structure of the Bald Eagle intrusion in the Duluth Complex and the East Pacific Rise: Institute on Lake Superior Geology, 51st Annual Meeting, Sault Ste Marie, Ontario, v. 51, 3 p. Weiblen, Paul W., 1965, A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota: Unpublished Ph.D. Thesis, University of Minnesota, 161 p. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-93/52, 90 p., 3 pls. ILSG07 108 Trip 4 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 5 Geologic Highlights of New Mapping in the Upper Southwestern to Northeastern Sequence of the North Shore Volcanic Group and Beaver Bay Complex Terrence J. Boerboom Minnesota Geological Survey University of Minnnesota James D. Miller, Jr. Minnesota Geological Survey & Department of Geological Sciences University of Minnesota Duluth John C. Green Department of Geological Sciences University of Minnesota Duluth View northeast across Pork Bay on the North Shore ILSG07 109 Trip 5 �INTRODUCTION The Minnesota Geological Survey is continuing to map the Keweenawan bedrock geology of 1:24,000scale quadrangles near the North Shore of Lake Superior, in northeastern Minnesota. The first phase of mapping from 1985-1993, partially supported by the USGS COGEOMAP program, focused on the Beaver Bay Complex in the central part of the region (Fig. 5-1; Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). More recent mapping, supported by the USGS STATEMAP program, has focused on quadrangles that intersect the shoreline, where the bedrock is predominantly volcanic rocks. This mapping phase started at the outskirts of Duluth in 2001, and to date has proceeded to the Lutsen quadrangle, having skipped past the COGEOMAP quadrangles. Ten quadrangle-scale geologic maps have been published to date from this work (Boerboom and others, 2002a, 2002b, 2003a, 2003b; 2006; 2007; Boerboom and Green, 2004; 2005, 2006; Miller and others, 2006), and at least two more are planned (Fig. 5-1). Figure 5-1. North Shore 7.5' quadrangles mapped by the MGS through the USGS-sponsored COGEOMAP and STATEMAP programs. M-numbers refer to MGS Miscellaneous Map Series. A trip similar to this, which covered the Upper Southwest sequence of the North Shore Volcanic Group (NSVG), was conducted for the 2004 Institute on Lake Superior Geology meeting held in Duluth. The trip for this year’s meeting will cross the transition from the uppermost Southwest limb to the uppermost Northeast limb, as well as the basal Schroeder-Lutsen sequence of the NSVG (Fig. 5-2). ILSG07 110 Trip 5 �Figure 5-2. Simplified geology of northeastern Minnesota showing the major sequences of the North Shore Volcanic Group (taken from Fig. 5.2 of Miller and others, 2002). VOLCANIC AND SEDIMENTARY ROCKS OF THE NORTH SHORE VOLCANIC GROUP This overview is a slightly modified excerpt from the MGS Report of Investigations 58, Chapter 5: "Volcanic and sedimentary rocks of the Keweenawan Supergroup in Northeastern Minnesota" by John Green (Miller and others, 2002). Magmatic activity related to the 1.1 Ga Midcontinent Rift produced a more than 10 kilometer thick edifice of lava flows and subvolcanic intrusions that are exposed along Minnesota's north shore of Lake Superior. The lava flows and minor sedimentary rocks are referred to as the North Shore Volcanic ILSG07 111 Trip 5 �Group (Goldich and others, 1961) and the intrusive rocks are variably assigned to the Duluth Complex, the Beaver Bay Complex, and miscellaneous intrusions of the Midcontinent Rift Intrusive Supersuite. Although previous publications have subdivided the North Shore Volcanic Group into informal volcanic suites and distinctive flows (Green, 1972, 1982), the 1:200,000-scale map (M-119) produced by Miller and others (2001) and accompanying report (Miller et al., 2002) represents the first time that the NSVG has been subdivided into coherent lithostratigraphic units on a geologic map. A brief description of the North Shore Volcanic Group and associated interflow sedimentary rocks of the Keweenawan Supergroup is given here. Rock classification, recognition, and textures As a coherent tholeiitic compositional suite, the volcanic rocks of the North Shore Volcanic Group can be described using only a few rock names (Fig. 5-3). The most primitive rocks are olivine tholeiites, which form an iron-enrichment trend with further evolution. They display ophitic textures and pahoehoe surfaces nearly everywhere. Most olivine tholeiites are aphyric, but those that are porphyritic contain dominantly plagioclase phenocrysts, less commonly olivine. Transitional basalts contain somewhat higher alkalies and other incompatible elements than the olivine tholeiites, but generally not enough to classify them as alkalic. Their texture is typically intergranular and fine- to medium-grained. Porphyritic varieties generally contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The reversed-polarity Hovland lavas, however, are characterized by transitional basalts (grading to basaltic andesites) that contain abundant, large, tabular, plagioclase phenocrysts. Transitional basalt flow surfaces are generally smooth (pahoehoe), although a few show breccia tops. The basaltic andesites and andesites (greater than 52 percent SiO2) are tholeiitic, rather than calcalkaline; they show iron enrichment and contain only anhydrous ferromagnesian minerals. These rocks are typically fine-grained and intergranular to felty or pilotaxitic, and many contain small phenocrysts of plagioclase, olivine, clinopyroxene, and magnetite. The andesites generally weather to a red-brown color, and have flow-brecciated (aa) tops but not bases. Many flows that contain 50 to 55 percent silica show millimeter-scale oxidation lamination (Green, 1989) parallel to the base. A few highly iron-enriched flows, separable only by chemical analysis, can be called ferroandesites. Figure 5-3. AFM compositional diagram for the lavas of the North Shore Volcanic Group (modified from Green, 1982). The boundary between tholeiitic and calcalkalic rocks is modified from Irvine and Baragar (1971). FeO* = FeO + 0.9Fe2O3. ILSG07 112 Trip 5 �Carmichael (1964) first used the name "icelandite" for rocks intermediate in character between andesites and rhyolites in the Tertiary lavas of eastern Iceland. They might be considered the tholeiitic equivalent of calc-alkaline dacite in orogenic suites. Other examples of these rocks have been described from the Galapagos (McBirney and Williams, 1969) and the Miocene of Nevada–Oregon (Wallace and others, 1980). Very large flows of similar composition in the Etendeka volcanics of Namibia have been referred to as quartz latites by Milner and others (1992). Icelandites in the North Shore Volcanic Group (Green and Fitz, 1993) are characterized chemically by SiO2 contents ranging from 60 to 68 percent, high FeO* (averaging 7 percent), K2O + Na2O values between 6.5 and 9 percent, a potassium/sodium atomic ratio of about 0.9, and an Mg number [Mg/(Mg + Fe) atomic] averaging 0.14. Petrographically, North Shore Volcanic Group icelandites grade continuously from the andesites to somewhat paler colors (brown or tan), but have a similar phenocryst assemblage. Quartz and alkali feldspar are common in the groundmass but never occur as phenocrysts. Flowtop features (crusty to coarsely brecciated) indicate that the icelandites were erupted as lavas. The rhyolites have higher silica and total alkali contents, and lower FeO than the icelandites. They are generally light gray, pink, or red. Several are very thick, extensive, and voluminous (up to several hundred cubic kilometers; Green and Fitz, 1993). Although most rhyolites are porphyritic (phenocrysts of quartz, alkali feldspar ± plagioclase, and altered fayalite ± ferroaugite), some lack quartz and alkali feldspar phenocrysts, and rare flows are aphyric. Groundmass textures are fine-grained holocrystalline, typically with a meshwork of platy quartz paramorphs after primary tridymite, which may show a flow structure. A "snowflake" texture is common, in which poikilitic quartz patches (coalesced ex-tridymite grains) enclose small, dusty alkali-feldspar grains. In quartz-phyric flows, these poikilitic quartz patches are in optical continuity with adjacent quartz phenocrysts (Green, 1990). Outcrop-scale flow structure, including folding, is common near flow tops and bases. In some flows, distinct fiamme, deformed to varying degrees, are recognizable. These imply an explosive eruption that produced a pyroclastic flow, which welded and remobilized to produce a rheoignimbrite. Structure and Lithostratigraphy As stated above, the Midcontinent Rift volcanic rocks and interbedded redbeds in northeastern Minnesota comprise the North Shore Volcanic Group (Goldich and others, 1961; Green, 1972, 1977, 1982; Basaltic Volcanism Study Project, 1981). In general, these rocks form an arcuate stack that is slightly tilted toward the southeast and forms the roof rocks into and under which the Duluth Complex and associated hypabyssal intrusions were emplaced. At the southwest end of the North Shore Volcanic Group near Duluth, the volcanic rocks strike north with a 10º to 20º easterly dip; at the northeast end at Grand Portage, the flows strike east–west with a 10º southerly dip (Fig. 5-2). Thus, traveling northeast along the Lake Superior shore northeast from Duluth, and southwest from Grand Portage, one encounters successively higher flows in the volcanic stratigraphy until the Tofte–Lutsen area in southern Cook County, where the highest exposed flows crop out. Exposure is generally excellent along the eroding lakeshore, and along the lower, high-gradient stretches of tributary streams, providing good control on the stratigraphy. A stack of volcanic rocks approximately 9.7-kilometers-thick has been measured in the “southwest limb” (Table 5-1), and another stack of volcanic rocks about 7.2-kilometers-thick has been measured in the “northeast limb” (Table 5-2, Miller and others, 2002). This implies nearly continuous subsidence during the rifting process. The difference in stratigraphic thickness between the two limbs reflects major complications in the central area, which appears not to have subsided at the same rate as in the limbs, and into which many of the subvolcanic intrusions were emplaced. Except for the capping Schroeder–Lutsen basalt sequence, no stratigraphic unit can be traced from one limb to the other; each limb has its own stratigraphic column. To aid in the correlation of intrusive and extrusive rock units throughout the Midcontinent Rift system, their paleomagnetic polarity has been used. Nearly all of the igneous and sedimentary rocks associated with the Midcontinent Rift were formed either during an earlier, reversed-polarity interval or a succeeding normal-polarity interval. Thus, in each limb of the North Shore Volcanic Group, the lower stratigraphic ILSG07 113 Trip 5 �units show reversed polarity, and the upper sequences show normal polarity. This polarity reversal forms the basis for distinguishing upper and lower sequences in the northeast and southwest limbs. U-Pb zircon dates demonstrate that the reversed-polarity magmatism occurred mainly in the time interval from 1108 to 1107 Ma; whereas, around the Lake Superior basin, normal-polarity magmatism occurred mainly in the interval from 1099 to 1094 Ma (for example Davis and Paces, 1990; Paces and Miller, 1993; Davis and Green, 1997). These two pulses were separated by a magmatically inactive time (at least in the upper crust), which appears to be expressed as a slight unconformity in the volcanic sequence on the north shore. However, because intrusions subsequently penetrated along this horizon in the North Shore Volcanic Group, this unconformity has not been recognized in outcrop. The Duluth Complex separates the upper and lower sequences of the southwestern limb (Fig. 5-2, Table 5-1). One other significant gap in the stratigraphic continuity of the North Shore Volcanic Group occurs near the stratigraphic top, where the Schroeder–Lutsen sequence overlies the upper units of the northeast and southwest sequences (Fig. 5-2, Tables 5-1 and 5-2). In the northeast limb, southwest of Grand Marais (Fig. 5-3), the Schroeder–Lutsen sequence unconformably overlies the Good Harbor Bay lavas, which are now recognized to include the Terrace Point basalt, as well as the Cut Face Creek Sandstone and the Good Harbor Bay andesites. The Terrace Point basalt contains a granite xenolith dated at 1096.7±0.9 Ma and thus must be younger than that. In the southwest limb, the Schroeder–Lutsen sequence overlies andesites and icelandites of the Onion River lavas (Boerboom and Green, 2006; Boerboom and others, 2006) as well as a sandstone and conglomerate unit (the Little Marais conglomerate) that in turn rests in sharp angular unconformity atop structurally disturbed flows of the Belmore Bay lavas near Little Marais. The Onion River lavas form a coherent package of dominantly intermediate-composition lavas that have only recently been mapped in detail, and their stratigraphic thickness is not taken into account in Table 5.1. They are located in approximately the same stratigraphic position as the Belmore Bay lavas, and may be continuous with them, but their relationship has been obscured by emplacement of the Beaver Bay Complex. Furthermore, the gently dipping Schroeder basalts have not been penetrated by the abundant hypabyssal intrusions of the Beaver Bay Complex that complicate the underlying volcanic sequence in this mid-shore area (Green, 1992; Miller and others, 1993). Attempts to date the Schroeder–Lutsen sequence have been unsuccessful to this point. The five major lithostratigraphic sequences comprising the two limbs of the North Shore Volcanic Group are further subdivided into informal lithostratigraphic units (Table 5-1 and 5-2). Some of these units are individual flows of distinctive lithology and/or substantial thickness and lateral extent. Most are suites of lava flows that have distinct lithologic characteristics or that are separated by intrusions. Some lava formations contain distinct flows or sedimentary rock units within an otherwise homogeneous package of lavas. Such units are given informal names (such as the Silver Beaver rhyolite within the Baptism River lavas, Manitou transitional basalt within the Schroeder basalts, and Indian Camp sandstone within the Lutsen basalts). See Chapter 1 of RI58 (Miller and others, 2002) for more details on stratigraphic nomenclature of Keweenwan rock in northeastern Minnesota. ILSG07 114 Trip 5 �Table 5-1. Generalized stratigraphy of the southwest limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) Lithostratigraphic units Lithologic character U/Pb age 9735 Total section basalt, andesite, and rhyolite flows 150 Carlton Quarry lavas (fault 1094.3±2.0 bounded) Schroeder–Lutsen sequence (normal polarity) 945 ophitic olivine tholeiite basalt flows; 900 Schroeder basalts <45 Little Marais conglomerate includes Manitou transitional basalt and Pork Bay breccia polymict volcanic conglomerate and sandstone angular unconformity 8275 Upper southwest sequence (normal polarity) 565 Bell Harbor lavas 100 Palisade Head rhyolite mostly quartz tholeiite basalt and basaltic andesite flows gray-pink, porphyritic rhyolite flow Beaver Bay Complex 700 Baptism River lavas 20 Silver Bay porphyritic basalt 730 Gooseberry River basalts 1096.6±1.7 ~1096 mixed lavas, mostly basalt; includes 165-meter-thick Silver Beaver rhyolite ophitic basalt flow with abundant large plagioclase phenocrysts mixed basalt flows, mostly ophitic Lafayette Bluff, Silver Creek diabase intrusions 315 Two Harbors basalts 550 Larsmont basalts mixed aphyric basalt flows; quartz tholeiite flows at base ophitic olivine tholeiite flows Stony Point–Knife Island diabase sheet 1500 Sucker River basalts mixed basalt flows, mostly ophitic 1350 Lakewood lavas mostly basalt flows; rhyolite, icelandite, and ferroandesite at base Lester River diabase sill 1285 mixed basalt, andesite, icelandite, and rhyolite flows Lakeside lavas 1098.4±1.9 Endion diabase sill 1160 Leif Erickson Park lavas ~1099 Duluth Complex 370 mixed basalts, andesites Lower southwest sequence (reversed polarity) 370 Ely's Peak basalts >8 Nopeming Sandstone porphyritic, diabasic, and ophitic basalts; pillowed and pyx-phyric basal flow white to tan quartzite and conglomerate angular unconformity Thomson Formation (Paleoproterozoic) ILSG07 115 Trip 5 �Table 5-2. Generalized stratigraphy of the northeast limb of the North Shore Volcanic Group showing U/Pb ages (Davis and Green, 1997; Green and others, 2001). Positions of intrusions denote approximate stratigraphic level affected and not age of emplacement. Thickness(m) ages 7359 325 Lithostratigraphic units Lithologic character U/Pb Total section Schroeder–Lutsen sequence (normal polarity) olivine tholeiite basalts, mostly Lutsen basalts ophitic; includes Indian Camp sandstone angular unconformity 3998 Upper northeast sequence (normal polarity) 130 Terrace Point basalt (within upper Good Harbor Bay andesites) 100 Cut Face Creek sandstone 131 Good Harbor Bay andesites ophitic, olivine tholeiite basalt younger than 1096.7±0.8 red, laminated, ripple-marked sandstone brown, porphyritic basaltic andesites Beaver Bay Complex – Beaver River diabase and Leveaux ferrodiorite brown, columnar-jointed basalt flow 122 Breakwater basalt 348 Grand Marais felsites pink to gray porphyritic rhyolite and felsite 335 Croftville basalts 250 70 Devil’s Track rhyolite Maple Hill rhyolite intergranular basalt and andesite flows, thick interflow sandstone aphyric, intergranular rhyolite flow porphyritic rhyolite flow 274 Red Cliff basalts 366 539 Kimball Creek rhyolite Marr Island lavas 198 235 900 Naniboujou basalts Devil’s Kettle rhyolite Brule River lavas ophitic olivine tholeiite flows, some plagioclase-phyric porphyritic rheoignimbrite mixed basalt, tholeiitic andesite, and icelandite flows intergranular basalt flows porphyritic lava flow interbedded basalt and rhyolite flows 1097.7±1.7 1100.2±1.9 Brule Lake-Hovland gabbro 3036 BASE 60 Lower northeast sequence (reversed polarity) 1932 Hovland lavas 67 92 945 Red Rock rhyolite Deronda Bay andesite Grand Portage lavas 60 Puckwunge Sandstone mostly plagioclase-phyric basalt flows, some rhyolite and andesite red/tan, porphyritic rhyolite tan/brown, porphyritic andesite basalt lava flows, pillowed at base 1107.7±1.9 1107.9±1.8 tan/white, cross-bedded quartz sandstone slight angular unconformity Rove Formation (Paleoproterozoic) ILSG07 116 Trip 5 �Physical Volcanology The volcanic rocks of the Midcontinent Rift, including the North Shore Volcanic Group, represent one of the world’s oldest and best-preserved examples of plateau lavas. However, they contain a greater thickness of flows and, in the North Shore Volcanic Group, a higher proportion of evolved compositions than typical plateau lavas. They are similar physically and chemically to the Tertiary lavas that make up eastern Iceland (Sigvaldason, 1974; Walker, 1974; Green, 1977; Wood, 1978), and they formed similarly over a plume at another major rift. The rocks also resemble the late Tertiary and Quaternary volcanic rocks of the southern Snake River Plain, Idaho and southeastern Oregon, because of their interbedded basalts and large rhyolites (for example Bonnichsen and Kauffman, 1987; Link and Hackett, 1988; Reidel and Hooper, 1989; Manley, 1996). The basalts range in character from typical flood flows as voluminous as tens of cubic kilometers to more modest, “plains-type” flows (Greeley, 1982) and thin flow units less than a meter thick. At Duluth in the southwest limb, and Grand Portage in the northeast limb, the lowest flows in the volcanic sequences are pillowed, and thus inferred to have erupted subaqueously; however, nearly all of the other flows were erupted subaerially. The flows show different physical characteristics, closely tied to their chemical compositions and viscosities (Green, 1977, 1989; Green and Fitz, 1993). Olivine tholeiites, which dominate the North Shore Volcanic Group, all have pahoehoe surfaces, with or without ropy structures. Other physical characteristics of the various rock types were previously discussed in the “Rock classification, recognition, and textures” section of this chapter. All of the flows ranging in composition from basalts to icelandites were erupted as lavas. The rhyolites are notable in their abundance relative to other plateau-lava sequences, their size (up to several hundred cubic kilometers), and extent (Green and Fitz, 1993). Several rhyolites show textural evidence of rheomorphic flow after eruption as ash-flow tuffs, though some were lavas. One of the largest, the Devil Track rhyolite in Cook County, which is as thick as 250 meters and can be traced for 40 kilometers along strike, has ambiguous features that make its mode of eruption difficult to discern; it may be a lava flow. Nearly all the icelandites and rhyolites show evidence of an unusually high temperature of eruption, such as magmatically crystallized groundmass tridymite. The evidently low viscosity of these large rhyolites is attributed to their high temperature, high iron and fluorine contents, and low oxidation state (Green and Fitz, 1993). Geochemistry and Chemostratigraphy The North Shore Volcanic Group constitutes a subalkalic, tholeiitic suite that ranges continuously from rather primitive olivine tholeiite to rhyolite, and shows a strong iron-enrichment trend (Fig. 5-3; also Basaltic Volcanism Study Project, 1981; Green, 1982; Brannon, 1984). However, relative abundances are strongly bimodal; basalts are greatly predominant, but rhyolites make up 10 to 25 percent of the section. The basalts show trace element and isotopic evidence of derivation mostly from a mantle plume (Nicholson and others, 1997), whereas most of the rhyolites include major contributions from partial melting of the Archean basement (Vervoort and Green, 1997). The most common basalt type, ophitic olivine tholeiite, is generally aluminum-rich (16 to 18 percent Al2O3). The most primitive flows have Mg numbers of about 0.65 to 0.68. The basal few flows in both limbs of the North Shore Volcanic Group have a unique geochemical and petrographic character. Typically they contain augite phenocrysts, are aluminum-poor, and are rich in both compatible (chromium and nickel) and incompatible elements (titanium, phosphorus, and lanthanum) with steep chondrite-normalized La/Yb ratios. This suggests derivation by relatively smallfraction melting of the initial plume head (Nicholson and others, 1991, 1997; Green, 1995). In general, there is little stratigraphic regularity of compositional change within the North Shore Volcanic Group, with the following exceptions. In the middle of the upper southwest sequence, there is a marked upward progression toward more primitive compositions through a 3.4-kilometer section from rhyolite east of the Lester River into a thick group of primitive olivine tholeiites in the Knife River–Two Harbors area (Brannon, 1984). This includes the Lakewood lavas, the Sucker River basalts, and the Larsmont basalts. In contrast, in the lower northeast sequence, the approximately 1-kilometer-thick basal ILSG07 117 Trip 5 �Grand Portage lavas progress upsection from basalt to increasingly evolved compositions, ending with Red Rock rhyolite (Green, 1995). As mentioned above, the Schroeder–Lutsen sequence, the youngest in the North Shore Volcanic Group, is composed almost entirely of olivine tholeiites. All of the North Shore Volcanic Group has been affected to some degree by hydrothermal/burial metamorphism. The more permeable (fractured, vesicular) tops and bases of the flows have undergone considerable mineralogical change (deposition of amygdule minerals, alteration of primary minerals), but in many cases the massive flow interiors are remarkably little-altered. Where alteration has approached equilibrium, mineral assemblages range from lower greenschist facies at the base of the North Shore Volcanic Group to zeolite facies at the top (Schmidt, 1993; Schmidt and Robinson, 1997). Interflow Sedimentary Rocks Clastic redbed strata occur at many horizons within the North Shore Volcanic Group (Jirsa, 1984). They are lenticular and range in thickness from a few centimeters to about 100 meters. As these rocks are relatively soft and erodable compared to the adjacent volcanic flows, they are mostly covered and are exposed only along actively eroding sites such as streambeds and the lakeshore. However several interflow sandstones have recently been recognized from water well cutting samples (Boerboom and others, 2007). Overall, the sedimentary rocks are predominantly red to brown, well sorted sandstone, with minor conglomerate, siltstone, and shale. Conglomerate beds are most abundant in the midshore area from Little Marais to Lutsen, either on or in close proximity to a prominent gravity low that is at a right angle to the North Shore. This gravity low is thought to reflect an uplifted crustal block (White, 1966), and intrusive rocks along this trend are known to contain large xenoliths of Archean crust (Boerboom, 1994). Although the relative timing and effects of uplift on the adjacent volcanic rocks are not fully understood, the association of interflow conglomeratic rocks with this structure implies that uplift may have been active during volcanism. Compositionally, these redbeds are mainly immature lithic arkose and feldspathic lithic arenite (see Fig. 5.3 in Miller and others, 2002). The angular to subrounded clasts are mainly plagioclase, mafic to felsic volcanic rock fragments, clinopyroxene, and Fe-Ti oxides; devitrified or replaced volcanic ash particles and shards are present in a few beds. Quartz is uncommon to absent. The framework grains have been variably cemented with hematite, calcite, prehnite, and a variety of zeolites, depending on the local hydrothermal/burial/contact metamorphic conditions. In some places hydrothermal minerals have replaced many or most of the clasts. A few of these redbed units have thicknesses in excess of 25 meters. These include a cross-bedded sandstone in Leif Erickson Park in Duluth (35 meters), which disconformably overlies an eroded basalt flow; the Little Marais conglomerate (and sandstone) exposed in the Manitou River area near Little Marais (as thick as 45 meters); the Indian Camp sandstone (68 meters) northeast of Lutsen; the Cut Face Creek sandstone southwest of Grand Marais (100 meters); and other newly recognized units to the north of and lower in the stratigraphy than the Cut Face Creek sandstone (See Boerboom – abstract volume for this meeting). The Cut Face Creek sandstone can be traced in outcrop for at least 4 kilometers along strike, and topographic and aeromagnetic data indicate the newly-recognized sandstones to the north extend for at least 10-15 kilometers along strike. Of these, the Little Marais conglomerate occurs at the base of the Schroeder–Lutsen basalt sequence. The Cut Face Creek sandstone underlies the Terrace Point basalt, which is now recognized to occur in the upper part of the Good Harbor Bay sequence (Boerboom and Green, 2007), and the sandstones intersected by well drilling to the north occur between basaltic lava flows tentatively grouped with the Croftville basalts. The sandstone in these units is typically planar- or cross-bedded, and some beds are ripple-marked or mud-cracked. The rocks are inferred to be dominantly fluvial, deposited by moderate-gradient, east- to southwest-flowing streams from sources nearly entirely within the subsiding Midcontinent rift basin (Jirsa, 1984). Many flow-top breccias of andesite and basalt with aa structure contain laminated red sandstone as a matrix because sand filtered down from the flow surface. Similarly, red, ILSG07 118 Trip 5 �laminated sandstone and siltstone form clastic dikes or crevice-fillings a few centimeters wide in the upper parts of some lava flows. GEOLOGY OF THE BEAVER BAY COMPLEX The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex that was emplaced into the upper part of the NSVG over a 600-km2 area in northeastern Minnesota (Fig. 5-4). Much of this area was the focus of detailed bedrock mapping by the Minnesota Geological Survey between 1985 and 1994 (Miller, 1988; Miller and others, 1989, 1993, 1994; Boerboom and Miller, 1994). Recent and ongoing mapping in the eastern BBC (Albers, 2006; Boerboom and others, 2006) will better established the geology of this part of the complex. Three general areas of the BBC, southern, northern, and eastern, are distinguished on the basis of distinctive rock types and intrusion form (Miller and Chandler, 1997). The relationship of BBC intrusions to other subvolcanic intrusions within the NSVG (Fig. 5-4) is unclear, because of poor exposure to the southwest and insufficient mapping to the northeast. Within the mapped area of the BBC, thirteen intrusive units have been identified that represent at least six major intrusive events (Miller and Chandler, 1997). Most intrusive activity forming the BBC occurred around 1096 Ma based on U-Pb dates of 1095.8±1.2 Ma for a Silver Bay intrusion, the youngest unit of the BBC, and Figure 5-4: Geology of the southern and northern Beaver Bay Complex (after Miller and others, 2001). Units labels are: nsl - NSVG Schroeder basalts; nsb - NSVG basaltic volcanics; nsf - NSVG felsic volcanics; asa - anorthositic series of the Duluth Complex; wlg - Whitefish Lake granophyre; slid Shoepack Lake inclusion-rich diorite; ccp - Cabin Creek porphyritic diorite; hct - Houghtaling Creek trocolite; blg - Blesner Lake gabbro; llg - Lax Lake gabbro; fg - Finland granophyre and qtz ferromonzonite; slg - Sonju Lake troctolite and gabbro; lvp – Leveaux porphyritic ferrodiorite; brd Beaver River diabase; sbi - Silver Bay intrusions. See map M-119 for more details. Locations of field trip stops are shown by black dots with corresponding stop number in white. ILSG07 119 Trip 5 �1096.1±0.8 Ma for the Sonju Lake intrusion (Paces and Miller, 1993). Whether activity overlapped the main stage of Duluth Complex magmatism at 1099 Ma is unknown, because attempts to date the oldest component of the BBC were not successful (Paces and Miller, 1993). The boundary between the BBC and Duluth Complex is generally marked by a northeast-trending keel-shaped intrusion in the northern BBC (Houghtaling Creek troctolite – hct on Fig. 5-4) that separates largely dike and sill intrusions of the BBC to the southeast from massive granophyric granite and extensive areas of structurally complex gabbroic anorthosite to the northwest that are typical of the roof zone of the Duluth Complex. The range of BBC parent magma compositions is similar to the olivine tholeiite and transitional basalt compositions that dominate the NSVG (Fig. 5-5A). Moreover, like the NSVG, the sequence of intrusion of BBC magmas generally involved progressively more primitive compositions. Compositional variations within the various intrusive units developed as a result of in situ magmatic differentiation (Fig. 5-5B), assimilation of footwall rocks, and/or composite intrusions of evolved magma from deeper staging chambers (Fig. 5-5C). The tightly clustered trend of BBC parent magma compositions evident on an AFM diagram (Fig. 5-5A) and the systematic variation of other elemental abundances suggest that all mafic BBC magmas evolved from a common olivine tholeiitic primary magma type. Such a primary composition, which is approximated by the most primitive, high-Al olivine tholeiites of the NSVG, is thought to have given rise to most MCR magmas, especially in later stages of magmatism (Green, 1983; Figure 5-5: AFM diagrams of Beaver Bay Complex (BBC) intrusions. A) Plot of estimated parental magma compositions to various BBC intrusions (see Miller and Chandler, 1997 for unit descriptions and details) compared to major NSVG lava compositions: OT - olivine tholeiite, TB - transitional basalt, A - andesite, FA - ferroandesite, I - icelandite, R - rhyolite (after Green, 1983). NSVG-pot is a primitive NSVG olivine tholeiite composition. The composition of the Leveaux porphyry is indicated by the label – lp. B) Calculated liquid line of descent of the Sonju Lake intrusion through troctolitic (slt), gabbroic (slg-slmd) and monzodiorite (slmd) intervals of the layered sequence. Also plotted are whole rock compositions of Finland granite (frg) and quartz ferromonzodiorite (frpm). C) Whole rock composition plots of Beaver River diabase (brd; ophitic margins and coarse subophitic interiors distinguished), Silver Bay intrusions (sbi; coarse marginal facies, layered ferrogabbroic cumulate interiors, and granophyric compositions distinguished), and composite intrusions from the northern BBC (nbbc). Sonju Lake intrusion differentiation trend (dashed line) is also shown. ILSG07 120 Trip 5 �Miller and Weiblen, 1990; Klewin and Shirey, 1992). That even the most primitive of the BBC intrusions, the Beaver River diabase, is significantly evolved from a primitive olivine tholeiite composition (Fig. 5.5C) indicates that all BBC parent magmas were generated in turn by magmatic differentiation of such a primary composition in deeper staging chambers. Petrologic modeling of some BBC intrusions and other hypabyssal bodies that intruded the NSVG (Jerde, 1991) suggests that most magmas experienced multistage, polybaric fractionation between their extraction from the mantle and their subvolcanic emplacement. Although the available radiometric ages do not indicate an overlap of magmatic activity between the BBC and the Duluth Complex (Paces and Miller, 1993), additional dates and more detailed petrologic studies may show that some Duluth Complex intrusions acted as the final levels of staging and differentiation of some BBC-bound magmas. The focus of emplacement of BBC intrusions appears to have migrated toward the rift axis and toward higher stratigraphic levels with time, perhaps reflecting plate drift and thickening of the volcanic pile. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge which trends northwest across the BBC. The presence of this buried crustal ridge is indicated by a pronounced saddle in the gravity high over northeastern Minnesota (Chandler, 1990) and the presence of Archean granitic gneiss, biotite schist, metagraywacke and granodiorite inclusions in early intrusions over the gravity low (Boerboom, 1994). The broad network of dikes and sheets characterizing the southern BBC becomes tightly focused into a narrow zone of subparallel dikes in the northern BBC which is situated over the gravity minimum (Fig.5-4). The eastern BBC opens up again into thick sheet intrusions. Based on geologic, geochronologic, geophysical, and geochemical evidence, Miller and Chandler (1997) suggested that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the formation and infilling of the western end of the Portage Lake Volcanic basin during the main to late stages of rift volcanism and graben formation. On the North Shore, these volcanic rocks are represented by the Schroeder-Lutsen basalts (Fig. 5-4). For more detailed information on these and other units of the Beaver Bay Complex see reports by Miller and Chandler (1997) and Miller and others (2002, Chapter 7) and the 1:24,000 bedrock geologic maps of the area (Miller, 1988; Miller and others, 1989, 1993a, 1994; Boerboom and Miller, 1994; Boerboom and others, 2006) This trip will have stops in the Beaver River diabase and the Leveaux porphyritic ferrodiorite. The Beaver River diabase typically forms prominent, lumpy topographic highs due in part to the large, resistant inclusions of anorthosite contained in it (e.g. Carlton Peak, stop 4B). It occurs as a series of interconnected set of dikes and sills composed predominantly of ophitic olivine gabbro. The northern and western limit is a dike that is sharply defined by outcrop and aeromagnetic data, and the area below this arc is a series of large, bifurcating dikes and thick, gently southeast-dipping sheets that are likely erosional remnants of a former semi-continuous sill (Figure 5-4). The upper and lower margins of the larger diabase sheets commonly contain inclusions of nearly pure anorthosite such as at Carlton Peak, and less commonly, large inclusions of granophyric granite. The Leveaux porphyritic diorite (LPD) is a semi-continuous, hypabyssal sill, at least 90 meters thick, that intruded into upper NSVG sequence rocks and was later intruded by the Beaver River diabase (Albers, 2006). The LPD makes up five prominent, cuesta-like ridges that trend parallel to Lake Superior over a 20 km distance and numerous smaller knob-like outcrops that occur as large inclusions within the younger Beaver River diabase. Examples of the Leveaux cuestas are Moose and Eagle Mountains, which can be seen uphill from the Lutsen Resort where this meeting is being held. Recent detailed bedrock geologic mapping by Albers (2006) and Boerboom and others (2006) has subdivided the LPD into four units: the Upper Contact Zone, the Porphyritic Zone, the Aphyric Zone, and the Sparsely Porphyritic Unit. The Upper Contact Zone is a thin (~10m), sparsely porphyritic (~5%), moderately vesicular, and locally amygdaloidal fine-grained ferromonzodiorite that forms the upper part of the LPD sill and is often not exposed due to erosion. The Porphyritic Zone is a thick (40-60m), fine- to ILSG07 121 Trip 5 �medium fine-grained ferromonzodiorite with 30-40% plagioclase megacrysts that composes the midsection of the LPD sill. The Aphyric Zone is a thin (4-19m), fine-grained ferromonzodiorite with rare plagioclase megacrysts (<3%) that occurs at the base of the sill. The Porphyritic-Aphyric Transition (PAT) is a gradational interval that separates the porphyritic and aphyric zones over a thickness of 10-40 cm. The Sparsely Porphyritic Ferrodiorite unit, which contains 5-20 % plagioclase megacrysts, occurs as outliers to the main LPD ridge. and may represent feeder dikes the LPD sill. The orientation of the PAT and sheet joints from the aphyric section indicate a gentle (10-20 degrees) southeast dip. Stops 6 and 9 will examine the porphyritic and upper contact zones of the Leveaux ferrodiorite. FIELD TRIP 5 STOP DESCRIPTIONS The general location of all stops are shown on Figure 5-6. A more detailed location map is included with each stop description, showing a portion of the appropriate 1:24,000 quadrangle map. Also listed are UTM coordinates (nad ’83), township-range-section, and the 7.5’ quadrangle name. Figure 5-6. Simple map showing relative locations of field trip stops, listed below. Stop 1. Kennedy Landing/Bell Harbor – conglomerate below SLB Stop 2 Caribou River - Pork Bay breccia and overlying basalt Stop 3. Sugar Loaf Cove – Schroeder-Lutsen sequence basalt Stop 4. Carlton Peak – Rhyolite and anorthosite Stop 5. Tofte Town Park – Schroeder-Lutsen sequence Stop 6. Springdale Hill – Porphyritic phases of Leveaux diorite Stop 7. Base of Schroeder-Lutsen sequence Stop 8. Onion River – Good Harbor Bay andesites Stop 9. Oberg Mountain -Leveaux porphyritic diorite and shoreline view Stop 10. Conglomerate Stop 11. Cut Face Creek – Terrace Point basalt, sandstone, and Good Harbor Bay andesite ILSG07 122 Trip 5 �---Leaving Lutsen Resort, head southwest on Hwy 61. Proceed approximately 26.5 miles to overpass over prominent creek. We will take the creek bed down to the shore and return to the road by private driveway. This is private property, so permission to access the shore is required. .--- Stop 5-1. Unconformity Conglomerate at Kennedy Landing/Bell Harbor. UTM coordinates (nad ’83) 640454E, 5274226N T.57N., R.6W., Sec.30, Finland quadrangle Highlights: ophitic basalt (Schroeder-type), polymict conglomerate, angular unconformity, felsic lithic tuff, agate-bearing quartz tholeiite flow, normal faults, interflow basalt clast conglomerates. U, 0 t z p.-, Stop 1 Description: From the cliff face at the west end of this bay, several volcanic and structural features are visible as noted in Figure 5-7. Exposed in the cliff face is an ophitic basalt which is the basal flow of the Schroeder-Lutsen sequence (OB). Underlying this basalt is a polymict volcanogenic conglomerate (PCg). Various types of basalt dominate this conglomerate, but it also contains clasts of felsite, granophyre, lithic tuff, and autolithic . Figure 5-7. Plan view of the geology exposed at Stop 5-1. Unit abbreviations are: OB – ophitic basalt, PCg – polymict conglomerate, B1-3 – basalt flows, FLT – felsic lithic tuff, BC1-2 – basalt clast conglomerate. Inset show fault relationships between OB and PCg units in western cove. ILSG07 123 Trip 5 �shale fragments. In the back wall of the cove between the ophitic basalt ledge and the point of conglomerate, two splayed faults are exposed (Fig. 5-7, inset). On the northeast side of the point of polymict conglomerate is in sharp angular contact with a thin sheet of basaltic andesite (B1) and and underlying a felsic lithic tuff (FLT) with imbricated clasts of porphyritic rhyolite. The contact between the polymict conglomerate and the basalt/tuff appears to be an unconformable erosional surface, but a fault contact cannot be discounted. . Continuing northeast, this basalt/lithic tuff sequence also abruptly give way across a small gap (fault?) to a basalt clast conglomerate (BC1). Near the outlet of a stream, the BC1 conglomerate grades downsection to a basalt breccia flow top of quartz tholeiite basalt flow (B2). This flow is colored a deep army green and contains abundant vugs and amygdules of agate and amethyst. Northeast of the point formed by flow B2, another basalt clast conglomerate (BC2) caps the upper part of the next underlying lava flow (B3). Discussion: The polymict conglomerate, which is termed the Little Marais conglomerate (Miller et al., 1993; Miller et al., 2006), is interpreted to occupy an angular unconformity that separates the SchroederLutsen sequence from the Upper Southwestern sequence of the NSVG. It can be traced at least 10 km to the northeast into the Little Marais quadrangle. Farther to the east, this unconformity develops a conformable relationship with the underlying volcanics and is marked by a thick sandstone unit (Cutface Creek Sandstone) (see Stop 5-11). ---Return northeast on Highway 61. Approximately 7.5 miles after crossing the Caribou River, turn left into parking area. Take the Superior Hiking trail path upstream about 300m to low-lying outcrops on the rivers edge .--- Stop 5-2. Pork Bay Breccia and Manitou River basalt. UTM coordinates (nad ’83) Area A-648250E, 5258850N; Area B-648480E, 5258950N T.58N., R.6W., Sec. 36, Little Marais quadrangle Highlights: Volcaniclastic breccia (Pork Bay breccia), thin olivine tholeiite flows, transitional basalt (Manitou River basalt) A Description: Location A- Exposed in low-lying outcrops on the south bank of the river is a brick red, polymict, matrix-supported, volcaniclastic breccia – the Pork Bay breccia. It is composed of subangular to subrounded volcanogenic clasts up to 40 cm across in an unbedded, sandy matrix (Fig. 5-8A). Clasts are composed dominantly of massive to scoriaceous and oxidized to fresh basalt and minor amounts (<5%) of other riftrelated rock types (e.g., gabbro, felsite, volcanic sandstone). The matrix consists of granule to medium sand-sized volcanic fragments cemented by calcite and zeolite. B Stop 2 Location B – Progressing upstream about 200 m to where the river make a sharp turn to the left (north), units overlying the Pork Bay Breccia are exposed (Fig. 5.8B). In the streambed are several thin (<1m) amygdaloidal olivine tholeiite flows oriented N50E/~20°SE. Exposures of volcaniclastic breccia occur ILSG07 124 Trip 5 �adjacent to the thin basalts, but contacts are not exposed. At the base of the cliff face on the southern shore are poor exposures of the breccia which presumably overlie the thin basalts. The overlying this breccia and exposed over most of the 8m-high cliff face is a massive, slightly Pl-porphyritic transitional basalt – the Manitou River basalt. At the basal contact, this flow is strongly chilled possibly irregular pockets of glass. We will not have time to continue upstream to Caribou Falls, however, a later visit is encouraged. The falls is interpreted to represent a major fault scarp that juxtaposes the Pork Bay breccia (downstream) with olivine tholeiitic basalts of the Schroeder-Lutsen sequence. Despite ample exposure in the falls area, a well-identified fault zone could not be identified leaving open the possibility that the contact may be steep lithologic contact rather than a structural one. MRB PBB OT Figure 5-8. A) Close-up photo of the polymict clasts in the Pork Bay breccia; whitish interstitial mineral is mostly calcite. B) Exposures in southern stream bank at Location B – OT – thin olivine tholeiite flows in streambed, PPB – breccia exposed in lower cliff face, MRB – massive, transitional basalt exposed in upper cliff face. Discussion: The origin of the Pork Bay Breccia is enigmatic. Various ideas put forth to explain its formation include that it is a lahar or some type of mass wasting deposit, that it represents a collapse breccia within a caldera, and that it represents deposits of an explosive cinder cone. None of these completely explain the salient attributes of the unit or its relationship with adjacent units. These attributes include: the wide variety of volcanic, intrusive and sedimentary clast types, the subrounded nature of the clasts, the porous sandy matrix, the lack of bedding, the thickness of the breccia (up to 60 m), the occurrence of variable thicknesses of tholeiitic basalts (2- 100m) between the breccia and the Manitou River basalt, and the occurrence of basalt flows within the upper parts of the breccia (as seen at location B). Given these attributes, we will discuss possible modes of origin on the outcrop. ---Continue northeast on Highway 61. Approximately 2.7 miles from the Caribou River, turn right into Sugar Loaf Cove Interpretive Center (look for a brown highway sign). Park in the lot and follow trails down to Lake Superior shore.--- ILSG07 125 Trip 5 �Stop 5-3. Olivine tholeiitic basalt flows at Sugarloaf Cove. UTM coordinates (nad ’83) 652044E, 5261120N T.58N., R.5W., Sec. 29, Little Marais quadrangle No hammers please! Note: This stop is on the property of the Sugarloaf Cove Interpretive Center Association and a State of Minnesota Scientific and Natural Area. It has been established to preserve the natural features of the beach and point for all to enjoy. Please leave your hammers behind and take only pictures, not samples. Highlights: Morphological features of olivine tholeiitic basalt flows. Description: The lava flows exposed at and near this stop are typical of the Schroeder-Lutsen basalts. Flows exhibit features characteristic of fluid, low-viscosity pahoehoe olivine tholeiites (Fig. 5-9). Columnar joints and sandstone-filled clastic dikes (sand-filled cracks) are common in the upper part of the massive flow interiors. Figure 5-9. Idealized internal structure of olivine tholeiite-composition lava flows. From Green, 1989. Six lava flows are exposed on the point (Fig 5-10A). The about 10 m of the upper part of flow 1, an ophitic basalt, is exposed on the southwest end of the point. Flows 2 and 3 are 1- to 2-meters thick and display bent pipe amygdules and oxidized ropey flow tops (Fig. 5-10B). Flow 4 is 2-3 meters thick and displays well developed columnar joints, ropey flow tops, and abundant clastic dikes at its eastern extent. Flows 5 and 6 define a trough structure atop flow 4 in the midsection of the point. At the northeast end, flow 5 occurs as two thin (~1m) flows and over 5 meters of ophitic basalt of represent the lower part of flow 6. ILSG07 126 Trip 5 �A. B. 4 3 35 2 Figure 5-10. A) Sketch of distribution and orientation of six flows on Sugarloaf Cove Point. B) Photo of thin lava flow 2-4 at the west end of the point. Discussion: These olivine tholeiite basalt flows are typical of the Schroeder-Lutsen sequence, the uppermost sequence of the North Shore Volcanic Group. In this area the Schroeder-Lutsen basalts unconformably overlie a thick sequence of mafic to felsic volcanic rocks, informally termed the Cross River lavas (Boerboom and Green, 2006), that strike approximately N55E and dip 10-20° SE, in contrast with the nearly shore-parallel strike of the Schroeder-Lutsen basalts (see Fig. 5-15, stop 7). The occurrence of these olivine tholeiite flows at the top of the NSVG highlights the fact that the NSVG generally becomes more primitive (less evolved) up section. ---Return to Highway 61 and drive NE approximately 7 miles to Temperance River. Turn left on small gravel road 0.8 miles NE of Temperance River across from National Forest Service Sign. Drive 0.95 miles up hill, over outcrops in road bed (Schroeder basalt) and park; Stop 4A outcrops just east of road.-- Stop 5-4A. Carlton Peak rhyolites. UTM coordinates (nad ’83) 660737E, 5271317N T.59N., R.4W., Sec. 20, Tofte quadrangle Highlights: Aphyric and porphyritic rhyolite flows. Description: There are several low outcrops in this area of rhyolite (at the south edge) and porphyritic rhyolite (at the north edge). There are also outcrops of andesite to the northeast of the rhyolite, and of ophitic basalt to the north of that. The porphyritic rhyolite is grayish-pink, with 58% phenocrysts of K-feldspar, quartz, oxidized iron silicate minerals, and magnetite. The aphyric rhyolite is light gray to purplish-red, with rare small phenocrysts of quartz, K-feldspar, altered mafic silicates, and magnetite. Small vuggy quartz-lined amygdules and tension gashes are common throughout. Discussion: The rhyolites here are part of the Carlton quarry sequence, which includes ophitic basalt at the top, ILSG07 127 Trip 5 �andesite, porphyritic rhyolite, and aphyric rhyolite at the base. This sequence has been intruded by the Beaver River diabase (stop 4B). This entire set of flows dips moderately northwest, and is presumably fault-bounded. The porphyritic rhyolite has an age of 1,094±2.0 Ma (Green and others, 2001), the youngest age obtained to date from Keweenawan rocks in Minnesota, and is comparable in age and stratigraphic position to rhyolite in the Porcupine Volcanics in Michigan and to the upper part of the Portage Lake Volcanics (Green, 2002 and references therein). The position of the Carlton Quarry lavas within the NSVG is somewhat enigmatic, given the age, and fortuitous gaps in outcrop preclude full understanding of the relationships of these lavas to the rest of the NSVG. ---Continue on up the gravel road to a large flat area below the main quarry face, or turn left on the way and wind your way up to the quarry wall. If you go up there, BE CAREFUL!--- Stop 5-4B. Carlton Peak anorthosite and Beaver River diabase. UTM coordinates (nad ’83) 661309E, 5272060N T.59N., R.4W., Sec. 20, Tofte quadrangle Highlights: Carlton Peak anorthosite, Beaver River diabase (BRD). Description: In the quarry staging area are multiple large blocks of pale green to locally purple, coarsegrained anorthosite, and blocks of weathered, dark green, Beaver River diabase. The anorthosite contains scattered 2-3 cm diameter dark clots of altered poikilitic olivine, but otherwise is composed of nearly 100% fresh, glassy plagioclase. The anorthosite is well exposed in natural outcrops at the top of Carlton Peak, which requires a moderately strenuous hike of approximately one mile on the Superior Hiking Trail, accessed by driving 2.8 miles north of Tofte on County Highway 2 (Caribou Trail) to the well-marked Superior Hiking Trail parking lot. An alternative short hike to the top of Britton Peak, located just east of and above the parking lot, leads to an inclusion of the same type of anorthosite, and a clear view of Carlton Peak. Other places on the shore where these anorthosite inclusions can be easily seen are at Split Rock Lighthouse State Park (see guidebooks by Miller and others, 1987, Stop 1 and Boerboom and others, 2004, Stop 2-8), and Tettegouche State Park. The easiest place of all is a newly-constructed road cut on Highway 61 in Silver Bay, which has clean exposures of anorthosite inclusions within the BRD. Discussion: Carlton Peak is a prominent topographic knob formed by the Carlton Peak anorthosite, which occurs as large inclusions in the BRD. Here and at many other places along the North Shore, anorthosite forms rounded, bare knobs because it is very massive, contains few fractures, and is more resistant to chemical weathering than the surrounding diabase (Fig. 5-11). Individual anorthosite inclusions at Carlton Peak range up to 1100 feet in diameter, and are among the largest of the known anorthosite inclusions in the BRD. The anorthosite inclusions throughout the BRD are round to angular in shape and vary in size from individual plagioclase crystals to large inclusions such as at Carlton Peak, and some are brecciated and recrystallized (Morrison and others, 1983). They are all composed of 90-99% course-grained bytownite to labradorite, and minor Mg-rich olivine, orthopyroxene, and less commonly clinopyroxene. The inclusions are most common near the bottoms of BRD sills, and in sharp contact with the BRD, but the BRD is not chilled against them. Anorthosite inclusions of this type are different from anorthositic-series rocks of the Duluth Complex, but there are similar inclusions of anorthosite within the anorthositic series. The highly disordered structural state of plagioclase and the absence of chilled margins in the BRD against the anorthosite indicate these inclusions were derived from a lower to mid-crustal source (Miller and Weiblen (1990). ILSG07 128 Trip 5 �Figure 5-11. Google earth oblique image of Carlton Peak anorthosite, looking northeast up the shore. From this perspective it appears to be a large slab-shaped inclusion. Isotopic and trace-element compositions of the anorthosite xenoliths imply that they may be preKeweenawan in age (Morrison and others, 1983), but the data are ambiguous. As an alternative, Miller and Weiblen (1990) suggest a crystal-mush theory in which significant proportions of Keweenawan anorthosite may have been generated by flotation of plagioclase under high pressure in the deep crust prior to Beaver Bay Complex magmatism at 1,096 Ma. Plagioclase cumulates formed under deep crustal conditions would probably be distinctive in character and composition, compared to anorthosite cumulates formed in the shallow crust. The ambiguous isotopic compositions of the inclusions may indicate that the anorthosites formed from Keweenawan magmas that were contaminated by older crust, rather than older anorthosites being contaminated by interaction with Keweenawan magmas. The Beaver River diabase (BRD) forms an extensive network of dikes and sills, and is one of the youngest intrusive phases of the Beaver Bay Complex. The BRD extends from Split Rock Point to just west of Grand Marais, in a large arc that is approximately 57 miles (90 km) long (Figure 5-4). Most of the diabase is fine-grained ophitic olivine gabbro, but the centers of some of the thicker sills (up to 150 meters) grade into coarse-grained subophitic to intergranular gabbro. Historic sidelight: In 1902, anorthosite from an inclusion similar to this, in present-day Split Rock Lighthouse State Park, was quarried by a group who mistakenly thought the anorthosite was corundum. In fact there is actually a high point of land on Lake Superior in the park, held up by anorthosite, known as Corundum Point. Another quarry site was near the Baptism River at Illgen City. Their plan was to sell the “corundum’ on the east coast, but they had to abandon that plan and turn to other ventures. This company went on to become the Minnesota Mining and Manufacturing Company, now simply 3M! ---Drive back to Highway 61 and head northeast towards Tofte for 0.8 miles, then veer right onto Tofte Park Road (24). Take first private drive to the right and head to shore OR continue to Tofte town park and boat access on right after about ¼ mile. From boat landing, head southwest about 1 km along semicontinuous outcrop of tholeiitic lava flows (~7 flows). Stop is 250m southwest of old boathouse.--- ILSG07 129 Trip 5 �Stop 5-5. Flow features and feeder dikes in Schroeder basalts UTM coordinates (nad ’83) 661800E, 5270200N T.59N., R.4W., Sec. 28, Tofte quadrangle Highlights: Ophitic olivine tholeiite basalt flows, columnar jointing, amygdaloidal feeder dikes. Description: Accessing the shore at the old boathouse (662000E, 52700350N), a smooth billowing flow contact dipping gently to the northeast is exposed at the boat ramp. Heading southwest about 30 meters, a north-trending 0.5m-wide feeder dike is exposed in the amygdaloidal flow. The dike is zoned in the degree of oxidation and in the concentration of vesicles and amygdules (Fig. 512A). Boat Landing Boat House Stop 5 Continuing about 130 meters to the southwest over the polygonally-jointed surface of an ophitic basalt flow, a domal flow contact with a thin flow is encountered. Passing across this dome back onto the ophitic basalt, one can see a feeder dike breaking through the amygdaloidal top of the underlying flow and feeding the ophitic basalt. The feeder dike is lined with pipe amygdules that are curved upward (Fig. 5-12B). A . B .. Figure 5-12. A) Feeder dike with zonation in oxidation and amygdule concentration. B) Feeder dike with upturned pipe amygdules. ILSG07 130 Trip 5 �---Continue northeast on Tofte Park Road to Highway 61. Turn right (northeast) and go three miles to Leveaux Ridge road on the left, across from the Chateau Leveaux. Go up Leveaux Ridge Road a short distance (~0.25 mi)., then turn right on Overlook Trail. Head uphill(~0.3 mi), park where road curves to west near driveway on right (21 Overlook Tr.). --- Stop 5-6. Porphyritic Phases of Leveaux Porphyritic Diorite (LPD) UTM coordinates (nad ’83) 665850E, 5274260N T.59N., R.4W., Sec. 14, Tofte quadrangle andesite Stop 6 - :i-çl " - H- ' -- Schroeder- Highlights: Amygdaloidal olivine tholeiite basalt of Schroeder Lutsen sequence, inferred fault zone, amygdaloidal, sparely porphyritic upper zone and porphyritic zones of the Leveaux porphyritic sill. Description: Exposures in a low ledge and the drainage ditch south of the driveway are moderately amygdaloidal intergranular basaltic andesite. Several mineralized northeast-trending shear zones cut the rock. On the opposite side of the driveway is a 5’ roadcut of sparsely plagioclase porphyritic and amygdaloidal, fine-grained ferrodiorite. Centimeter-sized plagioclase phenocrysts compose about 5% of the rock as does a similar abundance of quartz amygdules (Fig. 5-13A). This rock type continues along this roadcut to the next driveway (#33). Opposite the bridge across a substantial creek are pavement outcrops of strongly porphyritic (30-40%) ferrodiorite which are typical of the main Porphyritic Zone of the LPD sill (Fig. 5-13B) Figure 5-13. A) Outcrop appearance of sparsely porphyritic and amygdaloidal ferrodiorite typical of the Upper Zone of the Leveaux ferrodiorite. B) Outcrop appearance of strongly porphyritic ferrodiorite typical of the main Porphyritic Zone of the Leveaux ferrodiorite. Discussion: This exposure of the Leveaux porphyritic diorite sill is the only location where the upper contact zone is preserved. Along most of the cuesta mountain exposures, the upper part of the sill is ILSG07 131 Trip 5 �eroded into the porphyritic zone (see Stop 8). The vesicular/amygdaloidal character of the upper contact zone speaks to the shallow level of emplacement of the LPD into the volcanic edifice. The sparsely porphyritic nature of this zone is interpreted by Albers (2006) to have been caused by flow differentiation of phenocrysts out of the upper contact zone during emplacement and subsequent rapid crystallization. Also, this inferred fault is a major fault zone which can be traced for 10’s of kilometers subparallel to the shore. ---Go back to Highway 61 and continue NE 0.16 miles to roadcuts on uphill side of Highway. Park at edge of highway. WATCH OUT FOR HEAVY TRAFFIC!. --- Stop 5-7. Base of Schroeder-Lutsen Sequence and top of underlying andesites UTM coordinates (nad ’83) 666381E, 5274345N T.59N., R.4W., Sec. 13 Tofte quadrangle Highlights: Contact between base of Schroeder-Lutsen basalt sequence and underlying Good Harbor Bay andesites. This stop will be at a roadcut, but if permission can be obtained a better outcrop may be visited down on the lake shore. The description below is of the lakeshore outcrop, but most of the features in it can be see on the roadcut. Description: Upper part of andesites is composed of amygdaloidal andesite fragments up to 50 cm in size, surrounded by reddish-brown sandstone. The andesite clasts are angular, and vary in shape from equant to slab-like; the latter commonly oriented near vertical. Sand makes up 3040% of the volume of the upper 3 meters of the breccia, and the breccia is capped by a cross-bedded sandstone wedge that is cut off by erosion on a cliff corner (Fig. 5-14). Figure 5-14. Photographs of base of Schroeder basalt and underlying fragmental andesite, from outcrop on shoreline below stop 7. Hammer is approximately 45 cm long. ILSG07 132 Trip 5 �The base of the Schroeder basalt flow undulates slightly and contains pipe amygdules. In places, if the light is right, one can make out vague, bulbous lobe-like forms approximately 30 cm in diameter at the base of the flow. Discussion: There are two more exposures between here and Lutsen similar to this one, all characterized by large, boulder-sized and blocky amygdaloidal andesite clasts surrounded by sandstone, with a thin, discontinuous sandstone cap. These are also similar to andesite exposed beneath the Little Marais conglomerate, approximately 15 miles to the southwest (Fig. 5-15; Miller and others, 2006). The Little Marais Conglomerate is at least 60 feet thick, and composed mostly of sandstone, but contains some silt/shale beds, as well as large rounded boulders of basalt near the base and large angular blocks of basalt higher in the section that are ‘floating’ in sandstone. In all cases the underlying fragmental andesites are unusually thick, with substantially larger clasts than typical andesitic flow-top breccias. Despite the fact that the Schroeder-Lutsen sequence is clearly unconformable to the underlying rocks, as made evident by the discordant strike directions between the two (Fig. 5-15), the andesite flow tops below are all fragmental, at least in the available exposures. The upper surface of the Good Harbor Bay andesites may have been a slightly eroded surface made up of clinkery lava flows with an irregular topography, covered by sedimentary rocks of variable thickness. Local fault escarpments that formed prior to eruption of the Schroeder-Lutsen basalts may account for the unusually thick Little Marais conglomerate, with blocks of basalt fallen off the fault escarpment into the sedimentary pile. Figure 5-15. Simplified geologic map of a strip along Lake Superior from near Little Marais to Lutsen showing locations where unconformity beneath the SchroederLutsen basalts is exposed. ILSG07 133 Trip 5 �---DIRECTIONS: Continue northeast on Highway 61 about 0.7 miles. Just past the Onion River, pull into the Ray Berglund State Scenic Memorial Wayside. Park there and walk SW along highway to Onion River. An excerpt from MN Dept. of Transportation website: “The Onion River derived its name from a Paul Bunyan legend. The legend says that wild onions grew in such abundance in the area, that Paul Bunyan's loggers shed tears while cutting lumber”.--- Stop 5-8. Morphology of andesite lava flows in the Onion River valley. UTM coordinates (nad ’83) 667525E, 5275197N T.59N., R.4W., Sec. 12 Tofte quadrangle *The extent to which these flows can be seen is dependent upon the water level in the Onion River. If the river is high, some outcrops can be accessed on the highway and on the shoreline near the river mouth. Highlights: Andesite flow features, Terrace Point basalt, minor faults. Description: This stop will go through a multiple series of andesitic lava flows which exhibit classic flow features (Fig. 516). The andesite is porphyritic (plagioclase, clinopyroxene, some magnetite), with a matrix of fine felty plagioclase, interstitial K-feldspar, granular clinopyroxene, and blocky Fe-Ti oxides. A sand-filled flow-top breccia that caps the first flow in the river series is exposed on the roadcut just SW of the Onion River and on the shore at the river mouth. The base of the same flow, which exhibits classic andesite oxidation-lamination texture, from which the attitude of the flow can be reliably measured, is exposed in the river bed below the highway. A 3 meter wide brittle fault (attitude N45E, 85°N), with reverse sense of movement (north side down approximately 2 meters) is exposed about 200 feet north of the highway on the east bank. The fault zone contains brecciated andesite fragments surrounded by coarse white calcite and pinkish-orange laumontite. This fault projects back southwest across the river to a till-covered portion of the slope. Next to the fault, upstream, is another thick sand-filled flow-top breccia at the top of the next underlying flow. Proceed upstream past a total of five andesite flows. The sixth flow is the western end of the Terrace Point basalt flow (see also Stop 11 and Fig. 5-18). Although the top of the flow is fragmental similar to the andesite flows downstream, it is internally ophitic (very rare for an NSVG andesite), and contains rare fresh and glassy, blocky plagioclase phenocrysts. The next flow below this ophitic basalt is probably icelandite in composition, but lacking chemistry has been lumped with the andesites. After a series of andesite and icelandite flows, the river makes a sharp bend to the east where it intersects a prominent fault marked with slickenside surfaces that forms a high escarpment on the south side of the river. Above this is a narrow canyon that can be traversed in late summer when the water level is lowest, through a series of thin, irregular olivine tholeiite basalt flows. Traverse upstream until a snowmobile trail bridge is encountered; there is a small trail along the east side of the river that goes south from the snowmobile trail back to the parking area. Discussion: These flows were original termed the Onion River lavas by Boerboom and others (2006), with all the lava flows from Highway 61 to beyond the snowmobile bridge lumped together. Subsequent mapping to the east (Boerboom and others, 2007) has shown that the Onion River lavas are actually the western end of the Good Harbor Bay andesites. To the east the Terrace Point basalt is underlain by the Cut Face Creek sandstone, but that apparently pinches out and is no longer present here. ILSG07 134 Trip 5 �Figure 5-16. Idealized flow structure of quartz tholeiite to andesitecomposition lava flows. From Green, 1989. Figure 5-17. Geologic map of the Onion River lavas. ---Continue 0.4 miles northeast on Highway 61 to the Onion River Road. Take the Onion R. Rd 1.7 miles to parking area for the Superior Hiking Trail. Take Oberg Mtn trail about 0.6mi (~1000m) to the southwest overlook . --ILSG07 135 Trip 5 �Stop 5-9. Porphyritic Zone of Leveaux Porphyritic Diorite (optional stop) UTM coordinates (nad ’83) 666960E, 5276900N T.59N., R.4W., Sec. 1 Tofte and Honeymoon quadrangles Highlights: Porphyritic Zone of the LPD, vista of Leveaux Mountain and shoreline. Description: At the southwestern crest of Oberg Mtn, exposures of typical porphyritic ferrodiorite of the upper Porphyritic Zone of the LPD are exposed. The weathering of the outcrop accentuates the porphyritic texture of the white plagioclase against a brownish matrix. From this vantage point, the southeast-dipping sill can be seen in the topographic expression of Leveaux Mountain visible across the Onion River valley to the southwest. Farther southwest just off shore, Gull and Bear Islands can be seen (on a clear day) near the Taconite Harbor power plant, about 10 miles away. These islands are formed by outliers of LPD and represent the southernmost exposures of the Leveaux porphyritic diorite sill. ---DIRECTION: Continue east on Highway 61 for about 2.5 miles to the Poplar River. 600 feet past the Poplar River, veer right onto County Road 35 (gravel). Drive east on this road 1 mile and park at pull off where road joins back onto Highway 61. Walk straight down to lake through the woods to small cove.--- Stop 5-10. Interflow conglomerate. UTM coordinates (nad ’83) 673942E, 5279298N T.63N., R.3W., Sec. 24 Lutsen quadrangle Highlights: Interflow conglomerate, flow contact. Description: Small, 2 meter thick exposure of reddishbrown, poorly bedded, moderately well sorted and rounded, cobble conglomerate exposed below a thick flow of Lutsen basalt. The conglomerate is exposed as a small erosional window through the Lutsen basalts. Cobbles in the conglomerate are mostly less than 5 cm in diameter, in a matrix of coarse gritty sand and white calcite cement. In order of abundance, the clasts are composed of ambiguous brown, fine-grained mafic to intermediate volcanic rocks, flow-banded rhyolite, amygdaloidal basalt possibly similar to the adjacent Lutsen basalt, reworked sandstone, and rare agate. In thin section many of the clasts are composed entirely of a felty-matted, gray-birefringent mineral, presumably some type of secondary zeolite; others are replaced by ILSG07 136 Trip 5 �calcite in irregular alteration zones and clots. Less altered clasts exhibit relict flow banding typical of NSVG rhyolites, and relict textures typical of NSVG mafic to intermediate volcanic rocks. The contact with the overlying flow of dark green, ophitic Lutsen basalt is very sharp and straight, measured at N70E, 15° SE, typical for the NSVG. Discussion: Further northeast up the shore there are some reddish-brown, fine-grained, interflow sandstones contained entirely within the Lutsen basalts, as scattered 2-3 meter thick and one 75 meter thick bed. Although the conglomerate at this stop is likely not more than a few meters thick and may similarly be contained entirely within the Lutsen basalts, it does not outcrop in nearby stream exposures where anticipated if it were a conformable layer in the Lutsen basalts. More likely, this exposure is a window eroded through the base of the Schroeder-Lutsen sequence, and the conglomerate marks the unconformity between the Good Harbor Bay andesites and the Lutsen basalts (see Fig. 5-15, stop 7). The conglomerate contains few if any clasts derived from the texturally distinct Lutsen basalts, but rather is composed predominantly of felsic-intermediate composition volcanic rocks, basalt unlike Lutsen basalt, and some reworked sandstone. Thus the source for the conglomerate is apparently dominated by older volcanic and sedimentary rocks that are similar in composition to those observed inland from the Schroeder-Lutsen sequence. The conglomerate may represent local deposition along a minor fault escarpment, or a proximal-facies high-energy fluvial deposit filling topographic depressions on the andesites. ---Drive northeast on Highway 61 approximately 13 miles to Historical Marker/Geologic Marker at top of steep hill (Terrace Point) at curve in road. Park at wayside, stop is across Highway at large roadcut. BE CAREFUL CROSSING HIGHWAY.--- Stop 5-11A. Cut Face Creek Roadcut UTM coordinates (nad ’83) 691780E, 5289170N T.61N., R.1W., Sec. 34 Good Harbor Bay quadrangle Highlights: Thick interflow sandstone, deformation features in sandstone at base of flow, shale rip-up chips, dessication cracks, Terrace Point basalt flow, fragmental basalt intruded and overrun by basalt flow. Description: In high roadcut on north side of highway is an obvious contact between the Cut Face Creek Sandstone and the overlying Terrace Point basalt flow. The sandstone overlies the Good Harbor Bay andesites (stop 11b). Sandstone –Jirsa (1984) measured approximately 73 meters of sandstone and 3 meters of shale in this section of the Cut Face Creek sandstone, but reported overall that nearly 30 percent of it is composed of thinly bedded, graded layers of fine-grained sand, silt, and clay. He reports both symmetrical and asymmetrical ripple marks, and bimodal paleocurrent distribution, and concluded that the Cut Face Creek sandstone was deposited in a fluvial-lacustrine environment. Unlike nearly all the other exposed interflow sandstones within the NSVG, planar cross-bedding is predominant, but some trough cross beds are present near the top of the section. Other features that may be visible in ILSG07 137 Trip 5 �the sandstone are dessication cracks filled with sandstone or coarse pink zeolite minerals, and rip-up textures. The base of the sandstone is exposed to the north in the Cut Face Creek valley, where it overlies the Good Harbor Bay andesite (see stop 11B). Basalt – The Terrace Point basalt, which overlies the sandstone, is a major ridge-forming unit from here to the southwest, forming ‘sawtooth mountains’. It is a distinctive flow characterized by a dark green color, white thomsonite amygdules, uniform 3-4mm ophitic texture, and scattered small glassy plagioclase phenocrysts. In general the contact with the sandstone is sharp and straight, but in places the sandstone has been slightly deformed by the basalt flow. At the south end of the outcrop is a unit of scoriaceous, fragmental basalt that is intruded and by the Terrace Point flow. Similar rock types and relationships have been observed to the southwest, also near the flow base. The breccia contains 1-100 cm angular basalt fragments that are both massive and amygdaloidal, and scattered large blocks of massive basalt. At this locality and others, sub-volcanic dikes of Terrace Point basalt that intrude the fragmental basalt are slightly chilled, and contain small amygdules stretched parallel to, and columnar joints perpendicular to, curvilinear dike margins. The fragmental unit is interpreted as a cinder cone or lahar-type deposit that was subsequently intruded and overrun by the thick Terrace Point basalt flow. Figure 5-18. Simplified geologic map showing distribution of interflow sandstones and major volcanic units in the Lutsen to Terrace Point area. Discussion: The Terrace Point basalt flow has been considered (e.g., Green, 1972) to be the basal flow of the Schroeder-Lutsen sequence. However, recent detailed mapping to the southwest (e.g. Boerboom and Green, 2006, Boerboom and others, 2006 and 2007) has shown that the Terrace Point basalt is actually contained within the Good Harbor Bay lavas. On the Onion River continuous exposures show that the top of the Terrace Point basalt is fragmental and overlain by more andesite (Stop 8), and is clearly an ILSG07 138 Trip 5 �integral part of the flow sequence. Also, the base of the Schroeder-Lutsen sequence overlies both the Terrace Point basalt and andesites in the Good Harbor Bay lavas (Figure 5-15, stop 7). Thus, based on new mapping, the Terrace Point flow and the Good Harbor Bay andesites are now considered to be part of a continuous series termed the Good Harbor Bay lavas. The Cut Face Creek Sandstone was considered to represent clastic deposition during a hiatus in volcanism prior to eruption of the Terrace Point/Schroeder-Lutsen basalts. It was also considered to be somewhat unique in that it was one of the few thick sandstone units in the northeast limb of the North Shore Volcanic Group, along with the 68 meter thick Indian Camp sandstone (which is within the Schroeder-Lutsen basalts). However, recent remapping has shown that there are likely at least three more substantially thick sandstone units that occur within lower series of lava flows to the north of the Good Harbor Bay lavas (Fig. 5-18; Boerboom and others, 2007). Thus, it is now recognized that the Cutface Creek and Indian Camp sandstones, exposed because of more active erosion near the Lake Superior coast, are only one part of a larger set of thick sandstone units, some of which are located over 5 miles inland. The recognition of thick interflow sandstones throughout several different series of lava flows at different stratigraphic heights implies active sedimentation during a prolonged period of volcanism. Fragmental flow tops in the Good Harbor Bay Lavas contain abundant sandstone infillings, and thin, discontinuous, layers and crack fillings of sandstone are common in the Schroeder-Lutsen basalts. Elevated levels of clastic deposition during active volcanism may account for the relative abundance of sand at the tops of the lava flows, and the thick interflow sandstones may have formed during periods of volcanic quiescence, but continued basin subsidence. Available data seem to indicate that the thicker sandstones may vary in thickness along strike, consistent with deposition onto an irregular lava surface. ---Drive northeast on Highway 61 approximately ¼ of a mile to the Cut Face Creek wayside located to the right on the lake shore, just past Cut Face Creek. Park at wayside. Either cross highway to small roadcuts across from wayside (WATCH TRAFFIC), or if the creek is low walk back on beach to Cut Face Creek and proceed upstream in creek valley.--Stop 5-11B. Good Harbor Bay andesite UTM coordinates (nad ’83) 691953E, 5289526N T.61N., R.1W., Sec. 34 Good Harbor Bay quadrangle Highlights: Good Harbor Bay andesites, Cut Face Creek Sandstone Description: Small outcrop directly across highway from parking area and to northeast, and outcrops at the mouth of Cut Face Creek, are fine-grained, brownish-gray, sparsely porphyritic andesite. These are part of the Good Harbor Bay andesites, a series of andesitic flows that stretches from near Grand Marais to Tofte. The andesite here is overlain by the Cut Face Creek Sandstone, and the contact is exposed in numerous locations in the meandering Cut Face Creek valley. The basal contact appears to be sub-conformable with the andesite on a local scale, but on a regional scale the upper andesite surface probably undulates. Small pebbles of massive to amygdaloidal andesite are common in the lower 3 meters of the sandstone, typically in 3-25 cm thick planar cross-bedded pebbly beds. The upper part of the andesite typically contains stretched amygdules, and has cracks filled with sandstone (Fig. 5-19). Like the exposures in the Onion River (Stop 9), the andesites here have rubbly, sand-filled flow top breccias with fragments of stretched amygdaloidal andesite. Figure 5-19. Photograph of base of Cut Face Creek sandstone (white arrow), showing clastic dike (black arrow) filling a crack in underlying andesite. Hammer in circle is 45 cm long. ILSG07 139 Trip 5 �REFERENCES Albers, P.A., 2006, The geology and petrology of the Leveaux Porphyritic Diorite Intrusion: Investigating possible magmatic relationships to the anorthositic series of the Duluth Complex, Cook County, Minnesota. Unpublished MS thesis, University of Minnesota Duluth, 121 p. w/ map. Basaltic volcanism study project (BVSP), 1981, Pre-Tertiary continental flood basalts: in Basaltic Volcanism on the Terrestrial Planets, NY, Pergamon Press, p. 30-77. Boerboom, T. J., 1994, Archean crustal xenoliths in a Keweenawan hypabyssal sill, northeastern Minnesota. White was right!: Institute on Lake Superior Geology 40th Annual Meeting, Houghton, MI: Proceedings v. 40, pt. 1 Abstracts, p. 5-6. Boerboom, T. J., Green, J.C., and Albers, P., 2007, Bedrock geologic map of the Lutsen quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-1XX, scale 1:24,000. (in press). Boerboom, T. J., and Green, J. C., 2006, Bedrock geology of the Schroeder quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-170, scale 1:24,000. Boerboom, T. J., Green, J. C., Albers, P., and Miller, James D., Jr., 2006, Bedrock geology of the Tofte quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-171, scale 1:24,000. Boerboom, T. J., and Green, J. C., 2005, Bedrock geology of the Two Harbors NE quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-155, scale 1:24,000. Boerboom, T. J., and Green, J. C., 2004, Bedrock geology of the Split Rock Point quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-147, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002a, Bedrock Geology of the French River and Lakewood 7.5’ quadrangles, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-128, scale 1:24,000. Boerboom, T.J., Green, J.C., and Jirsa, M.A., 2002b, Bedrock Geology of the Knife River 7.5’ quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, Map M-129, scale 1:24,000. Boerboom, T.J., Miller, J.D., Jr., and Green, J.C., 2004, Field trip 2: Geologic highlights of new mapping in the southwestern sequence of the North Shore Volcanic Group and in the Beaver Bay Complex: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, Minn., Proceedings, pt. 2, Field Trip Guidebook, p. 45-85. Boerboom, Terrence J., Miller, James D. Jr., and Green, John C., 2003a, Bedrock geology of the Two Harbors 7.5’ quadrangle, Minnesota Geological Survey Miscellaneous Map Series Map M-139, scale 1:24,000. Boerboom, Terrence J., Miller, James D. Jr., and Green, John C., 2003b, Bedrock geology of the Castle Danger 7.5’ quadrangle, Minnesota Geological Survey Miscellaneous Map Series Map M-140, scale 1:24,000. Boerboom, T.J., and Miller, J.D., 1994, Bedrock geologic map of the Silver Island Lake, Wilson Lake, and western Toohey Lake Quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-81, scale 1:24,000. Bonnichsen, B., and Kauffman, 1987, Physical features of rhyolite lava flows in the Snake River Plain Volcanic province, southwestern Idaho: in The Emplacement of Silicic Domes and Lava Flows, J.H. Fink, ed., p. 119145. Brannon, J.C., 1984, Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group: unpublished PhD. Thesis, Washington University, St. Louis, 212 p. Carmichael, I.S.E., 1964, The petrology of Thingmuli, a Tertiary volcano in eastern Iceland” Journal of Petrology, v. 5, p. 435-460. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, v. 34, p. 476-488. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 5464. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H., and Krueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p. Greeley, R., 1982, The Snake River Plain, Idaho: Representative of a new category of volcanism: Journal of Geophysical Research, v. 87, N. B4, P. 2705-2712. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and ILSG07 140 Trip 5 �mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Green, J.C., 1995, Chemostratigraphy at the fringe of the Midcontinent Rift System: The northeast limb of the North Shore volcanic Group, Minnesota (ext. abs.): in Proceedings, Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System: 1995 IGCP Project 336 Field Conference and Symposium, p. 55-56. Green, J.C., 1992, Geologic map of the north shore of Lake Superior, Lake and Cook Counties, Minnesota: Part 1. Little Marais to Tofte: Minnesota Geological Survey Miscellaneous Map Series M-71, scale 1:24,000. Green, J.C., 1990, Primary tridymite crystallization and inferences from tridymite and quartz textures in high-T Keweenawan rhyolites and granophyres, Minnesota (abs.): Geological Society of America, Abstracts with Programs, v. 22, p. A289-290. Green, J. C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: The Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, p. 486-500, 21 figs., 2 tables. Green, J. C., 1983, Geologic and geochemical evidence for the nature and development of the middle Proterozoic (Keweenawan) Midcontinent Rift of North America. Tectonics, v. 94, p. 413-437. Green, J.C., 1982, Geologic map Atlas of Minnesota, Two Harbors Sheet: Minnesota Geological Survey, scale 1:250,000. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W.R.A., Coleman, L. C., and Hall, J.M., eds., Geological Association of Canada Special Paper 16, p. 407-422. Green, J.C., 1972, North Shore Volcanic Group, in Sims, P.K., and Morey, G.B., eds., Geology of MinnesotaA centennial volume: Minnesota Geological Survey, p. 294-332. Green, J. C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent Rift, North Shore, Minnesota: More data, more questions: Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wis., Proceedings, pt. 1, Programs and Abstracts, p. 29. Green, J.C., and Fitz, T.J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classifications of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, no. 5, p. 523-548 Jerde, E.A., 1991, Geochemistry and petrology of hypabyssal rocks associated with the Midcontinent rift, northeastern Minnesota: unpublished Ph.D. Thesis, University of California, Loa Angeles, 305. pp. Jirsa, M.A., 1984, Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p. Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift System: Tectonophysics, v. 213, p. 33-40. Link, P. K. and Hackett, W. R., editors, 1988, Guidebook to the Geology of Central and Southern Idaho. Idaho Geological Survey, Bulletin 27, 319 p. Manley, C.R., 1996, Physical volcanology of a voluminous rhyolite lava flow: The Badlands lava, Owyhee Plateau, southwestern Idaho: Journal of Volcanology and Geothermal Research, v. 71, p. 129-153 McBirney, A.R., and Williams, H., 1969, Geology and petrology of the Galapagos Islands: Geological Society of America Memoir 118, 197 p. Miller, J.D., Jr., 1988, Geologic map of the Split Rock Point NE and Silver Bay quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-68, scale 1:24,000. Miller, J.D., Jr., Boerboom, T.J., and Jerde, E.A., 1994, Bedrock geologic map of the Cabin Lake and Cramer 7.5’ quadrangles, Lake and Cook Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M82, scale 1:24,000. Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: in Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America”: geological Society of America Special Paper 312, p. 73-96. Miller, J.D., Jr., Green, J.C., Boerboom, T.J., and Chandler, V.W., 1993, Geology of the Doyle Lake and Finland quadrangles, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-72, scale 1:24,000. Miller, J.D., Jr., Green, J.C., and Boerboom, T.J., 1989, Geology of the Illgen City quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-69, scale 1:24,000. ILSG07 141 Trip 5 �Miller, J. D., Jr., Green, J.C., and Jerde, E.A., 2006, Bedrock geology of the Little Marais quadrangle, Lake and Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-172, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., ad Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. Miller, J. D., Jr., and Weiblen, P.W., 1990, Anorthositic rocks of the the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, v. 31, p. 295-339. Miller, J.D., Jr., Weiblen, P.W., and Green, J.C., 1987, Roadlog and stop descriptions for the Beaver Bay Complex, in Balaban, N.H., ed., Field trip guidebook for selected areas in Precambrian geology of northeastern Minnesota: Minnesota Geological Survey Guidebook 17, p. 55-60. Milner, S.C., Duncan, A.R., and Ewart, A., 1992, Quartz latite rheoignimbrites flows of the Etendeka Formation, northwestern Namibia: Bulletin of Volcanology, v. 54, p. 200-219. Morrison, D.A., Ashwal, L.D., Phinney, W.C., Shih, C., and Wooden, J.L., 1983, Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota: Geological Survey of America Bulletin, v. 94, no. 2, p. 206-221. Nicholson, S. W., Shirey, S. B., and Green, J. C., 1991, Regional Nd and Pb isotopic variations among the earliest Midcontinent Rift basalts in western Lake Superior (abstr). In Programs with Abstracts, GAC/MAC Annual Meeting, Toronto. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development Canadian Journal of Earth Science, v. 34, p. 504-520. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, np. B8, p. 13,997-14,013. Reidel, S. R. and Hooper, P. R., 1989, Volcanism and Tectonism in the Columbia River Flood-basalt Province. Geological Society of America Special Paper 239, 386 p Sigvaldson, G.E., 1974, Basalts from the centre of the assumed Icelandic mantle plume: journal of Petrology, v. 15, p. 497-524. Schmidt, S. Th., 1993, Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA: Journal of Metamorphic Geology, v. 11, p. 401-414. Schmidt, S. Th., and Robinson, D., 1997, Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition on the North Shore Volcanic Group, Minnesota” Geological Society of America Bulletin, v. 109, NO. 6, p. 683-697. Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent Rift System, northeast Minnesota: Nd-isotope evidence for melting of Archean crust” Canadian Journal Earth Sciences, v. 34, p. 521535. Walker, G.P.L., 1974, The structure of eastern Iceland. In Kristiansson, L., ed., Geodynamics of Iceland and the North Atlantic Area, D. Reidel Publishing Co., Dordrecht, p. 177-188 Wallace, A.B., Drexter, J.W., Grant, N.K., and Noble, D.C., 1980, Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon: Geology, v. 8, p. 380-384. White, W.S., 1966, Tectonics of the Keweenawan basin, western Lake Superior region. U.S. Geological Survey Professional Paper 524-E, 23p. Wood, D.A., 1978, Major and trace element variations in the Tertiary lavas of eastern Iceland and their significance with respect to the Iceland geochemical anomaly: Journal of Petrology, v. 19, p. 393-436. ILSG07 142 Trip 5 �53rd Annual Institute on Lake Superior Geology FIELD TRIP 6 GEOLOGY ALONG THE GUNFLINT TRAIL Mark A. Jirsa Minnesota Geological Survey University of Minnesota and Paul W. Weiblen Department of Geology and Geophysics University of Minnesota ILSG07 143 Trip 6 �Figure 6.1. Regional geologic map of northeastern Minnesota showing the location of Gunflint Trail (modified from Jirsa and Miller, 2005). Inset box outlines Figure 6.4. Field trip area lies at the junction of Neoarchean, Paleoproterozoic, and Mesoproterozoic terranes. INTRODUCTION The Gunflint Trail provides access to the Boundary Waters Canoe Area Wilderness (BWCAW), which lies a short distance off either side of the road. The western end of the trail also transects a nearly complete geologic section of Archean, Paleoproterozoic, and Mesoproterozoic rocks (Fig. 6.1), and a diversity of well displayed contact relationships. In this area, the Archean greenstone-granite terrane of the Wawa subprovince of Superior Province is represented by a succession of metavolcanic rocks (ca 2700 Ma) intruded by the Saganaga Tonalite (ca 2689 Ma). The Archean rocks are unconformably overlain by Paleoproterozoic sedimentary strata of the Animikie Group (ca 1870-1830 Ma), which includes the Gunflint Iron Formation. Mesoproterozoic rifting is manifest in hypabyssal dikes and sills of the Logan intrusions (ca 1115 Ma), and several phases of the Duluth Complex (ca 1100 Ma), emplaced into both Archean and Paleoproterozoic rocks. Despite this exceptionally well exposed record of Precambrian history, little geologic mapping has been conducted in this region since the 1970's. As a result, this field guide relies heavily on prior mapping (Grout, 1929; Mathez and others, 1977; Morey and Nathan 1977; 1978; Morey and others 1981) and field trip guides (e.g., Weiblen and others, 1971; Miller ILSG07 144 Trip 6 �and others, 1987). Although the individual stops are different, much of this guide is modified only slightly from earlier publications, with permission from the authors. Mapping in this region by the Minnesota Geological Survey has resumed, after a 30+ year hiatus. The new mapping will benefit from geological techniques and insights acquired during the intervening decades, particularly those resulting from the work of Canadian geologists. Furthermore, exposures in some areas are more accessible due to road building and recent forest fires. Regarding the latter, many parts of the BWCAW and adjacent lands experienced a megastorm in 1999 that resulted in large tracts of downed and tangled trees. Controlled burns and wild fires in some of the devastated areas—including the 2006 Cavity Lake fire—laid bare many outcrops that once were deeply concealed in forest, lichen, and moss. GEOLOGIC SETTING Neoarchean The oldest rocks exposed in the region are Neoarchean metavolcanic and metasedimentary rocks— probably equivalent to, but not demonstrably continuous with, the Ely Greenstone. Although the supracrustal successions are dissected by faults and intrusions, some correlation can also be made with adjacent terrane in Ontario (Fig. 6.2). Most recent mapping in this terrane was conducted by Jirsa and Miller (2004) in the 1:100,000-scale, Ely-Basswood Lake map sheet that lies just west of the Gunflint Trail. A mapping thesis by Vervoort (1987) delineated geology in the JAP Lake area, which lies between that map sheet and the Gunflint Trail exposures. Rocks of the terrane are divided into a number of faultbounded segments, each having distinct geologic characteristics that cannot be easily correlated from place to place. The Gunflint Trail area exposes the eastern end of what Gruner (1941) referred to as the Gabimichigami segment. Supracrustal rocks in this segment include variably pillowed, hypabyssal mafic metavolcanic flows (Stop 6-1), and andesitic pyroclastic rocks. The supracrustal rocks are cut by hornblende-phyric intrusions that are similar in composition to the associated andesite, and on this basis, inferred to be more or less contemporaneous with them (Feirn, 1977). Based on stratigraphic facing directions established from pillowed metabasalt flows, the supracrustal sequence forms an east-trending and steeply south dipping and younging homocline. The Saganaga Tonalite was emplaced along the northern edge of the supracrustal succession. The metamorphic grade of greenstone along this contact is locally increased from greenschist facies that is typical of much of the belt, to amphibolite grade, and foliation is well-developed. The western edge of the Gabimichigami segment is terminated by a northnortheast-trending fault that juxtaposed greenstone against sedimentary and volcanic rocks of the Knife Lake Group. The Knife Lake Group includes the Ogishkemunce conglomerate that contains detrital clasts of the Saganaga Tonalite. This distinctive sequence of conglomerate, sandstone, and alkalic rocks is interpreted to have been deposited in a complex array of successor basins developed along earlyformed faults (Jirsa and Miller, 2005) at some time after emplacement of the Saganaga Tonalite. ILSG07 145 Trip 6 �Figure 6.2. Geologic setting of Archean bedrock in northeastern Minnesota and adjacent Ontario (modified from Jirsa and Miller, 2005). Inset map shows location relative to subprovinces of the Superior Province. Lacking detailed geochronologic data for this immediate area, much of the temporal distinction between various geological elements of the Neoarchean bedrock is based on the correlation of fabrics that resulted from three major phases of deformation, denoted D1, D2, and D3, with U-Pb zircon dates acquired elsewhere. All three deformation events are the result of N-S- to NW-SE-directed compression. The timing of D1 deformation is bracketed between deposition of the volcanic and clastic rocks of the Wawa subprovince at about 2722 Ma (Peterson and others, 2001), and emplacement of the Saganaga Tonalite at about 2689 Ma (Corfu and Stott, 1998). Folds in the Ely Greenstone and related rocks attributed to D1 deformation are truncated by faults associated with Knife Lake strata, indicating that the latter is synchronous with or post-dates deposition and early deformation of the Ely. As such, the Knife Lake Group is inferred to be a Timiskaming-type sequence temporally equivalent to the Shebandowan assemblage exposed in adjacent parts of Ontario (Corfu and Stott, 1998). D2 deformation effected all of the Archean supracrustal units and can be crudely bracketed by U-Pb dates of intrusions in the Giants Range batholith to the southwest that place the regional deformation and metamorphic event between about 2674 Ma and 2685 Ma (Boerboom and Zartman, 1993). D3 deformation produced faults in the low grade supracrustal and intrusive rocks of the Wawa subprovince, and folds and metamorphic fabric in granitic and migmatitic rocks of the Quetico subprovince to the north. The batholith generally referred to as the Saganaga Tonalite was emplaced into metavolcanic and gneissic granitoid country rocks. Igneous fabric is best developed in the border zones; but overall, the intrusion is moderately foliated and has a shallow eastward lineation. U-Pb age dates include 2689±1 Ma for the tonalite; whereas, 2 phases of the Northern Light Gneiss, which lies in contact with the Saganaga along its northeastern boundary in Ontario, are dated at 2707±2 Ma and 2750±2 Ma (Corfu and Stott, 1998). On the basis of geochemistry, Vervoort (1987) infers that the Saganaga batholith represents the magma source for dacitic lavas that overlie mafic and ultramafic rocks in the JAP Lake area. The Saganaga batholith was divided by Grout (1929) into a medium- to coarse-grained tonalitic phase that makes up the majority of the batholith (Stops 6-2 and 6-3), and a more mafic border phase (Stops 6-4 and 6-5). Grout (1929, p.568) defined this border zone approximately one mile in width along the south, east, and south sides of the batholith. In the Gunflint Trail area, the border phase is more irregular in ILSG07 146 Trip 6 �distribution and consists of strongly foliated, medium to coarse-grained, quartz-bearing hornblende diorite to granodiorite that generally lacks quartz eyes. The border phase is also marked by a wide variety of inclusions. Grout (1929) suggested that some of the inclusions in the Saganaga batholith are the products of igneous processes that ranged from assimilation to the mixing of mafic and felsic magmas. Alternatively, Hanson (1972) suggested that some of the included mafic material could be a product of assimilation of greenstone country rocks during pluton emplacement. Field evidence showing the presence of nearly all gradations of inclusions between mafic and felsic magmatic-looking components, and the presence of quartz phenocrysts in some mafic inclusions, supports the former interpretation for many of the inclusions. The results of analyses of two mafic inclusions acquired recently by the authors are summarized in Figures 6.3A, B, and C. These data indicate that the mafic inclusions (Fig. 6.3D) have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. These data support Grout's earlier interpretations. This is not to say that country rock inclusions are not present—some fine-grained, dark-green, and variably cleaved inclusions do occur very near the contact with enclosing metavolcanic rocks (Stop 6-6a). To date there has been no systematic study of the mafic components of the Saganaga batholith. As pointed out by Didier and Barbarin (1991) in the conclusion of their multi-authored volume, further study of enclaves will contribute to the advancement of the overall petrogenesis of granites. The Saganaga batholith provides ample opportunity for such studies. Figure 6.3. Geochemical comparison of Saganaga batholith, mafic inclusions, and related rocks. A. SiO2-MgO plot. Abbreviations and data sources: TTG=Compositional field of the Archean TonaliteTrondhjemite-Granodiorite suite; S=Compositional field of primitive Archean sanukitoid samples from the southwestern Superior Province (Stern and others, 1989). Open circles: 1 Ely greenstone; 2 and 3 mafic inclusions at Stop 6-5; 4 border phase of the Saganaga Tonalite at Stop 6-5; 5, 6 Ely greenstone; 7-10 tonalite; 11 granodiorite. Analyses 1-4 were acquired for this study (on-file with Minnesota Geological Survey); analyses 5-11 are from Arth and Hanson, 1975, Table 2, p. 331, analyses 1-2, 19-22, and 24 respectively. Analyses 12-14 are from Stern and others, 1989, Table 1 p. 1695, samples 1-3. Crosses: analyses of the Roaring River Complex, Superior Province, Canada from Stern and Hanson (1991, Table 2, p. 220). They have demonstrated that incremental equilibrium crystallization of a melt ILSG07 147 Trip 6 �with the composition of analysis 15 (sample #7, op cit) can produce the three members of the TTG suite indicated. The two analyses of mafic inclusions at Stop 6-5 (#'s 2 and 3) plot in the primitive sanukitoid field, and their chemistry is similar to other chemical characteristics of the sanukitoid suite (Figs. 6.3B and C). These data imply that the mafic inclusions must either be deformed xenoliths of a sanukitoid body or segregations from a consanguineous melt. B. Chemical characteristics of selected sanukitoid suite rocks (data from Stern and others, 1989) and mafic enclaves in the Saganaga batholith (data from this study). Solid squares, dashed line = range of high-silica sanukitoid; Solid diamonds, dashed line = range of low-silica sanukitoid; Solid triangles = average of high- and low-silica sanukitoid; Pluses = high-silica mafic inclusion (#2 from Fig. 6.3A), in the Saganaga batholith; Crosses = low-silica mafic inclusion (#3) in the Saganaga batholith. The analyses are normalized to an average of high and low values for each chemical characteristic of several sanukitoid analyses. Data for the mafic inclusions lie well within the range of the sanukitoid values, except for Cr in mafic inclusion #3, which plots just above the high range of the sanukitoid rocks, and Ba and Sr for both mafic inclusions that plot slightly lower than the low range of the sanukitoid rocks. ILSG07 148 Trip 6 �C. REE analyses. Open squares = Saganaga mafic enclave from this study (#2 from Fig. 6.3A), Solid squares = Saganaga mafic enclave from this study (#3), Open diamonds = Saganaga tonalite this study (#4), Solid diamonds = Saganaga tonalite of Arth and Hanson, 1975, sample 19, Open circles = Saganaga tonalite of Arth and Hanson, 1975, sample 20. The REE data for the mafic enclaves in the Saganaga batholith are distinct from that of the tonalite samples, and lie roughly within the sanukitoid field (shaded) as defined by Stern and others (1989, Fig. 10, p.1702). These data support the conclusion from the chemical characteristics (Fig.6.3B), and the SiO2-MgO (Fig. 6.3A). D. Photographs of mafic inclusions analyzed for this study. Both inclusions occur in the strongly foliated, granodioritic border phase of Saganaga batholith at Stop 6-5. LEFT=inclusion #2, material for analyses shown on Fig. 6.3A-C was taken from circled (snow-filled) drill hole. RIGHT=Sample 3: material for analyses taken from bottom of sample below X. ILSG07 149 Trip 6 �Paleoproterozoic Animikie Group Sedimentary rocks of the Paleoproterozoic Animikie Group are exposed in an east-trending belt that extends from Thunder Bay on Lake Superior to a point 12 miles (19km) west of Gunflint Trail, where the belt is truncated by the Mesoproterozoic Duluth Complex (Fig. 6.4). The Animikie Group in this area consists of basal conglomerate and iron-bearing strata of the Gunflint Iron Formation, overlain by slate and siltstone of the Rove Formation. The sequence is broadly correlative with Animikie strata exposed to the southwest along the Mesabi Iron Range. The rocks form a homocline that dips gently southward, except where deformed by folding, faulting, and emplacement of Mesoproterozoic intrusions. For example, local folding, inferred to be associated with emplacement of the Logan intrusions, may be the product of magmatic shouldering into the relatively ductile sedimentary country rocks. Sequence "inflation" by the Logan sills may explain the observation that the dip of Paleoproterozoic rocks increases from 10 degrees away from the contact with Duluth Complex, to 60 degrees locally near it. Faults are present locally, but few have displacements greater than 50 feet. A notable exception is the block-shaped Lookout fault (Fig. 6.5) that lies adjacent to the Gunflint trail. As much as 200 feet of uplift on the west and south is speculated (Morey and others, 1981). In addition, the dip of Animikie strata on the west side of the fault is much steeper than that on the east, which explains in part the difference in the widths of map units apparent from the geologic map. Much of the complex-looking fold pattern east of the fault is likely an artifact of complicated surface topography and shallowly dipping units. The Lookout fault displaces both Animikie rocks and the Logan intrusions, but the timing of offset relative to emplacement of the Duluth Complex is unclear. Figure 6.4. Geologic map along the northwestern end of the Gunflint Trail, modified from published quadrangle maps (Morey and Nathan, 1977; 1978; and Morey and others, 1981), and mapping by Grout (1929). The locations of stops 6-2, 6-3, and 6-4 are shown—other field trip stops lie within the outline corresponding to Figure 6.5. Across the border in Canada, the Gunflint Iron Formation is essentially unmetamorphosed and consists of quartz, hematite, iron carbonates, greenalite, and magnetite. A thorough review and sedimentalogical interpretation of these rocks can be found in Pufahl and Fralick (2000), Fralick and others (2002), and references therein. In Minnesota, the original mineralogic character of the ironformation was modified by contact metamorphism adjacent to Mesoproterozoic intrusions, and many of the fine-scale textural and mineralogical attributes used to subdivide the iron-formation in Canada have ILSG07 150 Trip 6 �been destroyed. Consequently, a four-fold nomenclatural scheme that emphasizes various bedding aspects that are readily visible has been used for the Gunflint since the early work of Broderick (1920). The scheme was originally devised for the Biwabik Iron Formation on the Mesabi Iron Range the Mesabi Iron Range. It defines four informal members—Lower cherty, Lower slaty, Upper cherty, and Upper slaty. These terms emphasize the relative proportions of strata dominated by thickly bedded and granular (cherty) beds consisting of rounded sand-size intraclasts, vs. thinly bedded and very fine-grained (slaty) beds. The Lower and Upper members are grouped on the published geologic map that shows ironformation (Morey and others, 1981) and on Figure 6.5. The Lower cherty member ranges in thickness from 15 to 45 ft (4.5-13.5m). A conglomerate containing clasts of Archean granite and greenstone set in a matrix of feldspathic quartzite or stromatolite-bearing chert occurs locally (Stop 6-6b). The basal conglomerate is absent in most of the area, and a persistent thick-bedded to massive, magnetite-rich unit 5-15 ft (1.5-4.5m) thick (Stop 6-6c) composes the base of the Lower cherty member and unconformably overlies Archean rocks. The magnetite-rich layer is in turn overlain by a thick-bedded, chert-rich, magnetite-poor unit about 15 ft (4.5m) thick. The overlying Lower slaty member is 80-95 ft (24-29m) thick. The lowermost 10 ft (3m) is a black, thinly bedded, nearly magnetite-free argillite composed predominantly of volcanically derived material. This may equate with the Intermediate slate unit known on the Mesabi range. Massive and cherty beds that resemble the upper part of the Lower cherty member occur above the argillite, but they pass abruptly upward into a sequence of thick, chert- and silicate-rich beds with sparse magnetite intercalated with intervals of thinly laminated silicate-rich beds. The uppermost 50 ft (15m) of the Lower slaty member consists of thinly bedded to laminated slate containing various silicates and 20-35 percent magnetite (Stop 6-7a). The Upper cherty member is approximately 50 ft (15m) thick. The gradational contact between the Lower slaty and the Upper cherty members is marked by the appearance of irregularly bedded to lenticular chert-rich layers and by thin irregular layers consisting almost entirely of magnetite. The upper part of the Upper cherty member is characterized by several thick units of granular chert that contain microbial structures, intraformational conglomerate, and abundant magnetite. (Stop 6-7c). The Upper slaty member is approximately 150 ft (45m) thick. Thick lenticular beds of chert containing disseminated magnetite occur in the lower few meters, but most of the member consists of thin-bedded to laminated, silicate-rich units that are intercalated with intervals of thinly laminated graphitic argillite and centimeter-thick beds of relatively pure chert. The upper 10 ft (3m) are nearly magnetite-free and consist of limestone and chert interbedded with argillite (Stops 6-8a, 6-8b). The Rove Formation gradationally overlies the Gunflint Iron Formation and is at least 3,200 ft (970m) thick, although estimates of thickness are complicated by multiple Mesoproterozoic intrusions and likely fault repetition. It can be divided into a lower argillaceous unit, a transitional sequence, and an upper thin-bedded graywacke-rich unit (Morey, 1969). The lower argillite consists of intercalated argillaceous siltstone, silty argillite, and carbonaceous argillite (Stop 6-9). The transitional sequence separates dominantly argillaceous rocks below from an interval containing intercalated beds of coarseand fine-grained graywacke. The sandy beds contain many primary sedimentary structures indicative of deposition by turbidity currents that flowed dominantly southward. Contact metamorphic effects on the Gunflint Iron Formation were studied in detail by Floran and Papike (1975, 1978), and metamorphism of the Rove Formation was described by Labotka and others (1981). The sizes of the metamorphic aureoles adjacent to the Logan intrusions (as much as 33 ft (10m) wide) and their mineral assemblages are directly related to the thicknesses of the sills. Adjacent to the Duluth Complex, three metamorphic zones have been distinguished: 1) An outer zone of slightly metamorphosed iron-formation consisting of quartz, iron carbonate, minnesotaite, stilpnomelane, and hematite partially replaced by magnetite. 2) A 1.2 mile (2km) wide intermediate zone of moderately metamorphosed iron-formation containing grunerite-cummingtonite, hornblende, and actinolite, as well as quartz and finegrained magnetite. ILSG07 151 Trip 6 �3) A proximal zone of highly metamorphosed iron-formation occurring within 0.3 miles (0.5km) of the contact with the Duluth Complex and composed of quartz, magnetite, iron-rich pyroxenes, and fayalite. By contrast with the iron-formation, contact-related recrystallization of the Rove Formation is much less pronounced, even very near the Duluth Complex. The aureole is marked by a complex mixture of rock types suggestive of partial melting within a meter or so of the contact. In this contact zone, argillaceous rocks retain large-scale layering, but have a vague granoblastic texture. Individual layers contain cordierite and hypersthene locally, with minor biotite, ilmenite, plagioclase, K-feldspar, and olivine. At varying distances away from the contact, but generally no more than 100 ft (30m) away, biotite is the only well-developed metamorphic mineral in the pelitic rocks. Calcareous beds near the contact contain minor grossular garnet. A U-Pb age of 1878 ± 1 for the Gunflint Iron Formation was determined by Fralick and others (2002) from reworked volcaniclastic zircons in Ontario exposures. Subsequent geochronologic study of the Rove and equivalent Virginia Formations has considerably complicated and protracted the timeline of Animikie Group deposition. For example, dates by Addison and others (2005) of detrital zircons near the base of the Rove Formation yielded ages of 1827 ± 8 and 1836 ± 5 Ma, and 1832± 3 from a similar stratigraphic position in the Virginia Formation. All of these samples were acquired from several meters above what Addison and others infer is an ejecta layer derived from the Sudbury impact event (ca 1850). Recently acquired detrital zircon dates by Heaman and Easton (2005) indicate that upper parts of the Rove Formation are as young as 1777 Ma. Wirth and others (2006) report ca 1790 Ma detrital zircons in the Rove and equivalent Thomson Formations. Collectively, these data imply that deposition of strata assigned to the Animikie Group spanned some 100 million years, and hints at the likelihood of several major unconformities. The tectonic and depositional setting of the Gunflint and other semi-contemporaneous Lake Superior iron-formations remains somewhat contentious (summarized for all iron-formations in Clout and Simonson, 2005). Modern interpretations include deposition in one or more foredeep basins developed cratonward of crustal loading during Penokean orogenesis (Morey and Southwick, 1995). More recently, Pufahl and Fralick (2004) have inferred from sedimentalogical evidence that deposition occurred along a south-sloping continental margin undergoing subsidence in a back-arc extensional setting. Geochemical evidence for within-plate signatures of volcanic rocks associated with iron-formation in Michigan suggested to Schulz and Cannon (in press) a similar interpretation of back-arc spreading. From this and geochronologic work, they infer that deposition occurred in a broad foreland having local extensional basins hosting iron-formation. Following iron-formation deposition, turbidites of the Rove Formation and equivalent strata were deposited in a rapidly subsiding basin, with subsidence presumably driven by crustal loading by the northward migrating fold and thrust belt. A hiatal boundary between ironformation and overlying turbidites identified in Ontario and Minnesota (Addison and others, 2005), and Michigan (Schulz and Cannon, in press) roughly coincides with the ca 1850 Sudbury impact event. Mafic dikes Mafic dikes emplaced into the Saganaga Tonalite are prominent on aeromagnetic maps as two sets of positive linear anomalies trending northward and eastward. Exposures mapped along the north-trending magnetic trajectories (Fig. 6.4) show that the western of the two is diabasic, and the eastern consists of lamprophyre. The latter was the subject of work by O'Brien (1982) who mapped the 70 foot thick dike, reprocessed aeromagnetic data, and conducted petrologic work. He classifies the lamprophyre dike as camptonite, defined by its content of kaersutite (calcic amphibole), titanbiotite, and titanaugite phenocrysts in a plagioclase-rich groundmass. The precise age of these dikes is unclear, but a Paleoproterozoic age is likely. K-Ar dates of biotite by Goldich and others (1961) yielded an age of about 1750 Ma, which they attributed to late thermal resetting during alteration of the dikes. Petrographic and geochemical analyses by O'Brien indicate that much of the alteration was deuteric—having formed during cooling of the dike. On this basis, O'Brien speculated that the K-Ar date may be an accurate estimate for emplacement. A second set of aeromagnetic anomalies—trending east and slightly less pronounced than ILSG07 152 Trip 6 �the north-trending ones—have not been investigated in detail. The roadcut at Stop 6-4 exposes diabase inferred to lie along one of the anomalies. Mesoproterozoic Mesoproterozoic mafic intrusions comprise the remaining exposures in the Gunflint Trail area (Figures 6.4 and 6.5). The rocks represent the early magmatic stages of the Midcontinent Rift system. The apparently earliest of these are diabasic sills and dikes emplaced into the Animikie strata and collectively referred to as the Logan intrusions. A baddeleyite age of 1115 ± 1 Ma is reported from a sample of a Logan sill near Thunder Bay (Heaman and Easton, 2005). The sills are intruded, with slight angular discordance, by medium to coarse-grained gabbro and troctolite of the Poplar Lake and Tuscarora intrusions of the Duluth Complex. The Poplar Lake is part of the early gabbroic series of the complex. A basal gabbroic unit of the intrusion yielded a date of 1106.9 ±.8 (Miller and Severson, 2002). Field relationships indicate that the Poplar Lake is intruded by the Tuscarora intrusion, which is considered to be part of the layered series of Duluth Complex. Logan Intrusions The Logan intrusions are exposed along a series of prominent, east-trending ridges formed by the differential erosion of diabase sills and sedimentary rocks, particularly the Rove Formation. Individual sills are as much as 1100 ft (33m) thick, and can be traced along strike for several kilometers. Branching and merging of individual sills is common, and many sills thicken and thin down-dip. Some sills terminate against joints and inferred faults. Locally, fractures in the Rove Formation are occupied by thin dikes, which give a box-work configuration to the hypabyssal intrusions. Rock types include aphyric basalt, fine- to medium-grained diabase with ophitic clinopyroxene enclosing plagioclase, plagioclase cumulates, and granophyre (Jones, 1984). Chilled margins form sharp contacts with, and locally contain inclusions of, the country rocks (Stop 6-10). Diabase coarsens to medium-grained near the center of individual sills, and clinopyroxene is ophitic throughout. Minor differentiation is manifest in accumulations of plagioclase or granophyric intergrowths (quartz, sodic plagioclase, and orthoclase) in upper parts of sills. The configuration of Logan intrusions closely follows that of the gently folded Animikie Group strata, implying that minor ductile deformation occurred during the Mesoproterozoic. Duluth Complex The Duluth Complex is a sequence of generally discordant plutonic rocks consisting of many separate intrusions. Two of these intrusions, the Poplar Lake and Tuscarora, are exposed along the Gunflint Trail. The Poplar Lake intrusion, formerly referred to as Nathan's layered series, consists of interlayered gabbroic cumulates, with minor amounts of troctolitic and anorthositic cumulates. Rocks of the Poplar Lake have reversed magnetic polarity, and thus are broadly correlative with lower lavas of the North Shore Volcanic Group and with the Logan intrusions. As originally defined by Nathan (1969), the Poplar Lake intrusion is composed of at least 27 sheet-like units of mafic cumulates and intermediate to felsic rocks. More detail about the Poplar Lake intrusion can be found in this volume (Miller and Jerde, 2007; Field Trip #1). The Tuscarora intrusion irregularly cuts across the layered gabbro of the Poplar Lake intrusion (Morey and Nathan, 1978). The basal part of the Tuscarora intrusion consists of a fine-grained, augite-poikilitic, olivine gabbro (unit ttf on Fig. 6.5) (Stops 6-11 and 6-12). Within 0.3 mi (0.5km) of the basal contact, fine-grained troctolite coarsens to medium-grained (unit ttm). The troctolite units consist of 65-70 percent plagioclase and 10-15 percent olivine. Relative amounts of poikilitic augite and irontitanium oxides vary locally. Orthopyroxene mantles olivine and occurs in symplectic intergrowth with plagioclase. Biotite is locally present in association with iron-titanium oxides. Modal layering is well developed and generally concordant with unit boundaries that dip gently to the south—typically more shallowly dipping than the subjacent Animikie Group strata. The ttp and ttf units commonly contain chalcopyrite, pyrrhotite, and minor pentlandite interstitial to plagioclase and olivine. The sulfide concentrations are subeconomic, but locally form mappable zones (Stop 6-12). ILSG07 153 Trip 6 �FIELD TRIP 6 Stop Descriptions NOTES: 1) This group will not visit all of the stops described below. A number of stop descriptions are included in this guide to provide context and for future visits to the region. 2) Small maps showing location of individual stops are modified from USGS 7.5-minute (1:24,000scale) topographic maps, which are named below. 3) All UTM coordinates are given in NAD 83. Figure 6.5. Geologic map and schematic cross-section (A-A'), showing field trip stops 6-1 and 6-5 through 6-12 (geology modified from Morey and others, 1981). ILSG07 154 Trip 6 �STOP 6-1 Neoarchean pillowed basalt and basal Gunflint Iron Formation Location: T. 65 N., R. 4 W., sec. 28, NW, NE; Kekekabic Trail to Lookout (Fig. 6.5) Long Island Lake quadrangle 6-1a = So-called "Paulson Mine" at UTM 660,504E/5,328,673N (NAD83) 6-1b = Lookout at UTM 660,857E/5,328,364N [Requires 2 mile hike] Description: Follow the Kekekabic hiking trail from the parking lot, westward approximately 0.6 mi. to the junction with a north-bound trail that leads to the lookout. The main Kekekabic trail parallels the base of the Gunflint Iron Formation, and the north-facing slope immediately south of the trail contains exposures of the Lower slaty member. The iron-formation has been strongly metamorphosed in this area and now consists of various assemblages of quartz-grunerite-fayalite-magnetite and quartz-cummingtonitegrunerite-pyroxene-magnetite. 6-1a. Continuing westward about 800 ft from the trail junction, several test pits can be seen, including one that is fenced and labeled "Paulson Mine 1893," but probably isn't (the mine apparently lies some distance farther west). This and other test pits are developed in the Lower cherty member of ironformation, and waste piles contain abundant pyrrhotite, other sulfide minerals, and magnetite. 6-1b. Returning to the junction with the north-bound trail and following it up-slope, there are numerous exposures of Neoarchean metabasalt. Several small outliers of iron-formation containing iron-silicates and magnetite can be found along the route, implying that this south-facing surface is very near the unconformable contact between Archean and Paleoproterozoic rocks. About 120 feet north of the junction, a ledge of fine-grained diabase crosses the trail. Preliminary analyses (this study) indicate that it has a komatiitic composition. Intrusions of this composition are also found in the Newton Lake Formation, some 30 miles to the southwest, and high-Mg tholeiitic basalt flows and pyroxenitic to peridotitic sills were described by Vervoort (1987) in the JAP Lake area two miles to the west. At and near the lookout, metabasalt flows with pillow structures and autobreccia fabrics are exposed. Judged from pillows—which have been moderately flattened by regional D2 deformation—bedding trends eastnortheastward, is steeply south dipping and stratigraphic facing is southward. ILSG07 155 Trip 6 �STOP 6-2 Felsic phase of the Neoarchean Saganaga Tonalite cut by a small mafic dike. Location: T. 66 N., R. 4 W., sec. 31, NW, SW; Campsite #18 (Fig. 6.4) Munker Island quadrangle UTM: 656,333E/5,335,730N Description: Most of the outcrops in this area consist of gray, massive to trachytoid-foliated, medium to coarse-grained tonalite, having plagioclase in much greater abundance than microcline (Fig. 6.6). Large quartz phenocrysts, or eyes, as much as 1 cm in diameter are characteristic of this phase, which is typical of 90% of the batholith. Quartz eyes are polycrystalline aggregates, in which each crystal has a different optical orientation. Quartz also occurs as an interstitial mineral to subhedral plagioclase (An20-28). Small amounts of microcline occur as antipathetic exsolution in plagioclase, as rims on plagioclase, and as small interstitial grains. Hornblende is the dominant ferromagnesian mineral, together with minor amounts of augite, biotite, epidote, and chlorite. Figure 6.6. Texture typical of the Saganaga Tonalite, including lineated quartz "eyes" (medium gray). The small dike of aphanitic mafic rock in this exposure has not been sampled, but is inferred to be related to larger north-trending diabasic and lamprophyric dikes that form prominent northtrending anomalies on aeromagnetic maps. The pronounced foliation in the Saganaga Tonalite was inferred by Grout (1933) as a primary flow fabric (trachytoid). The tonalite is inferred to have been emplaced into Archean metavolcanic rocks shortly after early (D1) deformation, based a U-Pb date of 2689±1 in Canadian exposures (Corfu and Stott, 1998). As such, it experienced major regional metamorphism and transpression associated with D2 deformation. As with many large plutons in such terranes, the debate remains unresolved about whether the fabrics are wholly magmatic, wholly tectonic, or some hybrid of the two. ILSG07 156 Trip 6 �STOP 6-3 Neoarchean Saganaga Tonalite with rounded dioritic to granodioritic inclusions Location: T. 66 N., R. 4 W., sec. 31, SW, NW; Campsite #13 (Fig. 6.4) Munker Island quadrangle UTM: 656,219E/5,336,079N Description: Equigranular tonalite with characteristic quartz eyes, containing a wide variety of inclusions. The term "inclusion" has a tortured usage—we prefer to use the term to apply to material that has a contrasting composition or appearance from its host, regardless of origin, size, shape, degree of assimilation, or extent of equilibration with the enclosing host magma. Inclusions may represent xenolithic blocks of county rock incorporated into the Saganaga Tonalite, or cognate phases of the intrusion (autoliths). The inclusions found here have not been studied in detail. They present a unique opportunity for further petrologic and mineral chemistry research to explore their complex origin. The shape of inclusions at this stop is related to either magmatic or tectonic processes. In either case, one must keep in mind that 2dimensional shapes are dependent on the orientation of exposure-surface relative to that of the inclusions. This exposure, for example, provides only one surface—it is impossible to say from this whether the inclusions are discoid, sub-spherical, or egg-shaped. STOP 6-4 Granodioritic phase of Neoarchean Saganaga Tonalite with inclusions; cut by diabase dike Location: T. 66 N., R. 4 W., sec. 32, near center; Roadcut on both sides of Gunflint Trail (Fig. 6.4) Conners quadrangle UTM: 658,575E/5,335,837N Description: Exposures on the west side of the road consist of pinkish to gray hornblende granodiorite to granite inferred to be a border phase of the Saganaga Tonalite and containing inclusions that are both more felsic and more mafic than the enclosing rock (Fig. 6.7). For example, the large angular block shown in Fig. B ILSG07 157 Trip 6 �consists of quartz-eye-bearing tonalite, much like rock that is typical of the main phase of the intrusion. Foliation is poorly developed and likely magmatic in origin. This exposure demonstrates that the Saganaga is a composite batholith that, despite its apparent homogeneity, consists of quite varied magmatic phases, particularly near its border. Tonalite on the east side of the road is cut by a fine-grained diabasic dike several meters in width. Although this location lies some distance east of the prominent north-trending aeromagnetic trends associated with dikes that are described in the text above, the dikes presumably are related. Figure 6.7. Granodioritic phase of Saganaga batholith, containing many and varied inclusions. Large, lighter colored block in center of exposure is inferred to be an autolith of quartz-eye-bearing tonalite similar to the major phase of the batholith. STOP 6-5 Border phase Neoarchean Saganaga Tonalite with flattened inclusions and well-developed foliation in contact zone with Neoarchean metabasalt Location: T. 65 N., R. 4 W., sec. 22, SE, NE, SW; Roadcut on east side of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 661,834E/5,329,257N Description: This location exposes the border phase of the Saganaga batholith, characterized by a granodioritic composition, general lack of quartz eyes, and an abundance of dioritic inclusions consisting of varied proportions of hornblende, pyroxene, biotite, plagioclase, and minor quartz (Fig. 6.8). The irregular ILSG07 158 Trip 6 �ovoid and discoid shape of inclusions is oriented subparallel to well developed, steeply dipping and easttrending foliation. Hornblende crystals and aggregates define a prominent lineation plunging shallowly to the east. Petrology indicates that much of this fabric appears magmatic, yet foliation may be a hybrid of approximately coaxial magmatic flow and regional tectonic deformation (D2). This is typical of the border zone of the intrusion against Archean metabasaltic country rocks, which presumably lie in the low ground just to the south. A preliminary comparative geochemical study, summarized in Fig. 6.3 above, indicates that the mafic inclusions are not partially assimilated, recrystallized, and tectonically deformed country rock volcanic xenoliths as implied by some earlier workers. They have the common chemical characteristics of the sanukitoid suite of Archean granitoid rocks. Thus, we infer that the inclusions studied are autoliths derived from a separate, primitive sanukitoid-magma. Figure 6.8. Granodioritic border phase of Saganaga Tonalite containing mafic inclusions (see discussion associated with Fig. 6.3). STOP 6-6 Contact zone of Neoarchean Saganaga Tonalite and metabasalt; unconformably overlain by Paleoproterozoic conglomerate of the Lower cherty member of Gunflint Iron Formation Location: 3 exposures, all in T. 65 N., R. 4 W., sec. 22, NW, SW, SE; Bush path off Gunflint Trail [individual UTM coordinates given below] (Fig. 6.5) Long Island Lake quadrangle ILSG07 159 Trip 6 �Description: 6-6a [UTM: 661,925E/5,329,065N] Archean metavolcanic rocks containing abundant granitic sheets and dikes, presumably related to border phases of Saganaga Tonalite (Fig. 6.9). The boundary between tonalite and metabasalt has been mapped in many places as a fault (Weiblen and others, 1971). These exposures do not preclude that possibility, but they imply that passive emplacement of the intrusion has also occurred, at least locally. Figure 6.9. Outcrop of border zone of Saganaga Tonalite. Dark gray material probably represents various phases of the intrusion; lighter gray, wedge-shaped area in the foreground is inferred to be metavolcanic country rock. 6-6b [UTM: 661,965E/5,329,062N] Conglomerate developed at the gently southward dipping unconformity between Neoarchean intrusive and metavolcanic rocks and the overlying basal part of the Paleoproterozoic Animikie Group. The unit, regionally known as the Kakabeka Conglomerate, is present only locally on the western end of the Gunflint. In most places, the Lower cherty member of ironformation lies directly on eroded Archean surfaces. This small outcrop is one of the few places where the conglomerate is exposed and accessible along the contact. The conglomerate is greenish gray, poorly bedded, and contains subangular to subrounded fragments of Saganaga Tonalite and related granitoid rocks, metabasalt, and quartz, in a granular siliceous matrix. 6-6c [UTM: 661,965E/5,329,037N] Walking southward from the basal conglomerate is a low "step-up" onto southward dipping strata of the Lower cherty member of Gunflint Iron Formation. ILSG07 160 Trip 6 �STOP 6-7 Stromatolitic cherty Gunflint Iron Formation Location: 3 exposures, all in T. 65 N., R. 4 W., sec. 22 [specific locations given below], Magnetic Trail (Fig. 6.5) Long Island Lake quadrangle CAUTION and ADVICE: This is a fairly long hike, approximately 1 mile round-trip; please be prepared with water and other field needs. Although this is not in the BWCAW, it does lie within Superior National Forest and is frequented by hikers. For this reason, and to preserve scientific value, please be respectful in matters of hammering and sampling. Description: 6-7a [Sec. 22 SW, SE; UTM: 662,034E/5,328,8895N] Thin-bedded, fine-grained, chert-amphibolemagnetite-bearing strata assigned to the upper part of the Lower slaty member of Gunflint Iron Formation. Beds strike ENE and dip generally less than 8 degrees southward. 6-7b [Sec. 22 SE, SE; UTM: 662,401E/5,329,093N] Near the edge of this large controlled burn area are several exposures of stromatolitic iron-formation—inferred to be stratigraphically near the base of the Upper cherty member of the Gunflint Iron Formation. Irregular domal and laminar forms are present. Some float blocks of basal conglomerate can be found here, presumably glacially transported southward from the basal unconformity (Fig. 6.10). Figure 6.10. Boulder of basal conglomeratic unit of Gunflint Iron Formation. ILSG07 161 Trip 6 �6-7c [Sec. 22 NE, SE; UTM: 662,583E/5,329,257N] Inferred to be in the Upper cherty member of Gunflint Iron Formation. Crest of ridge exposes abundant 3-dimensional views of stromatolites, intraformational conglomerate, and stromatolite "hash," all in a peloidal to ooidal, cherty grainstone matrix. Given the apparent mineralogic replacement and moderate metamorphic grade, little of the original carbon-based material is likely present. Despite this, examples of nearly all morphological forms of stromatolites can be found, including columnar, domal, and laminar (Fig. 6.11). Figure 6.11. Cherty, stromatolitic Gunflint Iron Formation. A=Oblique surface of small columnar stromatolites; B=Horizontal surface of irregular domal stromatolites; C=Vertical section of laminar stromatolites; D="Stromatolite hash." ILSG07 162 Trip 6 �STOP 6-8 Upper member Gunflint Iron Formation Location: 2 exposures in T. 65 N., R. 4 W., sec. 26 SW, NE; Gravel pit north of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle Description: 6-8a [UTM: 663,754E/5,328,212N] Exposed here is a dip-slope composed primarily of granular (cherty) and laminated (slaty) strata of the uppermost Gunflint Iron Formation. The slope defines the southern limb of a large, east-plunging anticline, outlined by interdigitated iron-formation and nearly concordant Logan sills (Fig. 6.5). The gentle dip of this limb illustrates the observation that low angle folding and moderate-relief topography are responsible for the complex map pattern here on the east side of the Lookout fault. The bedding surface is marked by what we tentatively infer to be syneresis cracks. The cracks, now filled with quartz, occur both concentrically and radially around a central, apparently raised core within a single granular layer of siliceous iron-formation (Fig. 6.12). Syneresis cracks are defined generally as shrinkage cracks formed by dewatering in a gel or colloidal suspension. They differ from septarian cracks that may develop in a similar way, in that the latter typically occur in concretions. Surprisingly diverse interpretations can be found in the literature about syneresis cracks (summarized in Pratt, 2001). There is, however, general agreement that they represent localized tensional failure during sediment dewatering. The explanation for localized semi-brittle response to what likely were formation-wide stresses—caused by compaction or vibration due to syn-sedimentary earthquakes—is more contentious. It has been ascribed variously to the localization of cements, locally increased pore pressure, or zones of granular sediment made coherent by "microbial glue." It is interesting to note that syneresis structures are more prevalent in Precambrian and Cambrian rocks than younger ones. This may be due in part to more uniform organic bonding of clays in younger strata, which reduced the occurrence of stress-localization. Figure 6.12. Syneresis cracks on eroded bedding surface of thinly bedded Gunflint Iron Formation. 6-8b [UTM: 663,637E/5,328,055N] Metamorphosed carbonate-rich iron-formation. This small exposure is likely an outcrop, based on the observation that carbonate strata are seen elsewhere in the uppermost part of the Gunflint, and it is properly positioned relative to stratigraphy as depicted on the geologic map (Morey and others, 1981). The rock appears to be a metamorphosed breccia containing fragments of thinly layered carbonate. Mineralogy is not well known, as the exposure was only recently discovered. Based on published studies (Floran and Papike, 1975) it may contain some combination of ferrohypersthene, fayalite (±quartz), grunerite-cummingtonite and perhaps garnet (Fig. 6.13). ILSG07 163 Trip 6 �Figure 6.13. Uppermost carbonate facies of Gunflint Iron Formation. Large, light-colored crystals may be grunerite-cummingtonite; dark spots appear to be garnet. STOP 6-9 Paleoproterozoic slate of the Rove Formation Location: T. 65 N., R. 4 W., sec. 25 NE, NE; Road cut on County Road #46 about 0.5 mi. N of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 666,750E/5,328,753N Description: The small roadside outcrop exposes slate that is typical of the argillaceous parts of the Rove Formation. Bedding strikes eastward and dips gently to the south. ILSG07 164 Trip 6 �STOP 6-10 Mesoproterozoic Logan Intrusion Location: T. 65 N., R. 4 W., sec. 24 SE, SE; Bush trail off County Road #46 about 0.8 miles north of Gunflint Trail (Fig. 6.5) Long Island Lake quadrangle UTM: 665,674E/5,329,111N Description: Most of the Logan intrusions consist of medium-grained diabase, as seen along the trail leading to this outcrop. By contrast, this exposure—inferred to lie near the roof of the sill—consists of deformed and partially assimilated xenolithic fragments of Rove Formation slate and siltstone in white-weathering, feldspathic, locally porphyritic and granophyric rock (Fig. 6.14). Figure 6.14. Irregular border phase at upper part of one of the Logan Intrusions, containing irregular inclusions of Rove Formation slate country rock. ILSG07 165 Trip 6 �STOP 6-11 Mesoproterozoic Tuscarora intrusion of Duluth Complex—atypical border phase Location: T. 65 N., R. 4 W., sec. 27 SW, SE; Roadcut on Cross Lake road (CR#47) (Fig. 6.5) Long Island Lake quadrangle UTM: 662,074E/5,327,457N Description: A confusing exposure of the lower units (ttp, ttf) of the Tuscarora intrusion. The outcrop consists of intergranular to ophitic gabbro and augite troctolite, with pods and veinlets of coarse mafic pegmatite and shear bands containing sulfide mineralization. Spheroidal weathering produced "core stones" locally. STOP 6-12 Mesoproterozoic Tuscarora intrusion of Duluth Complex Location: T. 65 N., R. 3 W., sec. 30 SW, SE; roadcut on Gunflint Trail east of CR#50. (Fig. 6.5) Long Island Lake quadrangle UTM: 666,638E/5,327,433N Description: Just south of the parking pull-off is the rather poorly exposed intrusive contact between Rove Formation and units ttp and ttf of the Tuscorara Intrusion (Fig. 6.5 explanation; Morey and others, 1981). Unit ttf is a typical example of Cu-sulfide mineralized augite troctolite that is found at the base of the Duluth Complex here and in the Hoyt Lakes-Kawishiwi area to the southwest. It contains disseminated pyrite, pyrrhotite, and chalcopyrite. In the 1970's, International Nickel Company (INCO) drilled 7 holes in the basal Duluth Complex (Tuscarora and western Poplar Lake intrusions) to evaluate potential for Cu-Ni mining. All of these holes lie along the basal part of the intrusion within a few miles east and west of this stop. The archived drill cores were studied by Mogessie (1976) and Mogessie and others (1976). ILSG07 166 Trip 6 �REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N. J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology: 33:193196. Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the early Precambrian crust of northeastern Minnesota: Geochim. et Cosmo. Acta, 39:325-362, Table 2, p. 331-332. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, 30:2510-2522 Broderick, T.M., 1920, Economic geology and stratigraphy in the Gunflint iron district, Minnesota: Economic Geology 15:422-452. Clout, J.M.F., and Simonson, B.M., 2005, Precambrian iron formation-hosted iron ore deposits: in Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., Economic Geology 100th Anniversary Volume, p.643-679. Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin 110:1467-1484 Didier, J., and Barbarin, B., 1991, Enclaves and granite petrology: Elsevier, 625p, reference to p. 549. Feirn, W.C., 1977, The geology of the early Precambrian rocks of the Jasper Lake area, Cook County, northeastern Minnesota: Unpublished MS thesis, University of Minnesota, Duluth, 146p. Floran, R.J., and Papike, J.J., 1975, Petrology of the low-grade rocks of the Gunflint iron-formation, OntarioMinnesota: GSA Bulletin 86:1169-1190. Floran, R.J., and Papike, J.J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: Journ. Petrology, 19:215-288. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked volcanic ask: Can. J. Earth Sci., 39:1085-1091. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H.M and Drueger, H.W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, p. 57. Grout, F.F., 1929, The Saganaga Granite of Minnesota-Ontario: Journal of Geology 37:562-591. Grout, F.F., 1933, Structural features of the Saganaga granite of Minnesota-Ontario: Report of XVI International Geological Congress, Washington, p. 255-270. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642. Hanson, G.N., 1972, Saganaga batholith: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 102-107. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from UPb zircon and baddeleyite dating: in Easton, M., and Hollings, P., eds., Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, part 1, p. 24-25. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Jirsa, M.A., and Miller, J.D., Jr., 2005, Geologic implications of bedrock mapping in the Ely and Basswood Lake quadrangles, northeast Minnesota: in Easton, M., and Hollings, P. (eds.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, Part 1, p. 28-29. Jirsa, M.A., and Miller, J.D., Jr., 2005, Classic Precambrian geology of northeast Minnesota: Field Trip 2 in Robinson, L., ed., Field trip guidebook for selected geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook Series 21, p. 8-33. Jones, N.W., 1984, Petrology of some Logan sills, Cook County, Minnesota: Minnesota Geological Survey Report of Investigations 29, 40p. Lobatka, T.C., Papike, J.J., Vaniman, D.T., and Morey, G.B., 1981, Petrology of contact metamorphosed argillite from the Rove Formation, Gunflint Trail, Minnesota: Am. Mineralogist, 60:70-86. Mathez, E.A., Nathan, H.D., and Morey, G.B., 1977, Reconnaissance geology of the Hungry Jack Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map Series M-39, scale 1:24,000. Miller, J.D., Jr., and Jerde, E.A., 2007, Poplar Lake intrusion: this volume. ILSG07 167 Trip 6 �Miller, J.D., Jr., Morey, G.B., and Weiblen, P.W., 1987, Seagull Lake-Gunflint Lake area: A classical Precambrian stratigraphic sequence in northeastern Minnesota: in Biggs, D.L. ed., North-Central Section of the Geological Society of America Centennial Field Guide Volume 3, p.47-52. MillerJ.D., Jr., and Severson, M.J., 2002, Geology of the Duluth Complex: in Miller, J.D., Jr., Green, J.C., Severson, J.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 106-143. Mogessie, A., 1976 Petrologic study of copper-nickel mineralization in the Tuscarora intrusion Duluth Complex, northeastern Minnesota: M.S. thesis, University of Minnesota, Twin Cities, 137p. Mogessie, A., Stumpfl, E.F., and Weiblen, P.W., 1976, The role of fluids in the formation of platinum-group minerals, Duluth Complex, Minnesota: Mineralogic, textural, and chemical evidence: Econ. Geol. 86:15061518. Morey, G.B., 1969, The geology of the Middle Precambrian Rove Formation in northeastern Minnesota: MGS Special Publication 7, 62p. Morey, G.B., 1972, Gunflint Range, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 218-225. Morey, G.B., and Nathan, H.D., 1978, Geologic map of the Gunflint Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-42, scale 1:24,000. Morey, G.B., and Nathan, H.D., 1977, Reconnaissance geologic map of the South Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-38, scale 1:24,000. Morey, G.B., and Southwick, D.L., 1995, Allostratigraphic relationships of Early Proterozoic iron-formations in the Lake Superior region: Econ. Geol. 90:1983-1993. Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Misc. Map M-46, scale 1:24,000. O'Brien, H.E., 1982, Petrology, geochemistry, and magnetics of a lamprophyre dike intruded into the Saganaga Tonalite batholith, northeastern Minnesota: Senior Thesis, University of Minnesota, Twin Cities, 59p. Peterson, D.P., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.Canadian border: Phase I geochronology, (abstract): Institute on Lake Superior Geology Proceedings, 47th Annual Meeting, Madison, WI, 2001, Part 1, p.77-78. Pratt, B.R., 2001, Syneresis cracks: subaqueous shrinkage in argillaceous sediments caused by earthquake-induced dewatering: Sedimentary Geol., 117:1-10. Pufahl, P.K., and Fralick, P.W., 2000, Depositional environments of the Paleoproterozoic Gunflint Formation: in Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, Ontario, Part 2 Field Trip Guidebook, v. 51. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA: Sedimentology 51:791-808. Schulz, K.J., and Cannon, W.F., 2007 (in press), The Penokean Orogeny in the Lake Superior region: Precambrian Research. Stern, R.A., and Hanson, G.N., 1991, Archean high-Mg granodiorite: A derivative of light rare earth elementenriched monzodiorite of mantle origin: J. Petrol. 32:201-238, Table 2, p. 220. Stern, R.A., Hanson, G.N., and Shirey, S.B., 1989, Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province: Can. J. Earth Sci., 26:1688-1712; Table 1 p. 1695. Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the JAP Lake area, northeastern Minnesota: M.S. Thesis, University of Minnesota-Duluth, 193 p. Weiblen, P.W., Morey, G.B., and Mudrey, M.G., 1971, Guide to the Precambrian rocks of northwestern Cook County as exposed along the Gunflint Trail: in Davidson, D.M., Darby, D.G., Green, J.C., and Grant, J.A., eds., Technical Sessions, Abstracts and Field Guides, 17th Annual Institute on Lake Superior Geology, Annual meeting, Duluth, Minnesota, p.97-127. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior Region: Results of preliminary U-Bp detrital zircon studies: in Wilson, A.C., (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology Proceedings, 52nd Annual Meeting, Sault Ste Marie, Ontario, v. 51, Part 1, p. 69-71. ILSG07 168 Trip 6 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2007 Description An account of the resource Institute on Lake Superior Geology. Lutsen, Minnesota. May 8-13, 2007. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2007 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/749c5c6ee43eb071b1c6eea81168ff01.pdf 7cea60901a11884b9ca5c94d5c9eda94 PDF Text Text Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Pete Hollings, Mark Smyk, Bill Addison & Phil Fralick �Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist’s Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Bill Addison R.R. 2, Kakabeka Falls, Ontario, P0T 1W0, Canada Phil Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Cover Photos: Top - Shatter cone clast in impact breccia; Middle - Interflow sandstone unit in Paleoproterozoic basalts; Bottom - West coast of Patterson Island. �Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Reference to material in this volume should follow the example below: Hollings, P., Smyk, M., Addison, B. and Fralick, P., 2006. Field trip guidebook for the Slate Islands. Institute on Lake Superior Geology, Special Publication 1, p. 21. Published by the Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �ILSG Special Publication #1 - The Slate Islands Table of Contents Introduction..........................................................................................................................1 Safety Considerations..........................................................................................................1 Acknowledgements..............................................................................................................1 Regional Geology................................................................................................................2 One Archipelago, Two Possible Origins..............................................................................5 The Case for an Extraterrestrial Impact Origin for the Slate Islands Structure.........5 The Case for a Cryptoexplosion Origin for the Slate Islands Structure...................10 The Debate...............................................................................................................11 Economic Geology.............................................................................................................12 Stops...................................................................................................................................13 Stop A – “Honeymoon Bay” near Cove Island........................................................13 Stop B – Sunday Harbour.........................................................................................14 Stop C – Horace Cove..............................................................................................15 Stop D – Western shore of Patterson Island.............................................................16 Stop E – McGreevy Harbour....................................................................................18 References..........................................................................................................................20 -i- �ILSG Special Publication #1 - The Slate Islands Introduction Safety Considerations This volume is intended to serve not only as a guide for participants during the August 2006 field trip to the Slate Islands, but also as a reference for those planning to revisit the area at a later date. Consequently we have included UTM coordinates (NAD 83 datum) for stops. The Slate Islands are covered by southern boreal forest with some shoreline arctic-alpine disjunct flora and is protected as a Natural Environment Provincial Park with no visitor facilities. Rock collecting and sampling is prohibited throughout the entire archipelago unless a permit is first obtained from the Ministry of Natural Resources: Slate Islands Provincial Park Ministry of Natural Resources P.O. Box 970 Nipigon, ON P0T 2J0 A field trip to the Slate Islands creates a number of unique safety issues. Please exercise caution when getting in and out of the boats, as the outcrops are often sharp and extremely slippery. Personal flotation devices should be worn in the boats at all times. If you are planning to revisit these sites please be very careful. Lake Superior is a cold, dangerous lake; waves can often be metres high and even in mid-summer fog can appear very quickly. A GPS system, compass and maps should be utilized. We strongly encourage you to charter a large boat from the mainland rather than trying to make the trip to the islands yourself. Phone: (807) 825-3403 This is the first time a publication has been produced for a field trip that is not directly associated with an ILSG Annual Meeting. However, the location of the Slate Islands dictates that field trips to the islands are best made later in the summer when weather and lake conditions are more conducive to travel. Acknowledgements We would like to thank all those who provided comments on this guide and assisted with the running of the field trips, particularly Doug Caldwell and John Scott. Woodland Caribou on the Slate Islands -1- �ILSG Special Publication #1 - The Slate Islands Regional Geology The Slate Islands comprise a 7 km-wide archipelago of 17 islands located in northern Lake Superior approximately 12 km southeast of Terrace Bay (Fig. 1). The geology of the islands has been mapped by Coleman (1901), Parsons (1918) and by Sage (1975, 1991). The islands comprise both Archean and Proterozoic rocks. The Archean rocks are part of the Schreiber-Hemlo greenstone belt (Wawa Subprovince). Paleoproterozoic sequences include the Gunflint and Rove Formations of the Animikie Group. Mesoproterozoic Keweenawan basalts are interpreted to be an extension of the Osler Group of the Midcontinent Rift (Sage, 1991). Sage (1978, 1991) mapped greenschist facies Archean metavolcanic rocks and subvolcanic intrusive rocks ranging in composition from calc-alkaline dacite to tholeiitic basalt. The Archean supracrustal rocks consist of coarse felsic pyroclastic units, felsic to mafic tuffs, feldspar-phyric flows and amygdaloidal, pillowed and variolitic mafic flows with thin interbeds of argillite and siltstone (Fig. 2; Sage, 1991). On the basis of pillow facing directions Sage proposed that an antlicinal structure crosses the centre of Mortimer Island. The pillowed flows are most common on Mortimer and Delaute islands (Fig. 2); pillows are typically bun- to mattress-shaped and up to 2m across (Sage, 1991). In places on Mortimer Island the massive and pillowed basalts grade into flow breccias. Volcanic and intrusive rocks of more felsic compositions are found on Patterson, Dupuis, Spar and Leadman islands (Fig. 2) and have been interpreted by Sage (1991) to be highly sheared, amygdaloidal and porphyritic carbonatised sequences. Archean metasedimentary rocks are relatively rare and are predominantly volcaniclastic as they appear to interfinger with the volcanic flows (Sage, 1991). The Archean mafic volcanic rocks from the Slate Islands are can be subdivided into two distinct geochemical suites. One suite is characterized by flat primitive mantle-normalized patterns typical of tholeiitic rocks found in modern oceanic plateaus whereas the second, more abundant, suite is characterized by weakly LREE-enriched patterns with minor negative Nb anomalies, characteristic of rocks formed in an island arc setting (Fig. 3; P. Hollings, unpublished data). Similar assemblages have been reported in the Schreiber-Hemlo greenstone belt (Polat et al., 1998). Figure 1. Map showing the location of the Slate Islands. -2- �Figure 2. Geological map of the Slate Islands. Modified after Sage (1991). -3- 1 km Patterson Island Mortimer Island D Horace Cove C Edmonds Island McColl Island 87°00’ Sunday Harbour B McGreevy E Harbour Bowes Island 87°00’ A1 Dupuis Island Delaute Island A2 Cove Island Field trip stop 84°40’ Mafic volcanic rocks Felsic volcanic rocks Metasedimentary rocks Mafic intrusive rocks Felsic intrusive rocks Archean Animikie group Osler group Diabase dikes Breccias Post-Archean N ILSG Special Publication #1 - The Slate Islands 84°40’ �ILSG Special Publication #1 - The Slate Islands Figure 3. Primitive mantle normalised diagram showing representative samples of the two geochemical suites recognised amongst the Archean mafic volcanic rocks of the Slate Islands. ST33 = arc-type, ST38 = plateau-type. Normalising values from Sun and McDonough (1989). On the western shore of Patterson Island, Sage (1991) reported an ~20m thickness of iron formation of the Gunflint Formation lying unconformably on the Archean basement and below Mesoproterozoic Keweenawan basalts (Fig. 2). The lowermost three metres of the sequence consists of interbedded jaspilitic chert, hematite and carbonate overlain by a sequence of hematitic chert. The argillites are generally massive and only locally display well-developed bedding. Recent re-examination of outcrops on eastern Mortimer Island and Delaute Island mapped as Paleoproterozoic Rove Formation clastic sedimentary rocks, has resulted in them being reinterpreted as Archean metasedimentary rocks analogous to the McKellar Harbour turbidite sequence on the mainland and this is now reflected on Figure 2. Keweenawan basalts unconformably overlie the Gunflint rocks (Fig. 2) and form an ~120m thick flow sequence that dips ~80° at its base and diminishing to ~25° towards its top (Sage, 1991). This implies some degree of block rotation. Twenty-two individual flows can be recognized within the upper portion of the sequence. Interflow contacts are typically sharp and often marked by thin interflow sedimentary units (Sage, 1991). The basalts are typically vesicular and amygdaloidal, and in places show poorly developed ropy flow tops. The feldspar and pyroxene-phyric basalt flows are incipiently to completely altered, to sericite, carbonate and calcite (P. Hollings, unpublished data). However, even the relatively unaltered samples are significantly more altered than Osler basalts in the vicinity of Rossport (Hollings et al., 2006). Red, medium-grained, well-sorted, arkosic sandstone interflow units consist of sub-rounded to sub-angular grains of predominantly quartz, plagioclase, Kfeldspar, volcanic rock fragments and amphibole. The feldspars, especially the K-feldspar, are commonly intensely weathered (seriticised). Most grains have very fine-grained hematitic coatings. The sandstone is relatively matrix-poor with an earlier phase of radiating chalcedony and quartz fans to drusy cement overgrown by a later stage of blocky carbonate, void-filling cements. Halls (1974) proposed that the paleomagnetic signature of the basalts was comparable to the lower portions of the Osler volcanic group in northwestern Lake Superior. The aforementioned rocks are also intruded by a number of Keweenawan dikes and breccia bodies which commonly occupy and obscure lithologic contacts (Sage, 1991). Hinze et al. (1966), on the basis of aeromagnetic data, interpreted the presence of two major faults, which intersected to the south of the Slate Islands (Fig. 4). Sage (1991) has proposed that the onshore extension of the Figure 4. Faults of eastern Lake Superior with inferred directions of movement from Sage (1991). Modified from Hinze et al. (1966). -4- �ILSG Special Publication #1 - The Slate Islands northeast-trending Big Bay-Ashburton Bay Fault may be related to northeast-trending structures associated with the Mesoproterozoic Midcontinent Rift-related alkalic carbonatitic complexes at Deadhorse Creek and Prairie Lake. Sage (1991) reported that, in the vicinity of Patterson Island, breccia dikes cut and enclose blocks of lamprophyre with carbonatite affinity that have yielded a K-Ar age of ~300 Ma. Recently this unit has been dated using the U-Pb method, yielding a Keweenawan age of ~1100 Ma (L. Heaman, University of Alberta, personal communication, 1994, referenced in Dressler et al., 1999) suggesting the young K-Ar ages are likely the result of resetting. The breccias have been used to both argue for and against a meteor impact theory for the Slate Islands and are discussed further below. One Archipelago, Two Possible Origins In addition to the complex bedrock geology, the islands have also been the focus of interest and debate because they are considered by some to represent the “best-preserved, medium-sized, meteor impact structure on Earth” (V. Sharpton, Lunar and Planetary Institute, NASA, pers. comm. 1995). However, this theory is not universally accepted and Sage (1991, 1999) has proposed an endogenous cryptoexplosion process for formation of the islands. The Case for an Extraterrestrial Impact Origin for the Slate Islands Structure Before discussing the evidence for an impact, it is worth outlining the basic dynamics of a hypervelocity extraterrestrial impact. The continuous process that occurs during an impact is more readily understood if it is dealt with in stages (summarized by French, 1998, and outlined in particular for the Slate Islands by Dressler et al., 1998). 1) Contact/Compression Phase. As the impactor hits, hypervelocity shock waves are generated in both the impactor and the target rocks which forces the bedrock downwards and outwards, instantaneously vapourizing the impactor and the target rock near the point of impact, while further away the target melts as the shock pressures attenuate. 2) Decompression/Excavation Phase. The shock wave is immediately followed by a rarefaction or tensional wave, decompressing the remaining rock and allowing it to relax, opening fractures large and small, driving material downward, outward and upward, excavating an extremely short-lived, steep-sided, unstable transient crater. 3) Central Peak Formation. If the impactor and consequent forces are large enough, the unloading of deep bedrock by the removal of overlying rock plus the decompression following the shock wave, results in material in the bottom of the crater rebounding upward into a central peak within the transient crater (as when a drop of water hits the calm surface of a pond). 4) Transient Crater Collapse and Formation of the Final Crater. As the transient crater reaches its maximum size, the fractured and faulted oversteepened walls begin collapsing into the crater in a rush, meeting and mixing with the likewise collapsing central uplift, before settling into an approximation of the final crater form. The entire process from first contact by the impactor to this stage has not lasted much more than 5-10 minutes in most craters, perhaps 15 minutes in the very largest craters a couple of hundred of kilometres in diameter. 5) Long Term Adjustment. Then begins a long process of adjustment, lasting decades to many millennia, depending on many things, but primarily crater size. During this final stage, loose debris continues settling, aided by tremors as stresses are released, hydrothermal activity begins in medium-sized to large craters, cooling continues, and finally, consolidation and lithification of breccias takes place. A number of authors have proposed that the Slate Islands have preserved the site of a meteor impact (Halls, 1975, 1976; Robertson and Grieve, 1976; Halls and Grieve, 1976; Halls and Stesky, 1978; Dressler et al., 1995, 1998, 1999). The islands themselves have been identified as the central uplift of a mediumsized impact structure, which, bathymetry suggests, is surrounded by a submerged annular trough ringed by a ridge 30 to 32 km in diameter (Halls and Grieve, 1976; Dressler et al., 1995), representing the suggested final crater diameter. A crater this size implies an ~1.5 km diameter impactor with an arrival velocity of ~ 15 km/s. This circular feature was also transected and confirmed by the Great Lakes International Multidisciplinary Program of Crustal Evolution (GLIMPCE) seismic reflection line (Fig. 5; Mariano and Hinze, 1994). According to Dressler et al. (1998) almost all the rocks of the archipelago are somewhat brecciated and -5- �ILSG Special Publication #1 - The Slate Islands Figure 5. Reprocessed northern part of GLIMPCE Line A (courtesy of B. Milkereit, Geological Survey of Canada, 1994, published in Dressler et al., 1999). Left vertical axis is seconds of two-way time, right vertical axis is approximate depth in kiometres and horizontal axis shows shot points. X is the westward projection of the approximate centre of the central uplift. The distance from the centre of the central uplift (approximate geographic centre of the archipelago) to R is 15-16 km. R lies approximately where the rim of the structure is placed based on bathymetry. The strong reflections at 0.5 s may represent arenites of the Jacobsville Formation and not multiple reflections of the lake bottom which is at relatively shallow depth in the area investigated here. AB: Keweenawan basalt; BC: Jacobsville Formation. From Dressler et al. (1999). they propose that the bedrock can be considered a megabreccia, although the detailed mapping of Sage (1991) showed good structural coherence across the islands. For Sage (1999) this structural coherence between the blocks and with rocks on the mainland argues for an endogenous origin for the breccias on the islands. All local rocks have been intruded by a network of anastamosing breccia bodies ranging in colour from brick-red to greenish grey. The breccias consist of sharply angular to sub-rounded fragments up to four metres across derived from local Precambrian rocks. The breccias have been ascribed to both endogenous intrusive activity (e.g., Sage, 1991) and meteor impact (e.g., Sharpton et al., 1996) and have been used to argue both for and against the impact theory on the Slate Islands. The age of the Slate Islands structures and breccia bodies is poorly constrained (Table 1). Grieve et al. (1995) proposed an age of <350 Ma based on similarities in the erosional level between the Slate Islands and the ~350 Ma Charlevoix structure in Quebec. Sharpton et al. (1996) have proposed an age of 500-800 Ma based on the presence of clasts of the Mesoproterozoic Jacobsville sandstone (southern shore of Lake Superior, Michigan) and absence of any Devonian or Ordovician carbonates. However, the Slate Islands sandstone clasts are similar to sandstone interflow units found within the Osler basalts on the western shore of Patterson Island and, thus, may not be Jacobsville sandstone. More recent Ar-Ar age determinations on impact-generated pseuodotachylites have yielded spectra consistent with an age of ~450 Ma (Fig. 6; Sharpton et al., 1997; Dressler et al., 1999). Features that have been used in support of an impact event include dikes of clastic-matrix breccia Figure 6. 40Ar-39Ar release spectra. Samples 95SL103 and 95SL13:3e: dark gray, inclusion bearing “impact melts” (Keweenawan basalt). Sample 94B1D2, inclusion-poor pseudotachylite. From Dressler et al. (1999). -6- �ILSG Special Publication #1 - The Slate Islands Table 1. Stratigraphic age constraints on the Slate Islands impact. The age of the Jacobsville is ~1100 Ma, however these clasts may be interflow sandstones from within the Osler-like volcanic flows (see text). From Dressler et al. (1999). (Halls and Grieve, 1976; Sage, 1991), which include; pseudotachylites, polymictic allogenic breccias and monomictic autoclastic breccias (Sharpton et al., 1996) concentrated on the eastern shore of Patterson Island as well as Mortimer, Dupuis and Delaute islands. Shatter cones occur throughout the islands but are most obvious in the Keweenawan basalts. They are interpreted to have formed from the passage of a high-pressure shock wave (Dietz, 1964). They are characterized by a surface decorated with linear ridges and grooves (horsetail striations) that radiate from the apex of the cone. On the Slate Islands most shatter cones range from 2cm to ~30cm long; those in the Keweenawan rocks are 10 to 30 cm long (Sharpton et al., 1996). In addition Sharpton et al. (1996) reported a number of “mega cones” at least 10m long (and possibly up to 20m) in McGreevy Harbour (Fig. 2). As with the breccias the origin of the cones themselves remains controversial (Sharpton et al., 1996). Dressler et al. (1999) has suggested that the shatter cones formed during the compressional phase of the impact (Fig. 7) and indicate a minimum shock pressure in the target rocks of 3 GPa. Sage (1991) observed that shatter cones were most extensive close to breccia outcrops and used this to argue that the explosive emplacement of diatreme dikes was responsible for their formation. However, more detailed work (Sharpton et al., 1996) indicated that the shatter cones are ubiquitous on the islands. Dressler et al. (1995, 1999) have reinterpreted Figure 7. Formation of the Slate Islands impact structure. A) preimpact target; B) contact and compression; C) excavation; D) central uplift; E) central uplift collapse and modification; F) final structure; G) present structure, black areas indicate impact melt overlain by allogenic breccias (assumed, not shown in DF). a, Proterozoic and younger supracrustal rocks: Deformed Archean greenstone assemblage (assumed in annular trough); b, Mafic metavolcanics, minor metasediments and intrusive rocks; c, Intermediate and felsic metavolcanics, minor metasediments and intrusive rocks. From Dressler et al. (1999). -7- �ILSG Special Publication #1 - The Slate Islands Sage’s diatremes as impact breccia bodies (e.g., Bunte breccia, suevite). Microscopic planar deformation features (PDFs) in quartz and feldspar have been observed in rocks from the Slate Islands by Halls and Grieve (1976), Sage (1991) and Dressler et al. (1994). These planar lamellae are shock induced micro-melt zones <2-3 μm wide along crystallographic axes. They first appear in quartz at pressures of ~8 GPa along the {0001} and {1011} axes and at ~10 GPa they begin to appear along the {1013} axis (French, 1998). PDFs are considered diagnostic of the extremely intense shock waves produced during hypervelocity impacts. However, a single set of PDFs can easily be confused with Bohm lamellae and other planar features, and thus, two or more criss-crossing sets of PDFs along different measured crystallographic axes are the preferred diagnostic features. Crisscrossing sets of PDFs are seen both in the Slate Islands host rocks and in breccia components. In a detailed study of these features Dressler et al. (1998) showed a zone of maximum shock intensity on Patterson Island (Fig. 8), suggesting the location of the impact was slightly west of the center of Patterson Island. types cross-cut other breccias, plus features such as PDFs, allowed them to hypothesize when various features formed during the impact process. It is within this context that Dressler and Sharpton (1997) place their interpretations of the breccias (Figs. 7, 8 & 9; Table 2). The breccias identified by the authors include: A detailed study of the breccias on the Slate Islands has been undertaken by Dressler and Sharpton (1997) who have estimated that breccias make up ~15 to 25% of the Islands’ rocks. Interpretation of which breccia • Pseudotachylites which are thought to have formed as a result of brittle-or brittle-ductile seismic faulting and instantaneous melting due to the passage of the hypersonic shock wave during the compressional phase of the impact event. Pseudotachylites are relatively rare in the archipelago and occur as small veins and dikes. The early formation of these pseudotachylites is supported by the presence of clasts of pseudotachylite in the breccias. • Polymictic clastic matrix breccias are the most abundant breccia type on the islands but are more common on Patterson Island than on the outlying islands. The breccias contain a wide variety of clasts from all host lithologies, that are angular to sub-rounded, and range in size from <1mm to several metres. These are interpreted to have formed when decompression allowed opening of fractures within the crater walls and floor (Dressler et al., 1999) excavating the crater to a depth of ~1.5 km in approximately 1 minute. Figure 8. Sketch map of Slate Islands impact structure, located in northern Lake Superior. Dashed lines show concentric trends of coast lines and structural elements indicating crater center on western side of Patterson Island (approximate location is shown by cross). Previous estimates of crater center, based on shatter cone orientations (Stesky and Halls, 1983) or shock isobars deduced from planar deformation features in quartz (Grieve and Robertson, 1976), are shown as filled circles. Shatter-coned outcrops are shown as small unfilled circles. Filled diamond shows location of >10m shatter cone. Map is adapted from Sharpton et al. (1996). Shaded field is the area of highest shock values from Dressler et al. (1998). -8- �ILSG Special Publication #1 - The Slate Islands Table 2. Slate Islands impact breccias. From Dressler and Sharpton (1997). Figure 9. Section across the Slate Islands complex impact structure showing distribution of breccias investigated. Minor polymictic clastic matrix breccias are also present further away from the centre of the structure than shown here. Profile is based on bathymetric information from around the archipelago and on topographic maps of the islands. From Dressler and Sharpton (1997). -9- �ILSG Special Publication #1 - The Slate Islands The presence of Proterozoic clasts in breccias dominated by Archean material has been used to argue for downward movement and mixing of clasts over distances possibly as much as 5 km (Dressler and Sharpton, 1997). • Allogenic breccia deposits containing altered glass fragments (suevites) or with no glass fragments (Bunte breccia) are present on Dupuis and Patterson islands and have been used to argue for a shallower erosion level for the archipelago (Sharpton and Dressler, 1996). The Bunte breccias are interpreted to have formed either as fall-back deposits in the crater or as ground-surge deposits. The breccias contain mainly Proterozoic clasts, supporting their origin as fall-back deposits (Dressler et al., 1999). Dressler et al. (1999) have also reported the presence of suevite breccias and use the absence of aerodynamically shaped glass fragments to argue that they are also fall-back breccias. • Monomictic, autochthonous breccias are found on Mortimer Island and a number of the small outlying islands. The breccias comprise angular, densely packed fragments typically up to 20 cm in size within a matrix of similar clastic rock powder. These are interpreted to have formed late in the impact process during the crater modification phase (Fig. 7) as huge blocks of rock slumping off the transient crater walls ground together during their slide into the crater over several minutes (Dressler et al., 1999). The breccias are often autoclastic with transitional borders with their host rocks (Dressler and Sharpton, 1997). Halls and Grieve (1976) and Grieve and Robertson (1976) proposed that the Slate Islands represented uplifted basement that preserved breccias injected during impact into the crater subfloor (~0.5 to 1.5 km below the central peak). However, Sharpton et al. (1996) have suggested that the allogenic and autoclastic breccias indicate that the present exposure surface is only a few hundred metres below the original ground surface. This issue is not without controversy and is discussed further in Halls (1997), Grieve and Robertson (1997) and Sharpton and Dressler (1997). cone orientations and shock barometry (Stesky and Halls, 1983; Grieve and Roberston, 1976) suggested that it was closer to the center of Patterson Island (Fig. 8). However, given that both locations are within 1.5 km of each other and given that the impacting body was estimated to have a diameter of ~1.5 km, none of the proposed locations should be considered definitive (Sharpton et al., 1997). The Case for a Cryptoexplosion Origin for the Slate Islands Structure The title of Sage’s (1999) paper clearly stated his case: “The Slate Islands: A Uniquely Sited Cryptoexplosion Structure”. He noted that the Slate Islands are situated on or near the Proterozoic-Archean boundary and at the intersection of two major inferred faults, the Big Bay-Ashburton Bay Fault (or accommodation zone) and the Michipicoten Fault. He also noted that the Slate Islands lie on the flank of the Midcontinent Rift where crustal thickness reaches 50 km or more, and on a topographic ridge, extending southwest to Superior Shoals and northeast to the mainland, which bisects this thick crust. Perhaps most importantly to Sage, the Slate Islands are close to the Port Coldwell Alkalic Rock Complex, the Kilalla Lake Alkalic Rock Complex, Prairie Lake Carbonatite, Deadhorse Creek Diatremes and McKellar Creek Diatremes (Sage, 1999), all of Keweenawan age. Sage (1991) argued that “the possibility of a meteorite impact at this precise location – on a ridge traversing the Lake Superior Basin, on the nose of an Archean fold structure, at the precise location of the Proterozoic-Archean contact, at the precise location of two intersecting regional faults, and at the precise location of highly volatile alkalic magmatism – is too incredible to accept (Sage, 1991, p.56)”. Sage (1978) presented a number of geological observations favouring a non-impact origin many of which were elaborated upon in Sage (1991). These included: Current interpretations based on topographic and structural trends place the crater center in the westcentral part of Patterson Island (Fig. 8; Sharpton et al., 1996) whereas earlier interpretations based on shatter - 10 - • Clast size sorting in the diatremes from finegrained at the margins to coarse-grained at the center is typical of laminar flow (Sage, 1978) and more likely to occur in a diatreme than by downward intrusion. • Orientation of shatter cones was not consistent with a central impact structure. • Contact metamorphic effects between the breccia and alkalic diabase indicates that hydrothermal �ILSG Special Publication #1 - The Slate Islands activity accompanied breccia emplacement. • The presence or absence of an igneous matrix in breccia dikes on the islands and the mainland does not preclude an igneous origin. The Debate The debate – cryptoexplosion vs impact – over the origin of the Slate Islands structure largely mirrors (in a more genteel way) the vigorous debate which began when Dietz (1964) proposed that the Sudbury Structure was due to an impact. When Alvarez et al. (1980) proposed that the K/T extinction was caused by an impact, the debate became a nasty scientific controversy. Now, some 26 years later, the debates over Sudbury and the Chicxulub-K/T extinction crater are resolved in favour of an impact origin for both. The debate is summarized by Powell (1998) in an interesting, very readable popular book, “Night Comes to the Cretaceous”. To reiterate Sage’s statement, “the possibility of a meteorite impact at this precise location – on a ridge traversing the Lake Superior Basin, on the nose of an Archean fold structure, at the precise location of the Proterozoic-Archean contact, at the precise location of two intersecting regional faults, and at the precise location of highly volatile alkalic magmatism – is too incredible to accept” (Sage, 1991, p.56). Halls (1979) counters that the absence of complex overlapping shatter cone sets argues against the multiple emplacement events proposed by Sage (1978). On a larger scale Halls (1978) argued that the regional faults proposed by Sage (1978) are only inferred from geophysical data and the magnetic anomalies may also delineate the unfaulted margin of the Keweenawan basin. Halls (1979) also provided alternative explanations for the apparent coincidences suggested by Sage, observing that the lower and upper Precambrian contact predates the shock event and cannot be used to argue either for or against. The Slate Islands debate centres mainly on the interpretation of three sets of features: the breccias, shatter cones, and planar deformation features (PDFs). Sage (1991) has proposed that the breccias originated from the forcible emplacement of volatilerich magmas formed at depths > 35 km which have risen to a shallower level and exsolved a gas phase. The higher volatile contents of clasts and matrix have been argued to support this model. However, arguing against an origin at depths of ~35 km is the absence of deep-seated or magmatic material in the breccias (Robertson and Grieve, 1979). Another breccia problem is explaining how dikes containing upper level Paleoproterozoic fragments were emplaced into lower level Archean rock. Halls and Grieve (1976) were the first to suggest a downward injection of breccias into fractures (during the crater modification stage following the passage of the initial shock wave) as a result of an impact event (Robertson and Grieve, 1979). However, Sage (1991) has countered that the presence of stratigraphically high level clasts at depth could also be explained by collapsing fluid columns after the emplacement of diatremes. Today, the various breccia types (pseudotachylites, polymictic, allogenic, and monomictic autochthonous), their relationships within each other, and their locations within the Slate Islands structure seem to be best explained by their production during various phases of the impact process (Sharpton and Dressler, 1997; Dressler and Sharpton, 1997; Dressler et al., 1999). Sage (1991) has also advocated that the forceful emplacement of diatremes formed at depths > 35 km could account for the shock features – shatter cones and PDFs – preserved on the islands. Sage (1999) provides a number of examples of other occurrences of planar deformation lamellae and shock textures that may have been produced by kimberlite emplacement, however, he acknowledges that these features have also been interpreted as having been formed by impact events. Robertson and Grieve (1979) observed that shatter cone formation is a function of lithology as well as shock pressure and the fissile Archean metavolcanics would display more poorly developed cones than the structurally isotropic Keweenawan flows. The distribution of microscopic shock effects has been recorded by Grieve and Robertson (1976) who showed that the intensity of these features increase in a consistent fashion from the coast inward to the proposed impact centre (see Fig. 8). Roberston and Grieve (1979) also argued that diatreme emplacement is normally considered to be a process of “drilling and venting by gas streaming” rather than by violent explosions and that the pressures induced by this process are unlikely to exceed 1.5 GPa, whereas pressures of 2-6 GPa are required to generate shatter cones (French, 1998). Halls (1979) took issue with Sage’s measurements of shatter cones suggesting that he did not use the correct measurement procedure and did not properly correct - 11 - �ILSG Special Publication #1 - The Slate Islands his data and that stated that there is no convincing spatial correlation between shatter cones and breccia dikes, and furthermore, the breccias contain shatterconed clasts (Halls and Grieve, 1976). sampled by Resident Geologist staff in 1995 near Cove Island returned 604 ppm Zn, <100 ppm Cu and 157 ppm Pb (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). PDFs seem beyond debate, so long as they are really PDFs (and not Böhm lamellae), based upon measured widths and spacing of lines and particularly based upon their alignment along measured crystallographic axes. French (1998) has summarized a large body of laboratory experimental evidence and field evidence on PDFs, as do Dressler et al. (1998). Dressler et al. (1998) measured a large number of PDF orientations in Slate Islands quartz crystals. They found PDFs aligned along many different axes, notably the {1013} and {1012} axes, indicative of shock pressures as high as 18 GPa, pressures equivalent to those many hundreds of kilometers depth within Earth. Such observations cannot be explained by diatremes. A little discussed problem for the impact hypothesis is the apparent absence of evidence for an impact on the mainland north shore of Lake Superior, only 15 km from the proposed impact centre, which supposedly produced a crater ~15-16 km in radius. Thus, on both side of the debate, problems still have to be resolved, but the majority of the evidence described from the Slate Islands Structure is best explained by an impact. Economic Geology A synopsis of the mineral exploration history and mineralization is provided by Sage (1991). Two styles of mineralization in the Archean metavolcanic rocks have garnered the most exploration interest: 1) lode gold; and 2) volcanogenic massive sulphide copperzinc. Gold is associated with quartz-carbonate veins in deformed and altered (Fe-carbonate, sericite, chlorite, tourmaline) rocks. More than 20 occurrences of visible gold in float boulders of quartz vein material have been recorded on the islands (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Visible gold has also been noted in-situ near Horace Cove (aka St. Mary’s Bay). Massive sulphides occur in felsic metavolcanic rocks or as fragments in pyroclastic rocks. Massive pyrite sampled by Sage (1991) returned 0.11% Cu and 0.28% Zn. A sulphide-facies banded iron formation - 12 - �ILSG Special Publication #1 - The Slate Islands Stops are cut by a number of grey, heterolithic breccia dikes The field trip stops are labeled with letters rather than numbered in sequence as access to all stops, particularly on the outer shores of the islands, is weather-dependent. Many of the stops require some wading in order to reach all the outcrops so a change of footwear is recommended. Please take care when getting in and out of boats as outcrops are usually extremely slippery. with 0.5 to 5 cm, angular to sub-rounded clasts (Figs. 11 & 12). Narrow (<1 cm) dikelets may extend into the wall rock from the parent breccia dike. Stop A – “Honeymoon Bay” near Cove Island UTM coordinates – 0502004E 5386377N This small bay near Cove Island (Fig. 2) provides exposures of strongly sheared Archean felsic metavolcanic rocks at its northeast end (Stop A1). These metavolcanic rocks are phyllitic, displaying a pronounced west-trending, steeply dipping foliation with minor folds and kink bands. They are intruded by a 1 m wide, Paleoproterozoic diabase dike that zigzags across the outcrop (Fig. 10). A small (50 cm) wide breccia dike crosscuts the metavolcanic rocks. Metavolcanic rocks along the western shore of the bay Figure 12. Heterolithic breccia dyke at Stop A1. From this point groups will be shuttled out to a small island to the east of Honeymoon Bay (Stop A2; Fig. 2; UTM coordinates 0502426E 5386437N). This island consists of heterolithic breccia with clasts over 2 m across (Fig. 13). From the top of the island it is possible to look down upon a series of anastamosing dark grey breccia dikes under the water in the bay (Fig. Figure 10. Diabase dyke intruding Archean felsic metavolcanic rocks at Stop A1. Figure 11. Breccia dyke intruding Archean felsic metavolcanic rocks at Stop A1. Dykelets marked by arrows. Figure 13. Typical breccia exposed on the breccia island at Stop A2. - 13 - �ILSG Special Publication #1 - The Slate Islands from larger, parent dikes. Stop B – Sunday Harbour UTM coordinates – 0500801E 5386425N Figure 14. Anastamosing breccia dykes at Stop A2. 14) and across to a second island where an ~2 m wide, recessively weathered clast of reddish metavolcanic rock is clearly visible. You will need to get your feet wet to fully appreciate this outcrop. The majority of clasts typically range between <1 to 10 cm in size. They are derived from Archean metavolcanic rocks, Mesoproterozoic diabase and a variety of nondescript, fine-grained, variably altered rocks of indeterminate origin. Ragged, injected bodies of breccia may extend The beach consists of reworked glaciofluvial sediments characterized by a variety of locally derived and exotic rounded cobbles and boulders. Most of these are felsic plutonic and mafic metavolcanic rocks of the Schreiber-Hemlo greenstone belt. The exotic clasts are best exemplified by what have been termed “omars” (Prest, 1990), glacial erratics of massive, dark siliceous greywacke that contain light-toned (generally buff-weathering) calcareous concretions which are typically subspherical and weather recessively (Fig. 15). Omars, which commonly occur in and on eskers and outwash, but which also may be found in till and lacustrine deposits, are inferred to have been derived from the Omarolluk Formation of the Belcher Group in southeastern Hudson Bay (Prest et al., 2000). Most of the erratics were dispersed northwestward and westward across the Hudson Bay Paleozoic Basin by Labrador Sector ice, followed by westward and southwestward movement of ice across the Paleozoic and Archean terrain of northern Ontario, northern Manitoba and the upper Midwestern United States. A series of breccias are exposed on the eastern shore of Sunday Harbour. Dressler et al. (1999) reported the presence of two allogenic breccias at this outcrop: a Bunte Breccia is reported from the southern portion of the outcrop and a suevite breccia to the north. The southern end of the outcrop is a heterolithic grey breccia with clasts up to 50cm wide. It also contains clasts with well-developed shatter cones (Fig. 16). The grey breccia contains conspicuous reddish metavolcanic clasts and rare mafic to ultramafic clasts up to 50 cm across. At this location, the breccia is quite friable and easily dislodged from outcrop faces. Narrow diabase dikes with quartz-filled tension gashes intrude the metavolcanic rocks and are exposed just offshore in shallow water. Foliation orientations in the metavolcanic rocks are variable, suggesting either large-scale folding or rotation of large blocks of country rocks. Reddish alteration zones appear as dike-like bodies, cutting the grey breccia in places. Figure 15. Omars from the beach at Sunday Harbour (Stop B). - 14 - �ILSG Special Publication #1 - The Slate Islands chalcopyrite and hematite are noted. Grab sampling of vein material by Resident Geologist’s Program staff returned up to 3.95 ounces Au per ton and 0.2 ounce Ag per ton (Resident Geologist’s Files, Thunder Bay South District). The style of mineralization, alteration and deformation resembles that at Heron Bay, on the mainland shore of Lake Superior, approximately 50 km east of this location. The following synopsis of gold exploration at Horace Cove (aka St. Mary’s Bay) was modified from that of Sage (1991). Parsons (1918) concluded that the gold showing on the northwest corner of Horace Cove was the most promising of the known gold occurrences. From 1960 to 1963 Kimberly-Clark Pulp and Paper Company Limited conducted a mineral exploration program of the islands to test two gold showings. The main gold showing (St. Mary’s Bay occurrence) is on the northwestern corner of Horace Cove and the second occurrence (Cosen’s Showing) lies 240 m to the northeast. Figure 16. Shatter cone clast in breccia at Sunday Harbour (Stop B). Stop C – Horace Cove UTM coordinates – 0497386E 5387206N Shoreline outcrops at this location expose pervasively Fe-carbonatized and sericitized, schistose Archean metavolcanic rocks (Fig. 17). A strong, westtrending and steeply dipping foliation has resulted in the development of fissile, phyllitic rocks that also contain quartz, chlorite, green mica (chromian muscovite) and pyrite. Hydrothermal alteration and deformation preclude definitive recognition of the protolith. Sage (1991) has noted schistose basaltic to andesitic rocks, as well as dacitic to rhyolitic flows in the vicinity. Quartz- and feldspar-phyric units and sections containing quartz blebs (amygdules?) are also noted. Thin section analysis of this quartz-sericite schist by Nichols (1963) revealed a fine-grained groundmass of quartz blebs and scaly intergrowths of sericite that hosts siderite euhedra, altered albite and prochlorite and quartz amygdules. Along the shoreline, quartz-carbonate veins, with which most of the gold is associated, occupy a 050°trending fracture set. They range up to approximately 8 cm in width and are locally folded. Visible gold, pyrite, In 1960 Kimberly-Clark contracted an aeromagnetic and electromagnetic survey of the island. In 1961 and 1962 trenching, bulldozing, stripping, sampling and geologic mapping was done by the company over both the St. Mary’s Bay zone and Cosen’s showing. At St. Mary’s Bay bulldozer stripping to depths of 1.6 to 2.0 m exposed an area of approximately 18,900 m2; at Cosen’s showing 240 m to the north, approximately 5350 m2 of similar stripping was completed (G.E. Parsons, consulting geologist, personal communication, 1976). In 1963, Kimberly-Clark formed the Slate Island Mining Company Limited (The Northern Miner, September 19, 1963). Kimberly-Clark held a 50% interest, Junior Frood Mines Limited 25%, Upper Canada Mines Limited 12.5% and Cadamet Mines Limited 12.5% in Figure 17. Intensely sheared Archean metavolcanic rocks and folded quartz-carbonate vein at Horace Cove (Stop C). - 15 - �ILSG Special Publication #1 - The Slate Islands this new company (Financial Post Survey of Mines, 1964, p.169). In 1963, this company completed 20 diamond drill holes, totalling an estimated 1974 m on the St. Mary’s Bay zone (G.L. Puttock, personal communication, 1974). This work disclosed variable, but locally very high-grade, gold mineralization in quartz veins of short strike length and over narrow widths of 2 to 10 cm. Mineral exploration of the islands ceased with the termination of the efforts of KimberlyClark. In April 1973, finding the islands of no further use to them, Kimberly-Clark transferred its rights to the islands back to the Crown. Subsequently, in September 1973 the islands were removed from staking. The gold-bearing, quartz-carbonate veins of St. Mary’s Bay zone and Cosen’s showing display a strong southwesterly strike. Since the host rocks of the veins are folded into a northwest-trending sequence, these veins are approximately normal to stratigraphy as was observed in several places along the eastern shore of Patterson Island. Some evidence for shear folding of the quartz veins is indicated at the St. Mary’s Bay zone by the irregular “sawtooth” pattern of some of the veins. Nichols (1963) suggested that gold-bearing quartz veins on Patterson Island occurred in the nose of a fold and occupied shear and tension fractures. Based on samples and descriptions by G.E. Parsons (consulting geologist for Kimberly-Clark Pulp and Paper Company Limited, personal communication, 1974), gold locally occurs in three ways. These are: (1) in association with pyrite within the quartzcarbonate veins; (2) as flakes and thin sheets along the flanks of the quartz-carbonate veins; and (3) as thin sheets or flakes along schistosity planes of the rocks enclosing the quartz-carbonate veins. Sampling by Sage (1991) of various quartz veins returned nil to insignificant gold values except for the St. Mary’s Bay zone, where assays of 0.5 ounce Au per ton over widths of 2 to 3 cm were obtained. The quartz veins vary from tabular, lensoid, clearly defined veins to irregular anastomosing structures with no clearly discernible attitude. An average of 98 clearly defined veins gave an average width of 10.7 cm and a length of 5.5 m (Sage, 1991). Reddish-brown, coarse-grained carbonate is an ubiquitous, accessory to dominant mineral and pyrite is common to abundant. Rarely, black needle-like crystals of tourmaline were noted. A contoured stereonet plot of 167 quartz vein attitudes indicated a rather broad spread of attitudes with one and possibly two maxima. The strongest maximum defines a 070°-trending vein set dipping approximately 60° southeast. The second maximum defines a 035°trending, vertically dipping vein set. The intersection of these two trends would define a lineation striking 210°, plunging about 14° southwest. Brummer (1962) delineated an area of sericite schist and shearing extending for 300 m north-south and 570 m east-west at the northern end of Horace Cove. Pyroclastic rocks, diorites and porphyritic metavolcanic rocks were also noted. Three steeply dipping to vertical vein sets were identified: a major set at 035°; and minor sets at 063° and 050° to 060°. The 40 veins that had been discovered at that point ranged in strike length between 16 and 60 m and in width from 0.5 to 20 cm, averaging 5 cm. Brummer (1962) noted that approximately 80% of the gold occurred as this films along the outer vein margins. The altered wall rock was not sampled for assay; Nichols (1963) noted an absence of gold in wallrock. Stop D – Western shore of Patterson Island UTM coordinates – ca. 495965E 5387400N This location on the western shore of Patterson Island (Fig. 2) is a microcosm of Slate Islands geology, in that a variety of rock types and geologic features are exposed. The southernmost outcrops (UTM coordinates - 495965E 5387324N) are sheared Archean metavolcanic rocks which are unconformably overlain by hematite-jasper banded iron formation and ferruginous shales of the Paleoproterozoic Gunflint Formation (Fig. 18). Figure 18. Unconformable contact between Keweenawan basalts (left) and Animikie Gunflint Formation (right). - 16 - �ILSG Special Publication #1 - The Slate Islands The base of the Gunflint outcrop consists, very approximately, of 20m of interlayered grainstone and slaty iron formation. Diabase and red breccia dikes have obscured the lower contact of the Gunflint Formation. The grainstone layers are 1 to 30 cm thick and are composed of intraclasts of chert and jasper. Clast sizes range up to that of small pebbles but are dominated by medium-to coarse-grained sand. The rock has a pervasive quartz cement. The slaty iron formation layers form bundles less than one to several centimetrers thick. Individual layers are millimeter to sub-millimeter in thickness. They are composed of magnetite mixed with what is probably siliciclastic clay and silt. This unit is overlain by approximately 7 m of just the slaty iron formation. This denotes a rapid change from shallower, storm-dominated, bottom to deeper, more quiescent conditions, a trend similar to the Gogebic iron formation successions described from Wisconsin (Pufahl and Fralick, 2004). The exposed upper part of the Gunflint section dips approximately 20° to 30° to the north. The upper contact of the Gunflint with overlying Mesoproterozoic (Keweenawan) flood basalts is also obscured by diabase and breccia bodies. Small, delicate shatter cones (< 5 cm long) are developed in the argillaceous portions of the sedimentary rocks (UTM coordinates - 495964E 5387369N; Fig. 19). Figure 20. Amygdaloidal Keweenawan basalt. Stop D on Patterson Island. approximately 1m thick interflow sandstone unit can be accessed by wading across the small bay. Mediumgrained, interflow red sandstones form successions up to a couple of metres thick. Bed thicknesses vary from a few centimeters to approximately 1 m. The sandstones are massive; sedimentary structures, aside from upper flow regime parallel laminations, are not A series of north-striking basalt flows ranging from 1 to 2 m thick outcrop along the shoreline. Sage (1991) has noted 22 separate flows in this section. The basalts are vesicular and amygdaloidal and dip approximately 20° to 60° to the west. Pipe amygdules occur near flow bases; coalescing amygdules may form flow-parallel lenses and bands (Fig. 20). Ropy flow tops, characteristic of pahoehoe lava, are locally preserved (Fig. 21). An Figure 21. Pahoehoe texture developed on basalt flow tops at Stop D on Patterson Island. Figure 19. Shatter cones in the Gunflint Formation at Stop D on Patterson Island. well preserved. This may be the result of fluid escape, especially during heating by overlying basalt flows. Trough-like structures in the top of one bed overlain by basalt flows may represent gouge marks where blocks of solidified lava caught up in the overriding basalt flow has been dragged through the unlithified sand in a manner analogous to glacial striae (Figs. 22 & 23). The channels are oriented in an east-west direction, perpendicular to the strike of the flows. The interflow sandstone is thicker than interflow sedimentary rocks in Osler basalts on Wilson Island (Hollings and Fralick, 2005). In addition to the thick interflow unit, thin layers of baked interflow mudstone can be seen within and - 17 - �ILSG Special Publication #1 - The Slate Islands Figure 24. Shattercones in Keweenawan diabase, Stop D Patterson Island. Figure 22. Keel marks left in the upper surface of an interflow sandstone unit at Stop D, Patterson Island. Flow direction is parallel to black arrows. between the basalt flows. Shatter cones are particularly well-developed in the basalt flows and in the boulders and blocks that litter the beach (Fig. 24). In places the shatter cones exceed 20 cm in length. Stop E – McGreevy Harbour UTM coordinates – 500825E 5390752 N Dressler et al. (1999) have interpreted the structures preserved in the Archean felsic volcanic rocks at this site as large shatter cones, the largest being ~10m high (Fig. 25). To the west of the larger shatter cone a partial cone may be preserved that would imply a total length on the order of 20m. It is difficult to disembark at this site and equally hard to clamber on the steep talus cascading into the water. The scale of these large features is better appreciated from 15 to 20 m offshore. Figure 23. Close-up of keel marks left in the upper surface of an interflow sandstone unit at Stop D, Patterson Island. - 18 - �ILSG Special Publication #1 - The Slate Islands Figure 25. Large shatter cone visible in the cliff side in McGreevy Harbour (Stop E). - 19 - �ILSG Special Publication #1 - The Slate Islands References Alvarez, L.W., Alvarez< W., Asaro, F., andMichel, H.V. 1980. Extraterrestrial cause for the CretaceousTertiary extinction. Science, 269, 1095-1108. Brummer, J.J. 1962. Gold-bearing veins, St. Mary’s Bay, Patterson Island, Lake Superior, Ontario; unpublished report, Resident Geologist’s Files, Thunder Bay South District, 5p. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R. and Spencer, C., 1989. The North American Midcontinent Rift Beneath Lake Superior from GLIMPCE Seismic Reflection Profiling; Tectonics, v. 8, n. 2, p. 305332. Grieve, R. A. F., Rupert, J., Smith, J., and Therriault, A., 1995, The record of terrestrial impact cratering: GSA Today, v. 5, p. 189, 194–196. Halls, H.C., 1974. A Keweenawan sequence from the Slate Islands, Northern Lake Superior. Institute on Lake Superior Geology, 20th Annual Meeting, Sault Ste. Marie, Ontario, p.14. Halls, H.C., 1975. Shock-induced remanent magnetization in Late Precambrian rocks from Lake Superior. Nature, 255, 692-695. Halls, H.C., 1976. The Slate Islands: The Central Uplift of a meteorite impact crater. Institute on Lake Superior Geology, 22nd Annual Meeting, St Paul, Minnesota, p.27 Card, K.D., Sanford, B., and Davidson, A., 1994. Bedrock geology, Lake Superior, Ontario, USA. Natural Resources Canada, Map NL-16/17-G. Halls, H.C., 1979. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Discussion. Geological Society of America Bulletin, 90, 1084-1086. Coleman, A.P., 1901. The Slate Islands in Iron Ranges of Northwestern Ontario, Ontario Bureau of Mines, v. 11, 552-555. Halls, H.C., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 666. Dietz, R. S., 1964, Sudbury structure as an astrobleme: Journal of Geology, v. 72, p. 412–434. Halls, H. C., and Grieve, R. A. F., 1976, The Slate Islands: A probable complex meteorite impact structure in Lake Superior: Canadian Journal of Earth Sciences, v. 13, p. 1301–1309. Dressler, B.O. and Sharpton, V.L., 1997. Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada. Tectonophysics, 275, 285-311. Dressler, B.O., Sharpton, V.L., Schuraytz, B. and Scott, J., 1994. Bunte breccia, impact melt and suevite at teh SLate Islands impact structure, Ontario. Ontario Geological Survey, Miscellaneous Paper 163, 59-61. Dressler, B.O., Sharpton, V.L., Schnieders, B. and Scott, J., 1995. New observations at the Slate Islands impact structure, Lake Superior. Ontario Geological Survey, Miscellaneous Publication 164: 53-61. Halls, H. C., and Stesky, R.M., 1978. Paleomagnetic and shatter cone measurements from the Slate Islands, Northern Lake Superior. Canadian Geophysical Union, 5th Annual Meeting, London, Ontario, p.34. Hinze, W.J., O’Hara, W.N., Trow, J.W. and Secor, G.B., 1966. Aeromagnetic studies of eastern Lake Superior, in Steinhart, J.S., ed., The earth beneath the continents. American Geophysical Union, Geophysical Monograph, 10, 95-110. Dressler, B.O., Sharpton, V.L. and Schuraytz, B.C., 1998. Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada. Contributions to Mineralogy and Petrology, 130, 275-287. Hollings, P., and Fralick, P., 2005. A stratigraphic transect across the northern flank of the Midcontinent Rift near Rossport. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 57-70. Dressler, B.O., Sharpton, V.L. and Copeland, P., 1999. Slate Islands, Lake Superior, Canada: A mid-size, complex impact structure. Geological Society of America, Special Paper 339, 109-124. Hollings, P., Fralick, P. and Cousens, B., 2006. Geochemistry and sedimentology of the Osler Formation: Evaluating rifting in the Proterozoic. Canadian Journal of Earth Sciences, in press. French, B.M., 1998. Traces of catastrophe. Lunar and Planetary Institute, Contrib. 954, 120 pp. Mariano, J., and Hinze, W. J., 1994, Structural interpretation of the Midcontinental rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 31, p. 619– 628. Grieve, R. A. F., and Robertson, P. B., 1976, Variations in shock deformation at the Slate Islands impact structure, Lake Superior, Canada: Contributions to Mineralogy and Petrology, v. 58, p. 37–49. Grieve, R. A. F., and Robertson, P. B., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 666-667. Nichols, L.C. 1963. Gold occurrences on the Slate Islands of Lake Superior; unpublished B.Sc. thesis, Queen’s University, Kingston, Ontario, 61p. Norris, A.W. and Sanford, B.V., 1969. Paleozoic and - 20 - �ILSG Special Publication #1 - The Slate Islands Mesozoic geology of the Hudson Bay lowlands. In Hood, P.J., ed., Earth science symposium on the Hudson Bay. Geological Survey of Canada, Paper 68-53, 169-205. Parsons, A.L., 1918. Slate Islands, Lake Superior. Ontario Bureau of Mines, Annual Report, 27, 155-167. Polat, A., Kerrich, R., and Wyman, D., 1998. The late Archean Schreiber-Hemlo and White River-Dayohessarah greenstone belts, Superior Provnce: Collages of oceanic plateaus, oceanic arcs, and subductionaccretion complexes. Tectonophysics, 289, 295-326. Powell, J.L.,. 1998. Night comes to the Cretaceous. Harcourt Brace and Company, New York, 250 pp. Prest, V.K. 1990. Laurentide ice-flow patterns: A historical review, and implications of the dispersal of Belcher Island erratics; Géographie physique et Quaternaire, v.44, p.113-136. Sharpton, V., Dressler, B., Herrick, R., Schnieders, B., Scott, J., 1996. New constraints on the Slate Islands impact structure, Ontrio Canada. Geology, 24, 851-854. Sharpton, V.L., Copeland, P., Dressler, B.O. and Spell, T.L., 1997. New age constraints on the Slate Islands impact structure, Lake Superior, Canada. Lunar and Planetary Science Conference XXVIII, 1287-1288. Stesky, R. M., and Halls, H. C., 1983, Structural analysis of shatter cones from the Slate Islands, northern Lake Superior: Canadian Journal of Earth Sciences, v. 20, p. 1–18. Sun S. S., and McDonough., W.F., 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. In Magmatism in the ocean basins. Edited by A.D. Saunders, and M.J. Norry. Geological Society Special Publication. 42: 313-345. Prest, V.K., Donaldson, J.A. and Mooers, H.D. 2000. The Omar story: The role of Omars in assessing glacial history of west-central North America; Géographie physique et Quaternaire, v.54, no.3, p.257-270. Pufahl, P., and Fralick, P., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior Region, USA. Sedimentology, 51, 791-808. Roberston, P.B. and Grieve, R.A.E., 1976. Comparison of the distribution of shock metamorphism at Charlevoix, P.Q., and Slate Islands, Ontario. Geological Association of Canada – Mineralogical Association of Canada, Annual Meeting, Program with Abstracts, v. 1, p.42. Roberston, P.B. and Grieve, R.A.E., 1979. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Discussion. Geological Society of America Bulletin, 90, 10871088. Sage, R.P., 1975. Slate Islands. Ontario Division of Mines, Ministry of Natural Resources, Map P. 997, scale 1” to 0.25 miles. Sage, R.P., 1978. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior. Geological Society of America Bulletin, 89, 1529-1540. Sage, R. P., 1991, Precambrian geology, Slate Islands: Ontario Geological Survey Report 264, 111 p. Sage, R.P., 1999. The Slate Islands: A uniquely sited cryptoexplosion structure. In Summary of Field work and other activities 1999, Ontario Geological Survey, Open File Report 6000, 28-1 to 28-13. Sharpton, V. and Dressler, B., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 668-669. - 21 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Special Publication #1: Field Trip Guidebook for the Slate Islands, Ontario Description An account of the resource Institute on Lake Superior Geology: Special Publication #1: Field Trip Guidebook for the Slate Islands, Ontario Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2006 Contributor An entity responsible for making contributions to the resource Pete Hollings Mark Smyk Bill Addison Phil Fralick Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/10e97ee6c7c1c33da71620302b21e77e.pdf 3a06ee87c9410827142e7614a85509e2 PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 54TH ANNUAL MEETING MAY 6-10, 2008 MARQUETTE, MICHIGAN HOSTED BY: Michigan Technological University THEODORE J. BORNHORST AND JOHN S. KLASNER Co-Chairs Proceedings Volume 54 Part 1 – Program and Abstracts EDITED BY THEODORE J. BORNHORST AND GEORGE W. ROBINSON A. E. SEAMAN MINERAL MUSEUM, MICHIGAN TECHNOLOGICAL UNIVERSITY Cover Photos: Lake Superior region minerals in the collection of the A.E. Seaman Mineral Museum Top Left Clockwise: amethyst – Thunder Bay, Ontario; gold – Balmertown, Ontario; silver – Silver Islet, Ontario; copper – Phoenix, MI; goethite – Ishpeming, MI; gypsum – Crystal Falls, MI; chalcocite – Ladysmith, WI; rhodochrosite – Montreal, WI (photographs by George Robinson and John Jaszczak) �54TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 54 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE—SEISMITES FROM THE SUDBURY IMPACT? TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT TRIPS 4 AND 8: GEOLOGY OF THE EAGLE PROJECT. TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Reference to material in Part 1 should follow the example below: Cannon, W.F. and Schulz, K.J., 2008, Unusual features along the Archean/Paleoproterozoic unconformity at Silver Lake, Michigan—seismites from the Sudbury impact [abstract]: Institute on Lake Superior Geology Proceedings, 54th Annual Meeting, Marquette, MI, v. 54, part 1, p. 10-11. Published by the 54th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 54 PART 1— PROGRAM AND ABSTRACTS Institutes on Lake Superior Geology, 1955-2008........................................................................... iv Sam Goldich and the Goldich Medal................................................................................. vi Goldich Medal - Past Medalists and Committee ............................................................. viii Citation for 2008 Goldich Medal Recipient....................................................................... ix ILSG Student Research Fund............................................................................................. xi Student Paper Awards....................................................................................................... xii Eisenbrey Student Travel Awards ................................................................................... xiii Report of the Chair of the 53rd Annual Meeting............................................................. xiv 2008 Board of Directors..................................................................................................................... xvii 2008 Session Chairs........................................................................................................ xvii 2008 Student Paper Awards Committee ......................................................................... xvii 2008 Local Committees .................................................................................................. xvii Special Recognition ....................................................................................................... xviii 2008 Banquet Speaker ..................................................................................................... xix Program............................................................................................................................ xxi Abstracts ........................................................................................................................ xxix iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray iv �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 T.J. Bornhorst and J.S. Klasner Marquette, Michigan v �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the vi �second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1994 Cedric Iverson 1980 not awarded 1995 Gene La Berge 1981 Carl E. Dutton, Jr. 1996 David L. Southwick 1982 Ralph W. Marsden 1997 1983 Burton Boyum 1998 Zell Peterman 1984 Richard W. Ojakangas 1999 Tsu-Ming Han 1985 Paul K. Sims 2000 John C. Green 1986 G.B. Morey 2001 John S. Klasner 1987 Henry H. Halls 2002 Ernest K. Lehmann 1988 Walter S. White 2003 Klaus J. Schulz 1989 Jorma Kalliokoski 2004 Paul Weiblen 1990 Kenneth C. Card 2005 Mark Smyk 1991 William Hinze 2006 Michael G. Mudrey 1992 William F. Cannon 1993 Donald W. Davis 2007 Joseph Mancuso Ronald P. Sage 2008 GOLDICH MEDAL RECIPIENT Theodore J. Bornhorst Michigan Technological University Houghton, Michigan GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Doug Duskin (2005-2008) Richard Ojakangas (2006-2009) Terry Boerboom (2007-2010) Industry representative Academic representative Government representative viii �CITATION FOR GOLDICH MEDAL RECIPIENT Theodore J. Bornhorst, 2008 Goldich Medal Recipient It is my great pleasure to introduce Professor Ted Bornhorst as the recipient of the 2008 Goldich Medal. Ted is being honored with this award for his many contributions to the understanding of Lake Superior geology through his research and teaching and, most significantly, for his unparalleled service to the Institute over the past 25 years. Ted began (and will likely finish) his geological career at Michigan Technological University. He received his B.S. from Michigan Tech in 1974 and actually presented his first ILSG talk at that year’s meeting in Sault St. Marie on the topic of his senior research project the geology and geochemistry of the Fish Cove rhyolite in Keweenaw County. Ted headed to the University of New Mexico to complete his M.S. (1976) and Ph.D. (1980) degrees in economic geology only to return to Michigan Tech as an assistant professor in 1981. Ted received a full professor appointment in 1993 and was appointed director of the world class A.E. Seaman Mineral Museum at Michigan Tech in 2003. During his tenure at Michigan Tech, Ted has taught classes, advised students, and conducted research on a variety of topics and in a range of locations (e.g., gold in Finland, rhyolites in New Mexico, quaternary volcanic ashes off the coast of Guatemala). But by far, the main emphasis of his academic pursuits has been the Precambrian geology and mineral deposits of Upper Michigan. A true indication of his passion for Lake Superior geology is the fact that Ted has directed a five-week intensive Precambrian field course in the western UP almost every summer since arriving at Michigan Tech in 1981. This course has been taken by over 500 students. I would be willing to bet that the approximately 7000 students who have taken the introductory geology course from Professor Bornhorst since 1989 have probably received a hefty dose of Lake Superior geology, as well. Ted has advised (to completion) 4 Ph.D. dissertations, 24 M.S. theses, and 29 senior research projects, almost all dealing with Lake Superior geology. In addition to being the advisor and mentor for scores of Michigan Tech geology students, many of whom still do research and mineral exploration work in the Lake Superior region, Ted also serves the general public and the geological community by being a member of the State of Michigan's Mineral Well Advisory Committee, representing the western Upper Peninsula. And because of his knowledge of UP geology and mineral deposits, Ted has been appointed to many state committees and boards looking into rules and regulations for non-ferrous mineral exploration and mining in the state. Ted’s research interests in the Lake Superior region have focused on three general areas: the Cu deposits of the Keweenwaw Peninsula, the geology and mineral potential of the Ishpeming greenstone belt, and the petrology of granitoids in the Archean Northern Complex. Ted has been first or second author on over 50 journal publications. Of these, 19 were on topics dealing with Lake Superior geology. He has authored 16 field trip guides and 15 maps and technical reports on the geology and ore deposits of Upper Michigan. ix �Beyond this impressive resume of academic contributions to the geology of the Lake Superior region, perhaps the main reason that Ted is so deserving of the Goldich award is his unwavering commitment and very real service to the ILSG. Ted’s impressive list of contributions to the institute include: • Presenting 10 abstracts and coauthoring many more of his student’s presentations. • Organizing, writing guidebooks for, and leading 6 field trips; most impressive of these has been the Field Guide to the Geology of the Keweenaw Peninsula, by Bornhorst, Rose, and Paces. This now classic guidebook, which was originally published in 1983 and republished in 1994, has sold over 5,000 copies to geologists and non-geologists alike. • Serving as chairman or co-chairman of 4 annual meetings: 1983 (Houghton), 1994 (Houghton), 1999 (Marquette), and now 2008 (Marquette). • Serving as the Institute’s first webmaster beginning in 1995 and maintaining the ILSG webpage until 2004, when the position passed over to the Secretary-Treasurer. It is the exceptional contributions and service given by individuals like Ted Bornhorst that has sustained the ILSG over these past 54 years. As token of the Institute’s appreciation, it is altogether fitting and appropriate that Ted join the ranks of other prominent Lake Superior geologists who have been honored with this prestigious award. I know I speak for all the institute members when I say “thank you, Ted, for all you have given the Institute.” Jim Miller University of Minnesota Duluth x �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2006 and 2007 the ILSG Board of Governors awarded three $500 awards from the Student Research Fund. Cole Edwards (University of Wisconsin - Oshkosh) - Controls on the formation of the earliest marine phosphate deposits, Marquette Supergroup, Michigan Noah Planavsky (Rosenstiel School of Marine and Atmospheric Sciences Marine Geology and Geophysics, Miami) - Iron isotopes as oceanographic tracers in Animikie Basin Iron Formations Michael Taylor (University of Minnesota - Duluth) - Pleistocene glaciation as a mechanism for emplacement of high-salinity groundwater at anomalously shallow depths in the Lake Superior basin xi �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2007 the ILSG Student Paper Committee presented four awards from the ILSG Student Paper Fund. Two first place Best Student Paper awards: ($200 each) were given to: Sarah Nicholas (Macalester College) for her poster titled: Investigations of sulfide minerals leached in the presence of alkaline solids Noah Planavsky (Rosentiel School of Marine and Atmospheric Science) for his talk titled: Rare earth element patterns in Steep Rock Carbonates. Two Honorable Mention Best Student Paper awards ($100 each) were given to: Troy Boisjoli (St. Norbert College) Larissa Stevens (Lakehead University) xii �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2007 the ILSG awarded 15 travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Malcolm Alexander – Lakehead University Troy Boisjoli – St. Norbert College Dan Costello – St. Norbert College Clinton Forsha – Slippery Rock University Amanda Hogan – St. Thomas University Carissa Isaac – Lakehead University Travis Jacob – St. Thomas University Renata Jasinevicius – St. Norbert College Noah Planavsky – Rosentiel School of Marine and Atmospheric Science Patrick Quigley – University of Minnesota, Duluth Tommy Rodengen – St. Thomas University and Jody Rymaszewski – University of Wisconsin-Milwaukee Larissa Stevens – Lakehead University Michael Taylor – University of Minnesota, Duluth Stephanie Theriault – St. Thomas University xiii �REPORT OF THE CHAIRS OF THE 53RD ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY LUTSEN, MINNESOTA The U.S. Geological Survey with assistance from the Minnesota Geological Survey hosted the 53rd Annual Institute on Lake Superior Geology on May 8 – 13, 2007 at Lutsen Resort in Lutsen, Minnesota. The meeting consisted of two days of technical sessions with pre- and post-technical session field trips. Bill Cannon and Klaus Schulz helped with pre-meeting logistics. Gretchen Klasner provided valuable logistical assistance on-site at Lutsen Resort during the technical sessions. Connie Dicken and Bill Addison were media czars for the technical sessions, keeping all presentations on track with fewer glitches than normal. Pre-meeting registration was 114 students and professionals with an additional 69 on-site registrations, for a total of 183 registrants, an excellent turn-out. Proceedings Volume 53 was published in two parts. Part I – Program and Abstracts, edited by Laurel Woodruff, contains 49 published abstracts for 28 oral and 21 poster presentations; Part 2 – Field Trip Guidebook, edited by Jim Miller, contains descriptions of six field trips, three premeeting and three post-meeting. The 53rd ILSG marked the first time in its long history that an ILSG meeting was held in this part of Minnesota. Field trips visited areas new to the ILSG, which resulted in an excellent subscription for all the trips. On Tuesday, May 8, Jim Miller and Eric Jerde led an intrepid group of 8 participants on a two-day field trip into the Boundary Waters Canoe Area Wilderness to examine the mafic igneous rocks of the Poplar Lake Intrusion. Based out of the Rockwood Lodge off the Gunflint Trail, the trip required canoeing across Poplar Lake and several lakes farther into the BWCAW, only two weeks after ice-out! Thanks to warm weather, the couple of “soggy mishaps” served only as fodder for the Homer Award (way to go Steve!). On Wednesday David Cooper, from the National Park Service, Bill Cannon, and Brian Phillips guided 18 participants on a trip through the Grand Portage National Monument, looking at both the geological and cultural history of the area. Another trip on Wednesday into southern Ontario to look at Midcontinent Rift-related intrusions north of the international border involved 31 participants and was lead by Mark Smyk and Pete Hollings. A misstep on the outcrop on that trip resulted in an injury to former Goldich Medal recipient and long-time ILSG stalwart Dick Ojakangas, who unfortunately had to miss the rest of the meeting as he returned to Duluth for treatment. Throughout the meeting, the 60 people who had signed up for Mark Jirsa’s and Paul Weiblen’s Gunflint field trip were closely tracking the Ham Lake fire as it burned across the northern end of the Gunflint Trail. (The Poplar Lake Trip was far enough south of the fire to proceed, though the smoke plume was clearly visible). A final decision by the Forest Service closed the upper part of the Gunflint Trail on Friday and put the kibosh on any hope of running even a modified Gunflint Trail trip. Instead, Mark and Paul, with generous assistance from Bill Addison and Phil Fralick, quickly reconfigured the trip for a visit to the Thunder Bay area to see similar rocks as well as spectacular outcrops of Sudbury impact-related rocks. All of us involved appreciate the forbearance of seasoned and new ILSG attendees who were remarkably sanguine about the travails of the Gunflint Trail trip. Most of the people originally signed up for the Gunflint Trip xiv �stayed on for the substitute Thunder Bay trip, and we heard of no one being disappointed. One consolation from the cancellation is the Gunflint’s future potential as a field trip area for ILSG and enhanced exposure of rocks in burned areas, including newly discovered Sudbury ejecta layers found by Mark Jirsa while sneaking past the DO NOT CROSS signs on the Trail. Also on Saturday, Terry Boerboom, John Green and Jim Miller led 16 participants on a field trip along the spectacular shoreline geology of the North Shore from Little Marais to Grand Marias to highlight some recent detailed mapping. A very timely and popular two-day trip lead by Dean Peterson and Paul Albers (of Duluth Metals) set off for Ely on Saturday with a group of 20 participants. The trip looked at the geology and Cu-Ni-PGE mineralization of the Nickel Lake macrodike and South Kawishiwi Intrusion of the Duluth Complex. One hundred and fifty participants attended the banquet on Thursday night, even without the prospect of dessert. This year’s banquet speaker was Don Hunter, who is a Project Manager for the PolyMet Mining Corporation’s NorthMet copper-nickel project near Babbitt, MN. Mr. Hunter drew on his long international career in the mining industry for his talk on the ins-andouts of mineral exploration around the globe. The Powerpoint presentation of the informal ‘Homer’ award suffered from severe homerism itself due to an uncooperative projector, but eventually Steve Kissin was honored for his canoeing acumen. As always, a highlight of the banquet was the presentation of the 2007 Goldich medal to Joe Mancuso, retired professor from Bowling Green University in Bowling Green, OH. The medal was presented to Joe by Ron Seavoy, who ended his citation with “Joe, you deserve this,” a sentiment echoed by all who know Joe and his long-term contributions to the geology of the Lake Superior region. The student paper committee had its usual difficult job this year of selecting among 16 student oral and poster presentations. This year’s committee was Marcia Bjornerud from Lawrence University, Daniela Vallini of Woodside Energy, Ltd., and Graham Wilson from Magma Metals. In the end, two first place Best Student Paper awards ($200 each) were given to Sarah Nicholas (Macalester College) for her poster titled: Investigations of sulfide minerals leached in the presence of alkaline solids, and Noah Planavsky (Rosentiel School of Marine and Atmospheric Science) for his talk titled: Rare earth element patterns in Steep Rock Carbonates. In recognition of the excellent student presentations, two additional students were chosen for Honorable Mention ($100 each) – Troy Boisjoli (St. Norbert College) and Larissa Stevens (Lakehead University). Eisenbrey Student Travel Grants were given to 15 students: Patrick Quigley and Michael Taylor – University of Minnesota, Duluth; Malcolm Alexander, Carissa Isaac, and Larissa Stevens – Lakehead University; Troy Boisjoli, Dan Costello and Renata Jasinevicius, – St. Norbert College; Clinton Forsha – Slippery Rock University; Amanda Hogan, Travis Jacob, Tommy Rodengen, and Stephanie Theriault – St. Thomas University; Jody Rymaszewski – University of Wisconsin-Milwaukee; and Noah Planavsky – Rosentiel School of Marine and Atmospheric Science. All awards were presented at the conclusion of the technical sessions. Two of the presentations at the meeting were made by recipients of the 2006 ILSG Student Research Fund, Michael Taylor and Noah Planavsky. xv �The Institute’s Board of Directors met on May 8, 2007 and a brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 52nd ILSG from Ron Sage and minutes of last Board meeting from ILSG secretary, Pete Hollings. 2. Accepted the 2006-2007 ILSG Financial Summary from ILSG treasurer, Mark Jirsa. 3. Approved one co-chair from the 53rd meeting, Jim Miller, as on-going board member. 4. Nominated Terry Boerboom of the Minnesota Geological Survey to replace Tom Hart on the Goldich Committee, a position that Terry later graciously accepted. 5. Approved Marquette, Michigan as the location for the 2008 (54th annual) ILSG and co-chairs Ted Bornhorst and John Klasner. 6. Discussed the continued scanning of the ILSG publications and posting of those electronic publications on the ILSG website The 53rd ILSG meeting was a great success and we wish to thank all the people who contributed to that success. The staff of Lutsen Resort was professional and responsive to the needs of a large group. The setting on the shore of Lake Superior was stunning, and the weather was perfect (fireweather, apparently). The field trips this year had a large number of participants, and thanks are due to field trip leaders, van and bus drivers, and everyone else who stepped up when needed to drive, sort lunches, lay out core, or keep the crowds moving. As always, everyone who attended the 53rd ILSG was willing to help as necessary or adapt to any situation that developed. The meeting this year was well attended and we are heartened by the excellent student participation and attendance, a trend we hope continues. Jim and I were very pleased with the outcomes of the 53rd ILSG and hope that others agree that the meeting was a big success. Chairing a meeting requires a lot of organization and planning with a significant time commitment, and we thank our respective organizations for their recognition of the importance of the ILSG. We also thank the ILSG community and members who make the experiences of the co-chairs almost fun, especially once the meeting is over and we encourage others to take on the task. Laurel Woodruff and Jim Miller Co-Chairs, 53rd Institute on Lake Superior Geology xvi �2008 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Theodore J. Bornhorst and John S. Klasner, Co-Chairs 54th meeting (2011) Michigan Technological University, MI and retired Western Illinois University, IL Jim Miller (2010) University of Minnesota Duluth, MN Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Mark Smyk (2008) Ministry of Northern Development and Mines, Thunder Bay, ON Peter Hollings – Secretary (2008) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2009) Minnesota Geological Survey, St. Paul, MN 2008 SESSION CHAIRS Marcia Bjornerud, Lawrence University, Appleton, WI Milt Gere, Michigan DNR, Lansing, MI Elizabeth A. Gordon, St. Norbert College, De Pere, WI Joe Maki, Michigan DEQ, Office of Geological Survey, Gwinn, MI Penny Morton, University Minnesota, Duluth, MN Klaus Schulz, U.S. Geological Survey, Reston, VA Glenn Scott, Cliffs Mining Service Company George J. Hudak, University of Wisconsin, Oshkosh, WI 2008 STUDENT PAPER COMMITTEE Allan Blaske (Chair), STS, Lansing, MI Melanie Humphrey, Michigan DEQ, Office of Geological Survey, Gwinn, MI Laurel Woodruff, U.S. Geological Survey, Mounds View, MN 2008 COMMITTEES General Co-Chairs Theodore J. Bornhorst – Michigan Technological University, Houghton, MI John S. Klasner – Retired Western Illinois University, Macomb, Illinois Program and Abstracts Editors Theodore J. Bornhorst and George W. Robinson Michigan Technological University Field Trip Guidebook Editors John S. Klasner – Retired Western Illinois University Theodore J. Bornhorst – Michigan Technological University Local Registration Gretchen Klasner – Marquette, MI Registration Darlene M. Comfort – Michigan Technological University xvii �SPECIAL RECOGNITION The Co-Chairs of the 54th Annual Institute on Lake Superior Geology wish to give special recognition to several corporations for their generous financial support. Kennecott Minerals Company A Member of the Rio Tinto Group Eagle Project 1004 Harbor Hills Dr Marquette, MI 49855 Cliffs Mining Services Company Subsidiary of Cleveland-Cliffs Inc. 550 E. Division St. Ishpeming, MI 49849 Aquila Resources Inc. U.S. Office: Suite 310 - 314 W. Superior Street Duluth, MN 55802 STS 401 S. Washington Square, Suite 103 Lansing, MI 48933 1-800-959-4261 www.sts.aecom.com Other STS offices in the Great Lakes region: Marquette, Michigan; Green Bay, Wisconsin; Minneapolis, Minnesota xviii �2008 BANQUET SPEAKER Jon Cherry General Manager Kennecott Minerals - Eagle Project Marquette, MI The Kennecott Eagle Project xix �xx �PROGRAM xxi �TUESDAY MAY 6, 2008 8:00 a.m. FIELD TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT Tom Waggoner, retired, Cliffs Mining Services Company WEDNESDAY MAY 7, 2008 8:00 a.m. FIELD TRIP 1 CONTINUED: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT Tom Waggoner, retired, Cliffs Mining Services Company 8:00 a.m. FIELD TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE—SEISMITES FROM THE SUDBURY IMPACT? Bill Cannon, U.S. Geological Survey 8:00 a.m. FIELD TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT Tom Quigley and Bob Mahin, Aquila Resources Inc. 8:00 a.m. FIELD TRIP 4: GEOLOGY OF THE EAGLE PROJECT Andrew Ware and Jon Cherry , Kennecott Minerals Inc. Xin Ding, Indiana University 6:00 p.m. Return of Trips 1, 2 3, and 4 4:00 p.m. - 10:00 p.m. Registration at Ramada Inn 7:00 p.m. - 10:00 p.m. Ice Breaker Social and Poster Session THURSDAY MAY 8, 2008 Note: Asterisk * denotes a student eligible for Best Student Paper Award Presenter underlined 8:00 a.m. - 12:00 noon REGISTRATION 8:30 a.m. INTRODUCTORY REMARKS Theodore J. Bornhorst and John S. Klasner, Co-Chairs, 2008 ILSG xxii �TECHNICAL SESSION I Session Chairs: Milt Gere, Michigan DNR, Lansing, MI Penny Morton, University Minnesota, Duluth, MN 8:40 a.m. Peter Hollings and Mark Smyk Whatever Happened to the Logan Sills? Ongoing Research into the Geochemistry of Midcontinent Rift-related Mafic Intrusive Rocks South of Thunder Bay 9:00 a.m. Curtis D. Williams*, Edward M. Ripley, and Chusi Li The Effect of Magmatic Volatile Phase Separation Linked to Intrusion of the Duluth Complex: Solution to Anomalous Os Isotopic Compositions of the Virginia Formation? 9:20 a.m. Duncan J. Bain* The Shakespeare Cu-Ni-PGE Deposit: Evidence for a Two-Stage Emplacement Mechanism 9:40 a.m. Xin Ding*, Edward M. Ripely and Li C Geochemical and Stable Isotope Studies of Hydrothermal Alteration Associated with the Eagle Deposit, Northern Michigan 10:00 a.m. COFFEE BREAK AND POSTER SESSION 10:20 a.m. Natalie J. Pietrzak*, Norm Duke, Glenn Scott and Helen Lukey A Study of the Paragenetic Stages of Mineral Growth in Complex Iron Ores at the Tilden Mine and Development of a Mine Scale Model for Application to Ore Treatment Methods 10:40 a.m. Muatala H. Muvi-Tjikalepo, Theodore J. Bornhorst, George W. Robinson and W.C. Williams Multi-element Geochemical Signature of Copper Mineralization at the White Pine Mine, Midcontinent Rift System, Western Upper Peninsula, Michigan 11:00 a.m. Natalie King* Using Mineralization to Evaluate Small-Scale Controls on Shale Permeability in the Nonesuch Formation 11:20 a.m. Mary Louise Hill and Andrew Cheatle Iron-formation-hosted gold in the Superior Province of Northwestern Ontario 11:40 a.m. Lunch Break – 2008 ILSG Board Meeting (by invitation) xxiii �TECHNICAL SESSION II Session Chairs: Klaus Schulz, U.S. Geological Survey Glenn Scott, Cliffs Mining Service Company 1:00 p.m. T.W. Buchholz, A.U. Falster and Wm. B. Simmons Observations on Lanthanide Fractionation in the Wausau Complex, Marathon County, Wisconsin 1:20 p.m. G.J. Baldwin*, P.C. Thurston, B.S. Kamber, and M.G. Houle The Deloro-Tisdale SIZ of the Abitibi Greenstone Belt: An Example from McArthur Township, Ontario 1:40 p.m. Patrick Moran*, Philip Fralick, Mary Louise Hill, and Peter Hollings Geochemistry of Sedimentary Rocks associated with the Musselwhite Gold Deposit, Northwestern Ontario 2:00 p.m. Carrissa Isaac* and Pete Hollings Stable Isotope Geochemistry of the Musselwhite Au Mine, N. Ontario: Implications for Mineralization 2:20 p.m. COFFEE BREAK AND POSTER SESSION 2:20 p.m. Norman Duke Evidence for Reactivating Archean Structural Breaks During Paleoproterozoic Rift Sedimentation and Subsequent Accretionary History “Once a Fault Always a Fault” 3:00 p.m. Susan M. Karberg* Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota; Implications for Archean (2.7 Ga) Tectonics 3:20 p.m Emerald J. Erickson* and Vicki Hansen Structural and Kinematic Analysis of the Archean Shagawa Lake Shear Zone, Superior Province, Northeastern Minnesota 3:40 p.m. Sally Goodman* Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province: Insight on Granite-Greenstone Terrain Tectonics and Archean Crustal Evolution 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 55th Annual Meeting Location • 2008 Goldich Award Presentation to Ted Bornhorst • 2008 Banquet Address by Jon Cherry, Kennecott Minerals Company All registered participants are welcome to the banquet address xxiv �FRIDAY MAY 9, 2008 8:50 a.m. INTRODUCTORY REMARKS Theodore J. Bornhorst and John S. Klasner, Co-Chairs, 2008 ILSG TECHNICAL SESSION III Session Chairs: Elizabeth A. Gordon, St. Norbert College, De Pere, WI George J. Hudak, University of Wisconsin, Oshkosh, WI 9:00 a.m. M.G. Mudrey Jr., Peter Hollings, Lura E. Joseph, Mark Jirsa and Jo Kalliokoski On-line Electronic Access to Institute on Lake Superior Geology Publications 9:20 a.m. A.E. Hanson and B.A. Frey MN DNR Drill Core Evaluation Project—The Application of an XRF to Elucidate Gold Mineralization in the Vermilion Greenstone 9:40 a.m. E.M. Fein*, E.C. Ferré and D.K. Holm Flow Fabric Determination of Two Mesoproterozoic Midcontinent Rift Dike Swarms, Northeastern Minnesota 10:00 a.m. Val W. Chandler and Richard S. Lively Upgrade of Aeromagnetic Data at the Minnesota Geological Survey 10:20 a.m. COFFEE BREAK AND LAST POSTER SESSION 10:40 a.m. Lauri J. Pesonen Keweenawan Apparent Polar Wander Path: New Observations , New Ideas 11:00 a.m. James D. Miller, Dean M. Peterson, and George J. Hudak The Inaugural Season of the Precambrian Field Camp at the University of Minnesota Duluth 11:20 a.m. Rodney Johnson, Robert Seasor, and Tom Suszek An Archean-aged PGE-bearing Intrusion, Baraga County, Michigan 11:40 a.m. R.M. Easton and L.M. Heaman Detrital Zircon Geochronology of Hurnoian Supergroup Sandstones located within the Vernon Structure, North of Espanola, Ontario 12:00 p.m. Presentation of Student Awards Student Travel Awards Best Student Paper Awards 12:00 p.m. LUNCH BREAK xxv �TECHNICAL SESSION IV Session Chairs: Marcia Bjornerud, Lawrence University, Appleton, WI Joe Maki, Michigan DEQ, Office of Geological Survey, Gwinn, MI 1:00 p.m. L.G. Medaris Jr. and R.H. Dott Jr. The Seeley Slate and Baraboo Interval Sedimentation 1:20 p.m. Jared D. Lubben Stratigies for Drilling Unconsolidated Material and Historic Underground Mine Workings: Examples from Hibbing Taconite Company’s 2007 Diamond Drilling Campaign 1:40 p.m. W.F. Cannon and K.J. Schulz Unusual Features Along the Archean/Paleoproterozoic Unconformity at Silver Lake, Michigan—Seismites from the Sudbury Impact 2:00 p.m. K.J. Schulz and W.F. Cannon Geochemistry of the Sudbury Impact Layer, Northern Michigan: Implications for the Nature of the Source Materials 2:40 p.m Mark A. Jirsa, Paul W. Weiblen, Tatiana Vislova and Peter L. McSwiggen Sudbury Impactite Layer Near Gunflint Lake, NE Minnesota 3:00 p.m. FIELD TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY Bill Cannon, U.S. Geological Survey 6:00 p.m. Return of Trip 5 SATURDAY MAY 10, 2008 8:00 a.m. FIELD TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT Glenn Scott, Helene Lukey, Al Strandlie, and CCI/CCMO staff Cleveland Cliffs Inc. 8:00 a.m. FIELD TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Dean Rossell, Kennecott Minerals Inc. 8:00 a.m. FIELD TRIP 8: GEOLOGY OF THE EAGLE PROJECT Andrew Ware and Jon Cherry , Kennecott Minerals Inc. Xin Ding, Indiana University 6:00 p.m. Return of Trips 6, 7, & 8 54TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY ENDS xxvi �POSTER PRESENTATIONS Terra N. Anderson* and Dyanna M. Czeck Quartz Fabrics and Quantified Strains During Transpressional Deformation in the Seine Metaconglomerates Terrence J. Boerboom, John C. Green and Paul Albers Bedrock Geologic Map of the Lutsen Quadrangle, North Shore of Lake Superior, Minnesota Elizabeth Drommerhausen* and Steven Losh Properties of Fluid Involved in Formation of Natural Ore in the Mesabi Iron Range, Minnesota R.M. Easton, M.G. Houlé, D. Rowell and N.F. Trowell Toward a Common Map Legend for Ontario Shelby J. Frost*, Natalie A. Juda, and Jim Miller Student Capstone Map from the UMD Precambrian Research Center’s Field Camp: Bedrock Geology Map of Homer Lake and Adjacent Areas, Cook County, Minnesota Lynn Galston*, Karen G. Havholm and Stephen T. Hasiotis Reinterpretation of the Trace Fossil-Bearing Devils Island Sandstone, Keweenawan Rift, Northern Wisconsin Mark A. Jirsa, Edward Starns, Daniel E. Costello, Benedek Gal, Steven A. Hoaglund and Amanda J. Putz “Capstone” Geologic Mapping Near Gabimichigami Lake, Boundary Waters Canoe Area Wilderness, By Students of the Precambrian Research Center’s 2007 Field Camp J.N. Koester*, J.W. Goodge, and V.L. Hansen Structural and Metamorphic History of the Burntside Lake Shear Zone with Possible Implications for Archean Granite-Greenstone Formation Helene M. Lukey Mineral Zonation and Stratigraphy of the Tilden Hematite Deposit S. Moosavi*, T.K. Johnson, C. Wendland, A. Anderson, and G.J. Hudak Bedrock Geology of the Footwall to the Soudan Iron Formation South of Twin Lakes, St. Louis County, Northeastern Minnesota Northern Michigan Geologic Repository Association xxvii �Richard Patelke PolyMet Mining: NorthMet Cu-Ni-Co-PGE Project, Hoyt Lakes, Minnesota Dean Peterson Geological Map of the Northern South Kawishiwi Intrusion and Surrounding Areas, Duluth Complex: St. Louis and Lake Counties, Northeastern Minnesota A.K. Sartorelli*, A. Anderson and G.J. Hudak Pillow Morphological Studies Southwest of Fivemile Lake, Vermillion District, NE Minnesota K.J. Schulz, S.W. Nicholson and W.R. Van Schmus Penokean Massive Sulfide Deposits: Age, Geochemistry, and Paleotectonic Setting Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., Peterson, D. Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion: Lake County, Northeastern Minnesota xxviii �ABSTRACTS xxix �xxx �Quartz fabrics and quantified strains during transpressional deformation in the Seine Metaconglomerates Anderson, Terra N., and Czeck, Dyanna M. Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201; tna@uwm.edu. INTRODUCTION Previous studies have documented quartz fabrics in the field (e.g. Bestmann et al., 2004; Lisle, 1985; Stipp et al., 2002) and the lab (e.g. Heilbronner and Tullis, 2002; Hirth and Tullis, 1992; Mainprice and Paterson, 1984). However, these previous studies have not been conducted in areas with deformation under natural conditions where strain can be quantified and deformation mechanisms have been determined. In our study, we link quartz shape and crystallographic fabrics within a deformed metaconglomerate across a strain gradient. Fissler (2006) conducted strain analysis on the Seine Metaconglomerates from the Rainy Lake region in northwestern Ontario. She determined strain independently for various lithological groupings of clasts and determined various degrees of strain types and magnitudes throughout the region. Using her strain results as a framework for our fabric analysis, we use petrographic and Electron Backscattered Diffraction (EBSD) techniques to document the quartz grain fabrics throughout the region. METHODOLOGY Fissler (2006) classified twenty-six outcrops of the Seine Metaconglomerates as low, medium, or high strain. Oriented quartzite clasts and quartz veins were collected from several locations with different strain magnitudes. The quartzite clasts experienced the complete deformation history of the conglomerate whereas the quartz veins, having been intruded during the deformation, only exhibit evidence of later deformation stages. The quartzite clasts and quartz veins have a simple mineralogy that is readily compared to numerous results from deformation experiments. Thin sections were made from quartzite and quartz vein samples with the x-direction parallel to lineation and the y-direction perpendicular to foliation. Using a SEM-EBSD system and CHANNEL5 software produced by HKL Technology, an automatic map of the thin section was used to determine the crystallographic orientations of quartz grains. The map was constructed to maximize the area analyzed on the thin section with one measurement taken from each grain. The step size was determined by the average grain size and ranged from 150 to 200 microns. The data were entered into PFch5, a pole figure-plotting program developed by Mainprice (2005). Microstructures in the Seine have been studied in detail by Czeck (2001) and Fissler (2006). Both found evidence for very similar deformation mechanisms at all strain magnitudes in a range of clast types. Rocks were primarily deformed by diffusive mass transfer processes with some dislocation and micro-fracturing processes in certain phases. Using a petrographic microscope, we conducted additional microstructural analyses to infer deformation mechanisms within the quartz samples because the previous studies did not focus on quartz. PRELIMINARY RESULTS Petrographic analyses indicate the following microstructures. In low-moderate strained quartzite clasts, grain size is bimodal with larger grains 20 to 40 μm and smaller grains around 5 μm in diameter. Grain boundaries are irregular and show slight preferred orientation parallel to lineation, forming a shape preferred orientation (SPO). Within the grains, undulose extinction is common and some subgrains exist. Some recrystallized grains, primarily as “necklaces” around the larger quartz grains, can be observed at the edges of larger quartz grains. We interpret that the quartz underwent deformation primarily by dislocation creep with subgrain 1 �formation and recrystallization recovery mechanisms. Late stage fractures occurred which cut the main fabric and allowed fluid movement resulting in veins filled with calcite and some minor amounts of mica. In highly strained quartzite clasts, grain size is uniform ranging from 5 to 10 μm. The round, equant grains have regular smooth boundaries that often intersect at triple junctions. Internally, the grains often have undulose extinction throughout, and some subgrains are observed. Calcite fills in fractures that are oblique to lineation. The grain size near these zones is smaller, less than 5 μm in diameter. We interpret that the quartz grains are deforming primarily by dislocation creep, and that the reduced grain size is due to extensive recrystallization. The number and size of late stage fractures increased from the low strained examples. The fractures cut the main fabric and filled with calcite and more mica relative to the low strain clasts. In low-moderate strained quartz veins, grain sizes vary greatly from 20 to 500 μm. Grain boundaries are irregular and show a slight SPO subparallel to lineation. Some ~5 μm recrystallized grains are observed at the edges of the larger quartz grains. These are seen primarily as “necklaces” around the larger quartz grain edges. In moderate-highly strained quartz veins, grain size is fairly uniform ranging from 20-40 μm in diameter. Some larger grains that are 500 μm in diameter exist, often with “necklace” recrystallization of quartz and calcite around them. Boundaries are irregular and larger grains have undulose extinction. Calcite fills in fractures that are oblique to lineation. Preliminary results of the EBSD analyses show increase cluster of crystallographic preferred orientation (CPO) in the c-axis of quartz within quartzite clasts with increasing strain magnitudes. Similarly, in most cases there was an increase in CPO of quartz c-axes within quartz veins when strain magnitudes increased. These observations are consistent with deformation by dislocation creep with recovery primarily by recystallization. One interesting feature that stands out when comparing the microstructural and EBSD studies is that the SPO of quartz grains decreases with increasing strain, and the CPO of quartz grains increases with increasing strain. The increasing CPO without a corresponding increase in SPO is consistent with deformation by dislocation creep with recrystallization. We interpret the different microstructural observations between clasts of low to high strain to be consistent with dislocation creep that operated to a variety of strain magnitudes. REFERENCES Bestmann, M., Prior, D.J., Veltkamp, K.T.A., 2004. Development of single-crystal σ-shape quartz porphyroclasts by dissolution-precipitation creep in a calcite marble shear zone. Journal of Structural Geology, 26: 869-883. Czeck, D.M., 2001. Strain analysis, rheological constraints, and tectonic model for an Archean polymictic conglomerate : Superior Province, Ontario, Canada. Thesis, University of Minnesota. Fissler, D.A., 2006. A Quantitative Analysis of Strain in the Seine River Metaconglomerates, Rainy Lake Region, Northwestern Ontario, Canada. M.S. Thesis, University of Wisconsin-Milwaukee. Heilbronner, R., Tullis, J., 2002. The effect of static annealing on microstructures and crystallographic preferred orientations of quartzites experimentally deformed in axial compression and shear. Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives; Geological Society of London, Special Publications, 200: 191-218. Hirth, G., Tullis, J., 1992. Dislocation creep regimes in quartz aggregates. Journal of Structural Geology, 14: 145-159. Lisle, R.J., 1985. The effect of composition and strain on quartz-fabric intensity in pebbles from a deformed conglomerate. Geologische Rundschau, 74: 657-663. Mainprice, D. “PFch5” Petrophysical Software, Unicef Careware, 2005. <http://www.gm.univ- montp2.fr/PERSO/mainprice/index.html> Mainprice, D.H., Paterson, M.S., 1984. Experimental studies of the role of water in the plasticity of quartzites. Journal of Geophysical Research, 89: 4257-4269. Stipp, M., Stunitz, H., Heilbronner, R., Schmid, S.M., 2002. The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. Journal of Structural Geology, 24: 1861-1884. 2 �THE SHAKESPEARE CU-NI-PGE DEPOSIT: EVIDENCE FOR A TWO-STAGE EMPLACEMENT MECHANISM Duncan J. Bain, B.Sc., P.Geo., Department of Earth Sciences, University of Western Ontario, London, ON, N6A 5B7, dbain3@uwo.ca The Shakespeare Cu-Ni-PGE Project occurs in Shakespeare and Baldwin townships, located 70 km west-southwest of Sudbury, Ontario, Canada and is held by Ursa Major Minerals Incorporated. The property contains probable reserves of 7.3 million tonnes of 0.37% Ni, 0.39% Cu, 0.024% Co, 0.37 g/tonne Pt, 0.40 g/tonne Pd and 0.20 g/tonne Au and an indicated resource of 12.0 million tonnes of 0.35% Ni, 0.36% Cu, 0.02% Co, 0.34 g/tonne Pt, 0.38 g/tonne Pd and 0.19 g/tonne Au. It varies in width from 24 to 61 m and has an exposed strike length of 600 m but is open along strike and at depth. This mineralized zone occurs at the stratigraphic top of a Nipissing gabbro sill injected along the contact between metamorphosed hangingwall Mississagi Formation quartz arenites and footwall Pecors Formation wackes, both of the Paleoproterozoic age Hough Lake Group of the lower Huronian Supergroup. These units form the bulk of the Southern Structural Province. The deposit strikes ENE and dips steeply to the north. It consists of blebs, stringers and disseminated grains of pyrrhotite, chalcopyrite, pyrite, cobaltite and pentlandite within a melanogabbro to quartz diorite sill. Several mafic magmatic events overlap with deposition of the Huronian sediments. The 2490 Ma East Bull Lake Layered Intrusive Suite underlies the Huronian Supergroup and is cut by the 2473 Ma Matachewan and 2446 Ma Hearst dyke swarms. Bimodal mafic to felsic volcanism (Elsie Mountain, Stobie, Copper Cliff and Salmay Lake formations) and granite intrusions (Creighton and Murray) occurred between 2450 Ma and 2380 Ma. The Nipissing gabbro sills intruded Huronian sediments between 2220 and 2210 Ma. These magmatic events indicate a continental margin or intracratonic rifting environment, probably initiated by a mantle plume head. This re-activated the long-lived Great Lakes Tectonic Zone of Neoarchean age. The ~ 1870 to 1850 Ma Penokean Orogeny collisional event overprints the Southern Province, and late thermal nodes relate to 1750 to 1730 Ma intrusions such as the Cutler Granite. In the Shakespeare area the thermal effects imposed upper greenschist to lower amphibolite facies metamorphism. Post-Penokean mylonitic shearing re-activated regional scale northeast-trending Penokean structures such as the Murray and Hunter Lake faults. The East Bull Agnew Lake Intrusion occurs directly west of the Shakespeare deposit. Previous mapping by the Ontario Geological Survey shows the Nipissing gabbro at Shakespeare as a feeder system cutting up through both Archean basement rocks and Agnew Lake Intrusion. The 2003/2004 field seasons were spent detailed mapping and sampling the Shakespeare and Nipissing gabbro sills and Huronian sedimentary rocks. Examination of the deposit area and property grids to the ENE revealed “windows” of melanogabbro along a 10 km strike length under a thick cap of Mississagi Formation quartz arenites. This suggests the Shakespeare sill overlies much of the Nipissing sill throughout the immediate area. Locally the main Nipissing sill is a saucer shaped body dipping ~ 20o NNW but in the area of the deposit it and the overlying Shakespeare units dip 40o to 60o NNW. Cumulate layering of sulphides at the base of the Shakespeare melanogabbro indicates postmineralization tilting. Lidar imagery shows numerous property scale structures with a NNE to NE trend associated with the Hunter Lake Fault system; at least two are mylonite shears, Stumpy Bay and Spanish River. The thin 24 to 61 m Shakespeare sill overlies the thick 350 to 600 m Nipissing sill and is differentiated from basal melanogabbro through to upper quartz diorite, and melanogabbro is the main host for Shakespeare mineralization. 3 �Hand samples show color and texture differences between the melanogabbro at the base of the Shakespeare sill and the gabbro of the underlying Nipissing sill. Thin and polished section petrography of 45 samples of core and surface rock were studied to establish the degree of metamorphic overprinting. Most of the primary minerals, essentially plagioclase and augite, have been moderately to very strongly altered to chlorite, epidote, actinolitic hornblende and biotite, confirming the medium-grade regional metamorphism reported by others. Primary igneous textures are recognizable, a confirmation of essentially only thermal metamorphic effects. Apatite is a common trace mineral in thin section and accounts for the elevated P2O5 content. Polished sections showed fine blebs of pyrrhotite-chalcopyrite, with rare pentlandite. Minor amounts of ilmenite rim magnetite, accounting for a low TiO2 to FeO* ratio. However, both FeO* and TiO2 as well as V are higher in Shakespeare melanogabbro than in Nipissing gabbro, which indicates a Shakespeare melt more evolved than the main Nipissing magma. Major, minor, trace and REE geochemical studies were carried out to compare Shakespeare melanogabbro to the main Nipissing gabbro. Melanogabbro ranged down to 47.01% SiO2 whereas Nipissing gabbro is no lower than 50.23% and averages 52% SiO2. Al2O3 and CaO are also lower in melanogabbro, consistent with its lower plagioclase content. A lower Mg# indicates that melanogabbro is more differentiated than Nipissing gabbro. Rb/Sr ratios, Y and Zr are elevated in Shakespeare compared to average Nipissing gabbro values, reflecting contamination by continental crust. Chondrite-normalized REEs from Shakespeare melanogabbro and Nipissing gabbro show similar patterns but with higher abundances in Shakespeare, again indicating the Shakespeare melt is more evolved. No Eu anomaly is present in any of these rocks and Shakespeare and Nipissing are probably separate melt batches from the same chamber with Shakespeare more evolved than Nipissing. Ni-Cu-Co are erratically anomalous in both sill types. Agnew Lake mineralization has much lower Cu:Ni and Au/Pt+Pd ratios than the Shakespeare/Nipissing mineralization. These markedly differing metal signatures show the Shakespeare deposit cannot be derived by any direct remobilization process from Agnew mineralization that may have been encountered in the feeder system. The Shakespeare sill originated from the evolved melt capping the Nipissing magma chamber. The initial heave of this melt was intruded along the contact between hangingwall Mississagi quartz arenites and footwall Pecors wackes. Continental contamination, including sulphur from Pecors and silica mixing from Mississagi, lowered the temperature of sulphur saturation within a sulphurenriched Shakespeare melt, causing the rapid precipitation of Ni-Cu-Co sulpides and PGEs to the chamber floor. The large main heave of Nipissing melt was injected soon after, underplating the Shakespeare sill. Post-deposition extensional faulting caused the mineralized Shakespeare and Nipissing sills to be tilted along an ENE hinge line to produce the present day form of the deposit. 4 �The Deloro-Tisdale SIZ of the Abitibi greenstone belt: an example from McArthur Township, Ontario G.J. Baldwin1, P.C. Thurston1, B.S. Kamber1, M.G. Houle2 1 Dept. of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6 2 Ontario Geological Survey, Sudbury, ON P3E 6B5 The NeoArchean Abitibi Greenstone Belt encompasses 6 geochronologically resolvable tectonostratigraphic assemblages consisting of volcanic and minor sedimentary rocks. The assemblages were autochthonously deposited between 2750 Ma and 2696 Ma. Evidence for autochthonous emplacement is, for example, evident from zircon xenocrysts inherited from the 27302724 Ma Deloro assemblage in the 2710-2704 Tisdale assemblage, which are in direct, stratigraphic contact.. In the field, there is no evidence for an erosional unconformity between these units, but instead a series of sedimentary interface zones (SIZ’s) consisting primarily of Banded Iron Formation (BIF) and other related sedimentary rocks. South of Timmins, ON, in the Bartlett Dome area (McArthur, Bartlett, and English Townships) three of these SIZ’s can be observed in the Deloro assemblage. The uppermost SIZ follows the contact between the Deloro and Tisdale assemblages; however the best exposed and geochronologically constrained is the middle SIZ, particularly in McArthur Township. A nearly complete section of the ~50 m thick middle SIZ reveals 6 distinct lithologies from base to top; a substrate of felsic volcanic rocks; basal sulphide facies BIF; silicate facies- BIF; oxide facies BIF; debris flow bearing BIF; and oxide-sulphide facies BIF, with some of these lithologies recurring higher in stratigraphy.. Stratigraphy and primary structures represent features characteristic of submarine unconformities (Shanmugam, 1988), such as heterolithic debris flows, syn-sedimentary deformation features and cherty hardgrounds. Tabular chert breccia units were observed in stratigraphic association with debris flows, and chert-wacke units indicating synsedimentary production of the breccia units and early silicification prior to the debris flow event. Petrographic observations strongly suggest that many of the cherts within the BIF may in fact be silicified ash, an assertion that is being tested using REE+Y geochemistry. These geochemical studies will also aid in distinguishing between hydrothermal vs. seawater origin of Si in the formation of the BIF in the SIZ. U-Pb zircon dating of rhyolites immediately above and below the SIZ are used to help to constrain the deposition period. The underlying rhyolite yielded an age of 2728.1 +/- 1.6 Ma while the overlying rhyolite returned a slightly less precise age of 2724.5 +/- 2.1 Ma. The deposition of this particular SIZ could thus have lasted for a period of 1-6 million years. Taken at face value, the dates indicate a depositional period of 3.6 Ma and hence extremely slow sedimentation rates, or prolonged periods of water-rock interaction during the period of deposition. Despite the potential overlap in these dates, the overlying U-Pb age is similar to ages for felsic rocks overlying iron formation, further north in Deloro Twp (2724 +/- 3.7 Ma), as well as in the correlative Swayze greenstone belt (2724 =/2 Ma) (van Breeman et al, 2006), indicating a regional pattern. Further study is required at other localities along this and other SIZ’s in the Bartlett Dome area to confirm the interpretation that these SIZ’s represent a major gap in volcanism, possibly across the Abitibi greenstone belt. Shanmugan, G. 1988, Origin, recognition, and importance of erosional unconformities in sedimentary basins in Kleispehn, K.L. and Paola C., eds., New Perspectives in Basin Analysis (Frontiers in Sedimentary Geology): New York, Springer-Verlag, p. 84-108. van Breeman, O., Heather, K.B., and Ayer, J.A. 2006, U-Pb geochronology of the NeoArchean Swayze sector of the southern Abitibi greenstone belt. Geological Survey of Canada, Current Research 2006-F1, p 1-32. 5 �BEDROCK GEOLOGIC MAP OF THE LUTSEN QUADRANGLE, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu ALBERS, Paul, Duluth Metals, palbers@duluthmetals.com The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in eleven published 1:24,000 scale maps from Duluth to Lutsen, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Deer Yard Lake and Good Harbor Bay quadrangles will be published in July 2008 (Fig. 1A). All the maps in this series are available as printed maps, or as PDF and Arcview export files at the MGS website (http://www.geo.umn.edu/mgs/). With the exception of local thesis mapping by Albers (2006) and a regional-scale compilation map (Map M-119; Miller and others, 2001), the Lutsen quadrangle had not been published, and had been incompletely mapped. Approximately 30 sets of water well cutting samples, collected by Mckeever Well Drilling of Little Marais, Minnesota, were examined as part of the mapping effort. The area of this map lies in the uppermost portion of the Northeast sequence of the North Shore Volcanic Group (NSVG), and also the unconformably overlying Schroeder-Lutsen sequence (Fig. 1B). In addition, components of the Beaver Bay Complex, including the Leveaux ferrodiorite, the Lake Clara and Monker Lake diabases, and multiple phases of the Beaver River diabase are present in the map area. The new mapping has refined the volcanic stratigraphy of the NSVG in this area and has identified several previously unrecognized units and stratigraphic relationships. In particular it has been demonstrated that the Terrace Point basalt flow, which was formerly considered to be the lowermost unit of the Schroeder-Lutsen sequence, is instead part of the underlying, slightly older lava flows. In addition, three thick sandstone units were identified. One of these, the Eagle Mountain sandstone, is intersected in several water wells, and ranges from 6 to 18 meters in thickness; however much of the original stratigraphic thickness may have been removed by intrusion of the overlying Leveaux ferrodiorite sill, which apparently utilized the sandstone as a plane of weakness during emplacement. The other sandstone units are largely extrapolated from the north and east, where they have been identified in outcrops or water well cuttings as units up to 100 meters thick; they are extrapolated into this map area on the basis of linear topographic depressions and coincident linear negative aeromagnetic anomalies. Intrusive rocks of the Beaver Bay Complex are composed mainly of the Beaver River diabase, the porphyritic Leveaux ferrodiorite, and a newly-recognized ‘Silver-Bay’ type intrusion composed of a lower cumulate ferrogabbro and upper pyroxene-quartz ferromonzonite. The Leveaux ferrodiorite is a subvolcanic sill emplaced into volcanic rocks, and is intruded by the Beaver River diabase. The Monker Lake diabase dike (250 – 300 meters thick) is poorly exposed in the Lutsen quadrangle, but is easily demarcated by a sharp, linear positive aeromagnetic anomaly. The Lake Clara diabase, in the northwest corner of the map, is poorly constrained due to relatively poor outcrop and an indistinct aeromagnetic signature. This diabase locally forms a shallow-dipping sill capped by a thin quartz ferromonzodiorite which contains mafic enclaves interpreted as the product of melting of, and contamination by, the overlying rhyolite. References Albers, P.B., 2006, The geology and petrology of the Leveaux porphyritic dioritic intrusion: Investigating possible magmatic relationships to the anorthositic series of the Duluth Complex, Cook County, Minnesota: Duluth, Minn., University of Minnesota Duluth M.S. thesis, 206 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Map Series Miscellaneous Map M-119, sheet 1 of 2, scale 1:200,000. 6 �Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. B. Index map showing the locations of the major units mentioned in the abstract. 7 �OBSERVATIONS ON LANTHANIDE FRACTIONATION IN THE WAUSAU COMPLEX, MARATHON COUNTY, WI. T.W. Buchholz1, A.U. Falster2, Wm. B. Simmons2. 11140 12th St. N., Wisconsin Rapids, WI 54494; 2Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148. The Wausau Complex is composed of a series of four anorogenic intrusions ranging in age from 1565 Ma +3-5 for the alkalic Stettin pluton (Van Wyck, 1994) to 1505.9 ± 2.7 Ma for the Nine Mile Granite (Dewane & Van Schmus, 2007). The complex is usually considered to be a precursor to the younger Wolf River Batholith. Recent work on various sites in the Stettin and Nine Mile plutons has revealed interesting trends in lanthanide fractionation. While most REE bearing minerals are not overly unusual in their compositions, exceptions exist and may be clues to processes involved in the formation of these plutons. Minerals bearing the lanthanide elements, more commonly referred to as the Rare Earth Elements (REE), are relatively common in the Stettin and Nine Mile intrusions of the Wausau Complex (Falster et al, 2000). In general, minerals bearing REE elements will tend to accumulate either the light rare earth elements (LREE) or the heavy rare earth elements (HREE), and not both. LREE rich minerals are usually Ce-dominant, and HREE minerals are usually Y-dominant. Significant enrichments in other LREE elements are unusual, and in HREE elements even less common. LREE fluocarbonates, usually bastnaesite or related species, are usually A-site Ce-dominant with lesser levels of La, Nd and other LREE. However, it is not particularly uncommon to observe A-site Nd-dominance, and less commonly La-dominance in bastnaesite-group minerals from the Wausau Complex. This has particularly been noted in a small pegmatite designated the Ravine Pegmatite on the west side of the old Dehnel quarry in the Stettin intrusion, in a pneumatolytic-like assemblage in the Ladick quarry, Nine Mile pluton, and in a late-stage miarolitic phase in the County Materials quarry, Nine Mile pluton. LREE phosphates (typically monazite-(Ce)) are also usually A-site Ce-dominant. However, monazite-(Nd) has been noted in a small pegmatite exposed in a utility excavation on Evergreen Rd, and in mine dump material from the Summit Lake thorium prospect, both in the Stettin pluton. Additionally, minute crystals of rhabdophane-(Ce), rhabdophane-(Nd) and rhabdophane-(La) have been noted as late-formed minerals in a small miarolitic body briefly exposed in Wimmer Pit #3 in the Nine Mile pluton Rhabdophane is a hydrated LREE phosphate typically formed as a late-stage mineral in pegmatites, and may also form under weathering conditions. LREE-bearing members of the pyrochlore group associated with bastnaesite species in the Ravine Pegmatite in the Dehnel pit show LREE enrichment tendencies similar to trends in the associated bastnaesite-group minerals, as does closely associated fersmite (first report of this mineral for Wisconsin). 8 �It is less common to observe enrichment of specific HREE relative to Y, and no instances have yet been noted in the Wausau Complex of HREE minerals which are not Y-dominant. However, strong enrichment in Dy and Yb was observed in xenotime-(Y) intergrown with monazite-(Nd) in a small pegmatite exposed by utility work on Evergreen Drive in the Stettin pluton. Concentrations of Dy2O3 are consistently near 12 wt %, a level that has only been reported previously from the Annie #3 claim, Manitoba. Lesser degrees of Dy and Yb enrichment have also been noted in xenotime-(Y) from border phases of the Dehnel ravine pegmatite, Stettin Pluton, in xenotime-(Y) from a greisen assemblage in the Maguire Pit associated with cassiterite, ferberite/huebnerite and topaz in the Nine Mile pluton, and in xenotime-(Y) from a pegmatite exposed in the Red Rock North pit off of Spring Brook Rd, also in the Nine Mile pluton. Euxenite-(Y) associated with xenotime from the Red Rock North pegmatite shows a similar HREE enrichment pattern, and is also enriched in Ta and somewhat in Ti as well, approaching though not reaching the compositional field of tanteuxenite. Possible causes of fluctuations in relative abundances of LREE and HREE vary. Variations in LREE dominance in an oxidizing environment may result from oxidation of Ce3+ to Ce4+, facilitating retention of Ce4+ in solution and/or removal of Ce4+ from the system. Causes of variations in enrichment of various HREE elements is less certain; some authorities have suggested high levels of F may facilitate enrichment in even-numbered HREE (Dy, Er, Yb) relative to Y. Since F in the form of fluorite and F-rich micas is generally abundant in the Wausau Complex, why is such enrichment uncommon? References: Dewane, T. J., Van Schmus, W. R. (2007): U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma. Precambrian Research, V. 157, pp. 215-234. Falster, Alexander U., Simmons, Wm. B., Webber, Karen L., Buchholz, Tom (2000): Pegmatites and Pegmatite minerals of the Wausau Complex, Marathon County, Wisconsin. Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, V. XXX, pp. 13-28. Van Wyck, N. (1994): The Wolf River A-type Magmatic Event in Wisconsin: U/Pb and Sm/Nd Constraints on Timing and Petrogenesis. Institute on Lake Superior Geology, 40th Annual Meeting, Part 1, Program and Abstracts, p. 81-82 9 �Unusual features along the Archean/Paleoproterozoic unconformity at Silver Lake, Michigan—seismites from the Sudbury impact W.F. Cannon and K.J. Schulz, U.S. Geological Survey, Reston VA At Silver Lake, 50 km northwest of Marquette, Michigan, rocks along the unconformity between Archean crystalline rocks and the basal beds of Paleoproterozoic sedimentary rocks show an unusual type of deformation. About a meter of the Paleoproterozoic sediments, part of the Michigamme Formation, are preserved. They consist mostly of laminated mudstones and fine sandstone, with thin lenses of basal conglomerate and breccia. The Archean rocks are mostly granite with lesser metavolcanic rocks. Deformation in Archean rocks was brittle, varying from intense brecciation to small offsets between meter-scale joint blocks; in Paleoproterozoic sediments relict soft-sediment deformation indicates liquefaction and flow as well as injection as sediment dikes into Archean rocks and breccia zones. Fracturing and brecciation of the Archean rocks and consequent injection of dikes of the overlying Michigamme sediments occurs to some extent in all exposures. In places Archean granitic rocks are thoroughly broken into angular fragments ranging down to centimeter-scale or smaller (Figure 1). These fragments are now suspended in a matrix of clastic material derived from the overlying sedimentary beds. Brecciation and dike injection extends for at least 3-4 meters below the unconformity (Figure 2). Dikes with diverse orientations range from nearly a meter to only a few millimeters thick. In some exposures joint blocks of Archean basement rocks, some up to tens of meters long, show meter-scale displacements relative to each other. These displacements resulted in reshaping of the original nearly flat unconformity surface and molded the overlying sediments to this new shape to form an unusual array of small folds with very diverse orientations (Figure 3). We suggest that these unusual features are near-surface manifestations of an earthquake of about M10.5 on the Richter scale that was generated by the Sudbury impact, centered about 500 kilometers to the east, and record the passage of unprecedentedly intense seismic waves. The Sudbury ejecta layer is well documented in the lower part of the Michigamme Formation about 3 km from Silver Lake. We have tentatively identified ejecta particles within some of the sediment dikes indicating that the rocks at Silver Lake were very near the earth’s surface at the time of impact. Part of the energy released by giant impacts such as Sudbury is transmitted as seismic waves more intense than any generated by terrestrial events. The energy arrives first as a compressional wave which imparts energy to the rocks and is followed by the complementary rarefaction wave. During this rarefaction, rocks near the earth’s surface may respond by shattering and upward displacement creating open spaces into which overlying sediments can be injected. We interpret the breccia zones and abundant sediment dikes within the Archean rocks at Silver Lake to record this explosive rarefaction and dilation. The diversely oriented dikes occur in an abundance that requires substantial dilation and volume increase of the upper parts of the basement rocks to create the space now occupied by the dikes. Seismogenic disruption may be an important process in the formation of the Sudbury ejecta horizon and may be the cause of autochthonous breccias at the base of the ejecta-bearing layer at many other recently discovered localities. 10 �11 �UPGRADE OF AEROMAGNETIC DATA AT THE MINNESOTA GEOLOGICAL SURVEY CHANDLER, Val W., and LIVELY, Richard S., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114, chand004@umn.edu Between 2005 and 2007 the Minnesota Geological Survey (MGS) conducted an upgrade of the existing aeromagnetic data for Minnesota. These aeromagnetic data were acquired between 1979 and 1991 as part of a statewide survey, and data processing was limited by the computer capabilities of the time. Since then, significant technological improvements allow efficient handling of massive databases and post-processing tasks, such as line leveling and gridding, to be accomplished with small desktop computer systems. This project to upgrade the MGS aeromagnetic database was designed to reprocess and regrid the data using up-to-date software and hardware. As an added benefit access to the data was improved by making all the upgrades available via the internet. Considerable effort was directed towards recovering line data that were missing from the primary digital archive, a CD produced in 1992 by the National Geophysical Data Center of the National Oceanic and Atmospheric Administration. Approximately 2,700 line kilometers of missing data were fully recovered from digital media that were archived at the MGS and U.S. Geological Survey. Residual magnetic anomaly values for an additional 1,038 line kilometers were approximately recovered by digitizing the position of photo-spotted points and interpolating 100-meter-spaced magnetic profiles from the original state aeromagnetic grid (213.36 meter interval). Efforts were also made to mitigate line-leveling errors in the data, which locally caused striping artifacts, especially in derivative-enhanced grids. Selective leveling, using either statistical iteration or manual-editing, was applied to a total of 229 flight-lines and 101 tie lines, equating to approximately 21,000 line kilometers of adjusted data. In addition, approximately 522,933 line kilometers of data, nearly all of the available line data, were subsequently adjusted by micro-leveling. The revised line data were used to generate new aeromagnetic grids for Minnesota with a grid interval of 100 meters. To provide a regional context and minimize boundary discontinuities, gridded aeromagnetic data from areas adjoining Minnesota were also incorporated. To enhance the utility of the data for geologic mapping, filtered grids were produced, including aeromagnetic data reduced to vertical polarization (reduced to pole or RTP), the first vertical derivative of the RTP data (Fig. 1), and the second vertical derivative of the RTP data. The revised line data and the new statewide grids are downloadable from the MGS website at http://www.geo.umn.edu/mgs/magnetics.htm. Additional information available from this site includes copies of flight logs from the original surveys, and grid-based estimates of depth to magnetic basement derived using Euler’s equation. These products represent a major step forward in providing complete and up-to-date aeromagnetic coverage for Minnesota, and should serve the needs of the exploration and scientific communities in the region for many years to come. Acknowledgements: This study was supported by the Minnesota Minerals Coordinating Committee. Acquisition of most of the aeromagnetic data was supported by the Legislative Commission on Minnesota Resources. Additional data were contributed by the U.S. Geological Survey, U.S. Steel Corporation and the Geological Survey of Canada. David Dahl of the Minnesota Department of Natural Resources converted original line data to the NAD83 datum, and edited these data for repeats and other spurious features prior to this project. Robert Kuchs and Patricia Hill of the U.S. Geological Survey were instrumental in retrieving data originally acquired by their agency. Processing of the aeromagnetic data in this project was conducted using the OASIS/MONTAJ software system of Geosoft. 12 �Figure 1. First Vertical derivative of the revised aeromagnetic data reduced to pole. 13 �Geochemical and stable isotope studies of hydrothermal alteration associated with the Eagle deposit, northern Michigan Ding, Xin, Ripley, Edward M., Li C Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405 The Eagle Ni-Cu-PGE deposit occurs in olivine-rich melagabbro, melatroctolite, and feldspathic peridotite units that were emplaced during the early stage of development of the Midcontinent Rift. U-Pb dating of baddeleyite from a feldspathic peridotite indicates a crystallization age of 1107.2±5.7 Ma. Mineralization is characterized by the presence of two semi-massive sulfide zones in olivine-rich rocks that are linked by a zone of massive sulfides. Pyrrhotite, pentlandite, and chalcopyrite comprise the bulk of the sulfide mineral assemblage. Hydrothermal alteration in the igneous rocks is locally intense. Phase relations involving serpentine, chlorite, actinolite, prehnite and pumpellyite suggest the temperatures of alteration did not exceed 300℃. Olivine in the intrusive rocks has been variably converted to mixtures of serpentine and fine-grained magnetite (Fig.1). Interstitial pyroxene and plagioclase have been altered to mixtures of serpentine, chlorite, amphibole, and lesser amounts of talc. Disseminated sulfide minerals are locally replaced by magnetite, serpentine, and chlorite. Serpentine shows a broad and distinctive compositional spectrum, with variations linked to the parent mineral which is replaced. Serpentine which replaces olivine is enriched in Mg and depleted in Al and Fe relative to serpentine that replaces pyroxene and sulfide minerals (Fig.2). Aluminum is thought to be derived from clinopyroxene, and the relatively high Al content of serpentine associated with sulfide minerals suggests that Al was a mobile element during low-temperature hydrothermal alteration. δD value of serpentine range from -85 to -109, suggesting equilibration with fluids of δD values between -58 and 82 at 300℃. The δD values suggest that evolved, long path-length meteoric water characterized by a low time integrated water/rock ratio was responsible for the alteration. Oxygen isotopic analyses of serpentine are in progress; results will aid in further constraining fluid sources. 14 �Figure 1 Photomicrographs of serpentine replacing olivine, clinopyroxene and sulfide in Eagle deposit 15 �Properties of fluid involved in formation of natural ore in the Mesabi Iron Range, Minnesota Elizabeth Drommerhausen, Steven Losh; Dept of Chemistry and Geology, TN 242, Minnesota State University, Mankato MN 56001 Natural ore in the Mesabi Range has long been thought to have been formed by downward percolation of meteoric water, which dissolved non-oxide minerals (chiefly chert) from the Biwabik iron formation and oxidized, hydrated, and concentrated the remaining iron oxides. However, some investigators (e.g., Gruner, 1946) have suspected a hydrothermal origin for some if not all of the fluid that interacted with the iron formation. Morey (1999) hypothesized that the fluids responsible for formation of natural ore were driven northward and upward along an aquifer from the Penokean-aged collision zone ~100 miles south of the Mesabi Iron Range. We are evaluating the origin and the fate of fluids involved with leaching silica and producing natural ore by fluid inclusion and cathodoluminescence techniques. In the Hibbtac pit, brecciated quartz veins lie within and next to minor high-angle fault zones that experienced movement due to volume loss that accompanied chert dissolution from iron formation; oxidation and dissolution generally took place in the hangingwalls of the faults. Because the quartz is largely confined to faults that moved as a result of dissolution and the quartz is brecciated by movement on those faults, the quartz we sampled was very likely precipitated from the same fluid that dissolved chert from the iron formation. Thus temperatures and salinities of fluid inclusions hosted by this quartz may thus constrain the source of the fluid involved with formation of natural ore. We have also examined fluid inclusions in quartz from a 6-inch thick bedding-parallel quartz + bladed hematite vein from the LTV Number 6 pit near Aurora. The iron formation is unoxidized and gently-dipping, and bedding is slickensided, implying low-angle fault movement. Other veins in slickensided, unoxidized iron formation at this location contain carbonate minerals, but the vein we examined did not. Preliminary fluid inclusion data from two Hibbtac quartz vein samples from one high-angle fault indicates that the fluid that dissolved silica from the iron formation. Fluid inclusions in these samples are primary, simple liquid + vapor inclusions exhibiting the same liquid:vapor ratio throughout. They are not necked or otherwise deformed. The fluid was characterized by a wide range of temperatures as indicated by homogenization temperatures (Thomog): two vein samples from one fault give Thomog of 60º to 122º C (n=6). Salinities, determined from ice melting temperatures, range from 4.5% to 19 wt% NaCl equivalent. Fluid inclusion from the quartz + hematite vein at the LTV 6 mine have similar characteristics. Fluid inclusions from a second fault at the Hibbtac mine display significantly higher homogenization temperatures, from 164º to 240º C (n=5). Clearly, nearly all of these fluids are much hotter and have much higher salinity than expected for meteoric waters that percolated a short distance downward from the paleo-surface, even during a period of warm climate. Their characteristics are those of a relatively deeply-sourced fluid. 16 �The range in homogenization temperatures, which can vary by tens of degrees over a distance of millimeters, implies mixing of relatively warm and cool fluids during precipitation of quartz. Fluid mixing and/or episodic pulsing may manifest as alternating domains of contrasting cathodoluminescence. Cold cathodoluminescence reveals some growth zoning in quartz from the LTV Pit 6 samples. The preliminary data in hand support the notion that the rocks presently exposed in the Mesabi Iron Range were infiltrated by warm, saline fluids that are quite similar at two locations, and in two different stages of quartz paragenesis, that are about 50 miles apart. We are continuing to obtain more data from already-sampled localities and to expand the number of sample localities throughout the Mesabi Iron Range to evaluate the nature and variability, in space in time, of properties of fluid that interacted with rocks of the Biwabik Iron Formation. Gruner, J., 1946, The Mineralogy and Geology of the Taconites and Iron Ores of the Mesabi Range, Minnesota; St. Paul, Office of the Commissioner of the Iron Range Resources and Rehabilitation, 127 pp. Morey, G., High-grade iron ore deposits of the Mesabi Range, Minnesota – Product of a continental-scale Proterozoic groundwater flow system; Econ. Geol. v. 94, pp. 133-141 17 �EVIDENCE FOR REACTIVATING ARCHEAN STRUCTURAL BREAKS DURING PALEOPROTEROZOIC RIFT SEDIMENTATION AND SUBSEQUENT ACCRETIONARY HISTORY “ONCE A FAULT ALWAYS A FAULT” Norman Duke, Department of Earth Sciences, University of Western Ontario, London, ON, N6A 5B7, nduke@uwo.ca Isolated remnants of Paleoproterozoic cover sequences are occasionally preserved along structural breaks within Archean basement, even well removed from the major tracts of sedimentary cover. Excellent examples occur in the northeast Churchill Province where Hurwitz outliers are preserved along Pykes Break on the northern margin of the Rankin Greenstone Belt as well as along similar basement faults occurring within the Kaminak Greenstone Belt. Outliers of Huronian occur along segments of the Kirkland-LarderCadillac-Bousquet break and the Cobalt Plate is transected by the Temagami-Ville MarieBelleterre break in the southern Superior Province. A prime example from the Penokean Fold Belt is the outlier of Adjibik Quartzite preserved at Ropes, well interior to the Dead RiverIshpeming Greenstone Belt north of the Marquette Trough. That the Negaunee Iron Formation buttresses against the Southern Gneiss Complex, a wholey different Archean terrane-type compared to the bordering greenstones to the north, indicates reactivation of Archean subprovince boundaries played an important role in forming the major depositional troughs. The origin of the Archean breaks stem from the terminal Neoarchean cratonization event, i.e. complex fault zones accommodating the unroofing of the voluminous granite plutons and bounding local depocenters of orogenic ash tuff and polymictic conglomerate (see Figure A). Subsequent reactivation occurred during Paleoproterozoic rifting, accommodating the deposition of thick siliciclastic cover successions (see Figure B). Closing of rift basins during the Hudsonian/Penokean collisional events re-established the Archean basement to pre-rift thicknesses by reverse shear and lead to open folding and upward increasing tectonic telescoping within the overlying sedimentary cover successions (see Figure C). As a result, Proterozoic sedimentary outliers are preserved along reactivated Archean basement break environments, while abrupt stratigraphic/structural buttressing is commonplace along the boundaries of the major sedimentary troughs (see Figure D). Break reactivation well accounts for Proterozoic metamorphic overprinting of Archean lode gold mineralization, and may even have played a pivotal role in the late oxidation of the Negaunee siderite ores marginal to the Southern Shear, the southern boundary to the Marquette Trough. Strong brittle fracturing with associated chlorite that overprints the Southern Shear suggests this particular boundary fault had a protracted history culminating in normal displacement late in the Penokean. Could the oxidation of the Negaunee siderite bands to form supergene magnetite-martite-platey hematite ores relate to meteoric ingress as late as 1730 Ma? The primary lessons to be gleaned from the geological evidence for repeated displacements are: “once a fault, always a fault”; that these structures accommodated opposing normal/reverse displacements over time; and that they remained a focus for very different fluid types over their protracted geological history. 18 �A B C D Figures: A - 2695-2675 Ma Neoarchean Orogenic Break. B - 2450-2200 Ma Paleoproterozoic Rift. C - 1850 Ma Penokean Collision. D - Post-Penokean Thermal Domes. 19 �Toward a Common Map Legend for Ontario R.M. Easton, M.G. Houlé, D. Rowell, and N.F. Trowell, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, 7th Floor, Sudbury, ON P3E 6B5, Canada, mike.easton@ontario.ca Ontario encompasses over 1 million km2, 3 major Precambrian geological provinces (Superior, Southern, Grenville) and a variety of younger Paleozoic rocks and Quaternary sediments. The Ontario Geological Survey (OGS) typically produces maps at several scales: detailed (1:20 000), regional (1:50 000 and 1:250 000), or provincial (1:1 000 000). Historically, creation of map legends has been individualistic, dependent on both the mapping geologist and the character and scale of the map area. With the advent of increasing interjurisdictional collaborative mapping projects, digital collection of field data, and a focus on compilation maps, there is an increasing need for the OGS to develop a common map legend for all OGS Precambrian geology maps. Creating a common map legend is more challenging than simply creating a standard scheme for rock nomenclature, although the latter is a necessary prerequisite. A map legend must also be able to reflect relative age relationships, formal stratigraphic nomenclature where established, be applicable to areas of well known and poorly known geology, yet be independent of map scale. Furthermore, the methodology used must be relatively easy for the end user to understand and, for hard-copy maps, must contain as few characters as possible in order to avoid clutter and be comprehensible. This presentation presents a first attempt by the OGS to create a common map legend. At its essence is a basic 3 or 4 letter alphanumeric code: legend/colour code_rock class_root rock name[±key modifier]. For example, 5Ig[bi] is a biotite granite, assigned colour code of unit 5. Comments on the merits and drawbacks of this scheme are welcomed by the authors. During the course of developing a common map legend, specific problems were encountered with respect to existing nomenclature schemes for metamorphic, altered, and/or deformed rocks (a.k.a. composite-origin rocks) rocks. For example, is composition critical to the name (e.g., marble); or texture (e.g., schist); or protolith (e.g., metabasalt); or mineralogy (e.g., diopside marble); or a combination of factors (e.g., kyanite metapelite)? Unfortunately, all recent rock nomenclature schemes for compositeorigin rocks (e.g., Fettes and Desmons 2007) have focused on naming such rocks at the hand sample level, not at a macroscopic scale. To address this problem, a hierarchical nomenclature scheme, is proposed, as outlined in brief in the table below. Type Metamorphic Alteration Structural Subtype Root Name (examples) Structural Compositional schist, gneiss, granolite marble, amphibolite, pelite breccia, agmatite migmatite silicification fenite, skarn cataclastite and related rocks mylonite and related rocks Textural Hybrid Hydrothermal Metasomatic Brittle Ductile Impact Optional Modifier (textural, mineralogical, descriptive) kyanite schist calcitic marble fault gouge melt, breccia References Fettes, D. and Desmons, J. 2007. Metamorphic Rocks: A Classification and Glossary of terms, Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks; Cambridge University Press, Cambridge, UK, 244p. 20 �Detrital zircon geochronology of Hurnoian Supergroup sandstones located within the Vernon structure, north of Espanola, Ontario R.M. Easton, Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 mike.easton@ontario.ca and L.M. Heaman, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca Located north of Espanola, Ontario, the Vernon structure is a former depositional graben which preserves a near complete stratigraphic section through the Huronian Supergroup; from its contact with Neoarchean basement to the Lorrain Formation (upper Cobalt Group) (Easton 2005, 2006). The graben is bounded by Neoarchean monzogranitic rocks of the Birch Lake granite, part of the southern Abitibi subprovince, which has yielded a U-Pb zircon age of 2651±1 Ma (Kamo 2006). The depositional setting of the Huronian Supergroup in the Vernon structure provides an opportunity to investigate the influence of local versus regional provenance sources on the composition of Huronian Supergroup metasandstones. For comparison, Rainbird and Davis (2006) provide regional data on the detrital zircon geochronology of Huronian Supergroup metasandstones from the Elliot Lake and Cobalt areas. Samples were collected from 3 sandstone-dominated formations in the Vernon structure, from stratigraphically lowest to highest these are the Matinenda, Mississagi and Serpent formations. U-Pb zircon geochronology was conducted using laser-ablation analyses at the University of Alberta; 45 grains were analyzed from the Matinenda sample, and 36 grains each from the Mississagi and Serpent samples. The Matinenda Formation sample was collected from the base of the formation where it overlies a package of Huronian Supergroup mafic metavolcanic and volcaniclastic rocks in Baldwin Township. Most of the near concordant grains in the sample are Neoarchean, and define 2 groupings; 1) 2600 to 2690 Ma (n=18), and 2) 2690 to 2740 Ma (n=8). The latter age range is representative of the age of volcanism and plutonism in the Abitibi greenstone belt. The younger age range images emplacement ages representative of plutonic rocks of the southern Abitibi subprovince. In addition, 4 near concordant grains gave an average 207Pb/206Pb age of 2455±9 Ma, which is interpreted as representing the age of locally-sourced Huronian metavolcanic material. Rainbird and Davis (2006) document a similar population distribution of Neoarchean grains in their Matinenda Formation sample, but they did not find any Paleoproterozoic grains. The Mississagi Formation sample was collected from the uppermost part of the formation in Porter Township. This sample has an SiO2 content of 93 wt.%, and a Chemical Index of Alteration (CIA) value of 65. Again, the most concordant grains defined 2 groups, with 10 grains in the range from 2600 to 2690 Ma, and 5 grains in the 2690 to 2740 Ma interval. This sample also contained 4 grains in the range 2900 to 3000 Ma and 1 grain at 3320 Ma. This distribution is similar to that reported by Rainbird and Davis (2006), who also reported a population of very old grains, which only occurred in the Mississagi Formation. Roughly half of the analyzed grains in the Mississagi Formation sample were >10% discordant, in contrast to the other two samples, where significant discordance was much less common. The Serpent Formation sample was collected from the middle of the formation in Porter Township. This sample has a SiO2 content of 81.3 wt. %, a NaO2 content of 5.0 wt. % and a CIA value of 82; the high sodium content is typical of the formation. Although the two Neoarchean age groups are also present in this sample, the younger group (2600 to 2690 Ma) is dominant (22 grains), with only 2 grains present from the 2690 to 2740 Ma group, and 2 grains ranging from 2740 to 2790 Ma. This result differs from Rainbird and Davis (2006), who found the 2690 to 2740 Ma range dominant, as well as the presence of a sizable Geon 28 population. The dominance of the younger Neoarchean grains in the Serpent Formation in the Vernon structure may indicate that the sandstone there is sourced more locally than the Elliot Lake sample studied by Rainbird and Davis (2006). 21 �The geochemistry of the metasandstones provides limited provenance information. The Huronian metavolcanic rocks in the Vernon area lack any diagnostic trace element signatures (such as high Cr contents) that could be detected in the sandstones. The Birch Lake granite is characterized by high SiO2 (70-74 wt.%) and elevated thorium contents (20-50 ppm), and the metasandstones contain between 15 and 140 ppm thorium, with low uranium. Thus, plutons similar in composition to the Birch Lake granite could have provided a source for both quartz and thorium for the sandstones in the Vernon structure. Probably the most significant result from this study is the abundance of grains in the interval from 2600 to 2690 Ma (mostly 2660 to 2680 Ma) in all 3 samples, which appear to increase in abundance up section. If the sandstones were sourced solely from the northern Abitibi subprovince, the dominant population would be expected to range from 2690 to 2740 Ma. The limited geochronology available for intrusive rocks in the southern Abitibi subprovince range in age from 2660 to 2680 Ma (e.g., Prevec 1993, Krogh et al. 1984), suggesting that a significant portion of the detrital zircons in the sandstones are derived locally from the southern Abitibi subprovince. A titanite grain from the Matinenda Formation sample yielded a Pb207/Pb206 age of 2139±15 Ma, similar to regional Rb/Sr ages reported from Nipissing gabbro intrusions in the Southern Province (e.g., 2110±80 Ma, adjusted to current decay constants, Van Schmus 1965), perhaps reflecting postemplacement cooling of Nipissing gabbro intrusions that were emplaced into the Huronian Supergroup at circa 2215 Ma, or a regional metamorphic event. Attempts to date the timing of regional metamorphism using staurolite grains present in lower amphibolite facies metapelitic rocks of the McKim Formation, collected from classic localities near Agnew Lake, were inconclusive, due to low U contents in the staurolite grains coupled with the effects of metamorphic retrogression on the grains. References Easton, R.M. 2005. Geology of Porter and Vernon townships, Southern Province; in Summary of Field Work and Other Activities, 2005, Ontario Geological Survey Open File Report 6172, p.13-1 to 13-20. Easton, R.M. 2006. Geology and mineral potential of Southern Province rocks in Baldwin Township; in Summary of Field Work and Other Activities, 2006, Ontario Geological Survey Open File Report 6192, p.14-1 to 14-21. Kamo, S. 2006. Report on U-Pb geochronological data from the southern Abitibi Subprovince, Bannockburn– Montrose and Vernon townships, and the Grenville Front region, Thistle–Sisk townships, Ontario; internal U/Pb age report from the Jack Satterly Geochronology Laboratory for the Ontario Geological Survey, Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, 20p. Krogh, T.E., Davis, D.W., and Corfu, F. 1984: Precise U-Pb zircon and baddeleyite ages for the Sudbury Structure; in Geology and Ore Deposits of the Sudbury Structure; Ontario Geological Survey Special Volume 1, p. 431-446. Prevec, S.A. 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanics near Sudbury, Ontario; unpublished Ph.D. thesis, University of Alberta, Edmonton, Alberta, 223p. Rainbird, R.H. and Davis, W.J. 2006. Detrital zircon geochronology of the western Huronian Basin; Institute on Lake Superior Geology, Proceedings, 52, pt.1, Programs and Abstracts, 55-56. Van Schmus, W.R. 1965. The geochronology of the Blind River-Bruce Mines area, Ontario; Journal of Geology, v. 73, p. 755-780. 22 �Structural and kinematic analysis of the Archean Shagawa Lake shear zone, Superior Province, northeastern Minnesota Emerald J. Erickson., Dept. of Geology, Univ. of Minnesota-Duluth, eric1757@d.umn.edu Vicki Hansen., Dept of Geology, Univ. of Minnesota-Duluth, vhansen@d.umn.edu Granite-greenstone terrains exist in all Archean (3.8-2.5 Ga) cratons and are important to understanding Archean geologic processes. The debate about the formation of Archean granite-greenstone terrains features two hypotheses: 1) sagduction and diapirism caused by crustal scale density instabilities that led to sinking of greenstone basins and rise of granitic bodies and 2) arc-terrane accretion via plate tectonics processes that led to interlayered greenstone basins and volcanic arc plutons. The Superior Province in northeastern Minnesota is an Archean granite-greenstone terrain which is historically explained by arc-terrane accretion. The evolution of shear zones within greenstone terrains can provide critical insight between these hypotheses. The steeply dipping, east striking Shagawa Lake shear zone, northeastern Minnesota, lies within a greenstone belt bordered to the north and south by granite complexes. The Shagawa Lake shear zone lies within a greenstone belt consisting of Newton Lake formation (metadiabase and greenstone), Knife Lake group (felsic volcanic tuff and metasediments) and Ely Greenstone (greenstone and iron formation). The Shagawa Lake shear zone is bordered to the north by the Vermilion Granitic Complex and to the south by the Giants Range Batholith. I undertook a structural and kinematic study to understand the deformation within the Shagawa Lake shear zone. This work focuses on a ~18 km long and ~3-4 km wide portion of the shear zone (Fig 1.). I compiled structural data from published Minnesota Geological Survey maps and collected structural data and oriented samples for microstructural analysis along three north-south transects. The Shagawa Lake shear zone foliation averages 064o, 79oS. Elongation lineations are dominantly steeply plunging (90o ±30o) with subsidiary shallowly plunging lineations (15o ±10o). The average lineation is 143o, 84o. Locally dip-slip and strike-slip elongation lineations exist in the same rock. Shallow strike-slip lineations are few and overprint the penetrative elongation lineations. In the case of both strike-slip and dip-slip lineations, sections cut normal to foliation and parallel to lineation show fabric asymmetry, whereas sections cut normal to foliation and lineation display fabric symmetry. These relations are consistent with non-coaxial shear. Shear sense interpretations within the motion plane, normal to foliation and parallel to lineation, proved challenging. Of the 47 thin sections I was able to, with varying degrees of confidence, interpret 39. Microstructures within the Shagawa Lake shear zone 1) in the west, from north to south transition approximately from north-side-up to south-side-up, 2) towards the east both north-side up and south-side-up shear are recorded across the shear zone, 3) both right-lateral and left-lateral strike-slip shear occurs throughout and 4) zones of localized right-lateral and left-lateral strike-slip are not penetrative and probably postdate the later dipslip elongation lineation. The structural patterns within the Shagawa Lake shear zone are consistent with the rising of the northern and southern granitic complexes and concurrent sinking of greenstone basins forming north-side-up and south-side-up dip-slip displacement within the greenstones and volcanic sediments. 23 �24 �Flow fabric determination of two Mesoproterozoic midcontinent rift dike swarms, northeastern Minnesota Fein, E. M., (Department of Geology, Kent State University, Kent, OH 44242, efein@kent.edu), Ferré, E.C. (Department of Geology, Southern Illinois University, Carbondale, IL, 62901), & Holm, D. K. (Department of Geology, Kent State University, Kent, OH 44242) The planar geometry of mafic dikes provides an important constraint on the direction of magma flow, and their fast cooling preserves laminar flow fabrics of viscous magma, which aligns rigid grains (e.g., Cañón-Tapia, 2004). The aim of this study is to document regional-scale igneous flow patterns in two Mid-continent Rift System (MRS) dike swarms and to locate vent sources feeding these intrusions by obtaining a reliable measure of magmatic fabrics using Anisotropy of Magnetic Susceptibility (AMS). Previous thin section petrofabric analyses of aligned plagioclase phenocrysts suggested primarily lateral flow in both swarms, albeit from a relatively confined data set (Reichhoff, 1987). Another objective therefore is to assess the potential importance of lateral mass transfer in the process of intracratonic rifting and crustal evolution. The Carlton County (CC) and Duluth dike swarms, located in and around Duluth, MN, are geographically proximal but distinct in age, strike pattern, and chemical composition. The reversely polarized (older) CC dikes intrude Paleoproterozoic metagreywackes, and are subparallel in mapview. The younger, normally polarized Duluth dikes intrude MRS volcanics, and strike somewhat irregularly. The mean orientation measured for the CC swarm is N28°E, 89°NW. Duluth swarm’s mean orientation again strikes N28°E, although more variably, with shallower and more diverse dip directions ranging from ca. 70°NW to 70°SE (planes, Fig. 1). The dike geochemistry is consistent with a mantle plume source with some input from partial melting of the continental lithosphere (Seifert & Olmsted, 2004). Both dike swarms are basalt to basaltic andesite in composition. The CC dikes are predominantly quartz tholeiite, whereas the Duluth dikes are predominantly olivine tholeiite, with major primary minerals in both swarms including: plagioclase, olivine, augite, magnetite, and ilmenite (Reichhoff, 1987). The dikes in both swarms appear massive in outcrop and hand sample and no flow structures such as stretched vesicles were observed, making more traditional, macroscopic flow fabric measurement impossible. AMS, defined by preferred orientations of both crystallographic axes and shape preferred orientations of individual mineral grains and grain clusters, provides for delineation of rock fabrics in such macroscopically isotropic rocks (Rochette et al., 1992). The AMS technique is powerful and can measure the orientations of hundreds of grains in minutes, compared to hours of measurement using other approaches (Borradaile & Henry, 1997). This technique has been successfully applied to magma flow in dikes in studies since Knight & Walker (1988) empirically showed that AMS fabric is equal to magma transport direction in Hawaiian dikes. Since then, a basic understanding of the technique has been developed, and AMS is now accepted as a robust geologic tool for defining rock fabrics (Cañón-Tapia, 2004). We collected two oriented samples (at differing distances from the dike margins) from 26 dikes (13 from each swarm). To date, 21 oriented samples from 19 dikes have been analyzed. For each sample site an average of 16 cubes were analyzed, with a minimum of 9 per site. AMS measure-ment of each cube produces a magnetic susceptibility ellipsoid, represented by a second-order tensor describing the length and orientation of three principle axes, K1 > K2 > K3. K1 gives the average magnetic lineation, used as a proxy for magmatic flow fabric. K1 and K2 determine the plane of average magnetic foliation (Rochette et al., 1992; Ferré et al., 2004). Measurements were made using a Kappabridge KLY4-S susceptibility bridge at field intensity 300 A/m. The AMS results have a bulk magnetic susceptibility of about 1-3 × 10-2 SI, meaning that the magnetic signal is likely dominated by the fabric of ferromagnetic phases. The AMS signal is robust as the bulk magnetic susceptibility is well within the instrument sensitivity (2×10-8 SI at 300 A/m; Pokorný et al., 2004). The degree of magnetic anisotropy is on average 1.04 with a standard 25 �deviation of 0.03 for both swarms. Site means for 12 dikes in the CC swarm and 9 dikes in the Duluth swarm are plotted (Fig. 1). The open triangles indicate the swarm mean of the K1 site means, yielding average magnetic lineation orientations for each swarm. Results for the CC swarm cluster relatively consistently and include a magnetic lineation that is sub-vertical. In contrast, results for the Duluth swarm are complex and give poor statistics; however overall the Figure 1. Site averaged AMS results for all data collected to date in lower hemisphere stereoplots for each swarm (CC left; Duluth right): K1=squares; K2=triangles; K3=circles. Mean dike orientation for each swarm is also plotted. magnetic lineations are sub-horizontal or inclined. The preliminary AMS measurements suggest differing magmatic emplacement directions in the two dike swarms. The CC swarm was emplaced predominantly vertically, and the Duluth dike swarm displays more variable, oblique emplacement patterns, which may be influenced by inverse magnetic fabrics from single-domain magnetite in the Duluth swarm rocks. To independently corroborate these results, image analysis will be performed on three mutually perpendicular, oriented thin sections of a number of our samples using a software package to quantify mineral alignment as a proxy for magma flow. Borradaile & Henry, 1997, Tectonic applications of magnetic susceptibility and its anisotropy, Earth-Science Reviews, 42, 49-93. Cañón-Tapia, E., 2004, Anisotropy of magnetic susceptibility of lava flows and dykes: A historical account, from: MartínHernández, F., Lüneberg, C.M., Aubourg, C., & Jackson, M. (eds.) Magnetic Fabric: Methods and Applications, Geological Society, London, Special Publications, 238, 205-225. Ferré, E.C., Martín-Hernández, F., Teyssier, C., & Jackson, M., 2004, Paramagnetic and ferromagnetic anisotropy of magnetic susceptibility in migmatites: Measurements in high and low fields and kinematic implications, Geophysics Journal International, 157, 1119-1129. Knight, M.D. & Walker, G.P.L., 1988, Magma flow directions in Dikes of the Koolau Complex, Oahu, Determined from Magnetic Fabric Studies, Journal of Geophysical Research, v. 93, p. 4301-4319. Pokorný, J., Suza, P., & Hrouda, F., 2004, Anisotropy of magnetic susceptibility of rocks measured in weak magnetic fields using the KLY-4S Kappabridge, in: Martín-Hernández, Lüneberg, Aubourg, & Jackson, M. (eds.) Magnetic Fabric: Methods and Applications, Geol. Soc. London Spec. Pub 238, 69-76. Reichhoff, J., 1987, Two Keweenawan basaltic dike swarms in the Duluth area, Minnesota, Unpublished M.S. Thesis, University of Minnesota Duluth. Rochette, P., Jackson, M., & Aubourg, C., 1992, Rock magnetism and the interpretation of anisotropy of magnetic susceptibility, Reviews of Geophysics, v. 30, p. 209-226. Seifert, K.E. & Olmsted, J.F., 2004, Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example – the 47th Avenue sill, Can. J. Earth Science, 41, 829-842. This research is supported by ILSG, GSA, Sigma Xi, and Kent State University. 26 �Student Capstone Map from the UMD Precambrian Research Center’s Field Camp: Bedrock Geology Map of Homer Lake and Adjacent Areas, Cook County, Minnesota Shelby J. Frost1, Natalie A. Juda2, and Jim Miller3 1) Winona State University, Winona, Minnesota; 2) Macalester College, St. Paul Minnesota (presently at Golden Chalice Resources, Timmins Ontario); 3) Department of Geological Sciences, University of Minnesota Duluth One of the four capstone mapping projects conducted during the fifth week of UMD’s Precambrian Research Center’s 2007 summer field camp focused on an incompletely mapped Keweenawan gabbroic complex in far northeastern Minnesota (Fig. 1). This extensive gabbroic complex, named the Brule Lake/Hovland gabbro (BLHg) by Miller et al. (2001), occurs as an east-west trending, semicontinuous intrusive unit composed of an unknown number of separate intrusions. The structural position of the BLHg complex is generally above the mafic and felsic intrusions of the Duluth Complex and reversed-polarity lavas of the NSVG and below the Eagle Mountain and Pine Mountain granophyre bodies. Vervoort et al. (2007) reported U-Pb ages for the Eagle Mtn and Pine Mtn granophyres of 1098.6±3.6 Ma and 1095.3 ±3.8 Ma, respectively. Gradational contacts between the BLHg and the overlying granophyres suggest that the BLHg may have a similar age of emplacement. These possible ages and the high structural position of the BLHg are consistent with this intrusive complex being more similar to the Beaver Bay Complex (~1096 Ma) than to the deeper-seated and older Duluth Complex (1099-1108Ma, Paces and Miller, 1993). The western extent of the BLHg, which is where our mapping was focused (Fig. 1), was first reconnaissance mapped by Grout in the 1930’s and 40’s. Most of his mapping was published in MGS Bulletin 39 (Grout et al., 1959) as a series of small-scale (~1:100,000) township maps that show the generalized outcrop distribution of various rock types. In the Homer Lake area (T63N, R3W), Grout et al. (1959, Figure XXIV) documented the occurrence of locally foliated and layered gabbro, oxide gabbro, anorthositic gabbro, intermediate rocks, basalt, Figure 1. Generalized geology of northeastern Minnesota and hornfels. Davidson and showing the location of the field area. Intrusive units Brunell (1977) published mentioned in the text are labeled. another reconnaissance map of the area that focused mainly on the geology of the Brule Lake area, to the north of Homer Lake. Curiously, the geology of the Homer Lake area shown on the 1:24,000 scale Davidson and Burnell (1977) map does not show the various rock types that Grout et al. (1959) identified ringing Homer Lake. In fact, only one outcrop is shown in the Homer Lake area, which proved to misidentified. This suggests that these Davidson and Burnell did not actually map in this area. 27 �During the capstone week, we worked out of a base camp on Homer Lake just outside the Boundary Waters Canoe Area Wilderness (BWCAW) and made daily forays into the field by canoe. We completed five full days of field mapping as a single party of three. Jim Miller had also done two days of mapping in the previous summer to preview the area for the capstone project. Most mapping concentrated on the shoreline exposures of Homer, Axe and Whack Lakes. During the field work, over 110 outcrop stations were observed, the orientation of 50 structures were measured, and 41 handsamples were collected (petrographic study of these samples are planned for this spring). The field data were used to interpret a map area of about 3 miles, east to west, by 1 mile, north to south. In the reconnaissance map of Davidson and Brunell (1977), which was directly integrated into the regional Duluth Complex map (Miller et al., 2001), this area was interpreted to be underlain largely by gabbroic anorthosite related to the anorthositic series of the Duluth Complex. They show olivine gabbro in the northern part. Our mapping showed there to be few if any anorthositic series lithologies in the area, except for small inclusions. Rather, we found the area to be dominated by variably foliated oxide gabbro that dips moderately (15° - 35°) to the south . The most prominent and traceable feature in the area is sheet of hornfels basalt overlain by hornfels cross-bedded, volcanogenic sandstone. This volcanic-sedimentary unit can be traced along the north shore of Homer Lake for over two miles of strike length and forms a 100m thick screen that separates two distinct conformable gabbro sequence. The intrusive rocks north of (underlying) the screen is composed of mafic to intermediate rock types grouped as the Axe Lake gabbroic sequence. The gradual progression of mafic (diabase and oxide gabbro) to intermediate (ferromonzodiorite to granophyric gabbro) rock type from north to south (upsection) implies that this may represent a differentiated sequence formed by in situ fractional crystallization. South of (overlying) the volcanic screen is a sequence of oxide gabbro to gabbronorite that alternate between coarse-grained, poorly foliated and medium-grained, well foliated textures. This is informally termed the Homer Lake gabbroic sequence. What is perhaps the most interesting result of our mapping is that, while it shows Davidson and Brunell’s (1977) interpretation to be incorrect, it actually verifies the reconnaissance mapping of Grout et al. (1959). The main criteria for selecting a particular area for a capstone mapping project for the Precambrian field camp is that it be a generally a poorly mapped, but well exposed area, which previous reconnaissance mapping has shown to have a diverse and interesting geology. The Brule Lake/Hovland gabbro is one of the last poorly mapped parts of the greater Duluth Complex. Because of its location in and near the BWCAW, the BLHg will likely only become well mapped by academic-based mapping. To tease out the detailed geology of the BLHg, we plan to incrementally expand the map coverage of this intrusive complex over the coming years of the Precambrian field camp. Next year, we plan to work north and west of the current map area. References Davidson, D.M, Jr., and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-29, scale 1:24,000 Grout, F.F., Sharp, R.P., and Schwartz, G.M, 1959, The geology of Cook County Minnesota. Minnesota Geological Survey Bulletin, v. 39, 163 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Vervoort, J.D., Wirth K., Kennedy, B., Sandland , T., Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research 157, p. 235–268. 28 �REINTERPRETATION OF THE TRACE FOSSIL-BEARING DEVILS ISLAND SANDSTONE, KEWEENAWAN RIFT, NORTHERN WISCONSIN GALSTON, Lynn, Geology, University of Wisconsin - Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702, galstolm@uwec.edu and HAVHOLM, Karen G., Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54702 and HASIOTIS, Stephen T., Department of Geology, University of Kansas, Lawrence, KS, 66045 The Devils Island Sandstone is a fine-grained quartz arenite that is a late-stage Keweenawan rift-fill deposit. The unit is exposed on the coast of Lake Superior creating cliffs and sea caves on Sand and Devils Islands as well as the shoreline between Menard Road and Sand Point. There are also limited exposures on the Brule, Iron, and Siskiwit Rivers. Both the Devils Island Sandstone and the Outcrop locations, modified from Adamson, 1997 stratigraphically equivalent Hinckley Sandstone were interpreted as a shallow lacustrine environment (Tryhorn and Ojakangas, 1972; Adamson, 1997). Identification of adhesion structures in the Hinckley Sandstone, however, indicates an environment that experienced subaerial exposure. Further studies of sedimentary structures in the Hinckley Sandstone indicate a depositional environment ranging from fluvial to eolian dune and interdune (Johnson, Beaster, Kohn and Havholm, 2001). This revised interpretation of the Hinckley Sandstone prompted re-evaluation of the Devils Island Sandstone. Outcrops of the Devils Island Sandstone have been studied in some detail. A boat was used to access many of the outcrops, and stratigraphic sections were measured and described. Four out of five facies identified are similar to facies within the Hinckley Sandstone. Tangential cross-strata indicate a predominantly eolian dune environment with localized subaqueous features. Cross-strata dip primarily to the northeast, east and southeast. Trough cross-strata with rip-up clasts and coarser sand grains, including granules and pebbles in some localities, suggest a fluvial environment. Trough orientations indicate paleocurrent directions primarily to the northeastern quadrant. Sandy planar bedding contains sedimentary structures reflecting an environment of deposition that varied from dry (wind-ripple strata) to damp (adhesion structures) to wet (subaqueous ripple forms). Convolute bedding, including fluidescape structures and cm- to dm-scale faults as well as folds, appears to have resulted from rapid deposition onto a saturated substrate. These facies suggest a fluvial and dune-interdune environment. The fifth facies—silty planar bedding—contains laterally continuous, thinly laminated fining-upward sequences containing mm- to cm-scale silt-sand couplets with rippled to irregular silty capping laminae and, locally, tool marks and horizons with mudcracks. This facies indicates a predominantly subaqueous environment with fluctuating energy levels, possibly lacustrine. 29 �At one locality within the silty planar facies trace fossils are present including 1) curved to sinuous and Y-branching, cm-wide, meandering traces, and 2) sub-mm to mm-scale sinuous trails that overlap and cross over larger traces. Identification of trace fossils within the Devils Island Sandstone could serve either to constrain its age to Late Proterozoic, rather than Mid-Proterozoic, or provide rare evidence of Mid-Proterozoic multicellular life. REFERENCES Adamson, K.F., 1997, Petrology, Stratigraphy and Sedimentation of the middle Proterozoic Bayfield Group, northwestern Wisconsin. Unpublished Masters Thesis, University of Minnesota, 203pp. Johnson, A.D., Beaster, K.F., Kohn, J.D. and Havholm, K.G., 2001, Braided-stream/eolian environment of Proterozoic Hinckley Sandstone, Keweenawan rift, east-central Minnesota. Geological Society of America Abstracts with Programs, v. 33, p. A229. Tryhorn, A.D., and Ojakangas, R.W., 1972, Sedimentation and petrology of the upper Precambrian Hinckley Sandstone of east-central Minnesota. In P.K. Sims and G.B. Moey, eds., Geology of Minnesota: a Centennial Volume, Minnesota Geological Survey, Minneapolis, p. 431-435. 30 �Structural and Kinematic analysis of the Kawishiwi shear zone, Superior Province: Insight on granite-greenstone terrain tectonics and Archean crustal evolution Sally Goodman, University of MN Duluth, Dept. of Geological Sciences, good0491@d.umn.edu Two conflicting hypotheses have been invoked to explain the formation of Archean (~2.7 Ga) granite-greenstone terrains: 1) granite-greenstone terrains are collapsed back-arc or sutured forearc basins of subduction-driven accreted terranes, or 2) granite-greenstone terrains formed by density-driven sagduction of an insulating greenstone cover and concurrent rising of granite diapirs. Granite-greenstone terrains commonly preserve shear zones, which may reveal a history of Archean crustal evolution and tectonic processes. A structural and kinematic analysis of the Kawishiwi Shear Zone (KSZ), within the Vermillion District, Superior Province, was conducted to evaluate the tectonic processes of granite-greenstone formation. The KSZ, broadly bounded by the Vermillion Granitic Complex to the north and the Giants Range Batholith to the south. forms one of several east-striking, steeply dipping shear zones in the Vermillion District. Metamorphism of regional rocks is generally in greenschist and amphibolite facies. The ~30km KSZ length is truncated by younger (1.1 Ga) Keeweenawan intrusive rocks in the southeast and the north-striking Waasa fault in the west. Locally, the KSZ lies within Ely Greenstone; Knife Lake Group graywackes, tuffs, and conglomerates; and the Giants Range Batholith to the south and eastern extent. In order to understand the KSZ kinematic and structural evolution, major data collection and analysis included: 1) metamorphic foliation, elongation lineation and pitch domains from field mapping the eastern 24km x 10km KSZ and also existing geologic maps, 2) outcrop-scale and microstructural kinematic analysis from oriented rock samples, quartz c-axis petrographic fabric analysis, and 3) general metamorphic history from lithology and thin section petrography. The data show that the KSZ is ~3-5 km wide and steeply dipping; it has an average foliation of 253, 89 , and a regionally dominant steeply plunging mineral elongation lineation with some gently plunging lineations (fig, A). Pitch values are bimodal; pitch domains show a distributed region of high pitch values and a narrow zone of low pitch values along strike of the KSZ (fig, B). The pillowed Ely Greenstone commonly does not have a foliation or lineation, likely due to strain partitioning around pillows. Of 55 oriented thin sections, 21 had discernible asymmetric fabrics. Asymmetric fabrics occur in foliation perpendicular, lineation parallel planes and symmetric fabrics occur in foliation perpendicular, lineation perpendicular planes; therefore, mineral elongation lineations are parallel to the motion plane or vorticity normal section, which is consistent with noncoaxial shear with lineation forming parallel to shearing. It follows, in the regions with high pitch values, the displacement is vertical to oblique and shear recorded was both north-side-up and southside-up: along the narrow zone with shallow pitch, displacement is consistently dextral strike-slip. The structural and kinematic results point to two displacement events in the KSZ: one of distributed ductile dip-slip shear (both north- and south-side-up), and a subsequent, more focused dextral strike-slip shear. The distributed and dominant dip-slip shear favors the sagduction-diapirism hypothesis whereby a high Archean geotherm dictates widespread density-driven ductile deformation of rising granites and sinking greenstone/sediment packages. Green, J.C., Phinney, W.C., Weibler, P.W., 1966. Geologic Map of Gabbro Lake Quadrangle, Lake County, MN. MNGS: M-2, scale 1:31,680. Green, J.C. and Schulz, K.J., 1982. Geologic Map of the Ely Quadrangle, St. Louis and Lake Counties, Minnesota. MNGS, M-50, scale 1:24,000. Jirsa, M.A. and Miller Jr., J.D., 2004. Bedrock Geology of the Ely and Basswood Quadrangles, Northeast Minnesota. MNGS, M-148, scale 1:100,000. 31 �32 �MN DNR DRILL CORE EVALUATION PROJECT – THE APPLICATION OF AN XRF TO ELUCIDATE GOLD MINERALIZATION IN THE VERMILION GREENSTONE Hanson, A.E. and Frey, B.A., Minnesota Department of Natural Resources, Lands and Minerals Division, 1525 3rd Ave. E. Hibbing, MN 55746 The 2.7 Ga Vermilion Greenstone Belt is located in the southern extensions of the Wawa Subprovince, within the Superior Province of the Canadian Shield. It has a diverse setting that consists of volcanic-dominated lithologies including calc-alkaline and tholeiitic basalt flows, dacitic tuffs and banded iron formations (Larson 2004). This project focuses on a 90 square mile area of the Vermilion district from Tower, MN to just east of Ely, MN. As in the Canadian equivalents, there is evidence of lode gold deposits and volcanogenic massive sulphide deposits (Williams et al., 1991), however none of these is currently being mined in Minnesota. The main focus of this project is to look for gold mineralization potential. This includes the establishment of gold mineralization indicators, and mineralization patterns. To date, more than thirty drill holes have been logged, with additional samples taken for analyses. As the cores were being logged, the Innov-X Vacuum Portable X-Ray Fluorescence spectrometer* (XRF) was used to complement our other activities. The XRF provides us with real-time chemistry data and allows for quick correlation between chemistry, as well as the visible and invisible, core or cuttings features. The XRF input was also useful for indicating sample intervals for further chemistry work. The goal of this project focuses on sharing the information we obtain about the gold mineralization. The information will be provided in a digital format online, including GIS maps. There are some characteristics of the Innov-X XRF that we have to consider carefully, many of which involve interference. The XRF analyzes for multiple elements in one measurement. Hardware, software, and physics constraints require that trade-offs occur when obtaining results, however. Examples of interference include sample matrix effect, chemical matrix effect, instrument resolution, and calibration. The sample matrix effect takes the heterogeneity and/or homogeneity of the sample into account. The window through which the X-Rays are emitted is about ½ cm2, allowing only a small area to be analyzed. When we are examining drill core, this may be an advantage or disadvantage, depending on the sample characteristics. For example, we can analyze a small, unidentified grain, or do numerous analyses on an altered area leading up to a vein in order to see the chemical changes. This may be a disadvantage if we are trying to obtain a general composition of the matrix of the core. The XRF has been subject to chemical matrix effects, which occur as XRay absorption and enhancement phenomena, both of which involve iron in this case. We may have had lower than actual amounts of copper and elevated amounts of chromium due to the presence of iron. The effect of instrument resolution manifests when two elements have similar energy peaks, so it is especially prevalent with light elements. We cannot use the XRF for exact results because it is hard to recognize when this effect occurs. Calibration is extremely important, and can be manipulated in various ways to obtain various results, such as detection of lighter elements, or more accurate readings for specific elements. IF all samples have similar compositions, then calibration can be tweaked to optimize accuracy. This option is not available for a project such as ours with a wide range 33 �of greenstone belt rock compositions. Our XRF is calibrated for the optimized analysis of 20 transition metals specifically. The adjustable sampling time interval can be increased to increase sensitivity and lower the detection limit, but this also reduces the numbers of samples analyzed. The use of standards is important, and their general composition should approximate that of the samples, if possible. The software outputs analytical values for respective elements along with a “+/-“ value. The latter value will vary as the entire sample composition varies. Analytical values will not be produced unless analytical values are approximately 3 times the “+/-“ value, or more. The XRF may have a 1 ppm detection limit for gold, but it will not produce analytical values for a “+/-“ value of 3ppm, unless the sample actually has about 10 ppm Au. A lack of analytical values does not preclude gold amounts of interest. Overall, the XRF is a very useful real-time guide, but we will not use it as a precise quantitative instrument. Interesting Results. The XRF was first used with our previous work in the Virginia Horn near Gilbert, MN. In that case, we found elongate as opposed to equigranular arsenopyrite crystals, within the altered dacitic porphyry, which was found to correlate strongly with elevated gold amounts. The approximate ½ cm2 XRF sampling area is ideal for looking at the detailed mineral association related to gold mineralization. While working in the Vermilion area near Ely at the Raspberry Prospect, XRF analyses indicated that galena or rutile were the associations of choice for gold. Other areas of elevated base metals (such as molybdenum, copper, or zinc) lacked gold. Gold was also not arsenopyrite associated. XRF analyses also indicate the apparent regional tendency for fractures and shears to be associated with anomalous mercury. The XRF does not replace geologic observations, or laboratory assay and analyses, but it offers us one more portable tool in our arsenal. *The MN DNR does not endorse this product. The manufacturer’s name appears herein solely because it is considered essential to this report. References Larson, P.C., 2004, Regional Till Sampling of the Western Vermilion Greenstone Belt, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2004/23, 33p., 1 plate. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L., and Sage, R.P., 1991, Wawa Subprovince, in Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Geology of Ontario: Ontario Geological Survey Special Volume 4, Part 1, p. 485-539. 34 �IRON-FORMATION-HOSTED GOLD IN THE SUPERIOR PROVINCE OF NORTHWESTERN ONTARIO HILL, MARY LOUISE, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1 and CHEATLE, ANDREW, Chief Geologist, Musselwhite Mine, Box 7500, Thunder Bay, ON, Canada P7B 6S8 At Musselwhite Mine and in the Beardmore-Geraldton area, Archean iron formation hosts significant gold mineralization. Both Musselwhite and Beardmore-Geraldton lie within metavolcanic and metasedimentary belts of the Superior Province. In both locations, the iron formation is complexly folded and metamorphosed to the upper greenschist or lower amphibolite facies. These deposits can be compared to Archean iron-formation-hosted gold deposits that were mined at Homestake Mine, ND and Lupin Mine, NWT. What is the significance of iron formation as a host to gold mineralization in these deposits? Deposit-scale analysis at Musselwhite demonstrates that gold mineralization was synchronous with amphibolite facies metamorphism and deformation. The deposit is giant in scale (more than 2 million ounces mined to date and 2 million ounces remaining in mineral reserve) with an average grade of approximately 5.5 g/t. Gold occurs primarily in a semi-pelitic unit within the iron formation, with the richest ore typically found at intersections of narrow high-strain shear zones with this particular lithology. Heterogeneity of metamorphism and deformation within the iron formation appears to be a key factor in mineralization. Strain incompatibilities caused by competency contrasts at various scales have produced enough transient permeability to allow localized fluid migration and mineralization. In the Beardmore-Geraldton belt a network of anastomosing shear zones separates regional-scale lithons of metasedimentary and metavolcanic rock metamorphosed to the greenschist facies. The shear zones are steeply dipping with nearly horizontal transcurrent displacements. Here again, gold mineralization is linked to high-strain deformation zones. At this grade of metamorphism, iron formation has deformed in a more ductile manner than adjacent metavolcanic lithologies, and folding is common. Competency contrasts between lithologies are more pronounced during deformation at these temperatures, and this is significant to mineralization potential. Historic mining in this area has produced over 4 million ounces of gold in total, but no single mine has been a giant producer. Relationships seen at Musselwhite and other iron-formation-hosted gold deposits could have important implications for exploration in this area. The most significant contribution of iron formation to gold mineralization in Archean metavolcanic and metasedimentary belts may be the heterogeneities in deformation produced during synchronous metamorphism and deformation of contrasting lithologies in brittle-ductile shear zones. 35 �Whatever happened to the Logan sills? Ongoing research into the geochemistry of Midcontinent Rift-related mafic intrusive rocks south of Thunder Bay HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada. Until recently, the Mesoproterozoic Midcontinent Rift (MCR)-related gabbroic sill complexes around Thunder Bay and the Nipigon Embayment were described as the Logan Sills and considered to be a relatively homogeneous package of rocks (Stockwell et al., 1972). Recent work as part of the Lake Nipigon Region Geoscience Initiative has shown that the sills in the vicinity of Lake Nipigon can be subdivided into a number of discrete units with distinct geochemical and isotopic characteristics (Hollings et al., 2007a,b; Heaman et al., 2007). None of these sills were thought to have been part of the Logan Sill suite, which was restricted to the area south of Thunder Bay (Hollings et al., 2007a). As part of an ongoing investigation of the MCR-related rocks south of Thunder Bay, samples of gabbroic sills and dykes were collected and analysed in order to further characterize these spatially distinct suites. The sills share a number of common characteristics. Like the sills of the Nipigon embayment, the majority of sills from this study are characterized by negative Nb anomalies, consistent with pervasive crustal contamination at depth. The 2007 reconnaissance sampling shows that the southern sills can be divided into at least two, and possibly three, distinct suites. A mafic to ultramafic sill was identified in the Riverdale area in 2006 (Hollings et al., 2007c) and characterized by elevated Gd/Ybn ratios (Fig. 1), comparable to the Hele and Disraeli intrusions of the Nipigon embayment. Present levels of sampling suggest that this “Riverdale sill” is restricted to a relatively small area. If the samples are from a single sill, then it must be at least 60 m thick. At one location, the Riverdale sill is cut by a ~1 m wide vertical dyke that is geochemically similar to the Nipigon sills. Figure 1: Major and trace element variation diagrams illustrating the geochemical affinities of the sills south of Thunder Bay. Data from Logan sills are from Hart (2003, 2005). Nipigon data are from Hollings et al. (2007a). Data for dykes south of Thunder Bay are from L. Hulbert and R. Ernst (Geological Survey of Canada, pers. comm. 2006) from samples collected by M. Smyk and J. Scott (Ontario Geological Survey), Hollings et al. (2007c) and this study. 36 �A second sill suite is characterized by Gd/Ybn ratios between 2 and 2.5, comparable to the Jackfish and McIntyre sills of the Nipigon Embayment. However, this suite, like the Logan sills is characterized by somewhat higher La/Smn ratios (Fig. 1). The elevated TiO2 values of this sill are also consistent with the values reported for the Logan sills (Fig. 1). The third suite is geochemically similar to the main Nipigon suite of the Nipigon Embayment (Fig. 1). It is characterized by lower La/Smn ratios and lower TiO2 values for a given content of MgO. A single sample within this suite, collected from a ~1m thick sill that crops out in a package of Rove shale lies within the field of the Inspiration sills and may represent a distinct suite but additional sampling will be required to validate this. This sampling, planned for 2008, will also provide more comprehensive coverage of all the mafic intrusions south of Thunder Bay. This will help to elucidate boundaries between sill suites and individual intrusions and provide insight into mafic magmatism along the northwestern margin of the MCR. References Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; ILSG, Proceedings Volume 49, Part 1-Programs and Abstracts: 21-22. Hart, T.R. 2005. South Black Sturgeon River–Seagull Lake Area, Nipigon Embayment, Northwest Ontario: Lithogeochemical, Assay and Compilation Data. Ontario Geological Survey, Miscellaneous Release of Data 147. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., MacDonald, C.A., and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, 44, 1055-1086. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A., 2007c. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1087-1110. Hollings, P., Richardson, A., Creaser, R., and Franklin, J., 2007b. Radiogenic isotope characteristics of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1111-1129. Hollings, P., Smyk, M., and Hart, T., 2007c. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes. In Woodruff, L (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology 53rd Annual Meeting, Proceedings Volume 53, Part 1 – Program and Abstracts, 40-41. Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie, K.L. 1972. Geology of the Canadian Shield. In Geology and economic minerals of Canada. Edited by R. Douglas. Geological Survey of Canada Economic Geology Report 1: 838. 37 �STABLE ISOTOPE GEOCHEMISTRY OF THE MUSSELWHITE AU MINE, N. ONTARIO: IMPLICATIONS FOR MINERALIZATION ISAAC, Carissa, Lakehead University, Thunder Bay, Ontario cisaac@lakeheadu.ca and HOLLINGS, Pete, Department of Geology, Lakehead University peter.hollings@lakeheadu.ca The Musselwhite mine is located on the south shore of Opapimiskan Lake, approximately 720 km north of Thunder Bay. Hosted in the ~2.8Ga North Caribou Lake greenstone belt of the Sachigo Superterrane, Superior Province, the Musselwhite mine is classified as a shear hosted orogenic gold deposit, with an estimated reserve of 1.78 million ounces Au (Goldcorp, 2008). The amphibolite grade metabasalts and komatiitic metabasalts that host the mine have elevated gold concentrations with the highest gold grades hosted in iron formation contained within the volcanic pile(Hall and Rigg, 1986). Oxygen isotope signatures from quartz samples from Musselwhite range from +12 to +14.7 per mil. These values are consistent with previous work by Otto (2002) and indicate fluid compositions that fall within both the magmatic and metamorphic range. The large overlap between the magmatic and metamorphic fluid oxygen isotope signatures have led researchers to use nitrogen isotopes to investigate the source of gold bearing fluids in orogenic gold deposits (Pitcairn et al., 2005; Jia, 2000). Nitrogen substitutes for K+ as NH4+ in potassic minerals and has the advantage over oxygen and hydrogen isotopes in that it is present at low abundances in rocks. This reduces the effects of re-equilibration during successive thermal events such as metamorphism. Twenty biotite and 30 quartz samples have been analyzed for δ15N, δ18O and δD from the Musselwhite mine as well as 12 biotite samples from the surrounding granitoid rocks of the North Caribou Lake Greenstone Belt. Nitrogen isotopes in biotite from the Musselwhite mine are characterized by a δ15N range from 1.3 to 11.1 per mil, whereas biotite samples from granites and metasedimentary rocks adjacent to the deposit have a δ15N range of -6.9 to +6.1 per mil. The range of nitrogen isotopes at Musselwhite suggests mixing between magmatic and metamorphic fluids (Figure 1). Oxygen and hydrogen isotopic ranges for the granitoid plutonic rocks are +2.0 to +4.0 per mil and -59 to -80 per mil respectively; these values are typical of felsic plutonic rocks (Taylor, 1974). Oxygen and hydrogen isotopes of biotite samples from the mine range from +7.7 to +9.6 per mil for oxygen and -85 to -103 per mil for hydrogen. The anomalously low δD signature could be influenced by: 1) the iron content of the biotites, 2) degassing of the deposit at hypabyssal levels, 3) biologic activity. In conclusion, the stable isotopic data generated for the Musselwhite Mine suggest the possibility for fluid mixing and indicate that magmatic fluids may have had a role in transporting gold. References: Goldcorp, 2008. Goldcorp, inc. website – Musselwhite. http://www.goldcorp.com/operations/musselwhite/ Hall, R. S. and Rigg, D. M., 1986. Geology of the West Anticline Zone, Musselwhite Prospect, Opapimiskan Lake, Ontario, Canada. In Macdonald, A. J. Gold '86; an international symposium on the geology of gold deposits, pp. 124 -136. Hill, M.L.; Cheatle, A.; Liefrovich, R., 2006. Musselwhite Mine: an orogenic gold deposit in the Western Superior Province; In Programs with Abstracts, Geological Association of Canada Vol. 31, pp. 67. Jia, Y., 2000. Giant quartz vein systems in accretionary orogenic belts; the evidence for a metamorphic fluid origin from delta (super 15) N and delta (super 13) C studies,: Earth and Planetary Science Letters, December 30, 2000, Vol. 184, Issue 1, pp. 211-224. 38 �Otto, A. 2002. Ore forming processes in the BIF-hosted gold deposit Musselwhite mine, Ontario, Canada. M.Sc. thesis (unpublished) Freiberg University of Mining and Technology, Germany, 86 pp. Pitcairn I. K.; Teagle, Damon A. H.; Kerrich, R.; Craw, D.; Brewer, T. S.2005. The behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization; evidence from the Otago and Alpine schists, New Zealand , Pitcairn, I. K. In: Earth and Planetary Science Letters, April 30, 2005, Vol. 233, Issue 1-2, pp.229-246 Taylor, H. P., Jr. 1974. The Application of Oxygen and Hydrogen Isotope Studies to Problems of Hydrothermal Alteration and Ore Deposition; Economic Geology and the Bulletin of the Society of Economic Geologists, Vol. 69, Issue 6, pp.843-883 Figure 1: Plot of Nitrogen isotope date for the Musselwhite mine. Stars represent samples from Musselwhite Mine and hexagons represent regional granitoid samples. Fields for comparative data sets from Jia, 2000; Gsed = greenschist metasedimentary rocks, LG = lower Greenschist rocks, MG UG = middle-upper Greenschist rocks, GAT = Greenschist-Amphibolite Transition zone, LA = lower Amphibolite rocks, UAsed = upper Amphibolite metasedimentary rocks. 39 �“CAPSTONE” GEOLOGIC MAPPING NEAR GABIMICHIGAMI LAKE, BOUNDARY WATERS CANOE AREA WILDERNESS, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2007 FIELD CAMP Jirsa, Mark A., (jirsa001@umn.edu); STARNS, Edward, Costello, Daniel E., Gal, Benedek, Hoaglund, Steven A., and Putz, Amanda J. The Precambrian Research Center—a branch of the University of Minnesota, Duluth—conducted its first season of field camp in 2007. After 5 weeks of field training, students were assigned “Capstone Projects” that provide an opportunity to create new geologic maps in areas of poorly understood geology. The four students listed above worked with the first and second authors to map in an area of the 2006 Cavity Lake forest fire in the northeastern part of the Boundary Waters Canoe Area Wilderness. The fire was the delayed result of a mega-storm in 1999 that flattened thousands of acres, reducing the woods to a tangle of downed trees and stubby new growth. The fire greatly improved access, and the subsequent rains cleaned outcrops to reveal details about the bedrock geology not previously visible. Gabimichigami Lake straddles the contact between Neoarchean and Mesoproterozoic rocks. Specifically, the mapping occurred along the north, west and east shores of the lake in parts of the U.S.G.S. Gillis Lake and Ogishkemuncie 7.5-minute quadrangles. Previous mapping was regional in scale; and although fairly accurate, it lacks detail and is quite dated (Gruner, 1941). Students were divided into two teams, each tackling a different aspect of the geology. One team (Costello and Gal) mapped the basal contact zone of the Tuscarora Intrusion, a component of the Mesoproterozoic Duluth Complex; the other team (Hoaglund and Putz) mapped the structure and stratigraphy of Neoarchean volcanogenic sedimentary rocks that form the intrusion’s footwall. The map and geologic descriptions presented here are based solely on the observations from several days of field work. Although some samples were taken, no petrographic or geochemical data were acquired. The Neoarchean rocks consist of volcanic, volcaniclastic, and intrusive rocks informally called the Jasper Lake sequence. Regionally, the sequence consists of hornblende trachyandesitic flows and volcanogenic sedimentary strata derived from them, all intruded by dikes of similar composition. Though folded, the sequence appears to be generally southwestward-younging. It grades irregularly up-section from pyroclastic flows and breccia, to a thick package of poorly stratified volcanic conglomerate, to interbedded conglomerate, sandstone, and mudstone. Individual units of volcanic conglomerate are clastsupported, poorly sorted, and locally contain an abundance of one textural and compositional variant of trachyandesite over another. Taken together, the monolithic composition, generally poor sorting, disorganized bedding, and presence of dikes of similar composition imply that the poorly stratified conglomeratic units represent volcanic debris flows. The conglomeratic package grades irregularly to the south and west into a package of interbedded, moderately well-sorted conglomerate and volcanic sandstone, which grades further to predominantly volcanic graywacke, having graded bedding and mudstone drapes indicative of turbidite deposition. Mapping at Gabimichigami Lake was largely within the latter, presumably water-lain part of the sequence. Bedding dips steeply, and locally is overturned slightly to the south. Stratigraphic younging deduced from graded beds defines tight to open folds having northwest-trending axes, and plunges that range from northwest to southeast. No axial-planar cleavage was observed. Conglomerate layers in this package are white to brownish-gray, and clast-supported. Clasts are moderately to well rounded, moderately well sorted, and range from 1 cm to 20 cm in diameter. Clasts consist of porphyritic to aphyric hornblende dacite to trachyandesite, similar to the volcanic rocks near Jasper Lake. The presence of rare clasts of medium- to coarse-grained rocks having the same hornblende-rich composition may indicate erosion from hypabyssal intrusions in the otherwise largely volcanic source region. Graywacke is gray to light gray, fine- to medium-grained, and contains abundant feldspar and hornblende. Graded units of sandstone, siltstone, and mudstone are common. The well 40 �developed graded bedding, flame structures, ball and pillow structures, and mudstone rip-ups indicate that sediment was deposited by turbidity currents. Metamorphic grade in the Jasper Lake sequence appears to be low greenschist facies, except in the narrow contact aureole of the Tuscarora intrusion where biotite, staurolite, and possibly andalusite occur in metapelitic beds. Quartz-chlorite veins having variable trends are common near shear zones and fold axes. . The Mesoproterozoic intrusive rocks of the Tuscarora Intrusion are divided here into 4 units; a basal contaminated zone, a heterogeneous unit, the main augite troctolite, and a northwest-trending troctolite dike. With the exception of the dike, unit contacts, magmatic foliation, and oriented xenolithic inclusions strike northeast and dip shallowly to the southeast. The basal contaminated zone contains abundant inclusions of varied size and composition, including footwall metasedimentary rocks, in a fine-grained granodioritic, noritic, or possibly ultramafic matrix. Disseminated biotite is present near the footwall, and decreases in abundance stratigraphically upward. Mineral lineation and igneous layering are present locally. The basal contact is transitional, magmatically and structurally disturbed and brecciated, and is marked by local intrusions of fine-grained felsic material emplaced into the metasedimentary footwall rocks. The heterogeneous unit consists of fine- to coarse-grained, pegmatite-rich, subophitic augitetroctolite, mela-troctolite, and poikilitic anorthositic troctolite. The texture of the rocks is variable and taxitic. The contact with the underlying contaminated zone is abrupt. The main augite troctolite is medium- to coarse-grained, modally layered, and subophitic. It locally contains pegmatitic pods that are particularly abundant in the lower parts of the unit. Inclusions of poikilitic, troctolitic anorthosite are also present. Disturbed layers of pegmatite and varitextured rock, locally enriched in specific mineral phases (olivine, plagioclase, pyroxene and oxides) are common near the inclusions. The contact with the underlying heterogeneous unit is transitional. The troctolite dike consists of fine- to medium-grained, homogeneous, poikilitic troctolite and subophitic augite-troctolite. Trachytoid texture and weak modal layering occur locally near and parallel to the subvertical contacts. The unit contains small inclusions of poikilitic troctolitic anorthosite, which may have been derived from the Anorthositic Series rocks of the Duluth Complex. A preliminary geologic map of the Cavity Lake fire area that incorporates the student work described here will be available in July, 2008. This map will provide details about both the distribution and relative temporal framework of the Precambrian bedrock. Students enrolled in the 2008 field camp will continue mapping in the region. In addition, one student from the 2007 camp, Dan Costello, will begin thesis field work in the Tuscarora Intrusion during 2008. A final map of the area will be published after completion of these efforts. Support for mapping in the Cavity Lake fire area by the first and second authors is provided by the U.S. Geological Survey’s 2007 State Geologic Mapping Element (STATEMAP) of the National Geologic Mapping Program, and the State Special Appropriation to the Minnesota Geological Survey. REFERENCE Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin 52:1577-1642 41 �SUDBURY IMPACTITE LAYER NEAR GUNFLINT LAKE, NE MINNESOTA Jirsa1, Mark A., Weiblen2, Paul W., Vislova3, Tatiana, and McSwiggen4, Peter L. (1 Minnesota Geological Survey (jirsa001@umn.edu); 2 University of Minnesota-Twin Cities; 3 SUNY College-Oneota; 4 McSwiggen and Associates, St. Paul) We present here a preliminary, largely macroscopic description of rocks inferred to contain ejecta from the ca. 1850 Ma Sudbury meteorite impact event—following the discovery of similar deposits in Thunder Bay, Ontario and Michigan that are well documented by Addison and others (2005), Cannon and Addison (2007), and Pufahl and others, (2007). The Gunflint Lake exposures lie some 480 miles (770 km) west of Sudbury, making this one of the most distant sites known to contain what is considered “proximal ejecta” from the impact. It should be noted at the onset, that only a small portion of the material described here can be considered true ejecta—the great majority of the 7 meter-thick deposit is breccia that consists of thoroughly disheveled fragments that appear to have been derived from subjacent strata. Like the deposits near Thunder Bay, the breccia is sandwiched between Gunflint Iron Formation and sedimentary strata of what has traditionally been assigned to Rove Formation. Unlike deposits near Thunder Bay, the breccia lies within the metamorphic aureole of the Tuscarora and Poplar Lake Intrusions of the Duluth Complex (ca. 1100 Ma), and is intruded by diabasic sills of the Logan Intrusions (ca. 1115 Ma; Heaman and Easton, 2005). Pervasive carbonate mineralization and metamorphism has overprinted and obscured much of the original, delicate mineralogic features, but macroscopic textures and geochemical content that convey information about protolith and depositional mechanisms are preserved. The study of these rocks underway—in the context of the other ejecta sites—will provide a more detailed geologic history of the impact event. Figure 1—Composite and schematic sketch showing stratigraphy of Gunflint Lake impactite adjacent rocks. Approximate ages cited references. Symbols in breccia are broadly representative of fragment type and relative size: closed polygons=chert-rich formation; lines= layered, carbonate-rich formation; solid circles= and from ironironlapilli. To date, the Gunflint Lake breccia has been mapped within the broadly-folded, shallowly south-dipping sequence of Paleoproterozoic strata over a strikedistance of about 1.5 mi (2.4 km). The thickness, internal stratigraphy, and position relative to Logan Intrusions vary considerably along strike; however, the composite section shown in Figure 1 represents the gross sedimentalogical characteristics that are present in many exposures. Most of the deposit consists of very poorly sorted breccia containing blocks and slabs as large as 3 meters having nearly 42 �random orientations. The breccia is crudely graded; largest blocks occur near the base, and smaller and more rounded fragments occur up-section. The contact with underlying iron-formation is abrupt and clearly unconformable, but there is surprisingly little relief on the surface at outcrop scale. The ironformation footwall is not particularly fractured, nor is it ductily deformed: implying that neither seismic brecciation nor soft-sediment deformation, respectively, played a significant role in fragmentation. Most fragments appear to have been derived from the underlying iron-formation, though alteration and metamorphism likely destroyed some of the recognition criteria for smaller clasts in the “matrix” of large breccia blocks. The upper part of the breccia contains small to large blocks locally infilled with concentrically zoned, spherical structures as large as 1.5 cm. that are interpreted to be accretionary lapilli. The lapilli are the only fragments in the deposit presently considered to be part of the impact ejecta blanket—no grains of shocked quartz, microtektites, or other exotic clasts have been identified. A unit of bedded, and locally graded lapillistone and microbreccia caps the megabreccia in many locations. The unit contains both intact and abraded lapilli. It appears to have formed within channel-like depressions on the irregular surface of underlying breccia. The deposit is overlain by interbedded mudstone, limestone, and graded turbidite layers of the basal Rove Formation. The breccia marks an abrupt shift in depositional facies from iron- and silica-rich to iron- and silicapoor sediments, implying a catastrophic origin. Any interpretations at this time are tentative; however, the combination of fragment size, random orientation, chaotic distribution, and apparently local derivation counter-indicates deposition by common mechanisms such as sea cliff erosion or gravity-driven debris flow. Deposition may have been the result of impact-generated tsunamis, or be more directly linked to the turbulent impact ejecta plume. The deposit resembles those described in Thunder Bay and Michigan, to which a similar depositional history has been ascribed. Crude grading in the Gunflint megabreccia implies deposition by a single, intense, and waning event. The cap of bedded lapillistone and microbreccia may be the product of channelized back-flow following tsunami run-up, reworking by secondary or tertiary tsunami waves, or settling during collapse of the ejecta plume. Water depth at the time of impact is problematic—clearly the underlying iron-formation was deposited in fairly deep water, as was the overlying Rove Formation. However, the chaotic breccia and abundance of carbonate mineralization may indicate subaerial exposure during part of the depositional history. The presence of accretionary lapilli in only the upper part of the deposit is also problematic. In the tsunami scenario, this observation could reflect hydraulic sorting that precluded lapilli deposition during wave run-up, and instead selectively washed lapilli onto the brecciated landscape as the wave (s) receded. Many other questions remain, including the nature of the substrate at the time of breccia deposition, the content of exotic (ejecta) fragments, the precise mineralogic and geochemical composition, and the origin, paragenesis, and significance of carbonate. Work to address some of these questions and integrate the results into broader interpretations of the Sudbury impact event is underway. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology 33:193196. Cannon, W.F. and Addison, W.D., 2007, The Sudbury Impact layer in the Lake Superior iron ranges: A time-line from the heavens: ILSG Proceedings, 53rd Annual Meeting, v. 53, Part 1, p. 20-21. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: Constraints from UPb zircon and baddeleyite dating: in Easton, M., and Hollings, P., eds., ILSG Proceedings, 51st Annual Meeting, Nipigon, Ontario, Proceedings and Abstracts, v. 51, Part 1, p. 24-25. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology 35:827-830. 43 �AN ARCHEAN-AGED PGE-BEARING INTRUSION, BARAGA COUNTY, MICHIGAN JOHNSON, R., SEASOR, R. and SUSZEK, T., Rod Johnson and Associates, Inc., 1550 Baldwin Avenue, Negaunee, MI 49866 This paper describes the discovery, geology and exploration of an Archean-aged, PGE-bearing, ultramafic intrusion discovered in Baraga County, Michigan and unofficially named by the authors the Kortio Lake Intrusion (KLI). During 1997 a program was undertaken to determine if an analog to the Lac des Isle PGE deposit of northwestern Ontario, Canada might exist on the southern side of the Mid-continent Rift within the Northern Complex granite-greenstone terraine of the Upper Peninsula of Michigan. Initial inspection was concentrated upon known appinitic intrusions in northern Marquette County. Lack of any evidence of mineralization within the surface exposures of these bodies led to the decision to move the search to the far western boundary of the granite-greenstone terraine with the intent of prospecting the entire area, from western Baraga County to eastern Marquette County. Coarse-grained hornblendite intrusions were identified in an area previously believed to be underlain by amphibolites. Also discovered were glacial float boulders of coarse-grained hornblendite containing several percent pyrite and chalcopyrite. The outcrops that were the source of the glacial float were located. These outcrops exhibited easily identified copper staining, as malachite, along with varying amounts of pyrite, chalcopyrite and nickel-bearing sulfides. The northern edge of KLI is located immediately west and southwest of Kortio Lake (aka Little Summit Lake), 3 kilometers (2 miles ) south of Herman, Baraga County, Michigan A review of available geochemical and airborne magnetic surveys further supported the interpretation that the area had potential for hosting PGE and copper nickel mineralization. Reconnaissance mapping of the surrounding area indicated the potential for an intrusion of appreciable size. It was determined that the complex was approximately 4,300 meters (14,000 feet) long, in a northsouth direction and in excess of 3,200 meters (10,500 feet) wide, east to west. Detailed geologic mapping and outcrop prospecting and sampling was undertaken. That effort defined a hornblendite intrusion that has a surface expression of 4,300 meters (14,000 feet) long and a range in width of from 1,600 meters (5,250 feet) to 300 meters (1000 feet). Several areas of mineralization were identified in the prospecting and sampling phase. Field relationships indicate that the KLI is a late Archean-aged intrusive that is truncated, on its southern end, by the Great Lake Tectonic Zone (GLTZ). Mapping indicated that the hornblendite intrusion had been intruded by a later, but nearly contemporaneous, fine to medium grained diorite. The contact zone between the hornblendite and diorite ranges from sharp to gradational and numerous locations exhibit areas where the diorite contains variable-sized, autoliths of hornblendite. The KLI is intruded by dikes of probable Archean age (Metachewan?) and numerous east-west trending Keweenawan age diabase dikes. Also noted are narrow granitic pegmatite dikes and small intruded plugs ranging from granitic to dioritic in composition. The intrusion is also crossed by numerous shear zones and faults with unknown, but probable minor displacement. The structural relationships have yet to be determined. 44 �The hornblendite intrusion is a highly variable rock, both in texture and in content of primary and accessory minerals. Textures range in grain size from fine-grained, nearly aphanitic, to pegmatitic and include equigranular, porphyritic and breccia phases. Coarse-grained zones have hornblende crystals that range up to 4 cm in size. The coarse-grained zones generally contain an appreciable amount of biotite. Other primary accessory minerals of note occur within distinct zones. There are surface exposures of coarse-grained hornblendite that contain up to 25% of coarse-grained, interstitial carbonates. Other exposures of the coarse-grained hornblendite commonly contain visible apatite. Some areas of the hornblendite contain appreciable amounts of magnetite. Mineralization in outcrop and glacial float consists of pyrite and chalcopyrite. Grain size of the sulfides ranges from microscopic to blebs in excess of 2.5 cm in size. Analysis of the mineralized samples indicated the presence of nickel, platinum, palladium and gold in amounts that are well above background levels. The distribution of mineralization indicates magmatic segregation into layers. Although minor mineralization has been found within the diorite intrusion, and within veins and shear zones that cut it, the bulk of the mineralization is found within the hornblendite and that rock has been the focus of the exploration efforts. During 2004 a geochemical survey utilizing a field-portable X-ray fluorescence analyzer yielded metal contents in outcrop of up to 7% copper, 0.3% nickel, 0.6% chromium and 0.1% cobalt. Select hand samples have yielded up to 500 parts per billion of combined platinum group metals by fire assay. The project moved forward with an airborne survey in 2005 and a drilling project in 2006. During the summer of 2005 a helicopter-borne time domain EM and aeromagnetic survey was flown with excellent results in outlining the extent of the KLI with magnetics and the identification of numerous E-M conductors within the intrusive body. A total of 450 line kilometers were completed. During the summer of 2006 six diamond drill holes totaling 941 meters (3,088 feet), from three locations, were completed in the area of the original discovery of PGE mineralization. A seventh hole was drilled, with a total length of 365 meters (1,197 feet), to test an area of very high magnetism. Results of the diamond drilling confirmed the presence of copper and nickel mineralization that contains varying anomalous amounts of platinum, palladium and gold. PGE minerals have been identified by electron microscopy. The assay results indicated the potential for greater concentrations, at economic levels, within the body. 45 �Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota; Implications for Archean (2.7 Ga) Tectonics Karberg, Susan M., University of Minnesota – Duluth, Duluth, MN 55812, karbe002@d.umn.edu Granite-greenstone terrains preserve large tracts of stable Archean continental crust and as such they may hold the only tangible record of Archean tectonic processes. Granite-greenstone terrains include granitoid bodies surrounded by layered, folded, felsic to ultramafic volcanic and volcaniclastic greenstone belts, commonly cut by shear zones or faults. Researchers suggest two hypotheses for the formation of Archean continental crust: (1) sagduction/diapirism, in which crustal density instabilities cause greenstone sequences to sink into basins as granitoid bodies rise (Anheusser et al., 1969), and (2) volcanic arc accretion, in which modern-style plate tectonic processes drive the accretion and imbrication of island arcs and associated basins. Although the Superior Province is widely accepted as a result of transpressional volcanic arc terrane accretion, no robust evidence exists that dismisses the sagduction/diapirism hypothesis and there is a lack of strong arguments for the plate tectonics processes of volcanic arc terrane accretion. The Archean age Mud Creek shear zone, located between two Archean granitic bodies, offers an excellent location to examine the sagduction/diapirism and arc accretion hypotheses. The Mud Creek shear zone lies within a greenstone belt between the Vermilion Granitic Complex to the north and the Giants Range Batholith to the south. The supracrustal rocks between the two granitic bodies consist of basalt flows, tuff, iron formation, greywacke, slate, conglomerate, felsic lava and pyroclastics flows (Peterson and Jirsa, 1999). The Mud Creek shear zone defined on recent geologic maps is an area approximately 14 km in length and 1 km wide, however foliation and elongation lineation occur throughout the15 km by 7 km study area that encloses the shear zone. Therefore the Mud Creek Shear zone in this study is considered the larger deformed area. The goal of this study is to examine foliation, elongation lineation, and microstructural kinematic evidence to understand the process or processes which controlled the deformation of the Mud Creek shear zone. I compiled existing structural data including bedding, foliation and lineation orientations from geologic maps, collected additional new foliation and elongation lineation orientations, and oriented samples for microstructural analysis. Foliation orientations are consistently near vertical, ranging from steeply north-dipping to steeply-south dipping, trending northeast-southwest. Generally, bedding parallels foliation. Pillow basalts are not foliated, but pillow rims record evidence of shear, most likely due to strain partitioning around the strong pillow cores. Motion direction is unknown due to a general lack of the third dimension, but most pillows appear flattened into pancakelike shapes which are elongate in map view in the same orientation as other foliated rock types. In some locations outside of the study area pillows are stretched out into vertical pencil-like columns which parallel the vertical elongation lineation seen throughout the Mud Creek shear zone. Foliation trend lines are drawn in figure 1. The pattern shows the continuity of foliation orientation throughout the Mud Creek shear zone, dominantly northeastsouthwest. Elongation lineation occurs in all rock types, with the exclusion of some pillowed basalt. Pillow basalt may be too strong and strain is likely to be partitioned around pillows, leaving little to no record of visible elongation lineations. Orientations of elongation lineations range from shallow strike-slip to steep dip-slip. Shallow lineations are rare and occur as an overprint to dip-slip lineations or in confined zones. Vertical lineations (90º ± 10º) are dominant throughout foliated rock types, and also observed on the compositional layering of banded-iron-formation. Figure 1 contains a map of elongation lineation pitch showing the dominantly vertical aspect of displacement, but also shows pockets or zones of moderately east, moderately west, and shallow strike-slip displacement. Microstructural analysis reveals interesting spatial patterns (figure 2): 1) south-side-up displacement is dominant in the northwest portion of the study area, 2) north-side-up displacement is dominant in the southeast portion of the study area, and 3) a central zone records both displacement domains, including individual samples that record both domains. Individual samples that record both domains have a penetrative fabric (S-C foliations) that indicates south-side-up displacement, and a less penetrative fabric confined to small zones recording north-side-up displacement (S-C foliations). Results from microstructural analysis support the sagduction/diapirism hypothesis, with rise of the southern granitic complex, followed by rise of the northern granitic complex. Cross-cutting relationships indicate later strikeparallel dextral displacement occurred along narrow zones (<1km wide) of previously formed foliation planes. Other samples record shallow to moderate elongation lineation orientation that record sinistral displacement confined to the northwest corner of the study area. 46 �References: Anhaeusser, C.R., Mason, R., Viljoen, M.J., Viljoen, R.P., 1969. A reappraisal of some aspects of Precambrian shield geology. GSA Bulletin 80, pp.2175-2200. Peterson, D.M. and Jirsa, M.A., 1999. Bedrock geologic map and mineral exploration data, western Vermilion district, St Louis and Lake counties, Northeastern Minnesota. University of Minnesota. Figure 1. Schematic map of the Mud Creek shear zone with foliation trends (grey) and elongation lineation pitch zones (colors). Figure 2. Sample kinematics within the Mud Creek shear zone. D = down, U = up. Includes interpretive cartoon cross section from NW-SE. 47 �Using mineralization to evaluate small-scale controls on shale permeability in the Nonesuch Formation Natalie King Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482 Mineralization in the Nonesuch Formation, which hosts the White Pine stratiform copper deposit, is being used to evaluate controls on paleopermeability. Relatively little work has been done on characterizing the controls on permeability in shales, despite their importance in fluid migration and accumulation in sedimentary basins. This study will improve understanding of controls on shale permeability that affect both the formation of sediment-hosted ore deposits and the integrity of petroleum top seals. Cores through the Nonesuch Fm, three from northern Wisconsin, outside the main stratiform copper mineralization, and three from the upper peninsula of Michigan within the zone of mineralization of the White Pine stratiform copper deposit, were logged noting stratigraphy, color, texture, mineralogy, grain size, organic content, laminations, bedding, and other sedimentary structures. Four representative cores, two from the mineralized zone and two from the unmineralized zone, were sampled for petrographic and whole rock geochemical analyses. Initial results from geochemical analyses, indicate three geochemically distinct groups in the Michigan cores; above the ore zone, well-mineralized ore horizons, and poorly mineralized ore horizons. The poorly mineralized horizons show a positive Cu to S trend that increases downward in the lower stratigraphic sections in the Nonesuch Fm. Above the ore zone, As concentrations are slightly higher than in the ore zone. Preliminary point counting data show a positive correlation between organic content and Cu concentrations. These data support fluid movement through the lower Nonesuch Fm that in turn indicates that there was paleopermeability in the lower Nonesuch Fm. Further work will examine small-scale controls such as grain and organic orientation, grain size, and mineral content to characterize these paleofluid pathways and establish small-scale controls on shale permeability. 48 �Structural and metamorphic history of the Burntside Lake shear zone with possible implications for Archean granite-greenstone formation J.N., Koester, J.W., Goodge, V.L. Hansen, Department of Geological Sciences, University of Minnesota-Duluth, Duluth MN 55812, koest036@d.umn.edu There is much debate as to the origin of Archean granite-greenstone terranes. In an effort to understand the genesis of these ancient terranes, it is helpful to focus on the boundaries between greenstone and granite bodies. The Burntside Lake shear zone represents a good location to study the relative importance of vertical and horizontal displacement because it lies at the boundary between a greenstone terrane, the Newton Lake formation, and a granite body, the Vermilion granitic complex. In an effort to better understand the origin of granite-greenstone terranes in this area, my study addressed the structural and kinematic evolution of the shear zone and used metamorphic petrology to evaluate crustal displacements across the zone. The Burntside Lake shear zone lies at the boundary of the Wawa and Quetico subprovinces of the Superior Province. The Superior Province represents one of the largest tracts of Archean crust preserved in the rock record. The classic interpretation of the Superior Province is that it involved a process similar to island-arc accretionary tectonics occurring today (1-7). However, other Archean granite-greenstone terranes are interpreted as being born of saguduction/diapirism, in which greenstone units blanket underlying granites to induce a density instability. Due to this instability, granite bodies will rise while greenstone units sink into inter-granitoid basins (8-9). Through further study of structural boundaries in granite-greenstone terranes we can hope to learn more about processes active in the Archean. Fieldwork was conducted along an 8 km length of the Burntside Lake shear zone which stretches 50 km from the Vermilion fault in Minnesota northeastward to Ontario. The shear zone strikes between N30E to N50E. During fieldwork, orientation data were collected from both foliations and lineations to aid in understanding the structural regime recorded in rocks of the study area. In addition to these fabric data, 106 samples were collected for petrographic study as hand samples and thin sections. Of these, 19 were selected from different field locations to be cut into thin sections. Thin sections were used to study microfabrics and metamorphic mineral assemblages. My study of the Burntside Lake shear zone focuses on the interaction between a granitoid body and a greenstone body. The shear zone is characterized by vertical to nearly vertical south-dipping foliation orientations, only a down-dip lineation was observed (Fig. 1). Dominant rock types collected in the field are biotite schists and chloritic greenstones. On the thin section scale, foliations vary from planar to anastomosing. Foliation is defined by the alignment of biotite, chlorite and hornblende. Elongate needles of hornblende, biotite, and chlorite define lineations. However, in a few samples, quartz is elongated as well. S-C’ fabrics in biotite and chlorite were used to determine sense of shear, along with sigma grains when applicable. These microfabrics were poorly preserved and in some thin sections completely ambiguous, resulting in very few samples having definitive petrofabrics with which to interpret shear sense. However, north-side-up shear sense dominate rocks in the field area. Mineral assemblages present in the field area indicate a low to intermediate metamorphic grade associated with the shear zone. On the north side of the zone, peak mineral assemblages include garnet-biotite and hornblende-plagioclase. To the south of the fault, chlorite is the dominant peak metamorphic mineral. Using electron microprobe analysis to obtain mineral chemistry, I was able to determine metamorphic temperatures in the shear zone for eight samples. To the north of the fault, garnet-biotite pairs record a temperature of about 550 °C, whereas hornblende-plagioclase records an average temperature of 700 °C. The temperatures recorded by these two geothermometers show a 150 °C difference. This difference may be real, or may be an artifact of the calibrations chosen. Additionally, the plagioclase grains analyzed may record igneous temperatures or some intermediary temperature that 49 �represents neither a purely igneous or metamorphic temperature. To the south, chlorite compositions record temperatures between 150 – 330 °C. In conclusion, the Burntside Lake shear zone represents a nearly vertical to vertical zone of deformation approximately 1.5 km wide. Through kinematic analysis of microfabrics the zone appears to have a north-side-up sense of shear (please see caveats noted above), as interpreted from thin sections. Rocks to the north side of the shear zone were metamorphosed in the upper greenschist to lower amphibolite facies, whereas the south side of the shear zone records lower greenschist-facies. Thus, metamorphism recorded on both sides of the fault agrees with a north-side-up sense of shear, determined from microstructures. Further work on must be done to allow for a greater understanding of the complex history recorded in the rocks defining the Burntside Lake shear zone. Specifically, work to further constrain pressure/temperature, and kinematics including timing between the conflicting senses of shear apparent in some of the samples utilized in this study. And lastly, a comparison study of the many shear zones in the area to understand any relation/timing relationships that may exist. Figure 1: Equal area stereoplot of foliation orientations from the Burntside Lake Shear zone (compilation of my orientations as well as those complied by 10-11) References: (1) Card, K.D, 1990, P. Research, 48, 99-156. (2) Cawood, P.A., Kroner, A., Pisarevsky, S., 2006, GSA Today, 16, 7, 4-11. (3) Jirsa, M.A., Southwick, D.L., Boerboom, T.J., 1992, Can. J. E. Sci., v. 29, p. 2146-2155. (4) Tabor, J.R., Hudleston, P.J., 1991, Can. J. E. Sci., 28, 292-307. (5) Bauer, R.L., Bidwell, M.E., 1990, Can. J. E. Sci., 27, 1521-1535. (6) Hudleston, P.J., Schulta-El, D., Southwick, D.L., 1988, , Can. J. E. Sci., 25, 1060-1068. (7) Bauer, R.L., Hudleston, P.J., Southwick, D.L., 1992, Can. J. E. Sci., 29, 208 2103. (8)McGregor, A.M., 1951, Trans Geol. Soc. S. Africa, 54, 27-70. (9) Rey, P.F., Philippot, P., Thebaud, N., 2003, P. Research, 127, 43-60. (10) Green, J.C., Schulz, K.J., 1983, MGS Univ. MN. (11) Sims, P.K., Mudrey, M.G., Jr., 1978, MGS Univ. MN. 50 �Strategies for Drilling Unconsolidated Material and Historic Underground Mine Workings: Examples from Hibbing Taconite Company’s 2007 Diamond Drilling Campaign Jared D. Lubben1,2 1 Hibbing Taconite Company, P.O. Box 589, Hibbing, MN 55746 2 Cliffs Technology Group, 550 E. Division Street, Ishpeming, MI 49849 As mining efforts at Hibbing Taconite advance north-eastward along the strike of the Biwabik Iron Formation, diamond drilling information is critical for evaluating the potential of new mine areas. To the east of Hibbing Taconite’s current open pit mine operations, significant obstacles related to prominent rock stockpiles, strong oxidation trends, and historic underground mine workings are encountered during diamond drilling. These problematic zones typically overlie ore-strata and are encountered in almost every drill hole. Consequently, new ideas and drilling strategies are required to penetrate these zones. Over the summer of 2007, Hibbing Taconite completed its annual diamond drilling program within areas hosting the prominent historic underground mining operations. The focus of this report is two-fold: Different lines of evidence used for drill hole targeting and positioning will be explained while various drilling methods and strategies used to reach target depths for drill holes under the 2007 program will be discussed. 51 �Mineral Zonation and Stratigraphy of the Tilden Hematite Deposit Helene M. Lukey, Senior Staff Geologist. Cliffs Technology Group, Cliffs Mining Service Company, Ishpeming, MI 49849 Hypogene alteration zones have been described as a precursor to high-grade hematite deposits in Australia, Brazil and elsewhere. This alteration described as being below or distal and is typically magnetite-carbonate-silicates reflecting Mg-Fe metasomatism. This paper describes the mineral zonation and possible structural control on mineralogic variation within the Tilden Mine. In contrast to the high-grade deposits, these variations are not peripheral to the ore bodies but are the ore. The deposit occurs within the Negaunee Iron Formation, along the southern margin of the Paleoproterozoic Marquette Range Supergroup basin. The regional structure is a fault bounded west plunging syncline within the crustal scale Great Lakes Tectonic Zone, which forms the boundary between Archean granite-greenstone and gneissic terranes. Local structure consists of the second order anticlines and synclines with variable northwest and southwest plunges. The interaction of sedimentation and diagenesis within growth fault controlled basins, along with a metamorphic and/or hydrothermal overprint has resulted in a complex series of ore types. For modeling purposes, the deposit has been divided into geologic domains based on both lithology and metallurgical response in the laboratory and in the plant. Mineralogic variation is recognized within individual mineral grains; at blast pattern (10 to 15 meter) and development drill hole (100 meter) scales. A simplified stratigraphic column consists of the Main Pit Carbonate domain (martite-magnetitevarious carbonates±silicates) in the core of the Main Pit anticline overlain by the Martite domain (martite+local goethite) and the West Hematite domain (microplaty hematite+goethite). The Clastic domains consist of quartz-rich lenses with variable matrix material within a variety of iron formation types. Although variable, the domain stratigraphic thicknesses are 100 to 200 meters. These compositional and associated textural variations affect plant efficiency and product quality, and are critical factors for ore reserve modeling, ore control and mine planning decisions. 52 �THE SEELEY SLATE AND BARABOO INTERVAL SEDIMENTATION MEDARIS, L. G. Jr and DOTT, R. H. Jr., Dept. of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu The Baraboo Interval represents a major episode of sedimentation between 1700 and 1630 Ma in the Lake Superior region. In the Baraboo Range, the type area, the Baraboo Quartzite is overlain conformably by ~120 m of gray slate, the Seeley Slate, which in turn is overlain by ~300 m of dolomite and discontinuous iron formation, the Freedom Formation. These two formations occur only in the subsurface in the hinge of the Baraboo syncline and have been described previously from exposures in underground iron mines, now abandoned, and drill cores by Weidman (1904) and from drill cores by Leith (1935), Schmidt (1951), and Geiger (1986). Because information on the Baraboo Interval comes largely from the prominent Baraboo Quartzite and correlative quartzites, we have undertaken a new study of the Seeley Slate to gain further insight into the nature of Baraboo Interval weathering and sedimentation. During exploration for iron ore in 1907, the Oliver Iron Mining Company obtained numerous drill cores from the Freedom Formation and Seeley Slate in the western part of the Baraboo Range along the axis of the Baraboo syncline. Access to these drill cores was provided by the WGNHS, and samples of gray slate, shown by the driller logs as occurring beneath iron formation, were selected from nine drill cores for detailed investigation. The Seeley is a gray slate with cleavage discordant to stratification in most cores. At least a small percentage of quartz silt is dispersed among phyllosilicate minerals even in the most homogeneous appearing intervals, which range up to several centimeters in thickness. Lamination visible in most cores is defined by lighter-colored concentrations of quartz silt, which vary in thickness from 0.5 to about 1 cm. Rare, coarser laminae have quartz grains up to 0.2 mm in diameter. A small percentage of silty laminae show discernible graded bedding (e.g. Fig. 1, core 3691-2). Soft sediment deformation by depositional loading is evident in core 3601-6; some other laminae were contorted slightly by cross-cutting cleavage (core 4045-2). Although most deformation was ductile, brittle cracking is evident in a few silty laminae (core 3691-2). As determined by petrographic examination and XRD and EMP analyses, the Seeley slate consists predominantly of quartz and 2M1 muscovite, with subordinate rutile, pyrite, chlorite [repidolite, (Mg3.5-2.9Al2.82.9Fe5.7-6.2)12(Si5.3Al2.7)8O20(OH)16], and 53 �tourmaline [schorl, ( 0.2-0.3Na0.8-0.7)(Fe1.5-1.8Mg1.5-1.2)3Al6Si6(BO3)3(OH)4]. Locally, siderite [(Fe0.86Mn0.01Mg0.05Ca0.08)CO3] occurs with quartz and muscovite in fine laminae. The Seeley slate has an unusual composition for shale; four samples of typical slate are virtually devoid of CaO and Na2O, contain 4.0 - 6.8 wt% FeO, 1.9 - 2.0 wt% MgO, 0.60 - 0.69 wt% TiO2, and plot near the compo-sition of muscovite in an Al2O3-CaO+Na2O-K2O molar projection (Fig. 2). The composition of the Seeley slate may have resulted from extensive weathering of granite (Fig. 2). Although the Seeley Slate is relatively mature, having a CIA index of 70 to 74, it is much less mature than argillite and metapelite in the Baraboo Quartzite. The Seeley Slate contrasts markedly with the underlying and overlying formations. The red Baraboo Quartzite below was deposited initially by braided streams followed by shallow marine tidal and wave processes. The inferred marine transgression continued during Seeley deposition in a deeper, quieter setting on a marine shelf. Occasional density currents carried quartz silt and rare sand into the otherwise mud-dominated environment. The overlying Freedom Formation was probably deposited in the same setting, which was now starved of clastic input so that dolomite and iron-rich facies could accumulate chemically. Baraboo Interval strata accumulated as a sedimentary wedge on the southern passive margin of a Proto-North American craton. With a total thickness of about 1500 m and extending at least 1000 km east-west, they comprise a significant stratigraphic package, which includes one of the youngest known Banded Iron Formations in the world. The red color due to hematite cement in the Baraboo and correlative Sioux and Barron Quartzites indicates an oxygenic atmosphere, and the extreme compositional maturity of both the quartzites and their interstratified pelitic layers resulted from intense chemical weathering of the source terrain of these clastic sediments. In contrast, the gray color, mineralogy, and bulk composition of the Seeley Slate imply deposition of less intensely weathered detritus in a reducing environment as compared to the Baraboo Quartzite,. References Geiger, C.A. (1986) Geosci. Wis. 10, 28-36 Leith, A. (1935) Kansas Geol. Soc. Guidebook, 9th Ann. Field Conf., 320-322 Schmidt, R.G. (1951) M.S. thesis, Univ.Wis. – Madison, 40 pp. Weidman, S. (1904) WGNHS Bull. 13, 190 pp. 54 �The Inaugural Season of the Precambrian Field Camp at the University of Minnesota Duluth Miller, James D., Dept. of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 (mille066@umn.edu) Peterson, Dean M., Natural Resources Research Institute at UMD, 5013 Miller Trunk Hwy., Duluth, MN 55811 (dpeters1@nrri.umn.edu), and Hudak, George J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901(hudak@uwosh.edu) This past summer, the Precambrian Research Center at UMD ran the first season of a one-of-a-kind field camp. The camp teaches students mapping techniques that are best suited to field studies of Precambrian rocks of the Canadian Shield. By all measures, the camp was a rousing success. The Precambrian field camp was attended by 15 students - four from UMD and the remaining mainly from schools throughout Minnesota and Wisconsin. Jim Miller (MGS), Dean Peterson (NRRI) and George Hudak (UW-Oshkosh) served as full-time instructors, with assistance on various projects by Terry Boerboom (MGS), Val Chandler (MGS), John Goodge (UMD), Mark Jirsa (MGS), Howard Mooers (UMD), Mark Severson (NRRI), and Nigel Wattrus (UMD). The six credit course ran for six weeks from mid-July to late August. Although students worked six days a week, commonly 10-hour days, and often under very hot and humid conditions, they consistently kept a positive attitude throughout. The first two weeks of the camp were run out of UMD. Daily mapping exercises included 1) structural and outcrop mapping at Thompson Dam, 2) an introduction to geophysical field methods (gravity, ground magnetics, seismic), 3) mapping mafic cumulate rocks of the Duluth Complex at Spirit Mountain, and 4) mapping intrusive and volcanic rocks along the North Shore near Tettegouche State Park. The latter exercise included a day of canoe mapping along Lake Superior. For weeks 3 and 4, we moved the camp to Vermilion Community College in Ely. There students started with a stratigraphic correlation project along the Biwabik Iron-formation, that included core logging and measuring section in one of the taconite mine pits. Next, students conducted a two-day project of mapping along the basal mineralized contact of the Duluth Complex. At the end of the third week, we arranged an evening of mapping by lamplight along the main adit of the Soudan Iron Mine. During week 4, students conducted a multifaceted mapping project on greenstone belt geology and overlying glacial geology in the Fivemile Lake area west of Ely. The true highlights of the summer were the capstone mapping projects during Week 5. Here, students chose field mapping projects that entailed detailed bedrock mapping in previously unmapped wilderness areas, most in the BWCA. George Hudak mentored three students on mapping greenstone belt geology in the Twin Lakes area west of Ely. Mark Jirsa led four students into the BWCA off the Gunflint Trail, which had been intensely burned over in the Cavity Lake Fire of July 2006. Jim Miller worked with two students in the eastern prong of the Duluth Complex in the Homer Lake area off the Caribou Trail. Dean Peterson supervised a group of five students in mapping project of a gabbroic macrodike in the Nickel Lake-Gabbro Lake area of the BWCA. Upon returning to UMD for the final week, students worked on digitally compiling their capstone mapping data into ArcView and then creating geologic maps in Illustrator. These maps are on display in this year’s poster session. 55 �The true measure of the success of the camp is how the student shave taken advantage of this experience to better their academic and career goals. Of the 15 students, five have secured jobs with exploration companies, six are pursuing Master’s degrees (four at UMD, two at Lakehead University in Thunder Bay), three are currently finishing their undergraduate degrees with one planning to attend UMD next term. Many of the students have expressed to us how beneficial this camp has been to them, not only in teaching them particular field methods best suited to Precambrian terranes, but moreso in giving them the confidence to work with this type of geology, which we all know can be challenging. Being the only open field camp in the US that focuses on the unique attributes of Precambrian shield geology, we believe that UMD’s Precambrian field camp fills a very important niche in geological field education. Moreover, we hope this camp helps to reverse the alarming degradation of field geology as a primary component of geological education in many US schools. The art and science of observing and interpreting rocks in the field is still at the core of all geological endeavors and deserves a preeminent place in all geology curriculum. It is our mission to provide students with the tools and techniques that will start them on a life-long path of looking at rocks in the field, especially those of the Precambrian. To see photos from last year’s camp or download the geologic maps that were produced from the capstone projects and are on display at this meeting, please visit the PRC website (www.d.umn.edu/prc). 56 �BEDROCK GEOLOGY OF THE FOOTWALL TO THE SOUDAN IRON FORMATION SOUTH OF TWIN LAKES, ST. LOUIS COUNTY, NORTHEASTERN MINNESOTA MOOSAVI, S., JOHNSON, T. K., WENDLAND, C., ANDERSON, A., HUDAK, G. J., Precambrian Research Center, University of Minnesota – Duluth, Duluth, MN 55811, hudak@uwosh.edu The Ely Greenstone Formation of the Vermilion District of northeastern Minnesota is made up of a steeply north- to southwest-dipping sequence of Neoarchean supracrustal and associated intrusive rocks folded about the Tower-Soudan anticline. Recent mapping in the Vermilion District has concentrated in the area between the Soudan Mine (to the west) and Armstrong Lake (to the east; Hudak et al., 2002; Peterson and Patelke, 2003; Heine, 2005; Hoffman, 2007; Hudak et al., in prep.). The Lower Member of the Ely Greenstone (EGLM) is composed of submarine calc-alkalic and tholeiitic basalt and basalt-andesite sheet lava flows, pillow lava flows and pillow breccias with associated basalt to andesite tuffs and lapilli tuffs, along with subordinate rhyodacitic to rhyolitic lava flows, lapilli tuffs and tuffs, with minor intercollated epiclastic rocks and iron formations (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002, Hoffman, 2007). The EGLM has been subdivided into the older, largely shallow submarine Fivemile Lake Sequence (FLS) and the younger, deep submarine Central Basalt Sequence (CBS; Peterson and Patelke, 2003). The Soudan Iron Formation Member (EGSM) comprises Algoma-type cherty iron formation, massive to pillowed basalt lava flows, polymict epiclastic rocks and minor associated felsic tuffs (Peterson and Patelke, 2003). The Soudan Member is overlain by the Upper Member of the Ely Greenstone (EGUM). The EGUM is composed of poorly vesiculated tholeiitic basalt lava flows locally interlayered with Algomatype iron formation lenses (Schulz, 1980; Southwick et al., 1998). Near Tower and Soudan, the EGUM is commonly interstratified with the Lake Vermilion Formation (LVF: greywacke, slate, conglomerate, and dacite tuff), and the Gafvert Lake Sequence (GLS: subaerial to submarine dacite to trachyandesite lava flows, tuffs, and intrusions; locally, the LVF unconformably overlies EGLM and EGSM strata (Southwick et al., 1998; Peterson and Patelke, 2003). Detailed (1:5000 scale) geological mapping (Moosavi et al., 2007) occurred in the eastern part of the Vermilion District during August, 2007 as one of four capstone projects associated with the Precambrian Research Center field school. The mapping extended from about 1.5km south of Twin Lakes to the southern shoreline of Twin Lakes, and had the goal of increasing our understanding of the stratigraphy, hydrothermal alteration, and structural geology in this poorly understood part of the Vermilion District. The steeply north-dipping, east-west striking stratigraphic sequence is locally cut by east- east-northeast-trending, typically dextral shear zones composed of chlorite schist. The southwestern part of the field area comprises sparsely to moderately vesicular basalt and andesite lava flows which are correlative with the Eagle’s Nest Basalts (Jirsa et al., 2001). These lava flows are overlain by what we interpret as the CBS, which comprises a 1.0 to 1.5km thick sequence of exceptionally well-preserved sparsely- to moderately vesicular pillowed and massive basalt lava flows and associated hyaloclastite that are locally interlayered with 10-50m thick rhyodacite to rhyolite tuffs and lapilli tuffs. Locally, 2040m thick polymict breccias occur near the top of the CBS. These are compositionally and 57 �texturally similar to breccias observed by Hudak et al. (in prep.) near the top of the CBS south of Sixmile Lake. Overlying interbedded Algoma-type iron formations, felsic tuffs and felsic lava flows are correlative with the EGSM. A sharp contact exists between the EGSM and stratigraphically overlying dacite to rhyodacite tuffs which we interpret as the GLS. This supracrustal sequence is intruded by fine- to coarse-grained, ophitic gabbro sills. These sills are correlative with gabbro sills mapped in the Soudan Mine area (Peterson and Patelke, 2003) and near Needleboy and Sixmile Lakes (Hudak et al., in prep.), and may represent hypabyssal intrusions which fed the overlying EGUM basalt lava flows. Synvolcanic hydrothermal alteration south of Twin Lakes is variable. Basalt and andesite volcanic rocks are generally moderately altered to an epidote-chlorite-actinolite-quartz alteration assemblage. Felsic rocks are commonly sericite-altered. Near the top of the CBS, mafic to intermediate volcanic rocks are strongly altered to an epidote-quartz ± garnet (andradite) assemblage similar to that observed in the uppermost 50m of the CBS in the vicinity of Sixmile Lake (Hudak et al., in prep.). References Heine, J., 2005, Gafvert Lake Reconnaissance Mapping Project: Natural Resources Research Institute Technical Report NRRI/TR-2005/20, 12 p. Hoffman, A. T., 2007, Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: Unpublished M. S. thesis, University of Minnesota – Duluth, 295 p.. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Heine, J., Newkirk, T. T., Hocker, S. M., and Hauck, S., in prep., Comparative Geology, Stratigraphy, and Lithogeochemistry of the Needleboy Lake – Six Mile Lake Area, Vermilion District, NE Minnesota: Natural Resources Research Institute Geological Report of Investigation. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock Geological Map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-114, 1:24000 scale. Moosavi, S., Johnson, T., Wendland, C., Anderson, A., and Hudak, G., 2007, Bedrock Geology Map of the Footwall to the Soudan Iron Formation South of Twin Lakes, St. Louis County, Northeastern Minnesota: Precambrian Research Center, Geological Map Series Map 20074, Natural Resources Research Institute, University of Minnesota-Duluth. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL). Geological site investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003, 88p. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 58 �GEOCHEMSTRY OF SEDIMENTARY ROCKS ASSOCIATED WITH THE MUSSELWHITE GOLD DEPOSIT, NORTHWESTERN ONTARIO MORAN, PATRICK; FRALICK, PHILIP; HILL, MARY LOUISE, HOLLINGS, PETE, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, pcmoran@lakeheadu.ca The Musselwhite gold deposit, 100% owned and operated by Goldcorp Inc., has cumulatively produced in excess of 2 million ounces since 1997 and has a projected mine life through 2013. It is situated in the North Caribou Lake metavolcanic/metasedimentary belt in the central northwestern portion of Superior Province, approximately 430 km northwest of Thunder Bay. The belt occurs along the contact between the North Caribou Terrane and the Island Lake Domain, with a large, crustal-scale deformation zone forming its eastern margin. The Musselwhite gold deposit is hosted by amphibolite grade rocks dominated by banded iron formation (BIF). This study primarily focuses on the Northern Iron Formation (NIF) metasedimentary assemblage, host to the majority of gold mineralization at Musselwhite Stratigraphic and geochemical analyses suggest that the lithologies of the NIF assemblage were deposited on Mesoarchaen mafic to ultramafic volcanic rocks forming the ocean-floor. The NIF assemblage and another iron formation lower in the stratigraphy, the Southern Iron Formation (SIF), record hydrothermal regimes associated with, and interrupted by, eruptive volcanic activity. The stratigraphically lowest lithologies in the NIF assemblage, meta-argillite, quartzgrunerite BIF, and magnetite-dominant BIF, were deposited in deep, calm water, in association with venting hydrothermal fluids. These ancient chemical sediments are analogous in geochemistry to modern day deposits in places such as the Red Sea and East Pacific Rise. Differing Eu content between chert and magnetite layers indicate that rhythmically changing temperature variations drove the hydrothermal system, imparting the banded nature. The quite chemically pure chert and magnetite layers of the lower portion of the NIF assemblage contrast with silicate-dominant banded iron formation; the silicate-dominant BIF increases in importance stratigraphically upwards. It represents a decreasing hydrothermal system and/or an increase in the rate of clastic sedimentation. Hornblende-garnet and biotite-garnet schists were formed by metamorphism of mudstones composed of eroded material. The sediment that formed the hornblende-garnet schist is the same sediment that composes the siliciclastic component of the silicate-dominant NIF. Similarly the biotite-garnet schist represents a mudstone, but unlike the hornblende-garnet schist, it is primarily derived from intermediate to felsic source rocks. Lastly the garnet-quartzite represents metamorphosed sandstone eroded from the same intermediate to felsic igneous source rocks as the biotite-garnet schist. Just as there is an overall increase in clastic content upwards through the approximately 30 m thick sedimentary succession there is also a change from more mafic sourced debris to a more intermediate/felsic source. The majority of samples collected from Musselwhite did not experience significant remobilization of typically immobile elements. This is indicated by the relatively linear geochemical ratios between the immobile elements in question (Al2O3, TiO2, Zr, U, Th, etc). 59 �Even elements that are commonly more mobile (K2O, Na2O, etc) appear to have remained relatively immobile at Musselwhite. The only samples that show significant geochemical change are from shear zones. The gold mineralization is primarily associated with shear zones within the siliciclastic-rich, upper NIF assemblage, where pyrrhotite (possibly originally pyrite) replaced iron oxides and iron silicates. This indicates that the control on areas of gold mineralization was a combination of: 1) the presence of structural zones allowing gold-bearing fluids to move through the NIF, which could act as a geochemical trap for gold; and 2) structural conditions in the siliciclastic-rich NIF that favoured hydrothermal fluid involvement with this unit. 60 �ON-LINE ELECTRONIC ACCESS TO INSTITUTE ON LAKE SUPERIOR GEOLOGY PUBLICATIONS M.G. Mudrey, Jr., 106 Ravine Road, Mount Horeb, WI 53572 USA (mgmudrey@mhtc.net) Peter Hollings§ Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1,Canada (peter.hollings@lakeheadu.ca) (corresponding author) Lura E. Joseph, Geology Library, 223 Natural History Building, 1301 W Green, Urbana, IL 61801, USA (luraj@uiuc.edu) Mark Jirsa, Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 5514 USA 55114 (jirsa001@umn.edu) Jo Kalliokoski, 1010 7th Ave., Houghton, MI 49931 Authors are Present and Emeriti Secretaries/Treasurers of the Institute during the last 25 years and one librarian (Joseph) who helped the Institute to compile, index and create search mechanisms for its varied publications In the past 54 years, over 16 gigabytes of abstract, field trip guides and supplemental information from more than 100 individual documents have been produced as part of the annual proceedings of the Institute on Lake Superior Geology (ILSG). These include comprehensive volumes on mineral properties, combined abstract and field guides, and field guides published by sponsoring organizations. During the past three years students at Lakehead University and other volunteers have scanned and digitized these volumes. Use of optical character recognition (OCR) software has allowed the Institute to make the digital files searchable. By use of proprietary software from Cvision®Technologies, Inc, this quantity of information was compressed in PDF format to less than 600 megabytes, approximately 3600 compression ratio. The documents can be opened and read directly in Adobe Reader® and other software with read PDF compatibilities. The documents can be directly accessed for download at no cost at http://www.lakesuperiorgeology.org/ Work on the scanning consisted of direct scanning to PDF and scanning to tiff format images. PDF and tiff images were cleaned, and where necessary (particularly with older, yellowed, and mimeographed documents) extensively enhanced to improve readability. Where necessary, colored maps and illustrations were treated separately. The documents were then compressed. Compression to 5 megabytes was achieved from a 1.5 gigabyte original scan. To accompany the online collection of ILSG publications, a searchable index was created by Joseph - geology librarian at University of Illinois, Urbana-Champaign (UIUC) http://search.grainger.uiuc.edu/ilsg/ Work on the database project progressed in the following steps during summer and fall of 2007: 61 �• A bibliography was created from information available from GeoRef®. The references were subsequently parsed and imported into an Excel® database. Many entries were missing from GeoRef®, and gaps were filled in from the collection at UIUC, from PDFs as they were added to the ILSG web site, and via Interlibrary Loan. Each reference was checked for accuracy, and edited for consistency of the source field information for each volume. In addition, information was added to fields that are unlikely to be included in commercial indexes, such as type (oral presentation, poster, guidebook, address, or other), meeting location, and meeting date, and the speakers and titles of presentations for which abstracts are unavailable. • The completed Excel® database was imported into an Access database and loaded on a library server at UIUC. • A search engine that had been created in-house to search other library databases was modified to search the ILSG database. The Access® database can be easily updated. At some time in the future, key words could also be added to the database to enhance search results; however that would be a major project. There are currently over 2400 references in the index. The database can be searched by entering search terms into any or all of the following fields: author, title, source (including volume number), publication year, meeting location and/or meeting dates. In addition, there is a drop-down menu to search a type of reference (oral presentation, poster, guidebook, and so forth). The results can be sorted by author, publication year, type, meeting location, or meeting date. The default sort is by author. The default for type is “any.” If no search terms are entered, the results will include all references, one hundred at a time. This is useful for browsing. Each of the search fields can be searched for either “all of the terms” or “any of the terms.” Searching of phrases is beyond the capability of this search engine. Wildcards (truncation symbols such as *) are not used; however, truncation is automatic: that is, a partial word will return results. For example, “Eidu” in the author field will return references by Eidukat, and “mineral” in the title field will return references with “mineral,” “minerals,” “mineralization,” and so forth. Case (upper or lower) is not recognized, so is not important. The database format and search capabilities will be demonstrated in a poster presentation. A complete list containing such information as author, editor, chairperson, and sponsoring organization is maintained by: MTU Archives & Copper Country Historical Collections J. Robert Van Pelt Library Michigan Technological University Houghton MI 49931 Phone: 906-487-2505 e-mail: copper@mtu.edu Photocopies of most back volumes can be ordered from the MTU Archives at the prevailing copy rate. 62 �Multi-element Geochemical Signature of Copper Mineralization at the White Pine Mine, Midcontinent Rift System, Western Upper Peninsula, Michigan MUVI-TJIKALEPO, Muatala H., BORNHORST, Theodore J., and ROBINSON, George W., A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 and WILLIAMS, W.C., Phoenix, AZ 85048 The lower beds of the Nonesuch Shale and uppermost beds of the Copper Harbor Conglomerate host a giant stratiform copper deposit at the White Pine Mine. The ore body yielded roughly 2.0 billion kilograms of copper and 50 million ounces of silver from 198 million tons of ore from 1953 to 1996 (an average grade of 1.14 % copper and 0.25 ounces of silver per ton) (Johnson et al., 1995). Whereas the White Pine deposit has been the subject of both basic and in-depth geologic studies, heretofore there has not been a multi-element geochemical investigation. For this study, samples were selected from drill core to avoid the problems of geochemical mobility in surface exposures. A systematic vertical section through the ore body was obtained from 5 holes at the White Pine Mine. The holes were from both the center and edge of each deposit. A total of 387 samples (includes areas not discussed here) were analyzed for 64 elements by Activation Laboratories in Toronto, Canada. Copper ore samples are defined here as those samples with 0.2 % Cu or more. At the White Pine Mine, the weighted average for the samples used in this study have a copper content of 1.22 % Cu which is similar to the average grade in mined ore of 1.14 % Cu. The geochemical character of the White Pine copper ore is given in Table 1. Several elements (Cu, Ag, As, Hg, Ba, Br, Ca, Cd, Fe, Hg, K, Mn, Na, Ni, Pb, Re, S, V, Zn, U) were selected for more in-depth study (MuviTjikalepo, 2007). At the White Pine Mine, distribution of Ag, Hg, and Re closely parallel that of Cu. Arsenic does not appear to correlate well with the vertical changes in the amount of Cu. The vertical distributions of Pb and Cd appear to correlate well with each other, but not with Cu. The vertical variation of Zn shows no significant correlation to that of Pb-Cd. The contents of Zn and Ni appear to be closely related to one another and slightly resemble Cu. The vertical variations of Ba and Br appear to be irregular and/or nearly constant throughout and show no significant correlation to the distribution of Cu. The geochemical profiles of Ca and Fe appear to show an antithetic relationship with each other and appear unrelated to the distribution of Cu. The Mn concentration is more or less constant throughout the vertical ore column, and shows a weak relationship with Cu and a moderate connection with Ca. The vertical distribution of S displays a clear relationship with that of Cu. The geochemical profiles of both K and Na show no significant resemblance to the geochemical profiles of Cu, and their vertical distributions appear more or less homogenous throughout the ore column. The geochemical profiles of U closely parallel those of Cu, except in the uppermost part of the mineralized column where the U content appears to rise slightly with a corresponding slight drop in Cu. The V content on the other hand appears to be largely constant. In comparison to the Kupferschiefer ore horizon and the ore shale of the Zambian Copperbelt, the White Pine occurrence is geochemically dominated by Cu and Ag. In ore shale of the Zambian Copperbelt the metal association is Cu and Co. The association of metals in the Kupferschiefer shale and Zambian Copperbelt shale are relatively more polymetallic than White Pine. Acknowledgements This study was partially funded by Phelps Dodge Exploration (currently Freeport-McMoRan Copper & Gold), a Fulbright Fellowship to Muvi-Tjikalepo, and Michigan Technological University. The drill core from the White Pine Mine was obtained by Bornhorst during the time of mine closure through the generous assistance of Dr. Rod Johnson. 63 �Table 1: Weighted average values of elemental concentrations in copper ore from the White Pine Mine. Copper ore is defined as those samples with a minimum copper of 0.2 %. Weighted average determined by weighting using the total thickness of the interval sampled in the core. For the White Pine Mine, 49 samples covered 69 ft of core defined as ore. The number of digits listed is not necessarily significant on the basis of accuracy or precision. The QA/QC program included blind duplicate samples, a blind “blank” sample run with each batch, blind geochemical reference samples, laboratory duplicate samples, and laboratory geochemical reference standards. Raw and QA/QC data are given in Muvi-Tjikalepo (2007). Cu % 1.22 Al % Ag ppm As ppm Au ppb Ba ppm Be ppm Bi ppm Br ppm Ca % Cd ppm Ce ppm Co ppm Cr ppm Cs ppm Dy ppm Er ppm Eu ppm Fe % Ga ppm Gd ppm Ge ppm Hf ppm Hg ppb Ho ppm In ppm Ir ppb K% La ppm Li ppm Lu ppm Mg % 5.54 10.9 5.6 <2 621 2 0.2 4.5 2.10 7.8 52 33 79 6.1 6.6 3.9 1.63 4.58 22.2 7.0 0.3 6.1 92 1.3 <0.1 <5 2.14 25.7 43.7 0.5 2.16 Mn ppm Mo ppm Na % Nb ppm Nd ppm Ni ppm P% Pb ppm Pr ppm Rb ppm Re ppm S% Sb ppm Sc ppm Se ppm Sm ppm Sn ppm Sr ppm Ta ppm Tb ppm Te ppm Th ppm Ti % Tl ppm Tm ppm U ppm V ppm W ppm Y ppm Yb ppm Zn ppm Zr ppm 1216 1.1 1.47 16.3 31 51 0.08 11 7.8 46.6 0.15 0.32 0.6 17.0 1.3 7.2 3 120 0.9 1.1 0.3 6.8 1.77 0.82 0.46 2.4 133 <1 30.6 3.5 117 237 References Cited Johnson, R.C., Andrews, R.A., Nelson, W.S., Suszek, T., and Sikkila, K. 1995. Geology and mineralization of the White Pine copper deposits: unpublished Copper Range Company Report. Muvi-Tjikalepo, M.H. 2007. Stratigraphy and trace element distribution in the lower Nonesuch Formation of the Michigan segment of the North American Mid-continent Rift System, Gogebic-Ontonagon Counties, Michigan: M.S. Thesis, Michigan Technological University, 133 p. 64 �Northern Michigan Geologic Repository Association The geological repository facilities in Marquette are full-to-overflowing. There is no room for new cores or samples. Furthermore, the state cannot provide permanent, ongoing funding. The Northern Michigan Geologic Repository Association (NMGRA) is being created as a notfor-profit-organization to provide support for repository efforts in the Northern Peninsula to collect geologic cores, samples, specimens, and associated documents relating to the geology and house, materials collected in the State of Michigan. These materials will be catalogued and preserved for charitable, educational, scientific, and literary purposes. NWGRA will work in cooperation with scientific, professional, educational, governmental, social and philanthropic organizations to advance mutual concerns and activities. As a not-for-profit-organization, your contributions will be tax deductible. 65 �PolyMet Mining: NorthMet Cu-Ni-Co-PGE Project, Hoyt Lakes, Minnesota, USA Richard Patelke, Project Geologist, PolyMet Mining LOCATION: NorthMet is 70 miles north of Duluth in the heart of the world-class Mesabi Iron Range District of northeastern Minnesota. It is one of at least ten deposits in the region. STORY: Drilling for Cu-Ni-Co-PGE began with US Steel in 1969, with work by PolyMet since 1998. There are 371 drill holes over 285,757 feet with 34,186 multi-element assays. The Project includes the former LTV Steel Mining Company taconite iron ore concentrator, the “Erie plant,” idle since 2001 and now wholly owned by PolyMet. The Erie plant comprises a 100,000 tpd concentrator and associated facilities, such as a tailings basin with 28 years of capacity at PolyMet's intended initial production of 32,000 tpd. Also included are a rail fleet, shops, office buildings, and ready access to power and water. The plant is ready to run NorthMet ore after simple refurbishment, installation of flotation equipment, and construction of the hydromet facility. A bankable feasibility study was completed in 2006, permits and financing are expected in late 2008, with construction commencing immediately thereafter, concentrate production will begin in 2009, followed closely by hydrometallurgical processing, and ultimately copper metal production along with Ni-Co hydroxide and PGE concentrate. GEOLOGY: The NorthMet deposit is a large, disseminated sulfide, Cu-Ni-Co-PGE ore body in the Keweenawan Duluth Complex. It is hosted in grossly layered troctolitic rocks overlying metamorphosed greywackes. There are seven igneous stratigraphic units divided and recognized by texture and basal ultramafics. All intrusive and country rock units dip gently to the southeast. DEPOSIT: The main ore zone is in the basal igneous stratigraphic unit with local extension of mineralization into the overlying unit. A secondary ore zone (the "Magenta Zone") in the upper units in the western part of the deposit crosses stratigraphy. The Magenta Zone is copper and PGE-rich and sulfur poor relative to the rest of NorthMet. Mineralization is chalcopyrite, cubanite, pyrrhotite, and pentlandite. PGE are correlated with copper. The deposit is open along strike and down-dip, with continuing drilling expected to add resource in both directions. NorthMet has a resource (2007) of 638 million tons measured and indicated, 252 million tons inferred, and reserves of 275 million tons proven and probable at a grade of 0.28% Cu, 0.08% Ni, 0.008% Co, and 0.337 g/tonne Pt + Pd + Au. DISPLAY: Four core boxes displaying stratigraphic and mineralization suites, poster with crosssections, maps, and photos. 66 �Keweenawan apparent polar wander path: new observations, new ideas L.J. Pesonen Department of Physics, Laboratory for Solid Earth Geophysics, PB 64, 00014 University of Helsinki, Finland (lauri.pesonen@helsinki.fi) The Keweenawan apw track (the “Logan Loop“) of Laurentia has been considered to be the best documented Precambrian apw-sequence in the world. However, even after nearly 50 years of work, the cause of the pole motion is still debated. Here we present new observations extracted from the literature. We focus on two cases, (i) the Logan intrusions and (ii) the Mamainse Point volcanics. These units are of prime importance since, unlike many other Keweenawan units, they record two to three successive reversals (at least the latter case) thus being crucial in testing the various models of the Logan Loop. Mamainse Point Volcanics. Palmer (1970) pointed out that in the Mamainse Point section there are four “magnetostratigraphic” units, which are from oldest to youngest: R2-N2-R1-N1. Of the reversals the oldest one (R2-N2) is asymmetric in a similar manner as in most Keweenawan units: R inclinations are steep upward whereas N inclinations are moderate shallow downward. Recently, Swanson-Hysell et al. (2006) have given new evidences that the older reversal is not asymmetric if one considers the data in time progression (flow by flow) and not as mean values. Following this principle, they interpret the asymmetry as a rapid motion of Laurentia from high to nearly equatorial latitudes. This idea cast doubt that the asymmetry is due to a hypothetical non-dipole field as suggested by Pesonen and Nevanlinna (1981). There are some support for their idea. First, a close look of the Fig. 19 of Palmer (1970) shows also a streaking in reversed polarity (R2) inclinations, from steep values to shallow ones. Unfortunately, we don’t know if Palmer´s data correspond the stratigraphic order. There are more examples where time progression of the pole can be notified along the western arm of the Logan Loop, such as in Osler volcanics, where the pole of the lower R units is distinctly older than that of the upper R units (Fig. 1). Also, the pole of Logan R sills seem to be older than that of dykes (which cut the sills; Fig. 1). However, there are also contrasting evidences. For example, the N-poles of the Portage Lake lava sequence, when plotted in stratigraphic order (Books, 1972), do not plot along the track but make an oval across it, resembling typical secular variation pattern (Donadini 2007). Secondly, there are also some evidences of “back-and-forth” movement of the pole along the apw-track. Another problem is the age of the Mamainse Point volcanics, since according to Davis et al. (1995) the upper part of the lower R2 unit is ca. 1096 Ma, some 15-20 Ma younger than other R polarity poles, such as the Logan sills (ca. 1110 Ma). The only way to reconcile this is that the dated flow is an intrusive (sill) but even in this case, the pole will make back-and-forth movement along the track which is against the plate motion model. We have proposed that the major part of the loop, the asymmetry, and the back-and-forth movement can all be explained in terms of the fluctuating non-dipole field (Pesonen et al., 2006). Pigeon River dykes. Hollings et al. (2007) have pointed out that in the Pigeon River area (Ontario), there are three cross-cutting dyke swarms. Recently, one dyke from the the Pigeon River swarm has yielded an U-Pb age of 1141±20 Ma (Heaman et al., 2007) which is markedly older than the generally accepted age of the Keweenawan igneous units (1115-1087 Ma). In addition, Hollings et al. (2007) point out that geochemically the Thunder Bay (and south of it) sills are distinct from Lake Nipigon sills. These results call for a relook at the paleomagnetic data base if there are supporting evidences for their observations. Fig. 1 shows the poles from three, supposed to be nearly coeval (1115-1087 Ma), Keweenawan igneous areas in the northern part of the Lake Superior. These are Pigeon River (triangles), Thunder Bay (squares) and Lake Nipigon (circles). The dual polarity poles are derived of dykes (small symbol) and sills (large symbol), respectively. All the rock units have been paleomagnetically studied by three to six authors (Dubois, Robertson&Fahrig, Pesonen, Palmer, Halls and Stott, Middleton et al.; see references) thus allowing a consistency check to be made. The result is somewhat surpising: in the case of 67 �N-polarity data, there are no marked differences between poles from the three areas and the consistency check is excellent. In the case of R-polarity data, there is a slight tendency that the Nipigon sills and dykes are the oldest (in the sense of apw; Fig. 1), the Thunder Bay sills and dykes are next and the Pigeon River sills are the youngest units. Most important, the pole of the Pigeon River R-polarity dykes, recently dated at 1141±20 Ma, differ from other R poles. Unfortunately, data from only three dykes are so far available but the consistency between the two studies (Robertson and Fahrig, 1971; Pesonen, 1979) is good. It is noteworthy that this R polarity pole, if indeed of 1141 Ma old, cannot be matched with the coeval 1141 Ma Abitibi poles (either N or R). If the Pigeon River R-pole stands in future studies (more data urgently needed), the new pattern of Keweenawan-Abitibi poles does not allow a simple western arm of the Logan Loop to be drawn but requires, a more complex apwp. Alternatively, the complex pattern of poles may reflect a rapidly oscillating non-dipole field prevailing during the (oldest) R polarity epoch. The R2-N1 asymmetry in the Coldwell Complex (1108 Ma), and the new, surprisingly “young” age, 1104 Ma, for an R-polarity Nipigon sill with steep upward inclination (Halls and Stott, 2005; Fig. 1) requires also a back-and-forth movement of the pole along the apw-track, in support of an fluctuating non-dipole field model. The new poles from the coeval dykes from Central Arizona, being 14o apart from the Lake Superior area (Fig. 1) can also be interpreted in terms of an oscillating non-dipole field, although they are not yet dated with modern standards (Donadini, 2007). Figure 1. Paleomagnetic poles of Keweenawan intrusions from Pigeon River (PR), Lake Nipigon (NP) and Thunder Bay (TB) areas. Large (small) symbols denote sills (dykes) and open (closed) symbols denote R (N) polarities, respectively. Some key ages are added. Also shown are the ca. 1140 Ma old poles from the Abitibi dykes, Ontario (both polarities) and new results from the Central Arizona dykes (both polarities). The dotted swathe is the western arm of the Logan Loop. References Books, K.G. (1972). USGS Surv.Prof. Pap., 760, 42 p. Davis, D. et al., 1995. ILSG, 41, 9-10. Donadini, F. (2007). PhD thesis, Univ. of Helsinki, 188 p. DuBois, P.M., 1962. Geol.Surv. Can. Bull. 71, 1-75. Halls,H.C.&Pesonen, L.J (1982). GSA Mem., 156, 173-201. Halls,H.C&Stott, G (2005). OGS Open file report, 617, 52 p. Heaman, L. M. et al. (2007). CJES, 44, 1-32. Hollings, P. et al. (2007). CJES, 44, 389-412. Middleton, R.S. et al. (2004). JGR-B, 109, 2103, doi:10/1029 Palmer, H.C. (1970). CJES, 7, 1410-1436. Pesonen, L.J., 1979. Bull. Geol. Soc. Finl. 51, 27-44. Pesonen, L.J.&Nevanlinna, H., (1981). Nature, 294, 436-439. Robertson, W.A.&Fahrig, W., 1971. CJES, 8, 1355-1372. Swanson-Hysell, N. et al. (2006). GSA Abtracts, 38, 398 68 �GEOLOGICAL MAP OF THE NORTHERN SOUTH KAWISHIWI INTRUSION AND SURROUNDING AREAS, DULUTH COMPLEX: ST. LOUIS AND LAKE COUNTIES, NORTHEASTERN MINNESOTA PETERSON, Dean (Natural Resources Research Institute, University of Minnesota Duluth) The recent boom in metal prices has brought about a worldwide resurgence in the exploration for virtually all mineral commodities, and the Lake Superior district has certainly seen its share of these exploration dollars. In the Duluth Complex of northeastern Minnesota, five companies are actively working on Cu-Ni-PGE properties in the Partridge River (Polymet Mining, TeckCominco) and South Kawishiwi (Franconia Minerals, Duluth Metals Limited, and Encampment Minerals) intrusions. Such mineral exploration relies on published geological information (maps, reports) from geological surveys and/or academic organizations. The publication of a new bedrock geological map of the northern South Kawishiwi intrusion (SKI) and adjacent areas has been one of the author’s research projects over the last several years. Such work, if it is completed correctly, adds important knowledge that can be directly integrated into the detailed databases that the companies maintain for their properties. The caveat (if it is completed correctly) relates to how seamless the published data can be incorporated into exploration programs, resource calculations, and mine plans. For this project, geologic units that the companies use to define the igneous stratigraphy of the SKI (based on ~700 drilled within the map sheet) have been used to define the map units of the basal zone of the SKI. The author’s mapping has evolved from a geological mapping study of a small area (to understand magma inflow into the SKI) into a comprehensive geologic mapping and compilation project (~105,000 acres) to answer some of the fundamental questions on the origin of the extensive known and undiscovered Cu-Ni-PGE mineralization in the northern portion of the SKI. Such an increase in scope is needed due to the economic significance of the published resource estimates (>$140 billion) from this area. To date, over 15,000 outcrops, 1,400 structural measurements, and 12,500,000 meters of elevated contour lines have been integrated into the comprehensive GIS database. The new map area includes the geology from each of the seven major lithologic units in the area: the footwall Late Archean Giants Range batholith and Paleoproterozoic Biwabik Iron and Virginia Formations; the enclosing Mesoproterozoic Anorthositic Series rocks; and the Troctolitic Series Bald Eagle (BEI) and SKI intrusions and the arcuate Nickel Lake Macrodike that links the BEI and SKI. A new insight of this work has been the recognition that the northern SKI is not a shallowly dipping sill but rather a southwest trending funnel-like body. Such an interpretation leads to the conclusion that the eastern contact of the SKI, which previously was interpreted as the top of the intrusion, is a basal contact, and thus has great potential for hosting Cu-Ni-PGE mineralization at depth. The map presented in the 2008 ILSG poster session should be viewed as an update, as there are still several large areas that are yet to be mapped in detail. 69 �References Foose, M.P., and Cooper, R.W., 1978, Preliminary geologic map of the Harris Lake area, northeastern Minnesota: U.S. Geological Survey Open-File Report 78-385, 24 p., 1 plate, scale 1:12,000. Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Geologic map of Gabbro Lake quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-2, scale 31,680. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota: Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr., Severson, M.J. and Foose, M.P., 2005, Bedrock geology of the Babbitt Northeast quadrangle, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map M-160, scale 1:24,000. Peterson, D.M., Patelke, R.L., and Severson, M.J., 2004, Bedrock geology map and Cu-Ni mineralization data for the basal contact of the Duluth Complex west of Birch Lake, St. Louis and Lake Counties, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2004-02, scale 1:10,000. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock Geology of the Nickel Lake Macrodike and Adjacent Areas: Lake County, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. Peterson, D.M., 2006, Digital Base for Geological Mapping within the Northern South Kawishiwi Intrusion: Lake and St. Louis Counties, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2006-01, scale 1:20,000. Phinney, W.C., 1967, Reconnaissance geologic map of part of the Kangas Bay quadrangle: Minnesota Geological Survey file map, scale 1:24,000. Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., and Peterson, D., 2007, Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion, Lake County, Northeastern Minnesota: Precambrian Research Center Map Series PRC/MAP-2007-01, scale 1:10,000. 70 �A Study of the Paragenetic Stages of Mineral Growth in Complex Iron Ores at the Tilden Mine and Development of a Mine Scale Model for Application to Ore Treatment Methods Natalie J. Pietrzak1, Norm Duke1, Glenn Scott2 and Helen Lukey2 1 2 Earth Sciences Department, University of Western Ontario, London, Ontario N6A 5B7 Canada Cliffs Technology Group, Cliffs Mining Services Company, Ishpeming, 49849, Michigan, USA. The Tilden Mine in Negaunee, Michigan is owned and operated by the Cleveland-Cliffs mining company. The Tilden operation is mining a segment of the Negaunee Iron Formation that is stratigraphically located within the Proterozoic aged Marquette Range Supergroup (Lougheed, 1983). Cleveland-Cliffs currently experiences a reduction in pellet quality when processing ore mined from various locations within the Tilden pit. From a geological prospective, factors that can effect the plant operation and pellet quality are related to mineralogy and mineral textures. In previous work, the Tilden ore body has been divided into a series of geologic domains based on lithology, structure and metallurgy. Five principle domains have been outlined using the major metadiabase horizons as markers: 1) Northwest Domain; 2) CDIII-West Pit Domain; 3) Main Pit Domain; 4) Intrusive Domains and; 5) Main Pit Footwall Domain. These domains are then further subdivided based on ore type, ore lithology and metallurgy (Lukey et al., 2007). Focus of this Phd thesis is to characterize the mineralogical domains at pit scale. With the results of this thesis, the Tilden staff can determine the mineralogical variations that affect their pellet production quality. The main objectives of this project include: 1) to determine ore composition and textures of the metallurgically classified domains and; 2) characterize the mineralogical variations and separate primary, metamorphic and/or hydrothermal stages. Lougheed, M.S., 1983, Origin of Precambrian Iron-Formations in the Lake Superior Region: Geological Society of America Bulletin, v. 94, p. 325-340. Lukey, H.M., Johnson, R.C., and Scott, G.W., 2007, Mineral Zonation and Stratigraphy of the Tilden Haematite Deposit, Marquette Range, Michigan, USA: Iron Ore Conference. 71 �VERMILION DISTRICT, NE MINNESOTA SARTORELLI, A. K., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, sartoa61@uwosh.edu ANDERSON, A., Precambrian Research Center, University of Minnesota – Duluth, Duluth, MN 55811 HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901 Locating volcanic vents can be problematic because many of the physical features that characterize active volcanism (eruptions, volcanoes, hydrothermal and fumarolic activity) are no longer present. Ancient submarine lava flows appear to have formed with similar morphologies to modern submarine lava flows (Jones, 1968; Moore, 1975; Dimroth, et al., 1978; Cas and Wright, 1987; Walker, 1992; Kennish and Lutz, 1998; Batiza and White, 2000); therefore, detailed mapping and facies analysis are essential tools for evaluating: 1) proximity to ancient vent sites; 2) the water depth in which ancient submarine lava flows formed; and 3) the paleotopography associated with the genesis of ancient lava flows. Evaluating such features is necessary to accurately reconstruct ancient volcanic environments (Dimroth et al., 1978; Walker, 1992), and can provide important information useful for locating natural resources in ancient volcanic rocks (Cas, 1992; Gibson et al., 1999; Franklin et al., 2005). Detailed (1:500 to 1:5000 scale) volcanic facies mapping was performed in the Fivemile Lake Sequence of the Lower Member of the Neoarchean Ely Greenstone Formation (EGLM; Peterson and Patelke, 2003) immediately west of Fivemile Lake in the Vermilion District of northeastern Minnesota during August and September, 2007. The purpose of this mapping (which was performed as part of an Undergraduate Student – Faculty Collaborative Research Grant from the University of Wisconsin Oshkosh) was threefold: 1) to further document the distribution of various flow morphologies (sheet lavas, pillow lavas) in the EGLM; 2) to utilize facies analysis in an attempt to determine ancient volcanic vent locations in the EGLM; and 3) to further evaluate the hypothesis that increased thicknesses of hyaloclastite deposits (glassy fragmental rocks) associated with ancient submarine lava flows appear to correlate directly with increasing proximity to ancient vent sites and anomalous base metal mineralization (Newkirk et al., 2001; Hudak et al., 2002a). The mapping included making detailed observations, descriptions, and measurements of pillow morphology, pillow strike, pillow dip and topping directions, pillow bud orientations, pillow dimensions (maximum, minimum, and average pillow horizontal and vertical dimensions), and thickness of interpillow hyaloclastite deposits. Petrographic studies were performed after the mapping to further evaluate the mineralogical and textural characteristics of these extremely well preserved ancient pillow lavas. Our detailed mapping indicates that the region immediately south and west of Fivemile Lake is composed of pillow lavas which were erupted from two separate pillow volcanoes. Pillow budding directions indicate that the oldest pillows were formed in a shallow submarine environment and flowed east from a volcanic vent or vents located west of Fivemile Lake, whereas younger pillow flows flowed west from a shallow submarine volcanic vent or vents located east of the study area. Based on previous studies by Newkirk et al. (2001) and Hudak et al. (2002), this vent or vents may have been located along a north-northeast-trending synvolcanic structure immediately east and northeast of Fivemile Lake. It appears that our 72 �detailed pillow measurements are able to distinguish the products of overlapping pillow volcanoes that were active approximately 2.7 billion years ago. References Batiza, R., and White, J. D. L., 2000. Submarine Lavas and Hyaloclastite: in Sigurdsson, H. (editor in chief), Encyclopedia of Volcanoes: Academic Press, San Diego, p. 361-381. Cas, R. A. F., 1992. Submarine volcanism: eruption styles, products, and relevance to understanding the host-rock successions to volcanic-hosted massive sulfide deposits: Economic Geology, v. 87, p. 511-541. Cas, R. A. F., and Wright, J. V., 1987. Volcanic Successions: Chapman and Hall, London, 528 p. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1978. Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec, Canada: Canadian Journal of Earth Science, v. 15, p. 902-918. Franklin, J. M., Gibson, H. L., Jonasson, I. R., and Galley, A. G., 2005. Volcanogenic massive sulfide deposits: Society of Economic Geologists 100th Anniversary Volume, p. 523-560. Gibson, H. L., Morton, R. L., and Faculty Member, 1999. Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits: in Barrie, C. T. and Hannington, M. D., 1999, Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings, Reviews in Economic Geology, v. 8, p. 13-51. Hudak, G. J., Heine, J., Newkirk, T. T., Odette, J., and Hauck, S., 2002a. Comparative Geology, Stratigraphy, and Lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS Occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 p. Jones, J. G., 1968. Pillow lava and pahoehoe: Journal of Geology, v. 76, p. 485-488. Kennish, M. J., and Lutz, R. A., 1998. Morphology and distribution of lava flows on mid-ocean ridges: a review: Earth Science Reviews, v. 43, p. 63-90. Moore, J. G., 1975. Mechanism of formation of pillow lava: American Scientist, v. 63, p. 269277. Newkirk, T. T., Faculty Member, and Hauck, S. A., 2001. Preliminary lava flow morphology studies at the Fivemile Lake VMS prospect, Vermilion District, NE Minnesota: implications for volcanic processes, volcanic paleoenvironments, and VMS exploration: Geological Society of America Abstracts and Programs, v. 33, no. 6, p. A-398. Peterson, D. M., and Patelke, R. L., 2003. National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: NRRI Technical Report NRRI/TR-2003/29, 88p. Walker, G. P. L., 1992. Morphometric study of pillow size spectrum among pillow lavas: Bulletin of Volcanology, v. 54, p. 459-474. 73 �Geochemistry of the Sudbury Impact Layer, Northern Michigan: Implications for the Nature of the Source Materials K.J. Schulz and W.F. Cannon, U.S. Geological Survey, Reston, VA 20192 (kschulz@usgs.gov, wcannon@usgs.gov) We have analyzed selected materials from the Sudbury impact layer in Northern Michigan for major and trace elements including rare earth elements (REE). Samples include accretionary lapilli, black, possibly devitrified glass fragments, vitric-rich breccia, and matrix from chert breccia. Major element concentrations are variable in these samples; however, most element concentrations vary inversely with increasing SiO2 content. Matrix from a chert breccia at the base of the Hiawatha Graywacke in the Iron River-Crystal Falls district has the lowest SiO2 (61.1 %) and highest FeOt (24.5 %) and P2O5 (1.44 %) contents that probably reflect the presence of a significant component of the Riverton Iron-Formation that directly underlies the breccia. Two black, possibly devitrified glass fragments from drill core at L’Anse have the highest SiO2 contents (~ 95 %) and concomitantly low concentrations of all other major elements. The other samples show a range of SiO2 content from about 71 to 82 %. A distinctive characteristic of all but one of these impact layer samples is very low CaO (after removal of secondary carbonate) and Na2O contents (Fig. 1a). The trace element concentrations of the impact layer samples in Michigan also tend to decrease in abundance with increasing SiO2 content. However, the samples mostly have similar ratios of relatively immobile trace elements (e.g., Zr/TiO2, Zr/Nb, Th/Hf, Sc/Yb). Chondritenormalized REE patterns are similar for all impact layer samples and are characterized by enriched light-REE ([La/Yb]n = ~8 to 20) and no to moderate negative Eu anomalies (Eu/Eu* = 0.65 – 0.94). Chondrite-normalized extended trace element patterns also are similar for most of these impact layer samples and have prominent negative anomalies for Ba, Nb, Ta, Sr, and Ti, and no to negative P anomalies (Fig. 1b). The low Sr abundance complements the low CaO and Na2O content and suggests a very low plagioclase component in the impact layer samples. All of the samples have a relatively high U/Th ratio (0.46-7.43; mean = 1.94 versus 0.26 for average upper crust) that probably reflects secondary redistribution of uranium. In addition, the accretionary lapilli are anomalously enriched in Y, and two samples also are enriched in V and Cr. These enrichments may reflect secondary carbonate complexing, precipitation from seawater under appropriate redox conditions and/or prior enrichment of source materials. The most ironrich sample is also anomalously enriched in V. The compositional similarity of the accretionary lapilli and vitric-rich breccia samples (Fig. 1) suggests that the composition of the impact layer materials primarily reflects that of the source materials at the site of impact. Compared to the Onaping Formation, which represents the impact-related fallback breccia at Sudbury (Ames et al., 2002), the impact layer samples in Michigan have higher SiO2, FeOt and K2O contents, and lower abundances of the other major elements particularly CaO and Na2O (Fig. 1a). The abundances of most trace elements also are generally lower in the Michigan impact layer samples, but their chondrite normalized patterns are very similar to those of the Onaping Formation with the exception of prominent depletions of Ba, Sr and Ti (Fig. 1b). The low abundances of Ca, Na, Sr, and Ba in the Michigan impact layer samples are interpreted as a primary feature reflective of the original source material. Such low abundances are not common of most crustal rocks, but are characteristic of sediments derived from intensely 74 �weathered terranes in which Ca, Na, and Sr are selectively leached from weathering profiles. This suggests that the dominant source material for the impact layer was most likely sediments derived from an intensely weathered terrane. Carbonaceous shales in the Onaping are not depleted in Ca, Na, Sr and Ba, but shales in the Iron River-Crystal Falls district of Michigan are characterized by low abundances of these elements and have overall trace element patterns similar to those of the Sudbury impact layer samples (Fig. 1b). We suggest that similar sediments also were present in the Sudbury area at the time of impact and form the dominant component of the material ejected distally from the impact site. This conclusion is supported by the results of impact modeling studies that predict that the upper crustal portion of the shock-melted target rock volume at Sudbury would be ejected away from the crater area and produce lithological and chemical variations with radial distance from the crater in which deep basement components would dominate near the crater (i.e., Sudbury Igneous Complex) and upper crustal sedimentary components would increasingly dominate farther from the crater center. _______________________________________________________________________ Ames, D.E., Golightly, J.P., Lightfoot, P.C., and Gibson, H.I., 2002, Vitric compositions in the Onaping Formation and their relationship to the Sudbury Igneous Complex, Sudbury structure: Economic Geology, v. 97, p. 1,541-1,562. Figure 1. a) CaO-Na2O-K2O diagram for the Sudbury impact layer samples, Michigan and the Onaping Formation, Ontario, and b) chondrite-normalized extended trace element patterns for accretionary lapilli and vitric-rich breccia samples, Michigan. Also shown in 1b are fields for samples from the Onaping Formation, Ontario (Ames et al., 2002) (gray field) and shales from the Iron River-Crystal Falls (IR-CF) basin, Michigan (Schulz, unpublished data) (stippled field). 75 �Penokean Massive Sulfide Deposits: Age, Geochemistry, and Paleotectonic Setting K.J. Schulz1, S.W. Nicholson1, and W.R. Van Schmus2 1 U.S. Geological Survey, Reston, VA 20192 (kschulz@usgs.gov) (swnich@usgs.gov) 2 University of Kansas, Lawrence, KS 66045 (rvschmus@ku.edu) The Paleoproterozoic volcanic terrane that extends across northern Wisconsin eastward into northern Michigan hosts a number of copper-zinc±lead±gold massive sulfide deposits including Crandon and the recently discovered Back Forty deposit. These deposits collectively contain more than 100 million tons of identified base- and precious-metal mineralization and represent a large but still mostly undeveloped resource. The age of the volcanic terrane in northern Wisconsin is constrained between about 1890 and 1860 Ma by the dating of mostly intrusive rocks within the belt (Sims et al., 1989); however, published ages have not been available for the massive sulfide deposits. We have analyzed zircons from two samples from the Back Forty deposit, Menominee County, Michigan; a quartz porphyry from the footwall and a feldspar porphyry that intrudes the deposit. Zircon fractions from the two samples cluster within error along a common cord and regression through the origin defines a U/Pb age of 1874 ± 4 Ma (Fig. 1). This age is within error of an unpublished U/Pb zircon age (~1870 Ma) determined for the Lynne deposit in Wisconsin (R. Thorpe, personal communication to T. DeMatties, 1995) and firmly establishes that massive sulfide formation within the Wisconsin volcanic terrane was contemporaneous with iron formation deposition in the foreland basins in Michigan and Minnesota. To expand on our previous geochemical studies of the volcanic rocks hosting the Bend and Pelican massive sulfide deposits in northern Wisconsin, we have now analyzed volcanic rocks from the Flambeau, Thornapple (Eisenbrey), Lynne, Ritchie Creek, and Back Forty deposits. These results show that the felsic volcanic rocks hosting the Bend, Pelican, Ritchie Creek and Back Forty deposits, located in the central and eastern parts of the terrane, have similar orogenic calc-alkaline compositions and are characterized by strong enrichments in highly incompatible trace elements including Th (Fig. 2) and light REE ([La/Yb]n ~ 6-15). In contrast, the felsic volcanic rocks hosting the Flambeau and Thornapple deposits, both located in the western part of the volcanic terrane, are much less enriched in Th (Fig. 2) and other incompatible trace elements ([La/Yb]n ~ 3-6). The volcanic rocks hosting the Lynne deposit have compositions that are mostly transitional between the other two groups (Fig. 2). The enriched Th and light REE of the Bend, Pelican, Ritchie Creek and Back Forty felsic volcanic rocks are similar to calc-alkaline felsic volcanic rocks in continental back-arc and intra-arc rifts such as the Taupo volcanic zone in North Island, New Zealand; whereas, the compositions of the Flambeau and Thornapple felsic volcanic rocks are comparable to felsic rocks in oceanic (Kermadec) and evolved island arc (Kuroko) back-arc basins (Fig. 2). The enriched trace element chemistry of the volcanic rocks in the central and eastern portion of the volcanic terrane in Wisconsin and Michigan suggests that continental basement is more widespread in the terrane than previously recognized. Paleozoic massive sulfide deposits associated with similar enriched calc-alkaline felsic volcanic rocks that formed in extensional continental arc settings are generally characterized by a significant lead content (e.g., Bathurst deposits; Yang and Scott, 2003). In contrast, the Penokean massive sulfide deposits are generally lead-poor, with the 76 �exception of the Lynne deposit. The reason for lead-poor nature of the Penokean deposits remains a topic of investigation. References cited: Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2,145-2,158. Yang, K., and Scott, S.D., 2003, Geochemical relationships of felsic magmas to ore metals in massive sulfide deposits of the Bathurst Mining Camp, Iberian Pyrite Belt, Hokuroku District, and the Abitibi Belt: Economic Geology Monograph 11, p. 457-478 Figure 1. Concordia diagram for zircon fractions from footwall quartz porphyry (108415) and cross cutting feldspar porphyry (108408), Back Forty massive sulfide deposit, Menominee County, Michigan. Figure 2. Th vs Hf diagram for felsic volcanic rocks hosting Penokean massive sulfide deposits. Also shown are fields for more recent felsic volcanic rocks from extensional back-arc settings ranging from oceanic (Kermadec, SW Pacific) to evolved island arc (Kuroko, Japan) to continental (Taupo, NZ and Bathurst, NB). 77 �GEOLOGICAL MAP OF THE NICKEL LAKE MACRODIKE AND NORTHERN BALD EAGLE INTRUSION: LAKE COUNTY, NORTHEASTERN MINNESOTA THARALSON, Erik (Department of Geological Sciences, University of Minnesota Duluth, Minnesota)1 SWEET, Gabriel (Department of Geology, Macalester College, Minnesota)2 BOISJOLI, Troy (Department of Geology, St. Norbert College, Wisconsin)3 LENTZ, Brian (Department of Geoscience, Winona State University, Minnesota)4 FELLOWS, Tyler (Department of Geosciences, University of Wisconsin-Milwaukee, Wisconsin) PETERSON, Dean (Natural Resources Research Institute, University of Minnesota Duluth) 1 – Currently employed by Encampment Minerals, Ely Minnesota 2 – Currently MS student, Lakehead University, Thunder Bay, Ontario 3 – Currently employed by Cameco Mining, Saskatchewan 4 – Currently employed by Golden Chalice Resources, Timmins, Ontario The Precambrian Research Center’s (PRC) 2007 Precambrian field camp culminated in the mapping and publication of a series of bedrock geological maps from four areas in northeastern Minnesota. These “Capstone” projects were completed in areas of Minnesota where either previous mapping was only reconnaissance scale or in historically previously mapped areas (> 40 years ago) that are adjacent to ongoing research mapping projects. The new bedrock geology map of the Nickel Lake Macrodike and northern Bald Eagle intrusion of the Duluth Complex (presented in the poster session) is the result of seven days of field mapping by the authors in 2007. This map was created by PRC Field Camp students under the supervision and guidance of Dr. Dean Peterson, NRRI Senior Research Associate. The purpose of this map is to aid understanding the nature of the Nickel Lake Macrodike and its pertinence to Cu-Ni-PGE mineralization in the South Kawishiwi Intrusion (Peterson and Albers, 2007). Additionally, this map will extend the range of Dr. Peterson’s ongoing project to map in the northern half of the SKI. Access to the 641 outcrops mapped for this project was provided by extensive canoe shoreline mapping and traverses in the bush (approximately 300 kilometers of total mapping traverses. Mapping was completed at 1:5,000 and 1:10,000 scales and reduced for this map to 1:10,000. The new geologic information was integrated with previous data compiled from Green et al., (1966) and Peterson et. al., (2006) to form the foundation of the final published map. References Green, J.C., Phinney, W.C., and Weiblen, P.W., 1966, Geologic map of Gabbro Lake quadrangle, Lake County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-2, scale 1: 31,680. Peterson, D.M. and Albers, P.B., 2007, Geology of the Nickel Lake Macrodike and its association with Cu-Ni-PGE mineralization in the northern South Kawishiwi Intrusion, Duluth Complex, northeastern Minnesota: Institute on Lake Superior Geology, 53rd Annual Meeting, Trip #4, Field Trip Guidebook, Lutsen, Minnesota, Volume 53. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock Geology of the Nickel Lake Macrodike and Adjacent Areas: Lake County, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-200604, scale 1:10,000. 78 �The Effect of Magmatic Volatile Phase Separation Linked to Intrusion of the Duluth Complex: Solution to Anomalous Os Isotopic Compositions of the Virginia Formation? WILLIAMS, Curtis D., RIPLEY, Edward M., LI, Chusi, Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405, cuwillia@indiana.edu Both pyrite- and pyrrhotite-bearing pelitic rocks of the 1.85 Ga Virginia Formation show Re and Os isotopic ratios consistent with a mixing event at ~1.1 Ga. Initial 187Os/188Os ratios suggest that mixing must have involved a component with a near-chondritic Os isotopic ratio. The data can be simply interpreted to reflect the interaction of the Virginia Formation with mantle-derived (chondritic) magmas of the Duluth Complex. However, the Virginia Formation rocks in question occur one and one half kilometers or more from the Duluth Complex, are only slightly recrystallized and have never been in direct contact with magma. Fluid inclusions in layer-parallel quartz veins hosted by the Virginia Formation are characterized by pressure corrected (2 kb) homogenization temperatures between 500 and 600˚C. Together with oxygen isotope values of the quartz (5 ‰-6 ‰), the data suggest that magma-derived fluids may have been responsible for the Re-Os isotopic disturbance. In order to further constrain this premise we have undertaken LA-ICP-MS analysis of individual fluid inclusions and have evaluated the expected compositional variations of fluid derived from a mafic magma. We have utilized the program MELTS (Ghiorso and Sack, 1995) to evaluate the compositional variations expected in derivative melts and the program MagmaticVolatile-Phase (Candela and Piccoli, 1995) to estimate concentrations and ratios of elements in a fluid produced from H2O-saturated magma. A high-Al, olivine tholeiite was used as the parental magma, with initial H2O content varied between 0.2 and 2.0 wt%. At 75% fractional crystallization of a parental melt with 0.2 wt% H2O, the fluid produced is very Fe-rich (~4.9 wt%) with Fe≥Na>Ca>K>Mn≥Mg (assuming 1:1 Na melt-fluid partitioning). Fluids produced from 75% fractional crystallization and up to 1.0 wt% initial H2O show less pronounced differences between elements, with 1.2 wt% Fe, 1.1 wt% Na, 0.7 wt% Ca, 0.48 wt% K, 0.12 wt% Mn and 10ppm Mg. Fluids produced during equilibrium crystallization are also high in Fe, but Na and Ca display similar concentrations, and Mg concentrations for all starting H2O concentrations exceed 0.5 wt%. Relative concentrations also vary with Mg≥K>Mn. Between 1.0 and 1.2 wt% initial H2O, the derivative melts become saturated in H2O at variable crystallization intervals (85-51%, respectively) and exsolve a volatile phase. As in the case at 75% fractional crystallization, Fe is the dominant cation in these fluids, followed in order of decreasing concentration by Ca, K, Mn and Mg. As initial H2O is increased to 2.0 wt%, Fe concentrations rises (~2.2 wt%) such that Fe>>Ca>K>Mn>Mg. Modification of the starting melt composition was also varied via assimilation of Virginia Formation (melt minimum Qz(50)-Ab(24)-Mu(24)-H2O(2)). The assimilation of 10% Virginia Formation leads to the production of a volatile phase with less Fe (~3 wt%), but higher Ca (~1.1 wt%) and K (0.47 wt%). Both fractional and equilibrium crystallization of the contaminated 79 �starting composition show decreases between element concentrations as H2O content of the starting melt increases. To evaluate the initial Cl/H2O ratio of a melt, the program Magmatic-Volatile-Phase was used which incorporates this parameter along with the evolution of H2O in the melt. These models suggest a parental melt with 1.0 to 2.0 wt% H2O(i) will produce a Fe-rich vapor similar to that modeled indirectly by MELTS between 1.0 and 1.2 initial wt% H2O, with the exception of lower Ca concentrations. Our fluid inclusion analyses show Fe concentrations in excess of 1.3 wt%, while Na and K reach 0.16 wt% and 0.04 wt% respectively. Mg concentrations peak around 1700 ppm, while Ca is low or below detection limit. Compositions of the fluid inclusions are very similar to modeled results, which are consistent with the premise that interaction with magmatically-derived fluids was responsible for the anomalous Os isotopic signatures recorded in sulfide-bearing rocks of the Virginia Formation. 80 � https://digitalcollections.lakeheadu.ca/files/original/b76c560400c357210d234ab2d255788e.pdf d289755b7798e12879921a791be977d5 PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 54TH ANNUAL MEETING MAY 6-10, 2008 MARQUETTE, MICHIGAN HOSTED BY: Michigan Technological University THEODORE J. BORNHORST AND JOHN S. KLASNER Co-Chairs Proceedings Volume 54 Part 2 – FieldTrip Guidebook EDITED BY THEODORE J. BORNHORST AND JOHN S. KLASNER A. E. SEAMAN MINERAL MUSEUM, MICHIGAN TECHNOLOGICAL UNIVERSITY AND RETIRED WESTERN ILLINOIS UNIVERSITY Cover Photo: Brecciated banded iron formation from Ishpeming, MI. This photograph is similar to Plate XXV in Van Hise, Bayley, and Smyth, 1897, U.S. Geological Survey Monograph 28 “The Marquette Iron-Bearing District of Michigan.” (photograph by Tom Waggoner) i �54TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 54 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE—SEISMITES FROM THE SUDBURY IMPACT? TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT TRIPS 4 AND 8: GEOLOGY OF THE EAGLE PROJECT TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Published by the 54th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 54 PART 2— FIELD TRIP GUIDEBOOK TRIP 1: BANDED IRON FORMATION OF THE MARQUETTE DISTRICT .............................................1 TRIP 2: ARCHEAN-PALEOPROTEROZOIC UNCONFORMITY AT SILVER LAKE ............................45 —SEISMITES FROM THE SUDBURY IMPACT? TRIP 3: GEOLOGY OF THE BACK FORTY PROJECT ..........................................................................65 TRIP 4 AND 8: GEOLOGY OF THE EAGLE PROJECT ..........................................................................87 TRIP 5: THE SUDBURY IMPACT LAYER AT THE MCCLURE LOCALITY......................................115 TRIP 6: SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT .....................127 TRIP 7: GEOLOGY OF THE KEWEENAWAN BIC INTRUSION ........................................................181 iii �iv �54th Annual Institute on Lake Superior Geology Field Trip 1 BANDED IRON FORMATION OF THE MARQUETTE DISTRICT Tom Waggoner Consulting Geologist retired, Cliffs Mining Services Company The absence of evidence is not Evidence of absence Carl Sagan 1 �BANDED IRON FORMATION OF THE MARQUETTE DISTRICT This two day field trip within Marquette County will concentrate on the distinct types of iron deposits and clues to their formation. The trip will include both active and idle mining sites and outcrop locations never included in any prior field trip of the Marquette iron range. ClevelandCliffs will open their mines for the group. This field trip guide will include a narration of the overall Proterozoic geology of the Marquette Range. All Figures and Tables denoted with capital letters are part of the text. All figures denoted by a small “f” are found on the accompanying CD where full color will enhance their value in the discussion The CD also has plan geology maps of each of the stops and a photo gallery of old mining pictures taken throughout the history of mining on the range. Trip stops will include examples of banded magnetite, hematite, carbonate, silicates, clastics), hard ores (microplaty, specularite, magnetite) and supergene oxidation with enrichment. We will visit several examples of silica/hematite vents along with examples of hydrothermal alteration. We will also examine the late non ferrous metal overprint on the Negaunee iron formation. Acknowledgements I would like to thank Cleveland-Cliffs Inc. for reproducing the plan geology map of the Marquette range and for permission to attend their operations on the range. Appreciation is also expressed to all of Cliff’s personnel including Glenn Scott, Helene Lukey and Al Strandlie who helped on the field trip and in moving the drill core for examination during each day’s lunch session. USX is also acknowledged for allowing access to their Champion property The old photos presented in the CD have been graciously contributed by Cleveland-Cliffs, Jack Deo of Marquette, the Michigan Iron Industry Museum and the State Archives of Michigan. Both the Iron Industry Museum and the Cliffs Shaft Museum are acknowledged for hosting a lunch session on each of the field days. Thanks also go to the City of Negaunee for hydraulically cleaning outcrops in the area of the Jackson Mine. I am also indebted to John and Gretchen Klasner for their patient editing of the field trip guide. MARQUETTE RANGE SUPERGROUP Paleoproterozoic strata of the Marquette Range Supergroup (MRS) lie within the Marquette and Republic troughs that formed within the Archean basement (see plan geology foldout map). The three Groups (i.e. Chocolay, Menominee and Baraga) are related to plate tectonic activity associated with the Penokean and younger plate tectonic events (Schulz, 2007). They were drapfolded and faulted into the underlying troughs in the basement forming the gently west or northwest-plunging Marquette and Republic synclines respectively. The Chocolay Group includes a basal conglomerate overlain by quartzites, carbonates and slates. The Menominee group contains alternating slates and quartzites with a limited banded iron formation overlain by the economically significant Negaunee iron formation. The stratigraphic column includes the Hemlock Volcanic formation from the Amasa Oval where an overlying iron formation was emplaced at the close of volcanic activity. An unconformity separates the iron formation from the overlying Baraga Group. 2 �Chocolay Group Enchantment Lake Formation This group was named by Gair and Thaden (1968) for a sequence of conglomerate, greywacke and slate found at the eastern end of the Marquette Syncline. It lies on Archean basement or Mesnard quartzite. It has been described as lenticular by Gair (1975) who felt it was missing over topographic highs in the older terrain. The thickness of the basal unit varies from 0 to 600 feet. The unit correlates with the Fern Creek found in the Menominee and Felch Districts. In the Marquette area of the Animikie basin the earliest unit is a fairly thin sequence of conglomerate with minor sandstone and shale. The conglomerate contains clasts of the underlying local Archean terrain indicating limited movement of the clasts. Interpreted dropstone evidence convinced Pettijohn (1943), Gair (1975) and Ojakangas (2001) to conclude the unit was deposited in a glacial environment. However, Bayley et al., (1966), Gair and Thaden (1968) and LaRue (1980) favored an alluvial fan depositional environment. Van Hise and Lieth (1911) noted that the composition of the conglomerate is a function of the underlying rock upon which it rests. The conglomerate in section 22, T. 46 N., R. 26 W. consists of salmon colored cobbles or boulders (fig. 1). In the SE, NE, section 22, T. 47 N., R. 26 W. abundant octahedral and platy hematite replacing fine detrital grains found between the pebbles (fig. 2). Gair (1975, p.17 & 18) noted elevated potassium and sodium values for the slate component indicating that fluid influx caused the alteration. Mesnard Quartzite This unit was named by Van Hise and Bayley (1892) for the prominent outcrop in the Harvey area. The massive white vitreous quartzite attains a maximum thickness of 500 feet. It contains numerous ripple marks (fig. 3) and clasts and in places exhibits cross bedding. It is present only in the eastern half of the Marquette trough and has not been identified west of section 31, T. 48 N., R. 26 W. The predominant color of the well sorted quartz (+90% silica) is white. Numerous quartz veins and veinlets are common and some contain specular hematite. The unit is believed to be equivalent to the Sunday quartzite on the Gogebic Range, the Sturgeon quartzite on the Menominee Range and the Pokegama quartzite on the Mesabi Range. Kona Dolomite The major Kona outcrop area is on the eastern end of the Marquette syncline. The unit is not present in the central and west portions of the trough either due to faulting or non deposition. The Kona formation consists of dolomite (48%), argillite (42%) and quartzite (10%). The argillite was originally siltstone, mudstone or clay stone. Thickness of the unit varies between 0 and 800 meters. Pastel colors of cream, buff, pink, orange, salmon, tan, maroon, and purple characterize most of the carbonate series. The argillite is gray, green, tan, orange and chocolate brown. The quartzite exhibits shades of white to pink (fig. 4) to red. Minor brown chert beds are also observed. Bedding thickness varies from one centimeter to five centimeters. Some dolomites exhibit oolites. Quartz grains both in the dolomite and quartzite average .05 mm while 3 �the grain size of the dolomite averages .3 mm with some dolomite beds containing a uniform coarse crystal size that averages .25 cm. The Kona has been correlated with the Randville dolomite on the Menomonee Range and the Amasa uplift area. It is also correlated with the Bad River dolomite on the Gogebic Range based on its relative position in the stratigraphic column and the absence of dolomites in general in the younger sediments throughout the entire Lake Superior area. The Kona has many similarities to portions of the Lorraine formation north and east of Sault St. Marie, Ont. Taylor (1972) (Figure 1) identified eleven units that make up the Kona formation. His work helped to identify the distinctive members that in turn helped to resolve the structural movement along faults. Detrital quartz appears to originate from the south. The source direction of detritals for the younger iron formations is again shown to originate also from the south. Ripple marks (fig. 5) and other sedimentary features suggest a shallow or shore line environment. Bladed or fibrous rosettes of chert, irregular nodular quartz and cubic to rectangular bright red dolomite crystals indicate replacement of gypsum, anhydrite (fig. 6) and halite (fig 7). These occur in both the dolomite and pelitic sequences. 4 �Styolites can commonly be found between sedimentary units. Episodic super saturation could have caused the precipitation of the salts. There is abundant evidence of early life preserved in the Kona primarily in the form of stromatolites. These can be very large bioherms of the colleria type (fig. 8) or small thin undulating or corrugated mats (fig. 9) that have been replaced by silica and tend to weather out into thin wafer zones in the dolomite. Sharks Bay in Western Australia currently contains sub tidal stromatolites that grow under hyper saline conditions. Detail mapping utilizing known stratigraphy has confirmed a number of east-west striking high angle (75o to the north) normal faults with north side down parallel to the main syncline axis. The eastern terminus of the Marquette trough in the Harvey area appears to be a north-south flexure with the Proterozoic sequence being present to the east of Harvey under Lake Superior. 5 �Airborne magnetic data suggest a synclinal structure plunging to the east under the lake to the Keweenawan Rift hinge line east of the City of Marquette. Neither soft sediment slumping nor tectonic folding is prevalent in the Kona. Metamorphism, where it exists, is the low greenschist facies and lies within the chlorite zone (James, 1955). Intraformational brecciation is common (fig. 10) and may reflect a local tectonic event (Bayley et al, 1966) or could be due to a collapse in a supratidal environment (Larue, 1981). Isopach maps of six Kona members (Taylor, 1972) clearly illustrate a general thickening of the trough centering on section 6, T.47 N., R. 25 W. By the time the Negaunee iron formation was formed the largest basinal down warping had shifted southwestward to section 19, T. 47 N., R. 26 W. The general plunge of the Marquette syncline is to the west. The north side of the syncline exhibits a steep dip to the south with only minor north-south fault offsets. The south and southeastern areas exhibit domal flexures and complex fault offsets. A number of near-vertical, basic, fine-grained dikes (fig. 11) cut the Kona. Most are now primarily chlorite in composition. In addition there are abundant veinlets and joint fillings composed of quartz, carbonate, microcline and tourmaline. The Kona hosts strata bound copper sulfide resources, which have attracted exploration interest dating from 1888. Copper sulfides are found as disseminated grains, grain aggregates and shear zone fillings (fig. 12 & 13). Both vertical and lateral sulfide zonation have been identified. Over a 10 mile distance the sulfide assemblage shows a decrease in copper from east to west (Table 1). Table 1 Copper Assemblage of the Kona Dolomite from West to East WEST pyrite--------pyrite---------chalcopyrite-----bornite-------chalcocite chalcopyrite bornite chalcocite pyrite pyrite pyrite EAST Taylor (1972) suggested the copper was deposited in a Sabka environment involving connate waters that carried and deposited sulfides in a reducing environment. Since there is no evidence of a reducing environment existing at anytime, this is not a particularly appropriate process for the deposition of the Kona sulfides. An alternate source could be progressive hydrothermal precipitation much like that which occurred at White Pine. However, the only other alteration is silica flooding which does not follow the sulfides. The silica does carry specular and micaceous hematite. A resource of half a billion short tons of argillite/quartzite containing about 1% copper has been indicated. In addition to the copper mineralization there are concentrations of microplaty to specular hematite (fig. 14), the significance of which will be further discussed in the section covering the Negaunee, iron formation. The abundance of carbonate, algal features, psuedomorph quart/chert after evaporates, weathering, ripple marks and mud casts suggest a shallow lagoonal to open tidal environment with hypersaline water subject to intermittent subaerial exposure. Further, a warm to temperate climate is most likely to have accompanied this environment. 6 �Wewe Slate The Wewe occurs only on the eastern end of the Marquette Syncline and does not have an equivalent anywhere else in the Paleoproterozoic Lake Superior Basin. The unit is characterized as a green-gray-black, fine grained, faintly-laminated or banded fissile argillite. The major components are quartz, sericite and chlorite. The thickness ranges between 400 and 3000 feet. Some zones contain 2-5% pyrite as disseminations, cubes or concretions and contain the occasional calcite veins. Outcrops are too limited to define the internal stratigraphy but existing diamond drill holes would provide an excellent basis for developing the detailed stratigraphy. Menominee Group Ajibik Quartzite The Ajibik is a generally white vitreous quartzite with some graywacke components. It lies uncomfortably over the Wewe and is in turn conformably overlain by the Siamo slate. A basal conglomerate occurs in many places around the Wewe Hills uplift. Gair (1975) noted in section 23, T. 47 N., R. 26 W. that the basal conglomerate contains jasperoid (silicified Kona dolomite) and clasts of andalusite/chloritoid schist suggesting some of the hydrothermal alteration occurred prior to formation of the conglomerate. The north end of the quartz/hematite stockworks transects the Ajibik and caused brecciation in the Ajibik. It suggests the linear stockworks zone was relatively long lived starting before the Ajibik and lasting until after the Negaunee Iron formation. The thickness of the Ajibik ranges from 450 to 600 feet. Siamo Slate Rock types included in this formation are slate with lesser feldspathic quartzite and graywacke. Both chlorite and sericite are locally abundant. The first iron formation that occurs in the Marquette trough is the Goose Lake member which is up to 100 feet in thickness. It can be traced via outcrop and magnetics for at least 5 miles on the south and east side of the trough. Gair (1975) identified several lithologic sub units within the Siamo: lower laminated slate, Goose Lake iron member, middle slate, quartzite/graywacke and supper slate with chert bands near the top. The transition to the overlying Negaunee is gradual. The Goose Lake member was named by Tyler and Twenhofel (1952) for the banded-laminated iron formation which is primarily siderite, chert with minor magnetite, chlorite and stilpnomelane. Phosphorus values are four times higher than those found in the Negaunee. The Siamo ranges in thickness from 1000 to 3100 feet. Outcrops of the Goose Lake can be found along the shoulders of M-35 just northeast of the Empire Mine in sections 15, 16 and 20 T. 47 N., R. 26 W. 7 �Negaunee Iron Formation The Negaunee iron formation has been the focus of economic exploitation since its discovery in 1844. Early mining concentrated on the hydrothermal hematite and magnetite ore (hard ores) that outcrop at surface on topographical highs. Later during the 1800s shallow supergene enriched hematite (soft ores) deposits were brought into production via underground mining methods. By the middle of the 1900smost of these deposits were depleted. Throughout the period of hard and soft ore production, significant tonnages of banded iron formation were mined and shipped as siliceous ore that was used in the furnace operation to provide adequate slag production. During the 1950s concentration schemes were applied to the banded iron formation to produce an iron concentrate/pellet with relatively low silica and superior physical and chemical characteristics that improve furnace productivity. The Negaunee iron formation is one of many iron formations that occurs in the much larger Paleoproterozoic Animikie basin and whose remnants can be found around Lake Superior in Wisconsin, Minnesota, Michigan and Ontario. The collective iron deposits became known as the Lake Superior Type (LST) as opposed to the smaller Algoma Type. A comparison of LST and Algoma Type features of banded iron formations (bifs) (Table 2) would suggest more difference than actually exists. The argument that LST lack an igneous association is not valid. The Animikie basin contains some very extensive extrusive rocks in the form of the Clarksburg, Hemlock and Emperor volcanics. Similarly, bifs in both Western Australia and S. Africa contain many tuff intervals (called shales) within the iron formation (LaBerge, 1966, 1966). Table 2 Features Common to LST vs Algoma Type BIF LST low igneous association 1.8-2.4 b.y. rifting environment low S, Na, K, Al, P higher CO2 Algoma direct igneous association +2.5 b.y. volcanic arc environment higher S, Na, K, AL, P lower CO2 8 �More recent exploration worldwide has identified large iron formations in the Sokoman in Labrador, the Hamersley and Nubbaru in Australia, the Minas Gerais and Carajas in Brazil, the Kuruman/Griqualand deposits in South Africa and the Krivoy Rog in the Ukraine. It has long been debated as to whether the Negaunee is a time equivalent to the Biwabic, Gunflint, Ironwood, Fence, Riverton and Vulcan. Recent work on defining the existence and position of the Sudbury ejecta (Cannon, 2007) has established that these iron formations are essentially time equivalents. It also indicates the iron formation is older than 1850 Ma and slightly younger than 1875 Ma. Unlike the Gogebic and Mesabi Ranges where the stratigraphic units within the iron formation are traceable over many miles, the Negaunee is extremely variable over very short distances indicating variation in iron source, transportation, environment of deposition, diagenetic alteration and the degree of metamorphism. A general stratigraphic correlation (Figure 2) can be made by the commonality of certain features (Waggoner, 1972). For example the predominately clastic horizon present in the southeast corner of the Marquette trough and the Palmer fault block makes good marker horizon for the southeast end of the syncline. The original mineralogy of LST and Algoma banded iron formations are quite simple in that they contain silica in the form of chert and iron in the form of carbonate, oxides, silicates and sulfides. They also contain ubiquitous apatite and can also be associated with fine to medium clastic components. Common silicates present on the eastern portion of the Marquette trough are stilpnomelane and minnesotaite. Based on the presence of greenalite on the very western part of the Biwabic range where the metamorphic effect is non existent, it can be speculated that greenalite could be a precursor to either or both these silicates. Heat or pressure can transform 9 �the low metamorphic silicates to either cummingtonite or grunerite. Further heat and/or pressure can result in these silicates becoming amphiboles and pyroxenes. Indeed, in the vicinity of the large diabase sills on the eastern end of the Range the low grade silicates have been converted to coarse grunerite. Sodium rich zones in the Negaunee contain Riebeckite (fig. 15) and Acmite (fig. 16). Sodium rich minerals are present in LST bifs worldwide. Crocidolite and riebeckite are ubiquous to the iron formation in Western Australia (fig. 17) and South Africa (fig. 18). Influx of sodium into the early sediments translates into thee minerals during diagensis or low rank metamorphism. Calcium sulfate in the form of gypsum is quite common on the eastern end of the Marquette trough and eastern end of the Palmer Fault block. It can be present in both oxidized bif or as the soft ore matrix. Its presence is detrimental to ore quality and successful beneficiation applications. Quartz veins with micaceous hematite (fig. 19) are believed to have been emplaced during the formation of the hydrothermal hard ore. Cobbles of this material are included in the basal Goodrich conglomerate. The four major iron minerals we find today in the field do not necessarily reflect the original minerals. Banded iron formations on the Marquette Range have undergone diagenetic alteration. The majority of the magnetite (Han, 1971) and martite ore was originally carbonate silicate chert. Remnants of this lithology can be found at both the Empire and Tilden deposits. Han (1982) has shown that the core of most magnetite grains were originally very fine hematite that was altered to magnetite under a reducing diagenetic environment. The magnetite carbonate horizon at the Empire Mine was the exception as there was never any primary hematite and the magnetite formed was due to selective volume for volume replacement of iron carbonate. The seed hematite is found through the major bifs in the world (Han, 1988). The bulk of the magnetite, formed as a replacement to the iron carbonate, is fine grained (less than 50 microns) and exhibits a sooty gray black color. The carbonate is generally a mixture of siderite, ankerite, iron dolomite and calcite with variable amounts of Ca, Mg and Mn. With an abundance of Fe++ available much of the iron carbonate has been converted to magnetite with or without a fine hematite precursor seed core. Most of the iron silicates we see in the iron formation are either due to conversion of preexisting silicates/carbonate during digenesis ore metamorphic processes. Sulfides are conspicuous by their general absence from both the stratigraphic column and specifically in the Negaunee iron formation. However, at some places where hard ore is found, sulfides do occur and can be locally abundant. Pyrite and chalocopyrite predominate but bornite and pyrrhotite have been noted. A suite of sulfides representing nine geographical locations was analyzed for sulfur isotope values (Waggoner, 2006). Sulfur isotope values ranged from .02 to 6.8 %o indicating they most likely were hydrothermal in origin and support the hydrothermal origin of hard ores in general. Jasper may form by replacement of the gray/white chert with fine hematite resulting in orange jasper like that found at the Milwaukee-Davis south of Negaunee (fig. 20). This replacement method usually leaves “islands” of unaffected chert suggesting the replacement. Most of the bright red jasper on the eastern portion of the Marquette trough shows multi hued layers but they do not show any indication of replacement like cross cutting veins and gradation to gray/white chert. The bright jasper could be primary at a vent site that in turn grades laterally into normal color chert and carbonate as opposed to replacement of the chert carbonate. The presence of fine microplaty hematite as veins and diffusions (fig. 21) in the jasper shows definite signs of cross 10 �cutting and replacing the jasper. Replacement of the jasper proceeds from simple fracture and vein filling through brecciation and silica removal resulting in a high grade hard hematite ore. This sequence of replacement can be illustrated by a sequence of samples showing the stages of replacement (fig. 22-26). There is only a little existing evidence of life forms in the Negaunee unlike the Gunflint and Biwabic where there is evidence of extensive reef building. Mancuso, et al (1971) described some physical features that could be algal mats. Lougheed, et al (1973) described hematite framboid psuedomorph after pyrite that could have formed from a decaying biomass of some life form. Han et al (1992) described floating algae (fig. 27) that were preserved in the slaty layers of the magnetite-carbonate-silicate horizon of the Negaunee mined at the Empire Mine. Evidence suggests various life forms thrived in the environment associated with the generation of banded iron formations. Many workers have suggested life forms are integral to the precipitation of the iron minerals in bifs, however, preservation of the delicate banding of the chert in all color forms suggest life was not a major presence at the time of formation. Moreover life forms are not the only ways the banded iron formations could have formed. A combination of particulate matter and hydro gels can also be hypothesized. Iron formations have rationally been viewed as an entirely chemical precipitate. Since 1977 we now know sulfides can form instantly upon exiting vents (black smokers) as can calcium and silica (white smokers). It is quite possible that iron in the form of hematite and magnetite (temperature sensitive) can form in this same manner and accumulate as discrete bands. Both hydrothermal hematite and magnetite exhibit a metallic luster unlike hematite and magnetite generated by diagenetic replacement or supergene oxidation/enrichment seen on the eastern Marquette Range. James (1955) indicated primary hematite and magnetite are fairly inured to conversion to other forms during either diagensis or metamorphism other than expand in crystal size. In addition to diagensis and metamorphism there is supergene oxidation and enrichment of carbonate, silicates and magnetite to hematite, goethite and martite (James, 1953). Near surface oxidation and enrichment make a good case for top down oxidation by meteoric waters, possibly warmed by the exothermic reaction of oxidation of magnetite. The presence of certain clays (Bailey, 1960) can not easily be explained without a hydrothermal input. An example is the existence of dickite and high chrome nontronite (fig. 29) which does not generally form from weathering. The Negaunee iron formation on the northeast corner of the Marquette trough was originally iron carbonate silicate chert that has undergone supergene oxidation and enrichment to ore grade (fig.30 and 31). On the Marquette Range syncline axis form loci for ore formation floored by impervious slate or intrusives. Supergene ores generally form in structural lows where the underlying rock, in this case slate, is relatively impervious (Figure 3). In some instances basic dikes or faults can further constrain the formation of ore. On the Marquette Iron Range the bulk of supergene ore has been extracted by underground methods resulting in extensive caving of the surface due to poor structural integrity of the overlying rock. 11 �The chemistry of the various ore types shows the iron is usually increased during diagensis (Table 3). Table 3 MARQUETTE RANGE IRON ORE CHEMISTRY Carb Empire* Method Ore Type sol. Fe silica alum. phos lime magnes. Mn H2O s Silicous Richmond OP OP carb chert hem-mart. 30.5 39.5 28.9 37.3 1.37 0.98 0.1 0.042 1.29 0.23 4.6 0.37 0.6 0.09 0.7 3.22 0.018 Hard Ore Cliffs Sft Republic OP/UG fine hem. 61 8.61 1.24 0.163 0.07 1.21 0.029 UG spec. 64.3 5.35 1.38 0.074 0.46 0.36 0.5 0.5 0.01 Soft Ore Athens Salisbury UG UG earthy hem. 52.9 51.5 6.04 7.83 2.56 1.15 0.114 0.091 1.14 0.7 0.73 0.46 0.43 0.45 12.5 12.5 0.011 0.016 * Source-USGS PP 769 Source of others American Iron Ore Association Annual Analysis Book 12 �13 �Magnetite replacement of carbonate increases the iron while removing Ca, Mg and Mn. Oxidation of the ferrous iron to martite again increases the iron. Supergene oxidation and enrichment of the carbonate silicate chert increases the iron and reduces the other elements. Under some circumstances the manganese is also enriched as evidenced at the South Jackson and Lucy Mines in Negaunee. The uniformity in the chemistry for the Marquette Range soft ores (Table 4) suggests similar process acting on a common carbonate silicate chert protore. Table 4 Marquette Range Soft Ore Partial Chemistry Mine Ore Type Method Element Fe Silica Al2O3 Phos Lime Mag. Mn H2O Sul. Tracy* Salisbury* underground 54.1 6.57 2.24 0.08 0.29 9.45 0.315 underground Maas* Negaunee* underground underground 51.5 7.83 1.15 0.091 0.7 0.46 0.45 12.5 0.016 52.4 7.1 2 0.089 1.29 0.25 0.23 11.2 0.011 52 6.86 2.39 0.083 0.42 0.36 0.2 12.2 0.014 Athens* underground 52.9 6.04 2.56 0.114 1.14 0.73 0.43 12.5 0.011 *Data Source AIOA book Tracy--1962, Salisbury--1916, Maas--1916 Negaunee--1916, Athens--1930. From east to west there is a progressive change in the metamorphic mineral assemblage as indicated by James (1955). This is based primarily on the altered mineral assemblages in the iron formation. The associated stratigraphy both above and below the Negaunee do not necessarily show the same degree of alteration suggesting that much of the metamorphic effect is due to the late hydrothermal imprint of hematite and magnetite replacing earlier iron formation. Baraga Group Goodrich Quartzite The basal portions of the Goodrich in a few areas contain a unique conglomerate consisting of pebble of white vein quartz, jaspilite, oolitic jasper and hard hematite fragments. In some areas the interstices of the conglomerate are filled with hydrothermally emplaced hematite. Zones which have exhibited a high iron grade have been mined as iron ore (fig. 31). The Hard Ore Mine in Ishpeming and the Republic Mine have had considerable production from such deposits. 14 �The Goodrich Mine production came entirely from within the conglomerate. The schistose hematite pebbles where the hematite is arranged parallel to the chert banding now shows a chaotic arrangement indicating the schistose nature was present before being included in the conglomerate forming event. The presence of filled voids and replacement of fine detrital grains with hematite indicates the hydrothermal process continued throughout the period of conglomerate formation. The presence of the conglomerate would suggest that a period of tectonic change (Figure 4) preceded the formation of the conglomerate; an event that may have indicated a change from extensional basin development to one of compression, closing off the main iron mineralization and radically altering the nature of subsequent sedimentation. Figure 4 Possible Paleoproterozoic depositional environment for the basal Goodrich conglomerate that was facilitated by faulting. Modified after Grenne, et al, 1990 The remainder of the Goodrich is composed of a fine white quartzite with several minor argillite units. Ojakangas (1994) determined the material for the quartzite in eastern Baraga and western Marquette Counties came from the west-northwest and south east. The overall Goodrich varies from 300 to 1400 feet in thickness. 15 �Michigamme Formation This formation has an aggregate thickness ranging from 11,000 to 20,000 feet. Paleocurrent data recorded for the graywackes (Ojakangas, 1994) indicate the primary direction of sedimentation is from the southwest with lesser influx from the north. Lower Slate Member The slate unit is characterized by the presence of significant graphite (Fig. 32) and ultra-fine pyrite. The amorphous graphite as measured by the carbon content ranging from 4% to 20% as determined by Kramer (1987): Cannon et al. (1972) estimated the maximum thickness in the trough to be about 1500 feet and becoming non existent west of the Greenwood Quadrangle. In places the lower contact is phosphatic (fig. 33) and contains fluorapatite crystals as groups in a matrix or as fine phosphatic pebbles. Phosphate enrichment has been reported in nine separate areas with most sites occurring in the Dead River Basin located north of the Marquette trough. A major outcrop in the NE, section 15, T. 49 N., R. 28 W. contains a 15 meter thick areas of channel filled conglomerate. The sandy matrix contains abundant flattened and elongate pebbles of quartzite and black phosphatic slate. Drill hole and channel samples show the rock contains 15+% P2O5 as fluorapatite. The phosphatic pebbles weather quickly to produce negative relief on exposed surfaces. The US Bureau of Mines and Institute on Mineral Research at Michigan Technological University have been able to produce a suitable flotation weight recovery and grade on the limited resource. Greenwood Iron Formation Member The Greenwood was named by Swanson et al. (1930) for a laminated magnetite bearing argillite found in the lower Michigamme formation. It is present from West Ishpeming west to Humboldt. It varies from 600 feet thickness on the east end to over 1200 feet between the communities of Clarksburg and Humboldt. The laminated or bedded rock contains hornblende, biotite/chlorite along with grunerite, magnetite and quartz. Classic iron formation chert is missing and instead higher alumna, alkalies and calcium oxide are present representing clastic (or possibly pyroclastics) input. Clastic dilution is believed responsible for the lower than normal magnetite content. Chemical data is shown in Table 6. 16 �Table 6 Chemical Analysis Greenwood Iron Formation SiO2 Al2O3 Fe2O3 FeO MgO CaO Na2O K2O H2O+ H2OTiO2 P2O5 MnO CO2 Total 62.34% 6.65 6.22 15.76 2.30 1.26 .39 1.28 1.82 .07 .47 .22 .49 .07 Cannon, 1972, p. 90 N=4 99.34% Physical features of the magnetite suggest that it is primary (or possibly diagenetic) and not of clastic origin. The presence of the Greenwood in the same area of the Clarksburg volcanics found directly above is strong associative evidence that each emanated from the same vent area found near the community of Humboldt. Clarksburg Volcanic Member The Clarkburg is a member of the Michigamme Formation (James, 1958) and is composed of mafic pyroclastics, primarily tuffs (fig. 34) and agglomerated (fig. 35) with minor argillite and iron formation. It is estimated to be 2000 feet thick near the community of Clarksburg. Evidence of the center of extrusive activity around this community is present as a feeder zone that outcrops in section 18, T. 47 N., R. 28 W. The chemical composition indicates most rocks fall in the alkaline olivine basalt range according to Cannon et al. (1972). Much of the Clarksburg displays up to 20% iron calcite replacing fragments of devitrified glass shards and plagioclase. The member does not occur on the north limb of the syncline. Middle Graywacke/Slate Member Bijiki Iron Formation Member The iron formation can be characterized as a silicate sulfide chert unit with minor magnetite. It also contains the asbestos form of cummingtonite in the vicinity of the Peshekee River and US 41. 17 �The unit varies from 100 to 200 feet in thickness. Mining from the unit has produced over 4 million long tons of iron ore, primarily limonite (fig. 36) west of Lake Michigamme. Upper Slate Member The Upper slate is most prevalent in Baraga County where it consists of gray to black slate, impure quartzite and greywacke. The lower segments are carbonaceous with fine pyrite. Fairly abundant concretions (fig. 37) occur within the slate. The concretions are generally rich in carbonate (Table 7) as indicated by partial chemical analysis. They are believed to have formed diagenetically by replacing both the quartz and feldspar. Henrickson (1956) has used the mineralogy of the concretions to define the relative regional metamorphic rank. The chemistry of the slate itself is shown in Table 8. Table 7 Chemistry of Selected Upper Michigamme Concretions Metamorphic Zone Oxide Analysis* Chlorite Biotite N=2 N=1 SiO2 29.7% Al2O3 6.7 Fe2O3 .9 FeO 2.7 MnO .3 MgO 1.7 CaO 29.2 TiO2 .6 *Modified after Henrickson, 1956 64.8 11.5 2.1 4.0 2.5 7.2 .6 Garnet N=1 61.8 14.6 1.1 8.8 3.8 .7 .8 Staurolite N=1 60.2 12.7 2.0 3.4 .2 2.3 11.6 .5 chlorite samples: section 2, T 46 N., R. 37 W. biotite sample: section 19, T. 47 N., R. 32 W. garnet sample: section 19, T. 47 N., R. 32 W. staurolite sample: section 30, T. 48 N., R. 30 W. Table 8 Chemistry of the Upper Michigamme Slate* SiO2 Al2O3 Fe2O3 FeO MnO MgO CaO TiO2 P2O5 Carbon Sulfur 62.3% 15.3 1.1 6.2 .1 3.0 .5 .7 .1 3.0 .3 18 Modified after Henrickson, 1956, p. 71, N=25 �Near the Marquette/Baraga County line metamorphism of the Michigamme has resulted in quartz-bitotite garnet and staurolite schists (fig. 38) The Michigamme in Iron County hosts antahraxolite (fig. 39) as reported by James et al (1968) and Mancuso (1983, 1989) indicating biological activity. Paleoprotozoic Igneous Activity The Hemlock volcanic sequence centered on the Amasa Oval southwest of the end of the Marquette trough in Iron County is primarily basalt/andesite extrusives with lesser rhyolite extrusives. The volcanic pile exceeds 10,000 feet in thickness on the western flank of the Amasa Oval. The Fence River/Amasa iron formation found directly on top of the volcanics is equivalent with the Negaunee. The Hemlock volcanic event is believed to have provided the minerals and heat responsible for the bif. Schneider (2002) established a date of 1.87 Ga. for the rhyolite portion of the Hemlock. The Hemlock has been intruded by the large Kieranen sills which attain a thickness of 6000 feet. The Western Sill has undergone differentiation resulting in a basal peridotite grading upward into a gabbro complete with disseminated Cu+Ni sulfides to a top granophyre component complete with titaniferous magnetite. The Hemlock thins rapidly eastward and pinches out east of the Wilson Creek anticline and is not present in the south flank of the Republic Trough. Within the Negaunee there are many sills and dikes. Numerous small sheared chlorite dikes transect the iron formation, often along faults. The chemistry of the sheared dike shows low calcium and alkalies compared to the diabase sills (Table 9) suggesting a different source for the igneous activity. Supergene oxidation of the dikes is conspicuous by the presence of earthy red hematite. Where the oxidized dikes are in proximity to the soft ores they are difficult to distinguish save for the remnant cleavage of the original rock. The sills exhibit a diabasic texture and chemistry. Gair et al. (1970) and Simmons (1972) have described variations of gabbro, syenite and granophyres as minor and local variations. Diabase sills are resistant to erosion and form prominent ridges with iron formation occupying the erosional valleys. The USGS has named a number of the largest sills with extended surface exposure, some as long as 7 miles and a thickness of 900 feet. The varying texture and chemistry were not part of the naming process. In the Summit Mountain sill, in section 24, T. 47 N., R. 27 W. at least three separate intrusive events were recognized by contacts, textural variations and alterations. The complexity of episodic and closely timed events has yet to be satisfactorily resolved. All the diabasic minerals have been altered. Gair offered a (1975, p 122) description of the alteration observed. “Plagioclase is saussuritic and albitic, particularly in the large bodies, or is variably replaced by carbonate, chert, chlorite, sericite, biotite and clay minerals; in one place, replacement is by an unusual mixture and of biotite and epidote. Original pyroxene commonly is replaced by pale-green tremolite-actinolite and minor chlorite or biotite, or entirely by chlorite-biotite.” 19 �Sill and dike contacts generally exhibit a chill margin in the diabase while within the iron formation the carbonate was quickly altered to fine magnetite that was not affected by later diagenetic crystal overgrowth noted in the rest of the iron formation. Gair (1975) noted that intrusive dikes and sills were more altered when in contact with banded iron formation than with slates or quartzites. The iron silicates, minnesotaite and stilpnomelane are elevated to grunerite/cummingtonite at many intrusive contacts. The coarse diabase was found to contain partially assimilated xenoliths of fine grained chlorite dike. Both the field relations and analytical data indicate two intrusive events with the sheared chlorite preceding the massive intrusives. Epidote which is ubiquitous to the none sheared crystalline intrusive is absent from the smaller chlorite dikes. Within the Marquette trough the Clarksburg volcanic sequence found in the lower Michigamme strata is most likely to have provided the diabase found as dikes and sills in the underlying Archean, Chocolay group and Menominee group of the Marquette iron range. The volcanic center occurred on the south side of the trough in the vicinity of Clarksburg. A vent in section 18, T. 47 N., R. 28 W. coupled with an abundance of intrusive bodies and the thickest sequence would suggest this area was the center of greatest volcanic activity. The extrusive extends to Lake Michigamme on the west and to West Ishpeming on the east. They do not occur in the stratigraphic sequence on the north limb of the trough. Tuffs and agglomerates are common on the margins. The chemistry of the sills generally matches (Table 9) those of the Clarksburg diabase and basalts. Amygdaloidal basalts were recognized in the Negaunee at the Greenwood Mine (Cannon, 1974) and in the northwestern portion of the Palmer quadrangle (Gair, 1975). Amygdaloidal basalt is sandwiched between a footwall slate unit in section 27, T. 47 N., R. 27 W., suggesting that there may be syn-depositional origin for at least some igneous rocks. 20 �Table 9 Partial Chemical Analysis of Paleoprotozoic Intrusive RocksMarquette Range, MI 1 Oxides SiO2 Al2O3 Fe Fe2O3 FeO CaO MgO K2O Na2O TiO2 MnO P2O5 N= V Ni Cu Cr Co 3 14.41 11.5 18.57 14.4 0.34 7.74 0.7 0.51 1.68 8 283 103 184 60 43 Sample # 1 2 3 4 5 6 7 8 9 10 39.7 18.35 17.43 16.31 16.74 0.48 8.33 1.8 0.14 0.26 12 2 4 5 47.26 13.44 11.85 7.9 8.13 0.31 9.35 tr 0.48 1.33 6 43.11 48.13 14.8 16.31 12.02 8.67 1.89 1.75 13.76 9.57 0.41 1.34 8.77 2.68 11.62 7.47 2.5 0.64 0.4 2.15 2.05 2.5 3.08 0.96 0.41 0.17 0.41 0.17 3 1 1 6 Trace Metals in ppm 259 90 40 219 7 13.79 8.54 7.43 5.53 3.97 4.48 1.95 4 8 9 10 46.06 14.41 8.91 1.85 9.92 8.99 7.16 0.71 2.12 1.37 0.22 0.22 2 44.97 15.34 8.8 1.51 9.96 7.29 5.91 1.53 4.49 1.96 0.29 0.29 2 49.1 17 9.07 2.4 9.5 10 6.9 0.6 2.6 1.4 0.3 0.3 252 94 129 223 57 References Chlorite dikes at the Empire Mine. USGS PP 76- p. 123 Gair and Han 1975 Sheared un-oxidized chlorite dikes at the Tilden Mine, Sec 25 & 26 Sheared oxidized chlorite dikes at the Tilden Mine, Sec. 25 & 26 A 20 foot thick chlorite dike at the Tracy Mine, Sec. 8, 47-26 Mathias 1958 Clarksburg tuff, Cannon GQ-1168 p. 8 table 1 1974 Large meta-diabase sills T. 47N., R 26 & 27 W., Mathias table 3, 1958 Fine-medium unoxidized meta-diabase, Tilden Mine Meta-diabase Clarksburg Quadrangle, Cannon USGS Map GQ-1168, table 1 p.8 1974 Metabasalt Clarksburg formation. Cannon USGS Map GQ-1168, table 1, p.8, 1974 Typical basalt The latest intrusive event is marked by a few scattered quartz, ankerite and micaceous veins that cut both chlorite and meta-diabase dikes but do not cut the Keweenawan dikes. The large micaceous hematite crystals show the occasional striations that resemble twinning. The quartz is milk white and the carbonate is cream white to light brown in color. None of the veins persist over significant distances. 21 �Keweenawan Diabase Dikes (1108 Ma) Reversely polarized Keweenawan diabase dikes are present throughout the Marquette iron range. They exhibit an east-west trend with a near vertical dip. One narrow dike has been traced over a three mile strike length and down dip over 1200 feet. The diabase ranges in width from 6 inches to 80 feet and locally bifurcates over short strike distances. Where fresh the diabase is a dark green to black and with a clear dibasic texture. Where the diabase cuts iron formation the diabasic texture is obliterated by complete alteration to clay minerals. Where the Keweenawan dikes cut older sills and dikes in the iron formation, the diabase dikes appear fresh and unaltered suggesting contact with the evolving oxidation of the iron formation facilitated the dike alteration. The altered color is generally olive green with occasional oxidation of the contact zone to a slightly yellow color. Altered dikes are quite susceptible to weathering. Newly exposed surfaces usually disintegrate within two weeks. Rapid weathering explains why the dikes have not been observed in outcrop. Where the Keweenawan dikes have been observed in underground workings they are always argillized where they cut enriched natural ore bodies. Table 10 Selected Partial Analysis of Keweenawan Diabase Dikes, Marquette Iron Range, MI Sample I.D. T25-7-732 T26-75-1119 EM-1165-2 EM-1340-1 Coordinates NA NA 18,740S, 3,450W**** 20.570S. 2,370W NENW Sec.31 47-26 Description altered diabase altered diabase altered diabase altered diabase unaltered diabase OV-1 T25-7-732 T26-75-1119 EM-1165-2 EM-1340-1 OV- FeO* 9.08 21.09 25.46 16.08 14.66 SiO2** Al2O3 57.2 16.24 42.26 14.26 28.88 16.36 23.06 20.14 60.1 13.58 T25-7-732 T26-75-1119 EM-1165-2 Em-1340-1 OV1 P2O5 0.042 0.029 0.024 0.048 0.014 TiO2 0.57 0.85 0.3 0.57 0.24 Per Cent CaO MgO*** 1.55 3 2.6 7.5 0.62 7.8 1.07 6.3 8.64 3.3 MnO 0.01 0.02 0.06 0.01 0.02 K2O 0.24 0.6 0.1 1.01 0.06 Na2O 0.84 1.46 1.11 1 2.88 Cu 153 430 64 316 324 Cr 122 65 29 115 70 Co 37 60 50 50 70 1 ppm Ba 967 1660 1950 1180 1250 V 575 499 281 641 409 Ni 49 55 19 48 60 Sample Legend T = Tilden Mine DDH-Sec.-hole#-footage in hole Em = Empire Mine bench sample-footage OV-1 = Isabella dike in sec. 31, T. 47 N., R.26 W. * Soluble iron calculated as FeO ** Titration analysis *** Wet Chem. **** Mine triangulation coordinates ***** 1978 Barringer 42 element analysis-Co@, H2O and S not determined 22 �Although they do not control enriched ore concentrations like the older intrusives, they were definitely altered by the solutions acting in the porous oxidized and enriched zones. Gair (1975) suggested the solutions responsible for oxidation of the iron formation were active after Keweenawan time and were a causative effect in the alteration of the Keweenawan dikes. Table 10 shows the chemical variation found within the altered dikes. Alteration of the Keweenawan dikes resulted in a pronounced reduction of silica and calcium where the dike was in contact with ferrous iron formations (i.e. Empire) and to a lesser extent in a ferric environment (i.e. Tilden) Recent dating of the Eagle nickel-copper deposit indicates that it is 1107+-3.7 Ma. and establishes a good age for the general Keweenawan diabase event. Paleomagnetic data and chemistry of the abundant east-west Keweenawan dikes are similar to the Logan sills of the North Shore Volcanic group that have an age of 1108 matching the recent Eagle number. Northwest faults offset the Keweenawan dikes and indicate that the age of the offset is younger than 1108 Ma. The dikes are devoid of any metamorphic fabric. Structure Schulz (2007) summarized the Penokean Orogeny as starting with the collision of oceanic, island-arc terrain (the Pembine-Wausau terrain of Wisconsin) with the Archean Superior craton to the north. Upon collision the direction of subduction flipped to the north, creating extensive back-arc extension. This resulted in the development of the Animikie basin into which a thick group of sedimentary and volcanic rocks representing the Menominee Group were deposited. The accumulation included the Negaunee iron formation and its equivalents in other iron districts. The north-directed subduction brought the Marshfield terrain toward the Superior craton. The Pembine/Wausau island arc was thrust north onto the Superior craton resulting in subsidence of a foreland basin into which the Baraga group of sedimentary rocks, including turbidities, and volcanics were deposited. This took place between 1850 and 1835 Ma. Metamorphism occurred during folding, thrusting and tectonic thickening resulting in a metamorphic gradation of existing rocks accumulated during the Penokean Orogeny. The end of the Penokean was marked by emplacement of a number of plutons in the metamorphosed terrene. A north-verging fold-thrust system in the early Paleoproterozoic continental foreland in northern Michigan has been recognized by many workers (Cannon, 1973 and Klasner, 1978, 1991). Initial deformation caused thin skinned shortening of the Paleoproterozoic strata along decollements. This deformation also formed north-verging structures along with south-dipping foliation. Subsequent deformation involved block uplift of the Archean basement and formation of structures such as the Marquette and Republic troughs and possible development of Archean gneiss domes. One other structural event was active from early Chocolay sedimentation up through the end of Menominee sedimentation. A significant down warping occurred in the vicinity of section 6, T. 23 �47 N., R. 25 W. for the Chocolay sequence (Taylor, 1972) and migrated to the southwest to roughly the current position of sec. 19 T. 47 N., R. 26 W. where the Negaunee iron formation attains a thickness of over 1000 meters. The Wewe slate and Siamo slate, including the Goose Lake iron formation occur only in the eastern portion of the trough in the same down warping progression event. It is possible that a detailed study of the stratigraphy of either or both the Wewe and Siamo would reveal a similar thickening along the same NE-SW trend line. The north and south margins of the Marquette trough are bounded generally by high angle faults. The east-west Palmer fault, forming the south boundary of the Tilden ore body, dips uniformly at 55 degrees to the north. FIELD TRIP STOPS 1 THROUGH 8 IRON ORE HISTORY Although iron ore was discovered in outcrop in the vicinity of Negaunee in 1844, production was slow to materialize due to the poor to non existent transportation system. Early ore production went to a number of local charcoal furnaces that produced pig iron using charcoal produced from the local forests. As railroads were built and a system of boat locks installed at Sault Ste Marie in 1855 to accommodate large lake vessels, production of raw iron ore increased and moved to the lower lake steel mills. Initial iron ore mining concentrated on open pit hard ores that formed topographic highs. Discovery of soft enriched supergene ore on the eastern end of the range followed along with the application of underground extraction starting in 1880. These two types of ore were augmented by siliceous ore which was slightly enriched banded iron formation used for furnace slagging. By the 1950’s high grade ore reserves were almost depleted on most of the iron ranges in North America. This spurred the advent of beneficiating techniques to upgrade the banded iron formation into a high grade concentrate that could be pelletized for superior furnace operation. On the Marquette Range the first concentrating/pellet operation was the Humboldt Mine opened in 1954 followed by the Republic Mine opened in 1956. Both used an anionic hot oil flotation system to concentrate the specular hematite. The Empire Mine was opened in 1964 and used magnetite separation. The Tilden Mine followed in 1974 using a cationic silica flotation to concentrate the hematite. It should be mentioned that the soft ores from the Mather A underground mine were pelletized up until the mine closed in 1978. Thomas A. Edison invested the proceeds from the sale of his electric business to GE into iron mining and beneficiating schemes to upgrade magnetite banded iron formation to produce a salable concentrate product. In 1888 Edison provided the financial backing to Walter Mallory to construct a crushing/grinding/electro magnetic separation operation at Humboldt utilizing ore from the Sampson Mine (later to be included as part of the Humboldt Mine in 1952). The Edison Iron concentrating company began operation in 1889 and produced 893 long tons of magnetite concentrate up until a fire destroyed the mill in 1890. The mill was not rebuilt and Edison turned his attention to Fe-oxide deposits such as the magnetite in gneiss at Ogdensburg, New Jersey. He essentially went broke on his mining and concentrating activities 24 �partly because concentrate (small size) was not the preferred furnace feed. He was an inventor and his electro magnetic separation and roll crushing are integral to North American iron ore operations today. The last operating underground hard ore mines were the Cliffs Shaft and Champion Mine and they ceased operation in 1967 and 1968 respectively. The last underground soft ore mine on the range was the Mather B which ceased operation in 1972. The last siliceous mine was the Old Tilden Mine that shipped its last loads in 1973. The Humboldt ceased production in 1974 due to exhaustion of the ore while the Republic Mine shut down in 1981 due to difficult economic circumstances. Each field trip stop is identified by a latitude, longitude and elevation that can be used with a GPS to locate the outcrop on the ground. Stop 1 - Wewe Hills Vents section 21-23, 47-26 A- 46o 27’ 13.22” N, 87o 32’ 38.96” W, +1503 B- 46o 27’ 16.07” N, 87o 32’ 47.17” W, +1505 C- 46o 27’ 41.99” N, 87o 33’ 8.23” W, +1514 Stops 1 A-C are located on a continuous silicified/hematite zone adjacent to a fault, both of which trend N 30o W and is at least 6000 feet long. The stop exhibits similar features found in a number of other localities on the eastern portion of the Marquette Range. Most occurrences contain quartz and hematite stockworks that were emplaced by episodic pulses of silica usually followed by the emplacement of hematite/jasper. The hematite habit varies from botryoidal to microplaty through specularite to very coarse micaceous hematite. Rare earth analysis of the hematite shows high concentrations of LREE relative to HREE (Waggoner, 2003). The REE pattern of the vent hematite is mirrored by the hard ore hematite mined from the upper portion of the Negaunee Iron formation. 25 �Figure 1-1 Wewe Hills Stockworks. Stops A, B and C There has been considerable discussion in regard to the proper placement of these rocks within the stratigraphic section. Van Hise (1897), Van Hise & Leith (1911) and Gair (1975) thought the major slate lithology represented the Wewe slate. Taylor (1972) was able to identify the silicified rock in sections 23 & 24 as the big cusp algal dolomite member of the Kona formation making it more likely that the rocks directly above the basal conglomerate in sections 21 & 22 also belong to the Kona formation. Further east in section 23, directly above the silicified Kona, much of the slate has been hydrothermally altered. Gair (1975) has described chloritoid, andalusite (fig. 40), pyrophyllite and cordierite associated with sericite, chlorite and quartz alteration. In the venting process the elements of iron, silica, potassium, sodium, calcium manganese and aluminum are vented in solution or as fine particulates into a sea water environment. Settling and precipitation resulted in the formation of a banded sequence rich in one or more of the elements as carbonates, oxides or silicates. These same elements when confined to an intrusive or replacement environment resulted in zoned alteration of the host rock. The configuration of the venting in the Wewe Hills suggests the plumbing was linear that allowed for variable volume of discharge along its entire length. On the Marquette Range circumstantial evidence suggests the zone of weakness was present on or near the south side of the trough. Certainly the presence of the Clarksburg and several other Michigamme units only on the south side of the trough points to a weak zone in this location through which the fluids vented. 26 �During the venting process elements of iron, silica, potassium, sodium, calcium, manganese and aluminum are vented in solution or as particulates into a seawater environment. Differential settling and precipitation resulted in the formation of a banded sequence rich in one or more of the elements. These same elements when confined to an intrusive or replacement environment in rock result in zoned alteration. The configuration of the stockwork zone in the Wewe Hills suggests the plumbing was linear and allowed for variable volumes of discharge along the entire length. On the Marquette Range circumstantial evidence suggests the zone of weakness was present on or near the south edge of the trough. Clear timber cutting has aided visual inspection of silicified dolomite/slate units similar to those found to the east of Goose Lake Stop 1A. This outcrop consists of silicified slate and dolomite belonging to the undifferentiated Chocolay Group. The stockworks quartz (fig. 41) trends similar to that of the outcrop. Cross cutting hematite veins (fig. 42) with disseminated wall rock alteration can be distinguished by the red color. The dolomite contains breccia units where the silicified dolomite contains fine specular infill that in some areas resemble a crackly breccia. The breccia contains coarse quartz veins with minor micaceous hematite. Looking east across the valley the large outcrop is an orange to green boulder conglomerate (fig. 43) forming the basal Chocolay Group. Some parts of the conglomerate in sections 21 & 22 contain specularite and martite (fig. 44) replacing the fine fractions of the matrix in the conglomerate usually near shears and joints that acted as a fluid conduit. Stop 1B. The early exploration for iron ore resulted in the excavation of the shallow test pits where high concentrations of iron were encountered . The three to four foot near vertical vein of hematite jasper breccia was tested for size and grade at this location. The episodic injection/dilation caused openings to form into which fine hematite was precipitated as botryoidal hematite (fig. 45). This will be one of two places for Stop 1 where samples can be collected. Stop 1C. This is another early exploration where shallow excavation was used to examine both grade and size of the hematite concentration. The outcrop next to the road is an excellent example of the quartz stockworks overprinted by microplaty hematite and jasper veins. The red oxidation present in the gray green slate is defiantly later than the foliation and as such may be a later event not related to the quartz/hematite which occurred before the end of Goodrich time. This feature is mentioned because this kind of oxide staining at the Wernecke deposit in the NW Territories is believed to have occurred with the metallic oxide alteration. 27 �First day lunch stop will be at the Iron Industry Museum For any of the following Tilden and Empire ore types we are unable to access due to ongoing operations, core and samples will be on display at this lunch break. Core: magnetite silicate iron formation-Empire Mine magnetite carbonate iron formation-Empire Mine carbonate chert iron formation-Empire Mine clastic magnetite silicate chert-Empire Mine ultrafine magnetite at diabase contact soft hematite ore-Cascade Range “blue steel” ore-Cascade Range supergene enrichment-Cascade Range earthy hematite (like Mather ores)- Cascade Range goethite chert-Tilden Mine leopard goethite-Tilden Mine martite clastics-Tilden Mine magnetite carbonate chert-Tilden Mine martite hematitic chert-Tilden Mine martite chert-Tilden Mine Stop 2 - Tilden & Empire Mines section 18, 19, 20, T. 47 N., R. 26 W. and sections 22-27, T. 47 N., R. 27 W. The advanced development of these mines can afford many excellent features of the Negaunee Iron formation on the eastern end of the Range. The Negaunee is over 3000 feet thick in section 19. It has been divided into a lower transition unit that contains fine clastic material, a magnetite silicate horizon (fig. 46), a magnetite carbonate horizon (fig. 47), a clastic horizon (fig. 48) and finally an upper undifferentiated unit. All horizons have provided some ore but the largest sources are the magnetite silicate and magnetite carbonate horizons located in the center of the pit (see Table 2). The protore on the eastern end of the range was a fine iron carbonate silicate chert (fig. 49). Remnants of this type of material occur on the hanging wall of the Empire pit and the SW extension of the Empire pit, the deep portions of the Tilden pit and the Tilden CD III extension adjacent to the Keweenawan dike. Both mines exhibit good exposures of the clastic component of the iron formation. Good examples of the martite after magnetite are found in the Tilden ore horizon. 28 �Stop 3 - Marquette County Rd. 480 46*29”89.49”N 87o 35’ 51, 17”W +1391 Figure 3-1 County Road 480 road cut This stop illustrates the original protore to be found on the eastern end of the Marquette trough. The outcrop in the road cut is a gray to buff iron carbonate with minor silicates and some magnetite alternating with chert. Surface oxidation is very thin and tends to develop along exposed surfaces and bedding planes. This type of iron formation is very susceptible to supergene oxidation and enrichment. Stop 4 - North Jackson Mine 46o 29’ 53.98” N, 87o 37’ 20.48W +1428 The Jackson Iron Mine was opened in 1848. It provided iron ore for local pig iron furnaces and was the source of ore that fed the furnaces at Fayette down on the shore of Lake Michigan on the Stonington Peninsula. The site has been off limits for fifty year due to the underground Mather B operations which recovered the soft ore along the footwall of the Negaunee iron formation. 29 �The City of Negaunee purchased the lands recently and has opened part of it for a heritage park. This action allows our group to visit a classic mining site and examine the detail geology. We will enter the very eastern portion of the old North Jackson mine where both hard hematite and soft ore were mined during the middle of the 1800s. Figure 4-1 North Jackson Mine The access cut passes through soft ore on the south wall while the north wall is formed by a vertical sheared chlorite dike possibly of Clarksburg age. The cut opens into a two level pit where the lower level is under water. On the southeast part of the pit is a rubble pile of very large blocks of jaspilite that illustrates hematite replacing jasper. The majority of the blocks appear to be breccia with cross cutting veins of hematite and occasional magnetite. One large boulder has been cleared of organic material and should afford a good photographic opportunity. On the west wall is an exploration adit (screen covered) and the south pillar shows a vuggy metallic hematite with clay filling (probably kaolin). This showing illustrates that the 1.8 million year old hard hematite was further leached by the supergene process with resulting dissolution of the remaining jasper and formation of the classic porous soft ore texture. 30 �Stop 5 - Jasper Knob, Ishpeming 46o 29’ 12.43” N, 87o 39’ 15.01” W +1607’ The outcrop represents the very upper portion of the Negaunee iron formation that has undergone the initial stages of hydrothermal replacement of the jasper by microplaty hematite. The laminated jasper shows only minor disturbance including brecciation. Most of the brecciation is intraformational bedding breccias (or conglomerates). The jasper bands contain less than 0.5% iron oxide. Figure 5-1 Jasper Knob Some workers suggested oxidation and replacement of the siderite chert by jasper which in turn was replaced by hematite while others suggest a primary origin for the jasper and some of the banded hematite. Replacement of pre-existing chert/oxides by jasper (fig. 50) at both Tilden and Empire have left evidence of the process. Within the fine jasper in Ishpeming relic transitions are not in evidence. Note the undisturbed fine bedding in the jasper similar to both white and gray chert. If there was abundant fauna present, it would have disturbed the delicate banding or at least have left some evidence of its physical activity. To complicate matters supergene oxidation can produce a brick orange chert like that present at the Milwaukee-Davis Mine in Negaunee. The process usually leaves “islands” of uncolored chert (fig. 50a) Fold axis are horizontal to gently plunging to the west. Look carefully for isolated granular or oolitic zones. 31 �Stop 6 - Saginaw Mine 46o 27’ 26.48” N, 87o 43’ 43.36” W, +1648’ The Saginaw Mine is an interesting property due to the diverse types of banded iron formation to be found there. This is a transition area within the Negaunee iron formation on the Marquette range. To the east most of the iron formation was originally carbonate replaced by magnetite or oxidized to hematite and enriched. Figure 6-1. Saginaw Mine To the west the Negaunee is primarily banded specularite chert or banded magnetite silicate chert. To the east of the Saginaw the hard ore hematite is microplaty hematite. The lean magnetite silicate chert on the Saginaw property is very coarse and contrasts with the much finer martite goethite chert up section to the north underlying the jasper-hard ore deposit that was mined from 1872 to 1891 and produced 451,000 long tons. At the Saginaw the Negaunee begins a significant reduction in thickness to the west that may be either a function deposition or due to faulting (Simmons, 1972). Midway up the Negaunee section is a lean coarse magnetite cummingtonite chert iron formation overlain by earthy hematite-goethite chert iron formation (fig. 51) as a result of supergene oxidation of an original carbonate silicate chert iron formation. The grain size is much finer than the underlying 32 �magnetite horizon. Overlying the hematite-goethite are intermittent zones of jasper, microplaty and specular hematite. The high grade hard ore both at the top of the Negaunee and in the basal Goodrich was the target of mining at the Saginaw. Above the contact of the Negaunee with the Goodrich is a thick sequence of conglomerate consisting of jasper, microplaty hematite and minor vein quartz. The schistose nature of the hematite in the jaspilite pebbles and cobbles is parallel to bedding and occurs in random orientation in the conglomerate indicating the schistosity was present prior to incorporation of the pebble and cobbles in the conglomerate. The magnetite cummingtonite chert is fairly lean. Both the ground residual gravity and the total field magnetics indicate this unit forms a north facing thickening that could either be a fold or vent mound where coarse magnetite and iron silicates quickly accumulated in a pile. At the Little Commonwealth deposit in Florence Co., WI a small bif separates an up-slope quartzite from a down-slope slate indicating the iron formation acted as a barrier to sedimentation. This suggests positive relief to its surroundings. The Commonwealth also contains irregular shapes of silica globs that look like fiamme features. These features, however, are not present at the Saginaw. Looking northwest along the power line clearing an outcrop can be seen about half a mile to the northwest where the power line turns north. This is one of the best outcrops (fig. 52) of Goodrich conglomerate on the range. It contains cobble of microplaty hematite, banded jasper, oolitic jasper and white vein quartz. From this vantage point there is a large boulder of the Goodrich conglomerate that illustrates many of the features of the conglomerate portion of the Goodrich. Several hundred of feet into the woods line west of the turn in the power line is the Goodrich Mine where 50,000 long tons of enriched Goodrich conglomerate (fig. 53) was mined for iron from 1873-1882. The contact between the earthy hematite chert and the Goodrich conglomerate can be seen in the adit separated by a fault breccia (fig. 54). The basal Goodrich is an unconformity by definition but the same hydrothermal event responsible for hard ore formation was active after the conglomerate accumulated as evidenced by the hematite present in the interstices between pebble and cobbles. The movement that resulted in the accumulation of coarse clasts did no stop the hydrothermal process. The microplaty/specularite conglomerate juxtaposition to the earthy hematite suggests the specularite is primary and not a metamorphic product of preexisting earthy hematite. Although not apparent in outcrop the property contains a swarm of basic dikes and sills as determined from diamond drilling. Based on dump material at least one oxidized Keweenawan dike transects the property. From the rusty zones in outcrop very minor sulfides are present in the magnetite cummingtonite chert iron formation. Second day lunch stop at Cliffs Shaft Mine The Cliffs Shaft Mine Operated from 1848 to 1967 shipping 29 million long tons of high grade microplaty hematite with minor magnetite and siderite. The mine was accessed by a vertical shaft (Koepe hoist) to the 15th level-1250 feet below surface. The two cement obelisk shaped shafts were constructed in 1919. Ore was mined by room and pillar methods (see old photos on the CD). Copper sulfide veins are common throughout the ore body (fig. 55). The drainage ditch leading to the sump on the 10th level was lined by a thick layer of black copper oxide that assayed over 3% Cu. Samples of the hard ore can be found on the surface of the stocking area just west of Euclid St. (west of the shaft area). 33 �During lunch the field trip attendees will be able to view the Hawes mineral collection and examine examples of the following: Core Goodrich conglomerate – Saginaw area Soft supergene hematite-goethite – Saginaw area Goodrich quartzite contact with hard spec. ore – Humboldt Mine Samples Goodrich conglomerate ore-Republic Mine Goodrich conglomerate polished slab – Goodrich Mine Coarse magnetite – Champion Mine Coarse specularite – Champion Mine Greisen with molybdenite - Champion Mine Sulfides associated with hard ore – various sources Copper sulfide – Cliffs Shaft Stop 7 - Kloman Exploration, Republic Mine 46o 24’ 23.46”N 87o 59’ 10.94”W +1517 Figure 7-1 Kloman/Republic Mine The following production history for the Republic area was tabulated: Republic underground 1872-1926, 8.6 MLT (60-65% Fe) Republic open pit 1956-1981, 62 MLT from 145 MLT of specular hematite Kloman/Columbia underground 1873-1883, 95,000 LT 34 �The Republic ore outcropped in a bluff on a bend in the Michigamme River that corresponds to the mapped geology (Cannon, 1975). Initially mined from shallow pits, most production came from a number of shafts including the Pascoe (see old photo gallery on the CD) which was inclined 48 degrees down four thousand feet of the fold axis that bottomed 2900 vertical feet below the river. After being idle for many years the deposit was reactivated as an open pit to produce crude suitable for concentration. The crude feed had the following chemistry: Sol. Fe Silica Al2O3 MgO 38% 42.5% .72% .9% CaO Na2O K2O P .53% .03% .04% .033% The ore mined at the Republic Mine for taconite feed (1954-1981) originated from a specular jaspilite (fig. 56) found above the metadiabase sills and below the Goodrich quartzite on the southeastern end of the trough. The basal Goodrich is a conglomerate rich in specular fragments that were rich enough to constitute ore. The specularite generally contains less the 0.2% TiO2 but in the conglomerate area values of 0.50 to 1.65% TiO2 as rutile associated with hematite. The high grade specular hematite and magnetite occur at the top of the Negaunee as irregular replacement bodies. The magnetite may indeed be post tectonic as suggested by Cannon (1973). The banded iron formation consisting of specular hematite with minor magnetite does not show cross cutting veins or disturbances to the bedding, suggesting the specular chert is a primary facies generated when the hydrothermal hematite crystallized upon release into an aqueous environment and quickly settled as distinct bands. As seen in the Saginaw Mine area the presence of coarse specularite is not a metamorphic product of a preexisting oxide form because it coexists with soft oxide hematite and goethite. Examination of the magnetite silicate horizon of the lower Negaunee iron formation exhibits a uniform banding (fig. 57), unlike the Champion area where the magnetite silicate chert has undergone hydrothermal alteration as indicated by brecciation and clear evidence of solution replacement features corresponding to the change in mineralogy. Conceptually the magnetite silicate could have resulted from the same diagenetic replacement of the original carbonate silicate chert and subsequent metamorphism resulting in conversion of the low grade silicates to cummingtonite and grunerite with enhanced size to the magnetite. It could also be primary mineralogy enhanced by metamorphic overgrowth. The stratigraphy in the Republic trough consists of the undifferentiated Siamo and Ajibik formations, Negaunee iron formation, Goodrich conglomerate/quartzite and Michigamme slate (schist). A number of diabase sills parallel to the iron formation bedding have undergone the same folding flexure in creating the keel of the trough. The tight compressional folding that thickened the iron formation in the keel of the trough did not markedly impact the vertical and thinner iron formation on the north side of the fold. The keel plunges 48 degrees to the northwest. An excellent exposure of the upper portion of the Negaunee Iron formation can be found just north of the old pit on the Kloman property. Cannon (1972) provide a good stratigraphic section for this outcrop (Figure 7-2). The high grade lens was mined just at the Goodrich contact (fenced area). The iron formation consists of alternating bands of specularite 35 �and magnetite grading down section to more magnetite silicate chert. Compare the stratigraphy of Figure 2 with the stratigraphy shown for the Republic Mine and note the change in mineralogy over a relatively short distance. The Republic trough area was flown for gravity using Falcon method several years ago and a drilling program was conducted on a gravity target located several hundred feet east of Highway 95 and south of the Michigamme River. An economic target was not located. Klasner et al. (1974) using gravity indicted the depth of the Republic trough on Highway 95 to be 1,424 meters. A resource of 120 million long tons of oxide iron remains within the pit outline. 36 �Figure 7-2 Stratigraphic section-Kloman Mine. After Cannon (1972) 37 �Stop 8 - Champion Mine #7 46o 30’ 27.59”N, 87o 59’ 11.51”W, +1705’ Figure 8-1 Champion Mine The Champion Mine provides an excellent example of coarse specularite and magnetite hard ore similar to that found in the Humboldt, the Greenwood and the Republic Mines. Even though Babcock (1966, 1974) identified over 70 minerals at Champion, only the most common are available. The Champion Mine was operated spasmodically by multiple operators for over 100 years (1867-1967) and produced 6.5 million long tons of combined coarse specular hematite (fig. 58) and magnetite (fig. 59). The mine was accessed from seven shafts numbered progressively from east to west (see fig. 60 longitudinal section). The ore body is irregular, thin and dips 78 degrees to the north-northwest. The spindle shaped ore shoots rake steeply to the southwest. The lower level (26th) is at 2100 feet below the collar of shaft #7. Generally specular hematite occurs near or at the hanging wall Goodrich quartzite contact. In some cases iron ore has replaced some the overlying quartzite. Magnetite is more common on the footwall side and with increased depth. The iron formation is approximately 400 feet thick throughout the mine. Oxygen isotope studies have indicated iron oxides formed at 400-500oC. 38 �The original mineral composition of the banded iron formation is impossible to determine but we know the magnetite-silicate-chert was subjected to alteration that started with the formation of sericite and chlorite themselves replaced by specularite and magnetite respectively. Some of the magnetite postdates the specularite. Tourmaline and quartz followed while the last major addition included: quartz, jacobsite, manganese iron silicates, gold, pyrite (fig. 61), chalcopyite, bismuthinite, molybdenite and scheelite. Bodwell (1972) indicated the late mineral stage was prevalent between the #5 and #7 shafts associated with multiple quartz veining in massive magnetite. Alteration minerals are many but only a few are readily recognizable. Among them are: chloritoid, andalusite (fig. 62), andradite (fig. 63 ), garnet, tourmaline (fig. 64), sericite and chlorite. The manganese addition can be found in the minerals: jacobsite, mn-chloritoid, spassertine, kutnohorite, mn-cummingtonite, pyrophanite and mn-actinolite. L. Babcock’s studies of the mineralogy, using samples collected in the mine and from the dumps, reported the presence of gold. One 2 foot intersect in a diamond drill hole assayed 0.198 ounces per ton. Dump samples consisting of garnet-chloritoid-sulfides and massive magnetite with sulfides can assay 1-2 ppm gold. In some parts of the mine greisen has developed that contain visible molybdenite (fig. 65) that assays above 1% Mo. Subsequent workers have compared the Champion FeOx gold occurrence to the Tennant Creek and Starra deposits found in Australia. Taken by itself it could be just an anomaly, but coupled with widespread late sulfides associated with range wide hard ore, elevated hard ore REE values, the presence of copper and tungsten found on the south limb of the Republic trough and the presence of ferrites would suggest the iron formation and subsequent hydrothermal alteration are distal end products of a possible remote Iron Oxide Copper Gold (IOCG) feeder system (s). The high level metamorphic assemblage found at the Champion Mine is not seen in the surrounding sedimentary or volcanic rocks indicating the high grade metamorphic rank is site specific and not regional in nature. Bibliography for Banded Iron Formations of the Marquette District-2008 ILSG Field Trip Babcock, L.L., 1966, The Manganese-Bearing Silicate Minerals of Champion Mine Champion, Michigan, MS thesis, MTU, 70 p. Babcock, L.L., 1974, Mineralogy, Geochemistry, and Genesis of the MagnetiteJacobsite Mineral Series and Manganese-Ferrite-Bearing Iron-Formation From the Champion, Mine, Champion, Michigan, PhD thesis, MTU. Babcock, L.L., 1974, Mineralogy, Geochemistry, and Genesis of ManganeseFerrite-Bearing Negaunee Iron-Formation form Champion Mine, Champion, Michigan, pt II of PhD thesis, MTU. Bailey, S.W, and Tyler, S.A., 1960, Clay Minerals Associated with the Lake Superior Iron Ores, Econ. Geol. v. 55, p. 150-175. Bayley, R. W., and Tyler, S.A., 1966, Geology of the Menominee Iron-Bearing District,Dickinson County, Michigan and Florence and Marinette Counties, Wisconsin, USGS Prof. Paper 513, 96p. 39 �Bodwell, W.A., 1972, Geologic Compilation and Non-ferrous Metal Potential Precambrian Section, Northern Michigan, MS thesis, MTU. Cannon, W. F., and Klasner, J.S., 1972, Guide to Penokean Deformational style and Regional Metamorphism of the Western Marquette Range, Michigan, 18th Annual Institute on Lake Superior Geology, p. B1-B38. Cannon, W.F., 1972, Geology of the Greenwood 7 ½ Minute Quadrangle, Marquette County, Michigan, USGS unpublished Prof. Paper, 191p. Cannon, W. F., 1973, Penokean Orogeny in Northern Michigan, GAC Spec. Paper No. 12, p. 251-271. Cannon, W. F., 1973, High Grade Magnetite Deposits at Republic, Michigan: Their Bearing on the Genesis of the Marquette Range Hard Ore, 19th ILSG. Cannon, W. F., 1974, Bedrock Geologic Map of the Greenwood Quadrangle, Marquette, USGS Map GQ-1168. Cannon, W. F., 1975, Bedrock Geologic Map of the Republic Quadrangle, Marquette County, Michigan, USGS Map I-862. Cannon, W. F., and Addison, W.D., 2007, The Sudbury Impact Layer in the Lake Superior Iron Ranges: A Time-Line from the Heavens, 53rd ILSG. Gair, J.E., and Thaden, R.E., 1968, Geology of the Marquette and Sands Quadrangles, Marquette County, Michigan, USGS PP 397, 77 p. Gair, J.E., 1970, Metadiabase Sills in Negaunee Iron-Formation South of Negaunee, Michigan, USGS Bull. 1324A, p. 24-30. Gair, J.E., 1975, Bedrock Geology and Ore Deposits of the Palmer Quadrangle, Marquette County, Michigan, and Reflection on the Empire Miner by T.M. Han, USGS PP 769, 159p. Grenne, T., and Vokes, F.M., 1990, Sea-floor sulfides at the Hoydal Volcanogenic Deposit, Central Norwegian Caledonides, Econ. Geol. v. 85, p. 344-359. Han, T.M., 1971, Diagenetic-Metamorphic Replacement Features in the Negaunee Formation of the Marquette Iron Range, Lake Superior District, Soc. Mining Geology, Japan, Spec. Issue 3, p. 430-438. Han, T.M., 1982, Iron-Formation of Precambrian Age: Hematite-Magnetite Relationships in Some Proterozoic Iron Deposits-A Microscopic Observation, In Ore Genesis-The State of the Art, ed. Amstutz, G.C., et al, p.452-459. 40 �Han, T.M., 1988, Origin of Magnetite in Precambrian Iron-Formations of Low Metamorphic Grade, Proceedings of the Seventh Quadrennial IAGOD Symposium, p. 641-656. Han, T.M., and Runnegar, B., 1992, Megascopic Eukaryotic Algae from the 2.1 Billion-Year-Old Negaunee Iron-Formation, Michigan, Science v. 257, p. 232-235. Henrickson, E.L., 1956, A Study of the Metamorphism of the Upper Huronian Rocks of the Western Portion of the Marquette District, U.of Minn. PhD thesis James, H.L., 1953, Origin of the Soft Iron Ores of Michigan, Econ. Geol. v 48, p. 726-728. James, H.L., 1954, Sedimentary Facies of Iron-Formation, Econ. Geol. v. 49 p. 235-293. James, H.L., 1955, Zones of Regional Metamorphism in the Precambrian of Northern Michigan: Geol Soc of America Bull, v. 66, p. 1455-1488. James, H.L., 1958, Stratigraphy of pre-Keweenawan Rocks in Parts of Northern Michigan, USGS PP 314-C, p. 27-44. James, H.L., Dutton C.E., Pettijohn, F.A., and Wier, K.L., 1968, Geology and Ore Deposits of the Iron River-Crystal Falls District, Iron County, Michigan, USGS PP 570, 134p. Klasner, J.S. and Cannon, W.F., 1974, Geologic Intrepretation of Gravity Profiles in the Western Marquette District, Northern Michigan, GSA Bull. v. 85, p. 213-218. Klasner, J.S., and Cannon, W.F., 1978, Bedrock Geologic Map of the Southern Part of the Michigamme and Three Lakes Quadrangles, Marquette and Baraga Counties, Michigan, USGS Map I-1078. Klasner, J.S., Ojakangas, R.W., Schulz, K.J., and LaBerge, G.L., 1991, Nature and Style of Early Proterozoic Deformation in the Foreland of the Penokean Orogen, Michigan, USGS Bull 1904-K, 22 p. Kramer, R.S., Hwang, J.Y. and Johnson, A.M., 1987, A Mineralogical and Chemical Study of the Graphitic Lower Slate Member of the Michigamme Formation, Marquette and Baraga Counties, Michigan, Mich. DNR OFR, 97p. Larue, D.K., and Sloss, L.L., 1980, Early Proterozoic Sedimentary Basins of the Lake Superior Region, GSA Bull. v. 91, pt. I, p. 450-452, pt. II, p. 1836-1874. 41 �Larue, D.K., 1981, The Chocolay Group, Lake Superior Region, U.S.A.: Sedmentologic Evidence for Deposition in Basinal and Platform Settings on an Early Proterozoic Craton, GSA Bull. v. 92, p. 417-435. LaBerge, G.L., 1966, Altered Pyroclastic Rocks in Iron-Formation in the Hamersley Range, Western Australia, Econ. Geol. v. 61, p. 147-151. LaBerge, G.L., 1966, Altered Pyroclastic Rocks in South African Iron-Formation, Econ. Geol. v. 61, p. 572-581. Lougheed, M.S., and Mancuso, J.J., 1973, Hematite Framboids in the Negaunee Iron Formation Michigan: Evidence for their Biogenic Origin, Econ. Geol. v. 68, p. 202-209. Mancuso, J.J. and Lougheed, M.S. and Wygant, T., 1971, Possible Biogenic Structures from the Precambrian Negaunee (Iron) Formation, Marquette Range, Michigan, American Jour. of Sci. v. 271, p. 181-186. Mancuso, J.J., Kneller, W.A. and Quick, J.A., 1989, Precambrian Vein Pyrobitumen: Evidence for Petroleum Generation and Migration 2 Ga Ago, Precambrian Research v. 44, p. 13; 7-146. Mathias, D.L., 1959, The Basic Igneous Rocks of the Eastern Marquette Range, Michigan, PhD, Columbia Univ., 134p. Ojakangas, R.W., 1994, Sedimentary and Provenance of the Early Proterozoic Michigamme Formation and Goodrich Quartzite, Northern Michigan-Regional Stratigraphic Implications and Suggested Correlations, USGS Bull. 1904-R, 31p. Ojakangas, R.W., 2001, Paleoproterozoic Basin Development and Sedimentation In the Lake Superior Region, North America, Sedimentary Geol. v. 141-142 p. 319-341. Pettijohn, F. J., 1943, Basal Huronian Conglomerates of Menominee and Calumet Districts, Michigan, Jour. of Geol. v. 51, p. 387-397. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K. and Hamilton, M., 2002, Age of Volcanic Rocks and Syndepositional Iron Formations, Marquette Range Supergroup: Implications for the Tectonic Setting of Paleoproterozoic Iron Formations of the Lake Superior Region, Can. Jour. Earth Sci. v. 39, p. 999-1012. Schulz, K.J., and Cannon, W.F., 2007, The Penokean Orogeny in the Lake Superior Region, Precambrian Research v. 157, p.4-25. 42 �Simmon, G.C., 1972, Metadiabase Sills in Negaunee iron-formation Near National Mine, Michigan, USGS Bull. 1394A, p. A70-A75. Simmons, G.C., 1974, Bedrock Geologic Map of the Ishpeming Quadrangle, Marquette County, Michigan, USGS Map GQ-1130. Swanson, C.O., 1930, Report on a Portion of the Marquette Range Covered by the Michigan Geological Survey in 1929, Michigan Geological Survey in 1929, 15 p. Taylor, G.L., 1972, Stratigraphy, Sedimentology and Sulfide Mineralization of the Kona Dolomite, PhD, MTU, 112p. Tyler, S.A. and Twenhofel, W.H., 1952, Sedimentation and Stratigraphy of the Huronian of Upper Michigan, American Jour. of Sci., v. 250, Part I, p. 1-27 and Part II, p. 118-151. Van Hise, C.R., and Bayley, W.S., 1897, Marquette Iron-Bearing District of Michigan, USGS Monograph 28, 608 p. Van Hise, C.R., and Leith, C.K., 1911, The Geology of the Lake Superior Region, USGS Monograph 52, 641p. Waggoner, T.D., 1972, Stratigraphy, A tool in the Economic Development of the Marquette Iron Range, AIME meeting, Houghton, MI. Waggoner, T. D., 2003, A Hydrothermal Component o Iron Formations-A Marquette Range Perspective, 49th ILSG. Waggoner, T. D., 2006, Sulfur Isotopes from Pyrite in the Negaunee Iron Formation, 52nd ILSG. 43 �44 �45 �Introduction The Silver Lake area (Figure 2.1) lies along the north margin of the Dead River Basin, a structural outlier of Paleoproterozoic strata surrounded by Archean crystalline rocks. Silver Lake, a natural water body, was enhanced by an impoundment constructed in 1910 and served as a storage basin for downstream hydroelectric generation along the Dead River. Small outcrops along the north shore of the enhanced lake showed a variety of interesting and puzzling features at the Archean-Paleoproterozoic unconformity (Klasner and others, 1979). In May 2003, after very heavy rains, a segment of an earthen dam failed, resulting in catastrophic flooding downstream and a drop of the lake to near the original natural level. The current lake level is 25 to 30 feet below the former impounded level and about 1,000 acres of the previous lakebed are now dry land. This has resulted in reemergence of numerous outcrops on the former lake floor. These outcrops are along the Archean-Paleoproterozoic unconformity and are the focus of this trip. Because reconstruction of the dam and subsequent reflooding of the basin are planned, there is only a narrow time window in which to observe and study these unique features. Figure 2.1. Map of the Marquette region showing the location of Silver Lake and the driving route north from U.S. Highway 41 in Ishpeming. General Geology The Silver Lake area lies on the northern flank of the Dead River basin, which is a structural basin filled with Paleoproterozoic sedimentary rocks and surrounded by Neoarchean crystalline rocks of diverse lithology. The geology of the basin was mapped in detail during the 1970’s (Puffett, 1974; Clark and others, 1975; Klasner and others, 1979). The Paleoproterozoic rocks consist entirely of various informal units of the Michigamme Formation, a part of the Baraga Group. The Michigamme is volumetrically dominated by a thick succession of turbidites, which form the upper part of the formation. The lower units, however, including those seen at Silver 46 �Lake, consist of quartzite and conglomerate, laminated argillite, carbonaceous shale, and lean iron-formation or ferruginous chert. Recent studies also identified a layer of ejecta-bearing rocks in the lower part of the formation, which has been correlated with the Sudbury impact event (Cannon and others, 2006a, b; Cannon and Addison, 2007a, b; Kring and others, 2006; Pufahl and others, 2007). Figure 2.2. Map of the Silver Lake field trip area showing localities (1 through 17) described in this guide. General geologic relationships are generalized from Klasner and others (1979). The heavy dashed line is an unmaintained logging road drivable in some seasons by the stout of heart. This trip will hike the road from the point indicated. The extent of Silver Lake as shown is that prior to the 2003 dam failure. It’s configuration in 2008 is much smaller than shown. The structure of the Dead River Basin basin is complex as a result of both Penokean and Yavapai deformation. During the Penokean orogeny, between 1850 and 1830 Ma, thin-skinned deformation produced folds and slatey cleavage that are best developed in the upper part of the Michigamme. None of the structures seen at Silver Lake can be definitively assigned to the Penokean orogeny. The present structural basin is a result of differential movement between fault-bounded blocks of Archean rocks and the molding of Proterozic strata around the fault blocks. This deformation has long been interpreted to be a late phase of the Penokean orogeny, 47 �but recent geochronological data suggest that it is younger and equivalent in age to the Yavapai orogeny at appxoximately 1775-1750 Ma (see recent summaries by Holm and others, 2007; Schulz and Cannon, 2007). A low-temperature regional hydrothermal event has been documented to have occurred at nearly this same time during which xenotime cements formed in the basal Michigamme Formation. Such secondary xenotime is well developed at Silver Lake and material collected from Locality 16, described below, yielded a xenotime crystallization age of approximately 1785 Ma (Vallini and others, 2007). The geology in the area of this field trip is shown in Figure 2.2. Archean rocks, a diverse suite of metavolcanic and granitic rocks, form the prominent uplands whereas the Michigamme Formation underlies the lowlands. The very steep hillsides reflect the extreme contrast in erosional resistance of these two units. The area is divided into two structural panels by a prominent fault along the northeastern shoreline of Silver Lake. Northeast of the fault, Archean rocks are relatively uplifted and tilted toward the northeast. Along the steep hillside descending onto the Mulligan Plains there are sporadic exposures of the basal beds of the Michigamme Formation which dip 40° northeast and thus indicate the amount of rotation. Foliation in outcrops along the fault is nearly vertical. Southwest of the fault, including the area that is the focus of this trip, a block of Archean rocks has been uplifted and tilted slightly toward the southwest so that the unconformity between it and the Michigamme Formation forms a gently southeast-dipping surface along the lakeshore in the northwestern part of Figure 2.2. Draining of the lake exposed extensive new outcrops, such as shown in Figure 2.3, that consist of Archean rocks, mostly massive to foliated granite, and the basal beds of the Michigamme Formation. The outcrop surface closely mimics the unconformity surface so that discontinuous patches of the Michigamme are preserved in declivities on that surface. Lithology of basal Proterozoic beds. The maximum thickness of the preserved Paleoproterozoic strata at the field trip stops is only about one meter. A variety of rock types from conglomerate to fine-grained laminated sedimentary rocks are present and the rock type at any particular locality may reflect the micro-topography along the surface at the time of deposition. A few lenses of pebble conglomerate appear to be somewhat mature and consist of rounded and obviously waterworked debris, including rounded quartz pebbles. These may be lenses of wave-washed gravel that accumulated in depressions on the Archean surface during the earliest phase of marine transgression. More typically, basal beds are breccia consisting almost entirely of angular fragments of rock types contained in the immediately underlying Archean basement. They may be a residuum of physically weathered basement rock that experienced little or no wave action. Laminated fine-grained sedimentary rocks also are widespread and occur both above the basal conglomerate lenses or lie directly on the Archean basement where basal conglomerate is absent. 48 �Figure 2.3. Area near Location 17 showing the newly exposed outcrops of the former lake bottom. The land surface very closely mimics the unconformity between Archean granitic rocks and basal beds of the Michigamme Formation. Hundreds of individual vestiges of the basal sediments dot the surface of the granite. A significant aspect of the lowermost sediments is phosphatic material that occurs as masses of nearly pure carbonate flourapatite. These masses are typically from a few to as much as 10 cm in diameter and some have shapes and internal structures suggestive of stromatolitic growth. These were first described by Cannon and Klasner (1976) along with numerous other occurrences of phosphatic material within basal Baraga Group rocks in the Marquette area. A good example of these is shown in Figure 2.4A where stromatolite-like growths of apatite have repeatedly developed in the lowermost few centimeters of the Michigamme Formation immediately adjacent to the unconformity with Archean rocks and occur in three or four individual layers separated by fine-grained clastic sedimentary rocks. In this particular case, microtopography along the unconformity appears to have localized growth on a banded quartz vein which stood in relief above the surrounding granite. Phosphate masses grew on the relatively steep surface of the quartz vein, a situation apparently in some way physically favorable for phosphate accumulation. Other phosphate masses, such as shown in Figure 2.7B, occur directly on the unconformity with Archean granite. Some of this phosphatic material was also reworked into overlying conglomerate lenses. 49 �Figure 2.4. A- Masses of carbonate flourapatite (dark areas of negative relief just above unconformity) that appear to have grown in successive layers along steeply dipping microtopography on the unconformity. Host beds are laminated argillite. The immediately underlying rock is a banded quartz vein that cuts Archean granitic rocks. This feature can be seen at Locality 11. B- Small fold of Michigamme argillite between blocks of Archean granite. A vestige of the unconformity is seen at point A where the argillite was deposited in flatlying bedns on the Archean. The space not occupied by the fold was created by lateral opening of a gap between granite blocks B and C and slumping of the soft sediments into the new space. Other joints in the granite (D) are filled with injected sediments. Feature can be seen at Locality 16. 50 �Structure The excellent exposures created by the draining of Silver Lake reveal a set of unusual, intriguing, and puzzling small-scale structures along the unconformity. The gross structure is simple. The unconformity seen at localities 9 through 17 is gently inclined toward the south and southeast as a result of rotation of the Archean basement rocks, probably in the time interval 1775-1750 Ma. There are no penetrative fabrics within the Archean rocks that can be ascribed to this period of deformation; rather the Archean appears to have moved as a series of rigid blocks separated by faults and the overlying Paleoproterozoic strata moved passively with them. A variety of small-scale structures are also well exposed and appear to record an unusual structural event that we propose might have been a powerful earthquake caused by the giant Sudbury impact event. At several nearby localities within the Dead River Basin, a layer of ejecta-bearing breccia, interpreted to have been formed by the Sudbury impact is within the lower part of the Michigamme Formation (Cannon and others, 2006a, b; Kring and others, 2006; Cannon and Addison, 2007a, b; Pufahl and others, 2007). There is clearly a close temporal correspondence with the basal Michigamme beds exposed at Silver Lake and the time of the impact. In fact, our preliminary petrographic study of these rocks, discussed more fully below, found features that might be directly caused by the impact. Two types of structures are present: 1) drapes of Michigamme sediments around Archean blocks that have undergone small displacements relative to each other, and injection of sediments into joints within the Archean basement; 2) intense brecciation of the Archean rocks, soft-sediment flow of the basal Michigamme and intermixing of the two rock types. The first type of features, drapes of sediment around Archean blocks, is best seen at localities 14 through 17. Figure 2.4B shows the essential characteristics of this type of deformation. Finegrained laminated sediments were deposited unconformably on Archean granite in essentially flat-lying beds. This unconformiable surface is widely preserved (such as at A in Figure 2.4B). The tight syncline shown in figure 2.4B formed as these flat-lying sediments slumped and flowed into an open space created as blocks of granite (B and C) separated laterally. Numerous joint surfaces (D in Figure 2.4B) are also filled with sediments which apparently flowed into open spaces during this same event. Individual granite blocks ranged up to several meters or tens of meters in diameter and experienced relative displacements up to several meters. The result is a complex unconformity surface with structural relief of meters and complexly folded basal Michigamme strata. The second type of feature, brecciation of basement rocks and soft-sediment flow of the basal Michigamme, is very well displayed at localities 9 through 13 and illustrated in Figure 2.6 (Locality 9). All stages of brecciation of basement rocks are preserved, ranging from small movements on joint surfaces and infilling of the spaces thus created by sediments (Figure 2.9; Locality 13) to intense dismemberment of the Archean rocks into angular fragments which are intermixed with a matrix of clastic sediments (Figure 2.8; Locality 13). In the less brecciated granitic basement, clastic dikes are very common and range in thickness from nearly a meter to a few millimeters. The wider dikes commonly have an internal lamination (Figure 2.7A for example; Locality 11) indicating that the sediment fill was caused by an injection of originally overlying laminated sediments rather than an infiltration of individual clastic particles into open 51 �space. In places (locality 13 for example) a remarkable intersecting array of clastic dikes extends at least several meters below the unconformity showing that sediments were able to completely infiltrate a joint system well below the unconformity. Such features imply that a period of dilation affected the Archean rocks during which overlying soft sediments were injected into all available open spaces. Sudbury Seismites? Could the array of unusual features seen at Silver Lake have been caused by intense seismic shaking, and could that shaking have been caused by the giant impact at Sudbury? Giant impacts do generate exceptionally powerful earthquakes. For instance the Chicxulub impact in Mexico has been variously estimated to have generated a quake of M 10 to13 on the Richter scale, significantly more powerful than the largest known terrestrially generated earthquake. The Sudbury impact was a somewhat larger event. It too should be expected to have generated an earthquake of nearly unprecedented energy and to have left a unique imprint on rocks of the region. A calculation of the seismic intensity from the Sudbury impact using the on-line Earth Impact Effects Program (Marcus and others, 2004) indicates an intensity of 10.5 on the Richter scale, greater than any earthquake in recorded history, and a Mercalli Scale Intensity of X at Silver Lake (nearly total destruction of man-made structures in the modern sense). The Chicxulub impact has been shown to have produced seismic disturbance of sediments well over a thousand kilometers from the impact site (Norris and others, 2000; Terry and others, 2001). Thus it seems likely that the Silver Lake area, only about 500 km from Sudbury, was well within the range of significant seismic disturbance from the Sudbury impact. The intense shattering of Archean basement rocks and contemporaneous flow of overlying soft sediments are features that could have been caused by the passage of an impact-generated shock wave and consequent shaking. The complex array of sedimentary dikes that cut the Archean require a period of dilation during which fractures in the granite were opened and then filled by the injection of overlying soft sediments. Such features can form during passage of a seismic wave in which the leading edge of the wave is compressional and is followed by a dilational wave (Melosh, 1989). During this instantaneous dilation the Archean rocks may have expanded and formed open spaces along fractures. Overlying sediments would have been injected into the newly created space. Similar features have been reported from the Locke impact structure in Sweden (Sturkell and Ormo, 1997) where sediment dikes cut shattered granitic rocks surrounding this small Ordovician crater. Some of the material at Silver Lake is also similar to “clastic Sudbury breccia” (Rousell and others, 2003) that is found as much as several tens of kilometers outside of the present Sudbury Basin. A final piece of evidence that suggests a possible link to the Sudbury impact is possible impactrelated grains that have been found in some of the clastic dikes. Although our petrographic examination is very preliminary at this point, we have observed numerous millimeter-scale round to ovoid grains consisting of very fine-grained brownish clay. Many have abundant shrinkage cracks (Figure 2.5). These are very similar in appearance to grains that occur in some phases of the Sudbury impact layer at nearby occurrences and have been interpreted as altered microtectites formed from impact-generated vapor. These same rocks also contain a sparse collection of quartz grains that have planar features that may be shock-induced planar 52 �deformation features (Figure 2.5D), but we have not yet found truly definitive shock features. Nevertheless, there is at least suggestive evidence that ejecta material from the Sudbury impact was incorporated into the clastic injections. The distance which the Sudbury layer lies above the stratigraphy exposed at Silver Lake is unknown but it could have been very small. The nearest known exposure, Connors Creek, is only about 3 km to the south. There the Sudbury layer is about 150 meters above the Archean unconformity. Regional relationships suggest that underlying strata thin to the north and the layer may be very close to the level exposed at Silver Lake. Thus, we propose a model in which Sudbury ejecta material arrived at Silver Lake essentially contemporaneously with severe seismic shaking and deformation of Archean rocks that were overlain by only a meter or two of Paleoproterozoic sediments at the time. Both the sediments and ejecta particles were emplaced as sediment dikes in newly opened fractures. Figure 2.5. Photomicrographs of clastic material from sediment dikes injected into the Archean basement rocks. A, B- Broken and distorted spherules of aphanitic brown clay in siliceous finegrained matrix containing abundant quartz sand grains. These spherules are very similar in appearance to spherules within Sudbury ejecta deposits nearby. C- intact spherule of aphanaitic clay. D- quartz grain cut by planar features the may be shock induced planar deformation features. FIELD TRIP LOCALITIES The localities of principal interest for this trip are numbers 9 through 17 as shown on Figure 2.2. The easiest walking route to these localities is along the old shoreline on the northeast side of Silver Lake. Along this route numerous outcrops (localities 1 through 8) lie along the fault that juxtaposes Archean basement rocks and the Michigamme Formation. The route crosses the fault several times so exposures of both the Archean basement and Michigamme Formation can be examined. These eight localities are described briefly and localities 9 through 17 are described in 53 �more detail. Latitude and longitude values are given for each locality to assist in GPS navigation to them. In general, the photographs shown in Figures 2.6 through 2.11 are taken within a few tens of feet of the GPS locations. Locality 1.-- (46.6603, -87.8134) Small outcrop of sheared quartz and plagioclase phyric rhyolite; part of Archean basement. Locality 2.-- (46.6614, -87.8149) Long outcrop (300 ft) of massive to weakly foliated Archean biotite amphibolite. Foliation is irregular. Some features may be relict pillow selvages or cryptic pillows. The southeasternmost part of the outcrop is highly sheared, probably by movement on the fault. Locality 3.-- (46.6625, -87.8180) Michigamme Formation. Rusty-weathering, dark gray to black slate. A steep uniform cleavage is very well developed. In places bedding laminations from ½ to 1 inch thick are parallel or subparallel to cleavage. Locality 4.-- (46.6632, -87.8192) Michigamme Formation similar to Locality 3, except there are beds of coarser, more massive greywacke toward the north side of the outcrop. Locality 5.-- (46.6642, -87.8213) Michigamme Formation similar to locality 3. Locality 6. -- (46.6656, -87.8234) Long outcrop (300 feet) of highly sheared mafic volcanic rock (Archean). Way point is at northwest end of outcrop. Locality 7.-- (46.6667, -87.8273) Michigamme Formation as at Locality 3. Locality 8.-- (46.6674, -87.8282) Sheared Archean amphibolite with lesser felsic layers. Coarse amphibole crystals in places. Layering is parallel to nearly vertical shear foliation. Locality 9.-- (46.6652, -87.8319) After leaving locality 8 and turning southwest the route passes onto the Archean basement block south of the fault that has been tilted gently to the southeast. The unconformity with the base of the Michigamme Formation is well exposed here as a surface that dips about 15° to the southeast. The Archean rocks are sheared felsic metavolcanics with nearly vertical compositional layering and shear foliation (Figure 2.6A). They are overlain by a layer of breccia only an inch or two thick. Clasts are mostly very angular and appear to be in very large part of the same lithology as the immediately underlying Archean rocks (Figure 2.6B). Laminated gray argillite overlies the basal breccia. Only the lowest foot or two of this unit is exposed here. The relationships shown here leave no doubt that the intense penetrative deformation in the Archean rocks entirely pre-dates deposition of the Michigamme Formation. 54 �Figure 2.6. Features seen at Locality 9. A- unconformity between sheared Archean felsic volcanic rocks below and gray laminated argillite of the Michigamme Formation above. The base of the Michigamme Formation consists of a few inches of conglomerate with very angular fragments, mostly with lithology identical to the immediately underlying Archean rocks. The surface is cut obliquely through the strata which exaggerates the apparent thickness of the basal conglomerate bed. B- closeup view of the basal conglomerate showing the extreme angularity of most clasts and the essentially unsorted nature of the bed. Locality 10.-- (46.6649, -87.8333) Rocks here are entirely Archean and are mostly laminated metasedimentary rocks in beds ½ to 1 inch thick. Beds are highly folded and fold axes plunge about 55° to the southeast. The north edge of the outcrop is a sheared mafic rock, possibly a dike intruded into the metasedimentary unit. 55 �Locality 11 (46.6649, -87.8338) and Locality 12 (46.6642, -87.8337).-- These two localities are on the north and south ends respectively of a large outcrop area. Near locality 11, on the north part of the outcrop, a thin skin of basal conglomerate lies on the Archean basement. In places the Archean foliated granite is broken into large blocks and open spaces between the blocks are filled with sediment and small angular granite fragments. Lamination in the sediments is preserved in part and was deformed against the granite blocks (Figure 2.7A). At several places along the unconformity, light gray aphanitic masses of carbonate flourapatite lie directly on the granite (Figure 2.7B). Toward the south end of the outcrop, near Locality 12, one to two feet of breccia forms a layer down the east side of the exposure. This differs from most of the other nearby basal conglomerates in having a quartz-chlorite matrix and a diversity of lithic fragments of Archean rocks, including chert and quartz pebbles. Is some of this material Subury ejecta? The southern end of the outcrop is Archean granite. Locality 13.-- (46.6636, -87.8345) The outcrops here show the best examples of intensely brecciated Archean rocks, soft-sediment deformation and flow of Michigamme argillite, and intense development of sediment dikes in the Archean basement. On the northside of the outcrop the Archean rocks are brecciated to highly variable degrees. In places blocks of the Archean have moved apart on joint surfaces to create open space that was filled by clastic sediments. All variations can be seen from this relatively mild deformation to complete shattering of the Archean into centimeter-scale angular fragments that are suspended in a clastic matrix (Figure 2.8B). In some cases adjacent fragments can be fitted together to reconstruct the pre-brecciation geometry indicating that fragments have not moved far during the brecciation process. The unconformity is also well exposed here. Rather than a basal conglomerate, the base of the Michigamme is laminated argillite. Black to pinkish gray color banding emphasizes the bedding and readily shows intense soft-sediment deformation features (Figure 2.8A). There is an intermixing of the Michigamme argillite and Archean granite; granite fragments are incorporated into the basal foot of the sediments and masses of the argillite occur within the upper foot or two of the breccia. We interpret these relationships to indicate that the brecciation and soft-sediment deformation occurred simultaneously in response to the same seismic event. The south side of the outcrop provides a cross section of the upper 3-4 meters of the Archean basement granite below the unconformity. A remarkable array of sedimentary dikes is seen here at scales varying from about 0.5 m to a few millimeters (Figure 2.9). Virtually every joint surface appears to have a least a thin film of sedimentary material along it. The larger dikes generally have an internal layering shown by variations in grain size that is parallel to the dike margins. Such features indicate that the sedimentary material was forcefully injected into the joints rather than accumulating by settling of grains into open spaces. This in turn implies that there was a dilational event that simultaneously opened all of the fractures in this geometrically diverse fracture system to allow injection of the sediments. 56 �Figure 2.7. Features seen at Locality 11. A- Fractured Archean granite with gray laminated argillite injected between granite blocks. Numerous fragments of the country rock granite are incorporated into the argillite. B- View looking down on the surface of the unconformity. Archean rock is foliated granite. Several masses of carbonate flourapatite are directly on the unconformity surface and are overlain by gray argillite. 57 �Figure 2.8. Features seen at Locality 13. A- intensely brecciated granite. Angular granite clasts are suspended in a clastic sedimentary matrix. Overlying banded argillite is intensely deformed by soft-sediment flow. Note fragments of granite intermixed with basal beds of the argillite, and masses of argillite scattered through the granite breccia. B- brecciated granite showing varying degrees of fragmentation. Note rounded clast to right of scale with only slight separation of fragments and fractures filled with clastic sediment. Elsewhere angular clasts of various sizes are suspended in a clastic matrix. 58 �Figure 2.9. Features seen at locality 13. Views of the south side of the outcrop at various scales. The surface of the unconformity exposed on the north side of the outcrop projects to just above the top of the outcrop in A. Note the abundance if sedimentary dikes (darker), throughout the massive Archean granite. Dikes vary from about 2 feet wide (near top of outcrop in A) to paper thin seams (D). Locality 14.-- (46.6632, -87.8351) At this locality we begin to see a transition in the type of deformation from the brecciation and dike injection to the north to differential movement of larger Archean blocks, ranging in size up to tens of meters, and molding of soft sediments around these blocks. Although sediment dikes and brecciation are still fairly well developed here, there are also several examples of folds in the Michigamme where the sediments have been molded around or compressed between joint blocks of Archean granite. Note that where the basal Michigamme sediments are tightly folded the foliation in the adjacent Archean rocks is unaffected by the folds indicating that the Archean rocks moved as rigid blocks and the Michigamme was molded to the new shape of the top of the Archean. These features are very well exposed at Localities 16 and 17 to the south. Locality 15.-- (46.6630, -87.8355) This outcrop is entirely Archean rocks, mostly massive granite. On the north end of the outcrop there are many thin sediment dikes but their abundance diminishes to the south. Locality 16 (46.6613, -87.8365) and Locality 17 (46.6613, -87.8398).—Beginning in the vicinity of Locality 16 and continuing westward to Locality 17, the western limit of good 59 �exposures, there are a multitude of small-scale folds in the basal beds of the Michigamme Formation. These are mostly synclinal features with diverse orientations and plunges (Figures 2.10 and 2.11). They appear to have formed as the soft sediments were molded around blocks of Archean rocks as those blocks were structurally rearranged. Note numerous instances where tight folds in the Michigamme have no expression in the immediately adjacent Archean rocks, showing that the folds have formed in response to the newly acquired shape of the unconformity surface on top of the Archean by draping over that surface, or in some instances by being injected into open joints. Figure 2.10. Features seen at Locality 16. Examples of small folds in the basal beds of the Michigamme Formation caused by molding the sediments to the shape of blocks of Archean granite. These fine-grained sediments were no doubt deposited in flat-lying beds on a horizontal surface but were later distorted to their present configuration as blocks of Archean rocks were displaced relative to each other. 60 �Figure 2.11. Features seem between Localitites 16 and 17. A- synclinal folds of Michigamme Formation argillite with variable plunges formed between blocks of Archean granite. BFractures in granite filled with fine-grained sediment. C- fractured boulder with laminated argillite compressed into the open fracture. D- undulating unconformity surface on top of Archean granite with basal Michigamme sediments draped over it. References Cannon, W.F., and Addison, W.A., 2007a, Distal ejecta from the 1850 Ma Sudbury impact in the Lake Superior iron ranges: Geological Society of America Abstracts with Programs, v 38, p. 58. Cannon, W.F., and Addison, W.D., 2007b, The Sudbury impact layer in the Lake Superior iron ranges: a time-line from the heavens: Institute on Lake Superior Geology 53rd Annual Meeting, v. 53, p. 20-21. Cannon, W.F., Horton, J.W. Jr., Kring, D.A., 2006a, Discovery of the Sudbury impact layer in Michigan and its potential significance: Geological Society of America Abstracts with Programs, v. 38, no. 7, p.58. Cannon, W.F., Horton, J.W Jr.., Kring, D.A., 2006b, The Sudbury impact layer in the Marquette Range Supergroup of Michigan: Institute on Lake Superior Geology 52nd Annual Meeting, v. 52, p. 10-11. 61 �Cannon, W.F. and Klasner, J.S., 1976, Phosphorite and other apatite-bearing sedimentary rocks in the Precambrian of Northern Michigan: U.S. Geological Survey Circular 746, 6 p. Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Survey Geological Quadrangle Map GQ-1226, scale 1:24,000. Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution: Precambrian Research, v. 157, p. 106-126. Klasner, J.S., Cannon, W.F., and Brock, M, 1979, Bedrock geologic map of parts of Baraga, Dead River and Clark Creek basins, Marquette County, Michigan: U.S. Geological Survey Open File map 79-135, scale 1:62,000. Kring, D.A., Horton, J.W., Jr., and Cannon, W.F., 2006, Discovery of the Sudbury impact layer in Michigan, USA: Meteoritics and Planetary Science, v. 41, supplement, p. A100. Marcus, R, Melosh, H.J., and Collins, G., 2004, Earth Impact Effects Program: http://www.lpl.arizona.edu/impacteffects/ Melosh, H.J., 1989, Impact cratering: a geologic process: Oxford University Press, New York. Norris, R.D., Firth, J., Blusztajn, J.S., and Ravizza, G., 2000, Mass failure of the North Atlantic margin triggered by the Cretaceous-Paleogene bolide impact: Geology, v. 28, p. 1119-1122. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence for the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology, v., 35, p. 827-830. Puffett, W.P., 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U.S. Geological Survey Professional Paper 788, 53 p. Rousell, D.H., Fedorowich, J.S., and Dressler, B.O., 2003, Sudbury breccia (Canada): a product of the 1850 Ma Sudbury event and host to footwall Cu-Ni-PGE deposits: Earth-Science Reviews, v. 60, p. 147-174. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. Sturkell, E.F.F., and Ormo, J., 1997, Impact-related clastic injections in the marine Ordovician Lockne impact structure, central Sweden: Sedimentology, v. 44, p. 793-804. Terry, D.O., Chamberain, J.A., Stoffer, P.W., Messina, P., and Jannett, P.A., 2001, Marine Cretaceous-Tertiary boundary section in southwestern South Dakota: Geology, v. 29, p. 10551058. 62 �Vallini, D.A., Cannon, W.F., Schulz, K.J., and McNaughton, N.J., 2007, Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: evidence for a widespread 1786 Ma overprint based on xenotime geochronology: Precambrian Research, v. 157, p. 169-187. 63 �64 �54th Annual Institute on Lake Superior Geology Field Trip 3 GEOLOGY OF THE BACK FORTY PROJECT Tom Quigley Bob Mahin Aquila Field Office Geologic Staff 65 �54th Annual Institute on Lake Superior Geology  FIELD TRIP # 3    Back  Forty Geology and Mineralization  Tom Quigley   Bob Mahin    Aquila Field Office Geologic Staff      Gossan mineralization exposed at surface at the Back Forty project site     66 �Introduction  The Back Forty Volcanogenic Massive Sulfide (VMS) deposit located alongside the Menominee River in  the Upper Peninsula of Michigan is the most recent deposit of this type found in the Early Proterozoic  aged Penokean Volcanic Belt (PVB) which trends east west through Wisconsin and extends into the  Upper Peninsula of Michigan.  Numerous massive sulfide occurrences and several significant deposits  were discovered as a result of protracted exploration efforts focused on the Wisconsin portion of the  PVB during the 1960’s, 1970’s, and early 1980”s including the Crandon deposit (61 million tonnes 5.6%  Zn), Flambeau (5.8 million tonnes 4% Cu) and Lynne (6.1 million tonnes 8.7% Zn).    The Back Forty was discovered in 2002, and has a resource (current as of April 2007) of 6.6 million  tonnes with 5.3% Zn, 2.3 grams/tonne (g/t) Au, 29 g/t Ag, and 0.5% Cu in the measured and indicated  category, and an additional 1.75 million tonnes of 2.6% Zn, 2.8 g/t Au, and 32 g/t Ag in the inferred  category, making it the 2nd largest deposit found in the PVB to date and placing it in the upper 30th  percentile in size of VMS deposits worldwide.  The April 2007 resource was calculated on the basis of 151 diamond drill holes (35,000 meters).  Since  then an additional 150 holes (30,000 meters) of drilling has been completed in anticipation of a new  resource calculation, preliminary mine design, metallurgical testing, and pre feasibility studies planned  for 2008 and 2009.  Regional Geologic Setting of the Back Forty VMS Deposit  The Back Forty VMS deposit is one of a number of similar deposits located within the Ladysmith‐ Rhinelander volcanic complex in northern Wisconsin and western Michigan. The complex lies within the  Early Proterozoic Penokean volcanic belt (PVB), also known as the Wisconsin Magmatic Terrain, on the  western edge of the Paleozoic Michigan Basin (Figures 1 and 2).  Figure 1    67 �Figure 2.  Location of Back Forty project and other major VMS deposits      The PVB is characterized by volcanic island‐arc‐basin assemblages containing abundant calc‐alkaline  metavolcanic units, intrusive rocks and lesser amounts of sedimentary rocks, and is in structural contact  to the north, along the Niagara Fault zone, with a back arc basin sedimentary terrain containing  subordinate interbedded tholeiitic metavolcanic rocks and major Superior‐type, oxide‐facies iron  formations. This supracrustal sequence appears to correlate with the Marquette Range Supergroup in  Michigan (Dematties 2004).  The Back Forty project is located at the eastern edge of the PVB where the older volcanic supracrustal  rocks of the belt are covered by Paleozoic sedimentary rocks of the Michigan Basin.  Local Geologic Setting   Figure 3 shows the interpreted bedrock geology of the Back Forty area derived from published geologic  maps, airborne and ground geophysical data and sparse outcrops.  68 �    Figure 3.  Bedrock Geology of the Back Forty Project area.    Back Forty mineralization is hosted by dominantly felsic volcanic rocks which appear to be spatially and  possibly genetically related to a large intrusive complex of granite, tonalite and more mafic phases  exposed sporadically in Wisconsin and interpreted to extend into Michigan based on gravity and  magnetic data.  This central complex of felsic intrusive and volcanic rocks is flanked on the north and  69 �south by more mafic volcanic sequences as well as argillites and fine grained tuffaceous rocks to the  south.    Deposit Scale Geology  Geology of the host rocks  Back Forty mineralization consists of massive, semi massive, and stringer sulfide mineralization as well  as precious metal zones with sparse sulfides, developed within a highly altered sequence of rhyolite  breccias and pyroclastic rocks cut by dikes, sills and irregular intrusions of porphyritic dacite and  rhyodacite.  Late mafic dikes and at least one dioritic to gabbroic intrusive intrude the felsic sequence.   Figure 4 shows the bedrock geology of the immediate deposit area.                                    Figure 4 .  Bedrock geology of the Back Forty deposit.  Rhyolitic rocks comprise the main host for the massive sulfide mineralization and consist of three  chemically distinct sequences of rhyolite breccias, pyroclastic rocks and thin interbedded tuffaceous  rocks.  A well developed sequence of finely bedded tuffaceous sediments, including a cherty exhalative  70 �horizon, occurs at the break between the middle and upper rhyolite sequences.  Younger dacitic quartz,  feldspar porphyries intrude the entire sequence including the massive sulfides.    Structurally, this rhyolite sequence and associated massive sulfide mineralization has been deformed  into an asymmetric, moderately plunging (300 west‐southwest) anticlinal fold characterized by a gently  dipping north limb (30o northwest), a steeply dipping and sheared south limb (70o southeast).   The hinge  of the fold and associated massive sulfide mineralization have been breached by erosion in the vicinity  of the East Zone or near or at the fold’s subcrop to the east.  Folding has produced an axial planar  schistosity and faulting has offset lithologies and created zones of weakness for younger intrusive rocks.  Three chemically distinct (but not always visually distinct) rhyolite sequences have been identified in the  immediate area of mineralization.  The units are identified as Rhyolite 1, 2 and 3 from oldest to  youngest,  as illustrated in the generalized stratigraphic column shown in figure 5.    Figure 5.  Rhyolite stratigraphic section with major massive sulfide zones.  71 �Figure 6 shows two discrimination plots used to characterize Back Forty host rocks.  The largely rhyolitic  composition of the volcanic rocks is illustrated in the SiO2 vs Zr/TiO2 plot, and the immobile element plot  of Zr/TiO2 vs Al2O3/TiO2 clearly distinguishes the three rhyolites.    WR plot   Figure 6.  Discrimination diagrams for Back Forty host rocks.  72 �All three rhyolite types exhibit intense leaching of Na2O (feldspar breakdown) and concomitant increase  in K2O (sericitization) as a result of intense hydrothermal alteration.  Altered host rocks form  assemblages of quartz – sericite – pyrite throughout the drilled section hosting the massive sulfide  mineralization and throughout an extensive area surrounding the known mineralization.  The degree  and extent of this alteration is evidence for a large and long lived hydrothermal system and suggests the  potential for additional mineralization in the area.  Mineralization  Mineralization at the Back Forty project consists of base metal massive sulfide, semi massive sulfide and  stringer sulfide mineralization as well as precious metal (gold and silver) mineralization.  Base metal mineralization Massive, semi massive, and associated stringer sulfide mineralization occurs in at least 3 stratigraphic  intervals roughly 100 m apart within the altered rhyolite sequence (figure 5), originally occupying  horizons separating the major rhyolite eruptive events. Subsequent folding, shearing, faulting and  emplacement of younger intrusive rocks has complicated and disrupted this primary stratigraphy.  Massive sulfides are dominantly of the zinc‐rich variety although copper‐rich zones occur in some lenses  and copper‐rich stringer mineralization is locally common.  The felsic‐dominant volcanic stratigraphy  that is host to the Back Forty mineralization point to a bimodal‐felsic or a Kuroko‐style of mineralization,  defined by having >50% felsic volcanic rocks, and <15% siliciclastic rocks in the host stratigraphic  succession (Barrie, 2007).     Zinc‐rich massive sulfides at Back Forty consist of medium to coarse grained aggregates of pyrite,  sphalerite, and lesser chalcopyrite and galena, with varying amounts of silver and gold.  Pyrite is the  dominant gangue mineral with minor amounts of pyrrhotite and aresenopyrite.  Galena attains  potentially recoverable amounts in one of the zinc‐rich massive sulfides (the Tuff Zone), which also  contains elevated silver values relative to the other horizons.    Two massive sulfide lenses come to surface and have been intensely oxidized to form iron oxide‐rich  gossans which cap fresh massive sulfide.  The gossan mineralization consists  principally of botryoidal,  colliform and brecciated hematite and goethite, with lesser amounts of the minerals found in the  primary massive sulfide that have undergone partial to near complete replacement by the oxides. Minor  to trace amounts of bornite, gold, argentite, diaphorite, acanthite, ramdohrite (Ag3Pb6Sb11S24), Ni‐ skutterudite, eugenite, meneghinite, clausthalite (PbSe), cassiterite, and other trace phases are present  (Barrie, 2007).  Gold and electrum are present at grain boundaries of other minerals, and within colloidal  hematite.  Although recoverable amounts of copper are present in the Pinwheel gossan, base metal  tenors in gossan are generally very low, with gold and silver locally attaining very high grades.     Copper‐rich massive sulfides are limited in extent relative to the zinc‐rich variety, and are composed of  mainly pyrite and chalcopyrite, with some supergene enrichment to bornite in near surface zones  underlying gossan.  Like the zinc‐rich massive sulfides copper‐rich zones contain varying amounts of gold  and silver.  Textures in the massive sulfide lenses are massive to bedded with extremely variable bedding attitudes,  indicating post depositional deformation and remobilization.  73 �Stringer sulfide mineralization normally consists of cross cutting veins and fracture fillings of pyrite with  varying amounts of chalcopyrite and gold, and normally underlies massive mineralization but may be  laterally correlative with some massive lenses.  To better understand the geometry and stratigraphic position of the Back Forty massive sulfides, several  views and cross sections are presented below.  In plan view (Figure 4) the massive sulfide lenses occupy  the hinge and north and south limbs of the folded stratigraphy.  Figure 7 shows a longitudinal 3D section  of the mineralized zones viewed from the north, and  figures 8, 9, and 10 are cross sections (locations  shown on figure 4) which further illustrate the morphology of the massive lenses and host rocks.  Approx. 100 meters       Figure 7.  Three dimensional view of Back Forty sulfide and gossan mineralization.              74 �Figure 8.  Cross section of the near surface East Zone.  For legend see figure 4.  Scale is in meters.                    75 �Figure 9.  Cross section through the  Main Zone (hinge area and south limb) and Tuff Zone.  For legend  see figure 4.  Black diamonds represent gold intercepts.                    76 �Figure 10.  Cross section through the Pinwheel and 90 Gold Zone.  For legend see figure 4.              77 �Metal distribution is variable within massive sulfides and associated host rocks.  Figures 11 and 12  illustrate the typical patterns of base and precious metals in massive sulfides, stringer zones, altered  host rocks and younger intrusive porphyry.    Figure 11.  Metal distribution, Main Zone (Hinge area) massive sulfide and stringer zone.  This pattern  of metal distribution is typical of the Main Zone with zinc grades increasing towards the bottom of the  massive sulfide with strong and consistent gold mineralization in the underlying stringer sulfides.   Note also the gold values associated with the intrusive quartz – feldspar ‐ porphyry (QFP).  Au g//t Ag g/t 5 10 50 100 Zn % 5 10 15 Cu % Pb % 1 23 1 23 Massive Sulfide 65.7 m Au g/t Ag g/t Zn % 1.6 21 6.9 Cu % 0.3 Stringer Sulfides 31 m Au g/t Ag g/t Zn % Cu % 10.0 65 0.27 0.2   78 �Figure 12.  Metal distribution associated with Tuff Zone massive sulfide mineralization.  Note the  widespread gold and silver values in the tuffaceous sediment sequence hosting this massive sulfide  lens.  Siliceous Tuffaceous Sediments 65.1 m Au g/t Ag g/t Zn % Cu % 1.5 64.4 4.1 0.04 Massive Sulfide 9.5 m Au g/t Ag g/t Zn % Cu % Pb % 1.1 152.5 20.9 0.06 5.3   79 �Precious metal mineralization at Back Forty  Gold and silver occurs in all types of sulfide mineralization (massive sulfides, stringer zones), as well as  gossans, altered rhyolite host rocks, and younger intrusive porphyries which cut the host strigraphy and  sulfide mineralization.  Gold in massive sulfides and associated stringer zones is of variable grade, and although it can attain  very high grades locally (> 20 g/t), the average for all massive sulfides is 2.3 g/t.  A breakdown of metal  grades by individual sulfide zone is shown in figure 13.  Gold in sulfide mineralization is usually fine  grained and is closely associated with chalcopyrite and as native gold and electrum within pyrite and  along pyrite grain boundaries.      3D Model of Massive Sulfide and Gossan 16   Figure 13.  Tonnage and grade for individual sulfide zones (April 2007 NI‐43‐101 Resource)    Gossan mineralization derived from the oxidation of massive sulfide shows strong enrichment in gold  overlying the East Zone and gold and silver overlying the Pinwheel, as illustrated by the resource  numbers in Figure 13.  Fine grained, free gold in hematite is common in the gossan.  Silver occurs in  mercury silver minerals, eugenite (Ag11Hg2) and luanheite (Ag3Hg), and in acanthite (AgS), and locally as  coarse native silver.    80 �Not shown in figure 13 is precious metal mineralization associated with rocks surrounding massive  sulfide mineralization.  Numerous gold and silver intercepts in lithologies peripheral to the massive  sulfides and stringers prompted follow up drilling in 2007 and 2008 to target this style of mineralization.  As a result of this drilling, two zones of gold and silver mineralization – the 90 Gold Zone hosted by  siliceous sediments, and the PM Gold Zone hosted by quartz feldspar porphyry have been identified  (figure 14).  Both gold zones contain fine grained gold disseminated in silicified host rock with small  amounts of galena, aresenopyrite, chalcopyrite and pyrite.  Figure 14.  Plan view of massive sulfide lenses and gold zones        Expansion of all mineralized zones is ongoing at the project with 3 drills active.  A new resource estimate  is currently being prepared and preliminary mine planning, metallurgical studies, and environmental  baseline work is underway.    81 �82 �  FIELD TRIP DESCRIPTION AND STOPS  The field trip will concentrate on local and deposit geology to be illustrated by inspection of outcrops in  the vicinity of the Back Forty mineralization, drill core, and a review of technical data developed from  geophysical studies and drilling.  Stop 1   Field Office:   The Aquila field office is located directly adjacent to the Back Forty massive  sulfide discovery.  Field trip participants will assemble at the field office for an overview of the project  setting.  Stop 2  Porphyry Outcrops: From the field office, proceed across the River Road to outcrops of  quartz‐feldspar porphyry.  These outcrops are  a couple of hundred meters north of the main  mineralization but are typical of this rock type which occurs as dikes, sills and irregular intrusions  throughout the host rhyolite sequence.  Compositionally they are dacitic to rhyodacitic and typically fine  to medium grained with a dark ground mass of chlorite, biotite, amphibole, sericite and 5mm to 1 cm  phenocrysts of feldspar and lesser quartz.  These units are variably altered and locally gold mineralized –  especially along margins where they intrude rhyolite with heavy or massive sulfide.  Where they are  mineralized they are normally silicified with destruction of phenocrysts, and contain minor amounts of  chalcopyrite, sphalerite, galena and aresenopyrite and occasionally visible gold.    Stop 3  Rhyolite 2 Outcrops: Abundant outcrops of altered rhyolite occur in a broad area north  of the known massive sulfide mineralization, and are likely hanging wall (Rhyolite 2) to the Main Zone  (Hinge, South Limb and East Zone) massive sulfides, occurring on the northwest dipping, north limb of  the west south west plunging, asymmetric fold.  These outcrops contain quartz phenocrysts in a fine  matrix of sericite and quartz which is typical of Rhyolites 1 and 2, and are considered to be favorable  host rocks for massive sulfide mineralization.  They contain abundant disseminated pyrite and only  rarely sphalerite or other base metal sulfides and are considered part of the large hydrothermally  altered halo to the massive sulfide system.  The only obvious texture is a steeply dipping planar fabric –  probably an axially planar cleavage related to the fold system.  Stop 4 Pinwheel Gossan: Gossan outcrops here are the only exposure of massive sulfide  (formerly) at the project, and are completely to partially oxidized rocks composed principally of  botryoidal, colliform and brecciated hematite and goethite, clays, and chlorite, with lesser amounts of  the minerals found in the primary massive sulfide that have undergone partial to near complete  replacement by the oxides. Minor to trace amounts of bornite, gold, argentite, diaphorite, acanthite,  ramdohrite (Ag3Pb6Sb11S24), Ni‐skutterudite, eugenite, meneghinite, clausthalite (PbSe), cassiterite, and  other trace phases are present. Gold and electrum are present at grain boundaries of other minerals,  and within colloidal hematite.  These outcrops represent the up dip extension of the Pinwheel massive  sulfide, near the axis of the fold, where the Pinwheel Zone has been breached by erosion.  The Pinwheel  represents a stratigraphically higher sulfide horizon than the Main Zone massive sulfides which are  located about 100 meters below the outcrops of gossan.  This area of the Pinwheel gossan contains  significant magnetite and the resulting ground magnetic response clearly defines this portion of the  gossan.  Other parts of this gossan as well as the entire East Zone gossan however, are totally non  magnetic.    83 �                                    Reflected light (RL) and transmitted light (TL) photomicrographs of gossan samples. Top left: East  zone gossan: LK‐76 10.1‐10.85 ‐ gold in colloidal hematite;   0.8 mm; RL.  Top right: top of 90 zone: LK‐99 32.‐33.5 ‐ tiny gold‐electrum granules with pyrrhotite,  galena (grey) and acicular ramdohrite (grey) in vein in hematite matrix. 1.3 mm; RL. Middle left  and right: Pinwheel gossan: LK‐130P 10.2‐11.08 ‐zoned dolomite filling void between colloidal cpy  rimming UM9. 5.8 mm, TL (left) and RL (right). Bottom left and right: Pinwheel gossan: LK‐130P 14‐ 15.3: ‐mercury‐silver aggregates rimming an unidentified mineral and against zoned carbonate. 1.5  mm TL (left) and RL (right).  From Barrie (2007).    84 �Stop 5  Rhyolite 3 Outcrops:  Outcrops of rhyolite 3 are exposed south of the fold axis on the  south limb of the fold.  Unlike rhyolites 1 and 2, Rhyolite 3 is a non porphyritic rhyolite (usually) and  chemically distinct from the other rhyolites.  This unit is also highly altered – sericite, chlorite, silica, and  pyrite, and in drill core contains very distinctive round or ovoid, dark chloritic alteration spots.  It also  contains appreciable pyrrhotite and has a resulting positive magnetic signature.  The stratigraphic  position of this unit, although shown as the stratigraphically highest rhyolite (figure 5), is actually  uncertain.  It has not been identified on the north limb of the fold, and may represent a younger  intrusive unit into the mineralized rhyolite sequence.   Stop 6  Water Well Location:  This is the site of the original water well which encountered  massive sulfides.  The well was drilled by Kleiman Pump and Well from Iron Mountain Michigan.  The  drillers recognized heavy sulfides in the cuttings and subsequently contacted geologist Richard Lassin  who analyzed them and confirmed high (10%) zinc values.  Lassin and Kleiman also identified the gossan  outcrops and correctly speculated that the water well intercept represented the down dip, unoxidized  Pinwheel massive sulfide.  After contacting and partnering with Minerals Processing Corp. – a privately  held Michigan exploration company – a gravity survey was conducted down the River Rd. between the  gossan outcrops and water well, as well as on State of Michigan owned minerals to the east.  Both  surveys detected strong gravity responses.  A coincident electromagnetic response on the state ground,  prompted the initial diamond drilling program.  This is the site of the discovery hole, in what is now the  East Zone massive sulfide.  Stop  7  Field Office:  Drill core of representative mineralization and host rocks and other technical  information will be on display.  Stop 8  Daggett Core Facility:  More core will be on display with assay and other information at  the core storage facility in Daggett Michigan.    References  Barrie, C.T.,  2007  Petrography and mineral chemistry of the Back 40 VMS deposit, Menominee County,  Michigan: Initial Observations, Technical report prepared for Aquila Resources Inc., 27p    Dematties, T. A.,  2007, An evaluation of the Back Forty volcanogenic massive sulfide (VMS) deposit,  Menominee County, Michigan, U.S.A.:  Technical report prepared for Aquila Resources Inc., 52p    85 �86 �54th Annual Institute on Lake Superior Geology Field Trips 4 and 8 GEOLOGY OF THE EAGLE PROJECT Andrew Ware, Kennecott Minerals Company Jon Cherry, Kennecott Minerals Company Xin Ding, Indiana University 87 �Proterozoic high-MgO basaltic magmatism in the Midcontinent Rift system, northern Michigan: Precise baddeleyite U-Pb age and petrogenesis of the Eagle sulfide-bearing mafic-ultramafic intrusion Introduction The Eagle sulfide-bearing intrusion, first drilled in 2001 by Kennecott Mineral Company, consists of both disseminated and massive sulfides in most parts of the intrusion. The Eagle intrusion is part of the intrusive-extrusive association in the 1100 Ma Midcontinent Rift System (MRS). Major exposures of the volcanic rocks occur along the shores of Lake Superior, but not in the Eagle area. Instead, equivalent mafic dikes are abundant in the Eagle area. The styles of sulfide mineralization in the Eagle intrusion differ significantly from those associated with Duluth and Mellen Complex, the principle exposed plutonic rocks of the rift. In the Duluth Complex sulfide mineralization is restricted to the basal contact zones whereas Eagle sulfide mineralization is distributed throughout the host intrusion. The Cu, Ni, and PGE tenor of the sulfide ores from Eagle are also much higher. These features, together with higher olivine abundance and a lack of layering in the Eagle intrusion suggest that the Eagle intrusion may represent a dynamic magma conduit similar to the feeder dyke of the Voisey’s Bay Ni-Cu sulfide deposit in Labrador. The summary below represents our current understanding of the Eagle system based on the preliminary results of a collaborative study with Kennecott Minerals Company. Geological backgroud The Eagle Ni-Cu sulfide deposit occurs in the Baraga basin in northern Michigan (Fig.1). The Baraga basin was intruded by the Mesoproterozoic Baraga-Marquette dike swarm, which is considered to be related to the early stage basaltic magmatism in the MRS (Wilband and Wasuwanich, 1981, Green et al., 1987). Sulfide mineralization occurs in two intrusions referred to as the Eagle and East Eagle deposits. The western intrusion, which hosts the Eagle deposit, is ~480 m in length and 100 m wide near the 88 �surface. It narrows to ~10 m at the depth of ~340 m. The eastern intrusion is located 650 m to the east of the Eagle deposit. Fig.1 Map of the Lake Superior region showing major exposure of volcanic and plutonic rocks associated with the Midcontinent rift (after Davis and Green, 1997; Nicholson et al., 1997). The exposed volcanic rocks of the MRS are located around Lake Superior in southern Ontario, northern Minnesota, northern Wisconsin and Michigan. Volcanic rocks are also found in deep drill cores as far south as Kansas. Most of the volcanic rocks are thoeliitic in nature, with smaller amounts of intermediate and rhyolitic rocks (e.g. Nicholson et al., 1997). The principal intrusive rocks of the MRS are the Duluth Complex in Minnesota and the Mellen Complex in Wisconsin, both of which contain low-grade Ni-Cu sulfide mineralization. The Duluth Complex and associated subvolcanic intrusions comprise a large (5,000 km2) intrusive complex that represents a significant low-grade, but high tonnage, resource. The smaller Mellen Complex emplaced near the base of the Keweenawan volcanic section along the southeastern flank of Lake Superior, also 89 �contains low-grade mineralization. U-Pb dating of zircons from various intrusions in the Duluth Complex provides an age of 1099 Ma (Paces and Miller, 1993) and correlates with Keweenawan high Al olivine tholeiite basalts of the North Shore volcanic group (Chalokwu et al., 1996). The Mellen Complex was emplaced at 1102 Ma, and has been correlated with the Kallander Creek Volcanics of the Powder Mill group (Zartman et al., 1997). Fig.2 Stratigraphic diagram illustrates country rock around Eagle deposit, including Arhcean gneiss and Proterozoic sediment. The Eagle intrusion intruded Early Proterozoic sedimentary rocks of the Marquette Range Supergroup in the Baraga Basin (Fig.2). The Marquette Range Supergroup is divided into the Chocolay, Menominee, and Baraga Groups. The Baraga Group is thought to be contemporaneous with the Proterozoic Animikie Group in Minnesota (Ojakangas et al., 2001). In the Baraga Basin, the Baraga Group sediments are low-grade metamorphosed marine sediments that contain disseminated pyrite, pyrrhotite, or both. The Baraga basin is bounded to the north and south by Late Archean gneiss and 90 �granitoids, and to the east and southeast by Late Archean, low-grade metamorphosed volcanic and sedimentary rocks. The lowest member of the Baraga Group is the Goodrich Quartzite, which is overlain by a chert carbonate member. The chert carbonate member is overlain by the Michigamme Formation. Kennecott geologists informally divide the Michigamme Formation into three members: the Lower Slate, Upper Greywacke, and Fossum Creek. Sulfide and graphite-rich horizons are present in the Lower Slate and Lower Fossum Creek units. The sulfide assemblages are pyrrhotite-dominant, with lesser amounts of pyrite, chalcopyrite, and pentlandite. Lithology and sulfide mineralization of the Eagle intrusion The Eagle intrusion comprises feldspathic peridotite, gabbronorite, melatroctolite, melagabbro and olivine gabbro (Fig.3 and Fig.4). The basal contact occurs as an elongated feeder, which is composed of melatroctolite, which ranges in dip from steeply southeast to vertical. The melatroctolite is not restricted to the basal contact of the steeply dipping feeder, but can occur higher where it turns into a flat lying sheet. The melatroctolite is also discontinuously underlain by a thin (~25 m thick) olivine gabbro. In the central part of the Eagle intrusion, thick melatroctolite encloses ~60 m of melagabbro. However, there is no thermal contact or chilled margin between melatroctolite and melagabbro. In the upper part of the intrusion, the melagabbro is overlain by feldspathic peridotite. Gabbronorite occurs as enlongated lenses (a few meters thick), along the contact between feldspathic peridotite and melatrocolite. In general, lithological units of the Eagle intrusion show a broad range of orientations. Most strike east-southeast parallel to the trend of the Eagle intrusion and have flat to moderate dips to both north and south. 91 �Fig.3 Long section through the Eagle intrusion showing its stratigraph Fig.4 Section 431470E showing the stratigraphy of the Eagle intrusion 92 �Fig.5 Block diagram illustrating ore body distribution Three distinct types of sulfide mineralization occur at the Eagle deposit (Fig.5). They are described as disseminated, semi-massive and massive sulfide. Finely disseminated sulfide minerals can be found in most portions of the intrusion. The ore reserve is comprised of two semi-massive sulfide zones that are linked by a zone of massive sulfides. The mineralogy is typical of magmatic sulfides, and consists of pyrrhotite, chalcopyrite, pentlandite, and cubanite. The average grade of semi-massive sulfide ores are 2.1% Ni, 2.2% Cu, 0.5 g/t Pt and 0.3 g/t Pd. The average grade of massive sulfide ore is 6.1% Ni, 4.2% Cu, 1.1 g/t Pt and 0.8 g/t Pd. Petrography Modal proportions of minerals in rock samples from the Eagle intrusion have been estimated by point-counting. The results are shown in Fig.6. Olivine occurs as cumulus phases and pyroxene and 93 �plagioclase occur as interstitial phases in olivine-rich samples. But pyroxene and plagioclase occur as cumulus phases in olivine-poor or olivine-free samples. The percentage of granular, cumulus olivine grains increase from melagabbro, to melatroctolite, to feldspathic peridotite. Spinel occurs as inclusions in olivine suggesting that it is also an early cumulus phase. Minor amounts of amphibole and biotite occurs as interstitial phases in all samples. Fig.6 Modal proportions of the main rock types in Eagle intrusion plotted in Olivine-pyroxne-plagioclase phase diagram constructed after Morse (1980). Feldspathic peridotite (Fig.7a) consists of 50-65% cumulus olivine (average grain size < 5 mm), forming as large crystals. Intercumulus pyroxene (20-30%) commonly forms okiocrysts (3-5 mm). Intercumulus plagioclase (15-25%) occurs typically as euhedral to subhedral grains of variable size. Spinel (< 2%) occurs as inclusions in olivine and poikilitic pyroxene and plagioclase. Melatroctolite (Fig.7b) consists of 40-50% cumulus olivine (3~5 mm) occuring as medium to large elliptical grains, or as inclusions in pyroxene and plagioclase. Pyroxene (20-35%) occurs as euhedral to subhedral, intercumulus grains of variable size and plagioclase (20-30%) occurs as tabular, randomly 94 �oriented grains in the intercumulus space. Minor spinel inclusions are present in olivine, pyroxene and plagioclase. Fig.7 Photomicrographs showing typical textures of the main rock types. Photos (a) Feldspathic peridotite (b) melatrocotlite (c) melagabbro (d) gabbronorite (e) olivine gabbro 95 �Melagabbro (Fig.7c) consists of 30-40% cumulus olivine (0.5~3 mm), 30-45% cumulus pyroxene and 25-40% cumulus plagioclase. Olivine occurs as small as inclusions in pyroxene and plagioclase or relatively large crystals intergrown with pyroxene and plagioclase. Olivine-free gabbronorite (Fig.7d) is composed of euhedral pyroxene and plagioclase, showing preferred orientation. Pyroxene is present as large crystals and plagioclase normally occurs as euhedral to subhedral tabular crystals. Olivine gabbro (Fig.7e) consists of 20-40% cumulus olivine (< 2 mm), 40-50% cumulus pyroxene (< 1.5 mm) and 20-40% cumulus plagioclase (< 1.5 mm). Unlike other units, the olivine gabbro unit has very low sulfide concentration but high percentages of ilmenite and hematite. Baddeleyite U-Pb dating Fig.8 U-Pb isotopic data of baddeleyite from Eagle intrusion Results of U-Pb isotopic analysis for four abraded baddeleyite crystals and one unbraded zircon crystal from feldspathic peridotite are listed in Table 1 and illustrated in Fig. 8. The four baddeleyite fractions are concordant, but the zircon grain is discordant. The zircon grain defines a 207Pb/206Pb age of 96 �2623.3 Ma, which is consistent with the age of Archean basement of the Baraga basin. The weighted average of the 207Pb/206Pb ages for the four baddeleyite fractions is 1107.3±3.7 Ma. All baddeleyite fractions together yield a concordant age of 1107.2±5.7 Ma. The Eagle intrusion is now recognized as the second oldest intrusion in the southern part of the MRS and correlates with the eruption of the Siemens Creek volcanic suit and Mamainse Point volcanic suit (Fig.9). Fig.9 Chronostratigraphic correlation diagram for volcanic and plutonic rocks in western and eastern Lake Superior (after Davis and Green, 1997; Nicholson et al., 1997) Stratigraphic variations of olivine composition, whole rock Zr/Y and La/Yb ratios and δ34S The variation in the compositions of olivine in different rock types of the Eagle intrusion has been examined. The Fo contents of olivine in the sulfide-poor samples from the Eagle intrusion vary between 85 to 76 mod%. The contents of Ni in olivine are from 1,300 to 1,400 ppm. Compared to olivine from the olivine gabbro unit, olivines from other rock units are significantly depleted in Ni and exhibit a positive Fo-Ni correlation that is characteristic of fractional crystallization. In drill core 03EA034, the Fo contents of olivine decrease progressively with height in the melagabbro unit (Fig.10), stay relatively constant in the overlying melatroctolite unit, and reverse in the feldspathic peridotite unit. 97 �Fig.10 Stratigraphic variations of olivine composition, Mg# of clinopyroxene, plagioclase An number, incompatible element ratios, and S isotope in drill core 03EA034 Fig.11 Stratigraphic variations of olivine composition, incompatible element ratios, and S isotope in drill core YD0106 98 �The melagabbro unit is characterized by relatively low Zr/Y and La/Yb and elevated δ34S values. The melatroctolite unit has similar δ34S but distinctly higher Zr/Y and La/Yb ratios than the melagabbro. The Zr/Y and La/Yb ratios of the peridotite unit are similar to the underlying melatroctolite unit but the peridotite has distinctly lower δ34S values. In the stratigraphic diagram (Fig.10 and Fig.11), δ34S values are consistent from melagabbro through melatroctolite before a successive decrease towards the top of feldspathic peridotite. Controls on whole rock compositions In the plots of MgO versus FeO and MgO versus Al2O3 (Fig.12), the compositions of rocks from the Eagle intrusion are controlled by abundances of olivine and trapped liquid. Figure 13 illustrate chondrite-normalized trace element patterns for the Eagle intrusion and country rock. The slopes of trace element for Archean gneiss and Proterozoic sedimentary rock are much steeper than those of the Eagle intrusion. Feldspathic peridotite, melatroctolite, melagabbro and gabbronorite units generally have similar trace element slopes. The olivine gabbro unit has higher trace element abundances than the other rock types in the Eagle intrusion. All rock samples from the Eagle intrusion exhibit a negative Nb anomaly which is characteristic of crustal contamination. Fig.12 Variations of major elements, olivine, clinopyroxene, and plagioclase in Eagle intrusion 99 �Fig.13 Trace element abundance patterns for samples from (a) Eagle intrusion and (b) country rock, normalized to chondrite (values from McDonough and Sun, 1995) Fig.14 Frequency diagram illustrating Sulfur isotopic values of (a) feldspathic peridotite, melatrocotlite and melagabbrounits and (b) semi-massive and massive sulfides δ34S values of sulfide minerals from the Eagle intrusion vary between 1.0‰ and 4.3‰. The feldspathic peridotite and olivine gabbro samples have δ34S <3‰. Elevated δ34S values ranging from 3.6‰ to 4.3‰ are present in the melagabbro and melatroctolite units. Semi-massive and massive sulfide samples also have elevated δ34S values. Discussion Multiple pulses of magma and genetic relations The negative Fo-Ni correlation of olivine in melagabbro, melatroctolite, and feldspathic 100 �Fig.15 Plots of δ34S versus incompatible element ratios and olivine composition peridotite units is consistent with fractional crystallization. However, these different rock units have different La/Yb ratios and/or δ34S values (Fig.15). These variations are likely related to different degrees or different types of crustal contamination. The lack of systematic variations in La/Yb ratios and δ34S values with rock type (Fig.10) suggests that in situ contamination is not the main reason for the variations. It is more likely that those variations resulted from contamination at depth. The olivine gabbro unit has distinctly higher Ni content, which requires a Ni undepleted magma, unlike the depleted magma that formed other units. All these data suggest that at least three parental magmas were involved in the development of the Eagle intrusion: a Ni-undepleted magma, a Ni-depleted magma with δ34S <3‰, and a Ni-depleted magma with δ34S >3‰. They are likely related to each other by a differentiation process in a staging chamber such as: olivine crystallization, sulfide segregation or lack of sulfide segregation, or variation in crustal contamination history. Based on S field relations, it appears that Ni undepleted magma 101 �for the olivine gabbro unit intruded first, followed by the Ni-depleted magma with δ34S <3‰ to form the feldsapthic peridotite unit, and finally by Ni-depleted magma with δ34S >3‰ to form the melatroctolite and melagabbro units. Variations of La/Yb in each of these magmas suggest that additional, variable contamination took place during magma ascent and emplacement. Sulfide saturation and concentration The association of Ni depletion in olivine with elevated δ34S values in coexisting sulfides is consistent with the interpretation that sulfide saturation was caused by the addition of crustal S. However, some samples with Ni depleted olivine do not have elevated δ34S values. This may be due to variable δ34S value in the contaminant, or contamination with S-poor country rocks. The abundance of sulfide in the Eagle intrusion far exceeds the cotectic ratio during olivine crystallization. Some mechanism of sulfide concentration was required during magma emplacement. We envision that immiscible sulfide liquid droplets were carried along with olivine crystals by magma from a staging chamber. They settled out at the entrance of the subvertical feeder to the Eagle chamber due to a sudden decrease in velocity. In this model, the Eagle intrusion was a wider part of a dynamic conduit system that fed magma to overlying dykes or sills. Parental magma characteristics The FeO/MgO ratio of a parental magma can be estimated by using KD=(FeO/MgO)olivine/(FeO/MgO)liquid=0.3 (Roeder and Emslie, 1970). The calculated FeO/MgO for most primitive olivine from the Eagle intrusion is 1.04, which is similar to the values of picritic basalts in the LSCV suite and Group 1 of Mamainse Point. The Al2O3 contents of the liquids in equilibrium with spinels in the intrusion estimated using the relation of (Al2O3)spinel = 0.035(Al2O3)2.42 (Al2O3 in wt.%) by Maurel and Maurel (1982) are from 8.41 to 102 �Fig.16 Modeling curves of olivine fractionation with variable initial Ni contents Fig.17 Trace element abundance patterns for average Eagle intrusions, and volcanic rocks (data from Shirey et al., 1994). Model trace element compositions calculated from group 1 of Mamainse Point. 10.87 wt%. These values are also similar to those of picritic basalts in the LSCV suite and Group 1 of Mamainse Point. These similarities permit us to use the average compositions of picritic basalts to simulate fractional crystallization for the Eagle intrusion using the MELTS program by Ghiorso and Sack (1995). The results for olivine and trace element is shown in Fig.16 and Fig.17, modeled trends match the observed values well. 103 �Summary Mineral chemistry, whole-rock composition, and S isotopes indicate that the Eagle intrusion formed by multiple pulses of magma. The different magma pulses are different in the degrees of fractionation and type of contamination. Age correlation and phase relations suggest that the parental magma of the Eagle intrusion is similar to pictitic basalts found in the Lower Siemens Creek volcanic suite and the group 1 basalts at Mamainse Point, which both erupted during the early development of the Midcontinent rift system. The results of numerical modeling using the MELTS program indicate that the average picritic basalt can produce mineral assemblages and mineral compositions similar to those observed in the Eagle intrusion. Our current understanding of sulfide mineralization in the Eagle intrusion is that a mantlederived, high-MgO basaltic magma rose to a staging magma chamber, crystallized olivine and segregated immiscible sulfide droplets due to contamination with sulfide-bearing country rocks. The olivine- and sulfide- charged magma was then pushed up to a higher level at Eagle by new surges of magma into the staging chamber. Olivine and immiscible sulfide droplets became concentrated in the wider part of the Eagle conduit system where the silicate liquid continued to ascend. This process may have been repeated at least twice. References Chalokwu, C.I., Ariskin A.A. and Koptev-Dvornikov E.V. (1996) Magma dynamics at the base of an evolving mafic magma chamber: Incompatible element evidence from the Partridge River intrusion, Duluth Complex, Minnesota, USA. Geochim Cosmoch. Acta 60, 4997-5011. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Myers, P.E., Pesonen, L.J., and Wilband, J.T. (1987) Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterzoic Midcontinent rift of North American. In Halls, H.C. and Fahrig, W.F., eds., Mafic dyke swarms: Geological Association of Canada Speical paper 34, 289-302. Maurel, C. and Maurel, P., (1982) Etude expérimentale de la solubilité du chrome dans les bains silicatés basiques et sa distribution entre liquide et minéraux coexistants: conditions d’existence du spinelle chromifére, Bulletin Minéralogie 105, 197–202. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C. (1997) Rift-wide correlation of 1.1 Ga MRS basalts: implications for multiple mantle sources during rift development. Can. J. Earth Sci 34, 504520. Ojakangas, R.W., Morey, G.B., and Green, J.C. (2001) The Mesoproterzoic mid-continent rift system, Lake Superior region, USA. Sedimentary Geology 141-142, 421-442. 104 �Paces, J.B., and Miller, J.D. (1993) Precise U-Pb ages of Duluth Complex and related mafic intrusions, Northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga mid-continent rift system. Journal of Geophysical Research 98, 997-14013. Wilband, J.T. and Wasuwanich, P. 1981. Models of basalt petrogenesis: Lower Keweenawan diabase dikes and middle Keweenawan Portage Lake Lavas, upper Michigan. Contrib. Mineral. Petrol. 75, 395-406. Zartman, R.E., Nicholson, S.W., Cannon, W.F. and Morey, G.B. (1997) U-Th-Pb zircon ages of some Keweenawan supergroup rocks from the south shore of Lake Superior. Can. J. Earth Sci 34, 549-561. 105 �Eagle Project Area Quaternary Geology and Hydrostratigraphy* With the exception of the Peridotite outcrops in the Project area, the bedrock surface across the Plains is mantled by unconsolidated glacial deposits from the Quaternary period continental glaciation of the region. This surface forms the base of the Quaternary deposits. Hydrologically, this surface is considered to create a boundary to the movement of groundwater within the unconsolidated materials. The observed thickness of Quaternary deposits ranges from 0 ft (at the Peridotite outcrops) to greater than 200 ft. The deposit thickens in all directions away from the Peridotite outcrops, with the greatest thickness observed east and west of the Project area. The Quaternary deposits that define the Plains then thin toward the north and south, terminating at a boundary that is approximately coincident to the boundaries of the Baraga Basin metasedimentary rocks adjacent to the Archean bedrock formations that outcrop north and south of the Plains. Surficial geology is illustrated in Figure 1. A general hydrostratigraphic correlation nomenclature system was developed for the EBS and is summarized below. Surface Soil Layer A surface soil layer (black color with organic material and tree litter) was identified at most drilling locations. This layer is generally less than 1 ft thick (and mapped regionally as 0-2 in. thick on the Plains) and is classified as a sandy organic soil). Thin surficial layers of peat have also been identified in the area directly overlying the Eagle deposit ore body. Outwash and Beach Deposits (A Zone) The outwash and beach deposits are comprised of well-sorted, stratified fine- to medium-grained sand, with some gravel and minor quantities of silt and clay (less than 10%). The sand fraction of this material appears to be predominantly rounded quartz with trace to minor amounts of angular and sometimes platy mafic or fine-grained sedimentary rock grains. The unsaturated portion of this deposit is typically red to reddish brown and the saturated portion is brown. These surficial deposits are mapped regionally as having very rapid water infiltration rate characteristics (greater than 10 in./hr) (Twenter 1981). An unconfined water table defined as the A zone hydrostratigraphic unit occurs in the saturated portion of this deposit. The unsaturated zone is very thin in the southern portion of the Plains, where a large wetland complex exists. The unsaturated portion of the A zone then thickens significantly towards the northern edge of the Plains (up to 100 feet thick at wellQAL009 A/D northeast of the Peridotite outcrop). Generally a fining downward sequence is found in the A zone, with the fine sand fraction increasing with depth. A Zone groundwater elevation contours are shown in Figure 2. 106 �Transitional Deposit (B Zone) A gradational contact exists between the A zone outwash sand and a deeper transitional zone that contains a mix of fine sand, silt and clay, and typically continues to fine downward to predominantly silt and clay. While the A zone outwash and this transitional deposit may both be derived from melt water processes and could be lumped as outwash, the grain size characteristic change from predominantly sand to predominantly silt and clay. This transition is considered significant to primary conditions affecting groundwater flow as it indicates a decrease in permeability of the Quaternary formation from the coarse grained material to the fine-grained material. Directly above the Eagle deposit, the A zone coarse-grained materials are very thin (generally less than 5 ft in thickness) and the B zone fine-grained deposits form the bulk of the Quaternary deposits. As a result this area contains much more poorly drained surface soil and wetlands. Lacustrine Deposit (C Zone) A laterally extensive, massive clay deposit was identified in samples from most borings, and is found to be thickest south of the Peridotite outcrops, and thinnest north of the outcrops towards the north terrace. The clay deposit is easily recognized in soil sample cores as lean clay with medium to high plasticity. In some core samples it appears to be a massive deposit, while in other locations it contains thinly laminated and stratified layers of silt and clay. A sharp contact is typically observed at both the top and bottom of this deposit. On average the deposit contains 98% silt and clay. This deposit is defined as the C zone hydrostratigraphic unit. The clay deposit identified in soil borings ranged in thickness from 7-63 ft, thickest and most consistent in its elevation in the south/southeast part of the Plains (from locations QAL005A/D to QAL010A) and thinnest and less continuous towards the north and northeast, where this unit eventually pinches out near the edge of the north terrace. The pinch-out of the transitional and lacustrine deposits of the B zone near the north terrace is consistent with the glacial depositional model, as the transitional unit would be expected to pinch out at the edge of the moraine. This areal distribution pattern indicates that the fine-grained deposits were formed in ponded water between the bedrock highlands south of the Plains and glacial ice to the north, also consistent with the depositional model proposed by Segerstrom (1964). Outwash/Ablation Till (D Zone) A deposit of coarser-grained material was encountered beneath the C zone lacustrine deposit at most drilling locations. Samples from this deposit are predominantly fine- to medium-grained sand and are similar to samples of A zone material. This material appears to be outwash deposited prior to the glaciallake period on the Plains. This deposit is defined as the D zone hydrostratigraphic unit. Greater heterogeneity in grain size characteristics was observed within the D zone compared to the A zone. At 2 locations (QAL004A/D and QAL005A/D) south and southwest of the Peridotite outcrop, the D zone contains a layer with significant amounts of gneiss and granitoid cobble and 107 �gravel-sized outwash material indicative of high flow velocity glacial drainage channel deposits. At other locations (QAL001A/D, QAL002A/D and the base of QAL004A/D), the D zone contains a relatively high percentage of fine sand and silt, and generally becomes increasingly finer-grained toward its base. The finer grained portion is possibly derived from direct ice melt or sublimation (ablation till), since the base of this zone is most often identified in contact with a basal till deposit, described below. This outwash deposit is also discontinuous, interrupted by shallow bedrock and pinched out between the fine-grained units above and below. This deposit was not encountered beneath the C zone at well nests QAL006A/B and QAL010A. This deposit appears to be confined or partially confined, except at location QAL009A/D where the overlying C zone clay is absent. As a result of the pinch-out of the B and C zones in close proximity to the northern edge of the Plains, the A and D zone aquifers at this location become a single unconfined system. D Zone groundwater elevation contours are shown in figure 3. Basal Till (E Zone) Poorly-sorted basal till consisting of boulder- to sandy-sized clasts in a fine grained matrix is the lower most Quaternary deposit material identified in samples from all but one boring (QAL004A/D). This unit is substantially thicker east (QAL009A/D), west (QAL007A/D) and southeast (QAL010A) of the Project. Bedrock is encountered at greater depths at these locations, indicating that earlier glacial moraine deposition occurred in the bedrock valleys. Boulders are commonly present along the north terrace Lower Outwash Units (F Zone) At 2 locations (QAL007A/D and QAL010A), lower outwash deposits were found interlayered with E zone till. Representative samples of the lower outwash material are predominantly fine- to medium-grained sand. In QAL010A these units were found to be dry. The interlayered nature of the till and lower outwash units indicates fluctuations in glacial advances and retreats during earlier glacial depositional sequences. This lower outwash deposit is defined as the F zone hydrostratigraphic unit. *Text and figures extracted form Internal Company Report by Wiitala, D. et al. North Jackson Company. “Kennecott Minerals Company Eagle Project” Comprehensive Summary of Hydrologic Reports”. Feb, 2006 References Segerstrom, K. 1964. Negaunee Moraine and the Capture of the Yellow Dog River, Marquette County, Michigan. U.S. Geological Survey Professional Paper 501-C, pages C126-C129. Twenter, F. R. 1981. Geology and Hydrology for Environmental Planning in Marquette County, Michigan. U.S. Geological Survey Water Resources Investigations Report 80-90, Prepared in cooperation with the Michigan Department of Natural Resources, 44 pages. 108 �Figure 1. Regional Quaternary Geology. 109 �Figure 2. A Zone Groundwater Elevations (Summer Base Flow, August 2005) 110 �Figure 3. D Zone Groundwater elevation Contours (Summer base flow August 2005) 111 �Eagle Project - Field Trip Stops There are three field trip stops on the Eagle tour. Exposure in the Yellow Dog Plains is limited to two outcrops of ultramafic intrusives. Stop 4-1) Yellow Dog Peridotite – Eastern Outcrop. (UTM coordinates 432 440E 5 177 380N _ North side of County Road AAA) Travel north-west from Marquette on County Road 550. Turn on to County road 510 and then on to County Road AAA. Total distance from the 550/510 intersection is approximately 12 miles. The AAA turn will be flagged. Use precaution on the 510/AAA roads as logging trucks use these narrow roads for access. The main outcrop forms the western end of an inverted fin shaped intrusion that plunges to the east. Drilling has intersected feldspathic peridotite to a depth 720m below surface on the eastern end. In outcrop, both the eastern and western peridotites have distinctive, reddish brown, pitted weathered surfaces with rare bright red patches indicating oxidized pyrrhotite blebs. Weathering rinds are typically less than a centimeter thick, and relatively fresh looking sulfides can be seen within a few millimeters of the surface. Two primary lithologies, peridotite and pyroxenite, are recognized within both intrusions. Serpentinization of olivines, uralization of pyroxenes and chloritization of amphiboles are noted in thin section work. Stop 4- 2) Yellow Dog Peridotite – East end of Host Intrusive Complex for the Eagle Deposit. (UTM 431 720E 5 177 580N – South side of the County Road AAA) The eastern exposure is of an olivine gabbro. This particular phase represents one of the more primitive melts as defined by sulphur isotope data. Rare disseminated sulphide mineralisation can be observed in the outcrop. Stop 4-3) Kennecott Eagle Mineral Co Core storage Facilities 200 Echelon Drive, Negaunee. Turn Right of Highway 41 at the TV6 Studios. The Michigan State Police post is located opposite the turn off on the south side of the highway. Drive North for 200 yards and turn west through a set of large gates. The core storage buildings are located on the left hand side of the road. The turn off form the highway is located approximately 3 miles east of Negaunee. (Do not use MapQuest Directions). A review of core from the Eagle Project and The BIC project will be available for review. 112 �Eagle – Baraga Basin Exploration History The Baraga Basin region has until recently been subject to only sporadic exploration efforts. The earliest historical accounts of exploration in the basin date back to the mid- 1800’s when a group of investors tried to develop slate quarries along Slate River. Little documented exploration work took place in the Baraga basin between 1910 and 1950. During the 1950’s Jones and Laughlin conducted an exploration program along the northern portion of the East branch of the Huron River, investigating uranium-silver mercury mineralization associated with a graphitic shear exposed in the river. During the 1960’s and 1970’s, various interests conducted exploration programs on Ford mineral lands in the Baraga Basin and the western portion of the Marquette Trough. The programs were primarily focused on uranium and zinc. The U.S. Department of Energy provided funding to drill a number of deep holes in the Baraga Basin during the 1970’s presumably to provide stratigraphic information for the uranium exploration effort. Concurrently, the USGS began a bedrock-mapping program of the basin, focusing primarily on exposures in rivers, which produced an open file outcrop map with no report (Cannon, 1977). In 1976, Michigan Technological University drilled a 31-meter hole on the east end of the Yellow Dog (East Eagle) outcrop. The hole bottomed in coarse-grained peridotite with only traces of sulfide. In 1979, the Michigan DNR, in conjunction with the USGS, published a report on the Yellow Dog peridotite describing the results of geochemical, petrographic and geophysical studies of the peridotite (Klasner and others, 1979). The authors concluded that the anomalous sulfur and copper contents of the outcropping peridotite indicate a significant potential for copper-nickel ore deposits. Kennecott Exploration started working in the region in 1991 and actively explored for sedex zinc deposits through 1994. During the course of mapping, float boulders of peridotite with sulphides were discovered that indicated the potential for magmatic Ni-Cu sulphide mineralization. Kennecott partially shifted to magmatic nickel exploration in 1995 and drilled four holes to test the Yellow Dog peridotite (East Eagle). One hole (YD95-2) intersected 10 meters of moderate to heavy disseminated sulfide mineralization along the southern contact. Two more angle holes (YD95-3 and YD95-4) collared on the east end of the Yellow Dog East outcrop demonstrated the peridotite widened to the east but only intersected a meter or two of weak sulfide mineralization along the north and south contacts. The Michigan program was put on hold in the summer of 1996 and the Crystal Falls Office was closed as Rio Tinto reorganized the newly merged CRA, Kennecott and Rio Tinto exploration groups. The land position around Eagle was reduced to a core group of private and state leases in 1997 and 1998. Interest in the project was regenerated in 2000 through the persistent efforts of Kennecott geologist Dean Rossell who recognized the potential for the region to host significant nickel mineralization in light of recent published papers on Norils’k and Voisey’s Bay. The current nickel exploration program was started late in 2000. Drilling at East Eagle in July 2001 intersected 30 meters of disseminated, net textured and massive sulfides averaging 1.03% Ni and 0.75% Cu (YD01-01) and one of three holes on the east end of Eagle intersected 85 meters of disseminated sulphides averaging 0.6% Ni and 0.5%Cu (YD01-06). 113 �2002 drilling at Eagle targeted the center of a magnetic anomaly defined by ground surveys in 2001. The first hole, YD02-02, intersected 84.2 meters of massive pyrrhotite-pentlanditechalcopyrite averaging 6.3% Ni and 4.0% Cu, firmly establishing the presence of economic grade and width mineralization at Eagle. Subsequent definition drilling continued through the summer and fall of 2002 and has continued through to the present. References: Cannon, W.F., 1977, Bedrock geology in parts of the Baraga, Dead River, and Clark Creek Basins, Marquette and Baraga Counties, Michigan: U.S. Geological Survey Open-File report 77-467, scale 1:62,500. Klasner, J.S., Snider, D.W., Cannon, W.F., and Slack, J.F, 1979. The Yellow Dog Peridotite and a possible buried igneous complex of lower Keweenawan age in the northern peninsula of Michigan. State of Michigan, Dept. of Natural Resources, Geological Survey Division; Report of Investigation 24, 31 pp. 114 �115 �Introduction Note: As this guidebook is being prepared a substantial road construction project is underway to realign County Road 510 through the area of interest. Specifically, the road is being moved several hundred feet to the west of the location shown in Figures 2 and 3 and will connect to a new bridge being constructed over the Dead River. The descriptions and outcrop locations shown in this guide are those that existed through late 2007. When the field trip is conducted in May of 2008 the outcrop and access situation may be somewhat altered. The outcrops to the west of 510 are nearly all on land owned by Marquette County and are publicly accessible. Please observe private property boundaries to the far west and south of this area. The outcrops east of 510 are on private property to which the owners have granted access for scientific examination and reasonable sampling for research purposes. A set of outcrops near County Road 510, about 5 miles northwest of Marquette, Michigan (Figure 5.1) provides a complete section through the layer of debris deposited as a result of the giant impact at Sudbury, Ontario, which occurred about 500 km to the east at 1850 Ma . The Sudbury layer here is a breccia and sandstone unit about 40 m thick, which lies on banded ironformation and is overlain by pyritic black slate. Outcrops include: 1) the basal contact of the layer that consists of large rip-up clasts of the underlying iron-formation; 2) exposures of matrixsupported breccias in which most large fragments are chert, but many smaller fragments are impact glasses; 3) an upper massive sandstone with minor chert clasts and glass particles; and 4) the upper contact with black slate. The McClure site is the best-exposed section of the Sudbury layer currently known in Michigan and also is the thickest. In addition it is the closest exposure to the impact site at Sudbury. Because there are no preserved rocks of 1850 Ma age between here and Sudbury, the McClure site contains the most proximal ejecta that is likely to be found. General geology The McClure site is in the Dead River Basin, a structural outlier of Paleoproterozoic strata surrounded by Neoarchean crystalline rocks. The strata consist entirely of various informal units of the Michigamme Formation. The Sudbury layer at McClure was mapped as a chert conglomerate by W.P. Puffett (1974) who provided a detailed outcrop map of the immediate site as well as a 1:24,000 scale map of the Negaunee Quadrangle on which the layer was shown as a map unit. The unit was extended further west into the adjacent Negaunee SW quadrangle by Clark and others (1975). 116 �Figure 5.1. Map showing location of the McClure site. The Sudbury layer at McClure lies within a north-facing monoclinal succession of sedimentary rocks, all informal members of the Michigamme Formation, a part of the Baraga Group, which lies unconformably on Neoarchean granitic rock (Figure 5.2). The Michigamme Formation consists of a basal unit of quartzite and conglomerate, probably equivalent to the Goodrich Quartzite of the Marquette Range. The unit is about 60 meters thick and grades upward into a 150-200 meter-thick unit of impure quartzite and argillite. A 60 meter-thick unit of banded chert-hematite-goethite iron-formation overlies the impure quartzite and is the unit on which the Sudbury impact layer was deposited. Overlying the Sudbury layer with an apparent gradational contact is pyritic black slate. Thus the Sudbury layer at the McClure site lies about 250-300 meters above the base of the Baraga Group. This field trip will examine a set of outcrops that exposes a cross section of the impact layer as well as the upper and lower contacts with the adjacent stratigraphic units (Figure 5.3). Description of the Sudbury impact layer The rock layer here referred to as the Sudbury impact layer has been studied and described for nearly a century, but only in 2006 was it documented to be an impact-related unit. The most complete previous description was by Puffett (1974) who mapped and described the unit as a chert conglomerate containing many fragments of volcanic rocks. He interpreted it to have originated “during a period of volcanism in which thick tuff deposits accumulated, then was disturbed by landslides or other gravity-activated mechanisms that dumped material into the site of deposition.” Puffet clearly recognized the essentially instantaneous deposition of this massive, 117 �graded unit and the unusual mixture of volcanic fragments, chert clasts, and quartz sand grains, and called upon a reasonable combination of terrestrial geologic processes to have formed it. As an historic note, my former colleague, Willard Puffet, showed these outcrops to me in September 1967 on one of my first days of employment with the USGS in the Marquette Field Office. He asked if I could help explain these unusual features. Fortunately, he didn’t give me a deadline. Our current interpretation of the nature and origin of the Sudbury layer at the McClure site is based on examination of the outcrops and standard thin section petrography of a suite of samples collected at a regular interval across the unit. The definitive microscopic evidence for a link between the breccia bed at McClure and a major impact is the documentation of shock metamorphic features within it. A small percentage of the quartz grains within the breccia matrix contain relict planar deformation features (pdf’s) indicative of the extreme pressures generated instantaneously during a hypervelocity impact (Figure 5.4 A,B). There are no terrestrial processes capable of generating pressures remotely within the range needed to form these distinctive features. Figure 4 illustrates two examples of quartz grains with two sets of relict pdf’s. These planar features were originally lamellae of impact-generated glass resulting from breakdown of the quartz lattice along preferred crystallographic planes by extreme shock pressures. Over time the glass has recrystallized to quartz, but has left behind planes rich in inclusions, relict pdf’s, that mark the original shock lamellae. At the McClure locality the identification of true pdf’s is complicated by the occurrence of extraordinarily abundant Bohm lamellae, features produced by terrestrial deformation (sometimes referred to as metamorphic deformation lamellae). Apparently the temperature and pressure of deformation was optimum for development of these lamellae. Like pdf’s they occur as parallel lamellae within quartz grains (Figure 5.4 C,D) and can be difficult to distinguish with certainty from pdf’s. The most characteristic Bohm lamellae are thin planar features in which the quartz lattice has been slightly distorted so that the lamellae have extinction angles that vary by a few degrees from the host grain (seen best in Figure 5.4 C). Bohm lamellae are commonly somewhat curviplanar in contrast to unvaryingly planar pdf’s, and also commonly develop at approximately right angles to boundaries between individual crystallographic domains within strained quartz grains (also seen well in Figure 5.4 C). Bohm lamellae are also common in quartz grains in the underlying quartzite and greywacke so seem clearly to have developed in situ during deformation of the host rocks and are not related to the Sudbury impact. 118 �Figure 5.2. Geologic map of the area near the McClure site. Modified from Puffett (1974). 119 �Figure 5.3. Detailed map of the McClure site. Modified from Puffett (1974). Note that the location of County Road 510 is that prior to relocation in 2007-08. The new road is not shown. A cross section of the unit and modal compositions are shown in Figure 5.5. The most distinctive feature of the layer is the coarse chert breccia that makes up approximately the lower half of the unit. The breccia grades upward both in size and abundance of clasts, mostly chert. The basal unit is a framework of chert slabs up to a meter long surrounded by a matrix largely of clastic material and subordinate altered glass particles (Fig 5.6A). This grades up into matrix supported breccia (Figure 5.6 B,C,D,E) in which accretionary lapilli occur sparsely (Figure 5.6C). Clasts generally show little or no preferred orientation, but locally (Figure 5.6D) are well aligned. Most large clasts are chert, at least partly derived from the underlying iron-formation, but some phases have abundant exotic fragments, apparently volcanic rocks (Figure 5.6D). In some outcrops, many chert clasts have an alteration rim (Figure 5.6E) suggesting reaction between the clast and matrix. As shown by Figure 5, the breccia matrix has relatively constant composition expressed as the percentage of clastic quartz sand grains, altered glass particles, and fine groundmass. Glass particles account for 35-40% of the matrix. The glass particles are now mostly chlorite (Figure 5.7 A, B, C, E) in which relict vesicles are common. Many vesicles are flattened indicating considerable post-depositional distortion. Some particles have a complex intermixing of compositions (Figure 5.7A), possibly a result of immiscible melts. Other rock types, such as the quartzite clast in Figure 5.7F, are rare. The groundmass in the breccia matrix is aphanitic, 120 �apparently of felsic composition, and clouded with uniformly distributed opaque grains. Its nature is not clear at this point in our studies. Figure 5.4. A and B- quartz grains with two intersection sets of relict planar deformation features expressed by abundant fine inclusions. These are definitive indicators of intense shock pressures. C and D- Quartz grains containing Bohm lamellae showing slight variations in extinction angles from host grains, curvilinear nature, and right angle intersections with boundaries of deformation zones in strained grains (best seen in C). The upper part of the layer, beginning about 25 meters above the base, is a massive dark gray to black impure sandstone. Angular chert pebbles are sparse and much less abundant than in the underlying breccia. Glass particles are common but less abundant than in the breccia and average about 20% of the rock (Figure 5.7D). Rounded to subangular quartz grains are more abundant than in the underlying breccia and make up roughly 35% of the sandstone. The groundmass also differs from that of the breccia and appears to be fine clastic particles with a wide range of grain size in contrast to the very uniform groundmass of the breccia. The contact between the lower breccia and upper sandstone appears to be gradational over several meters. 121 �Figure 5. 5. Cross section of the Sudbury impact layer at McClure showing variations in the modal composition of the matrix of the lower breccia and the upper sandstone. The upper contact of the Sudbury layer with overlying black slate can be seen in very small exposures in the bed of the intermittent stream that is subparallel to Co. Rd. 510. When the stream is flowing, these outcrops are largely below the shallow water. The contact appears to be gradational over a meter or two in which fine-grained sandstone gives way to laminated carbonaceous slate. Interpretation Several features of the Sudbury impact layer at the McClure site provide clues to the processes responsible for its deposition: 1) Shock metamorphic features provide verification that it contains ejecta from a major extraterrestrial impact. Independent age constraints place the time of deposition within a roughly 40 million year time window that includes the 1850 Ma Sudbury event. No other major impact events of that age are known in the region, so a link to the Sudbury impact is deemed very likely. 2) The massive, graded and poorly sorted nature of the deposit and complete lack of internal bedding or laminations suggest the entire 40 m thickness records a single depositional event. 3) The high energy deposition indicated for the Sudbury layer is in sharp contrast to the very low energy environments indicated for the underlying even-bedded iron-formation and overlying laminated black slate. So deposition appears to be a unique instantaneous event. Both the underlying and overlying units were deposited in a marine setting with water depths greater than the depth of wave action. 4) The abundance of rounded quartz and chert sand grains throughout the unit indicates that, in addition to material ejecta from the crater at Sudbury, the unit contains a substantial component 122 �of material that was acquired by erosion of surficial materials that existed between Sudbury and the McClure site. 5) The abundance of altered particles of glass, a very high percentage of which are highly vesicular and of mafic composition, and have complex delicate shapes suggests that the particles were not derived by erosion of older volcanic rocks, which would have produced a variety of textures and compositions, but rather formed from solidification of highly gas-charged impactgenerated melts and acquired their present shapes in situ. 6) The very coarse breccia at the base of the unit, consisting of meter-scale slabs of the underlying iron-formation, indicates that the onset of deposition was a very high-energy event. Although studies of the Sudbury layer here are still in the early stages, a preliminary interpretation is presented based on current observations. Deposition began in relatively deep quiet water on a substrate of banded iron-formation. The basal beds are a result of highly energetic disruption of the iron-formation and may have been produced either by erosion caused by a fast-moving mass of ejecta or by seismic disruption of the surface sediments moments before the arrival of ejecta. The seismic shock wave generated by the impact would have arrived here within a minute or two after the impact, whereas the first ejecta may have arrived a few minutes later. Spaces between iron-formation slabs are filled with a mixture of clastic grains, particles of altered glass, and sparse accretionary lapilli indicating that the ejecta arrived while there was open space between the slabs. The remainder of the unit at McClure may record deposition from a single turbidity flow. Numerous numeric models of giant impacts have been published in recent years and all predict a rapid expansion of an ejecta cloud or ejecta curtain consisting of solid rock, impact melt, and vapor. Horizontal velocities of thousands of kilometers per hour are predicted. As this high velocity mass returns to the Earth’s surface, it continues to move at high velocities as a ground surge. This surging mass is capable of eroding and transporting surficial material and eventually incorporating it into hybrid deposits consisting both of ejecta and the eroded surficial materials. The mixture of ejecta material at McClure with quartz and chert sand and larger rock fragments, largely chert derived from the nearby iron-formation, suggests that a ground surge played a significant role in its formation. A less certain aspect of the interpretation is how a ground surge would have interacted with the ocean water that covered the area at the time. Did the surge ride atop the water column and eventually sink through it as it lost velocity, or did the entire water column become part of the surge. If the basal breccia is a result of erosion by the surge, then the water column must have been incorporated into the surge. If the breccia is a result of seismic disruption and later infiltration by ejecta an ocean-overriding mechanism is possible. Much additional work is required to understand the intriguing features so well exposed at the McClure locality. 123 �Figure 5.6. A- Coarse breccia at base of Sudbury layer containing meter-scale slabs of the inderlying iron formation. B- Typical lower breccia containing chert fragments supported in a matrix of sand-sized quartz grains and fragments of altered glass. C- Accretion lapilli in matrix of lower breccia. D- Elongated chert fragments showing preferred orientation. E- Chert fragment in lower breccia showing alteration rim. F- Lower breccia with an unusually high abundance of exotic (non-chert fragments. Coin is US penny in A-D. 124 �Figure 5.7. Photomicrographs of samples from the McClure site. A-Complex glass particle from lower breccia with two distinct compositions, possible immiscible melts. B- Complex fiamme of highly vesicular glass, now largely chlorite (trending upper right to lower left). C- Lower breccia matrix with numerous flattened particles of vesicular glass, now largely chlorite. D- Upper sandstone with numerous particles of altered glass, now largely chlorite; E- Vesicular glass particle in lower breccia and aphanitic matrix. F- Grain of quartzite in matrix of lower breccia. Note the abundance of rounded quartz grains in all samples. 125 �References Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Survey Geological Quadrangle Map GQ-1226, scale 1:24,000. Puffett, W.P., 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U.S. Geological Survey Professional Paper 788, 53 p. 126 �54th Annual Institute on Lake Superior Geology Field Trip 6 SUSTAINABLE RECOVERY OF IRON FROM THE MARQUETTE DISTRICT Glenn Scott, Helene Lukey, Al Strandlie, and CCI/CCMO staff Cleveland Cliffs Inc. 127 �Geology, Ore Processing and Reclamation at the Cleveland-Cliffs Michigan Iron Mines Ishpeming, Michigan 54th Annual Meeting of the Institute on Lake Superior Geology May 10, 2008 WELCOME 128 �Welcome to Cleveland-Cliffs Michigan Operations! In the early discussions of field trips for this conference, Ted Bornhorst suggested the inclusion of the processing plants in addition to the “standard” geologic tour. As we talked, this expanded into something with a decidedly broader scope to include a wide range of environmental quality and reclamation topics. Therefore, in contrast to the Institute field trip in 1999, this excursion does not focus on the geologic details of the Negaunee iron formation and the ore. Instead, the attention will be on the mining, processing and associated environmental aspects through to closure. Cleveland-Cliffs is proud of the environmental quality efforts at the Michigan Operations and we are looking forward to this opportunity to host the Institute. Please remember safety at all times by wearing your protective equipment, watching for hazards and paying attention to the suggestions of the guides We would like to thank Cleveland-Cliffs Inc and Cleveland-Cliffs Michigan Operations for supporting this visit. Special thanks to John Klasner for time and effort as editor. Field guides are: Helene Lukey Al Strandlie Al Koski Keith Kramer Karla Brudi John Meier Enjoy the tour – Glenn Scott FIELD TRIP STOPS NOTE: Unfortunately, CCI policy requests that visitors do not take photographs. 129 �Empire Pit Service building Pit operations for both the Empire and Tilden Mines are coordinated from the Empire pit service building. Empire overlook - Al Strandlie The view is to the north with the Empire Main Pit syncline plunging to the west. The CDV pit is to the northwest with the depleted CDI and CDII pits further north. Attachment 6-A Hematite overlook - Helene Lukey From the southwest end of the Tilden hematite pit, the mining operation and major geologic features can be seen (weather permitting). The view is to the east with the fault contact between the iron formation and the Archean to the south, with the martite and carbonate ore operations below. The large hill to the northeast shows a cross section of the intrusives on the north limb of the anticline. To the north, the “Slot” leads along strike to CDIII and the magnetite deposit. Attachment 6-A Ore Stockpiles If the pit access is such that only drive by viewings of the ore sites are possible, samples of the primary ore types can obtained at the stockpile area. Rock Stockpile reclamation - Al Koski As part of the reclamation plan, the rock (waste) stockpiles are being vegetated on an ongoing basis. The results can be seen in several areas; time and access will determine the exact locations. Attachment 6-B Tilden Plant - Keith Kramer The processes are described in the Attachment 6-C. The plant metallurgists and operators will lead the tour. 130 �Empire Tailings Basin - Gary Goodman The rejected material from the concentrating process is pumped to the tailing storage basins. Construction, maintenance and water balance will be discussed by Cliffs personnel as illustrated in Attachment 6-D. Access will be determined by weather and road conditions. Republic Wetlands Preserve - John Meier The Republic Wetlands Preserve is portion of Cleveland-Cliffs mitigation of impacts of the mining operations. The tour will be led by a representative of Cliffs Technology Group. The preserve is described in Attachment 6-E. Access will be determined by weather and road conditions. 131 �INTRODUCTION Cleveland-Cliffs Inc has been active on the Marquette Range since 1847 and has operated a series of underground and surface mines. Production in the early years was of from high grade natural ores but since 1967 production has been from low grade iron formation as pellets. The Marquette Range production began in 1846 on natural ores and pellet production began in 1956 (Boyum, 1979). Total production of the now depleted natural ore was over 300 million tons and pellets exceed 500 million tons. Pellet production has come primarily from the now exhausted Humboldt and Republic Mines and the presently operating Empire and Tilden Properties. On this tour, we will first visit the Empire and Tilden pits to observe the mining operations and examine the iron formation. We will then tour the Tilden concentrator and pellet plant followed with a visit to the active Empire tailings basin. Time and access permitting, we can view and discuss the reclamation of the rock stockpiles. The trip will conclude at the Republic Mine to view the reclaimed plant site and tailings area and to compare these to the active operations. AS WE WILL BE IN OR AROUND ACTIVE WORKING AREAS, PLEASE BE AWARE OF MOBILE EQUIPMENT AND OF THE POSSIBLITY OF SLIPS AND TRIPS. OVERVIEW The Tilden and Empire Mines are operated by Cleveland Cliffs and are located in the Upper Peninsula of Michigan, about 30 kilometers from the shore of Lake Superior (Figure 6-1). In the Lake Superior region, Tilden is unique in that the principle production (75%) is from a hematite deposit. The flotation process is complicated and can be sensitive to variations in mineralogy, chemistry and morphology of the iron and gangue minerals. The flotation ores are typically referred to as ‘hematite’. The actual minerals present and concentrated are hematite (both as martite and microplaty), magnetite, goethite/limonite and various carbonates including siderite, ankerite and dolomite. The common gangue minerals are quartz, chlorite and clays. Phosphorous occurs as apatite. Magnetite mineralogy is simpler as nonmagnetic species are (mostly) rejected in the concentrating process. Gangue minerals are quartz, hematite and carbonates. The 35% crude iron is upgraded to 65% before pelletizing. Annual production capacity is 8 million tons of pellets from 20 million tons of crude ore. Total production to date is 394 million tons of ore and 149 million pellet tons; published reserves are 717 million tons of ore and 260 million tons of pellets. Empire processes only magnetite with a capacity of about 5 millions tons of pellets per year. Total production to date is 777 million tons of ore and 220 million tons of pellets. 132 �Primary ore and waste parameters are crude to pellet weight recovery; concentrate chemistry (silica, phosphorous) and crude iron. These data are based on rather involved laboratory tests (Table 6-I) which may not directly reflect the plant response. REGIONAL GEOLOGIC SETTING The regional structures (Figure 6-2) are the Niagara Fault Zone, the collision zone between the Wisconsin Magmatic Terrane and the Superior craton (Schneider et al, 2002), and the Great Lakes Tectonic Zone, which forms the boundary between Archean granite-greenstone and gneissic terranes (Sims et al, 1980). In the Marquette Range area, deformation along the Great Lakes Tectonic Zone evolved from extension and deposition (Schneider et al, 2002) to closure and transpressional deformation and basin inversion (Cambray, 2002). The resulting faultbounded shallow west plunging asymmetric syncline contains a series of second-order growth fault basins that define the detailed stratigraphic variations. The Paleoproterozoic rocks in Michigan are termed the Marquette Range Supergroup (Cannon and Gair, 1970) and consist of three fining upward sequences (Map in Pocket). The lower portion has been correlated with the upper part of the Ontario Huronian (~2.2 Ga) and the upper parts, which contain the 1875 Ma iron formations, with the Mesabi Range of Minnesota. Simplistically, the sequence is from the Chocolay Group shelf facies quartzites and dolomite to the Menominee Group with argillites and the major iron formations to turbidites, greywacke and shale along with minor iron formation in the Baraga Group (Figure 6-3). Mafic igneous rocks with a continental tholeiite geochemical signature (Schulz, 1983) are present in the Menominee and Baraga Groups. Basal quartzites in each sequence are used as local structural and stratigraphic marker horizons. Metamorphic grades vary from sillimanite in the west to chlorite in the east and at the Tilden Mine (James, 1955) Negaunee Iron Formation and equivalents hosted the majority of the natural ore deposits and all of the concentrating grade production in Michigan. In the Marquette trough, the Negaunee reaches a thickness of 1300 meters without including the mafic igneous horizons. Due to the lack of correlative iron formation horizons, the igneous rocks, termed sills locally, are used for structural markers. There appears to be a poorly defined change from dominantly carbonate-chert on the north to magnetite-hematite-chert on the south (Waggoner, 2007). LOCAL GEOLOGY The Tilden and Empire Mines are located on the southern margin of the trough and are in fault contact with the Archean gneiss terrane (Attachment 6-A, Figure 6-3and 6-4). Local structure consists of upright to steeply inclined second order anticlines and synclines with low angle northwest and southwest plunges (Cambray 2002, Webster, 1999). At Tilden, due to the lack of clear marker horizons and rapid facies changes within the iron formation, igneous horizons are used for stratigraphic and structural correlation (Lukey, Johnson and Scott, 2007). At Empire, stratigraphy is determined by the igneous horizons and by the proportions of carbonates, silicates and clastics in the iron formation (Nordstrom, 1997; Han, 1975). See Figures 6-4 through 6-9 for mine geology. 133 �GEOLOGIC DOMAINS The domains are defined by geologic and metallurgical consistency (Table II, III) and are the basis for the resource modeling (Scott and Lukey, 1999; Nordstrom, 1999). Magnetite deposits are less variable (or perhaps the process is more forgiving). As there is no type example of iron formation within the mine, it is problematic if the mineralogic and textural variations reflect deposition in growth fault basins, diagenesis or hypogene events. IGNEOUS ROCKS Two ages of mafic rocks occur in the mine, the synsedimentary sills and associated dikes and a dike series of Keweenawan (~1000 Ma) related to the Midcontinent Rift. The older series vary from fine porphyritic to diabasic/ophitic and typically display chlorite-carbonate alteration assemblages, particularly in deformation zones. The younger series are typically unaltered diabase. The iron formation is variably altered along the intrusive contacts with the type and extent of alteration dependent on the thickness of the intrusive and the composition of the iron formation. FOLDING/FAULTING The major structures are the large (100s meters) scale Tilden Main pit anticline and Empire Main pit syncline; the fault that marks the contact of the Southern Complex and the iron formation; and the CDIII syncline. Smaller features (Figure 6-4) are the Section 20, CDI, II and V mining areas. Present geometry of these features is related to transpression during basin closure (Webster, 1999). The fault, initially a basin margin listric normal fault, was reactivated and is now a reverse fault that dips about 65° north (Cambray, 2002). At blast pattern level, faults and folds at the 1-20 meter scale tend to follow the trends seen in the larger structures. These features, while of relatively small amplitudes, can be significant in the detail block modeling and ore type boundaries. 134 �MINING AND PROCESSING The mining, concentrating and pelletizing processes are described in some detail in Attachment 6-B. The sequence begins with a scheduled mine plan and blast design. Holes are drilled, sampled and blasted. Broken ore is loaded into trucks to be transported to either the crusher and waste is taken to a rock stockpile. After initial crushing, the ore enters a series of autogeneous mills where it is ground to ~80% -31 microns (face powder) before the iron minerals are separated from the gangue. The magnetite process relies primarily on mechanical separation using the magnetic properties of the minerals. Hematite is processed by flotation and relies on chemical reagent selectivity. The tailings are pumped to the tailings basins and the water returned to the process (Attachment 6-D). Pelletizing is essentially the same in either ore type. The concentrate is “rolled’ into “green balls’ which are fired in the kilns to harden them for shipment. The kilns are heated by a combination of coal and natural gas. ENVIRONENTAL QUALITY AND RECLAMATION During operations, discharges of materials that might be harmful to the environment are monitored and prevented. This includes discharges into the water and air such as trace chemicals and particulate matter from stacks, tailings and roads. As stockpiles are completed the reclamation process is begun with introduced vegetation (Koski, 2007, Attachment 6-B). An example of closure reclamation will be seen at Republic Mine (Attachment 6-E). References Boyum, BH, 1979. The Marquette district of Michigan, 2nd edition, The Cleveland-Cliffs Iron Company: Ishpeming Cambray, FW, 2002. The evolution of a Paleoproterozoic plate margin, Northern Michigan, field trip guide for the Great Lakes Section–SEPM 32nd annual fall field conference Cannon, WF and Gair, JE. 1970, A revision of stratigraphic nomenclature of middle Precambrian rocks in northern Michigan, Geological Society of America Bulletin, 81: 2834-2846 Han, TM. 1975, Lithology, Stratigraphy and Petrology of Iron Formation at the Empire Mine, in Gair, JE, Bedrock Geology and Ore Deposits of the Palmer Quadrangle, Marquette County, Michigan, USGS PP 769, pp. 76-106 James, HL. 1955, Zones of regional metamorphism in the Precambrian of northern Michigan, Geological Society of America Bulletin, 66: 1455-1488 135 �Koski, AE. 2007, Reclamation of waste stockpiles at Cleveland-Cliffs Michigan Operations, n National Meeting of the American Society of Mining and Reclamation, June 2-7, 2007. Published by ASMR, 3134 Montavesta Rd., Lexington, KY 40502. Lukey, HM, Johnson, RC and Scott, GW. 2007, Mineral Zonation and Stratigraphy of the Tilden Hematite Deposit, Marquette Range, Michigan, USA, in Proceedings Iron Ore Conference2007, pp. 123130 (The Australian Institute of Mining and Metallurgy: Melbourne). Nordstrom, PM. 1999, Geologic field trip to the Empire Mine, in Institute on Lake Superior Geology Proceedings, 45th Annual Meeting, Marquette, MI, v. 45, part 2, p.129-134 Sims, PK, Card, KD, Morey, GB and Peterman, ZE. 1980, The Great Lakes tectonic zone – A major crustal structure in central North America, Geological Society of America Bulletin, Part 1, 91:690-698 Schneider, DA, Bickford, ME, Cannon, WF, Schultz, KJ and Hamilton, MA. 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron formation of the Lake Superior region, Can. J. Earth Sci. 39:999-1012 Schulz, KJ. 1983, Geochemistry of the volcanic rocks of northeastern Wisconsin [abs.], in Proceeding of the 29th Annual Institute on Lake Superior Geology, Michigan Technological University, Houghton Scott, GW and Lukey, HM. 1999, Geologic field trip to the Tilden Mine, in Institute on Lake Superior Geology Proceedings, 45th Annual Meeting, Marquette, MI, v. 45, part 2, p.114-128 Waggoner, TD. 2007, Personal communication 18 January Webster, CL. 1999, Structural analysis of a ductile shear zone within the Marquette Iron Range, Upper Peninsula, Michigan, MS thesis (unpublished), Michigan State University, East Lansing 136 �Table 6- I Glossary of Terms and abbreviations used at the mine and plant Natural Weight Recovery – The amount of material recovered from the material fed into the concentrator circuit. In other words, it’s the tons of concentrate made (measured as filter cake) from tons of crude ore used (measured by #3 belt scale) Metallurgical Weight Recovery (Met. Wt. Rec.) – Calculated by comparing the iron losses (as tailings) with the iron content of the crude ore fed into the concentrator (i.e., the head Fe). The formula used for this calculation is called the iron balance formula, or sometimes called the concentration formula. %Wt. Rec. = (Head Fe – Tail Fe) --------------------------- x 100 (Grade – Tail) Grade - Also called the concentrate grade, is a chemical measurement (assay) of the total iron oxide of the concentrate. Iron oxide is found in iron minerals such as hematite (Fe2O3), magnetite (Fe3O4), goethite (Fe2O3*OF) and iron carbonate (FeCO3). Concentrate Silica Grade - The chemical measurement (assay) of the % SiO2 in the concentrate. When a lower concentrate silica grade is achieved, the losses in iron units (tailings) increases. Head Grade - The assayed iron content of the crude ore fed into the concentrator circuit. Iron Recovery (Fe Rec.) – A calculation of the efficiency of the concentrator’s ability to recover the iron available. This is calculated by comparing the Met. Wt. Recovery, at some iron grade, with the head Fe of the crude. For example, %Fe Rec. = (Met. Wt. Rec. x Grade % Fe) --------------------------(Head % Fe) 137 �Percent Magnetic Iron Recovery (% Mag Fe Rec.) – The calculation of the efficiency of recovering the magnetic iron that was in the feed (crude ore). The Met. Wt. Rec., at some iron grade is compared with the magnetic potential (i.e. head) o the crude ore. For example, %Mag Fe Rec. = (Met. Wt. Rec. x Grade % Fe) --------------------------(Head Mag % Fe) Tailings - The product lost in the process. Tailings always include iron, because iron is always associated with many other minerals (silica, phosphate, carbonate, etc. or may be a liberation issue). Flot (flotation) – Flot ores are the martite, hematite, goethite, clastics and carbonates that are treated by selective chemical processes to achieve Fe and silica grade. The final stage of the magnetite process is flotation to achieve target silica grade. WIF (waste iron formation) – Iron formation that due to low weight recovery and/or high silica cannot be treated in the plant to produce economic concentrate. Rarely, phosphorous levels are too high to be treated. Magnetic Iron - The percent of the crude iron that is concentrated in the Davis Magnetic Tube Test (DMTT). %MagFe = DMTT Wt. Rec. x DMTT Grade The assumption is that all of this occurs as magnetite. However, in the Tilden ores an appreciable amount of hematite is locked up with the magnetite and is carried into the DMTT concentrate. This tends to over estimate the magnetic Fe content by 1-2% points and therefore overestimate the weight recovery. Satmagan - The Satmagan magnetic iron content is measured using susceptibility and is the actual magnetite content of the crude or concentrate. Domain – The deposit is divided into volumes of material with similar metallurgical response. These are usually stratigraphic horizons but may be fault bounded or nonconformable alteration/oxidation zones. The domains are the basis of the economic and planning models. 138 �139 �140 �141 �142 �143 �144 �145 �146 �147 �148 �149 �150 �151 �152 �153 �154 �155 �156 �157 �158 �159 �160 �161 �162 �163 �164 �165 �166 �167 �168 �169 �170 �171 �172 �173 �174 �175 �176 �177 �178 �179 �180 �54th Annual Institute on Lake Superior Geology Field Trip 7 GEOLOGY OF THE KEWEENAWAN BIC INTRUSION Dean Rossell Kennecott Minerals Company 181 �The Geology and Geologic Setting of the BIC Cu-Ni-PGE Prospect, Baraga County, Michigan U.S.A. Introduction The BIC mafic/ultramafic intrusion is located in Baraga County, Michigan, approximately 8 km southeast of the town of L’anse, Michigan. The roughly 1.1 km by 0.4 km, oval shaped intrusion forms a prominent hill with good exposures of the principle units that comprise the intrusion. The BIC intrusion has not been dated yet. However, based primarily on compositional similarities, Kennecott geologists believe it is similar in age to the mafic/ultramafic intrusion that hosts the Eagle Cu-Ni-PGE deposit, located ~35km to the east (fig 1), which has been recently dated at 1107.2+/- 5.7ma (Ding, 2007) The BIC intrusion has been the target of periodic exploration by Kennecott Exploration Company since the first discovery of Cu-Ni-PGE mineralized boulders near the intrusion in the mid-1990’s. The first drill hole into the intrusion, in 1995, was positioned at the south edge of the intrusion. The hole (BIC95-1, fig. 3) intersected ~3 m of disseminated sulfide mineralization in olivine melagabbro at the base of the intrusion, averaging 0.43%Cu, 0.32%Ni, 0.325ppm Pt and 0.345ppm Pd. Figure 1) Geology map of the northern portion of the Upper Peninsula of Michigan showing the location of the Baraga Basin and the BIC intrusion. Modified from Gregg (1993) 182 �No significant Cu-Ni-PGE resource has been identified at the BIC prospect yet. However, a drill hole completed by Kennecott Minerals Company in 2006 (07BIC-007), intersected 16.47m averaging 0.88%Cu, 1.00%Ni, 0.679ppm Pt, 0.991ppm Pd and 0.104ppm Au . This interval included a 2.8m interval with bands of massive sulfide, located in the meta-sediments immediately below the base of the intrusion, which averaged 1.66%Cu, 4.23%Ni, 1.383ppm Pt and 2.521ppm Pd. The metal tenor of the massive sulfide bands is comparable to some of the massive sulfides in the Eagle deposit. This could suggest that there is still some potential for a high grade massive sulfide body in the less explored portions of the BIC intrusion. Previous Geologic Studies No detailed geology map covers the area immediately around the BIC intrusion. The geology shown in Figure 2 is, in part, modified from data included in the USGS 1:62,500 scale open file geology map of the Precambrian geology of the Dead River, Clark Creek and Baraga Basins (Cannon, 1977). The area in figure 2 is also covered by the Iron River 1º x 2º quadrangle (Cannon, 1986). Geology in the Taylor Mine area (fig. 2) is compiled and modified from detailed mapping by Klasner (1972) and Klasner and others (1991). Ojakangas (1991) discussed stratigraphic correlations of Paleoproterozoic rocks in the area shown in figure 2. Gregg (1991) and Klasner and others (1991) described Penokean age deformation in the same area. The Archean geology to the southeast of the BIC intrusion is described in an unpublished master’s thesis by Turner (1979). A review of the Paleoproterozoic stratigraphy in the Baraga Basin, including the Taylor mine area, was recently undertaken by Gabe Nelson as part of a Masters thesis at Acadia University under Pier Pufal. The above data sources were supplemented by periodic reconnaissance mapping by me during the period 1999-1996. This work was augmented by regional geophysical studies and drilling programs carried out by personnel of Kennecott Exploration Company, Kennecott Minerals Company and various contractors. The more detailed geologic data from the BIC area is compiled from work by me, other Kennecott Exploration and Kennecott Minerals geologists, contract geologists and reports on petrography completed for Kennecott by Barnett (1995), Hauck (2001) and Johnson (2007). Regional Setting The BIC intrusion cuts Paleoproterozoic sediments in the southwestern portion of the Baraga Paleoproterozoic sedimentary basin (fig 1). The Baraga basin is bounded to the north and south, and underlain by Archean crystalline rocks. The Baraga basin merges with the Paleoproterozoic sediments of the Marquette Syncline southwest of the BIC intrusion (fig 1). The Archean, Paleoproterozoic and Mesoproterozoic geology is briefly summarized below. Archean The Archean terrane to the immediate south of the BIC intrusion (fig.2) is comprised largely of coarse grained, felsic gneiss and lesser amphibolite intruded by a variety of small mafic to ultramafic intrusions. Although there has been little mapping to confirm it, the gneissic rocks are most likely a continuation of the gneiss, intrusions and lower metamorphic grade supracrustal 183 �rocks (Marquette Greenstone Belt) that collectively comprise the Northern Complex (fig 1) to the east. A tonalitic intrusion dated at 2703 Ma and a rhyolite dated at 2780 Ma (Sims, 1993), are the only available age dates from the Northern Complex. Paleoproterozoic The recent discovery of the Sudbury ejecta horizon in the Baraga Basin (see below) constrains the bulk of Paleoproterozoic sedimentation to post 1850ma. Gregg (1993) divided the Baraga basin into two principle structural domains; the northern Huron River parautochthon and the southern allochthonous Falls River slice. Gregg proposed the boundary between the terrranes, which is marked by an abrupt change in structural style, is a south dipping thrust fault that he named the Falls River Thrust (fig. 2). Paleoproterozoic sediments to the north of the Falls River Thrust are characterized by weakly asymmetrical, relatively open folds with shallow axial plunges to the northwest or southeast. A single, southwest dipping, axial planar foliation is evident in most pelitic and siltstone horizons. Immediately south of the Falls River Thrust, folds are tight to isoclinal, generally overturned and often recumbent. In the Falls River slice, larger scale folds are overprinted by a second generation of folds with an associated crenulating foliation that is particularly evident in pelitic sediments. Boudinaged and folded quartz veins and lenses are prevalent in coarser-grained metagreywacke beds in the Falls River slice. Klasner and others (1991) mapped a thrust fault in the Komtie Lake area, south of the BIC intusion (fig. 2). They reported that a vertical exploration drill hole, located on the south side of Komtie Lake, penetrated 30 m of Archean gneiss followed by 3 m of mylonite before intersecting 45 m of Paleoproterozoic sediments. They proposed an approximately east-west striking and south dipping thrust fault that brought Archean gneiss over a thin veneer of the basal Paleoproterozoic sediments. They extended the fault westward to include strongly foliated rocks exposed along Plumbago Creek (fig 2). I extended the Komtie Lake thrust fault further to the northeast in figure 2, to an area where magnetic anomalies originating in the Paleoproterozoic sediments appear to continue under exposures of Archean gneiss. This extension has not been confirmed by mapping. Exposures of pelitic rocks in the immediate area of the Taylor mine (stop 3, fig. 2) generally lack the prominent crenulating cleavage seen in pelitic rocks exposed all along Taylor Creek further to the north (stop 4, fig. 2). Drill hole T-5, a 68.5 m deep vertical exploration hole collared northeast of the Taylor mine pit (fig. 2), bottomed in mylonitic rock. I propose that there is another generally east-west striking thrust fault north of drill hole T-5, separating the overriding Taylor Mine slice from the more deformed rocks of the Falls River Slice. Alternatively, the fault could be the westward continuation of the Komtie Lake thrust fault. Historically, deformation of the Paleoproterozoic sediments in the western portion of the Upper Peninsula has been attributed to a series of collisional events between 1888 Ma and 1830 Ma that collectively make up the Penokean orogeny (Schultz and Cannon, 2007). However, Schultz and Cannon (2007) point out that there is evidence of vertical faulting and uplift that significantly post date1830 Ma. They concluded that this younger deformation cannot be attributed to the Penokean orogeny and that it is more likely of Yavapai age. 184 �Mesoproterozoic Mesoproterozoic flood basalts associated with the Keweenaw Flood basalt Province are exposed along the length of the Keweenaw Peninsula and 30km southwest of the BIC intrusion at Silver Mountain, Michigan. The Keweenaw Flood Basalt province represents the exposed portion of the Midcontinent Rift system in the Lake Superior region. The Midcontinent Rift forms a prominent gravity anomaly that can be traced from the Lake Superior region southwest into central Kansas, and southeastward into southern Michigan. The total length of the geophysical feature is in excess of 2000 km (Hinze and others, 1997). Seismic data indicates the rift below Lake Superior is filled with more than 25km of volcanics buried beneath a total thickness of up to 8km of rift filling sediments (Bornhorst and others, 1994). The estimated volume of magmatic rocks associated with the rift is greater than 2 million cubic kilometers (Cannon, 1992). The Keweenaw Flood Basalt province was formed over an approximately 23 million year period, from ~1111 Ma. to ~1089 Ma. Volcanism was bimodal, but with preserved basaltic rocks much more abundant than rhyolitic rocks. Volcanism occurred in two distinct phases, with an approximately 5 million-year hiatus between phases (Miller, 1996). In Michigan and Wisconsin, the early phase volcanics are comprised of the Sieman’s Creek formation and volcanics of the Powdermill group (Wiband and Wasuwanich, 1980). The Portage Lake volcanics comprise the younger phase. The early phase volcanics are primarily reversely polarized. The Portage Lake volcanics are normally polarized. A mantle plume model has been widely evoked to explain the staged evolution and large volume of magmatic products associated with the Midcontinent Rift (Nicholson, 1997). Red bed sandstones (Jacobsville Sandstone) shed off the horst block formed during inversion of the Midcontinent Rift, cover Paleoproterozoic sediments west of BIC (fig. 2). Rift inversion may have begun as early as 1080 Ma and was completed by about 1040 Ma (Cannon, 1994). The probable cause of compression was continental collision in the Grenville province (Cannon, 1994). Paleoproterozoic Stratigraphy Archean rocks are either unconformably overlain by, or in fault contact with, Paleoproterozoic meta-sediments along the southern margin of the Baraga Basin. Ojakangas (1994) has correlated sediments in the Baraga Basin and western Marquette trough with the Baraga Group, the youngest of the three dominantly clastic sedimentary groups that comprise the Marquette Range Supergroup. He concluded, on the basis of paleocurrents, paleogeographic setting and isotopic data that the best tectonic model for Baraga Group sedimentation is a northward migrating foreland basin. Quartzites at the base of the Paleoproterozoic sedimentary sequence in the Baraga basin north of the Falls River thrust and in the Canyon Falls area (stop 1-fig. 2) are correlated with the Goodrich formation by Ojakangas (1994). The basal quartzites at both these localities appear to rest unconformably on Archean basement. The quartzites range from thickly to thinly bedded, with locally well developed planar and trough cross bedding. Quartzites in the Baraga basin are typically arkosic with conglomerate lenses. Ojakangas (1994) proposed that the Goodrich 185 �quartzites were deposited in a tidal environment. In the Baraga Basin, the Goodrich formation ranges in thickness from less than a meter in the eastern portion of the basin, to approximately 40 m in the western portion of the basin (Nelson, 2006). I interpret widely scattered outcrops of similar appearing quartzite exposed along the margins of the Archean to the south and east of the BIC intrusion as equivalents of the Goodrich quartzite described above. However, in most places they appear to be in fault contact with the Archean. Klasner and others (1991) interpreted strongly foliated, quartz rich schists along the north side of Plumbago Creek in the Taylor mine area (fig. 2) as mylonitic textured Archean gneiss. I have examined some of these outcrops and feel they could, in part, be strongly foliated arkosic Goodrich quartzite. The proximity of the sheared “quartzite” with iron formation exposed along the banks of Plumbago Creek has potential stratigraphic implications in the Taylor mine area. The Goodrich formation is overlain by the Michigamme formation, the uppermost formally recognized formation in the Baraga Group. Leith, et al (1935) divided the Michigamme formation into three principle members which, in ascending order are: the Lower Slate member, the Bijiki iron formation, and the Upper Slate member. Kennecott geologists have generally used this nomenclature for describing stratigraphic relationships in the Baraga Basin. However, in the western portion of the Baraga Basin, the Goodrich formation quartzites are immediately overlain by a thin interval (typically less than 20m thick) of inter-bedded chert and iron rich carbonate. Ojakangas (1994) suggested that this cherty horizon may be the equivalent of the Bijiki iron formation and that the Lower Slate member is missing in parts of the Baraga basin. However, Kennecott geologists believe this is a separate unit below the Lower Slate member and informally refer to it as the Chert Carbonate member. That informal designation is used in the rest of this field guide and in figure 2. William Cannon (personal communication) has identified layers with accretionary lapilli, pumice grains and, at one location, quartz grains, with shock lamellae from bedrock exposures and core samples of the Chert Carbonate member in the Baraga Basin. Cannon has proposed that these are ejecta from the 1850 Ma Sudbury impact event and correlated them with other ejecta horizons previously identified in Ontario and Minnesota (Addison et al, 2005). Kennecott drill hole 07BIC-033, the deepest hole completed at the BIC prospect, intersected intervals with probable accretionary lapilli and pumice fragments (Cannon, personal communication) in cherty rocks starting at a depth of 586 m. The likely presence of the Sudbury ejecta layer in the BIC drill hole provides confidence that the more deformed rocks in the southwestern portion of the Baraga basin (south of the L’anse thrust fault in figure 2) are stratigraphically correlative with the rocks in the northern portions of the Baraga Basin. The Chert Carbonate member and Sudbury ejecta layer is overlain by dominantly black to dark gray, thinly bedded, meta-siltstone and pelite in the Baraga Basin. The pelitic rocks are often graphitic and sulfide rich and contain only minor intervals of fine-grained greywacke. As mentioned above, Kennecott geologists believe this is the Lower Slate member of the Michigamme formation. This siltstone-pelite dominated interval increases from 20-90 m in the northern part of the Baraga Basin to thicknesses I speculate might be greater than 200 m in the vicinity of the BIC intrusion. However, structural complexities and insufficient drilling make 186 �accurate determinations of the thickness of this sequence currently impossible in much of the southern portion of the Baraga Basin. In the Taylor mine area (stop 3-fig.2) the Lower Slate member is overlain by the Bijiki iron formation. The Bijiki iron formation is primarily comprised of thinly bedded, black and white chert with lesser siltstone, iron carbonate and iron oxides (Ojakangas, 1994). In the immediate Taylor mine area the Bijiki iron formation ranges from 20-80m in thickness (Ford Motor Company reports). A Kennecott Exploration drill hole, ALB95-3, located approximately 2.7km west of the Taylor mine (fig. 2), intersected 280 m of banded iron formation, with lesser intervals of graphitic slate, starting at a depth of 110 m and continuing to the bottom of the hole. Bedding angles to core, along with the lack of any compelling evidence of fold or fault repetition, suggest that this is likely to be close to a true thickness. A second hole, ALB95-2, collared 1.1 km further to the west, intersected 194 m of iron formation. Both holes were terminated while still in iron formation so the total thickness of iron formation at this location is unknown. Kennecott geologists believe the iron formation in both holes is the Bijiki indicating a rapid westward thickening of the unit. This thicker part of the Bijiki is within a rhomb shaped magnetic and gravity high. The rapid westward thickening of the iron formation, and shape of the coincident geophysical anomalies, might be evidence of a fault bounded, second order basin that formed during deposition of the Lower Slate and Bijiki iron formation. The BIC intrusion cross cuts an approximately 15km long linear magnetic anomaly. Drilling and mapping by Kennecott geologists has confirmed that the linear magnetic anomaly is caused by abundant pyrrhotite in graphitic sediments. The sediments contain numerous thin bands of contorted quartz and 0.5-1cm thick bands and lenses of semi-massive pyrrhotite and pyrite with minor sphalerite and chalcopyrite. The ratio of pyrrhotite and pyrite varies considerably along strike, and within a drill intersection, significantly affecting its magnetic susceptibility. Similar sulfide rich sediments are seen immediately below the Bijiki iron formation at the Taylor mine and in a 25-35m interval immediately above the Bijiki iron formation in drill holes ALB95-2 and ALB95-3 (pyrite rich in hole ALB95-3 and pyrrhotite rich in hole ALB95-2). The author proposes that these sulfide rich, variably magnetic sediments are the continuation of the Bijiki iron formation member northward into the BIC area. However, this important marker horizon has not been identified anywhere else in the northern part of the Baraga basin. The Bijiki member is overlain by the Upper Slate member in the Taylor mine and BIC prospect areas. The Upper Slate member contains a significant percentage of greywacke inter-bedded with siltstone and pelite distinguishing it from the Lower Slate member. Ojakangas (1994) reported that greywacke beds made up 18% of a measured section in the Silver River north of the BIC intrusion. The greywacke beds are commonly graded and contain rip ups and other features indicative of deposition by turbidity currents. Baraga-Marquette Dyke Swarm The Baraga-Marquette dyke swarm is comprised of more than 150 diabase dykes (Green and others, 1987). The primarily east-west trending dikes form a belt that extends from the northern edge of the Baraga basin at least 75 km southward into southern Marquette County. Although 187 �most dykes in the swarm are less than 30 m thick, individual dykes are up to 185 m thick and can be traced for up to 59 km (Green et al., 1987). The majority of the known dykes are reversely polarized, forming prominent magnetic linear anomalies on magnetic maps. None of the diabase dykes have been dated. However, the measured diabase dyke paleomagnetic pole position in the Marquette area is virtually identical to that of reversely magnetized intrusions from the Thunder Bay area (Wilband and Wasuwanich, 1980). Sutcliff (1987) reported an age of 1109ma for the reversely polarized Logan sills in the Thunder Bay area. The dykes typically have subophitic to diabasic textures and contain 50-70% plagioclase, 3050% clinopyroxene and 1% or less olivine and Fe-Ti oxides. Most dykes are relatively fresh with little sign of alteration (Wilband and Wasuwanich, 1980). Most of the reversely polarized dykes have high TiO2 (3-5%), P2O5 (0.30-0.55%) and <15% Al2O3 (Wilband and Wasuwanich, 1980).The dykes also typically have high Cu (300-500ppm) and low Ni (<100ppm) contents (Kennecott data). Interestingly, no reversely polarized dykes are evident in magnetic data sets north of the Falls River thrust fault (fig. 2). This might suggest that the fault played some role in localizing the reversely polarized dykes of the Baraga-Marquette dyke swarm. The BIC Intrusion The BIC intrusion is located about 35km southwest of Eagle and 8km southeast of the town of L’anse, Michigan. The intrusion forms a prominent hill approximately 1100m long by 400m wide. Mapping, geophysics and drilling indicate the intrusion has roughly the same dimensions as the hill at bedrock surface (fig. 3). Although not well constrained along much of the intrusion, based on the drilling completed, the intrusion appears to be generally V shaped in cross section. Drilling and mapping in the eastern portion of the intrusion suggest the southern margin of the intrusion dips moderately to the north (fig. 4). Knowledge of the northern contact is limited, but it appears to be steeply, south dipping. A much smaller, shallow bowl shaped intrusion, referred to as Little BIC, was located just to the northwest of the BIC intrusion during 2006 drilling (fig. 3). The smaller intrusion is comprised mostly of relatively olivine rich lithologies very similar to those seen along the base of the main BIC intrusion. This smaller intrusion could be a fault offset of the larger BIC intrusion, or possibly a separate intrusion. The best mineralized intersections in drilling completed through 2007 have primarily come from this smaller intrusion. Unlike the intrusion hosting the Eagle ore body, the BIC intrusion is distinctly layered. Core logging, thin section work and very limited geochemistry show that the BIC intrusion can be subdivided into three principal units; an upper coarse-grained gabbro, a middle unit comprised of fine-grained gabbro and feldspathic clinopyroxenite, and a lower unit of feldspathic wehrlite and olivine melagabbro. All three units thicken toward the center of the intrusion and thin toward the margins. The following descriptions of the units are summarized from core logs and observations of outcrops and hand samples. Most of the descriptive mineralogy is taken from unpublished 188 �petrography reports prepared for Kennecott Exploration and Kennecott Minerals by Rod Johnson (2007) Steve Hauck (2002), and Bob Barnett (1995). Upper Unit - Gabbro The upper gabbro is the thinnest unit with no drill intersections exceeding 75 m (no upper contact has been located so this is only a minimum total thickness). It is exposed in a few scattered locations on the top of the hill. The best exposures are along the drill roads on top of the hill in the eastern portion of the Intrusion. The upper gabbro is an altered, medium to coarse-grained, oxide gabbro with 55% lath like plagioclase and 35% prismatic or granular clinopyroxene. The gabbro contains up to several percent titanomagnetite, minor apatite and trace olivine. The upper gabbro is moderately to strongly magnetic. Strong alignment of plagioclase laths, which can be up to 2cm in length, and prismatic clinopyroxnene creates a foliation in the gabbro in places. In other places, the crystals radiate, creating a stellate pattern. Small patches of granophyre are present in drill core and outcrop. The upper gabbro is moderately to intensely altered with plagioclase variably altered to sericite and clinopyroxene altered to amphibole and chlorite. Very fine grained hematite coats some plagioclase giving it a pinkish color and titanomagnetite is altered to martite and maghemite. Pyrite occurs as disseminations and rare veins (Hauck, 2002). Football size and shape pods of strong light green, epidote rich rock are common in outcrop and drill core of the upper gabbro. The pods, which have sharp contacts, can form up to 5% of some outcrops. The shape, size and distribution of the pods suggests that they might be preferentially altered xenoliths or autoliths. Middle Unit-Gabbro/Clinopyroxenite The middle unit is comprised of gabbro and clinopyroxenite which forms 3-10m high cliffs around the perimeter of the hill. The middle unit is by far the best exposed unit at the BIC prospect. Intersections in drill core of the middle unit reach 100m in drill holes in the eastern half of the intrusion but it appears to thin to the west. The unit is comprised of fine-grained, equigranular gabbro and feldspathic clinopyroxenite. The upper few meters of the unit is a fine-grained, strongly magnetic equigranular, oxide rich, cumulate textured gabbro with 40-50% granular clinopyroxene and 20-50% granular titanomagnetite and minor ilmenite. Plagiclase content varies, but is typically less than 40% in this oxide rich part. Biotite and amphibole are minor components in the upper portion of the unit. This magnetite rich interval is present in most holes and creates a distinctive spike in magnetic susceptibility profiles in most BIC drill holes (a magnetic profile is shown for hole BIC02-02 in figure 4) Magnetite content decreases rapidly with depth in the middle unit and most of the unit below the first few meters is weakly to non-magnetic. Clinopyroxene content increases downward and in the eastern portion of the intrusion much of the lower part of the middle unit is fine-grained, cumulate textured, feldspathic clinopyroxenite. The presence of cumulate clinopyroxenite is suspected in the western portion of the intrusion but not yet confirmed by thin section work. Alteration is similar to that seen in the upper gabbro with plagioclase largely altered to sericite, carbonate and actinolite and pyroxene is variably altered to chlorite, carbonate and amphibole. 189 �Fine-grained, disseminated chalcopyrite and trace bornite is found through out the unit, generally in trace amounts, but locally up to 0.5%. Minor pyrite and sphalerite are present in western outcrops of the middle unit, in addition to chalcopyrite. Lower Unit- Wehrlite/Olivine Melagabbro Unlike the upper two units, which contain only very rare olivine and orthopyroxene, the lower unit is relatively olivine rich and has up to 5% orthopyroxene in some thin sections. The lower unit is poorly exposed, with just a few outcropings along the south side and none on the north side. The unit is best exposed on the west end of the hill. Drilling indicates it is the thickest of the three units and has a thickness of greater than 200 m in drill hole BIC02-02 (fig 4). The upper portion of the lower unit is comprised of fine grained, moderately magnetic, feldspathic wehrlite and olivine melagabbro with 35-60% cumulate olivine, 10-20% clinopyroxene, 10-34% plagioclase and minor sulfide. Clinopyroxene is either granular or poikolitic on olivine and plagioclase is poikolitic on both olivine and clinopyroxene. Titanium rich phlogopite and amphibole are also minor (1-2%) primary mineral phases. Chromite occurs as grains within olivine and minor titanomagnetite and ilmenite occur as single or composite grains, often subpoikolitic on clinopyroxene. Barnett (1995) reported olivine compositions for outcrop samples of the lower unit that ranged from fo76 to 83. These values closely overlap with the range of fo76 to 85 reported for olivine melagabbro at the Eagle deposit (Ding, 2008). In most holes, olivine content decrease with depth in the lower unit, while clinopyroxene, plagioclase and sulfide increase. In the eastern portion of the intrusion, this change in mineralogy is accompanied by an increase in grain size in the lower 50m of the intrusion. Alteration is moderate to severe in the lower unit with olivine partially to completely altered to either iddingsite or serpentine and fine-grained magnetite. Both plagioclase and clinopyroxene are variably altered to chlorite and carbonate. The alteration tends to turn everything green in the most altered samples, often making visual determination of the primary mineralogy difficult in hand and core samples. Contact metamorphic Aureole Meta-sedimentary rocks peripheral to the BIC intrusion show the effects of low pressure contact metamorphism. Johnson (2007) studied thin sections cut from drill core samples of metasediments peripheral to the BIC intrusion. He divided metamorphic assemblages in the metasediments into a proximal granoblastic hornfels, a more distal porphroblastic spotted hornfels, and a regional green schist assemblage. Within two to three meters of the contact of the intrusion, primary structures and foliations in the meta-sediments are very poorly preserved. The regional metamorphic assemblage is overprinted by a granoblastic assemblage of cordierite, quartz, biotite, vesuvianite and sphene +/- andalusite, sillimanite, kspar and plagioclase. Scattered small pods and veins of coarser grained k-spar and quartz within the granoblastic hornfels suggest localized partial melting of the meta-sediments in close proximity to the intrusion. The granoblastic hornfels grades outward into spotted hornfels which in some drill holes can be recognized in the meta-sediments10 to 15m from the contact with the intrusion. The spotted hornfels is characterized by the growth of small (<0.5 mm) porphyroblasts in phyllosilicate rich 190 �beds. Johnson (2007) reported cordierite, andalusite and sillimanite as the principal prophyroblasts in the spotted hornfels. Johnson also reported that much of the high temperature metamorphic assemblage has been overprinted by a retrograde assemblage with porphyroblasts replaced by chlorite and white mica and biotite by chlorite. Mineralization Three types of sulfide mineralization related to the BIC intrusion have been recognized: disseminated chalcopyrite-pyrite mineralization in the middle unit, copper and PGE rich disseminated sulfide mineralization in the lower unit and thin bands of “Eagle like” massive sulfide in the hornfels beneath the intrusion. However, exploration work completed to date at BIC has not yet identified any significant Cu-Ni-PGE resource. Fine-grained chalcopyrite with trace pyrite, sphalerite and rare bornite is disseminated throughout the middle unit. Limited sampling of this interval in drill hole BIC01-01 gave Cu values up to 0.16% over 1.5 m. However, Ni values were all below 500ppm and Pt and Pd values were all at, or below, the detection limits (Kennecott Exploration data). Disseminated sulfides are erratically distributed throughout the lower unit In the BIC intrusion. However, sulfide abundance seldom exceeds 5% in most of the drill tested portions of the intrusion. The greatest abundance of sulfide is typically located within a 3-4m interval 1-2m above the base of the intrusion. In the Little BIC intrusion, the abundance of disseminated sulfides reaches 10% over short intervals. Continuous intervals with >4% disseminated sulfides exceeding 20 m have been intersected in some drill holes at Little BIC. Sulfides in the lower unit are comprised of irregularly shaped, composite grains of pyrrhotite, chalcopyrite and pentlandite that are subpoikolitic on olivine, clinopyroxene, plagioclase, amphibole, ilmenite and titanomagnetite (Hauck, 2002). Cubanite occurs both as lamellae in chalcopyrite and as irregular grains. Recalculating the metal contents of disseminated sulfides to 100% sulfide, BIC and Little BIC disseminated sulfide metal tenors in the lower unit average 12.77% Cu, 5.88% Ni, 10.5ppm Pt and 12.91ppm Pd (avg. 109 samples with 0.9-10% S). In contrast, disseminated sulfides in the Eagle deposit recalculated to 100% sulfide average 6.24% Cu, 6.39% Ni, 1.5ppm Pt and 0.9ppm Pd (avg. 2350 samples with 0.9-10% S). The significantly higher Cu:Ni ratio and greater PGE content of BIC disseminated sulfides compared to Eagle disseminated sulfides suggest a greater silicate melt to sulfide melt ratio (R factor) at BIC. Thin (<1m) bands of massive sulfide occur in the hornfels within a few meters of the base of the Little BIC intrusion, and in a few holes in the western portion of the BIC intrusion. Two samples of massive sulfide from hole 06BIC-007 (Little BIC intrusion- fig.3), selected to maximize sulfide content, averaged 2.72% Cu, 6.02% Ni, 1.8ppm Pt and 3.1ppm Pd (avg. 35.8% S). The significantly lower Cu and PGE tenors of the massive sulfides hosted in the meta-sediments suggests that they were not directly formed by gravitational settling of the overlying disseminated sulfides. Interestingly, the massive sulfides at BIC have metal tenors and Cu:Ni ratios very similar to Cu poor massive sulfides at the Eagle deposit. 191 �Acknowledgements I would like to thank Kennecott Exploration Company and Kennecott Minerals Company for granting permission to prepare this field guide. I would also like to thank the rest of the Kennecott North American Ni exploration team and the Kennecott Minerals exploration staff for the hard work they have put in over the years on the Michigan program including the BIC project. In particular, I would like to acknowledge the many contributions made by Andrew Ware and Steve Coombes. I would also like to thank Karen Rossell and Andrew Ware for reviewing the field guide. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33 pp. 193-196. Barnett, R.L., 1995, BIC Gabbro: Kennecott internal company report. Bornhorst, T.J., and Rose, W.I., 1994. Self-guided geological field trip to the Keweenaw Peninsula, Michigan. Institute on Lake Superior Geology, Proceedings 40th annual meeting, Houghton, Michigan. vol. 40, part 2, 185p. Corfu, F., and Lightfoot, P.C., 1996, U-Pb geochronology of the Sublayer environment, Sudbury Igneous Complex, Ontario: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 91 pp. 1263-1269. Cannon, W.F., 1977, Bedrock geology in parts of the Baraga, Dead River, and Clark Creek Basins, Marquette and Baraga Counties, Michigan: U.S. Geological Survey Open-File report 77-467, scale 1:62,500. Cannon, W.F., 1986, Bedrock geologic map of the Iron River 1x2 degree quadrangle, Michigan and Wisconsin: U.S. Geological Survey Misc. Investigations Series Map I-1360-B, scale 1:250,000. Ding, X., 2007, Geology of the Eagle nickel deposit: Kennecott internal company report. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987. Keweenawan dykes of the Lake Superior region: evidence for evolution of the Middle Proterozoic Midcontinent rift of North America. In `Halls, H.C. and Fahrig, W.F., eds., Mafic dyke swarms: Geological Association of Canada Special Paper 34, pp.-289-302. Gregg, W.G., 1993, Structural geology of parautochthonous and allochthonous terranes of the Penokean Orogeny in Upper Michigan-comparisons with northern Appalachian tectonics: U.S.Geological Survey Bulletin, 1904-Q, 28p. Hauck, S.A., 2002, Petrographic descriptions of samples from drill holes BIC-2 and RLP-95-1, Northern Michigan: Kennecott internal company report. Hinze, W.J., Allen, D.J., Braile, L.W., Mariano, J., 1997. The Midcontinent rift system: a major Proterozoic continental rift. In Middle Proterozoic to Cambrian Rifting. Eds. Ojakangas, R.W., Dickas, A.B, and Green, J.C., Geological Soc. of America Special Paper 312, pp. 7-35. Johnson, R.C., 2007, Petrography and petrology of selected igneous and meta-sedimentary drill core samples: Kennecott internal company report. 192 �Klasner, J.S., 1972, Style and sequence of deformation and associated metamorphism due to the Penokean orogeny in the Western Marquette Range, Northern Michigan: Houghton, Mich., Michigan Technological University Ph. D. dissertation, 132p. Klasner, J.S., Ojakangas, R.W., Schultz, K.J. and Laberge, G.L., 1991, Nature and style of deformation in the foreland of the Early Proterozoic Penokean Orogen, Northern Michigan: U.S. Geological Survey Bulletin. 1904-K, 22p. Miller, J.D., 1996. The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust (abst.); Institute on Lake Superior Geology, Proceedings 42nd annual meeting, Cable Wisconsin, vol. 42, part 1, p. 33 Nelson, G., 2006, Sedimentologic characteristics of the Baraga Basin: Kennecott internal company report Ojakangas, 1994, Sedimentology and Provenance of the Early Proterozoic Michigamme formation and Goodrich quartzite, Northern Michigan – regional stratigraphic implications and suggested correlations: U.S. Geological Survey Bulletin 1904-R, 31p. Turner, Thomas R.,1973, The geology of the northern complex near Herman, Michigan thesis (M.S.)--Michigan Technological University, 1973. Sutcliff, R.H., 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada:. Contrib. Mineral Petrology, vol.96, pp.201-211. Sims,P.K., 1996, Early Proterozoic Penokean orogen, in Sims, P.K. and Carter, L.M.H., eds. Archean and Proterozoic geology of the Lake Superior region, U.S.A., 1993, U.S. Geological Survey Professional Paper 1556,p.28-51. Wilband, J.T. and Wasuwanich, P., 1980, Models of basalt petrogenesis: lower Keweenawan diabase dikes and middle Keweenawan Portage Lake lavas, Upper Michigan: Contrib. Mineral. Petrol., v.75, pp.395-406. 193 �Field Trip Stops The first four stops on this trip are intended to highlight the variety of sediments that comprise the Paleoproterozoic Baraga Group in the vicinity of the BIC intrusion. They also provide an opportunity to see and discuss some of the structural complexity in this area. At stops 5 and 6 we’ll examine exposures of the BIC intrusion. Stop 7 will be at the Kennecott Minerals Company core shed near Negaunee, Michigan. Here we’ll have an opportunity to look at drill core form the BIC intrusion including mineralized intervals that are not exposed in the field. The location of field trip stops 1-6 are shown on figure 2. The locations of stops 5 and 6 are also shown on the more detailed BIC geology map. GPS coordinate locations provided for the stops are in UTM (Universal Transverse Mercator), zone 16. The datum is Nad 83. All of the field trip stops, except stop 1, are in areas of privately owned surface. Permission from the surface owners is required before accessing these areas. Some of the stops are along rivers and streams with high, often slippery banks and with potentially poor footing. Caution should be used in walking around these areas. Steep, cliff like outcrops are present in the vicinity of Stop 6, they provide great views but please stay well back from the edges. Stop 7-1 Canyon Falls on the Sturgeon River (UTM coordinates 386938E 5164275N) Good exposures of the Goodrich formation quartzites are exposed along the Sturgeon River at this location. To access the area, park at the Sturgeon River roadside park on the west side of US Highway 41 and follow the marked hiking trial south about 600m to the falls overlook. This area was a stop on a previous ILSG field trip led by Bill Cannon and John Klasner in 1972. The following stop description is an excerpt from that field guide. “This stop illustrates an anomalous structural style in that the rocks are relatively nonfolded as compared with the deformation style of nearby Precambrian X metasedimentary rocks, Here the quartzites, composed of quartz grains in a clay matrix with chlorite porphyroblasts, show very gentle N 70º W trending monoclinal folds. Ripple marks and sole marks are common on bedding surfaces. The more argillaceous layers show the development of a N 70º W cleavage” Ojakangas (1994) has correlated the thinly layered quartzite at this location with the Goodrich formation. 194 �Stop 7-2 Conglomerates on top of the Bijiki iron formation near the Taylor Mine. (UTM coordinates 388973E 5168500N) The stop is at rubble (subcrop) along the north side of a small drainage into Ogemaw Creek about 30m southeast of Old Hwy 41 (note: Old hwy 41 from the turn off of US highway 41 to the Taylor mine turnoff is a poorly maintained road that is often rutted and muddy and occasionally flooded. Klasner (1972) mapped a horizon of poorly exposed conglomerate and greywacke along the top of the Bijiki banded iron formation at this location. The reddish sandstone contains scattered matrix supported clasts of chert up to 10cm across. Drilling by Kennecott a few km to west of this location suggests that the Bijiki iron formation rapidly increases in thickness to the west. Perhaps, these conglomerates are additional evidence of a higher energy environment associated with the formation of a fault controlled sub-basin to the west. Stop 7- 3 Taylor mine site (UTM coordinates ~ 389660E 5169000N) The Taylor Mine site can be accessed by walking east from old hwy 41 along the old Taylor mine road. A trail to the north, along an old rail grade just before the old Taylor mine pit, leads to several good bedrock exposures. The Taylor Iron Co. shipped 32,970 tons of iron ore from the Taylor mine between 1880 and 1883 (Lake Superior Iron Ore Association, 1952). The property was explored by Ford Motor Company for iron ore during the 1950’s and 1960’s. Additional drilling was carried out on the property in the 1970’s as part of a regional uranium exploration program. John Klasner (1972) produced a detailed map of the mine area as part of his Ph.D. dissertation at Michigan Technological University. Kennecott acquired mineral title to the property as part of the purchase of all of the Ford Motor Company mineral title holdings in the Upper Peninsula. The mine site provides good exposures of the Lower Slate and Bijiki members of the Michigamme formation and diabase dykes of the Baraga-Marquette dyke swarm. Well exposed folds also contrast with the very weakly folded quartzite at stop 1. Klasner (1972) describes the folds at the Taylor mine as “asymmetric with slight overturning to the north and a recognizable S1 axial plane foliation. The folds have an amplitude of 400 feet (122 m) and a period of 600 feet (183 m). Minor folds are superimposed on the larger folds” 195 �Stop 7-4 Taylor Creek (optional) (UTM coordinates 390436E 5170300N) Good exposures of probable Upper Slate member of the Michigamme formation are found downstream along Taylor Creek from where old hwy 41 crosses it. However, in many places the banks of Taylor Creek are very steep and rocky. Access to this stop will depend on how high spring run off water level is. The banks of Taylor Creek at this stop are steep and the footing can be poor. Use caution when climbing down to view the exposures along the creek. Taylor Creek is within the Falls River slice, the allocthon proposed by Gregg (1993) south of the Falls River thrust fault (see fig. 2). Deformation evident in the bedrock exposures along Taylor Creek is different than that seen at either the Taylor mine or further north in the Baraga basin. In Taylor creek, small scale folds, where visible, are often nearly recumbent. In pelitic horizons, S1 foliations typically dip gently southward and are affected by a well developed crenulating cleavage associated with a second generation of folds. Stop 7-5 Exposures of the Lower and Middle Units on the west end of the BIC intrusion (UTM coordinates 396027E 5174514N) The west end of the BIC intrusion is accessible by hiking eastward from the Indian road along a series of old logging trails. The best exposures are located just below the top of the hill. The surface and mineral title are held by Kennecott Minerals Company at this stop and permission is required to access the area. At this stop, a natural flat terrace on the west facing slope of the prominent hill held up by the BIC intrusion, marks the unexposed contact between the Lower and Middle units of the BIC intrusion. Outcrops down slope from the terrace are comprised of rocks that range in composition from feldspathic werhlite to olivine melagabbro. They contain minor disseminate pyrrhotite, chalcopyrite and pentlandite. Nearly complete replacement of plagioclase by secondary minerals makes accurate determinations of modes very difficult in most hand samples of this unit. The Lower Unit of the BIC intrusion is compositionally similar to the olivine rich melagabbro that hosts much of the mineralization at the Eagle Ni-Cu-PGE deposit in the eastern end of the Baraga basin. Exposures upslope from the terrace are of equigranular, locally ophitic textured gabbros of the Middle unit. Unlike the Lower unit, neither olivine nor orthopyroxene appear to be present in the Middle unit. Minor pyrite and chalcopyrite are found as disseminations through out the unit. Hematite locally coats plagioclase giving it a pinkish hue. The contact between the olivine rich Lower unit and the olivine free Middle unit is relatively sharp. It is currently unclear if the change represents closed system fractionation or multiple pulses of different magmas. There is currently no recognized analog for the BIC intrusion Middle or Upper units at Eagle. More detailed descriptions of the units at BIC can be found in the first part of the guide. 196 �Stop 7-6 Upper Unit exposures on the east end of the BIC intrusion. (UTM coordinates 397013E 5174477N) The east end of the BIC intrusion is accessible by a series of logging and drill roads starting off the Silver River road north of the intrusion. The last part of the road to the top of the hill is typically deeply rutted and often not drivable. Walking the last part is recommended. Permission from Kennecott Minerals Company is required before accessing this stop. Glaciated exposures of the medium to coarse-grained oxide gabbro that comprise the Upper unit of the BIC intrusion are present in, and alongside the drill road going up the eastern end of the hill. Exposures of the gabbro near the top of the hill contain football size and shape patches with intense epidote alteration. The boundaries of the intensely altered rock are very sharp. It is currently uncertain if these are intensely altered xenoliths or cross sections of sub-parallel “pipe like” zones of hydrothermal alteration. Stop 7-7 Kennecott Minerals Company core shed. The Kennecott core shed is located 2.6 miles east of the town of Negaunee. Turn north off of US Highway 41 at the blue TV 6 building (across from the Michigan Police post) on to the old airport road. Follow the road around the curve to the west and proceed through the gate. The core buildings are the long sheds on the south side of the road just past the gate. Core from the BIC and Little BIC intrusion will available for viewing and discussion. 197 �198 �Figure 3) Geology map of the BIC intrusion showing the location of field trip stops 7-5 and 7-6. Figure 4) BIC intrusion cross-section A to A’ 199 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2008 Description An account of the resource Institute on Lake Superior Geology. Marquette, Michigan. May 6-10, 2008. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2008 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/049e0430282e1e4f5060cb8ecfd9dcd4.pdf 9b5e2fadcebef50097dce270ceefa13e PDF Text Text INSTITUTE ON LAKE SUPERIOR GEOLOGY 55TH ANNUAL MEETING MAY 5-10, 2009 ELY, MINNESOTA ORGANIZED BY: THE PRECAMBRIAN RESEARCH CENTER UNIVERSITY OF MINNESOTA DULUTH JAMES D. MILLER, JR., GEORGE J. HUDAK, AND DEAN M. PETERSON Co-Chairs Proceedings Volume 55 Part 1 – Program and Abstracts Edited by George J. Hudak, University of Wisconsin-Oshkosh and Precambrian Research Center, University of Minnesota Duluth Cover Photos: Various photos from Precambrian Research Center field areas over the past two years. Top row from left to right: Sunset over Ima Lake (BWCA), mapping anorthositic troctolite east of Jordan Lake (BWCA), moonrise over Ima Lake (BWCA). Center Row, left to right: Island within Twin Lakes (Superior National Forest), mapping north of Ima Lake. Bottom Row, left to right: Neoarchean pillow lavas in Ely, Soudan Member of the Ely Greenstone Formation east of Soudan Mine, and intermixed coarse-grained troctolite and medium-grained ophitic anorthositic olivine gabbro southwest of Alworth Lake (BWCA). Photos courtesy of George Hudak, Jim Miller, Dean Peterson and Eric Stifter. �55TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 55 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE TRIP 3: TOUR OF THE SOUDAN IRON MINE AND PHYSICS LAB TRIP 4: PIONEER MINE CANOE EXCURSION TRIP 5: GEOLOGY AND METAMORPHISM OF THE MESABI RANGE TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT Reference to material in Part 1 should follow the example below: Boerboom, T. J., and Green, J. C., 2009, Bedrock geological map of the Deer Yard Lake and Good Harbor Bay Quadrangles, north shore of Lake Superior, Minnesota [abstract]: Institute on Lake Superior Geology Proceedings, 55th Annual Meeting, Ely, MN, v. 55, part 1, p. 4-5. Published by the 55th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 55 PART 1— PROGRAM AND ABSTRACTS Previous Institutes on Lake Superior Geology, 1955-2009 ......................................................... iv Sam Goldich and the Goldich Medal ................................................................................. vi Past Goldich Medalists and the 2009 Goldich Medal Recipient ..................................... viii Goldich Medal Committee............................................................................................... viii Citation for 2009 Goldich Medal Recipient....................................................................... ix ILSG Student Research Fund............................................................................................. xi Student Paper Awards ....................................................................................................... xii Eisenbrey Student Travel Awards ................................................................................... xiii Report of the Chair of the 54rd Annual Meeting ............................................................. xiv 2009 Board of Directors..................................................................................................................... xvii 2009 Session Chairs ........................................................................................................ xvii 2009 Student Paper Awards Committee ......................................................................... xvii 2009 Local Committees .................................................................................................. xvii 2009 Meeting Sponsors.................................................................................................. xviii 2009 ILSG/Eisenbray Funds ............................................................................................ xix 2009 Banquet Speaker ..................................................................................................... xix Program ............................................................................................................................ xxi Abstracts ........................................................................................................................ xxix iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2009 ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray iv �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 Marquette, Michigan T.J. Bornhorst and J.S. Klasner 55 2009 Ely, Minnesota J.D. Miller, Jr., G.J. Hudak, D.M. Peterson v �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an AB from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the vi �second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1994 Cedric Iverson 1980 not awarded 1995 Gene La Berge 1981 Carl E. Dutton, Jr. 1996 David L. Southwick 1982 Ralph W. Marsden 1997 1983 Burton Boyum 1998 Zell Peterman 1984 Richard W. Ojakangas 1999 Tsu-Ming Han 1985 Paul K. Sims 2000 John C. Green 1986 G.B. Morey 2001 John S. Klasner 1987 Henry H. Halls 2002 Ernest K. Lehmann 1988 Walter S. White 2003 Klaus J. Schulz 1989 Jorma Kalliokoski 2004 Paul Weiblen 1990 Kenneth C. Card 2005 Mark Smyk 1991 William Hinze 2006 Michael G. Mudrey 1992 William F. Cannon 1993 Donald W. Davis 2007 Joseph Mancuso 2008 Theodore J. Bornhorst Ronald P. Sage 2009 GOLDICH MEDAL RECIPIENT L. Gordon Medaris, Jr. University of Wisconsin Madison, Wisconsin GOLDICH MEDAL COMMITTEE Serving for the meeting year shown in parentheses Richard Ojakangas (2006-2009) Terry Boerboom (2007-2010) Allan MacTavish (2008-2011) Academic representative Government representative Industry representative viii �CITATION FOR GOLDICH MEDAL RECIPIENT L. Gordon Medaris, Jr. - 2009 Goldich Medal Recipient Gordon Medaris’s many and diverse contributions to the geology of the Lake Superior region, as well as his long-continued participation in ILSG over the past five decades, are appropriately recognized by the Goldich Medal. Medaris’s broad interests make him difficult to pigeonhole. He is best known internationally as an igneous and metamorphic petrologist who has emphasized the study of eclogites and orogenic peridotites of the North American Cordillera, the Caledonides of Scandinavia, the Variscides of the central European Bohemian Massif and the Variscides of the southern Carpathians. He has also studied mantle xenoliths from California, central Europe and the Middle East. Gordon’s European contributions have been recognized with two awards from Charles University of Prague, the Gold Medal of Science in 1998 and the Boricky Medal in 2006. It is fair to say that Gordon has developed a Bohemian love affair. Better known to us is Gordon’s research in the Lake Superior region that began in the 1970s with a ground breaking study of the Wolf River batholith of east-central Wisconsin, which is part of a continental-scale Geon 14 magmatic event. That work was initiated in collaboration with Randy Van Schmus and Phillip Banks and was continued and expanded by his student J. Lawford Anderson. Next, Medaris studied with geochemist Robert Cullers the rare earth elements of the Seabrook Lake carbonatite and cogenetic alkaline rocks. By the 1980s, we find Gordon, Van Schmus, and student Randy Maass publishing syntheses of Penokean deformation and metamorphism across Wisconsin and adjacent areas. In 1983, Gordon was the principal convener and editor for an international symposium on Proterozoic Geology, which resulted in two GSA Memoirs, Number 160 being The Early Proterozoic Geology of the Lake Superior Region. Gordon, Dave Moecher, and others then studied the metamorphic conditions of Sam Goldich’s favorite high-grade gneisses in the Minnesota River Valley. Since retiring in 1998, Medaris has redoubled his research efforts in the Lake Superior region with collaborations that culminated in 2003 in the benchmark Journal of Geology article about the age, composition, and metamorphism of Baraboo Interval rocks and their tectonic significance. Gordon’s discovery of a paleosol beneath the Baraboo Quartzite and the previous recognition of paleosols beneath the Barron and Sioux Quartzites have important paleoclimatic implications, which he has discussed. He also has helped archaeologists to resolve pipestone artifact provenance by characterizing two distinct mineral assemblages in pipestone quarries -hematite-quartz-kaolinite in the Barron and hematite-muscovite-pyrophyllite-diaspore in the Sioux and Baraboo Quartzites. Six journal articles have appeared from these Baraboo Interval investigations and we are still counting. ix �Besides the full-length publications alluded to above, Gordon has contributed talks at no less than 21 ILSG meetings beginning in 1973. He also has been a major organizer of three different ILSG field trip guidebooks (1973, 1986, and 2001) and all of us have seen him on many other ILSG field trips. As many of you know, Gordon is a superb field and laboratory petrologist and mineralogist. Like Sam Goldich, he is gifted with the vision to spot a significant problem, to work out a research strategy, and to pursue it by whatever techniques are needed to answer critical questions. Gordon likes collaborative research, so does not hesitate to recruit colleagues from any specialty to work with him. All of us who have had the privilege to work with Gordon appreciate his vision and encouragement in these joint efforts. He never tries to dominate and is quick to give encouragement and credit, all with a wonderful quiet dignity. I know that I speak for many other co-workers in thanking Gordon for sharing the pleasure of his collaborations. For his many, varied and fundamental contributions to our knowledge of Lake Superior Geology and for his stimulation of the efforts of others, Randy Van Schmus, Daniel Holm, and Brad Singer join me in presenting L. Gordon Medaris, Jr. as the 2009 recipient of the Goldich Medal. R. H. Dott, Jr. April, 2009 x �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • The proposal application should be at least 500 words, and should have a statement of the research project, background information, a map of the research area, research steps necessary to complete the research, figures (if needed) , references, and a list of research expenses. The proposal should also include a proposed end date for the research. • The proposal will need to be signed by researcher’s supervisor. In 2008 the ILSG Board of Governors awarded four $500 awards from the Student Research Fund. Dan Costello (University of –Minnesota - Duluth) - Geology of the Tuscarora Intrusion, northeastern Minnesota, and its relationship to the Anorthositic Series of the Duluth Complex James Hiller (California State University Chico) - Detailed petrographic analysis of anthraxolite morphology in the Biwabik Iron-Formation, northern Minnesota Angela Hull (Kent State University) – Preliminary results of 40Ar/39Ar thermochronology from the Central Yavapai Province, U. S. Midcontinent Andrew Jansen (University of Wisconsin Oshkosh) - Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall of the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota xi �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: • The contribution must be demonstrably the work of the student. • The student must present the contribution in-person. • The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. • In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. • The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). • The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. • The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. In 2008 the ILSG Student Paper Committee presented three awards from the ILSG Student Paper Fund. Each of the following recipients received a $200 award: Elizabeth Drommerhausen (Minnesota State University) for her poster titled: Properties of fluid involved in formation of natural ores in the Mesabi Iron Range, Minnesota Carissa Isaac (Lakehead University) for her talk titled: Stable isotope geochemistry of the Musselwhite Au Mine, north Ontario: Implications for mineralization Natalie King (Colorado State University) for her talk titled: Using mineralization to evaluate small-scale controls on shale permeability in the Nonesuch Formation xii �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: • The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. • Students who are the senior author on either an oral or poster paper will be given favored consideration. • It is desirable for two or more students to jointly request travel assistance. • In general, priority will be given to those in the Institute region who are farthest away from the meeting location. • Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. • Successful applicants will receive their awards during the meeting. In 2008 the ILSG awarded 11 travel awards from the ILSG Eisenbrey Student Travel Fund. The awards were made to: Terra Anderson – University of Wisconsin Milwaukee Ding Xin – Indiana University Elizabeth Drommerhausen – Minnesota State University Emerald Erickson – University of Minnesota - Duluth Elizabeth Fein – Kent State University Shelby Frost – Winona State University Lynn Galston – University of Wisconsin – Eau Claire Sally Goodman – University of Minnesota - Duluth Susan Karberg – University of Minnesota - Duluth Natalie King – Colorado State University Curtis Williams – Indiana University xiii �REPORT OF THE CHAIRS OF THE 54TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY MARQUETTE, MICHIGAN The A.E. Seaman Mineral Museum of Michigan Technological University hosted the 54th Annual Institute on Lake Superior Geology on May 6 – 10, 2008 at the Ramada Inn in Marquette, Michigan. The Marquette/Ishpeming area has been the site of the ILSG annual meeting a total of 5 times out of 54. There were a total of 230 registrants for the meeting. Of these 37 were students and 215 pre-registered. The number of registrants for the meeting exceeded expectations. The Proceedings Volume 54 was published in two parts. Part 1 – Program and Abstracts, edited by Theodore J. Bornhorst and George W. Robinson. Part 1 contains 45 published abstracts for 29 oral and 16 poster presentations. The cover of Part 1 was highlighted by photos of Lake Superior minerals in the collection of the A.E. Seaman Mineral Museum and was designed by George Robinson. Part 2 – Field Trip Guidebook was edited by Theodore J. Bornhorst and John S. Klasner. Part 2 contains guides of seven field trips. The cover of Part 2 was highlighted by a photo of brecciated banded iron formation from Ishpeming, MI similar to one in Van Hise, Bayley and Smyth, 1897 and was provided by Tom Waggoner. Field trips were a dominant part of the 54th ILSG, consistent with ILSG tradition. There were four pre-meeting, one “syn-meeting” and three post-meeting trips. Participation in the field trips was excellent. Most were full to capacity and one even had names on a waiting list. This first of the pre-meeting trips, was a two day trip on Tuesday and Wednesday, May 6 and 7. Tom Waggoner led this trip of 27 participants to examine banded iron formation of the Marquette District. In addition to the printed field trip guide in the Proceedings Volume, all registrants received supplemental material related to this field trip on a CD and a colored map of the Marquette District. Cliffs Mining Services Company is thanked for providing financial support to distribute colored maps to all registrants and for access to the operating iron mines. On Wednesday, May 7, there were three concurrent one day field trips. Bill Cannon and Klaus Schulz lead a trip to inspect the Archean-Paleoproterozoic unconformity at Silver Lake and possible seismites from the Sudbury Impact. This was a once in a lifetime opportunity since beginning in 1910 these outcrops have been under the waters of Silver Lake, a natural body of water enhanced for hydroelectric generation. In May 2003 an earthen dam failed and exposed the outcrops making the trip possible for the 25 participants. The reconstruction of the dam will likely be completed in a few years, so once again the outcrops will be underwater. Tom Quigley and Bob Mahin of Aquila Resources Inc. led a field trip focused on the geology of the Back Forty project south of Marquette in Menominee County for 43 participants. Aquila Resources Inc. is thanked for providing this field trip for ILSG and for financial support for color printing of the Field Trip Guidebook. Andrew Ware, Jon Cherry, and Xin Ding led a field trip that concentrated on the geology of the Eagle Project. Kennecott Minerals Company is thanked for generously providing this field trip not just once before the technical sessions, but twice, before and after the technical sessions for a total of 87 participants. Kennecott also provided financial support for color printing of the Field Trip Guidebook. The Sudbury impact layer is a topic of high interest. Since a single locality with outcrops of this layer was available near Marquette, the xiv �organizers sought to have an abbreviated “syn-meeting” field trip. Bill Cannon, graciously agree to lead yet another field trip for the 54th ILSG. Thus, immediately following the technical sessions on Friday May 9 there was a 3 hour field trip to the McClure locality to examine the Sudbury impact layer. This trip was so popular with registrants, that in addition to the 52 participants, there was a waiting list and multiple people were not able to participate. There were three post-meeting field trips on Saturday May 10. One was a repeat of the pre-meeting Eagle trip. Glenn Scott, Helen Lukey, Al Strandlie and CCI/CCMO staff led a trip focused on sustainable recovery of iron from the Marquette District. This environmentally oriented trip had 13 participants. A color version of the printed field trip guide was provided to all participants on CD. Cliffs Mining Services Company is thanked for providing this field trip for the 54th ILSG. Dean Rossell led a trip to study the geology of the Keweenawan BIC intrusion. This trip, like the other trips, was well attended with 38 participants. Kennecott Minerals Company is thanked for making this trip possible for the 54th ILSG participants. The two days of technical sessions were held at the Ramada Inn of Marquette. The eight session chairs helped keep the presentations on track. There were the normal technical glitches. The student paper committee once again had a difficult job of selecting among 18 student oral and poster presentations. The committee awarded three Best Student Paper awards with a cash prize of $200 each: Elizabeth Drommerhausen (Minnesota State University) for her poster presentation titled: Properties of fluid involved in formation of natural ore in the Mesabi Iron Range, Minnesota, Carissa Isaac (Lakehead University) for her oral presentation titled: Stable isotope geochemistry of the Musselwhite Au Mine, north Ontario: Implications for mineralization, and Natalie King (Colorado State University) for her oral presentation titled: Using mineralization to evaluate small-scale controls on shale permeability in the Nonesuch Formation. One hundred and sixty-three participants attended the banquet on Thursday night. The 2008 banquet speaker was Jon Cherry, General Manager of Kennecott Minerals – Eagle Project. Jon brought participants up-to-date on the Kennecott Eagle Project with a well received Powerpoint presentation. A highlight of the banquet for me (Ted Bornhorst, C0-chair) was the presentation of the 2008 Goldich medal. Ted Bornhorst, Michigan Technological University was presented the medal by Jim Miller. Jim cited Ted for his contributions to Lake Superior geology and his service to ILSG. The Institute’s Board of Directors met on May 8, 2008. A brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 53th ILSG from Laurel Woodruff and Jim Miller. 2. Accepted the minutes of last Board meeting from ILSG secretary Pete Hollings. 3. Accepted the 2007-2008 ILSG Financial Summary from ILSG treasurer, Mark Jirsa 4. Accepted the motion to reappoint Pete Hollings as Secretary of the ILSG, and Mark Jirsa as the Treasurer of the ILSG. In keeping the ILSG constitution, the motion by the board to reappoint Hollings and Jirsa was brought forward to the membership of ILSG at the annual banquet. The membership passed the motion unanimously. 5. Ted Bornhorst, agreed to serve as on-going ILSG Board Member. xv �6. Nominated Al MacTavish of Magma Metals (Canada) Ltd. to replace Doug Duskin as the “industry member” on the Goldich Committee. The Board approved MacTavish as the new Goldich Committee member with a term of 3 years. 7. Received from Mike Mudrey a progress report on scanning initiative. 8. Discussed changes as proposed by Chair Bornhorst to the membership criteria as posted on the web site. A previous Board made email as the only contact to determine membership in ILSG. The proposed revisions returned the criteria to postal address and made “Member for Life” status truly member for life. Motion to accept the changes proposed by Bornhorst by Jim Miller, second by Mark Smyk, passed unanimously. 9. Approved Ely, Minnesota at the site for the 55th annual ILSG meeting with co-chairs Jim Miller, George Hudak, and Dean Peterson of the Precambrian Research Center. 10. The Board once again discussed special awards for contributions to ILSG. The Board agreed that the only award from ILSG will be the Goldich Award. Chairs of individual meetings can consider special awards or recognition of individuals, but only with prior consent of the Board. The Co-chairs, Ted and John, thank the participants, field trip leaders, and presenters for without you there would be no ILSG. And we also thank others, not already cited above, who played a role in this years meeting: Gretchen Klasner was invaluable to the success of the meeting as she did all of the on-site registration. The staff of Ramada Inn was professional and did an excellent job of responding to last minute requests. Undergraduate geo majors from Michigan Tech were drivers of vans for the field trips: Carla Alonso, Austin Andres, James Julip, Matt Laird, Eric Murray. Darlene Comfort, A.E. Seaman Mineral Museum, did a great job of keeping track of all of the registration details. This was a major effort for her! John and I hope that you agree that the 54th ILSG was a real success. Attendance in general and for the field trips was above our initial expectations. We are gratified that for all of the positive comments provided by many of you, thanks as it does make a difference. We are both happy to have the 54th annual ILSG in our past. Yes, being Chair of the annual meeting is a lot of work and added stress. But, it is worth it and we encourage others to try it out!! ILSG is a great professional organization with a long and rich history. We look forward to seeing you at the 55th ILSG and many more. Ted Bornhorst and John Klasner Co-Chairs, 54th Institute on Lake Superior Geology xvi �2009 BOARD OF DIRECTORS Board appointment continues through the close of the last meeting year, or until a successor is selected Jim Miller, Co-Chair, 55th Meeting (2010; joined board after 2007 meeting) University of Minnesota Duluth, MN / PRC, University of Minnesota Duluth, Duluth MN George J. Hudak, Co-Chair 55th Meeting (will continue on board until 2012) Univ. of Wisconsin Oshkosh / PRC, University of Minnesota Duluth, MN Dean M. Peterson, Co-Chair, 55th Meeting Duluth Metals Limited/ PRC, University of Minnesota Duluth, MN Theodore J. Bornhorst (2011) Michigan Technological University, MI Ann Wilson (2009) Ontario Geological Survey, South Porcupine, ON Peter Hollings – Secretary (2011) Lakehead University, Thunder Bay, ON Mark A. Jirsa – Treasurer (2011) Minnesota Geological Survey, St. Paul, MN 2009 SESSION CHAIRS Meghan Blair, Barr Engineering, Duluth, MN Dyanna Czeck, University of Wisconsin - Milwaukee Dave Dahl, Minnesota Department of Natural Resources, Hibbing, MN Dan England, Eveleth Fee Office, Inc., Eveleth, MN Mary Louise Hill, Lakehead University, Thunder Bay, ON Phillip Larson, Cliffs Natural Resources, Eveleth, MN Allan MacTavish, Magma Metals (Canada) Ltd., Thunder Bay, ON Greg Stott, Ontario Geological Survey, Sudbury, ON 2009 STUDENT PAPER AWARDS COMMITTEE Thomas Fitz (Chair), Northland College, Ashland, WI Dorothy Campbell, Ontario Geological Survey, Thunder Bay, ON John Gartner, Prime Meridian Resources Corp., Iron River, MI 2009 LOCAL COMMITTEES General Meeting Planning and Promotion James D. Miller, Jr., University of Minnesota Duluth Program and Abstracts Editor and Student Awards George Hudak, University of Wisconsin Oshkosh Field Trip Guidebook Editor Dean M. Peterson, Duluth Metals Limited Registration Julie Ann Heinz – Natural Resources Research Institute xvii �2009 MEETING SPONSORS The organizers wish to acknowledge and thank several companies and organizations who have contributed financial support to various components the meeting. Welcoming Reception Sponsor Guidebook Sponsor Student Sponsors American Institute of Professional Geologists – Minnesota Chapter Brooke Fahrendrog Angela Hull Kevin Kane Aaron Rowland Jeff Bruesewitz Kyle Makovsky University of Wisconsin - Eau Claire Kent State University Grand Valley State University University of Wisconsin - Eau Claire University of Wisconsin - Eau Claire Minnesota State University Minnesota Mineral Resource Education Foundation Kevin Kane Aaron Rowland Andrew Jansen Ryan Dayton Shelby Frost Tom Johnson Grand Valley State University University of Wisconsin - Eau Claire University of Wisconsin - Oshkosh University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth Northland Securities Brooke Fahrendrog Andrew Jansen Jessica Gary Steve Hoaglund Levi Markwood University of Wisconsin - Eau Claire University of Wisconsin - Oshkosh University of Minnesota Duluth University of Minnesota Duluth Slippery Rock University  Mesabi Range Geological Society Ryan Dayton Shelby Frost Tom Johnson Jessica Gary Steve Hoaglund Dan Costello Cara Leitheiser University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth University of Minnesota Duluth  xviii �2009 ILSG/EISENBRAY FUNDS Part of the registration costs for the 2009 meeting went toward establishing a fund to provide travel support for students. Eisenbray funds, totalling $1000, will be specifically distributed to the four recipients of 2009 ILSG Research Grants (* below). Another $1900 will be distributed to the other students listed below. Dan Costello* Michael DeAngelis Adam Fage Nathan Forslund Benedek Gal James Hiller* Angela Hull* Andrew Jansen* Maura Kolb Cara Leitheiser Natalie Pietrzak Victoria Stinson University of Minnesota Duluth University of Tennesse - Knoxville Lakehead University Lakehead University Eotvos Lorand University California State University Chico Kent State University University of Wisconsin -Oshkosh Lakehead University University of Minnesota Duluth University of Western Ontario Lakehead University 2009 BANQUET SPEAKER The Deep Underground Sky Dr. Marvin Marshak Professor of Physics/Director of Undergraduate Research at the University of Minnesota Founder of the University of Minnesota Underground Research Laboratory at the Soudan Mine xix �xx �PROGRAM xxi �TUESDAY MAY 5, 2009 8:00 a.m. FIELD TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX Rich Patelke – Polymet Mining Mark Severson – Natural Resources Research Institute, Univ. of Minnesota Duluth Dean Peterson – Duluth Metals Ltd. Tim Jefferson – Teck American Ernie Lehmann – Franconia Minerals 6:00 p.m. RETURN OF FIELD TRIP 1 TO GRAND ELY LODGE WEDNESDAY MAY 6, 2009 8:00 a.m. FIELD TRIP 1 (CONTINUED): CU-NI DEPOSITS OF THE DULUTH COMPLEX 8:00 a.m. FIELD TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE Phil Larson – Cliffs Natural Resources Howard Mooers – Department of Geological Sciences, University of Minnesota Duluth 6:00 p.m. RETURN OF FIELD TRIPS 1 AND 2 TO GRAND ELY LODGE 4:00 p.m. - 10:00 p.m. REGISTRATION AT GRAND ELY LODGE 7:00 p.m. - 10:00 p.m. ICE BREAKER AND POSTER SESSION THURSDAY MAY 7, 2009 8:00 a.m. - 12:00 noon REGISTRATION 8:45 a.m. INTRODUCTORY REMARKS Jim Miller, George Hudak, and Dean Peterson, 2009 ILSG Co-Chairs 8:55 a.m. REMEMBERANCE OF JOE MANCUSO (1934-2009) xxii �TECHNICAL SESSION I Session Chairs: Mary Louise Hill, Lakehead University Phillip Larson, Cliffs Natural Resources 9:00 a.m. Medaris, L. G., Jr., Jicha, B. S., Dott, R. H. Jr., and Singer, B. S. A 1465 Ma 40Ar/39Ar age for the Seeley Slate: implications for metamorphism and deformation in the Baraboo Range, WI 9:20 a.m. Addison, W. D., Brumpton, G. R., Fralick, P. W., and Kissin, S. A. The complex Gunflint-Rove Formations boundary at Thunder Bay, Ontario: Two disconformities and a base surge debrisite 9:40 a.m. Cannon, W. F., and Schulz, K. J. Reconstructing the Penokean Foreland Basin using the Timeline of the 1850 Ma Sudbury Impact Layer 10:00 a.m. COFFEE BREAK AND POSTER SESSION 10:40 a.m. Peitrzak, N. J.*, Duke, N., Scott, G., and Lukey, H. Ore Textures and Mineral Chemistry within the Oxide-Carbonate-Silicate Flotation Ores at the Cliffs Natural Resources’ Tilden Mine, Michigan 11:00 a.m. Walsh, James F. Hydrostratigraphy of the Biwabik Iron Formation – Implications for Current Groundwater Flow Patterns and Past Genesis of Natural Ore Bodies 11:20 a.m. Lunch Break (2009 ILSG Board Meeting - by invitation) _____________________________ TECHNICAL SESSION II Session Chairs: Dyanna Czeck, University of Wisconsin Milwaukee Greg Stott, Ontario Geological Survey 1:00 p.m. Gilbert, H. P. Bird River Belt in Southeastern Manitoba – A Nearchean Volcanic Arc in the Western Superior Province 1:20 p.m. Forslund, N. R.*, Hill, M. L., and Middleton, R. S. Alteration in the Southern Felsic Volcanics at Marshall Lake, Northwestern Ontario xxiii �1:40 p.m. Jirsa, M. A., and Driese, S. G. Neoarchean Weathering and Atmospheric pO2 Inferred from Paleosaprolite between Granite-Greenstone and Superjacent Conglomerate in the Boundary Waters Canoe Area, NE Minnesota 2:00 p.m. COFFEE BREAK AND POSTER SESSION 2:40 p.m. Stinson, V. R.*, Kolb, M. J.*, and Hill, M. L. Metamorphism and Deformation at Musselwhite Mine 3:00 p.m. Wendland, C., Fralick, P., and Hollings, P. Diamondiferous Mass-Flow and Placer Deposits Forming a Neoarchean Fan Delta, Wawa Area, Superior Province 3:20 p.m. Mudrey, M. G. Goldich Award Winners Who Have Passed On _____________________________ 6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION • Announcement of 56th Annual Meeting Location • 2009 Goldich Award Presentation to L. Gordon Medaris, Jr. • 2009 Banquet Address by Dr. Marvin Marshak, University of Minnesota All registered participants are welcome to the banquet address FRIDAY MAY 8, 2009 8:30 a.m. INTRODUCTORY REMARKS Jim Miller, George Hudak, and Dean Peterson, Co-Chairs, 2008 ILSG TECHNICAL SESSION III Session Chairs: Allan MacTavish, Magma Metals (Canada) Limited Dave Dahl, Minnesota Department of Natural Resources 8:40 a.m. Hansen, E., Reimink, J., and Harlov, D. Titanite, Pseudorutile, and REE-Minerals in the Allouez Conglomerate, Keweenaw Peninsula, Michigan 9:00 a.m. Hollings, P., Smyk, M. C., Halls, H., and Heaman, L. Mesoproterozoic Midcontinent Rift-Related mafic intrusions in Northwestern Ontario: continuing geochemical, paleomagnetic, petrographic, and geochronological studies xxiv �9:20 a.m. Chandler, V. W. Magnetic Anomalies from Pleistocene Sources in the Western Lake Superior Region: The Edge of Insanity or a Promising Threshold? 9:40 a.m. Verburg, R., and Dunlavy, P. Mine Water Quality Prediction and Environmentally-Responsible Mining -Yes We Can! 10:00 a.m. COFFEE BREAK AND LAST POSTER SESSION 10:40 a.m. Schulz, K. J., and Nicholson, S. W. Geochemistry of Midcontinent Rift-related Dikes and Mafic-Ultramafic Intrusions in the Baraga Basin, northern Michigan: Implications for the Nature of Rift Magmatism and Ni-Cu-PGE Mineralization 11:00 a.m. Watkins, K. P. Magma Conduit Hosted Platinum-Palladium-Copper-Nickel Mineralization at the Thunder Bay North Project, Northwest Ontario: Discovery, Exploration, Geology, and Resource Potential 11:20 a.m. Gál, B.*, Peterson, D. M., ad Molnár, F. Magmatic vs. Hydrothermal Processes in the South Filson Creek Mineralization, South Kawishiwi Intrusion, Duluth Complex 11:40 a.m. Peterson, D. M. The Nokomis Cu-Ni-PGE Deposit, Duluth Complex, Minnesota 12:00 p.m. LUNCH BREAK AND MEETING OF THE STUDENT PAPER COMMITTEE _____________________________ TECHNICAL SESSION IV Session Chairs: Mehgan Blair, Barr Engineering Co. Dan England, Eveleth Fee Office, Inc. 1:00 p.m. PRESENTATION OF THE STUDENT TRAVEL AND BEST PAPER AWARDS 1:20 p.m. Thorleifson, H. Options for Geologic Sequestration of Carbon in the Upper Midwest: Mineral Carbonation and Deep Injection 1:40 p.m. Arends, H., Johnson, R., Hanson, K., Friedrich, H., and Kostka, S. Structuring, Gathering, and Distributing Geological Data for Public Use 2:00 p.m. Miller, J. D, Carranza-Torres, C., Davis, R., and Hendrickson, D. New Educational Initiatives at the University of Minnesota Duluth: Preparing Students for Future Jobs in the Mining and Minerals Exploration Industries xxv �2:20 p.m COMPLETION OF 2009 ILSG TECHNICAL SESSIONS AND FINAL STATEMENTS BY THE 2009 ILSG CO-CHAIRS _____________________________ 3:00 p.m. FIELD TRIP 3: TOUR OF THE SOUDAN IRON MINE AND PHYSICS LAB Dean Peterson – Duluth Metals Ltd. James Pointer – Minnesota Department of Natural Resources, Parks and Recreation Marvin Marshak – Department of Physics, University of Minnesota 3:00 p.m. FIELD TRIP 4: PIONEER MINE CANOE EXCURSION Mark Jirsa – Minnesota Geological Survey 6:30 p.m. RETURN OF FIELD TRIPS 3 AND 4 TO GRAND ELY LODGE SATURDAY MAY 9, 2009 8:00 a.m. FIELD TRIP 5: GEOLOGY AND METAMORPHISM OF THE EASTERN MESABI RANGE Dick Ojakangas – Department of Geological Sciences, Univ. of Minnesota Duluth Mark Severson – Natural Resources Research Institute, Univ. of Minnesota Duluth Doug Halverson, Jeff Bird, Tom Campbell, Jarred Lubben, and Peter Jongewaard – Cliffs Natural Resources William Everett – Mesabi Nugget 8:00 a.m. FIELD TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE Jim Miller – Department of Geological Sciences, University of Minnesota Duluth 8:00 a.m. FIELD TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT Dean Peterson – Duluth Metals Ltd. Mark Jirsa – Minnesota Geological Survey George Hudak – Department of Geology, University of Wisconsin Oshkosh 6:00 p.m. RETURN OF TRIPS 5 AND 7 TO GRAND ELY LODGE SUNDAY, MAY 10, 2009 8:00 a.m. FIELD TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE (CONTINUED) 8:00 a.m. FIELD TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT (CONTINUED) 4:00 p.m. RETURN OF TRIPS 6 AND 7 TO GRAND ELY LODGE _____________________________________________ 55TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY ENDS xxvi �POSTER PRESENTATIONS Boerboom, T. J. and Green, J. C. Bedrock Geologic Map of the Deer Yard Lake and Good Harbor Bay Quadrangles, North Shore of Lake Superior, Minnesota Bruesewitz, J.* and Cameron, B. Geochemical and New SHRIMP-RG Zircon Age Constraints of the Cary Mound Granite, Wood County, Wisconsin Coleman, J., and Chiriboga, E. Methods for Estimation of Indirect Hydrologic Impacts on Wetland Plant Communities at Potential Hard Rock Mine Sites Costello, D. E.*, Miller, J. D. Jr., and Jirsa, M.A. Geology of the Tuscarora Intrusion, Northeastern Minnesota and its Relationship to the Anorthositic Series of the Duluth Complex Dayton, R. N.*, Miller, J. D. Jr., and Vervoort, J. D. Quantifying Assimilation vs. Fractional Crystallization using Sm-Nd, Lu-Hf and Pb Isotope Systems: The Geochemical Evolution of the Sonju Lake Intrusion, Finland, MN Diedrich, T., Brecke, D., Schreiber, M., and Zanko, L. Taconite-Derived Mineral Dust in Population Centers on the Mesabi Iron Range: Tracking Mineral Fibers from Ore to Air Fage, A.* and Hollings, P. Geology and Geochemistry of the Fearless-Python Property, Schreiber-Hemlo Greenstone Belt, Ontario Frey, B. A. Vermilion Greenstone Gold – New Data, Northeastern Minnesota Gary, J. L.*, Wattrus, N. J., Colman, S. M., and Voytek, E. B. Characterizing the Discharge Features of Glacial Lake Agassiz During the Post-Marquette Period Using Marine Seismic-Reflection Methods Gere, M. A., and Hoane, T. B. 2009 Update: Leasing State of Michigan Lands for Metallic and Nonmetallic Minerals Hage, M. M.*, and Fedo, C. M. Geochemistry and Petrology of Gunflint Iron Formation, Gunflint Trail, Minnesota Hauck, S. A., Heine, J. J., and Thorleifson, L. H. A Follow-up Glacial Till Indicator Mineral Survey in Minnesota: What Does It Indicate About Exploration for Diamonds And Other Mineral Deposits xxvii �Hefferan, K. P., and Heywood, N. C. Developing a 21st Century Geoscience Major: Melding the Old with the New Hiller, J. A.* and Shapiro, R. S. Detailed Petrographic Analysis of Anthraxolite Morphology in the Biwabik IronFormation, Northern Minnesota Hull, A.*, Holm, D., and Schneider, D. Preliminary Results of 40Ar/39Ar Thermochronology from the Central Yavapai Province, U. S. Midcontinent Jansen, A.C.*, Hudak, G. J., Heine, J. J., and Peterson, D. M. Lithogeochemical Evaluation of Neoarchean Mafic Volcanic Rocks Comprising the Footwall of the Soudan Member of the Ely Greenstone Formation, Northeastern Minnesota Jirsa, M., Cowan, H.*, Kowalik, J.*, and Niedermiller, J.* Geologic Mapping of Neoarchean Rocks Near Paulsen Lake, Boundary Waters Canoe Area Wilderness, by Students of the Precambrian Research Center’s 2008 Field Camp Johnson, T. K.*, Hansen, V. L., Hudak, G. J., and Peterson, D. M. Structural, Kinematic, and Lithogeochemical Investigation of the Murray Shear Zone, Northeast Minnesota Makovsky,K.*, and Losh, S. Fluid Movement through the Mesabi Iron Range, Minnesota Markwood, L. W.*, and Zieg, M. J. Interpretations of the Emplacement and Cooling History of a Thin Diabase Sill, Nipigon, Ontario Medaris, L. G. Jr., and Fournelle, J. H. Metamorphic Pseudorutile in the Seeley Slate, Baraboo Range, Wisconsin Meineke, D. G., and Djerlev, H. Geology and Magnetic Taconite Resources of Western Gogebic Iron Range, Wisconsin Shapiro, R.S. Alteration of Stromatolite Biosignatures in the Biwabik Iron-formation: Relevance to Astrobiololgy Stifter, E.*, Wartman, J.*, Gibbons, J.*, Kane, K.*, Murphy, L.*, Carlson, A.*, Mason, T.*, Hudak, G. and Peterson, D. Bedrock Geologic Map of the Disappointment and Ima Lakes Area, Lake County, Northeastern Minnesota xxviii �ABSTRACTS xxix �xxx �THE COMPLEX GUNFLINT-ROVE FORMATIONS BOUNDARY AT THUNDER BAY, ONTARIO: TWO DISCONFORMITIES AND A BASE SURGE DEBRISITE William D. Addison, Gregory R. Brumpton,2 Philip W. Fralick,3 Stephen A. Kissin3 1 R.R. 2, Kakabeka Falls, P0T 1W0, Canada (baddison@tbaytel.net). 2 211 Henry St., Thunder Bay, P7E 4Y7, Canada. 3Department of Geology, Lakehead University, Thunder Bay, P7B 5E1, Canada. Eight subaerially exposed chaotic debrisites containing ejecta from the 1850 Ma (Krogh et al., 1984) Sudbury impact event have been discovered in and near the City of Thunder Bay, Ontario. Ejecta features include planar deformation features (PDF) in quartz grains, vesicular devitrified glass clasts (DVG) and accretionary lapilli (Addison et al., 2005). Megascopic to microscopic Gunflint breccia clasts and ejecta are minor components embedded in an often recrystallized dominantly carbonate matrix. Seven sites are erosionally truncated. Only the Terry Fox site shows a complete profile extending from the Gunflint Formation up through the debrisite and into the overlying Rove Formation. The study area has had a complex history, summarized as follows. The Upper Gunflint Formation exhibits an ocean regression assemblage, terminating in stromatolites at most study localities, disconformably overlain by the debrisites. We postulate that the regression was completed at some unknown time before the 1850 Ma Sudbury impact event, leaving a subaerially exposed carbonate landscape. A sporadic, 0.3-1.2 m thick, iron-rich alteration profile found 0-1 m below the debrisite base at most sites may be evidence of a paleosol. If further work supports this hypothesis, it would confirm that the study area was subaerial prior to impact. Approximately two minutes after the Sudbury impact began, violent earthquakes fractured and delaminated lithified portions of the Upper Gunflint Formation, as evidenced by still in situ fractured rock at the Hwy 588 and GTP sites and by numerous mostly sharply angular sub-millimeter to meter size Gunflint clasts within the debrisite. The earthquakes were followed by massive base surges which stripped all unlithified material down to bedrock and ripped up and entrained most of the earthquake fractured Upper Gunflint Formation rock. The base surges then contained the following mixture of features in order of volume percent: 1) clasts of fractured carbonate in the silt to coarse sand size range; 2) ripped up clasts of Gunflint fractured chert, chert-carbonate and stromatolites; 3) ejecta consisting primarily of DVG, and much lesser volumes of accretionary lapilli, tektites and microtektites, quartz and feldspar grains, some of which show planar features and PDFs; and 4) small clasts of uncertain origin. The travel distance for most Gunflint chert and chert-carbonate clasts was relatively short as most are very angular. Slightly rounded chert-carbonate clasts are less common and probably travelled only slightly further from their source than the angular ones. No clasts show weathering rinds. The base surges contained sufficient water vapor that accretionary lapilli were able to form. Some accretionary lapilli passed through zones with varying water vapor concentrations allowing them to accumulate alternating coarser-grained layers and finergrained layers. 1 �The debrisites deposited by these base surges are chaotic and show significant changes in clast sizes and composition over meter scale and even centimeter scale distance within the deposits. The one exception to this chaos is a clear upward fining of Gunflint clasts within the debrisite, which can probably be attributed to the limited lifting and transporting power of a gas-supported fluidized flow like a base surge. These observations are consistent with base surge deposits described from the Chicxulub, Houghton and Ries impact events. The DVG shows varying degree of vesicle collapse ranging from none (round vesicles) to partial collapse (ovoid vesicles) to totally flattened vesicles. This suggests that DVG clasts arrived at varying temperatures and plasticities and had time to deform before cooling was complete. Had DVG landed in water, quenching would have been nearly instantaneous and all vesicles would have been round. We initially interpreted these debrisites as tsunami deposits. If tsunamis were ever involved, we would expect some sorting of DVG, with more of the least dense clasts (most vesicles) being deposited towards the top of the debrisites and with denser DVG clasts (few or no vesicles) being more common towards the base of the debrisite. That is not the case. The debrisites were then subaerially exposed for 15-18 Ma (Addison et al, 2005). It is improbable that an unlithified deposit could survive exposed for this long. The underlying Upper Gunflint Formation and overlying Rove Formation hint at a possible preservation mechanism. Volcanic tuffs are common in both the Upper Gunflint Formation and especially in the lower Rove Formation where there are seven tuff layers per meter on top of the debrisite. If similar tuffs were deposited on top of the debrisite during the period of subaerial exposure they may have provided sufficient protection to allow survival of some of the debrisite. Anastomosing chert and agate within the debrisite and centimeter-scale agate stalactites in debrisite vugs suggest that silica was leached from such tuffs and was redeposited within the debrisite until ocean transgression and deposition of the Rove Formation protected it until today. Subsequently ocean transgression deposited about one meter of carbonate before deposition of the lower Rove Formation organic-rich muds and interlayered volcanic ash. This abrupt transgression marks a disconformity at the debrisite-Rove Formation boundary. Large scale carbonate replacement and recrystallization during diagenesis destroyed or partially obscured many ejecta and non-ejecta features in the debrisite. Silica replacement did the same to a lesser extent. These observations and interpretations add heretofore unknown detail to what happened at the Gunflint-Rove boundary. Will similar sequences be discovered elsewhere in the Lake Superior region? References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Krogh, T. E., Davis, D. W., and Corfu, F., 1984, Precise U-Pb zircon and baddeleyite ages for the Sudbury area in Pye et al.., eds., The Geology and Ore Deposits of the Sudbury Structure: Ontario Geological Survey Special Volume 1, p. 431-446. 2 �STRUCTURING, GATHERING AND DISTRIBUTING GEOLOGIC DATA FOR PUBLIC USE Heather Arends, Minnesota Department of Natural Resources, Aggregate Resource Mapping Program, St. Paul MN, USA. E-mail: Heather.Arends@dnr.state.mn.us, Tel: 1 651 259-5376; Fax: 1 651 259-5939 Renee Johnson, Kevin Hanson, Hannah Friedrich, and Steven Kostka, Minnesota Department of Natural Resources, Aggregate Resource Mapping Program, St. Paul MN, USA The primary audience using aggregate resource maps and data are non-geologists, local units of government, and the general public. With increasing use of geospatial information and web-based mapping software, like Google Earth©, users expect information that is Internet accessible, interactive, easily compiled, and well documented. In addition, toolsets available for geologic mapping are evolving with the advancement of GIS (Geographic Information Systems) and GPS (Global Positioning Systems). With this evolution, governments and publicly financed institutions have a new responsibility to produce geospatial information. Over the past two years, the Aggregate Mapping Resource Program modified aspects of their data management, mapping methodology, and distribution of data to meet the growing demand of digital data. Internally, benefits of these changes include eliminating data entry redundancies, streamlining documentation, and an overall accelerated rate of mapping. Currently three geologists in our DNR program gather, enter, and produce geologic data. Databases are needed to ensure standards between geologists while accommodating different mapping styles and geologic settings. Standardization includes determined attribute widths and names, field order, information stored as text or numbers, and how people look at data versus how data is queried. Considering the pros and cons of database software, we prefer Microsoft Access© to ESRI© geodatabases for several reasons: the ability to develop one to many relationships, create new attributes on the fly, and programming flexibility. Data standardization expedites writing metadata and allows for the compilation of different project data with no additional processing for the user. Advancements in our methods of data collection include using a tablet computer with GIS software in conjunction with GPS in the field. Previous mapping methods consist of recording observations on a USGS (United States Geologic Survey) 24K quadrangle maps and/or within a fieldbook. Geologists then digitize location information and re-enter field descriptions into a database. Consequently, the same data is recorded twice and transcriptional error is potentially introduced into the dataset. Using a field computer and GPS provides better location accuracy, tracking capabilities, and eliminates redundant data entry processes. Furthermore, consulting data sets, such as aerial photography, parcel and ownership data, high-resolution elevation models, historical maps, and water well stratigraphy, in the field while simultaneously making observations provide additional benefits to the geologists. Various GIS software packages were tested. In our determination, the most stable configuration combines ArcGIS 9.3© with the GPS tacking toolbar for ArcGIS 9.2/9.3©. 3 �To improve distributing data, a DNR developed, web-based map server geospatially displays information used to map aggregate resources, aggregate resource data, base maps, and links to data documentation. The map server allows users to interact with GIS data without the having to download, install, and learn a new software program. By interacting with the data, a greater level of transparency exists on how different geologic units are delineated and classified given the available information at a single time, which reinforces the relationship between geology and distribution of mineral resources. 4 �BEDROCK GEOLOGIC MAP OF THE DEER YARD LAKE AND GOOD HARBOR BAY QUADRANGLES, NORTH SHORE OF LAKE SUPERIOR, MINNESOTA BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu GREEN, John C., University of Minnesota-Duluth, jgreen@d.umn.edu The Minnesota Geological Survey is continuing to map the bedrock geology of 7.5’ quadrangles near Lake Superior as part of the USGS STATEMAP program, resulting to date in thirteen published 1:24,000 scale maps from Duluth to Grand Marais, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Deer Yard Lake and Good Harbor Bay quadrangles are the most recent of these geologic maps (Fig. 1A). All maps in this series are available as printed maps, or as PDF and Arcview export files at the MGS website (http://www.geo.umn.edu/mgs/). Outcrop mapping was augmented by some 50 sets of water well cutting samples, collected at 10 foot intervals by Mckeever Well Drilling of Little Marais, Minnesota. These provided a crucial glimpse of the volcanic stratigraphy in the third dimension as well as information where the bedrock is poorly exposed. The area of this map lies near the top of the 7-10 km thick North Shore Volcanic Group (NSVG), and crosses the boundary between the upper portion of the Northeast sequence and the slightly discordant overlying Schroeder-Lutsen sequence (Fig. 1B). In addition there are several thick sandstone units, as well as components of the Beaver Bay Complex (Leveaux ferrodiorite, Murphy Mountain diabase, and Beaver River diabase) present in the map area. In keeping with prior work, the NSVG is subdivided into informal lithostratigraphic packages separated by major compositional changes, by intrusions or faults across which correlation is tenuous, or where thick flows or flow sequences form mappable units. The informal lithostratigraphic packages shown on this map include the Lutsen basalts, the Good Harbor Bay lavas (which include the Good Harbor Bay andesites, the Cutface Creek sandstone, and the Terrace Point basalt flow of Green, 2002), the Breakwater basalt, the Grand Marais felsites (rhyolite and icelandite), the Cascade River basalt, and the Croftville lavas (which includes the Pincushion Mountain trachybasalt of Green, 2002). The volcanic units immediately overlying (southeast of) the Leveaux ferrodiorite are poorly exposed and not named. The new mapping has refined the volcanic stratigraphy of the NSVG in this area and has continued previously mapped units, in particular thick interflow sandstone units, for several kilometers along strike from where mapped prior to the west (Boerboom and Green, 2007). Four thick sandstone units were identified, one within the Lutsen basalts (Indian Camp Sandstone), one below the Terrace Point flow (Cut Face Creek sandstone), and two other unnamed units identified mainly in water well samples. The Indian Camp and Cut Face Creek sandstones were already well known from exposures near the shore. Examinations of water well samples show that the Indian Camp sandstone is up to 170 feet thick and the Cut Face Creek sandstone more than 250 feet thick. The northern-most sandstone units were identified only in scattered water well cutting samples, but they correspond well with linear topographic depressions and coincident linear negative aeromagnetic anomalies, and can be confidently extended for some distance beyond the well intercepts (Boerboom, 2007). The Terrace Point basalt is a thick ophitic flow with scattered tabular phenocrysts and rare large megacrysts of glassy plagioclase, and contains rare but locally abundant xenoliths of granite, anorthosite, porphyritic ferrodiorite, rhyolite, conglomerate, andesite, and basalt, most of which are only tens of centimeters in size. However, near the base of the flow in the Cascade River the basalt contains a 60 m-diameter xenolith of coarsegrained biotite granite which yields a U-Pb zircon age of 1096.7±0.8 Ma (Green and others, 2001) as well as xenoliths of thermally metamorphosed sedimentary rocks, porphyritic ferrodiorite that matches the distinct texture of the Leveaux porphyry, and other volcanic rock types. The basalt here contains local hybrid pods contaminated by felsic material melted from the xenoliths, and exhibits a strong vertical to irregular flow banding. Overall, these features indicate this may be a feeder zone to the overlying Terrace Point basalt flow, a unique feature rarely found in the NSVG. The types of xenoliths and the plagioclase phenocrysts in the ophitic Terrace Point basalt are similar to those in the Beaver River diabase, leading to the speculation that it may be the extrusive equivalent to the diabase, which is also known from mapping to the southeast to intrude the Leveaux porphyry. Intrusive rocks of the Beaver Bay Complex in this map area consist of the Beaver River and Murphy Mountain diabase, and the eastern-most occurrence of the porphyritic Leveaux ferrodiorite. The latter is 5 �inferred from map distribution and measurements of aligned feldspar phenocrysts to form a southeast-dipping, funnel-shaped, subvolcanic sill-like intrusion. References Boerboom, T.J., 2007, Newly recognized thick interflow sandstones in the upper northeast limb of the North Shore Volcanic Group, Minnesota: Institute on Lake Superior Geology 53rd Annual Meeting, Lutsen, MN: Proceedings v. 53, pt. 1 – Programs and Abstracts, p. 8-9. Boerboom, T.J., and Green, J.C., 2007, Bedrock geology of the Lutsen quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous map M-174, scale 1:24,000. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Green, J.C., Davis, D.W., and Schmitz, M.D., 2001, Three new zircon dates for the Midcontinent rift, North Shore, Minnesota: More data, more questions: Institute on Lake Superior Geology 47th Annual Meeting, Madison, WI: Proceedings v. 47, pt. 1 – Programs and Abstracts, p. 28. Figure 1. A. Index map showing the location of mapped 7.5’ quadrangles along the North Shore of Lake Superior. M numbers refer to MGS Miscellaneous maps. B. Index map showing the locations of the major intrusions and volcanic sequences in part of northeastern Minnesota. 6 �GEOCHEMICAL AND NEW SHRIMP-RG ZIRCON AGE CONSTRAINTS ON THE CARY MOUND GRANITE, WOOD COUNTY, WISCONSIN Bruesewitz, Jeffrey1, and Cameron, Barry2, University of Wisconsin Milwaukee, Milwaukee, WI, 53201; bruesew3@uwm.edu1, bcameron@uwm.edu2 The Cary Mound granite is a late to post Penokean granite suite approximately 10 miles south of Marshfield, WI, and would be included with the 1835 Ma alkali-feldspar granite suite of Sims et al. (1989). Included within the suite are the alkali-feldspar granophyric granite, coeval rhyolite, diorite and mafic enclaves assumed coeval with the granite/rhyolite, and lamprophyre of uncertain younger age based on crosscutting relationships. Updated SHRIMP-RG zircon dates from USGS-Menlo Park have been obtained. The Cary Mound samples are characterized by euhedral zircon with homogenous cores with textural evidence for minor recrystallization surrounded by oscillatory zoned rims (Fig. 1). There is no difference in ages obtained for core versus rim domains. Using ten analyses from sample CMG-04 (county highway department quarry) give a concordia age of 1826 ± 9 Ma. Twelve analyses from sample CMG-15 (Haske quarry) give a Concordia age of 1827 ± 5 Ma (Fig. 2). These ages are statistically the same as the 1833 ± 4 Ma date reported by Sims et al. (1989). Previous interpretations of the 1835 Ma alkali feldspar granite indicate that it is most likely the result of crustal melting of a thickened post Penokean crust. The presence of significant amounts of diorite crosscutting and intermingled with the granite and mafic enclaves of basaltic nature that are related to the diorite indicate melting was more complex than a simple batch melt of thickened crust. The granite/rhyolite is likely a product of partial melting from a feldspar-rich continental crust as indicated by the strong depletions in Sr and Ti. The high concentrations of MgO, Ni, Cr, Zn, and V would be indicative of a mantle source for the mafic enclave with the diorite forming by fractionation from the parent basalt. The lamprophyre is characterized by an orthoclase and anorthoclase groundmass with abundant phlogopite phenocrysts and euhedral pseudomorphs of amphibole that have been replaced by montmorillonite. Using the classification of Rock (1991) the lamprophyre best falls into the calc-alkaline field and is termed a minette. It is enriched in Ba and Sr as is typical for lamprophyres. The lamprophyre is also enriched in the rare earth elements (REE’s), especially the LightREE’s. 7 �Figure 1. Representative cathodoluminescence images of zircons from samples (A) CMG05-04 and (B) CMG05-15. Ellipses indicate individual analysis spots for the sensitive high-resolution ion microprobe - reverse geometry (SHRIMP-RG). Each spot is labeled by grain number, alysis number (e.g. 4.1) and the corresponding 207Pb/206Pb age (± 1 σ Ma). Figure 2. Tera-Wasserburg plots of sensitive high-resolution ion microprobe – reverse geometry (SHRIMP-RG) U-Pb data of zircon for samples (A) CMG-05-04 and (B) CMG-0515. The data are presented as 1 σ error ellipses uncorrected for common Pb. References Rock, N. M. S., 1991, Lamprophyres: New York, Van Nostrand Reinhold, 285 p. Sims, P. K., Van Schmus, W. R., Schulz, K. J., Peterman, Z. E., 1989, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen. Can. J. Earth Sci. Vol 26, p. 2145-2158 8 �RECONSTRUCTING THE PENOKEAN FORELAND BASIN USING THE TIMELINE OF THE 1850 MA SUDBURY IMPACT LAYER Cannon, W. F., and Schulz, K.J., U.S. Geological Survey, MS 954, Reston, VA 20192 wcannon@usgs.gov, kschulz@usgs.gov The evolution of foreland basins, which are linear sedimentary basins formed on continental margins during terrane accretion, is highly variable both in time and space. Where terranes override thinned continental margins, a deep basin or foredeep forms near the margin and passes landward successively to a shallower outer slope, forebulge , and shelf. As accretion proceeds, these zones migrate toward the continent and are superimposed on previous sedimentary successions. Geologic and geochronologic studies (see Schulz and Cannon, 2007, for a review) have confirmed that the Paleoproterozoic Animikie Group and Marquette Range Supergroup in the Lake Superior region record the complex history of such a basin, the Penokean foreland. Unraveling the history of this complex basin is hampered by paucity of precise time lines. Only a few volcanic layers have been precisely dated and these are insufficient to reconstruct the basin history in more than a general manner. However, recently a bed of ejecta-bearing breccia that was deposited instantaneously across the Lake Superior region has been recognized as being related to the 1850 Ma meteorite impact at Sudbury, Ontario (Addison et al., 2005; Jirsa et al., 2008; Cannon et al., 2009). The Sudbury impact layer (SIL) can be traced regionally and provides a unique opportunity to reconstruct the Penokean foreland basin when the Penokean orogen was in a transformative state from a period of mild extension to the earliest stages of thrusting of volcanic arcs onto the continental margin (Schulz and Cannon, 2007). At the time of deposition of the SIL, the northern part of the Penokean basin near Thunder Bay was very shallow to subaerial as indicated by the occurrence of algal stromatolites beneath the SIL and local evidence of subaerial weathering. This shoaling of the basin probably accompanied the arrival of the forebulge. To the southwest conditions were different. At Gunflint Lake and along the Mesabi Range, the SIL lies conformably on the Gunflint and Biwabik Iron Formations for 250 km along strike. In this area, the stratigraphic succession records a progressively deepening basin with the upper cherty member of the Biwabik, a shallow-water, partly stromatolitc unit, overlain by deeper-water carbonate and silicate iron-formation of the upper slatey member. The SIL was deposited on the upper slatey member and was succeeded by black shale of the Virginia and Rove Formations. The shallow-water deposits of the upper cherty member may record the passage of the forebulge slightly before it arrived at Thunder Bay; the upper slatey member may mark the ensuing submergence on the outer slope prior to 1850 Ma. Farther south, deposition of the major iron-formations of the Gogebic and Marquette Ranges ended well before 1850 Ma. By then, both ranges had been uplifted, eroded, and resubmerged, recording passage of the forebulge (outer arch) and submergence onto the outer slope. As much as 500 m of clastic sediments, largely reduced-facies black shale, that represent temporal equivalents of the iron-formations of the Gunflint and Mesabi Ranges, covered the iron-formations of the Gogebic and Marquette Ranges by 1850 Ma. Just north of the Marquette Range, the SIL lies on a chert-carbonate unit of the Michigamme Formation, which is the southern temporal equivalent of the Gunflint Formation. Thus, at 1850 Ma iron- 9 �formations were being deposited on distal (shoreward) parts of the outer slope while black shales were being deposited on more proximal, deeper parts of the slope. Figure 1. Reconstruction of the Lake Superior region at 1850 Ma based on lithofacies immediately below the Sudbury impact layer. Area as shown has been foreshortened by 30 km across the Midcontinent Rift to restore relations prior to Mesoproterozoic extension. In the southernmost part of the basin, deep-water iron-formations of the Iron RiverCrystal Falls district occur directly beneath the SIL indicating that an additional ironformation facies was deposited deep within the axial zone of the basin near the advancing overthrusting arc terrane. Thus, the reduced shale facies deposited on the deeper portions of the outer slope passed southward to at least a brief period of ferruginous chemical sedimentation in the sediment-starved axial zone of the foreland basin. References Addison, W.A., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Cannon, W.F., Schulz, K.J., Horton, J.R., Jr., and Kring, D.A., 2009, The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan: Geological Society of America Bulletin, v. 121, in press Jirsa, M.A., Weiblen, P.W., Vislova, T., and McSwiggen, P.L., 2008, Sudbury impact layer near Gunflint Lake, NE Minnesota: Institute on Lake Superior Geology Proceedings, v. 54, p. 4243. Schulz, K.J., and Cannon W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25. 10 �MAGNETIC ANOMALIES FROM PLEISTOCENE SOURCES IN THE WESTERN LAKE SUPERIOR REGION: THE EDGE OF INSANITY OR A PROMISING THRESHOLD? CHANDLER, Val W., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114, chand004@umn.edu Over the last few decades high-quality aeromagnetic data in Minnesota, Wisconsin and Lake Superior has been crucial to interpreting the geology of the Precambrian bedrock, which lies concealed beneath a nearly continuous cover of Pleistocene glacial deposits. It has been generally assumed that Pleistocene deposits are non-magnetic, and thereby “transparent” to the magnetic data, but it is now appears that weak anomalies, generally on the order a few nanoTeslas to a few 10’s of nanoTeslas, are associated with the Pleistocene deposits themselves. Detection of these weak magnetic anomalies, which is usually most effective using derivative-enhanced data, requires that the underlying bedrock be non-magnetic, thereby providing a sufficiently quiet magnetic anomaly background. These quiet background conditions are best developed over thick basins of nonmagnetic sedimentary rocks, such as the Keweenawan sandstone sequences of the Mesoproterozoic Midcontinent Rift System and the slate-greywacke sequences of the Paleoproterozoic Animikie basin and associated outliers. The outlines of these basins are shown in Figure1, along with the interpreted traces of Pleistocene-related anomalies. These weak anomalies have straight to wandering forms that are reminiscent of stream channels, and they appear to be most closely associated with glacial deposits of the Rainy and Superior Lobes, both of which passed over magnetite-enriched bedrock at short distances up-ice. Magnetic susceptibility determinations of the glacial deposits, based either on model studies or on direct measurements of till and outwash samples, indicate that moderate values, generally in the 0.0025-0.0050 SI range, are common. Some of these weak anomalies can be directly related to topographic features, such as the Toimi drumlins of northeastern Minnesota and the Wadena drumlins of west-central Minnesota (Figure 1), but most show little or no correspondence to surface features. The causes of many of these weak anomalies remain unknown, but recent investigations indicate that at least some are related to bedrock valleys that are filled with relatively magnetic glacial materials. In east-central Minnesota these anomalies have been particularly useful in tracing buried valleys in areas with sparse drill-hole control. Weak anomalies in western Lake Superior have been used to trace several deep bedrock channels that may have developed as tunnel valleys beneath Superior Lobe ice. Although little geologic control exists in the Animikie basin, many stream-like anomalies parallel the expected east-west structural grain for the bedrock, and could therefore reflect bedrockcontrolled valleys. Although these channel-like anomalies are geophysical oddities that are somewhat restricted in their occurrence, they have proven to be pertinent to hydrogeologic and Pleistocene studies in the region, and further investigations are warranted. Aeromagnetic data used in this study were acquired with support form the U. S. Geological Survey, the Geological Survey of Canada, and the Minnesota Legislature through the Legislative Commission on Minnesota Resources,. Ship-borne magnetic data from Lake Superior were acquired by the Minnesota Geological Survey, in cooperation with the Large Lakes Observatory. Interpretive work supported by the Minnesota Geological Survey through the State Special Appropriation, the County Geologic Atlas Program, and appropriations from the Minnesota Minerals Coordinating Committee. 11 �Figure 1. Map showing the traces of magnetic anomalies that are interpreted to reflect Pleistocene deposits. Lighter lines designate anomalies that are related to drumlin fields. Stippled areas outline basins of Paleoproterozoic and Mesoproterozoic sedimentary rocks described in text. 12 �METHODS FOR ESTIMATION OF INDIRECT HYDROLOGIC IMPACTS ON WETLAND PLANT COMMUNITIES AT POTENTIAL HARD ROCK MINE SITES Coleman, J. and Chiriboga, E., Great Lakes Indian Fish and Wildlife Commission, Odanah, WI 54861, jcolema1@wisc.edu In areas of mineral development where wetlands are common it is frequently necessary to predict how mine development may affect wetlands through direct and indirect impacts. Direct impacts to wetlands are usually identified as filling or removal of wetlands during mine development, facilities construction, and waste storage or disposal. Indirect impacts to wetlands are less clearly defined but can result from, among other factors, modifications in physical hydrology. Developments in modeling of physical hydrology of ground and surface waters and a better understanding of how wetland communities are tied to physical hydrology allow for estimation of the indirect impacts of mineral development on wetland communities. As part of evaluation of several proposed mine projects, tools for evaluation of mine induced changes to surface and groundwater have been explored. These tools, primarily various surface and ground water models, integrate site specific geologic and hydraulic data into a framework that is based on the current understanding of how waters interact with the land surface and shallow and deep geology. Given an adequate understanding of a site's soils and geology these modeling tools can predict how modifications to the landscape through mining, facilities construction, or waste disposal may effect the level and rate of flow of waters in wetlands. Although these modeling approaches are fairly mature, their success depends on the type and quality of data available on which to base the analysis. Some of the most critical pieces of information for such modeling are the character of bedrock fracturing, the character of surficial materials, and hydraulic links between bedrock fractures and surficial materials. Existing drilling programs for mineral exploration can be adapted to provide some of this information by the analysis of fracture patterns and orientation in core and the retention of data on surficial materials that are penetrated prior to bedrock entry. The methods for integrating geologic and hydrologic information into ground and surface water models have been in use for many years. On the other hand, methods to predict the effect that a change in physical hydrology has on wetland plant communities has, up to this point, been less clearly defined. As part of the effort to evaluate the potential impacts of the proposed Crandon Mine in Wisconsin, methods were developed for identifying the sensitivity of plant communities to water level changes. These methods, while still being refined, present an opportunity to bridge the gap between expected changes in physical hydrology and effects on wetland plant communities. 13 �GEOLOGY OF THE TUSCARORA INTRUSION, NORTHEASTERN MINNESOTA AND ITS RELATIONSHIP TO THE ANORTHOSITIC SERIES OF THE DULUTH COMPLEX COSTELLO, DANIEL E.*, MILLER, JAMES D., Jr., Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812 (coste082@d.umn.edu), and JIRSA, M.A., Minnesota Geological Survey, University of Minnesota, St. Paul,MN 55455 The petrogenetic relationship between the layered series and anorthositic series of the Duluth Complex is not well understood. The Tuscarora Intrusion, located in the northeastern portion of the complex, is one of the best examples of this ambiguous relationship. Previous work in the Tuscarora within the Long Island Lake quadrangle (Morey et al. 1981) has described the troctolitic and anorthositic lithologies as interlayered on the scale of centimeters to meters. This observation is unique among the layered series of the Duluth Complex. This project seeks to take advantage of recent wildfires within the area to study the relationship between the Tuscarora Intrusion and the Anorthositic Series within the Gillis Lake quadrangle, through field mapping, petrographic observations, and geochemical analyses. The layered series occurs as a number of discrete mafic layered intrusions at the base and mid-levels of the complex, all of which are overlain by a structurally complex cap of anorthositic gabbros of the anorthositic series and granophyric rocks of the felsic series. Field relationships observed over many decades of study throughout the Duluth Complex typically show anorthositic series rock types as inclusions within layered series rocks or show layered series rock intrusive into anorthositic rocks. These observations, along with the very distinctive lithologies and internal structures of the two series, had long been interpreted to suggest that the anorthositic series was significantly older than the layered series (Miller and Weiblen 1990). However, U-Pb zircon ages show that the two units are essentially the same age (1099 Ma +/-0.5 Ma; Paces and Miller 1993). These age data imply not only that the two main stage rock series of the Duluth are approximately the same age, but also that they may be comagmatic or at least part of the same magmatic event. This possibility of a closer genetic relationship between the two series is actually supported by many gradational to ambiguous relationships between the two series, which in the past had been largely ignored as inconsequential anomalies (Paces and Miller, 1993). The Tuscarora Intrusion is one of the best examples of this ambiguous relationship, as described by Morey et al.(1981). This study found no direct evidence for interlayering between the troctolitic and anorthositic lithologies. Rather, the anorthositic series were found to be elongate inclusions within the troctolitic rocks of the Tuscarora Intrusion. The anorthositic inclusions are concentrated in the upper portion of the Tuscarora, as described below. Geochemical analyses of samples collected from both series show similar mineral chemistries and trace element behavior trends. This is interpreted to suggest that the two lithologies are closely related and may be part of the same magmatic event, even though they are not interlayered. These results agree with other studies where anorthositic rocks have been found as inclusions within the layered series. An unexpected discovery of this project is that the Tuscarora Intrusion can be divided into two distinct lithologic zones, based on modal mineralogy, textural patterns, internal structure, and inclusion type and amount (Fig. 1). Moreover, each zone can be divided into a couple of distinctive units. The lower zone (LZ) is somewhat heterogeneous, with compositions ranging from olivine gabbro to augite troctolite. A thin basal unit (Tbh) is very taxitic, with local biotite and orthopyroxene suggestive of footwall contamination. The augite troctolite unit (Tat) contains welldeveloped but variably oriented foliation and modal layering. Some of this structural variability may be due in part to the presence of very large (up to 100s of meters across) mafic hornfels inclusions (unit Thf, fig. 1), which occur throughout the Tat unit. These inclusions are interpreted to have been 14 �derived from the North Shore Volcanic Group, into which the Tuscarora and other Duluth Complex intrusions were emplaced. In contrast, the upper zone (UZ) is much more homogenous and consistently troctolitic. It can be subdivided into a thin melatroctolite basal unit (Tmt or Tum) that grades upwards into a finegrained, well-foliated troctolite to leucotroctolite (unit Ttr or Tut). This unit contains an abundance of anorthositic-type inclusions as described above, presumed to have been derived from the overlying anorthositic series (unit Aau). In addition, a large inclusion of the adjacent Poplar Lake inclusion (unit Pgb) has been identified by previous mapping (Morey 1981). A troctolitic dike (unit Ttd) has been mapped in the western portion of the map area extending from the upper zone, through the lower zone and into the footwall sedimentary rocks. This dike is approximately 80 meters across and contains several small inclusions of poikilitic troctolitic anorthosite. Based on results from field mapping, petrographic observations, and geochemical studies, the two zones of the Tuscarora Intrusion are interpreted to represent successive injections of melt within a single magma chamber. The Lower Zone was emplaced first, and encountered NSVG lithologies located between the recently formed Anorthositic Series and footwall rocks. This unit began to crystallize, followed by the introduction of a replenished melt to the chamber. The Anorthositic Series served as a hanging wall to this newly emplaced magma, and was incorporated as elongate inclusions in the roof portion of the crystallizing Upper Zone. This poster presentation focuses on the geologic map produced as part of this study. Support for mapping was provided by the Educational Component of the National Cooperative Geologic Mapping Program (EDMAP) of the United States Geological Survey. Geochemical studies were supported by grants from the Institute of Lake Superior Geology and from the Dept. of Geological Sciences and the Precambrian Research Center at UMD. References Miller, J.D., Jr., and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush: Journal of Petrology, v. 31, p. 295–339. Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: MN Geol. Surv. Misc. Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: New insights for physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent Rift system. Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14, 013. Pgb Figure 1: Generalized bedrock geologic map of the Tuscarora Intrusion and related rocks within the Gabimichigami and Gillis Lake quadrangles. Unit descriptions are provided in text. 15 �QUANTIFYING ASSIMILATION VS. FRACTIONAL CRYSTALLIZATION USING Sm-Nd, Lu-Hf AND Pb ISOTOPE SYSTEMS: THE GEOCHEMICAL EVOLUTION OF THE SONJU LAKE INTRUSION, FINLAND, MN DAYTON, R. N. and MILLER, J.D. Jr., Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 VERVOORT, J. D., Dept. of Geology, Washington State Univ., Pullman, WA 99164 The Sonju Lake Intrusion (SLI), located within the Beaver Bay Complex near Finland, MN, is the most completely differentiated intrusion related to the 1.1Ga Midcontinent Rift System (Miller and Ripley, 1996). The Finland granite, which is composed of micrographically-textured leucogranite to ferromonzonite, forms the hanging wall of the SLI. The SLI exhibits a cumulate stratigraphy consistent with closed system differentiation of tholeiitic magma by fractional crystallization (Stevenson, 1974; Miller and Ripley, 1996). Field relationships from outcrop and drill core through the SLI and the overlying Finland granite show a cyclic to irregularly gradational contact between the two bodies. This relationship, the smooth compositional variations across the contact and the parallel zonation of the two subunits of the Finland granite with the strike of the mafic cumulates of the SLI are consistent with the Finland granite being a late stage felsic differentiate of the SLI (Miller and Ripley, 1996). However, geophysical modeling of gravity and aeromagnetic data implies a volume of granite that approaches that of the SLI and therefore exceeds the volume of felsic material that could be accounted for by differentiation of a mafic body the size of the SLI (Miller et al., 1990). Miller and Ripley (1996) suggested that the earlier emplacement of the Finland granite acted as a density barrier to the upward movement of the mafic SLI magma. Underplating of the hot mafic SLI magma would be expected to lead to melting in the lower portions of the Finland granite. This poses a fundamental question of what portion of the apparent differentiation of the SLI is related to fractional crystallization, and how much is related to assimilation of a felsic partial melt from the granite. Major and minor element whole rock data are inadequate to the task of distinguishing these processes because the Finland Granite geochemically resembles an upper differentiate of the SLI. However, a radiogenic isotope study of these two systems has the potential to address the question, as reconnaissance isotopic data show that the granite has a radiogenic isotope signature that is distinct from the mafic rocks of the SLI (Vervoort, Unpublished data). To evaluate the roles of fractional crystallization and assimilation in the crystallization of the SLI, a total of 21 samples were collected from outcrop and drill core for analysis for Sm-Nd, HfLu, and Pb isotopes. The analyses were conducted at the radiogenic isotope facility at Washington State University using a Finnegan Neptune MC-ICPMS. Pb isotope compositions were analyzed for all 21 samples and 16 samples were chosen to measure Sm-Nd and Lu-Hf isotope compositions. These data were combined with Nd data from 8 samples in a previous reconnaissance study (Vervoort, 1996, unpublished data) to profile of the isotopic variation through the Sonju Lake intrusion and up into the overlying Finland Granite. The sample locations are shown on Figure 1. All of the samples collected from the exposed eastern area of the SLI and Finland Granite (Fig. 1) show initial epsilon Nd values for the SLI that are consistent with other uncontaminated, mantle-derived mafic volcanic rocks of the rift (epsilon Nd 0 ± 2, Vervoort et al., 2007). Samples from the Finland Granite yield moderately radiogenic initial epsilon Nd values of ≈ -3.5. Only minor contamination effects are evident in the uppermost SLI cumulates. However, a surprising result came from six samples collected from a drill core (SLI-1) that penetrates the transition zone between the SLI and the granite about 7 km to the west of the exposure area (Fig. 1). These samples, taken mostly from the well-foliated apatite ferrodiorite cumulates of the slad unit (Fig. 1), yield the most radiogenic initial epsilon Nd values, ranging from -4.1 to -5.2. This may imply that the Finland granite has isotopic heterogeneities, which has been shown by Beard (2008) to be possible in magmatic systems formed by partial melting. Several follow up samples were submitted to better 16 �understand this discrepancy. We hope to have these results and our best interpretation of these data available at the time of our presentation. References Beard, James S., 2008. Crystal-melt separation and the development of isotopic heterogeneities in hybrid magmas Journal of Petrology (May 2008), 49(5):1027-1041 Miller, J.D., Jr, Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: Geochemical and geophysical evidence of petrogenetic relationships. 36th Annual Institute on Lake Superior Geology, p. 66–68. Miller, J.D., Jr., Green, J.C., Chandler, V.W., and Boerboom, T.J., 1993, Geologic map of the Finland and Doyle Lake quadrangles, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-72, 1:24,000 scale. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G. (ed.):Layered Intrusions: Amsterdam, Elsevier, p. 257-301. Stevenson, R.J., 1974. A mafic layered intrusion of Keweenawan age near Finland, Minnesota. M.S. Thesis, University of Minnesota, Duluth, 160 pp. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T. and Harpp, K.S., The magmatic evolution of the Midcontinent rift: new geochronologic and geochemical evidence from felsic magmatism, Precambrian Research 157 (1–4) (2007), pp. 235–268 17 �TACONITE-DERIVED MINERAL DUST IN POPULATION CENTERS ON MESABI IRON RANGE: TRACKING MINERAL FIBERS FROM ORE TO AIR Tamara Diedrich, Devon Brecke, Megan Schreiber, Larry Zanko, Natural Resources Research Institute, University of Minnesota Duluth In an effort to address long-standing questions regarding the impact of dust derived from mining taconite on human health, the University of Minnesota is conducting multiple complementary health-related studies, including an exposure assessment, epidemiology studies, and exposure characterization research. As one of these studies, NRRI performing a detailed characterization of the dust that is produced from mining and processing Biwabik Iron Formation ore, with emphasis on any mineral fibers present and the elongated mineral particles that are produced from mining and processing activities. The characterization of mineral fibers and elongated particles begins with the examination of fibrous minerals in situ from thin sections of metamorphosed and unmetamorphosed ironformation (fig.). We are also looking at the crushed material corresponding to these thin sections (fig.), and the particulate matter present in the air of the taconite operations where it is mined and processed. Finally, we are conducting a three-year long, field-based study of taconite-derived mineral dust present in the air of communities directly surrounding the taconite operations of the Mesabi Iron Range. The community air sampling will result in long-term average mineral fiber concentrations in ambient air at these locations; characterization of any mineral fibers that are found using metrics relevant to their impact on human health (aerodynamic diameter, dimension, mineralogy, and chemistry); and average total particulate matter and its size distribution at these locations. Retrospective observations will be made using dated lake sediment cores from the region. Particulate matter (in ambient community air, within taconite operations, and aerosolized crushed material) is being collected using a Micro-Orifice Uniform-Deposit Impactor and on a total filter. These samples are then analyzed by a gravimetric method, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Figure: BSE SEM image of Minnesotaite needles in thin section of unmetamorphosed Biwabik Iron Formation (left); SEI SEM image of Minnesotaite particle liberated from crushing that same rock (right). 18 �GEOLOGY AND GEOCHEMISTRY OF THE FEARLESS-PYTHON PROPERTY, SCHREIBER-HEMLO GREENSTONE BELT, ONTARIO FAGE, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada The Fearless-Python property, owned by Metalcorp Limited, is located approximately 50 kilometres east of the town of Marathon, Ontario and is situated within the Schrieber-Hemlo greenstone belt of the Wawa subprovince of the Archean Superior Province. Fearless-Python has been extensively explored over the past 40 years, mainly being examined for Hemlo-type gold deposits. The property is sandwiched between the Cedar Lake Pluton (2688 – 2687 Ma, Corfu and Muir, 1989) to the north and the Pukaskwa Batholith (2719 and 2688 Ma, Corfu and Muir, 1989) to the south. The geology is dominated by generally east-west trending and northerly dipping metasedimentary rocks with mafic to intermediate metavolcanic rocks and minor felsic metavolcanic rocks. High level intrusive dykes and Proterozoic diabase dykes crosscut all lithologies (Thompson and Paakki, 2001). The entire Schreiber-Hemlo Greenstone belt has been affected by lower to mid amphibolite facies metamorphism (Pan and Fleet, 1993). The geology of the property is favorable for several deposit types, including; 1) the Gouda shear zone which hosts gold mineralization lies within the southern portion of the property and the major structural trend which is host to the Hemlo gold deposit, directly to the east, is also present within the property. The highest gold values are located within the Gouda Lake Horizon which occurs at the base of a well-developed, potassically altered quartz eye sericite schist hosting gold as well as disseminated and semi-massive to massive sulphides consisting of pyrite, pyrrhotite, sphalerite and lesser galena 2) VMS mineralization has been recognised at a number of locations within the property. Significant occurences of massive sulphides are present at two locations on the property, as well as several smaller zones of anomalous Zn, Cu, Pb values. 3) Molybdenite occurs as coarse aggregates in crowded feldspar porphyry and granite pegmatite dykes in the Duck Lake area north of the Gouda shear zone. 4) Possible Outokumpu-type Ni-Co-Zn-Cu mineralization has also been reported at two locations on the property. Prelimary analysis of trace element geochemistry shows that the metavolcanic rocks range from calc-alkaline to tholeiitic in composition. Primitive mantle normalized plots shown in Figure 1 indicate island arc and MORB-like affinities for the metavolcanic rocks. Felsic metavolcanic rocks have MgO contents of 0.2-1.4 weight percent, SiO2 of 68.3 weight percent and La/Sm, Gd/Yb ratios of 2.11-5.17 and 2.19-5.33 respectively. Intermediate and mafic metavolcanic rocks have MgO contents of 3.5-8.9 weight percent, SiO2 of 47.4-58.2 weight percent and La/Sm, Gd/Yb ratios of 0.64-3.31 and 1.04-4.72 respectively. Spatial relationships suggest that there may be a genetic relationship between the different deposit types at the Fearless-Python property. Hemlo-type gold and VMS mineralization occur together in the Gouda Lake Horizon. Molybdenite occurs in close proximity to the north of the Gouda shear zone. The CADI zone, which is a nickel prospect, occurs along strike to the west of the Gouda shear zone. Additional work in the summer of 2009 will 19 �utilise drill core and surface mapping to explore these relationships and test the model that the multiple mineralization events are genetically related. Figure 1. Representative primitive mantle normalised diagram for volcanic rocks of the Fearless-Python property (normalising values from Sun and McDonough, 1989). References Corfu, F. and Muir, T.L., 1989. The Hemlo-Heron Bay greenstone belt and Hemlo Au-Mo deposit, Superior Province, Ontario, Canada. I: Sequence of igneous activity determined by zircon U-Pb geochronology. Chemical Geology. 79:183-200. Fleet, M.E. and Pan, Y.,, 1991. Metamorphic Petrology of the White River Gold Prospect, Hemlo Area. Ontario Geological Survey, Grant 305, Final Report. 47 pp. Sun, S.-s., and McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Geological Society, Special Publication No.42: 313-345. Thompson M. and Paakki, J., 2001. Assessment Report on the 2000 Exploration Program on the White River Property, Bomby, Brothers and Laberge Townships, Ontario. Teck Exploration Ltd. Report No. 1340. 20 �ALTERATION IN THE SOUTHERN FELSIC VOLCANICS AT MARSHALL LAKE, NORTHWESTERN ONTARIO NATHAN R. FORSLUND1,2 (nforslun@lakeheadu.ca), MARY LOUISE HILL1 and ROBERT S. MIDDLETON2 1 Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada. 2 East West Resource Corporation, Thunder Bay, Ontario, Canada. The Marshall Lake property is a copper-zinc-rich volcanic-hosted massive sulfide (VHMS) deposit located approximately 255km northeast of the city of Thunder Bay, Ontario. The study area consists of a series of Archean rocks including metavolcanics that range in composition from mafic to felsic, and metasedimentary units, both clastic and chemical. The Summit Lake pluton is another significant lithology that may have played a role in driving the ore-bearing fluids, since it is penecontemporaneous with the surrounding metavolcanics. In the past, most work in the area has concentrated on the northern part of the sequence where most of the mineralization is known to occur. In 2006, East West Resource Corporation acquired the property, and since this time there has been an increased effort to understand the deposit as a whole; not only the area proximal to the mineralization, but also the distal rocks to the south. The alteration and subsequent metamorphism at Marshall Lake are typical for a bimodal mafic VHMS type deposit. The metamorphic assemblages present in the southern felsic metavolcanics represent depletion in sodium and silica and enrichment in magnesium and potassium when compared with an unaltered rock of similar composition. Most of the mapped area, especially along the contact between the metavolcanics and metasedimentary rocks, are dominated by the metamorphic assemblage garnet-amphibole. The intensity of this alteration decreases with distance from this contact. In the genesis of a VHMS deposit two types of fluids can contribute to the alteration geochemistry: the ore-bearing fluids that penetrate the siliceous cap rock underneath the precipitation site, and the convecting seawater that enters through fractures in the seafloor. The latter fluid type would result in the enrichment in potassium and magnesium that is seen. The evidence seen in the field, through petrography, and in the geochemistry is suggestive that seawater had more effect on the alteration of the southern volcanics than the ore-bearing fluids, however a background signature from the ore-bearing fluids is still present as can be seen with the depletion of sodium and silica. If this is indeed the case then it would indicate that the metasedimentary units to the south of the metavolcanics, consisting of banded iron formation within clastic sediments, may have been the paleoseafloor during the genesis of the deposit, and the volcanics to the north would have underlain these sediments. The sequence would then be regionally topping to the northeast, which agrees with the few structural measurements that are available. This would have some economic significance as well, since it may warrant exploration for the presence of a lead-zinc-rich horizon within (or in close proximity to) the banded iron 21 �formation, since lead and zinc would precipitate out at the top of the stratigraphic pile. Geophysical surveys used in the past to identify targets would have been difficult to interpret, since the iron formation itself is such a strong conductor and has such a high magnetic susceptibility. 22 �VERMILION GREENSTONE GOLD - NEW DATA, NORTHEASTERN MINNESOTA Frey, B.A., Minnesota DNR – Lands & Minerals Division, 1525 3rd Avenue East, Hibbing, Minnesota, 55746 Barry.Frey@dnr.state.mn.us A petrologic and geochemical reexamination of thirty archived sets of drill samples from the Vermilion Greenstone Belt in Northeastern Minnesota has revealed previously unrecognized gold-bearing intervals and mineral associations supporting new or additional gold mineralization models with individual prospects. Gold concentrations as high as 148ppm were observed during the collection of 3,772 semi-quantitative XRF analyses. The presence of acicular sodic-rich amphiboles in a sequence with gold-bearing chert-graphite-sulfide layers suggests that gold mineralization may be associated with sodic metasomatism. The Vermilion Greenstone Belt has long been associated with gold. In 1865, a “gold rush” occurred in the area around Lake Vermilion. Appreciable gold was not found at the time, but it did lead to the discovery of direct shipping iron ore at Soudan, Minnesota and eventually Ely, Minnesota. The Soudan Mine and the five major mines in Ely produced about 100 million tons or ore. Exploration also led to the developing of numerous smaller mines in the greenstone. Many of these did not even warrant a rail spur, but the ensuing exploration activities and relatively good outcrop exposure encouraged future work for other metals, including gold. Most Vermilion greenstone rocks have been subjected to greenschist facies metamorphism. Outcrop exposure is variable, but good compared with most of Minnesota. Past sampling, maps, and reports have been produced by the Minnesota Geological Survey, the Natural Resource Research Institute, and the Minnesota DNR. Exploration by at least fifteen exploration companies have produced historic drill hole samples, geochemistry, geophysical work and other data A known, widespread presence of anomalous gold within the Vermilion “greenstone” is one fruit of these efforts. Our work has included drill sample logging; semi-quantitative, real-time “hand-held” XRF chemistry; and limited assay and microprobe work in order to better elucidate the varied gold occurrences. The .76 cm2 XRF sample size complements the visual descriptions, and the grain size of most rocks. The sample size provides direct elemental associations with gold associated with discrete mineral grains. XRF traverses are also useful for zoning associated with veins and alteration fronts. The context of the sampling size, however, must be maintained when examining this data. Besides providing more detailed element associations with each prospect and encountered mineralization type, the XRF has also allowed for better physical placement of gold mineralization found within anomalous previous assays. Visible clues may be established since gold mineralization may be otherwise hidden. XRF element associations have been found with a summary in Table 1. Several gold mineralization types occur. Note, however, that the values for Foss Lake and Eagles Nest Prospects are based on a smaller number of semi-quantitative XRF readings with anomalous measurable gold. Also, the rutile association of the “Raspberry” is from visible logging. The rutile is found in close proximity to XRF 23 �anomalous gold, but not as intimately as with galena. All the prospects probably have several geochemical processes involved in determining the final gold locations and associations. Table1 - XRF Element Associations Prospect Raspberry Au Mineralization Type Intrusion hosted Semiquantitative XRF Value Au Association Pb (galena), Quartz veins, To 101 ppm Au Rutile(?); Fe, As, Mn, Cr, Se, Sn Raspberry Shear zone related (remobilization?) To 67 ppm Au Pb (galena), Quartz veins, Rutile?; Pb, Ag, Se Foss Lake Algoma BIF related Au To 28 ppm Au Fe oxide to Sulfide-graphite transition; As, Ba, Pb Shear zone related Pyrite; Mn, Sr, Ba, Mo?, Cu? Eagles Nest Shear Murray Shear To 4 ppm Au Volcanic Hosted Massive Sulfide? To 148 ppm Au Sphalerite, Cr, Zn, Sb, Cd, Hg Foss Lake Prospect drill core. The first Foss Lake Prospect drill core examined (DDH# 6314-36-1) contained a sequence of iron formation within basalts. Previously unknown gold was found within disturbed chert and graphite interlaminations at the broad transition from oxide-silicate-carbonate BIF to sulfide-graphite BIF. Away from the transition, the sulfide iron formation was heavily assayed with minimal Au. The elevated XRF gold was associated with elevated As, Cu, Co, Pb, and Mo. Chert layers within the sequence were locally noticeable because of a slight bluish cast exhibited in their appearance. Hand lens examination showed the presence of pale acicular fibers in and on the margins chert. Microscope examination indicated the minerals were amphiboles. Very fine grains were also scattered throughout the chert. The bluish minerals appear to be sodic amphiboles, probably crossite, and indicate probable sodic metasomatism. The nature of the association with the gold mineralization is unknown. A “dacitic” volcaniclast also had elevated gold. Minor dolostone and ultramafics also occurred within this drill core. 24 �MAGMATIC VS. HYDROTHERMAL PROCESSES IN THE SOUTH FILSON CREEK MINERALIZATION,SOUTH KAWISHIWI INTRUSION, DULUTH, COMPLEX BENEDEK GÁL, Eötvös Loránd University, Budapest, galbenedek@yahoo.com DEAN M. PETERSON, Natural Resources Research Institute, UMD, dpeterson@duluthmetals.com FERENC MOLNÁR, Eötvös Loránd University, Budapest, molnar@abyss.elte.hu The South Filson Creek (SFC) deposit (located in Sections 25 and 36, Township 62 North, Range 11 West) occurs above the basal units of the South Kawishiwi Intrusion (SKI), and represents a unique geological setting of Cu-Ni-PGE mineralization within the Duluth Complex (DC). It is located in an approximate stratigraphic high of 1000 m above the basal contact unlike all other known ore occurrences in the DC. Researchers have traditionally referred to this area as location of „structurally controlled” mineralization as signs of hydrothermal alteration has been described in previous papers (Kuhns et al. 1990, Severson & Hauck, 2003), however this attribute has to be revised in some points. The nature of hydrothermal processes overprinting the primary magmatic features and their significance in ore-genesis have been characterized in details during our current studies. Extensive field mapping has revealed mineralization both in the Layered Series troctolites and in the Anorthositic Series in the SFC. Sulfides in the Layered series appear as disseminated fine-grained patches and pockets in an area of approx. ¼ square kilometer only. This type of mineralization was in the main focus of exploration so far. Pyrrhotite, chalcopyrite, pentlandite and cubanite are the main ore-forming sulfide minerals with subordinate amount of other copper-bearing sulfides, presumably secondary in origin (bornite, covellite, talnakhite). Sulfides form interstitial blobs between cumulus silicates, fine-grained disseminations and microscopic veinlets. At least 5 different platinum group minerals have been distinguished in the samples, associated both with magmatic sulfides and secondary hydrothermal alteration products. Highest metal values for this type of mineralization were 1.25 wt% Cu and 0.2 wt% Ni and 2.4 ppm Pd, 1.2 ppm Pt and 0.4 ppm Au. Brittle lineaments in the area do not affect the distribution of sulfides. The magmatic mineralization in the troctolites of the SFC area are similar to the „confined”- style of mineralization (Peterson 2001, 2002). Mineralization in the Anorthositic Series is most likely hydrothermal in origin as its occurrence shows strong correlation to brittle structures and associated secondary alteration products. It shows only elevated copper-values (but still not as high as in Layered Series) up to 0.2 wt% Cu and some silver enrichment (up to 3 ppm Ag) but no PGE or Ni showings. Based on petrographic work, three types of hydrothermal alteration were possible to distinguish in the SFC area: • Alteration products of the first event can only be found in the Layered Series rocks. Serpentinization of olivine, chloritization of mafic minerals, albitization of plagioclase and several secondary sulfide minerals (bornite, covelline, talnakhite) are the product of this alteration event. Redistribution of PGEs have likely occurred due to the elevated Cl-content and salinity of the migrating fluids, however transportation of PGEs on bigger distances did not happen. Magmatic fluids containing high chlorine concentrations most likely were segregated from 25 �• • the crystallizing troctolitic melt, which process was documented in the Fenrichment and Cl-depletion trend of apatite in pegmatoidal samples and the presence of highly saline fluid inclusions in apatite. The second hydrothermal event can be observed both in the Layered Series and in the anorthosites and was most likely responsible for the formation of the mineralization in the Anorthositic Series rocks. Alteration products are chlorite, fibrous green amfiboles, sericite, prehnite, pumpellyite, carbonate and pyrite. Temperature of the fluids (based on chlorite thermometry and paragenesis) was between 250 and 350°C, pH was near neutral. The fluid was not capable of mobilizing precious metals. The third event is marked by rusty joints throughout the whole area. The joints are filled with rusty, serpentine-like material but alteration is not extensive around them and they do not have any significance regarding mineralization. References Kuhns, M.J.P., Hauck, S.A., and Barnes, R.J. (1990): Origin and occurrence of platinum group elements, gold and silver in the South Filson Creek copper-nickel mineral deposit, Lake County, Minnesota: Duluth – University of Minnesota, Natural Resources Research Institute, Technical Report, NRRI/GMIN-TR-89-15, 60 p. Peterson, D. M. (2001): Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex – Minnesota; Society of Economic Geologists, 2nd Annual PGE Workshop, Sudbury, Ontario, p.3 Peterson, D. M. (2002): Cu-Ni-PGE Mineralization in the South Kawishiwi Intrusion, Northeastern Minnesota; Variation due to Magmatic Processes – Institute on Lake Superior Geology, 48th Annual Meeting, Thunder Bay, Ontario, Proceedings vol. 48. Severson, M. J. & Hauck, S. A. (2003): Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex – Natural Resources Research Institute Technical Report NRRI/TR-2003/37, p. 296. 26 �CHARACTERIZING THE DISCHARGE FEATURES OF GLACIAL LAKE AGASSIZ DURING THE POST-MARQUETTE PERIOD USING MARINE SEISMIC-REFLECTION METHODS J.L. Gary, N.J. Wattrus, S.M. Colman, and E.B. Voytek, Large Lakes Observatory & Department of Geological Sciences, University of Minnesota – Duluth, Duluth, MN 55812 Glacial Lake Agassiz was the largest of the North American glacial margin lakes. Over its 4,000 year existence, Lake Agassiz varied substantially in aerial extent and volume. This variability was a function of the fluctuating retreat pattern of the Laurentide Ice Sheet’s southwestern margin, differential isostatic rebound of the North American crust, the topography of the land exposed by the retreating ice, and erosion of the various outlet channels draining the lake. These factors combined to form a history of Lake Agassiz punctuated by sudden and sometimes catastrophic rerouting of its drainage from one outlet channel to another (Teller, 2001). The amount and routing of Lake Agassiz discharge has become controversial. However, extensive onshore observations of Glacial Lake Agassiz discharge features have firmly established that northwestern Lake Superior was a major drainage route following the retreat of the Marquette glacial advance ca. 9,500 years 14C BP (Clark et al, 2001; Teller et al, 2002). We describe a high-resolution single channel seismic reflection dataset collected with a small airgun that we acquired to test our hypothesis that this drainage event (corresponding to the Nipigon Phase of Lake Agassiz) left diagnostic stratigraphic and geomorphic signatures beneath Lake Superior. The unique bathymetry of northwestern Lake Superior, where water depth plunges off Nipigon and Black Bays, makes this location ideal for the identification and characterization of the Post-Marquette depositional features. The steep and sudden dropoff from the shallow water bays into the deep offshore waters of the lake would have caused the high-velocity floods to slow and drop much of the sediment they were carrying. Our results confirm the existence of these sediment packages, which are now buried below a thin blanket of Holocene sediment. They form wedges of sediment that are thickest (some over 70 m thick) in the deep water area adjacent to the flood outlet. The apron of sediment thins lakeward and shore-parallel away from the outlet. The seismic character of the basal units of the apron, proximal to the outlet, is chaotic and only very weakly stratified suggesting that these deposits represent coarse sediment laid down during the initial stages of the flood when flow was presumably at its peak. These sediments are overlain and draped by a weakly stratified package that is more widely developed (extending lakeward beyond the bounds of our survey). We interpret this unit, which becomes more stratified and thinner lakeward, to represent the fine grained sediment associated with the latter stages of the flood when flow had eased. 27 �References Clark, P.U., Marshall, S.J., Clarke, G.K.C., Hostetler, S.W., Licciardi, J.M., and Teller, J.T., 2001, Freshwater forcing of abrupt climate change during the last glaciation: Science, v. 293, p. 283-287. Teller, J.T., 2001, Formation of large beaches in an area of rapid differential isostatic rebound: the three-outlet control of Lake Agassiz: Quaternary Science Reviews, v. 20, p. 1649-1659. Teller, J.T., Leverington, D.W., and Mann, J.D., 2002, Freshwater outbursts to the oceans from glacial Lake Agassiz and their role in climate change during the last deglaciation: Quaternary Science Reviews, v. 21, p. 879-887. 28 �2009 Update: Leasing State of Michigan Lands for Metallic and Nonmetallic Minerals Milton A. Gere, Jr. and Thomas B. Hoane, Michigan Department of Natural Resources, Forest, Mineral and Fire Management, P.O. Box 30452, Lansing, MI 48909-7952 The Department of Natural Resources offers leasing programs for state-owned mineral lands for the exploration and development of oil and gas, underground gas storage and metallic and nonmetallic minerals. 2009 Information on State of Michigan Metallic and Nonmetallic Mineral Leasing Programs: State Ownerships - Three major categories Fee - own both surface and mineral rights (4.0 million acres) Surface - own surface rights only (547,458 acres) Minerals - own severed mineral rights only (2.3 million acres) The state also owns 25 million acres of Great Lakes Bottomlands, which includes the mineral rights; however these are not open to exploration or development. Leases for Metallic and Nonmetallic Minerals – There were 45,240 acres under 200 State Metallic Mineral Leases and 3,688 acres under 48 State Nonmetallic Minerals Leases at the end of FY 2008. Leasing Process - Nominations – Sealed-bid Auctions - Direct Leases – Fees, other requirements: Lands nominated for leasing are field reviewed by Department of Natural Resources (DNR) foresters, wildlife biologists and fisheries biologists. Input is requested from other specialists. Lands are classified in four categories: 1) Leaseable; 2) Leaseable with Restrictions, 3) Leaseable, nondevelopment, and 4) Nonleaseable. Public notice is given, public input is requested, and any private surface owners are notified. Before classified lands are leased, final approval must be received from the DNR, and two state approval boards. Performance bonds and insurances are required. A mining permit and other permits, such as air quality, water discharge, and others, may be required by the Michigan Department of Environmental Quality (DEQ) and possibly other agencies prior to mining. Metallic Mineral Leases - Exploration and potential development - for any metallic mineral commodity found. Nomination fee is $300.00 for up to 640 acres within four contiguous sections. Lease terms are 10 years, with possible extension, or held while producing. The Direct Lease process is currently used. Upon leasing - Bonus fee (one-time) $3 per acre. Annual rental of $3 per acre increases to $6 on sixth year. (February 2009 rates, subject to revision) Approved exploration plans and surface use permits required for intrusive exploration. Approval to produce and approved mining and reclamation plan required. Production royalty rate is a percentage of selling price, by commodity as listed in the lease. Royalty may be renegotiated at a later date. See lease document on website for details. Nonmetallic Mineral Leases -- May be nominated by state or individual, currently there is no nomination fee. A. Production of a known nonmetallic commodity from a known location. 1. Sealed Bid Lease Auction - Usually on commodity-specific lease document. Fixed Annual Minimum Royalty. Bid on Royalty Rate per ton, subject to revision every three years based on U.S. Producers Price Index changes. Lease time terms vary, subject to possible extension. 2. Direct Leases --a. To County Road Commissions for sand and gravel from known locations at fixed CRC Royalty Rates, subject to revision every 3 years based on U.S. Producers Price Index b. To adjacent operations or private surface owners, in some cases. Commodity, Royalty rate and other terms negotiated. B. Exploration and potential development for any nonmetallic mineral commodity found, nominations may be for up to 640 acres within four contiguous sections. Sealed Bid Lease Auction --- Bid on the one-time Bonus rate per acre. Royalty rate fixed 29 �by percentage of selling price of commodity(s) produced, as listed in the Lease. Rate percentage may be renegotiated at a later date. C. Specific Commodity Exploration Lease ..Will vary with item. A lease for Potash is currently being developed and a sealed bid lease sale is expected to be held in late spring/early summer, 2009 --------------------------------------------------------------------------------------------------------------------------------------Note --Leases may also require various Surface Use Permits and Fees. Any Intrusive Exploration Activities require an approved Exploration Plan. Various State, local and Federal regulations may apply. Website Information - Go to the DNR Website --- www.michigan.gov/dnr Choose Doing Business with DNR - (on Left side) Open and scroll down to Minerals and open Metallic Minerals (M.M.) – Information, Procedure (policy), Rules, M.M. Lease Document. http://www.michigan.gov/dnr/0,1607,7-153-10368_11800_46635---,00.html Nonmetallic Minerals (N.M.) – Information, Procedure (policy), Rules, General N.M. Lease Document, Sand & Gravel Lease Document. http://www.michigan.gov/dnr/0,1607,7-153-10368_11800_46635---,00.html Choose Publications & Maps (on Left side), Choose On-line Maps ---open and view http://www.michigan.gov/dnr/0,1607,7-153-10371_14793---,00.html Land & Mineral Ownership (Shown as entire 40 acre blocks, number in upper right corner indicates how much land State owns and what type (surface, mineral, surface & mineral, mix.) http://www.michigan.gov/dnr/1,1607,7-153-10371_14793-31345--,00.html Mineral Leases (Left County List – County map with Lease numbers. Right County List List of Lease numbers and Lessee names for the County). http://www.michigan.gov/dnr/1,1607,7-153-10371_14793-30992--,00.html --------------------------------------------------------------------------------------------------------------------------------------Questions? Wish to discuss our Mineral Leasing programs? Contact us: Michigan DNR-- Forest, Mineral and Fire Management -- Mineral and Land Management Section, P.O. Box 30452, Lansing, MI, U.S.A., 48909-7952 Phone: 517-373-7663 Fax: 517-373-2443 Milt Gere, Geologist … Phone: 517-335-3249 E-mail: gerem@michigan.gov Tom Hoane, Geologist … Phone: 517-241-3769 E-mail: hoanet@michigan.gov --------------------------------------------------------------------------------------------------------------------------------------More info. … .For additional information about Michigan’s minerals, geology, Geological Sample and Drill Core Repository and required permits related to mining, etc. …Contact: Michigan Department of Environmental Quality, Office of Geological Survey. Lansing, MI: 517- 241-1515 Gwinn, MI: 906-346-8300 Website – www.michigan.gov/deq or www.michigan.gov/deqogs Choose Land (on Left side), Choose Geology in Michigan, Geological Mapping, or Gas, Oil, and Minerals, etc. (on Left side). A Public Benefit from the leasing and production of state-owned oil & gas and minerals --Most monies collected in State of Michigan Mineral Lease fees, rentals and royalties go to the Michigan Natural Resources Trust Fund (MNRTF) or the Fish and Game Fund, dependent upon origin of the State land ownership, about a 90/10 split. Local and State governmental units may apply to the MNRTF for grants for the purchase and development of public recreational properties. In Fiscal Year 2008, the State’s income from the leasing and production of state-owned minerals and oil and gas was approximately $104 million. About 90 percent, nearly $94 million was placed into the MNRTF. “Explore Michigan’s Minerals” 30 �BIRD RIVER BELT IN SOUTHEASTERN MANITOBA – A NEOARCHEAN VOLCANIC ARC IN THE WESTERN SUPERIOR PROVINCE GILBERT, H.P. Manitoba Geological Survey (360-1395 Ellice Ave., Winnipeg MB R3G 3P2). paul.gilbert@gov.mb.ca The Neoarchean Bird River Belt (BRB) in southeastern Manitoba is currently the focus of geological and geochemical investigations that have led to a revised interpretation of its tectonic setting and geological history. The BRB is part of a 150 km long, east-trending supracrustal belt that extends from Manitoba eastwards as far as Separation Lake in Ontario. It is located in the Bird River Subprovince within the southwestern Superior Province and occurs in a transitional oceanic–continental-margin setting between flanking older cratonic blocks — the 3.0-2.87 Ga North Caribou Superterrane to the north and the 3.4-2.8 Ga Winnipeg River Subprovince to the south (Percival et al., 2006). The predominant 2.724 Ga arc-type volcanic rocks of the BRB are compositionally and stratigraphically distinct from flanking, mid-ocean-ridge basalt (MORB)–type basaltic sequences that are probably relatively older than the arc-type rocks and may be associated with early arc rifting in a backarc setting (Gilbert et al., 2008). The MORB and arc-type volcanism together spanned at least 20 Ma; a mafic-ultramafic intrusion in the north part of the belt (2.745 Ga Bird River Sill) postdates the MORB-type volcanism but was emplaced prior to the arc-type volcanism. The arc-type volcanic rocks are divided into ‘north’ and ‘south’ structural panels that are each characterized by geochemically and stratigraphically distinct volcano-sedimentary sequences. The north panel rocks are akin to modern subduction-related rocks at active continental margins, whereas the sequence in the south panel documents incipient rifting in an extensional tectonic regime. Subsequent to arc volcanism, orogenic sedimentation (2.71– 2.70 Ga) resulted in the deposition of turbidites (Booster Lake Formation) and fluvialalluvial deposits (Flanders Lake Formation). Detrital zircon data indicate these orogenic sedimentary rocks may be stratigraphically equivalent to epiclastic rocks and metamorphic derivatives in the English River Subprovince, northeast of the BRB. Base-metal mineralization prospects in the BRB include both magmatic types and stratigraphically associated occurrences of probable hydrothermal origin (Gilbert, 2008). The Bird River Sill hosts base-metal and platinum-group-element (PGE) mineralization; elsewhere, base-metal mineralization commonly occurs at lithological contacts within the volcano-sedimentary sequences. The BRB also contains the TANCO mine at Bernic Lake, wholly owned by the Cabot Corporation. The mine produces Ta, Li and Cs from pegmatite and accounts for approximately 80% of global reserves of Cs. References Gilbert, H.P., 2008: Stratigraphic investigations in the Bird River greenstone belt, Manitoba (part of NTS 52L5, 6); in Report of Activities 2008, Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, p.121-138. 31 �Gilbert, H.P., Davis, D.W., Duguet, M., Kremer, P.D., Mealin, C.A. and MacDonald, J. 2008: Geology of the Bird River Belt, southeastern Manitoba (parts of NTS 52L5, 6); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Map MAP2008-1, scale 1:50 000 (plus notes and appendix). Percival, J.A., McNicoll V. and Bailes, A.H. 2006: Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and >2.98 Ga continent margin sequences and its implications for Archean terrane accretion, western Superior Province, Canada; Canadian Journal of Earth Sciences, v. 43, p. 895–927. 32 �GEOCHEMISTRY AND PETROLOGY OF GUNFLINT IRON FORMATION, GUNFLINT TRAIL, MINNESOTA HAGE, Melissa M.1 and FEDO, Christopher M.1 (1) Earth and Planetary Sciences, Univ. of Tennessee, Knoxville, TN 37996, mhage@utk.edu Near the NW shore of Lake Superior, along the Minnesota-Ontario boarder, the Gunflint Trail provides access to Paleoproterozic sedimentary rocks of the Animikie Group (ca. 18701830 Ma; Fralick et al., 2002), including the Gunflint Iron Formation. Animikie Group rocks crop out along a NE-SW-trending outcrop belt that extends approximately 175 km from ThunderBay, Ontario, where the unit is unmetamorphosed, to ~19 km west of the Gunflint Trail in northern Minnesota, where it is truncated by the Mesoproterozoic Duluth Complex. Metamorphic grade increases to upper amphibolite facies (Floran and Papike, 1978; Jisra andWeiblen, 2007). Although focusing research on metamorphosed Gunflint banded iron formation (BIF) might seem unusual from a sedimentalogical perspective when unmetamorphosed equivalents exist, many Archean BIFs the world over are highly metamorphosed, making an examination of the Gunflint Formation in a locale that has been metamorphosed very appropriate for making comparisons between BIFs of varying age. An ~ 8 m section of BIF from the lower slaty unit of the Lower Gunflint Formation was measured and samples representative of all the major lithologies were collected and facies logged. At field scale, the main lithologies include finely banded magnetite-quartz BIF and coarsely banded magnetite-quartz BIF, with less common centimeter-scale chert-dominated layers containing rip-up clasts of magnetite-rich layers. In thin section, samples range between Fe-silicate and oxide facies BIF, and contain varying amounts of quartz, magnetite, and amphibole. When quartz is present, it is as equant grains with 120º interlocking grain boundaries, which suggests recrystallization, and range in size from ~50 μm to 500 μm. If magnetite is present, it is typically anhedral, ranges in size from ~ 10 μm to 200 μm, and occurs either has disseminated grains between quartz or amphibole grains or as distinct bands. Amphiboles are found in all samples and typically occur as a clotted mass of fine (< 50 μm) equant grains or needles with a few larger (~ 200 μm) grains also present. Similar to the magnetite, the amphiboles occur either as disseminated grains and needles in-between quartz and magnetite grains, or as distinct layers. Some samples also contain trace amounts of very small (5-10 μm) grains of apatite. Samples from the measured stratigraphic section were also analyzed for bulk major-, trace- and rare earth element geochemistry. The major element chemistry of the Gunflint BIF is typical of other BIFs and is dominated by SiO2 (~ 36 to 82 wt%) and Fe2O3(T) (~ 16 to 57 wt%), with much lesser amounts of CaO (~ 0.6 to 3.0 wt%), MgO (~ 0.5 to 5 wt%), MnO (~ 0.1 to 0.5 wt%), Al2O3 (~0.3 to 1.7 wt%), Na2O (~ 0.1 to 0.2 wt%), K2O (~ 0.1 to 0.3 wt%), and P2O5 (~0.1 wt%). Although no evidence for clastic contamination was observed in outcrop or thin section scale, the geochemistry indicates slightly elevated abundances, relative to other very pure chemical sediments, of trace elements typically related to clastic detritus, such as Sc (1 to 3 ppm), Th (0.19 to 1.03 ppm), Hf (0.1 to 0.7 ppm), Zr (3 to 27 ppm), and Rb (1 to 35 ppm). Other evidence for aluminosilicate contamination is the lower Fe2O3/Pr ratios (30 to 76) relative to pure BIF (130 to 317) (Bau and Dulski, 1996). It has been suggested that Precambrian BIFs free from clastic contamination display a similar REE signature, regardless of provenance, age, and metamorphic grade: HREE enrichment, positive LaSN, EuSN, GdSN, and YSN anomalies, negative CeSN anomaly, (La/Sm)CN > 1, (Sm/Yb)SN < 1, and (Eu/Sm)SN >1 (Bau and Dulski, 1996; Bolhar et al., 2004). The PAASnormalized REE + Y (REYSN) plots of preliminary metamorphosed Gunflint samples show 33 �positive Ce, Eu, and Y anomalies (~1.1, 1.5 and 33, respectively) with an overall background slope that reflects depletion of the light REE and enrichment of heavy REE (Figure 1). Typically, seawater, and thus BIF, has a strong negative Ce anomaly, however the composition of chemical sediments also reflects the local redox conditions and is strongly influenced by post-depositional changes, which suggests that the Ce anomalies in these samples may not be primary (Derry and Jacobsen, 1990; Rollinson, 1993). The Eu anomalies are only weakly positive, ranging from 1.31.7, suggesting only minor hydrothermal input into the depositional basin (Klein, 2005). This is not unexpected as the size of the positive EuSN anomaly becomes smaller with decreasing age of deposition (Derry and Jacobsen, 1990; Bau and Dulski, 1996). Only one sample has a positive LaSN anomaly (1.01), with the others being only slightly negative (~0.8), and none of the samples have a positive GdSN anomaly (range from 0.03 to 0.25). Other BIF “fingerprints” are found in the amphibolite-grade Gunflint samples analyzed here, with (La/Sm)CN values ranging from ~2.7 to 3.5, (Sm/Yb)SN values ranging from ~0.7 to 1.1, and (Eu/Sm)SN values ranging from ~1.4 to 1.9. The one exception is the sample that has a (Sm/Yb)SN value slightly greater than 1 (1.14). In conclusion, preliminary analyses of metamorphosed Gunflint BIF geochemistry suggests that BIFs are capable of retaining near original compositions through diagenesis and metamorphism. This coincides with the findings of Frost et al., (2007) which found that Fe isotope heterogenetites in BIF are preserved during diagenesis and metamorphism. However, some of the traditional signatures used to fingerprint BIF, such as a negative Ce anomaly, require further investigation to test their veracity. Figure 1. PAAS-normalized REE + Y diagram comparing compositions from three preliminary samples collected from the amphibolite grade Gunflint Iron Formation, along the Gunflint Trail, Minnesota with BIF from amphibolite grade Isua BIF. Note positive Ce, Eu, and Y anomalies in the Gunflint samples. References Bau, M. and Möller, P., 1993, Geochimica et Cosmochimica Acta, 57, 2239-2249. Bohlar et al., 2004, Earth and Planetary Science Letters, 222, 43-60. Derry, L. and Jacobsen, S., 1990, Geochimica et Cosmochimica Acta, 54, 2965-2977. Floran, R. and Papike, J., 1978, Journal of Petrology, 19, 215-288. Fralick, P., Davis, D., and Kissen, S., 2002, Canadian Journal of Earth Science, 39, 1085-1091. Frost, D. et al., 2007, Contributions to Mineralogy and Petrology, 153, 211-235. Jisra, M. and Weiblen, P., 2007, 53rd Annual Institute on Lake Superior Geology Field Trip Guide 6: Geology along the Gunflint Trail. Klein, C., 2005, American Mineralogist, 90, 1473-1499. Rollinson, H., 1993, Using geochemical data: evaluation, presentation, interpretation, 133-142. 34 �TITANITE, PSEUDORUTILE, AND REE-MINERALS IN THE ALLOUEZ CONGLOMERATE, KEWEENAW PENINSULA, MICHIGAN Edward Hansen1, Jesse Reimink1, Daniel Harlov2 1 2 Geological and Environmental Sciences, Hope College, Holland, Michigan, 49423 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany The Allouez Conglomerate is an interflow sedimentary unit in the Portage Lake Volcanic Series that has undergone low grade metamorphism, copper mineralization and supergene alteration (Bornhorst, T.J., personal communication, 2008). Mineral associations and textures in 6 samples were investigated with the JEOL LV-4500 scanning electron microscope in the Department of Geophysical Sciences, University of Chicago and selected minerals were analyzed with the CAMECA SX-100 electron microprobe at the GeoForschungsZentrum, Potsdam. Titanite occurs in lamellae within oxide grains, in composite grains with hematite and/or magnetite (Fig.1), and in small independent grains in epidote-rich domains. It is frequently associated with a Ti-Fe oxide with an average Fe/Ti ration of 2/3 (Fig. 2) and electron microprobe totals of ~ 93%. This appears to be the oxihydroxide pseudorutile that commonly forms as an intermediate product, together with rutile or anatase, during weathering or diagenetic alteration of ilmenite (Schroeder et al.,2004). In the Allouez Conglomerate pseudorutile occurs in three different associations: 1. pseudorutile + titanite + TiO2, 2. pseudorutile + titanite and 3. pseudorutile + TiO2. These mineral associations are most easily explained by a model in which ilmenite is first altered to pseudorutile + TiO2 followed by the formation of titanite by the reaction: TiO2 + CaCO3 + SiO2 → CaTiSiO + CO2. Calculations done with the Perple_X program suggest an upper limit of 0.015 – 0.002 for xCO2 in the fluid phase during the formation of titanite at temperatures of 240 – 320 oC and inferred pressures of 150 MPa (Livnat et al. 1983) during metamorphism of epidote-bearing assemblages in the Portage Lake Series. Pure CaTiSiO5 – Fe oxide assemblages in very low-grade rocks require relatively CO2-poor, oxidizing conditions. Low-grade titanite also frequently contains significant amounts of CaAlSiO4(OH) (Enami et al., 1993) which may increase its stability. Aluminum concentrations in titanite from the Allouez conglomerate range from near 0 to 30% of the Ti site (Fig. 3). The wide range in Al values indicates disequilibrium. There is a strong correlation between Al and F concentrations and stochiometry (Fig. 3), indicates that between 50 and 100% of the Al takes the form of a CaAlSiO4F component. This suggests the presence of F in the metamorphic fluid phase. In medium to high-grade rocks titanite, together with allanite, can play an important role in the REE and Th budget. However both REE and Th were below electron microprobe detection limits in titanite from the Allouez conglomerate. Bright rims and patches evident on epidote grains in BSE images (Fig. 4) indicate an enrichment in LREE elements relatively late in the growth of the epidote. While cores of epidote grains generally contain no detectable LREE bright rims range up to 0.22 REE per 8 cations (Fig.5): close to the boundary between REE-rich epidote and allanite. The other major host for REE elements appears to be synchysite ((REE)CaF(CO3)2) which occurs as acicular crystals associated with calcite in veins and amygdules. REFERENCES Enami, M., Suzuki, K., Liou, J.G., Bird, D.K., 1993. Al–Fe3+ and F–OH substitutions in titanite and constraints on their P–T dependence. European Journal of Mineralogy 5, 219– 231. Livnat, A., Kelly, W.C., Essene, E.J., and Rye, R.O., 1983, P-T-X conditions of sub-greenschist burial metamorphism and copper mineralization, Keweenaw Peninsula, northern Michigan [abs.]: Geological Society of America Abstracts, v. 15, p. 629. Schroeder P.A., Pruett, R.J., and Melear, N.D. (2004) Crystal chemical changes in an oxidative weathering front in a Georgia kaolin deposit. Clay and Clay Minerals 52, 211-220. 35 �36 �A FOLLOW-UP GLACIAL TILL INDICATOR MINERAL SURVEY IN MINNESOTA: WHAT DOES IT INDICATE ABOUT EXPLORATION FOR DIAMONDS AND OTHER MINERAL DEPOSITS? Hauck, S.A., Heine, J.J. – Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Hwy., Duluth, MN 55811-1442 shauck@nrri.umn.edu and jheine@nrri.umn.edu, and Thorleifson, L.H. – Minnesota Geological Survey, University of Minnesota, Twin Cities, 2642 University Ave., St. Paul, MN 55144-4057, thorleif@umn.edu. Why should there be diamondiferous kimberlites in Minnesota? Minnesota has the following attributes that meet the requirements to host diamondiferous kimberlites; 1) an Archean-aged Superior Craton root that underlies 2/3s of MN, including a 3.2-3.8b.y cratonic fragment in SW MN that is required to produce diamonds (Helmstaedt, 2006), and diamondiferous kimberlites that have been found elsewhere within the Superior Craton in Ontario and Michigan; 2) major crustal structures cross-cut MN’s cratonic root, e.g., Vermilion Fault Zone, Great Lakes Tectonic Zone, Quetico Fault, etc., which are excellent kimberlite exploration areas; 3) KenoraKabetogama and Keweenawan dike swarms that intersect these and other structures and could have provided pathways for kimberlite emplacement, e.g., kimberlites found in the Kyle Lake and Attawapiskat kimberlite clusters in N. ONT.; 4) an Archean terrain with calc-alkaline lamprophyres and ultramafic volcanics that are time-equivalent with the Michipicoten greenstone belt, i.e., Wawa area, in ONT that has diamondiferous calc-alkaline lamprophyres/volcanics associated with diamondiferous heterolithic breccias and conglomerates. A diamondiferous ultramafic pyroclastic unit (Grassy Ultramafic Pyroclastic, MetalCORP Ltd.) occurs just north of the Minnesota border in Ontario (Ont. Geol. Survey, 2008). Figure 1. Location of various indicator mineral surveys. Since 2004, samples from the B- or C-horizon soils of various glacial tills throughout MN were collected for indicator minerals and/or related geochemistry, including pristine (1,189gr.), modified (321gr.), and reshaped (1,606gr.) gold grains (Heine et al., 2008). The six indicator mineral and geochemical sampling campaigns included: 1) one statewide C-horizon survey (WMC-MGS; Thorleifson et al., 2007); 2) two regional C-horizon surveys (NRRI-MGS, 2006-2007, in prep.; Heine et al., 2008, 2009); 3) two local B-horizon surveys (MnDNR; Dahl, 2005; Elsenheimer, 2006, respectively); and 4) Larsen (2004) reported on the -63µ geochemistry from Chorizon tills in the western Vermilion District in NE MN. Also, Martin (1995) reported on a select number of kimberlitic indicator minerals in MN, and additional till geochemistry on buried tills (Martin et al., 1988, 1989, 1991). As a follow-up to the 2004 WMC-MGS glacial till sampling program (30 km spacing) in MN, a jointly funded Minerals Coordinating Committee (MCC) and Permanent University Trust Fund (PUTF) project collected glacial till samples on a10 km spacing in NE and E-central MN. The WMC-MGS survey found carbonate clasts and carbonate matrix material throughout most of the State, except in the area of the MCC-PUTF-funded survey (Fig. 1). This thin drift area was deemed an area where closer spacing would be required in this heterogeneous area. The WMC-MGS survey collected 270 till samples, and the follow-up survey collected 79 samples. A 3rd survey collected 42 older till samples. The rationale for this survey was based upon a number of Cr-pyrope samples located during the initial WMC-MGS survey (Fig. 2). These samples were collected from rotosonic drill core in older glacial tills and exposures of Superior tills. Much of the previous sampling was in Des Moines tills. Also, unanalyzed indicator minerals from a previous Manitoba survey were analyzed using PUTF funds to understand the provenance of the G2, G7, G9, and G11 garnets found in southern MN (Figs. 1, 2; Thorleifson et al., 2009) and Mg-chromites and Mg-ilmenites found near MN northern border (Fig. 2). Data from the Manitoba Geological Survey indicator database will be combined with these data to better understand the provenance of MN Des Moines till samples. 37 �The data from these surveys indicate: 1) there is an anomalous gold, plus As, zone south of the Vermilion Fault in NE MN; 2) an anomalous area west of the basal contact of the Duluth Complex that extends into the Western Vermilion District, and is anomalous in Au, As, Ag, Pt, Pd, Cu, Co, Ni, chromite, gahnite, etc. (Fig. 2); 3) several concentrations of Crdiopsides in the 1st survey and 2nd survey that may relate to older tills; 4) the anomalous garnets in the 1st survey area are probably related to: a) reworked older till in MN; or b) they were transported from Manitoba; and 5) a combination of Mn-epidotes, gahnites, corundum, tourmalines, , and geochemical Cu, Co, and Ag anomalies in the old Figure 2. Results of indicator mineral and geochemistry surveys in MN. tills suggest a relationship the Wisconsin VMS belt and/or its MN extension. References Dahl, D.A., 2005, Results of glacial till sampling in the Vermilion greenstone belt, NE MN: MN Dept. Nat. Res., Div. Lands and Minerals, Project 365, 79 p. Elsenheimer, D., 2006, Results of glacial till sampling in the Virginia Horn Greenstone Belt, St. Louis County, Minnesota; St. Paul, MN Dept. Nat. Res., Div. Lands and Minerals, Project 370, Open-File Report, February 2008. Heine, J., Hauck, S., Thorleifson, H., Dahl, D., and Martin, D., 2008, Distribution of gold grains in Minnesota till: University of Minnesota Duluth, Natural Resources Research Institute, NRRI Poster-2008/02. Heine, J., Hauck, S., and Thorleifson, H., 2009, Selected indicator mineral and till chemistry results from multiple till surveys in MN: University of Minnesota Duluth, Natural Resources Research Institute, NRRI Poster-2009/01. Helmstaedt, H.H, 2006, From cratons to carats: Relationships between lithosphere-forming events and diamond growth episodes: Prospectors and Developers Association of Canada, PowerPoint presentation with voice over, CD #1. Larsen, P.C., 2004, Regional till geochemistry survey of the western Vermilion greenstone Belt, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2004/23, 33 p. Martin, D., 1995, A limited survey of selected kimberlite indicator minerals from glaciofluvial sediments across Minnesota: MN Dept. Nat. Res., Div. Minerals, Report 314, 29 p. Martin D.P., Dahl, D.A., Cartwright, D.F., and Meyer, G.N., 1991, Regional survey of buried glacial drift, saprolite, and Precambrian bedrock in Lake of the Woods County, MN: MN Dept. Nat. Res., Div. Minerals Report 280, 75 p. Martin, D.P., Meyer, G.N., Cartwright, D.F., Lawler, T.L., Pasitka, J.T., Jirsa, M.A., Boerboom, T.J., and Streitz, A.R., 1989, Regional geochemical survey of glacial drift drill samples over Archean granite-greenstone terrane in the Effie area, northeastern MN: MN Dept. Nat. Res., Div. of Minerals, Report 263, 2 vols., v. 1 , 59 p., v. 2, 323 p. Martin, D. P., Meyer, G. N., Lawler, T. L., Chandler, V. W., and Malmquist, K. L., 1988, Regional survey of buried glacial drift geochemistry over Archean terrane in northern MN: MN Dept. Nat. Res., Div. of Minerals, Report 252, Part I, 74 p., Part II, 386 p. Ontario Geological Survey, 2009, Diamonds 2009; Handout, Prospectors and Developers of Canada Association meeting, Toronto, Canada, March 1-3, 2009, 13 p. Thorleifson, L.H., Harris, K.L., Hobbs, H.C., Jennings, C.E., Knaeble, A.R., Lively, R.S., Lusardi, B.A., and Meyer, G.N., 2007, Till geochemical and indicator mineral reconnaissance of Minnesota: MN Geol. Surv. OFR-0701. Thorleifson, L.H., Matile, G.L.D., Keller, G.R., and Hauck, S.A., 2009, Till geochemical and indicator mineral reconnaissance of southeastern Manitoba (west half of NTS 52E AND 52L and all of 62H and 62I): final results: Manitoba Geological Survey, Open File Report 2009-13, 6 p., plus Access Database and plates. 38 �Developing a 21st Century Geoscience Major: Melding the Old with the New HEFFERAN, Kevin P. (kheffera@uwsp.edu) and HEYWOOD, Neil C. (nheywood@uwsp.edu), Department of Geography and Geology, University of Wisconsin-Stevens Point, Stevens Point, WI 54481 Many Geology Programs are facing uncertain futures due to budgetary constraints and poor communication of the role Geoscientists play in our world. According to the U.S. Bureau of Labor (http://www.bls.gov/oco/ocos288.htm), Geoscience—which incorporates fields such as geology, geography, biology, chemistry, physics, climatology and oceanography—is anticipating over 20% job growth during the next 15 years as existing workers retire, and energy and water resource needs expand. In response to this need, the University of Wisconsin-Stevens Point (UWSP) Department of Geography and Geology recently received a $1.7 million dollar grant to develop a GIS (Geographic Information Systems) Center for geospatial studies. This GIS Center will provide a means for undergraduate students to apply GIS, remote sensing, and other geospatial techniques to address local, regional, and global issues. UWSP’s Department of Geography & Geology has also implemented a new Geoscience Major for the Spring 2009 semester. Our goal is to retain fundamental elements of traditional geology programs and to incorporate high-technology geospatial skills applicable to the 21st Century workforce. Fundamental geology courses in physical geology, Earth history, Earth materials, structural geology, and sedimentary geology are coupled with remote sensing, GIS, environmental, and hydrogeology course offerings. Perhaps most important of all is a 3-credit field component within the U.S.A. that represents the keystone experience for Geoscience majors. Field research is a key training element for geoscientists, and is essential for understanding Earth processes. With respect to pedagogical developments, we present the recently-published Physical Geography Laboratory Manual (Lemke, Ritter and Heywood, 2008) and the imminent Earth Materials textbook (Hefferan and O’Brian 2010). Both texts expose students to the question, “What does contemporary society need, and expect, from Geoscientists?” We believe the new UWSP Major encompasses three critical elements to a contemporary undergraduate Geoscience Program: 1) fundamental Geoscience courses coupled with computer-based instruction; 2) active field experience; and 3) pedagogical innovations that reflect and adapt to the evolving field of Geoscience. Communication between the general public, industry and scientists are critical for successful Geoscience programs. At this interactive poster, we would appreciate a lively dialogue with other Geoscience faculty, students and professionals to learn of their approaches to future needs and demands. This is a Poster Presentation by Two UWSP Faculty Members 39 �DETAILED PETROGRAPHIC ANALYSIS OF ANTHRAXOLITE MORPHOLOGY IN THE BIWABIK IRON- FORMATION, NORTHERN MINNESOTA HILLER, James A. and SHAPIRO, Russell S. Dept. of Geological and Environmental Sciences, California State University, Chico; Chico, CA 95929 jamesashland@gmail.com Anthraxolite is a pyrobitumin composed of ~95% carbon (Morey 1994). It is widely believed to be the product of metamorphosed petroleum having had nearly all of its volatiles driven off. In the Biwabik Iron-Formation in northern Minnesota, anthraxolite has been found in a variety of locations but is generally constrained between the Intermediate Slate and the stromatolite layers of the Lower Cherty member (Morey 1994). This has led some to believe that the anthraxolite was sourced from the overlying Intermediate Slate, interpreted as a carbon-rich ash layer. The goal of this research is to study the morphology of anthraxolite at varying depths to provide a clearer explanation and understanding of the pattern of migration. This is accomplished by analyzing thin-sections at varying depth from core. The anthraxolite in core MGS-5 is located from 947'-995' (Severson 2005) and is found between the Intermediate Slate and the Lower Cherty stromatolites, consistent with Morey's (1994) observations. Five thin sections were made from several depths in order to better understand the events leading to the present distribution and morphology of the anthraxolite: 950', where thin sections were made parallel and perpendicular to bedding; 963', 988.5' and 995' where thin sections were made perpendicular to bedding. At 950', the anthraxolite ranges from 69-235 µm across (averaging 110 µm), is almost exclusively in the carbonate veins, and fills the void space between crystals. The anthraxolite is also found as rounded blebs when viewed parallel to bedding, with both concave and convex surfaces. Where it is found outside of the vein fill, the blebs range from 7-67 µm with an average diameter of 20 µm. These fragments are highly fractured and appear to be cutting the cherty matrix. At 963' the anthraxolite is in chert, between and overprinting 97-345 µm diameter grains of calcite. Here, the crescent-shaped blebs open in the upward direction with the upside fractured and the down-side cutting across older minerals. The crescents range in size from 268-843 µm in diameter and are spherical to crescent-shaped. The anthraxolite is also found in both cherty matrix and carbonate veins as spherical blebs. This distribution and morphology suggest movement as a liquid. Anthraxolite is also found as irregularly fractured grains suggesting movement after solidification. The crosscutting relationships associated with the anthraxolite in the Biwabik Iron- Formation is evidence of a complex history leading to its present location and morphology. There is, however, very little evidence of the original source of the anthraxolite despite previous hypotheses. Proposed future work includes analyzing pure anthraxolite samples that have already been isolated, using catalytic hydropyrolysis (HyPy) to drive off volatiles not removed during the solvent extraction. HyPy allows for more product to be analyzed by gas chromatography mass spectrometry (GC-MS) without damaging the larger organic ring structures (Marshall et al. 2007). The HyPy analysis should allow better identification of possible biomarkers and will significantly aid in the determination of a source. 40 �References Marshall, C.P., Love, G.D., Snape, C.E., Hill, A.C., Allwood, A.C., Walter, M.R., Van Kranendonk, M.J., Bowden, S.A., Sylva, S.P., Summons, R.E., 2007. Structural characterization of kerogen in 3.4 Ga Archaean cherts from the Pilbara Craton, Western Australia. Precambrian Research 155, 1–23. Morey, G. B., 1994, Anthraxolite in the Early Proterozoic Biwabik Iron Formation, Mesabi Range, northern Minnesota in Southwick, D. L. [editor] Short contributions to the geology of Minnesota. Minnesota Geological Survey, St. Paul, MN, United States), Report of Investigations 1994, 39–47. Severson, M. J., 2005. Preliminary correlation of submembers within the Biwabik Iron Formation as deciphered from geological descriptions obtained from various iron ore mines and other sources on the Mesabi range of Minnesota. Natural Resources Research Institute, Duluth, Minnesota, NRRI/MAP-2005-01 (draft). 41 �MESOPROTEROZOIC MIDCONTINENT RIFT-RELATED MAFIC INTRUSIONS IN NORTHWESTERN ONTARIO: CONTINUING GEOCHEMICAL, PALEOMAGNETIC, PETROGRAPHIC AND GEOCHRONOLOGIC STUDIES HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development and Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada, HALLS, Henry, Department of Geology, University of Toronto at Mississauga, Ontario L5L 1C6, HEAMAN, Larry, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada During the summer of 2008 a detailed sampling program of the Midcontinent Rift-related intrusions was initiated around Thunder Bay. This sampling built upon previous work undertaken between 1999 and 2007 by M. Smyk and/or P. Hollings (Hollings et al, 2007a; Hollings and Smyk 2008) who collected and analyzed over 100 samples. Over 150 additional samples were collected during the 2008 field season for geochemistry, petrography, geochronology, radiogenic isotopes and paleomagnetism studies. The whole rock geochemistry of the gabbroic sills to the south of Thunder Bay supports earlier observations by Hollings and Smyk (2008) that Logan-type sills predominate in this area, with the exception of the recently recognised Riverdale sill. Logan sills are typically composed of sub-ophitic gabbro and are distinguished geochemically by elevated Gd/Ybn (ca. 2.0 to 2.7) and La/Smn (2.0 to 2.5), as well as elevated TiO2 (ca. 3.0 to 4.5) as compared to Nipigon sills (Hollings et al. 2007b). Gabbro dykes containing anorthositic gabbro blocks crosscut Logan sills but share a similar rare earth chemistry. In contrast, the majority of the dykes analysed are geochemically comparable to the Nipigon sills suggesting that they do not represent feeders for the Logan sill complexes. However, limited data suggest that there is no significant geochemical difference between the Pigeon River, Cloud River and Mt. Mollie dyke suites, despite their different orientations and reported ages. Preliminary results suggest the presence of two other distinct dyke suites: one north-trending suite which intruded Rove Formation rocks southwest of Thunder Bay and a west-northwest-trending suite sampled on the Sibley Peninsula which intruded Sibley Group sedimentary rocks. Samples for paleomagnetic study were collected at four sites (Smyk et al. 2008). 1) The Riverdale sill and a geochemically distinct dyke which intruded both sill and Rove Formation sedimentary rocks exhibit reversed magnetic polarity. Directions obtained from the sill and the dyke are indistinguishable. 2) A 40 m wide, northeast-trending Pigeon River dyke near Arrow River (1078 + 3 Ma; Heaman et al. 2007) yielded a normal polarity. 3) An 85 m wide, northeast-trending Pigeon River dyke (Rita Bolduc locality; 1141 + 20 Ma, Heaman et al. 2007). Like the dyke near Arrow River, this dyke also exhibits normal magnetic polarity. Site directions in these two Pigeon River dykes are not significantly different at the 95% confidence level. 4) A 100 m wide, northwest-trending Cloud River dyke in Crooks Township yielded a reversed polarity. 42 �Samples were also collected for geochronologic study. Samples taken from the Riverdale sill yielded no dateable material and baddeleyite ages for the Cloud River dyke are pending. Preliminary data from baddeleyite from the Mt. Mollie dyke indicate an age of 1109.3 ± 6.3 Ma. This is older than the age of 1099.6 + 1.2 Ma (Heaman et al. 2007) that has been reported for the Crystal Lake gabbro with which the Mt Mollie dyke has been traditionally associated (Smith and Sutcliffe 1987). Additional geochronologic and radiogenic isotope data are pending. An enigmatic volcanic unit mapped by Tanton (1936) in central Devon Township was also sampled in 2008. Initially mapped as Rove Formation basalt, the unit consists of massive to columnar-jointed basaltic andesite flows and perhaps subvolcanic sills amongst Rove Formation clastic sedimentary rocks. Flows exhibit vesicular and amygdaloidal textures and locally have ropy tops. Although the rare earth element geochemistry of this volcanic unit is similar to that of the Riverdale sill, its magnesium content is lower. The remarkably coherent rare earth element geochemistry of this volcanic unit has been used to discriminate it from nearby Logan sills. References Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario; Canadian Journal of Earth Sciences, v.44, p.1055-1086. Hollings, P. and Smyk, M, 2008. Whatever happened to the Logan sills? Ongoing research into the geochemistry of Midcontinent Rift-related mafic intrusive rocks south of Thunder Bay; in Institute on Lake Superior Geology, Proceedings, v.54, part 1, p.36-37. Hollings, P., Smyk, M., and Hart, T., 2007a. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes; in Institute on Lake Superior Geology 53rd Annual Meeting, Proceedings, v.53, part 1, p.40-41. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A., 2007b. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, v.44, p.1087-1110. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan intrusive rocks of the Thunder Bay area; in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248255. Smyk, M., Hollings, P., Halls, H., and Easton R.M., 2008. Project Unit 08-021. Mesoproterozoic Midcontinent Rift-Related Mafic Intrusions near Thunder Bay: Geological, Paleomagnetic, Geochemical and Geochronological Studies. Summary of Field Work and Other Activities 2008, Ontario Geological Survey, Open File Report 6226, p. 18-1 to 18-6. Tanton, T.L. 1936. Pigeon River area; Canada Department of Mines, Geological Survey, Map 354A, scale 1:63 360. 43 �PRELIMINARY RESULTS OF 40AR/39AR THERMOCHRONOLOGY FROM THE CENTRAL YAVAPAI PROVINCE, U.S. MIDCONTINENT Angie Hull, Daniel Holm, Dept. of Geology, Kent State University, Kent, OH 44242; David Schneider, Dept. of Geological Sciences, Univ. of Ottawa, Ontario, Canada Much is known about the Precambrian U.S. tectonic and crustal evolution in regions where bedrock is exposed for direct observation (i.e. Rocky Mountains, Lake Superior region; Karlstrom & Keller, 2005; Holm et al., 2007). However, Phanerozoic sedimentary cover overlying the central Yavapai Province (YP), between the Rockies and the Great Lakes, limits our knowledge of that important region. Ar-Ar thermochronology from the upper Great Lakes region and western U.S. has proven critical for assessing Proterozoic tectono-metamorphic overprinting and cooling during and following growth and stabilization of southern Laurentia. Here we present the first results of Ar/Ar thermochronology on Paleoproterozoic basement drill core rocks in easternmost Colorado, Nebraska, and southern South Dakota (see figure). South Dakota. Hornblende from medium grained metabasalt in south central SD (SDBE) yields a complicated age spectra with a total gas age of 2268 Ma and a preferred age of 2449 Ma from selected steps constituting 31% of the gas released. Further east, a garnet rich gneiss (Woon W-1) yields a biotite plateau age of 1869 Ma (7 steps; 86% of total gas released). In easternmost SD, biotite from a granite gneiss (SDGT) yielded a plateau age of 1728 Ma (5 increments; 59% of total gas released). Southern Nebraska. Muscovite from a granite (NBDU) yields a plateau age of 1251 Ma (12 steps; 70% of the gas released). Farther north, biotite from a granite gneiss (NBCS-2) yields a plateau age of 1267 Ma (5 increments; 51% of the gas released). In the central region of southern NE, hornblende from a sheared tonalite (NBBF-1) yields a complicated age spectra with a total gas age of 1468 Ma and a preferred age of 1487 Ma from a large step constituting 36% of the total gas released. Nearby, biotite from a granite gneiss (NBDA-2) yields a plateau age 1222 Ma (6 increments; 75% of the total gas). In the southeast corner of NB, both biotite and muscovite from a granite (NBPN-1) were analyzed. The biotite yielded a slight saddle-shaped spectra, indicating possible excess argon, with a total gas age of 1138 Ma; muscovite yielded a plateau age of 1200 Ma (11 increments; 69% of the gas released). Slightly northeast of this locale, biotite from a granite yields a complex spectra and a total gas age of 1231 Ma. Eastern Colorado. Biotite from a deformed gabbro in Kit Carson Co. yielded a plateau age of 1238 Ma (7 increments; 85% of the total gas). In the north, the 1.86 Ga biotite age is consistent with rapid cooling after peak Trans-Hudson/ Penokean metamorphism. The 1728 Ma biotite age (SDGT) is similar to abundant 1750-1720 Ma hornblende/ biotite ages reported in east-central Minnesota, suggesting that geon 17 metamorphic affects extend westward into SD. Most surprising is the 2449 Ma hornblende age, which suggests the presence of Archean crust within the Proterozoic mobile belts of southern Laurentia. In the south, our results demonstrate the YP experienced the affects of younger, Proterozoic events largely unfelt throughout neighboring regions. Mica ages are consistently young across a >700 km swath of southern NE and eastern CO. Similarly young ages are reported from only a handful of the hundreds of samples dated in the Rockies and the upper Great Lakes region. Because Proterozoic crust in the southern Lake Superior were virtually unaffected thermally by 1.1 Ga rifting, we consider it unlikely that our 1130-1260 Ma mica ages represent widespread partial resetting during rifting. In CO and NM, similarly young mica ages are interpreted as partially reset by Tertiary igneous activity or representing deeper, and therefore more slowly cooled crustal levels (Shaw et al., 2005). However, both interpretations seem unlikely for our study area. We tentatively suggest that a regional thermal 44 �event possibly related to Neoproterozoic deformation may be responsible for the anomalously young Ar/Ar ages reported here. Holm, D.K., Schneider, D., and Chandler, V.W., 2007a, Proterozoic tectonic and crustal evolution of the Upper Great Lakes region, North America, Precambrian Research, v. 157. Karlstrom, K.E., Keller, G.R. (Eds.), 2005, Rocky Mountain Region: An Evolving Lithosphere. Am. Geophys. Union, Geophysical Monograph 54, pp. 421-441. Shaw, C.A., Heizler, M.T., and Karlstrom, K.E., 2005, 40Ar/39Ar Thermochronologic record of 1.45-1.35 Ga intracontinental tectonism in the southern Rocky Mountains: Interplay of conductive and advective heating with intracontinental deformation, in The Rocky Mountain Region: An Evolving Lithosphere Geophysical Monograph, the American Geophysical Union, Series 154, p. 163-184. Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, Sm-Nd, and Ar-Ar geochronology:Precambrian Research, v. 157, p. 80-105. 45 �LITHOGEOCHEMICAL EVALUATION OF NEOARCHEAN MAFIC VOLCANIC ROCKS COMPRISING THE FOOTWALL OF THE SOUDAN MEMBER OF THE ELY GREENSTONE FORMATION, NORTHEASTERN MINNESOTA JANSEN, A.C., HUDAK, G. J., Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI 54901, jansea73@uwosh.edu HEINE, J. J., PETERSON, D.M., Natural Resources Research Institute, University of Minnesota - Duluth, Duluth, MN 55811 The Ely Greenstone Formation (EG) comprises a steeply north- to southwest-dipping, north – to southwest younging sequence of Neoarchean supracrustal and associated intrusive rocks that are warped about the Tower-Soudan anticline in the Vermilion District of northeastern Minnesota. The EG has historically been broken up into three members. Upsection, these are: a) the Lower Member of the Ely Greenstone Formation (LMEG, composed of calc-alkaline to tholeiitic basalt and basalt andesite lava flows and tuffs with subordinate felsic lava flows, volcaniclastic and epiclastic rocks, and iron formations); b) the Soudan Iron Formation Member (SMEG, composed of Algoma-type banded iron formations, basalt lava flows, epiclastic rocks and minor felsic lava flows and tuffs); and c) the Upper Member of the Soudan Iron Formation (UMEG, composed of a monotonous sequence of tholeiitic basalt lava flows and local Algoma-type iron formation lenses (Schulz, 1980; Southwick et al., 1998; Hudak et al., 2002; Peterson, 2001; Peterson and Patelke, 2003; Hudak et al., 2007). Schulz (1980) and Hudak et al. (2007) interpret volcanic textures, sedimentary textures, and lithological characteristics to indicate a transition from a subaerial/shallow subaqueous setting to a deeper subaqueous environment during the temporal genesis of the EG. Southwick et al. (1998) indicate that a sharp transition from basaltic and basalticandesitic rocks with arc-like geochemical signatures in the LMEG, to basaltic rocks with MORB-like geochemical signatures in the UMEG, occurs abruptly at the top of the SMEG. More recent studies in the vicinities of Fivemile-Needleboy-Sixmile Lakes (Hudak et al., 2007) and Armstrong Lake (Jirsa et al., 2001) indicate that the transition from arc-like magmatism to MORB-like magmatism is more complicated than previously thought, with MORB-like basalts first occurring in the uppermost parts of the LMEG approximately 50100 meters into the footwall of the SMEG (Hudak et al., 2007), as well as locally within the SMEG (Jirsa et al. 2001). A model encompassing initial volcanic arc development followed by back-arc rifting immediately prior to the deposition of the SMEG has been proposed (Hudak et al., 2007). The purpose of this investigation was to evaluate, on a more regional scale than had been previously performed, the lithogeochemistry of mafic and intermediate volcanic rocks that occur in LMEG in the footwall to the SMEG. In addition to field and lithogeochemical data from the Fivemile-Needleboy-Sixmile Lake areas (Hudak et al., 2007) and Armstrong Lake areas (Jirsa et al., 2001), detailed mapping (1:2500-1:5000 scale) and sampling was performed south of Twin Lakes (on the northeastern part of the Vermilion District) as well as in the vicinity of Putnam Lake (in the southwestern part of the Vermilion District). Following field mapping, petrographic and lithogeochemical studies were performed. Petrographic work was utilized to distinguish between fin-grained massive flows and diabase sills and 46 �dikes. Whole rock major and trace element lithogeochemical analyses utilizing a wide variety of analytical methods (ICP/MS, instrumental neutron activation analysis, coulometry, and gravimetric) were employed. In the Twin Lakes area, mafic volcanic rocks with arc-like lithogeochemical signatures transition up-section into mafic volcanic rocks with MORB-like lithogeochemical signatures consistent with back-arc basin basalts in the immediate footwall (<100 meters) to the SMEG. The same lithogeochemical transition has also been documented in the immediate footwall rocks to the SMEG in the vicinity of Putnam Lake. Our results allow us to, for the first time, to document a change from arc-like to MORB-like (back-arc basin consistent) magmatism in the immediate footwall rocks to the SMEG on a regional basis. The development of a back-arc basin in a volcanic arc not only enables the occurrence of MORB-like volcanism, but also is commonly associated with the development of vigorous, regional hydrothermal activity which can produce chemical sedimentary rocks (e.g Algoma-type iron formations) as well as volcanogenic massive sulfide deposits (Franklin et al., 2005; Piercey et al., 2004). It is therefore not suprising that the SMEG occupies a position immediately up-section from the arc- to MORB lithogeochemical transition within the EG. The presence of this transition, apparently across the entire strike length of the SMEG may be explained if the current erosional surface is sub-parallel to a rift structure which occurred within the proposed back arc basin. References Hoffman, A. T., 2007. Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits; unpublished M.S. thesis, University of Minnesota-Duluth, 295 p. Franklin, J. M., Gibson, H. L., Jonasson, I. R. and Galley, A. G., 2005. Volcanogenic massive sulfide deposits: Society of Economic Geologists 100th Anniversary Volume, p. 523-560. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007. Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Institute on Lake Superior Geology, Proceedings Volume 53, Part 1, Proceedings and Abstracts, p. 42-43. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001. Bedrock geologic map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series Map M-114, 1:24000 scale. Peterson, D. M., and Patelke, R. L., 2003. National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: NRRI Technical Report NRRI/TR-2003/29, 88p. Piercey, S. J., Murphy, D. C., Mortenson, J. K., and Creaser, R. A., 2004. Mid-Paleozoic initiation of the northern Cordilleran marginal backarc basin: geologic, geochemical, and neodymium isotope evidence from the oldest mafic magmatic rocks in the Yukon-Tanana Terrane, Finlayson Lake District, Southeast Yukon, Canada: Geological Society of America Bulletin, v. 116, no.9/10, p. 1087-1106. Schulz, K. J., 1980, The magmatic evolution of the Vermilion Greenstone Belt, NE Minnesota: Precambrian Research, v. 11, p. 215-245. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geological setting and descriptive geochemistry of Archean supracrustal rocks and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 47 �GEOLOGIC MAPPING OF NEOARCHEAN ROCKS NEAR PAULSEN LAKE, BOUNDARY WATERS CANOE AREA WILDERNESS, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2008 FIELD CAMP Mark Jirsa, (jirsa001@umn.edu); Hugh Cowan, Jacqueline Kowalik, and John Niedermiller The Precambrian Research Center—a branch of the University of Minnesota, Duluth—conducted its second season of field camp in 2008. After 5 weeks of field training, students were assigned “Capstone Projects” that provide an opportunity to create new geologic maps in areas of poorly understood geology. Junior authors listed above are students who mapped Neoarchean bedrock in the Paulsen Lake area, which was burned by the 2006 Cavity Lake forest fire in the northeastern part of the BWCAW. The fire greatly improved access to and visibility of geologic features, allowing detailed mapping of rock units and complex contact relationships. The 2008 map area (Fig. 1) lies in the northeastern part of the U.S.G.S. Gillis Lake 7.5-minute quadrangle. Previous mapping was fairly detailed (Vervoort, 1987), but contacts were not well constrained. The map and geologic descriptions presented here are based on literature review and the observations from several days of field work— no petrographic or geochemical data were acquired. Figure 1. Simplified geology of the Cavity Lake fire area (fire=dotted outline), showing location of detailed mapping described here. Small inset map shows location within the Wawa subprovince of Superior Province. Geology modified from Jirsa and Starns, 2008. 48 �The principal Neoarchean rock units (from oldest to youngest) in the 2008 map area are: Paulson Lake sequence—a vertically dipping basement package of back-arc or ocean floor origin, consisting of pillowed and massive basalt flows, and hypabyssal, mafic-ultramafic sills. The age of the sequence is unknown. Well developed spherulitic and spinifex textures, and the presence of peridotite sills imply correlation with the largely ultramafic Newton Lake Formation, which lies to the west and southwest. Saganaga Tonalite (2689 Ma; Corfu and Stott, 1998)—coarse-grained, polyphase intrusion and apophosial dikes that intrude and cut out the base of the Paulsen Lake sequence. The tonalite has a distinctive texture marked by the abundance of quartz "eyes" and presence of hornblende. Jasper Lake sequence—hornblende- and pyroxene-porphyritic, dacitic to trachyandesitic volcanic and volcaniclastic rocks, cut by similarly porphyritic hypabyssal intrusions. Geochemical data and mineralogic similarity indicate that the Saganaga Tonalite may represent the magma chamber from which these rocks erupted. Bedding in supracrustal rocks dips steeply and stratigraphic younging is generally southward, as deduced from pillow morphology in flows and graded bedding and scour structures in conglomeratic rocks. Angular relationships of bedding imply that volcanic conglomerate units of the younger Jasper Lake sequence lie unconformably on the Paulsen Lake mafic-ultramafic rocks. Metamorphic grade appears to be low greenschist facies, except immediately adjacent to the Saganaga Tonalite where amphibolite facies assemblages are well developed and the rocks contain cleavage and a strong, shallow east-plunging mineral lineation. The boundary between rocks of contrasting metamorphic grades is a fault. We infer considerable uplift on the north side immediately adjacent to the Saganaga Tonalite. Conglomerate layers in the Jasper Lake sequence are white to brownish-gray, and clastsupported. Clasts are moderately to well rounded, moderately well sorted, and range from 1 cm to 20 cm in diameter. Clasts consist of porphyritic to aphyric hornblende dacite to trachyandesite, and some fragments have textures similar to phases of the Saganaga Tonalite—supporting the inference that it represents magmatic source to the Jasper Lake volcaniclastic strata. . A preliminary geologic map of the Cavity Lake fire area, created prior to the student work, was published as an open-file map (Jirsa and Starns, 2008). A final map that incorporates student work and recent thesis mapping will be published in coming months. Support was provided by the U.S. Geological Survey’s 2007 State Geologic Mapping Element (STATEMAP) of the National Geologic Mapping Program, the Precambrian Research Center (2007 and 2008 capstone projects), and the State Special Appropriation to the Minnesota Geological Survey. REFERENCES Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: GSA Bull. 110:1467-1484. Jirsa, M.A., and Starns, E., 2008, Preliminary bedrock geologic map of the Cavity Lake fire area, parts of the Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF-08-05, scale 1:24,000. Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the JAP Lake area, northeastern Minnesota: M.S. Thesis, University of Minnesota-Duluth, 193 p. 49 �NEOARCHEAN WEATHERING AND ATMOSPHERIC pO2 INFERRED FROM PALEOSAPROLITE BETWEEN GRANITE-GREENSTONE AND SUPERJACENT CONGLOMERATE IN THE BOUNDARY WATERS CANOE AREA, NE MINNESOTA Jirsa, Mark A., Minnesota Geological Survey (www.geo.umn.edu/mgs); and Driese, Steven G., Department of Geology, Baylor University, Waco, Texas Neoarchean rocks in parts of the Boundary Waters Canoe Area are exceptionally well exposed after recent forest fires that were the largest in Minnesota since 1894. New mapping in the burns reveals considerable detail, particularly about complex contact relationships (Jirsa and Starns, 2008). For example, the contacts between the Ogishkemuncie conglomerate and the volcano-plutonic country rocks from which it was derived are both faults and unconformities. Where not faulted, the contacts are marked locally by paleosaprolite developed in both the 2.689 Ga Saganaga Tonalite (Corfu and Stott, 1998), and the adjacent ca. 2.7 Ga metavolcanic rocks that it intruded. Petrographic and geochemical analyses of 2 suites of samples taken across the paleosaprolitic contact zones will constrain our understanding of environmental conditions during the Neoarchean. The Neoarchean rocks (from oldest to youngest) are: Paulson Lake sequence—a vertically dipping package of back-arc or ocean floor origin, consisting of pillowed basaltic to komatiitic flows, hypabyssal sills, and rare tuff, chert, and distal turbidite. Saganaga Tonalite—coarse-grained batholith and dikes that cut the Paulsen Lake sequence and have distinctive textures marked by abundant quartz "eyes" and hornblende. Jasper Lake sequence—hornblende- and pyroxene-porphyritic, dacitic to trachyandesitic, volcanic and volcaniclastic rocks cut by similarly porphyritic hypabyssal intrusions. Ogishkemuncie conglomerate—conglomerate and sandstone containing readily recognizable fragments of all the rock sequences described above. Sedimentary structures indicate deposition in coalescing alluvial fans, with fluvial transport locally into standing water. The strata are similar in many respects to other Timiskaming-type assemblages in the Superior Province that are inferred to represent deposition in successor-basins (e.g., Jirsa, 2000). Characteristics of the inferred 2.7 Ga paleosaprolite satisfy most of the 5 diagnostic criteria proposed by Rye and Holland (1998) for identification of pre-land plant paleosols. The unconformity dips 30-60o, in contrast to the vertical dip of underlying country rock. Macroscopic and microscopic features of the paleosaprolite include alteration, destruction of igneous texture, and diminution of feldspar expressed in increased content of quartz eyes in paleoweathered tonalite (Fig. 1); and oxidation, microfracturing, conversion of Fe-Mg minerals to chlorite, and semi-ductile (“soft”) deformation in metavolcanic rocks. The effects differ from those of tectonic cataclasis primarily in the irregularity of altered zones, locally producing equant paleocorestones of tonalite, with annealed concentric exfoliation structures. Figure 1. Photomicrographs showing textures of A. comparatively fresh Saganaga Tonalite 55 m below Ogishkemuncie conglomerate; B. weathered tonalite 15 m below conglomerate; and C. conglomerate 7 m above tonalite (note rounded to angular quartz and rock fragments). All in plane light; scale-bar 2 mm. 50 �Although primary paleosaprolite microfabrics and mineralogy of weathered granitic rocks are modified by low-to medium-grade metamorphism that is typical here, whole-rock geochemical patterns related to paleoweathering are commonly well-preserved, except for general K2O increases related to metasomatism (Driese et al., 2007; Driese and Medaris, 2008). Calculations of elemental gains and losses relative to fresh parent material for both the Saganaga Tonalite and the adjacent metavolcanic rocks will permit estimation of Neoarchean paleoatmospheric pO2, based on theoretical differences in the ratio of O2 demand to CO2 demand during weathering of granitic vs. mafic rocks, as well as Fe gains and losses (Pinto and Holland, 1988). Paleoatmospheric pO2 is inferred to have been quite low at 2.7 Ga, well before the circa 2.2 Ga “Great Oxidation Event” of Rye and Holland (1998), as illustrated in Figure 2. Additional information about pCO2 can also be extracted using mass-balance geochemical methods of Sheldon (2006). Analytical work is underway. Figure 2. Comparison of 2.7 Ga Ogishkemuncie paleosaprolite with other paleosols. (after Rye and Holland, 1998) REFERENCES Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: GSA Bull 110:1467-1484. Driese, S.G., and Medaris, L.G., 2008, Evidence for biological and hydrological controls on the development of a Paleoproterozoic paleoweathering profile in the Baraboo Range, Wisconsin, USA: Journal of Sed. Res. 78: 443-457. Driese, S.G., Medaris, L.G., Ren, M., Runkel, A.C., and Langford, R.P., 2007, Differentiating pedogenesis from diagenesis in early terrestrial paleoweathering surfaces formed on granitic composition parent materials: Journal of Geology 115: 387-406. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Can. Journal of Earth Sci. 37:1-15. Jirsa, M.A., and Starns, E., 2008, Preliminary bedrock geologic map of the Cavity Lake fire area, parts of the Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF-08-05, scale 1:24,000. Pinto, J.P., and Holland, H.D., 1988, Paleosols and the evolution of the atmosphere; Part II, in Reinhardt, J. and Sigleo, W.R. (eds.), Paleosols and Weathering through Geologic Time: GSA Special Paper 216: 21-34. Rye, R., and Holland, H.D., 1998, Paleosols and the evolution of atmospheric oxygen, a critical review: Am. Journal of Sci. 298:621-672. Sheldon, N.D., 2006, Precambrian paleosols and atmospheric CO2 levels: Precambrian Research 147:148-155. 51 �STRUCTURAL, KINEMATIC, AND LITHOGEOCHEMICAL INVESTIGATION OF THE MURRAY SHEAR ZONE, NORTHEAST MINNESOTA JOHNSON, Tom K. (University of Minnesota-Duluth, joh04310@d.umn.edu); HANSEN, Vicki L. (University of Minnesota-Duluth, vhansen@d.umn.edu); HUDAK, George J. (University of Wisconsin-Oshkosh, hudak@uwosh.edu); PETERSON, Dean M. (Natural Resources Research Institute, Duluth, MN, dpeters1@nrri.umn.edu) Archean (3.8-2.5 Ga) cratons host gold-bearing quartz vein systems in zones of inhomogeneous structural architecture. In the Superior craton, the largest and most goldproductive zones of multifarious structure include the Porcupine-Destor and Larder LakeCadillac breaks of the Abitibi Greenstone Belt, Canada. Previous geologic mapping and mineral exploration in southern reaches of the Superior craton of northeastern Minnesota revealed anomalous quartz vein-hosted gold in the Murray Shear Zone (Peterson and Patelke, 2003). This investigation of the Murray Shear Zone attempts to better understand unique characteristics shared with gold-rich belts in Canada, and the paucity of developed gold districts in northeastern Minnesota, through research into structural architecture, crustal kinematics, and geochemistry. The Murray Shear Zone cuts an arcuate succession of rocks comprising the Lower and Soudan Members of the Ely Greenstone, and the Lake Vermilion Formation. The 19-km shear zone strikes east-west, extending from the Tower-Soudan area to the Giants Range Batholith to the east. Investigation uncovers microstructural evidence for exclusively dipslip shear, parallel to steep-plunging mineral lineations (089/68) within steeply dipping foliation. Strain appears to be partitioned along east-striking, steeply-dipping metamorphic foliation planes that describe an anastomozing network. In the western portion of the study area curvilinear splays of focused strain fabric diverge to 4 km in width (map view) from 0.4 km width to the east. Strain heterogeneities exist within the Murray Shear Zone. Metamorphic foliations diverge around coherent lithologic blocks devoid of penetrative foliation. Brittle deformation features overprint ductile features in the form of extensional quartz veins that cut metamorphic foliations at high angles. Interpretations of instantaneous principal stress orientations attempt to correlate rheological behavior of rocks with field observations for a conceptual gold model. Strain asymmetry and steep planar and linear fabric coexist with quartz vein systems that host gold. Structurally-hosted gold materializes from: 1) differential stresses in the crust; 2) metamorphic devolatilization; and 3) corresponding fluid pressure fluctuations (deviatoric stresses) at the brittle-ductile transition. Geochemical data show zones of hydrothermal alteration enclosing gold-bearing quartz veins of anomalous gold mineralization in regionally-prevalent greenschist grade to locally amphibolite grade metamorphism. Alteration envelopes approaching the veins include chlorite schist, carbonate-chlorite schist, and carbonate-sericite ± green mica schist. Correspondingly, mass balance analysis utilizing the isocon method (Grant, 1986) indicates gains in Fe, Mg, Mn, Ca, Co, CO2, Cr, K, Ni, V, Sr, Sn, Rb, Ba, and Eu, and losses in Cu, Er, 52 �Na, SiO2, and Zn. Silica leaching occurs adjacent to gold-bearing quartz veins in a matter similar to that illustrated at the Yellowknife gold deposit, Northwest Territories, Canada (Boyle, 1955). References Peterson, D.M., Patelke, R.L., 2003, National underground science and engineering laboratory (NUSEL): geological site investigation for the Soudan Mine, northeastern Minnesota. Economic Geology Group, National Resources Research Institute, University of Minnesota Duluth: Technical Report NRRI/TR-2003/29. Grant, J.A., 1986, The isocon diagram—a simple solution to Gresens’ equation for metasomatic alteration: Econ. Geol. 81:1976-1982. Boyle, R.W., 1955, The geochemistry and origin of the gold-bearing quartz veins and lenses of the Yellowknife greenstone belt: Econ. Geol. 50:51-66. 53 �FLUID MOVEMENT THROUGH THE MESABI IRON RANGE, MINNESOTA Kyle Makovsky, Steven Losh; Dept of Chemistry and Geology, FH 145, Minnesota State University, Mankato MN 56001 The Mesabi Iron Range in northern Minnesota has been an important contributor of iron necessary for products that we use every day. The iron-rich sedimentary rocks were initially deposited in a shallow sea about 1.8 billion years ago. Later, fluids flowed through the rocks dissolving everything but the iron oxides and concentrating them into high-grade ore. Previous work has described these fluids as meteoric waters that have percolated downward through the rocks from the surface. G.B. Morey (1999) has stated that these fluids were driven up through the rocks due to the Penokean Orogeny. To determine the source of fluids associated with high-grade (natural) ore, we have sampled veins in low to high angle faults exposed in the Hibbtac, Thunderbird (UTac), and LTV #6 pits. Brecciated quartz cemented by quartz-hematite is found primarily in the high angle faults of this region. The sampled high-angle faults locally define boundaries between unoxidized and oxidized ore and are thought to have served as conduits for fluids that dissolved chert from taconite. Some of the movement on these faults was associated with collapse due to chert dissolution. Thus the quartz in these faults may have been precipitated from the same fluid that was responsible for leaching the taconite. The properties of the fluid that affected these rocks are being determined using cathodoluminescence and fluid inclusion techniques. Microscope examination of the iron ores revealed that the early-formed iron-rich mineral greenalite was replaced by other ironrich minerals, minnesotaite and stilpnomelane during diagenesis. Samples taken from the Thunderbird and Hibbtac mines have been analyzed using cathodoluminescence. Quartz in the veins and in the iron formation display growth banding produced by pulses of fluid moving through the rock and precipitating minerals episodically. Along with growth banding, differences in the vein quartz and matrix quartz due to fluid interactions with the rock can be seen. The source of fluids has been studied by analysis of fluid inclusions, which can give both the homogenization temperature and the salinity of fluids that are trapped in minerals. Fluids that ascended through the rocks would be expected to have a relatively high temperature and salinity, whereas meteoric fluids that descended from the surface would be expected to have much lower temperatures and salinities. One sample from a fault associated with high-grade iron ore in the Thunderbird mine near Eveleth showed high homogenization temperature values, 85.8-141.5°C (mean 125°C, n=26) and a high weight % NaCl equivalent (mean 3.98 weight %, n=12), suggesting the fluids that precipitated quartz in the faults ascended from depth. These values largely overlap temperatures and salinities of fluid inclusions from quartz breccia in faults in the Hibbtac Mine, as well as in bedding-parallel veins in the LTV6 samples. All of this data supports the idea of fluids rising from depth, not percolating downward. 54 �Geochemical data will also be presented for rocks in veins and in traverses from oxidized, leached iron formation into unoxidized iron formation to determine the nature and effects of the fluids responsible for the high-grade ore. References Morey, G., High-grade iron ore deposits of the Mesabi Range, Minnesota – Product of a continental-scale Proterozoic groundwater flow system; Econ. Geol. v. 94, pp. 133-141. 55 �INTERPRETATIONS OF THE EMPLACEMENT AND COOLING HISTORY OF A THIN DIABASE SILL, NIPIGON, ONTARIO MARKWOOD, Levi W. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA, 16057. lwm9100@sru.edu, michael.zieg@sru.edu Thin dikes and sills are often produced by single instantaneous injection events (e.g., Gray, 1978). Textural and compositional evidence of this type of formation typically includes the lack of internal chills or accumulation of phenocrysts into distinctive horizons. However, complex injection histories are being recognized in an increasing number of intrusions (e.g., White, 2007). In this study, we examine a thin diabase sill from the Nipigon embayment, Ontario. This sill is located beneath the main sill at Kama Hill, east of Nipigon. Textural and mineralogical evidence suggest that it was emplaced in a single injection, and thus represents an important end-member style of sill formation. Petrographic examination of the rocks in this sill focused on opaque oxides, as they can be characterized easily using automated image processing techniques. Digital photomosaics (transmitted, plane-polarized light) were prepared from thin sections of 18 samples spanning the 1.25 m thick sill. Randomly oriented test lines were used to determine mean crystal length using a method reviewed in Higgins (2006): Lmean= VV/PL, where VV is the modal fraction of the mineral of interest, as determined by both automated image analysis and by point counting, and PL is the number of crystals intersected along the test line. This analysis reveals smooth variations in grain size with distance, from smaller grains at the chilled margin to larger grains in the center of the sill, indicating that the magma cooled as a single unit. Unit cooling is consistent with emplacement of magma as a single injection event. The modal abundance of opaque oxide minerals in the sill was determined by digital threshholding and by point counting (N~1200). The abundance of oxides throughout the sill is statistically uniform. This lack of significant variation in mineral abundances is further evidence supporting unit emplacement: multiple injections would likely have disrupted the solidification fronts established in the cooling magma and resulted in identifiable mineralogical discontinuities. Using a numerical cooling model, we can extract crystallization kinetics (growth rates) from the grain size data, and predict texture variations in a variety of scenarios, in particular predicting the magnitude of the “chilling” signature in the case of reinjection events. Based on mineralogy and texture, the sill in this study was almost certainly emplaced in a single injection event. Using this as a baseline, we can identify complex injection histories as departures from the textural profile observed here. This makes it useful as a null hypothesis for testing formation history: unless textures depart significantly from those presented here, we must assume that the intrusion was formed in a single injection. Such textural criteria are particularly important when the magma composition remained constant through several injections, in which case the recognition of multiple injection events would not be reflected in the modal mineralogy of the rocks. References Gray, N.H., 1978. Crystal growth and nucleation in flash-injected diabase dikes. Canadian Journal of Earth Sciences, 15, 1904-1923. Higgins, M.D., 2006. Quantitative textural measurements in igneous and metamorphic petrology. Cambridge University Press, Cambridge, UK. 265 pp. White, C.M., 2007. The Graveyard Point Intrusion: an example of extreme differentiation of Snake River Plain basalt in a shallow crustal pluton. Journal of Petrology, 38, 303-325. 56 �57 �58 �59 �60 �GEOLOGY AND MAGNETIC TACONITE RESOURCES OF WESTERN GOGEBIC IRON RANGE, WISCONSIN Meineke, David G. (david.meineke@GlobalMineralsEng.com) and Djerlev, Henry (henry.djerlev@GlobalMineralsEng.com), LaPointe Iron Company, 3920 13th Avenue East, Hibbing, Minnesota 55746 The Gogebic Iron Range is 60 miles long, extending from the western Upper Peninsula of Michigan into northeastern Wisconsin. From 1886 to 1964 over 300 million tons of high grade, direct-shipping, natural iron ore was mined from the Gogebic Iron Range, largely by underground mining in Michigan and Wisconsin. The iron in these ores occurred primarily in hematite, goethite and limonite. Only one operation, the Berkshire Mine, in 1922-1924 mined and processed magnetite ores (4,000 tons) that required concentration to make a marketable grade product. The Berkshire operation, along with those early magnetic taconite operations in Minnesota and Michigan, marked the beginning of the Lake Superior taconite industry which now produces nearly all of the iron ore mined in the United States for the U. S. steel industry. The geology of the western Gogebic Iron Range has most recently been described by Cannon, LaBerge, Klasner and Schulz (2008). The iron ore-bearing members are part of the Paleoproterozoic Ironwood Iron Formation. The direct-shipping natural iron ores were mined along 25 miles of strike from Upson, Wisconsin, to Wakefield, Michigan. The natural ores were likely formed by circulating groundwater that oxidized the iron minerals and removed silica. A large magnetic taconite resource has been identified on the western Gogebic Iron Range from Upson to Mineral Lake (21 miles) in Wisconsin and a smaller, less defined magnetic taconite resource east of Wakefield, Michigan, west and east, respectively, of the 25 miles of the Gogebic Iron Range where natural iron ores were mined from Wakefield to Upson. The Ironwood Iron Formation underwent Mesoproterozoic deformation which tilted the strata 40° to 90° north (Cannon, LaBerge, Klasner and Schulz, 2008). For the 21 miles from Upson to Mineral Lake, we estimate an average dip of 65°, with the thickness of the potentially economic magnetic taconite 400 to 560 feet thick. The potentially economic magnetic taconite occurs in most of the lowest member (Plymouth) of the Ironwood Iron Formation and in the upper part of the Yale and lower part of the Norrie members, both of which occur stratigraphically above the Plymouth member, respectively. Diabase and gabbro dikes and sills have intruded the Ironwood Iron Formation in the western Gogebic Iron Range. LaPointe Iron Company has estimated the Wisconsin magnetic taconite resource located in the 21 miles from Upson to Mineral Lake to be 2.1 billion tons at a 1:1 maximum strip ratio which, based on Davis Tube magnetic concentrates from exploration drilling, could produce over 600 million tons of concentrate with greater than 65% iron and an average strip ratio of 0.40. Marsden (1978), in a study conducted for the U. S. Bureau of Mines using 1970’s technologies, costs and product prices, estimated that 3.7 billion tons of magnetic taconite could be profitably extracted over the same 21 miles. References Cannon, W.F., LaBerge, G.L., Klasner, J.S., and Schulz, K.J., 2008, The Gogebic Iron Range—A sample of the northern margin of the Penokean fold and thrust belt: U.S. Geological Survey Professional Paper 1730, 44 p. 61 �Marsden, R.W., 1978, Iron ore reserves of Wisconsin—A minerals availability system report, in Proceedings, American Institute of Mining Engineers, 51st annual meeting, Minnesota Section, Duluth, Minn., Jan. 11-13, 1978: Duluth, Minn., University of Minnesota, American Institute of Mining Engineers, no. 39, p. 24-1 to 24-28. 62 �NEW EDUCATIONAL INITIATIVES AT THE UNIVERSITY OF MINNESOTA DULUTH: PREPARING STUDENTS FOR FUTURE JOBS IN THE MINING AND MINERALS EXPLORATION INDUSTRIES James D. Miller Jr., Dept. of Geological Sciences, University of Minnesota Duluth mille066@umn.edu (218-726-6582) Carlos Carranza-Torres, Dept. of Civil Engineering, University of Minnesota Duluth carranza@d.umn.edu (218-726-7842) Richard Davis, Dept. of Chemical Engineering, University of Minnesota Duluth rdavis@d.umn.edu (218-726-6162) David Hendrickson, Coleraine Minerals Research Laboratory, Natural Resources Research Institute, University of Minnesota Duluth, dhendric@nrri.umn.edu (218-2454204) The departments of Geological Sciences, Civil Engineering and Chemical Engineering along with the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth are collaborating to develop new courses and new degree options that will prepare students for professional jobs in mining and minerals exploration industries, both locally and globally. These new course offerings and degree options are expected to be in place by the fall term of 2010 or soon thereafter. The Department of Geological Sciences will offer a BS degree with a mining and mineral exploration emphasis. To qualify for this option, students will be required to take the standard coursework required for a B.S. in geology (including two semesters each of calculus, chemistry, and physics and a six-week summer field camp), along with seven other required courses. These additional courses are Geologic Maps, Engineering Geology, Probability and Statistics, Economic Geology, Minerals Exploration, Mine Design and Operation, and Mineral Processing. Course proposals for the last three courses will be submitted this summer. The Geological Sciences department is also looking into developing a five-year professional masters degree in mining and exploration geology. Preliminary ideas for this degree program are that it will require advanced graduate level courses in geology, mining, exploration, and business and an internship with a mining or exploration company. The Civil Engineering department at UMD, which was initiated in 2008, has developed plans to offer a BS in Civil Engineering with a mining engineering minor. The mining minor will require students to take courses in soil mechanics, rock mechanics, engineering geology, mine safety, mine design and operation, excavation design, and mineral processing. Soil and rock mechanics are currently offered and the others will be developed in the coming year or two as faculty personnel allow. The Department of Chemical Engineering intends to offer a minor in Mineral and Material Process Engineering that builds on the department’s history of activity in education and research in this area. In developing this minor, the Chemical Engineering department is looking to partner with the Natural Resources Research Institute to offer courses in mineral processing and extractive metallurgy. 63 �Involving the experienced staff and research facilities of the NRRI in both teaching and research is a key component to these initiatives. The NRRI Coleraine Minerals Research Laboratory can provide a great resource for teaching and conducting applied research into mineral processing and extractive metallurgy. In addition, the economic geology group at the NRRI may be tapped for their experience in mineral exploration and ore deposits. In addition to teaching and research collaboration between our three academic departments and the NRRI, we are also hoping to tap into the practical expertise of local mining and exploration company personnel. We consider these course additions and curriculum modifications as a modest first step toward addressing the severe manpower and talent shortages that existed in the mining and minerals exploration industries prior to the recent economic downturn. When the recovery comes, we expect that these curricula changes will put UMD in a position to readily supply the exploration and mining industries with the well-trained geologists and engineers they will desperately need once again. We especially want to be the source of human capital for the venerable iron ore industry and the nascent base metal mining industry in northern Minnesota. Both of these industries can provide well paying and fulfilling jobs for decades to come. To fully serve the needs of the local, as well as the global mining industry, we are considering a grander plan that we are tentatively calling the Center for Mining and Mineral Exploration at UMD. The center would include a well integrated and more robust curriculum among our three departments and would seek support for basic and applied collaborative research by faculty and student for the benefit of the mining and exploration industries, especially locally. Of course, to fully realize this plan will require the support of the mining and exploration industries in the form of endowed faculty positions, funds to support faculty and student research, and in-kind contributions from local mining experts in teaching and research. So while these admittedly grandiose plans will have to at least wait for the mining industry and the rest of the economy to recover, the curriculum and program changes outline here will implemented regardless. Hopefully, these changes will be well received by the industry and will lead to support for our larger goal in the future. 64 �THE NOKOMIS CU-NI-PGE DEPOSIT, DULUTH COMPLEX, MINNESOTA Peterson, Dean M., Duluth Metals Corporation, 306 West Superior Street, Suite 407, Duluth, MN 55802. dpeterson@duluthmetals.com Duluth Metals Limited’s Nokomis deposit is the most recently discovered Cu-Ni-PGE deposit in the 1.1 Ga. Duluth Complex, Minnesota. The deposit was discovered utilizing a genetic ore deposit model that identified and back-tracked channelized magma flow within the South Kawishiwi intrusion (SKI). The model led to exploratory drilling in 2006, deposit discovery and initial resource estimation in 2007, and significant resource expansion in 2008, all in a period of 18 months. The deposit’s updated 2008 NI 43-101 compliant Resource Estimate, based on 108 holes drilled by Duluth Metals and 52 historic drill holes on and off the property, contains 449 million tonnes of Indicated Resources grading 0.624% copper, 0.199% nickel, and 0.600 grams per tonne of total precious metals (TPM = Platinum+Palladium+Gold), and an additional 284 million tonnes of Inferred Resources grading 0.627% copper, 0.194% nickel, and 0.718 grams per tonne of TPM. The combined Indicated and Inferred Resources contain approximately 10 billion lbs Cu, 3.1 billion lbs Ni, 165 million lbs Co, 4 million ounces Pt, 9 million ounces Pd, and 2 million ounces of Au. Within these NI 43-101 resources are large tonnages of higher grade material, and the company has commenced an internal research program to identify the geologic controls on the formation nickel-rich and PGE-rich mineralization in the SKI, as well as copper-PGE rich mineralization in the footwall Archean rocks. To date, Duluth Metals has drilled more than 500,000 Ft. (~152,000 m) of core in 154 holes into the deposit, and has only drilled about half of the property. The ore deposit model was developed in cooperation with researchers from the Natural Resources Research Institute of the University of Minnesota, Duluth. A fundamental aspect of the ever-developing ore deposit model is an understanding of the initial conditions of the magmatic system – its crystallinity, sulfur capacity, geochemistry, and geometry – and how the sulfur saturated SKI magma lived, worked, and died. Such understanding includes the realization that the magma was a crystal-liquid (silicate and sulfide liquids) slurry and the identification of magma channelways and sub-channels and their associated thermal anomalies. In addition, the SKI magmas locally melted the footwall granitoid rocks, and such melts have been incorporated into the sulfide-bearing troctolitic melts of the SKI. In the end, hard work and intellectual geologic thought has been used to identify one of the world’s largest resources of Cu-Ni-PGEs. 65 �66 �67 �68 �69 �ALTERATION OF STROMATOLITE BIOSIGNATURES IN THE BIWABIK IRONFORMATION: RELEVANCE TO ASTROBIOLOGY Russell S. Shapiro, Department of Geological and Environmental Studies, California State University, Chico, California, 95929 rsshapiro@csuchico.edu Stromatolites, the macrofossil evidence of microbial activity, are an important potential biosignature in the search for life on early Earth and in extraterrestrial missions, yet the taphonomic effect of metamorphism is poorly known. While broad regional metamorphism related to convergent tectonics may be largely restricted to post-Hadean Earth, alteration from volcanism, heated and reducing fluids, and impacts is quite common throughout the solar system. The present study describes and quantifies the effect of contact metamorphism on a biogenic stromatolite bed. While the specific conditions of this example may not be widely applicable in studies of current astrobiology targets, the pathways and resultant changes will serve as a valuable analog for developing tools for extraterrestrial stromatolite recognition. The stromatolites are constrained to two, meter-scale sequences in the Biwabik Iron-Formation of the Mesabi range on the western margin of the Animikie Basin. The Biwabik records shallow marine sedimentation during the Paleoproterozoic before and during a major collision, the Penokean Orogeny (Ojankangas et al., 2001). Deformation related to the Penokean orogeny and subsequent events lasted for nearly 30 million years based on faulting and synorogenic intrusions (see summary in Schulz and Cannon, 2007). However, it is assumed that no major mineralogical effects in the study area resulted from this event. The next major phase of alteration was related to mid-continental rifting at 1,100 Ma. Though ultimately a failed rift, conspicuous volumes of basalt flows and subjacent gabbro, troctolite, granodiorite and anorthosite formed along the rift. The contact aureole extends for approximately five kilometers in the Mesabi range with temperatures near the contact in excess of 850 degrees C (Hyslop et al., 2008). Mineralogic changes were detailed by French (1968), Loughheed (1983), Floran and Papike (1978), Frost et al. (2007), Hyslop et al. (2008) and others, defining isograds within the contact aureole. Stromatolites were compared from the each of the two Biwabik beds from both outside and inside the contact aureole. Petrographic thin- and thick-sections were studied with standard transmitted and reflected light microscopy. The unmetamorphosed representative samples came from core at U.S. Steel Minntac (basal stromatolite layer) and Minnesota Geological Survey deep core 2 (upper stromatolite layer from the Upper Cherty member). Samples from within the contact aureole were collected from taconite mine exposures in Polymet (formerly Cliffs-Erie / LTV) Area 5 (basal) and Northshore block 20 (upper stromatolites). Based on isograds presented in French (1968) and modified by Frost et al. (2007), the Area 5 locality is between zones 5 and 6 and Northshore 20 is between zones 7 and 8. Zone 5 records formation of ferrohypersthene with graphite, zone 6 is defined by hedenbergite, zone 7 by fayalite, and zone 8 by orthopyroxene. Results Minntac (basal stromatolites, outside aureole)—The stromatolites in this least altered location are composed of sideritic laminae. The laminae are defined by 0.5 mm thick bundles composed of bands averaging 25 μm thick of organics and hematite. Lenses of microquartz occur in shelter porosity. Granules between stromatolite columns are composed of quartz with thin magnetite rims. Filamentous microfossils comparable in size and density to Gunflintia minuta from the Gunflint IronFormation are rarely preserved in hematite. Early diagenetic features include rare stilpnomelane and ankerite either as late-stage spar or replacing late-stage silica cement. MGS-2 (upper stromatolites, outside aureole)—These stromatolites are composed of very fine (10-20 μm) laminae and ministromatolites. The thin, dark laminae are defined by organics. Granules are rare and composed of the iron phyllosilicate greenalite. Most grains are ooids rim-replaced by 70 �subhedral magnetite. Early marine cements are preserved by quartz though most of the stromatolites are neomorphosed into mosaic microquartz. Diagenesis is recognized by blocky euhedral ankerite that crosses fabrics. PolyMet 5 (basal stromatolites, within aureole)—The thin-sections show prevalent destruction of laminae, replaced by microquartz. Secondary removal of microquartz is defined by intrusions of magnetite and calcite. Where preserved, the dark laminae are leached, leaving 1-2 μm thick bands of iron oxides. Veins through the stromatolites are filled with magnetite, calcite, and pumpellyite. Radiating rosettes of either grunerite or minnesotaite are found within laminae and among basal quartz sand grains. Granules of epidote occur at the base of stromatolite columns. Northshore 20 (upper stromatolites, within aureole)—This location is closest to the gabbro. The stromatolites are still recognizeable with laminae defined by single crystals of magnetite preserved within mosaic microquartz. Crystals of microquartz average 40 μm across. Fabric-destructive, larger subhedral magnetite ~7-20 μm across, occurs throughout the stromatolite. Alteration History 1) Stromatolites formed under normal marine conditions as alternating thin laminae of sideritic mud and organic-rich, hematite layers. Greenalite granules washed in from deeper waters. 2) Early marine silica formed as rim and shelter porosity cements. 3) Burial diagenesis led to reduction of hematite, formation of magnetite rims on grains and in stromatolitic laminae. Some destruction of laminae through replacement by microquartz. Ankerite formed as small crystals irregardless of fabric. Growth of stilpnomelane. No increase in crystal sizes. 4) Intrusion of gabbro led to further reduction and loss of hematite and remaining iron-carbonate. Laminae now composed of single crystal-thick bands of magnetite. Complete replacement by microquartz. Formation of fabric-destructive calcite, usually in association with magnetite. Pumpellyite formed in veins associated with calcite and magnetite. References Schulz, K. J., and Cannon, W. F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, 1578:4—25. Okjakangas, R. W., Morey, G. B., and Southwick, D. L., 2001, Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America. Sedimentary Geology, 141142:319—341. Lougheed, M. S., 1983, Origin of Precambrian iron-formations in the Lake Superior region, Bulletin of the Geological Society of America, 94:325—340. French, B. M., 1968, Progressive contact metamorphism of the Biwabik Iron-formation, Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 45, 103 p. Floran, R. J., and Papike, J. J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: correlation of compositional and assemblage variations at low to moderate grade: Journal of Petrology, 19:215—288. Frost, C. D., von Blanckenburg, F., Schoenberg, R., Frost, B. R., and Swapp, S. M., 2007, Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation: Contributions to Mineralogy and Petrology, 153:211—235. Hyslop, E. V., Valley, J. W., Johnson, C. M., and Beard, B. L., 2008, The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota: Contributions to Mineralogy and Petrology, 155:313—328. 71 �BEDROCK GEOLOGIC MAP OF THE DISAPPOINTMENT AND IMA LAKES AREA, BOUNDARY WATERS CANOE AREA, LAKE COUNTY, NE MINNESOTA Stifter, E., Wartman, J., Gibbons, J., Kane, K., Murphy, L., Carlson, A., Mason, T., Hudak, G., and Peterson, D., Precambrian Research Center, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812, ecstift@yahoo.com The “capstone” project for the Precambrian Research Center summer field camp encompasses one week of detailed field mapping in small groups with faculty from the summer field camp. During the fifth and sixth weeks of the 2008 field camp, seven students mapped in the vicinities of Disappointment and Ima Lakes (located within the Boundary Waters Canoe Area Wilderness) under the direction of PRC Faculty Dr. Dean Peterson and Dr. George Hudak. Geological maps of the area were originally published in reports by Van Hise (1901) and Gruner (1941). The purpose of this capstone mapping project was 1) to better understand the nature of the contact between the Mesoproterozoic Duluth Complex and Neoarchean supracrustal strata; 2) to better understand the compositional make up of the Duluth Complex in this area; and 3) to better understand the stratigraphic and structural characteristics of the Neoarchean rocks in this locale. The final map was published at 1:10000 scale and covered an area of approximately 15 square miles. Prior to mapping, detailed field mapping sheets were constructed. Field mapping sheets were produced at 1:5000 scale. One side of each field mapping sheet consisted of digitized topographic maps, topographic contours and bathymetric contours (digitized in 3D). The second side of each field mapping sheet consisted of an air photo and aeromagnetic data (Chandler, 1991). Mapping was primarily done from canoe, but several difficult traverses through blow-down were also accomplished. For example, there were several traverses completed by faculty and students where our feet never touched the ground for several hundred meters, as we were climbing over dead and fallen trees. Each night, each field party copied their field data on to a master field map, so that by the end of the week, the field map was essentially completed. During the sixth week of the field camp, students digitally produced the field map utilizing a wide variety of software (including ArcView, AutoCad, Surfer, and Adobe Illustrator). Neoarchean strata (~2.72-2.67Ga) in the study area varied from steeply southwest-dipping to steeply northeast-dipping. The base of the stratigraphic section is interpreted to comprise the Knife Lake Group, and is composed, from oldest to youngest units, of a) massive and pillowed basalt lava flows ; b) interbedded rhyodacitic to dacitic tuff/ lapilli tuff and polymict lapilli tuff/tuff-breccia deposits; c) andesitic lapilli tuff/tuff- breccias; d) interbedded mustone, chert, and Algoma-type oxide-facies banded iron formation; and e) interbedded mustones and greywackes with minor Algoma-type oxide-facies banded iron formation. Subsequent regional D2 deformation led to the development of west-northwest – eastsoutheast-trending zones of chlorite schist that are up to 50 meters thick and that can be followed along strike for 500-800 meters. Timiskiming-type metasedimentary strata composed of polymict conglomerates and conglomeratic sandstones occur north of the chlorite schist zones, and are believed to comprise the Ogishke conglomerate (Jirsa and Miller, 2004). Locally, Neoarchean intrusive rocks are locally present and include 72 �synvolcanic diabase dikes, and post-volcanic quartz-feldspar porphyry dikes and diorite stocks. The Mesoproterozoic (~1.1 Ga) Duluth Complex in the vicinity of Ima Lake is composed of early Anorthositic Series rocks (anorthosite and anorthositic gabbro) that were subsequently intruded by the new, informally named Ima Lake Intrusion. The Ima Lake Intrusion is broadly layered, and comprises a) a basal unit composed primarily of oxide- and sulfidebearing gabbros with local zones comprising sulfide-bearing norite; b) an extensive unit of augite troctolite; c) and an upper zone composed of anorthositic troctolite. The sulfidebearing gabbroic base of this intrusion is akin to Early Gabbro Series intrusions to the eastnortheast and differs markedly to the classic sulfide-bearing troctolitic intrusions of the Troctolite Series to the southwest (ie., the South Kawishiwi and Partridge River intrusions). A two kilometer long by up to 500 meter thick zone of pyroxene hornfels occurs along the southwestern margin of the Ima Lake Intrusion, suggesting that Neoarchean supracrustal strata were thermally metamorphosed during the emplacement of the Duluth Complex. This detailed mapping project, combined with analysis of aeromagnetic data, has resulted in the relocation of the contact between the Duluth Complex and the adjacent Neoarchean supracrustal strata approximately 1.6 kilometers from previous mapping (Miller et al., 2001). References Chandler, V. W., 1991. Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map Series S-17, scale 1:500,000. Gruner, J. W., 1941. Structural Geology of the Knife Lake Area of Northeastern Minnesota: Geological Society of America Bulletin, v. 52, p. 1577-1642. Jirsa, M.D., and Miller, J. D., 2004. Bedrock geology of the Ely and Basswood Lake 30’ x 60’ quadrangles, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Miller, J. D. Jr., Green, J. C., Severson, M. J., Chandler, V. W., and Peterson, D. M., 2001. Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, scale 1:200,000, two sheets. Van Hise, C. R., 1901. The iron-ore deposits of the Lake Superior region: 21st Annual Report of the U. S. Geological Survey, Part III. 73 �METAMORPHISM AND DEFORMATION AT MUSSELWHITE MINE Stinson, Victoria R. vrstinso@lakeheadu.ca, Kolb, Maura J., and Hill, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1 Musselwhite Mine is a shear-zone-hosted orogenic gold deposit located within the Superior Province in Northwestern Ontario, 480 km north of Thunder Bay. An Archean metamorphosed and deformed banded iron formation is host to the economic gold mineralization. Metamorphism and deformation are contemporaneous with gold mineralization at Musselwhite Mine. Most of the gold mineralization is within garnet-grunerite-biotite schist. Associated garnet-staurolite-biotite schist and sillimanite-garnet-staurolite-biotite schist indicate metamorphism within the staurolite or sillimanite zone of the amphibolite facies. True amphibolites are absent within the mine due to dominance of metasedimentary lithologies. Grunerite schist and biotite-grunerite schist, often logged as mafic volcanics, are interpreted as metasedimentary. As expected at amphibolite facies metamorphic conditions the rocks have mainly undergone ductile deformation. At the microscopic scale, strain is heterogeneous across compositional bands within the metamorphosed banded iron formation. Quartz bands have relatively larger grains with undulose extinction and irregular grain shapes typical of grain boundary migration recrystallization. Adjacent to grunerite, quartz is finer grained, implying some difference in strain rate or deformation mechanisms along phase boundaries. Within iron-rich bands, grunerite, biotite, and garnet are products of metamorphic reaction. Finegrained biotite and grunerite define the foliation. Strain partitioning between these iron-rich bands and the quartz bands is likely. Although most minerals exhibit evidence of ductile deformation mechanisms at these metamorphic conditions, garnet does not. The garnet crystals have undergone brittle deformation. This small amount of brittle deformation in otherwise ductile conditions may create temporary porosity in a region which otherwise has none. This seems to be important to gold mineralization because gold is found in and around areas where both brittle and ductile deformation structures are present. 74 �OPTIONS FOR GEOLOGIC SEQUESTRATION OF CARBON IN THE UPPER MIDWEST: MINERAL CARBONATION AND DEEP INJECTION Harvey Thorleifson, Minnesota Geological Survey, 2642 University Ave W, St Paul, MN 551141057 USA; Telephone 612-627-4780 ext 224; Fax 612-627-4778; thorleif@umn.edu Increasing concern about climate change has necessitated assessment of ways to reduce emissions, while increasing our preparedness to adapt. Emissions reductions can be achieved by reducing combustion of fossil fuels, by reducing other activity that generates greenhouse gases, and by increasing carbon storage in vegetation and soils. In addition, the technology to capture CO2 from sources such as electrical generating stations and ethanol plants is available, allowing geologic sequestration through methods such as deep injection or mineral carbonation to return carbon to the geosphere. Options within Minnesota therefore are being assessed, as an alterative to eventual transportation of CO2 by pipeline to a jurisdiction such as North Dakota or Illinois. In relation to deep injection, the most prospective rocks in Minnesota at the 1 km depth required to maintain CO2 in a liquid-like state are sedimentary basins of the Midcontinent Rift, present in two north-south belts on either side of the Twin Cities, running from Pine County and Washington County, south to Iowa. Currently available data, however, indicate that there is a very low probability of success in confirming suitable geologic conditions in these rocks, due to likely lack of adequate porosity and permeability, as well as inadequate seal integrity - while at the same time, it is recognized that drilling may be required to adequately clarify options prior to major expenditures on other options (Thorleifson, 2008). Another geologic technique is mineral carbonation, in which CO2 is reacted with olivine-rich material from mining, producing mineral products for disposal or use in construction (Metz et al., 2005). A clear advantage of this method is the lack of risk due to leakage. Minnesota may eventually be well positioned to utilize the mineral carbonation method of geologic carbon sequestration, given the presence of large tonnages of appropriate rock material near Duluth that may be mined for copper, nickel, and platinum group elements, in proximity to well developed infrastructure. In this method, CO2 is reacted with minerals such as olivine, yielding carbonate and quartz. Should these deposits go into production, it is possible that the slurry of minerals produced as a waste product from the mines could be suitable for mineral carbonation of CO2, and in a future carbon-trading scenario, these mines could obtain significant revenue by selling carbon credits. The principal constraint to mineral carbonation at present, however, appears to be cost. According to the Intergovernmental Panel on Climate Change (IPCC), costs for deep injection of CO2 into saline formations are estimated at 0.5 to 8 US$/tCO2, while their estimate for mineral carbonation is 50 to 100 US$/tCO2 (Metz et al., 2007). Furthermore, although the process has been demonstrated experimentally, it has not been tested at a scale that approximates field conditions. Nevertheless, there could be developments in the method, and there could be circumstances in which a particularly favorable mineral carbonation opportunity could coincide with constraints to other sequestration options, such as transportation, thus possibly making mineral carbonation a conceivable option. Minnesota agencies therefore are preparing to conduct an analysis of the mineral carbonation option, to place available information into a Minnesota context, largely by modeling a scenario related to Duluth-region mining in order to evaluate the magnitude of this potential opportunity. A needed aspect of the analysis will be an approximation of the foreseen waste rock composition and production rates at the proposed and contemplated mines, based on literature, analogy to mines elsewhere, and data made available by the project proponents. The analysis will seek to identify the amount of CO2 that could be stored, given well-outlined assumptions, resulting in an estimate of the potential that can readily be updated as needed information is enhanced. This information will enable 75 �Minnesota public and private agencies to be aware of the likelihood that mineral carbonation could be an option for the state, the approximate magnitude of the potential in terms of the amount of carbon that could be sequestered at what rate and cost, and the factors that will govern whether implementation can be anticipated. By doing so, an activity that could possibly bring significant economic benefits to the state in association with anticipated climate change policy initiatives will be clarified. Should the literature review phase be encouraging, further analysis of mineral processing considerations would be called for, to be conducted by parties with the required expertise. References Metz, B., O. Davidson, , H. de Coninck, M. Loos, and L. Meyer, eds., 2005, IPCC Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, 431 p. Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, L.A. Meyer, eds., 2007: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 851 pp Thorleifson, L. H., ed., 2008, Potential capacity for geologic carbon sequestration in the Midcontinent Rift System in Minnesota, Minnesota Geological Survey Open File Report OFR-08-01, 138 p. 76 �MINE WATER QUALITY PREDICTION AND ENVIRONMENTALLYRESPONSIBLE MINING – YES WE CAN! Rens Verburg Golder Associates, Redmond, WA E-mail: rverburg@golder.com, Tel: 425 883 0777; fax: 425 882 5498 Patrick Dunlavy Golder Associates, Roseville, MN E-mail: pdunlavy@golder.com, Tel: 651 697 9737; fax: 651 697 9735 Minnesota is entering a new era of mine development as economic prospects associated with the Duluth Complex are being pursued with considerable vigor. Equally vigorous has been the opposition to such developments, fueled in large part by the concerns related to potential environmental impacts. Coupled with the notion that prediction of such impacts is considered by some to be exceedingly difficult and frequently flawed, it would seem that exploitation of the Duluth Complex faces an uphill battle. In reality, prediction and prevention of environmental impacts has made significant strides in the last decade. Current available guidance on sample collection, geochemical characterization methods, interpretation of results, water quality prediction, and evaluation of potential environmental impacts is based on best practices that have met with demonstrated success. In particular, the most recent guidance, the Global Acid Rock Drainage Guide (GARD Guide), developed by the International Network for Acid Prevention (INAP, 2009), a consortium of mining companies dedicated to reducing liabilities associated with sulfide mine materials, contains a wealth of information in this regard, and presents a roadmap to evaluate and manage environmental issues related to generation of acid rock drainage and metal leaching (ARD/ML). Our general understanding of geologic materials, mine wastes, and hydrogeochemical factors that govern mine water quality continues to advance through the implementation of laboratory and field experiments. Similarly, ongoing characterization and monitoring of mine facilities allows for development of improved scaling factors needed to extrapolate results from smaller-scale tests to an operational level. Further, the necessary tools required for geochemical, hydrological, and hydrogeological modeling in support of water quality prediction exist. It should further be noted that the use of more sophisticated tools does not necessarily equate to more accurate and precise water quality predictions. The nature and sophistication of a prediction effort should vary depending on the desired outcome. For instance, a prediction exercise aimed at answering a “yes/no” question (for example: will the water quality criterion for constituent X be exceeded?) requires less a priori understanding of the system being evaluated, in which case the use of a relatively basic prediction approach may suffice. In contrast, when a more quantitative answer is required (for example: what is the expected concentration of constituent X), the complexity of the prediction effort may be quite significant, requiring both a detailed conceptualization of the system being modeled as well as use of advanced modeling codes. Therefore, tools should be selected that suit the need of a particular application and are compatible with the range and quality of the input data. Also regulatory expectations with respect to water quality prediction should become more realistic. It should be recognized that massive sampling and testing sampling campaigns coupled with extensive, expensive, and frequently impenetrable water quality modeling are not always a guarantee for more precise and accurate water quality predictions, but in fact may provide a false sense of security. Instead, use of simpler and more robust approaches, that take into account accumulated knowledge complemented by a targeted verification campaign, may be just as reliable and useful for decision making. A good example of such a robust approach is the use of geo-environmental models. Geo-environmental models provide a very useful way to interpret and summarize the environmental signatures of mining and mineral deposits in a systematic geologic context (Plumlee, 1999), and can, therefore be used to anticipate water qualities and potential environmental problems at future mines, operating mines, and orphan sites. 77 �Figure 1 shows ranges of pH and trace metals concentrations for mine water discharging from two types of ores of interest to Minnesota: banded iron formation ores (i.e., the taconite ores), and magmatic sulfide deposits such as the Duluth Complex. Without having collected a single sample, stakeholders can readily identify potential water quality issues associated with the two different ore types. Through a limited sampling and characterization program, these water quality ranges can be refined and verified. Information presented in Figure 1 can also be used to identify and evaluate prevention and mitigation alternatives. Although some site-specific testing will be needed, such a program can focus from the outset on appropriate and relevant methods, bounded by pragmatic constraints. Figure 1 identifies that exploitation of the Duluth Complex may generate mine water that is of different quality than the taconite operations have traditionally produced. However, the modern mining industry is committed to and capable of minimizing environmental impacts. Kennecott’s Flambeau Mine in Wisconsin is a good example of such an approach. Although, due to the nature of the ore deposit, potential water qualities at this site were predicted to be much poorer than expected from the Duluth Complex, proper waste management and reclamation have lead to successful prevention of water quality impacts while the site has been restored to beneficial use References INAP (2009). The Global Acid Rock Drainage Guide http://www480.pair.com/aturner/gardwiki/index.php/Main_Page Plumlee, G.S. (1999). The Environmental Geology of Mineral Deposits. In: The Environmental Geochemistry of Mineral Deposits, Part A: Processes, Techniques and Health Issues (Eds.: Plumlee, G.S., and M.J. Logsdon). Reviews in Economic Geology Vol 6A. Society of Economic Geologists, Inc. Figure 1. Water quality ranges for magmatic sulfide deposits and banded iron formation 78 �HYDROSTRATIGRAPHY OF THE BIWABIK IRON FORMATION – IMPLICATIONS FOR CURRENT GROUNDWATER FLOW PATTERNS AND PAST GENESIS OF NATURAL ORE BODIES Walsh, James F., Minnesota Department of Health, james.f.walsh@state.mn.us Recent well logging studies conducted by the Minnesota Geological Survey and the Minnesota Department of Health on the west-central Mesabi Iron Range suggest that groundwater flow within the Biwabik Iron Formation is influenced by internal stratigraphic contacts. Groundwater flow appears to be focused in distinct zones within the Upper and Lower Cherty members. The strongest evidence for flow appears near the top of these members, close to the contact with overlying slaty strata. The Upper Cherty flow zone is characterized by groundwater that is young in recharge age, warm and high in total dissolved solids relative to the Lower Cherty flow zone. These findings have several implications regarding mine dewatering and other environmental considerations. In addition, they may reflect on the origins of the natural ore bodies that were exploited in the early years of mining on the Mesabi Range. These data support a model whereby vertical movement of oxidizing fluids along faults and fractures was accompanied by lateral flow along contacts between slaty and cherty strata. 79 �MAGMA CONDUIT HOSTED PLATINUM-PALLADIUM-COPPER-NICKEL MINERALIZATION AT THE THUNDER BAY NORTH PROJECT, NORTHWEST ONTARIO: DISCOVERY, EXPLORATION, GEOLOGY AND RESOURCE POTENTIAL Keith P. Watkins, Magma Metals Limited, Level 3, 18 Richardson Street, West Perth, Western Australia WA6005, Australia, keith.watkins@magmametals.com.au Glacially transported Pt-Pd-Cu-Ni mineralized peridotite boulders were found on the west shore of Current Lake, approximately 50 km northeast of Thunder Bay, by two geologists prospecting through the area in 2001. Magma Metals Limited optioned the claims in 2005 and found a larger occurrence of mineralized boulders on the east shore of the lake in mid2006. Magma drilled its first hole, TBND001, under these boulders in December 2006 and intersected 10.5 m @ 2.69g/t Pt+Pd, 0.45% Cu & 0.34% Ni from 72 m down-hole. Subsequent geophysical surveys and over 45,000 m of drilling have delineated a large mineralized intrusive complex within a 50 km2 area. The complex forms a network of maficultramafic magma conduits associated with the Keweenawan-age Midcontinent Rift. The mafic-ultramafic magmas were emplaced within late-Archean granitoids and metasediments of the Quetico subprovince of the Superior craton. Most of the exploration work has focused on the Current Lake Intrusive Complex (CLIC), the 5 km-long easternmost conduit, which is described here. Several intrusive phases have been identified and there are strong structural controls on the emplacement of the conduits. The earliest intrusive phase in the CLIC is a minor pegmatoid unit which intrudes flat structures. This is followed by gabbro phases which have assimilated much country rock and show a variety of textures and compositions; these intrude flat and steep structures. Finally, several pulses of sulphide-rich melagabbro to peridotite intrude the earlier phases and form the main conduits. The dominant style of mineralization is disseminated sulphide, principally pyrrhotite and chalcopyrite. Thick zones have been intersected in systematic drilling over a strike length of 3 km in the CLIC, some of these zones are relatively high-grade, e.g. 61.7 m @ 2.87g/t Pt, 2.74g/t Pd, 0.66% Cu & 0.38% Ni from 29.3 m down-hole, including 35.5 m @ 4.52g/t Pt, 4.31g/t Pd, 1.04% Cu & 0.57% Ni in drill-hole TBND061. Zones of net-textured (semimassive) sulphide also occur. Some narrow intervals of high-grade massive sulphide mineralization have also been intersected near the base of the conduit, including 0.4 m @ 13.80g/t Pt, 10.75g/t Pd, 3.70% Cu & 2.91% Ni from 315.15 m down-hole in drill-hole BL08-61. There is generally good correlation between pyrrhotite-chalcopyrite abundance and grade of mineralization. Sulphide tenors (estimated grades in 100% sulphide) are consistently in the ranges 3-4% Ni, 6-8% Cu, 24-38g/t Pt and 22-37g/t Pd. There is excellent correlation between Pt, Pd, Cu and Ni in the melagabbro-peridotite indicating a pristine magmatic system with little alteration or re-distribution of metals. Preliminary petrochemical analysis indicates the mineralized melagabbro-peridotite had a tholeitiic parent magma (~6% MgO) and there is evidence of homogenized crustal contamination in addition to localized marginal contamination. 80 �Recent reconnaissance drilling in other parts of the magma conduit network to the west of the CLIC has confirmed the potential for further mineralization outside of the area currently being systematically drilled. There is potential for an aggregate multi-million ounce resource of platinum-group metals with substantial copper and nickel credits within the magma conduit network. There is also potential for associated Ni-Cu massive sulphide deposits. A major drilling program is in progress to define initial resources within the northwestern and central parts of the CLIC over a strike-length of 3 km. 81 �DIAMONDIFEROUS MASS-FLOW AND PLACER DEPOSITS NEOARCHEAN FAN DELTA, WAWA AREA, SUPERIOR PROVINCE FORMING A WENDLAND, Corey, FRALICK, Philip, and Hollings, Peter, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1, philip.fralick@lakeheadu.ca Diamond bearing Neoarchean metaconglomerates are present in the Michipicoten Greenstone Belt, Wawa-Abitibi Subprovince, near Wawa, Ontario. They form a portion of the Dore Metasediments in the Arliss Lake Subbasin, and unconformably overlie a succession of mafic metabasalts. The conglomerates are transitional to the south into argillite and are overlain by argillite. This in turn is conformably overlain by metabasalts. The conglomeratic succession under study here has a maximum thickness of 454 meters and is confined in what appears to be a deformed paleovalley at the base of the sedimentary succession. It pinches out against basement on its northern margin and is terminated by a fault on its southern margin, where it is 200 meters thick. Both matrix supported and clast supported beds characterize the unit. Dominant clast types include basalt, rhyolite, gabbro, diorite and sandstone. Clasts are generally angular to sub-rounded, and more rarely rounded to well-rounded. Matrix compositions and textures are variable from mud- and silt-sized material dominating within the matrix-supported, cobble to boulder conglomerates, to fine-grained to very coarse-grained feldspathic, quartz-rich and mafic sands within the clast-supported, cobble to pebble conglomerates. Boulder conglomerates are poorly sorted, while cobble to pebble conglomerates are poorly to moderately-well sorted with a decrease in clast size generally corresponding to better sorting. The unit can be divided into two main lithofacies associations. The dominant lithofacies within Association 1 is massive, matrix-supported cobble to boulder conglomerate, characterized by an abundance of mud-sized matrix material that is disorganized to swirlly. This is interbedded with massive to crudely horizontally layered, and more rarely trough cross-stratified, cobble to pebble conglomerate and minor horizontally layered coarsegrained sandstone. Contacts between these lithofacies are commonly planar and rarely erosive. This facies association is most prevalent near the base of the succession in the center of the apparent paleovalley. Many of the features of the mud-rich conglomerates suggest weakly sheared and highly viscous debris-flows. The poor-sorting, high mud matrix content, angularity of clasts and lack of sedimentary structures implies that few grain-to-grain bedload collisions occurred (weakly sheared) and the transportation mechanism was incapable of winnowing fine sediments and sorting the clasts. Highly viscous debris-flows are characterized by a muddy matrix and often occur in the proximal reaches of alluvial fans, whereas sandy matrix is typical of less viscous, distal-fan debris-flows. Some units containing disorganized clast fabrics may have formed from non-sheared (high strength) plug flow, or only weakly sheared, high viscosity flow on the upper fan. The trough cross-stratified and horizontally layered conglomerates interlayered with the massive conglomerates are the product of active bedload traction transport that was more efficient at winnowing fine sediment and sorting clasts. The sporadically developed erosive bases exhibited by these units and their upward fining is typical of turbulent fluidal flow or heavily sediment laden stream flow following a debris-flow event. Where the stratified conglomerates exhibit more extensive erosional bases they probably represent later erosive reworking of debris-flow material by stream activity. The horizontally stratified sandstones present capping the layered conglomerates were deposited by waning flow resulting in cessation of movement of clasts, falling of sand from the saltation and suspension populations into the traction population, and its deposition. Composite units with thick, crude to distinct internal layering or with the presence of thin, discontinuous sandy zones may result from rapidly surging flows. Lithofacies Association 2 occurs in the middle and upper portions of the unit. It is composed of interbedding of massive, clast-supported conglomerate and horizontally laminated and trough crossstratified sandstone. Other minor lithofacies that occur locally are horizontally layered and trough 82 �cross-stratified conglomerate; planar cross-stratified, ripple laminated and scour-fill sandstones: and horizontally laminated and massive mudstones. The major difference between the massive conglomerates of this Association and Association 1 is that the former is generally finer-grained, dominated by cobbles and pebbles, is clast supported and has a coarse-grained sand to granule matrix. This association was deposited by traction currents in braided fluvial channels of the Scott type. These channels were dominated by gravel longitudinal bars with sandy lenses formed by the infilling of chute channels and scour hollows during falling stage and low water. Sand and gravel bar edge sand wedges were also present in the system and major channels were probably mostly dominated by gravel, as more extensive sandstone successions, typical of sandy large channels, are rare. Association 1 was the product of proximal alluvial fan debris-flows that would have occurred on a fairly steep gradient allowing for such rapidly surging flows to result in an increase in sediment instability causing the coarse-grained sediment and mud-charged debris-flows to move down the alluvial fan-delta. As the gradient decreased the highly viscous debris-flows would deposit almost instantaneously resulting in the immature nature of the fabric and large boulder sized clasts being left in suspension in the muddy, swirlly textured matrix. Once the initial surge of the debris-flow had been deposited the fluvial activity that remained would, on occasion, be strong enough to actively transport bedload material depositing stratified gravels and sands as the stage fell. Further down fan was subjected to less debris-flow activity and it developed an extensive network of gravelly braided channels and gravel bars. It is likely that the upper fan-delta was dominated by off channel processes, whereas in the mid-fan-delta, where the main channel was no longer entrenched, water and sediment was delivered to the fan’s surface on a more consistent basis leading to a better developed fluvial network. A transgressive event resulted in a rapid drowning of the fan delta and deposition of finegrained sediment. Whole-rock geochemistry was conducted on samples of metasedimentary rock from the unit. Ratio plots utilizing immobile elements clearly indicate that the sediment was mostly derived from mafic rock mixed with an ultramafic igneous source. A minor number of samples had a significant felsic source component. The CIA values for the sandstones indicate most samples have undergone a moderate amount of weathering with some reflecting fairly intense weathering. Elements typically contained in heavy minerals, and therefore enriched in placer deposits, (i.e. Zr, Ti, Nb, Y, REEs, Cr and Ni) do not show any systematic enrichment in the sandstones sampled that cannot be accounted for by the composition of the source rocks. This strongly implies that preferential heavy mineral enrichment did not occur in the sandstones. The conglomerates are still under investigation. Diamond concentrations are correlated with increased amounts of Ni, Cr, Co, Ti, Fe and Mg in the samples. When the sandstone samples were plotted on a TiO2/Zr, Nb/Y diagram they defined a more alkalic trend than the felsic to mafic volcanic rocks in the area. This is in agreement with an alkalic ultramafic source rock for the diamonds. The source could either be ultramafic, diamond-bearing lamprophyre dikes present in the area or as yet undiscovered or eroded kimberlites. Whatever the source, a prolific amount of it must have been exposed on surface during formation of the fan delta. 83 �84 � https://digitalcollections.lakeheadu.ca/files/original/d69a6dae6ff77240acdeb4f1d620f717.pdf 86b54caaca65e5dfc5993a8fdcf2341f PDF Text Text 55TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ELY, MINNESOTA, MAY 5 – 12, 2009 PROCEEDINGS VOLUME 55 PART 2 – FIELD TRIP GUIDEBOOK �INSTITUTE ON LAKE SUPERIOR GEOLOGY 55TH ANNUAL MEETING MAY 5-10, 2009 ELY, MINNESOTA HOSTED BY: Precambrian Research Center, UMD JAMES D. MILLER, GEORGE J. HUDAK AND DEAN M. PETERSON Co-Chairs Proceedings Volume 55 Part 2 – FieldTrip Guidebook EDITED BY DEAN M. PETERSON DULUTH METALS LIMITED AND THE PRECAMBRIAN RESEARCH CENTER Cover Photo: Photographs taken in recent years during geological mapping, ore deposit research, fishing for dinner, and educational activities for undergraduate geology students in northeastern Minnesota. (photographs by Dean Peterson) i �55TH INSTITUTE ON LAKE SUPERIOR GEOLOGY PROCEEDINGS VOLUME 55 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE TRIP 3: SOUDAN IRON MINE AND PHYSICS LAB TOUR TRIP 4: PIONEER MINE CANOE EXCURSION TRIP 5: GEOLOGY & METAMORPHISM OF THE EASTERN MESABI RANGE TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT Published by the 55th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 55 PART 2— FIELD TRIP GUIDEBOOK TRIP 1: CU-NI DEPOSITS OF THE DULUTH COMPLEX ............................................................ 1 TRIP 2: GLACIAL GEOLOGY OF THE VERMILION MORAINE ............................................ 81 TRIP 3: SOUDAN IRON MINE AND PHYSICS LAB TOUR ..................................................... 100 TRIP 4: PIONEER MINE CANOE EXCURSION............................................................................. 110 TRIP 5: GEOLOGY & METAMORPHISM OF THE EASTERN MESABI RANGE ........... 116 TRIP 6: GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE .................................. 156 TRIP 7: ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT ................................... 178 The editor (Dean M. Peterson) wishes to thank the field trip guidebook authors for their contributions to the 55th Annual ILSG conference Field Trip Guidebook tome. This annual meeting is especially built around excellent field trips for ILSG participants that mineral industry companies and local geologists in the Lake Superior region provide to the Institute. I believe that this field guide continues that tradition, and will be useful for professional geologists and students of geology for many decades to come. The editor also wishes to tell the authors of the guidebook that some of the text and figures have been edited for clarity (which I guess is the role of a compiler and editor), and I hope that such editing better conveys the message that the written descriptions and especially the rocks in the Lake Superior District are trying to tell us. The institute wishes to thank Duluth Metals for their $2,000 contribution to the 55th ILSG Field Trip Guidebook. These monies have been used to pay for the numerous color figures and thus lower the cost to participants of the printed version. iii �55th Annual Institute on Lake Superior Geology Field Trip 1 CU-NI-PGE DEPOSITS OF THE DULUTH COMPLEX GEOLOGY AND DEVELOPMENT Rich Patelke (PolyMet Mining) Dean Peterson (Duluth Metals Ltd. & PRC) Mark Severson (Natural Resources Research Institute) Tim Jefferson (Teck American) Ernie Lehmann (Franconia Minerals) 1 �COPPER-NICKEL-PGE PROJECTS GEOLOGY PROGRESS AND DEVELOPMENT INSTITUTE ON LAE SUPERIOR GEOLOGY, ELY MINNESOTA, MAY 2009 Field Trip Coordinator: Dean Peterson Field Guide Compiled By: Richard Patelke Written Contributions By: Richard Patelke-PolyMet Steve Geerts-PolyMet Mark Severson-Natural Resources Research Institute Tim Jefferson-Teck American Dean Peterson-Duluth Metals Richard Routledge-Scott Wilson RPA for Franconia Presenters of Introductory Talks: Kevin Pylka-PolyMet Dean Peterson-Duluth Metals William Brice-Franconia Paul MacRobbie-Teck American Mark Severson-Natural Resources Research Institute Richard Patelke-PolyMet Field Trip Leaders: Richard Patelke PolyMet Mark Severson-Natural Resources Research Institute Tim Jefferson-Teck American Ernie Lehman-Franconia Stuart Behling-Franconia Dean Peterson-Duluth Metals 2 �PART 1: INTRODUCTION, DEPOSITS, AND REGIONAL GEOLOGY OF THE FIELD TRIP AREA By: Richard Patelke, Mark Severson, Tim Jefferson, and Dean Peterson PURPOSE OF TRIP Over time there have been many trips to drill sites, core sheds and outcrops of the mineralized parts of the Duluth Complex. As the current projects move toward development it becomes appropriate to visit the sites, assess the development potential, and see how that potential has changed over time. We will visit core displays for four of these projects, PolyMet’s NorthMet project, Teck American’s Mesaba deposit, Franconia’s Birch Lake deposit, and Duluth Metals Nokomis deposit. PolyMet is in the permitting process with state and federal agencies; Franconia and Duluth Metals have commissioned scoping studies on their deposits and are on the cusp of proceeding to development. Teck does their assessment work internally and is taking a different and less public route through economic review and development. These write-ups are not core specific and are intended to give a sense of the general geology of these projects. Besides the four companies whose projects we will visit, Encampment Resources, Cardero Resources, Prime Meridian Resources, and Kennecott have been active in the area. Technical reports from most of these companies are also available on “SEDAR” at http://www.sedar.com. These reports will give good background on the reserve and resource calculations, as well as development planning. Throughout these descriptions, the term “PGE” means platinum group elements (mostly platinum and palladium) and gold. Since these deposits were first known in the 1950's and 1960's, market and technological changes have made older assessments of the projects obsolete. The big market change has been the rise of PGE as important products, providing about a fourth of the revenue for these projects. Technological change has included: the ability to routinely and inexpensively analyze for PGE; improvements in flotation processes that allow the capture of almost all sulfide (minimizing sulfur in tails as well as improving recoveries), allow for the production of saleable split (i.e., separate copper and nickel) concentrates; and the refinement of hydrometallurgical processes that are able to cleanly recover not just copper and nickel, but PGE, gold, cobalt, and silver. Better flotation and the polymetallic nature of these deposits have pushed the ore vs. waste cut-off much lower than in historic estimates. The change in environmental aspects of these projects due to the recent market and technological changes cannot be overstated. Much of the research on these issues over the years has assumed metal and sulfur contents in waste rock and tailings far above expected ore versus waste cut-off grades for these deposits today, when hydrometallurgical processes are considered. EXPLORATION AND DEVELOPMENT BACKGROUND Large resources of low-grade copper-nickel sulfide ore that locally contain PGE concentrations are well documented by drilling in the basal zones of the Partridge River and South Kawishiwi intrusions. At least eleven occurrences of significant mineralization have been delineated in the basal 300 to 1000 feet of both intrusions. Of these eleven occurrences, three projects are currently undergoing deposit definition drilling, including the Mesaba (Teck American), Birch Lake (Franconia Minerals), and Nokomis (Duluth Metals Ltd.) deposits. A fourth project, the NorthMet deposit (PolyMet Mining) is currently undergoing environmental review and mine permitting. Overall, the copper-nickel mineralization consists predominantly of disseminated sulfides that collectively constitute over 4.4 billion tons of material 3 �averaging 0.66% Cu and 0.20% Ni at a 0.5% Cu cut-off, according to an earlier study by Listerud and Meineke (1977). Serious exploration for Cu-Ni deposits at the base of the Duluth Complex began in 1948, about 8 miles to the southeast of Ely, MN, when strongly mineralized rocks were uncovered in an excavation used to source road material for Spruce Road. Local prospector Fred S. Childers of Ely noted copper stains in the material and he, along with Roger V. Whiteside of Duluth, began searching along the basal contact in the vicinity of the Kawishiwi River. In 1951, they diamond drilled a 188 foot deep hole and intersected mineralized gabbro that averaged 0.36% Cu and 0.13% Ni. In 1952, both Bear Creek Mining Company (BMC) and the International Nickel Company (INCO) began intensive exploration efforts along a 38 mile-long zone that coincided with the basal contact. INCO eventually picked up the Childers-Whiteside properties (Spruce Road and Maturi deposits); whereas, BMC concentrated most of their effort near the town of Babbitt which resulted in the discoveries of the Babbitt (formerly called Minnamax and now known as Mesaba) and Serpentine deposits. By 1960, these exploration efforts indicated that very large tonnages of disseminated Cu-Ni mineralization were present; however, the low-grade nature of the deposits and the unavailability of state-owned mineral lands at the time led to suspension of activities. In 1966, state mineral leases were offered by the Minnesota Department of Natural Resources (DNR) and were awarded to successful bidders, resulting in renewed exploration activity (including the return of BMC and INCO). Since 1966, over 20 companies have been actively involved in exploration for Cu-Ni and Fe-Ti-V deposits along the basal contact of the Complex. Over 2,000 holes, totaling over 2 million feet of core, have been drilled. Exploration efforts during this period also defined several more deposits including: Dunka Road (now NorthMet) and Wyman Creek (United States Steel Corp.), Birch Lake (Duval Corporation and Newmont Mining), South Filson Creek (Hanna Mining), Dunka Pit (Erie Mining, BMC, and Exxon), and Wetlegs (BMC and Exxon). AMAX Exploration Inc. leased the Babbitt deposit from BMC and renamed it the Minnamax deposit in 1973. During mid to late 1970s, the Spruce Road and Minnamax deposits came closest to development. Mining plans were submitted, test shafts were sunk (one each at the Maturi and Minnamax/Mesaba deposits), surface bulk samples were collected from three sites, and various land-use and water-use permits were requested from State and Federal agencies. In 1974, the Minnesota Environmental Quality Board required that a regional Environmental Impact Statement (EIS) be conducted prior to acceptance of any site-specific EIS mining-related proposals. The DNR discontinued lease sales of State lands (1974-1982) until completion of the regional EIS. However, by the time the regional EIS was submitted in 1979, development of the Cu-Ni deposits was put on hold by the most of the mining companies involved due to weakened copper and nickel markets and the inability to make marketable (i.e., "smeltable") separate copper and nickel concentrates. Amax abandoned their plans to develop an underground high grade ore zone within the Minnamax/Mesaba deposit (known as the Local Boy ore body) in late 1982. Then starts the “PGE” era. During the early period of drilling (prior to 1980), all of the exploration companies recognized that the Cu-Ni deposits had some potential for hosting PGEs. Based on very limited sampling, the companies assumed that the typical Cu-Ni ore contained no more that a few hundred parts per billion (ppb) combined platinum and palladium. In 1985, the DNR and Minerals Resource Research Center (MRRC of the U of M) conducted a geochemical evaluation of portions of a Duval drill hole (DU-15), from the Birch Lake area, and found significant values of up to 9 parts per million (ppm) combined Pt and Pd (Sabelin and Iwasaki, 1985, 1986). This was at a time when demand for these elements was increasing due to their use in automotive catalysts. A short time later, Morton and Hauck (1987) compiled all of the known PGE data for the Complex and reported the presence of anomalous PGE values, often associated with high Cu values, at several other Cu-Ni deposits. These discoveries sparked renewed interest in the Cu-Ni deposits as potential polymetallic deposits (Miller et al., 2002; and references therein). E.K. Lehman and Associates of Minnesota obtained mineral leases from the state of Minnesota and began drilling wedges off the discovery hole (DU-15W) in the Birch 4 �Lake area. These Lehmann leases were later incorporated into Franconia Minerals holdings. Additional drill holes were sampled and analyzed for PGEs by several other companies throughout the Duluth Complex, and as a result, significant PGEs were found at many deposits. The occurrences of PGE mineralization for each deposit will be more thoroughly discussed later in this guidebook. Enter the “Hydromet” era. Early development of the deposits was hampered both by state leasing issues, complex metallurgy that resulted in an inability at the time to make marketable separate Cu and Ni concentrates, and by general environmental concerns regarding sulfide mining and conventional pyrometallurgical processes. In the mid to late 1990s, the potential of developing the Cu-Ni deposits using hydrometallurgical techniques once again sparked renewed activity in the Duluth Complex. PolyMet plans to use the PlatSol technique, developed and patented by SGS Lakefield on NorthMet ores, to recover Cu, Ni, Co, and PGE at the NorthMet deposit (Dunka Road). Teck American is conducting tests on Mesaba ore to utilize its patented Cominco Engineering Services Laboratory (CESL) process to recover metals. Both Duluth Metals and Franconia have PlatSol licenses. The PlatSol and CESL processes are of similar concept, both utilizing an autoclave (pressure oxidation) process wherein sulfides are converted to sulfates and metals put into, and then recovered from, solution. Thus the sulfur air emissions of conventional smelting are eliminated and an inert and potentially marketable by-product of gypsum (calcium sulfate) is produced. Other residues from the processes are easily isolated for landfill disposal or other containment. REGIONAL GEOLOGIC SETTING, DULUTH COMPLEX The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion years) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 2,200 square mile (5,700 square km) arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, that imply intrusive roots more than 8 miles (13 km) deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying much of northeastern Minnesota were emplaced during the development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 1250 mile (2,000 km) long, segmented, arcuate path to Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >170 miles (275 km) in an arcuate fashion from Duluth nearly to Grand Portage (Fig. 1-1). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift related flood basalts and hypabyssal intrusions of the Beaver Bay Complex (Fig. 1-1). In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice between 1108 and 1098 Ma. The geology of the Duluth Complex and adjacent areas has recently been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al., 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Readers’ interested in more detailed descriptions of the geologic setting of the Duluth Complex should begin their quest for knowledge by downloading these publications from the MGS website (ftp://mgssun6.mngs.umn.edu/pub2/). Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. 5 �Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semicontinuous mass of intrusions strung along the eastern and central roof zone of the complex, emplaced during an early stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, emplaced during early stage magmatism (~1108 Ma). Anorthositic series—A structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—A suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). Figure 1-1. Generalized geologic map of northeastern Minnesota. (modified from Miller et al., 2002). LOCAL GEOLOGIC SETTING-PARTRIDGE RIVER, SOUTH KAWISHIWI, AND BATHTUB INTRUSIONS The four deposits under review for this trip are located in three of the oldest intrusions in the Complex. NorthMet and parts of the Mesaba deposit in the Partridge River intrusion, Parts of Mesaba in the newly defined Bathtub intrusion, and Birch Lake and Nokomis in the South Kawishiwi intrusion (Fig. 1-2). 6 �Figure 1-2. Location of Cu-Ni±PGE sulfide deposits, Fe-Ti±V oxide deposits (Oxide-bearing Ultramafic Intrusion - OUI), and other exploration areas along the western edge/base of the Duluth Complex. Note that the NorthMet deposit was referred to as the Dunka Road deposit and the Mesaba deposit was referred to as the Babbitt deposit; the most recent names for these two deposits are used in this guidebook. 7 �Partridge River intrusion The Partridge River intrusion (PRI) consists mainly of troctolitic cumulates, dips gently to the southeast, and is exposed in an arc-shaped area that extends from the Water Hen deposit, on the southwest, to the southern edge of the Mesaba/Babbitt deposit, on the northeast (Fig. 1-3). Footwall rocks include the Paleoproterozoic Virginia Formation and locally the Biwabik Iron Formation. The basal 3000 ft. (900 meters) are known in great detail from studies of abundant drill core (Severson and Hauck, 1990; Severson, 1988) and are subdivided into seven or more units that can be traced over a strike-length of 15 miles (24 kilometers). The units of the Partridge River intrusion (PRI) are recently described in Miller and Severson (2002) and are depicted in Figure 1-3. At the base of the PRI is Unit I which consists of a suite of heterogeneoustextured troctolitic rocks that contain the vast majority of disseminated sulfide-mineralized zones. The top of Unit I is characterized by a fairly persistent ultramafic horizon, which in actuality is at the base of Unit II. Within Unit I are several laterally-discontinuous ultramafic horizons and abundant footwall sedimentary inclusions of the Virginia Formation. Noritic rocks are common at the basal contact and adjacent to the inclusions. Unit II consists of more homogenous-textured troctolitic rocks with minor sulfide-bearing zones. However, at the Wetlegs deposit, both Units I and II contain abundant laterallydiscontinuous ultramafic horizons, interbedded with troctolitic rocks that are collectively referred to as the Wetlegs Layered Interval (Fig. 1-3). Unit III is a major marker bed throughout much of the PRI (Wetlegs to Mesaba deposits - Fig. 1-3) in that it is characterized by a poikilitic leucotroctolite with olivine oikocyrsts that are randomly dispersed throughout the rock giving it a mottled appearance. This mottled-appearance, and the relatively finegrained nature of Unit III, give it a distinct appearance in drill core and it is easily identified. Unit III pinches out to the west of the Wetlegs deposit and is present on only the southern fringe of the Mesaba deposit. The rapid pinch-out of Unit III to the north within the Mesaba deposit appears to be related to emplacement of a distinctly different sub-intrusion herein referred to as the Bathtub intrusion (see discussion below). Figure 1-3. Generalized stratigraphy of the basal zone of the Partridge River intrusion (modified from Severson, 1994). Roman numerals (I through VIII) denote igneous units in the Partridge River intrusion; BT1 and BT4 denote igneous units in the Bathtub intrusion; and OUI denotes Oxide-bearing Ultramafic Intrusions. 8 �Overlying Unit III in the PRI are units IV through VIII. Unit IV varies from a troctolite to augite troctolite, often contains an ultramafic base, and commonly grades upward into Unit V which is coarsergrained and varies from a troctolite to troctolitic anorthosite. Units VI and VII, and additional units above VII, are generally homogenous-textured troctolitic to anorthositic troctolitic rocks; each with a persistent ultramafic base that record magma injection events. Bathtub intrusion The Bathtub intrusion (BTI) is wholly contained in the central portion of the Mesaba (Babbitt) deposit. It has recently been singled out as a separate intrusion to explain the abrupt change from typical Partridge River intrusion stratigraphy in the southern part of the deposit to a completely different stratigraphy, to the north, in the remainder of the deposit. There are three structural features that are pertinent to understanding the intrusive history of the BTI that include (Fig. 1-4): 1) an east-west trending paired syncline and anticline in the footwall rocks referred to as the Bathtub Syncline and Local Boy Anticline; 2) a zone that is closely associated with the Local Boy Anticline, referred to as “The Hidden Rise,” that separates the PRI and BTI; and 3) a north-trending zone fault zone, referred to as the Grano Fault, that has been postulated to have been the feeder zone for the BTI and footwall-injected massive sulfides of the Local Boy ore zone. The “Hidden Rise” is a loosely-defined zone wherein scattered hornfels inclusions, and associated noritic rocks, are fairly common. When viewed collectively, the inclusions in “The Hidden Rise” define an eastwest trending “ridge” that is roughly positioned at the contact between the PRI and BTI. Thus, “The Hidden Rise” is used to both define this hornfels-bearing “ridge” and to artistically, and conveniently, divide the BTI from the PRI. The morphology of this feature suggests that it may have originally served as the floor and/or north edge of an earlier intruded PRI and later served as a wall along the south edge of the BTI as it was emplaced. The BTI has been subdivided into two main units, BT1 and BT4, each of which contain several internal subunits (Fig. 1-4). In the vicinity of the Bathtub Syncline, ultramafic layers and modally-bedded rocks are extremely common within the BT4 Unit and have been collectively referred to as the Bathtub Layered Interval (BTLI). Figure 1-4. Schematic “type-section” cross-section, looking east, through the Mesaba deposit that crudely displays the spatial distribution of most of the igneous units in the Bathtub intrusion and pertinent structural features. Note that not all of the PRI units are shown on the right side of the figure. Cu-rich massive sulfides are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. Local Boy is positioned along the crest of the Local Boy Anticline, in close to proximity to “The Hidden Rise,” and just west of the Grano Fault. Most of the massive sulfides are associated with 9 �either hornfelsed sedimentary inclusions above the basal contact or with footwall rocks below the contact while the interfingering intrusive rocks are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A possible feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. West-directed increases in Cu and PGE, associated with the massive sulfides at Local Boy, suggest that the immiscible sulfide melt fractionally crystallized and became progressively enriched in Cu and PGE as it was deposited in an east-to-west direction. Partridge River and Bathtub intrusion footwall rocks Because the footwall at NorthMet and Mesaba is so similar, following is a generic description appropriate to both deposits. The drilled footwall rock types at Mesaba and NorthMet consist mainly of the Virginia Formation and Biwabik Iron Formation. Both are Paleoproterozoic in age (approximately 1.9-1.8 Ga) and are the two upper units of the Animikie Group . Any discussion on these two formations must include a description of their type-section on the Mesabi Range, as well as, a description of them as related to the metamorphism and partial melting that was produced during emplacement of the Complex. Lying beneath the Biwabik Iron Formation, but encountered only in a few drill holes are the Paleoproterozoic Pokegama quartzite (also of the Animike Group), along with granites and gneisses of the Archean Giant’s Range Batholith. Biwabik Iron Formation The Biwabik Iron Formation (BIF) exposed on the nearby Mesabi Range has typically been subdivided into four informal lithostratigraphic members (Wolff, 1917) that are, from the bottom up: Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty. Diamond drill holes at Mesaba and NorthMet generally pierce the top submembers of the Upper Slaty, and end in submember C or D: Submember A is comprised of a pale to white chert and marble, submember B is characterized by alternating bands of green diopside and chert, and submember C is a green to gray thin-bedded rock consisting of chertfayalite-ferrohyperstene with wispy black bands of magnetite (taconite). The upper contact of the BIF is gradational with the overlying Virginia Formation. Virginia Formation Below the PRI and BTI The Virginia Formation is a thick sequence of argillite, siltstone, and graywacke at the top of the Animikie Group. On the basis of lithotypes present in five drill holes, Lucente and Morey (1983) divided the Virginia Formation into two informal members – a lower argillaceous lithosome and an upper silty and sandy lithosome. The lower lithosome is approximately 600 feet (180 meters) thick and contains common intervals wherein black, thin-bedded, carbonaceous argillite is the dominant rock type (Lucente and Morey, 1983); visible sulfides are locally present. The lower lithesome is the unit which underlies the Mesaba deposit, and the base of the Virginia Formation is what is most commonly intersected below the Bathtub intrusive, prior to reaching the Biwabik Iron Formation. In close proximity to the Duluth Complex, the Virginia Formation is described as being a hornfels that, as defined by Turner (1968), is a nonfoliated rock composed of a mosaic of equidimensional grains. In reality, many of the “hornfels” textures exhibited by the metamorphosed Virginia Formation do not meet this criterion, but as the term has been widely used in descriptions of the Virginia Formation in the vicinity of the Complex, it is retained in this guidebook. Mineral assemblages in the hornfels, in both the footwall and in inclusions within the Complex, consist of varying mixtures of cordierite, quartz, K-spar, and biotite with lesser amounts of chlorite, muscovite, plagioclase, orthopyroxene, and minor graphite and sulfides. 10 �The effects of partial melting are profound and portions of the hornfelsed Virginia Formation no longer even remotely resemble a sedimentary rock. Severson et al. (1994a) subdivided the hornfelsed Virginia Formation, in both the footwall and in inclusions within the Duluth Complex, into at least five informal units based largely on metamorphic attributes, which are each related to varying degrees of partial melting. These members, and a pre-Duluth Complex sill, are described below and are schematically portrayed in Figure 1-5 - although in real occurrence, this idealized metamorphic progression is more erratic, often with rapid lateral and vertical changes between the four metamorphic units discussed below. Figure 1-5. Schematic cross-section showing the general relationships of the metamorphosed footwall rocks beneath the Duluth Complex at the Mesaba, NorthMet, Wetlegs, and Serpentine deposits. Cordieritic hornfels Directly beneath the basal contact of the Duluth Complex, the adjacent Virginia Formation typically consists of massive/nonfoliated, cordierite-rich hornfels that display a bluish-gray color in drill core. The rock is generally fine-grained, granoblastic, and biotite-poor (due to loss of water into the Complex) and locally may contain porphyroblastic and/or poikiloblastic cordierite. Original bedding planes are preserved in some localities, but mostly the bedding planes have been obliterated by contact metamorphism. Recrystallized unit (RXTAL) Beneath the cordieritic “capping” the next metamorphic variant of the Virginia Formation nearest to the Duluth Complex is a rock that is referred to as the RXTAL unit. The RXTAL unit is properly classed as a diatexite and is characterized by fine- to medium-grained cordierite, plagioclase, biotite, quartz, and Kspar with lesser amounts of Opx and opaques. Bedding planes of the original argillaceous rocks are obliterated and what remains is a massive recrystallized rock with decussate biotite that contains enclaves (blocks and folded boudins) of more structurally competent calc-silicate hornfels and thin-bedded siltstone Disrupted unit (DISRUPT) With increased distance from the Complex, the RXTAL unit progressively grades into the DISRUPT unit which is a thin-bedded rock that is visibly deformed and underwent less degrees of partial melting. 11 �Textures that characterize the DISRUPT unit are bedding planes that are extremely chaotic and random in orientation due to pervasive small-scale folding, faulting, and brecciation. Superimposed on this chaotic pattern are abundant zones of leucocratic partial melts that are also chaotic and folded. The rock consists of varying amounts of quartz, cordierite, K-spar, biotite, plagioclase, and muscovite with leucosome veins and patches containing quartz, K-spar (microperthite), plagioclase, and muscovite (Duchesne, 2004). The DISRUPT unit is properly classed as a metatexite. Graphitic argillite and Bedded Pyrrhotite (BDD PO) units Carbonaceous argillite of the lower lithosome of the Virginia Formation is commonly preserved as either the BDD PO unit, or graphitic argillite, in close proximity to the Duluth Complex. This rock commonly contains over 5% disseminated pyrrhotite and/or extremely thin-bedded pyrrhotite laminae (hairlinethick), and variable amounts of graphite, staurolite(?) and sillimanite. Wherever the unit contains conspicuous and regularly-spaced laminae of pyrrhotite (0.5-3.0 mm thick at 1-20 mm spacings) it is informally referred to as the bedded pyrrhotite unit (BDD PO unit). VirgSill The VirgSill is generally present in the bottom 0.5-130 feet of the Virginia Formation, and as local apophyses into the top of the Biwabik Iron Formation. The VirgSill was intruded along the contact between the Virginia Formation and Biwabik Iron Formation and exhibits a granoblastic texture indicating that it was metamorphosed by the Duluth Complex (and thus the VirgSill is pre-Duluth Complex in age). On this basis, the VirgSill is inferred to be equivalent to the Logan sills (circa 1,109 Ma); as is another sill, the BIFSill, in the C submember of the Biwabik Iron Formation (Hauck et al., 1997). However, the VirgSill and BIFSill are different chemical entities (the VirgSill is much more Crenriched), and thus, these two sills may be related to at least two different intrusive events. When present, the VirgSill ranges from a few centimeters to several meters thick. Identification of the VirgSill in drill core is hampered by the fine-grained granoblastic texture that makes it difficult to distinguish from the enclosing hornfelsed Virginia Formation rocks; both were metamorphosed by the Duluth Complex. The VirgSill is subdivided into two textural varieties (Severson et al., 1994a; Park et al., 1999): 1. MG unit – fine-grained, massive, gray-colored unit (massive gray unit or MG unit) that appears to be a border phase or chill zone - albeit quite thick at some localities (up to 200 feet-thick in drill hole B1-264). In some drill holes the entire interval of the VirgSill consists of the MG unit. 2. Coarser-grained interior – medium- to coarse-grained, green- to brown-colored, olivine- and hornblende-bearing interior of the sill that is easily identified as an intrusive rock in drill core. The coarse-grained interior is not always present, and when present, may be up to 80 feet thick, and occurs as either a single lense within a thick MG unit or as several vertically-stacked lenses within the MG unit. Both the MG unit and coarser-grained interior of the VirgSill contain variable amounts of plagioclase, olivine, hornblende, clinopyroxene, orthopyroxene, and biotite with local sulfides (pyrrhotite, chalcopyrite, and bornite). The presence and metamorphic effect of this sill has caused an “armoring” wherein previously metamorphosed Virginia Formation has resisted assimilation by the Duluth Complex. Across much of the lower parts of the NorthMet and Mesaba deposits there is a persistent thin (<10ft.) rind of Virginia Formation associated with this sill. South Kawishiwi intrusion The South Kawishiwi intrusion (SKI) consists mainly of troctolitic cumulates and dips gently to the southeast. The SKI is exposed in an arc-shaped area that extends from the Serpentine deposit, on the southwest, to the Spruce Road deposit, on the northeast (Fig. 1-2). Footwall rocks include the Paleoproterozoic Virginia Formation, Biwabik Iron Formation and Archean Giants Range Batholith, the latter is the dominant footwall rock type. The presence of Biwabik Iron Formation as inclusions, from the 12 �Birch Lake deposit to as far north as the Spruce Road deposit, indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the South Kawishiwi intrusion (Severson et al., 2002). The basal stratigraphic section is known in great detail from studies of abundant drill core and is subdivided into 17 different units (Fig. 1-6) that are present over a strike-length of 19 miles (31 kilometers). The lowermost units are unevenly distributed along the strike length of the intrusion in a “compartmentalized” fashion, suggesting a complicated intrusive history (Miller and Severson, 2002). A few salient features to keep in mind regarding the igneous stratigraphy of the SKI include: • The vast majority of sulfide mineralization is confined to the BH (Basal Heterogeneous Unit), BAN (Basal Augite Troctolite and Norite Unit), UW (Updip Wedge Unit), and U3 (Ultramafic 3 Unit); • Major marker beds include three horizons that contain abundant cyclic ultramafic layers (U1, U2, and U3 Units) and a pegmatite-bearing unit (PEG Unit - originally recognized by Foose, 1984). The U1, U2 and U3 Units represent periods of rapid and continuous magma replenishment that crystallized more primitive ultramafic layers before mixing with the resident magma (Severson et al., 2002); • The U3 Unit is unique in that it contains several massive oxide pods (titanomagnetite-rich), as well as, recognizable inclusions of bedded Biwabik Iron Formation. The spatial correspondence between the U3 Unit and footwall iron-formation suggests that most of the massive oxide pods are iron-rich “restite” produced by assimilation and partial melting of the iron-formation (Muhich, 1993; Severson, 1994; Severson et al., 2002); • The U3 Unit contains the vast majority of high PGE values, especially within the Birch Lake area and possibly at the Nokomis deposit. However, high PGE values are also present in the PEG Unit (Birch Lake area and Nokomis deposit), the top of the BH Unit (Maturi deposit and Nokomis deposit), and in very locally in troctolitic rocks situated well above the basal contact (South Filson Creek deposit); and • A large inclusion/pillar of anorthosite is present at the Nokomis deposit. This pillar, and possible proximity to a vent area and magma flow paths (see discussion for Nokomis deposit) are the inferred reasons for high PGE values at the Nokomis deposit. Figure 1-6. Generalized stratigraphy of the basal zone of the South Kawishiwi intrusion (modified from Severson, 1994; and included in Miller and Severson, 2002). The lowermost igneous units are: BAN = Basal Augite Troctolite and Norite; BH = Basal Heterogeneous; U3 = Ultramafic 3; PEG = Pegmatitic unit of Foose (1984); U2 = Ultramafic 2; U1 = Ultramafic 2; AT-T = Anorthositic Troctolite to Troctolite; UW = Updip Wedge; Main AGT = Main Augite Troctolite. 13 �REGIONAL ECONOMIC GEOLOGY While Minnesota is home to the United States iron ore industry, development of its known non-ferrous deposits has been hampered by industry downturns, remoteness from the rest of the base metals industry, a complex land situation, and a (wrongly) perceived environmental risk. The state has done much to support non-ferrous exploration, in particular maintaining an impressive drill core and data library in Hibbing, and sponsoring extensive mapping and sampling projects. Research arms of the MDNR and the University have contributed much knowledge to mineral processing methods for these ores. In general this work has not been well publicized. This short discussion on regional mineral potential focuses on rocks in the north part of the state, but there has been recent exploration for nickel and diamonds throughout the state. For detailed discussion of potential in Archean rocks see Peterson (2001a) and for Duluth Complex rocks see Miller et al., 2002. Mesabi Range Iron Mines-Status There are six operating iron mines employing about 3,500 people in the region. They all produce taconite pellets from low grade magnetic ore. Four are captive to steel companies, two produce pellets for market, generally through long term contracts. Product shipping is largely by rail to one of four ports on Lake Superior, then by boat to mills on the lower lakes. About thirty eight million tons of pellets were produced in 2007. Total material movement (ore and stripping) was on the order of two hundred million tons. The one-hundred plus year history of this world class mining district means that there is an extensive developed infrastructure and service industry for these mines. Two iron related projects are in development in 2009: Mesabi Nugget, located near the PolyMet plant site, will initially use purchased concentrate to produce iron nuggets suitable for electric furnace production of steel, the company has submitted plans to the state to re-open some of the LTV pits and produce their own concentrate; and Essar Steel (Minnesota Steel Industries, formerly Minnesota Iron & Steel) plans to re-open a closed taconite mine (Butler Taconite) at the western end of the Mesabi Range and produce direct reduced iron from taconite pellets on site. That direct reduced iron will in turn be used to make steel slab for shipment. This will be the only mine to steel, single-site production facility in North America. Both of these projects are under construction. Other Regional Economic Geology Two broad age groups dominate the other rocks with mineral potential in northern Minnesota. Archean rocks represent possible hosts for lode gold, Volcanogenic Massive Sulfide (VMS), and diamond prospects. North of the Duluth Complex is extensive terrane of exposed Archean rock, similar to that in Ontario (Wawa and Quetico subprovinces). Over the years various prospects for gold and base metals have been delineated, but follow up work has been sporadic and generally short lived. These prospects include, for gold: Raspberry, Murray Shear zone, Spaulding Bay, Mud Lake, Pac Man Pond, and Section 6, investigated by Goldfields, Newmont, Kerr McGee, and Noranda among others. For base metals the deposits include: Clear Lake, Skeleton Lake, Fivemile Lake, and Purvis Road worked by the above companies as well as Exxon, Teck, Rendrag, and Lehmann. A 2001 PhD. thesis (Peterson, 2001a) is an excellent review of the Archean mineral potential of the region. That report makes detailed analytical comparisons between producing gold and base metals camps in Canada and prospects in Minnesota. Diamond work in Archean age rock of Minnesota includes Exmin (DeBeers), WMC, and others; as well as the Minnesota Geologic Survey. Proterozoic rocks in northern Minnesota include Animikian Basin (Paleoproterozoic, ~1.8 Ga) sedimentary rocks (Pokegama Quartzite, Biwabik Iron-Formation, and the Virginia Formation). Besides current iron ore production, past exploration has focused mostly on zinc and other base metals in the Virginia. Anomalous mineralization (sphalerite, molybdenite) has been found in the Virginia, but no prospects have been defined. 14 �Titanium and Other Oxide Mineral Potential in the Duluth Complex There are at least four titanium deposits within 12 miles (19 kilometers) of PolyMet’s Hoyt Lakes plant site (out of 13 known titanium deposits in the area). They are all located in the Duluth Complex and locally called “OUI’s” for “Oxide Ultramafic Intrusions”. These are titanium rich plugs which cross-cut the rocks of the Complex. All are greatly undersampled, especially for PGE and other oxides besides titanium (chromium, vanadium, etc.) Bulk samples have been processed from two of these titanium prospects. The titanium in these deposits is in magnesium-rich ilmenite which is not easily processed by current commercial methods. BHP, Coleraine Minerals Research Laboratory of the NRRI, and others have done extensive process testing towards adding value to these prospects. All 13 have been drilled, but generally not to a point where a legitimate (i.e., NI43-101 compliant) resource can be declared. Two OUI’s with reasonable historical resource estimates are Longnose, with 50 million tons averaging 21% TiO2 based on 11 drill holes and Water Hen, with 62 million tons averaging 14% TiO2, based on 37 drill holes. At present, Cardero Resources has leased Longnose, as well as the Section 34 deposit about halfway between Duluth and the Iron Range. This group of deposits represents great potential for undiscovered copper-nickel-PGE, silver, as well as oxide rich “plug-like” intrusions with known titanium resources and possibilities for chromium, vanadium, etc. It is obviously unknown what is undiscovered, but historic exploration on the Duluth Complex and associated rocks has focused almost exclusively on the large copper-nickel deposits along the northwestern contact with virtually no work done on the interior of the Complex. There is a large collection of core available for all of these projects, and in general a good collection of related data stored at the Minnesota Department of Natural Resources in Hibbing. NRRI has logged most of this core, and consolidated whatever assay data is available (Patelke, 2003). 15 �PART 2: POLYMET NORTHMET DEPOSIT By: Richard Patelke, Steve Geerts, Mark Severson NORTHMET PROJECT SUMMARY NorthMet, located in the Partridge River intrusion of the Duluth Complex, is a large, disseminated sulfide deposit in heterogeneous troctolitic rocks associated with the 1,100 million year old Mid-Continent rift. Metals of interest are copper, nickel, cobalt, platinum, palladium, and gold. The majority of the metals are concentrated in four sulfide minerals: chalcopyrite, cubanite, pentlandite, and pyrrhotite, with platinum, palladium and gold also found in bismuthides, tellurides, and alloys. NorthMet is one of eleven coppernickel-PGE deposits along the northern margin of the Complex (PGE: platinum, palladium, gold). All of these share grossly similar geologic settings–disseminated sulfides with minor local massive sulfides in heterogeneous rocks forming the basal unit of the Duluth Complex along the contact with older rocks. The deposit is on the southern flank of the Mesabi Iron Range, which is host to six large operating taconite mines, the closest of which is less than two miles (3.2 km) north of the planned NorthMet pits (Figure 1-7). Ore from NorthMet will be processed at a rate of 32,000 short tons per day through the former LTV Steel Mining Company iron ore concentration plant (“Erie Plant) with new facilities for processing of the NorthMet copper-nickel-PGE concentrates through a hydrometallurgical method day to produce copper metal and various hydroxide and concentrate products of nickel-cobalt-PGE (Figure 1-8). EXPLORATION and DEVELOPMENT There have been four major drilling programs since 1969, re-sampling for PGE began in 1989, three PolyMet joint ventures were pursued and dissolved in the 1990's, processing technology was developed in the late 1990's, the former LTV Steel Mining Company concentrator and other property was optioned in 2003, and the metallurgical process was refined in 2005-2008. Drilling programs have been conducted by United States Steel (USS, 1969-1974) and PolyMet Mining Inc. (Reverse Circulation or “RC” drilling and core drilling in 1998-2000 & two phases of core drilling in 2005 and 2007), plus two (actually two pairs of twins) holes by NERCO Minerals Company in 1991. This drilling encompasses 285,756 feet over 371 holes as of May 2008. Over 35,973 acceptable assays have been taken from this drilling (216,344 feet assayed). Table 1-1 gives a breakdown of years, footages, and number of assays for all project drilling. United States Steel (USS) began core drilling at NorthMet (as the Dunka Road project) in 1969. Drilling targeted a conductor that turned out to be in the footwall metasedimentary rocks, but the first drill hole hit massive sulfide in the Duluth Complex. Drilling continued over five years for 112 holes with 133,716 feet of intercept. The working assumption was to mine the deposit from underground, sampling was limited to the most continuous zones with strong visible copper-nickel mineralization, and only about 2,200 samples representing about 22,000 feet were taken. USS assayed only for copper, nickel, sulfur, and iron. PGE presence was known from sampling on concentrates, but the economics of PGE recovery were apparently not pursued. Project work stopped while apparently incomplete and was not restarted. USS did not do much follow-up, but kept their land ownership, core, pulps, coarse rejects, and records for the project. In the mid 1980's the Minnesota Department of Natural Resources (MDNR) began sampling various historic drill core intervals in the Duluth Complex for PGE and got some good, but localized, results. In 1989 Fleck Resources (Fleck) leased the Dunka Road property from USS and began a program of re-assaying USS pulps and coarse rejects with a much more extensive multi-element suite, as well as 16 �adding in some new samples from existing core through cooperative work with the Natural Resources Research Institute (NRRI), associated with the University of Minnesota Duluth. The results were very positive in showing elevated PGE values in the deposit and confirming the previous copper-nickel assays. Fleck partnered with NERCO in 1991 for some bulk sample work, mine plans, environmental reviews etc., done through Fluor Daniel Wright engineers, but the partnership was eventually dissolved. In 1995 Fleck joined with Argosy Mining Corp. to do more work on the project, again with no major progress towards production. In June 1998, Fleck became PolyMet and focused their resources on Dunka Road, which was renamed NorthMet. Without partners, except for a brief venture with North Mining (North), PolyMet drilled and sampled 87 holes in 1998-2001, and sent two large bulk metallurgical samples to Lakefield Laboratories (now SGS) in Lakefield, Ontario for development and refinement of the PlatSol hydrometallurgical process and began some environmental background work. In the summer of 2000, North was taken over by Rio Tinto. The joint venture agreement was terminated by PolyMet upon consideration that NorthMet appeared to be a low priority to Rio Tinto. The main concern was that other partnership opportunities might be missed during the time that Rio Tinto assessed and prioritized the ongoing North projects. However, much of the North funding was already in place and was used to partially finance the 2001 pre-feasibility study. After release of the pre-feasibility study (2001), a brief hiatus, and a major re-evaluation of how the project should proceed, PolyMet became active again in 2003 with new management and a new development plan. This plan involves integrating the former LTV Steel Mining Company iron ore concentration plant (“Erie Plant) with new facilities for processing of the NorthMet copper-nickel-PGE concentrates through a hydrometallurgical method at rate of 32,000 short tons of ore per day to produce copper metal and various hydroxide and concentrate products of nickel-cobalt-PGE. Geologic work towards this end began in 2004 and first focused on a careful and total re-compilation of the historic NorthMet project drill hole related data. This effort organized and verified all drilling metadata, location, downhole survey, lithology, and assay data, and cataloged all paper (and digital) records for the project. Of note is that this resulted in an increase in the number of acceptable assays from 12,000 to around 17,200 and an improved geologic picture from careful consolidation of existing records. This work was used as background for a revised resource estimate in January 2005 and planning of a drill program for 2005. The 2005 program entailed drilling and sampling 109 holes (77,000 feet), collection of a forty ton metallurgical bulk sample for pilot scale testwork, geotechnical (oriented core) drilling, in-fill sampling of previously drilled core, and extensive collection of waste characterization data. The 2005 drilling program added 13,450 multi-element assay records to the existing database. A PolyMet report covers the details of historic drilling and assaying (Patelke & Geerts, 2006). 17 �Figure 1-7. PolyMet NorthMet project site. 18 �Figure 1-8. Detail of Erie Plant site showing existing facility and new construction. 19 �Drilling in 2007 for 24,530 feet with 3,546 assays concentrated on defining mineralization in the upper units in the west part of the deposit (the “Magenta Zone”). This drilling and the subsequent re-modeling of the deposit turned about 50 million tons of material previously classed as waste to ore. There is also over 34,000 feet of hydrogeology drilling and “stratigraphic holes” (drilling by other companies not done as part of the NorthMet project). No assays are in use from these 44 holes which are used for geologic control. Approximately 89.5% of Unit 1 and about 57% of the upper units have been sampled across the deposit. The sampled percentages are higher in the anticipated area of mining. Table 1-1. Total drilling and assaying for NorthMet project Company Drilling years Assaying years No. of drill holes Total footage for group No. of assay intervals used in “accepted values” tables Assayed footage used in final database Assay Laboratories US Steel 19691974 1969-1974, 1989-1991, 1999-2001, 2005-2006, 2008 112 133,716 11,259 73,303 USS, ACME, ALS-Chemex NERCO 1991 1991 2 (4) 842 165 822 ACME PolyMet reverse circulation drilling 19982000 1998-2000 52 24,650 4,765 23,767 ACME PolyMet core drilling 19992000 2000-2001, few in 32 22,156 4,058 20,727 ALS-Chemex PolyMet RC drilling deepened with AQ core tail 2000 2000 3 2,696 524 2,610 ALS-Chemex PolyMet core drilling 2005 2005-2006 109 77,166 11,656 71,896 ALS-Chemex PolyMet core drilling 2007 2007 61 24,530 3,456 23,310 ALS-Chemex Totals for Exploration Drilling: 371 285,756 35,973 216,344 1970's? none 6 9,647 none none INCO 1956 none 3 2,015 none none Humble Oil / Exxon 19681969 none 3 9,912 none none Bear Creek / AMAX 19671977 none 11 8,893 none none PolyMet / Barr Engineering (hydrologic testing) 20052007 none 21+ 3,459+ none none US Steel stratigraphic holes 20 �Table 1-2. Large metallurgical samples collected at NorthMet Bulk Sample Year Tons Location of sample USS Bulk sample pit No. 1 1971 Unknown, but small Pit in center of property USS Bulk sample pit No. 2 1971 300 Pit at east end of property USS Bulk sample pit No. 3 1971 20 Pit at east end of property NERCO PQ drill core 1991 Estimated at 4.5 tons or less by drill core size One PQ drill hole from each end of property Argosy Mining 1995 Unknown, but small Composited from USS coarse rejects PolyMet RC drill cuttings 1998 26 One composite, mostly from what is now considered east part of 10 year pits PolyMet RC drill cuttings 2000 33 One composite, mostly from what is now considered east part of 10 year pits PolyMet 4 inch and PQ core and coarse reject 2005 10.5, 21.5, and 10.7 Three composites from within ten year pits across property PolyMet coarse reject 2006 4.2 and 4.94 One composite from 10 year east pit, one from 20 year pit across property PolyMet ¼ core from 2005 and 2007 Drilling 2007 500 kg One composite, from east and west pit areas PolyMet ¼ core from 2005 and 2007 Drilling 2008 4.44 One composite, from east and west pit areas PolyMet ¼ core from 2005 and 2007 Drilling 2008 4.48 One composite, from east and west pit areas Sampling in Unit 1 (the main mineralized zone) is now mostly continuous through the zone for all generations of drilling. The PolyMet RC and core holes have continuous sample through the upper waste zones (which do have some intercepts of economic mineralization). Work in 2005 through 2008 essentially completed the sampling of historic USS core within the area likely to be mined. This broad sampling limits the possibility of location bias in the sample set. While not all of the USS core has been sampled, there is no known unsampled mineralized core. There have been numerous bulk samples taken at NorthMet (Table 1-2). Samples have been representative by Unit and rock type. Agreement between calculated grades (based on core sampling) and analyzed grades of final sample has been excellent. Earlier bulk samples represented the first ten years of production, more recent samples used material from across the deposit. Each bulk sample has built upon the previous, and work has progressed to the point where PolyMet has confirmed the ability to make separate, saleable, copper and nickel concentrates. This will allow the company to develop cash flow from sales much earlier in production while completing construction of the hydrometallurgical facility. The planned hydrometallurgical process (PlatSol) was developed on NorthMet ores. The process uses pressure oxidation (225°C, over 30 atmospheres) in the presence of chloride to capture all base and precious metals in the concentrate. Hydromet process recoveries are all over 98%. Other geologic data collected includes: recovery and RQD measurements on all core, over 7,000 specific gravity measurements, over 900 whole rock analyses, over 300 Rare Earth Element packages and a large amount of microprobe data collected for waste characterization purposes. 21 �GEOLOGY OF THE NORTHMET DEPOSIT NorthMet consists of seven igneous units that dip southeast, with most economic sulfide mineralization in the top parts of the lowermost unit (Unit 1). The following is a summarized description of the geology of the deposit, based on observations from drill core and limited outcrop mapping. Quaternary Geology In general the Quaternary geology of the region is a thin (0-30 feet or 0-10 meters., but locally thicker) blanket of glacial deposits including till, lacustrine materials, and outwash. Low spots are usually peat bog or open wetland. Topography is subdued and drainage is poor. Site specific geologic studies of the drift have not been done, though a series of geophysical soundings were carried out in 2006 to better define drift thickness outside the area to be mined (Ikola, 2006). Lehr and Hobbs (1992) mapped the area as part of the Wampus Lake Moraine. Minnesota Geologic Survey map 164 (Jennings and Reynolds, 2005, includes GIS database) categorizes all drift materials as Rainy Lobe till and re-sedimented glacial deposits, overlain locally by post glacial peat. Test pits for preliminary PolyMet engineering studies and informal observations of sumps and other small excavations bear this out. Most areas consist of unsorted sand / silt / clay with cobbles and boulders. Boulders on surface can be greater than 10 feet in size and there may be a boulder lag horizon just below the ground surface in some areas. As measured from drill holes, thickness of the drift ranges from 0 to 50 feet (mostly less than 20 feet) and averages about 12 feet. The 2006 geophysical soundings measured thicknesses up to 60 feet past the western margins of the drilled area. Structural Geology The general structure of the NorthMet deposit, as defined by igneous contact dips, foliation in serpentinized zones, bedding trends in the Biwabik Iron Formation (BIF) and in the Virginia Formation, is dominated by an overall dip ranging from 15-25° to the southeast, striking about N56°E. Dips in the seven igneous units are grossly similar, but dips of the mineralized zone are up to 60° in the east pit area. Dips in both the Animikian and the Duluth Complex rocks can be attributed to crustal loading, associated with the input of large volumes of magma originating from the Mid-continent Rift System (Sims and Morey, 1972). Numerous faults have been proposed across the NorthMet Deposit, based largely on reconciling dips in the footwall rocks. Unfortunately, not enough evidence has been established through drilling to indicate with certainty the exact location of major offsets or faulting within the igneous rock units or the footwall rocks on a hole-to-hole basis. This definition difficulty is compounded by the fact that over time the fault representations have been extended vertically from ground surface to footwall, though many were originally thought to only show offset in the footwall, or were based solely on limited outcrop evidence. Clearly however, offset or faulting exists, at least within the footwall rocks, due to substantial offsets in the BIF (assuming an average 20° dip) as evidenced between drill holes portrayed in cross-sections. Many of these same offsets can be correlated in adjacent cross-sections. Fault zones are apparent in drill core and show up as brecciated intervals (up to several feet thick), including gouge mineralization (clay, calcite, quartz, etc.), slickensides on serpentinized fracture faces, and/or severely broken (rubble) core. However, the exact location of all faults/offsets at the NorthMet Deposit on a hole to hole basis has only been approximated, due to the sparse structural information as so far provided by drilling. Extensive angle drilling in 2005 and 2007 (142 of 170 holes) brought no great clarity to this issue (virtually all previous drilling was vertical). The current geological model and working cross-sections are therefore constructed with minimal faulting influence, especially within the igneous rock units of the Partridge River intrusion, until more evidence clarifies this issue. 22 �Logging and Mapping Units A summary of the general stratigraphy of the NorthMet Deposit is outlined below. Rock units and formations are listed in descending order, as would be observed from top to bottom in drill hole. NorthMet units are labeled as Units 1 through 7 (Units I through VII in Severson’s terminology), bottom to top. Unit 3 is probably the oldest, the intrusion sequence of the other units is not clear. The broad picture is of a regular stratigraphy of troctolitic to anorthositic rock units, dipping southeast at 20° to 25°, with basal ultramafic units defining the boundaries of some of these units. The basal ultramafic zones tend to have diffuse tops, sharp bases, and are commonly serpentinized and foliated. Geologists have generally picked the unit boundaries at the base of these ultramafics though there are local exceptions. Economic sulfide mineralization is ubiquitous in the basal igneous unit (Unit 1) and is locally present, but restricted, in the upper units. Rock Type and Unit Classification Igneous rock types in the Complex are classified at NorthMet by visually estimating the modal percentages of plagioclase, olivine, and pyroxene, using a rock classification scheme (Figure 1-9) modified from Phinney (1972). Due to subtle changes in the percentages of these minerals, a variation in the defined rock types within the rock units may be present from interval to interval or hole to hole. This is especially true for Unit 1. Figure 1-9. Modified Phinney (1972) diagram for rock type classification Unit definitions are based on: overall texture of a rocktype package; mineralogy; sulfide content; and context with respect to bounding surfaces (i.e., ultramafic horizons, oxide-rich horizons). Unit definitions are not always immediately clear in logging, but usually clarified when drill holes are plotted on crosssections. In other words, to correctly identify a particular stratigraphic unit, the context of the units directly above and below should also be considered. Based on drill hole logging, the generalized rock type distribution at NorthMet is about 83% troctolitic, 6% anorthositic, 4% ultramafic, 4% sedimentary inclusions, 2% noritic and gabbroic rocks, and minor pegmatite, breccia, basalt inclusions, and others. Unit Definitions and Descriptions Descriptions of the general igneous Stratigraphy for the NorthMet deposit is described below and presented in a stratigraphic column in Figure 1-9. 23 �Figure 1-9. Generalized stratigraphic column for NorthMet units (modified after Geerts, 1994) Unit 7 Unit 7 (Figures 1-9, 1-10, and 1-11) is the uppermost unit intersected in drill holes at the NorthMet Deposit. It consists predominantly of homogeneous, coarse-grained anorthositic troctolite and troctolitic anorthosite, characterized by a continuous basal ultramafic subunit that averages 20 ft. thick. The ultramafic consists of fine- to medium-grained melatroctolite to peridotite and minor dunite. The average thickness of Unit 7 is unknown due to erosion removing the upper parts. Unit 7 is generally not mineralized. Unit 6 Very similar to Unit 7, Unit 6 is composed of homogeneous, fine- to coarse-grained, troctolitic anorthosite to troctolite. It averages 400 ft. thick and has a continuous basal ultramafic subunit that averages 15 ft. thick. Overall, sulfide mineralization is minimal, although a number of drill holes in the southwestern portion of the NorthMet Deposit contain significant sulfides and associated elevated PGEs 24 �(Geerts 1991, 1994). Sulfides within Unit 6 generally occur as disseminated chalcopyrite/cubanite with minimal pyrrhotite. This mineralized occurrence, the “Magenta Zone”, transitions into Units 3, 4, and 5, and is discussed in greater detail below. Unit 5 Unit 5 exhibits an average thickness of 250 ft. and is composed primarily of homogeneous, equigranulartextured, coarse-grained anorthositic troctolite. Anorthositic troctolite is the predominant rock type, but can locally grade into troctolite and augite troctolite towards the base of the unit. The lower contact of Unit 5 is gradational and lacks any ultramafic subunit, therefore the transition into Unit 4 is a somewhat arbitrary pick. Due to the ambiguity of this contact, thicknesses of both units vary dramatically. However, when Units 5 and 4 are combined, the thickness is fairly consistent deposit-wide. Aside from Magenta Zone mineralization in the west, Unit 5 is not mineralized. Unit 4 Being somewhat more mafic than Unit 5, Unit 4 is characterized by homogeneous, coarse-grained, ophitic augite troctolite with some anorthosite troctolitic. Unit 4 averages about 250 ft. thick. At its base, Unit 4 may contain a local thin (usually no more than 6 inch) ultramafic layer or oxide-rich zone. The lower contact with Unit 3 is generally sharp. Unit 4 is rarely mineralized outside the Magenta Zone. Unit 3 Unit 3 is used as the major “marker bed” in determining stratigraphic position in the PRI. It is composed of fine- to medium-grained, poikilitic and/or ophitic, troctolitic anorthosite to anorthositic troctolite. Characteristic poikilitic olivine gives the rock an overall mottled appearance. On average Unit 3 is 300 ft. thick. As with Units 4 and 5, the thickness of Units 2 and 3 tend to be highly variable, whereas if combined into one unit, it is more consistent deposit-wide (though not as consistent as Units 4 & 5). Unit 2 Unit 2 is characterized by homogeneous, medium- to coarse-grained troctolite and augite troctolite with a consistent basal ultramafic subunit. The continuity of the basal ultramafic subunit, in addition to the relatively uniform grain size and homogeneity of the troctolite, makes this unit distinguishable from Units 1 and 3. Unit 2 has an average thickness of 100 ft. The ultramafic subunit at the base of Unit 2 is the lowermost continuous basal ultramafic horizon at the NorthMet Deposit, averages 25 ft. thick, and is composed of melatroctolite to peridotite and minor dunite. In some ways the characteristics of Unit 2 and how it fits into the stratigraphy are ambiguous. It can be interpreted as the lower part of Unit 3, the upper part of Unit 1, or a separate unit. Based on continuity of the ultramafic boundary it seems to be a lower, more mafic, counterpart to Unit 3 or a separate unit. However, even though Unit 2 has been historically described as barren, in the western part of the deposit it appears to have mineralization grossly continuous with that at the top of Unit 1. The general lack of footwall inclusions would argue against Unit 2 being older than Unit 1. Unit 1 Of the seven igneous rock units represented within the NorthMet Deposit, Unit 1 is the only unit that contains significant deposit-wide sulfide mineralization. Sulfides occur primarily as disseminated interstitial grains between a dominant silicate framework and are chalcopyrite > pyrrhotite > cubanite >pentlandite. Unit 1 is also the most complex unit, with internal ultramafic subunits, increasing and decreasing quantities of mineralization, complex textural relations and varying grain sizes, and abundant sedimentary inclusions. It averages 450 ft. thick, but is locally 1,000 feet thick and is characterized lithologically by fine- to coarse-grained heterogeneous rock ranging from anorthositic troctolite (more abundant in the upper half of Unit 1) to augite troctolite with lesser amounts of gabbro-norite and norite (becoming increasingly more abundant towards the basal contact) and numerous sedimentary inclusions. 25 �By far the dominant rock type in Unit 1 is medium-grained ophitic augite troctolite, but the textures can vary wildly. Two internal ultramafic subunits occur in drill holes in the southwest, and have an average thickness of 10 ft. Footwall rocks are covered in the Partridge River intrusion description. Inclusions Two broad populations of inclusions occur at NorthMet: hanging wall metabasalts (Keweenawan) and footwall metasedimentary rocks. Basalts are fine-grained, generally gabbroic, with no apparent relation to any mineralization. Footwall inclusions may carry substantial sulfide (pyrrhotite) and often appear to contribute to the local sulfur content. Footwall inclusions are all Virginia Formation, no iron-formation, Pokegama Quartzite, or older granitic rock has been recognized as an inclusion at NorthMet. Sedimentary inclusions make up about 4% of the logged rocktypes, and basalt inclusions sum to less than 1% of the drilling footage. Inclusions and Timing Generally, hanging wall inclusions are restricted to Unit 3 and the units above, while footwall inclusions are most abundant in Unit 1. This zoned distribution of inclusions indicates that one possible scenario for order of intrusion is that Unit 3 intruded first, created space between the basalt and the Virginia Formation, then portions of the hanging wall basalts collapsed into the Unit 3, but for some reason Unit 3 was not able to dissagregate or assimilate much of the footwall rock (due to temperature, viscosity of magma or ductility of the footwall). Unit 1 however, intruded between Unit 3 and the footwall and was able to assimilate large portions of the footwall and thus contaminate itself with both sulfur and silica. In this scenario Unit 2 is intruded after Unit 1, between Units 1 and 3, as Unit 2 has limited footwall inclusions. Unit 3's intrusion would have separated the footwall and Unit 1 from later Units 4 through 7, which never reacted with the footwall at the NorthMet site. Therefore, any footwall inclusions seen in Units 4 through 7 (and probably those seen in Unit 2) can be interpreted as being carried in from some other part of the magmatic system. Note that basalt overlies and is in direct contact with the Virginia Formation at the Wetlegs deposit to the west of NorthMet, implying that the starting conditions for this chain of events are plausible. Other Igneous Units Quadrangle scale outcrop mapping indicates that other igneous stratigraphic units are present above Unit 7. These units are similar to Units 6 and 7 in that they consist of homogeneous-textured troctolitic rocks with basal ultramafic members. There are minor, unmineralized, pre-Complex sills in both the Virginia Formation and Biwabik Iron Formation at NorthMet (VirgSill and BIF Sill in footwall descriptions above). In neither case is there any apparent relation to Duluth Complex mineralization. Early sills in the Virginia probably metamorphosed the Virginia, forming a zone that resisted assimilation during later intrusion of the Complex–hence leading to the thin “rind” of metamorphosed Virginia on top of the BIF seen in the deeper downdip drill holes at NorthMet. Alteration The vast majority of rock within the NorthMet Deposit would be considered fresh and is unaltered or only weakly altered. Types of alteration most commonly observed in NorthMet rocks are serpentinization / chloritization of olivine, sericitization and saussuritization of plagioclase, and uralitization of pyroxenes. Most alteration is related to close proximity of fractures and/or joints that cross-cut the troctolitic rocks. Likewise, on a microscopic level the center of alteration is focused around microfractures. This pattern suggests that both fracturing and accompanying alteration of the rock occur as a result of the migration of late-stage deuteric fluids during the cooling phase. The vast majority of sulfide mineralization is 26 �independent of alteration. Nickel in Silicates (Lab Assay Nickel vs. Recoverable Nickel) It has been characteristic of NorthMet and other Duluth Complex deposits to show lower nickel recoveries in process test work than would be expected from laboratory assays on drill core. Generally there is a loss of about 25-35% of the nickel compared to drill core assays when concentrating sulfides. From previous work, it is known that small amounts of unrecoverable nickel occur as a magnesium-ironnickel silicate [(Mg,Fe,Ni)2 SiO4] that is tied up in the mineral olivine, which is one of three significant gangue minerals that occur across the NorthMet deposit. Testwork has shown that most of the very small amount of nickel contained in silicates would not be recovered during the autoclaving process proposed. For example, mineralogical studies show that approximately 25% to 35% of the rock in NorthMet is composed of olivine. Previous microprobe study, plus work by PolyMet in 2006, has shown an average of about 0.10% nickel in olivine. The approximate nickel grade of the PolyMet metallurgical bulk samples is 0.10%. Because the average nickel in the olivine is the same as the average nickel in the bulk samples, the unrecoverable nickel in the olivine would be expected to reduce nickel recovery by the amount of olivine in the bulk sample - 25% to 35%. Nickel recoveries on the six PolyMet metallurgical bulk samples have ranged from 69% to 77%. This is in line with an approximate 25% to 35% loss of nickel to silicate. 27 �Figure 1-10. Geologic map of NorthMet Deposit, all units dip southeast, Magenta Zone is projected upward, does not actually subcrop 28 �Figure 1-11. Cross section 35700 at west end of property and 45600 at east end. Purple shading indicated ore zones, bar graphs along holes indicate grades expressed as dollar values, where red = $7.42 cut-off to average grade (~$14.39), and purple shows above average grade, blue are zones of potential lean ore should metals prices rise. 29 �ECONOMIC MINERALIZATION The majority of economic mineralization (copper, nickel, cobalt, platinum, palladium, and gold) at NorthMet occurs in the upper parts of basal Unit 1, with copper and nickel in chalcopyrite, cubanite, and pentlandite, all in the presence of pyrrhotite. Cobalt is contained in sulfides. Platinum, palladium, and gold, while showing good correlation with sulfur and the other metals, are also in a variety of tellurides, bismuthides, and alloys, as well as associated with the major and minor sulfides. Table 1-3 shows correlation of metals values in drill core data. Table 1-3. Simple correlation ® table for economic metals and sulfur Cu % Cu % Ni % S% Pt ppb Pd ppb Au ppb Pt+Pd+Au Co ppm Zn ppm Ni % S% Pt ppb Pd ppb Au ppb Pt+Pd+Au Co ppm Zn ppm 1.000 0.860 1.000 0.541 0.572 1.000 0.568 0.508 0.195 1.000 0.750 0.635 0.292 0.673 1.000 0.591 0.472 0.250 0.482 0.699 1.000 0.760 0.645 0.292 0.778 0.983 0.755 1.000 0.544 0.704 0.621 0.217 0.281 0.241 0.288 1.000 -0.021 -0.004 0.286 -0.041 -0.037 -0.017 -0.039 0.093 1 The simple correlation table above (number of samples=19,516) shows the strong relation of copper, nickel, and palladium, and a somewhat surprising relation of cobalt to sulfur. Zinc’s low factor is probably related to its multiple origins as either magmatic or derived from assimilation of footwall rock, hence representing two populations of data. The sulfur vs. metal correlation is probably greatly affected by iron, the presence of which is not shown here, but is in excess in all rocks. Grades are highest at the top of Unit 1 and fade going down hole. Grades appear to be higher down-dip though this may be an artifact of less dense sampling. There is a smaller zone of economic mineralization (about 50 million tons) at the western end of the property in the upper units, known as the “Magenta Zone.” This zone is generally copper and PGE-rich (sulfur-poor relative to metals) and of “average” reserve grade. The minerals of interest from a waste characterization perspective are the same as above, but pyrrhotite is expected to be the main mineral affecting water quality in regards to waste rock, though the traces of chalcopyrite, cubanite and pentlandite will require study for waste rock storage. Trace pyrite and pyrrhotite are the main sulfide minerals found in the tailings. Pyrite is largely from joint faces and other secondary sources-it is rarely seen in polished section or core. Most sulfide mineralization at NorthMet is of a distant source (but sedimentary?), some is locally modified by sulfur derived from footwall metasedimentary rocks (Virginia Formation). Minor veins and other crosscutting relations indicate some movement of sulfides within the deposit, but there is no evidence recognized for large scale relocation of sulfides, nor any macroscopic evidence for any hydrothermal event that may have remobilized PGE’s or sulfides. Virtually all sulfide mineralization at NorthMet moved in with magmatic pulses, and metal enrichment of the magma happened in a deeper chamber. Therefore, the main controls on the location of mineralization within the deposit may be the specific magmatic pulse or pulses making up the individual units. While textures in Unit 1 are described as heterogeneous, there is also a broad homogeneity in regards to mineral 30 �occurrence, mineral chemistry, whole rock and REE chemistry, and gross rock type that all reinforce the view of a large system of magma pulses replenishing the resident magma at the NorthMet site. The exception to this is that some sulfur, particularly in Unit 1, was derived locally from assimilation of footwall rocks (evidenced by high pyrrhotite content nearer footwall inclusions). The main effect of this assimilation has been to dilute the sulfide grade with additional pyrrhotite in Unit 1, rather than this sulfur scavenging more base metals from the magma. RESOURCE The PolyMet resource and reserve (Table 1.4) models have been done in cooperation with several consultants, most recently PEG Mining of Toronto. PolyMet supplies the geologic solids model, database, and block model geometry. Geostatistics and population of the block model, and hence the resource estimate, are done in consultation, with finalized resource block models then sent forward to engineers for reserve calculation and mine planning. Resource geologic modeling treats the NorthMet deposit as five separate domains: 1. Virginia Formation footwall rocks; 2. a domain including the upper, higher grade parts of Unit 1, locally merged with the higher grade zones at the base of Unit 2; 3. the remainder (lower part) of Unit 1; 4. the Magenta zone in Units 3, 4, 5, & 6 in the western part of the deposit; 5. and the remaining, less mineralized, parts of Units 2 through 7. Unit 1 is mineralized throughout the deposit area, with other units (2 through 6) showing some economic mineralization in the western and central parts of the deposit, but essentially no continuous zones in the east. There is no known economic mineralization in the footwall rocks. Deposit wide, Unit 1 has the highest grades near its top. Though grades vary, Unit 1 is also mineralized to the east of the deposit, down-dip (south) to depths of at least 2,500 feet, and past the limits of expected pit development in the west. The development of waste rock stockpiles over these areas in the east and south is not expected to encumber any material that could reasonably be classed as ore because the upper units are barren and the Unit 1 mineralization is from 1,700 to over 2,500 feet below ground surface. For modeling purposes, Unit 1 is bounded by both “hard” and “soft” geologic surfaces. A “hard” boundary is one where the interpolation of drill hole data into the block model does not cross geological surfaces, a soft boundary is one where interpolation crosses geological boundaries. The top of Unit 1 (i.e., the ultramafic at the base of Unit 2) is a soft boundary for mineralization estimation as the mineralized domain model crosses from Unit 1 into Unit 2. The base of Unit 1, where it contacts the Virginia Formation, is a hard boundary for estimation and metals values, with virtually all sulfide in the Virginia Formation below as pyrrhotite. No data from Unit 1 is used in estimating grades in the Virginia Formation, or vice versa. In the up-dip, west half of the deposit there is an arbitrary and diffuse geologic boundary within Unit 1 that vanishes to the east. This is roughly equal to the top of a petrological contamination zone where large quantities of the footwall metasedimentary rocks have been assimilated. This zone is informally called the “front” or “norite zone” by PolyMet geologists. Precious metals values drop off in this zone and pyrrhotite becomes the dominant sulfide. Moderate copper values may persist below this line, but this is essentially a lower physical limit to combined polymetallic grades above the likely project cut-offs. 31 �Table 1-4. NorthMet resource and reserve values. Work done by Wardrop Engineering 2007. Cut-off based on “Net Metals Value” per ton, accounting for grade, average flotation and hydromet recovery, realization costs, metal prices, and other factors. See Desaultels and Patelke, 2008 for resource calculation details. Enough reserve has been shown for 24 years of production. RESERVES-2007 Cut-off Million Cu Ni Co Pt Pd Au value Tons % % ppm ppb ppb ppb Proven Probable Proven and Probable $7.42 $7.42 $7.42 118.1 156.5 274.6 0.30 0.27 0.28 0.09 0.08 0.08 75 72 73 75 75 75 275 248 260 38 37 37 Cut-off value Million Tons Cu % Ni % Co ppm Pt ppb Pd ppb Au ppb $7.42 $7.42 $7.42 $7.42 202.5 491.7 694.2 229.7 0.285 0.256 0.265 0.273 0.083 0.075 0.077 0.079 74 70 71 56 71 66 68 73 258 231 239 263 36 34 35 37 RESOURCES-2007 Measured Indicated Measured & Indicated Inferred ASSUMPTIONS Metal and Units Assumed Metal Price Average % recovery, as used in DFS Cu % Ni % Co ppm Pt ppb Pd ppb Au ppb $1.25 lb 92.33 $5.60 lb 70.34 $15.25 lb 40.75 $800 oz 75.74 $210 oz 72.69 $400 oz 67.04 In the center of the deposit the highest, near surface, Unit 1 grades transition into the middle of the unit, while in the east, mineralization is strong and vertically persistent throughout the unit. The top of the merged Unit 1 and Unit 2 mineralized domain (domain 1) forms a hard boundary that, combined with the bedrock ledge (depth to bedrock) surface, forms the bottom and top estimation boundaries for the upper units (exclusive of the “Magenta Zone”, which is internal to this domain). There is no conclusive relation between specific Unit 1 specific rock type and presence or grade of mineralization except that noritic rocks are generally of lower grade. Units 2 and 3: These units are treated as one unit in the geologic model, with PolyMet geologists considering them as a single package grading from an ultramafic base to an anorthositic top for modelling purposes. The thickness of the package stays relatively constant, though the thickness of the two individual units varies, primarily due to Unit 2 locally thinning. While generally barren, Unit 2 has mineralization at its base in the western half of the deposit. These zones may not be strictly equivalent to Unit 1 type mineralization. Copper and nickel values are lower, as is pyrrhotite, but behavior of other metals is inconsistent, with PGE (Pt + Pd +Au) content varying locally relative to nearby grades at the top of Unit 1. Above the basal zone of Unit 2 it is usually barren, mediumgrained, and homogenous in texture. Average PGE in Unit 2 is slightly above that of Unit 1. Unit 3 shows mineralization in the west, in the middle of the unit and near the top. This occurrence is merged into the Magenta Zone. Units 4 and 5 are also modeled as a geologic package. There is no compelling geologic reason to fully separate these units, the boundary between them being an arbitrary pick based on overall changes in texture from homogenous to heterogeneous, grain size, and plagioclase content, but without a well defined 32 �bounding horizon. The top boundary of Unit 5 is the basal ultramafic of Unit 6, which is an unused hard boundary in grade modelling. The bottom boundary of Unit 4 is a discontinuous ultramafic horizon. There are also discontinuous oxide-rich zones along the contact between Units 3 and 4. Metals and sulfur grades in Unit 4 are proportional to Unit 1, but consistently lower. Unit 4 has few high copper or sulfur assay intervals. There is some near surface mineralization, modelled as a part of the Magenta Zone, described below. Otherwise there is only low grade, discontinuous material at the base. Unit 6 and Unit 7: These units are very similar in nature. Both are homogenous anorthositic troctolites with well defined ultramafic bases. No top for Unit 7 has been seen in drill hole. Units 3, 4, 5 and 6 host a zone of mineralization, modeled as the Magenta Zone. Unit 6 material was described by Geerts (1994) as the “Magenta Horizon” when originally found in six drill holes. Further drilling has extended these copper rich, sulfur poor zones (of moderate overall grade) into more than fifty drill holes in Units 3, 4, 5, and 6. The zone transitions across the ultramafic base of Unit 6 and into Units 3, 4 and 5, (i.e., does cross the igneous stratigraphy) which is problematic if the emplacement model of these units representing individual pulses of magma is correct. There is no gross evidence for this mineralization being hydrothermal, which could cross boundaries, but would presumably alter large masses of rock. Unit 7 has a few good assay intercepts, but no apparent continuity for sulfides. Table 1-5. Average values for assays by unit after removal of the less than 0.05% copper intervals (drill core samples). Unsampled zones not accounted for here. Data complete through 2006. Cu % Ni % S % Pt+Pd+Au Co Cu+Ni Cu/Ni Cu/S ppb ppm % Total % of unit sampled Average sample length-feet Unit 1 0.3 0.09 0.83 349 76 0.39 3.35 Unit 2 0.2 0.07 0.39 365 73 0.27 2.74 0.43 90 5.3 0.61 80 5.6 Unit 3 0.19 0.05 0.5 286 62 0.25 3.19 0.53 71 7.2 Unit 4 0.21 0.06 0.58 269 66 0.28 3.40 0.44 51 7.6 Unit 5 0.27 0.07 0.54 398 65 0.35 3.64 0.54 41 7.8 Unit 6 0.33 0.08 0.48 532 69 0.41 3.74 0.69 27 7.2 Unit 7 0.2 0.06 0.32 330 83 0.26 3.60 0.72 11 8.4 Copper, nickel, and sulfur values in Table 1-5 are calculated after removing samples with less than 0.05% copper. Samples removed are generally those collected for waste characterization purposes, many well outside the expected mining area, and these low values can somewhat obscure the ore chemistry / mineralogy relations in the “ore.” Ratios are calculated on all raw data, not on the copper-nickel-sulfur values shown here. • No lateral or vertical zonation has been recognized in sulfide or silicate mineral Chemistry; • Gatehouse (North Mining) did report some geochemical cyclicity in unit 1, but this has not been revisited with the larger data set; • Poor assay grades in the noritic rocks are related to footwall assimilation and contamination, otherwise there is little connection between grades and specific rock type. About 83% of the igneous rocks at NorthMet are troctolites, 6% anorthositic rocks, 4% ultramafic rocks, and 4% footwall inclusions. The remainder are norites, gabbros, and other; • Within Unit 1 copper:sulfur ratio tends to be highest at top, then diminishes with depth, following the pattern of PGE’s; 33 �• • • The upper units have higher copper:sulfur ratios than Unit 1 (i.e., more chalcopyrite rich), but lower overall copper values; Ratio of PGE to copper is lowest in Unit 1, but Unit 1 has greatest quantities of both; Chalcopyrite is the dominant sulfide in the upper units regardless of total sulfur content; Sulfide (Ore) Mineral Proportions Various metallurgical test programs have been conducted on NorthMet ores since the 1970's. Reported sulfide mineral proportions have not been entirely consistent between these tests. Table 1-6 shows well characterized sulfide mineral proportions for waste rock from studies done by PolyMet in 2006 and results from various previous studies. Sulfide mineralogy within the NorthMet Deposit has been described in detail through petrographic observations and microprobe analysis. Approximately 95-98% of all sulfide mineralization consists of 4 predominant species, in decreasing order of abundance: chalcopyrite (cp) > pyrrhotite (po) > cubanite (cb) > pentlandite (pn). In general, Po:Cp+Cb ratios increase towards the basal contact or in proximity to sedimentary inclusions. Likewise, Cp:Cb ratios increase with increased distance away from the footwall rocks. In core logging and other work, chalcopyrite is often not distinguished from cubanite. The results show a fairly wide range of values, which may not be totally representative of the deposit as a whole. It is important to note that these discrepancies may be the results of differences in composites, mixed intervals from multiple units including both waste rock and ore, or variations in petrography or laboratory procedures. Some of the composite samples submitted for metallurgical studies were prepared from relatively limited representative core (NERCO samples from 2 drill hole locations), while others were prepared from multiple locations evenly distributed across the deposit. Table 1-6. Sulfide Average Percentage (recalculated to 100 % Sulfide) SGS Lakefield 1991 (NERCO L1) Chalcopyrite % 44 Cubanite % 12 Cp:Cb ratio 4:1 Pyrrhotite % 2 Pentlandite % 9 SGS Lakefield 1991 (NERCO H1) 36 18 2:1 3 8 SGS Lakefield 2000 (PolyMet Conc.) 32 14 2:1 27 7 SGS Lakefield 2005 (Comp. 1) 37 9 4:1 38 16 SGS Lakefield 2005 (Comp. 2) 42 7 6:1 36 15 SGS Lakefield 2005 (Comp. 3) 36 7 5:1 41 16 - - - - - 54 15 4:1 21 3 METALLURGICAL STUDIES: INDEPENDENT STUDIES: Geerts 1994 (Unit 1 – Ore) - - - - - Unit 6 76 6 12:1 5 4 Unit 5 55 3 17:1 17 5 Unit 4 41 5 8:1 32 16 Unit 3 48 6 9:1 35 5 Unit 2 52 13 4:1 24 9 Unit 1 39 7 6:1 44 9 POLYMET WASTE ROCK STUDY 2006: Mining Mining (Figure 1-12) at NorthMet will begin with contractor clearing and overburden stripping of the pit and stockpile areas. Engineered stockpile bases and liner systems must be in place before mining begins, 34 �as does the overall water collection system for treatment and pumping to the tailings basin. Ore and waste production will start in the east pit, with production from the west pit ramping up soon afterwards. Up through about year 11 or 12 production from both pits will be equal until the east pit is mined out. At that point, backfilling of the east pit will begin, with the ultimate goal of constructing a wetland in that pit. The central pit area will be mined last. Ore will be moved at a rate of 32,000 tons per day. Waste to ore strip ratio will be about 1.46:1. Ore will be moved by truck to the “superpocket” and loaded to 100 ton capacity side dump rail cars by pan feeder. There will be twenty trains per day of 16 cars each. Trains will go to the crushers via the existing rail line parallel to the Dunka Road and a short run of new track through the former LTV pits. The equipment fleet is expected to be eight 240 ton haul trucks, two electric hydraulic shovels, one large wheel loader, two rotary blasthole drills, plus bulldozers, rubber tired dozers, road graders, water trucks, and smaller fleet vehicles. GPS control for dispatch is planned for all equipment, with high precision GPS planned for the shovels and drills. High precision GPS may also be installed on the haul trucks. Bench heights will be forty feet. Ore blast patterns will have blasthole burden and spacing of 25' by 28', and the holes will have 5 ' of subdrilling and 17' of stemming. Waste rock blast patterns will have blasthole burden and spacing of 29' by 34', 6' of subdrilling and 20' of stemming. ANFO and emulsion blends will be the explosives used. Ore and waste categorization (“ore control”) will be by assay of core and / or blast holes and careful pit mapping. Waste material will be sorted to stockpiles, and stockpile liners will be built, according to the sulfur and metals content of the waste rock. Over 80% of the waste rock will never produce acid drainage, and about 4% will produce acid drainage within one year. Safe final disposition of this rock will be a permit condition, but as yet the state has not concluded what that condition will be. PolyMet has proposed to place some under capping systems, and to place some underwater. Figure 1-12. Layout of mine site showing 20 year pits, stockpiles for various waste categories, haul roads, railroad, pipelines, lease boundary, and loadout. 35 �PolyMet Process Plant The Plant Site consists of three areas: ● Process Plant - discussed in this section – beneficiation and hydrometallurgical processing facility including Tailings Basin and Hydrometallurgical Residue Facility ● Area 1 Shop – mine mobile equipment major repair facility ● Area 2 Shop – base of mine and railroad operations, locomotive service facility, rail car maintenance facility The Process Plant design is based on key parameters determined by the characteristics of the deposit to be mined and the beneficiation and hydrometallurgical processes that will extract the metals from the ore. On average, 32,000 short tons of ore will be processed each day. This results in annual metal production of 38,821 short tons of copper, 9,037 short tons of nickel, 400 short tons of cobalt, 22,184 ounces of platinum, 87,129 ounces of palladium and 13,824 ounces of gold. The process plant is located at the LTVSMC plant site. The entire Process Plant is in an area that was previously disturbed by mining operations. The Beneficiation Plant will use the Coarse Crusher Building, Fine Crusher Building and Concentrator Building that were part of the LTVSMC taconite plant. The Hydrometallurgical Plant will be located in the three new buildings constructed at the LTVSMC site - the Hydrometallurgical Facility, the Cu Solvent Extraction Building and the Cu Electrowinning Tank House. The Beneficiation Plant consists of the following processing steps: ● Ore Crushing – reducing 48” ore from the mine to 2.5” in the Coarse Crusher Building and from 2.5” to 0.315” in the Fine Crusher Building ● Ore Grinding – reducing the 0.315” ore from the Fine Crusher Building to 120 microns in the Concentrator Building – Flotation – separating the ore into ƒ Concentrate containing the minerals with the metals to be extracted in the Hydrometallurgical Plant ƒ Tailings containing the rock in the ore that is not valuable to be pumped to the Tailings Basin ● Concentrate Regrinding – reducing the size of the concentrate particles from 120 micron to 15 micron The flotation process has been designed to recover virtually all of the sulfide minerals to the concentrate and minimize the amount of sulfide minerals remaining with the tailings. This process has been tested with ore samples representing the NorthMet ore in a pilot plant set up to represent the proposed grinding and flotation process. The flotation tailings generated by this work has been subjected to rigorous waste characterization. Collectively this has demonstrated that the flotation tailings can be placed in the LTVSMC Tailings Basin. The Hydrometallurgical Plant consists of the following processing steps: ● High Pressure Oxidation Autoclave. – Oxidize the copper, nickel, cobalt, iron sulfides and the gold and platinum group minerals (AuPGMs) contained in the concentrate to liberate the contained metal and drive those metals into a leach solution. – Generate a leach residue containing the portions of the concentrate that did not dissolve. ● AuPGM Precipitation – precipitate the AuPGMs from the leach solution and filter the precipitated solids to be sold as a product for further refining. 36 �● ● ● ● ● ● ● ● Solution Neutralization – neutralize the leach solution (minus AuPGMs) by adding limestone and generating a solution neutralization residue. Copper Solvent Extraction – separate the leach solution (minus AuPGMs) into two components. – A strip solution containing the copper. – A raffinate solution containing the non copper portions of the leach solution plus residual copper. Copper Electrowinning – transfer the copper from the strip solution onto plates as a very pure final product. Raffinate Neutralization with Iron and Aluminum Removal. – Neutralize the raffinate solution (minus AuPGMs and copper) by adding limestone and generating a raffinate neutralization residue. – Precipitate iron and aluminum (which behave as impurities in downstream processing) with the raffinate neutralization residue. Residual Copper Removal – precipitate residual copper from raffinate solution and recycle. Mixed Hydroxide Precipitation – precipitate the nickel, cobalt and zinc from the raffinate solution and filter the precipitated solids to be sold as a product for further refining. Magnesium Removal – precipitate magnesium from the raffinate solution and pump the precipitated solids to the Hydrometallurgical Residue Facility along with the leach residue, solution neutralization residue, and raffinate neutralization residue. These residues are not hazardous waste individually or combined. The total annual residue generation is 794,000 short tons. The Hydrometallurgical Residue Facility will be located within the LTVSMC Tailings Basin. The facility will have engineered lined containment areas designed for the residues to be disposed. Water management at the Plant Site will: ● Segregate water that has contacted hydrometallurgical residues in the Hydrometallurgical Plant and the lined Hydrometallurgical Residue Facility and recycle it to the Hydrometallurgical Plant. ● Collect water that has seeped from the Tailings Basin and return it to the Tailings Basin for reuse. ● Use treated water from the Mine Site (via the Tailings Basin) as make-up water. ● Supplement the Mine Site make-up water with new water from Colby Lake. The result is that there is no surface discharge of process water at the Plant Site and that the requirement for make-up water via water appropriation from Colby Lake is minimized. The Plant Site already has required service infrastructure available: ● County Road 666 ends at the Main Gate for the industrial area that included the NorthMet Process Plant, Area 1 Shop and Area 2 Shop. ● The Canadian National Railroad serves the industrial area that included the NorthMet Process Plant, and existing PolyMet track connects to the Area 1 Shop and the Area 2 Shop. ● Three Minnesota Power 138Kv transmission lines serve the NorthMet substation. ● Access to railroad line from mine to plant site The closure plan for the Plant Site consists of standard mineland reclamation practices and landfill closure practices (for the Hydrometallurgical Residue Facility). Original watersheds will be restored to the extent practicable. Structures will be removed and the structure areas covered with soil and revegetated. 37 �PART 3: DUNKA ROAD FIELD TRIP STOPS By: Mark Severson INTRODUCTION In transiting from the PolyMet plant site to Babbitt, we will follow the Dunka Road, a private mine road originally used as a service road for the LTV railroad and to access the Dunka Pit iron mine east of the Northshore mine. The road is now controlled by PolyMet, Cliffs Erie, and Minnesota Power. Access is restricted. Note that UTM locations given for the field trip stops (Fig. 1-13) are estimates. NORTHMET AND MESABA DEPOSITS As the bus proceeds to Babbitt, we will make a few stops, time and weather permitting, and traverse across the southernmost limits of the NorthMet deposit and the Mesaba deposit. There is very little to see in the way of mineralized exposures for either of these deposits. However, one point to keep in mind is the immense size of these low-grade deposits. The NorthMet deposit extends for about 3 miles (4.8 km) along the road, and the Mesaba deposit extends for another 2.5 miles (4 km) along the road. The deposits are separated by undrilled and untested area along the Partridge River that is about 1 mile 1.6 km) wide. Most of the undrilled area is within the PolyMet lease area. Figure 1-13. Locations of field trip stops along Dunka Road. LTV Mine Just outside the town of Hoyt Lakes, the bus will enter and proceed east through the now inactive LTV taconite mine. Founded in 1957 by the Erie Mining Company, this mine was the second taconite production facility on the Mesabi Iron Range. Ore was mined from a series of pits in the Paleoproterozoic Biwabik Iron Formation. At least four horizons were mined: one from the Upper Slaty Member (submembers D and E); two from the Upper Cherty Member (submembers G-H-I-J, and submembers LM); and one from the Lower Cherty Member (submembers S-T-U). Crude ore, which contains about 30% iron, was typically crushed and ground to a powder-like consistency and passed through several stages of magnetic separation and silica flotation to form a magnetite concentrate with 67% iron content. It was then mixed with a bentonite binder and rolled in balling drums to produce “green pellets” (or balls about the size of a marble). The pellets were then conveyed to a furnace where they were heat-hardened at about 2,400° Fahrenheit (1,300° C). After the pellets were 38 �hardened they were able to be easily handled and transported by rail to the Taconite Harbor Pier, and shipped via the Great Lakes to iron mills. In 1973, a record high of 13,104,000 tons of pellets was shipped from Taconite Harbor. In its last years of full production, the LTV Mining Company produced about 8 million tons of taconite pellets annually. In May of 2000, the LTV Steel Corporation announced its intent to close the mine and facilities, including a power plant on Lake Superior. Over 1,400 people were laid off in January 2001. The mine, facilities, and tailings basin were acquired by Cliffs Management Services in late 2001. PolyMet bought the concentrator and tailings basin from Cliffs for use in processing Cu-Ni material from the NorthMet Deposit. As the bus progresses down the Dunka Road (a private mine-related road) through the mine area note the undulating nature of bedding planes in the iron-formation (LTV Area 3 where iron ore was mined from the Lower Cherty member). This area is located near the base of the Biwabik Iron Formation and it does not take much imagination to envision deposition of the gently undulating beds in shallow waters along the shores of the Animikie Basin. To the north (left side of bus), the iron-formation conformably overlies beach sands of the Pokegama Quartzite (bottom of the Animikie sediment package). To the south, the iron-formation is conformably overlain by deep water sediments and turbidites of the Virginia Formation (top of the Animikie sediments). After going through Area 3 the road takes a sharp jog to the south (right). At this locale, the surface trace Siphon Fault approximates the road and iron-formation is present to the west of the road and the Virginia Formation is present across the swamp on the east side of the road. Just before the road curves back to the east to climb a low hill, take the immediate left and go down a secondary mining road for about 500 feet (150 meters). Proceed to the north on foot along a flagged trail to the next stop. STOP 1-1: Virginia Formation near Siphon fault No hammering please! Location: Cliffs-Erie site, T. 59 N., R. 14 W., sec. 26, SE of SE of NE Allen quadrangle; UTM: 569,505E/5,271,610N (NAD 83) Description: This is the only natural exposure of the Virginia Formation on the Mesabi Iron Range. Unfortunately, it is only a few feet thick. A total of 1,443 feet (440 meters) of the formation is present in drill cores from holes drilled south of the range. Note the graded beds, mud chips, concretions, and loading at the bases of these beds. The bedding is near vertical in this location due to proximity to the north-trending Siphon fault—an inferred growth fault (Graber, 1993) wherein the iron-formation decreases in thickness to the east (across the fault) by about 100 feet (30 meters). STOP 1-2: Wetlegs Cu-Ni Prospect / Partridge River Intrusion Location: Rail tracks just south of Dunka Road T. 59 N., R. 13 W., sec. 18, NE of SE of SE Allen quadrangle; UTM: 572,72E/5,271,178N (NAD 83) A west to east traverse along the LTV railroad tracks will be conducted at this field stop. The traverse, approximately 0.5 miles (0.8 km) long, will start at the basal contact and progress upwards through the igneous stratigraphy of the Partridge River intrusion. Outcrops of Units I and III will be viewed. A generalized geologic map of the Wetlegs prospect is shown in Figure 1.13. Detailed descriptions of the railroad cuts are listed in Table 1-7 below. 39 �Figure 1-13. Geology of the Wetlegs Cu-Ni deposit area and the location of Stop 1-2 (Severson and Miller, 1999). 40 �Table 1.7. Wetlegs Prospect outcrop descriptions of Stop 1-2. Feet (W to E) Description of Outcrops 0 ft. Railroad culvert – Longnose Creek 100-110 ft. (left side of tracks) Sulfide/gossan-cemented till consisting mostly of large boulders of a pyrrhotite- and graphite-rich member of the Virginia Formation (the BDD PO unit). Note the rounded granite cobbles beneath the BDD PO boulders. Drilling indicates that the BDD PO subcrops very close to this location. 337-355 ft. (left side of tracks) Very fine-grained (chilled) gabbronorite at basal contact with 60% plagioclase, 30% orthopyroxene, 5 % clinopyroxene, trace to 2 % biotite, and 3 % oxides. The outcrop contains well assimilated “streaks” of Virginia Formation inclusions (very hard to see recently). 400-1025 ft. (both sides of tracks) Unit I – taxitic medium- to coarse-grained ophitic augite troctolite (POcf) to olivine gabbro (PcOf) with patches and lenses of augite-rich pegmatite. The outcrops are sulfide-bearing with the sulfides unevenly distributed along patches, spots, lenses, and joint faces (at numerous orientations!). Trace amounts to 5% uralite due to pervasive deuteric alteration is present. 1245-1355 ft. (two outcrops on north side of tracks) Unit I – taxitic, pervasively uralitized, medium- to coarse-grained ophitic olivine gabbro (PcOf) to augite troctolite (POcf)with trace to 1% sulfides. Also present within these outcrops are very coarse-grained anorthosite inclusions that vary from 10 cm across to 3x5 meter blocks. The edges of the inclusions are sharp, and straight to highly lobate. Within the inclusions, the plagioclase foliation is subvertical and highly variable. 1385-1685 ft. (many outcrops on both sides of tracks) Unit I – taxitic, medium- to coarse-grained ophitic augitic troctolite (POcf) with abundant irregular pegmatitic patches and lenses. The pegmatites contain variable amounts of saussurized plagioclase, clinopyroxene, oxides, biotite, and uralite, with minor quartz, kfeldspar, graphic granite, and sulfides. Most of the sulfides are either within or adjacent to the pegmatites. Uralite is common throughout the outcrops as pervasive replacement products in irregular patches and along joints. 2355-2480 ft. (four outcrops on both sides of the tracks) Unit III – mottled, poikilitic/ophitic troctolite (Po(cf)) to augite troctolite (Pocf). This is a major marker bed due to the presence of medium- to high-density olivine oikocrysts up to 10 cm across. This unit is easily recognized in drill core due to the mottled texture, and the relatively finer-grained plagioclase (1-5 mm). STOP 1-3: Dunka Railroad Junction-PolyMet Discussion Stop Location: Where rail line splits along Dunka Road, T. 59 N., R. 13 W., sec. 13, NW of NE Babbitt SW quadrangle; UTM: 578,358E/5,273,795N (NAD 83) Weather permitting we will stop here to look across a clear cut where some of the PolyMet facility will be located, talk about development plans, note Dunka Junction as a landmark on most regional maps and air photos, and look at outcrops of the upper units of the PRI (Unit VIII or 8?), barren of sulfides, but very typical. Some basalt inclusions are in the flat outcrop to the south of the road. 41 �STOP 1-4: Dunka Railroad Hornfels and Late OUIs Location: Abandoned rail grade, south and parallel to the Dunka Road T. 60 N., R. 12 W., sec. 33, NE of SW, Babbitt SE quadrangle; UTM: 584,808E/5,276,122N (NAD 83) The Dunka Railroad hornfels is a large mafic volcanic inclusion located near the eastern margin of the Partridge River intrusion and just south of the Mesaba deposit. Outcrops and drill hole information indicates that the inclusion is about 900 x 1,500 meters across. The hornfels is a fine-grained, granoblastic to poikiloblastic basaltic hornfels that contains variable amounts of plagioclase, augite, hypersthene, inverted pigeonite, and olivine. Both massive and meta-amygdaloidal varieties are present. In addition to the basaltic hornfels, several small bodies of late intrusive OUIs (Oxide Ultramafic Intrusions-cross-cutting plugs of oxide-rich rock) are also present in the railroad cuts. The abundance of OUI at this locality is related to proximity to the north-trending Grano Fault, which may have served as a feeder vent for the massive sulfides at the Local Boy area of the Babbitt/Mesaba deposit. These OUIs, and a wide variety of granitic rocks, occur as lenses and bodies that cut the troctolitic rocks of the Partridge River intrusion. They are extremely common in drill holes, within a 1,300- 1,900 foot (400-580 meter) wide zone, on the west side of the Grano Fault. The OUIs at this locality are characterized by medium- to coarse-grained clinopyroxenite with 75-80% augite, 0-15% olivine, <5% interstitial plagioclase, and 515% oxides (ilmenite is dominant). Contact relationships with the basaltic hornfels are sharp but highly irregular and lobate. Granitic dikes and veins, present within a NNE-trending zone, are also evident in the exposures. They are rarely observed in close proximity to the OUI, but where they are, the dikes crosscut the OUI. Four sites, shown on Figure 1-14, can be observed at this locality. Figure 1-14. Geology of the Dunka Railroad hornfels and the location of Stop 1-4. (From Miller, Severson, and Foose, 2002). 42 �PART 4: MESABA DEPOSIT – TECK AMERICAN INCORPORATED By: Tim Jefferson and Mark Severson BACKGROUND Previous exploration and development work The Mesaba deposit was first discovered along the base of the Duluth Complex in 1958 by Bear Creek Mining Company (BMC) which targeted a geophysical anomaly. Between 1958 and 1960 BMC completed 55 shallow drill holes for 43,000 feet (13,952 meters) on their discovery located 5 miles (8 km) south of Babbitt. BMC renewed drilling activities in 1967-1971 completing 149 additional holes. Drill hole B1105 intersected substantial amounts of semi massive to massive sulfide mineralization between 1,400 and 1,800 feet (425 and 550 meters) below surface in footwall rock. Subsequent drilling defined a high grade zone appropriately named the Local Boy deposit after BCM geologist Stuart Behling, the “local boy,” who encouraged BMC to continue drilling this site. Bear Creek defined the overall tonnage and grade for the deposit to be 851 million tons (772 million tonnes) grading 0.46% Cu, 0.12%Ni at 0.25% Cu cut-off. In late 1973, AMAX Exploration, Inc. agreed to take over BMC’s state and private leases. During the next four years (1974-1978) AMAX continued drilling over the entire deposit (completing 228 drill holes), and evaluated whether an underground operation was feasible (Watowich, 1978). In particular their focus was drawn to the Local Boy ore body, and following successful permitting, they sank a shaft in 1976-1977. Four drifts totaling 3,800 feet (1,160 meters) were developed and 218 underground holes were completed. This detailed definition resulted in an overall underground resource of 364 million tons (330.2 million tonnes) averaging 0.84% Cu and 0.19% Ni , with a Local Boy-only resource of 5 million tons (4.54 million tonnes) grading 1.89% Cu, 0.36% Ni. Both underground resources were estimated based on a 0.60% Cu cut-off. Due to weakening copper and nickel markets and the inability to produce separate high grade Cu and Ni concentrates, Amax abandoned their plans to develop the deposit in late 1981. Rhude and Fryberger obtained leases and evaluated the Local Boy deposit circa 1990. Arimetco Inc, picked up the Babbitt deposit leases, renamed the Mesaba deposit, and evaluated the property circa 1994-1996. They did not complete any drilling but collected two bulk samples for metallurgical test work. Arimetco upgraded the resource estimate to 3,300 million tons (2,993.7 million tonnes) grading 0.46% Cu, 0.12% Ni, cut-off 0.38% Cu (Miller et al, 2002). Arimetco Inc. declared bankruptcy in late 1996. Present exploration and development work Teck American Incorporated picked up a package of state and private leases covering the Mesaba deposit in 1997. In 2001 the evaluation of the resource included collection of a 5,511 ton (5,000 tonnes) bulk sample. Sulfide concentrates were made from this bulk sample and tested at their Cominco Engineering Services Lab near Vancouver, BC, using their patented CESL hydrometallurgical pressure oxidation process. New definition drilling began in the fall of 2007 and continued until June, 2008 for a total of 67,430 feet (20,560 meters) in 64 drill holes (Fig. 1.15). This drilling was concentrated on the western portion of the deposit to complete a 400 foot (120 meter) grid infill program. In addition to this work, a new 4,409 ton (4,000 tonnes) bulk sample was mined in the fall of 2008. Sulfide concentrates from this ore were made at NRRI’s Coleraine Mineral Research Laboratory. The concentrates were shipped to the CESL facilities for a new round of hydrometallugical tests, including the successful recovery of an intermediate mixed nickel-cobalt hydroxide. While Teck has not released a new reserve estimate pending completion of additional definition drilling, they are continuing their evaluation of the deposit on several fronts, including baseline environmental studies, geophysical surveys, and flotation and ore beneficiation / 43 �recovery test work at their Applied Research and Technology division at Trail, BC, along with other engineering studies. Figure 1-15. Drill hole location map for Mesaba Deposit. Grid north is about 33° west of north. Recent re-logging of historic holes at the Mesaba deposit, in addition to information gained from logging of holes completed in 2007-2008 by Teck American geologists and Mark Severson (NRRI), indicates that the deposit is primarily hosted by a previously unrecognized intrusion within the Duluth Complex (Severson and Hauck, 2008). It is believed that this intrusion, informally named the Bathtub intrusion (BTI), lies between the Partridge River intrusion (PRI), to the south, and the South Kawishiwi intrusion (SKI) to the north and east. This intrusion is believed to have been fed by a vent in the Grano Fault area on the east side of the Mesaba deposit. The BTI is believed to pre-date the South Kawishiwi intrusion, and is coeval in age to the Partridge River Intrusion. It is further believed, based on drill hole evidence, that igneous units of the PRI and BTI overlap and co-mingle, with the upper units of the PRI overlying BTI units. Supporting evidence for this new interpretation is based on igneous units that are unique to either the PRI or BTI, and different styles of sulfide mineralization between the two. The following geologic discussion is largely based on the work of Severson and Hauck (2008) but is condensed and summarized. The reader is referred to the regional geology section for further description of the Partridge River and South Kawishiwi intrusions. 44 �Figure 1-16. Preliminary geologic map of the Mesaba deposit showing major geologic units of the Bathtub, Partridge River, and South Kawishiwi intrusions. Major structural features associated with the deposit are also shown. Outline of ore deposit and Teck property boundaries are indicated. 45 �GEOLOGIC SETTING Footwall Rocks As there is great commonality between the footwall rocks at the NorthMet Deposit and the Mesaba Deposit, they are discussed in the regional geology section for the PRI Structure There are three structural features that are pertinent to understanding the intrusive history of the BTI that include (Fig. 1.16): 1. an east-west trending paired syncline and anticline in the footwall rocks referred to as the Bathtub Syncline and Local Boy Anticline; 2. a zone that is closely associated with the Local Boy Anticline, referred to as “The Hidden Rise,” that separates the PRI and BTI; and 3. a north-trending fault zone, referred to as the Grano Fault, that has been postulated to have been the feeder zone for the BTI and footwall-injected massive sulfides of the Local Boy ore zone. The paired Local Boy Anticline and Bathtub Syncline have been determined to be pre-complex, and pre-or syn-deposition of the BIF and Virginia formations by the following evidence: 1) BIF submembers A and B are notably thin or absent along the trough of the syncline. There is always a thin layer of Virginia Formation between the BIF and the overlying Bathtub intrusion in the synclinal trough (indicting that the Duluth Complex has not assimilated BIF A and B in the syncline). 2) The VirgSill also is thickest along the axis of the anticline and limbs of the syncline, and thins or is absent in the trough of the syncline. This attests to a more open structural setting along anticlinal folding for injection of the sill. It is thus interpreted that the thinning of BIF within the trough is due to lack of deposition of these more calcareous units in deeper water (Severson et al., 1994a). The “Hidden Rise” is a loosely-defined zone wherein scattered hornfels inclusions of footwall Virginia Formation, and associated noritic rocks, are fairly common. When viewed collectively, the inclusions in “The Hidden Rise” define an east-west trending “ridge” that is roughly positioned at the contact between the PRI and BTI. Thus, “The Hidden Rise” is used to both define this hornfels-bearing “ridge” and to artistically, and conveniently, divide the BTI from the PRI. The morphology of this feature suggests that it may have originally served as the floor and/or north edge of an earlier intruded PRI and later served as a wall along the south edge of the BTI as it was emplaced. Along the far eastern edge of the Mesaba deposit is the north-trending Grano Fault, so named for the abundant and sometimes voluminous amounts of late granitoid and oxide rich pyroxenitic lenses (OUIs) associated with the fault zone (Severson, 1994). The late intrusive lenses are interpreted to have vertical configurations. They were injected along subsidiary fault zones parallel to, and immediately west of, the Grano Fault. The late intrusives cut the troctolitic rocks and thus, demonstrate that the fault was active during and after emplacement of the PRI, BTI and SKI and may represent a rift related transform fault. Other features that are associated with the Grano Fault include: • a steep drop in the basal contact (down to the east); • abundant pre-Complex sills are common within the Biwabik Iron Formation in a limited area at the Serpentine deposit - the localized increase in the sills outlines the fault trace at Serpentine and suggests that the fault was activated prior to emplacement of the SKI and PRI (see also Zanko et al., 1994); and • a well-defined topographical lineament occurs along the trace of the fault to the south of the Mesaba deposit. Along the eastern edge of Mesaba, a buried valley, defined by contouring the top 46 �of the ledge (Plate IV, Severson et al., 1994b), is also present on the northern extension of the same topographical lineament. BATHTUB INTRUSION The newly named Bathtub intrusion (BTI) is wholly contained in the central portion of the Mesaba (Babbitt) deposit. The BTI has recently been singled out as a separate intrusion to explain the abrupt change from typical Partridge River intrusion (PRI) stratigraphy, in the southern part of the deposit, to a completely different stratigraphy to the north in the remainder of the deposit. The BTI has been divided into two major units, BT1 and BT4, each of which contain several subunits. These units, in addition to footwall rocks, and structural features, are portrayed in Fig. 1.17. Figure 1-17. Schematic “type-section” cross-section, looking east, through the Mesaba deposit that crudely displays the spatial distribution of most of the igneous units in the Bathtub intrusion. Note that not all of the PRI units are shown on the right side of the figure. BT1 Unit The lowermost unit of the BTI is referred to as the BT1 Unit. It is very similar to Unit I of the nearby PRI in that it is heterogeneous-textured at all scales, contains abundant hornfels inclusions near the basal contact, and is the main sulfide-bearing unit at Mesaba. However, there are some important differences between Units I and BT1 that include: • Augite troctolite is the dominant rock type in the bottom half of BT1 (as is also the case for Unit I) but in many of the cross-sections the entire up-dip portion of BT1 consists of augite troctolite; • Massive sulfide occurrences are more common near the basal contact in the BT1 than in Unit I (excluding the unique Local Boy ore zone) indicating that sulfide settling may have been a more important mineralization mechanism in the BTI; • Coarse- to very coarse-grained disseminated sulfides (up to several centimeters across) are exceedingly common in the lowermost portions of BT1; whereas, this same relationship is not so obvious in Unit I – this again implies the importance of a sulfide settling origin, and; • Ultramafic horizons and patches are very common in portions of the BT1; whereas, similar ultramafic horizons are not as common in Unit I of the PRI. The BT1 Unit has been further subdivided into several internal subunits that are discussed below. BT1-a 47 �This subunit of the BT1 is a heterogeneous-textured augite troctolite grading to olivine gabbro. The BT1-a subunit is more common in the bottom half of the BT1 Unit and increases up dip (to the north) at the expense of most other subunits of the BT1. BT1-c At the base of the BT1 there is significant silica contamination of the magma, due to assimilation of the footwall rocks, and noritic rocks (norite to gabbro norite), with common hornfels inclusions, are the dominant rock types with lesser amounts of augite troctolite. The BT1-c subunit spatially occurs as a rind or coating along the basal contact of the BTI. BT1-uz Wherever olivine-rich ultramafic rocks are common over appreciable intervals in the BT1 Unit this subunit is used to designate ultramafic zones. The morphology of the ultramafic rocks in these zones ranges from well-defined layers to zones where irregular ultramafic patches are presumably peppered throughout a troctolitic host rock. BT1-at This subunit of the BT1 is used to denote areas where anorthositic troctolite is the dominant rock type. The BT1-at zone is located at the very top of the BT1 Unit in the cross-sections of this report. It is a small unit that is locally present in only the central portion of the Bathtub ore zone. BT-sli A few holes in the extreme western end of the BTI exhibit well-defined modally-bedded rocks consisting of alternating troctolitic and olivine-enriched ultramafic rocks. These intervals are designated as BT-sli for the Bathtub Side Layered Interval (Figures 8 and 9a). The BT-sli subunit occurs about in the center of BT1 unit where it comes in close proximity to “The Hidden Rise.” While the BT-sli can be readily mapped out and correlated between cross-sections, it is difficult to tell if this subunit is a downward continuation of the BTLI or "± Picrite" (as depicted in Fig. 1.17). BT4 Unit The uppermost unit of the BTI is referred to as the BT4 Unit. It was originally correlated with Unit IV of the PRI. However, the BT4 Unit is distinctly different from Unit IV in that the BT4 Unit at Mesaba is: • heterogeneous-textured at all scales and composed of many alternating rock types; • sulfide-bearing whereas Unit IV is mostly sulfide-barren – the sulfides in BT4 are generally, finergrained and generally of lower ore grade tenor in comparison to sulfide-bearing zones in the underlying BT1 Unit; • floored by a semi-persistent ultramafic layer termed the "± Picrite" (see discussion below) in the central portion of the Bathtub ore zone; and • ultramafic layers and modally-bedded zones, termed the Bathtub Layered Interval (BTLI), are common in the central portion of the Bathtub ore zone. The BT4 Unit as been further subdivided into several internal subunits based on the presence of a dominant rock type. The various subdivisions of the BT4 Unit are briefly discussed below. BT4-a This subunit of the BT4 on the cross-sections denotes areas where heterogeneous-textured augite troctolite is the dominant rock type. BT4-at This subunit of the BT4 is used to denote areas where anorthositic troctolite is the dominant rock type. Thick zones of BT4-at are common to some cross-sections through the Mesaba deposit and show relatively good correlation and predictability with similar zones in adjacent cross-sections.. "± Picrite" 48 �At the base of BT4 is a semi-persistent olivine-enriched ultramafic horizon referred to as the "± Picrite." It is present in about 70% of the drill holes in the BTI-portion of the Mesaba deposit. The "± Picrite" is generally absent in the up dip direction (to the north) and is variably present to the south in the contact zone between the PRI and BTI. Where present, the "± Picrite" is about 1-15 feet thick, but exceptions are locally present. In some areas, the "± Picrite" consists of several stacked ultramafic horizons, or modal beds, that are interlayered with troctolitic rocks, and thus, the zone represents a collection of several cyclic layers. In other areas of the Mesaba deposit, the "± Picrite" is not always easily singled out as it occurs in close proximity to a downward thickening BTLI with similar ultramafic layers and modal beds. Therefore, in some instances it is difficult to pick the "± Picrite" out of a myriad of ultramafic horizons associated with either the BTLI or BT-sli. Bathtub Layered Interval (BTLI) In the vicinity of the Bathtub Syncline, ultramafic layers are extremely common within the BT4 Unit. The ultramafic layers may represent repetitious cyclic layers and can be correlated in drill holes as an overall rock package. This package of abundant cyclic layers, present in the BT4 Unit, is referred to as the Bathtub Layered Interval (BTLI). In the eastern half of the Mesaba deposit the BTLI appears to be present in a subhorizontal saucer-shaped morphology. Conversely, in the western half of the deposit, the BTLI is confined to one or two cylinder-shaped zones, albeit with irregular edges, that are positioned in close proximity to “The Hidden Rise.” Overall, the ultramafic rock types of the BTLI are characterized by alternating assemblages of either/or: melatroctolite (picrite), feldspathic peridotite, peridotite, dunite (minor), olivine-rich troctolite, and troctolite with modal beds of olivine-rich layers. One or more of these rock types may be stacked above the other in no particular order, and the thickness of this assortment may be highly variable between drill holes. The number of individual ultramafic layers present within the BTLI for any particular drill hole varies drastically. In some holes, over 75 individual ultramafic layers and modal beds are intersected, whereas in other holes only a few scattered ultramafic beds are encountered. The range in thickness for each of the individual ultramafic beds also shows considerable variation, ranging from a few inches to over tens of feet thick. Although the BTLI can be correlated as a package of alternating troctolitic and ultramafic layers, each of the individual ultramafic layers cannot be correlated on a hole by hole basis. This situation indicates that the ultramafic layers either: 1) commonly bifurcate - thick ultramafic layers may divide into many thin ultramafic layers; 2) some may actually represent dike-like features (filter pressed?); 3) some may pinch out or have very limited spatial extent due to localized crystallization or other deposition-related origins; or 4) combinations of the above. Gradational tops and sharp bases are commonly present, indicating that crystal settling may have been important (this is especially true in the eastern half of the Mesaba deposit). However, the reverse (gradational bottoms and sharp tops) is also locally present. In addition, the inclination of contacts and modal bedding associated with the ultramafic layers are highly variable, ranging from 5°-80° (with localized overturned beds). This variation in inclinations can even be present in even a single drill hole. For the most part, the bedding and contact inclinations in the BTLI are steeper higher up in the drill hole and gradually shallow with depth. The shallow to steep angles exhibited by the BTLI may reflect that the ultramafic layers originated via a variety of mechanisms that include: 1) crystal settling to form subhorizontal layers (dominant in the eastern half of the deposit); 2) filter-pressing to form localized dikelike morphologies; 3) slumpage and folding of the beds took place before they were fully crystallized to form highly irregular and overturned beds; 4) compaction differences took place during lithostatic loading of the crystal pile to form steep and irregular beds; 5) cooling and crystallization took place along, and parallel to, the southern wall of the BTI (up against “The Hidden Rise”); or 6) combinations of all of these mechanisms. Whatever their origin, the steep beds displayed by the BTLI in the western half of the Mesaba deposit are inordinately associated with “The Hidden Rise.” 49 �PARTRIDGE RIVER INTRUSIVE (PRI) AT MESABA Many of the igneous rock units that are present at the nearby NorthMet deposit are also present along the southern edge of the Mesaba deposit and are believed to represent units of the Partridge River Intrusion (see previous geologic setting discussion). Additionally, Units IV through VI of the PRI appear to extend northward and overlie the heterogeneous-textured BT4 Unit. This relationship, also depicted in Figure 1.17, suggests that the BTI was eventually over-ridden/overlain by the upper units of the PRI. The overall timing of emplacement for the PRI versus the BTI is unknown but correlations in the cross-sections crudely suggest the following: • Units I through III were intruded first along the southern edge of the Mesaba deposit with a vent area located somewhere to the southwest. “The Hidden Rise” generally marks the northern extent of this intrusive activity and originally formed as part of the floor to these units. Unit III may have been intruded as thin lenses across and north of “The Hidden Rise” – this may explain the local presence of Unit III-like inclusions in the BTI. • Concurrent with or after the above activity, the BT1 Unit was intruded from a vent area located somewhere to the east, possibly from the Grano Fault area. “The Hidden Rise” formed the southern wall of this particular magma chamber. • The BT4 Unit was intruded into the same magma chamber but was emplaced above the BT1 Unit. • Concurrent with or after the above activity, Units IV through VII+ of the PRI were intruded from a vent area located somewhere to the southeast. These upper units were emplaced over the BT4 Unit. MINERALIZATION AT MESABA The Mesaba deposit is characterized by disseminated sulfide mineralization, which occurs most commonly as intercumulus accumulations of chalcopyrite, cubanite, and pyrrhotite. Pentlandite crystals are less commonly identified megascopically. Additionally, less common occurrences of talnakite and bornite have been noted. Short intercepts of semi-massive to massive sulfide mineralization are often encountered in drill core in the Virginia footwall rocks immediately adjacent to noritic intrusive rock. Sulfur isotope analyses have indicated that the source of the sulfur used in the formation of the sulfides of the Mesaba deposit is the pelitic sediments of the Virginia Formation (Ripley, 1986). The model of sulfide deposition entails turbulent injection of units of the BTI wherein immiscible sulfide droplets coalesce within the silicate melts and attract the chalcophile elements (chiefly copper and nickel) through magma mixing. Thus, the most contaminated magma (from assimilation of footwall Virginia formation) hosts basal sulfides that contain excess sulfur relative to intrusive units higher above the footwall. The sulfide content of the rock increases, often dramatically as the footwall is approached. This sulfide content increase is accompanied by the increasing presence of pyrrhotite and a subsequent change in the copper bearing sulfides (cubanite is dominant over chalcopyrite). The disseminated mineralization is generally composed of 1-4% sulfides, but can reach upwards of 8-12 % sulfides as the footwall is approached. In addition to the interstitial disseminated sulfide and semi-massive to massive sulfide mineralization, sulfides may locally occur as clots up to several cm in diameter, and are seen occasionally as chalcopyrite rich vein fillings indicative of a late sulfide-rich fluid origin. The most important mineralized zone at Mesaba is the basal zone, starting at the footwall Virginia Formation contact, that commonly ranges between 200 and 400 feet thick (60 and 125 meters thick). Higher up in the intrusive package, often overlapping the BT1-BT4 unit boundary, is a second zone of disseminated sulfide mineralization which is more erratic and discontinuous in nature. 50 �The Mesaba deposit (hosted by the BTI) displays significant differences with the nearby NorthMet deposit (hosted by the PRI). At NorthMet, the ore zone lower in the deposit is more stratiform and near the top of PRI Unit I, while at Mesaba the main mineralized zone starts immediately at the footwall contact zone. As noted in earlier discussions, units of the BTI are more erratic and chaotic than those of the adjoining PRI intrusion. This is also true of the sulfide distribution which is often locally quite chaotic, and variable but overall the basal zone is tied together by adjacent drill holes to define a strongly mineralized ore body of considerable extent. The footprint of the Mesaba deposit is an oblong to arcuate shape, 3,000 by 13,000 feet (925 by 4,000 meters) in approximate dimension, cropping out to surface on the northern/up-dip side and extending to approximately 1,650 feet (500 meters) below surface in the southern/downdip direction. The strongest basal mineralization is often localized within the Bathtub Syncline. Here, concentration of sulfides by gravitational settling into the footwall depression has likely occurred. Teck American has not released any new reserve/resource estimates, and the reader is referred to historic reserve numbers as reported at the start of the Mesaba section. Three geologic cross sections from the western half of the Mesaba deposit depicting composited Cu-Ni grades from historic and recent drilling are displayed below (Figures 1.18, 1.19, and 1.20). PGE Mineralization at Mesaba Platinum group metal mineralization (PGE) occurs at Mesaba and the other Duluth Complex copper-nickel deposits. Along with analyzing all new drill core for precious metals (Pt-Pd-Au), Teck American is in the process of cataloging and analyzing pulps from historic drill holes (BMC, AMAX) for precious metals at this time as well. Analytical results of this data collection (in progress) have not been released to the public. As a general statement, the western side of the Mesaba deposit (hosted by the BTI) generally contains very low PGE values, while there are occurrences of anomalous PGE values in the eastern half of the Mesaba deposit. It is postulated that the higher values to the east may be dependent on a more proximal distance from the hypothesized vent area of the BTI. Analytical results of limited analysis of historic pulps for PGEs from the Local Boy ore body and pulps from historic drill holes intersecting upper portions of Unit I of the PRI in the southern portions of the Mesaba lease have been previously published and are summarized below: Local Boy PGE Occurrences Numerous anomalous PGE and precious metal values are confirmed to be present within the massive sulfide ores (Severson and Barnes, 1991; Hauck and Severson, 2000). Maximum values include: Pd – 11,100 ppb, Pt – 8,300 ppb, Au – 13,100 ppb, and Ag – 62 ppm (note that these values are present in sampled intervals that range from 5 feet to 15 feet thick). The majority of the anomalous PGE values are spatially distributed along the axis of the Local Boy anticline with the highest Cu and PGE values occurring in the west half of Local Boy. The Grano Fault may have served as a feeder zone to the massive sulfides that were injected into the footwall rocks along the Local Boy Anticline as an immiscible sulfide melt. This melt fractionally crystallized in an east-to-west direction and progressively became enriched in PGE towards the west (see discussion below). PGE Mineralization at the Top of PRI Unit I (along the Southern Margin of the Mesaba Deposit Drill holes along the southern margin of the Mesaba deposit intersect an igneous stratigraphic section similar to the section present at the nearby NorthMet deposit. Limited sampling for PGE at the top of Unit I (the equivalent of the Red Horizon of Geerts (1991, 1994) at NorthMet) has taken place in a few holes at the Mesaba deposit (Severson and Hauck, 2003). For the most part, the Pd contents at the top of Unit I in the sampled holes are similar to Pd contents of the Red Horizon at the NorthMet deposit. Publicly available data indicates a maximum of 1,267 ppb Pd is present at the top of Unit I in the southern portion of the Mesaba deposit (Severson and Hauck, 2008). 51 �MASSIVE SULFIDES AT THE LOCAL BOY ORE ZONE OF THE MESABA DEPOSIT Cu-rich massive sulfides near the basal contact of the Complex are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. In 1976, AMAX Inc. completed a 1,700-foot-deep exploratory shaft (Minnamax shaft) down to massive sulfides of the Local Boy ore zone and, in 1977, completed four drifts (A, B, C, and D; Figures 1.21 through 1.24). Underground Fan drilling (217 holes) was completed in 1978 to further define the massive sulfide distribution. Sulfide minerals include pyrrhotite, pentlandite, chalcopyrite, talnakhite, cubanite, maucherite (nickel arsenide), sphalerite, bornite and late mackinawite, chalcocite, covellite, godlevskite, and native silver. A more detailed description of these minerals, along with microprobe compositions, microphotographs, and possible paragenetic sequence, are presented in Severson and Barnes (1991). The Grano Fault may have served as a feeder zone to both the BTI and to massive sulfides of the Local Boy ore zone. Severson and Hauck (2003) speculated that magma that issued from the Grano Fault may have been initially enriched in PGE, forming the PGE-enriched massive sulfides at the base of the PRI in the Local Boy ore zone, but as the magma intruded in an east-to-west direction [to form the BTI] it became progressively impoverished with respect to PGE in such a manner that rocks at the extreme western end of the Bathtub ore zone contain very little PGE. Footwall Structures in the Local Boy Ore Zone Several investigators have recognized that pre-existing structural conditions in the footwall rocks strongly influenced the basal contact of the Duluth Complex (Mancuso and Dolence, 1970; Watowich, 1978; Holst et al., 1986; Martineau, 1989; Severson and Barnes, 1991). Major irregularities in the basal contact are generally related to folds in the underlying country rock indicating that intrusion proceeded more or less along bedding planes in the footwall rocks (Holst et al., 1986). This is readily expressed by a major eastwest -trending trough and ridge in the basal contact at Mesaba that coincides exactly with a synclineanticline that is defined by the top of the Biwabik Iron Formation (BIF). The thickness of preserved Virginia Formation between the Complex and the BIF is variable due to the amount of material assimilated by the Complex. The Local Boy ore zone is also situated over this anticlinal ridge. The majority of massive sulfide ore zones, hosted mainly by the Virginia Formation (Severson and Barnes, 1991), are broadly coincident with the axis of the anticline. The contoured top of the BIF in the Local Boy area is shown in Figure 1.21. Similar anticline geometries are also present for the basal contact as shown in Figure 1.21. All the data indicate that an EW-trending anticline is the major structural feature present within the footwall rocks of the Local Boy area. 52 �Figure 1-18. Cross-section 68+00W through the west end of the Mesaba deposit showing grades of significant intervals. Drill holes MB-08-38 from this cross-section will be on display at Teck’s core shack. 53 �Figure 1-19. Cross-section 44+00W through the west end of the Mesaba deposit showing grades of significant intervals. Drill holes MB-07-15 and MB-08-37 from this cross-section will be on display at Teck’s core shack. 54 �Figure 1-20. Cross-section 36+00W through the west end of the Mesaba deposit showing grades of significant intervals. Drill hole MB-08-36 from this cross-section will be on display at Teck’s core shack. 55 �Figure 1-21. Contoured top of the Biwabik Iron Formation at Local Boy (left), and the contoured top of the basal contact between the footwall Virginia Formation and the Partridge River intrusion at Local Boy (right). The spacing of the contours in Figure 1.21 suggests that the anticline is asymmetrical with a steeper flank to the immediate south of the anticlinal crest. Also, fault zones in drill core, as well as recognizable fault offsets of correlative units, are most commonly present on the south flank of the anticline. Taken collectively, all these data suggest that additional structural features, in the form of increased faulting and shearing, are more important on the south flank of the anticline in the Local Boy area. The northeasttrending Kulas Fault is also shown in the Figure 1.21. This fault was initially mapped by Jim Kulas in the underground drifts at Local Boy. The cross-sections included in Severson and Hauck (2008) also recognized the fault, with an offset of 10-20 feet, and named the fault after Kulas. Mineralization Trends in the Massive Sulfide at the Local Boy Ore Zone The vast majority of massive sulfides at Local Boy are contained within the Paleoproterozoic Virginia Formation. Even though the massive sulfides straddle the basal contact, most of the massive sulfides are associated with either hornfelsed sedimentary inclusions above the contact or with footwall rocks below the contact while the interfingering intrusive rocks are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed in this area by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A similar mechanism is proposed for the Norilsk-Talnakh deposits in Russia. Even though the basal contact of the Complex with the Virginia Formation is highly undulatory, the massive sulfides exhibit a definite top and bottom. The ore is distributed such that most of it is contained within a zone between 20 feet and 300 feet above the top of the Biwabik Iron Formation. The geologic constraint for the bottom of the ore zone generally corresponds to the top of the VirgSill. The constraints for the upper portion of the ore zone are unknown and may have been obliterated during emplacement of 56 �the Complex. Figure 1.22 is an attempt to show, in a plan view, where massive sulfide zones are present. Also shown in the figure are the different massive sulfide types (ranging from pyrrhotite-dominant to Curich) relative to structural features. The relationships shown in Figure 1.22 indicate that: 1) semicontinuous massive sulfide zones are present, mainly to the south of the Kulas Fault; and most important 2) the massive sulfides show a progressive change in an east-to-west direction from Cu-poor massive sulfides to Cu-rich massive sulfides in the vicinity of the Local Boy anticline. These relationships suggest that the injected immiscible sulfide melt underwent fractional crystallization and progressively became more Cu and PGE enriched as it moved through the footwall rocks in an east-to-west direction. Figure 1-22. Potential distribution of semi-massive to massive sulfide types (Cu-poor versus Cu-rich) at the Local Boy ore zone (left); and an isopach map of the cumulative thickness of the massive sulfide zones at the Local Boy ore zone (right). Note that the massive sulfides are not present as a continuous blanket, but rather, as one or more stacked disjointed/separated multiple horizons near the basal contact. A possible feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. Other data that indicates that the Grano Fault was a potential feeder vent include: 1) the massive sulfides are more common, and thicker (Figure 1.22), close to the Grano Fault (feeder) and along the axis of the Local Boy anticline (structurally-prepared site); 2) the VirgSill rarely contains significant amounts of disseminated sulfides – except near the Grano Fault; and 3) the Biwabik Iron Formation rarely contains sulfides – except near the Grano Fault. In summary, the massive sulfides at the Local Boy ore zone are interpreted to be structurally controlled in that they are situated along the axis of the Local Boy anticline. The massive sulfides are Cu-rich (5-25% Cu) and are almost exclusively hosted by the Virginia Formation. Sulfide textures suggest that the massive sulfides were injected as an immiscible sulfide melt into the footwall rocks. The overall pattern of sulfide types and PGE contents suggest that the sulfides formed via a process of fractional crystallization of an immiscible sulfide melt as it migrated into the footwall rocks. The Grano Fault is inferred to represent the potential feeder zone in this scenario. 57 �PART 5: FRANCONIA BIRCH LAKE PROJECT Extracted from Routledge, 2004; some contribution by Mark Severson Birch Lake Deposit A few widely-spaced holes were drilled in the Birch Lake area by Duval Corporation in the 1970s. Some Cu-Ni mineralization was intersected at great depths in many of these holes; however, exploration companies were not impressed with the Cu-Ni grades and largely abandoned the area - the potential for PGE mineralization was never considered. In the mid-1980s, the MDNR and Mineral Resources Research Center (MRRC) conducted analyses of iron-rich intervals in the basal portion of drill hole Du15. PGE values as high as 9,123 ppm Pd+Pd, associated with high Cr2O3 contents (5.3%) were documented (Sabelin and Iwasaki, 1985; 1986). This discovery marked the start of serious PGE exploration in the Duluth Complex and a multitude of holes and wedges have since been drilled at Birch Lake. Property Geology The Birch Lake property is entirely underlain by the South Kawishiwi intrusion (SKI) that is itself bordered on the southwest by the Partridge River body and on the southeast by the Bald Eagle pluton. The SKI extends approximately 25 miles (40 km) northeast-southwest and is up to 4.5 miles (7 km) wide. The footwall Middle Precambrian metasediments outcrop less than a kilometer west of the property boundary in the area of the Birch Lake deposit, and are best exposed in the Dunka pit area. Figure 1.23 shows the property geology. On the Birch Lake property, the SKI ranges in vertical thickness from 1150 feet to 4,420 feet as interpreted from 34 pilot hole drill intercepts of the footwall metasedimentary rocks or Giants Range monzonite. The lithology and igneous stratigraphy of the SKI (Fig. 1-6) been has been simplified to seven principal units found on the property as summarized in Table 1.8. The Cu-Ni bearing sulfides and associated PGE mineralization at Birch Lake occur consistently in the upper portion of the “U3” ultramafic unit below its contact with pegmatitic phases of sulfide-barren, hanging wall troctolites, gabbros, and anorthosites. The U3 unit is mainly troctolite phases with compositions ranging to anorthosite. It is variable in modal mineralogy, composition and texture over short distances but can be distinguished by the presence of sulfides, cumulus olivine with interstitial plagioclase, and olivine rich ultramafic intervals (dunites, melatroctolites, picrites) ranging from less than 1 foot to tens of feet (0.3 m to tens of meters). Late granitic and felsic dikes cut the SKI and U3 unit. The U3 unit has been intersected at depths as shallow as 1,125 feet (343 m) and up to 3000 feet (915 m) deep in 31 pilot holes on the property. Intercept thicknesses of the U3 range from 48 feet to 375 feet (14.6 m to 114.3 m) and vertical thickness averages 175 feet (53 m) based on statistics for 31 pilot holes. Strong PGE enrichment is associated with late stage Cu-Ni sulfides, particularly with chalcopyrite, talnakhite, and bornite where they occur as replacement mineralization. Weaker PGEs are found where chalcopyrite and pyrrhotite are the primary sulfides. Saussauritization and serpentinization are common deuteric alteration in the U3 unit. Retrograde alteration and schistosity accompany the east west and northwest faults. The Complex has not been significantly deformed since magma consolidation, but it has been subjected to displacements along reactivated basement faults as well as cross faults. Mapped structures are mostly sub-vertical north-northeasterly faults and fault zones that are evident as linears on airphotos and topographic maps. Rowell (2002) believes that these faults have been active pre-, syn- and postemplacement of the SKI and offset the mineralized U3 unit. Where exposed in parts of the SKI and footwall rocks, movement on these faults ranges from 10 feet to 400 feet (3 m to 120 m). 58 �Table 1.8. Stratigraphic Section for South Kawishiwi intrusion at Birch Lake. Unit Thickness Remarks Hanging Wall Rocks AT&T Main AGT AT-T Thick Averages 275 m 21 m to 365 m; average 115 m Thin picrite units at top or middle of unit Cu-Ni-PGE Mineralized Units PEG U3 BH BAN 3 m to 80 m; averages 28 m 1 m to 125 m; averages 30 m Extremely variable 3m to 115 m; averages 38 m Sulfides first appear near base Up to 20 ; Cu-Ni-PGEs-oxides enriched Primary host for Cu-Ni sulfides in intrusion 120 m Contact metamorphosed to pyroxene hornfels Melted and recrystallized Footwall Rocks: BIF GRB West-northwest trending faults cut the northeasterly faults and show left lateral displacements in the south portion of the property and right lateral offsets under Birch Lake (Rowell, 2001). These late faults have vertical displacements in the order of 10m and may be akin to transform faults that accompany rifting elsewhere. The Bob Bay Fault zone trends north through Bob Bay but northeasterly south and north of Bob Bay. The faulting appears to have influenced the localization of sulfides and higher PGEs and it effectively cuts off the Birch Lake deposit on the west. Drill holes in this fault zone commonly intersect massive and disseminated sulfides in the footwall rocks and/or felsic dikes that cross-cut the intrusion. The sense of displacement between holes 88-1 and DU-15 is in the order of 200 to 300 feet (60 m to 90 m) down on the east side. A regional west-northwest fault diminishes mineralization and metal grades along its length and divides the deposit into a north, main segment and a small south segment. The spatial distribution of faults on the property is such that the Birch Lake deposit is displaced laterally and vertically by the sub-vertical fault sets. However, at the relatively wide spacing of the mostly vertical drill holes that test the deposit, the location of most of the interpreted faults is not known with the degree of confidence generally required for reserve estimation and mine planning. Deposit Types The Duluth Complex hosts four types of magmatic mineralization at or near its footwall: • Large, low grade disseminated Cu-Ni sulfide deposits that are locally enriched in PGEs. • Localized high grade zones of massive Ni-Cu sulfides, which maybe enriched in PGEs. • Disseminated, PGE enriched, Cu-Ni sulfides associated with specific types of phase-layer transitions, and in this sense are stratabound deposits. • Titanium and vanadium oxides-rich ultramafic plugs that are, in some cases, potentially deposits. The Birch Lake deposit is an example of the third- type of mineralization: 1% to 5% disseminated copper and nickel sulfides bearing significant palladium, platinum, and gold values with lesser silver, cobalt and rhodium. The mineralization is stratabound in that it is consistently associated with the top of the U3 unit and pegmatite marker horizon that is traceable hole to hole as the deposit hanging wall. Palladium and platinum enrichment and the Pt:Pd ratio is greater near top of the U3 unit; copper and nickel grades are variable as is the Ni:Cu ratio. The base of the deposit within the U3 unit is determined by assay cut-off. 59 �Figure 1-23. Birch Lake property geology (from Routledge, 2004). Mineralization Information on mineralization has been obtained from core logging by personnel of the BBJV, its partners and State geologists, laboratory analysis and the detailed mineralogical investigation of five core samples from five drill holes. The latter consisted of reflecting light microscope and scanning electron microscope 60 �study of polished thin sections prepared from heavy minerals concentrated by heavy liquid separation of crushed and ground core (Cabri, 2002). These samples had relatively high PGE grades. Detailed petrography and electron microprobe work on drill core from four holes has also been done by the UMN and the NRRI (Marma et al., 2002). Sulfides are disseminated interstitially in the rock matrix and mirror the size of rock forming mineral grains: coarser sulfides with coarse grained to pegmatoid fabrics, finer sulfides with medium grained rocks. The sulfides occur: • intergrown as eutectic and replacement textures • as triple point exsolution between rock mineral grains • intergrown with silicates • rarely as sulfide seams or veinlets Microscope study by Cabri (2002) has identified the major ore minerals to be chalcopyrite and undefined members of the chalcopyrite family, possibly one or more of talnakhite, mooihoekite, putoranite and haycockite; the oxide minerals chromian spinel, ilmenite, magnetite, chromite and native copper and troilite. Common minerals found are the copper sulfides bornite, chalcocite, and cubanite as well as nickel sulfide minerals heazlewoodite and pentlandite. Trace amounts identified are altaite, digenite, frobergite, galena, mackinawite, millerite, sphalerite and nine different PGM-bearing minerals, native silver, silver telluride and alloys of silver and gold. Cobalt is analyzed from pentlandite up to 2.12 wt%. Iron sulfide gangue is pyrrhotite and troilite. The PGEs occur as various fine grained Pd tellurides with other Pt, Os, Ru, Au, Ag, Te and B minerals. Ninety percent of the PGMs are associated with copper sulfides (Cabri, 2002) as discrete grains attached to sulfides, as sulfide inclusions and at the margins between sulfides and gangue silicates. The PGMs may form halos around, or be included in, interstitial copper sulfides, pyroxenes, secondary amphiboles and biotite. PGEs are also remobilized in chlorite, serpentine or secondary magnetite. High PGE values were first analyzed from an interval characterized by poikilitic chromite in plagioclase feldspar known as " 2 in one texture". Pd minerals occur at twice the frequency of Pt minerals and this is reflected in drill core analyses. Native silver is generally occluded in sulfides with Ag and Au-Ag alloys found as discrete grains and inclusions. Geologic controls on spatial distribution of metals and mineralization are not fully understood (Lehmann, 2002b). Lehmann (2002a) suggests that partially crystallized ultramafic magma was injected into the crystallizing SKI from vents associated with faults at the base of the Duluth Complex. This produced thick localized piles of dunites and melatroctolites in the SKI along the margin of the Duluth basin. Metal enriched residual fluids introduced with the ultramafic magma migrated upwards and laterally to be trapped and precipitate sulfide mineralization where permeability was disrupted horizontally, such as elevation changes and changes in magma composition. An initial pulse of mineralization introduced widespread chalcopyrite and cubanite and pyrrhotite with low, but anomalous, PGE values at the base of the Complex. A subsequent more localized mineralization event precipitated sulphur deficient copperiron sulfides and troilite accompanied by low grade PGEs that were trapped stratigraphically higher in the SKI at the pegmatite hanging wall unit at the Birch Lake deposit. PGEs and copper may have been redistributed by cooler, late stage, magmatic fluids that caused the deuteric alteration of the ultramafic host rocks. Further complicating this picture, the presence of chlorine-rich drops (formed by a deliquescent process) on the drill core suggests that a hydrothermal model of concentrating the PGEs could also be invoked. 61 �PART 6: DULUTH METALS DRILL CORE REVIEW Dean M. Peterson - Duluth Metals Limited INTRODUCTION AND COMPANY HISTORY Wallbridge Mining Company Limited began active mineral exploration in the Duluth Complex in 1998 when it reviewed data on the adjacent Maturi and Spruce Road deposits. The company completed limited field work including geological mapping, geophysical surveys and a three hole drill program. A review of existing data allowed an estimation of an underground Inferred Resource of copper, nickel, cobalt and PGE of the Maturi deposit and its extension to the east. Based on this initial work, the company elected to proceed to acquire properties immediately to the east of the Maturi Deposit, previously referred to as the Maturi Extension Property but currently referred to as the Nokomis Property. Duluth Metals was incorporated under the laws of the State of Delaware on January 18, 2000 under the name of Wallbridge America Corporation. During 2000 and 2001 Duluth Metals acquired four state leases and two federal prospecting permits within the Nokomis Property for future exploration. The initial exploration work in 2001 and 2002 included contracting a 3D computer model of the geology (completed by individuals at the Natural Resources Research Institute or NRRI) and reviewing the exploration potential of Duluth Metals' holdings on the basis of this new 3D geological model. Resampling was done on drill core from seven holes on the property and one hole immediately adjacent to the property stored at the core storage facility in Hibbing, Minnesota. Results of this new geochemical data provided the foundation of the initial magma flow model for the deposit (Peterson, 2001b). During 2003 and 2004 Duluth Metals conducted no significant exploration and merely maintained its properties in good standing. Increased development activity in the area and increasing commodity prices in 2005 resulted in Wallbridge activating Duluth Metals. Also in that year Wallbridge concluded that the most effective way to finance the exploration and possible development of these properties was to "spin off" Duluth Metals by vending it into a free standing company to be run independently. In early 2006 a fully equipped field office was set up in Ely, Minnesota located approximately 15 minutes drive from the project site. Facilities including three bedroom accommodation, computerized data handling, core analysis and storage have been organized at the Ely field office. A seven hole drill program completed in the spring of 2006 extended the mineralization to the east into the property acquired in January 2006 from a private party. The results of the drilling also confirmed the grade and consistency of the mineralization located beneath a very large anorthosite block in the middle of the Nokomis Property. Through sound geologic analysis of the public domain data and the fortuitous open properties available between the known Maturi and Spruce Road deposits, the foundation for a world-class Cu-Ni-PGE discovery was put into place. However, it requires more than a bit of well positioned property to make a world-class discovery. The favorable alignment of world metals demand, increasing metals prices, new hydrometallurgical extractive technologies, and a bit of luck provided the strong market conditions facilitating the successful Initial Public Offering (IPO) of Duluth Metals stock in October of 2006. The financial underpinnings of the IPO provided the fuel for a systematic exploration drilling of the property. This approximately 500,000 foot drill program was incredibly successful, with every one of the 155 drill holes intersecting Cu-Ni-PGE mineralization. This extensive drill program provided the cornerstone of the development of the giant Nokomis Cu-Ni-PGE deposit. In tandem with the drilling efforts were property acquisition efforts (still ongoing), resulting in the purchase of private surface lands, leasing of additional State lands, and the optioning of the Dunka Pit property as a potential process and tailings site (Fig. 1-24). 62 �Figure 1-24. Simplified terrain and geology map with overlays of identified Cu-Ni-PGE & TiO2 deposits and Duluth Metals properties. 63 �NOKOMIS DEPOSIT MILESTONES With the success of drilling efforts (Fig. 1-25) came National Instrument 43-101 (NI 43-101) compliant resource calculations, showing that within the Duluth Complex the Nokomis stands out with its continuity of grade and overall dimensions. NI 43-101 is a mineral resource classification scheme used for the public disclosure of information relating to mineral properties owned by, or explored by, companies which report these results on stock exchanges within Canada. Figure 1-25. Defined resource blocks categorized as Indicated and Inferred, Duluth Metals drill holes, and the location of selected drill holes on display for this field trip (ILSG, 2009). With the positive resource picture, metallurgical testing was warranted on the Nokomis data and a study was contracted with SGS Lakefield. Here again the Nokomis deposit proved itself by demonstrating exceptionally high recoveries of Cu, Ni, and precious metals (Table 1-9). These high recoveries are above typical metal recoveries of conventional smelting, therefore a technology agreement was signed by Duluth Metals for the use of the patented Platsol process. Table 1-9. Nokomis metallurgical test results, completed by SGS Lakefield. Metal / Recovery Cu Ni Pt Pd Au Flotation / Concentration Recovery 95.3% 72.4% 86.0% 87.0% 73.0% Hydrometallurgical (Platsol) Recovery 99.6% 99.2% 97.6% 98.1% 84.1% Combined Recovery 94.9% 71.2% 83.9% 85.4% 61.3% 64 �With the positive metallurgical testing, two scoping studies were completed on the Nokomis Project by the consulting firm Scott Wilson Roscoe Postle Associates Incorporated (SWRPA) resulting in very positive outcomes. Based on indicated and inferred resources, the Nokomis Deposit ranks as one of the largest combined base and precious metal resources discovered in North America in decades. For comparison, the Nokomis deposit contains a higher than average grade for deposits within the Duluth Complex. Specifically, the Nokomis Deposit contains significantly higher grade zones of copper, nickel, platinum, palladium, gold and silver zones in large coherent portions of the deposit. One of the main objectives of the Company is to quantify these higher grade zones and appraise the potentially positive impact on future mining scenarios. An updated interim resource estimate was received on June 4, 2008 from SWRPA based on drilling completed through April 2008 (Fig 1-25). The increased resource estimate update at a 1% copper equivalent cut-off and various copper cut-off grades is shown in Table 1-10. Based on SWRPA review of metal prices, process recoveries, refining costs and underground mine operating costs likely to apply at the Nokomis deposit site, the 1.0% copper equivalent cut-off grade is reasonable for the statement of indicated and inferred resources at this time. Table 1-10. Indicated and inferred resource estimate for the Nokomis Deposit. Cut-off 1.0% CuEq 0.5% Cu 0.6% Cu 0.7% Cu 0.8% Cu Cut-off Grade 1.0% CuEq 0.5% Cu 0.6% Cu 0.7% Cu 0.8% Cu Tonnes (000's) 449,413 376,306 247,149 112,035 41,078 Tonnes (000's) 284,230 236,102 155,743 72,418 33,292 Cu % Ni % 0.624 0.658 0.714 0.794 0.883 0.199 0.206 0.216 0.233 0.255 Cu % Ni % 0.627 0.667 0.725 0.817 0.900 0.194 0.198 0.201 0.214 0.215 Indicated Resources Co Au Pt % g/t g/t 0.010 0.011 0.011 0.011 0.011 0.084 0.090 0.103 0.123 0.152 0.159 0.172 0.199 0.240 0.293 Inferred Resources Co Au Pt % g/t g/t 0.01 0.01 0.01 0.01 0.01 Note: 1. 2. 0.096 0.105 0.120 0.144 0.173 0.191 0.210 0.239 0.280 0.320 Pd g/t TPM g/t CuEq % 0.358 0.390 0.452 0.549 0.679 0.600 0.653 0.753 0.912 1.124 1.46 1.54 1.66 1.86 2.09 Pd g/t TPM g/t CuEq % 0.431 0.475 0.542 0.640 0.739 0.718 0.790 0.902 1.065 1.231 1.50 1.58 1.69 1.88 2.03 CIM definitions were followed for Mineral Resource estimation and classification. Mineral Resources are estimated at a zone definition (wireframe) cut-off grade of approximately 1.0% Cu equivalent grade (CuEq). 3. The approximately 1.0% CuEq cut-off grade includes all material in the wireframed zones. 4. Bulk density is 3.01 t/m3 5. Resources were estimated to a maximum depth of approximately 1,350 m. 6. Copper equivalent (CuEq%) is based on Net Smelter Return Factors as determined for the Preliminary Economic Assessment by Scott Wilson RPA dated January 18, 2008. 7. Metal Prices used were $1.75/lb copper, $7.00/lb nickel, $10.00/lb Co, $600/oz Au, $1100/oz Pt and $350/oz Pd. 8. Copper equivalent (CuEq%) = Cu% + 3.03 x Ni% + 0.63 x Co% + 0.30 x Au g/t + 0.76 x Pt g/t + 0.24 x Pd g/t based on expected metal prices and process recovery and refining charges. 9. TPM is Au g/t + Pt g/t + Pd g/t. 10. Co, Au, Pt, Pd grades, that are lacking in historic drill holes, have been entered in the resource database based on regression of assay grades from DML drill hole assays 65 �The indicated and inferred resource estimates for base and precious metal content in the Nokomis deposit truly are enormous (Table 1-11). It is perhaps more revealing to compare these Nokomis numbers on the world stage of magmatic Cu-Ni-PGE sulfide deposits. This comparison (Fig. 1-26) is based on two types of data for this class of ore deposit. Historic metal contents for mining camps and individual deposits are taken from Anthony Naldrett’s book titled “Magmatic Sulphide Deposits: Geology, geochemistry, and exploration” published in 2004. Publically reported metal contents for these same mining camps and deposits, as well as the Nokomis deposit, are from SEC filings, NI 43-101 reports, and related audits available online. Review of this figure clearly displays the size and importance of the Nokomis deposit on the world stage. For example, a simple comparison of the publically reported data for the Sudbury mining camp versus Nokomis shows that the Nokomis deposit contains more copper and PGE than is left in all of the active and developing mines in Sudbury. Precious Base Table 1-11. Calculated base and precious metal content of the Nokomis Deposit. Metal Indicated Inferred Copper 6.18 Billion lbs. 3.93 Billion lbs. Nickel 1.97 Billion lbs. 1.21 Billion lbs. Cobalt 103.00 Million lbs. 62.80 Million lbs. Platinum 2.30 Million ozs. 1.75 Million ozs. Palladium 5.17 Million ozs. 3.94 Million ozs. Gold 1.21 Million ozs. 0.88 Million ozs. TPM (Pt+Pd+Au) 8.68 Million ozs. 6.57 Million ozs. Figure 1-26. Chart of the contained base and precious metals in world class Cu-Ni-PGE mining camps and deposits. 66 �On January 12, 2009, Duluth Metals announced the receipt of a new independent NI 43-101 Preliminary Assessment ("PA" or Scoping Study) on its Nokomis Project from SWRPA. This report (available online at SEDAR) provides an updated preliminary assessment of the Nokomis Project, based on the June 2008 mineral resource estimate. This PA is based on an expanded 40,000 tonne per day ("tpd") production rate scenario which doubles the January 2008 PA production rate. The report confirms positive economics for the Nokomis Deposit even at today's lower metal prices with the potential to be one of the world's low cost copper-nickel producers (Table 1-12). Table 1-12. Nokomis production rate scenarios. Base Case1 Scenarios Production Rate2 @ 20,000 tpd Production Rate3 @ 40,000 tpd Undiscounted Net Present Value $4.328 Billion $8.214 Billion Net Present Value @ 10% $792 Million $1.598 Billion Average Annual Cash Flow $205 Million $434 Million Internal Rate of Return (IRR) 21.0% 23.0% Capital Cost $795 Million $1.332 Billion Payback Period 4 years 4 years Annual Metal Production 102.1 million lbs Cu 23.8 million lbs Ni 121,000 ozs TPM 181.7 million lbs Cu 42.3 million lbs Ni 251,000 ozs TPM Note, all monetary units are in $US: 1 Scott Wilson RPA: Base Case Prices of $1.75/lb Cu; $7.00/lb Ni; $10.00/lb Co; $1,100/oz Pt; $350/oz Pd; $600/oz Au January 18, 2008 Scott Wilson RPA Preliminary Assessment on the Nokomis Project, Minnesota, U.S.A. 3 January 8, 2009 Scott Wilson RPA Preliminary Assessment on the Nokomis Project, Minnesota, U.S.A. 2 The new PA for the Nokomis Project envisions and includes costing for a fully integrated mining and processing facility that encompasses the following general process flow: Ore production rate of 40,000 tonnes per day (14 million tonnes per year); Underground mining by blasthole open stoping with partially recoverable pillars; Underground access by shaft and ramp; Underground ore handling by conveyor systems; Underground primary crushing, further crushing and grinding on surface; Transfer from mine to concentrator via 10 km slurry line; Agitated holding tanks at mill with a minimum capacity of 11,000 m3; Flotation concentration, producing a bulk copper-nickel-cobalt-PGM-gold concentrate; Hydrometallurgical processing using PLATSOL™ process; Production of saleable copper and nickel metal via standard electrowinning and production of cobalt and PGM-gold products to shipped to refineries for final processing to metal; and a brownfields tailings disposal facility within three kilometers of the processing site. Table 1-13 shows unit cash costs (operating and capital), calculated for either nickel or copper. This measure provides a means to compare costs in dollars per pound of metal to individual metal prices (also in dollars per pound of metal). SWRPA notes that the calculation is net of byproduct credits, which in this case amount to approximately 60% of revenue. Since no single metal is a dominant contributor, unit cash costs calculated in this manner appear very low, or negative in most cases. 67 �Table 1-13. Unit cash costing chart for evaluating the Nokomis deposit as a Copper or Nickel mine with all other metals as byproducts. Assumptions Low Case Base Case Market Case Key Variable - Cu Price ($/lb) Key Variable - Ni Price ($/lb) Capital Cost, including Contingency (Billion $) Conceptual Projected Mine Life $1.55 /lb. $4.90 /lb. $1.332 22 years $1.75 /lb. $7.00 /lb. $1.332 22 years $3.31 /lb. $12.70 /lb. $1.332 22 years $(0.09) /lb. $0.42 /lb. $0.32 /lb. $(0.72) /lb. $0.42 /lb. $(0.30) /lb. $(2.36) /lb. $0.42 /lb. $(1.94) /lb. $(2.15) /lb. $1.79 /lb. $(0.36) /lb. $(3.61) /lb. $1.79 /lb. $(1.82) /lb. $(11.62) /lb. $1.79 /lb. $(9.83) /lb. Nokomis as a Copper Mine Unit Cash Costs - Operating ($/lb Cu) Unit Cash Costs - Capital ($/lb Cu) Total Unit Cash Costs - Operating + Capital ($/lb Cu) Nokomis as a Nickel Mine Unit Cash Costs - Operating ($/lb Ni) Unit Cash Costs - Capital ($/lb Ni) Total Unit Cash Costs - Operating + Capital ($/lb Ni) Low Case - reflecting recent low metal prices of 1.55/lb Cu; $4.90/lb Ni; $10.00/lb Co; $795/oz Pt; $295/oz Pd; $600/oz Au. Base Case - same base case prices used in the previous Technical Report (January 2008 PA), based on long-term average price forecasts of the past several years. ($1.75/lb Cu; $7.00/lb Ni; $10.00/lb Co; $1,100/oz Pt; $350/oz Pd; $600/oz Au). Market Case - prices as of January 13, 2008*, for direct comparison to the previously-reported 20,000 tpd scenario contained in the January 2008 PA. ENVIRONMENTAL STUDIES With the scoping studies completed, Duluth Metals initiated baseline and preliminary Environmental Studies. Duluth Metals recognizes environmental guardianship as an important corporate priority, and is working closely with State and Federal agencies to establish policies, programs and practices for conducting its business in an environmentally sound manner. Our company realizes its long-term success depends on the well being of the environment and the community as a whole. Duluth Metals goal is to not only remain in compliance with all State and Federal regulations governing their operations but to exceed them. Duluth Metals has an environmental policy under which it has made a number of commitments consistent with responsible environmental stewardship. The Company is committed to: 1. 2. 3. 4. 5. 6. 7. 8. 9. Retain qualified, professional employees and consultants to design and implement their field programs; Develop, design and operate facilities in a socially and environmentally friendly manner in order to mitigate environmental impacts; Support research to advance understanding of industry's impact on the environment and to reduce harmful effects through improved practices and technologies; Contribute to the dissemination of environmentally sound technology and management methods; Work with government and the public to develop effective, efficient, and equitable measures to protect the environment based on sound science; Require contractors to comply with our Company's environmental policy requirements; Encourage dialogue on environmental issues with employees and the concerned public groups and to be responsive to their concerns; Ensure that all employees understand and are able to fulfill their environmental responsibilities; and, Reclaim work sites in accordance with site specific criteria in a planned and timely manner. 68 �The numerous studies need to be completed (over a number of years) in order for Duluth Metals to gain the right to mine the deposit in an environmentally sound manner is perhaps best viewed in a graphical form (Fig. 1-27). This chart clearly displays the complex interplay of geology, geoengineering, metallurgy, mine planning and design, and extensive environmental studies that must mesh together into a coherent plan to put the Nokomis Deposit into production. Duluth Metals has initiated baseline environmental and engineering studies in support of the Nokomis environmental review and permitting process. Examples of baseline environmental studies completed include: Wetlands Delineations and Functional Assessments; Stream Morphology Assessments; Wildlife Habitat Surveys; Sensitive Plant Species Surveys; and, Phase 1a Cultural Resources Searches. In addition, several preliminary environmental engineering studies have been completed to help with siting potential facilities. Figure 1-27. The environmental review and permit process utilized in Minnesota for the Cu-Ni-PGE projects in the Duluth Complex. Dark grey boxes represents work that the company has to complete, light grey boxes are completed by the state and federal agencies, and white boxes represent times the public has input into the process. GEOLOGY OF THE NOKOMIS DEPOSIT Duluth Metals Limited’s Nokomis deposit is the most recently discovered Cu-Ni-PGE deposit in the 1.1 Ga. Duluth Complex of northeastern Minnesota. The deposit is located near the north end of the South Kawishiwi intrusion (SKI) west-southwest of the junction of the Nickel Lake Macrodike (NLM) and the SKI (Peterson et al., 2006; Peterson and Albers, 2007; Tharlason et al., 2007; Peterson, 2008; White, in prep; Gal, 2008). The deposit was discovered utilizing a genetic ore deposit model that identified channelized magma flow within the SKI under a large xenolith/pillar of anorthosite. The model led to exploratory drilling in 2006, deposit discovery and initial resource estimation in 2007, and significant resource expansion in 2008, all in a period of 18 months. The regional scale magma flow model that is being used by Duluth Metals to interpret the origin of the Nokomis Deposit is presented in Figure 1-28. Duluth Metals has come to the realization that the initial basaltic composition SKI magmas that ultimately created the Nokomis deposit intruded as sulfide-bearing, crystal-laden slurries (olivine and plagioclase crystals). Therefore the company has reinterpreted the regional basal stratigraphy (PEG, U3, BH, BAN) of the SKI (Fig. 1-6) at Nokomis into the Basal Mineralized Zone, or BMZ. The company believes that 69 �the geometry of the system (sill-like sub-horizontal intrusion) led to crystal sorting and melting the footwall granitic rocks to create the heterogeneous lithologies of the BMZ (Fig. 1-29). Figure 1-28. Regional scale magmatic flow model for the northern SKI. Modified after Peterson, 2008. Figure 1-29. Simplified crystal-liquid slurry model for the SKI in the Nokomis area. 70 �The SKI is a shallow dipping (~24º east-southeast) sill-like troctolitic intrusion exposed in an 8- x 32kilometer arcuate band along the northwestern margin of the Duluth Complex. Lithologic units within the Nokomis deposit include Mesoproterozoic rocks of the SKI and Anorthositic Series of the Duluth Complex as well as basalt xenoliths of the North Shore Volcanic Group. At Nokomis, SKI magmas intruded between hangingwall anorthositic rocks and footwall granitic rocks of the Neoarchean Giants Range batholith (Fig. 1-29). Brief descriptions of the map units that Duluth Metals recognizes on the property are given in Table 1-14. Table 1-14. Lithostratigraphic units within the Nokomis deposit. Duluth Complex and related rocks (1.1 Ga.) SKI Anorthositic troctolite to troctolite (ATA Series) - Medium to coarse-grained, homogeneous, wellfoliated and locally layered anorthositic troctolite, troctolite, and ophitic troctolitic rocks. In the field, this unit is commonly referred to as the “sea of troctolite”. Augite-bearing troctolite (Main AGT) - Heterogeneous, coarse-grained, subophitic to ophitic, poorly foliated augite troctolite characterized by scattered augite-rich pegmatitic clots and patches. Commonly capped by hanging wall inclusions (HB & Ai) and interpreted to be the solidified basaltic liquid that carries the crystals and sulfides of the BMZ. Xenoliths in the SKI Sulfide-bearing troctolite (BMZ) - Heterogeneous, sulfide-bearing, vari-textured troctolite, augite troctolite, anorthositic troctolite, and olivine gabbro with 0.5 - 5% disseminated chalcopyrite, cubanite, pentlandite and pyrrhotite. Anorthosite (AN-G & Ai) - Undifferentiated Anorthositic Series inclusions. Includes well-foliated anorthosite, troctolitic-anorthosite, poikilitic troctolitic anorthosite, gabbroic anorthosite, and rarely gabbro and troctolite. Inclusions range from a few cm’s to elongate bodies measured in km’s. Anorthositic gabbro to gabbro (Upper Gabbro) - Mixed group of Anorthositic Series rocks that occur in the central portion of the map area. Includes well-foliated anorthositic gabbro, gabbro, anorthosite, hornfelsed basalt, and augite troctolite. Basaltic hornfels (Upper Basalt, HB) - Fine-grained, granoblastic to poikiloblastic basaltic hornfels; consists of variable amounts of plagioclase, augite, olivine, hypersthene, and inverted pigeonite. Commonly associated with Anorthosite xenoliths (unit Ai). Footwall Giants Range Batholith (2.68 Ga.) Porphyritic quartz monzonite (GRB) - Pink, coarse-grained, hornblende-phyric, quartz monzonite with large (1-2 cm) orthoclase phenocrysts. Also contains irregular zones of aplite and supracrustal xenoliths. Strongly recryatallized and partially melted locally anong the contact with the SKI. Two detailed geologic cross sections through the Nokomis Deposit are presented in Figure 1-30. These sections display the continuity of the basal mineralization as well as the differences in the hangingwall stratigraphy from west to east through the deposit (see Fig. 1-6). In the east, the deposit is located under an extremely thick (>1000m) pillar of Anorthosite Series rocks, and in essence the basal SKI can be viewed as a thin sill-like body. To the west, the anorthosite pillar ends and the immediate hangingwall rocks to the deposit are troctolites of the Main AGT unit. We interpret that the Main AGT as the 71 �solidified troctolite melt (see Fig. 1-29) that carried the crystals and sulfide droplets of the magmatic slurry. Figure 1-30. Geologic cross sections through the Nokomis Deposit. 72 �NOKOMIS ORE DEPOSIT MODEL Duluth Meals is utilizing advanced geological modeling and analysis of the Nokomis deposit to identify and characterize higher grade copper, nickel, platinum, palladium and gold zones within the deposit. This same modeling defines contiguous, large tonnage, higher grade zones that will be important in the evaluation of future mining scenarios. The geologic modeling also provides an innovative way to understand the origin of, and the controls on, grade distribution within the deposit. As well, the model will assist on directional search parameters and variography of subsequent block models and grade tonnage estimates. Duluth Metals is commissioning a new 43-101 resource estimate which will include 155 drill holes and 63 wedges as part of its higher grade zone targeting effort. The results of our models show four distinct types of higher-grade Cu-Ni-PGE mineralization within the Nokomis Deposit. These four distinct types of mineralization are briefly described below and are presented graphically in Figure 1-31. 1) PGE- rich disseminated mineralization on the Eastern side of the Nokomis Property. This area is known as the Eastern High Grade Corridor (21 drill holes as press released October 27, 2008). This large coherent zone of significantly higher grade PGE mineralization is the result of the initial constriction of the magma channel beneath a large block of older Anorthositic Series rocks within the Nokomis Deposit. Such a blockage impedes the flow of crystals and sulfide droplets. However, the liquid portion of the magma continues to flow along with its dissolved Platinum Group Elements ("PGE"). Once these dissolved and flowing PGE's come in contact with a stuck sulfide droplet, they dissolve into the sulfide droplets and increase the PGE tenor of the sulfide. 2) Ni-Co enriched semi-massive sulfides at, or immediately below the base of the magma channel. This mineralization is believed to have formed by continuous flushing of hot magma through the channelway which melted footwall granitic rocks. These granitic melts were incorporated into the magma and induced the formation of new sulfide minerals beneath the crystal slurry. These new sulfides scrubbed the melt of nickel and cobalt and settled to the bottom of the system. 3) Cu-PGE enriched disseminated mineralization deep in the footwall below the magma channel. The ever-deepening (into the melting footwall) magma channel induced and pinned hydrothermal convection cells in the footwall beneath the magma channel. The water in the system was released from the footwall granitoids by thermal metamorphism and circulated Cu-PGE downwards beneath the channelway. 4) Cu-PGE enriched disseminated mineralization at the top of the mineralized zone (Top-Loaded). To the sides of the magma channelway, the sulfide-bearing crystal-liquid slurry was intruded as batches of magma out and to the sides of the channel. Since these highly crystalline magmas are distal to the very hot main magma channel, they solidified quickly into sulfide-bearing troctolite. Once the silicates (olivine+plagioclase) are solid, buoyant fractionated sulfide (Cu-PGE) liquid and magmatic waters moved upwards through the solidifying crystal pile and precipitated Cu-PGE sulfide and hydrous silicate minerals beneath the overlying crystal-poor, liquid silicate magma. Company sponsored and internal research on many aspects of the mineralization within the Nokomis Deposit is continuing. Models based on normative mineralogical compositions have shown that the composition of Cu-Ni-PGE mineralization reflects local segregation and fractionation of the sulfide liquid as well as processes involved in the transportation upwards and/or downwards of the evolved sulfide liquid through the solidified host rock (Figs. 1-32 and 1-33). Further details of segregation-fractionationinfiltration and transportation processes of evolved Cu-Ni-PGE sulfide liquids/fluids are currently being explored by Duluth Metals. Such studies include detailed petrography, fluid inclusion studies, researchgrade total PGE analyses, whole-rock analyses, and stable isotope geochemical studies. 73 �Figure 1-31. The integrated ore deposit model for the Nokomis deposit. Figure 1-32. Copper-Nickel assays (A), normalized sulfide mineralogy calculated at 100% sulfide (B), and sulfide fractionation model for Nokomis drill hole MEX-084. 74 �Figure 1-33. Copper-Nickel assays (A), normalized sulfide mineralogy calculated at 100% sulfide (B), and sulfide fractionation model for Nokomis drill hole MEX-109. DRILL CORE DISPLAY Intervals from four drill holes will be on display in the Duluth Metals drill core logging shed in Ely Minnesota. These holes were chosen to display typical aspects of the four styles of mineralization outlined above that occur throughout the deposit. The locations of the four holes (MEX-084, MEX-109, MEX-141, and MEX-154) are given in Figure 1.25 and press released assay data for these holes are given in Table 1-15. The only drill hole on display that has exceptional textures and/or Cu-Ni-PGE grades for the defined style of mineralization (Fig. 1-31) is MEX-084, which is perhaps the finest example of mineralization in the footwall Giants Range batholith ever drilled in the Duluth Complex. The other drill holes (MEX-109, MEX-141, MEX-154), display typical textures and Cu-Ni-PGE grades (Table 1-15) within the Nokomis Deposit. 75 �We at Duluth metals invite you to examine these drill hole intervals and think about the physical and chemical processes that came together to create truly one of the world’s largest resources of Cu-Ni-PGE (Fig. 1-26). Herein at the Nokomis deposit, a channelized and crystal-laden (plagioclase and olivine phenocrysts) basaltic melt containing sulfide droplets intruding sub-horizontally as a sill-like body between a granitoid footwall and anorthosite hangingwall. The physical magmatic processes that the system underwent imparted discrete thermal and chemical processes onto the system. Many of these combined processes can be seen as snapshots in time and place in the drill hole intervals presented. Table 1-15. Composite assays from the four selected Duluth Metals drill holes on display. Details of the Interval From (ft) To (ft) Int. (ft) Cu (%) Ni (%) Au (g/t) Pt (g/t) Pd (g/t) TPM (g/t) Ag (g/t) MEX-141 Drill hole in the magma channel in the PGE-rich Eastern High Grade Zone 0.3% Cu cut-off 4032.0 4202.0 170.0 0.779 0.246 0.131 0.296 0.718 1.146 2.981 0.5% Cu cut-off 4032.0 4192.0 160.0 0.809 0.251 0.136 0.309 0.747 1.192 3.152 including 4147.0 4182.0 35.0 1.201 0.436 0.158 0.352 0.957 1.468 3.907 including 4177.0 4182.0 5.0 2.050 1.280 0.134 0.700 2.080 2.914 5.300 MEX-84 Drill hole beneath the magma channel with abundant footwall mineralization 0.3% Cu cut-off 2849.7 3115.4 265.7 0.691 0.250 0.097 0.184 0.423 0.704 2.452 0.5% Cu cut-off 2859.6 3110.4 250.8 0.705 0.258 0.100 0.187 0.433 0.720 2.515 including 3041.5 3110.4 68.9 1.033 0.270 0.146 0.286 0.637 1.068 4.082 including 3041.5 3071.0 29.5 1.209 0.367 0.188 0.334 0.670 1.192 4.559 MEX-109 Drill hole into Top-Loaded mineralization in the center of the property 0.3% Cu cut-off 3483.0 3668.0 185.0 0.760 0.227 0.130 0.260 0.602 0.991 2.903 0.5% Cu cut-off 3493.0 3648.0 155.0 0.828 0.247 0.143 0.286 0.660 1.089 3.150 including 3498.0 3538.0 40.0 0.955 0.278 0.213 0.414 0.901 1.527 3.863 MEX-154 Drill hole into thick Top-Loaded mineralization in the Western portion of the property 0.3% Cu cut-off 1627.0 1937.0 310.0 0.609 0.179 0.065 0.117 0.260 0.443 2.408 0.5% Cu cut-off 1627.0 1797.0 170.0 0.690 0.196 0.073 0.129 0.287 0.490 2.757 including 1667.0 1682.0 15.0 1.020 0.289 0.156 0.188 0.446 0.790 3.833 0.5% Cu cut-off 1882.0 1937.0 55.0 0.582 0.166 0.072 0.120 0.268 0.460 2.141 76 �REFERENCES Desaultels, P., and Patelke, R., 2008, Updated Technical Report on the NorthMet Deposit, Minnesota, USA, by Wardrop Engineering, (Toronto) for PolyMet, 109 p. 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Graber, R., 1993, LTV Steel Mining Company, (abst.), Institute on Lake Super Geology, Geology and Taconite Mines of the Mesabi Range, Field Trip Guide, pages 39-45. Hauck, S.A., Severson, M.S., Zanko, L.M., Barnes, S.-J., Morton, P., Aliminas, H.V., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 137-185. Holst, T.B., Mullenmeister, E.E., Chandler, V.W., Green, J.C., and Weiblen, P.W., 1986, Relationship of structural geology to the Duluth Complex to economic mineralization: Minnesota Department of Natural Resources, Division of Minerals Report 241-2. 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Lehr, J.D., and Hobbs, H.C., 1992, Glacial geology of the Laurentian divide area, St. Louis and Lake Counties, Minnesota: Minnesota Geological Survey Field Trip guidebook series, 18, Prepared for the 39th Midwest Friends of the Pleistocene Field Trip, Biwabik, Minnesota, 1992, 73 p., map, 1:250,000. Listerud, W.H., and Meineke, D.G.., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74p. Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and Sedimentology of the Lower Proterozoic Virginia Formation, northern Minnesota: Minnesota Geological Survey Report of Investigations 28, 28 p. Mancuso, J.D., and Dolence, J.D., 1970, Structure of the Duluth Gabbro Complex in the Babbitt area, Minnesota (abs.): 16th Annual Institute on Lake Superior Geology, Thunder Bay, Ontario, p. 27. Marma, J.C., Brown, P.E., and Hauck, S.A., 2002, Magmatic and hydrothermal PGE mineralization of the Birch Lake Cu-Ni-PGE deposit in the South Kawishiwi intrusion, Duluth Complex, northeast Minnesota: Paper No. 52-2 presented at the 2002 Annual GSA meeting, Denver, October 27-30, 2002. 77 �Martineau, M. P., 1989, Empirically derived controls on Cu-Ni mineralization: A comparison between fertile and barren gabbros in the Duluth Complex, Minnesota, U.S.A.: in Prendergast, M. D., and Jones, M. J., eds., Magmatic Sulphides - The Zimbabwe Volume: Inst. Min. Metall., London, p. 117-137. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota; Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr. and Severson, M.J., 2002, Geology of the Duluth Complex in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 106-143. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, 207 p. Miller, J.D., Jr., Severson, M.J., and Foose, M.P., 2002, Geologic map of the Babbitt NE 7.5’ quadrangle, Lake and St. Louis Counties, Minnesota. Minnesota Geological Survey Open-file Map, 1:24,000 scale. Muhich, T.G., 1993, Movement of titanium across the Duluth Complex–Biwabik Iron Formation contact at Dunka Pit, Mesabi Iron Range, northeastern Minnesota: Duluth, Minn., University of Minnesota Duluth, M.S. thesis, 154 p. Naldrett, A.J., 2004, Magmatic Sulphide Deposits: Geology, geochemistry, and exploration: Springer-Verlag, Berlin, Germany, 727 pages, ISBN 3–540–22317–7. Park, Y.R., Ripley, E.M., Severson, M.J., and Hauck, S.A., 1999, Stable isotopic studies of mafic sills and Proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota: Geochimica et Cosmochimica Acta, v. 63, no. 5, p. 657-674. Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-2003/21, 96 p. Patelke, R.L., and Geerts, S.D.M., 2006, PolyMet NorthMet Drill Hole / Geological Database Recompilation: Location, Downhole Survey, Assay, Lithology, Geotechnical Data, and Related Information, 2004 to 2006, PolyMet Mining Inc. Internal report, 86 p. Peterson, D.M. and Albers, P.B., 2007, South Kawishiwi Intrusion Cu-Ni-PGE mineralization in association with the Nickel Lake Macrodike, Institute on Lake Superior Geology, 53rd Annual Meeting, Field Trip Guidebook, Lutsen, Minnesota, Volume 53. Peterson, D.M., 2001a, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Peterson, D.M., 2001b, Copper-Nickel-PGE mineral potential of the eastward extension of the Maturi Cu-Ni deposit, Duluth Complex, Lake County, Minnesota; Natural Resources Research Institute, Confidential Report of Investigations NRRI/RI-2001-02, 29 pages, 15 plates, 1 CD-rom. Peterson, D.M. and Severson, M.J., 2002, Chapter 4, Archean and Paleoproterozoic rocks forming the footwall of the Duluth Complex, in Geology and mineral potential of the Duluth Complex and related intrusions of northeastern Minnesota, Minnesota Geological Survey, Report of Investigations 58, pp. 76-93. Peterson, D.M., 2008, Bedrock geologic map of the Duluth Complex in the northern South Kawishiwi intrusion and surrounding area, Lake and St. Louis Counties, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2008-01, scale 1:20,000. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock geology of the Nickel Lake Macrodike and adjacent areas: Lake County, Northeastern Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. 78 �Phinney, W.C., 1972, Duluth Complex, history and nomenclature, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minn. Geol. Survey, pp. 333-334. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones – a dilemma: Economic Geology, v. 81, p. 974-978. Routledge, R.E., 2004, Review of the mineral resources of the Birch Lake Property, Minnesota, U.S.A. prepared for Franconia Minerals Corporation: Roscoe Postle Associates Inc. NI 43-101 technical report filed on SEDAR, January 22, 2004, 92 p. Sabelin, T., and Iwasaki, I., 1985, Metallurgical evaluation of chromium-bearing drill core samples from the Duluth Complex (Contract report of Minnesota Department of Natural Resources, Division of Minerals): Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 58 p. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Internal report, Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 23 p. Severson, M.J., 1988, Geology and structure of a portion of the Partridge River Intrusion, northeastern Minnesota: Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Rept., NRRI/GMIN-TR-88-08, 78 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., and Barnes, R.J., 1991, Geology, mineralization and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part II: Mineralization and geostatistics: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-91/13b, 216 p. Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Severson, M.J., and Hauck, S.A., 2003, Platinum group elements (PGEs) and platinum group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2003/37, 296 p. Severson, M.J., and Hauck, S.A., 2008, Finish logging of Duluth Complex drill core (And a reinterpretation of the geology at the Mesaba (Babbitt) deposit): University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2008/17, 68 p. Severson, M.J., and Miller, J.D., Jr., 1999, Bedrock geologic map of Allen quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-91, scale 1:24,000 Severson, M.J., Miller, J.D., Jr., Peterson, D.M., Green, J.C., and Hauck, S.A., 2002, Mineral potential of the Duluth Complex and related intrusions in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 164-200. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994a, The Babbitt copper-nickel deposit, Part B: Structural datums: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-94/21b, 48 p. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994b, The Babbitt copper-nickel deposit, Part C: Igneous geology, footwall lithologies, and cross-sections: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR-94/21c, 79 p. Sims, P.K., and Morey, G.B., eds. 1972, Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey. Tharalson, E., Sweet, G., Boisjoli, T., Lentz, B., Fellows, T., and Peterson, D., 2007, Geological Map of the Nickel Lake Macrodike and Northern Bald Eagle Intrusion, Lake County, Northeastern Minnesota: Precambrian Research Center Map Series PRC/MAP-2007-01, scale 1:10,000. 79 �Turner, F.J., 1968, Metamorphic Petrology – mineralogical and field aspects: McGraw-Hill Book Company, New York, 403 p. Watowich, S.N., 1978, A preliminary geological view of the Minnamax copper-nickel deposit in the Duluth Gabbro at the Minnamax project: in Graven, L.K., compiler, Productivity in Lake Superior Mining, 39th Proc., Annual Mining Symposium, University of Minnesota, Minneapolis, p. 19.1-19.11. White, C., in prep, The Nokomis Deposit, a Masters of Geology thesis: University of Minnesota, Duluth. Wolff, J.F., 1917, Recent geologic developments on the Mesabi range, Minnesota: American Institute of Mining and Metallurgical Engineers Transactions, v. 56, p. 142-169. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/GMIN-TR-93-52, 90p. 80 �55th Annual Institute on Lake Superior Geology Field Trip 2 GLACIAL GEOLOGY OF THE VERMILION MORAINE 3D interpretation of the location of the Laurentide ice sheet in the Soudan Mine area at the time of the development of the Vermilion Moraine (modified from Peterson and Patelke, 2003). Phil Larson (Cliffs Natural Resources) Howard Mooers (Department of Geological Sciences, UMD) 81 �FIELD TRIP 2 GLACIAL GEOLOGY OF THE VERMILION GREENSTONE BELT Phillip C. Larson (Cliffs Natural Resources) Howard D. Mooers (Department of Geological Sciences, University of Minnesota-Duluth) INTRODUCTION This field trip provides an introduction to the glacial geology and glacigenic sedimentary environments of the Vermilion district of northeastern Minnesota. The bedrock geology and topography of this area are characteristic of millions of square km of glaciated Precambrian shield terranes in Canada, Scandinavia, and the United States. Overlying bedrock in the belt is an assemblage of glacigenic sediments characteristic of a great portion of the glaciated shields on earth. In recent years, these characteristics have allowed the glaciated Vermilion greenstone belt to serve as a virtual laboratory for investigating the nature and scale glacial erosion and entrainment, and transport processes. Metavolcanic and metasedimentary rocks of the Vermilion district are also notably prospective for volcanogenic massive sulphide and shear-hosted lode gold deposits (Peterson and Jirsa 1999; Peterson, 2001; Peterson and Patelke 2004). Mineralized clasts and geochemical anomalies related to such deposits are evident in glacigenic deposits overlying and down-ice of the greenstone belt. The presence of anomalous mineralization, combined with the well-described nature of glacigenic sediment dispersal, in the district provides a unique opportunity to demonstrate the techniques by which till geochemical and mineralogical data can be incorporated into an integrated mineral exploration program. This field trip visits a selection of sites that illustrate the range of glacigenic sediment types present in this typical glaciated shield terrane, as well as their compositional heterogeneity. A second theme of the trip is the collection and manipulation of the types of data useful to mineral exploration in glaciated terranes. REGIONAL BEDROCK GEOLOGY (The following overview of regional bedrock geology is largely taken from Peterson, Jirsa, and Hudak (2009), included in this guidebook.) Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield (Fig. 2-1). Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The two belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. The boundary between these contrasting structural panels occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. 82 �To the south, supracrustal rocks of the Soudan belt are intruded by syn- to post-tectonic granitoid rocks of the Giants Range batholith. At the northern edge of the Vermilion district, the Vermilion fault marks the boundary with granite, granite-rich migmatites, migmatites, paragneisses, and biotite schists of the Quetico subprovince of the Superior Province of the Canadian Shield (Jirsa and Boerboom, 2003). Figure 2-1. Bedrock geology of the Vermilion district, modified from Morey and Meints, 2000. GLACIAL HISTORY OF THE VERMILION DISTRICT In common with most of the high latitude regions of North America, the Vermilion district was repeatedly glaciated during the ice ages of the Pleistocene Epoch. Glacigenic sediments and landforms in the Vermilion district are associated with the Rainy Lobe of the Laurentide ice sheet. While there are a number of possible definitions of what constitutes the Rainy Lobe – sedimentological, textural, compositional, and association with particular geomorphic features – a definition rooted in glacial dynamics perhaps works best. In this sense, the Rainy Lobe refers to that portion of the Laurentide ice sheet lying northwest of Lake Superior (occupied by the Superior Lobe), and east of the Winnipeg Basin and Red River Valley (occupied by the Red River Lobe). In common, Rainy Lobe landforms and glacigenic sediments reflect a general northeast to southwest ice flow direction, and a Labradoran (northeastern) sediment provenance. Preglacial Environment In common with much the Canadian Shield, glacial erosion has nearly completely stripped preglacial regolith from bedrock in the Vermilion district. However, preglacial saprolites are a common occurrence 83 �underlying glacigenic sediments in central and western Minnesota; the nearest such occurrences are exposed in open pit mines of the Mesabi Range, on the south flank of the Giant’s Range. Massive iron oxide mineralization exposed in surface workings of the Consolidated Vermilion mine (near Stops 2-6 and 2-7) contains voids lined by botryoidal goethite. The botryoidal goethite occurs as dripstone-like structures, suggesting that it precipitated in open spaces in the vadose zone. Inasmuch as the water table of the preglacial peneplained Canadian Shield was likely similar to the modern, this occurrence supports the hypothesis that little erosion of unweathered bedrock was accomplished by multiple glaciations through the course of the Pleistocene. The topography observed in the Vermilion district today therefore likely represents that of the base of the preglacial weathering regolith. Pre-Late Wisconsinan Throughout the field trip area, no till or glaciofluvial sediments older than those deposited during the Late Wisconsinan retreat of the Laurentide ice sheet have been described. However, circumstantial evidence of the past existence of pre-Late Wisconsinan glacigenic sediments are present, and pre-Late Wisconsinan glacigenic sediments are preserved a few 10s of km to the south. Winter (1971) and Winter and others (1973) described a dark-colored, sandy-silty calcareous till in exposures in open pit mines on the Mesabi Iron Range. Inasmuch as this till, where present, occurred immediately above bedrock, they referred to it as the “basal till”. Stark (1977) and Lehr and Hobbs (1992) described occurrences of Winter’s basal till in exposures in the Dunka Mine, approximately 20 km south of the area encompassed by the current field trip. The matrix of Winter’s basal till is calcareous, and the pebble fraction contains carbonate clasts in addition to the granitic and metamorphic lithologies typical of Rainy Lobe tills. A northeast-southwest pebble fabric in Winter’s basal till strongly supports a northeastern provenance for this till. In this case, carbonate in pebbles and till matrix is derived from Paleozoic carbonates in the Hudson Bay Lowlands. Surficial Rainy Lobe tills in the region are typically non-calcareous, leading some workers to suggest that calcareous tills in the region must have a Keewatin (northwestern) provenance. However, it is more likely that calcareous Labradoran (northeastern) provenance tills are the norm in northeastern Minnesota, with the relatively thin, noncalcareous tills deposited during the Late Wisconsin the exception (Mooers and Lehr 1997, Larson and Mooers 2005, Larson and Mooers 2008). Mooers and Lehr (1997) and Larson and Mooers (2008) noted the closed associated between carbonate and distinctive clasts of Paleoproterozoic greywacke in HBL provenance tills. In many cases, the greywacke clasts are able to survive englacial comminution and weathering better than the carbonates, and only greywacke clasts remain. The presence of greywacke clasts in till is therefore strong evidence that the material forming the till was once calcareous. The presence of calcareous Labradoran provenance tills to the southwest (Mooers and Lehr 1997), and to the northeast (northwestern Ontario) (Karrow and Geddes 1987), suggests that a continuous sheet of calcareous till once extended across the Vermilion greenstone belt. However, by Late Wisconsinan time, much of this calcareous till had been remobilized and removed from the Canadian Shield, including the field trip area (Larson and Mooers 2008). Late Wisconsinan By no earlier than 15 ka, the Rainy and Superior Lobes began to retreat from the St. Croix Moraine in central Minnesota (Clayton and Moran 1982). By the time the St. Louis Sublobe advanced from the northwest into the basins of Glacial Lake Upham I and Aitkin I, the southern margin of the LIS was roughly coincident with the Giant’s Range (Fig. 2-2). 84 �Figure 2-2. Surficial geology of the Vermilion district. Prior to retreating to the Giant’s Range and the Laurentian divide, the topography generally sloped away from the margin of the LIS. Consequently, meltwater tended to be efficiently channeled away from the ice margin. However, once the margin retreated to the north of the height of land, meltwater ponded in front of the ice sheet, forming proglacial lakes, with the surface elevation of these lakes controlled by the elevation of the outlet spilling over the height of land. The first proglacial lake formed north of the Giant’s Range – Glacial Lake Norwood – was the first in a succession of lakes and lake stages that characterized the margin of the LIS until the final drainage of Glacial Lake Ojibwe. The retreat of the LIS margin north of the height-of-land marked a momentous change in the nature of sediment and landforms associated with the ice margin. South of the height-of-land, ice margin sedimentlandform assemblages are dominantly hummocky moraines composed of till and outwash channels, fans, and plains composed of glaciofluvial sediment. North of the height-of-land, ice margin sedimentlandform assemblages are dominantly sharp-crested moraines composed of glaciofluvial sediments and proglacial plains composed of glaciolacustrine sediment. Glacial Lake Norwood Once the margin of the LIS retreated north of the Giant’s Range, meltwater ponded to form Glacial Lake Norwood (GLN) (Winchell 1901), which drained to the south through the Embarrass Gap. The initial level of GLN was approximately 1475 feet, indicating that the area south and west of Big Rice Lake was 85 �still occupied by ice (Lehr and Hobbs 1992). This stage of the lake likely corresponds to the ice marginal position marked by the Big Rice moraine. Retreat of the LIS from the Big Rice moraine to the Wahlsten moraine roughly coincided with a drop in the level of GLN to 1450 feet (Lehr and Hobbs 1992). The drop in lake level was triggered in part by downcutting of the outlet through the Embarrass Gap. Further downcutting of the Embarrass Gap outlet was prevented since the outlet was draining into Glacial Lake Upham II at the time, also at roughly 1450’ elevation. Continued outflow through the Embarrass Gap indicates the presence of glacial ice in the area north of Nashwauk and Hibbing. GLN persisted at the 1450 foot level during the retreat and re-advance of the LIS to the Vermilion moraine. As ice and debris covered ice melted north of the Giant’s Range the lake expanded to the west. This larger lake is referred to as Glacial Lake Koochiching (GLK) (Hobbs 1983), in reference to its being fronted by Keewatin (northwestern) provenance ice in southern Koochiching County. The highest level of GLK (Mizpah stage) formed strandlines at 1430 feet elevation as far west as southwestern Koochiching County (Hobbs 1983). Lowering of the level of Glacial Lake Upham II allowed continued downcutting of the Embarrass Gap outlet to 1430 feet, and later to 1400 feet (Lehr and Hobbs 1992). Melting of stagnant ice and debris-cored ice in northeastern Itasca County ultimately opened a new outlet to GLN/GLK southward through the Prairie River. Opening and downcutting of the Prairie River outlet rapidly lowered GLK to about 1350 feet elevation (Gemmell stage) (Hobbs 1983). The LIS probably did not retreat from the Vermilion moraine prior to opening of the Prairie River outlet, as glaciolacustrine sediments are not found north of the moraine. Persistence of a proglacial lake draining through the Embarrass Gap resulted in much of the area between the Vermilion moraine and the Giant’s Range being wave-washed, or mantled by glaciolacustrine sediment. Big Rice moraine The first moraine to the north of the Giant’s Range is the Big Rice moraine. The moraine orientation is roughly east-west, indicating ice flow was from north to south (180°). The LIS margin was for the most part subaerial while this moraine formed, as GLN was of limited extent. Consequently, most of this moraine is comprised of hummocky meltout till. Björck (1990) radiocarbon dated basal organic sediment from Heikkila Lake at 12,100±150 ka (LU-2556) (14.1±0.5 ka cal yr bp). Heikkila Lake is a kettle lake hosted in the moraine, so this date represents a minimum age on moraine stabilization. Wahlsten moraine The second moraine to the north of the Giant’s Range is the Wahlsten moraine. Similar to the Big Rice moraine, the Wahlsten moraine’s orientation is roughly east-west, indicating continued ice flow to the south (180°). The Wahlsten moraine is generally lower in elevation is lower than the Big Rice, so a significantly greater portion of the LIS margin fronted GLN. To the west, where the ice margin fronted the lake, the moraine is composed of coalescing ice-contact deltas. Here the moraine has a sharp, symmetrical cross sectional profile, and is composed predominantly of glaciofluvial sands and gravels. To the east, the moraine is composed of hummocky meltout till, fronted by proglacial outwash fans. Basal organic radiocarbon dates from Lempia Lake, a kettle lake immediately south of the moraine, are 12,050±240 ka (Lu-2555) and 11,550±550 ka (Lu-2502) (14.1±0.7 ka cal yr bp) (Björck 1990). As with the Heikkila Lake date, this like represents a minimum age of landscape stabilization. Vermilion moraine The Vermilion moraine is the third prominent moraine to the north of the Giant’s Range. Unlike the Big Rice and Wahlsten moraines, Vermilion moraine orientation is roughly west-northwest to east-southeast. 86 �This change in moraine orientation is reflected in a change in ice flow direction, to approximately 210°. Reorientation of the ice flow direction was possibly triggered in part by faster recession of the LIS from the Wahlsten moraine in the area fronted by GLN. Asymmetric retreat and re-advance to the Vermilion moraine resulted in a slight inflection in the ice margin orientation in the vicinity of Bear Head Lake. This inflection in turn produced a slight trough in the surface of the LIS, extending in the up-ice direction from Bear Head Lake, including past the general vicinity of Ely. This slight trough in the LIS resulted in enormous volumes of surface meltwater and glacial debris, captured from a disproportionately large area of the ice sheet, being channeled to a single discharge point. A large sediment fan, composed of sandy, pebbly, cobble, and boulder gravel was deposited in front of the Vermilion moraine (Lehr and Hobbs 1992), in part by evacuating stagnant ice north of the Wahlsten moraine, and a segment of the Wahlsten moraine itself. West of Soudan, the LIS margin fronted GLN, and the moraine is composed predominantly of coalescing ice-contact deltas. In this area, the moraine has a sharp, symmetrical cross-sectional profile., and is composed primarily of glaciofluvial sands and gravels. East of Soudan, the LIS margin was subaerial, and the moraine is composed of hummocky meltout till and associated small, proglacial outwash fans. The age of the Vermilion moraine is poorly constrained, however a basal radiocarbon date on woody material from a kettle lake in the Vermilion moraine southeast of Ely is 12,000±85 ka (ETH-29845) (13.8± 0.1 ka cal yr bp) (Lowell and others, in press), placing a minimum age on moraine stabilization. The next prominent moraine set formed by the LIS after the Vermilion greenstone belt are the EagleFinalyson-Steep Rock moraines of northwestern Ontario, 100 km to the northeast (Zoltai 1965). Björck (1985) dated obtained a bulk sediment radiocarbon date of 11,100±110 ka (WIS-1375) (13.0±0.2 ka cal yr BP) from a lake on the down-ice of these moraines, suggesting the area to the north of the Vermilion moraine was deglaciated by that time. GLACIGENIC SEDIMENT IN THE VERMILION DISTRICT In the Vermilion district, Rainy Lobe tills are pebble- and boulder-rich, with a sandy matrix. The lithologic and geochemical composition of tills and associated glaciofluvial sediments is highly heterogeneous, reflecting the similarly heterogeneous regional bedrock composition. Tills generally lack a significant component of clay-size grains, or clay minerals. The coarse-grained nature of tills, and lack of clay-size material, result in high permeability and poor compaction – even in lodgment tills. Spatially, the distribution of glacigenic sediment is quite variable, with till, glaciofluvial, and even glaciolacustrine sediment widely distributed through the region. Sediment cover is patchy, with abundant bedrock outcrop, particularly in the areas north of the Vermilion moraine and above the level of Glacial Lake Norwood. Although glacigenic sediment may locally be several meters thick, it typically averages ~1 m. Rainy Lobe provenance tills in the Vermilion district are essentially non-calcareous, and have a very low acid buffering capacity. This, combined with their high permeability, results in deep weathering, particularly of labile sulphide minerals. The spruce, fir, and pine forests common to the area favor formation of podzolic soils. Most soil profiles evidence an Of horizon overlying a bleached Ae horizon. Even the thickest soil profiles in till lack a true C horizon, with most displaying weak iron oxide staining (presumably due to weathering of iron sulphide minerals). In many cases, a C horizon is completely absent, with a vibrant red Bf horizon resting directly on bedrock. When sampling till for geochemical 87 �analysis, the weathered C horizon is the preferred sampling media. Where the C horizon is absent or inaccessible, care is taken to sample the Bf horizon. Erosion rates Larson and Mooers (2004) estimated a bedrock erosion and entrainment rate of ~10 mm·a-1 during the period the Laurentide ice sheet was actively eroding, entraining, and transporting sediment to the Vermilion moraine, a rate comparable to that of modern temperate glaciers in Alaska (Hallet et al. 2006). Assuming active erosion and entrainment occurred at that rate over a modest mean flowline length of 10 km, as much as 300 tons of debris per meter length per year were delivered to the Vermilion moraine. This debris delivery rate is sufficient to construct the observed Vermilion moraine in as little as 100 years. Clearly, such high erosion and entrainment, and sediment delivery rates could not have acted over either a large area of the ice sheet’s bed, or over a long period of time. This provides strong support to the hypothesis that till and glaciofluvial sediment deposits and landforms associated with the Vermilion moraine formed over a relatively short period of the history of the Laurentide ice sheet – likely only a few centuries. In this sense, surficial sediments in the VGB and elsewhere on glaciated shield terranes represent a snapshot in time in the long history of both the Pleistocene CHARACTERIZATION OF GLACIGENIC SEDIMENTS FOR DRIFT EXPLORATION Incorporation of geochemical and mineralogical data collected from glacigenic sediments into an integrated mineral exploration program is a key to successful exploration in glaciated terranes. In common with surficial sediment sampling programs in other geomorphic environments, the three critical questions an explorer seeks to answer over the course of a drift exploration program are: • • • What is the significance of a particular anomaly? How far has the material been transported? What direction was the material transported? The answers to these questions can be more systematically determined in a glaciated terrane than in any other surficial environment. In this sense glaciation, far from obscuring bedrock mineralization, provides an important tool for detecting and vectoring to bedrock mineralization. Mean transport length Till in the VGB is characterized by extreme compositional variability, reflecting the composition of local bedrock. A useful concept for quantifying this variability is the concept of a mean transport length – the distance from within which 50% of the material in the till has been derived. Mean transport length has been characterized for the VGB on two different size fractions of till. This is accomplished by plotting the concentration of a till component derived from a known source (indicator) against the distance along a glacial flow line down-ice of the source. In the case of the VGB, the extreme contrast in composition between granitoid and high-grade metamorphic rocks of the Vermilion Batholith to the north of the Vermilion fault, and the greenschist grade metavolcanic and metasedimentary rocks of the VGB to the south of the Vermilion fault provides a widespread, but easily recognizable, indicator in the form of the lithic and geochemical components derived from the Vermilion Batholith. In particular, potassium feldspar-bearing coarse-grained granitoid clasts in tills from the VGB are uniquely derived from north of the Vermilion fault. In addition, rocks of the Vermilion Batholith are characterized by relatively high potassium content (~3.4% K2O) relative to those of the VGB (~1.7% K2O). The concentration of both indicators in till decrease systematically with distance from the Vermilion fault; this decrease can be mathematically approximated using the equation: 88 �ciT >0 = ciT =0 e − aT , (1) Where ci is the indicator concentration, T is the transport length, and a is a dimensionless constant. Solving for T at the point at which ciT>0 = 0.5ciT=0 gives an estimate of the transport length from within which 50% of the material in the till is derived. In the case of K2O in the -63 micron fraction of till, this relationship is: K 2 O = 1.73 + 1.66e −0.225T , (2) where T is transport length in km, indicating a mean transport length for the -63 micron fraction of ~3.0 km (Fig. 2-3) (Larson 2004). (The factor 1.73 is the apparent mean K2O concentration in weight percent of rocks of the greenstone belt.) 4.5 4.0 K2O (wt %) 3.5 3.0 2.5 2.0 1.5 1.0 -2000 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Transport Length (m) Figure 2-3. K2O concentration in the -63 micron fraction of tills in the VGB, plotted against distance in the ice-flow direction from the Vermilion fault. (Larson 2004.) 0.7 y = 0.592e-0.270x Granitoid fraction 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -2 0 2 4 6 8 10 12 14 Transport Length (km) Figure 2-4. Granitoid concentration in the 2-4 mm fraction of tills in the VGB, plotted against distance in the iceflow direction from the Vermilion fault. (unpublished data from 2008 PRC Field Camp students) 89 �In the case of granitoid clasts in the 2-4 mm fraction of till: c gran = 0.592e −0.270T , (3) where the background granitoid fraction of tills in the greenstone belt is 0.0, indicating a mean transport length for 2-4 mm pebbles of ~2.6 km (Fig. 2-4) (unpublished data from 2008 PRC field camp students). These two independently derived estimates provide a robust assessment of the mean transport length of till-forming material in the Vermilion district. Ice flow direction Measurements of striae and grooves in bedrock indicate preservation of a number of predominant orientations, corresponding to a number of paleo ice flow directions. Between the Vermilion and Wahlsten moraines, striae cluster between 180-185°, roughly normal to the east-west orientation of the Wahlsten moraine. Immediately south and to the north of the Vermilion moraine, striae cluster between 200-225°, with a predominant mode between 210-215°; this orientation is roughly normal to the WNW to ESE orientation of the Vermilion moraine. Scattered throughout the area, a very few striae have been measured oriented ~240°; where present, these striae orientations are overprinted by the 180-185° and 210-215° striae sets (Fig. 2-5). Figure 2-5. Rose diagram of ice flow indicator (striae and grooves) orientations in the Vermilion district. The multiple ice flow directions recorded in striae and moraine orientations are interpreted to record changing ice flow directions during retreat of the Rainy Lobe. The older 240° orientation is interpreted to represent ice flow direction when the area was completely covered by the Laurentide ice sheet, and the ice margin was located a significant distance to the southwest. The 180° orientation reflects the ice flow direction when the ice margin was located at the Big Rice and Wahlsten moraines, while the 210° orientation reflects the ice flow direction during the readvance to the Vermilion moraine. North of the Vermilion moraine, striae in the 200-225° cluster varies. At a given outcrop, or cluster of outcrops, individual striae orientations typically vary over a range of ~5°. However, over length scales of ~1 km, striae cluster orientations may vary up to ~20°. This may be the product of spatial and temporal variation in erosion rates as ice flow direction changed during retreat and re-advance of the Rainy Lobe to the Vermilion moraine. 90 �Mineral endowment Considered at a sufficiently large scale, the composition of till broadly reflects that of the substrate underlying the ice sheet. In the case of a system where mean transport length is relatively short, till composition reflects that of the local bedrock. A sufficiently large sample of till compositional data from such an area therefore represents a statistically meaningful characterization of bedrock composition. In addition to providing important information regarding threshold values for background and anomalous concentrations, comparison of a sample population from a particular area to a reference population may provide information regarding the relative endowment of elements of economic interest. Rencz and others (2006) present a useful compendium of the statistical characteristics of a number of geochemical variables for the -63 micron fraction of tills in Canada. They included in excess of 13,000 till samples compiled from over 50 separate surveys. Bajc (1999, 2000) collected nearly 700 -63 micron till geochemical samples from the Shebandowan greenstone belt, a Wawa subprovince supracrustal sequence located 150 km northeast of the Vermilion district. Larson (2004) reported 150 -63 micron till geochemical analyses collected in the Vermilion district. Comparison of these three data sets provides an assessment of the relative thresholds for background and anomalous geochemical values of tills in the Vermilion district. 1.00 Percentile rank 0.95 0.90 Shebandow an Vermilion 0.85 Canada 0.80 0.75 10 100 1000 Ni (ppm ) -63 m icron till Figure 2-6. Relative abundance of nickel in the -63 micron fraction of till. Data from Rencz and others (2006), Bajc (1999,2000), and Larson (2004). Nickel displays high abundance in the fine fraction of Vermilion district tills relative to both the larger Canadian data set (Rencz et al., 2006) and the Shebandowan greenstone belt (Bajc 1999, 2000) (Fig. 2-6). However, in both the Canadian and Shebandowan datasets, the 98th percentile values are higher than in the Vermilion dataset. This suggests that while tills in the Vermilion district are characterized by high background nickel values, no truly anomalous samples indicative of potential economic nickel mineralization have been collected. Copper displays a slightly elevated abundance in the fine fraction of Vermilion district tills relative to both the larger Canadian data set (Rencz et al., 2006) and the Shebandowan greenstone belt (Bajc 1999, 2000) (Fig. 2-7). In both the Canadian and Shebandowan datasets, only the maximum values are higher than in the Vermilion dataset. This suggests that tills in both the Vermilion district and Shebandowan belt are characterized by similar background values somewhat higher than Canada as a whole. However, the highest copper values in the Vermilion district are lower than both other data sets, suggesting somewhat lower potential for economic copper mineralization than in the Shebandowan belt, or Canada as a whole. 91 �Zinc displays high abundance in the fine fraction of Vermilion district tills relative to both the larger Canadian data set (Rencz et al., 2006) and the Shebandowan greenstone belt (Bajc 1999, 2000) (Fig. 2-7). In both the Canadian and Shebandowan datasets, only the maximum values are higher than in the Vermilion district. This suggests that tills in the Vermilion district belt are characterized by background zinc values higher than the Shebandowan belt and Canada as a whole. The significantly higher 98th percentile zinc values in the Vermilion district are significantly higher than for both other data sets, suggesting higher potential for economic zinc mineralization than in the Shebandowan belt, or Canada as a whole. Figure 2-7. Relative abundance of copper (left) and zinc (right) in the -63 micron fraction of till. Data from Rencz and others (2006), Bajc (1999,2000), and Larson (2004). Integrating ice flow directions, mean transport length, and geochemical data: The Probability Window Integration of the three parameters can serve as a powerful tool for vectoring to economic mineralization in glaciated terrane. When a mineralized clast or anomalous geochemical sample is found in till or other glacigenic sediment, the only statement that can be said with any confidence about its origin is that it did not come from bedrock underlying the sample site. However, careful observation of ice flow direction indicators, calculation of mean transport length, and assessment of the significance of anomalies can be used to constrain the potential source to a probability window. The probability window (a polygon) is constructed by extending side lines in the general up-ice direction of a sample location. The exact orientations of the lines are opposite the limits of the potential range of ice flow directions determined for the area in question. The up-ice limit to the probability window is imposed by constructing a third line at a distance equal to the mean transport length. The polygon thus defined represents an area within which there is theoretically a 50% probability of the anomalous sample originating (Fig. 2-8). Clearly, given the inherently heterogeneous nature of till composition, relatively low confidence can be placed any single such determination of a potential source area for an anomalous sample. However, multiple probability windows calculated from multiple anomalous samples, augmented by iterative sampling, offer to provide increasing confidence in identifying potential source areas. This information, when integrated with geologic and geophysical data, can provide a powerful tool for vectoring toward economic mineralization in glaciated terrane. 92 �Figure 2-8. Illustration of the use of probability windows to constrain the potential source area of mineralized clasts or anomalous till geochemical samples(data from Larson 2004). FIELD TRIP STOPS Note: All location coordinates are given as UTM Zone 15, NAD 83 datum STOP 2-1. Hidden Valley Recreation Area Location: 588490E, 5306530N Ice-contact glaciofluvial deposits. The Hidden Valley exposure reveals poorly sorted coarse gravel glaciofluvial sediments of the Rainy lobe. In this vicinity, the lithologic composition of the sediment reflects a predominance of granitoid rocks eroded from the Quetico subprovince of the Canadian Shield. This deposit is part of a broad zone of voluminous glaciofluvial deposits extending northeast from Bear Head Lake, likely deposited in a subtle trough in the surface of the ice sheet extending up-ice of an inflection point in the orientation of the ice margin. The enormous quantity of glaciofluvial sediment deposited on this trend is a reflection of the enormous surface meltwater discharge captured by the trough in the ice sheet. STOP 2-2. Wolf Creek Road gravel pit Location: 570310E, 5304700N Ice-contact glaciofluvial deposits. The Wolf Creek gravel pit reveals 40 feet of glaciofluvial sediment deposited subglacially as an esker. On the flanks of the esker, a stony diamict is exposed; this may be a till deposited by the LIS during the re-advance to the Vermilion moraine. This locality lies essentially on the Vermilion fault, and the 93 �lithologic composition of the sediment is overwhelmingly dominated by granitoid and high-grade metasedimentary rocks of the Vermilion Granitic Complex. This lithologic composition represents the composition of sediment in transport by glacier ice as the Rainy Lobe begins flowing across greenschistgrade metasedimentary and metavolcanic rocks of the greenstone belt. The concentration of greenstone lithologies in tills down-ice of this locality provides a quantitative assessment of the rate at which greenstone rocks are eroded and entrained by the Rainy Lobe. STOP 2-3. Vermilion moraine (subaqueous segment) In this vicinity, the Vermilion moraine is characterized by a proximal (northeast) ice-contact scarp and a distal subaqueously-deposited outwash apron. The crest of moraine is generally ~1450’ elevation, corresponding to the level of Glacial Lake Norwood. The bulk of the moraine at this location is composed of well-sorted sand and gravel, suggesting much of the sediment was deposited in water, either directly from supra- and englacial meltwater, or from resedimented ice-contact debris. Pinching and swelling of the moraine along it length reflects the spacing of meltwater drainage channels on the ice surface. Locally, where the crest of the moraine exceeds 1450’ elevation, sediment at the moraine crest is composed of diamict, presumably meltout till, formed by release of englacial debris on the surface of the ice sheet. This suggests as significant fraction of the sediment delivered to the ice margin was englacial debris. STOP 2-3A. Forest Lane Resort Location: 548090E, 5298730N Ice-proximal scarp of moraine. The area on the up-ice side of the scarp is characterized by an abundance of large boulders. Large boulders are common in deposits of the Rainy Lobe, and result from the isotropic nature of the granitoid bedrock and widely spaced fractures, which favors glacial quarrying of large boulders. The boulders were of sufficient size that meltwater draining from the ice surface was unable to transport them beyond the ice margin. STOP 2-3B. Holmes Excavating Gravel Pit Location: 547250E, 5298480N Ice-distal slope of moraine. At this locality, the Vermilion moraine is composed of subaqueously-deposited sand and gravel representing a system of prograding deltas along the ice margin. Sediment size range from silt to coarse sand and gravel indicating a variety of depositional energies, and the sediments are often highly deformed. The deformation suggests that the deposition was rapid. Loading was apparently fast enough that finer-grained sediments were not efficiently dewatered whereas coarser sediments dewatered effectively. The strength of soft sediment is a function of water content; consequently, deformation of the coarser gravel lenses tends to be brittle in character (Fig. 2-9), while in the finer grained sediment deformation tends to be ductile in character. Deformation of the sediments ranges from small-scale faults and folds to large-scale (10s of meters) folds and detachment structures (Fig. 2-10). 94 �Figure 2-9. View of brittle deformation features in sand and gravel of Vermilion moraine, Holmes Excavating Gravel Pit. (photo from fall of 2002) Note that the coarser gravel beds behave as rigid blocks, while the interbedded silts and sands deform in a ductile fashion. Figure 2-10. View of nappe structure and recumbent fold nose in sand and gravel of Vermilion moraine, Holmes Excavating Gravel Pit. (photo from fall of 2002). 95 �STOP 2-4. Breitung Township gravel pit Location: 558020E, 5295840N Vermilion moraine at strandline of Glacial Lake Norwood. At this location on the Vermilion moraine, the margin of the ice sheet was positioned roughly at the level of Glacial Lake Norwood. Rather than building a steep-crested moraine, sediment delivered to the margin by meltwater formed a prograding flat-topped fan delta. Visible in the walls of the pit are channel scourand-fill structures. Not coincidentally, this particular fan delta is located at the discharge point of an esker system traceable for several km up-ice (see Stop 2-8). STOP 2-5. Murray Road Location: 562380E, 5294540N Vermilion moraine (subaerial segment) At this location on the Vermilion moraine, the ice margin was position well above the level of Glacial Lake Norwood, and a relatively steep subaerial moraine formed. Here the moraine is composed of a complex association of sand, gravel, and meltout till. Large boulders are common in the moraine. One boulder in particular is a felsic tuff, correlative with felsic Metavolcanic bedrock mapped only a few km up-ice. On subaerial segments such as this, the moraine is fronted by small outwash fans and aprons on the distal side. STOP 2-6. Mud Creek Road Location 563680E, 5303070N Till on bedrock. This exposure provides an opportunity to examine basal till of the Rainy lobe. It is difficult to determine the nature of the till deposition, however, the sediments and the local geomorphic expression suggest a lodgment origin. The lithologic composition of the till is a direct reflection of the local bedrock, and the till has a sandy loam texture with a slightly higher proportion of silt than typical exposures. STOP 2-7. Mud Creek Road Location: 563970E, 5302860N Thick till. At this location, a relatively thick sequence of till is exposed in a roadcut. Its position in the lee side of an east-west trending bedrock ridge may be responsible for its preservation. The lithologic composition of the till varies along the exposure and directly reflects the lithology of local bedrock immediately beneath the till. At the eastern end of the exposure, the till is composed of a predominance of iron formation, incorporated from bedrock immediately underlying the till. Also at this location, a single clast of Hudson Bay-provenance Paleoproterozoic greywacke was recovered. Its occurrence lends further support to the idea that a continuous sheet of carbonate-greywacke bearing drift at one time extended over the Vermilion district. It also suggests that a drift sheet deposited early in the Wisconsinan glaciation was largely removed by remobilization later in the glacial cycle. 96 �STOP 2-8. Mud Creek Road Location: 564020E, 5302360N Esker. This gravel pit contains the remnants of an esker; the same esker system responsible for deposition of the fan delta at Stop 2. This esker system, like most of those associated with the Vermilion Moraine, is discontinuous. Scour and polish are visible on striated bedrock, suggesting sediment-laden meltwater may have been eroding bedrock. STOP 2-9. Trygg Road gravel pit Location 567060E, 5300080N Sulphide mineralized clasts in glacigenic sediment. The Trygg Road pit exposes glaciofluvial sediment and basal till of the Rainy Lobe, and lays approximately 5 kilometers downglacier from the Vermilion fault. This gravel pit is notable in that the till and glaciofluvial sediments exposed here contain an abundance of sulphide mineralized metasedimentary clasts, ranging in size up to bouders. These clasts are likely derived from cherty metasediments hosted by the Gafvert Lake felsic complex. Both the size of the largest mineralized clasts, as well as their abundance, reflect their relative short transport distance from their bedrock source, perhaps no more than 1 km up-ice. Therefore this pit offers an opportunity to visualize clearly the rapid rate of entrainment of local lithologies and the concept of calculation of the mean transport length from the ratio of locally derived lithologies to those derived north of the Vermillion fault. REFERENCES Bajc, A.F. 1999. Results of regional humus and till sampling in the eastern part of the Shebandowan greenstone belt, northwestern Ontario. Ontario Geological Survey Open File Report 5993, 92 p. Bajc, A.F. 2000. Results of regional till sampling in the western part of the Shebandowan greenstone belt, northwestern Ontario. Ontario Geological Survey Open File Report 6012, 82p. Björck, S., 1985, Deglaciation chronology and revegetation in northwestern Ontario, Canadian Journal of Earth Sciences 22: 850-871. Björck, S., 1990, Late Wisconsin history north of the Giants Range, northern Minnesota, inferred from complex stratigraphy. Quaternary Research 33: 18-36. Clayton, L., and Moran, S. R. 1982. Chronology of late Wisconsinan glaciation in middle North America. Quaternary Science Reviews 1: 55–82. Jirsa, M.A., and Boerboom, T.J. 2003. Bedrock geology of the Vermilion Lake 30' x 60' quadrangle, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-141, scale 1:100,000. Hallet, B., Hunter, L., and Bogen, J. 1996. Rates of erosion and sediment evacuation by glaciers: a review of field data and their implications. Global and Planetary Change 12: 213–235. Hobbs, H.C. 1983. Drainage relationship of glacial Lake Aitkin and Upham and early Lake Agassiz in northeastern Minnesota. In: J. T. Teller and L. Clayton, Editors, Glacial Lake Agassiz, Geological Association of Canada Special Paper 23, pp. 245–259. Hudleston, P.J., Schultz-Ela, D.D., and Southwick, D.L. 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences 25: 1060-1068. 97 �Jirsa, M.A., Southwick, D.L., and Boerboom, T.J. 1992. Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression. Canadian Journal of Earth Sciences 29: 2146-2155. Jirsa, M.A., and Boerboom, T.J., comps. 2003. Bedrock geology of the Vermilion Lake 30’ X 60’ quadrangle, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map M-141, scale 1:100,000. Karrow, P.F. and Geddes, R.S. 1987. Drift carbonate on the Canadian Shield. Canadian Journal of Earth Sciences 24: 365-369. Larson, P.C. 2004. Regional Till Sampling of the Western Vermilion Greenstone Belt, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2004/23, 33 p., 1 plate. Larson, P.C., and Mooers, H.D. 2004. Glacial indicator dispersal processes: a conceptual model. Boreas 33: 238249. Larson, P.C. and Mooers, H.D. 2005. Comment on ‘‘Subglacial erosion and englacial sediment transport modeled for North American ice sheets’’ by D.H.D. Hildes, G.K.C. Clarke, G.E. Flowers, S.J. Marshall. Quaternary Science Reviews 25: 233-234. Larson and Mooers. 2008. Control on Laurentide ice sheet dynamics by unconsolidated sediment in the Hudson Bay lowlands. Geol. Assoc. Canada Min. Assoc. Canada Joint Annual Meeting Abstracts 33: 92-93. Lehr, J.D. and Hobbs, H.C. 1992. Field trip guidebook for the glacial geology of the Laurentian Divide area, St. Louis and Lake Counties, Minnesota. Prepared for the 39th Midwest Friends of the Pleistocene Field trip, Biwabik, Minnesota. University of Minnesota, Minnesota Geological Survey, St. Paul, Minnesota, Guidebook Series No. 18, 73 p. Lowell, T.V. and Fisher, T.G. In press. Radiocarbon deglaciation chronology of the Thunder Bay, Ontario, area and implications for ice sheet retreat patterns. Quaternary Science Reviews. Mooers, H. D., and Lehr, J. D. 1997. Terrestrial record of Laurentide Ice Sheet reorganization during Heinrich events. Geology 25: 987-990. Morey, G.B., and Meints, J., comps. 2000. Geologic map of Minnesota, bedrock geology(3rd edition): Minnesota Geological Survey State Map Series S-20, scale 1:1,000,000. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D.M., and Jirsa, M.A., comps. 1999. Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-98, scale 1:48,000. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, Northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p. Peterson, D.M., and Patelke, R.L. 2004. Bedrock geology and lode gold prospect data map of the Mud Creek Road area, northern St. Louis County, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Map NRRI/MAP-2004/01, scale 1:12,000. Rencz, A.N., Garrett, R.G., Adcock, S.W., and Bonham-Carter, G.F. 2006. Geochemical Background in soil and till. Geological Survey of Canada Open File 5084. Sims, P.K., and Southwick, D.L. 1985. Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U.S. Geological Survey Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A. 1998. Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan–Bigfork area, northern Minnesota: Implications for metallic mineral exploration: Minnesota Geological Survey Report of Investigations 51, 69 p. 98 �Stark, J.R., 1977, Surficial geology and groundwater geology of the Babbitt-Kawishiwi area, northeastern Minnesota, with planning implications [M.S. thesis]: University of Wisconsin, Madison, 104 p. Winchell, N.H. 1901. Glacial lakes of Minnesota, Geological Society of America Bulletin 12: 109-127. Winter, T.C. 1971. Sequence of glaciation in the Mesabi-Vermilion iron range area, northeastern Minnesota: U.S. Geological Survey Professional Paper 750-C, p. 82-88. Winter, T.C., Cotter, R.D., and Young, H.L. 1973. Petrography and stratigraphy of the glacial drift, MesabiVermilion iron range area, northeastern Minnesota: U.S. Geological Survey Bulletin 1331-C, 41 p. Zoltai, S.C. 1965. Glacial features of the Quetico-Nipigon Area, Ontario. Canadian Journal of Earth Sciences 2: 247–269. 99 �55th Annual Institute on Lake Superior Geology Field Trip 3 SOUDAN IRON MINE AND PHYSICS LAB Rail depot at the Soudan Mine circa 1925, © Minnesota Historical Society/CORBIS. Dean Peterson (Duluth Metals Ltd. & PRC) James Pointer (MN Dept of Natural Resources, Parks and Trails) Marvin Marshak (Department of Physics, Univ. of Minnesota) 100 �FIELD TRIP 3 SOUDAN IRON MINE AND PHYSICS LAB Dean Peterson (Duluth Metals Ltd. & PRC) James Pointer (MN Dept of Natural Resources, Parks and Trails) Marvin Marshak (Department of Physics, Univ. of Minnesota) INTRODUCTION The Soudan Mine was the first iron ore mine in Minnesota when it opened in 1882. Since then the Minnesota Iron Range’s have contributed billions of tons of iron ore to the industrialization of America and the world. Iron mining at Soudan began in surface pits but due to the rock formation, the miners quickly went underground. By 1892 the entire mining operation was underground. The mine remained open for 80 years, but technology changes caused the mine to cease production in 1962. The Soudan Mine, a registered National Historic Landmark, is currently owned and operated by the State of Minnesota, Department of Natural Resources, Division of State Parks and Trails. Each year approximately 30,000 visitors go underground at Soudan, mostly during the summer season from Memorial Day weekend through the end of September and the first three weekends in October. This figure includes over 5,000 K-12, college, and other formal groups that visit the mine throughout the year. In February 2004, Dr. Michael Turner, Assistant Director of the National Science Foundation (NSF), announced the NSF’s intent to pursue a three-phase process leading to the establishment of a Deep Underground Science and Engineering Laboratory (DUSEL) in the United States (see Bachall et al., 2001). DUSEL would provide a program and one or more locations to study the deep (to depths of ~2.5 km) underground geological and biological environment and use the isolation of that environment to frontier physical science and engineering initiatives. The goals of NSF’s deep underground geological and biological science initiative were outlined in the 2003 document “EARTHLAB: A Subterranean Laboratory and Observatory to Study Microbial Life, Fluid Flow, and Rock Deformation” (McPherson et al., 2003). The Soudan Mine in northeastern Minnesota was one of eight sites under consideration by the NSF for hosting DUSEL under the leadership of Dr. Marvin Marshak of the University of Minnesota (Marshak et al., 2003, 2005, 2007) Although the University of Minnesota’s three proposals for the Soudan Mine ultimately did not win the competition for hosting the DUSEL, the collaboration between the universities’ School of Physics & Astronomy in Minneapolis and the Natural Resources Research Institute in Duluth laid the foundation for this field trip. SOUDAN UNDERGROUND MINE STATE PARK The promise of gold brought the early settlers to the Vermilion Iron Range in 1865. These early pioneers discovered economical quantities of iron ore while prospecting. One of these prospectors was George Stuntz, a government surveyor. Stuntz discovered the existence of commercial value iron ore on the South shore of what is now known as Stuntz Bay on Lake Vermilion. From 1865-75 Stuntz made many trips back to the Lake Vermilion area collecting numerous samples. Some of these specimens were shown to George C. Stone, a Duluth banker and Minnesota legislator. He became interested in the prospect of iron mining on the Vermilion. George Stone convinced Charlemagne Tower, a lawyer in Pennsylvania, that the mining operation would be a good business venture. At the time a new steel making process, known as the Bessemer process, was being introduced in the United States, creating a need for high grade iron ore. Tower dispatched Professor Albert Chester, a renowned geologist, to the Lake Vermilion region 101 �as head of an exploring expedition. Chester returned with a detailed account of the mineral lands. In his report Chester mentioned large quantities of iron ore and little or no gold, thus ending Minnesota's brief and dramatic gold rush. When the mine opened in 1882, the miners used a rock quarrying method of mining. The basic tools were hand drills, pry bars, black powder and bare hands. The railroad finally reached Tower in 1884, allowing the first shipment of ore from the Soudan Mine to be made on July 31st. The ore was railed to Two Harbors, Minnesota, where it was shipped to the steel mills in the East. As the ore was removed and the mine pits were sunk deeper into the hill, it became necessary to send the miners, equipment, and supplies down ropes hung over the side of the pit. Like all mining, the work was dangerous. Masses of earth and rock kept caving in, which resulted in heavy losses of lives and halts in production. These tragic incidents prompted the decision to work the mine by underground methods. By 1892, the entire operation had been moved underground. Of the thirteen shafts on property, eventually only two shafts remained: the No. 8, which is in use today for tours, and the Alaska shaft, which is about 1700 feet to the east. The No. 8 reaches a depth of almost 2,400 feet with horizontal drifts extending more than three-fourths of a mile east and west to various ore bodies. It connects with the Alaska shaft on a variety of levels. The year 1924, marked the end of an era for the Soudan Mine, because the mine was changed from a "mule and man-powered mine" to a modern electrified operation. At this time a new head frame and hoisting system were constructed, which is the equipment that is in use today. After World War II, the Soudan Mine, like other underground operations on the Vermilion Range, started to slow down. The 27th level was the last to be opened for mining. The level was never fully developed because of the advancement of the new taconite process and changes in smelting methods detracted from the usefulness of Soudan ores. In the early 1960's, increasing costs of production began to reduce profits. Soon the Vermilion ores could not compete economically with the newly developed pelletized taconite ores of the giant Mesabi Range, just to the south. The Soudan Mine ceased operations on December 15, 1962. The last shipment from the stockpile was made in August 1963. The United States Steel Corporation generously donated the mine to the State of Minnesota for creation of the Soudan Underground Mine State Park. The Department of Natural Resources, Parks and Trails Division, administers the park. The park is a testimonial to the men who labored for us all. The Park maintains the entire underground tour route, which includes the No. 8 shaft, the west section of the 27th level, the physics lab, and the rescue chamber. In addition there are pumping stations at the 12th, 22nd and 27th levels, and a reservoir for a sprinkler system on the 25th level. The remaining areas of the mine are monitored and research is conducted in a variety of these areas. Researchers are taking water samples, soil samples, and gas samples in hopes of learning more about their properties. Bat research is also conducted in the mine, since the mine is a large hibernacula for bats. The State Park also has a number of historic surface buildings, many of which are open for public visitation. These buildings include the “dry house”, the engine house, the drill shop, and the crusher. The historic “dry house” includes exhibit space and a gathering area for the tours. The State Park’s Historic Underground Mine Tour leads visitors through the world of underground mining. Visitors don hard hats and enter a "cage" for the descent into the mine. The typical 90-minute mine tour takes visitors nearly half a mile down into the Earth to the 27th level of the mine. Once underground, a 3/4 mile train ride takes visitors to the Montana ore body, the last and deepest area mined at Soudan. Public tours run daily from Memorial Day weekend through the end of September and the first three weekends in October. The park offers group tours to schools, colleges, organizations and 102 �businesses. In addition to the traditional underground mine tour, the Park is working to offer a geology tour for groups that includes a walking tour of the 27th level drift. The historic underground mine tour was made ADA accessible in 2008, with the addition of ramps and a new elevator. There is a charge for the underground mine tours. SOUDAN UNDERGROUND LABORATORY The University of Minnesota has operated the Soudan Underground Laboratory since 1980. Soudan’s existing laboratories are 710 m deep and include two caverns (MINOS and Soudan 2), each 15 m wide by 15 to 16 m high. About 250 scientists and engineers and a support staff of 9 members now work on experiments at the Soudan Laboratory. Neutrinos and WIMPS Neutrinos are elementary particles that often travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter ν (nu). Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos; each type also has an antimatter partner, called an antineutrino. Electron neutrinos or antineutrinos are generated whenever neutrons change into protons or vice versa, the two forms of beta decay. Interactions involving neutrinos are generally mediated by the weak force. Most neutrinos passing through the Earth emanate from the Sun, and more than 50 trillion solar electron neutrinos pass through the human body every second. Observations of the large-scale structure of the universe show that matter is aggregated into very large structures that would not have time to have form under the force of their own self-gravitation. It is generally believed that some form of missing mass is responsible for increasing the gravitational force at these scales, although this mass has not been directly observed. This is a problem; normal matter in space will heat up until it gives off light, so if this missing mass exists, it is generally assumed to be in a form that is not commonly observed on earth. A number of proposed candidates for the missing mass have been put forward over time. Early candidates included heavy baryons that would have had to be created in the big bang, but more recent work on nucleosynthesis seems to have ruled most of these out. Another candidate are new types of particles known as weakly interacting massive particles, or "WIMP"s. As the name implies, WIMPs interact weakly with normal matter, which explains why they are not easily visible. Detecting WIMPs thus presents a problem; if the WIMPs are very weakly interacting, detecting them will be extremely difficult. Detectors like CDMS at Soudan and similar experiments measure huge numbers of interactions within their detector volume in order to find the extremely rare WIMP events. MINOS Experiment The Main Injector Neutrino Oscillation Search (MINOS) Far Detector Laboratory is 82 m long and houses the 5,500 tonne MINOS Far Detector. The MINOS detector at Soudan is the target of the 735 km Fermilab Neutrinos at the Main Injector (NuMI) beamline. The $125 million NuMI beam first produced neutrinos in January 2005. A deeper understanding of lepton physics is the goal of a multi-decade program using the NuMI beam line, the MINOS Detector to measure 2-3 neutrino oscillations, the NOνA Off-Axis Detector to study 1-3 neutrino oscillations and a Proton Driver, to significantly upgrade the NuMI intensity. MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by Super-Kamiokande experiment in 1998. Neutrinos produced at Fermilab by the NuMI (Neutrinos at Main Injector) beamline are observed at two detectors, one very close to where the beam is 103 �produced (the near detector), and another much larger detector 735 km away in northern Minnesota (the far detector). Both MINOS detectors are steel-scintillator sampling calorimeters made out of alternating planes of magnetized steel and plastic scintillator. The magnetic field causes the path of a muon produced in a muon neutrino interaction to bend, making it possible to separate neutrino and antineutrino interactions. This feature of the MINOS detectors allows MINOS to search for CPT violation with atmospheric neutrinos and anti-neutrinos. The MINOS experiment started detecting neutrinos from the NuMI beam in February 2005. On March 30, 2006, the MINOS collaboration announced that the analysis of the initial data, collected in 2005, is consistent with neutrino oscillations, with the oscillation parameters which are consistent with Super-K measurements. The NuMI beam is the only long baseline neutrino beam in the United States and one of only three such beams in the world. NuMI and the CERN-Gran Sasso beam are both >700 km in length. The K2K and T2K beams in Japan are less than 300 km long. NuMI has the highest flux of any of the three beams at the energy required for the first maximum of 2-3 neutrino oscillations. The NuMI beam represents a substantial functioning asset to neutrino physics in both the United States and the world. To produce the NuMI beamline, 120 GeV Main Injector proton pulses hit a water-cooled graphite target. The resulting interactions of protons with the target material produce pions and kaons, which are focused by a system of magnetic horns. The neutrinos from subsequent decays of pions and kaons form the neutrino beam. Most of these are muon neutrinos, with a small electron neutrino contamination. Neutrino interactions in the near detector are used to measure the initial neutrino flux and energy spectrum. Because they are so weakly interacting, the vast majority of the neutrinos travel through the near detector and the 735 km of rock, then through the far detector and off into space. On the way toward Soudan, about half of the muon neutrinos oscillate into other flavors. CDMS II Experiment The Cryogenic Dark Matter Search (CDMS), housed in the 70 meter long Soudan 2 laboratory, is a series of experiments designed to directly detect particle dark matter in the form of WIMPs. Using an array of semiconductor detectors (silicon and germanium) at near absolute zero temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with terrestrial materials. The first experiment, CDMS 1, was run in a tunnel under the Stanford University campus. The current dark matter search experiment at Soudan (CDMS 2) has been recently upgraded with almost double the detector mass and is now called SUPER CDMS. NOνA Experiment The University of Minnesota will receive a grant of $40.1 million for the NOνA neutrino experiment construction. Funds are part of $1.2 billion from Recovery Act to be disbursed by Department of Energy’s Office of Science. The NOvA funding for Minnesota will generate an estimated 60-plus construction jobs and procurements for concrete, steel, road-building materials and mechanical and electrical equipment from U.S. firms. The NOνA Experiment will construct a detector optimized for electron neutrino detection in the existing NuMI neutrino beam. The primary goal of the experiment is to search for evidence of muon to electron neutrino oscillations. This oscillation, if it occurs, holds the key to many of the unanswered questions in neutrino oscillation physics. The NOνA far detector will be located in northern Minnesota between Orr and International Falls. The groundbreaking for construction will take place on May 1, 2009. The NOνA detectors will be constructed from liquid scintillator contained inside extruded PVC modules. The far detector will have a total mass of 15 kilotons and be 15.7 meters wide, 15.7 meters tall, and 78 meters long. A smaller copy of the far detector will be constructed in the NuMI beam on the Fermilab site to measure the neutrino event rates prior to oscillation. 104 �SOUDAN GEOLOGY The Soudan mine is located in the Neoarchean (~2.7 Ga) Vermilion Greenstone Belt of the Wawa subprovince of the Superior Province of the Canadian Shield. Supracrustal rocks in the greenstone belt consist of volcanic-dominated stratigraphic sequences that are divided on the basis of stratigraphic and structural setting into: (1) the southern Soudan belt and (2) the northern Newton belt (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the Soudan Mine area, this discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), immediately north of the Soudan Mine (Fig. 3-1). The Soudan belt, which hosts the Soudan Mine contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences that locally contain komatiitic flows and peridotitic sills. The two belts are faultbounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. To the east, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced posttectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calcalkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neoarchean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 3-1. Figure 3-1. Simplified correlation map of Neo-Archean assemblages across the U.S. - Canadian border, modified from Peterson et al., 2001). Inset shows major subprovinces of the southwestern Superior Province. 105 �Local Geological Setting A detailed geological study of the Soudan DUSEL area was recently completed by Peterson and Patelke (2003) and is available online at http://www.nrri.umn.edu/egg/REPORTS/TR200329/TR200329.html. Detailed field mapping ranging from 1:100 to 1:2000 scale (Peterson and Patelke, 2003; Hudak et al., 2002, 2003), petrographic studies (Hudak et al., 2002, 2003), whole rock lithogeochemical studies (Hudak et al., 2002, 2003), and electron microprobe investigations (Hocker et al., 2003) have led to a more complete understanding of the tectonic environment, stratigraphy, structure, hydrothermal processes and mineralization episodes associated with the diverse strata in the Soudan Mine vicinity. The local geological setting has been subdivided into three stratigraphic sequences (Fig. 3-2). The older Fivemile Lake Sequence is composed dominantly of coherent and volcaniclastic, dominantly calcalkaline to transitional basalts and andesites, minor intercalated coherent and volcaniclastic rhyodacite and rhyolite, associated chemical exhalites (including subeconomic VMS horizons) and epiclastic strata. Facies analysis indicates the FLS formed within a relatively shallow submarine arc environment. The younger Central Basalt Sequence (CBS) comprises an extremely texturally well-preserved sequence of sparsely amygdaloidal calc-alkaline, transitional, and tholeiitic basalt and andesite pillowed and massive lava flows; in-situ and resedimented rhyodacite to rhyolite lava flows, lava domes, and tuffs; as well as volcanic-derived mudstones, sandstones and breccias intercalated with chert, exhalites, and Algoma-type iron formations. Facies analysis indicates the CBS formed in a deeper water submarine environment which, based on lithogeochemical data, may be transitional between a volcanic arc and the opening of a back-arc rift. Overlying the CBS are sedimentary-dominated rocks of the Upper Sequence (US), which in the Soudan DUSEL area consists dominantly of Algoma-type iron-formation; dacitic epiclastic rocks, crystal tuff and tuff breccia; mixed basaltic rocks and iron-formation; and greywacke. The contact between the CBS and the overlying US is transitional over approximately 100-150 meters, and marks a stratigraphically upward increase in the proportion of chemical sediments and decrease in volcanic rocks. The supracrustal strata in the immediate area have been intruded by numerous synvolcanic – diorite and gabbro sills, diabase and feldspar-porpyhry dikes – and post-tectonic – lamprophyre dikes – intrusions. A simplified geologic map of the Soudan DUSEL area is presented in Figure 3-2. Periods of generally N-S directed compression resulted in three Neoarchean deformation events in the Soudan area. The earliest deformation (D1) produced the Tower-Soudan Anticline (Fig. 3-1), which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally, and Peterson and Patelke (2003) have described D1 flexural-slip shear zones immediately north of the Soudan Mine. A second deformation (D2) associated with synchronous regional metamorphism resulted in foliation development and shear zones having largely dextral asymmetry. D2 is constrained in the Vermilion greenstone belt to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993). The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event (Jirsa et al., 1992). Major shearing that produced the Mud Creek and related shear zones, e.g., the Murray, Mine Trend, and Linking shear zones in the Soudan Mine area (Fig. 3-2), is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation (D3) include abundant NEand NW-trending faults (Fig. 3-2). 106 �Figure 3-2. Simplified bedrock geologic map and stratigraphic section of the Soudan Mine area, Sections 26 and 27, T62N, R15W. Modified from Peterson and Patelke, 2003. 107 �SOUDAN FIELD TRIP The short time-frame of the 2009 ILSG field trip to the Soudan Mine only allows for quick tours of the facility and the scientific treasures it holds. The excursion begins at the surface of the Soudan Mine and will include a quick overview of the local geology based on geologic maps and observations of the region from the head frame vista. The three minute trip underground in the inclined shaft will take us 2,341 feet underground. We will gather in the main hall of the MINOS cavern for brief PowerPoint presentations about the Soudan Mine from our field trip leaders. The presentations will pertain to the following topics: the State Park (James Pointer), geology of the area (Dean Peterson), and the various physics experiments (Marvin Marshak). The group will be split in half and rotate through a tour of the Underground Physics Laboratory and a historic mine tour, which takes us via a 3/4 mile train ride to the Montana ore body, the last and deepest area mined at Soudan. REFERENCES Bahcall, J., Barish, B., Calaprice, F., Conrad, J., Doe, P.J., Gaisser, T., Haxton, W., Lesko, K.T., Marshak, M., and Robinson, K., 2001, Underground Science: University of Washington Institute for Nuclear Theory report, 36 p. Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Science, v. 30, p. 2510-2522. Hocker, S.M., Hudak, G.J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Five Mile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion District of NE Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Report of Investigation NRRI/RI-2003/17, 49 p. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion district, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hudak, G.J., Heine, J., Newkirk, T., Hocker, S., and Hauck, S., 2003, Comparative geology, stratigraphy, and lithogeochemistry of the Needleboy Lake – Six Mile Lake area, Vermilion District, NE Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Report of Investigation, NRRI/RI-2003/18, 22 p. Hudak, G.J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002, Comparative geology, stratigraphy, and lithogeochemistry of the Five Mile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report, NRRI/TR-2002/03, 390 p. Hudleston, P.J. 1976, Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Hudleston, P.J., Schultz-Ela, D., and Southwick, D.L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1,060-1,068. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression: Canadian Journal of earth Sciences, v. 29, p. 2,146-2,155. Marshak, M.L., Cushman, P.B., Heller, K., and Peterson, E.A., 2003, SOUDAN: A Proposal for a National Underground Science and Engineering Laboratory (NUSEL): National Science Foundation, submitted proposal, 146 p. 108 �Marshak, M.L., Peterson, D.M., Peterson, E., Kieft, T.L., and Cushman, P.B., 2005, Site and Conceptual Design for the Soudan Deep Underground Science & Engineering Laboratory (DUSEL), a proposal submitted to the National Science Foundation, 15 pages. Marshak, M.L., Peterson, D.M., Peterson, E., Alexander, C.A., and Cushman, P.B., 2007, Frontiers of Underground Science, a proposal submitted to the National Science Foundation, 100 pages. McPherson, B.J., and the EarthLab Steering Committee, 2003, EarthLab: A subterranean laboratory and observatory to study microbial life, fluid flow, and rock deformation: Geosciences Professional Services, Inc., 62 p. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S. - Canadian border; Phase I geochronology, abstract and oral presentation, Institute on Lake Superior Geology, 47th Annual Meeting, Thunder Bay, Ontario, v. 47, p. 77-78. Peterson, D.M and Patelke, R.L., 2003, National Underground Science and Engineering Laboratory NUSEL); Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 97 p., 3 plates, 1 CD-rom. Southwick, D.L., Boerboom, T.J., and Jirsa, M.A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: Implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. 109 �55th Annual Institute on Lake Superior Geology Field Trip 4 PIONEER MINE CANOE EXCURSION Sibley Mine headframe, Ely Minnesota 1905, Minnesota Historical Society. Mark Jirsa (Minnesota Geological Survey) Mike Hillman (Ely Historian) 110 �FIELD TRIP 4 PIONEER MINE (Miners Lake) CANOE EXCURSION Mark Jirsa—Minnesota Geological Survey, Mike Hillman—Ely historian Figure 4-1. General geology of Archean rocks in the western half of the Vermilion district (after Clements, 1903). Black= iron-formation; diagonal rule=metasedimentary rocks; white=metavolcanic rocks (and lakes); plus symbol=intrusions. MINERS LAKE (information from Minnesota Department of Natural Resources website) Miners Lake was formed by the flooding of the open pit portion of a series of mostly underground iron mines after mining was discontinued in 1967. The lake has a maximum depth of about 140 feet. The flooded basin of Miners Lake includes the formerly mined area and a shallow bay to the east that was not mined. Brook and Rainbow trout are stocked at the rate of about 3000/year. Bass, pan fish, and white suckers are also present. The 2009 fishing season starts May 9. INTRODUCTION The Pioneer Mine was one of the richest underground mines on the Vermilion Iron Range from its opening in 1888, until closure in 1967. Mining extracted crystalline hematite from jaspilitic ironformation, and the underground workings eventually attained a depth of more than 1500 feet. The ironformation occurs as discontinuous layers and lenses within the Neoarchean Ely Greenstone along the Vermilion Iron Range or Vermilion district, as it was known (Fig. 4.1). The geology at Pioneer Mine is much like that at other mines of the district, including the Soudan Mine about 20 miles to the west. The Pioneer Mine and the adjacent Chandler, Sibley, Savoy, and Zenith mines (Fig. 4.2) lie within a complex isoclinal fold—known as the Ely Trough—having steeply dipping limbs. The brief geologic description presented here is based on historical records, primarily from mining company geologists of what was then the Oliver Iron Mining Division, U.S. Steel Corporation. This trip starts with a visit to the Pioneer Mine Dry with local historian “Iron-Mike” Hillman. We then embark on a short canoe trip down the axis of the trough, exploring exposures along the mine walls. 111 �Steeply dipping, pillowed metabasalt flows, lean and brecciated iron-formation, and a complex array of intrusions are visible from the gunnels. Figure 4-2. Geology of Ely trough mines (from Reid, 1956). EXPOSURES No descriptions of individual field “stops” are given here—instead, participants are encouraged to paddle along at a safe distance from the shoreline and observe, using the following general information as a guide. Bedding trends more or less parallel to the long axis (E-W) of the lake and dips steeply. Exposures at the west end of the lake show layered jaspilitic and slaty iron-formation capping pillowed, fragmental, and amygdaloidal metabasalt; overlain to the north by massive metabasalt. This is the west edge of the Chandler Mine. The south shore presents dip-slope exposures of pillowed metabasalt, mafic intrusions, and lenses and layers of lean iron-formation, all trending subparallel to the shoreline. These exposures lie within the Chandler and Pioneer Mines. Stratigraphic younging based on pillow morphology is northward, consistent with the interpretation that this shoreline exposes the south limb of the Ely trough. Easternmost exposures the north shore are within the Zenith Mine, and show liesegangbanded, altered (leached, oxidized) iron-formation. CAUTION should be taken to avoid disturbing the metastable wall rocks! 112 �LOCAL MINING HISTORY Time-line for development of the Vermilion Iron Range (modified from Skillings, 2004) 1865 Iron ore discovered at Soudan by George R. Stuntz 1883 Iron ore discovered at Ely by H.R. Harvey 1884 Completion of Duluth and Iron Range Railroad line to Tower 1884 First shipment of ore from Soudan mine 1888 First shipment from Chandler and Pioneer Mines 1901 Largest annual shipment of >2 million tons 1967 Last shipment from Vermilion (at Pioneer Mine—289,000 tons of gravity concentrate) 1884-1967 Total iron ore shipped from Vermilion = 103,752,604 tons Last iron ore reserve estimate calculated in 1985 for the Vermilion Iron Range = 6,237,076 tons. Ely Trough mining and production (Skillings, 2004): Mine Name Operation Yrs. ~Total (million tons) Chandler N 1891-1942 9 Chandler S 1888-1957 2 Pioneer 1888-1967 41 Zenith 1892-1964 21 Sibley 1899-1954 10 Savoy 1899-1916 2 GEOLOGY The main rock types in the mine, as described in previous literature (Reid, 1956) include: Greenstone (pillowed and massive metabasalt, minor hyaloclastite and mafic tuff) Porphyry intrusions (plagioclase and quartz phenocrysts) Basaltic intrusions (metagabbro, metadiabase) Jaspilite Slaty iron-formation Graphitic and sericitic schist “Paint rock” (an alteration product, apparently of any of the rocks types above) The large deposit of iron ore at Ely is found in the basal part of a trough-like lens of iron-formation about 1.75 miles long and .25 miles wide at the surface. Mines within the trough structure included the Chandler North and South, Pioneer, Zenith, Sibley, and Savoy. The structure is blunt U shape at the west end (Chandler) and narrows to the east. The ore body plunges eastward from the Chandler into the Pioneer mine, and is abruptly split by a broad mass of paint rock. The ore continues along both N and S limbs of the trough, but the north limb ore pinches out eastward into the Zenith. Most of the Zenith, and all of the Sibley and Savoy ore bodies lie within the south limb of the trough, and the north limb is presumably truncated by a fault. The ore is fragmental hematite, cemented to varying degrees that increase with depth by secondary hematite. The limbs of the trough, as defined by iron-formation, converge at about 1500 depth, and the ore-bearing structure is less than 500 feet wide. The trough roughly parallels the east-northeast-trending regional strike of the Vermilion district. Its axis is warped, with a steep north to vertical dip on the west, and a steep southerly dip on the east. Footwall rocks surrounding the trough consist of metabasalt of the Ely Greenstone and several types of intrusive rocks. Least-altered metabasalt exposed outside of the mines consists of actinolite, epidote, chlorite, quartz, and albite. 113 �Near the ore bodies, the rocks contain a more pronounced cleavage and consist of chlorite, fine-grained muscovite (sericite), and quartz. The trough interior is composed largely of jaspilite (alternating layers of hematite and chert), with lesser amounts of argillaceous iron-formation, mafic sill-like intrusions, irregular plagioclase-quartz porphyries, and lenses of graphitic and sericitic schist. Felsic porphyry intrusions were encountered during mining, and mafic sills and dikes were reported to cut iron-formation locally. Both of these rock types are common in the regional geology. The precise age of folding responsible for the Ely trough is unknown. Jirsa and Miller (2004) speculate that the Ely trough and other similarly trending structures were deformed during D1, much like the deformation that formed the Tower-Soudan anticline to the southwest. This is largely based on the observation that fold axes near Ely are truncated by D2 structures related to deformation along the Knife Lake-Burntside Lake trend. ORE DEPOSITS The major hematite ore bodies occur near the base of the trough. It was considered a “rubble ore,” because the bulk of it consisted of small angular fragments of hard blue hematite. The ore was relatively soft in the upper parts of the mine, but hardness increased with depth as it became partially to completely cemented by secondary hematite. In many places the ore was as hard as the “lump” ores at Soudan. The high grade ore bodies are more or less conformable with layering in jaspilite and greenstone, implying that the ores occupy the stratigraphic position of lean iron-formation. The ores contained 55-65 percent iron, low silica, P, Mn, and Al2O3. An increase in pyrite content with depth was observed, and carbonate minerals increased eastward within the trough. Alteration is more intense than at any other deposits in the Vermilion district. The alteration includes oxidation and leaching, and involves the loss of silica, lime, magnesia, soda, and potash, and the addition of iron and oxygen. “Paint rock” is the ultimate extent of that alteration, resulting locally in material consisting almost entirely of kaolin and ferric oxide. The origin of hematite ores on the Vermilion Range is controversial. speculated historically: Three scenarios have been 1) Leaching of silica from the primary iron-formation resulted in the formation of a residual hematite body, which was later compacted, hardened, and fractured by deformation and metamorphism: 2) Post-metamorphic leaching of silica and introduction of iron by downward-moving surface water; 3) Post-metamorphic leaching of silica and simultaneous introduction of iron by rising heated waters A detailed study of mineralogy and structure at the Zenith Mine, which lies just east of the Pioneer, produced the following conclusions (Machamer, 1968): 1) The ore deposits are post-metamorphic; 2) They formed by the replacement of silica in iron-formation by iron oxides; 3) The replacement resulted from rising hydrothermal fluids, most likely generated from higher metamorphic grades at depth. REGIONAL MINING HISTORY Although the 6 mines described here were by far the most productive, at least 12 iron mines were explored and developed in the area from Ely; westward to Armstrong Lake and Mud Creek Road; and eastward along the Fernberg Road (Stenlund, 1988). Only 3 of these mines actually shipped ore, and their life-spans were short due to high costs in this rugged terrane and low tonnage. The local mining and 114 �logging history is displayed in the Vermilion Interpretive and History Museum of the Ely-Winton Historical Society, housed on the campus of Vermilion Community College. Overall the Soudan iron-formation member and other iron-formations in the Ely Greenstone consist of lens shaped bodies interleaved with volcanic strata. The most productive mines occur at Tower-Soudan and Ely. It is interesting to note that these are the two localities along the Vermilion range where ironformation is “doubled” by folding. At the least, the folding consolidated otherwise attenuated iron-rich units. It is also likely that deformation played an important role in the alteration of the iron-bearing units to high-grade hematite deposits. REFERENCES Clements, J.M., 1903, The Vermilion Iron-bearing district of Minnesota: USGS Monograph 45, 463p. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Machamer, J.F., 1968, Geology and origin of the iron ore deposits of the Zenith Mine, Vermilion District, Minnesota: Minnesota Geological Survey Special Publication SP-2, 56p. Reid, I.L., 1956, Geology of the Ely Trough: in Schwartz, ed., Guidebook for Field Trips, Geological Society of America Guidebook Series No.1 Precambrian of northeastern Minnesota, p.135-148. Skillings Mining Review, 2004 Minnesota Mining Directory; Westmorelandflint Publishing, Duluth, Minnesota. Stenlund, Milt, 1988, Ghost Mines of the Ely Area: Ely-Winton Historical Society, Ely, MN, 44p. 115 �55th Annual Institute on Lake Superior Geology Field Trip 5 GEOLOGY & METAMORPHISM OF THE EASTERN MESABI RANGE Dick Ojakangas (Dept. of Geological Sciences, UMD) Mark Severson (Natural Resources Research Institute) Doug Halverson (Cliffs Natural Resources) Jeff Bird (Cliffs Natural Resources) Tom Campbell (Cliffs Natural Resources) Jared Lubben (Cliffs Natural Resources) Peter Jongewaard (Cliffs Natural Resources) William Everett (Mesabi Nugget) 116 �FIELD TRIP 5 GEOLOGY AND METAMORPHISM OF THE EASTERN MESABI IRON RANGE Richard W. Ojakangas, Professor Emeritus, University of Minnesota Duluth Mark J. Severson, Natural Resources Research Institute, Thomas J. Campbell, Cliffs Technical Group, Cliffs Natural Resources Jared D. Lubben, Cliffs Technical Group, Cliffs Natural Resources Peter K. Jongewaard, Cliffs Technical Group, Cliffs Natural Resources Douglas G. Halverson, Northshore Mining/Cliffs Natural Resources Jeff Bird, Northshore Mining/Cliffs Natural Resources William Everett, Mesabi Mining INTRODUCTION Iron-formation was described as early as 1866 by Henry Eames on what was to become the Mesabi Iron Range. Several attempts were made by individuals to find ore on the Mesabi range on their way north to the iron mines of the Vermilion range (Soudan to Ely, Minnesota); however, it was not until November 16, 1890 that the first rich iron ore on the Mesabi range was discovered by the Merritt brothers near what is now Mountain Iron, Minnesota. In 1892, the first shipment from this mine was 4,245 tons of ore (White, 1954). Exploration for iron ore ensued and within the next few years, most of the productive parts of the Mesabi Range were discovered. The Mesabi Iron Range is the largest iron range in the United States and is one of the largest in the world. It is 0.25 to 3.0 miles wide and 120 miles long (Fig. 5-1). The Biwabik Iron Formation, as thick as 750 feet, in general dips gently to the southeast at an angle of about 7° to 15°. The iron-formation, called taconite, typically contains 30 to 40 percent iron and 40 to 50 percent SiO2, plus other components (Morey, 1992). In numerous places along the length of the range, silica was leached out, thereby enriching the iron content to over 55 percent. These pockets became the high-grade natural ore mines; there were more than 500 individual mines prior to merging into larger mines as the ore between adjacent properties was removed. These were very important in making the United States an industrial giant, and were instrumental in providing raw material for World Wars I and II. As the high-grade ore was depleted, the taconite process was developed. In 1967, taconite production exceeded natural ore production. Currently, six taconite plants are in production (Fig. 5-2). The name of Biwabik Iron Formation was chosen by Van Hise and Leith (1901, p. 356), "…because the word Biwabik is the Chippewa word for a piece or fragment of iron." The word taconite is also used in discussions pertaining to hard, unoxidized portions of the iron-formation. H.V. Winchell (1882, p. 135) originally called portions of the Biwabik Iron Formation "taconyte" because he thought the rocks correlated with lower Cambrian rocks in the Taconic Mountains in northern New England. Since that time, many geologists have used taconite in their descriptions of the iron-formation and it has thus become firmly established. Perhaps a more proper definition for taconite is an economic term for ironformation from which iron can be profitably extracted after fine-grinding, followed by magnetic separation and pelletizing (Morey, 1993). 117 �Figure 5-1: Generalized map of the Mesabi Iron Range (cross-hatched). Note the Duluth Complex (Keweenawan, 1.1 Ga) on the east side. REGIONAL GEOLOGY The peneplaned Archean craton in the Lake Superior region formed a platform upon which a Paleoproterozoic continental margin assemblage was deposited in Minnesota, Michigan, and Wisconsin. Extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform. Seas transgressed onto the continent one or more times and an ocean basin opened south of present-day Lake Superior. Island arcs that formed during southward subduction collided with the craton margin to the north as the ocean basin closed. A remnant of this oceanic crust is poorly preserved as a dismembered ophiolite sequence in Wisconsin (Schulz, 1987, 2003). The arc volcanics are preserved as the Wisconsin magmatic terranes. The collision resulted in a fold-and-thrust belt known as the Penokean orogen. To the north of the fold-and-thrust belt, a northwardmigrating foreland basin—the Animikie basin—developed as the stacked thrusts weighed down the crust (Fig. 5-3). Thick turbidite successions were deposited along the basin axis, and terrigenous clastics and Lake Superior-type iron-formation were deposited on the shelf along the northern margin (the foreland or peripheral bulge) of the basin. See Ojakangas and others (2001) and Severson and others (2003) for more detailed summaries on Paleoproterozoic basin development in the Lake Superior region. The development of the Midcontinent Rift System at 1.1 Ga severed the basin into northwestern and southeastern segments (Fig. 5-3). If the Midcontinent Rift System rocks are removed from the geologic map, the different portions of the Animikie basin become contiguous and the fold-and-thrust belt rocks of Minnesota, Wisconsin, and Michigan become continuous (Fig. 5-4). Figure 5-5 is an interpretive cross-section of the Animikie basin during its formative stages, with sediments derived from the Archean basement to the north and from the fold-and-thrust belt to the south. The Paleoproterozoic supracrustal rocks in the northwestern segment, including east-central and northeastern Minnesota and the adjoining part of Ontario, are for the most part poorly exposed. However, 118 �mining of iron ore on the Mesabi and Cuyuna ranges and continued mining of taconite on the Mesabi Range have resulted in excellent artificial exposures and an abundance of drill hole information. Geophysical surveys and stratigraphic test drilling by the Minnesota Geological Survey have also been major sources of information (for example Southwick and others, 1988). Figure 5-2: Generalized map of the Mesabi Iron Range. Inset A - Aerial distribution of taconite pits and cities. Inset B - Longitudinal section of the Biwabik Iron Formation showing: average thickness of the iron-formation at each taconite operation (along with the thickness of the various submembers at each operation), and mined taconite intervals (as black columns adjacent to the sections). From Severson and others (in prep.) Figure 5-3: Generalized geologic map showing the distribution of Precambrian rocks and structural elements of the Lake Superior region, modified from Ojakangas 1994 and references therein (from Ojakangas and others, 2001). 119 �Figure 5-4: Schematic hypothesized paleogeography at the time of sedimentation of the Paleoproterozoic Animikie Group turbidites that overlie shelf deposits in the Animikie basin. The rocks of the 1,100 Ma Midcontinent Rift System have been removed from the map, and Michigan and Wisconsin are thus positioned 60 miles closer to Minnesota-Ontario than they were after the formation of the Midcontinent Rift System. Arrows denote generalized transportation directions of sediment from major source areas. Compare with Fig. 5-3. Modified from Ojakangas (1994) and references included therein (from Ojakangas, et al., 2001). Figure 5-5: Schematic cross-section depicting deposition of the Animikie Group turbidites that overlie shelf deposits in the Animikie basin, with sediment derived from both the north south. The southern area, the fold-andthrust belt, comprises a complex assemblage including: 1) accreted Paleoproterozoic volcanic and plutonic rocks and volcanic rocks of the Wisconsin magmatic terranes; 2) accreted Archean miniplate terranes; 3) older Paleoproterozoic passive-margin sedimentary rocks and volcanic rocks produced during initial rifting of the continental margin, both scraped off the southward-subducting Archean Superior craton; and 4) recycled initial foredeep deposits, possibly including basal shallow water sandstones deposited in the transgressing sea of the northward-migrating foreland basin. The peripheral bulge comprises a source-rock assemblage of Archean granitic rocks and Archean volcanic-sedimentary (greenstone) belts. Scale is approximate. Compare with Fig. 5-4. Modified from Ojakangas (1994) and references included therein (from Ojakangas and others, 2001). 120 �Animikie Group The Paleoproterozoic Animikie Group unconformably overlies the Mille Lacs and North Range Groups to the south and the Archean basement to the north (see Fig. 5-6; Southwick and Morey, 1991). Magnetic data show North Range structures are present beneath Animikie strata to the east of the exposed North Range Group (Chandler, 1993). The group consists of three conformable major formations on both the Mesabi and Gunflint ranges. The respective units on the two ranges are the Pokegama Formation and the Kakabeka Quartzite (the lowest units), the Biwabik and Gunflint Iron Formations (the middle units) and the Virginia and Rove Formations (the upper units, composed of graywacke and shale). The Thomson Formation in the northern part of east-central Minnesota is correlative with the Virginia and Rove Formations. The Biwabik and Gunflint Iron Formations are on strike with each other and were probably continuous prior to the intrusion of the Duluth Complex at about 1,100 Ma. In the model presented here, the Animikie Group in Minnesota and Ontario on the Mesabi and Gunflint ranges and the Baraga Group of Michigan and Wisconsin on the Gogebic Range were both deposited in the Animikie foreland basin. The basal units comprised of siliciclastic sediment derived from the Archean basement, and the overlying iron-formation, were deposited in a shallow sea on the northern edge (the peripheral bulge or foreland) of the northward-migrating Animikie basin (for example Ojakangas, 1994). Additional details are provided below in the section titled "Environments of deposition, Animikie Group." The siliciclastic and iron-formation units are exposed on the Gogebic Range of northern Michigan and Wisconsin (the Palms Quartzite and Ironwood Iron Formation), on the Mesabi Range of northern Minnesota (the Pokegama Formation and the Biwabik Iron Formation), and on the Gunflint Range of northeast Minnesota and Ontario (the Kakabeka Quartzite and the Gunflint Iron Formation), and are lithostratigraphic equivalents. They probably were continuous from south to north prior to development of the Midcontinent Rift System in Mesoproterozoic time. A consequence of this model is that they are diachronous, with the units in Michigan and Wisconsin (located about 60 miles to the south of the Mesabi Range during deposition) thus somewhat older than those in Minnesota and Ontario. The thickest and uppermost units in the basin, essentially lithostratigraphic correlatives but probably differing somewhat in age, are the Michigamme, Tyler, and Copps Formations of the southeastern segment and the Thomson, Virginia, and Rove Formations of the northwestern segment. These are typical turbidite-shale (flysch) sequences, with graded beds and intercalated muddy "rain-out" sediment (Fig. 5-6). Figure 5-6: Generalized correlation chart of Paleoproterozoic strata in the Lake Superior region. Note that recently obtained age dates are shown for the Gunflint Iron formation, Mahnomen Formation, Hemlock Volcanics, and the Rove and Virginia Formations (sources listed in the references). The position of the Sudbury layer is from Cannon and Addision (2007). 121 �Ages Along the Mesabi Range, the Pokegama Formation rests unconformably on diabase dikes of the Kenora– Kabetogama dike swarm that give a Rb-Sr isochron age of 2,125 ± 45 Ma (Southwick and Day, 1983; Beck, 1988), and this provides a maximum age for deposition of the Pokegama Formation. A minimum age of 1,930 ± 25 Ma (Pb/Pb) for the Pokegama Formation was obtained by Hemming and others (1990) from quartz veins that cut the Pokegama Formation. A U/Pb age on euhedral zircons from an ash layer in the upper Gunflint Iron Formation of Ontario is 1,878 ± 2 Ma (Fralick and Kissin, 1998; Fralick and others, 2002). A similar age of 1,874 ± 9 Ma was obtained on zircon from rhyolite in the Hemlock Formation that is adjacent to (and is possibly interlayered with) the Negaunee Iron Formation in the Marquette Range Supergroup of Michigan (Schneider and others, 2002). A zircon age from an ash layer near the base of the Virginia Formation is 1,850 Ma (Hemming and others, 1996), and an age of 1,821 ± 16 Ma has been obtained from an ash layer in the Rove Formation about 70 meters above the Gunflint Iron Formation (Kissin and others, 2003). Several of these ages are shown on Figure 5-6. It has been recently postulated that deposition of the iron-formations in the Lake Superior region was affected by the Sudbury impact event at 1850 Ma (Cannon and Addison, 2007; Jirsa, 2008). Field and petrographic evidence for this event are clear in the Gunflint Iron Formation (Addison and others 2005; Jirsa, 2008) and the iron-formations of Michigan (Cannon and Addison, 2007). However, an indication of this event in the top of the Biwabik Iron Formation is more subtle (Addison and others, 2007), possibly due to deeper water conditions that are recorded in the presently mined and drilled stratigraphic section on the Mesabi Range. Pokegama Formation This formation has long been called the Pokegama Quartzite, but because it contains appreciable argillite and siltstone, the name Pokegama Formation is more appropriate. It has been studied by several workers since it was named by Winchell (1893) for exposures at the western end of the Mesabi Iron Range. Much of the previous work has been summarized by Morey (1972, 1973, 2003). Few natural exposures exist, as thick glacial drift generally covers the formation. Outcrops, road cuts, and mine cuts occur at a few places along the length of the range, but most exposures are in the central portion of the range. A few drill holes have penetrated the entire formation. One is located just south of Eveleth (T. 57 N., R. 17 W., sec. 5, NE, NE) and another is southwest of Mountain Iron (T. 58 N., R. 18 W., sec. 8, SE, SE); the thicknesses are 167 feet and 85 feet, respectively (Fig. 5-7). Other drill cores, some recently rediscovered and some recently drilled, have not yet been studied in detail. Numerous drill holes have penetrated only the upper few feet of the formation, as the drilling was generally undertaken in relation to iron ore exploration and development. The Pokegama Formation is thin at the eastern end of the range and thickens to the western end where it may be more than 300 feet thick. The formation is composed of three main rock types—argillite, siltstone, and quartzite. The quartzite is generally silica-cemented quartz sandstone, and is therefore an orthoquartzite rather than a metaquartzite. Morey (2003) determined that mineralogical changes in the Pokegama Formation and the Biwabik Iron Formation are the result of diagenesis rather than metamorphism, except at the eastern end of the range adjacent to the Duluth Complex. These three rock types make up three gradational members—lower, middle, and upper—respectively, as shown in Figure 6-7. Minor thin conglomerates occur at the base of the formation, and seem to represent a weathered residuum on the surface of Archean rocks, perhaps reworked by fluvial processes. The Pokegama Formation unconformably overlies Archean metavolcanic, metasedimentary, and plutonic rocks. There may be as much as 100 feet of relief on the Archean surface (Grout and Broderick, 1919), 122 �but the surface was, nevertheless, essentially a peneplain. Some Archean "knobs" were islands when the Pokegama Formation was being deposited, and are present in the wooded areas between Eveleth and Virginia where they have been re-exhumed. The Pokegama Formation–Biwabik Iron Formation contact is gradational, with some cherty horizons in the upper Pokegama Formation and some sand grains of quartz in the lowest bed of the Biwabik Iron Formation. Various geologists have placed the contact at different stratigraphic levels. Figure 5-7: Measured sections from two drill holes that penetrate the entire Pokegama Formation. Dark shading represents shale, thin blank units represent siltstone, the slanted pattern represents sandstone and siltstone, and the dotted pattern represents sandstone. Modified from Ojakangas (1983). Biwabik Iron Formation This is one of the world's major iron-formations, and the largest in the United States. The formation is 200 to 750 feet thick and consists of four divisions as defined by Wolff (1917). These lithostratigraphic units, now informal members, are from the bottom up, the Lower Cherty, the Lower Slaty, the Upper Cherty, and the Upper Slaty (these are miners' terms, and do not indicate metamorphism; Fig. 5-2). The cherty members are dominantly granular (sand-textured), thick-bedded (several inches to a few feet), and are largely composed of chert and iron oxides. The slaty members are dominantly fine-grained (mudtextured), thin-bedded (less than 1 inch), and composed mostly of iron silicate and iron carbonate with local chert beds. However, these two rock types are interbedded on all scales and are generally gradational. They contain about the same high quantities of silica, 42 to 47 percent (Morey, 1992). The Lower Slaty member is not present at the far western end of the range. There are some diagnostic marker units within the formation. Two stromatolite-bearing intervals several feet thick are present, one at the base of the Lower Cherty member and the other in the middle of the Upper Cherty member. The black "Intermediate Slate" at the base of the Lower Slaty member is reportedly an ash-fall tuff containing about 4 to 5.5 percent aluminum oxide (Morey, 1992). At the top of 123 �the Upper Slaty member are several feet of limestone and dolomite. Most of these marker units, which are prominent in the eastern and central parts of the range, pinch out to zero in the vicinity of Hibbing, about 60 miles from the west end of the range (Severson and others, in press). Virginia Formation There are rare exposures of the Virginia Formation in mines at the east end of the Mesabi Range where it has been metamorphosed by the mafic intrusions of the Mesoproterozoic Duluth Complex. Several holes drilled south of the range to study the underlying iron-formation have been drilled through the Pleistocene cover and have intersected as much as 1,443 feet of the preserved lower part of the formation (Lucente and Morey, 1983). The lower portion of the formation in the drill holes is dominantly black shale. The upper portion of the drill core, while still dominantly shale, contains beds of siltstone and fine-grained feldspathic graywacke comprising thickening- and coarsening-upward turbidite sequences. Ash-fall tuff, cherty sideritic ironformation, chert, and limestone are minor rock types low in the formation. The contact with the underlying Biwabik Iron Formation is gradational. The clastic rocks were largely derived from the Archean rocks to the north, with some contributions from lower Proterozoic rocks to the south (Lucente and Morey, 1983). The Virginia Formation is correlated with the Thomson Formation (Morey and Ojakangas, 1970) that is exposed 60 miles to the south in the vicinity of Carlton and Cloquet, Minnesota, and also with the Rove Formation in northeast Minnesota and adjacent Ontario (Morey, 1967). ENVIRONMENTS OF DEPOSITION, ANIMIKIE GROUP The Pokegama Formation is interpreted to have been deposited in a tidally influenced shallow marine setting near the shoreline, having received clastics from the Archean basement to the north (Ojakangas, 1983). In this model of a transgressing sea, the lower (argillaceous) member was deposited at the shoreline in the upper tidal flat, the middle member of intercalated argillaceous and silty sediment was deposited seaward in the middle tidal flat, and the upper member of quartz sand was deposited still further seaward in a lower tidal flat/subtidal environment. This is illustrated in Figure 5-8. Walther's Law is applicable here, with the vertical facies showing the relationships of the lateral facies. The lowermost Pokegama Formation contains siltstone beds that contain alternating thicker and thinner laminae that have been interpreted as evidence of the diurnal inequality, and are being investigated further for possible clues to the Paleoproterozoic lunar orbit (Ojakangas, 1996). The Biwabik Iron Formation is interpreted to have been deposited seaward of the Pokegama Formation on a shallow marine, tidally dominated shelf (Fig. 5-8). Precipitation of iron minerals including iron carbonate, iron silicate, chert, and perhaps some hematite, occurred on the outer shelf in waters below wave base, giving rise to the mud-textured (slaty) iron-formation. These minerals were likely related to upwelling waters from the deeper part of the basin. 124 �Figure 5-8: Sedimentation model showing lateral relationships of the siliciclastic tidal facies of the Pokegama Formation, the two main facies of the Biwabik Iron Formation, and the Virginia Formation (on the slope?). Thicknesses and geography are not to scale; modified from Ojakangas (1983). The two sand-textured members (Lower Cherty and Upper Cherty) formed in a shallow-water, highenergy environment, as indicated by stromatolites, cross-bedding, and rounded (locally oolitic) grains of iron minerals and chert. Shoreward-moving tidal currents (flood tides) and/or storms may have disrupted the mud-textured sediment (precipitates) and transported sand-sized aggregates into shallower water where they were altered by seafloor processes and early diagenetic processes. Thus these granules are interpreted as "intraclasts" derived from within the basin. Shallow channels up to a mile wide and tens of feet deep were cut into the Lower Slaty member and filled with sand-textured grains of iron minerals and chert in the Virginia horn area. These grains apparently were derived from shallow water and carried seaward into the deeper water environment in which the iron minerals were precipitating. Ebb-flow tidal currents, or offshore flowing storm-generated currents, are interpreted as the erosion and transportation agent. A plot of 102 cross-bed measurements in the Minorca Mine (Fig. 5-9) on the northeast edge of the Virginia horn shows 90 percent of the readings making a very prominent mode to the north–northeast and a minor, broader mode to the south (Fig. 5-9). This distribution is interpreted as the product of a strong flood tide toward the paleogeographically determined northern shoreline and a much weaker ebb tide. A study of the orientations of stromatolite mounds in the stromatolite horizon within the Upper Cherty member was conducted by Boerst (1999). His map is presented in Figure 5-10. A paleocurrent plot of mound elongation (Fig. 5-10) is interpreted as the result of shore-normal tidal currents and shore-parallel longshore currents in shallow water. The repetition of the cherty and slaty members has long been interpreted as the result of transgression and regression (White, 1954). The Virginia Formation was deposited seaward of the iron-formation, probably in a slope-type environment (Fig. 6-8) where episodic turbidity currents deposited graded beds. Some volcanic ash falls 125 �evidently settled into the basin forming graded beds with a totally volcanic composition. The dominance of black, fissile shale suggests the "raining out" of clay (such as settling through the water column) and deposition in deep, anoxic water below the wave base. Minor, thin, sandstone lenses were deposited by bottom currents (Lucente and Morey, 1983). N 102 Figure 5-9: A. Paleocurrent rose diagram of 102 cross-bed measurements from the Lower Cherty member (LC-4 submember) in the Minorca Mine. Inset B - Photo of cross-bedding in the Minorca Mine. Inset C - Photo of herringbone cross-beds in the Minorca Mine 126 �Figure 5-10: Mapped stromatolite mounds in the algal submember (I submember) in the Upper Cherty of the LTV 2E pit. The rose diagram represents the elongation of the mounds, with each elongate mound plotted on both sides of the rose diagram. From unpublished work by Kevin Boerst (1999). MINERALOGY Details on the origin of the primary iron minerals of the Biwabik Iron Formation, or any BIF, are exceedingly complex and controversial and are beyond the scope of this guidebook. In brief, Eh and pH are major controls on the stability of the various iron minerals and possible colloidal precipitates in the depositional environment and subsequent diagenetic environment. A variety of primary chemical precipitates for iron-formation in general have been postulated by an assortment of authors and include siderite, iron hydroxide/oxyhydroxide, iron silicates (Konhauser et al., 2002; Rajan et al., 1996), nontronite and iron oxides (Hiemenz, 1997), and colloidal iron silicates (Lascelles, 2007). Metamorphic effects, especially in the easternmost Mesabi Range, modified the mineralogy through progressive devolatilization reactions and modification or near obliteration of primary textures, especially as the contact of the Duluth Complex is approached. Recrystallization and replacement of primary iron minerals and granules during diagenesis and metamorphism has been extensive, and consisted of a number of discrete events. Earlier work on the oxidized taconites of the western Mesabi Range was accomplished by Bleifuss (1964). He showed that late hematite was developed by the oxidation and pseudomorphic replacement of magnetite octahedra, that layers of goethite were precipitated from solutions likely derived from the oxidation of siderite, and that some goethite formed by the oxidation of acicular iron silicate minerals. Additional work was done by Ojakangas in Zanko and others (2003). All of the magnetite grains are euhedral and are interpreted as late diagenetic in origin. Some of the hematite inclusions and crystals in magnetite are similar to those illustrated by Han (1982). He proposed that much of the magnetite formed by the replacement of, and overgrowth on, pre-existing hematite that served as nuclei. Han further suggested that ionic diffusion of ferrous iron was a key process in the formation of the magnetite. Organic carbon may have acted as a reductant in this process. Some of the magnetite may also originate from replacement of precursor siderite as observed in many examples in drill core and polished thin section from the western and central Mesabi Range. The genesis of the high-grade (natural) ore bodies that occur as pockets along fault zones in the Biwabik Iron Formation has long been debated. It is clear that a major hydrologic event removed 40 to 60 percent 127 �of the silica and oxidized the iron minerals to hematite and goethite. However, it is unknown whether these fluids were descending, cool, meteoric waters or ascending hydrothermal waters related to igneous activity. Did this event occur during the Cretaceous (the age of conglomerates that contain clasts of highgrade hematite), or prior to that time? Morey (1999) provided an excellent review of the arguments. He then proposed that a large-scale, topography-driven, hydrothermal ground-water system moved waters northward, during the Paleoproterozic, through the sands of the underlying Pokegama Formation, from the vicinity of the regional Penokean orogenic uplift in northern Wisconsin and east-central Minnesota. Graber and Strandlie (1999) questioned this concept and pointed out that the lack of metamorphosed natural ore bodies in the eastern Mesabi Range proves that they were formed long after emplacement of the Duluth Complex rather than during the Paleoproterozoic. The mineralogy of the Eastern Mesabi Range is reviewed in detail by Gundersen and Schwartz (1962) and that of the Dunka Pit area by Bonnichsen (1975). The following discussion serves to compliment the work of these researchers with work performed to date by Cliffs Natural Resources geologists in the area of Northshore Mining’s Peter Mitchell Mine. In contrast to iron formation of the Western Mesabi Range, which is dominated by greenschist metamorphic facies assemblages, rocks of the eastern Mesabi have been subjected to middle greenschist through upper amphibolite facies metamorphic conditions. Hornblende and pyroxene hornfels facies are also observed at the contact of iron formation with the Duluth Complex. Progressive effects of contact metamorphism toward the contact of the Duluth Complex have produced distinct mineral assemblages that document changing metamorphic conditions and concomitant changes in mineralogy. These changes are readily apparent in the highly reactive mineral assemblages of the Biwabik Iron Formation. The Biwabik Iron Formation in the vicinity of the Peter Mitchell Mine consists of higher grade metamorphic assemblages that include magnetite, iron-rich chain silicates, and quartz. Minerals found to occur in the Biwabik Iron Formation from the Peter Mitchell Mine and Dunka Pit are given in Table 5-1 below. The overlying pelites of the Virginia Formation are generally quartz-muscovite-biotite-microcline-plagioclase+/-chlorite schist with local zones of carbonaceous+/pyrite+/-pyrrhotite phyllite. Minerals found in the Virginia Formation in the area of the Peter Mitchell Mine are provided in Table 5-2. Excluding Cu-Ni PGE mineralization, the mineralogy of gabbroic rocks of the Duluth Complex in this area is relatively simple: labradorite, anorthite, augite, olivine group minerals, with accessory magnetite, ilmenite, and apatite. Table 5-1. Minerals found in the Biwabik Iron Formation in the area of the Peter Mitchell Mine and Dunka Pit. Amphibole Group Grunerite Ferro-actinolite (minor) Ferrohornblende (minor) Ferro-pigeonite Cummingtonite Pyroxene Group Hedenbergite (minor) Ferrohypersthene (minor) Ferroselite Diopside Olivine Group Fayalite Other Silicates Quartz Oxides/ Hydroxides Magnetite Carbonates Phosphates Siderite Hydroxylapatite Ferroan Clinochlore Hematite (minor) Ankerite Pyrite Biotite Goethite Rhodocrosite (minor) “Anthraxolite” Oligoclase Ilmenite Ferroan Dolomite Ferroan Kutnahorite Calcite Andesine Almandite Andradite Cordierite Sekaninaite? Wollastonite Epidote Titanite 128 Other Minerals Pyrrhotite �Table 5-2: Mineralogy of the Virginia Formation in the vicinity of the Peter Mitchell Mine. Silicates Muscovite Biotite Quartz Microcline Anorthite Oligoclase Andesine Anthophyllite Cordierite Staurolite Wollastonite Ferroan Clinochlore Andalusite Sillimanite Almandine Epidote Titanite Other Minerals Ilmenite Pyrrhotite Pyrite Chalcopyrite Rutile Ankerite Calcite Graphite/Carbonaceous material Quartz is the most abundant mineral in the Biwabik Iron Formation and constitutes layers of recrystallized chert. It also occurs pervasively in the matrix of the iron formation admixed with varying amounts of other fine-grained minerals such as magnetite, and minerals of the amphibole group. Quartz grains are equant to irregular and granoblastic. Fayalite also occurs as equant, granoblastic grains with quartz along with fine-grained magnetite, especially in the M, N, and O submembers (see later discussions on submembers of the iron-formation). In addition to quartz, fayalite is commonly associated with ferrohypersthene, cummingtonite, grunerite, magnetite, and rarely cordierite. Sekaninaite, the Feanalogue of cordierite, has been tentatively identified in one sample by x-ray diffraction methods. Magnetite is the second most abundant mineral in the Biwabik Iron Formation and is the ore mineral of interest in taconite units of the entire Mesabi Range. The occurrence of magnetite in the Eastern Mesabi Range is described in detail by Gundersen and Schwartz (1962) and Bonnichsen (1975) and will be only briefly reviewed here. In the Peter Mitchell Mine and at Dunka Pit, magnetite is abundant in the F through O taconite submembers where it constitutes ore. Equant magnetite grains range from submillimeter to locally 5 mm in size and are invariably euhedral due to their high placement in the idioblastic series due to their high force of crystallization, similar to garnet group minerals. Magnetite occurs in distinct laminae and in thin to thick, planar to wavy layers of variable continuity that consist of profusely intergrown grains with minor amounts of fine-grained quartz. Many magnetite-rich layers in the Upper Cherty member exhibit parting surfaces containing coarse-grained aggregates of magnetite crystals. Magnetite also is found as fine- to coarse-grained isolated clusters or “granules” in recrystallized chert, and as fine-grained replacement of algal stromatolites (I submember) and intraformational conglomerate clasts. Magnetite-rich layers alternate with quartz-rich (recrystallized chert) layers and amphibole-rich (generally cummingtonite-grunerite) layers. Poikiloblastic textures of magnetite are only seen upon microscopic examination of grains in polished thin section and reveals poikiloblasts enclosing quartz with very minor amounts of amphibole group minerals and fayalite. Minerals of the amphibole group are found throughout the formation with grunerite and cummingtonite most prevalent. These amphiboles are very fine- to very coarse-grained, subhedral, prismatic, isolated 129 �crystals or occur as intergrown crystals forming divergent fascicles, rosettes and radial, acicular aggregates in the matrix or in distinct bands of variable thickness. Poikiloblastic crystals of cummingtonite-grunerite can be observed macroscopically and microscopically to enclose grains of quartz and magnetite. Ferrohornblende and ferroactinolite, although relatively abundant, are difficult to distinguish from minerals of the cummingtonite-grunerite series macroscopically. Cummingtonite, grunerite, ferrohornblende, and ferroactinolite have all been observed replacing retrograded pyroxenes. Minerals of the pyroxene group are dominated by ferrohypersthene and hedenbergite in most of the submembers. Bonnichsen (1975) reports ferro-pigeonite and diopside locally at Dunka Pit. All of the pyroxenes in iron formation are medium- to very-coarse-grained, subhedral, variably poikiloblastic and exhibit replacement by amphiboles locally. Pyroxenes increase in both modal abundance and grain size as the contact with the Duluth Complex is approached (west-northwest to east-southeast across the Peter Mitchell Mine). Bonnichsen (1975), and Cliffs Natural Resources observations, show that ferrohypersthene crystals are disproportionately large compared to other mineral species when the mineral is found in abundance. Ferrohypersthene crystals, from 2-7 cm, are not uncommon in close proximity to the Duluth Complex and can be coarsely poikiloblastic containing quartz, magnetite, fayalite, hydroxylapatite, and amphiboles. Growth of these pyroxenes is not encumbered by previously existing or slower growing crystals. Almandine garnet occurs locally in the Biwabik Iron Formation, largely restricted to the Lower Slaty and Lower Cherty members, as 2 mm to 2 cm, red-brown subhedral to euhedral crystals generally associated with amphiboles, biotite, and ferroan clinochlore. Andradite garnet is commonly found in the A and G submember, in similar habit to almandine, at the Peter Mitchell Mine. Most of this garnet is a constituent of the silicate-rich bands, but it is also locally found in late-stage calcite-quartz veins with diopside and ferrohornblende. Layer silicates are subhedral and more common in the “slaty” layers imparting a schistosity. Oligoclase, andesine, rare carbonate group minerals, ilmenite, epidote, and hydroxylapatite are generally very finegrained and comprise matrix constituents. METAMORPHISM Contact metamorphism has affected the Biwabik Iron Formation and Virginia Formation disproportionately; assemblages in the iron-formation are very reactive and sensitive to more subtle changes in P-T compared to the Virginia Formation. The pelitic Virginia Formation lacks diverse mineral assemblages and records few reactions because many of the minerals are stable over large ranges in temperature and fH2O. In contrast, the Biwabik Iron Formation preserves prograde metamorphic reactions several kilometers west and north of the contact with the Duluth Complex; whereas, contact effects within the Virginia Formation, including partial melting, are limited to within 200 meters from the Duluth Complex. A very visual and pronounced example of the contact effects of the Duluth Complex is observed in the A submember at the top of the Biwabik Iron Formation. The A was originally a thin, “dirty” limestone-dolomite layer that was drastically transformed mineralogically into an off-white and dark-speckled rock with an assemblage containing typical calc-silicate contact metamorphic minerals that include wollastonite, diopside, andradite, ferrohornblende, titanite, andesine, epidote, fluorapatite, and recrystallized calcite and dolomite. Profound thermal effects of the Duluth Complex on adjacent metasediments are largely due to its high emplacement temperature. Chalokwu and others (1993) estimated emplacement temperatures of the Partridge River intrusion of the Duluth Complex to be approximately 1,150°C. Klein (1973), and subsequent researchers, have noted that olivine-bearing assemblages in iron-formation have largely been described from contact metamorphic occurrences involving high temperature intrusions. 130 �Taconite units of the Biwabik are characterized by regular interlayering of magnetite-rich and quartz-rich layers ranging from <1cm up to 12cm thick, locally. Outcrops farthest from the Duluth Complex display prominent banding with sharp contacts between magnetite, quartz, and quartz-amphibole layers. Boudining of quartz-rich layers, re-healed structural discontinuities, and intraformational conglomerate clasts are observed locally within taconite submembers. The degree of metamorphism and deformation increases with proximity to the Complex. Outcrops nearest the Compex lack pronounced banding compared to exposures that are more distal in the aureole. Exposures in close proximity to the Complex exhibit larger grain size, increased reaction band thickness, and more diverse mineral assemblages that can include quartz, magnetite, ferrohypersthene, hedenbergite, ferroselite, ferrohornblende, ferroactinolite, cummingtonite, grunerite, oligoclase-andesine, ilmenite, titanite, and pyrrhotite. Magnetite grain size is essentially unaffected by increased metamorphic grade, but does exhibit coarsening locally near the contact with the Duluth Complex along with increased Ti content. In the relative close proximity to the Duluth Complex, French (1968) delineated four zones of progressively metamorphosed iron-formation (Fig. 5-11) as follows. Zone 1 = “Unaltered” taconite with quartz, magnetite, hematite, siderite, ankerite, talc, chamosite, greenalite, minnesotaite, and stilpnomelane. Contrary to French (1968) and McSwiggen and Morey (2008), “unmetamorphosed iron-formation” in the Mesabi Range does not exist. All rocks have been subjected to diagenetic effects as well as burial and/or regional metamorphism, not to mention contact metamorphic effects due to the Duluth Complex in the Eastern Mesabi Range. Zone 2 = Transitional taconite with the above minerals but with extensive replacement by quartz and ankerite. French (Fig. 23; 1968) portrays zone 2 as occurring 2.8-10.0 miles from the Duluth Complex but notes that the first widespread metamorphic affects to the iron-formation occur three miles from the Duluth Complex. Zone 3 = Moderately metamorphosed taconite marked by the appearance of iron-rich amphibole (grunerite) at the expense of original iron carbonates (ankerite and siderite) and iron silicates (minnesotaite and stilpnomelane). Zone 3 is present 1.7-3.0 miles from the contact with the Duluth Complex and the temperature of formation was probably 300-400° C. Zone 4 = Highly metamorphosed taconite (sillimanite grade) is well expressed by the appearance of iron pyroxenes and complete recrystallization of the taconite. The taconite consists of quartz, iron amphiboles (cummingtonite and blue-green hornblende), iron pyroxenes (hedenbergite, and ferrohypersthene), and magnetite, with variable amounts of fayalite, garnet, and calcite. Zone 4 occurs within 1.7 miles of the contact with a minimum temperature of formation around 600° C (as indicated by the presence of wollastonite in the A submember). Morey and others (1972) agreed with French (1968) but show the divisions between the four zones to be positioned much closer to the Duluth Complex (see Fig. 5-11). Griffin and Morey (1969) found that hedenbergite is the dominant pyroxene phase in the Zone 3 / Zone 4 transition. Frost and others (2007) have since revised and expanded these zones/isograds to specifically include the formation of ferrohypersthene with crystalline graphite, hedenbergite, fayalite, and orthopyroxene. The reader is referred to this reference for details as well as an extensive list of possible metamorphic reactions in the Biwabik Iron Formation. 131 �#3 R.15W. re y R.14W. French #2 30 36 12 7 13 18 8 4 9 Aurora 27 31 #4 35 34 33 32 2 1 10 11 4 5 6 13 24 25 18 8 17 C h t lu Du le p om x 36 3 Trend of Biwabik Iron Formation Fault 9 8 12 7 Hoyt Lakes 17 14 7 Mo r ey 26 Drill Hole 17700 Biwab ik 11 12 35 Biwabik 10 28 29 31 4 5 23 22 36 30 French #2 Metamorphic zones of French (1968) 22 R.16W. R.15W. R.14W. Mo r ey #3 one mile T.58N. Mo 5 6 #2 Mo 1 Mo r ey #1 re y T.59N. 2 25 34 3 3 26 21 35 34 25 ault on F 33 32 20 19 27 28 29 24 14 26 27 19 24 6 Siph French #1 23 15 16 33 32 31 1 11 17 18 22 21 36 2 13 20 28 23 T.60N. French #4 French #3 22 T.59N. T.60N. R.13W. Metamorphic zones of Morey and others (1972) Figure 5-11: Approximate boundaries of four zones of metamorphosed iron-formation after French (1968) and Morey and others (1972). French (1968) found that metamorphism of the iron-formation was largely isochemical with a progressive loss of H2O and CO2 towards the Duluth Complex; with no change in the siderite and ankerite compositions towards the Complex. Possible reactions in Zone 4 (French, 1968) include: 1. 2. 3. 4. 5. hedenbergite formed by grunerite+calcite=hedenbergite+quartz+H2O+CO2; hedenbergite formed by ankerite+quartz=hedenbergite+CO2; ferrohypersthene formed by grunerite=ferrohypersthene+quartz+H2O; fayalite formed by grunerite=fayalite+quartz+H2O; and fayalite formed by magnetite+quartz=fayalite+O2. Bonnichsen (1968) felt that the metamorphism at Dunka Pit was also isochemical. He suggested that all of the CO2 was driven from the iron-formation; whereas, a portion of the H2O remained behind, although a mechanism of this partial segregation of volatiles was not proposed. Bonnichsen (1968) indicated this water reacted with the various pyroxenes and fayalite to form hydrous minerals, mainly amphiboles, until the supply of H2O was exhausted. However, textural evidence indicating recrystallization of siderite, and some greenalite, directly to hedenbergite and fayalite suggests the overstepping of intermediate minnesotaite and grunerite producing reactions. This indicates a steep thermal gradient over a protracted time interval during emplacement of the Duluth Complex and the local occurrence of pigeonite indicates temperatures of at least 800°C (Vocke, 1981). A rapid rise in temperature during emplacement of the Duluth Complex and overstepping of lower temperature metamorphic reactions would result in a virtually complete devolatilization of the iron formation. Dehydration and decarbonation reactions during prograde metamorphism of iron formation are responsible for a 10-50% volume reduction of the original lithology, depending on the original bulk chemistry (Floran, 1975; Caddey and others., 1990). Volume loss in the Biwabik Iron Formation due to metamorphic devolatilization has enhanced the thinning of this unit eastward across the Mesabi Range. 132 �Closer to the Duluth Complex, Bonnichsen (1968) found that metamorphic grade of the Biwabik Iron Formation reached pyroxene hornfels facies in the Dunka Pit area. At this locale, the iron-formation is positioned ≤¼ mile from the Duluth Complex. It is mainly composed of recrystallized quartz, magnetite, and orthopyroxene – the orthopyroxene is mostly ferrohypersthene with inverted pigeonite (characterized by ferrohypersthene with hedenbergite lamellae). Also present are lesser and variable amounts of Ca pyroxene (hedenbergite, diopside), fayalite, cummingtonite, talc, and hornblende±actinolite (mostly formed during retrograde metamorphism). Bonnichsen (1968) describe poikilitic textured minerals (ferrohypersthene, fayalite, and cummingtonite) in the iron-formation enclosing quartz and magnetite. Perry and Bonnichsen (1966) determined the maximum temperature of metamorphism at Dunka Pit, as determined by O18/O16 ratios of quartz and magnetite, was estimated to be 700-750° C. This was confirmed by Hyslop and others (2008) in their recent O and Fe isotope study of the Biwabik Iron Formation. They showed that isotopically determined temperatures decreased from about 700°C at the contact of the Duluth Complex, to 375°C at a distance of 2.6 km from the contact. Pigeonite in ironformation in contact with the Partridge River intrusion of the Duluth Complex suggests peak metamorphic temperatures in excess of 825°C, as determined by Chalokwu and others (1993). At the Wyman Creek Cu-Ni deposit, Perry and Bonnichsen (1971) estimated a temperature range of 400-650° C for metamorphism of the iron-formation in drill hole #17700 (Fig. 5-11), in which the top of the ironformation is located 525 feet below the Duluth Complex. A more recent study by Muhich (1993) at Dunka Pit, found that where the Biwabik Iron Formation is in direct contact with the Duluth Complex (the intervening Virginia Formation is absent), metamorphism was not isochemical and that some metasomatic transfer of elements took place in a thin zone that spans the contact. This transfer is illustrated by significant gains in titanium in the iron-formation. Overall, within a 25 foot wide zone on one side of the contact, Muhich (1993) found that the iron-formation showed gains in TiO2, V, Al2O3, CaO, Na2O, K2O, Ba, Rb, Sr, MgO, Cu, Ni, and H20; and a loss of SiO2 and P2O5. On the other side of the contact, also in a 25 foot wide zone, the Duluth Complex showed gains in K2O, Rb, S, Fe2O3, and H2O; with a loss of TiO2 and MnO. A temperature of metamorphism for the iron-formation, as determined from coexisting titanomagnetite/ilmenite pairs, was found to be in the range of 651-689° C. PRODUCTION FIGURES—IRON ORE AND TACONITE The annual amounts of direct-shipped ore and taconite produced from the Mesabi Iron Range are shown in Figure 5-12. Production and shipping of direct ore started in 1892 and rose steadily until 1953 when a maximum 76 million tons were produced in one year (note the precipitous drop in direct ore production corresponding to the Great Depression). At around 1955, there was a dramatic decrease in the amount of direct ore as the various mines became depleted. This also corresponds to the initial start-up of taconite mining, using a concentrating and pelletizing method developed by E.W. Davis of the University of Minnesota. Reserve Mining opened the first taconite operations in 1955 (Peter Mitchell Mine) and was shortly followed by Erie Mining in 1957 (the old LTV site). Six more taconite operations were added in the 1960s, and by 1967, annual taconite production exceeded direct ore production. The mid-1980s marked a serious depression in the iron ore and steel industry that resulted in the closure of one operation (Butler Taconite) and the bankruptcies of two other taconite producers including Reserve Mining Company – the former operator of the Peter Mitchell Mine. The Peter Mitchell Mine reopened as Cyprus Northshore Mining in 1989. It was subsequently purchased by Cleveland-Cliffs in 1994 and has been operated as Northshore Mining since that time. More recently, LTV Steel and Eveleth Taconite have closed; Evtac has since reopened as United Taconite. 133 �Figure 5-12: Annual production figures for direct ore (includes all forms of direct ore) and taconite for the period 1892-2008. Data and graph from James Sellner, Minnesota Department of Natural Resources, Lands and Minerals Division, Hibbing, MN WHAT'S IN A NAME? (THOSE CONFUSING IRON-FORMATION SUBMEMBERS) The four-fold stratigraphy of Lower and Upper Cherty and Lower and Upper Slaty members (Wolff, 1917) is still used at each of the currently operating (and inactive) taconite mines on the Mesabi Iron Range. However, each of the mining companies further subdivides the Biwabik Iron Formation into several submembers based on bedding types (Fig. 5-13) and mineral assemblages. It is at this point that the Biwabik Iron Formation stratigraphy becomes very complicated and at times confusing. This is mainly due to the following reasons: • • • • There are localized lateral facies changes between mines (and even within a single mine). Some mines reconcile these differences by splitting out numerous submembers (each with a distinct bedding type, texture, ore grade, and/or mineral assemblage), whereas other mines lump many of these same differences within a single submember. There are significant lateral facies changes over several miles between mines. For example, a particular horizon may be massive-bedded at one location but is regular-bedded a few miles away. This is particularly troublesome within the Upper Cherty member in the western two thirds of the Mesabi range. Not all mines use the same numbering system—some use abbreviations (for example LC—Lower Cherty member) followed by a number (as in LC-5 at the top of the Lower Cherty member). However, other mines use an alphabet system, devised by Gundersen and Schwartz (1962), starting with the A submember at the top of the Upper Slaty member (in this system the top of the Lower Cherty member corresponds to the R submember). And further still, another mine refers to the Lower Cherty member as the number 1 unit and subdivides it into eight submembers, with 1-8 at the top of the Lower Cherty member. Some mines label downward in their numbering system, whereas other mines label upward in their numbering system. 134 �Figure 5-13: Textural characteristics of the Biwabik Iron Formation (from Severson and others, in prep. – modified from Pfleider and others, 1968). The submember nomenclature that is used at each of the mines is summarized in Figures 5-15 through 518. It can readily be seen on these summary charts that submember names change nomenclature from one mine to the next. This is because there are few good marker horizons within the Biwabik Iron Formation, and even these can exhibit gradual lateral facies changes or pinch-and-swell relationships to each other. A few of the potential marker horizons within the Biwabik Iron Formation are presented below. • Top contact of the Biwabik Iron Formation with the Virginia Formation—In the eastern half of the Mesabi Iron Range a carbonate horizon is present at the very top of the Upper Slaty member and the contact between the Biwabik Iron Formation and Virginia Formation is easily recognized (Gruner, 1924). However, to the west of Hibbing, the carbonate layer is absent and lenses of thin-bedded iron carbonate iron-formation are present in the Virginia Formation, and the top of the Biwabik Iron Formation is not easily discerned. • Algal horizon (submember I) – A thin unit containing algal stromatolites and jasper-bearing intraformational conglomerate is present near the top of the Upper Cherty member. This submember is easily recognized but is not present west of Hibbing. • Lower Cherty member - The Lower Cherty member is remarkably homogeneous over most of the Mesabi Range. The major units that are present, depicted in Figure 5-15, range from thin-bedded rocks at the base (Basal Red Unit of Figure 5-15) that is overlain successively by Regular-Bedded, Wavy-Bedded, and Irregular-Bedded & Mottled units respectively. Each of these units have been recently correlated in over 380 dill holes along the Mesabi Range and are so named for the bedding type that is dominant (Severson and others, in prep.). These major units (left side of Figure 5-15) are suggestive of a transgressive period at the beginning of iron-formation deposition (Basal Red Unit) followed by a regressive period wherein the overlying units were deposited in progressively shallower water. As can also be seen in Figure 5-15, each of these units has been called a plethora of submember names by the each of the various taconite mines. 135 �• Base of the Biwabik Iron Formation—The base of the Lower Cherty member is generally characterized by thin-bedded iron-formation (also called the Basal Red Unit - Fig. 5-15) with localized algal stromatolite and basal conglomerate horizons. However, at many localities the base of the Biwabik Iron Formation exhibits a gradational contact with the underlying Pokegama Formation. In the Virginia horn area, the base of the Biwabik Iron Formation contains an ironbearing sandstone (White, 1954) that some mines include with the iron-formation, whereas others lump this type of material with the Pokegama Formation. Figure 5-14: Correlation chart of submembers at each of the mines/areas within the Biwabik Iron Formation (correlations from Severson and others, in prep., except for the Evtac and Butler mines). All columns are hung on the base of the Lower Slaty member. Bars to the left of the columns indicate mined taconite ore zones. Note that correlations at the Thunderbird South Mine (Evtac) and the MSI mine (MSI-Mesabi Steel and Iron) are tenuous. From the above description it is evident that there are few good marker horizons within the upper portions of the iron-formation. Even the top and bottom contacts of the iron-formation are gradational and subject to various interpretations. The "Intermediate Slate," the algal horizon in the Upper Cherty member, and most of the units in the Lower Cherty member are the only easily recognizable marker units. However, even using these horizons as markers, one can see from the correlation charts that there are problems. Clearly, much additional work needs to be done to understand how submembers at one mine correlate with submembers at an adjacent mine. These types of studies could inevitably be important in determining why ore grades, and waste rock characteristics, change between mines and even within a 136 �single mine. Such studies have been initiated by geologists at the Natural Resources Research Institute (Severson and others, in prep.) and over 380 drill holes have been logged and correlated in an area extending from Biwabik to Coleraine, MN. W E Correlation Chart of Lower Cherty submembers at various taconite mines/areas on the Mesabi Range Red bars represent portions of the Lower Cherty that are mined at the various taconite operations. No Scale Implied Lower Cherty thins to the west and is not recognized in Cass County LC-3 Ox Bold Striped LC-4A-upper Variably-bdd, Mott Reg/Irreg-bdd, halos LC-4A-lower Wavy-bdd Wavy-Bdd Unit LC-4B Wavy-bdd w/thick wavy LC-3 Wavy-bdd FeSil LC-4C Wispy/Wavy S&P 1-6 Wavy-bdd w/thick wavy LC-5A Local Thin-bdd Unit not present LC-5B 1-4 1-3 8-3 LC-6 1-2 LC-2 Basal Red Unit LC-1B Basal Contact Units LC-1A no submember designation approx. 300' 280' Avg. thickness of Lower Cherty (in feet) - in the mine handouts rarely drilled LC-5B BoldStriped Mesabi Select Laurentian LC-8 Mesabi Select LC-7 Bold Striped BoldStriped LC-5A Irreg/Congl FeSil (mott) LC-5A Irreg/Congl FeSil (mott) LC-6 Mott LC-4 Wavy-bdd LC-4 Wavy-bdd LC-5 Wavy-bdd no halos LC-3 Wispy Wavy S&P LC-1 bottom of LC-1 or top of basal units algal/congl/basal Ss 250' BoldStriped LC-4 LC-4 3-37' S T halo distribution unknown R through W R halo distribution unknown no halos @ depth LC-3 LC-3 U LC-2 LC-2 V LC-1 LC-1 W 225' 190' 110' LC-1 waste bottom of LC-1 or top of basal units algal/congl/basal Ss rarely drilled "Footwall IF" rarely drilled no submember designation approx. 250' 200' 200' Mesabi Select LC-5A Mott LC-4 Wavy-bdd S&P Northshore LC-5B LC-5A Mott LC-2 ore bottom of 1-0 BoldStriped Mesabi Select Cliffs/Erie (old LTV) LC-3 waste LC-2 LC-1 McKinley/ East Reserve LC-5B incr Si LC-3 Wispy Wavy S&P LC-2 1-0 "Basal Red" rarely drilled LC-5B Utac ? 1-5 Wispy/Wavy S&P not present Reg-Bdd Unit Minntac East Pit 1-7 Wavy-bdd Transition Zone Wk Wavy/Reg-bdd Minntac West Pit Mesabi Select 1-8 Med/Reg-bdd (mott) LC-4A-middle LC-4 Wavy-bdd (w/halos unless otherwise noted) LC-2 oxidized LC-5 Thick/Reg-bdd (Mott) oxidized oxidized Irreg-Bdd & Mott Unit (FeSil) Hibtac LC-1 Ox Mesabi Select Mesabi Select Unit Bold Striped Unit Keetac Siphon Fault Generalized Units Coleraine (OxTac) Figure 5-15: Correlation chart of Lower Cherty submembers at the various taconite mines/areas along the Mesabi Range. Contacts between all submembers are transitional/gradational! Note that the MSI mine area (old Butler mine) is not portrayed on this figure; however, the submembers are similar to those at Keetac. “S&P”= salt-andpepper texture due to disseminated magnetite in chert. From Severson and others (in prep.). W E Correlation Chart of Lower Slaty submembers at the various taconite mines/areas on the Mesabi Range Red bars represent portions of the Lower Slaty that are mined at the various taconite operations. No Scale Implied! Generalized Units Coleraine (OxTac) Keetac Minntac West Pit Hibtac Minntac East Pit Utac Laurentian McKinley/ East Reserve Cliffs/Erie Northshore, & Dunka Pit considered to be ore all the way to top of Upper Cherty Alternate Contact for top of Lower Slaty probably similar to contact of Gruner (1924) and recognized by Severson and others (in prep.) Uppermost Thin-bedded UC-3 LS-10 Middle IBC Middle Thin-bedded 90-115' Possible top of Lower Slaty at Utac 10-65' No Lower Slaty west of Grand Rapids probably similar to contact of White, 1954 10-80' LS Traditional Contact for top of Lower Slaty 15-20' 5-25' gap 25' LS LS all or bottom half Intermediate Slate Total Thickness of Thin-bedded rocks (including IBCs) to bottom of Lower Slaty member LS-9 "Interbedded Cherts" (IBCs) Reg-bedded w/wavy-bedded zones "Intermediate Slate" UC-3 (bottom) UC-2 LUC-2 40-80' LUC-1 Thin-bdd zone observed in hole MGS-2 80-110' UC-1 Traditional top of Lower Slaty 30-90' 85-130' 110-145' LS LS-6 LS-2 LS-6 LS 50-85' P P (mixed thin-bdd, "Mesabi Select equivalent" & slate) 15-20' 8-45' 0-26' LS-1 3' 1-10' 15-20'gap <1.5' locally present 1-2' Thin-bedded iron-formation UC-3A UC-3 UC-2 UC-1 LUC-3 LS-7 25-45' algal layer UC-3 (middle) 100-140' LS-8 LS-7 Traditional Contact for top of Lower Slaty 145-200' LS-9 215'approx. UC-4 LS-10 90-115' LS-8 Lower IBC Lowermost Thin-bdd UC-13 UC-12 UC-11 UC-13 UC-12 UC-11 Upper IBC 230-260' 145-240' 145-200' <6' locally present LS 10-20' Q 25' Q locally present Chalcedonic Chert ("Flint") at Base and top of Lower Slaty Conglomerate Algal columns and/or jasper-bearing conglomerate Paint Rock "Mesabi Select equivalent" (Similar to Mesabi Select Unit but in Lower Slaty !) NOTE: Specific correlations of the IBCs are tenuous as they occur as large lenses in the thin-bedded rocks and are not necessarily laterally-continuous from one mine to the next. No Scale Implied! Figure 5-16: Correlation chart of Lower Slaty submembers at the various taconite mines/areas along the Mesabi Range. Note the three optional contacts for the top of the Lower Slaty. From Severson and others (in prep.). 137 �W E Correlation Chart of Upper Cherty submembers at various taconite mines/areas on the Mesabi Range Red bars represent portions of the Upper Cherty that are mined at the various taconite operations. (dashed red bars represent portions of the Upper Cherty that are possible taconite ore zones) 25'/15 150'/11 145'/12 Generalized Units Minntac West Pit Minntac East Pit Utac Average Thickness of Upper Cherty 140'/15' 125'/10 100'/7 (130-160') (100-160') (90-100') (130-190' downdip) Upper Cherty merges with Lower Cherty at Grand Rapids; both pinch out in Range 26 (White, 1954) (in feet/no. of holes) (120-165') (3-40') (195-100') (Thickness Range) (in feet) downdip 190' Conglomerate Lower Reg-bdd Unit Algal/ conglomerate UC-2 plus Thick-bdd and mottled submarine valley? Thinbedded Upper Cherty Alternatingbedded based on oxidation textures Wavybedded Algal Units UC-3 UC-8 Upper Reg-bdd Unit ? Upper Alt-bdd Unit Algal Unit UC-16 UC-15 UC-15 Bottom Alt-bdd Unit UC-1 UC-16 UC-14 UC-14 UC-7 UC-6 UC-5 upper pit limit ? UC-3 (upper 1/3) Reg/Medbedded 105' Cliffs/Erie (old LTV) Northshore approx. 215' approx. 140' McKinley/ East Reserve Laurentian ? based mostly on units intersected in MGS-2 Hibtac Drastic change in bedding types of Upper Cherty in vicinity of Biwabik, MN Keetac Coleraine (OxTac) G Reg/Wavy H thin Wavy I algal I algal J Thin Wavy J Thick-bdd K Wavy/Mott K Wavy-Bdd w/congl L wide Wavy L Wavy-bdd FeSil Thick-bdd M Reg M Reg-bdd w/ congl N Wavy N Fayalite-Qtz rock low Mgt content O Fayalite-Qtz rock Mgt granules Thick/Irreg-bdd algal Wavy-bdd H algal ? ? O ? Reg-bdd Eastern Mesabi Range No Scale Implied Strongly oxidized - may not always be present wide Wavy Wavy-bdd another 2' thick algal about horizon 5-8 feet below base of Upper Cherty Figure 5-17: Correlation chart of Upper Cherty submembers at various taconite mines/areas along the Mesabi range. Note that the Upper Cherty thins drastically in the central portion of the Keetac area in a valley-like morphology. Note also the distinct change in bedding types that constitute the Upper Cherty in the eastern Mesabi Range and the two possible algal units. Upper Cherty submembers at Coleraine are based solely on oxidation characteristics that have been described in Zanko and others (2003). Submembers at the Cliffs-Erie site, and Northshore and Dunka Pit mines, are described by Gundersen and Schwartz (1962). Modified from Severson and others (in prep.). W E Correlation Chart of Upper Slaty submembers at various taconite mines/areas on the Mesabi Range Generalized Units Average Thickness of Upper Slaty - in feet/no. of holes (Thickness Range/ no. of holes) Minntac West Pit Minntac East Pit up to 260' 115'/10 100'/3 80'/4 (30' to West) (153' @ west end) (98-104'/3) (64-102'/4) Coleraine (OxTac) 25'/9 Keetac (13-33'/9) Hibtac (145 & 205') (89-153'/10) 2 holes - east end Utac Laurentian Cliffs/Erie (old LTV) McKinley/ East Reserve Northshore 120' 93' in MGS-2 (74' &134') in 2 holes Virginia Formation top not drilled not exposed Not Reg-Bedded Unit Alt-Bedded Unit Thin-Bedded Unit Chalcedonic Chert ("flint") unit D very s in o rille d bu sim ne ho ilar le to u (MG t ... nits S-2) at C a liffs re -Eri e A Dolomite/ Limestone Unit valley fill material? Upper Slaty thickens and is mixed with voluminous argillite to the west (White, 1954) - difficult to pick top contact!! Red bars represent portions of the Upper Slaty that are mined at the various taconite operations. B A Dolomite/Marble Vague-bdd B Chert & Diopside Thin-bdd C Thin-bdd D Wavy-bdd E F G Mass-bdd w/ syneresis cracks Dolomite/Marble C D Thin & Wavy E Mass-bdd w/ syneresis cracks F Thin/Alt-bdd Thin & Wavy-bdd Mass-bdd Eastern Mesabi Range No Scale Implied Figure 5-18: Correlation chart of Upper Slaty units and submembers at the various taconite mines/areas along the Mesabi Range. Note that the Upper Slaty exhibits drastic changes in thickness across the Mesabi Range. Submembers at the Cliffs-Erie site, and Northshore and Dunka Pit mines, have been described by Gundersen and Schwartz (1962). Modified from Severson and others (in prep.). 138 �Cliffs-Erie Site (old LTV) Stratigraphy modified from Gundersen and Schwartz (1962) and stratigraphic column in Morey (1993) Stratigraphic Columns for taconite mines on the east end of the Mesabi Range Total iron-fm thickness = 350-470 feet (Grout and Broderick, 1919) Modified from Gundersen and Schwartz, 1962 Virgina Fm Total iron-fm thickness = 350-470 feet (Grout and Broderick, 1919) 35-55' Limestone A Vague BDD, B Chert + Fe Sil C Upper Slaty Virginia Formation Virgina Fm Thin BDD Keweenawan? Sill delaminated A TAC ORE 25-55' D Thin&wavy BDD +/- chert pebbles E Mass w/syneresis cracks, High Phos (columns hung on base of Lower Slaty) Northshore 3-6' A Chert/marble 13-20' B Chert + Diop 26-43' 35-50' C Dunka Pit Thin BDD Keweenawan Sill Thin BDD F +/- gran jasp beds modified from Bonnichsen, 1968 Total iron-fm thickness = 175-300 feet 8-10' D Wavy BDD 2-6' E Sept. cracks 13-20' F Thin/Wavy-bdd w/ Sept. cracks 25-34' G Med-bdd Mgt-rich Beds w/Irreg-bdd zones <5' Algal congl 8-12' H Reg-bdd 3-10 I Algal/congl 15-24' Wavy-bdd w/ thick cs-grn J Mgt beds 30-47' 15-40' Reg BDD w/ wavy-straight BDD Algal 20-35' O wide spaced wavy beds 85-130' P Thin BDD Non-mag Lower Slaty Aurora Sill (also intrudes submembers I through O) B Chert + Diop ? C Thin/Vague-bdd Wavy-bdd thin-spaced F Reg/Med-bdd (slumped), Garnet Alt-bdd w/dk grn G hedenbergite beds (garnet) 30-60' L Wavy-bdd thick bnds @ irreg spacings 9-20' Reg-bdd M w/fayalite 2-5' N Fayalite-qtz 6-17' O Thick-bdd 60' P Vague/Thin-bdd green w/minor Mgt bands Q 10-20' S&P R Thick-bdd, mott 11' S TAC ORE Lower Cherty T Vague, wavy BDD +/- mott U Thin-reg BDD Thick + thin BDD 25-45' Thin BDD V + Arg + jasp W 3' V Wavy BDD +/- mott Thin-reg (Hem) Algal / congl MGT granules Pokegama Fm O Thin-bdd sim ilar to P but better-defined bedding & w/m ore mgt Vague/Thin-bdd P Green (FeSil) wk-mod mag 50-70' Lower Cherty Granule/qtzose Congl Mass/Vague-bdd P Green (FeSil) nil-wk mag Q Thin-bdd & black w/ sulfides & (garnet) Lower Slaty Lower Cherty Pokegama Fm 50' Submember Name: Upper Cherty 3 UC-3 Bedding Descriptions (see abbreviations) 0 Upper Cherty M Straight/Wavy-bdd Average Thickness (in feet) Vertical Scale in feet 50 25 K Wavy-bdd (congl) GraphiticArgillaceous IF R Thick BDD 8' S ? BDD/MGT rich 5' T Granules 10' U Reg BDD? Reg/Mass-bdd w/ H pinch & swell beds I Intraform. Congl Mott, Mixed bdd = J Thin/Med-bdd K' Wavy-bdd (congl) 3-35' 10-20' Q carbonaceous 2' 5-15' Low MGT content The most metamorph & reconstituted 45-95' Upper Slaty E Mass-bdd (no sept. cracks) K 26' "Intermediate Slate" Keweenawan Sill C Thin/Vague-bdd D Thin/Lenticular-bdd 20-40' Wavy BDD w/ intraform rip ups L M SIPHON FAULT TAC ORE 40-80' Keweenawan? Sill TACONITE ORE K wide spaced thin wavy beds T ACONITE ORE Upper Cherty Keweenawan Sill iron-formation metamorphosed by Duluth Complex and thusly contains superimposed metamorphic textures A Chert/marble 20-40' I J Wavy BDD, mott Virg. Fm delaminated A 25-85' H Thin wavy BDD Duluth Compex 20-40' TAC ORE 40-75' Green, wavy reg G BDD Variable!! R thru V can't be distinguished Columns denote submembers of the Biwabik Iron Formation as defined by this investigation coupled with individual taconite mine descriptions along the length of the Mesabi Range. Figure 5-19: Submember nomenclature for the Biwabik Iron Formation as used at the various taconite mines and idled mine sites that will be visited on this field trip. Modified from Plate II in Severson and others (in prep.). Mined horizons are shown by a bar on the side of the column. 139 �Figure 5-20: Bedrock geology at the Mesaba Nugget project. Note the locations of two cross-sections (shown in yellow) that are portrayed in Figures 5-21 and 5-22. Figure 5-21: Cross-section A-A’ at the Mesabi Nugget project. Figure 5-22: Cross-section F-F’ at the Mesabi Nugget project. 140 �Figure 5-23: Modeled geologic section of Northshore Block 21. Diamond drill hole traces and relative unit thickness are illustrated on the section. The general geology of rocks seen at stops 5-6 through 5-11 is shown. Figure 5-24: Modeled geologic section of Northshore Block 1E. Submember thickness is illustrated on the section. The overlying Virginia Formation (VF) and intruding Duluth Complex (GB) are viewed along the south end of the section. 141 �FIELD TRIP STOPS Figure 5-25: Location of field trip stops in the eastern Mesabi Iron Range of northeastern Minnesota. Dark black lines outline the margins of the Biwabik Iron Formation. DIRECTIONS: Drive south from Ely on highway 21 through Babbitt (take a right at the T intersection at Babbitt) and Embarrass to the Four Corners intersection. Take a left at the intersection and go south on 135 about 5.3 miles to a gated entrance for PolyMet Mining and Mesabi Nugget (also known as the Cliffs-Erie site or old LTV mine). Continue down this private drive about one mile to the construction entrance to the Mesabi Nugget plant and turn right – proceed down this road and check in at the guard shack. STOP 5-1: Mesabi Nugget Project Site Location: T. 59 N., R. 15 W., Sections 13 and 24, Allen quadrangle Description: Steel Dynamics Inc. and Kobe Steel Ltd. have jointly partnered in the Mesabi Nugget Project to construct and operate the world’s first large scale Rotary Hearth Furnace (RHF) on the Mesabi Iron Range. The Mesabi Nugget Project is under construction on a portion of the former Erie Mining Company / LTV Steel Mining Company property. The construction site is located on the footwall of the old Area 1 mine site. The ITmk3 RHF technology was developed by Kobe Steel and will be used to produce a domestic supply of high-quality iron nuggets for Steel Dynamics’ electric arc furnace facility in Butler, Indiana. The technology will combine iron concentrates and non-coking coal into a simple onestep process that reduces and melts the iron concentrate within the rotary furnace, producing a 97 percent pure iron nugget. The second phase of the project involves permitting and reactivation of several taconite pits adjacent to the construction site to extract iron ore for the nugget process. The Mesabi Iron Range was initially identified by the first Minnesota State Geologist, Henry Eames, in 1866 at the northeast end of Embarrass Lake, roughly two miles to the west of this project site, at a point where the Biwabik Iron Formation intersected the Embarrass River. The eastern end of the Biwabik Iron Formation was first mapped from Embarrass Lake to Birch Lake by the Minnesota Geological Survey during the years of 1879 and 1881. By 1884 the Duluth & Iron Range Railroad had been built to provide access to the newly discovered Vermilion Iron Range at the southeastern end of Lake Vermilion. The railroad provided access for iron ore shipments from Tower to Lake Superior. The original rail route runs north–south along the eastern boundary of the Mesabi Nugget project site. The Village of Mesaba, shown in Figure 5-21, was the first incorporated community on the Mesabi Range. The village’s rail station, 142 �Mesaba Junction, provided a departure point for the early mineral exploration and new arrivals to the region. Today the rail access into the Mesabi Nugget plant intersects the Canadian National rail line near the site of the original Mesaba Junction. The structural geology and outline of the four informal members of the Biwabik Iron Formation are shown in Figure 5-21 overlying a current aerial photograph of the project area. Four mining areas of the former Erie/LTVSMC comprise the Mesabi Nugget project area: Area 1 to the north, Area 6 & 9 to the west, and Area 2wx to the south. The western mining area is dominated by the northwest striking Donora fault and several northeast trending faults. To the south the Aurora Sill was intruded into the lower portion of the Upper Cherty member and caused a pronounced steepening of the dip in Area 6, as shown in Figure 5-22. The structural complexity of the area has resulted in a diverse and long history of both natural ore and taconite mining. By the early 1900’s there were eight underground and open-pit mining operations within the boundary of the Mesabi Nugget project. The early mining operations extracted natural ore from oxidized zones in the Upper Cherty member associated with the fault structures of the region. The prominent mining operations are shown in Figure 5-21. Figure 5-23 is a cross-section through the Area 2wx mining area. It identifies the gently sloping Biwabik Iron Formation, the Wentworth Fault, a diabase dike intersecting Area 2wx, and the Duluth Complex to the south with associated metamorphic isograds. In the early 1950’s Pickands Mather & Co. constructed and operated a taconite pilot plant adjacent to the present Area 9 pit on the western edge of the project area. During the ensuing years Erie Mining Company was constructed to utilize the extensive ore reserves of high grade taconite of the eastern end of the Mesabi Iron Range. Figure 5-19 is a generalized geologic column of the Biwabik Iron Formation in the Aurora area. NEXT: Return to private drive and proceed approximately 2.5 miles east to the guard shack near the office buildings of PolyMet Mining Corp. After receiving permission to enter the property, go straight and follow this road about 2.3 miles to a T-intersection with Dunka Road (another private company road). Continue north (left) at this intersection and drive about 3.5 miles along various mining company roads to the Pit 5E location. The stratigraphic nomenclature for the Biwabik Iron Formation at the Cliffs-Erie site is presented in Figure 5-19. STOP 5-2: Lower Cherty and Lower Slaty members Location: Pit 5E, Cliffs-Erie site, T. 60 N., R. 14 W., sec. 36, SE, SE Allen quadrangle; UTM: 571,040E/5,275,876N (NAD-83) Description: At this inactive mine pit the entire stratigraphic section of the Lower Cherty and Lower Slaty members can be viewed. Also present at this site are localized exposures of the underlying Pokegama Formation and granitic rocks of the Archean Giants Range Batholith. Both are exposed in the floor of the mine where they are present as several small domal features giving the overall impression of an egg-carton morphology. The Giants Range Batholith at this site is characterized by a porphyritic quartz monzonite that contains an unusual amount of chlorite. The Lower Cherty member at this site is about 80 feet thick. Most of the units are regular- to mediumbedded and constitute taconite ore. At the bottom of the mine bench, the W submember that is characterized by interfingering conglomerate, algal stromatolite horizons, and pale gray chalcedonic chert bands with quartz-filled syneresis cracks; all three can occur in direct contact with either the underlying Pokegama Formation or granitic rocks of the Giants Range Batholith. The stromatolites are recognized by bright red jasper columns. The conglomerate contains a variety of iron-formation clasts (cherty and 143 �thin-bedded iron carbonate) in a chalcedonic matrix with detrital quartz grains. Above the W submember is the V submember which is typically thin-bedded, locally contains chalcedonic chert bands (up to a few inches thick), and appears to be less than one foot thick. The V submember is not exposed in the mine walls (covered by mining rock debris) but can be found as loose pieces on the mine floor in close proximity the first stromatolite horizon that will be visited. These pieces are typically thin-bedded, nonmagnetic, and exhibit fine-scale cross-beds that could be likened to hummocky cross-stratification (HCS). At the bottom of the mine face is a Regular-Bedded Unit that is correlative with the U submember. A one foot thick imbricated conglomerate is present about three feet above the stromatolite horizon in the U submember. This unit transitions upward into a Wavy-Bedded Unit (the T submember), which in turn, grades upward into the Mottled Unit (submember S). Both are similar in that they exhibit magnetite-rich wavy beds that terminate, bifurcate, and pinch and swell in a semi-random manner. The S submember contains conspicuous iron carbonate mottles (ankerite?) that are pink and <1 cm across. The WavyBedded Unit (submember T) at this locality is identical to all of the other wavy-bedded units at all of the taconite mines in the Mesabi Range. Within the T submember the magnetite is in: 1. the wavy beds, 2. disseminated throughout the cherty bands, and 3. within mottles that are generally less than 1 centimeter in diameter and cored by iron-carbonate. The T submember is about 50 feet-thick and is easily recognized in drill core due to the wavy beds and a salt-and-pepper texture that is defined by disseminated magnetite. Cross-beds are observed in some of the fallen blocks that are associated with this unit. Crossbedding measurements taken elsewhere (Ojakangas and others, 2005), although not definitive, are suggestive of a tidally-influenced marine environment. This, coupled with Walther's law of succession of sedimentary facies (the facies observed vertically are also similarly related laterally), places the deposition of the iron-formation seaward of the Pokegama Formation. The Lower Slaty member at this site is 80 feet thick and consists entirely of thin-bedded, weakly to moderately magnetic rock. At the base of the Lower Slaty member is the "Intermediate Slate" which is characterized by thin-bedded, black, organic-rich mudstone that is about five feet thick at this locality. The "Intermediate Slate" is extremely fissile and locally exhibits bright, shiny graphitic surfaces with bedding-parallel slickensides. Pyrite is common to this submember and is present as both disseminated fine- to medium-grained cubes and as thin disks (marcasite/pyrrhotite) along bedding planes. All of the Lower Slaty member constitutes waste rock at this mine. Note that the name "slate" has been applied to all thin-bedded rocks in the Biwabik Iron Formation, but this term is a misnomer, because these rocks are essentially unmetamorphosed and do not have the cleavage of a true slate but merely a parting parallel to bedding (White, 1954). Morey (1992, 1993) reported that the "Intermediate Slate" possibly contains an ash-fall component. It has the highest Al2O3 content of any analyzed sample of the Biwabik Iron Formation. NEXT: From Stop 5-2, return to the south to the intersection with Dunka Road. Take a left and head east along Dunka Road about 1.8 miles. While continuing to the next stop the bus will go through Pit 3 where ore was mined from the Lower Cherty member. Note that the undulating floor of this pit was exposed during striping by following the bedding trends of one or two bed forms. Imagine that this roll-and-swell topography may mimic the original bedding surface of the unlithified iron-formation as it was deposited in shallow water. After going through Pit 3 the road takes a sharp jog to the south. At this locale, the surface trace Siphon Fault is present along the east side of the road. Iron-formation is present to the west of the road and the Virginia Formation is present across the swamp on the east side of the road. Just before Dunka Road curves back to the east take the immediate left and go down a secondary mining road for about 500 feet. Proceed to the north on foot along a flagged trail to the next stop. 144 �STOP 5-3: Virginia Formation near Siphon fault No hammering please! Location: Cliffs-Erie site, T. 59 N., R. 14 W., sec. 26, SE, SE, NE Allen quadrangle; UTM: 569,505E/5,271,610N (NAD 83) Description: This is the only natural exposure of the Virginia Formation on the Mesabi Iron Range. Unfortunately, it is only a few feet thick. A total of 1,443 feet of the formation is present in drill cores from holes drilled south of the range. Note the graded beds, mud chips, concretions, and loading at the bases of these beds. The bedding is near vertical in this location due to proximity to the north-trending Siphon fault—an inferred growth fault (Graber, 1993) wherein the iron-formation decreases in thickness to the east (across the fault) by about 100 feet. NEXT: From Stop 5-3, proceed back to, and angle across Dunka Road to a gated entrance (not locked). Go down this secondary mine road about 0.5 miles. Park and walk about 300 feet to the south. STOP 5-4: Algal submember (I submember) near the top of the Upper Cherty member Location: Pit 2E, Cliffs-Erie site, T. 59 N., R. 14 W., sec. 23, N 1/2, NW Access to this site is via Dunka Road, which is a private mining company road. Allen quadrangle; UTM: 568,202E/5,2706,25N (NAD 83) Description: Algal structures were first described by Leith (1903) as "contorted bedding." Grout and Broderick (1919) were the first who assigned an organic origin to them. The algal submember within the Upper Cherty member consists of mounds of fossilized algal colonies that are separated by jasper-bearing intraformational conglomerate; the overall thickness of this unit is 2 to 20 feet. This horizon occurs only in the eastern half of the range (not present west of Hibbing). However, there is some ambiguity in the positioning of this unit within the Upper Cherty member. In the Virginia Horn area, this horizon is positioned near the base of the Upper Cherty and can be traced upwards in a series of drill holes (Severson and others, in prep.) to a position near the top of the Upper Cherty near Hibbing (see Fig. 517). To the east of the Virginia Horn, the horizon can be traced at the base of the Upper Cherty to a position near Biwabik, MN, where two algal horizons are reported (in drill hole MGS-2 and in the old LTV submember nomenclature). These two horizons occur in the bottom and top of the Upper Cherty member. It is difficult to tell which of these algal horizons is the one that was traced eastward from the Virginia Horn. The lower algal horizon pinches out further east and the upper horizon can be traced as far east as Dunka Pit. The algal horizon that will be visited on this trip occurs near the top of the Upper Cherty member. This locality is an excellent place to view a nearly horizontal portion of the iron-formation that contains abundant individual mounds of algal stromatolites. Stripping of glacial overburden in this area once revealed a dip slope the size of a football field that contained stromatolite mounds (Graber, 1993). Figure 5-10 illustrates a large portion of that exposure that has since been mined. The present stop is at an area located several hundred feet west of that site. Internally, the mounds are characterized by many individual, columnar, finger-like structures that are convex upward. The mounds protrude up through a thin veneer of the overlying thin- to wavy-bedded H submember. Measurements on a nearby mine face in this horizon showed that all the columnar stromatolites were inclined at 30° to the vertical; unfortunately, that site has also been removed by mining. At the extreme eastern edge of this exposure are the J and H submembers. Both of these submembers 145 �contain rare anthraxolite, which is an organic bitumen containing 95 percent or more carbon that is black with a vitreous luster and conchoidal fracture and resembles obsidian (Morey, 1994). Morey (1994) reported that anthraxolite is present throughout the iron-formation but is most common beneath the carbon-rich Intermediate Slate. Furthermore, he suggested it formed via a mechanism of concentrating carbon from a mass-kill phenomenon, followed by later migration of a carbon-rich liquid to form the anthraxolite. NEXT: Return to Dunka Road and proceed eastward about 14.6 miles (through a locked gate about midway down the road) to the entrance and guard shack of the Peter Mitchell Mine. After receiving permission to enter the property, proceed down the road to the offices of Northshore Mining. The next ten stops are on the mine property. The stratigraphic nomenclature of submembers in the Peter Mitchell Mine is portrayed on Figure 5-20. STOP 5-5: Submembers I and G (and lunch) Location: Peter Mitchell Mine, Block 35, T.60N, R.13W., Section 33 Babbitt quadrangle; UTM 574,773E/5,276,403 (NAD-83) Description: This site was chosen as an excellent area to collect unique garnets, up to several inches across, from the G submember (located at the top of the mine bench) and to take another look at the I submember (exposed at the base of the bench near the water). At this locale, the I submember is 3-5 feet thick and consists almost entirely of intraformational conglomerate, often with bright red jasper clasts, and rare algal stromatolites. The G submember contains coarse-grained, brown to brown-red, euhedral andradite garnet. The garnets occur as single crystals and as clusters of crystals. Both exhibit dodecahedral forms – especially when associated with patches that are rich in calcite, quartz, and actinolite. NEXT: The following series of stops offer a review of several submemers of the Upper Slaty and Upper Cherty members. The samples examined to date from these stops exhibit middle to upper greenschist facies metamorphism based on the identified gangue mineralogy. Thin sections reveal gangue mineralogy mostly consisting of quartz-cummingtonite-grunerite. Reflected light studies of ore from these stops indicate a wide range of magnetite grain size and liberation characteristics, which are important parameters for ore grading and classifying. It is important to note that rocks from most of these stops are lower metamorphic grade when compared to the metamorphic grades in the extreme northeast end of the Peter Mitchell Mine and Dunka Pit. STOP 5-6: Submembers K and L Location: Peter Mitchell Mine, Block 21, T. 60 N., R. 13 W., sec. 26, NE, NW Babbitt quadrangle; 578,337E/5,278,665N (NAD-83) Description: Both the K and L submembers of the Upper Cherty member constitute taconite ore. Both are wavy-bedded and contain isolated black chert intraclasts. The K submember is slightly different in that the wavy beds are thinner and more closely-spaced than the wavy beds of the L submember. Petrographic review of samples from the L submember at this stop indicate abundant subhedral magnetite grains (5-15 microns) mainly confined to well-defined bands parallel to bedding. Further contributing to the high-grade nature of this submember are coarse assemblages (granules and pebbles) of similar subhedral magnetite grains disseminated within the cherty beds of the rock. X-ray analysis of a sample from this location indicates gangue mineralogy of quartz, cummingtonite, and grunerite. 146 �Magnetite grains in the overlying K submember typically occur as granules and assemblages of euhedral grains ranging up 200 microns in diameter. The assemblages are randomly distributed within the matrix; however, diffuse bands of magnetite assemblages are observed. X-ray analyses of these samples indicate a gangue mineral suite of quartz, cummingtonite, and grunerite that is consistent with the suite identified for submember L. The magnetite textures in both submembers observed at this location can differ greatly from textures to the east where coarser assemblages are commonly observed closer to the Duluth Complex. These changes in texture can affect magnetite grade and the liberation/grindability performance of individual ores. STOP 5-7: Submembers I, J, and K (all constitute taconite ore) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W., section 24, S ½, NE Babbitt Quadrangle; UTM 578,505E/5,278,416N (NAD-83) Description: Submember K (described above), as well as, submembers J and I, are viewed at this site. The I submember is present at the top of the mine bench but can be readily spotted in loose pieces in the floor of the mine where it is distinguished by: 1. pink coloration; 2. conglomeratic nature; and 3. the presence of ribbon-like stringers of quartz veins that parallel bedding. Metamorphism is higher in this area relative to the previous stops and the I submember does not display the striking algal features, but rather a more “washed-out” appearance. The J submember is typically regular-bedded to weakly wavybedded with close-spaced magnetite-rich beds. A northeast-trending fault is present in this area and is characterized by chloritic breccia. Some weak oxidation (brown-stained zones) is associated with the fault; however, these zones are still strongly magnetic. STOP 5-8: Submembers G, H, and I (all constitute taconite ore) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, N ½, NE Babbitt Quadrangle; UTM 578,617E/5,278,464N (NAD-83) Stay away from the steep wall (loose blocks above)! Description: Submember H at this location displays thin-bedded to weakly wavy-bedded quartz taconite with abundant magnetite. Along with large accumulations of magnetite, a gangue assemblage of quartzgrunerite-ferroactinolite is also displayed within the exposure. The top of the H can host a thin (~1 foot) intraformational conglomerate that separates the H and G submembers (not seen at this location). This feature can be used to map the contact between the Upper Slaty and Upper Cherty members. Note that Gundersen and Schwartz (1962) interpret the G submember as occurring in bottom of the Upper Slaty; whereas the G submember is considered to be at the top of the Upper Cherty at the Cliffs-Erie site and Dunka Pit mine (Figure 5-19). The G submember differs from H in that it is wavy-bedded and contains lensoidal chert bands that often exhibit bulb-shaped pinch-and-swell margins. Reflected light analysis of this exposure displays well-defined bands of coarse magnetite (>50 microns), which also follow the wavy bedded nature of the iron formation typically associated with the H submember. The I submember is subtly noticed as you walk to the north and displays several examples of jasper-bearing algal quartz taconite. Magnetite grains range between fine (<5 microns) and coarse (> 50 microns) mostly occurring as disseminations within the matrix of the unit. Both submembers are typically classified as ore at the Peter Mitchell Mine. Within the G submember, medium to coarse-grained andradite is observed. Bedding in the G submember 147 �is significantly thicker than other submembers at the Peter Mitchell Mine and shows well-developed pinch and swell characteristics. STOP 5-9: Submembers F (waste at this site but ore to the east) and G (taconite ore) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, NE, NE Babbitt Quadrangle; UTM 578,818E/5,278,516N (NAD-83) Description: The F and G submembers of the geologic section at the Peter Mitchell mine can be viewed at this stop. The G submember is described above. The F submember typically consists of alternating magnetite-rich, thin-bedded sets, and thicker-bedded cherty sets (alternating bedding of Figure 5-13). Locally present in the F submember are small septaria-like structures, or syneresis cracks, that consist of whitish quartz-filled subvertical circular and radial fractures in the granular cherty layers. As can be seen at this location, contacts between submembers are typically gradational and are best determined in drill core where changes in bedding characteristics and mineralogy can be readily observed. The F submember can contain appreciable magnetite; however, it is typically classified as waste due to the fine grain size making it difficult to liberate the iron. Magnetite grains observed under reflected light range between <5 microns to 50+ microns in submember F while analysis of submember G identifies a coarser magnetite grain size range of 10 – 50+ microns occurring in easy-to-liberate assemblages. Further micro-examination of the F and G at this location confirm the presence of quartz and ferroactinolite, while common gangue minerals can also include the andradite, cummingtonite, and hedenbergite. At this location in particular, thin zones of cummingtonite crystal growth interrupt the thicker cherty beds within the iron formation. STOP 5-10: Submembers C and D (waste) Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, S1/2, NE Babbitt Quadrangle; UTM 578,681E/5,278,133N (NAD-83) Description: The contact between submembers C and D of the Upper Slaty member can be viewed at this location. Both the C and D submembers are thin-bedded units of the Upper Slaty member; however, the D submember is different in that it contains slightly thicker beds and lenses of chert. A 20-foot-thick Keweenawan sill is also present near this stop (see Stop 5-11) and is positioned towards the top of the C submember. Petrographic review of samples from this area display minor magnetite with a gangue mineralogy of cummingtonite-grunerite series amphiboles as well as cordierite and ferroactinolite. The C submember also contains magnetite, fayalite, ferrohypersthene, and chert. X-ray analysis of submember D at this location indicates gangue mineralogy of quartz, cummingtonite, and hedenbergite. Magnetite, although scarce in the C submember, is fine-grained (5-10 microns) and confined to fractures and open-spaces within the matrix of the recrystallized slaty beds. Magnetite grains identified within the D submember are very-fine grained (~5 microns) and disseminated within the quartz-rich matrix of the rock. The C and D submembers are typically categorized as waste based mostly on low magnetic iron values, however, high phosphorous concentrations measured within both submembers also limits them as waste. 148 �STOP 5-11: Keweenawan Sill in the C submember Location: Peter Mitchell Mine, Block 21, T.60N., R.13W, Section 24, N1/2, SE Babbitt Quadrangle; UTM 578,702E/5,278,080N (NAD-83) Description: A 2- to 18-foot-thick sill is present towards the top of the C submember across most of the strike length of the Peter Mitchell Mine and Dunka Pit. A sill with identical chemistry and textures is present within the J submember much further to the west in Pit 2E at the Cliffs-Erie Site (Severson and Hauck, 1997). The sill at this locality, informally referred to as the BIFSill or C-Sill, is generally fine- to medium-grained with locally very coarse-grained plagioclase phenocrysts and vertical columnar jointing. A granoblastic texture is evident in thin-section, indicating that the sill was emplaced in the early Keweenawan and was later metamorphosed by intrusion of the Duluth Complex. Hauck and others (1997) noted that the BIFSill is chemically similar to the Logan sills to the northeast in the Rove Formation, and have informally called this sill a "Logan-type" sill. X-ray diffraction data collected from samples of the sill at this site yield a mineral assemblage of quartzalbite-actinolite-antigorite-illite with magnetite. In reflected light, magnetite is abundant and occurs as randomly oriented lath-shaped crystals believed to represent the pseudomorphic replacement of preexisting ilmenite crystals by magnetite. STOP 5-12: Keweenawan Sill (in the Virginia Formation) and A and B submembers Location: Peter Mitchell Mine, Block 11, T.60N., R.12W, Section 19, SW, SE Babbitt Quadrangle; UTM 581,691E/5,278,881N (NAD-83) Description: At the very top of the Biwabik Iron Formation is a 2- to 6-foot-thick chert and marble unit (A submember) that corresponds to the carbonate horizon that is present in only the eastern half of the Mesabi Range. This unit is locally absent in some areas (non-depositional unconformity) and extremely thick in other areas. The B submember is characterized by alternating chert and diopside bands up to one foot thick; marble layers are locally present. In some areas, pink granophyric veins locally cut the B submember. These veins exhibit pinch-and-swell relationships in that the veins thicken within the diopside bands and pinch in the chert bands. The mineralogy of the marble beds represents an assemblage of calcite-ankerite-wollastonite-quartz, while the B submember consists of a diopside-ankerite-wollastonite-magnetite mineral assemblage. Magnetite grains are scarce within these beds and are very fine grained when present. At the very base of the Virginia Formation is a 2- to 100-foot-thick sill, informally called the VirgSill, that consists of a fine-grained, granoblastic rock with varying amounts of plagioclase, clinopyroxene, orthopyroxene, hornblende, olivine, and biotite. The informal term of "Cr-bearing sill" was first used by Hauck and others (1997) to highlight the relatively high chromium contents (600 to 1,200 parts per million) that are characteristic of this sill. This sill exhibits two varieties: 1) a fine-grained, massive, gray-colored unit (this exposure) that is extremely difficult to distinguish from the hornfelsed Virginia Formation in drill core, and 2) a medium- to coarse-grained unit that is olivine- and/or hornblende-rich and is easily recognized. A Keweenawan age is inferred for the VirgSill, but this sill is decidedly different than the BIFSill and age dates are needed for both of these sills. STOP 5-13: Duluth Complex and metamorphosed Virginia Formation (optional) Location: Peter Mitchell Mine, Block 0, T.60N., R.12W, Section 16, SW, SW 149 �Babbitt Quadrangle; UTM 584,266E/5,280,874N (NAD-83) Description: In close proximity to the Duluth Complex, the well-bedded sediments of the Virginia Formation are typically transformed into a rock that at first appearance looks like an intrusive rock due to the presence of randomly oriented biotite. This rock is informally referred to as the "recrystallized unit," but is more properly classed as a diatexite (Sawyer, 1999). During emplacement of the Duluth Complex, the sediments of the Virginia Formation were heated, generating 20 to 40 percent pervasive partial melts, that enabled these rocks to literally flow in response to stresses that were applied during emplacement. All bedding planes are obliterated and what remains is a medium-grained recrystallized rock that contains plagioclase, cordierite, orthopyroxene, and randomly oriented biotite. Within this recrystallized matrix are blocks/boudins of more structurally competent siltstone and calc-silicate hornfels (originally limey layers). Also at this locality are gabbroic rocks at the basal contact of the South Kawishiwi intrusion. The rocks exposed at this stop consist of weakly to moderately mineralized, fine- to medium-grained, ophitic augite troctolite to olivine gabbro. Copper-nickel values are unknown for this exposure. As you walk to this site you will notice numerous blasted blocks of the C submember with shiny dark green faces along bedding planes and on joint faces. This coating is an iron silicate known as hisingerite (Fe3+2Si2O5(OH)4·2H2O). Also present are blasted blocks with coarse-grained magnetite, and coarsegrained pyroxene that has grown along bedding planes during metamorphism by the Duluth Complex. STOP 5-14: Submembers F-P (Blasted Rock) Location: Peter Mitchell Mine, Block 3E-4E, T.60N., R.12W., Section 9,SW, SE Babbitt Quadrangle; UTM 5282276N, 585022E (NAD 83) Description: This stop was chosen to show the highly metamorphosed nature of the iron formation in close proximity to the Duluth Complex. The iron formation in this area of the pit contains abundant coarse grained iron pyroxenes, iron amphiboles, fine grained fayalite, garnets, and coarse grained granular magnetite. The F and G submembers in this area of the pit are hard to distinguish in drill core and in broken form because the original bedding textures, primarily in the F, have been overprinted or altered by recrystallization of the iron formation. In situ, and in blasted form, the presence of red to brown-red garnet, which is abundant in the G, is the best indicator to use in distinguishing between the two submembers. The I submember in this area is a 1’ to 2.5’ thick quartzite conglomerate retaining no stromatolite textures due to overprinting caused by the intense recrystallization of the chert. In the J through M submembers little has changed with the banding characteristics of the magnetite allowing the submembers to be readily identified in the field. The biggest change seen in the J through M submembers, other than the increase in grain size, is the overall decrease in thickness of each unit. This change in thickness may in part be explained by a decrease in the average thickness seen in the chert bands separating the magnetite bands. This gives each submember an overall condensed appearance as the magnetite bands become more closely spaced. The O and P submembers can be tough to identify in drill core and in the field due to their rather massive appearance. The best indicator to distinguish the two units is the abundant disseminated granular magnetite which occurs in the O. NEXT: Leave the Peter Mitchell Mine and return to Dunka Road. Take a left after the guard shack and cross the Tomahawk Trail. Continue north to a locked gate (permission is needed to enter) and follow several mining roads to the next stop. 150 �STOP 5-15: Dunka Pit (Optional) Location: Dunka Pit T. 61 N., R. 12 W., section. 35, SE, NW Babbitt NE quadrangle; UTM 587,337E/5,286,348N (NAD-83) Description: At the extreme north end of the pit, the iron-formation is in fault contact with the Duluth Complex. The fault is near-vertical, north-trending, and juxtaposes iron-formation, west of the fault, against gabbroic rocks of the South Kawishiwi intrusion (SKI) to the east of the fault. The age of the fault is not easily determined, but faulting appears to have been initiated before emplacement of the SKI. In the immediate area of the fault, a westward change in the character of the iron-formation is obvious over a 20-25 foot wide zone. This change consists of well-bedded iron-formation grading into a banded plagioclase-magnetite rock, that in turn, grades into a troctolitic rock with magnetite lenses. Emplacement of the SKI induced upper-greenschist to middle-amphibolite facies metamorphism of the iron-formation resulting in coarse grain sizes that can be viewed within both ore and gangue minerals. To the immediate west of the fault, the iron-formation contains high amounts of titanium due to the metasomatic transfer of Ti across the fault (Muhich, 1993). Due to its high titanium content, which is detrimental to making steel, the iron-formation immediately adjacent to the fault was discarded during mining. END OF TRIP (return to Ely) REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1840 Ma Sudbury impact event: Geology 33, p. 193-196. Addison, W.D., Cannon, W.F., and Brumpton, G.R., 2007, How to identify Sudbury impact ejecta in the Lake Superior Region: Institute of Lake Superior Geology, 53rd Annual Meeting, May 8-13, 2007, Lutsen, Minnesota, v. 53, Part 1-Proceedings and Abstracts, p. 1-2. Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Minneapolis, University of Minnesota, Ph.D. dissertation, 262 p. Bleifuss, R.L., 1964, Mineralogy of oxidized taconites of the western Mesabi and its influence on metallurgical processes: Transactions of AIME, v. 229, p. 235-244. Boerst, K., 1999, Stromatolites in the LTV 2E pit, Mesabi range, northeastern Minnesota: Unpublished Undergraduate Research Opportunity Project, College of Science and Engineering Office, University of Minnesota Duluth, 4 p. Bonnichsen, B., 1968, General geology and petrology of the metamorphosed Biwabik Iron Formation, Dunka River area, Minnesota: Unpublished Ph.D. dissertation, University of Minnesota, Minneapolis, 240 p. Bonnichsen, B., 1975, Geology of the Biwabik Iron Formation, Dunka River area, Minnesota: Economic Geology, v. 70, no. 2, p. 319-340. Caddey, S. W., Bachman, R. L., Campbell, T. J., Reid, R. R., and Otto, R. P., 1991, The Homestake Gold Mine, an Early Proterozoic iron-formation-hosted gold deposit, Lawrence County, South Dakota. U. S. Geological Survey Bulletin 1857-J. 151 �Cannon, W.F. and Addison, W.D., 2007, The Sudbury Impact layer in the Lake Superior iron ranges: A time-line from the heavens: Institute of Lake Superior Geology, 53rd Annual Meeting, May 8-13, 2007, Lutsen, Minnesota, v. 53, Part 1-Proceedings and Abstracts, p. 20-21. Chalokwu, C. I., Grant, N. K., Ariskin, A. A., and Barmina, G. S., 1993, Simulation of primary phase relations and mineral compositions in the Partridge River intrusion, Duluth Complex, Minnesota: implications for the parent magma composition. Contrib. Mineral. Petrol., v. 114, p. 539-549. Chandler, V.W., 1993, Geophysical characteristics, in Sims, P.K., ed., The Lake Superior region and Trans-Hudson Orogen, Precambrian: Conterminous US: Geologic Society of America: The Geology of North America, v. C-2, p. 81-89. Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, v. 39, p. 1089-1091. Fralick, P.W., and Kissin, S.A., 1998, The age and provenance of the Gunflint lapilli tuff [abs.]: Institute on Lake Superior Geology, 44th Annual Meeting, Minneapolis, Minn., Proceedings, v. 44, pt. 1, p. 66-67. French, B.M., 1968, Progressive contact metamorphism of the Biwabik Iron Formation, Mesabi Range, Minnesota: Minnesota Geological Survey Bulletin 45, 103 p. Frost, C. D., von Blackenburg, F., Schoenberg, R., Frost, B. R., and Swapp, S. 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Gruner, J.W., 1924, Contributions to the geology of the Mesabi range: With special reference to the magnetites of the iron-bearing formation west of Mesaba: Minnesota Geological Survey Bulletin 19, 71 p. ———1946, The mineralogy and geology of the taconites and iron ores of the Mesabi range, Minnesota: Office of the Commissioner Iron Range Resources and Rehabilitation, St. Paul, in cooperation with the Minnesota Geological Survey, 127 p. Gundersen, J.N., and Schwartz, G.M., 1962, The geology of the metamorphosed Biwabik Iron Formation, eastern Mesabi district, Minnesota: Minnesota Geological Survey Bulletin 43, 139 p. Han, T.M., 1982, Iron formations of Precambrian age: Hematite-magnetite relationships in some Proterozoic iron deposits—microscopic observations, in Amstutz, G.C., El Goresy, A., Frenzel, G., Kluth, C., Moh, G., Wausehkuhn, A., and Zimmerman, R.A., eds., Ore genesis: The state of the art: New York, SpringerVerlag, p. 451-459. 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Rajan, S., Mackenzie, F. T., and Glenn, C. R., A thermodynamic model for water column precipitation of siderite in the Plio-Pleistocen Black Sea. Amer. Jour. Sci., v. 296, p. 506-548. Sawyer, E.W., 1999, Criteria for the recognition of partial melting: Physics and Chemistry of the Earth, v. 24, no. 3, p. 269-279. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: Implications for tectonic setting of Paleoproterozoic iron formations of the Lake Superior Region: Canadian Journal of Earth Sciences, v. 39, p. 999-1012. Schulz, K.J., 1987, An Early Proterozoic ophiolite in the Penokean orogen [abs]: Geologic Association of Canada Program Abstracts 12, p. 87. ———2003, A Paleoproterozoic suprasubduction zone ophiolite-island arc complex in northeastern Wisconsin [abs]: Institute on Lake Superior Geology, 49th Annual Meeting, Iron Mountain, Mich., Proceedings, v. 49, pt. 1, p. 71-72. 154 �Severson, M.J., and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen Quadrangle, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-97/19. Severson, M.J., Heine, J.J. and Patelke, M.M., in prep., Geologic and stratigraphic controls of the Biwabik Iron Formation and the aggregate potential of the Mesabi Iron Range: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2009/09. Severson, M.J., Zanko, L.M., Hauck, S.A., and Oreskovich, J.A., 2003, Geology and SEDEX potential of Early Proterozoic rocks, east-central Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2003/35, 160 p. Southwick, D.L., and Day, W.C., 1983, Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario: Canadian Journal of Earth Sciences, v. 20, p. 622-638. Southwick, D.L., and Morey, G.B., 1991, Tectonic imbrication and foredeep development of the Penokean orogen, east-central Minnesota—an interpretation based on regional geophysics and the results of test-drilling: U.S. Geological Survey Bulletin 1904, 17 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. Vallinni, D.A., McNaughton, N.J., Rasmussen, B., Fletcher, I., and Griffin, B.J., 2003, Using xenotime U-Pb geochronology to unravel the history of Proterozoic sedimentary basins: a study in Western Australia and the Lake Superior Region [abs.]: Institute on Lake Superior Geology, 49th Annual Meeting, Iron Mountain, MI, Proceedings, v. 49, pt. 1, p. 79-80. Van Hise, C.R., and Leith, C.K., 1901, The Mesabi district: U.S. Geological Survey Annual Report, v. 21, pt. 3, p. 351-370. Vocke, C. M., 1981, T-fO2 conditions of the metamorphism of the Stillwater Iron-Formation, Montana. M.S. Thesis. State University of New York, Stony Brook. White, D.A., 1954, The stratigraphy and structure of the Mesabi range, Minnesota: Minnesota Geological Survey Bulletin 38, 92 p. Winchell, N.H., 1882, The Potsdam sandstone: Minnesota Geological and Natural History Survey Annual Report, v. 10, p. 123-136. ———1893, Twentieth annual report for the year 1891: Minnesota Geological and Natural History Survey, 344 p. Wolff, J.F., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers, Transactions, v. 56, p. 229-257. Zanko, L.M., Severson, M.J., Oreskovich, J.A., Heine, J.H., Hauck, S.A., and Ojakangas, R.W., 2003, Oxidized taconite geological resources for a portion of the western Mesabi Range (west half of the Arcturus Mine to the east half of the Canisteo Mine), Itasca County, Minnesota—a GIS-based resource analysis for land-use planning: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2001/40, 85 p. 155 �55th Annual Institute on Lake Superior Geology Field Trip 6 GEOLOGY OF THE LAKE ONE TROCTOLITE BY CANOE Jim Miller (Department of Geological Sciences, UMD) 156 �FIELD TRIP 6 GEOLOGY OF THE LAKE ONE TROCTOLITE (BY CANOE) NORTHWESTERN DULUTH COMPLEX Jim Miller (University of Minnesota Duluth) INTRODUCTION The Duluth Complex and associated Keweenawan intrusions in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa. 2 These rocks cover an arcuate area of over 5,000 km (Fig. 6-1) and give rise to two strong gravity anomalies (+50 & +70 mgal) that imply intrusive roots to more than 13 kilometers depth (Allen and others, 1997). The intrusive rocks of northeastern Minnesota were emplaced into a comagmatic volcanic edifice during formation of the Midcontinent Rift between 1108 and 1095 Ma (Paces and Miller, 1993). The Duluth Complex is physically defined as a more or less continuous mass of mafic to felsic plutonic rocks that extends in an arcuate fashion from Duluth to nearly Grand Portage (Fig. 6-1). It is bounded by a footwall of predominantly Paleoproterozoic and Archean rocks, a hanging wall of largely mafic volcanic rocks and hypabyssal intrusions, and internally, it contains scattered bodies of strongly recrystallized mafic volcanic and sedimentary hornfels. Defining the Duluth Complex more genetically, it is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a comagmatic volcanic edifice in two general stages - an early stage at about 1108 Ma and a main stage at 1099 Ma. Within the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position. Felsic Series – massive granophyric granite and smaller amounts of intermediate rock that occurs as a semi-continuous mass of intrusions strung along the eastern and central roof zone of the complex and was emplaced during early stage magmatism (~1108 Ma). Early Gabbro Series – layered sequences of dominantly gabbroic cumulates that occur in two major intrusions along the northeastern contact of the Duluth Complex and were also emplaced during early stage magmatism (~1108 Ma) Anorthositic Series – a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism (~ 1099 Ma). Layered Series – a suite of stratiform troctolitic to ferrogabbroic cumulates that comprise at least 11 variably-differentiated mafic layered intrusions and occurs mostly along the base of the Duluth Complex. These intrusions were emplaced during main stage magmatism, but generally after the anorthositic series. The Lake One troctolite refers to layered series rocks that occur along the base of the Duluth Complex in the Lake One area (Fig. 6-1). In contrast to most other layered series bodies, it is not designated as an “intrusion” because its eastern extent is not known because of inadequate mapping in the heart of the Boundary Waters Canoe Area (BWCA). This trip will focus on three general aspects of the geology of the Lake One Troctolite: 1) the contact relationships between the Lake One troctolite and Archean rocks of the footwall and anorthositic series of the hanging wall; 2) the general lithostratigraphic variations in through the troctolite and 3) the characteristics of large volcanic hornfels inclusions within the troctolite. The descriptions of the Lake 157 �One troctolite that follow are based largely on field mapping conducted in 1981, 1982, and 1984 as part of my PhD dissertation (Miller,1986). Lake One Troctolite Figure 6-1. Generalized geology of the Duluth Complex and related Keweenawan rocks of northeastern Minnesota highlighting the location of the Lake One Troctolite. Other layered series intrusions are: TI – Tuscarora intrusion, WLI – Wilder Lake intrusion, SKI – South Kawishiwi intrusion, BEI – Bald Eagle intrusion, PRI – Partridge River intrusion, GLI – Greenwood Lake intrusion, OSI – Osier Lake intrusion, WMI – Western Margin intrusion, BLI – Boulder Lake intrusion, DLS – Layered Series at Duluth. Felsic Series intrusions are: CL – Cucumber Lake granophyre, MH – Misquah Hills granophyre, BL - Beth Lake granophyre (and Wine Lake monzodiorite), WL – Whitefish Lake granophyre, I – Isabella granophyre, MW - Mt. Weber granophyre, and FB – Fairbanks/Brimson granophyres. Geology modified from MGS Misc. Map M-119 (Miller et al., 2001). PREVIOUS STUDIES OF THE LAKE ONE TROCTOLITE The geology of the northwestern Duluth Complex was first examined during the early state geological survey conducted from 1875 – 1901. In the fourth volume of the final report, Winchell (1899) accurately showed the contact between gabbroic rocks and older greenstone, metasedimentary rocks, and granite in the Lake One-Snowbank Lake area (Chapter XXII, Winchell, 1899, p. 420). After the discovery of Cu- 158 �Ni mineralization in the Spruce Road area of the basal Duluth Complex in 1948, exploration activity ensued all along the basal contact of the complex, including the Lake One area. William Phinney (University of Minnesota), who was responsible for mapping most of the Duluth Complex in the Gabbro Lake 15’ quadrangle in the early 1960’s (Green et al., 1966), continued reconnaissance mapping to the east in the late 1960’s into what is now the BWCA. Although geologic mapping in this area was conducted entirely by canoe, Phinney + was able to cover considerable ground with the aid of floatplane access to areas deep within the wilderness. (The present strict restrictions on motorized vehicles within the BWCA, which was established in 1978, make such easy access impossible today.) From 1966 to 1970, Phinney reconnaissance mapped parts of nine 7.5’quadrangles in that four year period: Snowbank Lake, Quagda Lake, Isabella Lake, and Lake Insula (all part of the Forest Center 15’ quadrangle), Alice Lake, Lake Polly, Ensign Lake East, and Kekekabic Lake (Fig. 6-2). Unfortunately, no published or open file maps were produced from this effort. Rather, the only documentation of this mapping is a small figure included in Phinney’s (1972) article on the northwestern part of the Duluth Complex contained in the Geology of Minnesota Centennial Volume (Fig. 6-2). Snowbank Lake  7.5’ quadrangle  Figure 6-2. Generalized geology of the northwestern part of the Duluth Complex from detailed mapping in the Gabbro Lake 15’ quadrangle (Green et al., 1966) and reconnaissance mapping elsewhere. Figure from Phinney (1972, figure V-26). Detailed mapping the southern half of the Snowbank Lake quadrangle (NW quad of the Forest Center 15’) was conducted by the author for his PhD dissertation (Miller, 1986). Although most of the mapping focused on the anorthositic series rocks in the quadrangle, the troctolitic rocks exposed between the basal contact and the anorthositic series hanging wall in the Lake One and Lake Two areas was also mapped in detail. This mapping showed the presence of a multiply emplaced suite of troctolitic cumulates centered 159 �on Lake One, which were referred to as the Lake One troctolite in the thesis and in a subsequent paper written from the thesis research on the anorthositic series rocks (Miller and Weiblen, 1990). Although generally troctolitic rocks extend along the basal contact from the Lake One area to nearly the Gunflint Trail, it is not clear whether this is one continuous intrusion or several discrete bodies. ROCK NOMENCLATURE Rock names used in this guidebook will follow the modal rock classification scheme recommended by Miller, Severson, and Green (2002) and shown in Figure 6-3. The modal rock names will also commonly be modified with textural terms that indicate the general grain size and the intergrowth relationship between clinopyroxene and plagioclase, namely ophitic (pyroxene oikocrysts enclose many plagioclase chadacrysts), subophitic (pyroxene encloses only a few plagioclase), and intergranular (pyroxene and plagioclase are both anhedral to subhedral granular). The rock terms may also be modified to indicate the presence of minor or accessory mineral phases (e.g., biotitic, apatitic, olivine-bearing, …). Figure 6-3. Modal classification scheme for gabbroic rocks of the Duluth Complex based on modal proportions of plagioclase (Pl), olivine (Ol), and clinopyroxene + Fe-Ti oxide (Cpx+Ox) (from Miller et al., 2002). Rock type abbreviations at the plagioclase-rich end are: LT – leucotroctolite, ALT – augite leucotroctolite, OLG – olivine leucogabbro, LG – leucogabbro, GA – gabbroic anorthosite, OGA – olivine gabbroic anorthosite, TA – troctolitic anorthosite, A – anorthosite. An alternative, short-hand rock classification scheme is used here that incorporates both the mode and textures of the major minerals into a rock code. Basically, the scheme lists the minerals above 2 modal % in decreasing order of abundance. Uppercase abbreviations indicate a granular habit and lowercase abbreviations indicate an intergranular habit. This scheme is outlined in Table 6-1. Although based on non-genetic descriptions of mode and texture, one can interpret the rock code in terms of a cumulate rock 160 �type whereby granular minerals may be inferred to be cumulus and intergranular minerals may be interpreted to be intercumulus. Table 6-1. “Cumulate” Classification Scheme Granular (Cumulus) / Interstitial (Intercumulus) Mineral Codes PP*/P/p- plagioclase F/f - Fe-Ti oxide O/o - olivine A/a - apatite C/c - clinopyroxene (augite) −/b −/α −/g - biotite I/i - inverted pigeonite H/h - hypersthene, bronzite - amphibole - granophyre * used with anorthositic group rocks Cumulate Code Translation of Some Common Rock Types in the Duluth Complex Ophitic augite troctolite POcf Augite-bearing oxide troctolite POFc Olivine gabbroic anorthosite with poikilitic olivine PPoc Ophitic olivine gabbro PcOf Biotitic, feldspathic dunite with poikilitic plagioclase Opb Intergranular, apatitic oxide olivine gabbro PCFOA Ophitic biotitic augite leucotroctolite POcb ARCHEAN FOOTWALL ROCKS The footwall of the Lake One troctolite in the Snowbank Lake quadrangle is composed of a variety of Archean rocks including granite, greenstone, and metasedimentary rocks (Gruner, 1941; Sims, 1985; Jirsa and Miller, 2004). In the Lake One area, the basal contact projects across the northern channel of Lake One where it outflows into the Kawishiwi River (Fig. 6-4). Along this stretch, calc-alkaline to tholeiitic metavolcanic and metavolcaniclastic rocks with a strong east-west penetrative foliation occur. These units have been interpreted to belong to the upper member of the Ely Greenstone (units Aeu and Aec, Fig. 6-4). Younger metasedimentary rocks including conglomerates, arkosic sandstones and siltstone of the Knife Lake Group lie unconformable on the Ely Greenstone units (unit Akc, Fig. 6-4) and are interpreted to be Timiskiming-equivalent sediments (Jirsa and Miller, 2004). Another major feature of the footwall is the east-west-trending North Kawishiwi fault, which is a major splay of the regional transpressional Vermilion Fault system. The fault projects at an acute angle into the base of the Duluth Complex in the northern Lake One area (Fig. 6-4). That the fault appears to merge into the basal contact and does not obviously cut the Duluth Complex suggests that it may have been reactivated during emplacement of the Duluth Complex and thus controlled the shape of the basal contact. 161 �Figure 6-4. Geology of the Lake One troctolite in part of the Snowbank Lake quadrangle (after Miller, 1986). Geology of the Archean footwall taken from Jirsa and Miller (2004). The red line shows the approximate profile line along which the stratigraphic column in Figure 6-6 is constructed. ANORTHOSITIC SERIES The hanging wall of the Lake One troctolite is composed of various plagioclase-rich rock types of the anorthositic series. The contact between the troctolitic rocks and the anorthositic series is very irregular in shape and in character (Fig. 6-4). In the Snowbank Lake quadrangle, the contact curves southeast to Rock Island Lake and the western shore of Lake Three and then irregularly back to the north through Lake Two. From Lake Two, it curves to the east and more or less parallels the basal contact. Despite the irregular shape of the contact, the troctolitic rocks consistently strike to the northeast (i.e., roughly parallel to the basal contact) and dip to the southeast, except in the Rock Island Lake area where the dip locally reverses to the north to define a synform/antiform structure (Fig. 6-4). Where the contact between the Lake One troctolite and the anorthositic series is structurally disconformable, the change in lithology is abrupt but the two rocks are rarely observed to be in contact. This suggests possible fault contacts, especially in places where the troctolite strikes at high angles to the inferred contact. However, in a few places where the contact can be approximated to within a several meters, troctolitic rocks are observed to 162 �be somewhat finer grained, nonfoliated and hosting scattered anorthosite inclusions, thus implying an intrusive contact. In one area of Lake Two, contacts are marked by zones of troctolite with abundant anorthosite inclusions (unit Dmx, Fig. 6-4). And in another area of Lake Two where the internal structure of the troctolite is semi-conformable to the contact, a more gradational contact relationship is observed. The anorthositic series of the Duluth Complex is composed of a structurally complex suite of medium– to coarse-grained, variably foliated, plagioclase-rich cumulates that occurs over the entire breadth of the complex from the western edge of the Poplar Lake intrusion of the early gabbro series to the southern terminus of the Duluth Complex at Duluth (Fig. 6-1). Over the entire exposure area of the anorthositic series, its lithologic character, internal structural, and contact relationships with adjacent rocks remain fundamentally constant. This is also true of its age of emplacement as two anorthositic series rocks from different parts of the complex have nearly identical U-Pb ages - 1099.1±0.6 Ma and 1099.3±0.6 Ma (Paces and Miller, 1993). The average rock type of the anorthositic series is an altered, coarse-grained, moderately foliated, ophitic olivine leucogabbro composed of about 80% plagioclase. With the exception of some occurrences of granular olivine in sub-cotectic abundance, subhedral to euhedral plagioclase is the only cumulus phase in these rocks. An average rock type is somewhat misleading, however, since lithologic variability on a small (outcrop) scale is a ubiquitous feature of the anorthositic series (Miller and Weiblen, 1990). The range of the anorthositic series rock types results from variations in plagioclase mode from 70-99%; mafic mineral proportions; olivine texture from granular to poikilitic (oikocrysts up to 15 cm across); grain size from medium to very coarse; and abundance of felsic mesostasis up to 20%. Moreover, moderate to nearly complete hydrothermal alteration of olivine, pyroxene and, to a lesser degree, plagioclase is evident in many anorthositic series rocks. Despite variations in texture and modal mineralogy and the complex zoning of individual crystals, one consistent feature of anorthositic series rocks is the constancy of average plagioclase compositions (An65-62; Miller and Weiblen, 1990). This constant An content contrasts markedly with considerable variability in the mg# of mafic silicates and defines a distinct trend of An-Fo variation compared to layered series rocks, which more closely resemble normal gabbroic differentiation trends (Fig. 6-5). The An-Fo variations of anorthositic series rocks are similar in trend to anorthositic rocks of the Middle Banded Zone of the Stillwater Complex, though the latter have overall greater Fo and An contents (MBZ, Fig. 6-5). In general, mineralogical and chemical variations indicative of in situ fractional crystallization are generally lacking in anorthositic series rocks (Miller and Weiblen, 1990). Complex internal structure is perhaps the most cogent feature of the anorthositic series in understanding its formation. Although rarely layered, anorthositic series rocks typically have some degree of plagioclase alignment. The attitude of this foliation, however, is extremely variable on an outcrop scale. Another structural complication is presented by changes in rock type that occur across sharp (but unchilled) to gradational boundaries. Discrete inclusions of one type of anorthositic rock (typically a more leucocratic composition) in another are common (Taylor, 1964; Miller and Weiblen, 1990). In the Snowbank Lake quadrangle, at least three intrusive episodes in the formation of anorthositic series rocks can be distinguished, with each successive episode producing less leucocratic compositions (Miller, 1986; Miller and Weiblen, 1990). Because of these outcrop-scale structural complexities, it is difficult to divide the anorthositic series into lithologic subunits. What is clear however, is that the anorthositic series was emplaced in multiple intrusive events. Nearly all workers who have investigated the anorthositic series have come to the common conclusion that these rocks formed from the emplacment and static crystallization of plagioclase crystal mushes (Grout ,1918; Phinney, 1969; Davidson, 1972; Miller and Weiblen, 1990 ). Miller and Weiblen (1990) cited field, petrographic, and mineral chemical evidence to support the interpretation that the anorthositic 163 �series was formed from multiple intrusions of variably differentiated basaltic magma physically enriched in intratelluric plagioclase of a near constant composition. However, because of their high viscosity, these mushes did not experience significant differentiation after being emplaced into the Duluth Complex. Rather, differentiation and the physical enrichment of plagioclase in anorthositic series parent magma/mushes was accomplished in a deeper staging chamber. Under higher pressures of the lower crust, plagioclase would be bouyant in basaltic magmas. Anorthositic Series Figure 6-5. Fo-An variations of coexisting olivine and plagioclase in gabbroic and anorthositic rocks from the Duluth Complex and other mafic layered intrusions (Skaergaard and Stillwater). Plotted are the average (symbols) and range (lines) of Fo-An values determined from microprobe analyses of 21 anorthositic series rocks from Snowbank Lake quadrangle. (see Miller and Weiblen, 1990) Most studies of the Duluth Complex interpreted the unique attributes of the anorthositic series as indicating an origin that was early, similarly unique, and unrelated to that of the layered series. In most areas of the complex, inclusions of anorthositic series rocks within the layered series abound as we will see in the Lake One area. In contrast, inclusions of troctolite have not been reported within anorthositic series rocks. These observations and a well-documented chilled margin of the layered series against the anorthositic series at Duluth (Taylor, 1964) seemed to imply that, in fact, anorthositic rocks were considerably older (and colder) than the layered series (Miller and Weiblen, 1990). However, this interpretation, which came to develop the status of a paradigm (Miller, 1992), was challenged by precise U-Pb ages that showed the anorthositic series and layered series to be essentially the same age of 1099 Ma (Paces and Miller, 1993). 164 �Miller and Severson (2002) suggested that the anorthositic series and layered series were likely tapped in succession from a lower crustal chamber that was initially highly charged with suspended plagioclase. Whereas early extractions from this deep staging chamber was highly enriched in plagioclase, such crystal suspensions were progressively flushed out, resulting in successively less crystal-rich magmas. This model seems best able to explain the lithologic and structural attributes and contact relationships of the Duluth Complex at Duluth and along the northwestern contact area. Moreover, with the 1099 Ma age of the Duluth Complex approximately marking the transition from the latent stage to the main magmatic stage of Midcontinent Rift evolution, this model fits well with the latent stage being a period of extensive underplating of the crust (Miller and Vervoort, 1996; Vervoort et al., 2007). LITHOSTRATIGRAPHY OF THE LAKE ONE TROCTOLITE The Lake One troctolite is composed of a 1.5 kilometer-thick stratiform sequence of troctolitic cumulates exposed between the basal contact and the axis of the synformal structure in the vicinity of Rock Island Lake (Fig. 6-4). Within this stratigraphic section, seven stratiform map units can be distinguished (Figs. 6-4 and 6-6). Figure 6-6 shows the general modal variations across this stratigraphic section as well as map units originally defined by Miller (1986) and their equivalents in Figure 6-4. The basal unit (D1cz) is a texturally and modally heterogeneous (i.e. taxitic) unit that occurs in the lower 50 meters of the intrusion. Modal mineralogy varies from troctolite to olivine oxide gabbro that locally contains signficant amounts of orthopyroxene and biotite. This is commonly interpreted as indicating contamination from the siliceous and volatile-rich footwall (Severson, 1994; Miller and Severson, 2002). Modal layering and igneous foliation is variably developed. This basal contact zone is similar to contact zones of other layered series intrusions in terms of its heterogeneity, however, it is notably poor in Cu-Ni sulfide which is the hallmark of the contact zones on the Partridge River and South Kawishiwi intrusions to the southwest (Fig. 6-1). Between 50 and 300 m above the basal contact, the heterogeneous nature of the contact zone gives way to a more homogeneously textured zone of medium-grained ophitic olivine gabbro to augite troctolite (PcOf to POcf cumulates; unit D1og). The rocks are typically moderately to well foliated and show subtle modal layering. Biotite and orthopyroxene gradually decrease in abundance through the unit. The contact between the D1og and D1at unit is chosen where the augite to olivine ratio consistently drops below 1:2 and isomodal olivine layering becomes intermittently developed. This transition give rises to a monontonous sequence of augite troctolite to troctolite. The largest unit of the Lake One troctolite, D1at, is a consistently medium-grained, moderately to well foliated, ophitic troctolite to augite troctolite (PO, POcf) that is locally well layered. It is also rich in mafic hornfels inclusion as will be described below. Near the top of the unit exposed at the southeast end of Lake One, this unit becomes more consistently layered, enriched in ophitic augite, and coarser grained (Fig. 6-6). This augite troctolite abruptly gives way to a narrow interval of medium-grained troctolite with augite troctolite inclusions and then to rhythmically layered troctolite and melatroctolite (PO/OP). This unit is labeled the D1t’ unit (Figs. 6-4 and 6-6). The abrupt transition between the D1at and D1t’ units seems best explained as marking major recharge event of more primitive magma into the Lake One magma chamber. This D1t’ troctolite unit (D1t’, Fig. 6-4) grades upward into a medium-grained, but coarsely ophitic augite troctolite (POcf) containing augite oikocrysts up to 15 centimeters across (unit D1at’). Together, the D1t’ and D1at’ units are about 150 m and may represent a differentiation cycle. The augite troctolite of the D1at’ unit then abruptly gives way to a homogeneous, well foliated, locally oxide-bearing troctolite (POF-PO cumulate; unit D1t”), which likely represents another recharge event. . Unlike the D1t’ unit, 165 �this troctolite unit is rarely layered in the Lake Two area. The oxide troctolite quickly grades into a medium-grained ophitic augite-bearing troctolite to form the uppermost unit of the Lake One troctolite (unit D1at”). Both the D1t” and D1at” units contain anorthositic series inclusions. Again the area in southern Lake Two is so rich in anorthositic inclusions that it is distinguished as a separate map unit (Dmx). Foliation and intermittent layering in the D1ta” unit defines a synformal structure north of Rock Island Lake that trends to the NE (Fig. 6-6). The oxide troctolite that occurs on the south limb of this synformal structure is presumably the D1t” unit reappearing, however in this case, it displays a very well developed modal layering. Farther southeast of Rock Island ake, the troctolite returns to a southeasterly dip, thus defining an anticlinal structure before descending below the anorthositic series cap. Figure 6-6. Idealized stratigraphic variation in modal mineralogy through the Lake One troctolite approximately along the red profile line shown in Fig. 6-4 (modified from Miller, 1986). Spikes between Plag and Ol fields schematically portrays intervals of olivine isomodal layering. Units defined by Miller (1986) are correlated with map units shown in Figure 6-4. Arrows show the stratigraphic intervals formed by three successive intrusive pulses and subsequent crystallization that are interpreted to have created the intrusion. The monotonous troctolitic composition of the Lake One troctolite is typical of other layered series intrusions comprising the northern and northwestern margin of the Duluth Complex - specifically the Tuscarora Intrusion (Morey et al., 1981; Costello et al., 2009); the South Kawishiwi intrusion (Phinney, 1972; Severson, 1994; Miller and Severson, 2002), and the Partridge River Intrusion (Severson and Hauck, 1990; Miller and Severson, 2002) (Fig. 6-1). Moreover, the cyclical attributes of the Lake One troctolite wherein augite troctolite abruptly gives way to well layered oxide-bearing troctolite/ melatroctolite is commonly observed in these other intrusions. These troctolite-dominated intrusions contrast with other well-differentiated systems like the Layered Series at Duluth and the Greenwood Lake intrusion. (Miller and Ripley, 1996). The igneous stratigraphies of these layered series intrusions 166 �demonstrate a general sequence of cumulus mineral crystallization that is typical of complete fractional crystallization of tholeiitic magmas (Wager and Brown, 1968): Ol(±CrSp) —> Pl+Ol —> Pl+Cpx+Ox(±Ol±Opx)—> Pl+Cpx+Ox+Ap(±Ol), That the Lake One troctolite and its kindred intrusions only display the first part of this crystallization sequence is best explained as indicating that these were systems open to repeated and significant recharge of primitive magma. The cyclical changes in mode and texture in the Lake One troctolite, along with a limited range in mineral compositions (Miller, 1986), suggest that a least three major episodes of magma emplacement were responsible for its formation. INCLUSIONS AND STYLE OF EMPLACEMENT Two general types of inclusions occur in the Lake One troctolite – mafic hornfels inclusions and various types of anorthositic inclusions. Anorthositic series inclusions were described above. I will concentrate on the mafic hornfels inclusions here and discuss what the distribution of the inclusions implies about the emplacement of the Lake One troctolite. The mafic hornfels inclusions range from meters to kilometers in size, but tend to be elongate in the plane of foliation and layering in the troctolite (unit Nmh. Fig. 6-4). The hornfels displays a fine-grained granoblastic texture and mineralogically ranges from troctolite to olivine gabbro. Oxide and orthopyroxene are also locally observed, but no systematic petrographic study of the inclusions has been conducted to determine the most common mineralogic types. Some inclusions contain well distributed coarse gabbroic knots that have been interpreted elsewhere (Miller and Severson, 2002) to represent meta-amygdules (some even show pipe amygdule shapes). Similar inclusions occur throughout the lower sections of layered series intrusions and are generally interpreted to be thermally metamorphosed basalt flows from the lower section of the North Shore Volcanic Group (NSVG). The largest inclusion (or group of inclusions) in the map area occurs between the northeastern part of Lake One and the north arm of Lake Two (Fig. 6-4). Troctolitic rocks in the vicinity of inclusions are coarser and enriched in augite and iron oxide to create olivine gabbro to augite troctolite (mapped as unit D1og). Perhaps this gabbroic aureole formed from volatiles and partial melts driven off the large inclusion. Alternatively, perhaps it represents a fragment that was delaminated from a position originally closer to the lower contact. Interestingly, mafic hornfels inclusions of significant size are almost exclusively contained within the lower two cycles of the troctolite (up to unit D1at’). The only exception to this is the mafic hornfels that will be investigated at Stop 6-13. Also, although anorthositic inclusions occur throughout the Lake One troctolite, large anorthositic series inclusions occur exclusively in the upper cycle (D1t” and D1at”). This strongly suggests that the anorthositic series was emplaced not directly at the unconformity between the Archean and basal lava flow, but instead some unknown distance up into the volcanic pile of NSVG. Interpreting the stratigraphy of the three successive cycles of the Lake One troctolite as indicating emplacement from bottom to top, implies that the first injection of Lake One troctolite magma was at the Archean-volcanic unconformity. This large scale intrusion effectively delaminated the basal lava flows that remained between the Archean footwall and the anorthositic series. The second recharge event was also was largely into the basaltic flows, but the third cycle was emplaced into the base of the anorthositic series. Recent mapping in the Tuscarora intrusion to the east shows a similar relationship (Costello et al., 2009). Two general troctolitic zones, comparable to the Lake One cycles, can be distinguished in the Tuscarora 167 �with the lower one rich in mafic hornfels inclusions and the upper zone exclusively containing anorthositic series inclusions. This provides supporting evidence for the Lake One troctolite being the western extension of the Tuscarora intrusion. STOP DESCRIPTIONS DAY 1 GEOLOGY OF THE LOWER LAKE ONE TROCTOLITE Figure 6-7. Geologic map of the lower Lake One troctolite showing the field stop locations for Day 1. Red dashed line shows the travel route to and from the Kawishiwi Lodge (KW). 168 �Departing from the Kawishiwi Lodge, we will canoe west into the Kawishiwi River channel to the basal contact of the L1T. The geology between the lodge area and the BWCA canoe landing is dominantly metasedimentary rocks (conglomerates, arkosic sandstone and siltstone) of the Knife Lake Group. Downstream of the canoe landing, metavolcanics and metavolaniclastics of the Upper Ely Greenstone predominate. STOP 6-1: Contact of Lake One Troctolite (D1cz) and Archean Metavolcaniclastic Rocks (Aec) Location: NW bank of Kawishiwi River (UTM83: 613225/5309950) Exposed in outcrop forming the point to the southwest is a medium- to coarse-grained, poorly to moderately foliated, subophitic biotitic oxide olivine gabbro (PcOfb) with local patches of pegmatitic gabbro. An irregularly-shaped, meter-sized inclusion of gabbroic anorthosite is exposed in this area. Back to the northeast and inland about 5 meters is a ledge outcrop of a very dense, metasedimentary hornfels. Continuing NE back toward the shore, is more olivine gabbro. Here, the vari-textured nature of the basal contact zone is more evident with grain size ranging from fine to coarse. An interesting feature exposed here are spaced stringers of coarser gabbro at right angles to the moderately developed foliation. Perhaps this represents vapor streaming out of the footwall. At the far northeast end of the outcrop, the contact with metasedimentary hornfels is inferred within a meter of exposure. Canoe SW to the portage. Cross over the portage and leave canoes. We will walk to the SW to view outcrops STOP 6-2: Augite Troctolite of the Lower D1og Unit Location: NW bank of Kawishiwi River at SW end of portage (UTM83: 612927/5309765) Exposed in a pavement outcrop SW of the portage trailhead is a homogeneous medium- to medium coarse-grained, moderately to poorly foliated augite troctolite (POcf). This rock and more gabbroic varieties are typical of the D1og unit. Augite and oxide comprise about 8% of the rock here, but gradually increase in abundance to the NW to about 15%. About 1-2% biotite is also present. Follow the north shore of the channel to the next portage. STOP 6-3: Contact of Lake One Troctolite (D1cz) and Archean Metavolcaniclastic Rocks (Aec) Location: NW bank of Kawishiwi River at E end of portage (UTM83: A – 612560/5309866; B 612658/5309802) Area A - Exposed in along the north bank of the rapids to the west of the portage is a strongly foliated, well-bedded metavolcaniclastic. Foliation and bedding are oriented ~N75E/80SE. The metasedimentary rock is cut by abundant quartz and chlorite veins. Granoblastic texture is locally evident. Area B - Exposed in outcrop forming the point to southeast of the portage landing is a medium coarsegrained, poorly to moderately foliated, biotitic ophitic oxide olivine gabbro (PcOf) to augite troctolite (POcf). High density oxide oikocrysts (<1 cm) stand out in relief and augite oikiocrysts are 1-2 cm across. This rock is unusually homogeneous for being so close to the basal contact. Note also that it is barren of sulfide. Canoe SW to the portage at the west end of the bay. Cross over the portage and continue up the meandering channel about 600 m to an island in the channel. 169 �STOP 6-4: Oxide Troctolite in the Lower D1at Unit Location: Island in river channel (UTM83: 612348/5309210) The exposures on this island are medium-grained, moderately foliated, augite-bearing oxide troctolite (POfc). Subpoikilitic to anhedral granular oxide is about as abundant as ophitic augite (each about 5%). Typically the augite:oxide ratio is 2-3:1. The greater abundance of oxide and its partially granular habit suggest that it is cumulus in part. Analyses of these granular oxide elsewhere indicate that they are complex Cr-Mg-Al-Ti spinels. Note that the rock is subtly plagioclase porphyritic. Also, subtle olivine layering is locally evident, especially on the NW corner of the island. All of these characteristics are common in rocks of the D1at unit. Canoe S to the portage. Store canoes here while we explore the outcrop along the trail and exposures to the east. STOP 6-5: Oxide-rich Melagabbro Intrusion/Inclusion? in the D1ta Unit Location: North end of portage to Lake One (UTM83: 612387/5309040) Outcrops on the west side of the portage trail are altered medium-grained augite troctolite. The mafic phases have been replaced by actinolite and chlorite and plagioclase is partially sericitized. The source of this alteration be related to the rock seen to the east along the river channel. Here we find a coarsegrained to pegmatitic amphibole-oxide-plagioclase rock. Unfortunately, a thin section was not made of this rock to determine whether the green amphibole is primary (hornblende?) or secondary (actinolite?) after pyroxene. Moreover, it is not clear whether this rock is an inclusion (perhaps of recrystallized iron formation) or is an oxide ultramafic intrusion (OUI’s of Severson and Hauck, 1990). The alteration near the portage trail and the abundance of chloritic veins in the area suggest it may be a volatile-rich intrusion. However, the distortion of foliation to a dip of 70° in troctolite on the opposite shore suggests that it might be an inclusion. If the latter, it further implies that Paleoproterozoic strata existed on top of the Archean basement in this area when the Duluth Complex intruded. The last exposure of Biwabik Iron-formation observed in the footwall of the Duluth Complex occurs 30 km to the SE in the Dunka Pit area (Fig. 6-1). Portage canoes to S end of the portage trail. Store canoes here while we explore the outcrop in the campsite area to the west. STOP 6-6: Intermittently Layered Oxide Troctolite (D1ta Unit) Location: Campsite west of south end of portage trail near Lake One Dam (UTM83: 612340/5308833) This stop is mostly intended as a lunch stop. The pavement outcrops at the campsite are medium-grained, layered and moderately foliated oxide troctolite (POFc). Granular (cumulus?) iron oxide up to 3 modal% is present in greater abundance than interstitial augite (1-2%). Olivine-rich layers are common throughout the outcrop. Canoe from portage past island to the S, then head E to SW end of large island. STOP 6-7: Mafic Hornfels (Nmh) in the D1ta Unit Location: Large island in central area of Lake One (UTM83: 612820/5308566) Exposures on the SW end of this island show a mafic hornfels with a generally troctolitic modal composition. Locally, patches and stringers of coarse altered (amphibolitized) gabbro define crude layering that trends NE, roughly parallel to foliation and layering in the bounding troctolite. These 170 �gabbroic areas are interpreted to be metamorphosed amygdaloidal zones of the original basalt flows. As is typical of mafic hornfels, they are cut by a network of chlorite-quartz veins. Canoe into the northern basin of Lake One and head east to the outlet for the north arm of Lake Two. STOP 6-8: Large Mafic Hornfels inclusion (Nmh) in Olivine Gabbro (D1og) Location: Large outcrop knob north of outlet for the north arm of Lake Two (UTM83: 615090/5309395) Exposures in the lower part of the slope are medium- to medium coarse-grained, non-foliated, subophitic olivine oxide gabbro. Progressing NE up the large knob, the gabbro coarsens and then a complex contact with mafic hornfels is encountered. The mafic hornfels is generally fine-grained, granoblastic and varies in mineralogy from olivine gabbro to troctolite. Crs clots of gabbro (meta-amygdules?) are dispersed throughout and the inclusion is cut by abundant chlorite veins. The inclusion on this knob is about 30 meters across, but it is probably part of a high concentration of hornfels inclusions in this area (greatly simplified in Fig. 6-7). Northeast of the inclusion is more olivine oxide gabbro that locally displays a plagioclase porphyritic texture. It is not clear whether this olivine oxide gabbro is a contamination halo affecting troctolitic rocks around the inclusions or whether it is “attached” to the hornfels that collectively comprise a larger inclusion mass included in the D1at unit. Canoe west back into the northern basin of Lake One to an island near the entrance to the northern arm. STOP 6-9: Well-layered Troctolite of the D1at unit Location: Island near the entrance to the northern arm of Lake One (UTM83: 613938/5309517) Exposed along a NE –trending ridge on northern side of this island is medium-grained, well-foliated augite-poor troctolite (PO) that commonly displays modal layering. Olivine enriched layers are 2-10 cm thick and contain 40-45% olivine compared to normal troctolite with 25-30% olivine. Low angle crossbedding is noted locally. In one location, a small (<1 m) anorthositic inclusion is present. A similarly well layered troctolite is exposed on the northwestern projection of the island. Canoe into the northern arm to exposure on E side of channel just before it opens into the northern bay. STOP 6-10: Rhythmically-layered Augite Troctolite of the D1og unit Location: Cliff face on east bank of channel (UTM83: 614164/5310251) Exposed in the cliff face is a rhythmic layering in a medium-grained, poorly foliated, biotitic ophitic augite troctolite (POcfb). The layering is differentially weathered such that the more plagioclase-rich intervals stand out in relief. The dip of the layering is about 40° to the southeast. Some of the layering seems to be graded with olivine-rich intervals having sharp bases and gradational upper parts that grade to normal augite troctolite. Some subtle cross-bedding may also be recognized. Below this layered interval, the rock becomes considerably more gabbroic and vari-textured typical of the contact zone (D1cz). If time permits, we may investigate the contact zone again which is exposed on the west side of the channel. It is similar to the relationships seen at stop 6-1. 171 �DAY 2 GEOLOGY OF THE UPPER LAKE ONE TROCTOLITE Departing from the Kawishiwi Lodge, we will canoe south down the northern arm of Lake One and into the southern basin of Lake One. There we head east toward to the portage to Lake Two. Figure 6-8. Geologic map of the upper Lake One troctolite showing the field stop locations for Day 2. Red dashed line shows the travel route. 172 �STOP 6-11: Autolithic Inclusions near the D1at-D1t’ Contact Location: Small pavement outcrop at NE shore of southern basin of Lake One west of portage entry to Lake Two (UTM83: 613670/5307863) In this small outcrop, four different blocks of troctolitic rock types are exposed in sharp contact with one another as shown in the sketch in Figure 6-9. Based on the cross-cutting relationships of foliation by sharp contacts, rock type 1 is the youngest and rock type 2 is the next oldest. Just to the east of this outcrop, a well layered oxide troctolite of the D1t’ unit occurs that is interpreted to indicate an new magma recharge cycle. Perhaps this auto-brecciation represents the structural disruption that accompanied this recharge event. Figure 6-9. Sketch of contact relations among troctolitic rock types exposed at Stop 6-11 (from Miller field notes, Station 450, Aug. 8, 1981) Head east to the portage to Lake Two. Store canoes and investigate outcrops in landing area. STOP 6-12: Layered Troctolite of the D1t’ Unit Location: Outcrop at portage landing and in stream to the north of the portage. (UTM83: 613925/5307821) The small outcrop at the landing is a medium-grained, augite-poor troctolite (PO) with very subhedral granular olivine. Progressing into the stream, the troctolite here shows rhythmic to intermittent isomodal olivine layering. Olivine layers range from 2 to 20 cm thick. Upstream, the rock gradually increases in augite and oxide mode. At the SE end of the portage, the rock is an augite troctolite (POcf) containing inclusions of mafic hornfels. Head across the two small portages to Lake Two. The next stop will be at the SE end of the second portage. STOP 6-13: Meta-amygdaloidal Mafic Hornfels Inclusions in Oxide Troctolite of the D1t” Unit Location: Outcrop at portage landing and in stream to the north. (UTM83: 614126/5307391) 173 �Pavement outcrops at the landing into Lake Two show a complex mix of mafic hornfels inclusions that locally display coarse clots and stringers of gabbro (meta-amygdules) hosted in a medium-grained, augitebearing oxide troctolite (POFc). An anorthositic inclusion also is exposed in this outcrop. In the troctolite, oxide occurs as small subhedral grains and is equal to more abundant than augite. The troctolite is near the base of the D1t” unit. These mafic hornfels represent the highest stratigraphic occurrence of volcanic inclusions in the Lake One troctolite. Head south from the portage to the SW arm of Lake Two and to the portage to Rock Island Lake. Cross the portage and canoe into the east corner of Rock Island Lak.e STOP 6-14: Northwest-dipping Layered Troctolite of the D1t” Unit Location: Outcrop at the east end of Rock Island Lake. (UTM83: 614038/5305437) Rhythmic isomodal olivine layering is exposed throughout the eastern end of Rock Island Lake. Olivine layers range are 1-10 cm thick. Small granular (cumulus) oxides are present in both the melatroctolite layers and the intervening troctolite. Pyroxene is rare throughout (<1%). The northwesterly dip of the layering here defines a synform structure in this area. The cause of this deformation is not clear (sag over a feeder?), but it likely occurred when the rocks were still semi-molten or at least at a high temperature since no deformation features are noted on a handsample or thin section scale. Canoe along SW along shoreline of Rock Island Lake to bay at south end. STOP 6-15: Anorthositic Series (Das) Location: Sloping pavement outcrop on east side of bay. (UTM83: 613847/5305200) Three different anorthositic rock types are displayed on this outcrop. At the north end is a medium coarsegrained poikilitic olivine gabbroic anorthosite with 5-8 cm oikocrysts of olivine. Across an obscured, but presumably sharp contact is a coarse grained troctolitic anorthosite with large anhedral granular olivine, ophitic augite, and subpoikilitic oxide collectively totaling about 10% of the rock. Across a clearly sharp contact is a medium-grained leucotroctolite containing 15-20 % olivine and very little augite or oxide. This latter rock contains inclusions of the other varieties and is thus the younger intrusion. The outcrop scale variability in anorthositic rock types and the sharp unchilled contact relationships displayed here are typical of the anorthositic series. Canoe and portage back into Lake Two. Head to the campsite on the eastern point of land that separates the SW arm of Lake Two from the main part of the lake. STOP 6-16: Mixed Troctolitic and Anorthositic Rocks of the Dmx Unit Location: Campsite in SW Lake Two (UTM83: 614250/5306703) Exposed over the clean pavement surface at this campsite is a complex mix of anorthositic series rock types as inclusions in a fine- to medium-grained matrix. The anorthositic inclusions range from cm to tens of meters across and show a range of mineralogic and textural varieties typical of the anorthositic series. The intervening troctolite appears to be of two generations, an early finer grained troctolite that is locally Pl-phyric and a later medium-grained type. The contacts between the troctolite and anorthositic inclusions range from sharp to gradational over a several centimeters. This suggest that some of the anorthositic inclusions may have been partially molten when intruded by the troctolite and subsequently became disaggregated. A meter-sized mafic hornfels inclusion is exposed in one area of the outcrop. 174 �Head east through the cluster of islands in southern Lake Two to an island in the east central part of the lake. STOP 6-17: Mixed Troctolitic and Anorthositic Rocks of the Dmx Unit Location: Island in east central Lake Two (UTM83: 615570/5307100) Exposures around the margins of this small island provide another view of the scale and complexity of mixing between anorthositic and troctolitic rock types in the Dmx unit. Anorthositic varieties, which tend to be medium- to coarse-grained, include poikilitic olivine gabbroic anorthosite, granular troctolitic anorthosite, leucotroctolite and nearly pure anorthosite. The troctolite is variable in texture and mode as it locally seems to become hybridized with the anorthositic inclusions. Head north to the entrance to the northern arm of Lake Two. STOP 6-18: Contact between Augite Troctolite (D1ta’) and Oxide Troctolite (D1t”) Location: Point on east side of entrance to north arm of Lake Two (UTM83: 615488/5307646) Shoreline outcrops here display medium-grained, moderately foliated oxide troctolite containing up to 3% anhedral granular to subpoikilitic oxide (POF-POf). Some subtle olivine layering is also evident. Progressing to the north, this rock abruptly transitions in to a coarsely ophitic augite troctolite with pyroxene oikocrysts up to 4 cm diameter (POcf). This transition is taken to define the D1ta’-D1t” contact and is interpreted to represent the third major recharge event that created the Lake One troctolite. Canoeing into north arm of Lake Two to the next stop, we can observe several interesting features in the outcrops lining the western side of the channel. Specific points of interest are labeled on Fig. 6-8. These include: O (615456/5307950) – coarsely ophitic augite troctolite w/ 5cm oikocrysts; also gabbroic pegmatite P (615473/5308241) – gabbroic pegmatite cutting augite troctolite. L (615596/5308493) – modal layering dipping S (sheet jointing dipping N); this is in the D1t’ unit STOP 6-19: Augite Troctolite of the Upper D1ta Unit Location: Sloping pavement at campsite on east side of channel (UTM83: 615488/5307646) Pavement outcrops here are medium-grained, moderately foliated, ophitic augite troctolite (POcf) corresponding to the upper part of the D1ta unit. Augite oikocrysts are present, but do not stand out as much as to the south at the previous stop (6-18). Head north to the landing adjacent to the old dam. Investigate outcrop to the north. STOP 6-20: Olivine Oxide Gabbro Aureole (D1og) Location: Landing on east side channel at northern end of the north arm of Lake Two (UTM83: 615572/5308640) Outcrop ledges along the east bank of the channel show a vari-textured (medium – pegmatitic), subophitic olivine? oxide gabbro similar to that seen yesterday at Stop 6-8. Some streaky textural layering is evident in some exposures. The occurrence of gabbro here demonstrates that the gabbro forms a complete 175 �aureole around the mafic hornfels block (or blocks) that are abundant in this area. The gabbroic pegmatite cutting troctolitic rocks upsection from this (as seen in the channel exposures to the south) may also have emanated from the gabbroic aureole. If so, this would argue for the aureole having developed by contamination of volatile-rich partial melts driven from the hornfels inclusion(s). Return to Kawishiwi Lodge. End of field trip. REFERENCES Allen, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In: Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p.47-72. Costello, D. E, Miller, J.D., Jr., and Jirsa, M.A., 2009, Geology of the Tuscarora intrusion, northeastern Minnesota and its relationship to the anorthositic series of the Duluth Complex. 55th Annual Institute on Lake Superior Geology, Program and Abstracts vol. 55, p 14-15. Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Green, J.C., Phinney, W.C., & Weiblen, P.W., 1966. Gabbro Lake quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-2, scale 1:24,000. Grout, F.F., 1918, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458. Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin, v. 52, p. 1577-1642. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake 30’ x 60’ quadrangles, northeast Minnesota. Minnesota Geological Survey Miscellaneous Map M-148, scale 1:100,000. Miller, J.D., Jr., 1986, The geology and petrology of anorthositic rocks in the Duluth Complex, Snowbank Lake quadrangle, northeastern Minnesota. unpublished Ph.D. dissertation, University of Minnesota, 280 p. Miller, J.D., Jr., 1992, The need for a new paradigm regarding the petrogenesis of the Duluth Complex. 38th Annual Institute on Lake Superior Geology,p. 65-67. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. In Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., Jr. and Severson, M.J., 2002, Chapter 6. Geology of the Duluth Complex. In Miller et al., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D., Jr., and Vervoort, J.D., 1996, The latent magmatic stage of the Midcontinent Rift: a period of magmatic underplating and melting of the lower crust. 42nd Annual Institute on Lake Superior Geology, Proceedings Volume, Part I – Program and Abstracts, p. 33-35. Miller, J.D., Jr. and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology 31, p. 295-339 Miller, J.D. Jr., Green, J.C., and Severson, M.J., 2002, Chapter 1. Terminology, nomenclature, and classification of Keweenawan igneous rocks of northeastern Minnesota. In Miller et al., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. 176 �Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 Morey, G.B., Weiblen, P.W., Papike, J.J., and Anderson, D.H., 1981, Geologic map of the Long Island Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-46, scale 1:24,000 Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,997-14,013. Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota. Minnesota Geological Survey Report of Investigation 9, 20 p. Phinney, W.C., 1972, Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A centennial volume. Minnesota Geological Survey, p. 335-345 Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report NRRI/TR 93/34, 210 p. (with plates) Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Sims, P.K., 1985, Generalized bedrock geologic map of the west-central Vermilion district, northern Minnesota. U.S. Geological Survey Miscellaneous Investigations Map I-1529, scale 1:48,000. Taylor, R. B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 pp. Vervoort et al., K. Wirth, B. Kennedy, T. Sandland and K.S. Harpp, 2007, The magmatic evolution of the Midcontinent rift: new geochronologic and geochemical evidence from felsic magmatism, Precambrian Research 157 (1–4) (2007), pp. 235–268 Wager, L.R. and Brown, G.M., 1968. Layered Igneous Rocks. San Francisco: W.H. Freeman, 588 pp. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/ 100 plates. 177 �55th Annual Institute on Lake Superior Geology Field Trip 7 ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT: STRATIGRAPHY, STRUCTURE, MINERALIZATION Graded bedding in greywacke of the Lake Vermilion Formation, Pike River dam (stop 7-14) "In the region occupied by these rocks are found immense bodies of magnetitic and hematitic iron ore. In the talcose slates and primitive schistose rock are veins of quartz, carrying auriferous and argentiferous sulphides of iron and copper." Henry H. Eames, 1866 Dean Peterson (Duluth Metals Ltd. & PRC) Mark Jirsa (Minnesota Geological Survey) George Hudak (Dept. of Geology, UW-Oshkosh & PRC) 178 �FIELD TRIP 7 ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT: STRATIGRAPHY, STRUCTURE, AND MINERALIZATION Dean M. Peterson (Duluth Metals Limited & PRC) Mark A. Jirsa (Minnesota Geological Survey) George J. Hudak (University of Wisconsin Oshkosh) INTRODUCTION Archean greenstone belts are one of the world's premier geological settings for hosting a variety of economically important mineral deposit types. These deposits include high-grade iron ore, lode-gold, volcanogenic massive sulfide, komatiite associated nickel, magnesite, and a number of others. The origin of these deposits are intrinsically linked to the architecture of the greenstone belt, namely the interrelationships between stratigraphy, structural setting, and multiple generations of hydrothermal fluids. The Vermilion district of northeastern Minnesota contains one of the classic granite-greenstone terranes in the United States. This district comprises the south-central part of the Wawa subprovince of the Superior Province of the Canadian Shield, and has been broadly correlated with the Saganagons Assemblage of the Wawa subprovince in northwestern Ontario (Peterson et al., 2001; Peterson and Patelke, 2003). In Canada, the Wawa subprovince hosts numerous lode gold (e.g. the Hemlo and Renabie districts) and volcanic-hosted massive sulfide (VMS) ore bodies (e.g. the Winston Lake, Willroy, Big Nama Creek, Willecho, and Geco deposits; Fyon et al., 1992). The Vermilion district is well known for its numerous, previously mined, massive hematitic iron ore deposits. These iron deposits were discovered in the early 1880s, and virtually all-subsequent exploration efforts in the region were targeted on similar iron-formation hosted hematite deposits. However, the discovery of world-class ore deposits in Ontario (the Kidd Creek VMS deposit in 1964 and the Hemlo gold deposit in 1980) led to short periods of both base metal and gold mineral exploration in the Vermilion district. To date, no lode gold and/or VMS ore bodies have been discovered in the Vermilion district, although abundant evidence exists that future exploration may result in the discovery of economically important deposits. GREENSTONE BELTS A strong debate continues on the origin, development, and architecture of Archean greenstone belts, particularly with regard to the roles of subduction, plume magmatism, rifting, diapirism and autochthonous vs allochthonous development (e.g. de Wit, 1998; Hamilton, 2003). Studies in the Superior and Slave Provinces of Canada indicate that strongly contrasting tectonic styles may have been in operation at the same time. For example at ca. 2.7 Ga, large diapiric batholiths and synclinal greenstone keels may suggest that diapirism was an important tectonic process in the Slave Province (Bleeker, 2002), whereas Stott (1997) suggests that the linear distribution of belts suggests that accretionary tectonics (i.e. plate tectonics) may have dominated in the Superior Province. Neither theory precludes the other, and in developing models for Archean tectonic evolution, no one model will be equally applicable to all areas. Hoffman (1990) presents a model of greenstone belt formation via arc-trench progradation as an application of the principle of lateral and temporal equivalence, also known to sedimentologists as "Walther's Law". In this model (Fig.7-1), accretion of the overriding plate in a subduction zone involves scraping of material off the downgoing plate and arc magmatism. The off-scraped material consists of sediment and the tops of igneous bathymetric highs (ie. island arcs, remnant arcs, seamounts, oceanic island chains, submarine plateaus, fracture zones and microcontinents). 179 �Figure 7-1. Prograding arc-trench model for the generation of granite-greenstone terranes. On the left is an idealized cross-section of an arc-trench system: AS, asthenosphere; CUP, cumulate underplating; LM, lithospheric mantle; MA, magmatic arc; MM, magma melting and mixing; PM, zone of partial melting; TR, subduction trench. Fore-arc accretion is achieved by scraping of sediment and topographic highs off of the downgoing plate. Examples of bathymetric highs that could be scraped off and eventually form greenstone belts are depicted on the right. Modified from Hoffman, 1990. We hope to portray our present understanding of the Vermilion district in the context of its geological architecture—highlighting the interrelationships between stratigraphy, structure, and mineralization. The trip will revisit many outcrops on which so many historical discussions, many of them heated, pertaining to the regional geologic setting of the Archean occurred. Many of the stop descriptions in this field trip have been modified from the Field Trip Guidebook of the 50th Annual Meeting of the Institute on Lake Superior Geology, which was held in Duluth in May of 2004. In particular, volcanology and hydrothermal alteration within the Lower Ely Greenstone was described in detail by Hudak et al. (2004), gold mineralization north of the Mud Creek shear zone was described in detail by Peterson and Patelke (2004b), and classic outcrops in northeastern Minnesota were described by Jirsa et al (2004). Readers of this guidebook should review these documents for detailed descriptions. REGIONAL GEOLOGICAL SETTING Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa subprovince of the Superior Province of the Canadian Shield. Rocks of the Wawa subprovince in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota, and was designated informally as the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of the Soudan Mine, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic 180 �rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are fault-bounded, and the relationship between stratigraphic units within each belt is largely conformable, although faults obscure contacts locally. In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 7-2. Figure 7-2. Simplified correlation map of Neoarchean assemblages across the U.S. - Canada border (modified from Peterson et al., 2001). Inset illustrates major subprovinces of the southwestern Superior Province. Lithostratigraphic units in the western Vermilion district include: (1) the Lower member, Soudan Iron Formation member, and Upper member (Upper Ely) of the Ely Greenstone, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999) and the Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons. Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Rock types associated with the lithostratigraphic units in the area are presented in Table 7-1. 181 �Table 7-1. Lithostratigraphic units within the western Vermilion district (from Peterson and Jirsa, 1999). Intrusive Rocks Late Intrusions Vermilion Granitic Complex Plutons and stocks of syenite, monzonite, diorite, and lamprophyre Granite, schist, amphibolite, and schist-rich migmatite Giants Range Batholith Granite, granodiorite, monzodiorite, and schist-rich migmatite Supracrustal Rocks Newton Belt Newton Lake Formation Tholeiitic and komatiitic basalt flows, intrusions, and clastic strata Bass Lake Sequence Tholeiitic basalt lava flows, iron-formation, and felsic porphyries Soudan Belt Knife Lake Group Lake Vermilion Formation Graywacke, slate, conglomerate, & sheared equivalents Graywacke, slate, dacitic tuff, and minor conglomerate Gafvert Lake Sequence Dacitic to trachyandesitic lava flows, tuffs, and intrusions Britt Sequence Tholeiitic basalt lava flows Upper Ely Greenstone Tholeiitic basalt lava flows and iron-formation Soudan Iron Formation Layered cherty iron-formation, epiclastic rocks, and tuff Lower Ely Greenstone Calc-alkalic & tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks and minor iron-formations STRUCTURAL GEOLOGY Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskamingtype clastic sedimentary sequences in local fault-bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion district is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion district to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation event (D3) include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and 182 �Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary. ECONOMIC GEOLOGY Since the mid-1860’s, numerous mineral exploration programs have been conducted in the Vermilion district. Most of these exploration programs focused on identifying minable deposits of massive hematitic iron ores, such as those mined between 1883 and 1962 in the Soudan Iron Formation at the Soudan Mine. During the 1980s and early 1990s, subeconomic lode gold mineralization was discovered in close proximity to the east-west-trending Murray shear zone, which dissects the Lower Ely, and in close proximity to the Mud Creek shear zone, which separates the Soudan belt from the Newton belt to the north. Four VMS prospects occur within the Lower member of the Ely Greenstone, and occur in close proximity up-section from a semiconformable quartz-epidote alteration zone that extends for at least 19km along strike in the north limb of the Tower-Soudan Anticline (Peterson, 2001). These VMS prospects include the Skeleton Lake prospect (drilled by Exxon, 1972), the Eagles Nest prospect (drilled by Newmont, 1988), the Fivemile Lake prospect (drilled by Teck, 1994), and the Purvis Road prospect (drilled by Rendrag, 1999). Recent studies of these three types of mineral deposits in the Vermilion district link stratigraphy and structure, and thus help unravel the architecture of the greenstone belt. Brief descriptions of hematite, VMS, and lode gold mineralization in the field trip area are presented below. Origin of Massive Hematite from Algoma-Type Iron Formation Most iron ores mined today comprise the iron oxide minerals magnetite, Fe3O4 (72% Fe); hematite, Fe2O3 (70% Fe); goethite, Fe2O3(s) * H2O, (63% Fe); and limonite, a mixture of hydrated iron oxides (up to 60% Fe). The world’s most important iron ore resources occur in iron-rich sedimentary rocks (20% - 40% Fe) known as banded iron-formations (BIFs), which occur on all continents and are almost exclusively of Precambrian age. In many iron-mining districts, e.g., the Mesabi Range of northern Minnesota, the BIFs are mined as iron ore with the iron content concentrated into pellets (~65% Fe) in large on site facilities. In other districts, e.g., the historic Vermilion Range of the Soudan Mine area; Hamersley District, Western Australia, the BIFs are the source rocks for large, natural high-grade concentrations of iron that typically occur as bodies of massive hematite and/or hematite-goethite with >60% Fe. The origin of these important natural concentrations of iron minerals remains highly debated. The Fe atoms in hematite are all Fe3+, whereas in magnetite they are comprised of two Fe3+ and one Fe2+ atoms. Therefore, the transformations of magnetite to hematite, or hematite to magnetite, in Fe-conservative systems is always a redox reaction, with the Fe2+ atoms in magnetite oxidized to Fe3+ atoms, or the Fe3+ atoms in hematite reduced to Fe2+ atoms, by reactions such as: 2Fe3O4(mt) + 1/2O2(g) → 3Fe2O3(hm) (1) and 3Fe2O3(hm) + H2(g) → 2Fe3O4(mt) + H2O (2) In the past, the study of the transformation of magnetite to hematite (1) and conversely hematite to magnetite (2) in natural systems has largely focused on these reactions, which require either an oxidizing or reducing agent and an occurrence under specific redox environments. Since virtually all of the known concentrations of high-grade iron ores are hematite-dominant, the exploration for such deposits has concentrated on supergene enrichment of magnetite-rich BIFs. In this model, magnetite-rich BIFs are uplifted and subjected to weathering under oxygenated conditions to form goethite-rich ores; and are subsequently buried and metamorphosed to hematite-rich ores (Morris, 1985). Problems with this model have recently been discussed by Ohmoto (2003) for the Tom Price Mine of the Hamersley district, and are applicable to the origin of the massive hematite ores of the Soudan Mine. Ohmoto (2003) has proposed an alternative mechanism for the transformation of magnetite-rich iron-formations to massive hematite ores by the acid-base reaction: Fe3O4(mt) + 2H+ ↔ Fe2O3(hm) + Fe2+ + H2O (3) 183 �Similar to most acid-base reactions, reaction (3) would be most efficient at high temperatures, and such hydrothermal fluids are capable of leaching silica as well as Fe2+ from magnetite. In addition, the conversion of magnetite to hematite by reaction (3) produces a volume decrease of 32%, greatly increasing permeability of the rocks, which would facilitate further water-rock reactions and enhance conversion of banded chert-magnetite to massive hematite. VMS-Associated Volcanic and Hydrothermal Alteration Processes, Lower Ely Greenstone The Lower Ely Greenstone is composed of calc-alkalic and tholeiitic basalt and basalt-andesite, lava flows, tuffs, and lapilli tuffs with subordinate felsic lava flows, tuffs, epiclastic rocks, and iron formations (Schulz, 1980; Southwick et al., 1998; Jirsa et al., 2001; Hudak et al., 2002a, 2002b; Hoffman, 2007; Jansen et al., in press). The Lower Ely Greenstone has been subdivided into the older Fivemile Lake Sequence and the younger Central Basalt Sequence (Peterson and Patelke, 2003). The Central Basalt Sequence is correlative with the Armstrong Lake Basalts (Jirsa et al., 2001). The Fivemile Lake Sequence is characterized by abundant primary mafic and felsic volcaniclastic strata, highly amygdaloidal basalt and basalt-andesite pillow lavas and sheet flows, multiple selvege pillows, and epithermal-like zinc stringer mineralization that, taken together, indicate a shallow subaqueous depositional setting (Hudak et al., 2002a; Hudak et al., 2002b; Hudak et al., 2007; Hoffman, 2007). The overlying Central Basalt Sequence/Armstrong Lake Basalts comprise exceptionally well-preserved, generally sparsely amygdaloidal sheet flows and single selvege pillow lavas which are locally interstratified with subordinate banded iron formation and chert exhalite horizons, mafic tuff (interpreted primarily to be resedimented hyaloclastite), lithic wackes and polymict breccias that have been interpreted to have formed in a deep subaqueous setting (Peterson and Patelke, 2003; Peterson et al., 2005). Major and trace element data (Hudak et al., 2007; Hoffman, 2007; Jansen, in press) indicate that the lithogeochemistry of volcanic rocks in the Lower Ely Greenstone is more complicated than previously thought (e.g. Southwick et al., 1998). In the Fivemile Lake Sequence, basalt and basalt-andesite flows and volcaniclastic rocks illustrate arc-like lithogeochemical characteristics, and rhyodacites and rhyolites have FI- and FII chemistries. In the Central Basalt Sequence, basalt and basalt-andesites with arc-like lithogeochemical signatures transition up-section into basalts with MORB-like trace element patterns and rhyodacites and rhyolites illustrate FI, FII, and FIII lithogeochemical characteristics. Hudak et al. (2007), Hoffman (2007) and Jansen et al. (in press) have proposed a tectonic model for the development of the Lower Ely which encompasses initial arc development followed by back-arc rifting and associated MORB-type and FIII-type volcanism immediately prior to the deposition of the overlying Soudan Iron Formation. This model appears to be most consistent with the observed stratigraphic, volcanological, and lithogeochemical characteristics of the Lower Ely, as well as studies of both ancient and modern back-arc basins which illustrate that the development of back-arc basins in oceanic volcanic arc settings are commonly associated with the presence of vigorous submarine hydrothermal activity which may produce Algoma-type iron formations and/or volcanogenic massive sulfide deposits (Franklin et al., 2005; Piercey et al., 2004). Geological mapping by Peterson (2001) has indicated the presence of a regional semiconformable quartzepidote alteration zone extending for at least 19 km along strike within the Lower Ely along the north limb of the Tower-Soudan anticline. This type of alteration is a common feature in many Archean VMS camps (e.g. Noranda (Gibson, 1989) and Snow Lake (Skirrow and Franklin, 1994)), and is attributed to silica- and calcium-dumping that occurs in the deep, sub-seafloor as downwelling hydrothermal fluids are heated to temperatures in excess of 350°C (Franklin, 1986; Franklin, 1993; Franklin et al., 2005). Semiconformable alteration zones associated with VMS systems are generally much larger in area than their associated mineralization, and therefore provide exploration geologists regional areas in which to concentrate more detailed, follow-up field mapping, geochemical studies, and geophysical surveys for identifying VMS targets. 184 �The composition and distribution of hydrothermal alteration mineral assemblages in the Lower Ely is similar to that described in major lava-flow dominated VMS mining districts worldwide (e.g. the Noranda Camp, Quebec; Morton and Franklin, 1987; Franklin, 1996; Gibson et al., 1999; Hudak and Morton, 1999; Peterson, 2001; Hudak et al., 2002a,b). Results of recent studies in the Vermilion district (Hudak et al., 2002a; Hudak et al., 2002b; Hocker et al., 2003; Hudak et al., 2004; Hudak et al., 2006; Hoffman, 2007) indicate that not only are the compositions and geometries of the regional alteration mineral assemblages identical to those present in many lava flow dominated massive sulfide mining districts, but that detailed alteration mineral chemistries (Hocker et al., 2003) are also consistent with those associated with the VMS ore deposits in these mining camps. These two observations suggest that the processes that formed the alteration mineral assemblages in the Lower Ely were similar to those that formed equivalent alteration zones in well-established VMS mining camps. A general genetic model for the formation of VMS deposits and associated hydrothermal alteration zone, as recently presented by Franklin et al. (1998), requires convective metalliferous hydrothermal fluid generation in the sub-seafloor environment via heating of down-welling seawater and leaching of metals from the enclosing volcanic and sedimentary strata (Fig. 7-3). The size of a convective hydrothermal system is a function of the abundance of heat in the upper two kilometers of the sub-seafloor crust (Franklin, 1996; Franklin et al., 1998). The intrusion of hypabyssal synvolcanic dikes and/or sills into the shallow sub-seafloor may vigorously enhance the dynamics of convective hydrothermal cells (Campbell et al., 1981). On reaching a critical reaction temperature of ~ 350°C, sustained acid pH in the hydrothermal fluid (evolved fluid) is achieved, and metals are leached from the rocks into the evolved fluid via primary mineral breakdown by calcium metasomatism, silicification, and hydrolysis reactions (Seyfried et al., 1999). In basalt-dominated systems (such as that in the Lower Ely), leaching-related alteration of mafic "source" zones (lower semi-conformable alteration) forms a mineral assemblage composed of albite-epidote-zoisite/clinozoisite-actinolite-quartz. These zones are variably metal-depleted, and are characterized by patchy silicification and epidotization associated with areas metasomatically enriched in silica and calcium. In lava flow-dominated stratigraphic sequences, regionally confined discordant “pipe-like”, and more regionally extensive “semiconformable” alteration zones are present (Morton and Franklin, 1987). The “pipe-like” semi-conformable alteration zones are closely associated with zones of cross-stratal permeability (e.g. synvolcanic fault zones), and are characterized by well-defined vertically extensive alteration zones containing anomalous abundances of sericite, chlorite (both Fe- and Mg-rich varieties), actinolite/ferroactinolite, quartz, pyrite, and locally, chalcopyrite and/or pyrrhotite. Semiconformable alteration zones extend for several kilometers to tens of kilometers in the rocks stratigraphically beneath and adjacent to VMS mineralized horizons (Santaguida et al., 2002a; Santaguida et al., 2002b). In maficdominated volcanic environments, such alteration typically is associated with regional zones of spilitization (an alteration assemblage composed of albite + quartz + Mg-rich chlorite ± sericite), silicification (quartz ± albite), and epidote-quartz alteration (epidote + quartz ± actinolite ± carbonate) (Morton and Franklin, 1987; Gibson et al., 1999; Santaguida et al.. 2002a, Santaguida et al., 2002b). Regional semiconformable alteration zones in felsic rocks in VMS producing camps such as Noranda (Quebec) or Sturgeon Lake (Ontario), typically comprise extensive zones of spilitization, silicification, and sericitization (sericite + quartz ± Mg-rich chlorite) (Morton and Franklin, 1987; Gibson et al., 1999). Both discordant and semiconformable alteration zones have been discovered in the Vermilion district and have been described by Hudak and Morton (1999), Peterson (2001), Odette et al. (2001), and Hudak et al. (2002a). Semiconformable alteration zones in the Lower Ely are dominated by mineral assemblages containing various proportions of quartz, epidote, zoisite/clinozoisite, Fe-chlorite, Mg-chlorite, actinolite, ferroactinolite, sericite/pyrophyllite, and albite. Odette et al. (2001a, 2001b) and Hudak et al. (2002a) have shown via mass balance analysis that semiconformable quartz + epidote ± actinolite ± albite ± chlorite alteration mineral assemblages in the Fivemile Lake area are metasomatically enriched in calcium 185 �and silica, and depleted in base metals (copper and zinc) by 50-90%. Pipe-like, northeast-trending disconformable alteration zones in the Lower Ely are largely composed of Fe-rich chlorite, sericite/ pyrophyllite, actinolite and/or ferroactinolite. Pipe-like alteration zones that have been mapped upsection have, to date, not led to the discovery of economically significant VMS deposits, but have been instrumental in locating potential base metal sulfide-bearing stratigraphic horizons and localized chemical exhalites. Figure 7-3. Simplified schematic model of a convective hydrothermal system associated with the formation of Noranda–type (Morton and Franklin, 1987) or lava-flow dominated-type (Gibson et al., 1999) VMS deposits (modified from Franklin , 1996). Lode Gold Ore Deposit Model and Gold in the Vermilion District The brief description of Archean lode-gold deposits that follows is presented as both a basic reference and also to highlight the important features of the model that will be seen during the field trip. Archean lodegold deposits are one category of ore deposit classified as mesothermal lode-gold deposits (Hodgson, 1993). This deposit type has also been called orogenic gold (Groves et al., 2000), greenstone gold (Robert et al., 1991), Archean lode-gold, mesothermal gold-quartz veins, shear-hosted gold, low-sulfide gold-quartz veins (Berger, 1986b), lode-gold, Mother Lode veins (Bohlke and Kistler, 1986), and iron formation-hosted gold deposits (Berger, 1986a; McMillan, 1996; Rye and Rye, 1974; Fripp, 1976; Kerswill, 1993; Thorpe and Franklin, 1984; and Vielreicher et al., 1994). Whatever the name, they are a widespread group of epigenetic ore deposits that have formed in similar settings throughout geologic time. In general, the deposits form during compressional or transpressional deformation at convergent plate margins in accretionary or collisional orogens (Fig.-7-4). They form over a large crustal-depth range (2 to 20 km) from deep-seated, low-salinity H2O-CO2 ± CH4 ± N2 ore fluids, with Au transported as reduced sulfur complexes. The ore fluids are generated during lower 186 �crustal metamorphism from dehydration reactions. Regional structures provide the main control on distribution of lode gold deposits and mining camps. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, subsidiary faults, shear zones and/or large folds. Highly competent and/or chemically reactive rocks are the most common hosts to the larger deposits. Gold deposition occurs late during the evolutionary history of the host terranes, normally within D3 or D4 in a D1-D4 deformation sequence. Absolute ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the evolutionary history of hosting orogens. Figure 7-4. Generalized tectonic model for the formation of mesothermal gold deposits, after Groves et al., 2000. The late timing of lode-gold deposits is critical to geology-based exploration methods, and hence mineral potential evaluations for these deposits. The late timing is critical because of the present structural geometry of: (1) the deposits, (2) the mining camps, and (3) the enclosing geologic terranes are essentially all similar to the structural geometry during gold mineralization. Therefore the interpretation of bedrock geological maps and cross-sections can be used to discern the physical conditions that existed at the time of ore deposition. Exploration for mesothermal lode-gold deposits should incorporate various aspects of the ore deposit model into criteria that can vector into the most favorable areas for hosting such mineralization. The most fundamental characteristic of this class of deposit is the spatial association of the deposits to regional structures. Zones of widespread carbonate alteration (adjacent to regional structures) should be identified and used to focus subsequent exploration. Within carbonate alteration zones, gold is typically only in areas containing quartz veins, silicification, and/or sericite alteration (with or without sulfides). Two general structural controls on the orientation of lode gold ore shoots include deflections and curvatures of shear zones, and where high strain zones intersect favorable geological elements (Poulsen and Robert, 1989). A widespread area of gold mineralization occurs in numerous prospects east of Lake Vermilion, within the Vermilion greenstone belt of northeastern Minnesota. The mineralization occurs in rocks of the Neoarchean (~2.7 Ga) Bass Lake sequence (Peterson and Jirsa, 1999) of the Wawa subprovince of the Canadian Shield. This zone of abundant gold mineralization is bounded to the south by the Mud Creek shear zone and to the north by the Vermilion fault. The main access to these prospects is along the Mud Creek road (St. Louis County 38). A brief period of mineral exploration for lode-gold deposits in this immediate area of the Vermilion district occurred in the mid 1980s to early 1990s. These programs typically consisted of grid-based geologic mapping, bedrock sampling, ground geophysics, and the completion of soil geochemical surveys. Conversations with many of the people involved in gold exploration programs in the immediate field trip area (centered on Section 6, Township 62 North, Range 14 West), and compilation of all exploration data from the district as a whole (data from the terminated lease files of the Minnesota DNR), has led to the conclusion that interpretation of linear structural elements exposed in outcrops were not used in designing exploratory drilling plans. Therefore, many of the prospects discovered as a result of these exploration programs remain untested by drilling. 187 �FIELD TRIP All stop locations are given in Universal Transverse Mercator coordinates (UTM), Zone 15, using the North American Datum of 1983 (NAD83). Section subdivisions read from smallest to largest quarter; e.g., “NW, SE” should be read “NW quarter of the SE quarter.” The small map insets showing stop locations are taken from USGS 7.5-minute topographic quadrangles listed with each stop. The first day of the field trip will include stops located between the towns of Tower and Ely. A more detailed geological map with stop locations is given in Figure 7-5. Figure 7-5. Simplified geological map of the field trip stops in the Vermilion district. Geology modified from Peterson, 2001. FIELD TRIP STOPS From Ely, our route into the Vermilion district will include driving west along Minnesota Highway 169 to the Bear Head Lake road (County road 128). Turn left (south) and continue approximately 1.6 miles to an outcrop on the east side of the roadway. 188 �STOP 7-1. Silicified Fivemile Lake sequence pillow lavas / Regional semi-conformable alteration Location: T.62N., R.14W., sec.22 SE, SE; roadside outcrop along Highway 128 (Bear Head Lake State Park road) Eagles Nest 7.5-minute quadrangle UTM: 567,810E/5,297,800N DESCRIPTION: At this location we can observe part of the regionally extensive quartz-epidote alteration zone. The outcrop contains relatively undeformed bun- and mattress-shaped pillows. Interpillow hyaloclastite zones are generally pale to dark green in color, and are chlorite and/or actinolite-rich. Minor red-brown staining locally occurs in these zones, and is indicative of the presence of trace amounts of pyrite and/or chalcopyrite. Pillow selvedges commonly contain up to 10% round to oval, pipe-like quartz-epidote and/or actinolite chlorite amygdules. The cores of the pillows are typically pale green gray in color due to nearly wholesale replacement of the original igneous minerals by quartz and epidote. This quartz-epidote alteration is typical for much of the Lower Member of the Ely Greenstone (Lower Ely), and is one of the most important components of possible VMS exploration in the district (leaching of Cu & Zn) out of a large volume of rock due to hydrothermal alteration. NEXT: Continue on Highway 128 southeast for approximately 2.2 miles to the Purvis Forest Management Road. Turn left and continue about 0.4 mile to logging road on the right side (south). Follow logging road approximately 0.15 miles to a series of outcrops. STOP 7-2 Xenolithic hornblende diorite, Purvis pluton Location: T.62N., R.13W., sec.30 NE, SW. Eagles Nest 7.5-minute quadrangle UTM: 571,805E/5,296855N DESCRIPTION: The Purvis pluton is an east-west trending, moderate-sized (~3km3), sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness that ranges from 100-1200 meters 189 �(Peterson, 2001). This intrusion occurs in the lower stratigraphic section of the north limb of the TowerSoudan anticline (Peterson and Jirsa, 1999; Jirsa et al., 2001). Recent work by Drexler and Hudak (in press) indicate that the intrusion has several phases, including 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes. Detailed field mapping by Peterson (2001), Hovis (2001) and Drexler et al. (2004) suggest that the Purvis pluton is a synvolcanic intrusion based on the following characteristics: 1) it lacks a contact metamorphic aureole; 2) its uppermost contact is proximally associated with intense, semiconformable quartz + epidote alteration zones; 3) D2 deformation fabrics occur in both the intrusion and the surrounding country rocks; and 4) early xenolithic diorite phases are cross-cut by thin, commonly D2-deformed dikes of younger tonalite and trondhjemite phases. Galley (2002) and Galley (2003) have indicated that these characteristics are key features of synvolcanic intrusions temporally associated with the genesis of many Precambrian VMS deposits. Peterson (2001) has suggested that the Purvis pluton may have been the heat source that drove hydrothermal systems that produced the Eagles Nest and Purvis Road VMS prospects. This locale offers an opportunity to investigate the xenolithic hornblende diorite phase of the Purvis pluton. The outcrop adjacent to the road predominately contains four types of xenoliths: 1) dark green xenoliths of amygdaloidal (5-8%) basalt-andesite pillow lavas which locally have preserved selvedges and interpillow hyaloclastite, and are locally contact metamorphosed along their margins; 2) pale green epidote + quartz-altered basalt-andesite lava xenoliths that are up to 15 cm in diameter; 3) rare coarsegrained gabbro/diorite xenoliths up to 3 cm in diameter; and 4) rare <1-2 cm diameter subangular chert xenoliths. Large iron-formation xenoliths up to several meters in diameter, and amphibolite xenoliths up to several centimeters in diameter, may be observed in xenolithic hornblende tonalite outcrops least of this location. Iron-formation xenoliths present in outcrops east of here were likely derived from iron-formation horizons that occur immediately southwest of Purvis Lake. Basalt and altered basalt fragments also were derived from the surrounding Lower Ely. The presence of epidote-quartz altered mafic xenoliths suggests that this phase of the Purvis pluton stoped its way upward into an earlier-formed proximal zone of quartzepidote alteration formed from high temperature seawater-rock interaction (e.g. Galley, 2003). Amphibolite xenoliths are believed to represent contact metamorphosed basalt fragments based on petrographic similarities (Drexler et al., 2004). Drexler et al. (2004) have shown that coarse-grained gabbro/diorite fragments likely represent xenoliths of the earliest phases of the pluton. Studies of ancient VMS deposits has documented that the deposits commonly occur in depressions on the paleo-seafloor (3rd-order basins) while modern deposits on the seafloor are found on high-standing structures, such as ridges. These differences are probably more apparent than real, in that the modern deposits are generally confined to the axial graben, or depression, of what are otherwise are high-standing structures. In addition, both ancient and modern deposits occur in areas of anomalously high heat flow, generally linked to synvolcanic intrusions beneath the hydrothermal systems. The recent mapping in the Purvis Road area has shown the presence of all of the attributes of typical VMS-forming hydrothermal systems. These attributes include a synvolcanic intrusive heat source (the Purvis pluton), a paleotopographic high-standing structure, VMS-style alteration mineral assemblages, and the presence of Cu and Zn-rich massive sulfide (recent logging in this area has exposed numerous angular boulders of massive sulfide in the basal till). NEXT: Return to Highway 169, turn left (west) and travel approximately 4.4 miles to the junction with the Murray Forest management road. Turn left (south) and travel approximately 0.2 miles to south verging bend in the road. Get out and walk along the old logging road to the west. 190 �Stop 7-3. Shallow water volcanic rocks of the Fivemile Lake sequence. Andesite, Rhyolite, and Scoria Location: T.62N., R.15W., sec.25 SW, NE Soudan 7.5-minute quadrangle UTM: 560,980E/5,297,025N DESCRIPTION: At this stop, we’ll be examining a series of outcrops that display the varied geology of the shallow-water Fivemile Lake sequence (Peterson and Patelke, 2003). The short traverse will include outcrops of highly vesicular/amygdaloidal basaltic andesite, rhyolite lava flows and breccias, and a unit of andesitic scoria. Rocks of similar texture occur throughout the central core of the Lower Ely greenstone. NEXT: Return to the bus and continue south along the Murray Forest Management road for approximately 0.6 miles to the junction of the old DM&IR rail line. Get out and walk west along the rail line for 50 meters to the outcrops on the north and south side of the rail line. Stop 7-4. Gold prospect along the Murray Shear Zone Location: T.62N., R.14W., sec. 30, SW, SW Soudan 7.5-minute quadrangle UTM: 561,490E/5,296,205N DESCRIPTION: This is one of the few gold prospects in the lower member of the Ely Greenstone. South of the old rail line, intense shearing associated with the north edge of the Murray shear zone culminated with the formation of chlorite-ankerite-sericite schists. In the mid-1980s, Newmont Exploration discovered lode gold mineralization along the northern margin of the Murray shear zone. Gold mineralization in this area, named the Murray prospect by Newmont, is associated with quartz-carbonate-pyrite-galena-tetrahedrite veins in strongly sheared and carbonatized rocks. Newmont reported values up to 12.5 g/t gold during the course of their exploration. 191 �An estimate of the amount of displacement within the panel of rocks bounded by the Murray shear zone is given in Table 7-2 (Peterson and Patelke, 2003). These values were calculated geometrically by using the average plunge of measured lineations (71°) and two measured lines of possible correlative stratigraphy offset by the bounding shear zones. The calculated total displacement values (net slip) are quite large (up to 13.8 km, or 43,000 feet of net slip), but the displaced rocks would still fall within the range of depth generally associated with greenschist facies metamorphism. Table 7-2. Calculated displacement along the Murray shear zone Calculated Displacement (Kilometers) Strike Slip Dip Slip Net Slip 71° 4.5 13.1 13.8 71° 3.0 8.7 9.2 Lineation Plunge . NEXT: Return to Highway 169 via the Murray Forest Management Road. Turn right on Highway 169 and travel approximately 0.7 miles to the junction of a logging/gravel road on the south side of the highway. Walk up the road to the southwest to the very large outcrop on the top of the hill. Stop 7-5. Central Basalt Sequence Sheet Flows, Pillow Lavas, and Perlitic Hyaloclastite Location: T.62N., R.14W., sec 19, SE, SW, Soudan 7.5-minute quadrangle UTM: 562,000E/5,297,800N DESCRIPTION: The Central Basalt sequence (Peterson and Patelke, 2003) comprises a steeply north-dipping (75°vertical), north-facing sequence of sparsely amygdaloidal pillowed and massive lava flows of basalt andesite to basalt composition that are believed to be correlative with the tholeiitic Armstrong Lake volcanic sequence mapped in the Eagles Nest quadrangle (Jirsa et al., 2001). Hudak et al. (2007) and Jansen et al. (in press) have shown that the lowermost sections of the Central Basalt Sequence is composed of submarine basaltic andesite to basalt lava flows that have rare earth element lithogeochemical patterns similar to mafic rocks in oceanic volcanic arcs. However, locally, submarine basalt lava flows that occur within 50-200m stratigraphically below the contact between the Central Basalt Sequence and the overlying Soudan Member of the Ely Greenstone Formation illustrate MORB-like lithogeochemical patterns, and have suggested that this may document a change from an oceanic arc to back-arc environment immediately prior to the deposition of the Soudan Member. Relative to massive and pillowed basalt and andesite flows in the Fivemile Lake sequence, Central Basalt sequence lavas flows are notably less amygdaloidal, and lack multiple pillow rind structures. In addition, the Central 192 �Basalt sequence lacks the thick sequences of scoriaceous basalt-andesite lapilli tuffs that are commonly interstratified with lava flows in the Fivemile Lake sequence. These characteristics of the Central Basalt sequence indicate eruption and deposition in a deeper submarine environment than the stratigraphically older Fivemile Lake sequence, and suggest overall increasing water depth during the temporal development of the Lower Ely. Deepening of the water column could be accommodated by extensional tectonics and normal faulting associated with the development of the proposed back-arc environment. The outcrop comprises two east-southeast striking massive basalt flows, ranging from at least five to nine meters in thickness, that are separated by a ten meter thick flow unit comprising pillows and pillow lobes (Fig.7-6). All three lava flows at this vicinity illustrate tholeiitic, MORB-like lithogeochemistries (Hudak et al., 2007). Figure 7-6. Detailed geological map of sheet flows, pillow lavas, in-situ hyaloclastite and associated “self-peperite”. Flow 1, at the southern part of the outcrop, is composed of a pale- to dark green, faintly feldspar-phyric (~10% 0.5-1 mm laths), sparsely amygdaloidal, basalt sheet flow that locally exhibits tortoise-shell jointing formed in response to contraction during cooling. The uppermost 10-40 cm of the coherent part 193 �of Flow 1 is generally silicified and epidotized. Petrographic observations indicate that this section of the flow also contains up to 70% <0.1 cm round spherulites. An irregular contact occurs between the coherent basalt flow and an overlying one- to two meter thick unit of dark green, exceptionally well-preserved perlitic in-situ hyaloclastite and associate self-peperite (c.f. Batiza and White, 2000). The hyaloclastite formed from non-explosive fracturing of the basalt glass developed on the flow top due to quenching by water, whereas the perlite formed following deposition by hydration of volcanic glass. An irregular contact occurs between the hyaloclastite and Flow 2, which is composed of north-facing mattress- to bunshaped pillow lavas and pillow lobes with numerous “neck and knob” structures. Individual pillows have well developed perlitic hyaloclastite margins that range from 1-4 cm in width. Pillow buds indicate propagation from east to west, suggesting the volcanic vent was located east of this location. The coherent pillows and lobes are overlain by up to 2.5 meters of hyaloclastite breccia that contains 20-40% subround to subangular pale gray green basalt lapilli in a jigsaw puzzle-fit dark green perlitic hyaloclastite matrix. The upper contact of Flow 2 and the overlying basalt sheet flow (Flow 3) is irregular, and is marked by thin (1-8 cm thick), sheet-like basalt fragments that are up to 1.6 meters in length. These fragments locally appear to be isoclinally folded about an east-west-trending fold hinge. Although the genesis of this structure is currently not well understood, it may be due to syneruptive deformation of either thin slabs of hot, basal flow margin crust from the overlying flow, or thin injections of basalt magma into the hyaloclastite from either the underlying pillows or the overlying sheet flow. Flow 3 comprises an at least ten-meter thick pale green-gray, slightly feldspar-phyric, sparsely amygdaloidal sheet flow. Steep, NNEtrending west dipping D3 joints are well developed in this unit, as are lens-shaped psuedo-pillows that are up to 50 cm in diameter. NEXT: Return to Highway 169, turn right and travel approximately 3.1 miles to the junction of the Mud Creek road (County 38). Turn left (north) on 38 and travel approximately 1.5 miles to a series of low-lying outcrops on the east side of the road. Stop 7-6. Fragmental rocks of the Gafvert Lake sequence. Location: T62N., R14W., sec. 10, NE,SW Eagles Nest 7.5-minute quadrangle UTM: 567,000E/5,301,490N DESCRIPTION: The informally named Gafvert Lake sequence (Peterson and Jirsa, 1999) is interpreted to represent an Archean stratovolcano of andesitic to dacitic composition that stratigraphically overlies rocks of the Ely Greenstone. The complex includes lava flows, fragmental rocks (tuff, lapilli tuff, tuff breccia, debris flow deposits) and porphyritic intrusions. The widespread nature of dacitic fragmental rocks of Gafvert affinity in the Vermilion district indicates that repeated episodes of explosive volcanism (Crater Lake type caldera formation) occurred in the area. Capping the central portion of the Gafvert Lake sequence are a number of thick massive pyrite horizons, that have metal signatures associated VMS, epithermal, 194 �and biologic affinity (Peterson, 2001). We will examine a series of outcrops of fragmental dacitic deposits located on the east side of the Mud Creek road. NEXT: Continue northwest on 38 for approximately 2.1 miles to the boat landing parking area along Mud Creek. Walk approximately 100 meters north along road to the outcrop along the east side of the roadway. STOP 7-7. Mud Creek shear zone Location: T.62N., R.14W., sec. 5, SE, SE; Outcrop just northwest of Mud Creek near road. Chad Lake 7.5-minute quadrangle. UTM: 564,230E/5,302,800N DESCRIPTION: The regional scale Mud Creek shear zone occupies the east-northeast trending valley of Mud Creek, which is clearly visible at this location. This shear zone separates rocks of the Newton Belt (here the Bass Lake sequence) to the north and rocks of the Soudan Belt (Gafvert Lake sequence and the Upper Ely Greenstone Formation) to the south. Development of this shear zone is a product of largely dextral transpressive deformation that has been partitioned into discrete zones, presumably late in D2 deformation. It is generally believed that gold-bearing mineralization was introduced during these later deformation events, and the Mud Creek shear zone and environs continue to attract considerable attention as a gold target. The Mud Creek shear zone is analogous with major faults (Destor-Porcupine fault) and “breaks” (Cadillac-Larder Lake break) of major lode-gold mining districts in Canada. Historic gold assays taken from rocks of the shear zone itself are essentially devoid of gold, as is the case for most major structures within Archean lode-gold mining camps. This series of outcrops are located within the northern margin of the internal highly strained zone of the shear, and include outcrops of: (1) ankeritesericite-quartz-green mica-pyrite schist with quartz and tourmaline knots, and (2) highly folded and compositionally banded phyllites with quartz veins. The protolith for these rocks are unknown, because of the intense deformation, but could be any of several rock types in the region, including quartzofeldspathic porphyry, metavolcanic rock, or graywacke. NEXT: Drive northwest along County 38 approximately 1.1 miles to widened portion of the road. 195 �Stop 7-8. Sheared quartz-feldspar porphyry, basal till, and detailed mapping interpretations Location: T62N., R14W., sec. 5, SW, NW. Chad Lake 7.5-minute quadrangle UTM: 563,190E/5,303,615N. DESCRIPTION: East of Lake Vermilion, the geology of the Bass Lake sequence is dominated by six basic rock types, which include: (1) Tholeiitic pillowed basalt flows interpreted to have formed in a deep-water setting based on volcanic textures; (2) Gabbro sills interpreted as synvolcanic in age due to their stratigraphic continuity and similar deformation as the enclosing pillowed basalts; (3) Felsic porphyries (feldspar porphyry and quartz-feldspar porphyry) interpreted to have intruded during late stages of D2 deformation based on field relationships and geochronology (quartz-feldspar porphyry from the Pac Man Pond prospect returned a 207Pb/206Pb age of 2683.0 +/- 1.4 Ma (Peterson et al., 2001)); (4) Algoma-type ironformation; (5) Thinly-bedded argillite and siltstone; and (6) Sheared rocks, which are dominated by chlorite-rich schist, phyllite, and phyllonite. In addition, localized areas of fragmental felsic volcanic rocks occur stratigraphically below distinct iron-formation horizons. In the last twenty years, numerous gold prospects have been discovered in the eastern portion of the sequence. These prospects generally fall into one of three categories; (1) auriferous quartz-carbonatepyrite veins and sulfidized zones in iron-formation; (2) auriferous quartz-sericite-ankerite-pyrite schists; and (3) felsic intrusive-hosted auriferous quartz veins and stockworks. All of the prospects are found within areas of moderate to strong iron-carbonate alteration, with the best mineralization commonly found within sericitic alteration zones. Numerous equigranular and porphyritic felsic intrusions occur within the areas of alteration and gold mineralization, and are a good guide for locating mineralized structures. The gold mineralization is generally related to deformation in subsidiary structures associated with movement along the D2 Mud Creek shear zone. Widening of the roadbed of County 38 in 2003 exposed a number of new outcrops and cuts into the basal till in this area. Detailed geologic mapping of gold prospects north of the Mud Creek shear zone by Peterson and Patelke (2004a) included mapping these new exposures of the Bass Lake sequence. For this stop, we will traverse along County 38 and look at these new exposures. NEXT: Continue northwest along County 38 approximately 0.55 miles to a small yellowish outcrop on the east side of the road. 196 �Stop 7-9. The Kerr McGee gold prospect Location: T63N., R14W., sec. 31, SE,SE Chad Lake 7.5-minute quadrangle UTM: 562,480E/5,304,610N DESCRIPTION: The Kerr McGee gold prospect is hosted within an extensive zone of highly strained rocks, interpreted to be a subsidiary structure associated with the Mud Creek shear zone. Moderate to high-grade gold mineralization at the Kerr McGee prospect occurs within multiple thin (0.2 – 2.0 meter) zones of quartzsericite-ankerite-pyrite ± green mica ± tourmaline schist hosted by an extensive zone of essentially goldbarren chlorite-rich schist. Thin and probably boudined iron-formation horizons occur locally in the chlorite-rich schist, and locally are strongly mineralized in this area. Mineralized zones locally contain extensive foliation and shear parallel quartz, ankerite, and/or quartz-ankerite veins, and may widen in zones of silicification. The style of gold mineralization exposed in the Kerr McGee prospect is similar to both the Clear Cut (~½ mile west) and Railroad Zone (1½ miles east) prospects. In fact, the sericitic zone that hosts the mineralization may have continuity to both of these other prospects. Three-dimensional visualization (Fig. 7-7) of the detailed lithological and structural mapping by Peterson and Patelke, (2004a) within the Kerr McGee prospect area reveals important information that can be used to design drilling plans that significantly increase the chance of intersecting gold mineralization exposed in outcrop at the surface. For example, drill hole RC-3, which is located 100 meters east of the main gold showing on the eastern side of this knob, was drilled due north (at a dip of 45º) and targeted to intersect the mineralization exposed in outcrop at the Kerr McGee showing. Chevron Resources drilled this hole in 1987, at the western boundary of their lease property (the Kerr McGee prospect was then held by Kerr McGee). Detailed structural mapping in these outcrops reveals that the rocks within the mineralized zone have moderate to strong elongation and intersection (foliation and shear planes) lineations trending 60º and dipping northeast at 72º. The best interpretation of the down-dip orientation of the mineralized zone is this lineation trend and plunge, and drill hole RC-3 never intersected the mineralized zone. 197 �Figure 7-7. Three-dimensional view of the relationship between structural boundaries, the mineralized zone exposed on the surface at the Kerr McGee prospect, and drill hole RC-3. Upward extension to the surface of the two anomalous zones (> 1,000 ppb gold) intersected in hole RC-3 would place these zones in the black spruce and cedar swamp located south-southeast of the prospect. NEXT: Return to Highway 169 via County 38. Turn right (west) and travel approximately 7.3 miles to the junction of Jasper road in the town of Soudan. Turn right on Jasper road and follow road to the Tjunction (~0.5 miles). Turn right, go up the hill, disembark at mine buildings and walk about 150 feet north and up-hill to outcrop on the right. STOP 7-10. Archean Soudan iron-formation member of Ely Greenstone No hammering please! Location: T.62N., R.15W., sec. 27, NE, NE; Soudan Mine State Park. Soudan 7.5-minute quadrangle UTM: 557,120E/5,296,660N DESCRIPTION: This classic exposure of the Soudan iron-formation member of the Ely Greenstone lies on the north limb of the Tower-Soudan anticline, and at the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone. The outcrop displays two generations of tight folding in 198 �delicate laminae of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silvercolored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend east, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of style and orientation; implying they formed by layer-parallel, soft-sediment slumping (Fig. 7-8). Lundy’s mapping of this outcrop is an interesting demonstration of unraveling details at a single outcrop that led to recognition that D1 deformation was not systematic here, but likely soft sediment. Furthermore, it is a microcosm of regional-scale deformation Figure 7-8. Outcrop map showing bedding trajectories and several generations of folds and faults (from Lundy, 1985). F1 folds are nonsystematic and include nappe and sheath fold geometries. It is interesting to observe the rhythmic microlaminae (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling, tidal, climatic, annual, or some other repetitive influence in the depositional environment. What is known about units of iron-formation in the Ely Greenstone, of which there are many, is that deposition occurred during periods of relative volcanic and tectonic quiescence by the slow subaqueous “rain” of chemical precipitates. The deep excavations in this area are the early workings of the Soudan iron mine, the first in Minnesota. The mine produced about 16 mt of high-grade hematite ore (60-63 percent iron content) from 1884 until 1962, when the land was deeded to the State of Minnesota and converted to a park. Most of the production came from underground workings that began here in 1900, and which now can be visited on guided tours. The mine also houses an underground physics research facility at 2340 feet below the surface. A massive expansion of that facility is under consideration to create a national underground laboratory at considerably greater depths (Peterson and Patelke, 2003). 199 �NEXT: Return to Highway 169 and turn right. Follow 169 through the town of Tower to the large outcrops immediately west of tons (approximately 3.1 miles). STOP 7-11 Archean fragmental volcanic rocks Location: T.62N., R.15W., sec. 32, SW, SW; Highway 169 road cut, west edge village of Tower. Tower 7.5-minute quadrangle. UTM: 553,380E/5,294,430N DESCRIPTION: This outcrop consists of fragmental, variably reworked volcanic conglomerate and tuffaceous rocks of the Gafvert Lake sequence of the Lake Vermilion Formation. The rock is composed of about 85-95 percent dacitic detritus, 3-5 percent gray clasts of graywacke, slate, and basaltic andesite, and a small percentage of magnetic and sulfidic fragments. Fragments range in size from a few millimeters to 20 cm. The generally poorly developed sorting and bedding, together with varied clast composition, implies a debrisflow origin. Compare these rocks with those of Stop 7-6. NEXT: Continue west on Highway 169 approximately 1.5 miles to road cut. STOP 7-12. Archean dacitic tuff/ Paleoproterozoic or Mesoproterozoic diabase dike Location: T.61N., R.16W., sec. 1, SW, NE; Highway 169 road cut. Tower 7.5-minute quadrangle. UTM: 551,160E/5,293,960N DESCRIPTION: These road cuts expose outcrops of white, dacitic tuffaceous sedimentary rock, a component of the Lake Vermilion Formation. Regionally, the formation consists of all compositional gradations between what 200 �appears to be first-cycle tuff, tuffaceous greywacke, and mixed-source greywacke, interbedded on all scales. In a general way, the tuffaceous component increases in proportion to the east toward the TowerSoudan anticline. The presumed source of the dacitic volcanic detritus exposed in this area is stratovolcanos of Gafvert Lake affinity, which overlie the composite volcanic shield complex of the Ely Greenstone. Ring plains and irregular basins composed of detritus shed from the high-standing volcanic complex are now represented by the Lake Vermilion Formation. The northeast-trending, steeply dipping, seven-meter wide diabase dike that cuts tuffaceous rocks has been the source of considerable debate. It’s petrographic (olivine-bearing) and geochemical (silica undersaturated) composition is similar to Mesoproterozoic dikes (Schmitz, 1994); yet it lies nearly along strike with, though east of, dikes of the Paleoproterozoic Kenora-Kabetogama dike swarm. NEXT: Continue west on Highway 169 approximately 0.7 mile to Stop 7-13. STOP 7-13. Multiply folded Archean greywacke Location: T.61N., R.16W., sec. 2, NE, NE; south side of Highway 169 just east of CR 526. Tower 7.5-minute quadrangle. UTM: 550,050E/5,294,000N DESCRIPTION: This outcrop at the road and several smaller ones in the bush nearby show the superposition of two generations of folds in thin-bedded, well-graded graywacke of the Lake Vermilion Formation. The second-generation folds (F2) are associated with a regional axial plane cleavage in which sedimentary clasts are flattened. The earlier F1 folds have no associated cleavage and tend to be erratic in form, trend, and distribution. Folds display “eye” and “mushroom” shapes that locally are interpreted to be sheath folds (Hudleston and others, 1987). These characteristics are consistent with deformation of poorly lithified sediment. The superposition of deformation events is manifest in the transection of F1 folds by cleavage related to D2. In this area and to the west, one can find anticlinal synclines and synclinal anticlines, indicating stratigraphic inversion prior to D2 folding. NEXT: Continue approximately 1.7 miles to the west to the junction of 77. Turn right on 77 and follow for approximately 0.6 miles to the outcrop on the left side of the road on the north side of the Pike River Dam. 201 �STOP 7-14. Archean greywacke at Pike River Dam Location: T.61N., R.16W., sec. 3, NW, SW; west side of County Road 77, on N side of river. [Note that Fortune Bay Casino—the overnight hotel—lies to the north off of CR 77]. Tower 7.5-minute quadrangle. UTM: 547,300E/5,293,340N DESCRIPTION One of the truly classic outcrops of greywacke of the Lake Vermilion Formation is beautifully exposed at this stop. Prior to about the 1950s, no depositional mechanism could satisfactorily explain the coincidence in graywacke of; 1) coarse sand derived from a source many kilometers distant and having an altered clayey matrix; 2) interbedded black slate; and 3) the lack of evidence for reworking in shallow water (indicative of deposition below wave base). This was changed when the concept of turbidity currents was introduced to the geological profession by Kuenen and Migliorini (1950). Despite widespread publication on turbidites in more modern geologic settings through the 1950s and 1960s, the facies model was not refined and applied to Archean and Proterozoic strata in the Lake Superior region until somewhat later (Morey, 1965; Ojakangas, 1966). The geology of field trip stops in the Virginia Horn area is presented in Figure 7-9. NEXT: Return to Highway 169 and turn west. Travel south on Highway 169 approximately 28 miles to the junction with Highway 53; follow 53 south approximately 0.7 miles to the Laurentian Divide wayside rest Stop. 202 �Figure 7-9. Generalized geologic map of the Virginia horn area (modified from Jirsa and others, 1998) showing details of field trip stops 9-15 to 9-19. 203 �STOP 7-15. Archean Giants Range batholith at “Confusion Hill,” Laurentian Divide Location: T.59N., R.17W., sec.19 SE, SE; wayside off Highway 53. Virginia 7.5-minute quadrangle. UTM: 534,337E/5,269,458N DESCRIPTION: Exposed near this wayside and in road cuts on both sides of the highway is an array of variably layered intrusions having both tonalitic (white) and dioritic (black) compositions. A cursory look shows intrusive relationships that conclusively demonstrate that diorite was emplaced into tonalite at one locality, and at another, tonalite was emplaced into diorite. In detail, all compositions intermediate between the two end members are also present locally. Although the dioritic component is abundant here, the bulk of the mapped unit is tonalitic. Emplacement of this unit, now known as the Lookout Mountain tonalite, probably involved some degree of magma mingling. Dikes of tonalite that cut the adjacent high-grade supracrustal rocks of the Minntac sequence contain metamorphic fabrics, yet little evidence of metamorphic origin can be seen in the interior of the body, implying it is syntectonic with respect to D2 deformation. U-Pb zircon dates (Boerboom and Zartman, 1993) of two components of the batholith exposed to the north bracket the age of D2 deformation between about 2674 and 2682 Ma. Exposures at Confusion Hill are a small part of the Giants Range batholith, which forms the core bedrock of the Laurentian (drainage) divide. The batholith is a 40-mile wide belt of intrusions that can be traced on geophysical maps and outcrop east to the Mesoproterozoic Duluth Complex, and west beyond the western border of Minnesota. It separates Archean supracrustal sequences in the Virginia horn from those of the Tower-Soudan area—making stratigraphic correlation between the two districts speculative. NEXT: Follow Highway 53 south through the town of Virginia. Take the exit for Highway 135 (east) approximately 0.5 miles to Bourgin Road. Turn right (south) on Bourgin Road and continue about 0.4 mile to large cut on left (east) side of road. 204 �STOP 7-16. Archean graywacke and slate, intruded by quartzofeldspathic porphyry. Location: T.58N., R.17W., sec.21 SW, SW; road cuts on east side of the Bourgin Road. Eveleth 7.5-minute quadrangle. UTM: 536,311E/5,260,659N DESCRIPTION: Outcrops along this side of the road expose quartzofeldspathic porphyry (QFP) intruded into variably deformed graywacke, siltstone, and slate of the Mud Lake sequence. The sedimentary rocks here are moderately deformed, but much of that deformation is inferred to predate the main cleavage-forming event D2, and some may be soft-sediment in origin. The QFP is large and continuous to the east, but at this locality it appears to be segmented into a zone of multiple dikes. Both graywacke and QFP are intensely altered to some combination of iron-carbonate minerals (ankerite, ferroan dolomite) and sericite. Regionally, this style of alteration is commonly, though not always associated with QFP intrusions— presumably because the QFP remained more structurally rigid than the enclosing sedimentary rocks during the shear-related deformation event that accompanied alteration late in D2. Most gold mineralization in the area is closely allied to this alteration, yet this outcrop is surprisingly barren. One of the earliest gold discoveries in Minnesota was made by J.W. Gruner (in Grout, 1937) in a railroad cut not far from stop 7-16. The cut exposes graywacke intruded by quartzofeldspathic porphyry, having visible gold associated with small quartz veins. Despite several episodes of mineral exploration in this area (most notably be Newmont Exploration in the 1980s) no economic gold deposits have been discovered. NEXT: Follow Bourgin road to the south and west to a frontage road on the east side of Hwy 53. Turn north (right) on the frontage road and travel about 0.2 miles to first road to right, turn up-hill and continue to #7 Mesabi Lane. 205 �STOP 7-17. Archean conglomerate Private driveway! Location: T.58N., R.17W., sec.20 SW, SE, No. 7 Mesabi Lane; village of Midway. Eveleth 7.5-minute quadrangle. UTM: 535,713E/5,259,459N DESCRIPTION: Archean conglomerate and lithic sandstone that form the driveway here are part of the northeast-trending Midway sequence, containing these strata types locally interbedded with subaerially deposited, calcalkalic (trachyandesitic) volcanic rocks. The sequence is inferred to have formed after earliest deformation (D1) of the enclosing graywacke and basaltic rocks of the Mud Lake sequence, but before the cleavage-forming D2 deformation that affected both sequences. The conglomerate contains clasts of basalt, graywacke, porphyritic trachyandesite, and quartzofeldspathic porphyry (QFP). This provenance indicates that the older Archean rocks of the Mud Lake sequence were intruded by QFP, deformed, and uplifted, to provide detritus to what was probably a successor or “pull-apart” basin developed along a major structure now occupied by the Pike River fault zone. Midway sequence conglomerate has previously been interpreted as a basal sediment (Sutton, 1963), and as a proximal turbidite fan deposit (Levy, 1991), depositionally transitional with graywacke and slate of the Mud Lake sequence. Subsequent work (Jirsa, 2000) indicates that the conglomerate is part of a Timiskaming-type clastic and volcanic sequence that unconformably overlies the older volcanic strata. Deposition of the Midway sequence required uplift, subaerial erosion, continental volcanism, and deposition in isolated basins along a major structural break. NEXT: Return to Highway 135. Turn right and follow 135 approximately 2.4 miles to a residential street on the northwest side of the town of Gilbert. Turn right and go 5 blocks and park by the Gilbert Junior High School. 206 �STOP 7-18. Archean pillowed and massive greenstone Location: T.58N., R.17W., sec.23 NW, SE, SW; north edge of athletic fields, Gilbert Junior High School. Gilbert 7.5-minute quadrangle DESCRIPTION: Outcrop of pillowed and massive basalt is part of the Archean Mud Lake sequence, metamorphosed to low greenschist-grade. Pillow shapes indicate stratigraphic facing is to the northwest, which places this outcrop on the south side of a major D1 structure known as the Mud Lake syncline. Note also the presence locally of fractures filled with reddish jasper, presumably deposited in depressions on the rock surface by overstepping of Paleoproterozoic seas during deposition of the Biwabik Iron Formation. NEXT: Follow residential roads to Highway 37 in the center of Gilbert. approximately 3.3 miles to the on/off ramps onto Highway 53. Turn right and travel on 37 STOP 7-19. Paleoproterozoic Pokegama Quartzite (A) and Biwabik Iron Formation (B) Location: T.58N., R.17W., sec.32 SE, SE, and adjacent, junction of Highways 37 and 53. Eveleth 7.5-minute quadrangle. UTM: scattered outcrops extend from 535,956E/5,256,913N on the north (stop 9-19A), to 536,263E/5,256,200N on the south (stop 9-19B). DESCRIPTION 7-19A: Unconformably overlying the Neoarchean rocks of the Virginia Horn area is the Animikie Group sediments of Paleoproterozoic age. Coarse grain size and massive beds as thick as 1.5 m characterize this outcrop of the sandy, upper member of the Pokegama Quartzite. Thin beds of shale and siltstone separate the massive beds. Ojakangas (1993) interpreted the deposition of this facies as within high-energy, lower tidal or subtidal environment. 207 �DESCRIPTION 7-19B: This exposure of gently southeast-dipping strata is part of the Lower Cherty member of the Biwabik Iron Formation. It overlies and is generally in transition with the Pokegama Quartzite at stop 5-5A. Notice that both formations have sandy textures and cross-bedding, implying a moderately high-energy depositional environment. The most significant difference between these two units is the abrupt change in sediment source from the extrabasinal quartz grains in the Pokegama, to recycled, chemically precipitated chert in the Biwabik. Measurements of cross-bedding in the iron-formation are bimodal, implying deposition in a tidally influenced marine environment (Ojakangas, 1993). OPTIONAL FIELD TRIP LOCATIONS Highlighting Neoarchean bedrock in the Vermilion District: 3 driving/hiking traverses within striking distance of Ely Figure 7-10. Geologic map of the Ely to Moose Lake area (clipped and modified from Jirsa and Miller, 2004) KAWISHIWI FALLS TRAVERSE (approximate round trip from Ely = 1-2 hours) Metabasalt and iron-formation of the Neoarchean Ely Greenstone. Stop KF-1—Pillowed metabasalt; UTM: 588,370E/5,307,100N (NAD 83) Directions: Drive east of Ely (set odometer at junction hwy 1 with hwy169 or County road 18Fernberg Trail) and drive east for about 1.3 miles. Outcrop on north (left) side of road is classic example of pillowed metabasalt; note radiating chlorite amygdules and concentric jointing in pillows. Stratigraphic younging is to the north. Stop KF-2—Iron-formation lens; UTM: 592,785E/5,309,288N (NAD 83) Directions: Continue east on hwy 169 (also County road 18) to crossing of Kawishiwi River at about 4.5 miles from starting mileage; park near bridge and walk farther east to first outcrop on south (right) side of road. Outcrop consists of gray, magnetite-hematite-chert iron-formation with minor pyrite. Stop KF-3—Kawishiwi Falls, metabasalt; UTM: 592,440E/5,309,755N (NAD 83) Directions: Cross bridge over Kawishiwi River; continue 0.3 mile farther east to driveway north of trail that leads to the falls parking lot. Follow walking trail northwest of parking lot for about 0.35 miles to falls. ECHO TRAIL TRAVERSE (approximate round trip from Ely = 1-2 hours) Neoarchean Newton Lake Formation and the Wawa/Quetico subprovince boundary. 208 �This traverse visits newly created roadside outcrops along the Echo Trail north of Ely. The Newton Lake Formation is a package of presumably thrust-stacked tholeiitic to komatiitic lava flows, mafic to ultramafic sills, and thin felsic tuff. It differs from the Ely Greenstone in the abundance of ultramafic rocks and general lack of iron-formation. Judging from geochronologic work in the adjacent Shebandowan Greenstone Belt in Ontario (Corfu and Stott, 1998), the two formations are approximately coeval at ca. 2720 Ma. Metasedimentary rocks of the Quetico subprovince to the north represent graywacke deposited as foreland-basin fill during D1 orogenesis and eventual accretion of the Wawa subprovince to the evolving craton at ca. 2699-2696 Ma. Directions to start of Echo Trail: Drive east of Ely along hwy 169 to County Road 88 about 0.9 mile east of junction of highways 1 and 169. Turn north (left) on 88 and follow it around the east end of Shagawa Lake for 2.2 miles to Echo Trail (County Road 116) turn-off on the right (north). Set odometer here and drive northward on Echo Trail to stops. Stop ET-1—Spherulitic pillowed basalt of the Newton Lake Formation; UTM: 585,745E/5,309,470N; 0.5 miles north of CR #88 on west (left) side of Echo Trail. Steeply dipping pillowed basalt, with stratigraphic younging to the south. Metamorphic grade is low greenschist. Note local presence of large vacuoles in pillows showing “drain-back” features. Stop ET-2—Metadiabase and felsic tuff of Newton Lake Formation UTM: 585,748E/5,310,655N; 1.25 miles north of 88 on west (left) side of Echo Trail. Roadcut exposes about 100 foot-thick unit of cherty-looking to granular felsic tuff displaying delicate pristine bedding features. This unit may be a block “floating” in metadiabase. Small outcrop on east side of road shows metadiabase cut by coarse grained hornblende-pyroxenebiotite lamprophyre containing inclusions of metadiabase. Stop ET-3—View of Burntside fault (a drive-by). UTM: 583,254E/5,311,035N; 3.25 miles north of 88 at junction with Somero Road (Tnsp #4651). Prominent NE-striking linear depression separates steep rock walls of low greenschist grade Newton Lake Formation of the Wawa subprovince on the south, from higher metamorphic grade rocks of the Quetico subprovince on the north. Stop ET-4—Quetico subprovince metasedimentary and intrusive rocks. UTM: 581,590E/5,312,295N; 4.0 miles north of 88 on east (right) side of Echo Trail about 0.1 mile north of junction with Passi Road (CR #803). Note that relict graded bedding and other sedimentary features are preserved locally, despite amphibolite grade metamorphism and local migmatization. MOOSE LAKE TRAVERSE (approximate round trip from Ely 2-4 hours) Neoarchean Knife Lake Group Directions: Drive about 16 miles east of Ely on hwy 169-Fernberg Road to left turn at Moose Lake Road (CR #183); go 2.5 miles northeast along Moose Lake Road to parking area on the right for the SecretBlackstone Lakes trail area. Hike eastward along a series of trails that cross outcrops, and veer off the trails onto some of the larger exposures. The Knife Lake Group contains the youngest supracrustal strata in the Vermilion district, exposed as thin layers on and tectonic wedges within older volcanic units (Ely and Newton Lake). Two major sequences are recognized; an older package of alkalic hornblende-phyric volcanic, volcaniclastic, and derived sedimentary rocks; and a younger sequence of polymictic conglomeratic strata known locally as the Ogishkemuncie conglomerate. It appears that parts of both sequences are exposed along the multitude of 209 �trails. Though both sequences are fragmental at this location; the Ogishkemuncie can be distinguished by the presence of clasts of red and black iron-formation, black and white chert, vein quartz, and quartzphyric granitoid derived from the Saganaga Tonalite exposed about 20 miles to the northeast. Lithology and sedimentary structures indicate alluvial fan, fluvial, and fluvial-lacustrine deposition; though some apparently deeper water turbiditic sediments occur locally. Moose Lake, just to the west, is one of a linear chain of lakes that parallels faults associated with the Shagawa Lake-Knife Lake trend. This fault system appears to have been long-lived, and may initially have controlled development of a successor basin into which at least some of the Knife Lake strata were deposited. These supracrustal rocks can be correlated on the basis of lithologic and relative temporal setting with the Shebandowan Assemblage in adjacent Ontario(Corfu and Stott, 1998). There, the alkalic volcanic rocks and Saganaga Tonalite were dated at about 2689 Ma, and the younger conglomerate units (like the Ogishkemuncie) have dates in the range of 2684-2682 Ma. The latter evolved during regional transpressive deformation (D2) at about 2685-2680 Ma. REFERENCES Batiza, R., and White, J. D. L., 2000, Submarine lavas and hyaloclastite: in Sigurdsson, H., Encyclopedia of Volcanoes: Academic Press, San Diego, CA, p. 361-381. 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Galley, A., 2002, Characteristics of composite subvolcanic intrusive complexes associated with Precambrian VMS districts: in Galley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Parquette, F., Paradis, S., Santaguida, F., and Taylor, B., 2002, CAMIRO Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration zones, and VMS deposits: Geologic Survey of Canada Open-File Report 94E07, p. 1-40. Galley, A., 2003, Composite synvolcanic intrusions associated with Precambrian VMS-related hydrothermal systems: Mineralium Deposita, v. 38, p. 443-473. Gibson, H. L., 1989, The Mine Sequence of the Central Noranda Volcanic Complex: Geology, Alteration, Massive Sulfide Deposits, and Volcanological Reconstruction: unpublished Ph. D. dissertation, Carleton University, Ottawa, Ontario, Canada, 800 p. Gibson, H. L., Morton, R. L., Hudak, G. J., 1999. Submarine volcanic processes, deposits and environments favorable for the location of volcanic-associated massive sulphide deposits: Reviews in Economic Geology, v 8, p. 13-51. Grout, F.F., 1937, Petrographic study of gold prospects of Minnesota: Economic Geology 37:56-68. Groves, D.I., Goldfarb, R.J., Knox-Robinson, C.M., Ojala, J., Gardoll, S., Yun, G.Y., and Holyland, P., 2000, Latekinematic timing of lode-gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia; Ore Geology Reviews, v. 17, Issues 1-2, Pages 1-38. Hamilton, W.B., 2003, An Alternative Earth: GSA Today, 13, 4-12. Hocker, S. M., Hudak, G. J., and Heine, J., 2003, Electron microprobe analysis of alteration mineralogy at the Archean Fivemile Lake volcanic-associated massive sulfide mineral prospect in the Vermilion district of northeastern Minnesota: Natural Resources Research Institute Report of Investigations NRRI/RI-2003/17, 49 p. Hodgson, C.J., 1993, Mesothermal lode-gold deposits: in Mineral Deposit Modeling. Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., Duke, J.M. eds., Geological Survey of Canada, Special Paper 40, p. 635–678. Hoffman, A. T., 2007. Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: unpublished M. S. thesis, University of Minnesota – Duluth, 295 p. Hoffman, P.F., 1990, On accretion of granite-greenstone terranes. In: Robert, F., Sheahan, P.A., and Green, S.B. (eds.), Nuna Conference on Greenstone Gold and Crustal Evolution. Proceedings of a workshop, Val d’Or, Quebec, May 24-27, 1990, Geological Association of Canada, Mineral Deposits Division, 32-45. 211 �Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion district, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hovis, S. T., 2001, Physical volcanology and hydrothermal alteration of the Archean volcanic rocks at the Eagles Nest volcanogenic massive sulfide prospect, northern Minnesota: unpublished M. S. thesis, University of Minnesota – Duluth, Duluth, Minnesota, 137 p. Hudak, G. J., and Morton, R. L., 1999, Mineral Potential Study, Minnesota Department of Natural Resources Project 326, Bedrock and Glacial Drift Mapping for VMS and Lode Gold Alteration in the Vermilion – Big Fork Greenstone Belt, Part A: Discussion of Lithology, Alteration, and Geochemistry at the Fivemile Lake, Eagles Nest, and Quartz Hill Prospects: Minnesota Department of Natural Resources Division of Minerals Project 326 Report, 136 p. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, 2002b, Geological mapping of the Needleboy Lake – Sixmile Lake area, northeastern Minnesota: a summary of volcanogenic massive sulfide potential: Natural Resources Research Institute Report of Investigation NRRI/RI-2002/14, 15 p. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002a, Comparative geology, stratigraphy, and lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion district, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Hocker-Finamore, S. M., and Heine, J., 2006, Field distribution, petrography, and lithogeochemistry of epidosites in the vicinities of Fivemile, Needleboy, and Sixmile Lakes, Vermilion District, NE Minnesota: 52nd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 57, Part 1 – Programs and Abstracts, p. 30-31. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: 53th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Programs and Abstracts, p. 42-43. Hudak, G.J., Heine, J., Jirsa, M.A., and Peterson, D.M., 2004, Volcanic stratigraphy, hydrothermal alteration, and VMS potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake area: 50th Annual Meeting, Institute on Lake Superior Geology, Field Trip Guidebook, Volume 50, p. 1-45. Hudleston, P. J., Schultz-Ela, D., and Southwick, D. L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1060-1068. Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district, north-eastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Hudleston, P.J., Bauer, R.L., Southwick, D.L., Schultz-Ela, D.D., and Bidwell, M.E., 1987, Structural geology of the boundary between Archean terranes of low-grade and high-grade rocks, northern Minnesota: in Balaban, N.H., ed., Field trip guidebook for selected areas in Precambrian geology of northeastern Minnesota, Geological Society of America north-central section meeting, St. Paul, Minnesota; Minnesota Geological Survey Guidebook Series n.17, p. 1-42. Jansen, A. C., Hudak, G. J., Heine, J. J., and Peterson, D. M., in press, Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall to the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 1 – Programs and Abstracts Jirsa, M. A., 2000. The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa subprovince, Minnesota: Canadian Journal of Earth Sciences, v. 37, p. 1-15. Jirsa, M. A., Boerboom, T. J., and Peterson, D. M., 2001, Bedrock geological map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map M-114, scale 1:24,000. Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake (U.S. portion) 30’ x 60’ quadrangles, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. 212 �Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geologic map of the Virginia Horn, Mesabi Iron Range, St. Louis County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-85, scale 1:48 000. Jirsa, M.A., Boerboom, T.J., Green, J.C., Miller, J.D., Morey, G.B., Ojakangas, R.W., and Peterson, D.M., 2004, Classic Outcrops of Northeastern Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Field Trip Guidebook, Volume 50, p. 129-169. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes during D2 transpression: Canadian Journal of earth Sciences, v. 29, p. 2146-2155. Kerswill, J.A., 1993, Models for Iron-formation-hosted Gold Deposits: in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., eds., Geological Association of Canada, Special Paper 40, p. 171-200. Kuenen, P.H., and Migliorini, C., 1950, Turbidity currents as a cause of graded bedding: Journal of Geology, 58:91127. Levy, E.R., 1991, The geology and sedimentology of the Archean metasedimentary rocks of the Virginia horn area, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Duluth, 199 p. Lundy, J.R., 1985, Clues to structural history in the minor folds of the Soudan Iron Formation, northeastern Minnesota: Unpublished M.S. thesis, University of Minnesota, Minneapolis, 144p. McMillan, R.H., 1996, Iron formation-hosted Au, in Lefebure, D.V. and Hoy, T, eds., Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, British Columbia Ministry of Employment and Investment, Open File 1996-13, p. 63-66. Morey, G.B., 1965, The sedimentology of the Precambrian Rove Formation in northeastern Minnesota (abs.): Institute on Lake Superior Geology, 11th Annual Meeting, St. Paul, Minnesota, Proceedings p. 25-26. Morris, R.C., 1985, Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes – a conceptual model, in Wolf, K.H., ed., Handbook of strata-bound and stratiform ore deposits: Amsterdam, Elsevier, v. 13, p. 73-235. Morton, R. L., and Franklin, J. M., 1987, Two-fold classification of Archean volcanic-associated massive sulphide deposits: Economic Geology, v.82, p. 1057-1063. Odette, J. D., Hudak, G. J., Suszek, T., and Hauck, S. A., 2001a, Preliminary evaluation of hydrothermal alteration mineral assemblages and their relationship to VMS-style mineralization in the Fivemile Lake area of the Archean Vermilion Greenstone Belt, NE Minnesota: Institute on Lake Superior Geology, 47th Annual Meeting, Proceedings Volume 47, Part 1-Program and Abstracts, p. 75-76. Odette, J. D., Hudak, G. J., Suszek, T., and Hauck, S. A., 2001b, Preliminary evaluation of hydrothermal alteration mineral assemblages and their relationship to VMS-style mineralization in the Fivemile Lake area of the Archean Vermilion Greenstone Belt, NE Minnesota: Geological Society of America Abstracts and Programs Volume 33, No. 6, p. A-420. Ohmoto, H., 2003, Nonredox transformations of magnetite-hematite in hydrothermal systems: Economic Geology, v. 98, p. 157-166. Ojakangas, R.W., 1966, Precambrian stratigraphy and structure of the Tower, Minnesota quadrangle (abs): Institute on Lake Superior Geology Proceedings, 12th Annual Meeting, Sault Ste. Marie, Michigan, Proceedings p. 17. Ojakangas, R.W., 1993, Pokegama Quartzite: in Institute on Lake Superior Geology Proceedings, 39th Annual Meeting, Eveleth Minnesota, v. 39, Part 2, p.19-21 and 46-48. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. 213 �Peterson, D. M., and Jirsa, M.A., 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, Northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p. Peterson, D. M., and Patelke, R. L., 2004b, Economic geology of gold occurrences in the Vermilion district, northeast of Soudan, Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Field Trip Guidebook, Volume 50, p. 200-226. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., 2001, Correlation of Archean assemblages across the U.S.- Canadian border: Phase I geochronology: 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 – Programs and Abstracts, p. 77-78. Peterson, D. M., Jirsa, M. A., and Hudak, G. J., 2005. Field Trip 9: Architecture of an Archean Greenstone Belt: Stratigraphy, Structure and Mineralization: in Robinson, L., ed., 2005, Field Trip Guidebook for Selected Geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21, p. 154-180. Peterson, D.M and Patelke, R.L., 2004a, Bedrock Geology and Lode Gold Prospect Data Map of the Mud Creek Road Area, Northern St. Louis County, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2004-01. Piercey, S. J., Murphy, D. C., Mortenson, J. K., and Creaser, R. A., 2004, Mid-Paleozoic initiation of the northern Cordilleran margin backarc basin: geologic, geochemical, and neodymium isotope evidence from the oldest mafic magmatic rocks in the Yukon-Tanana Terrane, Finlayson Lake District, southeast Yukon, Canada: Geological Society of America Bulletin, v. 116, no. 9/10, p. 1087-1106. Poulsen, K.H., and Robert, F., 1989, Shear zones and gold: Practical examples from the southern Canadian Shield: in Bursnall, J.T. ed., Mineralization and Shear Zones, Geological Association of Canada, Short Course Notes Volume 6, p. 239-266. Robert, F., Sheahan, P.A., and Green, S.B., eds., 1991, Greenstone gold and crustal evolution: NUNA Conference Volume, Geological Association of Canada, Mineral Deposits Division. Rye, D. M. and Rye, R. O., 1974, Homestake Gold Mine, South Dakota: I. Stable Isotope Studies; Economic Geology, v. 69, p. 293-317. S Santaguida, F., Gibson, H. L., Hannington, M. D., Watkinson, D. H., 2002b. Part II. Scaled metasomatic changes associated with epidote-quartz hydrothermal alteration in the Noranda Volcanic Complex, Quebec: in Galley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F., and Taylor, B., eds., Database for CAMIRO Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration, and VMS Deposits: Geological Survey of Canada Open File Report 4431, p. 181241. Santaguida, F., Gibson, H. L., Watkinson, D. H., and Hannington, M. D., 2002a, Part I: Semiconformable epidotequartz hydrothermal alteration in the central Noranda Complex, Canada: relationship to volcanic activity and VMS mineralization, in Galley, A., Bailes, A., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F., and Taylor, B., eds., Database for CAMIRO Project 94E07: Interrelationships between subvolcanic intrusions, large-scale alteration, and VMS Deposits: Geological Survey of Canada Open File Report 4431, p. 139-180. Schmitz, M.D., 1994, Origin and petrogenesis of the Early Proterozoic Kenora-Kabetogama mafic dike swarm: unpublished seniors thesis, Macalester College, 110 p. Seyfried, W. E. Jr., Ding, K., Berndt, M. E., and Chen, X., 1999, Experimental and theoretical controls on the composition of mid-ocean ridge hydrothermal fluids: Reviews in Economic Geology, v. 8, p. 181-200. Sims, P. K., and Southwick, D. L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U. S. Geological Survey, Miscellaneous Investigations Map I-1527, scale 1:48,000. 214 �Skirrow, R. G., and Franklin, J. M., 1994, Silicification and metal leaching in semiconformable alteration beneath the Chisel Lake massive sulfide deposit, Snow Lake, Manitoba: Economic Geology, v. 89, p. 31-50. Southwick, D. L., (compiler), 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-79, scale 1:100,000. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. Stott, G.M., 1997, The Superior Province, Canada, in de Wit, M.J., and Ashwal, L.D., eds., Greenstone belts: Oxford, Clarendon Press, p. 480-507. Sutton, T.C., 1963, Geology of the Virginia horn area: Minneapolis, University of Minnesota, M.S. thesis, 97 p. Thorpe, R.I and Franklin, J.M., 1984, Chemical-sediment-hosted Gold: in Canadian Mineral Deposit Types: A Geological Synopsis, Eckstrand, O.R., Editor, Economic Geology Report 36, Geological Survey of Canada, p. 29. Vielreicher, R.M., Groves, D.I., Ridley, J.R. and McNaughton, N.J., 1994, A Replacement Origin for the BIF-hosted Gold Deposit at Mt. Morgans, Yilgarn Block, W.A; Ore Geology Reviews, v. 9, p. 325-347. 215 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2009 Description An account of the resource Institute on Lake Superior Geology. Ely, Minnesota. May 5-10, 2009. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2009 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/0db73ce293c8674f66874392c1ce55d2.pdf 288aabd621a6c1c067a07e901099871b PDF Text Text 56TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY INTERNATIONAL FALLS, MINNESOTA MAY 19-22, 2010 PROCEEDINGS VOLUME 56 PART 1 – PROGRAMS AND ABSTRACTS ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 56TH ANNUAL MEETING MAY 19-22, 2010 INTERNATIONAL FALLS, MINNESOTA PETER HOLLINGS, PETER HINZ, MARK SMYK, MARK JIRSA, AND TERRY BOERBOOM Co-Chairs Proceedings Volume 56 Part 1 – Program and Abstracts Edited by Terrence J. Boerboom, Minnesota Geological Survey Cover Photos: Conglomerate in Seine Group (Field Trip 6) �56TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 56: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN) TRIP 2: GEOLOGY OF AN ARCHEAN SUCCESSION AT ATIKOKAN (CAN) TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT (CAN) TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX (US) TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN) Reference to material in Part 1 should follow the example below: Boerboom, T. J., 2010, Latest bedrock geologic map of Carlton County, east-central Minnesota and revisions to the southern edge of the Animikie basin [abstract]: Institute on Lake Superior Geology Proceedings, 56th Annual Meeting, International Falls, MN, v. 56, part 1, p. 1-2. Published by the 56th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 56 PART 1— PROGRAM AND ABSTRACTS Previous Institutes on Lake Superior Geology, 1955-2010 ...................................................... iv Sam Goldich and the Goldich Medal .............................................................................. vi Past Goldich Medalists and the 2010 Goldich Medal Recipient .................................... viii Goldich Medal Committee ........................................................................................... viii Citation for 2010 Goldich Medal Recipient .................................................................... ix ILSG Student Research Fund ........................................................................................ xii Student Paper Awards .................................................................................................. xiii Eisenbrey Student Travel Awards ................................................................................. xv Report of the Chairs of the 55th Annual Meeting .......................................................... xvi 2010 Board of Directors ................................................................................................................. xix 2010 Session Chairs ..................................................................................................... xix 2010 Student Paper Awards Committee ....................................................................... xix 2010 Meeting Sponsors ................................................................................................. xx 2010 Banquet Speaker .................................................................................................. xxi Program ..................................................................................................................... xxiii List of Poster Presentation Abstracts .......................................................................... xxx Summary of Oral Presentation Schedule ................................................................... xxxii Abstracts .................................................................................................................. xxxiii Location map for Frances Memorial Sports Center (starting point for several field trips ............................................................... 75 Location map for Backus Community Center (banquet location) ................................... 76 iii �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2009 ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray iv �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 Marquette, Michigan T.J. Bornhorst and J.S. Klasner 55 2009 Ely, Minnesota J.D. Miller, Jr., G.J. Hudak, D.M. Peterson 56 2010 International Falls, Minnesota P. Hollings, P. Hinz, M. Smyk, M. Jirsa, and T. Boerboom v �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam‘s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92 nd birthday. In the late 1970‘s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for ―outstanding contributions to the geology of the Lake Superior region.‖ Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 vi �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vii �PAST GOLDICH MEDALISTS 1979 Samuel S. Goldich 1995 Gene La Berge 1980 not awarded 1996 David L. Southwick 1981 Carl E. Dutton, Jr. 1997 1982 Ralph W. Marsden 1998 Zell Peterman 1983 Burton Boyum 1999 Tsu-Ming Han 1984 Richard W. Ojakangas 2000 John C. Green 1985 Paul K. Sims 2001 John S. Klasner 1986 G.B. Morey 2002 Ernest K. Lehmann 1987 Henry H. Halls 2003 Klaus J. Schulz 1988 Walter S. White 2004 Paul Weiblen 1989 Jorma Kalliokoski 2005 Mark Smyk 1990 Kenneth C. Card 2006 Michael G. Mudrey 1991 William Hinze 2007 Joseph Mancuso 1992 William F. Cannon 2008 Theodore J. Bornhorst 1993 Donald W. Davis 1994 Cedric Iverson 2009 L. Gordon Medaris, Jr. Ronald P. Sage 2010 JOINT GOLDICH MEDAL RECIPIENTS William D. Addison and Gregory R. Brumpton Thunder Bay, Ontario GOLDICH MEDAL COMMITTEE Serving for the meeting years shown in parentheses Terry Boerboom (2007-2010) Government representative Allan MacTavish (2008-2011) Industry representative Mary Louise-Hill (2009-2012) Academic representative viii �CITATION FOR GOLDICH MEDAL RECIPIENTS William D. Addison and Gregory R. Brumpton 2010 Goldich Medal Recipients Bill Addison and Greg Brumpton are the first joint recipients of the Goldich Medal of the Institute on Lake Superior Geology. It is my privilege and pleasure to make this presentation in recognition of their remarkable contributions to the geology of the Lake Superior region. Bill and Greg worked as an inseparable team for more than a decade in the search for, and eventual discovery of the Sudbury impact layer, a bed containing material ejected from the crater near Sudbury, Ontario by the giant impact event at 1850 Ma. Their discovery and resulting publications constitute a fundamental contribution to the geology of the region and have ignited a renewed interest by many colleagues in studies of the classic geology of the iron ranges of the region where extensions of their original discoveries have since been documented. Greg Brumpton (left) and Bill Addison in their natural habitat near Thunder Bay. Before further discussing their scientific contributions I would like to present some biographical information on each of our medalists. Unlike so many of our career-long members, Bill and Greg appeared on our scene with a bang within the past decade, as mature, fully-formed researchers with something truly important to tell us. In assembling this biographical sketch it became apparent that virtually no one in the geologic community knew much of their pre-impact existence and they were truly international men of mystery to all of us. Even Google failed to provide enlightenment. ―William D. Addison‖, for instance, yields 18,800 hits suggesting that Bill clearly has multiple personae. So I was forced to go directly to the source and ask Bill and Greg themselves to give some pertinent information. Here are their abridged autobiographies. Bill Addison was born at an undisclosed location on April 27, 1939. His lifelong fascination and love of nature was instilled at an early age by his parents through many outdoor activities and their encouragement to not only enjoy nature but understand what he was experiencing in the outdoors. Bill‘s formal education was at the University of Toronto where he received a B.Sc. in forestry in 1963 and M.Sc. in fish physiology in 1966. Bill married his wife, Wendy, in 1966 and promptly treated her to a long honeymoon wilderness adventure in a remote corner of the Northwest Territories. Bill‘s early career was with the Ontario Department of Lands and Forests as a walleye research biologist. In 1970 he began his 28 year career as a science teacher as Westgate High School in Thunder Bay. ix �Greg Brumpton was born in Windsor, Ontario on July 16, 1941. His family‘s business in horticulture lead to an interest in agriculture and biology and Greg received his undergraduate degree in agriculture from the Ontario Agricultural College in Guelph, Ontario. He then studied at the University of Wisconsin-Madison, where he received his M.S. degree in entomology in 1966. Greg became a high school teacher in Windsor, Ontario in 1966 and married his wife, Carol Ann, in 1968. In 1970 Greg and Carol Ann moved to Thunder Bay and Greg continued his science teaching career at Westgate High School, arriving coincident with Bill Addison, thus beginning their lifelong collegial partnership. Greg, like Bill, retired in 1998 and the two soon took up their hobby of revolutionizing our understanding of the Paleoproterozoic history of the Lake Superior region. The discovery by Bill and Greg of the ejecta-bearing bed produced by the Sudbury impact, documented in their Geology paper in 2005, was a revelation to many of us with long experience in the geology of the Lake Superior region, as well as to the broader community of impact geologists. But it was a quiet labor of dedication and perseverance by our two medalists that spanned more than a decade of frustrations and false leads before their ultimate success and their sudden appearance in our midst with their remarkable discovery. Greg became interested in the topic of impact-induced mass extinctions in the 1980‘s and by 1991 had recognized that, in the Thunder Bay area, the highly fossiliferous Gunflint Formation was overlain by the nonfossiliferous Rove Formation and that the Gunflint/Rove boundary might be the same age as the Sudbury impact. He proposed therefore to examine that boundary for signs of ejecta and a possible mass extinction event. The hunt proved more difficult than it might have seemed because of sparse exposures and few drill holes to provide samples from the critical horizon. But perseverance ultimately paid off with discoveries of ejecta both in outcrop and drill cores. For many of us in the ILSG community who have followed Bill and Greg‘s discovery and subsequent investigations for the past five years, its importance to the geology of our favorite region is clear. To a great extent ―You heard it here first‖ because they have honored the Institute by presenting their findings through a series of papers at ISLG meetings. But, we also need to appreciate how their work has been received by the broader community of impact researchers. Bevan French, one of the most eminent researchers in the field of impact geology for many decades, and a pioneer in documenting the Sudbury impact itself, wrote the following in support of Bill and Greg‘s nomination. ―I consider Addison and Brumpton's study to be one of the most exciting discoveries in geology in recent years. It has provided one of those rare and unexpected major insights which permanently change our existing picture of geology, and, at the same time, indicate new and unexpected directions of research.” You may have noticed that I have not mentioned the word ―geology‖ anywhere in Bill‘s and Greg‘s professional training or experience. This is not an oversight. These two remarkable gentlemen are entirely self taught in our field of research. Their curiosity and intellectual enthusiasm have brought them to a level of geologic knowledge such that even close colleagues might not realize this lack of formal training were it not for their modesty in frequently pointing out this ―flaw‖ in their backgrounds. I believe that never in the history of ILSG have two self-taught individuals had such a profound impact on understanding the geologic history of the Lake Superior region or stimulated as much new research as our two medalists have done. x �It was my honor to have nominated Bill Addison and Greg Brumpton for the 2010 Goldich Medal. But I also would like to acknowledge the strong support of that nomination provided by Klaus Schulz and Laurel Woodruff of the USGS, Phil Fralick of Lakehead University, John Klasner of Marquette, Michigan, Jorma Kalliokoski of Houghton, Michigan, David Kring of the Lunar and Planetary Institute in Houston, Texas, and Bevan French, of NASA (retired) and scientist emeritus at the Smithsonian Institution Museum of Natural History in Washington, DC. On behalf of these supporters, and with great personal pleasure, I congratulate William D. Addison and Gregory R. Brumpton as recipients of the Goldich Medal of the Institute on Lake Superior Geology. William F. Cannon May, 2010 xi �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute‘s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive onehalf of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31 st of each year Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. Details of the application process can be found on the ILSG web site. The proposal will need to be signed by researcher‘s supervisor. In 2009 the ILSG Board of Governors awarded two $500 awards from the Student Research Fund. The winners are listed below, along with the title of the presentation they will be giving this year. Amanda van Lankvelt (Lawrence University) – Baraboo-interval quartzite breccias. Shelby Frost (University of Minnesota – Duluth) – Effects of contact metamorphism by the Duluth Complex on Proterozoic footwall rocks in northeastern Minnesota xii �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper— last modified by the Board in 2001—follow: The contribution must be demonstrably the work of the student. The student must present the contribution in-person. The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. (continued next page) xiii �STUDENT PAPER AWARDS (CONTINUED) In 2009 the ILSG Student Paper Committee presented several awards from the ILSG Student Paper Fund. Each of the following recipients received awards ranging from 80 to 100 dollars: Best oral presentations: Benedek Gal (Eötvös Lorând University, Budapest)) – Magmatic vs. hydrothermal processes in the South Filson Creek mineralization, South Kawishiwi Intrusion, Duluth Complex. Natalie Pietrzak (University of Western Ontario) – Ore textures and Mineral Chemistry within the oxide-carbonate-silicate flotation ores at the Cliffs Natural Resources’ Tilden Mine, Michigan Best poster presentations: Ryan Dayton (University of Minnesota-Duluth) – Quantifying assimilation vs. fractional crystallization using Sm-Nd, Lu-Hf and Pb isotope systems: The geochemical evolution of the Sonju Lake Intrusion, Finland, MN James Hiller (California State University) – Detailed petrographic analysis of anthraxolite morphology in the Biwabik Iron-Formation, northern Minnesota Andrew Jansen (University of Wisconsin-Oshkosh) – Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall of the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota Kyle Makovsky (Minnesota State University-Mankato) – Fluid movement through the Mesabi Iron Range, Minnesota Dan Costello (University of Minnesota-Duluth) – Geology of the Tuscarora Intrusion, northeastern Minnesota and its relationship to the anorthositic series of the Duluth Complex xiv �EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. Students who are the senior author on either an oral or poster paper will be given favored consideration. It is desirable for two or more students to jointly request travel assistance. In general, priority will be given to those in the Institute region who are farthest away from the meeting location. Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. In 2009 the ILSG awarded 12 travel awards from the ILSG Eisenbrey Student Travel Fund, to students: Dan Costello Michael DeAngelis Adam Fage Nathan Forslund University of Minnesota – Duluth University of Tennessee – Knoxville Lakehead University Lakehead University Benedek Gal James Hiller Angela Hull Andrew Jansen Maura Kolb Cara Leitheiser Natalie Pietrzak Victoria Stinson Eötvös Lorând University California State University Chico Kent State University University of Wisconsin – Oshkosh Lakehead University University of Minnesota – Duluth University of Western Ontario Lakehead University xv �REPORT OF THE CHAIRS OF THE 55TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ELY, MINNESOTA The Precambrian Research Center (PRC) of the University of Minnesota Duluth hosted the 55 th Annual Institute on Lake Superior Geology on May 5 – 10, 2009 at the Grand Ely Lodge in Ely, Minnesota – Gateway to the Boundary Waters Canoe Area Wilderness. Despite this area being well known for its classic Precambrian geology, this was the first time that the ILSG has been held in Ely. We are pleased to report that this was one of the best attended meetings in the 55 year history of ILSG with a total of 234 registrants, including 45 students. The organizing committee for the meeting was comprised of the three PRC directors: Jim Miller (meeting and field trip logistics, promotion, fundraising, and registration), George Hudak (technical program, student travel, and best paper awards), and Dean Peterson (field trip guidebook). Julie Ann Heinz, an executive office administrator at the UMD Natural Resources Research Institute, provided valuable assistance with meeting registration and planning. During the meeting, Julie Ann and Ginny Aldag handled on-site registration and other institute business. The two-day technical session held at the Grand Ely Lodge on Thursday and Friday (5/7 and 5/8) included 21 talks and 24 posters presentations, including 4 oral and 14 poster presentations by students. The meeting opened with a remembrance of Joe Mancuso who passed away in April of 2009. Joe was a professor of economic geology at Bowling Green State University (OH) and a long-time ILSG member (attended the 1st meeting in 1955). He received the Goldich medal in 2007. This year‘s Goldich medal recipient was L. Gordon Medaris of the University of Wisconsin-Madison. Gordon was recognized for his long and productive career studying the Paleoproterozoic of Wisconsin, in particular the Baraboo interval, and for his many contributions to the Institute. Gordon was presented the medal at the annual banquet by Bob Dott, his renowned colleague at UW Madison. The evening banquet talk was presented by Marvin Marshak, a physics professor at the University of Minnesota – Twin Cities and founder of the Soudan Underground Physic Laboratory. Dr. Marshak gave a very humorous and informative talk on the search for subatomic particles entitled “The Deep Underground Sky”. The meeting offered seven field trips that highlighted various aspects of the geology and ore deposits in the Ely area. Most trips were filled to capacity with a cumulative total of 325 field trip attendees. Pre-meeting field trips included a very popular two-day trip on the CU-NI DEPOSITS OF THE DULUTH COMPLEX led by Rich Patelke (Polymet Mining), Mark Severson (Natural Resources Research Institute, UMD), Dean Peterson (Duluth Metals Ltd.), Tim Jefferson (Teck American), and Ernie Lehmann (Franconia Minerals) and a one-day trip on GLACIAL GEOLOGY OF THE VERMILION MORAINE led by Phil Larson (Cliffs Natural Resources) and Howard Mooers (Department of Geological Sciences, UMD). On Friday afternoon following the completion of the technical session, two field trips were offered. One gave participants an underground tour of THE SOUDAN IRON MINE AND PHYSICS LAB led by Dean Peterson (Duluth Metals Ltd.), James Pointer – (Minnesota Department of Natural Resources, Parks and Recreation) and Marvin Marshak (Department of Physics, University of Minnesota). The other had Mark Jirsa (Minnesota Geological Survey) lead a PIONEER MINE CANOE EXCURSION around the perimeter of Miners Lake, just across the road from the meeting site in Ely. xvi �Several post meeting trips completed the meeting. A one-day trip on the GEOLOGY AND METAMORPHISM OF THE EASTERN MESABI RANGE led by Dick Ojakangas (Department of Geological Sciences, UMD), Mark Severson (Natural Resources Research Institute, UMD), Doug Halverson, Jeff Bird, Tom Campbell, Jarred Lubben, and Peter Jongewaard (Cliffs Natural Resources), and William Everett (Mesabi Nugget). A two-day trip on the ARCHITECTURE OF AN ARCHEAN GREENSTONE BELT was led by Dean Peterson (Duluth Metals Ltd.), Mark Jirsa (Minnesota Geological Survey), and George Hudak (Department of Geology, University of Wisconsin Oshkosh). Jim Miller (Dept. of Geological Sciences, UMD) led a twoday canoe excursion into the BWCA on the GEOLOGY OF THE LAKE ONE TROCTOLITE. The student paper committee comprised of Tom Fitz (Northland College), John Gartner (Prime Meridian Resources), and Dorothy Campbell (Ontario Geological Survey) once again had a difficult job of selecting among the 18 student oral and poster presentations. More than half of the students received financial aid to defray travel and registration expenses. A total of $5400 was distributed to 23 students from 13 different schools. This generous aid was provided by funds contributed by meeting registrants and several corporations and organizations. Specifically, meeting registrations surcharges generated $2000 and the ILSG Eisenbrey Fund contributed $1000. The American Institute of Professional Geologists (AIPG-Minnesota Chapter) contributed $1000, the Minnesota Mineral Resource Education Foundation (affiliated with Mining Minnesota) contributed $500, Northland Securities gave $500, and the Mesabi Range Geological Society contributed $500 (to UMD students). In addition to these contributions for student support, the meeting benefitted greatly by a $2000 contribution from Duluth Metals Ltd., which allowed the guidebook to be printed in color. Also, Barr Engineering contributed $1000 that went to sponsor the welcoming reception on Wednesday evening. See the 55th Program and Abstract volume for more details on student funding. . The Institute‘s Board of Directors met on May 7, 2009. The meeting was called to order by Chair George Hudak. The meeting was attended by the three co-chairs of the Ely meeting (Miller, Hudak, and Peterson), Treasurer Mark Jirsa, Secretary Peter Hollings, and Board Members Ted Bornhorst (2008 chair) and Ann Wilson (2006 chair). Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Report of the Chairs (Bornhorst and Klasner) for the 54th ILSG, Marquette, Michigan was received and accepted. 2. Received, discussed, and accepted 2008-2009 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2008-2009 ILSG Secretary‘s report (Hollings) 4. Appointed meeting chair (George Hudak) as on-going ILSG Board Member. 5. Discussed application process for ILSG Student Research Awards. Hollings to prepare a two-page form and circulate to the board 6. International Fall, MN was approved as the site for the 2010 (56 th Annual) meeting location with Mark Jirsa (MGS) and Pete Hollings (Lakehead) serving as co-chairs, and Terry Boerboom (MGS) and Mark Smyk (OGS) serving on the organizing committee.. xvii �7. Approved replacement of Richard Ojakangas as ―academic member‖ on Goldich Committee (end of term 2009) with Mary Louise Hill (Lakehead U). 8. Discussed moving toward advertising of the annual meetings exclusively by email and website. Miller gave a positive report on how this approach worked for the Ely meeting. Miller reported compiling an email list of over 750 contacts. 9. It was agreed that Jirsa would order 10 new Goldich medals. It was also agreed that Hollings would prepare a letter that could be sent to new Goldich committee members explaining the process. This letter will be distributed to the Board prior to being sent out 10. Hudak suggested that a notice page be added to the web site to be used to post job and graduate opportunities. It was agreed that the proceedings of the institute would now be posted immediately after each meeting. The hosting of the ILSG web site was discussed, Miller to investigate the possibility of hosting the site through the Precambrian Research Center. 11. The board thanked Mudrey and Hollings for their work on digitizing the volumes. These have proved a useful resource for researchers The co-chairs would like to thank the participants, especially the students, for their enthusiasm, the field trip leaders for their hard work, the presenters for their informative talks and posters, Marvin Marshak for his classic banquet talk, the session chairs and subcommittee members for their important contributions, and the meeting sponsors for their generosity. A big thank you to Julie Ann Heinz for her cheerfulness and organization in handling the registrations. Thanks also to Sue Marturano, Sue Salveson, and Denise Endicott for handling the budgeting. Thanks to our wives, Ginny Aldag, Rachel Pastoor and Deb Rausch, for their support and understanding. Thanks to Barb Lyke, Tara Akeman, and the entire staff of the Grand Ely Lodge for being accommodating, flexible and very professional. Although there were some things that we could have improved upon (e.g. a projector that showed true colors), we are pleased with how the meeting went. We are particularly gratified by the near record attendance, the popularity of the field trips, and the generally favorable comments we receive about the technical meeting. It made the planning, organizing, and oversight of the meeting worth the effort. You all should try it sometime…really. See you all in International Falls. Respectfully submitted, Jim Miller, George Hudak, and Dean Peterson Co-Chairs, 55th Institute on Lake Superior Geology xviii �2010 BOARD OF DIRECTORS Board appointment continues through the close of the meeting year indicated in parantheses, or until a successor is selected Peter Hinz, Co-Chair, 56th Meeting Ontario Geological Survey Peter Hollings, Co-Chair, 56th Meeting Lakehead University, Thunder Bay, ON Mark Smyk, Co-Chair 56th Meeting Ontario Geological Survey Mark Jirsa, Co-Chair, 56th Meeting Minnesota Geological Survey, St. Paul, MN Terry Boerboom, Co-Chair, 56th Meeting Minnesota Geological Survey, St. Paul, MN Jim Miller (2010) University of Minnesota – Duluth, MN Peter Hollings – Secretary (2010) Lakehead University, Thunder Bay, ON Theodore J. Bornhorst (2011) Michigan Technological University, MI Mark A. Jirsa – Treasurer (2011) Minnesota Geological Survey, St. Paul, MN George Hudak (2012) Precambrian Research Center and Natural Resources Research Institute 2010 SESSION CHAIRS Peter Hinz Ontario Geological Survey Mark Smyk, Ontario Geological Survey Peter McSwiggen, McSwiggen and Associates Charlie Blackburn, Blackburn Geological Leonard Espinosa, Retired, Michigan Department of Natural Resources 2010 STUDENT PAPER AWARDS COMMITTEE Graham Wilson (Chair), Turnstone Geological Services Ltd. Jim Miller, University of Minnesota – Duluth Anthony Pace, Ontario Geological Survey xix �2010 MEETING SPONSORS The organizers wish to sincerely thank the several companies and organizations who have contributed financial support and support in kind to various components the meeting. American Institute of Professional Geologists (AIPG) – Minnesota Atikokan Mineral Development Initiative Brett Resources Canadian Institute of Mining, Metallurgy, and Petroleum (CIM) – Thunder Bay Mesabi Range Geological Society Metalcorp Minnesota Geological Survey Numax Ontario Geological Survey Prime Meridian Resources Q-Gold Rainy River Resources Society for Mining, Metallurgy, and Exploration (SME) – Minnesota Voyageurs National Park William Miller – University of Minnesota School of Physics and Astronomy NOvA project xx �2010 BANQUET SPEAKER The Importance of Being Cratered: The New Role of Meteorite Impact as a „Normal‟ Geological Process Dr Bevan M. French Adjunct Professor Smithsonian Institution National Museum of Natural History Planetary geologist, petrologist and author xxi �xxii �PROGRAM xxiii �WEDNESDAY MAY 19, 2010 8:30 a.m. FIELD TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN) Wally Rayner – Rainy River Resources CJ Baker – Rainy River Resources Craig Ravnaas – Ontario Geological Survey 77 passenger tour bus Return to rec centre approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE FORT FRANCES MEMORIAL SPORTS CENTER*, FORT FRANCES, ONTARIO *SEE MAP ON PAGE 75 IN BACK OF THIS VOLUME FOR LOCATION --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 2: GEOLOGY OF ARCHEAN SUCCESSION AT ATIKOKAN Phil Fralick – Lakehead University 77 passenger tour bus Return to White Otter Inn approximately 5:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE WHITE OTTER INN, ATIKOKAN, ONTARIO --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT Dyanna Czeck – University of Wisconsin - Milwaukee Howard Poulsen – Consultant 9 – passenger vans Return to rec centre approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE FORT FRANCES MEMORIAL SPORTS CENTER*, FORT FRANCES, ONTARIO *SEE MAP ON PAGE 75 IN BACK OF THIS VOLUME FOR LOCATION 4:00 p.m. - 10:00 p.m. REGISTRATION AT RAINY RIVER INN**, INTERNATIONAL FALLS, MN 7:00 p.m. - 10:00 p.m. ICE BREAKER AND POSTER SESSION **THE RAINY RIVER INN IS THE FORMER HOLIDAY INN xxiv �THURSDAY MAY 20, 2010 A.M. 8:00 a.m. - 12:00 noon REGISTRATION 8:30 a.m. INTRODUCTORY REMARKS Mark Smyk and Peter Hinz, Ontario Geological Survey TECHNICAL SESSION I Session Chairs: Mark Smyk and Peter Hinz, Ontario Geological Survey 8:40 a.m. Miller, James D., and Stifter, Eric Cyclical Phase Layering in the Duluth Complex at Duluth – Evidence for Periodic Magma Venting from a Shallow Staging Chamber? 9:00 a.m. Hoaglund, S.A.*, Miller, J.D., Crowley, J.L., Schmitz, M.D. U-Pb zircon geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift 9:20 a.m. Cundari, Robert*, Hollings, Peter, and Smyk, Mark Geology and geochemistry of the Rove basalts, a possible Midcontinent-Rift related extrusive unit south of Thunder Bay, Ontario 9:40 a.m. COFFEE BREAK AND POSTER SESSION 10:20 a.m. Magnus, Seamus*, Kissin, Stephen Assimilation and petrogenesis in the Navilus and Terry Fox sills, Thunder Bay, Ontario 10:40 a.m. Puchalski, Raya*, Hollings, Peter, and Smyk, Mark The petrology and geochemistry of the Riverdale Sill 11:00 a.m. Hollings, Peter, and Smyk, Mark Radiogenic isotope characteristics of Midcontinent Rift-related intrusions south of Thunder Bay 11:20 a.m. Lunch Break (2010 ILSG Board Meeting - by invitation) xxv �THURSDAY MAY 20, 2010 P.M. TECHNICAL SESSION II Session Chair: Peter McSwiggen, McSwiggen and Associates 1:00 p.m. van Lankvelt, Amanda*, and Bjornerud, Marcia Revisiting the Baraboo breccias 1:20 p.m. Pietrzak, Natalie*, Duke, Norm, Scott, Glenn, and Lukey, Helene, Hydrothermal Overprint by Greenstone Sills within the Tilden Pit, Michigan 1:40 p.m. Noah Planavsky*, Andrey Bekker, Olivier J. Rouxel, Balz Kamber, Axel Hofmann, Andrew Knudsen, Timothy W. Lyons The rare earth element and yttrium composition of Archean and Paleoproterozoic iron formations revisited: A new perspective on significance and mechanisms of iron formation deposition 2:00 p.m. O’Hare, Sean* and Fralick, Philip Sedimentological study of the glaciogenic Bruce Formation, Huronian Supergroup, in the North Shore area, Ontario 2:20 p.m. COFFEE BREAK AND POSTER SESSION 3:00 p.m. Jirsa, Mark A. Stratigraphy of Sudbury “impactite‟ near Gunflint Lake, NE Minnesota 3:20 p.m. Weiblen, Paul, W., Jirsa, Mark A., and McSwiggen, Peter Sudbury impact ejecta – amphiboles in samples from the vicinity of the Gunflint Trail, Minnesota 3:20 p.m. Cannon, William F., and Slack, John F. Global effects of the Sudbury bolide impact at 1850 Ma: Potential link to the evolution of marine mineral deposits ----------------------------------------------------------------------------------------------------------------6:00 p.m. ICE BREAKER – MIXER – CASH BAR 7:00 p.m. ANNUAL BANQUET AND AWARD PRESENTATION AT BACKUS COMMUNITY CENTER (SEE MAP ON PAGE 76 IN BACK OF THIS VOLUME FOR LOCATION) • Announcement of 57th Annual Meeting Location • 2010 Goldich Award Presentation to: • William D. Addison and Gregory R. Brumpton • 2010 Banquet Address by Dr. Bevan M. French, Smithsonian Institution All regstered participants are welcome to the banquet address xxvi �FRIDAY MAY 21, 2010 A.M. 8:30 a.m. INTRODUCTORY REMARKS Mark Jirsa, 2010 ILSG Co-Chair TECHNICAL SESSION III Session Chair: Charlie Blackburn, Blackburn Geological 8:40 a.m. Kolb, Maura J.* and Hill, Mary Louise A microstructural study of gold mineralization at Musselwhite Mine and Hammond Reef shear-zone-hosted gold deposits 9:00 a.m. Stinson, Victoria R.*, and Hill, Mary Louise Structural control at Hammond Reef gold deposit north of Atikokan, Ontario 9:20 a.m. Siemieniuk, Steven*, Hollings, Pete, McCuaig, Cam, and Porwal, Alok Gold in the Wabigoon Subprovince – GIS-based Mineral Potential Mapping 9:40 a.m. Scott, Robert J.* and Hill, Mary Louise Quartz eyes of the Moose Lake Porphyry Complex, Hemlo, Ontario 10:00 a.m. COFFEE BREAK AND LAST POSTER SESSION 10:40 a.m. Gilbert, Paul Geology and geochemistry of arc and ocean-floor volcanic rocks in the Northern Flin Flon Belt, Manitoba 11:00 a.m. Fralick, Philip Anatomy of an Archean carbonate platform: Geology of the Steep Rock Group, Canada 11:20 a.m. Smyk, Mark, Stott, Greg, and Atkinson, Brian The Ring of Fire: An Overview of the Geology, Mineral Deposits and Exploration History of the McFaulds Lake Area 11:40 p.m. LUNCH BREAK AND MEETING OF THE STUDENT PAPER COMMITTEE xxvii �FRIDAY MAY 21, 2010 P.M. TECHNICAL SESSION IV Session Chair: Leonard Espinosa Retired - Michigan Department of Natural Resources 1:00 p.m. Boerboom, Terrence J. Latest bedrock geologic map of Carlton County, east-central Minnesota and revisions to the southern Animikie Basin edge 1:20 p.m. Chandler, Val W.. The Search for Magnetic Basement in Minnesota: Sliding Along with the Euler Equation 1:40 p.m. Waggoner, T.D. Post Penokean Age of Mineralization on the Marquette Range, Michigan 2:00 p.m. PRESENTATION OF THE STUDENT TRAVEL AND BEST PAPER AWARDS 2:20 p.m COMPLETION OF 2010 ILSG TECHNICAL SESSIONS AND FINAL STATEMENTS BY THE 2009 ILSG CO-CHAIRS 3:00 p.m. FIELD TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) Chris Hemstad – Boreal Explorations Return to Rainy River Inn approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE RAINY RIVER INN**, INTERNATIONAL FALLS, MN 3:00 p.m. FIELD TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX Mark Jirsa – Minnesota Geological Survey Ash River laboratory staff Return to Rainy River Inn approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE RAINY RIVER INN**, INTERNATIONAL FALLS, MN **THE RAINY RIVER INN IS THE FORMER HOLIDAY INN xxviii �SATURDAY MAY 22, 2010 8:30 a.m. FIELD TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) Mark Jirsa – Minnesota Geological Survey Chris Hemstad – Boreal Explorations 47 passenger tour bus Return to Rainy River Inn approximately 5:30 p.m. TRIP LEAVES FROM AND RETURNS TO THE RAINY RIVER INN**, INTERNATIONAL FALLS, MN **THE RAINY RIVER INN IS THE FORMER HOLIDAY INN --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) Peter Hinz – Ontario Geological Survey Other industry geologists 9 – passenger vans Return to rec centre approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS TO THE FORT FRANCES MEMORIAL SPORTS CENTER*, FORT FRANCES, ONTARIO *SEE MAP ON PAGE 75 IN BACK OF THIS VOLUME FOR LOCATION --------------------------------------------------------------------------------------------------------------------8:30 a.m. FIELD TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN) Andrew Conly – Lakehead University Rob Purdon – Ontario Ministry of Natural Resources Mini-vans Return toWhite Otter Inn, Atikokan, approximately 6:00 p.m. TRIP LEAVES FROM AND RETURNS THE WHITE OTTER INN, ATIKOKAN, ONTARIO _____________________________________________ 56TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY ENDS xxix �POSTER PRESENTATIONS Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller J.D., Jr., Mesoproterozoic Bedrock Geology of the Homer Lake – Vern Lake Area, Cook County, Northeastern Minnesota Boerboom, T. J., Magnuson, A., Creighton, Dalyice, McGinn Mapping of Mesoproterozoic bedrock in the Tait Lake quadrangle, Northeastern Minnesota Buchholz, T. W., Falster, A. U. and Simmons, W. B. Additions and New Data on the Mineralogy of the Keweenaw Copper District, MI. Bultman, M. W. Preliminary analysis of ground-based magnetic profile data aimed at understanding the concealed mineral resource potential of the Wisconsin magmatic terranes Fage*, A. and Hollings, P. Geology and Geochemistry of the Hemlo East Property, Schreiber-Hemlo Greenstone Belt, Ontario Fahrenkrog, Brooke, Totenhagen, Mike, Jaret, Steven, Watson, Tabitha, and Jirsa, Mark A. Geologic mapping of Archean and Proterozoic rocks near Bingshick Lake, Boundary Waters Canoe Area Wilderness, by students of the Precambrian Research Center‟s 2009 field camp Fralick, P., Brumpton, G.R., Jirsa, M.A., Kissin, S.A., Severson, M.J. Sedimentology of the Paleoproterozoic ejecta layer from the Sudbury impact event Frost*, S. J Effects of contact metamorphism by the Duluth Complex on Proterozoic footwall rocks in northeastern Minnesota Hudak, G., Diedrich, T., Monson-Geerts, S., Schreiber, M., Zanko, L., and Schwanke, A. Taconite-derived mineral dust in population centers on Mesabi Iron Range: an update Markwood*, Levi W. and Zieg, Michael J., Textural analysis of a simple igneous intrusion, Nipigon, Ontario Medaris, L. G. Jr. and Koellner, S. E. Ferromagnesian Minerals in the Stettin Syenite Complex, Marathon County, Wisconsin: Compositions and Contrasts with the Wolf River Batholith Mulvey, Lucy, Ross, Cabin, Zeitler, Joseph, Pendleton, Matthew, McCarthy, Andrew, Copp, Lee, Nowak, Robert, Hudak, George, and Peterson, Dean, Bedrock geologic map of the Disappointment Lake area, Lake County, Northeastern Minnesota Patelke, R.L., and Severson, M.J. Virginia Formation outcrop (New outcrops versus old concepts) Ryan*, A. J. and Zieg, M. J. Petrographic and geochemical analysis of a Nipigon diabase sill xxx �Triplett*, J., Saini-Eidukat, B., Feit, S., and Dolezal, D. Identification and Characterization of Fibrous Zeolites in Western North Dakota Weiblen, Paul W. New ways to study old problems. White, Chris R., and Albers, Paul B. Geologic map of the central Seine Bay/ Bad Vermilion Lake Intrusion with accompanying airborne Geophysics, relating to Numax Resources, Inc., Mine Centre Property, Mine Centre, Ontario xxxi �Thursday a.m. Thursday p.m. Technical Session I Session chair: Mark Smyk and Peter Hinz Technical Session II Session chairs: Peter McSwiggen Session 1a. Midcontinent Rift 8:30 AM introductory remarks 8:40 AM Miller 9:00 AM Hoaglund* 9:20 AM Cundari* Session 2a. Misc. Paleoprot. 1:00 PM VanLankveldt* 1:20 PM Pietrzak* 1:40 PM Planavsky* 2:00 PM Ohare* 9:40 AM Coffee break and poster session 2:20 PM Coffee break and poster session Session 1b. MCR Mafic Sills 10:20 AM Magnus* 10:40 AM Puchalski* 11:00 AM Hollings Session 2b. Sudbury Impact 3:00 PM Jirsa 3:20 PM Weiblen 3:40 PM Cannon 11:20 AM Lunch break 2010 ILSG Board Meeting Friday a.m. Friday p.m. Technical Session III Session chairs: Charlie Blackburn Technical Session III Session chairs: Leonard Espinosa Session 3a. Archean Econ. Geol. 8:40 AM Kolb* 9:00 AM Stinson* 9:20 AM Siemieniuk* 9:40 AM Scott* Session 4a. Archean Geology 1:00pm Boerboom 1:20 PM Chandler 1:40 PM Waggoner 2:00 PM Student paper, poster, and travel awards 10:00 AM Coffee break and poster session Session 3b. Proterozoic 10:40 AM Gilbert 11:00 AM Fralick 11:20 AM Smyk Session 4b. Afternoon Field Trips 3:00 PM depart for field trips 11:40 PM Lunch break xxxii �ABSTRACTS xxxiii �xxxiv �MAPPING OF MESOPROTEROZOIC BEDROCK IN THE TAIT LAKE QUADRANGLE, NORTHEASTERN MINNESOTA Boerboom, T.J., Magnuson, A., Creighton, D., and McGinn, K. Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 The Precambrian Research Center (PRC) field camp culminates in several ‗capstone‘ mapping projects located in different geographical areas/geological terranes in northeastern Minnesota. One of the 2009 PRC capstone projects was focused on mapping an area within the North Shore Volcanic Group that is intruded by components of the Beaver Bay Complex. Compared to the other capstone projects, which were largely canoe-based and in areas of plentiful and clean bedrock exposures, this map area was just the opposite – no lakeshore outcrops, overgrown clearcuts, and relatively small and scattered outcrops located mostly near the apex of topographic knobs. As such, the participants in this camp gained a different perspective of ‗real-life‘ field geology: the reality of field work in most northern forested areas – thick brush, insects, heat, and lack of water. Nevertheless, the mappers persevered and were able to provide significant refinements to our understanding of the bedrock geology of the southwestern Tait Lake quadrangle. Prior to the PRC effort, the only mapping in the area was reconnaissance outcrop studies by John Green from the University of Minnesota-Duluth. This information was used in the construction of a regional, 1:200,000-scale geologic map, which portrayed the bedrock units as largely undifferentiated volcanic rocks (Miller et al., 2001). Based on the results of the PRC capstone project, within the chosen map area we have defined a southeast-dipping series of volcanic rocks that includes quartz- and feldspar-phyric rhyolite, sparsely plagioclase-porphyritic andesite, and porphyritic basalt. These volcanic rocks are intruded by the Lake Clara diabase, a southeastdipping semi-conformable sill which was known only in a general fashion from prior work. Our new mapping has identified a thin ferrodiorite layer that is apparently semi-conformable to the base of the diabase sill, which may be the product of partial melting of the rhyolite below the sill and contamination of the adjacent diabase. Tentative plans for the 2010 capstone project are to continue mapping in the northwest corner of the Tait Lake 7.5‘ quadrangle, in a zone that transitions from the Eagle Mountain granophyre (1098.6±3.6 Ma; Vervoort and others, 2007) into volcanic rocks of the North Shore Volcanic Group. This mapping would be continuous with prior detailed mapping to the north in the Brule Lake quadrangle (Davidson and Burnell, 1977). However, difficult access may present a serious impediment to this plan in which case work may proceed elsewhere. References: Davidson, D.M., Jr., and Burnell, J.R., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map M29, scale 1:24,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism, Precambrian Research 157 (2007) 235–268. 1 �LATEST BEDROCK GEOLOGIC MAP OF CARLTON COUNTY, EAST-CENTRAL MINNESOTA AND REVISIONS TO THE SOUTHERN ANIMIKIE BASIN EDGE Boerboom, Terrence J., Minnesota Geological Survey (boerb001@umn.edu) As part of the Minnesota Geological Survey‘s ongoing County Geologic Atlas Program, a revised bedrock geologic map has recently been released for Carlton County, in east-central Minnesota. As with other county atlases produced by the Minnesota Geological Survey, this atlas was funded by a combination of state and county funds. Carlton County straddles the southern edge of the Animikie basin (Figure), which contains strata dominated by graywacke-slate, and includes the Virginia Formation to the north and the Thomson Formation to the south. To the north the Virginia Formation overlies the Biwabik Iron Formation and the Pokegama Quartzite, the latter of which was deposited directly on Archean basement as well as eroded ca. 2.1 Ga Kenora-Kabetogama dikes. In contrast, the Thomson Formation at the southern margin unconformably overlies metamorphosed sedimentary and igneous rocks of the Paleoproterozoic Mille Lacs Group. This paper focuses on the Thomson Formation at the southern margin of the Animikie basin. The bedrock geology of east-central Minnesota has been mapped numerous times, each version taking into account the latest geologic concepts and newly-acquired geophysical and drill-hole data. In early regional compilations the Thomson Formation included some rocks now assigned to the Mille Lacs Group, and the Cuyuna North and South Range strata were correlated to those in the lower part of the Animikie basin. However, more recent geologic interpretations associate the Cuyuna ranges with the older Mille Lacs Group, as part of a belt that was deformed and uplifted during the Penokean Orogeny, and upon which the Animikie strata were deposited in a foreland basin setting. This scenario is supported by the ca. 1,850 Ma metamorphic ages obtained in both the Mille Lacs Group and the Cuyuna North Range, and ca. 1,800 Ma detrital zircons in the Thomson Formation. Figure showing extent of Animikie basin (gray) in Minnesota. Twice-deformed Thomson in dark gray. Carlton County outlined by heavy line. 2 �Holst (1982) documented two structural domains in the Thomson Formation, separated by what is colloquially known as ‗Holst‘s line‘. Slate and graywacke south of this line (southern domain) contain a subhorizontal S1 foliation overprinted by regional F2 folds and accompanying S2 crenulation cleavage, whereas rocks north of this line (northern domain) were deformed and folded only once by the regional D2 deformation. Winchell (1899) described an outcrop of conglomerate with clasts of metagraywacke, slate, and black chert, in the vicinity of what is now ‗Holst‘s line‘. Although this outcrop has apparently been buried by road construction, its reported location and lithology imply a possible unconformity at the base of the northern domain. A second conglomerate, intersected by a deep drill hole near the base of the southern domain, contains clasts of granitoid rocks, metagraywacke, chert, and marble; this may represent a second, lower unconformity between the base of the southern Thomson domain and the older Mille Lacs Group. A possible scenario is that the southern domain may represent an early protoAnimikie depositional basin marked by a basal conglomerate, which was deformed and uplifted to provide sediment to the main, northern domain of the Animikie basin, the base of which is marked by a second conglomerate. Based on examination of drill cores and outcrops, multiple east-west trending, low-amplitude aeromagnetic anomalies over the southern domain of the Thomson Formation are considered to be caused by sulfidic intervals spatially associated with thin to thick beds of chert as well as local metagabbro sills. The aeromagnetic anomalies, which coincide with electromagnetic and gravity anomalies, continue west into areas formerly designated as Cuyuna South Range. It is proposed that much of what has mapped as Cuyuna South Range is actually infolded remnants of the lowermost Animikie basin strata. In this case the lower Animikie basin would include a substantial proportion of mafic volcanic rocks and hypabyssal intrusions. Geophysical modeling implies that the magnetic source to these anomalies is a north-dipping sheet, consistent with the inferred geometry of the Animikie basin. These sulfidic horizons may be analogous to thin oxide- and sulfide-facies iron-formations in the lower part of the Michigamme Formation in Michigan, which is correlative with this part of the Animikie basin (Rossell, 2008; Ware and others, 2008). Metamorphic ages in the Mille Lacs Group and the Cuyuna North Range show a Penokean metamorphic event (ca. 1,850 Ma), and a later Yavapai metamorphic overprint (ca. 1,760 Ma). However no metamorphic ages have been obtained from the Cuyuna South Range or from the Thomson Formation; an older ca. 1850 Ma age in the Cuyuna South Range would disprove the proposed correlation of the south range with the Animikie basin. The once-deformed Thomson Formation at its type locality contains detrital zircons as young as 1,800 Ma, the same age as the Hillman tonalite to the south. If that were the source for the zircons, deep erosion may have accompanied Yavapai uplift and deformation. References: Holst, T.B., 1982, Evidence for multiple deformations during the Penokean Orogeny in the Middle Precambrian Thomson Formation, Minnesota: Canadian Journal of Earth Sciences v. 19, p. 2043-2047. Rossell, D., 2008, Trip 7: Geology of the Keweenawan BIC intrusion, in 54th Annual Institute on Lake Superior Geology Proceedings Vol. 54, part 2 – Field Trip Guidebook; Bornhorst, T.J., and Klasner, J.S., eds., p. 181-199. Ware, A., Cherry, J., and Ding, X., 2008, Trips 4 and 8: Geology of the Eagle Project: in 54th Annual Institute on Lake Superior Geology Proceedings Volume 54, part 2 – Field Trip Guidebook; Bornhorst, T.J., and Klasner, J.S., eds., p. 87-115. Winchell, N.H., 1899, The geology of the Carlton plate, in Winchell, N.H., and others, Geology of Minnesota: volume 4 of the final report, 1896-1898: Minnesota Geological and Natural History Survey, p. 550-565. 3 �ADDITIONS AND NEW DATA ON THE MINERALOGY OF THE KEWEENAW COPPER DISTRICT, MI. Buchholz, Thomas W., 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, Falster, Alexander U. and Simmons, Wm. B., Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148 Recent work on material from several former mines on the Keweenaw Peninsula, MI has revealed the presence of several minerals not previously reported from the district. Heinrich & Robinson (2004) mentioned the possible occurrence of anilite (Cu 7S4) near Painesdale, MI. In an attempt to search for possible anilite and associated minerals, carbonate matrix was partially removed from samples of chalcocite-bearing vein material from the nearby Baltic Mine in order to better expose the sulfides. Small grains of a soft, deep blue sulfide showing excellent cleavage and evidence of multiple twinning were noted on a few samples. XRD analysis (Gandolfi camera) of a small crystal fragment yielded a pattern showing good agreement with anilite. The anilite seems restricted to quartz-rich portions of thin carbonatequartz-sulfide veinlets, is intimately associated with chalcocite, and is usually spatially associated with late cross-cutting fractures. Associated minerals in the veinlets include native copper, metallic grains of Cu-As alloys showing highly variable Cu-As ratios and spongy appearing masses of Cu-arsenide phases that seem to be composed of fine-grained domains with widely variable compositions. Work continues to attempt to characterize these phases. Several of the samples from the Baltic Mine dump revealed crude wires of native silver associated with chalcocite, quartz and minor barite. Small patches of a Cu-Ag sulfide were noted closely associated with the native silver during examination of this material using SEM/EDS. The Cu-Ag sulfide is identical in overall appearance to chalcocite and was analyzed via electron microprobe. The result, [(Cu1.024 Fe0.012Mn0.001)Ʃ1.037(Ag0.955) (S0.998 As0.009)Ʃ1.007)], is in good agreement with stromeyerite, CuAgS, and is the first report of this mineral for Michigan. It has been suggested that alteration of silver-bearing sulfides and sulfosalts may promote the growth of wire silver; if correct, it seems possible that Ag released by stromeyerite alteration supported the growth of these wire silvers. A small sample from the Wolverine #2 mine contained silvery inclusions in calcite that were confirmed as arsenopyrite by EMP: (Fe0.990 Cu0.003 Co0.001)Ʃ0.994 As0.997 S1.009), in good agreement with the accepted formula FeAsS. The arsenopyrite crystals were perched on a silvery to dark gray colored matrix. Portions of the matrix were analyzed by EMP and are an intergrowth of cobaltite, [(Co0.771 Fe0.158 Ni0.047 Cu0.023)Ʃ0.999 As0.994 S1.007) (analyzed) vs CoAsS], skutterudite, [(Co0.825 Fe0.189 Cu0.006 Ni0.003)Ʃ1.023 (As2.800S0.177)Ʃ2.977) (analyzed) vs CoAs3], and other As-S bearing phases. Minute well-formed crystals of probable safflorite, [(Co 0.587 Fe0.378 Cu0.059 Ni0.002)Ʃ1.026 (As1.850S0.119Se0.004)Ʃ1.973 (analyzed) vs CoAs2], are exposed on surfaces of the matrix and exhibit good orthorhombic morphology. Work continues to characterize additional phases in the matrix. Small grains and crystals of silver (0.12 wt% Hg, 0.21 wt% Se) are present throughout and upon the matrix. Neither arsenopyrite, cobaltite nor safflorite have been previously reported from the Keweenaw Peninsula, though undefined Co-arsenides have been reported from the Mohawk #2 mine, part of the same mineralized trend as the Wolverine mines (Moore, 1962; Heinrich & Robinson, 2004; Butler & Burbank, 1929). 4 �References: Heinrich, E. W., Robinson, George W. (2004) Mineralogy of Michigan. A. E. Seaman Mineral Museum Publishing, MI. 252p. Moore, P. B. (1962) Copper arsenides at Mohawk, Michigan. Rocks & Minerals 37:24-26. Butler, B. S., and Burbank, W. S. (1929) The Copper Deposits of Michigan. U S Geological Survey Professional Paper 144, 238p. 5 �PRELIMINARY ANALYSIS OF GROUND-BASED MAGNETIC PROFILE DATA AIMED AT UNDERSTANDING THE CONCEALED MINERAL RESOURCE POTENTIAL OF THE WISCONSIN MAGMATIC TERRANES BULTMAN, Mark W., U.S. Geological Survey, 520 N. Park Ave, Suite 355, Tucson, AZ, 85719 The Wisconsin magmatic terranes host numerous volcanogenic massive sulfide (VMS) mineral deposits ranging in size from uneconomic to the world class Crandon deposit with 72.5 million tons resource (DeMatties, 1994). While there has been extensive exploration in the terranes, an understanding of the total VMS resource is hindered by thick glacial deposits and/or relatively thin Cambrian sandstones that cover most of the area where permissive host rocks might occur. No medium size VMS deposits (10 to 60 million tons resource) are known in the area, and it is likely that some undiscovered deposits in this size range occur here (DeMatties, 1994). All known economic VMS deposits are associated with syngenetic and epigenetic strata-bound to stratiform mineralization within, along the flanks of, or near the top of felsic volcanic centers (DeMatties, 1994). Felsic centers are found within more mafic metavolcanic rocks creating a bimodal assemblage of volcanic rocks. The ability to distinguish concealed felsic centers from more mafic metavolcanic rocks would contribute to knowledge of the potential distribution and extent of these deposits and serve as an exploration tool. In order to discriminate concealed felsic centers from more mafic concealed metavolcanic lithologies, Earth‘s total intensity magnetic field profile data were acquired by a truck-mounted cesium vapor magnetometer. The magnetometer is mounted at 3.5 meters height and acquires data at measurement intervals of 1.5 to 2.0 meters. These data include information on the spatial distribution of near surface magnetic dipoles that in many cases are unique to a specific lithology. These magnetic signatures have distinct textures (the physical appearance of the data, especially groups of anomalies) that can be compared visually and characterized by several descriptive statistics. Ground-based magnetic profile data have been used successfully in the Basin and Range Province of North America for the identification of bedrock lithology concealed by 100 meters or more of Cenozoic basin fill (Bultman, 2009). In the Basin and Range Province, candidate lithologies (lithologies that may be found concealed by basin fill sediments) often outcrop in the mountain ranges directly adjacent to the basin. This allows the magnetic signature of a candidate lithology to be field calibrated and precisely defined. In areas with thick (10‘s of meters) continuous cover of glacial sediments, it is impossible to acquire the magnetic signature of a candidate lithology. As felsic rocks are generally less magnetic, they display a low amplitude signal in the ground-based magnetic profile data. The mafic metavolcanic rocks displays a much higher amplitude signal that is easily visible in profile data and is easily discriminated from felsic center profile data. Figure 1 displays approximately 6 km of ground-based magnetic profile data plotted along the road over which it was acquired. Numerous cultural features are identified within the profile, which is also plotted on a background image of the aeromagnetic data (Daniels and Snyder, 2002). A significant change in texture is indicated on the profile data and defines the location of a concealed 6 �Figure 1: Ground-based magnetic profile data (thin black line) acquired from west to east along County Roads I and V (black line), Rusk County, Wisconsin plotted on top of the Wisconsin aeromagnetic map (Daniels and Snyder, 2002). Location of a concealed contact between more mafic concealed lithology to the west and a more felsic concealed lithology to the east is indicated along with anomalies due to cultural features. Mapped contacts (Sims, 1989) are shown by white lines. Mapped units are: Cs= Upper Cambrian Mount Simon Sandstone and Xtg=Early Proterozoic felsic volcanic and volcanogenic rocks (after Sims, 1989). contact between more mafic rock to the west and felsic rock to the east. There is no indication of this change in concealed lithology on the aeromagnetic image, displayed in the background of Figure 1. This change in lithology can also be identified using statistical analysis of the groundbased magnetic profile data. Using this technique, known felsic centers have been identified and partially mapped at the Flambeau, Bend, Schoolhouse, Lynne, and Crandon deposits. In addition, several areas with very low amplitude magnetic signals in the over 1000 km of ground-based magnetic profile data acquired to date may represent additional concealed felsic centers. Presently, it is impossible to differentiate felsic metasediments from felsic centers, and it may be difficult to reconcile this issue. Future work includes the addition of more ground-based magnetic profile data in the Wisconsin magmatic terranes, acquisition of data on thin Cambrian sandstones that overlie the western end of the terrane, and acquisition and analysis of data over other terranes concealed by glacial drift in the upper Midwest. References: Bultman, M.W., 2009, The utility of ground-based magnetic profile data in concealed mineral resource investigations. Technical session abstract book: Northwest Mining Association Annual Meeting, Reno, Nevada, Nov. 29-Dec. 4, 2009, p. 43. Daniels, D.L. and Snyder, S.L., 2002, Wisconsin aeromagnetic and gravity maps and data, a web site for distribution of data, U.S. Geological Survey Open-File Report 02-493. http://pubs.usgs.gov/of/2002/of02-493/ DeMatties, T.A. , 1994, Early Proterozoic Volcanogenic Massive Sulfide Deposits in Wisconsin: An Overview: Economic Geology, Vol. 89, 1994, pp. 1122-1151. Sims, P.K., 1989, Geologic map of Precambrian rocks of Rice Lake 1x2° quadrangle, northern Wisconsin: U.S. Geological Survey Map I-1924. 7 �GLOBAL EFFECTS OF THE SUDBURY BOLIDE IMPACT AT 1850 MA: POTENTIAL LINK TO THE EVOLUTION OF MARINE MINERAL DEPOSITS Cannon, William F., and Slack, John F., U.S. Geological Survey, Mail Stop 954, Reston, VA 20192 The bolide impact at Sudbury, Ontario at 1850 Ma is of comparable magnitude to the Chicxulub impact that produced profound and well-documented global biological and environmental changes at the K-P boundary. Like Chicxulub, Sudbury probably produced global effects and we have speculated that it caused changes in ocean chemistry and structure which, in turn, influenced the nature of marine mineral deposits (Slack and Cannon, 2009). Several studies (Figure 1) show that circa 1850 Ma was a time of substantial changes in the occurrence and global abundance of sedimentary manganese deposits, volcanogenic massive sulfide (VMS) deposits, sedimentaryexhalative (SEDEX) sulfides, and deep-water (Cu-rich) VMS-related exhalites, in addition to Superior-type banded iron-formations (BIF), which is our focus here. From these changes it appears that 1850 Ma was a pivotal moment in marine metallogeny. The deposition of BIF required a stratified water body in which deep anoxic waters allowed high concentrations of dissolved ferrous iron, which was then deposited by oxidation to insoluble ferric iron where the chemocline intersected the shallow seafloor. Deposition of BIFs on continental shelves and in foreland basins implies that the entire ocean was stratified for the long periods of Earth history during which BIFs were deposited (Fig. 1). We suggest that the disappearance at 1850 Ma of BIFs (and granular iron formations) from open-marine settings marks the end of anoxic deep water conditions. The change from mainly reduced (sulfide) facies to oxidized (hematite, magnetite) facies in deep-water exhalites at approximately this time is consistent with loss of the deep mass of anoxic seawater. Secular correspondence of several fundamental changes in marine metallogeny shown in Figure 1 and the Sudbury impact is striking, a timing that we suggest is not coincidental. We propose that the Sudbury impact disrupted the previously stratified ocean and mixed deep anoxic water with shallow oxygenated water thus ending the requisite conditions for BIF mineralization. At 1850 Ma the Earth‘s surface (Fig. 2) contained a single supercontinent, Columbia, surrounded by a single ocean (Zhao, et al., 2004), a situation that is critical for evaluating global effects of the Sudbury impact into an epicontinental sea near the margin of Columbia. We propose three mechanisms that might have broken down the preexisting ocean stratification: 1-Giant tsunami: Impacts of large bolides into oceans produce giant tsunamis. Impacts may excavate a crater in the ocean water with a diameter 1.5 times that produced in the underlying crust; the crater is caused by both vaporization of water and physically impelling the water laterally. Tsunami waves result both from lateral expulsion of water and return flow into the crater. Models indicate that in open oceans with depths typically greater than 4 km, these waves can be hundreds of meters high, capable of disturbing the entire water column, and scouring the sea floor over large regions. The Sudbury impact into continental crust, although probably covered by an epicontinental sea, may have produced tsunami waves of lower amplitude. Data to estimate maximum wave heights from the Sudbury impact are lacking. Thus, the capability of Sudburyrelated waves to mix the global ocean cannot be predicted accurately by models. 2-Shelf collapse and debris flows: Earthquakes generated by the Chicxulub impact caused massive failure of much of the outer continental shelf of eastern North American, and spread a layer of debris as much as several meters thick across the western North Atlantic seabed (Norris and Firth, 8 �2002). Shelf failure was common for at least 4,000 km from the impact site; lesser failure is known at distances up to 7,000 km. A 4,000 km circle centered on Sudbury (Fig. 2) indicates that virtually the entire continental margin of Columbia was likely susceptible to shelf failure from the Sudbury impact. Debris flows radiating out from Columbia may have stirred and mixed ocean waters, creating a new suboxic state for deep seawater by mixing with oxygenated surface water. Antipodal effects: The massive earthquake caused by the Sudbury impact may have had global consequences, but the phenomenon of antipodal focusing of seismic energy should be considered in addition to simple hemispheric transmission and attenuation of seismic energy. Axial focusing of seismic energy and the delivery of a strong seismic signal at the antipode of an earthquake‘s origin is widely observed and well understood in theory. Large-scale deformed terranes are known on both Mercury and the Moon that are antipodal to major impact sites. We suggest that the intuitive notion of decreased seismicity away from Sudbury needs to be modified and the possibility of significant antipodal effects thoroughly evaluated. For instance, if the Sudbury impact produced major surface deformation at its antipode, probably on the deep seabed, significant tsumanis could have been generated there that were transmitted back toward the Columbia landmass, with physical mixing of ocean waters occurring at the antipode as well as closer to Sudbury. We propose that these three phenomena, all likely consequences of the Sudbury bolide impact, possibly acting in unison, resulted in profound changes in the nature of the 1850 Ma global ocean and in the numerous observed changes in the character of marine mineral deposits at that time. References Norris, R.D., and Firth, J.V., 2002, Mass wasting of the Atlantic continental margin following the Chicxulub impact event: Geological Society of America Special Paper 356, p. 79-95. Slack, J.F., and Cannon, W.F., 2009, Extraterrestrial demise of banded iron formations 1.85 billion years ago: Geology, v. 37, p. 1011-1014. Zhao, G., Sun, M., Wilde, S.A., and Sanzhong, L., 2004, A Paleo-Mesoproterozoic supercontinent: Assembly, growth, and breakup: Earth-Science Reviews, v. 67, p. 91-123. 9 �THE SEARCH FOR MAGNETIC BASEMENT IN MINNESOTA: SLIDING ALONG WITH THE EULER EQUATION Chandler, V.W., Minnesota Geological Survey, 2642 University Ave. St. Paul, MN, chand004@umn.edu Among the most commonly derived estimates from aeromagnetic data are depths to the top of magnetic sources, or ―magnetic basement‖. Originally these estimates were done graphically using profile records or contour maps, but in recent decades a variety of computer-based techniques have been developed. One particularly robust approach is based on Euler‘s homogeneity equation; which is iteratively solved within windowed subsets of gridded aeromagnetic data, thereby providing estimates of source locations in three-dimensional space (Reid and others, 1990). The Euler method requires no a priori information regarding source magnetization, and only requires selecting a structural index (SI), which is the power of the rate of field change that is sensitive to source-type. For example a SI=1 is consistent with a thin sheet (dike or sill) whereas a SI of 0.5 is commonly used to approximate contact solutions. The Euler Method is used here to investigate the Precambrian geology and upper crustal structure of Minnesota and adjacent areas. This study employs the ―located‖ Euler approach (Geosoft Inc.), in which the analytical signal is used to focus the Euler analysis to areas maximum anomaly gradients, where solutions are most likely to be accurate. Euler analysis of an aeromagnetic grid that has been continued to a relatively low level of 300 meters yields abundant information about sources at or near the Precambrian bedrock surface. In addition to providing thousands of estimates of depth, the Euler solutions form numerous chains of solutions that can facilitate geologic mapping. Not surprisingly, areas of near-surface Precambrian rocks are usually associated with noticeable concentrations of shallow depth estimates, but other solutions in these and in deeper bedrock areas indicate that some anomaly sources may actually lie 100‘s of meters below the Precambrian surface. Euler analysis of aeromagnetic data that were smoothed by upward continuing to a level of 1000 meters was useful for investigating deeper (1000-5000 meters) features, such as the Paleoproterozoic Animikie basin and the upper Keweenawan basins of the Mesoproterozoic Midcontinent Rift. Euler analysis of the Animikie basin using a structural index of 1 yields results that are comparable to earlier magnetic-based estimates, and indicates that the deeper parts of the Animikie basin contain 2500 to 4500 meters of non-magnetic strata. The deepest part of the Animikie basin lies to the east, along the basal contact of the Duluth Complex, perhaps reflecting a regional tilting related to Keweenawan rifting. Estimate of dip along this apparent slope averages around 5 degrees. A shelf along the northwestern margin of the Animikie basin corresponds to the west-southwestward extension of the Virginia Horn. Along the Midcontinent Rift, Euler analysis with a structural index of 0.5 indicates that sedimentary basins east and west of the St. Croix Horst typically contain 3-5 km and 2-3 km thick packages of strata, respectively (Figure 1), whereas the basins atop the horst typically have 1-2 km (Ashland Syncline) and 3-5 km (ancestral Twin Cities basin) of strata. These estimates are generally consistent with those using seismic data (Chandler, 1989; Allen, 1994), and they provide some new insights on those parts of the rift that lack seismic data. For example, the Euler results imply an unusually deep (>5000 meters) basin in south-central Minnesota along the Belle Plaine Fault (Figure 1), and further investigation is warranted. 10 �The results of this study indicate that the Euler method is a viable geophysical tool that will help in understanding Minnesota‘s geology in all three dimensions. References Allen, D.J., 1994, An integrated geophysical investigation of the Mid-continent Rift System: Western Lake Superior, Minnesota and Wisconsin: Lafayette, Ind., Purdue University, Ph.D.Dissertation, 267 p. Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J., and Anderson, R.R., 1989, Interpretation of seismic reflection, gravity and magnetic data across Middle Proterozoic Mid-Continent Rift System, Northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, p. 261-275. Mooney, H. M., Farnham, P. R., Johnson, S. H., Volz, G., and Craddosck, C., 1970, Seismic studies over the Midcontinent Gravity High in Minnesota and northwestern Wisconsin: Minnesota Geological Survey Report of Investigations 11. Reid, A. B., Allsop, J. M., Gassner, H., Millet, A. J., and Somerton, I. W., 1990, Magnetic interpretation in three dimensions using Euler deconvolution: Geophysics, v. 55, p. 80-91. Figure 1. Depth to magnetic basement of the Midcontinent Rift in Minnesota and Wisconsin, based on Euler analysis of 1000 meter level aeromagnetic data with a structural index of 0.5. Contour interval is 1000 meters, heavy lines represent seismic refraction profiles by Mooney and others (1970), and lighter lines represent seismic reflection lines used by Allen (1994). SCH, and BPF designate the approximate location of the St. Croix Horst, and the Belle Plaine Fault, respectively. 11 �GEOLOGY AND GEOCHEMISTRY OF THE ROVE BASALTS, A POSSIBLE MIDCONTINENT RIFT-RELATED EXTRUSIVE UNIT SOUTH OF THUNDER BAY, ONTARIO CUNDARI, Robert, HOLLINGS, Peter Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, and SMYK, Mark Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada A unit of mafic rock in Devon Township, south of Thunder Bay, was mapped by Tanton (1931) and was termed Rove Formation Basalt. Geul (1970) mapped this unit as a Logan diabase sill. The primary objective of this study was to characterize the unit and test whether it is of intrusive or volcanic origin. This was achieved by mapping the unit at a scale of 1:20 000 (Fig. 1), and through petrographic and geochemical analysis. The unit is exposed on a plateau 7 km long and 0.8 to 1.0 km wide. The unit is 4 to 6 m thick and is in apparent conformable contact with the underlying shales of the Paleoproterozoic Rove Formation, where a pronounced chilled margin consists of variolitic material up to 20 cm thick. The flow-top also exhibits a variolitic texture up to 15 cm thick. The presence of ropy flow top and amygdules, as well as quench textures, support a volcanic origin. Figure 1: Map of the Study Area at 1:20000 scale (modified after Tanton, 1931) Major element chemistry reveals a tholeiitic, intermediate composition with samples plotting in the basaltic andesite to andesite fields as well as in the basaltic trachy-andesite to trachy-andesite fields on a TAS diagram. The unit typically has an intergranular texture consisting of randomly oriented plagioclase laths with interstitial chlorite, an alteration product of primary augite. Most samples contain minor serpentine (after olivine), opaque minerals, secondary quartz, oxides, pyrite and calcite. Amygdules are present in most samples and are infilled with some combination of calcite, quartz, chlorite and pyrite. Lower flow contacts and flow-tops are typically glassy with abundant spherulites that sometimes coalesce into bands. Rare-earth element geochemistry shows the unit to be relatively enriched in both HREEs and LREEs, similar to the ultramafic sills of the Nipigon Embayment as well as the Riverdale Sill 12 �(Hollings et al., 2007, 2009) (Fig. 2). A primitive mantle-normalized REE plot shows that the volcanic unit is characteristic of an Ocean-Island Basalt (Fig. 3), but with a negative Nb anomaly, most likely the result of lower crustal contamination. This evidence is further supported by an epsilon Nd(T=1100Ma) of -3.48, which also suggests contamination of the unit by a lower crustal source. The trace element characteristics of the volcanic unit suggest an origin in Keweenawan time as they are geochemically similar to volcanic units of the Midcontinent Rift (Hollings et al., 2007) rather than Paleoproterozoic volcanic units of the Gunflint Formation. Figure 2: Gd/Ybn versus La/Smn showing comparison between the study unit and other Keweenawan Midcontinent rift-related units. Fields from Hollings et al., (2007, 2009). Figure 3: Comparison of primitive mantlenormalized plots from the study unit (A) with Ocean Island Basalt (OIB) (B) and Gunflint volcanic rocks (C). OIB data from Hollings et al. (2007), and Gunflint data from Fralick (unpublished). Field, petrographical and geochemical analysis suggests that the unit is of volcanic origin with emplacement likely during Keweenawan Midcontinent rifting. Thus, we are proposing that the unit be renamed the Devon Volcanics. References Geul, J.J.C., 1970. Geology of Devon and Pardee Townships and the Stuart Location; Ontario Department of Mines, Geological Report 87, 52 p. Hollings, P., Hart, T., Richardson, A. And MacDonald, C.A., 2007a. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayement, northwestern Ontario. Canadian Journal of Earth Sciences 44: 1087-1110. Hollings, P., Smyk, M., Halls, H. and Heaman, L. 2009. Mesoproterozoic Midcontinent Rift-related mafic intrusions in northwestern Ontario: continuing geochemical, paleomagnetic, petrographic and geochronologic studies; in Institute on Lake Superior Geology, Proceedings and Abstracts, v.54, part 1, p.42-43. Tanton T.L., 1931. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63360. 13 �GEOLOGY AND GEOCHEMISTRY OF THE HEMLO EAST PROPERTY, SCHREIBER-HEMLO GREENSTONE BELT, ONTARIO FAGE, Adam and HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada The Hemlo East property, owned by Metalcorp Limited, is located approximately 50 kilometres east of the town of Marathon, Ontario and is situated within the Schrieber-Hemlo greenstone belt of the Wawa subprovince of the Archean Superior Province. Hemlo East has been extensively explored over the past 40 years, mainly being examined for Hemlo-type gold deposits. The property is sandwiched between the Cedar Lake Pluton (2688 – 2687 Ma, Corfu and Muir, 1989) to the north and the Pukaskwa Batholith (2719 and 2688 Ma, Corfu and Muir, 1989) to the south. The geology is dominated by generally east trending and north dipping metasedimentary rocks with mafic to intermediate metavolcanic rocks and minor felsic metavolcanic rocks. High level intrusive dykes and Proterozoic diabase dykes crosscut all lithologies. The entire SchreiberHemlo Greenstone belt has been affected by lower to mid amphibolite facies metamorphism (Pan and Fleet, 1993). Tholeiitic basalts are the prevalent volcanic rock type. Mg# varies from 64 to 36 over a SiO 2 range of 45-54 wt%. Cr and Ni correlate with Mg# while Fe, Ti, Zr and all REE have a negative correlation (Fig. 1). These rocks are characterized by a near-flat pattern on a primitive mantle normalized plot and have been described as having a plume-derived oceanic plateau association by Polat (2008). Tholeiitic to calc-alkaline basalts, andesites, rhyodacites, and rhyolites make up a minor portion of the volcanic rocks and are characterized by LREE enrichment as well as Nb depletion; relative to Th and La (Fig. 1). Rhyolites also exhibit HREE depletion. These rocks posess a subductionderived oceanic island arc association (Polat, 2008). Preliminary zircon 206Pb/238U age dating on a rhyolite yielded an age of 2694.5±1 Ma. Depleted initial Nd isotopic compositions (εNd =+1.76 to +2.11) are consistent with long-term depleted heterogeneous mantle sources (Polat, 2008). The presence of both oceanic plateau and island arc associations of the volcanic rocks can be explained with a subduction zone at the edge of an oceanic plateau which led to rifting and the formation of a backarc basin; this is consistent with the findings of Polat (2008). 14 �Rock/Primitive Mantle 1000 Ocean Plateau Basalts Andesites Ocean Island Arc Basalts Rhyolites and Rhyodacites 100 10 1 0.1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Figure 1. Primitive mantle normalised diagram for average compositions of volcanic rocks of the Hemlo East property (normalising values from Sun and McDonough, 1989). References: Corfu, F. and Muir, T.L., 1989. The Hemlo-Heron Bay greenstone belt and Hemlo Au-Mo deposit, Superior Province, Ontario, Canada. I: Sequence of igneous activity determined by zircon U-Pb geochronology. Chemical Geology. 79:183-200. Fleet, M.E. and Pan, Y.,, 1991. Metamorphic Petrology of the White River Gold Prospect, Hemlo Area. Ontario Geological Survey, Grant 305, Final Report. 47 pp. Sun, S.-s., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Geological Society, Special Publication No.42: 313-345. Polat, A., 2008. The geochemistry of the Neoarchean (ca. 2700Ma) tholeiitic basalts, transitional to alkaline basalts, and gabbros, Wawa Subprovince, Canada: Implications for petrogenetic processes. Precambrian Research. 168:83-105. 15 �GEOLOGIC MAPPING OF ARCHEAN AND PROTEROZOIC ROCKS NEAR BINGSHICK LAKE, BOUNDARY WATERS CANOE AREA WILDERNESS, BY STUDENTS OF THE PRECAMBRIAN RESEARCH CENTER’S 2009 FIELD CAMP FAHRENKROG, Brooke1, TOTENHAGEN1, Mike, JARET1, Steven , WATSON1, Tabitha, and JIRSA2, Mark A. 1 2009PRC Students 2 Minnesota Geological Survey The Precambrian Research Center—a branch of the University of Minnesota, Duluth—conducted its third season of field camp in 2009. After 5 weeks of field training, students were assigned ―Capstone Projects‖ that provide an opportunity to create new geologic maps in areas of poorly understood geology. The students mapped Archean and Proterozoic bedrock in the Bingshick Lake area (Figure 1) that was impacted by forest fires in 2006 and 2007. The fires greatly improved access to and visibility of geologic features, allowing detailed mapping of rock units and illuminating complex contact relationships. Figure 1. Regional geology (simplified from Jirsa and Starns, 2008) showing map area of 2009 PRC students. Listed below, from youngest to oldest, are the principle map units: Mesoproterozoic Heterogeneous troctolite of the Tuscarora Intrusion of Duluth Complex: Light grey to grey, medium to coarse grained, heterogeneous troctolite with localized pegmatitic patches and ophitic clinopyroxene. The 16 �highly variable unit contains localized zones of augite-troctolite, mela-troctolite, and troctolite, with sulfide-rich patches occurring locally. Light orange oxidation staining occurs on weathered surfaces. Diabase dike (age uncertain): Consists of fine- to medium-grained, dark-colored mafic rock, with aphanitic chilled contacts against country rocks. Paleoproterozoic Gunflint Iron Formation: Grey to white, fine to coarse grained, Superior-type banded iron-formation. The unit is weakly to strongly magnetic. It consists of alternating medium regular-bedded slaty magnetite rich beds and massive cherty beds locally containing magnetite halos, diffuse structures and intraclasts of slate. Coarse magnetite and quartzite grains were observed near baked zones along the contact with younger troctolite. Red oxidation surfaces occur along the baked zone and on weathered surfaces. Rove Formation: Light to medium gray, brecciated or conglomeratic, biotite-rich schist. Neoarchean Trachyandesite of the Jasper Lake sequence (of Jirsa and Starns, 2008): Tan, fine to medium grained, heterogeneous trachyandesitic volcanic conglomerate and breccia. Angular to subangular fragments (up to 1 foot in diameter) ranging widely in composition from felsic to intermediate to mafic. Interbedded layers (as much as to 1 foot in thickness) of siltstone occur locally. Light chlorite and actinolite alteration occur throughout unit. Metagabbro of the Paulsen Lake sequence (of Jirsa and Starns, 2008): Grey, medium-grained, subophitic metagabbro. The unit displays what appears to be chlorite-actinolite alteration that locally replaced as much as 40% of precursor pyroxene. Metabasalt of the Paulsen Lake sequence: Light green to grey, aphanitic to fine-medium grained, metabasalt, with heavy chlorite and quartz-epidote alteration present throughout. The unit is weakly foliated and alternates between pillowed, massive, and pillow breccia flows. The most significant findings of our field research involved more accurate positioning of previously defined contacts, and investigating those contacts to shed light on the apparent age relationships between units. The main findings are as follows: 1) Discovery of an angular unconformity between pillowed and autobrecciated mafic-ultramafic flows of the Archean Paulsen Lake sequence and younger, but also Archean volcanic conglomerate of the Jasper Lake sequence. Some of the alteration in the older volcanic strata beneath the unconformity may be the result of paleoweathering during the hiatus. These attributes are similar to those described by Jirsa and Driese, 2009 (ILSG) for the post-2689 Ma erosional unconformity between the Ogishkemuncie conglomerate and Saganaga Tonalite. 2) Discovery of breccia at the stratigraphic top of Gunflint Iron Formation that may be related to 1850 Ma Sudbury meteorite impact event. The breccia occurs within a few hundred feet of the basal Duluth Complex, and therefore is highly metamorphosed. 3) The observation at the western edge of the map area that cherty units of the Gunflint Iron Formation lie directly on eroded Archean bedrock, with no intervening clastic horizon. This contrasts with areas to the east near the Gunflint trail and west on the Mesabi Iron Range where clastic units (Kakabeka Conglomerate and Pokegama Quartzite) occur at the unconformity. REFERENCES Jirsa, M.A., and Driese, S.G., 2009, Neoarchean weathering and atmospheric pO2 inferred from paleosaprolite between granite-greenstone and superjacent conglomerate in the Boundary Waters Canoe Area, NE Minnesota: Institute on Lake Superior Geology Proceedings 55:50-51. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Lake fire area, parts of Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake 7.5 minute quadrangles, northeastern Minnesota: Minnesota Geological Survey Open-File Report OF08-05, scale 1:24,000. 17 �SEDIMENTOLOGY OF THE PALEOPROTEROZOIC EJECTA LAYER FROM THE SUDBURY IMPACT EVENT Fralick, Philip, Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Brumpton, G.R., 211 Henry St., Thunder Bay, ON, P7E 4Y7 Jirsa, M.A., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN, 55114-1057 Kissin, S.A., Department of Geology, Lakehead University, Thunder Bay, ON, P7B 5E1 Severson, M.J., Natural Resources Research Inst., 5013 Miller Trunk Hwy., Duluth, MN 55811 Assistance in the field was provided by: W.D. Addison, W.F. Cannon, K.J. Schulz and L.G. Woodruff At 1850 Ma an object, probably in excess of ten kilometers in diameter, struck the edge of Superior Craton in the area that is now Sudbury, Ontario. The impact produced the second largest known crater on the surface of the Earth, propelling an immense amount of material into the atmosphere. The area four to seven hundred kilometers west of the impact site consisted of a peneplained terrain that was sub-aerial in its northern extremity, with lithified Gunflint Formation bedrock, and possibly flooded with very shallow marine conditions in its central and southern portions. The ejecta cloud swept across this setting, entraining locally derived material. In places the impact layer deposited by this surge was altered by possible tsunami activity, sub-aerial reworking and diagenesis, and fifteen million years later was buried by sediment of the transgressing ocean. Addison et al. (2005) first discovered this layer, which has since been described at other locations by Pufahl et al. (2007), Cannon et al. (2010) and Addison et al. (in press). To date over twenty outcrop and core sections through the layer have been found. It is extremely laterally variable, ranging from totally absent in some sections to tens of meters in thickness in others. Sections within a few hundred meters of one another can be composed of different material with contrasting textures and sedimentary structures. Common components of the deposits are: 1) pebbles to boulders of predominantly locally derived chert and carbonate, 2) pebble to sand-sized devitrified glass, 3) 3 to 20mm accretionary lapilli, 4) unshocked quartz and feldspar grains, and 5) quartz with planar deformation features (PDFs). Where pebble-sized devitrified glass is common accretionary lapilli are usually absent. Conversely, sections with lapilli in grain-support commonly do not contain devitrified glass. Massive carbonate and silica replacement has occurred. In sub-aerially exposed sections blocky calcite cements developed first, then this was overprinted (including lapilli) by quartz and iron carbonate. This lithified material occasionally locally collapsed forming boulder piles along what were probably fault scarps. The two most typical types of successions are: 1) up to 7 meter thick boulder conglomerates with devitrified glass +- lapilli, and 2) thin (averaging approximately 50 cm) graded sand- to granule-sized material with lapilli. The thick successions commonly overlie fractured to shattered bedrock with large rotated blocks. In other areas the boulder conglomerates overlie what had been water-saturated sediment that liquefied allowing sinking of interstratified, brecciated, chert layers. Models suggest this area would have experienced an earthquake of approximately magnitude 10, quite capable of producing these effects. The overlying disorganized pebble to boulder conglomerate contains clasts up to 3.5 meters in length that are commonly in matrix support, but in some portions of the sections are in clast support. Pebbles of devitrified glass can be mixed throughout the matrix or may appear only in the upper half of the conglomerate where they are commonly small pebble- to granule-sized. In some sections the conglomerates are organized as a series of stacked lenses that decrease in size upwards with some small lenses near the top filled with the first appearance of accretionary lapilli. In other sections lapilli are scattered between the disorganized boulders of the massive conglomeratic deposit. At some locations, approximately one meter of clast-supported, parallel laminated to trough cross-stratified lapilli, with possible chute and pool structures, interlayered with sand- to granule-sized material, overlies the conglomerate. Alternatively, at some locations it is overlain by a massive layer of devitrified glass 18 �pebbles or a thick assemblage of massive or large-scale cross-stratified, sand-sized material. These thicker successions appear to fill depressions. The thinner sections of impact related debris mostly overlie intact bedrock, which apparently was swept clean of the earthquake rubble. They commonly show grading, consisting of either: 1) one or more successions that are normally graded from sand and granules, with lapilli, to silt, or 2) are reverse then normally graded with the granules and lapilli concentrated in the middle of the tens of centimeters thick unit. Without modern analogues interpretation of these successions is difficult. The presence of areas with slickensides and chute and pool structures in some of the outcrops denotes deposition by a high velocity base surge, whereas multiple graded units in some drill-holes are possibly the product of tsunami activity. It is unlikely some other sections represent tsunami deposits as the accretionary lapilli and glass fragments would have arrived in the area via ballistic trajectories and the surge cloud prior to arrival of the tsunami. Thus, any tsunami deposit should have some of this material incorporated throughout it. However, in many sequences the glass and lapilli are not present near the base, making tsunami deposition of sediment in these sections unlikely. Alternatively, in base surges, steam with few solid particles forms the leading front of the cloud followed by the ejecta-rich portion. It is reasonable to visualize the earthquake-generated loose debris being initially redeposited in hollows by the steam-surge, with glass and lapilli deposited from the main surge cloud mixed with the local sediment capping this. As the velocity of the debris cloud decreased and the terrain developed at least a partial covering of impactrelated sediment the abundance of locally derived chert and carbonate grainstone clasts diminished and better sorted layers rich in either glass or accretionary lapilli developed. A further decrease in the velocity of the blast cloud resulted in a transition from pebble- and granule-sized material to sand-sized glass fragments and other debris. Tsunami reworking of these successions into more chaotic masses in some areas is possible. Deposition on a locally fault brecciated erosional surface with topographic relief led to rapid lateral changes in lithofacies and thickness. It is likely that the thicker, coarser basal units are confined to topographic lows, as, in places, they can be seen to pile-up against bedrock cliffs facing the impact site. The thin, reverse-graded ejecta layers are typical of small volume base surges. These probably formed from the lower density tail of the flow on topographically higher areas that were erosively swept clean by the main flow. Reworking of some of the deposits in whole or in part by gravity sliding or possibly fluvial activity is also indicated. References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, v. 33, p. 193-196. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W. and Kissin, S.A., in press. Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: are they base surge deposits or tsunami deposits? In, eds. W.U. Reimold and R.L. Gibson, Large Meteorite Impacts and Planetary Evolution IV, Geological Society of America Special Paper 465. Cannon, W.F., Schulz, K.J., Horton, J.W. and Kring D.A., 2010. The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, U.S.A. Geological Society of America Bulletin, v. 122, p. 50-75. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J. and Edwards, C.T., 2007. Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan. Geology, v. 35, p. 827-830. 19 �ANATOMY OF AN ARCHEAN CARBONATE PLATFORM: GEOLOGY OF THE STEEP ROCK GROUP, CANADA FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, philip.fralick@lakeheadu.ca And RIDING, Robert, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA Carbonate platforms capping oceanic islands and plateaus are not uncommon in the modern ocean. However, they are rare in the rock record of the Archean. The 200 to possibly 500 meter thick succession of calcium carbonate rocks present at the long abandoned Steep Rock iron mine, Ontario, Canada, is one of the few such examples of an extensive Mesoarchean-Neoarchean limestone platform. The approximately 2.8 Ga sedimentary assemblage is dominated by limestones that unconformably overlie 3.0 Ga tonalitic gneiss incised by a paleochannel network backfilled with fluvial sandstones and conglomerates. The lower meters of the limestone contains scattered tonalitic sand and gravel, but the remainder has very low amounts of siliciclastic material. Wilks and Nisbet (1985, 1988) have provided excellent descriptions of the Steep Rock platform succession; from Stratifera-like and Irregularia-like structures that pass upward to laterally linked hemispherical stromatolites, branching walled and unwalled columnar forms and giant domes. They also noted the presence of crystal fans, which are probably the deposits originally described as the supposed fossil Atikokania by Walcott (1912). The present study adds details of lithotypes, and suggests the presence of grainstones in the interval underlying the giant domes and possibly interlayered with some domes. Mudcrack-like structures were observed on the upper surface of one dome. Giant domes dominate the upper 70 m of the succession. They are persistently elongated, suggesting current-influence, and range up to 5 m in length, 2 m in width and 1.4 m in height. Inter-dome sediment is usually lacking. The giant domes are distinctly banded and composed of centimeter-decimeter layers of crystal fans alternating with cuspate fenestral fabric (fenestrate microbialite, Sumner, 2000; Sumner and Grotzinger, 2000). Large domes are not developed in lithologies that lack cuspate fenestral fabric, and these show a variety of centimetric interlayers that include wavy, irregularly fenestral, and possibly granular fabrics, together with thin sheet cracks. Twenty-seven samples from various lithofacies were analyzed for major and trace elements and carbon isotope ratios. Carbon varies from delta 0 to 2.7, with the lightest values consistently present in the columnar stromatolites and the heaviest in the crystal fans. There is a progression from lighter carbon isotope values at the base of the section upward to heavier values at the lowest zone of crystal fans. Above this the values rapidly decline to near 0 where the columnar stromatolites are located, and then gradually rise again to the heavy carbon enriched upper third of the section dominated by the crystal fans. Strontium and barium concentrations have similar trends, whereas the concentrations of iron, and to a lesser extent manganese, behave in an inverse manner. Iron carbonate present in iron formation overlying the limestone has low levels of Sr and Ba, and contains significant amounts of Fe and to a lesser degree Mn. Its carbon isotopic ratio is -5.6, which is similar to values derived from other iron formations of similar age. 20 �Fralick et al. (2008) suggested that the oceanic plateau was drowned during isostatically driven subsidence. It is clear that carbonate deposition was able to keep pace with sea-level during most of the subsequent load-driven subsidence of the carbonate platform. The dominance of calcite rather than an iron carbonate in the sequence indicates lower levels of Fe+2 dissolved in the water over the shallow plateau than deeper portions of the plateau where siderite was accumulating. This possibly reflects a sub-oxic, shallow water zone created by photosynthetically released oxygen together with restricted circulation above a chemocline. Heavier carbon isotopic ratios could reflect evaporitic loss of C12 leading to aragonite crystal fan precipitation. Low Sr contents indicate that the non-crystal fan carbonates were deposited as calcite, except for the columnar stromatolites that are dolomite and may have been deposited as dolomite in the low sulfur Archean ocean. These stromatolites also have the lowest C isotopic ratio and high concentrations of iron and manganese. This suggests less saline conditions and better connection with the open ocean during deposition of the columnar stromatolites. A possible relationship between stromatolite morphology and salinity further indicates the organisms that constructed these features may not have been very salt tolerant. Conversely, they may have been iron or manganese oxidizers that could not survive when water on the shallow portions of the plateau became more stagnant and concentrations of Fe and Mn in solution dropped. The emerging picture is of an oceanic platform where carbonate deposition outpaced subsidence, sustaining very shallow water conditions that limited incursion of Fe- and Mn-rich open ocean water, increased salinity, and promoted precipitation of aragonitic fans. Carbonate deposition ended when the balance controlling water level switched in favor of relative sea-level rise. Deeper water conditions allowed the chemocline to advance over the area and iron formation deposition ensued References Fralick, P., Hollings, P. and King, D., 2008. Stratigraphy, geochemistry and depositional environments of Mesoarchean sedimentary units in western Superior Province: Implications for generation of early crust. In, Ed. K.C. Condie and V. Pease, When did Plate Tectonics Begin on Planet Earth? G.S.A. Spec. Paper 440, p. 77-96. Sumner, D.Y., 2000. Microbial vs environmental influences on the morphology of late Archean fenestrate microbialites. In, Ed. R.E. Riding and S.W. Awramik, Microbial Sediments, Springer, Berlin, p. 307-314. Sumner, D.Y. and Grotzinger, J.P., 2000. Late Archean aragonite precipitation: petrography , facies associations, and environmental significance. In, Ed. J.P. Grotzinger and N.P. James, Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World. SEPM Spec. Pub. 67, p. 123-144. Walcott, C.D., 1912. Notes on fossils from limestones of Steeprock Series, Ontario. Geol. Sur. of Can., Mem. 28, p. 16-23. Wilks, M.E. and Nisbet, E.G., 1985. Archean stromatolites from the Steep Rock Group, northwestern Ontario, Canada. Can. Jour. of Earth Sc., v. 22, p. 792-799. Wilks, M.E. and Nisbet, E.G., 1988. Stratigraphy of the Steep Rock Group northwestern Ontario: A major Archean unconformity and Archean stromatolites. Can. Jour. of Earth Sc., v. 25, p. 370-391. 21 �EFFECTS OF CONTACT METAMORPHISM BY THE DULUTH COMPLEX ON PROTEROZOIC FOOTWALL ROCKS IN NORTHEASTERN MINNESOTA FROST, Shelby J.1, GOODGE, John W.1, SWENSON, John B.1, (1) Department of Geological Sciences, University of Minnesota – Duluth, 1114 Kirby Drive, Duluth, MN 55812, Frost113@d.umn.edu The Duluth Complex is composed of numerous mafic intrusions that were emplaced in northeastern Minnesota during formation of the Midcontinent Rift approximately 1.1 Ga (Miller et. al., 2002). When it intruded, the heat of this igneous body significantly affected the wall rocks around it, including those in the Animikie Group, and created a contact metamorphic aureole. The purpose of this research is to understand crustal conditions associated with emplacement of a large igneous complex, and develop a better idea of the thermal state of the crust during the time of rifting. To understand crustal conditions we must determine the effects that intrusion of the Duluth Complex had on adjacent wall rocks, in particular the Thomson and Virginia formations of the Animike Group. These effects include the extent and grade of metamorphism. Optical petrography was used to determine mineralogical and textural relationships between minerals. The Animikie Group was subjected to two metamorphic events based on sedimentary features such as relict sedimentary bedding, regional metamorphic features such as disrupted bedding, and contact metamorphic features such as porphyroblasts and mortar texture. Scanning electron microscopy and EDS analysis helped identify major metamorphic minerals including rare high-K cordierite. A thermal model was constructed to model heat flow from the Duluth Complex into the wall rocks, and mimic a thermal gradient similar to the one observed in nature. The two-dimensional model was a very simplified version of the parameters of the intrusion observed in nature, and ltimately we obtained discordant results that we can‘t explain. Based on this petrographic study, the contact metamorphic aureole extends from the Duluth Complex into the Animikie Group for approximately 30 m. This coincides with estimates made by Severson (1995) and Duchesne (2004). The aureole is so thin because the wall rocks were fairly cool when the Duluth Complex intruded, and probably had a temperature around 75 C. The peak metamorphic mineral assemblage includes minerals such as ferrosilite, gedrite, and cordierite, which occur at temperatures around 600-700 C and a pressures around 2.5 Kbar (Duchense, 2004). References Duchesne, L., (2004). Fusion partielle et microstructures associees dans l‟aureole de contact du complexe igne de Duluth, Minnesota. University of Quebec Masters thesis, 217 p. Miller, J. D. Jr., and Severson, M. J., (2002) Geology of the Duluth Complex in Miller, J. D. Jr., Green, J. C., Severson, M. J., Chandler, V. W., Hauck, S. A., Peterson, D. M., and Wahl, T. E., (2002). Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. University of Minnesota Report of Investigations 58. 207 p. Severson, M. J., (1995). Geology of the southern portion of the Duluth Complex. Natural Resources Research Institute Special Report, 217 p. 22 �GEOLOGY AND GEOCHEMISTRY OF ARC AND OCEAN-FLOOR VOLCANIC ROCKS IN THE NORTHERN FLIN FLON BELT, MANITOBA. GILBERT, Paul Manitoba Geological Survey (360-1395 Ellice Ave. Winnipeg Manitoba R3G 3P2) The Flin Flon Belt is one of the most prolific mining districts in Canada, with combined reserves and production to 2007 of 183 million tonnes of Cu-Zn-Au-Ag ore from 26 different deposits. The Flin Flon Belt has a protracted magmatic history spanning approximately 60 million years (Stern et al., 1999). Juvenile arc magmatism (1.90–1.88 Ga) in an intra-oceanic setting was characterized by tholeiitic to calcalkaline and late alkaline (shoshonitic) volcanism, accompanied by rifting and subsequent back-arc basin development. Collisional tectonism, development of a 1.87–1.84 Ma ‗successor arc‘, and late brittle deformation resulted in the present complex structural geometry of the belt. Detailed geological mapping and numerous research projects have focused largely on the central part of the belt where almost all of the ore deposits are located. This paper is based on mapping of the northern Flin Flon Belt (NFFB), in which an approximately 15 km by 35 km area contains more than 20 tectonically distinct blocks or fault slices of diverse volcanic, related intrusive and subordinate sedimentary rocks. Most of these components are akin to modern analogues of arc and MORB volcanic rock types (Figure 1) and are part of the Flin Flon arc assemblage (Stern et al., 1995a). Mid-ocean ridge (MORB)–like volcanic rocks are structurally intercalated with juvenile arc rocks, and an extensive area of depleted-MORB-type rocks (Dismal Lake Terrane) extends along and across the northern boundary of the Flin Flon Belt, where it is locally in fault contact with paragneiss and orthogneiss of the Kisseynew Domain to the north (Gilbert, 2004). Arc-type rocks in the NFFB are assumed to be associated with subduction at a former oceanic arc, whereas MORB and depleted-MORB types are interpreted to have been associated with rifting and emplacement in a back-arc basin environment. Volcanic rocks of the Flin Flon arc assemblage are 1890 to 1878 Ma in age (Lucas et al., 1994; Stern et al., 1999; N. Rayner, pers. comm., 2009). MORB-type rocks in the NFFB may be roughly coeval with the arc assemblage, based on the 1.90 Ga age of synvolcanic intrusions in MORB-type volcanic rocks elsewhere in the Flin Flon Belt (Elbow-Athapapuskow ocean-floor assemblage; Stern et al., 1995b). Six lensoid enclaves of turbiditic sedimentary rocks are structurally intercalated with the volcanic fault blocks in the NFFB. These turbidite deposits, apparently devoid of volcanic interlayers, are distinguished from lithologically similar but older, fine-grained sedimentary rocks that occur within 1890-1878 Ma, juvenile arc volcanic sequences. Detrital zircon data from greywacke within the westernmost turbidite enclave suggest the depositional age likely does not exceed 1860 Ma (N. Rayner, pers. comm., 2009) and that the turbidites are thus of successor-arc age. Turbidite fault slices elsewhere in the NFFB may also be of successor-arc age or relatively older, penecontemporaneous with juvenile arc volcanism (e.g., the 1885 Ma Vick Lake tuff; Stern et al., 1993). Fluvial-alluvial conglomerate and cross-bedded sandstone (Missi Group) at the west margin of the Flin Flon Belt are the youngest known supracrustal rocks in the NFFB. These sedimentary deposits and associated volcanic rocks are unconformable or in fault contact with volcanic rocks of the Flin Flon arc assemblage and were deposited between 1845 and 1835 Ma (Ansdell et al., 1999). 23 �Figure 1: Th-Hf-Nb diagram showing the distinctive fields of arc and MORB-type volcanic rock suites in the northen Flin Flon Belt. Compositional fields of modern volcanic rocks after Wood (1980). Abbreviations: E-MORB, enriched mid-ocean ridge basalt; N-MORB, normal mid-ocean ridge basalt. References Ansdell, K.M., Connors, K.A., Stern, R., and Lucas, S.B., 1999; Coeval sedimentation, magmatism, and fold- thrust belt development in the Trans-Hudson orogen: Geochronological evidence from the Wekusko Lake area, Manitoba, Canada: Canadian Journal of Earth Sciences, vol. 36, p. 293-312. Gilbert, H.P. 2004: Geological investigations in the northern Flin Flon Belt, Manitoba (parts of NTS 63K13NE and 63K14NW); in Report of Activities 2004, Manitoba Industry, Economic Development and Mines, Manitoba Geological Survey, p. 9–23. Lucas, S.B., Stern, R.A., Syme, E.C., Reilly, B.A. and Thomas, D.J. 1996: Intraoceanic tectonics and the development of continental crust: 1.92–1.84 Ga evolution of the Flin Flon Belt, Canada; Geological Society of America Bulletin, v. 108, p. 602–629. Stern, R.A., Lucas, S.B. Syme, E.C., Bailes, A.H., Thomas, D.J., Leclair, A.D., and Hulbert, L. 1993. Geochronological studies in the NATMAP Shield Margin Project area, Flin Flon Domain: results for 1992–1993. In Radiogenic age and isotopic studies: Report 7. Geological Survey of Canada, Paper 93-2, p. 59-70. Stern, R.A., Machado, N., Syme, E.C., Lucas, S.B. and David, J. 1999: Chronology of crustal growth and recycling in the Paleoproterozoic Amisk Collage (Flin Flon Belt), Trans-Hudson Orogen, Canada. Canadian Journal of Earth Sciences, v. 36, p. 1807-1827. Stern, R.A., Syme, E.C., Bailes, A.H. and Lucas, S.B. 1995a: Paleoproterozoic (1.90–1.86 Ga) arc volcanism in the Flin Flon Belt, Trans-Hudson Orogen, Canada; Contributions to Mineralogy and Petrology, v. 119, p. 117–141. Stern, R.A., Syme, E.C. and Lucas, S.B. 1995b: Geochemistry of 1.9 Ga MORB- and OIB-like basalts from the Amisk Collage, Flin Flon belt, Canada: evidence for an intra-oceanic origin; Geochimica et Cosmochimica Acta, v. 59, p. 3131–3154. Wood, D.A. 1980: The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters, v. 50, p. 11-30. 24 �U-PB ZIRCON GEOCHRONOLOGY OF THE DULUTH COMPLEX AND RELATED HYPABYSSAL INTRUSIONS: INVESTIGATING THE EMPLACEMENT HISTORY OF A LARGE MULTIPHASE INTRUSIVE COMPLEX RELATED TO THE 1.1 GA MIDCONTINENT RIFT Hoaglund, S.A.a*, Miller, J.D.a, Crowley, J.L.b, and Schmitz, M.D.b a Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 USA b Department of Geosciences, Boise State University, Boise, ID 83725 USA Previous geochronology of mafic intrusive magmatism associated with the 1.1 Ga Midcontinent Rift in NE Minnesota by Paces and Miller (1993) established, with high precision, the main intrusive periods that created the Duluth Complex and related intrusions. This study did not, however, resolve differences in emplacement ages within and between major intrusive units due to a small number of dated samples. We present new high-precision isotope dilution U-Pb baddeleyite and CA-TIMS zircon ages from four mafic intrusions that build on the work of Paces and Miller (1993). Three of these are from troctolitic intrusions (Partridge River, Bald Eagle, Tuscarora) belonging to the layered series of the Duluth Complex (DC) that, based on field relationships, appear to represent the entire range of layered series intrusive activity. They yielded irresolvable weighted mean Pb207/Pb206 ages between 1098.8 ± 0.3 and 1098.0 ± 0.7 Ma (Table 1). These ages fall in line with the 1098.6 ± 0.5 Ma and 1099.3 ± 0.3 Ma ages reported by Paces and Miller for two layered series intrusions (Figure 1). Moreover, these ages are irresolvable from the ages of two DC anorthositic series samples analyzed by Paces and Miller (1099.0 ± 0.6 Ma and 1099.1 ± 0.5 Ma) and recently confirmed by unpublished data from the Boise State University isotope laboratory. An early intrusion from the Beaver Bay Complex (BBC), the Houghtaling Creek troctolite, was also dated in this study to test the implication from Paces and Miller‘s study that a 3 Ma magmatic hiatus exists between emplacement of the deeper seated DC and the more hypabyssal BBC. The Houghtaling Creek sample yielded an age, which is irresolvable from the Duluth Complex ages and thus implies that BBC and DC magmatism likely overlaps. The evidently synchronous emplacement of layered series and anorthositic series intrusions indicate that voluminous main stage magmatism occurred during an extremely short interval of within 1 Ma. Reasonable estimates of the geometry of the dozen or so layered series intrusions that comprise the DC indicate that over 15,000 km3 of mafic magma intruded during emplacement of the layered series alone. The total volume of intruded magma is likely much higher when erosion is factored in and rocks of the anorthositic series and early BBC are included. 25 �Table 1. Summary of high-precision U-Pb zircon/baddeleyite data for the Duluth Complex and Beaver Bay Complex intrusions. All errors are reported at the 95% confidence interval. Sample Namea WLFG HCT BEI PRI TI ES Stratigraphic Unit Rock Type Early Phase (BBC) Early Phase (BBC) Late Phase (DC) Early Phase (DC) Early Phase (DC) Misc. intrusion ferrogabbro augite troctolite olivine gabbro augite troctolite troctolite augite troctolite Grain Type zrc./bad. zrc. zrc. zrc. bad. zrc. Pb207/Pb206 age (Ma) 1095.75 ± 0.92 1098.62 ± 0.50 1097.97 ± 0.72 1097.98 ± 0.37 1098.81 ± 0.32 not defined Pb206/U238 age (Ma) 1091.88 ± 0.35 1095.31 ± 0.25 1095.64 ± 0.19 1095.94 ± 0.18 not defined not defined a WLFG = Wilson Lake ferrogabbro; HCT = Houghtaling Creek troctolite; BEI = Bald Eagle intrusion; PRI = Partridge River intrusion; TI = Tuscarora intrusion; ES = Endion sill Figure 1. Summary of weighted mean Pb207/Pb206 ages for Duluth Complex and Beaver Bay Complex magmatism from Paces and Miller (1993) and this study (modified from Miller, 2008, unpublished figure). 26 �RADIOGENIC ISOTOPE CHARACTERISTICS OF MIDCONTINENT RIFTRELATED INTRUSIONS SOUTH OF THUNDER BAY HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada, SMYK, Mark C., Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada. New geochronological, geochemical and paleomagnetic data for Mesoproterozoic Midcontinent Rift-related dikes and sills located in and south of Thunder Bay, Ontario demonstrate the presence of three sill suites: an earlier, spatially restricted mafic to ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the northern part of the study area (Hollings et al., 2010). In addition, three predominant dike sets are recognized: the northeasttrending Pigeon River swarm, the northwest-trending Cloud River dikes and the Mt. Mollie dike (Fig. 1). Figure 1: Geological map of the study area showing the location of major intrusive suites. Modified after Pye and Fenwick (1965). Thirty-four Rb-Sr and Sm-Nd isotope analyses were undertaken on the intrusive units south of Thunder Bay with the full data set reported in Smyk and Hollings (2009). Initial 87Sr/86Sr ratios (Sri) for the Logan Sills range from 0.7048-0.7115 and epsilon Nd(T=1100Ma) from -1.4 to +0.4, whereas the Riverdale Sill is characterised by smaller range of Sr i (0.7037-0.7055) and more negative epsilon Nd(T=1100Ma) (-1.6 to -1.9). The epsilon Nd values for both suites are broadly similar to those of the Nipigon sills reported by Hollings et al. (2007) but the Logan sills are characterised by significantly higher Sri values (Fig. 2). TDM values for the Logan and Riverdale sills are in the range of 1750-1850 Ma which are considerably younger than the generally Archean TDM values of the sills of the Nipigon embayment. The younger TDM and the elevated 27 �Sri of the southern sill suites are consistent with contamination by Rove shales which have Sr i of up to 0.7207 and epsilon Nd(T=1100Ma) of -9.3 (Fig. 2). All three dike suites are characterised by highly variable Sr i and epsilon Nd(T=1100Ma) values with epsilon Nd ranging from positive to negative in all three suites (Fig. 2). Samples with strongly negative epsilon Nd values are also characterised by strongly enriched Th abundances, negative Nb anomalies and older (~2200 Ma) TDM ages, consistent with contamination by a deeper and older source, likely Archean basement. The dikes with less negative epsilon Nd values have similar major and trace element geochemistry to the strongly contaminated samples suggesting they were derived from the same mantle source but were emplaced after limited interaction with the underlying crust. This supports geochronological data presented by Hollings et al. (2010) which suggested that the dike swarms were emplaced over an extended period. Figure 2: 87 Sr/86Sr i and eNd(T=1100Ma) for intrusive Midcontinent Rift rocks south of Thunder Bay. Fields from Hollings et al. (2007). References Hollings, P., Richardson, A., Creaser, R., and Franklin, J., 2007. Radiogenic isotope characteristics of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1111-1129. Hollings, P., Smyk, M., Halls, H., and Heaman, L., 2010. The geochemistry, paleomagnetism and geochronology of the dykes and sills associated with the Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, in press, doi:10.1016/j.precamres.2010.01.012. Pye, E. G., and Fenwick, K. G., 1965, Atikokan-Lakehead sheet, Kenora, Rainy River and Thunder Bay districts; Ontario Department of Mines, Geological Compilation Series, Map 2065, scale 1 inch = 4 miles. Smyk, M. and Hollings, P., 2009. Project Unit 08-021. Mesoproterozoic Midcontinent Rift–Related Mafic Intrusions Near Thunder Bay: Update. Summary of Fieldwork and Other Activities 2009, Ontario Geological Survey, Open File Report 6240, p. 11-1 to 18-5. 28 �TACONITE-DERIVED MINERAL DUST IN POPULATION CENTERS ON MESABI IRON RANGE: AN UPDATE HUDAK, George, DIEDRICH, Tamara, MONSON GEERTS, Stephen, SCHREIBER, Megan, ZANKO, Larry, and SCHWANKE, Alanna, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 The Natural Resources Research Institute (NRRI) is conducting a detailed characterization of dust that is produced from mining and processing Biwabik Iron Formation ore. This three-year long study is an effort to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR). NRRI‘s research will characterize airborne particulate matter within taconite operations, in communities surrounding taconite operations on the Mesabi Iron Range (MIR) and in population centers in other regions of northeastern Minnesota, as well as particulate matter deposited in lake sediments. NRRI‘s sampling and characterization work represents the community/environmental component of a larger University of Minnesota (U of M) effort that is addressing long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore) on human health. The U of M School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Air sampling is performed within taconite operations and communities by NRRI scientists during both winter and summer seasons. MIR community sampling sites include: 1) Keewatin Elementary School; 2) Hibbing High School; 3) the Virginia Courthouse; 4) the Babbitt Municipal Building; and 5) Silver Bay High School. Population centers in northeastern Minnesota include: 1) the Ely Fernberg site; and 2) the Natural Resources Research Institute in Duluth, MN. Sampling sites within taconite operations include crushers, magnetic separators/concentrators, agglomerators/ball drums, and the kiln/pellet discharge area. Taconite processing plant sites, from west to east, include: 1) U.S. Steel‘s Keewatin Taconite (Keetac); 2) Cliff‘s Natural Resources‘ Hibbing Taconite (Hibtac); 3) U.S. Steel‘s Minntac; 4) Cliff‘s Natural Resources‘ United Taconite (UTAC); 5) ArcelorMittal‘s Minorca Mine; and 6) Cliff‘s Natural Resources‘ Northshore Mine. Particulate matter from air samples is collected using a microorifice uniform deposit impactor (MOUDI) (Marple et al., 1991) in conjunction with a total suspended particulate filter (TSP). Size-fractionated samples collected from the MOUDI, as well as the total filter, are evaluated via gravimetric analysis and subsequently subjected to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health‘s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization‘s Method 10312 Ambient air – Determination of Asbestos Fibres – Direct-Transfer TEM Method (ISO 10312, 1995). 29 �Additional analyses are completed on relevant metamorphosed and unmetamorphosed Biwabik Iron-Formation diamond drill core. Analyses include: 1) optical and electron microscope studies of polished thin sections; 2) TEM studies of liberated elongated mineral particles produced by the modified elutriator method (Berman and Kolk, 2000); and 3) x-ray diffraction evaluations of crushed material. The lake sediment component of NRRI‘s work relies on studying cores collected from the eastern and western regions of the MIR to offer insights into historic production of mineral dust in the MIR. Core samples obtained from ―North-of-Snort‖ Lake near Babbitt and Silver Lake near Virginia are being analyzed for elongated mineral particles and are being age dated utilizing the 210Pb method to ensure that the oldest sediment samples pre-date MIR mining activities. The NRRI has completed numerous study tasks over the past year-and-a-half. Community air sampling is nearly completed and includes forty-eight sampling events in MIR communities and seven sampling events in the NE Minnesota communities. All operating taconite processing facilities are being sampled, and additional sampling events are planned for the mid-2010, including taconite mine road dust, drilling dust, and mine blasts. Figure 1: Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) References Berman, D. W., and Kolk, A. J., 2000, Draft: Modified elutriator method for the determination of asbestos in soils and bulk materials Revision 1: Submitted to the U.S. Environmental Protection Agency Region 8, May 23, 2000. ISO 10312, 1995, Ambient air – determination of asbestos fibres – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A microorifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRIRI-2006-02, 10 p. 30 �STRATIGRAPHY OF SUDBURY ―IMPACTITE‖ NEAR GUNFLINT LAKE, NE MINNESOTA Jirsa, Mark A., Minnesota Geological Survey (jirsa001@umn.edu) The impactite near Gunflint Lake is sandwiched between underlying, gently dipping, parallel-bedded Gunflint Iron Formation (~1878 Ma), and overlying slate and siltstone of the Rove Formation. The term ―impactite‖ is a very general one used informally here to include all facies of sedimentary rocks inferred to have been formed or deformed in response to the ca. 1850 Ma Sudbury meteorite impact. By that definition, it includes autochthonous material interpreted to be seismically folded and shattered ironformation (ejecta-absent), and overlying strata composed largely of allochthonous material derived from the impact site (ejecta-bearing) (Fig. 1). This discussion uses stratigraphic observations from recent mapping to devise a sequence of depositional events that is consistent with experimental and empirical evidence of impact processes (e.g., www.lpl.arizona.edu/impacteffects). It builds on—and in some cases should supersede—preliminary results presented in 2008 by Jirsa and others (ILSG v. 54). The impactite locality at Gunflint Lake is one of several similar exposure areas in the Lake Superior region (see Fralick and others, and Cannon and Slack; this volume). All of these deposits exhibit extreme lithologic variability from place to place within each exposure area, and between exposure areas, which is not surprising given the chaotic nature of impact processes. It should not be inferred, therefore, that the interpretations presented here necessarily apply to successions in other areas. Figure 1. Stratigraphic framework derived from 8 exposures along a 2 mile strike-length near Gunflint Lake; hung from the contact between ejecta-bearing and ejecta-absent facies of the impactite horizon (bold dashed line). 31 �Because the impactite deposits near Gunflint Lake lie within the contact metamorphic aureole of Mesoproterozoic intrusions, much of the original mineralogic features are obscured (see Weiblen and others, this volume). Despite this, macroscopic textures and sedimentological details are sufficiently preserved to convey information about protolith and depositional mechanisms. Figure 1 depicts the apparent stratigraphic relationships. In no single outcrop are all facies present; nevertheless, an approximation of temporal relationships can be inferred from the juxtaposition of two or more facies in individual outcrops. In the following section, facies are described in apparent stratigraphic order from oldest to youngest: Ejecta-Absent Contorted iron-formation facies: The uppermost layers of iron-formation are chaotically folded and exhibit both ductile and brittle behavior in close proximity at the scale of individual outcrops. The rheologic response depends on the apparent rigidity of material at the time of deformation. Silica-rich layers display brittle, shattered to semi-ductile, boudinage-like textures. By contrast, much of the ironsilicate mudstone layers behaved in a ductile fashion, locally showing evidence of fluidization and injection into superjacent strata. Folds are non-systematic in trend and style, and multiple hinge detachments occur. These attributes counter-indicate a regional tectonic origin, and instead are best viewed in the context of impact as the result of impact-generated seismicity imposed on semi-lithified substrate. Parautochthonous breccia facies: At several locations, straight-bedded iron-formation passes laterally along strike into irregular zones in which the silica-rich layers have been broken and disheveled, while still retaining some semblance of jigsaw-puzzle fit. Megabreccia facies: This term is used for breccia composed of unsorted slabs (as large as 5 m), blocks, and smaller fragments of iron-formation. The fragments are angular and in most places have random orientations. Fragments of iron-silicate mudstone typically show some evidence of semi-ductile behavior, and locally this material was fluidized to form irregular matrix. Ejecta-Bearing Mesobreccia facies: This is fragmental rock containing angular, subrounded, and amoeboid clasts (up to 5 cm long) of dark green material and scattered accretionary lapilli. Petrography shows that much of the original structure of the clasts has been metamorphically recrystallized and annealed; however, relicts of amygdaloidal and fluid-looking textures remain—implying a glassy protolith. Lapillistone-gritstone facies: Accretionary lapilli as large as 1.5 cm occur as irregular masses and layers interbedded with sandy to silty gritstone. In areas least affected by metamorphism, several grains of shocked quartz with planar deformation features have been identified in thin section. Spherule, pellet, small lapilli facies: The upper parts of ejecta horizons locally contain layers, lenses, and interbeds of accretionary or relict glass grains that are smaller than typical lapilli and generally lack concentric zonation. These may represent waning ejecta plume deposition. Ejecta-bearing conglomerate facies: In a few localities, the uppermost part of impactite deposits consist of conglomerate containing subrounded fragments of iron-formation (in contrast to breccias described above), and matrices containing variably abraded lapilli. Nearly all contacts between individual facies are gradational, with one very important exception—in all exposures, the boundary between ejecta-absent and ejecta-bearing facies is extremely sharp. This is interpreted to reflect a fundamental shift in geologic process from intense seismic perturbation of uppermost iron-formation represented by the ejecta-absent facies, to deposition by the passing ejecta plume. The uppermost conglomerate facies represents mixing of local and exotic detritus, presumably by tsunamis or post-impact fluvial or marine processes. This interpretation was strongly influenced by field discussions with William Cannon, William Addison, Phillip Fralick, and Christian Koeberl, and the pioneering work of Bevan French. 32 �GEOCHEMICAL CHARACTERISTICS OF CADMIUM-RICH SOILS OF THE PEMBINA ESCARPMENT, NORTH DAKOTA JYOTI, Vijaya and SAINI-EIDUKAT, Bernhardt, Department of Geosciences and Environmental & Conservation Sciences Program, North Dakota State University, Fargo, ND 58108, HOPKINS, David and DESUTTER, Tom, Department of Soil Science, North Dakota State University, Fargo, ND 58108. Investigations of topsoils in northeastern North Dakota (Hopkins et al., 1999) revealed that trace element levels of cadmium (Cd), are 10 to 20 times higher than the baseline level of 0.3 mg/kg total Cd established by Garrett (1994) for northern prairie soils of Canada and the adjoining states of United States. North Dakota produces 96% of the nation‘s flaxseed, 50% of all durum wheat, and is the top U.S. producer of confectionary sunflower (NDASS, 2008). All these crops are known to bioaccumulate Cd. Naturally occurring Cd concentrations were determined for soils on the Pembina Escarpment in Cavalier County, NE North Dakota. These soils overlie the Cretaceous Pierre Formation and the glacial till parent materials include considerable shale lithic fragments. Surface and core sampling locations from both residual and transported soils included an active agricultural field, a field currently in the Conservation Reserve Program (CRP), and former agricultural fields that are now grassland fields in a North Dakota State Wildlife Management Area (WMA). Samples were analyzed for Cd and other trace elements using nitric acid digestion followed by ICP–OES at NDSU. Results revealed marked differences in measured Cd values between different sampling sites, e.g., values for WMA fields range from 0.3 mg/kg to 7.0 mg/kg (n=139) for the 0-30 cm depth, whereas values for the agricultural field and CRP area range from 3.0 mg/kg to 16.4 mg/kg (n=25), for samples ranging in depth from surface soils to 1.5 m. Values for core samples from the WMA measured at 15 cm intervals down to 225 cm depth range from 0.01 mg/kg (detection limit) to 1.9 mg/kg (n=188). The measured values for Cd concentrations in several samples in this study confirm earlier results showing significantly higher concentrations of Cd compared to the ‗environmental baseline‘ value of 0.3 mg/kg reported by Garrett (1994). Chemical analyses for cores were grouped by soil horizon and lithology and correlation matrices were calculated for each class. A subset of results illustrates the correlation of Cd concentrations with other analytes for each sample class (Table 1). In bedrock material, the highest positive correlations for Cd are with Ca, Fe, Mn, organic matter, and electrical conductivity, while a weaker positive correlation is seen for Cd-Ni. Cd-pH shows a weak negative correlation. In sand and silt layers, Cd is strongly positively correlated with As. Interestingly, most of these Cd-analyte pairs (with the exception of organic matter) are negatively correlated in topsoil. References Garrett, R.G., 1994. The distribution of cadmium in A horizon soils in the prairies of Canada and adjoining United States. In Current Research, B1994, 73-82. Geological Survey of Canada. Hopkins, D.G., Norvell, W.A., Wu, J., 1999. Formation and distribution of trace-element-enriched soils near the Pembina Escarpment, Cavalier County, North Dakota. Proc. 91st ND Acad. Sci., Grand Forks, ND. NDASS, 2008. North Dakota Agricultural Statistics Service. USDA. Fargo, ND. 33 �Table 1. Correlations of Cd with other analytes in soil horizons and lithologic units of cores taken from formerly cropped fields on the Pembina Escarpment in eastern Cavalier County, ND. Top value: Pearson correlation coefficients; second value: Prob > |r| under H0: Rho=0. Analyte pairs with p < 0.05 are in bold. EC: Electrical Conductivity; OM: Organic Matter; n: no. of observations. Argillic Shale/ Claystone Contact Shale Bedrock Carbonate -0.4520 0.1401 -0.0024 0.9872 -0.0332 0.9273 -0.0270 0.8076 0.2902 0.5769 -0.7948 0.0588 0.4462 0.1265 0.8443 0.0006 0.0029 0.9848 -0.3489 0.3231 0.2248 0.0398 0.2970 0.5677 -0.5338 0.2754 Ca -0.7071 0.0069 -0.0559 0.8630 -0.0596 0.6940 0.2294 0.5239 -0.1151 0.2970 0.9326 0.0067 0.6525 0.1602 Co -0.0347 0.9103 -0.4021 0.1951 -0.0232 0.8781 -0.0143 0.9688 -0.2762 0.0110 0.4110 0.4183 -0.2300 0.6610 Cr -0.4866 0.0917 0.3111 0.3250 -0.0537 0.7231 -0.1552 0.6685 -0.0036 0.9739 -0.5916 0.2161 -0.8484 0.0327 Cu -0.5146 0.0720 0.5872 0.0447 0.1572 0.2968 -0.2217 0.5382 0.1411 0.2004 0.5623 0.2454 -0.6847 0.1335 Fe 0.0130 0.9664 -0.0642 0.8430 0.0677 0.6549 -0.2225 0.5367 0.2971 0.0061 0.9542 0.0031 -0.8845 0.0192 Mn -0.5024 0.0802 -0.3479 0.2679 -0.3838 0.0085 0.0972 0.7894 -0.3030 0.0051 0.9724 0.0011 0.3275 0.5264 Mo 0.0323 0.9165 0.7907 0.0022 0.1808 0.2292 -0.2604 0.4675 0.0565 0.6099 0.2543 0.6267 -0.5448 0.2636 Ni -0.4427 0.1298 -0.3163 0.3165 0.0312 0.8372 0.21408 0.5526 -0.3201 0.0030 0.7446 0.0895 -0.5448 0.2636 Zn -0.3397 0.2562 -0.2677 0.4003 0.5441 <.0001 -0.08486 0.8157 -0.0158 0.8863 0.4436 0.3783 -0.9208 0.0092 OM 0.3361 0.2615 0.4367 0.1558 0.4244 0.0033 0.1346 0.7109 0.3723 0.0005 0.8919 0.0169 -0.9449 0.0045 pH -0.4932 0.1231 -0.4500 0.1177 0.0225 0.8876 0.5565 0.0948 -0.1676 0.1324 -0.9958 0.0587 0.9069 0.0126 EC -0.7882 0.0040 0.4638 0.1508 -0.2525 0.1067 0.2002 0.5791 -0.2152 0.0521 0.9976 0.0441 0.4759 0.3400 13 12 46 10 84 6 6 Topsoil Sand & Silt Al 0.5223 0.0671 As n Support from NIH grant P20 RR016471 from the INBRE program of the National Center for Research Resources is gratefully acknowledged. 34 �A MICROSTRUCTURAL STUDY OF GOLD MINERALIZATION AT MUSSELWHITE MINE AND HAMMOND REEF SHEAR-ZONE-HOSTED GOLD DEPOSITS Kolb, Maura J. and Hill, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada, maurajoy@gmail.com Musselwhite Mine and Hammond Reef are shear-zone-hosted gold deposits located in the Western Superior Province of the Canadian Shield. Like other ―lode-gold‖ or ―orogenic gold‖ depostis these two deposits are proximal to major regional shear-zones. Musselwhite Mine is located adjacent to the North Caribou-Totogan Lake shear zone, while Hammond Reef is adjacent to the Marmion shear zone. The importance of deformation and metamorphism in shear-zone-hosted gold systems is investigated by comparing microstructures associated with gold mineralization from two very different gold deposits. Musselwhite Mine is hosted by amphibolite facies metamorphosed banded iron formation while Hammond Reef is hosted by greenschist facies metamorphosed tonalite. For both deposits gold mineralization can be seen associated with deformational structures where nearby minerals demonstrate very different responses to strain due to differing competency. Examples include brittle failure of competent minerals while matrix minerals deform ductilely, as well as strain shadows formed around these competent minerals Strain shadows around garnet crystals are commonly observed at Musselwhite Mine. At Hammond Reef gold mineralization is often associated with strain fringes around pyrite crystals. These strain fringes show varying degree of recrystallization due to ductile deformation after formation. Quartz textures observed within these recrystallized strain fringes are consistent with textures observed in quartz within the greenschist facies of metamorphism. Gold inclusions have been seen within the metamorphic minerals grunerite and garnet at Musselwhite Mine as well as muscovite at Hammond Reef. This demonstrates that gold mineralization occurred during or before metamorphism. Gold mineralization is also commonly associated with sulphide minerals in both deposits (pyrrhotite at Musselwhite Mine and pyrite at Hammond Reef). Many similar microstructures host gold mineralization at Musselwhite Mine and Hammond Reef. Finding gold mineralization within common structures indicates the importance of deformation and metamorphism in the formation of shear-zone-hosted gold deposits. Textures and mineral assemblages observed in this study support the interpretation that gold mineralization occurred as an ongoing event for both Musselwhite Mine and Hammond Reef. 35 �ASSIMILATION AND PETROGENESIS IN THE NAVILUS AND TERRY FOX SILLS, THUNDER BAY, ONTARIO MAGNUS, Seamus, KISSIN, Stephen, Department of Geology, Lakehead University, 955 Oliver Rd., Thunder Bay, ON, Canada, P7B 5E1, seamusmagnus@hotmail.com The numerous diabase sills in the Thunder Bay area were called the Logan sills by A.C. Lawson at the end of the 19th century. They were first studied in detail by Blackadar (1956), particularly with regard to the pink granophyre associated with some of the sills. He attributed this pink granophyre, especially in the case of the Navilus sill, located immediately east of Thunder Bay, to in situ assimilation of granitic rock. Differentiation of diabase was not considered capable of producing the large volumes of granophyre present in some of the Logan sills. Textural evidence suggests, as discussed by Philpotts (1978), that on cooling, the magma separated into two immiscible liquids, one iron-rich and one silica-rich. This has resulted in a zone of quartzo-feldspathic granophyre containing minute hematitic inclusions, which produces the pink colour seen in hand sample. However, some reaction with xenoliths is also present as seen in the rounding of immiscible carbonate xenoliths and the formation of granophyric zones surrounding quartzo-feldspathic xenoliths. Geochemical evidence indicates that late-stage differentiation by fractionation is the primary cause of variation within the Navilus sill. The uppermost samples of the Navilus sill, which typically contain xenoliths, display this late fractionation through enrichment of light REEs and depletion of heavy REEs (Figure 1a). A Nb-Ta trough on the chondrite-normalized spider diagram, as well as enrichment in LILs, indicates assimilation of crustal material has most likely occurred. (a) (b) Figure 1 – Chondrite-normalized REE diagram (a) and chondrite-normalized spider-diagram (b) for the Navilus Sill. Note the clockwise rotation of uppermost samples (squares) due to fractionation. The Terry Fox sill, located 4 km west of the Navilus sill, shows REE and Spider patterns almost identical to the normal diabase in the Navilus sill. Although the sample of the basal contact between the Terry Fox sill and Rove shale shows some variability in the REE and Spider diagrams, a more detailed analysis is required to determine whether they are the effects of in-situ assimilation or fractionation. 36 �Figure 2 – Plot of Gd/Yb vs La/Sm for several Keweenawan Intrusive Suites, with data from Hollings et al. (2007) and Hart (2002). Note that the Navilus and Terry Fox samples plot within the Nipigon Sill Complex samples. Comparison of the Navilus and Terry Fox sills to other Keweenawan intrusions and intrusive complexes in the Thunder Bay area using trace-element discrimination diagrams (Figure 2) indicates that these sills are grouped with the Nipigon sills. In summary, textural evidence has shown that assimilation of granitic xenoliths is not likely the cause of the formation of the pink granophyre, but the development of magmatic immiscibility has produced this feature. Geochemical evidence indicates that assimilation of crustal material has most likely occurred, though chemical variations seen within the sills appear to have been caused by in-situ fractionation. A more detailed study on the possible effects of assimilation at the base of assimilation at the base of the Terry Fox sill is required, although assimilation of xenoliths has possibly contributed to geochemical differences as seen in the Navilus sill spider diagram (Figure 2). References Blackadar, R.G., (1956). Differentiation and Assimilation in the Logan Sills, Lake Superior District, Ontario. American Journal of Science, v. 254, p. 623-645. Hart, T.R. (2002). Proterozoic Volcanic and Intrusive Whole Rock Geochemical Data Associated with the Keweenawan Midcontinent Rift, Lake Superior Area, Ontario. Ontario Geological Survey, Miscellaneous Release of Data 114. Hollings, P., Hart, T., Richardson, A., and Macdonald, C.A., (2007). Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Science, v. 44, p. 1087-1110. Philpotts, A.R., (1978). Textural Evidence for Liquid Immiscibility in Tholeiites. Mineralogical Magazine, v. 42, p. 417-425 37 �TEXTURAL ANALYSIS OF A SIMPLE IGNEOUS INTRUSION, NIPIGON, ONTARIO MARKWOOD, Levi W. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA, 16057. lwm9100@sru.edu, michael.zieg@sru.edu In this study, I examined the textural variations in a small (1.4 m) diabase sill from Nipigon, Ontario, which was apparently formed by a single, instantaneous injection event, in order to develop a baseline to test against other sills with unknown injection histories. With an understanding of the textural characteristics of a simple intrusion, we can recognize the variations in more complex intrusions with complicated thermal histories and make comparisons between observed textural variations and the predictions of theoretical models. Such textural criteria are particularly important when the magma composition remained constant through several injections, in which case reinjection events would not be reflected in the chemical composition or modal mineralogy of the rocks. Petrographic examination of the rocks in this sill focused on opaque oxides, as they can be characterized easily using automated image processing techniques. Samples were collected from a 1.4 meter thick diabase sill in Nipigon, Ontario spanning the entire vertical cross-section of the sill. Thin sections were prepared at 5 cm intervals throughout the sill, yielding 28 thin sections between chill margins. Randomly oriented test lines were used to determine mean crystal length using the intercept method discussed by Higgins (2006): Lmean= VV/PL, where VV is the modal fraction of the mineral of interest, as determined by automated image analysis, and P L is the number of crystals intersected along the test line. To determine P L, a hundred test lines were randomly oriented in the field of view of a petrographic microscope for each sample and grain intersections along the test lines were averaged. To determine V v, digital threshholding was used to determine the average area of opaque minerals present in each sample and divided by the entire area of the slide. Once Vv and PL are known, Lmean can be determined for any position in the sill. Figure 1 shows the modal abundance of opaque oxides with varying position in the sill and Figure 2 shows the variations in L mean through the sill. Based on the observed textural variations, namely a smooth transition from smaller grains at the chilled margins to larger grains in the sill‘s center, this sill was almost certainly emplaced in a single injection event. However, questionable data points and the large statistical uncertainties in calculate mean lengths prevent us from using this intrusion as a baseline for confidently identifying complex injection histories. Further investigation and refinement of this textural profile could ultimately make it useful as a null hypothesis for testing formation history: unless the textural profile in the intrusion of interest differs significantly from the profile in this sill, we must assume that it was formed as a single injection. 38 �Figure 1: Modal abundance of opaque minerals. Figure 2: Mean crystal lengths of opaque minerals. References Higgins, M.D., 2006. Quantitative Textural Measurements in Igneous and Metamorphic Petrology, p. 34-35. 39 �BEDROCK GEOLOGIC MAP OF THE DISAPPOINTMENT LAKE AREA, LAKE COUNTY, NORTHEASTERN MINNESOTA MULVEY, Lucy, ROSS, Cabin, ZEITLER, Joseph, PENDLETON, Matthew, McCARTHY, Andrew, COPP, Lee, NOWAK, Robert, HUDAK, George, and PETERSON, Dean, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811 The ―capstone‖ project for the Precambrian Research Center (PRC) summer field camp encompasses one week of detailed field mapping in small groups with faculty from the field camp. During the fifth and sixth weeks of the 2009 field camp, seven students mapped Neoarchean and Mesoproterozoic rocks in the vicinity of Disappointment Lake (located in the Boundary Waters Canoe Area Wilderness) under the direction of PRC faculty Dr. Dean Peterson and Dr. George Hudak. Geological maps of this area were originally published by Van Hise (1901) and Gruner (1941). This capstone mapping project sought to: 1) define and characterize the nature of the contact between the Mesoproterozoic Duluth Complex and Neoarchean supracrustal strata; 2) to better understand the lithological components of the Duluth Complex and the Neoarchean supracrustal strata in this area; 3) to obtain a better understanding of geological structures and their orientations within the Neoarchean supracrustal stata; and 4) to identify and map out regions of Mesoproterozoic (Duluth Complex associated) and Neoarchean (Snowbank Lake Granite-associated (Miller et al., 2001; Jirsa and Miller, 2004)) contact metamorphism within Neoarchean strata. Field mapping was completed at 1:5,000 scale, with the final map being published at 1:10,000 scale. Mapping was completed by means of lakeshore mapping from canoes and traverses through the bush. The north-central and northeastern regions of the field area are composed principally of Neoarchean intrusive and supracrustal strata. The Neoarchean supracrustal sequence is interpreted to represent the Knife Lake Group (Jirsa and Miller, 2004), and is composed, from oldest to youngest units, of: a) massive and pillowed basalt lava flows; and b) thick-bedded to massive rhyodacitic to dacitic tuffs locally interbedded with matrix- to nearly clast-supported, medium-bedded to massive polymict volcaniclast-dominated epiclastic deposits. Regional D2 deformation led to the development of localized west-northwest – east-northeast-trending chlorite schist zones which can be traced for at least 300 meters. Southwest of the chlorite schist zones, greenschist facies metamorphosed Timiskiming-type interbedded polymict conglomerates, conglomeritic sandstones, and mudstones interpreted as the Ogishke Conglomerate (Jirsa and Miller, 2004) appear to unconformably overlie the mafic lava flows and felsic volcaniclastic rocks and associated epiclastic sedimentary rocks described above. These sedimentary strata have been deformed into a series of relatively tightly folded anticlines and synclines characterized by moderately- to steeply- dipping (60°-90°) fold limbs with westnorthwest – east-southeast trending fold axes. Folding predated the intrusion of medium- to coarse-grained granite and hornblende granite (the Snowbank Lake Granite (Jirsa and Miller, 2004) and associated aplitic to pegmatitic dikes which occur in the westernmost one-third of the field area. Locally, xenoliths of sedimentary rocks, and sedimentary rocks along the margin of this intrusion, have been recrystallized into fine-grained to medium-grained granoblastic to granular hornblende hornfels. 40 �The southeastern one-third of the field area comprises Mesoproterozoic intrusive rocks that comprise the Duluth Complex. A marginal phase composed of medium- to coarse-grained oxide gabbro and poikilitic olivine gabbro transitions into medium- to coarse-grained troctolite that locally displays modally-layered subhedral olivine crystals and ophitic augite crystals. In the northeastern part of the field area, a southeast- northwest-trending wedge-shaped marginal phase of diorite and hornblende diorite occurs in an area roughly 1.5 km long by 1.0 km wide. Three distinctive zones of Mesoproterozoic thermal metamorphism have also been identified and mapped in the field area. Neoarchean sedimentary strata have been thermally metamorphosed to the pyroxene hornfels facies for up to 250 north of the Duluth Complex contact to produce fineto coarse-grained norite. Basalt hornfels xenoliths are locally present in the troctolitic phase of the Duluth Complex. In the east-central region of the field area, approximately 1 to 3km northwest of the contact between the Duluth Complex and Neoarchean supracrustal strata, a southeast - northwest-trending 1.5km by 2.0km zone of hornblende hornfels facies metamorphosed Neoarchean conglomerate, sandstone, and mudstone occurs. This southeast – northwest-trending zone has the same orientation as the wedge-shaped zone of marginal diorite and hornblende diorite phase of the Duluth Complex. The spatial distribution of the hornblende hornfels facies metamorphosed rocks, as well as the marginal hornblende diorite phase of the Duluth Complex, suggest the former presence of a shallowly-dipping southeast- northwest trending sill of Duluth Complex that has subsequently been eroded away. References Chandler, V. W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map Series S-17, scale 1:500,000. Gruner, J. W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: Geological Society of America Bulletin, v. 52, p. 1577-1642. Jirsa, M. D., and Miller, J. D., 2004, Bedrock geology of the Ely and Basswood Lake 30‘ by 60‘ quadrangles, northeast Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-148, scale 1:100,000. Miller, J. D., Jr., Green, J. C., Severson, M. J., Chandler, V. W., and Peterson, D. M., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-119, scale 1:200,000, two sheets. Van Hise, C. R., 1901, The iron-ore deposits of the Lake Superior region: 21st Annual Report of the U. S. Geological Survey, Part III. 41 �FERROMAGNESIAN MINERALS IN THE STETTIN SYENITE COMPLEX, MARATHON COUNTY, WISCONSIN: COMPOSITIONS AND CONTRASTS WITH THE WOLF RIVER BATHOLITH MEDARIS1, L. Gordon Jr. and KOELLNER2, Susan E. 1 Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 2 20509 Hillside Trail, Lindale, TX 75771 medaris@geology.wisc.edu, susankoellner@gmail.com The Stettin syenite complex is the northernmost of four overlapping, concentric intrusions, which are located immediately west of Wausau in central Marathon County, Wisconsin (Myers et al., 1984). The intrusions decrease in age from north to south, with zircon U-Pb ages of 1565 ± 4 Ma in the Stettin complex, 1522 ± 6 Ma in the Wausau complex, and 1506 ± 3 Ma in the Ninemile complex (Van Wyck et al., 1984; Dewayne and Van Schmus, 2007). Previously, these intrusions were thought to be associated with geon 14 Wolf River magmatism, but their ages are significantly older than the 1468-1484 Ma Wolf River batholith and preclude a cogenetic relation between them. The Stettin complex consists of concentrically arranged syenite bodies, the most important of which are syenite, tabular syenite, nepheline syenite, and quartz syenite (Myers et al., 1984). The latter two units illustrate the divergent silica differentiation trends in the complex, i.e. silica undersaturation in nepheline syenite and silica oversaturation in quartz syenite. Values for the Thornton-Tuttle Differentiation Index (normative qtz+or+ab+ne) range from 71 to 94 in nepheline syenite and from 82 to 87 in quartz syenite. The complex is markedly Fe-rich, with Fe#'s ranging from 89.3 to 99.8. Stettin syenites are more alkaline than Wolf River granites, A/CNK = 0.84 ± 0.08 (n=10) vs. 1.00 ± 0.08 (n=38), have higher Agpaitic Indices, (Na+K)/Al = 1.44 ± 0.10 vs. 1.21 ± 0.10, and are more sodic, Na/K = 1.77 ± 0.15 vs. 0.88 ± 0.22 (Myers et al., 1984; Anderson and Cullers, 1978). Another important distinction is the presence of titaniferous magnetite in the Stettin complex and ilmenite in the Wolf River batholith. 42 �Clinopyroxene occurs in all four major syenite units in the Stettin complex, ranging in composition from Di55Hd44Acm1 to Di8Hd90Acm2 in syenite, from Di12Hd70 Acm18 to Di6Hd39Acm55 in tabular syenite, from Di11Hd81Acm8 to Di10Hd35Acm55 in nepheline syenite, and from Di32Hd64Acm4 to end-member acmite in quartz syenite. Overall, clinopyroxene in the Stettin complex displays one of the most pronounced agpaitic compositional trends reported among alkaline complexes worldwide. Olivine occurs in syenite and nepheline syenite, ranging in composition from Fa 77 to 97, and having a consistently higher Fe# than coexisting clino-pyroxene. Amphibole in syenite, nepheline syenite, and tabular syenite is largely hastingsitic, but in quartz syenite reaches near end-member riebeckite, which has been stabilized under igneous conditions by the presence of F (>2 wt.%). Amphibole in the Wolf River batholith consists of ferroedenite to hastingsite (Anderson, 1980), devoid of the marked alkaline trend in Stettin amphibole. Biotite approaches annite (or ferriannite) in composition and is relatively poor in Al. Many samples contain insufficient Si+Al to fill the tetrahedral position, which is a common feature in biotite from alkaline rocks. Wolf River biotite is also relatively Fe-rich, but is more aluminous than Stettin biotite (Anderson, 1980). The compositions of Stettin and Wolf River ferromagnesian minerals are dependent in part on intensive crystallization conditions, e.g. relatively reducing conditions, at or below that of the FMQ buffer, allow the crystallization of hedenbergite and fayalite. However, the occurrence of sodic pyroxene and amphibole in the Stettin complex, as in many other alkaline complexes, is more a function of bulk composition, i.e., a relative increase in Na, combined with an increase in Fe3+/Fe2+, during differentiation. The contrasts in Stettin and Wolf River amphibole and biotite compositions are also due to differences in bulk composition between the two bodies, reflecting their different degrees of alkalinity. References Anderson JL (1980) American Journal of Science, v. 280, 289-332. Anderson JL and Cullers RL (1978) Precambrian Research, v. 7, 287-324. Dewane TJ and Van Schmus WR (2007) Precambrian Research, v. 157, 215-234. Koellner SE (1974) M.S. thesis, University of Wisconsin-Madison, 155 pp. Myers PE et al. (1984) Institute on Lake Superior Geology, v. 30, Field Trip #3, 58 pp. Van Wyck N et al. (1984) Institute on Lake Superior Geology, v. 30, Part 1 Program and Abstracts, 81-82. 43 �CYCLICAL PHASE LAYERING IN THE DULUTH COMPLEX AT DULUTH – EVIDENCE FOR PERIODIC MAGMA VENTING FROM A SHALLOW STAGING CHAMBER? Miller, James D., and Stifter, Eric, Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 Igneous layering is an almost ubiquitous feature in mafic intrusive bodies and yet remains an enigma. One of the more intriguing forms of layering in the 1.1 Ga Duluth Complex of NE Minnesota is cyclical phase layering exhibited by the Layered Series at Duluth (DLS). The DLS is a well-exposed, well-differentiated, north-south striking, eastward-dipping, sheet-like intrusion. Its cumulate stratigraphy ranges from troctolite (Pl+Ol) upward to olivine oxide gabbro (Pl+Cpx+Ol+Ox) and ultimately to intermediate and felsic rock types as final differentiates. Detailed field mapping by Miller and Green (2008) separated the DLS into five distinct zones: the basal contact zone, the troctolitic zone, the cyclic zone, the gabbroic zone, and the upper contact zone. The cyclic zone is composed of least 5 macrocycles wherein troctolitic (Pl+Ol) cumulates grade upward into olivine gabbroic (Pl+Aug+Ol+Ox) cumulates. The 4phase gabbros are then abruptly overlain by troctolitic cumulate marking the base of the next macrocycle. A conventional interpretation of such layering may be that it formed by progressive differentiation followed by magma recharge. However, cryptic layering of olivine and pyroxene across the cumulate regressions marking each macrocycle boundary show no significant increase in mg#, which such a model would predict. Instead, Miller (Miller and Ripley, 1997; Miller et al.. 2002) proposed that decompression due to magma venting at shallow depths might better explain this layering by significantly changing phase equilibrium without changing magma composition. Exposed in the Spirit Mountain ski area and within the second macrocycle of the cyclic zone is a large outcrop exhibiting meter-scale mesocyclic layering that mimics the textural and mineralogical changes seen in the larger scale macrocycles (Fig. 1A & B). Detailed field, petrographic, and geochemical studies of these mesocycles were performed to further test the plausibility of magma recharge as the mechanism responsible for the cyclic phase layering. Analyses of olivine and augite revealed that mg# (=Mg/(Mg+Fe)*100) remained within 2% of the mean throughout individual cycles and throughout the entire mesocyclic system of 16 cycles (Fig. 2). Despite this minor variation, there is a systematic difference in the mg# of olivine, such that the olivine in troctolite cumulates is consistently more fosteritic than gabbroic cumulates. Some cycles show a similar shift in augite, though this is less consistent. This may at first glance seem to argue for recharge of more primitive magma as the cause for the phase layering. However, the shift in mg# in olivine may also reflect a shift in the Fe-Mg equilibrium between melt and olivine when augite becomes a cumulus phase. Trace element data, particularly REE, show a significant difference between the troctolite and gabbro layers, but this can be attributed to differing partition coefficients for the main cumulate phases. In conclusion, the field, petrographic, mineral chemistry, and lithochemical attributes of the mesocyclic layering observed as Spirit Mountain does not unambiguously favor either a magma recharge or decompression venting model for its origin. More data and evaluation are needed to definitely determine the origin of the mesocyclic and larger macrocyclic layering of the Cyclic Zone of the Duluth Layered Series. 44 �References Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. Miller, J.D., Jr., and Green, J.C., 2008, Bedrock geology of the West Duluth and eastern portion of the Esko quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Misc. Map M-183, scale 1:24,000 Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G. (ed.), :Layered Intrusions: Amsterdam, Elsevier, p. 257-301. Olivine Augite Figure 1. Mesocyclic layering of troctolite (Ol+Pl) and Ox-Ol gabbro (Pl+Cpx+Ol+Ox) cumulates in outcrop (A) and in thin section (B). Figure 2. Stratigraphic changes in the physical and compositional characteristics of mesocycles in the Spirit Mtn area. 45 �MESOPROTEROZOIC BEDROCK GEOLOGY OF THE HOMER LAKE – VERN LAKE AREA, COOK COUNTY, NORTHEASTERN MINNESOTA Jim Miller, Sam Blakely, Amy Brown, Dan Foley, Aaron Rowland, and Eric Stifter Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As one of the capstone mapping projects for the 2009 Precambrian field camp, a crew of four students (Blakely, Brown, Foley, and Rowland), a teaching assistant (Stifter), and an instructor (Miller) conducted five days of field mapping bedrock geology of the Homer Lake and Vern Lake areas of northeast Minnesota. This area comprises intrusive and volcanic rocks formed during the 1.1 Ga Midcontinent Rift in northeastern Minnesota. This mapping project expanded on a capstone mapping project conducted in 2007 (Frost et al., 2007). A 1:12,000-scale bedrock geologic map that integrates the previous mapping and new mapping from last season will be displayed as a poster presentation. A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/capstone. Prior to the detailed mapping conducted by the PRC field camp students, the general geology of the Homer Lake area was poorly known. Township-scale mapping by Grout et al. (1959) showed the area to be dominantly composed of gabbroic to felsic intrusive rocks, mafic volcanic rocks and minor volcaniclastic sedimentary rocks. They noted that the gabbroic rocks are well layered and portions are particularly rich in Fe-Ti oxide. Curiously, Davidson and Burnell (1977) did not integrate Grout et al.‘s (1959) mapping into their reconnaissance geologic map of the Brule Lake 7.5‘ quadrangle, which contains Homer Lake. Showing only one outcrop on Homer Lake, they portrayed the geology as being composed of exclusively gabbroic and anorthositic rocks. The regional geologic map of northeastern Minnesota focused on the geology of the Duluth Complex (Miller et al., 2001) incorporates Davidson and Burnell‘s (1977) interpretation, which current mapping shows to be largely incorrect. In fact, Grout et al.‘s (1959) mapping has been shown to be quite accurate. Capstone mapping in August of 2007 by Shelby Frost, Natalie Juda, and Jim Miller and reconnaissance mapping in 2006 by Miller resulted in a geologic map of the Homer Lake area (Frost et al., 2007). This map interpreted the area to be composed of two main gabbroic suites separated by a thin screen of basaltic and sedimentary hornfels that runs along the north shore of Homer Lake (and noted by Grout et al., 1959). The gabbroic suites and the hornfels units have an east-west strike and gentle (15-25°) dips to the south. The basaltic hornfels has some remnant volcanic features such as flow top breccias and amygdaloidal zones and the sedimentary hornfels commonly displays cross-bedding. These units are interpreted to be basalt flows and interflow sediments of the North Shore Volcanic Group. Gabbroic rocks north of (below) this hornfels screen include olivine diabase, oxide gabbro, ferromonzodiorite, and granophyric gabbro, which were collectively termed the Axe Lake gabbroic sequence. South of the hornfels units are a series of oxide gabbros that were termed the Homer Lake gabbroic sequence. Two textural variants of the oxide gabbros were distinguished – a medium-grained, well-foliated and intergranular rock type and a coarse-grained, poorly foliated, and ophitic to intergranular lithology that can locally contain up to 25% interstitial granophyre. These two variants were found to be interlayered in 4-5 cycles over a stratigraphic thickness of about 600 meters. Capstone mapping during the 2009 field camp extended the mapping of Frost et al. (2007) to the south and west of Homer Lake. This mapping expanded the total map area from 3 square miles to about 10 square miles. The screen of volcanic and sedimentary hornfels was traced across the map area, though at its western extent where it crosses Vern Lake, the hornfels occurs as inclusions in the base of the Homer Lake sequence. The Axe Lake sequence was traced to the west and east of Axe Lake The more intermediate lithologies recognized in the Axe Lake area during 2007 mapping were found to persist to the east, but were not observed to the west. West of Axe Lake, the main rock type observed is olivine 46 �gabbro to augite troctolite. Exposures along Vern Lake at the western extent of the map area, troctolite and olivine gabbro contain abundant inclusions of anorthositic gabbro. Mapping to the south of Homer Lake to East Pipe and Pipe Lakes revealed that the oxide gabbros that compose most of the Homer Lake gabbroic sequence grade upward into an apatitic oxide gabbro, a ferromonzonite, and then into a granophyric leucogranite. In addition to recognition of these new map units, the 2009 map simplified the multiple coarse and medium grained oxide gabbro units of Frost et al. (2007) into two main alternating cycles. Also, the lowermost coarse oxide gabbro unit of Frost et al. (2007) was renamed as the heterogeneous oxide gabbro to highlight the variable texture and granophyre content of this unit. One the more interesting discoveries was the occurrence of nearly pure layers of FeTi oxide in the upper oxide gabbro cycle in the vicinity of Vern River. Grout (1949-50) noted such layers of nearly pure ilmenite in this area and to the west, which he speculated may be an economic Ti resource. Although too little of the Axe Lake sequence has been mapped to adequately speculate as to its petrogenesis, the Homer Lake sequence has been revealed as a rather complete mafic to felsic differentiated sequence. In contrast to other mafic layered intrusions in the Duluth and Beaver Bay complexes, however, the differentiation of the Homer Lake sequence appears to start from a very evolved oxide gabbro (Pl+Cpx+Ox) cumulate rather than more primitive troctolitic (Ol+Pl) cumulates. Moreover, granophyre capping the Homer Lake sequence is part of the massive Eagle Mountain granophyre (Miller et al., 2001). Therefore, this much granophyre could not have fractionated from the oxide gabbro. Rather, as found with other granophyre masses in the Duluth and Beaver Bay complexes, the felsic rocks likely served as a density barrier to the mafic magmas of the Homer Lake sequence. Underplating of the Eagle Mountain granophyre by the Homer Lake mafic magmas would have caused partial melting of the lower part of the granophyre, which may have assimilated with the upper part of the Homer Lake sequence to create the ferromonzonite unit. If this scenario is correct, then the recent U-Pb age of 1098.6±3.6 Ma reported by Vervoort et al. (2007) for the Eagle Mountain granophyre gives a maximum age for the Homer Lake sequence. A geochemical study through the Homer Lake sequence into the overlying Eagle Mtn granophyre is necessary to discern the extent to which this apparent differentiation sequence is the result of fractional crystallization or contamination by the granophyre. Plans for the 2010 capstone mapping project in this area are to follow the stratigraphy of the Homer Lake sequence to the west. Aeromagnetic data indicates that the sequence should not only persist to the west, but should also thicken. References Davidson, D.M. and Burnell, J.R., 1977, Brule Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-29, 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000. Grout, F.F., 1949-50, The titaniferous magnetites of Minnesota: Department of Iron Range Resources and Rehabilitation, 117 p. Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism. Precambrian Research 157, p. 235-268. 47 �SEDIMENTOLOGICAL STUDY OF THE GLACIOGENIC BRUCE FORMATION, HURONIAN SUPERGROUP, IN THE NORTH SHORE AREA, ONTARIO O’HARE, Sean and FRALICK, Philip Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada The Bruce Formation is a diamictite succession located in the lower-most portion of the Paleoproterozoic Quirke Lake Group, of the Huronian Supergroup. The Formation is exposed on the north shore of Lake Huron, and covers an area east of Sault Ste. Marie to east of Sudbury, Ontario, Canada. This assemblage of rocks represents deposition on a passive margin after initial rifting and volcanism at approximately 2450 Ma (Fralick and Miall, 1989). The passive margin succession is composed of three glaciogenic cycles beginning with outwash sandstones, which are overlain by glacial diamictites and capped by marine deposits. The Bruce Formation is situated above the Mississagi Formation, which commonly shows evidence of extensive soft sediment deformation of its cross-stratified sandstones (Fig. A). These are also more rarely intruded by clastic dykes (Fig. B), orientated perpendicular to bedding and filled with diamictite. The upper contact with the Espanola Formation is sharp and conformable. The Espanola represents the oldest cap carbonate known in the worldwide geologic record and may be evidence of a snowball Earth event. The Bruce Formation was studied in detail in an attempt to understand the ambient conditions that were present at the time of its deposition. The study included measuring and describing 16 detailed sections in the field, plus petrological and geochemical analysis of samples collected from the field areas. Glaciogenic deposits can be either terrestrial or marine and marine deposits can be further divided into three lithofacies zones: 1) the grounding ice line zone, 2) the floating ice-shelf zone, and 3) the proximal iceberg zone. Deposits in each of these environments have distinctive characteristics that, where present, can be used to identify the depositional setting. Most outcrop areas in the sections studied consisted of massive diamictite that contained considerable quantities of sand and only scattered pebbles and cobbles. The thickness of the assemblage of diamictites making up the Formation varied drastically from section to section, with three less than 10 meters thick and others hundreds of meters in thickness. Within these massive diamictites are areas with more distinctive features. Lenses and layers of moderately well-sorted sandstone are not uncommon. In other portions of the section mud layers and mud wisps highlight layering in the diamictite, and at one location a fine-grained succession within the diamictite contained a dropstone (Fig. C). If other dropstones exist in areas of the diamictite they are commonly impossible to recognize without layering to deform. However, where clay-rich layers and wisps are present in the diamictite they can be seen to bend under and be terminated by penetration of dropstones. Layers of pebbles and cobbles in clast support were more rarely present (Fig. D). These represent deflation lags produced by current activity removing the finegrained material. Rarest of all were flat bottomed and convex-up mounds of pebbles and cobbles in clast-support approximately one meter across, although a larger deposit of this type may also have been present in one section. Similar features are produced in modern environments by uneven melting of icebergs causing flipping and dumping of melt-out debris on their surface. In Pleistocene successions diamictite deposited near the grounding ice line commonly contains subaqueous outwash sand layers and lenses. The diamictite deposited below floating ice shelves is generally massive but has features, such as mud-rich layers and sand-rich layers, which indicate current activity. Dropstones and deflation lag layers can also develop in this zone. The 48 �proximal iceberg zone can also be dominated by massive diamictite, which has a lower clast concentration, and also contains dropstones. The most conclusive evidence of the iceberg zone is the presence of mound shaped iceberg dumps, which are created when an iceberg overturns. This implies that large segments of the glaciogenic succession were deposited under floating ice with open water present in the basin. The open water is necessary to develop current activity. Although this data does not negate the development of a snowball earth at some point during this glacial period it does not support it. 49 �VIRGINIA FORMATION OUTCROP (New Outcrops Versus Old Concepts) Patelke, Richard L. (PolyMet Mining, 6500 County Highway 666, Hoyt Lakes, MN 55750) and Severson, Mark J., Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 In their 1983 study of Virginia Formation stratigraphy, Lucente and Morey noted the absence of natural exposures of the formation and the temporary nature of a few Virginia Formation exposures related to iron mining. Their stratigraphic study was based solely on four deep core holes that were drilled as part of the ―Mesabi Deep Drilling Project.‖ The statement (taken on faith?) that the Virginia Formation doesn‘t outcrop is a truism that may actually have discouraged geologists from looking for outcrop. However, several areas of actual outcrop have been found in the last 15 years. While not technically on the Iron Range, there are long known exposures of weakly to strongly partially melted Virginia Formation in close proximity to the Duluth Complex at Linwood Lake (Severson 1995). These particular outcrops were originally found by a minerals exploration company in the 1970s and their rumored occurrence was eventually followed up. As part of a mapping project for the Minnesota Geological Survey, Severson and Miller, (1999) routinely visited areas as part of outcrop searches and recorded a small area of Virginia Formation outcrop in the area of the Siphon Fault (related to structure) at the east end of the LTV taconite mine in the Allen Quadrangle. Recent low water in that same area has expanded the outcrop area to about 200 ft. by 1200 ft of scattered exposure, ranging from intact outcrop, inplace blocks, and a large area of frost shattered angular blocks. About one and a half miles east of this location, along the Dunka Road, is another area of frost shattered angular boulders of Virginia Formation that was discovered in early April, 2010. Again, these subcrops were recently exposed by low water and presumably represent outcrop. In the fall of 2009, upon the insistence of some of PolyMet employees, who are residents of Hoyt Lakes and Aurora, we canoed into an area on the St Louis River, just south of Whitewater Lake, that the locals call ―The Slates‖. Here we found flaggy outcrop of Virginia Formation in floor of the river, exposures up to 12 ft. high at the riverside, and exposures in the woods near the river. The area extends about 750 ft. along the stream. Unlike the other areas of outcrop, metamorphism is not readily apparent. Sedimentary features include bedding, cross-bedding, ripple marks, and load casts. Rock types are siltstones intermixed with graywackes. Compass issues precluded quality measurements, but overall strike is SW to NE, with gentle dips to the southeast. Sulfide (pyrite) is present, but rare, in these exposures. Another area to the south of Hoyt Lakes reported by the same locals has yet to be visited. This experience would indicate the value of pursuing that lead. In summary, a reported paucity of outcrop of any unit should never be taken at face value. Outcrops can be found in some of the most unlikely areas if enough effort is expended in systematic searches. This search should also include conversations with local outdoor enthusiasts, woodsmen, loggers, and hunters. 50 �References: Lucente, M.E., and Morey, G.B., 1983, Stratigraphy and Sedimentology of the Lower Proterozoic Virginia Formation, Northern Minnesota, Minnesota Geological Survey Report of Investigations 28, 28 pages. Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR95/26, 185 p. Severson, M.J., and Miller, J.D. Jr., 1999, Bedrock Geologic Map of Allen Quadrangle: Minnesota Geologic Survey, Miscellaneous Map —91, 1:24,000. 51 �HYDROTHERMAL OVERPRINT BY GREENSTONE SILLS WITHIN THE TILDEN PIT, MICHIGAN PIETRZAK, Natalie, University of Western Ontario, DUKE, Norm University of Western Ontario, SCOTT, Glenn, Cliffs Natural Resources and LUKEY, Helene, Cliffs Natural Resources The Negaunee Iron Formation in the Paleoproterozoic Marquette Range Supergroup contains numerous greenstone sills and dykes. Relict gabbroic textures within these bodies identify their mafic igneous origin. Direct dating has been unsuccessful due to the pervasive retrogression of the primary silicate mineralogy. Schneider et al., (2002) suggests the Negaunee and ~1.8 Ga Hemlock volcanics interfinger and therefore date the Negaunee between 1880 and 1860 Ma. However, Hans and Runnegar (1992) consider the Negaunee to be 2.1 Ga based on Sm-Nd isotopes and identification of fossils. The lowest greenstone sill exposed in the Tilden Pit is termed the ―Pillar Intrusive‖ and this separates the Martite Domain from the overlying Hematite Domain (Lukey et al., 2007). Three strategic drill cores have been studied to document detailed petrography from the base of the pillar to the lowest Negaunee clastics. Microscopic examinations supported by microprobe analyses reveal that primary iron formation textures have been hydrothermally overprinted up to 100 meters below the pillar. The chilled margin of the pillar is mineralogically complex and comprises chlorite, sericite, albite and veinlets of carbonate, quartz, minor rutile and apatite: Fe-oxides are absent. Proximal to the chilled margin, iron formation textures are overprinted by massive recrystallized quartz with trace to minor disseminated Fe-oxides and K-feldspar. Vugs, occasionally filled with specularite and commonly associated with goethite patches, increase towards the sill contact. With increasing depth away from the sill, Fe-oxides increase in size and abundance whereas the goethite patches decrease in size and abundance. Within the sill chilled margin chlorites are fine grained and show moderately high iron content (Figure 1A). Chlorite and carbonate are absent in the recrystallized quartz zone and reappear in veinlets cross-cutting the primary oolitic-chert textures 100 meters below the pillar. Below the recrystallized aureole the chlorite shows progressive increasing iron content from low iron in the oolitic-chert lithologies to higher iron contents in the lower iron formation bands and basal clastic units (Figures 1B-D). Carbonate shows a similar compositional trend as chlorite. Within the chilled margin the carbonate is fine grained calcite (Figure 2A), while ankerite-dolomite species occur in distal oolitic-chert. Within the lower iron formation bands and basal clastic lithologies the iron content of the carbonate increase dramatically to form siderite-magnesite species (Figures 2B-D). The emplacement of greenstone sills and related dykes was accompanied by hydrothermal overprinting of the enveloping iron formation as well as the mafic intrusions. The intense chloritization of the sills and dykes suggest they intruded into still hydrous sediments and autohydration resulted in near-complete chlorite replacement of the chilled margins. Domains of chlorite schist that develop along the contact clearly demonstrate hydrothermal alteration predated Penokean folding. The recrystallized massive quartz aureole, with vuggy textures, represents a dehydration zone that underwent loss of iron and CO2. The increase in porosity and permeability proximal to the sill allowed ingress of cooler fluids, giving rise to the goethite patches. Evidence for direct involvement of dehydration of enveloping sediments and pervasive retrogression of the greenstones indicates the sills are likely of similar age to the deposition of the Negaunee Iron Formation. 52 �Figure 1: Chlorite chemistry of grains defined by ellipses: A) the chilled margin; B) oolitic-chert unit; C) chertcarbonate- banded unit; D) clastic unit. Figure 2: Carbonate chemistry defined by ellipses from: A) the chilled margin; B) oolitic-chert unit; C) chert-carbonate banded unit; D) clastic unit. References Hans, T. and Runnegar, B. 1992. Megascopic Eukaryotic Algae from 2.1 Ga Negaunee Iron Formation: Science 257: 323-235 Lukey, H.M., Johnson, R.C., and Scott, G.W. 2007. Mineral Zonation and Stratigraphy of the Tilden Haematite Deposit, Marquette Range, Michigan, USA: Proceedings of Iron Ore (AusIMM:Melbourne) Schneider, D., Bickford, M., Cannon F., Schulz, K. and Hamilton, M. 2002. Age of volcanics and syndepositional iron formations, Marquette range Supergroup: implications for the tectonic setting of the Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Sciences 39: 999-1012 53 �THE RARE EARTH ELEMENT AND YTTRIUM COMPOSITION OF ARCHEAN AND PALEOPROTEROZOIC IRON FORMATIONS REVISITED: A NEW PERSPECTIVE ON SIGNIFICANCE AND MECHANISMS OF IRON FORMATION DEPOSITION Noah Planavsky1,2*, Andrey Bekker3, Olivier J. Rouxel2,4, Balz Kamber5 Axel Hofmann6, Andrew Knudsen7, Timothy W. Lyons1 1 Department of Earth Sciences, University of California, Riverside, Riverside, CA 92521, USA 2 Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA 3 Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada 4 University of Brest, European Institute for Marine Studies, Technopôle Brest-Iroise, Place Nicolas Copernic, 29280 Plouzané - France 5 Department of Geology, Laurentian University, Sudbury, ON P3E 2C6, Canada 6 School of Geological Sciences, University of Kwazulu-Natal, Durban, 4000, South Africa 7 Department of Geology, Lawrence University, Appleton, WI 54912, USA The ocean and atmosphere were largely anoxic in the early Precambrian, which dictated a dramatically different iron cycle than today. Extremely iron-rich sedimentary deposits, iron formations, are the most conspicuous evidence of a disparate iron cycle in the early oceans. Rare Earth Element (REE) systematics have long been used as a tool to understand the genesis of iron formations and the chemistry of ancient oceans. However, many earlier REE studies of iron formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered formations. We present new chemical analyses of iron formation samples from 18 units, but with a focus on Lake Superior iron formations. These new analyses allow a reevaluation of the mechanisms and significance of Precambrian iron formation deposition. There are several temporal trends in our REE and Y dataset that we propose to reflect shifts in marine redox conditions. In general, Archean iron formations do not display significant shale-normalized negative Ce anomalies and only iron formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger iron formations but are essentially absent in their Archean counterparts. These marked differences in REE+Y patterns of late Paleoproterozoic and Archean iron formations can be explained in terms of varying water column REE cycling. Similar to modern redox-stratified basins, the REE+Y patterns in late Paleoproterozoic iron formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater. Oxide dissolution— mainly of Mn oxides—in an anoxic water column lowers the Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to neighboring REE (La and Pr) values/concentrations. In contrast, Archean iron formations do not display REE+Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Fe-Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. These results question classical models for deposition of Archean iron formations that invoke oxidation at or above a redoxcline. In contrast, we suggest that metabolic iron oxidation is a more likely oxidative mechanism for these iron formations, implying that the iron distribution of Archean oceans could have been controlled by microbial iron uptake and ecosystem stratification rather than the oxidative potential of shallow-marine environments. 54 �55 �THE PETROLOGY AND GEOCHEMISTRY OF THE RIVERDALE SILL PUCHALSKI, Raya, HOLLINGS, Pete Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, and SMYK, Mark Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada The Riverdale Sill is located within the southern city limits of Thunder Bay. Although the sill consists mainly of gabbronorite, olivine gabbro can be found in a number of localities. The intrusive contact between the sill and the Paleoproterozoic Rove Shale is sharp and the chilled margins are only a few centimetres thick. The sill is exposed over an area approximately 6 km long and 2 km wide but the true thickness is unknown because the upper contact is not visible. In comparison to other Midcontinent Rift-related intrusions, the Riverdale Sill is geochemically similar to ultramafic intrusions such as the Hele and Disraeli (Hollings et al. 2007). Although it is found south of Thunder Bay it most closely resemble the Jackfish Sill of the Nipigon Embayment rather than the Logan and Nipigon Sills which surround it. The Riverdale Sill crops out in a quarry on West Riverdale Road where it has an exposed thickness of 10 m. Detailed sampling was carried out every metre up through the cross section of the sill to investigate geochemical variations within it. There is no geochemical evidence for fractionation within the sill (Fig. 1) which is supported by the absence of cumulate textures. Fig. 1 Geochemical data from a transect through the Riverdale Sill. SiO2, TiO2 and MgO in weight percent Cr and Ni in ppm. Geochemical evidence for contamination of the sill is localized to within less than 1m of the contact with Rove Shale, as shown by the higher SiO 2 values in the gabbronorite at the contact (Fig. 1). On a primitive mantle-normalised plot, a number of samples of the Riverdale Sill broadly resemble Ocean Island Basalts (OIB; Fig. 2). However, some samples within the sill have negative niobium anomalies which are best interpreted to be the result of crustal contamination at depth. The less-contaminated samples are typically found towards the center of the sill. The geochemistry of the center of the sill varies slightly from that near the sill margins. Samples of gabbronorite taken adjacent to a shale xenolith within the sill do not display a negative Nb 56 �anomaly. The lack of this anomaly, combined with the lack of contamination above the contacts in the sill at the quarry, supports a model in which contamination is occurring at depth rather than during emplacement. ЄNd(T=1100Ma) values of -1.6 to -1.9 for the Riverdale Sill are consistent with this model (Smyk and Hollings, 2009). Fig. 2 Rare earth element primitive-mantle normalised plot (Modified after Hollings et al. 2007 and Sun et al.1989) Geochemical variations within the sill can be interpreted as the result of two pulses of magma, with the first more-contaminated pulse intruded by the second, less-contaminated magma shortly after the emplacement within the shale. The less-contaminated magma may have pushed the contaminated magma to the edges of the sill, leaving a less-contaminated core. References Hollings, P., Hart, T., Richardson, A. and MacDonald, C.A., 2007. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayment, north western Ontario. Canadian Journal of Earth Sciences 44: 1087-1110 Smyk, M. and Hollings, P., 2009. Project Unit 08-021. Mesoproterozoic Midcontinent Rift-Related Mafic Intrusions Near Thunder Bay: Update. Summary of Field Work and Other Activities 2009, Ontario Geological Survey, Open File Report 6240, p. 11-1 to 18-5. Sun, S.-S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalt: implications for mantle composition and processes. Magmatism In the Ocean Basins. Geological Society, London: Special Publication No. 42, pp. 313-345. 57 �PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF A NIPIGON DIABASE SILL RYAN, Andrew J. and ZIEG, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA 16057 ajr2727@sru.edu; michael.zieg@sru.edu In this study, we present preliminary results of a detailed stratigraphic analysis of a Nipigon diabase sill. This sill has been sampled in a drill core (BSE-07-01) that has been made available for study by RPT Uranium Corp and is archived by the Ontario Geological Survey. The sill is intersected over 266 m in the core. In this preliminary investigation, we report on the stratigraphic variations in modal mineralogy (by point counting), geochemical composition (by XRF), and plagioclase composition (by optical methods). The ultimate goal of this study is to examine the relationships between mineralogical, chemical, and textural variations in mafic sills as indicators of emplacement and differentiation processes. In a companion study (Markwood and Zieg, this volume), textural and mineralogical variations are examined in a much smaller (~1.5 m) sill. The small sill was emplaced in a single injection event, while we are hypothesizing that the larger (266 m) sill was emplaced in a series of smaller injections (as was originally proposed for the petrographically similar Logan sills by Blackadar, 1956). By comparing the variations in these two sills, we hope to develop techniques to better recognize reinjection events and characterize the petrologic evolution of mafic intrusions. Of particular interest in the differentiation of this sill is the evidence for post-injection settling of entrained olivine phenocrysts. The settling of phenocrysts to form discrete olivinerich horizons is well-known from, e.g., the Palisades sill (Gorring & Naslund, 1995). In this sill, the distribution of olivine-rich zones (Fig. 1) is similar to the variation trends in plagioclase composition (Fig. 2), for example between 75 and 110 m above the basal contact. Based on this petrographic evidence, it is proposed that this sill is a composite intrusion, built up by a sequence of at least three separate injections. This type of composite intrusion has been recognized in the Beacon sill of Antarctica (Zieg, 2007) that was emplaced without phenocrysts, and in this study we are specifically interested in examining the additional effects of phenocryst settling on the geochemical and textural variations. Future work will focus on further refining the spacing and timing of these reinjection events through additional sampling of olivine-rich zones and textural analysis of potential reinjection horizons. References Blackadar, R.G., 1956. Differentiation and assimilation in the Logan sills, Lake Superior District, Ontario. American Journal of Science, 254, 623-654. Gorring, M.L. and Naslund, H.R., 1995. Geochemical reversals within the lower 100 m of the Palisades sill, New Jersey. Contributions to Mineralogy and Petrology, 119, 263–276. van der Plas, L. and Tobi, A.C., 1965. A chart for judging the reliability of point counting results. American Journal of Science 263, 87-90. Zieg, M.J., 2007. Geochemical, mineralogical, and textural characterization of the Beacon Sill, McMurdo Dry Valleys, Antarctica. EOS, Transactions of the American Geophysical Union, 88(52), 2007 Fall Meeting Supplement, Abstract #V43A-1119. 58 �Figures Figure 1. Stacked modal abundances of major minerals at ~10 m intervals in stratigraphic height. Abundances determined by 1000-1100 grain point count, yielding maximum error of ~ ±3% (van der Plas & Tobi, 1965). Figure 2. Plagioclase composition as a function of stratigraphic height in sill, determined using Carlsbad-Albite method. 59 �QUARTZ ―EYES‖ OF THE MOOSE LAKE PORPHYRY COMPLEX, HEMLO, ONTARIO SCOTT, Robert J. and HILL, Mary Louise, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 rjscott@lakeheadu.ca. The Hemlo gold deposit, Ontario, Canada is located in the Archean Schreiber-Hemlo greenstone belt, Wawa subprovince, of the Superior Province. The deposit has been subjected to a complex history of deformation, metamorphism and intense hydrothermal alteration. The host lithology to most of the gold at Hemlo is the Moose Lake Porphyry Complex (MLPC), a felsic orthogneiss, characterized by the occurrence of quartz ―eyes‖. These quartz ―eyes‖ are actually deformed quartz porphyroclasts. The matrix of the MLPC is mylonitic, composed of quartz, microcline, minor biotite and white mica. The micas define the weak to strong foliation. Other minerals present are calcite, plagioclase, and pyrite, with trace gold, sphene and tourmaline. The porphyroclasts of quartz range in size from 1mm to 5mm and are typically elongated to form the characteristic eye shape. Rarely, feldspar porphyroclasts are present in addition to quartz porphyroclasts. Although common in association with gold deposits, quartz ―eyes‖ are generally rare. These occurrences of quartz porphyroclasts in a mylonitic quartz/feldspar matrix is curious and challenging to explain. Microscopic observations reveal the brittle deformation and alteration of feldspar porphyroclasts in spatial association with ductile deformation of quartz porphyroclasts. The existence of quartz porphyroclasts in a mylonitic matrix could be dependent on the protolith of the MLPC, which is the subject of some debate. Was the protolith a volcanic rock with quartz phenocrysts or is the MLPC a deformed mylonitized granitic intrusive rock? 60 �GOLD IN THE WABIGOON SUBPROVINCE – GIS-BASED MINERAL POTENTIAL MAPPING SIEMIENIUK, Steven, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, sesiemie@lakeheadu.ca HOLLINGS, Pete, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, pnholling@lakeheadu.ca McCUAIG, Cam, University of Western Australia, Centre for Exploration Targeting – M006, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia PORWAL, Alok, University of Western Australia, Centre for Exploration Targeting – M006, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia Using a Geographical Information System (GIS) it is possible to analyze multiple large datasets to identify and quantify spatial relationships. The rationale behind this project is to identify areas of potential gold mineralization in the Wabigoon Subprovince using automated empirical and conceptual techniques in a GIS environment. Specific objectives include defining empirical gold associations in the Wabigoon Subprovince; creating a template for conceptual gold exploration using a mineral systems approach; and generating empirical and conceptual maps of gold mineralization potential. While referred to as the traditional Wabigoon Subprovince, the chosen study area was defined as being the Archean rocks between the English River and Quetico sedimentary basins. The study area excludes the Proterozoic rocks of the Nipigon Basin, and is bounded on the west by the Manitoba border and on the east by the Paleozoic rocks of the James Bay Lowlands (Figure 1). Figure 1: 1:250,000 bedrock geology with study area shaded in black. (Modified from Ontario Geological Survey, Miscellaneous Release Data 126 – Revised) 61 �Mineral potential mapping is applied using either empirical (data driven) or conceptual (knowledge driven) techniques. Recent developments in mathematical geological modeling are beginning to combine the benefits of both techniques but are outside the scope of this project. Weights of Evidence (W of E) and Neural Networks will be used to perform the empirical analyses, with fuzzy logic being the technique applied to do the conceptual targeting. The mineral systems approach that will be used to do the conceptual targeting allows for expert input into both defining the critical and constituent processes in gold mineralizing environments and their subsequent ranking. The mineral systems approach to characterizing ore deposition processes stems from the oil and gas industry and the widely applied source rock-migration-trapseal methodology in petroleum exploration plays. Applied to gold mineralization, this would translate to source-active pathway-physical trap-chemical trap. While many small low-grade small-tonnage gold deposits exist, it is believed that each of the above 4 processes (source-active pathway-physical trap-chemical trap) must be present and active to generate a large gold deposit. At the scale of the Wabigoon Subprovince it is important to select datasets that have relatively uniform coverage across the study area so as to not introduce bias. To achieve this goal, publically available geosciences datasets were examined and various layers representing proxies to critical gold mineralization processes were determined with a mineral systems framework in mind. Once the data compilation and generation stage is complete, data analysis and mineral potential mapping can begin. It is important to realize that these techniques will not provide drillable targets, but rather allow for a quantifiable and justifiable means to examining multiple geosciences datasets in a common framework. Mineral potential mapping forces the examination of the spatial relationships between objects, documents the reasoning behind expert opinions and helps to eliminate ―armwaving‖ in the decision making process. Useful not only in mineral exploration, mineral potential mapping and the techniques used during its application can be useful in environmental risk assessments, land-use planning, and many other related disciplines. References Joly, A., McCuaig, C., Porwal. A., and Bagas, L. (2010) 4D integrative-approach for target generation, GranitesTanami Orogen, Western Australia, Public Lecture, Lakehead University Ontario Geological Survey (2006) 1:250 000 Scale Bedrock Geology of Ontario, Ontario Survey, Miscellaneous Release Data 126 - Revised. Geological Peterson, D. M. (2001) Development of Archean Lode-Gold and Massive Sulfide Deposit Exploration Models using Geographic Information System Applications: Targeting Mineral Exploration in Northeastern Minnesota from Analysis of Analog Canadian Mining Camps, Unpublished PhD Thesis, University of Minnesota, 503 pages. Porwal, A. K. (2006) Mineral Potential Mapping with Mathematical Geological Models, Published PhD Thesis, Utrecht University, 289 pages. 62 �THE RING OF FIRE: AN OVERVIEW OF THE GEOLOGY, MINERAL DEPOSITS AND EXPLORATION HISTORY OF THE MCFAULDS LAKE AREA SMYK, Mark, Resident Geologist Program, Ontario Geological Survey, Thunder Bay ON STOTT, Greg, Precambrian Geoscience Section, Ontario Geological Survey, Sudbury ON, and ATKINSON, Brian, Resident Geologist Program, Ontario Geological Survey, South Porcupine ON The McFaulds Lake area of northern Ontario has been the focus of intense exploration activity since 2002, already resulting in the discovery of a remarkable number of varied metallic mineral deposits, including: Volcanogenic massive sulphide (VMS) copper-zinc-silver; o e.g. McFaulds #3 deposit (Indicated: 802 000 t @ 3.75% Cu, 1.1% Zn) o e.g. McFaulds #1 deposit (Inferred: 839 700 t @ 1.03% Cu, 2.38% Zn) Orthomagmatic copper-nickel-PGE; o e.g. Eagle‘s Nest deposit (Indicated: 6.9 Mt @ 2.04% Ni, 0.95% Cu, 1.3 g/t Pt, 3.4 g/t Pd; Inferred: 4.3 Mt @ 1.42% Ni, 0.87% Cu, 0.8 g/t Pt, 3.4 g/t Pd) Orthomagmatic chromium + iron + vanadium + titanium + PGE; o e.g. Black Thor deposit (Inferred Mineral Resource: 69.5 Mt @ 31.9% Cr2O3 ; 25% Cr2O3 cut-off); and Lode gold o e.g. Triple J occurrence: (>1.0 km strike length, depth >300 m.; grades ranging from 0.3 to 30.0 g/t Au) The arcuate area termed the ―Ring of Fire‖ is underlain by the folded Neoarchean McFaulds Lake greenstone belt and subvertically dipping mafic to ultramafic intrusions, at least some of which are layered and crosscut the western portion of the belt. An altered, feldspar-phyric dacite associated with the VMS mineralization returned an age of 2737 + 7 Ma (Rayner and Stott 2005). Limited radiometric dating, field constraints indicate that there might be at least two main ages of layered intrusions preserved in this region: an older, Mesoarchean suite, ca. 2808 Ma (e.g. Highbank Lake intrusion), close to the southwestern boundary of the Oxford-Stull domain, and a younger, Neoarchean suite of intrusions (e.g. Ring of Fire Intrusion, 2735 Ma; Gowans et al. 2010) hosting the copper-nickel-PGE and chromite deposits. This latter intrusive event is synchronous with local VMS mineralization and may represent the synvolcanic heat source for the VMS system. Over 40 companies have been involved in exploration in the Ring of Fire, spending approximately $100 million since 2002. Approximately 32 000 claim units, totaling over 5000 km2, were staked. The scarcity of outcrop and Paleozoic cover in the eastern portion of the area have created a reliance on detailed geophysical surveys and diamond drilling to identify exploration targets, delineate deposits and elucidate Archean basement geology. Gowans, R., Spooner, J., San Martin, A.J., and Murahwi, C. 2010. Technical report on the Mineral Resource Estimate for the Blackbird chrome deposits, James Bay Lowlands, northern Ontario, Canada; unpublished company report, Noront Resources Limited, 188p. Rayner, N. and Stott, G.M. 2005. Discrimination of Archean domains in the Sachigo subprovince: a progress report on the geochronology; in Summary of Field Work and Other Activities 2005; Ontario Geological Survey, Open File Report 6172, p.10-1 to 10-21. 63 �STRUCTURAL CONTROL AT HAMMOND REEF GOLD DEPOSIT NORTH OF ATIKOKAN, ONTARIO STINSON, Victoria R. and HILL, Mary Louise, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1; vrstinso@lakeheadu.ca Hammond Reef is located 22km north of Atikokan, Ontario, within the Marmion Lake batholith adjacent to the Finlayson Lake greenstone belt. Gold mineralization at Hammond Reef is commonly found associated with sulphides, such as pyrite, and in its native form. The main ore zones, and historical mine workings, are primarily composed of muscovite schist with greater than 20cm quartz veins within a northeast trending shear zone. The main ore zone exhibits ductile, brittle-ductile, and brittle deformation, chloritization, sericitization, and carbonitization. Intensely sheared muscovite schist hosts gold when adjacent to lithons which provide competency contrasts. Areas outside of the main ore zones also show prospect for mineralization as they too underwent similar deformation, metamorphism, and alteration histories. In granitoids the pyrite mineralization is found most abundantly in areas of rheological contrast, such as adjacent to competent lithons. The areas with the greatest amount of pyrite and gold mineralization have the largest and most intense ductile shear zones and the greatest abundance of lithons. In pegmatites the sulphide mineralization and alteration is generally absent but gold is found within structures similar to the gold-bearing granitoid. In the field, areas of highest competency contrast that are the most prospective for gold can be identified based on intensity, size, and spacing of ductile shear zones and lithons. At Hammond Reef there is a correlation between width of shear bands and intensity of ductile strain. Strain can be described as weak if shear bands are greater than 5cm wide, moderate with shear bands 5 to 2cm wide, strong with shear bands 2cm to 2mm wide, and intense with shear bands less than 2mm wide. Lithons are described based on size, lithology, composition, and characteristics which make the rock more competent than the surrounding shear zone, such as differing lithology, composition, folding, increased quartz veins, etc. Where outcrop is scarce, or pyrite is absent in areas of intense deformation adjacent to lithons, it is difficult to locate mineralized zones and other structures are required to locate areas of highest strain. Structures to be noted in the field which indicate high strain ductile and brittle-ductile deformation, and therefore the greater potential for mineralization, are pinch-and-swell, boudinage, foliation, lineation, pressure shadows of amphibole, quartz, and pyrite, drag and sheath folds, lithons, and S-C fabrics. Low-lying areas and rivers are also geomorphological indicators of eroded shear zones at Hammond Reef. Structures that have undergone the greatest amount of deformation, and therefore areas that have a history of deformation in a ductile, brittle-ductile, and brittle manner, have the greatest potential for mineralization at Hammond Reef when formed adjacent to a competent lithon. Long deformation histories result in the greatest amount of porosity and permeability, however brief, and therefore gold mineralization. Ductile structures which have been crosscut by brittleductile and brittle structures are the best indicators for mineralization as they have been reopened repeatedly through time. Prospecting along the splays of the regional shear zone utilizing knowledge of competency contrasts that have undergone brittle-ductile and ductile deformation histories should be noted in the Finlayson Lake greenstone belt and Marmion batholith area. 64 �IDENTIFICATION AND CHARACTERIZATION OF FIBROUS ZEOLITES IN WESTERN NORTH DAKOTA TRIPLETT, Jason1, jason.triplett@ndsu.edu, SAINI-EIDUKAT, Bernhardt1,2, FEIT, Sharon1, and DOLEZAL, Dillon1 1Department of Geosciences, 2Environmental and Conservation Sciences Program, North Dakota State University, Fargo, ND 58108-6050. The naturally occurring, non-asbestos, fibrous zeolite mineral erionite is a concern due to its potential for causing pleural malignant mesothelioma (PMM) in humans (Baris et al., 1987). PMM is a rare cancer usually associated with exposure to fibrous asbestos minerals. Studies have shown that low-level environmental exposure to erionite-bearing altered volcanic bedrock may be the explanation for a high PMM rate in certain locations (Metintas et al., 1999). Epidemiologic studies suggest erionite could potentially be more active and more toxic than some forms of asbestos (Metintas et al., 1999). The presence of erionite in North Dakota was reported by Forsman (1986) as occurring in the Killdeer Mountains (KDM), one of a number of buttes in western North Dakota. It was discovered in volcanic tuffs of the late Oligocene to early Miocene Arikaree Formation. Other formations throughout western North Dakota, South Dakota, and Montana have been recognized as erionite-bearing (Goodman and Pierson, 2010). Due to the possible health risks associated with inhalation of the fibers, hazard mapping (Forsman, 2006) was undertaken by the North Dakota Department of Health (NDDoH), in cooperation with the North Dakota Geological Survey (NDGS) and the Environmental Protection Agency (EPA). These investigations led to gravel quarry restrictions, gravel use restrictions, dust control measures, and guidance plans to control and reduce the overall exposure by businesses and private landowners working in close proximity to the bedrock formations and/or gravel quarries which potentially contain erionite (NDDoH 2009). For the study reported here, samples were collected from regions in ND where erionite is known or suspected to be present. A total of 37 rock and/or soil samples were taken from North and South Killdeer Mountains in Dunn County, West and East Rainy Buttes in Slope County, and White Butte, also in Slope County. Sample preparation consisted of breaking down each sample and placing it in a water column to separate any zeolite fibers from larger size fractions. Suspended materials, including the zeolite fibers, were collected from the water column and vacuum filtered. All 37 filtered samples were analyzed using powder X-ray diffraction (XRD). Of those, the 8 which showed the strongest zeolite patterns were analyzed using scanning electron microscopy (SEM). Visual inspection of the samples was conducted (Fig. 1), as well as determination of a preliminary chemical composition of the suspected zeolite fibers using SEM/EDS. The remaining 29 samples, which did not show as strong XRD indications of erionite or offretite, were also scanned by SEM to identify any other potential zeolite containing samples. The SEM/EDS chemical composition data of fibers were plotted for comparison with data from Forsman (1986) and Passaglia et al. (1998) (Fig. 2). The results of this study are consistent with those of Lowers and Meeker (2007), who analyzed fibers separated from ND soil and roadbed samples. SEM/EDS data plot in both regions of erionite and offretite. Ongoing and future work includes stratigraphic correlation, additional powder XRD and SEM analysis, and electron microprobe analysis. 65 �Figure 1: Concentrated zeolite fibers from South KDM. Left: sample jwt080602-01. Right: sample jwt080603-03 Figure 2: Comparison of SEM/EDS analyses. Passaglia et al. (1998): erionite (open diamond) and offretite (open box). Forsman (1986): erionite (bold, open box). This study: erionite/offretite (filled box). References Baris, I., Artvinli, M., Saracci, R., Simonanto, L., Pooley, F., Skidmore, J., and Wagner, C., 1987, Epidemiological and environmental evidence of the health effects of exposure to erionite fibers: A four-year study in the Cappadocian region of Turkey. International Journal of Cancer, 39:1, 10-17. Forsman, N.F., 1986, Documentation and diagenesis of tuffs in the Killdeer Mountains, Dunn County, North Dakota. Report of Investigation No. 87, North Dakota Geological Survey, 13 p. Forsman, N.F., 2006, Erionite in tuffs of North Dakota: The need for erionite hazard maps. GSA Abstracts with Programs, 38:7, 366. Goodman, B.S. and Pierson, M.P., 2010, Erionite, a naturally occurring fibrous mineral hazard in the tri-state area of North Dakota, South Dakota, and Montana. GSA Abstracts with Programs, 42:3, p. 5. Lowers, H.A., and Meeker, G.P., 2007, Denver microbeam laboratory administrative report 14012007: U.S. Geological Survey Administrative Report, 11 p. Metintas, M., Hillerdal, G., and Metintas, S., 1999, Malignant mesothelioma due to environmental exposure to erionite: follow-up of a Turkish emigrant cohort. Eur. Respiratory Journal 13, 523-526. North Dakota Department of Health, http://www.health.state.nd.us/EHS/Erionite/. Accessed Aug. 17, 2009. Passaglia, E., Artioli, G., and Gualtieri, A., 1998, Crystal chemistry of the zeolite erionite and offretite. Amer. Mineral. 83, 577-589. Support from NIH grant P20 RR016471 from the INBRE program of the National Center for Research Resources is gratefully acknowledged. 66 �REVISITING THE BARABOO BRECCIAS VAN LANKVELT, Amanda, and BJØRNERUD, Marcia, Geology Department, Lawrence University, 711 E Boldt Way, Appleton, Wisconsin 54911 USA. Many outcrops of Baraboo-interval quartzites in Wisconsin contain brecciated zones with angular quartzite fragments in a network of quartz veins. The formation of the breccias has generally been assumed to coincide with the folding and deformation of the quartzite at 1630 Ma (Holm et al 1998), but muscovite in some of the quartz veins yields well-defined 40 Ar/39Ar ages of ~1450-1460 Ma, approximately coeval with the emplacement of the Wolf River Batholith (Medaris et al 2002; 2003). By re-examining the textures in the breccias, we seek to resolve some of the ambiguity about their formation. Samples of breccia and quartz veins were collected from five sites: three in the Baraboo Syncline itself (Ableman‘s Gorge and Williams Quarry on the north limb; Devil‘s Lake on the south limb); and two from other Baraboo-interval quartzites (Necedah Mound and Waterloo). Muscovite grains in samples from Williams Quarry, Devil‘s Lake and Waterloo have yielded ‗Wolf River‘ ages (Medaris et al. 2002; 2003). Dateable muscovite is not found in the veins at Ableman‘s Gorge and is present but has not been analyzed at Necedah Mound. The outcrop-scale geometry of the breccias and veins is not the same at every site, and their relationship to other structures is not always clear. The brecciated zone at Ableman‘s Gorge is tens of meters thick, traceable for km along strike, and generally bedding-parallel. The breccias from Williams Quarry and Devil‘s Lake, in contrast, are relatively localized and have more complex geometries not obviously related to bedding or to other secondary structures. The breccias at Necedah and Waterloo are meters thick and can be traced for up to 100 m, but their relationship with primary layering, which is difficult to discern in the massive quartzite, is not known. Internally, the breccias are quite similar to each other. The breccia fragments are angular and appear to be purely dilational, since there is no evidence for cataclasis or shear offset. The fragments of quartzite have a ‗jigsaw‘ appearance and seem to have been separated almost isotropically through the growth of vein materials. The breccia vein material is composed primarily of coarse-grained quartz with small amounts of muscovite and/or kaolinite. Primary void space is present in some of the breccias. Several types of textural evidence point to multiple stages of mineral growth in the veins. Pronounced differences in the size and shape of quartz grains within individual samples suggest distinct episodes of growth, and the coarsest, commonly euhedral, quartz crystals are optically zoned, with growth bands marked by planes of fluid and mineral (mainly quartz and muscovite) inclusions. In optical cathodoluminescence microscopy, the zoned vein quartz, as well quartz grains in the host rock, are entirely dark, indicating either that the all of the quartz is compositionally uniform and/or that no ‗activator‘ trace elements are present. The muscovite grains from which Ar ages have been obtained have generally grown epitaxially on the vein quartz although in some cases (e.g., Devil‘s Lake) the textural relationships point to multiple, possibly alternating, episodes of quartz and muscovite growth. In samples from all localities, some or most of the vein quartz exhibits undulose extinction and quasi-ductile microfractures, indicating that it was subjected to significant deviatoric stress (> ~100 MPa), but small finite strains, some time after its formation. 67 �Provisional Interpretations: The dilational nature of the breccias and textural evidence for multiple, distinct episodes of mineral growth points to a sustained period of elevated fluid pressures that transiently and repeatedly reached supralithostatic values. While the vein quartz could easily have formed from locally derived silica-rich fluids, there is no source of potassium in the mineralogy of the supermature host rock (Medaris et al, 2003) to allow formation of the observed muscovite. This indicates that some of the fluids responsible for the veining came from a remote source. Although some of the breccias do occur along layer-parallel zones, this is unlikely to reflect fluid flow along primary, stratigraphically controlled channels of high permeability since the breccias formed during or after the folding event. It is more likely that flexural slip between stratigraphic layers during folding created layer-parallel zones of increased permeability that provided channels for vein-forming fluids. The textural and deformational observations in combination with the tectonic history of the Midcontinent provide several possible interpretations. One is that the breccias and most of the vein fillings formed during the Baraboo event at 1630 Ma, and that the plastic deformation in the vein quartz represents stresses late in the orogenic cycle. In this interpretation, the muscovite must have formed later, probably from fluids related to the emplacement of the Wolf River Batholith (WRB). This scenario requires some mechanism for generating elevated and fluctuating fluid pressures, and large fluid pressure gradients, to create a pervasive ‗Wolf River‘ signature in Baraboo-type quartzites (including the Sioux Quartzite of SW Minnesota; Naymark et al., 2001). Another permissible (if radical) interpretation is that the Baraboo deformation occurred at 1450 Ma, and that the WRB was not ‗anorogenic‘ as traditionally believed, but emplaced at the end of an orogenic cycle. This is consistent with evidence for a 1.4 Ga tectonic event in New Mexico (Daniel and Pyle, 2006) and would provide a source for both the K-rich fluids and the larger deviatoric stresses needed for deformation of the vein quartz. A third possible interpretation is that the vein-forming fluid flow events occurred in Wolf River time but that the vein quartz was deformed by far-field stresses from the Grenville orogeny at ca. 1 Ga. References cited Daniel, C. and Pyle, J., 2006. Monazite-xenotime thermochronometry and Al2SiO5 reaction textures in the Picuris Range, northern New Mexico: New Evidence for a 1450-1400 Ma Orogenic event. Journal of Petrology 47, 97-118. Holm, D., Schneider, D., & Coath, C., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907-910. Medaris, L.G., Jr., Singer, B.S., Brown, P.E., Jicha, B.R. & Smith, M.E., 2002. Wolf River-age brecciation in the Baraboo Quartzite, Wisconsin: Implications for Proterozoic tectonics in the Lake Superior region. Proceedings of the Institute on Lake Superior Geology 48, 24-25. Medaris, L.G., Jr., Singer, B.S., Dott, R.H., Jr., Naymark, A., Johnson, C.M. & Schott, R.C., 2003. Late Paleoproterozoic climate, tectonics and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites. Journal of Geology 111, 243-257. Naymark, A., Singer, B., Medaris, L.G., Jr., 2001. Recognition of post-1630 Ma fluid-driven metamorphism in Baraboo interval quartzites by means of laser probe geochronology. Proceedings of the Institute on Lake Superior Geology 47, 66-67. 68 �POST PENOKEAN AGE OF MINERALIZATION ON THE MARQUETTE RANGE, MICHIGAN T. D. Waggoner, Geological Consultant Email: thomaswaggonergeo@hotmail.com The Champion Mine located on the south limb of the Marquette Syncline in Western Marquette County, Michigan is noted for the diverse hydrothermal mineral assemblage that includes molybdenite, native gold, boron and bismuth. The molybdenite is found associated with greisen and with the magnetite ore. Moly occurs as coarse flakes and books that can be readily separated from the gangue. Test work was conducted under contract by David Selby in England at Durham University. The technique utilized is found in the reference listing below. With the advent of the age determination technique using isotopic Rhenium (Re) and Osmium (Os) it was natural that molybdenite would be tested as it contains an abundance of both. The decay constant of 187 Re to 187Os is the key to the age determination. Re-Os date delineation utilizes the isochron dating method assuming 187Os did not exist at the time of emplacement. The isochron plots the ratio of radiogenic 187Os to non-radiogenic 188Os against the ratio of the parent isotope 187Re to the non-radiogenic isotope 188Os. It was long suspected the alteration minerals were emplaced at the end of the Negaunee iron formation ~1874 Ma or at least no younger than the Humboldt granite (~1760 Ma) located five miles to the southeast of the Mine. It then came as some surprise when the molybdenite associated with the greisen showed an age of 1672+or-7 Ma and the molybdenite in the magnetite yielded an age of 1570+or-7 Ma. It may be that a continuum of geological events is present in the Superior Province of the Canadian Shield. Further age dating of the sulfides associated with the hard ores on the Animikie Basin will help further define the hydrothermal events. References Stein, H.J., 1999, The Status of the Re – Os Chronometer for Dating Sulfides and Oxides, in Mineral Deposits: Processes to Processing, Stanley et al. (Eds), p. 1291-1294. Selby, D. et al., 2001, Re – Os Geochronology and Systematics in Molybdenite from the Endako Porphyry Molybdenite Deposit, British Columbia, Canada, Econ. Geol. v. 96, p. 197-204. Selby, D. et al., 2001, Late and Mid-Cretaceous Mineralization in the Northern Canadian Cordillera: Constraints from Re – Os Molybdenite Dates, Econ.Geol. v. 96, p. 1461-1467. Selby, D. et al, 2009, Re – Os Sulfide (Bornite, Chalcopyrite and Pyrite) Systematics of the Carbonate-Hosted Copper Deposits at Ruby Creek, Southern Brooks Range, Alaska, Econ. Geol. v. 104, p. 437-444. 69 �NEW WAYS TO STUDY OLD PROBLEMS WEIBLEN, Paul W., Minnesota Geological Survey, pweib@umn.edu This poster shows how two recently developed instruments, the Electric-Pulse Disaggregator, and the Tabletop scanning electron microscope, provide new methods to characterize the textures, minerals, and mineral compositions of rock samples See http://www.marinereef.com and http://www.ndsu.edu/pubweb/~sainieid/PPD/Curator_ms.shtml. Samples from the Paleoproterozoic Sudbury Ejecta Layer are used as examples. The scanning electron microscope (SEM) and the electron microprobe (EMP) are widely used to augment the petrographic microscope as tools to characterize rocks. The tabletop scanning electron microscope (TSEM) provides a new and simpler tool than the SEM for obtaining images and energy dispersive x-ray (EDX) spectra (including more accurate analyses of Carbon). Since it is not necessary to evaporate a carbon film on polished thin sections or mineral grains, samples can be transferred directly from a binocular, or petrographic microscope. The convenience and time saved is significant when studying many samples (Figs. 1 and 2). Electric pulse disaggregation (EPD) provides a mechanism for liberating individual mineral grains while preserving their three dimensional surfaces. This method of mineral separation has several additional advantages over conventional mineral separation methods. First, the separated grains provide clean samples (Figs. 3 and 4) for energy dispersive x-ray, electron microprobe and bulk chemical analysis (for the two first methods boundary effects of adjacent phases in polished thin sections are eliminated). Secondly, EPD products provide samples in which interesting minerals of low concentration, e.g., zircon, and platinum group minerals can be efficiently concentrated and recovered for further study. Thirdly, concentrates of EPD products can provide samples of minerals of very low concentration or small grain size that may not be recognized in thin section or only postulated on the basis of bulk chemistry. Fig. 1. Photomicrograph of Sudbury Ejecta containing glassy fragments and spherules in a carbonate matrix. Cross polarized transmitted light. Fig. 3. EPD products of sample OB KS GTP. Note the glass spherules are cleanly separated from the carbonate matrix. 70 Fig. 2. Photomicrograph of fine-grained matrix in iron formation breccia, that cannot be resolved optically. Plain polarized light. Fig. 4. EPD Products of Sample G 652. The fine-grained matrix actually consists of euhedral crystals. Some as small as 0.050 mm. �SUDBURY IMPACT EJECTA – AMPHIBOLES IN SAMPLES FROM THE VICINITY OF THE GUNFLINT TRAIL, MINNESOTA WEIBLEN, Paul W., Minnesota Geological Survey, pweib@umn.edu JIRSA, Mark, Minnesota Geological Survey, jirsa001@umn.edu MCSWIGGEN, Peter, McSwiggen Associates, PMcS@McSwiggen.com Over two dozen outcrops of Sudbury Meteorite Impact Ejecta have been found and mapped in the vicinity of the Gunflint Trail in Minnesota (Fig. 1). A generalized stratigraphic column has Fig. 1. Geologic map depicting the Sudbury Meteorite Impact Layer, in the Gunflint Trail area. The layer lies within the contact aureole of the Mesoproterozoic intrusive rocks. Hence contact metamorphism combined with weathering (See Figs. 4 and 7) make comparison of mineral assemblages, textures, and compositions with Sudbury ejecta elsewhere problematic. been derived from these exposures (See Jirsa, this volume). The complete column is not exposed at any of the exposures. We have studied the occurrence of amphibole in the four samples noted on Fig. 1. See Figs. 2 -4 for photos. These samples provide a first look at differences in occurrence, textures, and compositions of amphibole in the basal breccia (G652), the zone of cmsized accretionary lapilli (G033 and G498, same outcrop). and an upper zone of smaller (< 3mm), pellets (G508) that are generally not zoned except for finer-grained outer rims. Optical and scanning electron microscopy (SEM), electron microprobe analysis (EPA), x-ray diffraction (XRD), and electric-pulse disaggregation (EPD) (See Weiblen this volume) have been used to characterize textures and mineral compositions. Chemical analyses have been obtained on select samples. A sampling of the results thus far is presented in Figs. 5-11. Fig. 2. Outcrop photo G 652. Fig. 3. Outcrop photo G033 Fig. 4. Sample photo G508 The photomicrographs below are all cross polarized light images. Fig. 5. G652Z-Porphyroblast Fig. 6. G498-Lapilli Matrix interface 71 Fig. 7. G508A-Pellet in Matrix �Fig. 9. Back Scatter Electron Image of EPD products from G498 Z. Euhedral crystals as small as 0.005 mm are found. Fig. 8. A portion of a 1.5 mm electron microprobe traverse across a matrix pellet interface in G 508A.. See text for discussion. G 652 Amphibole-Pyroxene Traverse Mole Fraction 1 0.8 Si 0.6 Ca 0.4 Fe Mg 0.2 142 143 135 139 140 144 0 Selected Points Fig. 10 A portion of a 1.5 mm electron microprobe traverse from the center of one accretionary lapilli across matrix into the center of an adjacent lapilli in G 498Z.. See text for discussion. Fig. 11 Selected analyses across the Amphibole (144-139)-Pyroxene (135-142) interface shown in Fig. 5. We have found it useful to distinguish the small ~< 3mm, generally unzoned ―lapilli‖ with the term ―pellet‖ (Figs. 4, 7 & 8). Unaltered/weathered pellets are just discernible in the alteredweathered rim of sample G508 A (Fig. 4). We have separated intact pellets from the matrix of altered-weathered rims by EPD. Since the pellets and the matrix have the same quartzcarbonate-amphibole mineral assemblage, it is curious that the pellets have remained impervious to alteration-weathering in contrast to the matrix. The data in Fig. 8 indicates negligible Ca in matrix amphibole in contrast to the Ca bearing pellet amphibole. Is this difference a metamorphic or an alteration-weathering effect? EPD traverses across accretionary lapilli indicate the zoning is defined by variations in quartz, carbonate, and amphibole. If amphibole formed from a Mg,Fe carbonate as found in lapilli elsewhere, we thought it remarkable that the original zoning would be preserved. However we find that the reaction products are fine-grained euhedral crystals (Fig. 8) and the zoning is preserved on a scale of 0.010-0.030 mm! The products of the reaction Mg carbonate + quartz + water + oxygen are Mg,Fe amphibole + calcite + carbon dioxide. [ 7((Mg,Fe)Ca(CO3)2) + 8SiO2 +H2O+7O2 = (Mg,Fe)7Si8O22(OH)2 +7CaCO3 +14CO2 ] This reaction could be the origin of some of the relatively pure calcite found in Gunflint ejecta samples (Incidentally, thermodynamic data indicate that dolomite + quartz + water break down to tremolite + calcite at ~ 190 oC and 1 atm.). Another example of contact metamorphism in the Gunflint area is the occurrence of porphyroblasts in some samples (Fig. 5). This preliminary data indicates that the ejecta samples contain much information on the contact aureole in the Gunflint area. For further examples of how our studies augment petrographic examination (Figs.2-4) are presented in an accompanying poster session (See Weiblen, this volume). 72 �GEOLOGIC MAP OF THE CENTRAL SEINE BAY/ BAD VERMILION LAKE INTRUSION WITH ACCOMPANYING AIRBORNE GEOPHYSICS, RELATING TO NUMAX RESOURCES, INC., MINE CENTRE PROPERTY, MINE CENTRE, ONTARIO WHITE, Chris R., 1776 W. Chig-A-Big Rd., Ely, MN 55731, white812@d.umn.edu, and ALBERS, Paul B., 3925 Willowwood St. SW, Prior Lake, MN 55372, pbalbers@gmail.com Numerous massive oxide (Fe-Ti-V) outcrops were discovered in 2004 by Numax Resources, Inc. on their Mine Centre property, which is located in northwestern Ontario. This was followed up by prospecting, mechanical trenching, channel sampling, two ground geophysical surveys (walking magnetic and VLF electromagnetic), and three diamond drill holes over the next five years. Numerous channel and drill core samples assayed greater than 60% Fe2O3, 15% TiO2, and 3000 ppm V. This encouraging data prompted Numax Resources, Inc. to requisition a detailed (1:5000 scale) geologic map of the property. Along with the property map, ten previously excavated trenches were mapped in greater detail (1:400 scale). Numerous 1 to 30 m-wide continuous to semi-continuous, massive to semi-massive oxide layers (Figs. 1 and 2) trend in an east-west to northeast-southwest orientation across the 12.5 km-long Mine Centre property (Bernatchez, 2009). These massive and semi-massive oxide layers correlate well with magnetic highs discovered during the Ontario Geological Survey airborne geophysical survey (OGS, 2009). The massive oxide layers are bound and interlayered with various medium- to coarse-grained mafic intrusive rocks consisting of gabbro, pyroxenite, and anorthosite (Albers and White, 2010). In the western portion of the property, the rocks are subvertical and dip steeply to the north. In the eastern part of the property, the rocks dip 30-60 degrees to the northwest. Tops are interpreted to the north based on graded modal layering of silicates. The northern contact of the Seine Bay/Bad Vermillion Lake intrusion is bound by felsic intrusive rocks consisting of granodiorite, quartz diorite, tonalite, and trondhjemite; while the southern contact consists of mafic volcanic rocks including massive, amygdaloidal, and plagioclase-phyric basalt. Locally, subvertical shear zones are adjacent to major lithologic contacts and massive oxide layers, though individual massive oxide layers appear to be largely undeformed. Upcoming work includes systematically-spaced diamond drilling of high grade Fe-Ti-V layers, petrographic analysis through numerous sections of the mafic intrusion, electron microprobe study of the massive oxide mineralization, 3D geologic modeling, and continued detailed geologic mapping. 73 �Figure 1. One of numerous massive oxide outrops (1.5 m wide) within sheared and/or lineated gabbro in Trench L28. Figure 2. Hematite alteration of massive oxide in Turtle Trench. Selected References Albers, P.B., and White, C.R. 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario. Unpublished report prepared for Numax Resources, Inc. Bernatchez, R.A. 2009. A Report on the Fe-Ti-V and Cu-Ni-PGM and other Mineral Potential for the Mine Centre Property. Unpublished report prepared for Numax Resources, Inc. Poulsen, H.K. 2000. Precambrian Geology and Mineral Occurrences, Mine Centre – Fort Frances area; Ontario Geological Survey, Map 2525, scale 1:50,000. Ontario Geological Survey. 2009. Ontario airborne geophysical surveys, Magnetic and Electromagnetic Data, Grid and Profile Data (ASCII and Geosoft formats) and Vector Data, Mine Centre Area; Ontario Geological Survey. Geophysical Dataset 1061a ISBN 978-1-4435-0854-4 [DVD] ISBN 978-1-4435-0855-1 [ZIP FILE]. 74 �Map showing location of Frances Memorial Sports Centre, starting point for several of the field trips. 75 �Map showing location of Backus Community Center in Relation to the Rainy River Inn (formerly the Holiday Inn). The banquet will be at the Community Center, located at 900 5th Street, International Falls. 76 � https://digitalcollections.lakeheadu.ca/files/original/32399adc6c38eb74de1ea21245ef73a3.pdf c2bc448b184da4557bd192d8d0e7624f PDF Text Text 56TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY INTERNATIONAL FALLS, MINNESOTA MAY 19-22, 2010 PROCEEDINGS VOLUME 56 PART 2 – FIELD TRIP GUIDEBOOK ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 56TH ANNUAL MEETING MAY 19-22, 2010 INTERNATIONAL FALLS, MINNESOTA PETER HOLLINGS, PETER HINZ, MARK SMYK, MARK JIRSA, AND TERRY BOERBOOM Co-Chairs Proceedings Volume 56 Part 2 – Field Trip Guidebook Edited by Terrence J. Boerboom, Minnesota Geological Survey Cover photo: Moderately deformed Seine conglomerate containing metavolcanic and granitoid clasts, with clast tiling due to dextral deformation. Hwy 11, 1 km east of Horsecollar Junction �56TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 56: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN) TRIP 2: GEOLOGY OF AN ARCHEAN SUCCESSION AT ATIKOKAN (CAN) TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT (CAN) TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX (US) TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN) Reference to material in Part 1 should follow the example below: Published by the 56th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ii �TABLE OF CONTENTS PROCEEDINGS VOLUME 56 PART 2— PROGRAM AND ABSTRACTS TRIP 1: MINERAL DEPOSITS OF THE RAINY RIVER AREA (CAN)………………. .................... 1 TRIP 2: GEOLOGY OF AN ARCHEAN SUCCESSION AT ATIKOKAN (CAN) ………………. ..... 14 TRIP 3: STRUCTURAL GEOLOGY ALONG THE QUETICO FAULT ((CAN) ………………. ........ 47 TRIP 4: ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND LITTLE AMERICA GOLD MINE (US) ............................................... ……………….76 TRIP 5: ASH RIVER NEUTRINO DETECTOR LABORATORY AND THE ARCHEAN VERMILION GRANITIC COMPLEX (US) ………………........................................... 79 TRIP 6: TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY (US) ………………. ................................................... 81 TRIP 7: MINERAL DEPOSITS OF THE MINE CENTRE – RAINY LAKE AREA (CAN) .. ………….91 TRIP 8: GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE (CAN)……….126 The editor (Terry Boerboom) would like to sincerely thank all of those who contributed to this field trip guidebook. The time and effort that went into the preparation of the field trip stops and write-ups are greatly appreciated by all. iii �. iv �Field Trip 1 Mineral Deposits of the Rainy River area, Ontario N.W. (Wally) Rayner and staff Rainy River Resources Ltd. Suite 303 – 1620 West 8th Avenue Vancouver, BC V6J 1V4 Visible gold in drill hole NR09-446 (returned 1088.45 g/t Au over 1.5 m), courtesy www.rainyriverresources.com 1 �Field Trip 1 MINERAL DEPOSITS OF THE RAINY RIVER AREA, ONTARIO B.W. (Wally) Rayner, Rainy River Resources, Ltd. INTRODUCTION Rainy River Gold Deposit, Rainy River Resources Limited (modified from: http://www.rainyriverresources.com/The-Project/Rainy-RiverOverview/default.aspx) Overview The Rainy River Gold Project (RRGP) is an advanced-stage gold exploration project situated in the southern half of Richardson Township, approximately 50 km northwest of Fort Frances (Fig. 1). In June 2005, Rainy River Resources Limited acquired a 100% interest in the project from Nuinsco Resources Limited and has since added approximately 4 million ounces in total gold resources to the deposit. The property has year-round road access, power lines in close proximity and a railway 21 km to the south. With over $65 million in its treasury, Rainy River plans to carry out aggressive exploration in 2010. In addition to over 70,000 m of planned drilling, the company will be completing a Preliminary Economic Assessment in the third quarter of 2010. The company is also conducting Metallurgical Testing and Environmental Baseline and Geotechnical Studies. (Portions of the following text have been directly extracted from SRK Consulting‘s Mineral Resource Evaluation Report dated July 10, 2009.) History of the Rainy River Gold Project (RRGP) The Rainy River Project has attracted exploration interest since 1967. Various companies including Noranda, International Nickel Corporation of Canada (INCO), Hudson‘s Bay Exploration and Development and Mingold Resources operated in the area centered on the RRGP between 1967 and 1989. The Ontario Geological Survey undertook geological mapping in 1971 and again in 1987-88 in conjunction with and a rotasonic overburden drilling program. Nuinsco undertook exploration activities between 1990 and 2004, with Rainy River continuing from 2005 onwards. Nuinsco drilled a series of widely spaced reverse circulation drill holes from 1994 to 1998, defining a 15 km long ―gold-grains-in-till‖ dispersal train emanating from a 2 �thickly overburden-covered, 6 km2 "gold-in-bedrock" anomaly. Nuinsco completed a series of diamond drilling programs to assess the mineral potential of the above anomalies which led to the initial discovery of the 17 Zone in 1994. Nuinsco subsequently discovered the 34 Zone in 1995 and 433 Zone in 1997. Between 1994 and 1998, Nuinsco drilled 597 reverse circulation holes and 217 diamond drill holes (49,515 m). These were mostly in the Richardson area. The 34 Zone was further drill-tested between 1999 and 2004. In June 2005, Rainy River completed the acquisition of a 100% interest in the project from Nuinsco. In the same year Rainy River re-logged key sections of the historical core drilled on the property and then input all of the data into a GIS database. Rainy River subsequently drilled in excess of 100 reverse circulation holes in three phases to better define the ―gold-in-till‖ and ―gold-in-bedrock‖ anomalies. In 2007, Rainy River discovered the ODM Zone, which is correlated as being the western extension of the 17 Zone. Rainy River completed a fourth phase of exploratory reverse circulation drilling at this time. Between 2005 and 2007 an additional 209 diamond drill holes for 95,340 m were drilled. In April 2008, a mineral resource estimate for the RRGP was completed. At a cut-off grade of 0.5 g/t Au, an Indicated resource of 34,238,000 t at 1.26 g/t Au and an Inferred resource of 67,564,000 t at 1.03 g/t Au were reported. Figure 1. Location of the Rainy River property (from http://www.rainyriverresources.com/Theme/RainyRiver/files/Technical_Reports/July%2010,%2 02009.pdf) 3 �In 2008, Rainy River drilled an additional 112 diamond drill boreholes for 59,719 m and completed a fifth phase of reverse circulation drilling near the resource area totaling 47 holes. During this time a better understanding of the deposit was gained with additional zones of mineralization recognized, including the HS zone. The addition of 124 diamond drill holes (68,453 m) from the 2009 drilling campaign has resulted in an overall increase in gold resources to 2.37 M ounces gold in the indicated category and 2.66 M ounces gold in the inferred category. The company‘s focus of defining mineralization below the limits of the proposed open pit has successfully increased total underground gold resources to 935,000 ounces in all classes, a 298% increase over the 2009 NI 43-101-compliant estimate. Regional and Local Geology The Rainy River property falls within the 2.7 Ga Rainy River Greenstone Belt (―RRGB; Fig. 2‖) which forms part of the Wabigoon Subprovince. The Wabigoon Subprovince is a 900 km long east-trending area of komatiitic to calc-alkaline metavolcanic rocks which are in turn succeeded by clastic and chemical sedimentary rocks. Granitoid batholiths have intruded into these rocks, forming synformal structures in the supracrustal rocks which often have shear zones along their axial planes. Figure 2. Regional geology of the Rainy River Gold Property (RRGP) (modified from http://www.rainyriverresources.com/Theme/RainyRiver/files/Technical_Reports/July%2010,%2 02009.pdf and from OGS Map 2443) 4 �The Wabigoon Subprovince is host to the Sturgeon Lake volcanogenic massive sulphide (VMS) deposits to the northeast in addition to several other, but smaller, orebodies. It is a very prospective terrain in which to conduct exploration. The Wabigoon Subprovince was locally overlain by Mesozoic (Jurassic and Cretaceous) sedimentary rocks and were subjected to deep lateritic weathering followed by Quaternary glaciation. Limited preservation of the Mesozoic cover sediments and saprolite occurs in localized palaeolows. The Wabigoon basement rocks and remnant Mesozoic sedimentary rocks are overlain by Labradorian till of northeastern provenance. This till has been found to contain anomalous concentrations of gold grains, auriferous pyrite and Cu-Zn-sulphides. It is overlain by a glaciolacustrine clay and silt horizon and by argillaceous and calcareous Keewatin till of western provenance. The Rainy River area therefore was covered successively by the Labradorean and Keewatin ice sheets. The Property is centered on Richardson Township with the Sabaskong granitoid batholith to the north and the Black Hawk Stock to the east. A package of metasedimentary rocks is found south of Richardson Township. Wedged in between these lithologies are a conformable series of tholeiitic mafic and overlying calc-alkalic intermediate to felsic metavolcanic rocks. These strike almost east and dip to the south (Fig. 3). Figure 3. Bedrock geological interpretation for the area surrounding the Richardson caldera (from: http://www.rainyriverresources.com/Theme/RainyRiver/files/ Technical Reports/July%2010,%202009.pdf) 5 �Intermediate metavolcanic rocks (dacites) host most of the Rainy River gold mineralization. A well-defined penetrative fabric is commonly observed on a regional scale. This foliation is approximately parallel to the trend of the metavolcanic rocks on the Rainy River Property, which strike at approximately 300° and dip 50° to 70° to the south. A steep, southwest-plunging stretching lineation is recorded in all lithologies within the main mineralized zones, with kinematic indicators suggesting south-over-north, reverse-sinistral deformation. The regional-scale, east-trending Quetico Fault has been interpreted to not extend as far west as the Richardson Township area, not being detected in diamond and reverse circulation drilling programs to date. No other thoroughgoing shear zones have been identified; significant shearing appears to be restricted to gold-bearing rocks that were weakened by the earlier volcanogenic alteration. Late, broadly north-trending, brittle faulting produces small offsets in the mineralized horizons. Deposit Types and Mineralization Early exploration work on the Rainy River Project was based on the premise that the gold mineralization was of a shear-hosted, epigenetic type. As exploration activities progressed, a volcanogenic massive sulphide model was proposed. A volcanogenic caldera model (termed the ‗Richardson Caldera‘ model) was proposed in 1997 to explain the gold and sulphide mineralization observed at Rainy River. Recent studies of all available exploration data support a model of sulphide and gold mineralization being of early volcanogenic rather than later epigenetic (shear-hosted) origin, belonging to the gold-rich subclass of the volcanogenic massive sulphide spectrum although the sulphides are mainly disseminated rather than massive. Several mineralized zones have been delineated by exploration on the RRGP to date (Fig. 4). Gold mineralization is found in the southern Cap Zone, the central ODM/17, Beaver Pond and West Zones and the northern HS and 433 Zones. Recent studies suggest that there are at least two stages of gold mineralization within the Rainy River Project, an early pervasive disseminated stage as well as a later stage of cross-cutting veinlets that often contain visible gold. The lowest-grade, disseminated gold mineralization is mostly associated with deformed volcaniclastic (permeable) dacites. Zones of higher gold mineralization are often associated with strong silicification and finely layered, foliation-parallel sphalerite and pyrite. Visible gold is typically associated with narrow (<2 cm thick) quartz veinlets, narrow pyrite veins or a sphalerite- / pyrite-rich breccia matrix. Detailed mineralogy studies have revealed that gold and electrum occur as inclusions in pyrite or in close association with sphalerite, carbonates (ankerite) and fine-grained silicates. The gold-bearing pyrite occurs as relict, anhedral grains, whereas the gold-rich sphalerite represents late-stage sulphides that formed rims on or infilled fractures in pyrite. Magmatic Ni-Cu-Co-PGE-Au-Ag mineralization is found in the 34 Zone which is associated with a tubular, late-stage pyroxenite-gabbro intrusion that crosscuts the ODM/17 Zone. The magmatic sulphides vary from massive to net-textured and disseminated. 6 �Figure 4. Resource zones location map with Digital Elevation Model backdrop (http://www.rainyriverresources.com/Theme/RainyRiver/files/images/Plan_map_april.jpg) 7 �FIELD TRIP STOPS UTM Coordinates are nad ‘83 Zone 15 Rainy River Gold Project – Richardson Twp Ontario ** Please refer to map at end of field trip stop descriptions** MAFIC UNITS: *Stop 1A 1B UTM- 422272E/5411907N UTM- 422222E/5411941N Local Information: outcrop mapped by Blackburn (1976) is adjacent to reverse circulation (RC) drill hole #358 and identified as andesite, where as bedrock in RC holes #357 and #358 was identified as dacite, indicating that this first stop is very close to a lithological contact. Lithology: definite mafic rock containing (>40 % actinolite, chlorite). Southern part of outcrop is thick lava flow while the northern part is partly pillowed. Flows face southwestward and foliation trend is 055deg dipping 85 deg SE. Comments: The orientation of the lava flow is compatible with the regional strike of the predacite mafic units on the east limb of the Dearlock syncline as shown by Blackburn (1976). *Stop 2 UTM-425740E/5411640N Local information: Isolated exposure on the north side of sand and gravel pit. Lithology: Medium grained mafic rock that can be either part of thick lava flow or intrusion. Pillowed mafics present. Mafics are intruded by several generations of porphyritic, felsic to intermediate dykes up to 2 m wide. Comments: Part of the pre-dacite mafic basement sequence on the east side of the eastern inner caldera fault. Several 10-20 cm wide fault zones marked by brecciation and increased schistosity trending 040-050 degrees. No stripped outcrop. Stop 3A 3B UTM 427048E/5411103N UTM 426607E/5411108N Local Information: Exposures on Roen pasture. Lithology: Medium grained mafic rock that could be either part of thick lava flow or intrusions. Pillows present locally trend NE and face SE. Minor porphyritic felsic to intermediate dykes. Comments: Part of the pre-dacite basement mafic sequence, east of the eastern inner caldera fault. The base of the dacite is inferred to be 150m above to the SE of the outcrop area. 8 �*Stop 4A 4B 4C UTM 425580E/5409350N UTM 425423E/5409413N Local Information: These series of stripped areas follow the strike of a mafic lava flow sequence and metagabbro occurrences within the dacite caldera sequence. 4A was stripped by Nuinsco as a possible site for a portal for underground exploration. 4B and 4C were stripped as part of a Nuinsco exploration program. Lithology: 4A- mafic lava flows vary between porphyritic and non-porphyritic and have been cut by fine grained quartz phyric intermediate dykes. Flow top breccia zones up to 50 cm thick are observed in the trenches. 4B- mafic lava flow with quartz carbonate and quartz carbonate sulphide veining host the Cap mineralization. Vein sulphides are primarily pyrite along with chalcopyrite and sphalerite. 4C- mafic lava flow in contact with dacite, possible fault contact. Comments: This mafic sequence is an inter-caldera flow that may have formed a cap on the main dacite sequence hosting the Rainy River Gold deposit. Within the mafic sequence are sills of metagabbro. *Stop 5 UTM-423174E/5410172N Local Information: Mafic outcrop exposed on Highway 600 toward Dearlock. Lithology: North trending ophitic metagabbro sill (possible thick mafic flow) 100 m thick, 400m strike length. Western contact not exposed. On the east, the metagabbro is in sharp contact with massive and pillowed mafic lava flows. Pillow shapes indicate that the flows face westward. Comments: Attitude of the mafic flows is compatible with the regional structural trend of Blackburn(1976) but is not compatible with the rock units deduced from bedrock encountered by RC drilling by ODM (Averill 1997). INTERMEDIATE UNITS: *Stop 6 UTM-426650E/5410563N Local Information: Davis Pasture, partly cleaned outcrops. Lithology: Dominantly a subaqueous, felsic-intermediate lava flow characterized by white weathered flow lobes > 10 meters thick, 50 m long at 035 degrees. In the NW corner of the outcrop there is a contact between a felsic to intermediate dyke on the SE and brecciated dacite on the NW at 030 degrees. Comments: Rusty matrix is probably an autoclastic breccia or hyaloclastite produced by quench fragmentation of felsic magma during the advance of the lava flow under water. The intermediate dyke is only sparsely porphyritic and phenocrysts are generally > 2 mm in size. 9 �*Stop 7 UTM-426148E/5410326N Local Information: These outcrops occur behind Alvin McClain‘s residence and on strike with the units at Station #6. Lithology: The lithologies exposed are complicated by a number of faults. Volcaniclastic conglomerates occur at the north end of the outcrop up to 5 m thick and comprised of 75-90 % sub-rounded to rounded felsic to intermediate clasts in a coarse sandy matrix. Rare fuchsite and chlorite rich pebbles are observed. Several brecciated lava flows occur in sequence; these units contain rounded-angular lapilli and blocks from 5mm to 10‘s of cm. Quartz crystal rich dacite is most abundant lithology in this outcrop. Comments: this is a key outcrop as it exposes a number of lithologies and relationships between them. The brecciated units and the quartz crystal rich dacite may represent a fall deposit or pyroclastic flow. Alternately this area may represent similar quench fragmentation of subaqueous dacite flows as seen at Stop 6 or fragmented lava domes within the caldera complex. *Stop 8 UTM- 426273E/5409497N Local Information: A well exposed, cleaned, discontinuous outcrop area south of the drill road. ATV outcrop. Lithology: Most of the outcrop is quartz crystal bearing dacite that contains about 5% medium grained rounded quartz crystals as well as rare rounded rock fragments. Within this unit are discontinues beds of sandstone and conglomerate. The massive dacite shows well developed late jointing and minor displacement faulting. The mafic dyke that strikes E-W across the top of the outcrop shows the style of displacement that is observed in drill core along main litho contacts as well as in the surface projection of the ODM/17 zone mineralization. Comments: this outcrop represents the upper part of the thick dacite unit which is the hanging wall to the main ODM/17 zone mineralization. Based in the presence of sparse rock fragments and the variability in crystal content across the unit, Ayres interpreted this quartz bearing dacite unit as a pyroclastic flow deposit. Drill hole NR97-43 is in this outcrop. Stop 9 UTM-426074E/5408994N Local Information: Large lichen covered outcrop west of Highway 600 north of the Teeple farm drainage ditch. Lithology: Relatively uniform quartz crystal bearing dacite that contains 5% quartz and 15-20% plagioclase crystals of similar size. The unit is intruded by several metagabbro dykes. Comments: Intermediate unit of upper dacite that occurs stratigraphically above the capping mafic unit to the northwest. This dacite is probably of crystal tuff fall deposit or pyroclastic flow unit. 10 �*Stop 10 UTM-426230E/5409875N Local Information: West of Highway 600, opposite the junction with Roen Road. Lithology: Relatively uniform quartz crystal poor dacite. No clast or internal features were observed in this exposure. In the NE corner of the outcrop, there is a 2 m wide zone with strong planar fabric trending 140 degrees. This fabric is parallel to the trend of the inferred east boundary caldera fault. Comments: Rocks are part of the thick eastern dacite sequence on the east site of the eastern inner caldera fault, but they are relatively close to the inferred location of the fault. This unit is a dacite tuff fall or pyroclastic flow. Stop 11 UTM-426320E/5409850N Local Information: Large, lichen covered outcrop east of Highway 600 Lithology: This large outcrop area contains lithologies that are mainly dacite quartz crystal tuff and or flows. Locally pebble to cobble conglomerate units contain rounded felsic to intermediate clasts. The pyroclastic rocks appear to be pumice rich. The dacite is intruded by a number of metagabbro (mafic) dykes that have diverse trends and cut by a number of small scale faults. The faults are in places several metres across and are hosts to diversely orientated quartz veins. Comments: This unit weathers with positive relief and may represent the up lift part of the eastern caldera block. Stop 12 UTM-426900E/5410000N Local Information: Large lichen covered outcrop immediately east of station 11 and east of the junction with the Roen Road. Lithology: Quartz crystals are ubiquitous and variable abundance, exposures are similar to the outcrop at stop 11. Outcrops in this area weather with higher relief, due to less alteration and the presence of metamorphic biotite and hornblende. Comments: These rocks probable fall within the metamorphic halo resulting from the emplacement of the Blackhawk Stock. Stop 13 UTM-427536E/5409453N Local Information: Large outcrop area north and east of the Teeple farm. Lithology: Outcrops are mainly quartz and feldspar porphyritic flow and tuffs and autoclastic breccias interbedded with lens of cobble to boulder conglomerate, locally with exotic clasts of unknown origin. Comments: Ayres mapping in 2005 indicated that conglomerate beds range up to 120m thick, this unit was not located in subsequent mapping of this area. 11 �Stop 14 UTM-427300E/5410175N Local Information: Series of poorly exposed small outcrops along a highlevel swamp within the same outcrop area as stop 13. Access is from the Clark Road that runs along the east side of the Teeple Farm. Lithology: Pyritic mafic tuff or flow, possibly hyaloclastite about 50 m thick. Comments: Strong AEM conductor drilled by Nuinsco. Both pyrite and pyrrhotite occur in this unit and explains the conductive nature of the lithology. This unit is either a mafic flow within the dacite or part of the underlying basement sequence displaced into its current position by unrecognized fault. FELSIC UNITS: *Stop 15 UTM-426632E/5408608N Local Information: Small outcrop in Teeple pasture about 30m north of Highway 600. Lithology: Quartz and plagioclase crystal bearing unit with no primary structures. Possible felsic lava flow northeast of the dyke. Comments: Outcrop cut by northwest trending diabase dyke. Tour note: The stops mark with * are recommended the other are of general interest. Lunch break for the tour group will be at the core shack where core from the central part of the ODM/17 zone will be on display as well as other polished core of high grade mineralization. 12 �13 �Field Guide Addendum Rainy River Resources OFF Lake STOP 1: Cunningham Farm (PRIVATE PROPERTY) Two outcrops in the south part of the OLFC contain extensive quartz vein systems. The vein systems were discovered by Dr. Lorne Ayers in 2006 during the course of geological mapping. The host rock in both outcrops is a suite of white to pale grey, to pale brown, to rusty weathering, quartzphyric felsic dikes that contain 1 to 3%, 1- to 4-mm, quartz phenocrysts. Groundmass of the dikes has been recrystallized and is now fine grained. Dikes vary from foliated to massive. The two outcrops may be part of a single vein system. A. Northern (Lorne’s) Outcrop (436670E; 5412250N) The vein system in the northern outcrop (Figure 1), was mechanically cleaned and washed in October, 2007, and has since been channel sampled. This is a relatively small, subcircular outcrop, the cleaned part of which is 35 m (east-west) by 25 m (north-south). On this outcrop, there is evidence of at least three intrusive events, three vein injection events, several periods of deformation and sulphide mineralization. Felsic dike complex The first intrusive event was emplacement of felsic dikes. Because of early deformation of the dikes, resulting in a well developed fabric, injection of voluminous quartz veins, and other stages of intrusion, vein injection and deformation, it was not possible to identify more than one phase of dike emplacement on the northern outcrop. However, based on observations made elsewhere (Ayres, 2007), including the larger southern outcrop, dike emplacement was a multiple intrusive event with some dike emplacement possibly postdating mineralization. The early fabric, which trends 000° and has a vertical dip, is a combination of sericitic foliation, fractures and <0.5-cm-wide quartz veins. Mineralization The quartz-phyric, felsic dikes contain as much as 10% pyrite and minor chalcopyrite. The sulphide minerals occur as disseminated grains and aggregates, which are as much as 1 cm in diameter and in narrow, sericitic, shear zones. Anomalous gold values occur in a sample collected in 2006 (Ayres, 2007). As noted below, sulphide mineralization also occurs in early quartz veins. Early quartz veins At the northern outcrop, the felsic dike complex was injected by an early stockwork of pale-grey, glassy, quartz veins that contain <5% pyrite and have localized malachite staining. Most veins are 1 to 5 cm wide and are typically 5 to 30 cm apart, but locally veins are 40 to 120 cm wide; the wider veins appear to be discontinuous pods and/or the result of amalgamation of veins. Although the veins are interconnected to form a stockwork, there is, in most places, a preferred orientation to many veins. Vein orientation ranges from 100 to 160°, and the veins dip between 45° SW and 90°; connector veins are as much as 90° divergent to this trend. Variations in vein trend are a function of both differences in orientation from place to place across the outcrop and warping of veins. The preferred orientation of veins may be a function of a dominant fracture system and associated subsidiary fracture systems, flattening, or a combination of these factors. Some vein deformation is suggested by localized warping of the veins. In most of the northern outcrop, the early quartz veins range in abundance from 10 to 70%, although there are local areas where only a few veins were observed. Veins are most abundant in the central 14 �part of the outcrop and decrease in abundance toward the southwest and northeast margins. Where veins are abundant, the quartz-phyric, felsic, host rock forms angular, elongated fragments surrounded by quartz. Where veins are less abundant, vein habit is variable, and this variation is, at least in part, a function of vein abundance: 1. in some places, particularly where vein width is variable, the host rock has a brecciated appearance with rounded to angular, more equidimensional fragments separated by 1- to 5cmwide veins; <1cm-wide, subsidiary veins extend into, and across the larger fragments; and 2. in other places, particularly where vein abundance is only 10 to 15%, veins have a relatively consistent trend, and few veins deviate more than 20° from this trend; these veins are partly interconnected and, in places, bifurcate and rejoin, surrounding lenticular rock areas, but there is no obvious stockwork or breccia pattern. On the southwest side of the outcrop, there is a relatively abrupt boundary, with a general trend of 120°, between an area that contains abundant quartz veins on the northeast and an area with only sparse veins on the southwest. This boundary is gradational over 1-2m, and the trend of the boundary is irregular; the irregularities have amplitudes of several metres. From this boundary, the vein zone extends northeastward for 23 m to the edge of the cleaned exposure. On the northeast side of the exposure, vein abundance is less than in the central part of the stockwork zone, but quartz veins are still present in a small outcrop, which was not cleaned, about 15 m northeast of the cleaned exposure. Near the east side of the outcrop, there are two fault-bounded blocks that are 1 to 3 m wide and contain only sparse, early quartz veins. Boundary faults truncate early quartz veins in adjacent parts of the outcrop. These blocks are wedge-shaped on horizontal outcrop surfaces, consist of the same quartz-phyric, felsic host rock as elsewhere on the is 4 m long and 1 m wide; it has a general northerly trend, subparallel to the well developed foliation. A second northerly trending block at least 3 m wide is incompletely exposed in the southeast corner of the outcrop. Early faults and veins The early quartz veins are offset along faults that typically trend 020 to 030°; in most places the offset is only a few centimetres. The faults vary from ductile to brittle structures. On the outcrop surface, ductile faults lack obvious fault lines whereas brittle faults have obvious fault lines, some of which are filled by relatively straight, quartz + chlorite veins that are as much as several centimetres wide. Locally, early quartz veins have been dragged and rotated against faults that are now filled by quartz + chlorite veins. The dragging indicates a left-lateral sense of movement and some dragged veins now almost parallel the early faults. Late quartz veins On the northwest side of the outcrop, several, white to locally rusty, quartz veins as much as 40 cm wide transect both the early quartz veins and the quartz + chlorite veins; no sulphide minerals were observed in these veins. These late veins have an average trend of 045°, and dip ranges from 40°NW to 70°SE. Vein margins vary from straight to irregular with narrow vein offshoots extending as much as several metres subparallel to the main veins and 40 cm perpendicular to the main veins. Late granitoid dikes Two distinct ages of granitoid dikes were intruded into the late quartz veins and all older structures and rock units. The granitoid dikes are distinguished by colour and trend, and they comprise an earlier, texturally variable, pink- to pale-grey-weathering phase and a younger pale-grey- to cream- 15 �weathering phase. Dikes of both phases extend completely across the outcrop. These dikes have not been metamorphosed, and they are probably related to the late tectonic Finland stock. As mapped by Blackburn (1976), the margin of this stock is 600 m west of the northern outcrop. Alternatively, the dikes could be related to the Fleming batholith, the western edge of which is 1200 m east of this outcrop. Three, pink- to grey-weathering, leucocratic dikes, ranging in width from 1 to 15 cm, were observed; the dikes are slightly sinuous, but the average trend is 060°, and the dip is 75°NW. The dikes contain at least 25% quartz and <1% mafic minerals, and, texturally, they vary from aplitic to pegmatitic. The pegmatitic dike, which ranges in width from 1 to 3 cm, has a central, 5- to 10-mm-wide, quartzmuscovite zone with a grain size of 5 to 8 mm. The younger, pale-grey- to cream-weathering dikes have straight to sinuous to zigzag habits, and they trend 020° to 030°. They include a >3-m-wide dike that is incompletely exposed at the southwest corner of the exposure, and two, closely spaced, 10- to 20-cm-wide dikes in the centre of the outcrop; one of the narrow dikes bifurcates, but the second branch terminates within 2 m. These dikes have a grain size of 1 to 2 mm, and they contain 25 to 30% quartz and 5% biotite. The wider dike contains sparse, 1- to 2-cm-wide, unmineralized, quartz veins, and it is weakly foliated parallel to the contact, which is a 2-mm-wide, pinch and swell shear zone. B. Southern outcrop: (436648E; 5411951N) In this outcrop, which is considerably larger than the northern outcrop, the quartz vein system occurs along the relatively poorly exposed northwest edge; the vein system has been stripped in a number of places by Joe Hackl and the author. The quartz vein system is very similar to that in the better exposed northern outcrop. Pale-grey, quartz veins that are generally <5 cm wide, but locally are as much as 50 cm wide, occur in foliated, rustyweathering, quartz-phyric, felsic dikes. Most veins preferentially trend between 030° and 100°, but the veins are interconnected to form a stockwork within an in situ breccia. The vein system is at least 30 m wide, and it was traced about 100 m to the northeast, along a trend of 045°, before disappearing in an area of poor exposure about half way across the outcrop. To the northeast, the vein system appears to decrease in width, and the system here is less obvious because there is less rusty weathering. On the southeast side of the quartz-injection zone, foliated, rusty-weathering, quartz-phyric, felsic dikes that contain the vein system are in sharp contact with a more massive, quartz-phyric, felsic phase that lacks quartz veins and rusty weathering. The contact is covered by a 1m width of overburden, and it was not actually observed. However, there is good control on the contact location, and the contact trends 030°, subparallel to the vein system. The more massive phase could be a younger intrusion, although still part of the Off Lake felsic dike complex. The vein system in this outcrop was intruded by two dikes that may be related to the late tectonic, Finland stock. These include: 1. a 30cm-wide, pink-weathering, feldspar-phyric, granitoid dike that contains sparse, 1cm-long, feldspar phenocrysts in a medium-grained groundmass, and 2. a 20cm-wide, zigzag, pegmatite dike.The granitoid dike trends 030° and the average trend of the pegmatite is 060°. In the southern outcrop, the veins are on the west side of the outcrop and appear to form a generally northerly trending zone as much as 10 m wide; within this zone, individual veins are as much as 1 m wide and have diverse trends. 16 �Genesis of the vein systems If the pale-grey vein system in the southern outcrop is projected along the apparent trend of the system, as established by the dominant trend of the veins and tracing of the system, it would be close to the early vein system in the northern outcrop, although the vein system in the northern outcrop has a different average trend. This spatial relationship in addition to the lack of vein systems on outcrops farther north suggest that the two vein systems are genetically related. The early quartz veins predate several generations of later quartz veins, both ductile and brittle faults, and two periods of granitoid intrusions. The early age of the veins and the sharp contact on the southern outcrop between foliated, quartz-phyric, felsic rocks containing the quartz stockwork and a more massive, quartz-phyric, felsic unit that lacks quartz veins suggests that the vein system may be related to emplacement of the Off Lake felsic dike complex. The only similar quartz stockwork observed to date in the area is associated with the Off Lake fault. The vein system on the Cunningham option has a distinctly different trend to that of both the Off Lake fault, the inferred extension of which is about 800 m to the east, and the Potts fault, which is about 1 km to the south. However, the quartz stockwork on the Cunningham option may be related to an undiscovered early fault in the southern part of the Off Lake felsic dike complex. 17 �Field Trip 2 GEOLOGY OF THE STEEP ROCK GROUP: ANATOMY OF AN ARCHEAN CARBONATE PLATFORM Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1 philip.fralick@lakeheadu.ca Part A – Geology of the Steep Rock Group: Anatomy of an Archean Carbonate Platform. Part B – Description of Hammond Reef Gold Deposit, Brett Resources Inc. INTRODUCTION The Steep Rock Group is located near Atikokan, Ontario where it lies unconformably on Mesoarchean tonalite (Figures 1 and 2). It was first described by Smyth (1881) as consisting from bottom to top of: 1) Conglomerate, 2) Lower Limestone, 3) Ferruginous Formation, 4) Interbedded Crystalline Traps, 5) Upper Calcareous Greenschist, 6) Upper Conglomerate, 7) Greenstones, 8) Agglomerate, and 9) Dark Grey Slate. Lawson (1913) reduced this to four units by combining the upper six into one unit that he described as interbedded crystalline traps. Jolliffe (1966) applied the term Group to the succession, thus changing its status from a Series (chronostratigraphic unit), which it had been termed before, to a lithostratigraphic unit. He also separated the volcanic rocks into a lower Ashrock and an upper assemblage of flows, tuffs and sedimentary rocks. Wilks and Nisbet (1988) were the first to formally define the Steep Rock Group according to the procedures put forward by the International Commission on Stratigraphic Classification outlined in Hedberg (1972, and subsequent publications of the ICSC). Their formal classification remains in place and should be followed when referring to the rock units that were defined. From bottom to top the Steep Rock Group consists of (Wilks and Nisbet, 1988): 18 �Wagita Formation – predominantly conglomerate and sandstone with some pelite (mudstone). It is 0 to 150 m thick and appears to occupy depressions (paleovalleys) in the unconformably underlying Marmion Complex. It is overlain conformably by the Mosher Carbonate Mosher Carbonate – This unit is composed mainly of calcite with minor amounts of dolomite, ferronian dolomite (ankerite), quartz, pyrite and carbon (in the form of kerogen, Rothpletz 1916; Hayes et al, 1983). It is up to 500 meters thick with an irregular upper surface. Jolliffe Ore Zone – The lower member consists of the unconsolidated, earthy, Manganiferous Paint Rock Member. It is composed of fragments of goethite, hematite, chert and quartz with a matrix consisting of these minerals plus kaolinite, illite, calcite, gibbsite and pyrolusite (Huston, 1956). It is 100 to 300 meters thick and averages 3.8 % manganese. The Goethite Member overlies the Manganiferous Paint Rock Member. It is composed of goethite (67%) and hematite (21%) with accessory quartz and kaolinite. Where the ore is not brecciated and altered it resembles banded iron formation. This unit is 50 to 100 meters thick and is overlain by the Dismal Ashrock. Dismal Ashrock – This unit is predominantly a mafic to ultramafic pyroclastic rock. Most clasts are rounded and less than 1 centimeter in diameter but fragments up to seven meters occur. Thin volcanic flows interlayer with the pyroclastics. Lenses of massive bedded pyrite and carbonaceous chert also occur in this Formation and the Jolliffe Ore Zone. The ashrock is 100 to 400 meters thick and is possibly structurally overlain by the Witch Bay Formation. Witch Bay Formation – Mafic and intermediate volcanic and sedimentary rocks make up this unit. It is extensively deformed with a minimum, and poorly constrained, thickness of 1000 meters. Its lower contact may be a structural discontinuity and it is only tentatively included in the Steep Rock Group. Its upper contact is either intrusive or structural. All units in the above description have been metamorphosed and the prefix meta is implied to apply to all rock names. Kusky and Hudleston (1999) believe that both the Dismal Ashrock and the Witch Bay Formation are allochthonous, having been tectonically transported to their present positions. They interpret the Dismal Ashrock as a mélange with a ductile high strain zone at its top. They further believe that deposition of the Steep Rock Group began at 3.0 Ga as the Marmion Gneiss Complex rifted, the Jolliffe Ore Zone was formed by the sub-aerial exposure and weathering of the Mosher Carbonate and the Steep Rock Group may have formed a portion of a linear array of 3.0 Ga platform sequences along with the Sachigo and La Grande River Subprovinces. The highly oxidized state of the Jolliffe Ore Zone and its forming under an Archean anoxic atmosphere is difficult to reconcile. This combined with the huge amount of limestone that would need to be dissolved to form a terra rosa up to 300 meters thick make their model for the formation of the Jolliffe Ore Zone untenable. In addition, age determinations obtained since this paper was published throw other portions of their conclusions into serious doubt. The age of the Steep Rock succession remained conjectural for most of its history in the literature. The first step towards understanding the age relationships occurred in 1988 when 19 �Davis and Jackson published a U-Pb zircon age of approximately 3000 Ma for the Marmion Batholith. This age was refined to 3001.6 1.7 Ma by Tomlinson et al. (2003). A U-Pb zircon age of 2780.4 1.4 Ma was also obtained by Tomlinson et al. (2003) for the Dismal Ashrock, though the age of this zircon may reflect inheritance. Based on correlations between the Steep Rock area and the Finlayson and Lumby areas to the north, which were put forward by Fralick and King (1996), Fralick et al. (2008) bracketed the age of the Steep Rock assemblage between 2828 Ma, the age of volcanics in the Lumby Lake belt that underlie the sedimentary rocks, and 2780 Ma, the youngest zircon in the overlying Dismal Ashrock. Recently age determinations on detrital zircons obtained from basal siliciclastics of the Wagita Formation provided a youngest zircon age of 2779 22 Ma (Denver Stone, O.G.S., personal communication). Unlike age determinations on igneous rocks, where the statistically most likely age is given as the probable age, ages on detrital zircons indicate the unit is probably younger than a specific age and, therefore, the older age of the range is quoted. Thus, the Wagita Formation is likely younger than 2801 Ma. This agrees well with the age put forward by Fralick et al. (2008) of between 2828 and 2780 Ma. REGIONAL SETTING The following is taken largely from Fralick et al. (2008). Examining the Steep Rock Group in isolation, without considering its place within the framework of correlative units in the area does not allow a regional picture to emerge. The Steep Rock Group forms a portion of a once continuous rock assemblage that has been dissected by northeast trending faults (Figures 1 and 2) (Fralick and King, 1996; King, 1998; Wyman and Hollings, 1998; Tomlinson et al., 1999). Figure 1. Generalized geology map of Superior Province showing the belt structure and the location of the Atikokan area on the southern edge of Wabigoon Subprovince. 20 �Figure 2. Geology of the Steep Rock region, including correlative units in the Finlayson and Lumby Lake greenstone belts. 21 �Figure 3. Stratigraphy of the volcanic and sedimentary units in the Atikokan area (ages in millions of years). Consistency of younging indicators throughout the stratigraphic successions is supported by U-Pb zircon age determinations and appropriate stratigraphic position of correlative units in adjacent areas. Units have been tectonically flattened. For sources of age determinations see Fralick et al. (2008). 22 �Sedimentary rocks of the Finlayson Lake Group and Upper Lumby Lake Group overly a thick volcanic succession that began erupting before 3014 Ma (Tomlinson et al. 2003) (Figure 3). Tomlinson et al. (1999) proposed that the mantle source of the Mesoarchean volcanic assemblages in the Steep Rock and Lumby Lake areas was geochemically similar to the source of basaltic rocks that form the Cretaceous Ontong Java oceanic Plateau and that eruption probably occurred in a continental setting following rifting of the crust. Wyman and Hollings (1998) had previously proposed that most of the volcanic units in the Lumby Lake belt have an ocean plateau affinity. The combination of interpretation of depositional environments with sediment geochemistry and precise U-Pb geochronology of volcanic units enables reconstruction of the paleogeography for this area spanning a 230 m.y. period from the Mesoarchean to the Neoarchean (Fralick et al., 2008). Thick successions of tholeiitic basalt older than 2997 Ma. are present at the base of the Lumby and Little Falls belts. This implies that a sub-aqueous lava plateau existed in the area by 3.0 Ma (Figure 4). Figure 4. Mafic volcanism in the Steep Rock–Lumby Lake area prior to 3.0 Ga begins building a basaltic plateau. At 3000 Ma tonalitic magmas intruded the tholeiitic basaltic plateau. In both the Lumby and Little Falls areas, Marmion magmas erupted at surface, resulting in felsic volcanism and deposition of clastic aprons surrounding the volcanic centers (Little Falls Group; Figure 5). 23 �Ongoing mafic volcanism throughout this time interval resulted in ash layers from this source interlayered with the felsic volcanic debris. From 3000 to 2920 Ma mafic volcanism continued to thicken the volcanic pile. Periodic bursts of felsic volcanism linked to tonalitic intrusions also occurred (Figure 6). Throughout the next 100 m.y. mafic volcanism sporadically continued with felsic volcanic pulses, such as the one that erupted the 2898 Ma rhyodacites in the Lumby Lake belt and supplied the sub-aqueous clastic apron forming the Lower Lumby Lake Group. With the cessation of volcanism throughout the area sometime after 2828 Ma, minor block faulting, possibly triggered by adjustment to thermal decay in the region, created localized depocenters for epiclastic sediment in the northern Finlayson area and possibly upraised the tonalities of the Steep Rock area (Figure 7). These tonalities and mafic volcanic country rocks underwent erosion, and sediment was channeled through paleovalleys to the shoreline where deltaic deposits accumulated in the shallow areas and mass-flows off the deltas dominated the deeper regions. Subsidence of the area, due to isostatic adjustment to the weight of the ocean plateau, led to submergence of abandoned portions of the delta top, as seen in the upper section of the Finlayson Lake Group. As subsidence and erosion lowered the source area to base-level, the bedrock channel system backfilled, forming the basal Steep Rock clastics of the Wagita Formation, and starved the now largely submerged Finlayson deltaic complexes of sediment. Continued subsidence led to the development of carbonates in the shallower areas, which was succeeded by cherts and iron formation as the area continued to deepen as it isostatically adjusted (Figure 8). Previously, the sedimentary rocks in the Steep Rock – Little Falls – Finlayson – Lumby Lake areas have been ascribed to a rift setting (Wilks and Nesbet, 1988; Thurston and Chivers, 1990; Tomlinson et al., 1996). The major problem with this interpretation is that some of the volcanic units and associated sedimentary aprons are as old as the supposedly rifted tonalitic basement. This still allows the younger, upper sedimentary units to be rift related. However, recent investigations of the igneous geochemistry have indicated that the mafic to komatiitic volcanic pile associated with the sedimentary rocks was erupted in an ocean-plateau setting (Wyman and Hollings, 1998; Hollings et al., 1999; Hollings and Wyman, 1999). An excellent modern analogue for the evolution of the regional geological setting through time is the Cretaceous of the Kerguelen Plateau off Antarctica. As volcanism ceased in the Kerguelen Plateau 120 m.y. ago (Schlich, 1989) the sub-aerial portion remained above sea-level for 40 m.y. leading to erosion of up to 8000 meters of igneous rock (Coffin et al., 1990; Coffin, 1992). Similar erosive downstripping in the Steep Rock area would account for exposure of the Marmion Batholith. The basal units of the sedimentary succession capping the Kuerguelen Plateau are composed of fluvial units analogous to the Wagita Formation. These are overlain by algal limestones, which developed in shallow marine environments (Barron et al, 1989) as sub-aerial deposition failed to keep pace with subsidence driven by isostatic adjustment to the thick volcanic pile. The clastic succession at Steep Rock is capped by stromatolitic limestones deposited after base-level rise flooded source areas. As load and thermal decay induced subsidence continued to outpace sedimentation on the Kurguelen Plateau, the calcareous shallow water successions were succeeded by marine chalk and marl. Deep-water cherts and iron formation record the same 24 �Figure 5. At 3000 Ma tonalitic magmas, possibly generated by partial melting at depth of the thick plateau assemblage, intruded into the Steep Rock-Lumby Lake ocean plateau. The resultant surface volcanism built positive relief features from which sediment aprons composed of eroded felsic volcanic debris spread out over the plateau. Figure 6. The Steep Rock-Lumby Lake ocean plateau continued to grow during the 3000 to 2920 Ma time period. Sporadic tonalitic intrusion fed minor intermediate to felsic volcanic events, but the dominant volcanism erupted tholeiitic basalt. 25 �Figure 7. Volcanism ceases on the Steep Rock-Lumby Lake ocean plateau between 2820 and 2780 Ma, and the major sedimentary sequence is deposited. Localized relief produces slope failures and resultant agglomerates in the north Finlayson area. These are overlain by a deltaic clastic wedge prograding from the incised channels of the Steep Rock area. Figure 8. The siliciclastic system depicted in Figure 7 is succeeded by chemical/biochemical deposits as transgression causes clastic sediment starvation. With flooding of the source terrain stromatolitic carbonates developed in shallower areas and iron formation, which had dominated the deeper regions to the north, migrated first over the adjacent siliciclastic deposits and then blanketed the carbonate assemblage as well. 26 �process on the Archean Steep Rock – Finlayson – Lumby Lake Plateau. The striking analogies between the sedimentary units at Steep Rock and the sediments of the Kerguelen Plateau reinforce interpretations based on igneous petrochemistry (Wyman and Hollings, 1998; Hollings and Wyman, 1999; Hollings et al., 1999) that this assemblage represents an oceanic plateau. THE CARBONATE PLATFORM Carbonate platforms capping oceanic islands and plateaus are not uncommon in the modern ocean. However, they are rare in the rock record of the Archean. This may be the result of low preservation potential. However, Phanerozoic carbonate platforms suffer from this problem due to their oceanic location and there are still plentiful examples of these preserved in the geologic record. Thus, the Steep Rock carbonate succession represents one of the few opportunities to decipher the paleoecology and paleohydrology of an Archean carbonate platform. Wilks and Nisbet (1985, 1988) have provided an excellent description of the paleoecology of the Steep Rock platform. The following is largely taken from their work. Layering within the Mosher Carbonate exhibits a systematic change in form upward through the unit (Figure 9). The physical and chemical changes in the water mass that must have governed this transition in environments may have been related to a deepening trend from the shoreline environments represented by carbonates on top of fluvial conglomerates and sandstones to the overlying iron formation that was deposited in quite, deeper water (Figure 10) (Fralick et al., 2008). Alternatively, environmental change may have been related to periods of open verses more restricted circulation of water on the plateau leading to evaporation driven precipitation. We will discuss these possibilities in the field. -------------------------------------Figure 9. Variation in carbonate lithosomes with height in the section. 27 �Figure 10. A possible model for organization of carbonate deposition on the Steep Rock ocean plateau. Transgression produced a vertical stacking of the lithofacies. The upward changes in lithosomes present in Figure 9 may be caused by shifts in salinity rather than the lateral variation combined with transgression depicted here. The carbonate succession begins with an interlayered to sheared originally conformable contact with the underlying fluvial, valley-fill Wagita Formation or the Mesoarchean tonalite gneiss. The best representation of this contact is in an exposure near the bottom of the pit that is now under more than a hundred meters of water. Here decimeter-scale carbonate layers are interbedded with sandstone layers, with similar thickness, over a vertical distance of approximately half a meter. At the first major carbonate outcrop we will visit the basal contact with the 3001 Ma tonalite is sheared, however, pieces of weathered tonalite are present in the carbonate. If the contact is traced laterally it is quite evident that the limestones are onlapping the basement and the tonalitic debris was not carried very far away from the gneiss into the nearshore. The carbonates near the base of the succession have fairly flat layering. The layering is highlighted by changes in organic carbon (kerogen), iron oxides (recent weathering of iron carbonates?) and chert content. What follows, with the exception of the Figures, which are new, is the description provided by Wilks and Nisbet (1985) of the stromatolite succession: ―Near the base of the unit, simple Stratifera-like stratiform structures are characteristically present (Figure 11). These have flat to undulatory laminae, occasionally with the development of pseudocolumnar, laterally linked structures, which occur throughout the carbonate member. These Irregularia-like pseudocolumnar structures become more common upwards, and columns obtain heights of up to 2 cm (Figure 12A). Above this are hemispherical, laterally linked stromatolites (Figure 12B-D). These cumulate linked structures are similar to those figured by Hofmann (1971) as Cryptozoon walcotti. Laminae are wavy and 0.5-3.5 mm thick, and the structures are 5-15 cm high and in basal diameter. 28 �Figure 11. Stratiform layering, which dominates the lowest portion of the section. A) Undulatory, disrupted layering is further disrupted by the small faults. B) This sample has more consistent layering. Note the presence of white cement following some layers. C) Close-up of B. White calcite cement filling void with a flat bottom and undulating top. This shape is typical of gas bubbles that form in present day bacterial mats. D) Laterally consistent layering with small pseudocolumnar stromatolites. 29 �Figure 12. Moving stratigraphically up-section from Figure 11 the stromatolites increase in relief. A) Well developed pseudocolumnar stromatolites. B) Next hemispherical stromatolites begin to appear. C) Close-up of B. Successive hemispherical stromatolites are easily visible growing on top of one another. D) Laterally linked hemispheres in places resemble digitate forms. 30 �Figure 13. Columnar and branching columnar forms, such as those depicted here, are striking but are only developed in a vertically limited area. 31 �In places these ―cryptozoon‖ structures are succeeded by branching walled and unwalled columnar forms (Figure 13 and 14A). Branching furcate columns with ragged margins occur with a y style of parallel branching (Walter 1972). Height is up to 20 cm. Above these, simple, small conical structures occur, with diameters of 2-10 cm.‖ Wilks and Nesbit (1985). The 3.5 meters below the branching columnar stromatolites contains the first appearance of layers containing the outlines of fans of crystals. In some layers these crystal fans are growing upwards in others they are growing downwards (Figure 14B). The following is a description of crystal fans by Sumner (2000): Figure 14. A) Cross-sectional views of columnar forms in outcrop often appear as oncolite-like in shape. B) The first appearance of crystal fans immediately below the columnar stromatolites. Both upward and downward radiating fans are present. ―Microbialites commonly are cross-cut by fibrous mineral pseudomorphs. These pseudomorphs are replaced by calcite, but preserve their primary fibrous character with blunt crystal bundle terminations which suggests an aragonite primary mineralogy (e.g. Sandberg, 1985). The psuedomorphs form fanning bundles of crystals that grew upward. Laterally, neighboring fans of crystals interfered with each other‘s growth (Figure 14). Wavy bedding in the microbialites is due to doming of microbialites over areas with abundant fibrous pseudomorphs. This geometry suggests that fibrous mineral precipitation and microbialite growth were contemporaneous (Sumner and Grotzinger, 2000).‖ Sumner (2000). The upper half of the carbonate succession contains ―giant stromatolites‖ (Figure 15). These are composed of 1 to 15 cm thick layers of crystal fans commonly radiating upwards (Figure 16 A and B). They are overlain by darker layers of fenestrate microbialite. The microbialites consist of stacked, concave-up thin dark lines, which give the appearance of stacks of upside-down fingernails (Figure 16C and D). In some layers the dark filaments forming the stack are supported and separated by thin support columns (Sumner and Grotzinger, 2000; Sumner, 2000). 32 �Figure 15. Giant domes in the upper portion of the carbonate succession. A) Giant domes in plan view. Elongation may reflect current activity (elongate parallel to current). B) Giant domes in cross-sectional view. C) Crystal fans dominate this succession and the domal structure, as seen here in cross-section, is very subdued. D) Close-up of silicified crystal fans in C. The spaces between the microbialite layers are filled with white calcite. Sumner (1997, 2000) believed that the cuspate laminae and the support structures were formed by two different microbial communities. Where early calcification of the support structures occurred, the open, net-like morphology with white calcite cement in the pores was preserved (Figure 16D, upper 33 �portion). Where the support structures were not mineralized during formation the structure was Figure 16. A and B) Crystal fans underlain by a thin dark layer (iron- and/or organic-rich?). In A the lowest layer appears to be an open-structure microbialite but in B it may be a coarse grainstone. C) Finestrate microbialite with a collapsed structure. D) The lower portion is a finestrate microbialite with a collapsed structure, overlain by one with an open structure. compacted and the darker layers dominated by cuspate layering were formed (Figure 16C and D lower portion) (Sumner, 2000). The combination of crystal fans and in some layers cuspate layering, which appears in places to be draping the fans, produces irregular bed surfaces that have an appearance similar to an egg carton (Figure 17A). In other less common areas, the presence of hemispherical stromatolites in the upper layer of the giant domes creates an upper surface composed of small elongate mounds (Figure 18A). More rarely, some surfaces are similar to small mudcracks with turned-up edges (Figure 17B). The dominance of the crystal fans in the layers forming the large domes indicates that these structures probably should not be referred to as stromatolites. They do appear to have layering formed through microbial processes, but whether these layers cause the shape of the structures is not known. In areas where microbial layers are not present and crystal fans dominate domes are not well developed (Figure 15C and D), indicating that possibly the organically produced layers are the determining factor in the development of domes. 34 �Figure 17. Plane views of bedding surfaces in giant domes. A) The upper ends of crystal fans cause an irregular egg carton-like pattern on most surfaces. B) Much more rarely surfaces have what appear to be mud-cracks (photo is approximately 15 cm across). Grainstones have not been previously described from the Steep Rock limestones. However, clastic carbonate layers with grainsizes from micrite to granules and small pebbles are present (Figure 18C). They probably dominate the succession between the columnar stromatolites and the large domes. They are also found interlayered with the large domes and forming the base on which some of the fans in the domes are growing. The micrite and grainstone layers are 1 to tens of centimeters thick, massive to parallel laminated and in one instance trough cross-stratified. The clastic beds commonly appear as non-descript, massive continuous, parallel-sided layers and this blandness may have contributed to their non-recognition in the past by researchers primarily interested in the stromatolites. Though metamorphism and structural deformation in the Steep Rock Group is not intense the carbonates have been subjected to post-depositional alteration. Wegenast (1954), Jolliffe (1955, 1966), Wilks (1986), Kusky and Hudleston, (1999) and Sumner (2000) believed that a karst topography developed on the upper surface of the Mosher Carbonate during sub-aerial exposure. They view the manganiferous paint rock as a residual soil composed of goethite, gibbsite, hematite, and kaolinite with blocks of carbonate and lenses of pisolitic ferruginous bauxite, which formed during this interval. The Jolliffe Ore Zone would represent an iron formation that was deposited on top of the thick soil as marine transgression once again flooded the area. Alternatively, Kimberley and Sorbara (1976) believed that aqueous fluids acidified by the sulfide-rich dismal ashrock were responsible for the alteration. Some serious problems exist with the former more widely held hypothesis. 1) The manganiferous paint rock is consistently described as unconsolidated and earthy, yet authors who believe it is a residual soil that formed upon exposure of the Mosher Carbonate require it to have gone through the Kenoran Orogeny. It is difficult to believe that it was metamorphosed to greenschist facies but not lithified. All other paleosols of this age in the Canadian Shield are lithified and deformed. 2) This same problem applies to the karst collapse breccias as well. Although Kusky and Hudleston (19999) stated that these were weakly deformed the numerous breccias I have examined are not (Figure 18B). The delicate cements formed surrounding the breccia blocks show no evidence of deformation or 35 �Figure 18. A) Where hemispherical stromatolites are present near the surface of the domes the surface acquires an elongate bulbous appearance. B) Non-deformed breccia with two generations of cement. C) Cross-stratification in a grainstone tempestite layer. D) Iron content increasing towards the top of a layer. This type of iron enrichment is not uncommon. 36 �metamorphism making their existence before the Kenoran Orogeny unlikely. 3) Alteration zones in the carbonate appear to be vertical and some are not associated with the contact. These ironenriched altered areas are stratigraphically overlain by unaltered carbonate, but the alteration does continue vertically where outcrop allows observation. 4) The type of alteration implies highly oxidizing fluids. As well, the manganiferous paint rock ―soil‖ is up to 300 meters thick. Where did the large concentrations of oxygen come from in the Archean? Certainly not the atmosphere, as other Archean weathering profiles exhibit little to no oxidation. 5) The same succession of siliciclastics with conglomerates and sandstones overlain by carbonates and in turn overlain by iron formation is present in the Lumby Lake area, as previously described. There is no evidence of a period of exposure between the carbonate and iron formation present there. 6) The iron formation overlying the Mosher Carbonate is also brecciated and altered (it has goethite whereas hematite and/or magnetite are standard for oxide facies iron formation in Superior Province). Late mafic and lamprophyre dikes that cut lower units are involved in this brecciation (Shklanka, 1972). Thus, if the paint rock is a soil on a karst two episodes of brecciation substantially separated in time are needed. 7) Lastly, pieces of wood have been found in with the ferruginous pisoliths (R. Bernatchez, personal communication, in Wilks and Nisbet, 1988), and Machado (1987) believed that there was evidence of termite activity during formation of the ore. This placed the age of the ore as younger than Jurassic (Machado, 1987). This evidence overwhelmingly does not support the dissolution and alteration occurring in the Archean, but rather indicates it occurred in approximately the last 100 Ma. The scenario for the development of the carbonate platform is summarized in Figure 10. Here Walthers Law has been used to juxtaposed horizontally the lithofacies that succeeded one another vertically. This is based on the alteration in the upper carbonate and overlying iron formation developing in the Phanerozoic and the succession representing a transgression culminating in deeper water where the iron formation was deposited. This may not be accurate as the up-section change in lithosomes in the limestone may reflect open circulation evolving to more closed evaporitic conditions leading to precipitation of the crystal fans, but it does reflect a possible scenario. In this depiction the grainstones are in the higher energy areas of the platform close to the deeper water conditions where shoaling waves would be expected. Also, upwelling of deeper water in this zone may have lead to the precipitation of the crystal fans. The stromatolite-dominated lithofacies are in the platform interior where quieter water conditions should have prevailed. Eventually the entire assemblage subsided below the photic zone and deposition of iron formation replaced carbonate precipitation. 37 �FIELD TRIP STOPS When looking at the carbonate outcrops it is essential to get up close to them. There is a reason sedimentologists usually have their face within a few decimeters of the rock. You will miss most of the detail if you do not do this. As well the outcrops on this trip will seem to be repetitive if you do not look at the detail, and think about what it might be inferring about how the rock formed. Do not use hammers on outcrops. You can beat and sample loose material all you want. And, most importantly, be careful and always mindful of your surroundings. We will be walking on slopes and near cliffs all day. BRING YOUR CAMERA Stop #1: Overlooking northeast end of Hogarth Pit. The basement tonalite is visible at this stop. We will take a few minutes to look at the tonalite and discuss the overall setting of the Steep Rock Group. Special emphasis will be placed on the igneous and sedimentary history of the area from 3.0 to 2.8 Ga, prior to deposition of the Steep Rock Group. This will develop the context of the paleogeographic setting as deposition began in this area. Stop #2: One quarter of the way down the east side of Hogarth Pit from stop #1 (locations #3, 4 and 5 of Wilks and Nesbit, 1985, 1988). We will probably be at this location from one to two hours. After disembarking from the bus we will walk down a hill towards the pit for approximately 15 minutes. Notice how the small stream following the path changes from precipitating white material to orange material. The white precipitate is a mixture of calcium carbonate with minor aluminum hydroxide. The stream is spring fed and as the water flows onto the surface CO2 is degassed, which leads to a decrease in acidity by driving the formula: H 2O + CO2 --- HCO3- + H+ to the right. This, in turn, causes supersaturation of the Ca and Al in solution and their precipitation. As the stream continues to flow down the slope the decrease in acidity and the presence of oxygen result in the precipitation of iron hydroxides, causing the orange colour. A large cliff with boulder debris is present near the bottom of the hill. The rocks forming this cliff are mostly composed of carbonate collapse breccia fragments with calcite or dolomite cement. Some of the cement filling voids is intricately banded. Are there examples of miniature stalactites and stalagmites or dripstone indicating carbonate precipitation above the water table? Is this breccia deformed, i.e. did it go through the Kenoran Orogeny? Stay back from the cliff as you examine the boulders as loose material periodically falls. 38 �Next we will walk across the large flat area to the high ground to the south. We need to make our way up the sand and gravel slope to the glacially polished outcrop. The contact between the limestone, which forms most of this outcrop, and the tonalite is exposed to the left (NE). A weak schistosity is developed along it, but if you get up close to it tonalitic debris is visible in the limestone. Actually, the top 60 cm of the tonalite is not tonalite. It is sandy tonalitic debris. If you follow the contact 50 meters up the hill you will notice that it is at an angle to the layering in the limestone and the limestone layers were progressively burying this paleohill. The presence of tonalite debris in the basal limestone and a regolith in the upper tonalite strongly infers that this contact is original not a later structural discordance. Moving up-section in the lower portion of the outcrop you will be in the stratiform stromatolitic layers for the first 5 meters. Then there is an area where chert replacement has preserved thumbsized columnar stramatolites growing upwards from the flat-mat forms. Bulbous forms the size of an orange start to appear in this zone. Some of the flat mat layers and bulbous forms have white calcite cement filled vugs that have flat bottoms and arched tops. These are the classic shape of gas bubbles that form and are trapped in leathery mats as the microorganisms rot. They are an indication that flat layering is stromatolitic in origin. The stratiform stromatolites and small mounds continue for 40 meters from the base of section. The first crystal fans appear at approximately the 40 meter mark. They average 5 cm high and are along certain horizons, with the fans in some layers orientated upwards and in other layers downwards. The zone containing columnar stromatolites overlies the area with the crystal fans. These are obliquely growing out towards us so they are truncated forming nests of elipses. They appear to be composed of ferruginous dolomite (ankerite) or dolomite with hematite. The spaces between columns are filled with white cement, grainstone? and draping microbial laminations. (Grainstone is carbonate sandstone.) This is the only known layer of large columnar forms and can be used as a marker horizon. Probable grainstones are present approximately 10 meters above the columnar forms. The finegrained grainstones are approximately 5 cm thick and the granule layers average 10 cm thick. After approximately 20 meters of probable grainstones the layering is disrupted by a zone of iron alteration and brecciation. The breccia varies from clast to matrix support. How did this breccia form? Is it related to Archean karst on the upper surface of a then horizontal Mosher Carbonate or did it form after folding and metamorphism? Now we will walk around the hill and pick up the section approximately 50 meters higher. Stay away from the drop-off and aware of where you are relative to hazards at all times. The outcrop begins with approximately 14 meters of grainstone. A layer approximately 6 meters above the base of this section has possible trough cross-stratification. The large carbonate domes lie on top of the grainstone. Notice that the 1 to 15 cm thick layers are composed of dark (organic-rich), radiating crystal fans, with light calcite cement between individual crystals, overlain by fenestrae microbialite composed of thin dark lines resembling stacks of upside-down fingernails next to one another. These linked concave-up structures are not easy to see. In other fenestrae microbialite the dark lines form a small net design with light calcite cement between the lines. These latter ones display more of the original form as they have not collapsed due to early compaction. Why are the top surfaces of the layers so irregular? From here we will proceed back to the bus. 39 �Stop #3: Southeast end of Hogarth Lake (South Roberts Pit, Site #7 of Wilks and Nisbet, 1988). This is our lunch stop. We will eat our lunch in the clearing next to the road. A path goes to the lowest bench where large domes are exposed, but internal structure is poorly preserved. We will not spend much time at this outcrop. Stop #4: About 100 meters down the hill from stop #3 a field strewn with boulders and debris piles is down a slope to the left (east). It is a difficult walk down this slope and across the field to the path on the other side but that‘s where we are going. The path leads to benches overlooking Errington Pit. Once on the path the walking is easy. Take the right fork and after approximately ten minutes an outcrop will appear on your right. The first 45 meters consists of centimeter thick layers of dark and light banded limestone. Then there is a 6 meter thick chlorite-rich diabase dike. This is followed by 9 more meters of the no-stramatolitic, banded limestone, then 15 meters of large (0.75 m high by 1.5 to 2 m across) domes. Some delicate laminations are preserved here. The next 10 meters is sheared with a dike following the fault zone. The rock on the other side of the fault appears to be a repetition of the lower portion of this section with approximately 50 meters of cm- to dm-thick layers of dark limestone with white calcite cement void fills. There is a four meter wide dike in the upper portion of this assemblage. As you go around the curve in the road keep away from the far edge where there is a cliff going down and do not walk close to the end of the road where it has collapsed into the pit! Seventy meters of large (5m x 2m x 1.4m high) domes caps this succession. We will need to go to a lower bench to see the top of this assemblage. Smaller cabbage-sized stromatolites and flat layering with white cement lenses are also common in this upper unit. One of the large domes has an overturned edge. After looking at this section we will walk up the road the way we came in to the flagging tape. Those with a reasonable fitness level can cut through the woods here to the lower road. Those wishing not to climb down a steep slope should continue up the road to the fork and take the fork on your right that we have not yet been down. As we walk through the woods nearing the road to a lower bench the slope of the ground will increase. We will reach a point where we have to carefully pick our way and here a glacially polished outcrop will open up before us. This consists of what were originally fairly flat, sharp-sided, continuous layers. Some of these have silicified upright crystals that are leaning slightly due to deformation. They give the appearance of layer upon layer of 10 to 15 cm tall crystals growing from the bottom. Be very, very careful at this exposure as the polished surface has small pebbles on it that make it very easy to slip over the cliff below your feet!! Do not go near this outcrop unless you are holding a tree or something similar that will keep you in place if you lose your footing. The layers curve at one spot showing that relief was developed. Abundant styolites are also present here. Have a look at the large boulder sitting next to the base of the outcrop as well. It appears that the ankeritic area dropped into the rest bending the layering. Is there another explanation? The group will reform at the base of this hill on the lower road. We will continue along this road and cut down to a still lower road when we near the pit. Walk with caution through this area as the various benches all have drop-offs at their outer edges and all eventually end in drop offs. Do not roll boulders, etc., over the edges, you do not know if someone is below. These lower benches provide access to the best preserved large domes. Here the internal crystal fan 40 �structure is easily visible. Of particular interest is the three dimensional relationship between the domes. Look at where they come together. Are the layers continuous from one to the other? Do the layers in one dome truncate those in another or do the domes bud off one another? Some of the crystal fans are growing from clastic layers of grainstone and rudstone (pebble conglomerate). The fenestral microbialite layers also appear to be rare here, with crystal fans in places overlain by dark micrite. Where microbial layering is present in the upper portion of a layer in a dome note how it controls the surface topography of the bed. Some very good samples of crystal fans in the loose material can be obtained here. From here we will walk back along the lower road and across the boulder field and up the slope to the bus. Stop #5: We will drive to the confluence of the roads at the south end of Hogarth Lake, park and walk a very short distance around the bottom of the lake to the south western corner. Here debris of the Dismal Ashrock litters the ground. 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Stratigraphy of the Steep Rock Group, Steep Rock Lake, Ontario. In, ed. by A.M. Goodwin, Precambrian Symposium, Geological Association of Canada, Special Paper Number 3, p.75-98. Kimberley, M.M. and Sorbara, J.P., 1976. Post-Archean weathering of Steep Rock Group iron formation. Proceedings, 1976 Geotraverse Conference, University of Toronto, Toronto, Ontario, p. 128-136. King, D., 1998. Depositional environments of the 3.0 Ga Finlayson and Lumby Lake greenstone belts, Superior Province, Canada. M.Sc. thesis, Lakehead University, Thunder Bay, 200p. Kusky, T.M. and Hudleston, P.J., 1999. Growth and demise of an Archean carbonate platform, Steep Rock Lake, Ontario, Canada. Canadian Journal of Earth Sciences. v. 36, p. 565-584. Lawson, A.C., 1913. The Geology of Steep Rock Lake, Ontario. Geological Survey of Canada, Memoir 28, p. 7-15. Machado, A. de B., 1987. On the origin and age of the Steep Rock buckshot ore. Chemical Geology, v. 60, p. 337-349. Sandberg, P., 1985. Aragonite cements and their occurrence in ancient limestones. In, ed. by N. Schneidermann and P.M. Harris, Carbonate Cements, S.E.P.M. Speceal Publication 36, p. 33-57. Schlich, R., et al., 1989. Principal results and summary. In, ed. by Schlich et al., Proceedings of the Ocean Drilling Program, Initial Reports, Volume 120, p. 73-85. Shlanka, R., 1972. Geology of the Steep Rock Lake area, District of Rainy River. Ontario Department of Mines and Northern Affairs, Geology Report 93. Smyth, H.L., 1891. Structural geology of Steep Rock Lake, Ontario. American Journal of Science, v. 43, 3rd Series, p.317-331. Sumner, D.Y., 1997. Late Archean calcite-microbe interactions: Two morphologically distinct microbial communities that affected calcite nucleation differently. Palaios, v. 12, p. 300-316. Sumner, D.Y., 2000. Microbial vs environmental influences on the morphology of late Archean fenestrate microbialites. In, ed by R.E. Riding and S.M. Awramik, Microbial Sediments, Springer-Verlag, Berlin, p. 307-314. 42 �Sumner, D.Y. and Grotzinger, J.P., 2000. Late Archean aragonite precipitation: petrography, facies associations and environmental significance. In, Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, S.E.P.M. Special Publication 67, p.123-144. Thurston, P.G. and Chivers, K.M., 1990. Secular variations in greenstone sequence develop-ment emphasizing Superior Province, Canada. Precambrian Research, v. 46, p. 21-58. Tomlinson, K.Y., Thurston, P.C., Hughes, D.J. and Keays, R.R., 1996. The central Wabigoon region: Petrogenesis of mafic-ultramafic rocks in the Steep Rock, Lumby Lake, and Obonga Lake greenstone belts (continental rifting and drifting in the Archean). In, ed. by R.M. Harrap and H. Helmstaedt, Lithoprobe Report 53, Lithoprobe Secretariat, University of British Columbia, p. 65-73. Tomlinson, K.Y., Hughes, D.J., Thurston, P.C. and Hall, R.P., 1999. Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, western Superior Province. Lithos, v. 46, p. 103-136. Tomlinson, K.Y., Davis, D.W., Stone, D. and Hart, T.R., 2003. U-Pb age and Nd isotope evidence for Archean terrain development and crustal recycling in the south-central Wabigoon subprovince, Canada. Contributions to Mineralogy and Petrology, v. 144, p. 684-702. Walter, M.R., 1972. Stromatolites and the biostratigraphy of the Australian Precambrian and Cambrian. Special Papers in Palaeontology, Number 11. Wegenast, W.G., 1954. The footwall rocks of the Steep Rock ore zone. M.Sc. thesis, Queens University, Kingston, Ontario. Wilks, M.E., 186. The geology of the Steep Rock Group, northwestern Ontario: A major Archean unconformity and Archean stromatolites. M.Sc. thesis, University of Saskatchewan, Saskatchewan, Canada, 206 p. Wilks, M.E. and Nisbet, E.G., 1985. Archean stromatolites from the Steep Rock Group, northwestern Ontario, Canada. Canadian Journal of Earth Science, v. 22, p. 792-799. Wilks, M.E. and Nisbet, E.G., 1988. Stratigraphy of the Steep Rock Group, northwestern Ontario: A major Archean unconformity and Archean stromatolites. Canadian Journal of Earth Sciences, v. 25, p. 370-391. Wyman, D. and Hollings, P., 1998. Long-lived mantle plume influence on an Archean protocontinent: geochemical evidence from the 3.0 Ga Lumby Lake greenstone belt, Ontario, Canada. Geology, v. 26, p. 719-722. 43 �Hammond Reef Gold Deposit, Brett Resources Inc. Mark Smyk, Ontario Geological Survey (modified from http://www.brettresources.com/s/HammondReef.asp) INTRODUCTION The Hammond Reef project is the flagship project of Brett Resources Inc.. In 2006, Brett agreed to earn a 60% option on the property from Kinross Gold Corp. Brett management recognized that this project has significant upside potential. Prior to agreeing to the option, Kinross Gold Corp. outlined a 1.8 million ounce inferred resource on the property. Subsequent to earning their 60% interest in 2008, Brett acquired the remaining 40% of the project from Kinross. In the summer of 2009, Brett announced their second resource estimate for the property - 155 Mt at 1.04 g/t Au in the Inferred category, using a 0.6 g/t Au cut-off, for a base case of 5.2 million ounces of gold. The majority of the ground surrounding the known deposit remains untested by modern exploration methods. Small-scale mining took place on the property directly to the northeast of the current deposit, the Manley area, at the turn of the 19 th century and again in the 1930's. Small-scale mining focused only on the free gold found in quartz veins, which records indicate had grades of approximately 0.3 ounce Au per ton. The Hammond Reef property hosts widespread gold mineralization within a 100 to 300 m wide, northeasterly-trending corridor. Gold mineralization is associated with quartz vein stockworks. Mineralization defined in the current deposit is relatively flat-lying, dipping between 35° to 45° and seems to flatten out at depth. The favourable geometry of the deposit and the fact that 97% of the inferred resource is found within 300 m of surface, makes the deposit amenable to openpit mining. LOCATION AND INFRASTRUCTURE The Hammond Reef Project is located in the Sawbill Bay-Marmion Reservoir area of the Thunder Bay Mining District, approximately 170 km west of Thunder Bay, and 23 k northeast of Atikokan. The property can be accessed both by water and by gravel road and there is a power line approximately 10 km to the west of the property. GEOLOGY AND MINERALIZATION The Hammond Reef property is predominantly underlain by the Marmion Batholith granitoid suite, characterized by fresh to intensely altered tonalite-trondhjemite; subordinate, unaltered granitoid gneiss; and minor mafic lenses (typically highly altered). Minor pegmatite dykes and pegmatite segregations are present. Quartz stockwork overprints all phases but is only weakly developed in the mafic lenses. The quartz stockwork hosts the gold mineralization. The stockwork is confined to a broad, anastomosing envelope of alteration measuring up to 6000 m wide on surface exposures and has a northeast strike over a length of approximately 40 km. The trend of the alteration system, major quartz veins, gneissic enclaves, and mafic lenses parallel the dominant east-northeast-trending regional fabric. This alteration comprises a gradual increase in thicknesses of halos of saussurite surrounding fractures. The gradually coalescing halos of alteration, with increased frequency of quartz veining, control the development of pervasively altered granitoid. Gold values show a gradual increase once weak but continuous areas of veining and alteration are observed. 44 �Strong alteration and foliation develops into a schist zone without appreciable veining (Hammond Reef Schist Zone-HRSZ). A discrete zone of strongly foliated Fe-carbonate with variable amounts of sericite, chlorite, hematite, limonite and pyrite lenses is easily mapped and has previously been described as highly foliated, tectonized granitoid breccia or breccia zone. Veining can be grouped into two styles -- 5 cm to 50 cm wide, straight and generally undeformed "leader" veins and millimetre- to centimetre-thick, densely spaced, randomly oriented "stockwork" veins -- and suggests extensional dilation during formation. The veins are coeval, with no clear crosscutting features or consistent overprinting observed. The Hammond Reef property exhibits a number of similarities to the Fort Knox deposit in Alaska. Hosted within the Fort Knox Pluton which intrudes the Fairbanks schist, gold occurs in the margins of stockwork veins and veinlets, quartz-filled shear zones, and along fractures within the granite. RESOURCES Brett announced an updated resource estimate for the Hammond Reef Deposit (July 23, 2009 news release). Subsequently, on November 12, 2009, Brett announced the results of a Preliminary Assessment Study which uses the new Base Case Inferred Mineral Resource of 259.4 Mt at 0.80 g/t Au, totaling 6.70 million ounces of gold at a 0.3 g/t Au cut-off. The Preliminary Assessment Study was prepared by Scott Wilson RPA under the supervision of Mr. Richard J. Lambert, P.E., a registered professional engineer and an independent Qualified Person under the standards set forth by National Instrument 43-101. *The Study is preliminary in nature, it includes Inferred Mineral Resources that are considered too speculative geologically to have the economic considerations applied to them that would enable them to be categorized as Mineral Reserves, and there is no certainty that the Preliminary Assessment will be realized. A table of the Inferred Resource at various cut-off grades can be found below. Cut-off Tonnes Grade Gold Oz Au g/t (000,000) Gold g/t (000,000) 1.00 60.2 1.46 2.83 0.90 77.1 1.35 3.34 0.80 98.4 1.24 3.93 0.70 124.6 1.14 4.56 0.60 155.0 1.04 5.19 0.50 188.5 0.95 5.78 0.40 227.0 0.87 6.34 0.30 259.4 0.80 6.70 0.20 281.9 0.76 6.89 45 �METALLURGY In September 2009, Brett announced the results of a metallurgical test program conducted at SGS-Lakefield Research on samples from both the "A" and "41" Zones. Head grades of the test composites varied from 0.60 to 3.4 g/t Au. The result of the program was a new flowsheet that is simple and will reduce capital and operating costs while improving recoveries: Coarse primary grind of 180 microns Bulk sulphide flotation to pre-concentrate the gold to a stream of less than 10% by weight of the plant feed with a gold recovery of over 95% Fine regrinding of the flotation concentrate Cyanidation of the reground flotation concentrate, recovering 97.8% of the contained gold Overall gold recovery 93% All test composites were found to be likely non-acid generating as a result of Acid Base Accounting (ABA) and Net Acid Generation (NAG) analyses. The sulphide sulphur content of the deposit is low, and there appears to be adequate carbonate to neutralize any potential acid generation. EXPLORATION HISTORY The Sawbill Bay Gold District has been the focus of intermittent gold exploration since the original discovery of the Hammond Reef property in 1895. Small-scale underground mining and milling occurred between 1896 and 1900, and again from 1937-1941, with the property changing hands several times. Active "modern" exploration programs were conducted at Hammond Reef by Falconbridge Gold in the 1980's and by Pentland Firth in the 1990's. This work resulted in the delineation of two separate low-grade gold mineralized zones, the "A" and "41" Zones, delineated by a total of 83 diamond drill holes (18,060 m). A mineral inventory was prepared by Pentland Firth which indicated that the two zones contained an estimated 88 Mt grading 0.93 g/t Au, for a total of 2.64 million troy oz. gold*. (*This is a historic, non- NI 43-101-compliant resource provided for informational purposes only.) Since first optioning the property from Kinross in March 2006, Brett Resources has used systematic exploration to expand the known areas of mineralization at the Hammond Reef Project. In 2006, Brett started their work with a geological exploration/sampling program, including geological mapping, prospecting, systematic sampling of outcrops, soil sampling, core sampling for geochemical signatures, and magnetometer surveys. Initial success came as soil sample results outlined three strong new northeast-trending anomalous gold trends west of Snail Bay area, northeast of the claims known as the Manley Patents. The Manley Patents are the site of at least nine historic shafts mined by small-scale mining in the early 1900's, but were until recently privately held, and have not been subject to modern exploration methods. A trenching program was conducted over the new anomalous gold area and results outlined three separate mineralized zones indicating the presence of a new wide gold mineralized horizon open to the north, and more than 2 km from the A and 41 Zone areas. An additional 50 to 60 m wide mineralized zone was exposed over 450 m of strike length through trenching on the optioned Sande & Stewart property. It remains open and bears the same orientation and appearance as the mineralized zone north of Mitta Lake. 46 �From the end of 2006 to the end of 2008 Brett completed 34,145 m of drilling at Hammond Reef. Step-out drilling around the A Zone encountered significant new mineralization to the southwest, southeast and northeast. Of particular interest were new intersections to the northeast, indicating that mineralization continued along strike towards the 41 Zone. The continuity both at depth and laterally was found to be remarkably good. In-fill drilling in the 41 Zone confirmed the continuity of the zone on a 50 m spacing, as well as expanded mineralization westward. By the middle of 2008, Brett had sufficient new drill intersections to complete a NI 43-101compliant resource estimate for the Hammond Reef deposit, incorporating the contiguous A, 41 and Mitta Lake zones of mineralization. The result was a resource of nearly 5 million ounces of gold in the inferred category (see details in Resources section), nearly double the estimate by previous operators. Eight holes drilled late in 2008 were not included in the resource estimate, and some holes intersected well developed gold mineralization in areas that will add to the gold inventory in future resource estimates. The first phase of the 2009 diamond drilling program started in February, with 16 step-out drill holes for a total of approximately 5000 m of core drilling. Results of the spring drilling program were reported in late May, with the program having successfully extended the Hammond Reef resource in three directions: to the west; down-dip by over 150 m; and northeastward by confirming that the strike extent of the gold-bearing structures extend past the Manley Patents by more than 2 km northeast from the currently defined resource. Previous exploration identified broad low-grade mineralization in two surface trenches located in the Snail Bay area, northeast of the Manley Patents and more than 2 km along strike to the northeast of the current gold resource. One of the primary objectives of the spring drilling program was to test the depth extension of the mineralization in these trenches. Two holes were drilled and both holes returned mineralized intersections. These new discoveries of gold mineralization in the Manley area have the same style of mineralization found in the A Zone and remain open along strike and at depth. No modern exploration techniques have been applied to this area and that was one of the main objectives of the exploration program in the summer and fall of 2009. A second objective of the 2009 spring drilling program was to provide information for a scoping study expected to be completed during the fourth quarter of 2009. In particular, drilling was to test the mineralization up- and down-dip of the currently defined resource area. The holes stepped out from 50 to 275 m along sections and tested both deep and shallow targets and extended the resource at depth by as much as 150 m down-dip to the south. The program has also intersected additional shallow gold-bearing structures that are not in the current resource. These new structures have excellent potential to allow Brett to expand the current resource and positively impact the strip ratio of the pit. The scoping study will incorporate an updated Hammond Reef resource. The best results from the drilling program were from hole BR 126 which intersected 40.5 m of 1.95 g/t Au and from hole BR 127 which returned 159 m of 0.97 g/t Au. Brett's scoping study will evaluate the economic viability of the Hammond Reef gold deposit. A major part of the study will require engineering reviews of mining methods, pit design and location and processing plant design. 47 �Brett conducted widespread field exploration work at Hammond Reef in 2009. With a combination of geophysics, soil sampling, prospecting, mapping and trenching. Brett hopes to detect new anomalous gold mineralization and to identify new exploration targets for drill investigation. Brett recently announced that Osisko Mining Corporation mailed shareholders of Brett and filed with the Canadian securities regulatory authorities and the U.S. Securities and Exchange Commission, the offer and take-over bid circular, formally commencing Osisko's friendly offer to acquire all of the outstanding common shares of Brett on the basis of 0.34 of an Osisko common share and $0.0001 in cash for each common share of Brett (Brett Resources Inc., news release, April 13, 2010). Osisko and Brett entered into a support agreement on March 21, 2010 providing for the terms of the Offer and the agreement by the board of directors of Brett to recommend that Shareholders accept the Offer. The consideration under the Offer represents a premium of 52.5% using the 20-day volume weighted average prices of Osisko and Brett quoted on the Toronto Stock Exchange and TSX Venture Exchange, respectively, for the 20 trading day period ending March 16, 2010. 48 �Hammond Reef property location and regional geology (from http://www.brettresources.com/i/maps/091202_Regional_Geology.jpg) 49 �General geology of the Hammond Reef property, showing location of altered and goldmineralized zones (from http://www.brettresources.com/i/pdf/2009-11-27_HR_NI43-101.pdf). Each UTM square is 5 km wide. 50 �Field Trip 3 DEFORMATION IN THE RAINY LAKE REGION: A FABULOUS DISPLAY OF STRUCTURES CONTROLLED BY RHEOLOGICAL CONTRASTS Dyanna M. Czeck University of Wisconsin-Milwaukee Howard Poulsen Consultant, Ottawa Photo: Flattened Seine conglomerate 51 �1. Introduction The Rainy Lake area has been the locus of geologic studies for more than a century, starting with the pivotal early work by Lawson (1887; 1913). Over the decades, it has seen its share of controversy from Lawson‘s classic arguments for the existence of crystalline rock that was demonstrably younger than supracrustal rocks (Lawson, 1887) to the long-lived ―SeineCoutchiching Problem‖ stemming from cross-border disputes about relative ages of sedimentary sequences (Van Hise et al., 1905; Van Hise and Leith, 1911; Lawson, 1913; Grout, 1925; Tanton, 1927; Hawley, 1930; Gill and Hawley, 1931; Merritt, 1934; Ojakangas, 1972; Wood, 1980). The stratigraphic arguments have largely ceased due to an understanding of the dynamic tectonic framework responsible for the juxtaposition of the disparate rock types (Poulsen et al., 1980; Davis et al., 1989; Poulsen, 2000a) and the application of high precision U-Pb dating to key units (Davis et al., 1989; Fralick and Davis, 1999). During our fieldtrip, we will focus on analyzing key structural features that will help us understand the kinematics of the regional and local deformation. As the region straddles a significant boundary within the Superior Province, the kinematic history of the area may be used to help elucidate Archean tectonic styles and patterns of continental growth. The complex juxtaposition of various lithological units makes for intriguing geology in general, and structural geology in particular. The various rock units provide a study in contrasts where the structural styles and strain intensities are largely controlled by the rheological contrasts of adjacent rock types. When a region is tectonically deformed, deformation is preferentially localized in weaker rocks, leaving stronger rocks to remain relatively undeformed with weak to nonexistent deformation fabrics (analogous to embedding a marble into modeling clay and squishing the whole package- all of the deformation is in the modeling clay and the marble stays undeformed). In the field, we can see these rheologic contrasts at a variety of scales, from cm scale pebbles with contrasting strengths in deformed conglomerates, to km scale strong plutonic rocks surrounded by weaker volcanic rocks. Indeed, we can even see evidence for strength inversions. For example, when granites intrude, they are molten and weaker than their host rocks, but as they cool, they gradually become stronger than their hosts, resulting in an inversion in the relative strengths over time. A general theme amongst the stops on this trip will be to evaluate the relative strength, or competence, of adjacent rock types and evaluate how the competence contrasts are manifested into diverse styles of structures. 2. Geologic Setting and Geochronology The large Archean Superior Province (Fig. 1) can be divided into subprovinces based on lithostratigraphy and metamorphic grade (Card and Ciesielski, 1986). The subprovinces were assembled by repeated island arc, microcontinent collisions. Several pieces of evidence are consistent with the island-arc accretion model. First, the microcollisions are evidenced by rocks that can be interpreted as arc sequence subprovinces (metavolcanic rocks of the greenstone belts) and their corresponding accretionary prism subprovinces (metasedimentary rocks) (Langford and Morin, 1976; Hoffman, 1989; Percival and Williams, 1989; Card, 1990; Hoffman, 1990). Second, the ages in the greenstone belts are generally similar along strike, but differ systematically across strike (Hoffman, 1989). Third, the deformation characteristics found 52 �Figure 1. Superior Province based on divisions by Card and Ciesielski (1986). across the province are best explained by contractional deformation. A history of subduction and southward accretion was responsible for the juxtaposition of Superior Province subprovinces (Langford and Morin, 1976; Percival and Williams, 1989; Card, 1990). The Rainy Lake zone (Fig. 2) lies on the western part of the boundary between two lithotectonic subprovinces of the Superior Province: the Wabigoon granite-greenstone terrane to the north and the Quetico metasedimentary terrane to the south (Card and Ciesielski, 1986). Oblique collision between the Wabigoon and Wawa volcanic-plutonic terranes during the late Archean approximately 2.69-2.7 Ga (Stockwell, 1982; Davis et al., 1989; Fralick and Davis, 1999) most likely formed the tectono-metamorphic features of this boundary zone (e.g. Poulsen, 1986; Percival and Williams, 1989). The metavolcanic subprovinces (Wabigoon, Wawa) were subsequent island arcs that collided due to ongoing subduction, and the metasedimentary subprovince (Quetico) is comprised from sedimentary basin deposits between the arcs (Percival, 1989; Blackburn et al., 1991; Williams, 1991). 53 �54 Figure 2. Simplified Rainy Lake region geologic map modified from Davis et al., (1989). Full detailed map is Poulsen (2000c). �Poulsen (1986) recognized the Rainy Lake boundary area as a complex transpressive deformation zone. The area is bounded by two major structures, the Quetico and the Rainy Lake-Seine River faults, which have most likely behaved as both dextral faults and shear zones during their deformation histories (Fig. 2). The two faults/ shear zones converge to the east into a single structure and delineate a wedge-shaped area between them (Poulsen, 1986, 2000a, b, c). Within the wedge, a complex pattern of anastomosing subvertical shear zones and faults wrap around gneissic domes and granitoid plutons (Poulsen, 1986, 2000a, b, c; Druguet et al., 2008). Flattening fabrics combined with the evidence for dextral shearing indicate an overall dextral transpressive tectonic regime with subhorizontal NW-SE shortening (Poulsen, 1986; Tabor and Hudleston, 1991; Borradaile and Dehls, 1993; Borradaile et al., 1993; Poulsen, 2000a, b; Czeck and Hudleston, 2003; Druguet et al., 2008; Czeck et al., 2009). The main lithological units in the study area can be summarized based on dates obtained by Davis et al. (1989) and Fralick and Davis (1999), as follows: (1) Keewatin Group tholeiitic and calcalkaline metavolcanic rocks. These form interlayered sequences of ultramafic to felsic compositions, although basalts and andesites are most common. Crystallization ages range from 2728+4/-3 to 2725±2 Ma. (2) Metagabbroic rocks with metadolerites and metaanorthosites. They are found as sills and larger intrusions of variable dimensions, generally showing compositional and textural layering. Includes the Seine Bay- Bad Vermillion Complex (2728+/-2 Ma for the trondhjemitic Mud Lake component) and the Grassy Portage Sill (2727+/-2 Ma). (3) Gneisses. Most gneisses in the region are orthogneisses corresponding to synvolcanic ―Laurentian‖ plutonic rocks, about 2725±2 Ma that vary compositionally from granitic to tonalitic and form the core of the Rice Bay dome and the gneisses near Black Sturgeon Bay. (4) Coutchiching Group metasedimentary rocks. These are the sedimentary rocks located south of the Quetico Fault and north of the Seine-River Rainy Lake Fault and are bracketed in age between 2704±3 and 2692±2 Ma by detrital zircon and a crosscutting intrusion. They contain metagreywackes and metapelites, especially biotite schists. They locally contain porphyroblasts of garnet, andalusite and staurolite. (5) Quetico metasedimentary rocks. The Quetico turbiditic metasediments are found to the south of the Seine River-Rainy Lake Fault. The age of the youngest analyzed detrital zircon is 2699±1 Ma. (6) Seine River Metasedimentary Group. Deposited partly during and after the nearby Coutchiching metasedimentary rocks. The depositional age is bracketed between 2693±1-2692±1 Ma, (Fralick and Davis, 1999). This unit formed as one of the Superior Province‘s molasse basin-fill units (Corcoran and Mueller, 2007) and includes the Seine River conglomerates, well developed near Mine Centre (Wood, 1980; Frantes, 1987; Czeck, 2001; Czeck and Fralick, 2002; Czeck and Hudleston, 2003; Fissler, 2006; Czeck et al., 2009). (7) ―Algoman‖ calk-alkaline granitoids. These are relatively unmetamorphosed and poorly deformed granite, granodiorite, and quartz monzonite rocks that form several individual plutons (age of the Ottertail pluton: 2686+2/-1 Ma). They were previously interpreted as 55 �postkinematic and their age of emplacement was used as a constraint for the ending of regional deformation (Davis et al., 1989; Poulsen, 2000a). However, by means of field relationships, microstructural analysis, anisotropy of magnetic susceptibility (AMS), and gravity inversion techniques, Czeck et al. (2006), demonstrated that these plutons formed syntectonically, late within the Archean transpressional event. 3. Structural Framework Many authors have found evidence for a three phase (D1, D2 and D3) deformational history (Poulsen, 1986; Davis et al., 1989; Tabor and Hudleston, 1991; Poulsen, 2000a; Druguet et al., 2008). All of these phases are associated with oblique convergence related to the island arc collisions. While the structures are similar in style and sequence to other features found throughout the central Superior Province, the phases are unlikely to temporally correlate across subprovince boundaries, and may not even correlate within the Rainy Lake region due to the dynamic nature of the tectonic setting. D1 is characterized by early recumbent folding that produced the inversion of the stratigraphic sequence and S1, the first regional schistosity (Poulsen et al., 1980). The combination of several observations including the present-day juxtaposition of adjacent right-way-up and overturned folds (Poulsen et al., 1980; Borradaile, 1982; Poulsen, 2000a), suggest that the original orientations of many of the folds were nappe-like. S1 is generally subparallel to the layering in the metavolcanic and metasedimentary rocks. A gneissic foliation attributed to D 1 is well developed in the gneiss domes. D1 is likely to have included significant thrust or oblique faulting that juxtaposed allochthonous units. There is less direct evidence for thrusting, due to a general lack of sedimentary markers that would enable one to recognize duplicated sequences of rock. However, the hypothesis of early faulting is corroborated by the strain patterns within this and other Archean belts, which are generally too low to account for the now largely steeply dipping units (e.g. Schultz-Ela, 1988; Hudleston and Schwerdtner, 1997). This early history of thrusting and recumbent folding can be thought of as ―stacking‖ (Davis et al., 1989; Czeck, 2001; Czeck and Fralick, 2002). As the crust was ―stacked‖ in D1, rocks that were once near the surface became buried. In these rocks, D1 deformation was followed by D2, which is characterized by folding and dextral shearing under a transpressional regime. F2 folds resulted in steep to subvertical bedding subparallel to a regional subvertical S2 foliation with a mean ENE-WSW strike. The Seine River Group was deposited in a fault-related basin during early D2 (Poulsen, 1986, 2000a), which explains their strong D2 fabrics and minor amounts of internal folding (Czeck, 2001; Czeck and Hudleston, 2003; Czeck et al., 2009). However, it is important to note that the transition of D 1 to D2 was not necessarily contemporaneous across the Rainy Lake region and the vertical bedding in the Seine is likely a result of initial internal stacking followed by D 2 deformation styles. The Algoman plutons were emplaced and cooled during late stages of D 2 (Czeck et al., 2006). Many leucocratic and quartz veins were emplaced throughout D2 (Druguet et al., 2008). D2 was responsible for the penetrative foliation and lineation fabrics and much of the recorded strain throughout the region. In general, the foliation is subvertical (Fig. 3a), and where strain 56 �Figure 3. Equal area stereonets representing structural fabric in the Rainy Lake region from Czeck and Hudleston (2003). (a) Stretching (mineral) lineations. (b) Poles to foliation. can be measured, such as within the Seine conglomerates, strain is largely of a flattening type (Czeck, 2001; Czeck and Hudleston, 2003; Fissler, 2006; Czeck et al., 2009). Dextral shear sense indicators in the form of asymmetric folds, conglomerate strain shadows, clast tiling, and other features are often observed on subhorizontal planes regardless of lineation orientation (Czeck and Hudleston, 2003). Most of these fabric features along the boundary are consistent with a type of ductile transpression first described by Sanderson and Marchini (1984). This style of transpression is a mathematical description of a specialized case of ―transpression‖ that in its simplest form just means oblique convergence (Harland, 1971). The style of transpression described by Sanderson and Marchini (1984) involved homogeneous deformation consisting of orthogonal simple shear and pure shear components with constant volume in a vertically bounded zone (Fig. 4a). Such an idealized scenario has since been termed ―monoclinic transpression‖ (e.g. Lin et al., 1998) and is perhaps most likely to correspond to strain in deep, vertical ductile shear zones during oblique convergence. The structural fabrics for this model include vertical foliations, flattening fabrics, and asymmetric shear sense indicators on the horizontal plane (Fossen and Tikoff, 1993) (Fig. 5). The consistencies of fabrics observed along the Wabigoon–Quetico boundary with the homogeneous transpression model suggest that at least the Wabigoon–Quetico boundary as a whole has undergone quasi homogeneous transpression. Figure 4. Transpression Models. a) Homogeneous monoclinic transpression (Sanderson and Marchini, 1984). b) Triclinic transpression with oblique simple shear component (Lin et al., 1998). c) Triclinic transpression model with horizontal simple shear component and oblique extrusion direction (Czeck and Hudleston, 2003, 2004). 57 �Figure 5. Schematic deformation fabrics and strain predicted for monoclinic transpression (Fossen and Tikoff, 1993). Planes shown are foliations, lines shown are stretching lineations, and schematic ―pebble‖ indicating flattening strain and asymmetric strain shadows from Czeck and Hudleston (2003). The lineation orientation (Fig. 3b) is one major aspect of the structural fabric in the Rainy Lake region that does not mesh with the monoclinic transpression model, which predicts either vertical or horizontal lineations (Fossen and Tikoff, 1993). The orientations of the stretching lineations, as defined by the preferred orientation of metamorphic minerals, varies at the hectometric scale; they plunge between 0-90° in both east and west directions (Czeck and Hudleston, 2003). The subhorizontal asymmetric shear sense indicators combined with the greatly variable stretching lineation orientations prove to be difficult to explain with most kinematic models. In shear zones that deformed via simple shear, we would expect to find asymmetric shear sense indicators on the horizontal plane, but the lineations should be horizontal. In shear zones that deformed via homogeneous monoclinic transpression models (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993), we would expect to find asymmetric shear sense indicators on the horizontal plane, but the lineations should be only vertical or horizontal depending on the amount of pure shear versus simple shear and the strain magnitude. In many triclinic transpression models that can explain obliquely plunging lineations (Jiang and Williams, 1998; Jones and Holdsworth, 1998; Lin et al., 1998), the simple shear component is no longer horizontal, so the asymmetric shear sense indicators should be found on a corresponding oblique plane (Fig. 4b). For these reasons, most kinematic models for shear zones are incompatible with structures found in the region. The best model proposed for D2 that explains the subhorizontal asymmetric shear sense indicators and the highly variable stretching lineation orientations is a triclinic transpression with horizontal simple shear and variable extrusion direction (Czeck and Hudleston, 2003, 2004) (Figs. 4c, 6). In the original monoclinic transpression and most subsequent triclinic models (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; Robin and Cruden, 1994; Dutton, 1997; Jones and Holdsworth, 1998; Lin et al., 1998), material was assumed to extrude vertically upwards. This assumption is logical because, in general, one would expect the direction towards the earth‘s surface to provide the least resistance for material movement. However, rocks at depth may have other boundary conditions imposed by anastomosing shear zone geometries or large scale rheology contrasts due to a diverse lithological assemblage that cause localized deviations in the extrusion direction. Certainly, the Rainy Lake region has both complex three58 �Figure 6. Schematic deformation fabrics and strain predicted for transpression with oblique extrusion from Czeck and Hudleston (2003). Planes shown are foliations, lines shown are stretching lineations, and schematic ―pebble‖ indicating flattening strain and asymmetric strain shadows. (a) Simple view of transpression with nonvertical extrusion. (b) Schematic view of transpression with overall bulk vertical extrusion and localized zones of nonvertical extrusion. Dark colored ellipses represent schematic ‗hard‘ zones that influence local extrusion directions. The relative location of (a) is indicated. dimensional anastomosing shear zones and lithologies with strong competence contrasts that could account for local variation in the extrusion directions. Domains of higher strain are localized in the relatively incompetent metavolcanic and metasedimentary rocks and channeled around the competent gneissic domes, metagabbros, and Algoman plutons. Two-dimensional strain was estimated using folded and boudined dikes, and the areas of lowest strain are located on the ―strain shadow‖ flanks of the gneiss domes and plutons (Castaño, 2007; Druguet et al., 2008). As a result, the dominant, composite S1+S2 foliation displays a complex heterogeneous pattern both in orientation and intensity. Dextral transpression continued as the rocks were exhumed. It is evidenced by retrograde metamorphism and D3 structures (Poulsen et al., 1980; Poulsen, 2000a). During D3, strain localized along major mylonitized dextral shear zones and faults such as the Quetico and Seine River-Rainy Lake Faults, narrow mesoscale shear zones, and crenulation bands. This late stage of deformation is more brittle-ductile in nature than D2, and thus represents rock exhumation. Progressive D2 to D3 dextral transpressional deformation was likely controlled by the distribution of lithological units with contrasting competence, the previous structure, and the emplacement of the Algoman plutons. Many of the late quartz veins may have been emplaced during D 3, but this inference needs to be more rigorously tested. Some authors note that the final D3 stage of deformation involved amplification of strike-slip motion along wrench zones (Bauer et al., 1992). The latest brittle faulting has largely been 59 �described as dextral strike-slip motion (Kennedy, 1984), but local late-stage brittle faults associated with N-S shortening also have been observed (Tabor and Hudleston, 1991). In all likelihood, the last stages of deformation associated with the oblique convergence may have been strongly partitioned with the major strike-slip faults accommodating the boundary-parallel motion and the domains between the major faults accommodating the boundary-perpendicular motion (Tabor and Hudleston, 1991). 4. Field Trip Stops We have arranged the stops in a sequence that will allow us to unpeel the stages of deformation. We will look at the least deformed, youngest rocks first (the Ottertail Pluton), followed by syndeformational metasedimentary rocks (the Seine Conglomerates), followed by older rocks that have seen most/ all of the deformation and, in many cases, are intruded by many generations of syntectonic veins. In this way, we will see rocks with progressively more complicated (and exciting!) structures. There are a few bonus stops at the end in case we have time on our way back to Fort Frances. The locations of most stops can be found on Figure 2. Stops 3 and 4 can be found on Figure 14 which is a map drawn for the entire Seine Basin (overlaps with map on Fig. 2). Stop 5 can be found on both Figs. 2 and 14. Stops are located by UTM coordinates based on NAD 83, UTM Zone 15 Stop 1: Ottertail Pluton Margin From the Recreation Center in Fort Frances, drive east along Hwy 11 for approximately 45 km. Outcrop located on both sides of road. (0506850E, 5396300N) The Ottertail Pluton is one of the calcalkaline granitoids of the Algoman type according to Lawson (1913) that range in composition from diorite-granite. More recent studies have linked them to sanukitoid magma suites (Shirey and Hanson, 1984). They are compositionally distinct from the older orthogneisses in the area, such as in the core of the Rice Bay Dome, which typically have a tonalitic composition (Poulsen, 2000a). All of the Algoman type intrusions have relatively discrete discordant boundaries and are generally semi-elliptical on map view. Although the Ottertail is obviously relatively late as judged simply by looking at the map, the details of how late is a bit more complicated and requires a look at both its margins and interior. The Ottertail itself is mostly granite to granodiorite. The granites are two-feldspar subsolvus granites, containing both K-feldspar and plagioclase along with quartz, hornblende, and biotite. Across its extent, it has a heterogeneous composition and grain size distribution (Czeck et al., 2006). At this stop, we will view agmatitic breccias that are common at the margins of the Ottertail (Poulsen, 2000a). Agmatites are defined by breccia- like mesosomes intruded by a leucosome. The breccia like character implies hydraulic fracturing at the pluton edge 60 �(Kornprobst, 2002). The xenolithic blocks within the breccia are foliated country-rock remnants that were likely deformed during D1 and D2. The Ottertail pluton‘s composition near the boundaries has significantly higher percentages of chlorite, biotite, and hornblende, presumably input from the country rock. Much of the chlorite is a retrograde reaction product of either biotite or hornblende. Together, the percentages of these three minerals range from 2%–15% in the center of the pluton, grading to 34%–58% along the eastern and western margins (Czeck et al., 2006). Feldspars are more sericitized in these boundary regions (Czeck et al., 2006). Stop 2: Ottertail Pluton Continue driving east on Hwy 11 for approximately 4 km. Approximately 1 km east of Pearson’s Road turnoff, outcrop is located on both sides of Hwy 11. (0510221E, 5397524N) Here we are located in the center of the Ottertail Pluton with the typical granitic chemistry. The rocks appear largely unmetamorphosed in outcrop, but were subjected to lowgrade, often hydrous metamorphism (Fig. 7a). Chlorite is the primary metamorphic mineral present, which formed by retrograde metamorphic reactions of biotite and hornblende (Czeck et al., 2006). Trace evidence of other metamorphic minerals including epidote, clinozoisite, and zoisite are often present (Czeck et al., 2006). The pluton is not highly deformed, but a range of deformation microstructures can observed throughout, either to subtle or intense degrees (Fig. 7b, c). The amount of grain scale deformation varies non-systematically throughout the pluton. In most samples, quartz microstructures, such as undulose extinction and subgrain formation, indicate that some solidstate deformation accommodated by dislocation creep mechanisms was active (Czeck et al., 2006). In the samples with the most deformation microstructures, a weak to moderate S–C fabric, quartz ribbons, fractured feldspars, and kinked biotites have also been observed (Czeck et al., 2006). Variations in the microstructural assemblages are most likely due to strain localization or possibly different cooling histories. The microstructures indicate that the granite was emplaced and cooled enough to deform in the solid state (in at least some portions) during deformation. Well-defined planar anisotropy of magnetic susceptibility (AMS) fabrics within the Ottertail Pluton are roughly coincident with the steep foliations measured in the surrounding rocks, indicating that the AMS fabrics were most likely caused by regional deformation (Czeck et al., 2006). Gravity inversion analysis indicates that the pluton is shallow, with a maximum depth of approximately 4.5 km (Czeck et al., 2006). The shallow nature of the pluton supports the conclusion that the steep internal fabrics are unlikely to be purely flow from a feeder zone, but rather a result of the regional strain. The pluton was likely syntectonic, late in D 2. Its geometry is consistent with intrusion into a transpressional setting, possibly into shallow rhombochasms created by en echelon P shears related to the major shear zones or along oblique thrusts (e.g. Tikoff and Teyssier, 1992). 61 �Figure 7. Photomicrographs of Ottertail Pluton from Czeck et al. (2006). a) Typical thin section with low-grade metamorphic mineralogy roughly consistent with regional greenschist-facies metamorphism (cross-polarized light). b) Undulose extinction in quartz (cross-polarized light). c) Subgrain formation in quartz (cross-polarized light). Stop 3: Moderately strained Seine Metaconglomerates Continue east along Hwy 11 for approximately 33 km to Horsecollar Junction . Outcrop located along north side of Highway 11. (0542130E, 5398538N) The Seine Group was interpreted to have been deposited in a fluvial system by Lawson (1913), Wood (1980) and subsequent workers (e.g. Czeck and Fralick, 2002). This formation undergoes a gradual transition from conglomerate dominated near its base to sandstone dominated near its top. The lithofacies found within the Seine match various environments of braided stream systems including channels and bars. Conglomerate dominated longitudinal bar-channel sequences are the most common. However, sandier channel sequences are not rare and gain importance higher in the formation (Czeck and Fralick, 2002). Lawson (1913) suggested the Seine Group was deposited at the site of an ancient fault zone, the strike of which is approximated by the current distribution of the conglomerate and sandstone. 62 �Within the conglomerates, asymmetric shear sense indicators are prevalent. In general, these are either in the form of asymmetric pressure shadows at the ends of clasts, wrapped foliation indicating rotation of the most rigid clasts, and clast tiling. All of these shear-sense indicators suggest dextral shear and are most evident on the subhorizontal plane, regardless of lineation orientation. The best model proposed for the deformation in the metaconglomerates that explains the subhorizontal asymmetric shear sense indicators, the dominant flattening fabrics, and the highly variable stretching lineation orientations is a transpression with horizontal simple shear and variable extrusion direction (Czeck and Hudleston, 2003, 2004). The specific clast strain combined with the other structural evidence should allow for a more rigorous test to this model (Fernández et al., in preparation). The metaconglomerates within the Seine Group provide an excellent opportunity to observe competence contrasts and their effects on deformation. The deformation here is typical of much of the Seine: the granitoid clasts being fairly undeformed while the volcanic clasts reflect significant flattening. Minor amounts of quartzite, BIFs, and other lithologic clast types are also observable. Czeck et al. (2009) used the clast shapes on the three-dimensional road outcrops to calculate strain within the Seine‘s various clast populations using the Rf/ method for two-dimensional strain (Ramsay, 1967; Dunnet, 1969; Lisle, 1985), a bootstrap statistical technique (McNaught, 2002; Mulchrone, 2005; Yonkee, 2005), and best-fit three-dimensional ellipsoid calculations (Launeau and Robin, 2003, 2005) (Fig. 8). In most cases, the calculated long axes of the strain ellipsoid match the mineral lineation and short axes are normal to the foliation, indicating that primary clast fabrics have not significantly affected the results. The strain magnitude is strongly dependent on lithology and varies greatly and nonsystematically throughout the region. In general, the mafic and felsic clast populations have similar strain magnitudes at a particular outcrop and granitoid clasts have consistently smaller strain magnitudes. The regional-scale variations in strain magnitude cannot be linked to changes in the concentrations of clast types within the outcrops (Fissler, 2006), but are likely related to spatial proximity to the minor shear zones. Of the 26 outcrops evaluated, 19 have an overall flattening strain, five exhibited a constrictional strain, and two exhibited plane-strain. Flattening dominates throughout the field area while constriction exists in the western and easternmost stations. Strain magnitude does not correlate to strain shape. At this location, long/short strain axes ratios were approximately 2:1 for granitoid clasts, 4:1 for felsic clasts, and 6:1 for mafic clasts. Czeck et al. (2009) also used detailed finite strain measurements of the clasts to calculate effective viscosity contrasts using established methods (Treagus and Treagus, 2002). The two volcanic clast populations have similar effective viscosities, regardless of strain magnitude, with ratios of felsic/mafic ranging from 0.27 to 2.12 and average ratio of 1.19. The granitoid clasts are significantly more competent than the mafic clasts with effective viscosity ratios ranging from 2.35 to 12.39 and average ratio of 5.61. The results are consistent with a qualitative competence hierarchy of granitoid > felsic ≥ mafic, although the quantitative effective viscosity ratios change with strain magnitude: for small strains, granitoid >> felsic ≥ mafic, but for large strains, granitoid > felsic ≥ mafic. The variations in these ratios throughout the field area indicate that the effective viscosity contrast between clast types is strain dependent and possibly deformation path dependent (Czeck et al., 2009). 63 �Figure 8. Plots indicating a) strain magnitudes ( s west to east across basin (stations A-Z) from Czeck et al. (2009). 64 �Stop 4- Highly strained Seine Metaconglomerates Continue east along Hwy 11 for approximately 19 km . Outcrop located along north side Highway 11. (0559619E, 5398914N) The Seine here is extremely deformed (amongst the highest strain found within this unit), reflected in the strained shapes of both the volcanic and plutonic clasts. Many clasts are deformed beyond recognition, giving the rock a striped appearance. Is it possible to accurately estimate strain in a rock that is this deformed? Most likely, all strain estimates are minimum estimates because only the least deformed clasts can be measured. Czeck et al. (2009) estimated strain for the various clast types at this outcrop and found the most disparate strain measurements in their study. Long/short strain axes ratios were approximately 3:1 for granitoid clasts, 11:1 for mafic clasts, and 136:1 for felsic clasts! The errors on the best fit ellipsoids were high, leading to doubts on the veracity of these results (don‘t put much faith in these numbers!). The lineation plunges ~44°E. While this lineation orientation is the approximate ―average‖ stretching lineation for the Seine, it cannot be thought of as ―typical‖ because the lineation orientations are so inconsistent. This outcrop is a key location to demonstrate that the lineation orientation is not directly related to the amount of strain (Czeck, 2001; Fissler, 2006; Czeck et al., 2009). This is important because most transpression models predict subvertical-vertical lineations in highly strained outcrops, and outcrops such as this one allow us to rule out many kinematic models. Again, the dextral asymmetric indicators are most prominent on the subhorizontal plane which allows us to rule out many triclinic transpression models. The combination of subhorizontal asymmetric shear indicators and variable lineation orientations makes a deformation model of quasi homogeneous transpression with a variable extrusion direction most likely (Czeck and Hudleston, 2003, 2004). This outcrop also displays some crenulation cleavage. In general, crenulations formed in the Seine when the clasts are stained enough to create a layering effect. Therefore, crenulations formed relatively late within the strain history, probably during D 3. Crenulations of both dextral and sinistral senses can be observed, indicating that there was likely still a pure shear component to the strain at the time of crenulation formation. The extensive silica veining and carbonate alteration in this rock suggests that fluids infiltrated the rock during deformation. The relative timing of the carbonate fluid and quartz veins is ambiguous in thin section, suggesting that they were at least at times, coeval (Czeck, 2001). Fluid flow in shear zones is often associated with the possibility of preferential volume loss of soluble material, although the largely constrictional strains measured here (Czeck et al., 2009) are not consistent with typical volume loss apparent strains which fall in the flattening field (Ramsay and Graham, 1970). There was probably a symbiotic relationship between fluid localization and enhanced deformation. With such instances of fluid infiltration in highly deformed zones, people often ask the ―chicken vs. egg‖ question of whether the fluid caused the high deformation through softening of the rock or whether the high deformation caused paths for fluids to infiltrate. We will leave the ―chicken vs. egg‖ question open for discussion. 65 �Stop 5 – Two-dimensional horizontal view of Seine Metaconglomerates Continue on Hwy 11, turn south on to Forest Tour Road (dirt track overgrown with weeds). (0536650E, 5398850N) This outcrop allows us to see a large two-dimensional view of the Seine Metaconglomerates with many beautiful examples of clast tiling, asymmetric strain shadows, and other kinematic indicators. Many sandy beds and channels, often with graded bedding, can be located within this outcrop. Is it possible to determine stratigraphic facing here? As a note of historical interest, A.C. Lawson recognized cross-beds in the Seine sandstones as early as 1911, only a year or so after their significance as a ―way-up‖ indicator was appreciated in younger, flat-lying rocks elsewhere. Lawson‘s work in the Seine appears to have been the first use of observed reversals in stratigraphic facing to define traces of folds in a Precambrian setting. Stop 6- Pseudoboudins Return west on Hwy 11 for approximately 66 km. Outcrop is located at junction of Hwy 11 and dirt road turnoff (south side of Hwy). Also small outcrop on north side of Hwy. (0497963E, 5393628N) At this outcrop of pelitic–psammitic schists, we can see fine examples of segmented pegmatite veins resembling boudin trains. Some of these segmented veins are indeed boudins, and they can be distinguished based on: 1) clearly defined edges, often with straight sides formed by fracturing that are oriented at high angle to the host rock fabric and 2) clear fabric deflection of the host rock into the boudin neck regions. However, within other segmented veins, we can see distinct examples of apparent boudinage or ―pseudoboudinage‖ (Bons et al., 2004; Bons and Druguet, 2007; Druguet et al., 2008) (Fig. 9). In pseudoboudinage, a string of pegmatite beads is oriented at a low angle to the foliation and layering. Although resembling boudinage, this structure more likely formed by irregular inflation and collapse during vein intrusion (Bons et al., 2004) or intrusion into dilational jogs (Bons and Druguet, 2007). This interpretation is based on: 1) the irregular ―cauliflower‖ shape of the individual segments and 2) the large amount of extension required to form these structures by boudinage, an amount that is incompatible with strain determined from nearby boudins (Druguet et al., 2008). Subsequent vein-parallel tectonic extension may have produced further separation between segments. Good examples of pseudoboudinage can be seen on the outcrop off of the south side of the highway. Good examples of folded and boudinaged veins can be seen on the small outcrop on the north side of the highway. 66 �Figure 9. Schematic illustration of geometrical differences between apparent boudinage and actual boudinage from Bons et al. (2004). a) Collapse and inflation of a pegmatite dyke upon emplacement, without any dike-parallel stretching. b) Boudinage of a solidified pegmatite dyke, due to dike-parallel stretching. Stop 7- Shear zones within the Grassy Portage Sill Continue west along Hwy 11 for approximately 4 km to the junction with Hwy 502. Turn right and drive north along Highway 502 for approximately 2 km. Turn east along unnamed road and drive for approximately 4.5 km. Outcrop is a small mound on north side of road. (0498897E, 5399146N) The Grassy Portage intrusion is a large metamorphosed layered gabbroic sill, which is exposed approximately 20 km along its strike. It has been overturned so that it dips steeply to the northwest (Poulsen, 2000a). Within the sill, layering is formed by variations in mineralogy and chemistry, both regionally (km scale) and locally (cm scale) (Poulsen, 2000a). Compositions range from melagabbro to anorthosite (Poulsen, 2000a). The margins have been exploited for disseminated chalcopyrite and pyrrhotite mineralization (Poulsen, 2000a). We will observe the sill along its base (northwest margin) on a subhorizontal glaciated surface. This lower portion of the sill is dominated by compositions of gabbroic and anorthositic rocks that either may have formed by gravitational setting or autointrusion (Poulsen, 2000a). The metagabbroic rocks range from leucogabbro to gabbro in composition and contain hornblende and plagioclase with minor amounts of chlorite and biotite (Poulsen, 2000a). Most of the gabbros have a poikilitic texture with hornblende megacrysts, but some have an equigranular texture. 67 �Within the sill, the anorthositic rocks are often found as lenticular pods within the gabbros that range in size up to 50 cm maximum diameter. They have adcumulate textures of fine-grained andesine (Poulsen, 2000a). At this particular outcrop, the pods have adcumulate textures, but are composed of fine-grained zoisite. The zoisite textures indicate that they were formed by hydrous alteration of plagioclase as evidenced by some relict plagioclase grains with quartz and minor amounts of calcite (Carreras et al., in press). The metagabbros contain a wavy pre-shearing weak foliation formed by aligned and linked amphiboles, which strikes approximately 110° with steep-subvertical dip. The orientation of this local foliation is consistent with the prevailing foliation in the immediate region, which is deflected from the typical NE-SW regional strike to a WNW-ESE orientation due to deflection around the Rice Bay Dome. Both the gabbros and anorthosites are affected by subsequent small-scale (cm-dm) shear zones, defined by deflection and intensification of this prevailing foliation and studied in detail by Carreras et al. (in press) (Fig. 10). They are narrow and discrete bands, with a maximum width of a few centimetres, that define an anastomosed pattern with orientations dominated by NNWSSE striking, moderately to steeply dipping strands (Carreras et al., in press). Inside the shear zones, the rocks are well-foliated and fine-grained mylonites. The shear zones are preferentially within the gabbros and often localize at the gabbro/pod margins. This prevalence of ductile deformation within the gabbros indicates that the gabbros were less competent than the anorthosites during much of the deformation. However, the shear zones also cut across some pods where they are defined by mylonitic banding with finer grain size and segregated compositional banding. Through detailed kinematic analysis including foliation deflection patterns and relative timing criteria outlined in their paper, Carreras et al. (in press) determined evolution of the shear zones including the relative timing of initiation and subsequent rotation of the features (Fig. 11). They distinguished two main sets of shear zones, dextral and sinistral, which each exhibit a range of orientations. The final angular pattern between dextral and sinistral shears is not an original feature. Dextral and sinistral shears formed together at nearly right angles, and the angles progressively opened towards the extension direction as a result of increasing strain. The obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge-shaped domains of lesser deformed rock. Through time, there was an increasing prevalence of dextral shears over sinistral ones . The studied pattern and sequential analysis indicate that bulk deformation was noncoaxial with a deformation regime evolving from a pure shear-dominated dextral transpression to a higher vorticity dextral transpression (Carreras et al., in press). 68 �Figure 10. Detailed outcrop map of the shear zones within the Grassy Portage Sill from Carreras et al. (in press). 69 �Fractures are the latest structures, cutting both the foliation and shear zones. They are located exclusively in the anorthosite (zoisite) pods and are chiefly oriented transverse to the mylonitic foliation. The regularity of the fracture orientations suggests that they formed in response to the final state of instantaneous strain because one would only expect them to be subparallel if they formed without any subsequent rotation. They are interpreted as extensional (Mode I) fractures. The incidence of the fractures exclusively within the pods indicates that the pods were relatively more competent than the surrounding gabbros at this final stage of deformation. Stop 8- Folds and syntectonic veins within mafic metavolcanics Return to Hwy 502 and drive north for approximately 3 km. Outcrop is located along west side of Highway 502. (0496149E, 5401196N) At this stop, we are located in relatively incompetent mafic metavolcanic rocks that are channeled between more competent units: the Black Sturgeon Bay gneiss dome to the northwest, the Rice Bay gneiss dome to the southwest, and the Baseline Bay (Algoman) pluton to the east. The foliation at this outcrop is roughly striking NE/SW, in an orientation that is consistent with the foliation in the incompetent units being channeled around the more competent units. Here, the banded mafic metavolcanic rocks are isoclinally folded (F2). A sequence of leucocratic veins are intruded into the host rock. The leucocratic veins are syntectonic with D2 because many such veins that cross-cut F2 folds are folded and co-planar with them (axial planes in veins are subparallel to those in the metasedimentary rocks), although in a more open geometry. This relationship reveals that the host rocks underwent folding prior to vein intrusion, but both vein and host rock underwent additional D2 folding. Druguet et al. (2008) used veins such as these to estimate two-dimensional strain in metavolcanic and metapelitic host rocks surrounding the competent Rice Bay gneiss dome. This outcrop is located in one of the highly strained zones (Fig. 12). The veins and their host rocks underwent a competence inversion. When the veins were first intruded, they were less competent than the host rock, but as they cooled, they eventually became more competent. We can see evidence for the early less competent veins with cuspate margins and irregular veins boundaries. We can see evidence for the late more competent veins with folding. 70 �Figure 11. Two-dimensional (horizontal) kinematic model for shear zone set formation from Carreras et al. (in press). a) Mean orientation of the early and late dextral (D) and sinistral (S) sets of shears and corresponding Rose diagrams. The finite bulk shortening direction faces the obtuse angle between the sets. In both stages, there are more dextral shears than sinistral ones. The orientation and corresponding Rose diagram of the late fractures (F) is also included. b) Model for the sequential development of shears, drawn on the XZ plane with north upwards. In addition to shears, the orientation of the bulk X and Z directions, the assumed shear plane and the trend of the pre-existing foliation (dashed lines) have been drawn. It is assumed that the initial angle that faced the bulk shortening direction between sets was ~90°. As deformation progressed, the two sets rotated in opposite directions, and the angle between them increased. The later development of new dextral and sinistral sets led to complex coalescences and crosscutting relationships. 71 �Stop 9- Folding and syntectonic veins in alternating pelitic and psammitic schists Turn around and return south on Hwy 502. Outcrop is located along south side of Hwy 11 immediately east of Hwy 502 turnoff. (0495680E, 5396171N) At this stop, we are located in relatively incompetent metasedimentary rocks that are channeled between more competent units: the Rice Bay gneiss dome to the northwest and the Grassy Portage gabbroic sill to the east. The foliation at this outcrop is striking approximately NE/SW, parallel with the map expression of this channeled metasedimentary unit. The rock consists of alternating pelitic and psammitic schists with F2 isoclinal folds easily seen in the metapsammitic layers and early quartz veins. There are multiple veins ranging from quartz to leucocratic in a variety of orientations. We can see folding and boudins in the veins depending on their orientation. The folds in the veins have a tighter geometry in the pelitic schists compared to the psammitic schists due to the higher competence contrast between the veins and the metapelites (e.g. Ramsay, 1967; Cruikshank and Johnson, 1993). Based on two-dimensional strain analysis of vein deformation with assumed constant volume, this outcrop is located in a moderately strained zone with long/short axial strain ratios of approximately 2:1 and plane strain (Druguet et al., 2008) (Fig. 12). Stop 10- Decimeter scale upright folds Located along north side of Hwy 11 immediately west of Hwy 502 turnoff. (0495180E, 5396173N) The next stop is right near the last within the layered metasedimentary rocks. Here we can observe decimeter scale low amplitude folds within the sedimentary layers. Note that the F2 folds plunge eastward and display an asymmetry consistent with the location on the south-east flank of the Rice Bay Dome. Two outcrop localities convincingly demonstrate downward facing in this metasedimentary (metagreywacke) unit: near Pocket Pond (pillow lavas) and Bear Pass Boat Launch (graded beds) (Poulsen et al., 1980). In both cases, the metagreywackes are structurally beneath, but stratigraphically above the metavolcanics, a relationship that is supported by zircon ages (Davis et al., 1989). Given that these metasedimentary rocks are younger than both the mafic-ultramafic rocks to the east and the felsic rocks in the core to the dome to the northwest, there is a problem in interpreting their configuration prior to development of the F2 ―dome.‖ The possible solutions are shown in Fig. 13, all resulting in ―stacked‖ tectonostratigraphy. Therefore, in the big-picture view, down-plunge and eastward from this locality is an antiformal syncline. 72 �Figure 12. Schematic structural map of the study area from Druguet et al. (2008) with localities where strain estimations were performed (A–G). Diagrams of azimuth distribution of veins from four different areas. Pale grey: shortened veins; black: extended veins and, where possible; grey: shortened and extended veins. (LSZ = from a low strain zone, MSZ= from a moderate strain zone, HSZ = from a high strain zone.). Fieldtrip Stop 8 is from the HSZ at location 5e in the north-central portion of the map. Fieldtrip Stop 9 is from the MSZ at location 8d in the northcentral portion of the map. Fieldtrip Stop 11 is from the HSZ at location 5a in the western portion of the map. Figure 13. Schematic illustration showing alternative explanations for stacking of tectonic slices in Rice Bay – Bear Pass area. From Davis et al., (1989). 73 �74 Figure 14. Simplified map of Seine Basin including Stops 3-5. Modified from (Czeck, 2001; Czeck and Hudleston, 2003; Czeck et al., 2009) based on maps from (Wood et al., 1980b, a; Stone et al., 1997a, b) . �Stop 11- Four consecutive sets of strongly deformed leucocratic veins within a foliated metasedimentary rock Drive west along Hwy 11 for approximately 17 km to junction with Rocky Inlet Road. Outcrop is located on the SE corner of the intersection. (0479790E, 5391262N) This outcrop is a strongly foliated metasedimentary rock of the Coutchiching Group located between various Algoman granitic intrusions to the north and south. Four consecutive sets of strongly deformed leucocratic veins intrude the host rock and are boudinaged and folded depending on orientation. When veins intruded at angles higher than 30° with respect to the foliation, they were folded. When intruded at lesser angles, they were boudinaged or folded then boudinaged. There is an increasing measurable strain in progressively older veins, which indicates multiple synkinematic D2 intrusions (Druguet et al., 2008). Based on two-dimensional strain analysis of vein deformation with assumed constant volume, this outcrop is located in a highly strained zone with long/short axial strain ratios of approximately 5.5:1 and flattening strain (Druguet et al., 2008) (Fig. 12). In this highly strained zone, the folds are tighter than at lesser deformed outcrops. The folds are tight to isoclinal and classified as Class 1C and 2 folds characterized by relative thickening of the hinge zones, which is compatible with flattening strain. Lastly, the veins are offset by discrete brittle faults, primarily ductile, that presumably occurred during D3. Bonus Stop 12 If we have time for this stop, it is located approximately 4 km east of Stop 11 on north side of Highway 11, immediately next to convenience store with Bear statues. (0484020E, 5393045N) Here, we can observe metabasalts with prominent pillows in three-dimensions. Stretching lineations defined by alignment of mafic minerals and long axes of pillows are near vertical. Strain appears to be constrictional, although detailed strain measurement has not been performed. Leucocratic intrusions have cuspate structures that preserve magmatic relationships and weak internal fabrics, suggesting syntectonic intrusion. Bonus Stop 13 From Bonus Stop 12, drive west along Hwy 11 for approximately 8 km to the western lookout point on north side of Noden Causeway, Hwy 11, immediately under powerlines. (0477336E, 5388940N) Here, we can observe well-foliated granitoid rock with large K-feldspars that has been intruded by many felsic and mafic dikes. Fabrics include a foliation oriented approximately N86W, 88NE defined by alignment of plagioclase and a steep (80°W pitch) mineral lineation. The granitoid contains many semi brittle- semi ductile shear zones. The highlights of the stop are exceptional small brittle faults that cut the tectonic fabric and shear zones at a high angle. 75 �References Bauer, R. L., Hudleston, P. J., Southwick, D. L., 1992. Deformation across the western Quetico subprovince and adjacent boundary regions in Minnesota. Canadian Journal of Earth Sciences 29, 2087-2103. Blackburn, C. E., Johns, G. W., Ayer, J., Davis, D. W., 1991. Wabigoon subprovince. In: Thurston, P. C., Williams, H. R., Sutcliffe, R. H. & Stott, G. M. (Ed.), Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, 303-381. Bons, P. D., Druguet, E., 2007. Some misleading boudin-like structures. Geogaceta 41, 31–34. Bons, P. D., Druguet, E., Hamann, I., Carreras, J., Passchier, C. W., 2004. Apparent boudinage in dykes. Journal of Structural Geology 26, 625–636. Borradaile, G. J., 1982. Comparison of Archean structural styles in two belts of the Canadian Superior Province. Precambrian Research 19, 179-189. Borradaile, G. 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Minnesota Geological Survey, St. Paul, 163-171. Percival, J. A., 1989. A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada. Canadian Journal of Earth Sciences 26, 677-693. Percival, J. A., Williams, H. R., 1989. Late Archean Quetico accretionary complex, Superior province, Canada. Geology 17, 23-25. Poulsen, K. H., 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J. & Ashwal, L. D. (Ed.), Tectonic evolution of greenstone belts Technical Report 86-10. Lunar and Planetary Institute, Houston, TX, 177-179. Poulsen, K. H., 2000a. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266. Poulsen, K. H., 2000b. Geological Setting of Mineralization in the Mine Centre–Fort Frances Area. Ontario Geological Survey Mineral Deposits Circular 29. Poulsen, K. H., 2000c. Precambrian geology and mineral occurrences, Mine Centre-Fort Frances area, 1:50000. Poulsen, K. H., Borradaile, G. J., Kehlenbeck, M. M., 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, 1358-1369. Ramsay, J. G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York. Ramsay, J. G., Graham, R. H., 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences 7, 786-813. Robin, P.-Y., Cruden, A. R., 1994. Strain and vorticity patterns in ideally ductile transpression zones. Journal of Structural Geology 16, 447-466. 78 �Sanderson, D. J., Marchini, W. R. D., 1984. Transpression. Journal of Structural Geology 6, 449-458. Schultz-Ela, D., 1988. Strain patterns and deformation history of the Vermilion district, northeastern Minnesota. Ph. D. thesis, University of Minnesota. Shirey, S. B., Hanson, G. N., 1984. Mantle-derived Archaean monzodiorites and trachyandesites. Nature 310, 222-224. Stockwell, C. H., 1982. Proposals for time classification and correlation of Precambrian rocks and events in Canada and adjacent areas of the Canadian shield. Part 1: time classification of Precambrian rocks and events(Ed.), Geological Survey of Canada Paper 80-90, 135. Stone, D., Hallé, J., Murphy, R., 1997a. Precambrian geology, Mine Centre area. Ontario Geological Survey Preliminary Map P. 3372, scale 1:50,000. Stone, D., Hallé, J., Murphy, R., 1997b. Precambrian geology, Mine Centre area. Ontario Geological Survey Preliminary Map P. 3373, scale 1:50,000. Tabor, J. R., Hudleston, P. J., 1991. Deformation at an Archean subprovince boundary, northern Minnesota. Canadian Journal of Earth Sciences 28, 292-307. Tanton, T. L., 1927. Stratigraphy of the northern subprovince of the Lake Superior region. Bulletin of the Geological Society of America 38, 731-748. Tikoff, B., Teyssier, C., 1992. Crustal-scale, en-echelon "P-shear" tensional bridges: a possible solutoin to the batholithic room problem. Geology 20, 927-930. Treagus, S. H., Treagus, J. E., 2002. Studies of strain and rheology of conglomerates. Journal of Structural Geology 24, 1541-1567. Van Hise, C. R., Adams, F. D., Bell, R., Lane, A. C., Leith, C. K., Miller, W. G., 1905. Report on the special committee for the Lake Superior region. Journal of Geology 13, 89-104. Van Hise, C. R., Leith, C. K., 1911. The geology of the Lake Superior region. United States Geological Survey Monographs 52. Williams, H. R., 1991. Quetico subprovince. In: Thurston, P. C., Williams, H. R., Sutcliffe, R. H. & Stott, G. M. (Ed.), Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, 383-403. Wood, J., 1980. Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas; a comparison. Precambrian Research 12, 227-255. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980a. Mine Centre Area (Eastern Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2202, scale 1:15840. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980b. Mine Centre Area (Western Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2201, scale 1:15840. Yonkee, A., 2005. Strain patterns within part of the Willard thrust sheet, Idaho-Utah-Wyoming thrust belt. Journal of Structural Geology 27, 1315-1343. 79 �FIELD TRIP 4 ARCHEAN GEOLOGY OF VOYAGEURS NATIONAL PARK AND THE LITTLE AMERICAN GOLD MINE Chris Hemstad (Boreal Explorations) and Mark Jirsa (Minnesota Geological Survey) This boat trip through a very small part of Voyageurs National Park highlights the complex Archean geology of the southern Wabigoon and northern Quetico subprovinces of Superior Province (Fig. 4.1). The tour begins at the landing located within metasedimentary rocks of the Quetico subprovince. The boat travels northward, up Black Bay to Little American Island, then northeastward to Dryweed Island. Due to limited time and docking capabilities, the only actual stop will be at the historic Little American Gold Mine—the first and only operating gold mine in Minnesota. Elsewhere the geology will be viewed and interpreted ―over the gunnels.‖ Little American Island lies within the broad zone of dextral shear associated with the Rainy Lake-Seine River Fault, shown by wavy dashed lines on Figure 4.1. Gold-bearing quartz veins on the island were emplaced into variably deformed metabasalt and mafic schist of the Wabigoon subprovince. Dryweed Island, to the northeast, forms a lozenge of comparatively less deformed rocks between the Rainy Lake-Seine River and Tilson Bay fault zones. Most of the exposures on Dryweed Island are components of the Seine Group, a sequence of quartzofeldspathic sandstone, conglomerate, and minor felsic tuff. Stratigraphic facing in the Seine is consistently southward, so this exposure appears to represent a fault-bounded, southward-tilted block. See Jirsa and Hemstad, Field Trip 6, this volume for more details. Figure 4.1—Geology of Little American Island and Dryweed Island in Rainy Lake (modified from 1:24,000-scale maps of Hemstad and others, 2000, 2001). 80 �Figure 4.2—Portion of an historic geologic map showing Little American Island (circled), and the town site of Rainy Lake City on the mainland to the southeast (cross-hachured). Figure 4.3—Plat map of Rainy Lake City and historic photo (from Perry, 1993) 81 �Gold was discovered on Little American Island in 1893 by George Davis, a prospector en route to exploration in Canada, who camped on the well-used spot (Fig. 4.2). There he discovered a quartz vein, cracked some off, crushed and panned it. Later assays returned estimates of gold contents in the range of $50 to $100 per ton. The rush was on! Investors from Duluth and elsewhere quickly amassed $300,000 to capitalize a mining venture. By 1894, Rainy Lake City was platted on the mainland to the southeast, where a mill for stamping and processing the ore was constructed. In a short time, the city had several sawmills, 8 dry-goods stores, 8 grocers, three hotels and restaurants (Fig. 4.3), seventeen saloons, two newspapers, a livery barn, three laundries, a school, a bank, a doctor, and a lawyer (Perry, 1993). The mine produced $4,635 worth of gold in 1893 from the 2 meter wide composite quartz vein. However, its operation was plagued with episodes of failing capitol and production calamities. An incompetent amalgamator sent $10,000 worth of gold into the lake, and the collapse of a loading dock sent a season‘s ore to the bottom (Perry, 1993). Mining continued intermittently, and the mine eventually reached a depth of 210 feet. At today‘s prices, an estimated total of $45,000 worth of gold was extracted during its operation, but the immense cost of transportation and milling brought the mine to a close by 1895. Several attempts to reopen it were made until 1898, when the County seized the property. By that time, the lure of the Klondike sapped much of the excitement from this remote endeavor. The town site of Rainy Lake City was abandoned and dismantled. By contrast, other mines in what was called the "Rainy Lake Gold Region" were very productive, especially those in the nearby Mine Center area of Canada. Acknowledgement: This boat tour is courtesy of the National Park Service, Voyageurs National Park. We extend our thanks to their staff, including Mike Ward-Park Superintendent and Tawnya Schoewe-Head Park Interpreter, for making it possible. REFERENCES Hemstad, C.B., Ojakangas, R.W., and Southwick, D.L., 2000, Bedrock geologic map of the Island View quadrangle, Koochiching County, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-105, scale 1:24,000. Hemstad, C.B., Ojakangas, R.W., Southwick, D.L., and Nemitz, M., 2001, Bedrock geologic map of Cranberry Bay quadrangle, Koochiching and St. Louis Counties, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-110, scale 1:24,000. Perry, David E., 1993, Gold town to ghost town: Boom and bust on Rainy Lake: Lake States Interpretive Association, HCR 9, Box 600, International Falls, MN 55649. 82 �FIELD TRIP 5 ASH RIVER NEUTRINO DETECTOR LABORATORY (NOVA) AND THE ARCHEAN VERMILION GRANITIC COMPLEX Mark Jirsa (Minnesota Geological Survey*) and William Miller (University of Minnesota School of Physics and Astronomy) *This manuscript was not reviewed to conform to editorial standards of the Minnesota Geological Survey This field trip visits the NOvA project, under construction off the Ash River Trail southeast of International Falls. The project is part of a wide ranging effort to understand the fundamental make-up of matter. The acronym ―NOvA‖ stands for NuMI, Off-axis, electron neutrino (ve), Appearance. ―NuMI‖ refers to Neutrinos at the Main Injector facility at Fermi National Accelerator Laboratory (Fermilab) in Chicago. More than 180 scientists from 28 national laboratories and institutions around the world are working on the NOvA neutrino experiment that will take place at the Ash River site. The experiment will investigate the role of subatomic particles called neutrinos in the origin and shaping of matter in the universe. Fermi National Accelerator Laboratory, which manages the project, will generate a beam of neutrinos to send to a 15,000-ton detector at the new facility. Despite traveling through the earth‘s crust (Fig. 5.1), the particles will complete the 500 mile interstate trip in less than a hundredth of a second. Scientists will study changes that the particles undergo as they travel. Figure 5.1—Schematic diagram of neutrino trajectories. 83 �The $278 million NOvA project is funded by the Office of Science of the U.S. Department of Energy and Fermi National Accelerator Laboratory. The NOvA experiment at Ash River will be operated by the University of Minnesota School of Physics and Astronomy. The American Recovery and Reinvestment Act provided $40.1 million to the project. For more information, contact Bill Miller via e-mail at Miller@soudan.umn.edu or visit the website at wwwnova.fnal.gov/fermilab_nova.pdf. Excavations and outcrops near the NOvA site expose leucogranite-rich migmatite of the Vermilion Granitic Complex in the heart of the Quetico subprovince of Superior Province. See Jirsa and Hemstad, Field trip 6, stop 6-10 this volume for more details. The narrow, darker colored unit shown on the Figure 5.2 near the site is biotite-hornblende diorite. Figure 5.2—Geologic map of the NOvA site (modified from Southwick and Ojakangas, 1979). REFERENCE Southwick, D.L., and Ojakangas, R.W., 1979, Geologic map of Minnesota, International Falls sheet: Minnesota Geological Survey, scale 1:250,000. 84 �FIELD TRIP 6 TRANSECT THROUGH THE QUETICO-WABIGOON SUBPROVINCE BOUNDARY Mark Jirsa (Minnesota Geological Survey*) and Chris Hemstad (Boreal Exploration) *This manuscript was not reviewed to conform to editorial standards of the Minnesota Geological Survey GEOLOGIC SETTING This trip examines the geology along the boundary between two subprovinces of the Archean Superior Province—the volcanoplutonic Wabigoon subprovince to the north, and the largely metasedimentary and migmatitic Quetico subprovince to the south. The boundary is somewhat transitional, as it is marked by a broad zone of shear associated with the Rainy Lake-Seine River Fault and several other fault structures. The two terranes have contrasting lithologic and metamorphic attributes. The Wabigoon portion of the area consists of tightly folded volcanic rocks and metagraywacke, unconformably overlain by and structurally interleaved with conglomerate and quartzite of the Seine Group and metamorphosed to greenschist facies. The Quetico portion contains biotite schist of graywacke protolith and migmatitic plutonic rocks that are multiply deformed, metamorphosed to amphibolite facies, and intruded by younger granite. Although the subprovince boundary is clearly a metamorphic one, work by Devaney and Williams (1989) on exposures along strike to the northeast implies that the coincident lithologic shift reflects a depositional transition during Neoarchean terrane accretion. The Wabigoon represents oceanic plateau and island arc deposition; the Quetico represents the transition arc to submarine margin and trench-fill deposition. Seismic reflectivity indicates north-dipping structures are present at depth along the subprovince boundary in Canada (Percival and others, 2006). These structures may represent the accretionary boundary that has been over-steepened and segmented locally by subsequent north-south crustal shortening and dextral wrench faulting. The Seine Group, which lies within a fault- and unconformity-bounded block, appears to represent continental sedimentation into one or more successor basins developed along the subprovince boundary. All of these Archean rocks were intruded by diabase dikes of the Kenora-Kabetogama swarm at about 2067 Ma (Wirth and others, 1995). 85 �WABIGOON SUBPROVINCE Only a sliver of the subprovince is exposed on the Minnesota side of the border, and therefore, much of what is known about it is based on work to the north. Volcanism in the subprovince as a whole occurred in the range of 2745-2720 Ma (Corfu and Davis, 1992); with rare older and younger volcanic sequences locally. Metavolcanic rocks exposed on the mainland and islands in Minnesota are variably schistose and include pillowed, massive, and tuffaceous basalt; felsic tuff, and hypabyssal intrusions. Schistose metasedimentary rocks of apparent turbiditic origin are infolded with the volcanic strata. Plutonic rocks vary from synvolcanic batholiths consisting of tonalite, diorite, and gabbro; to younger granodiorite batholiths and plutons (~2710 Ma), monzodiorite (sanukitoid) plutons (~2698-2690 Ma), and monzogranite (~2690-2660 Ma). Younger clastic successions, including the Seine Group described below, sit with local unconformably on older volcanic strata and typically have ages in the range of 27112686 Ma. Seine Group The Seine Group exposed in Minnesota (or Seine conglomerate, as it is referred to in Canada) consists of interbedded conglomerate, quartzofeldspathic and lithic sandstone, and minor tuffaceous strata. Although moderately to strongly deformed, sedimentary structures are locally preserved and include trough cross-bedding and normal and reversed grading. Collectively, these characteristics indicate fluvial deposition. Conglomeratic beds contain pebble- to cobble-size, rounded to subrounded, polymictic clasts that include fragments of subjacent rocks and a variably porphyritic volcanic component of intermediate to felsic composition. The contrast between the relatively deep water, pillowed volcanic and turbiditic rocks, and the superjacent quartzose and conglomeratic strata of the Seine is striking and implies that significant uplift by tectonic shortening must have occurred. The unit trends discontinuously northeastward across the mainland and islands and continues into Canada. In Minnesota, it appears to be bounded on the north by the Tilson Bay Fault, and stratigraphic younging is consistently southward. Additional details about the sedimentology and deformation of the Seine conglomerate can be found in Czeck and Fralick, 2002; and Poulsen and Czeck, field trip 3, this volume. The Seine Group is one of a series of similar deposits throughout the Superior Province represented by subaerial unconformities overlain by fluvial and lacustrine or marine basin-fill sediments. The relative temporal framework of these deposits indicates that they are timetransgressive, generally younging to the south, which is consistent with arc collisional events (Corcoran and Mueller, 2007). Czeck and Fralick 2002 suggested that the Seine conglomerate was deposited following collision of the southern Wabigoon terrane with the Wawa and intervening Quetico terranes. The age of the Seine is bracketed between about 2696 Ma, the age of a clast, and 2686 Ma, the age of an intrusion which cuts the Seine (Davis and others, 1990). QUETICO SUBPROVINCE The Quetico is one of a number of east-trending, largely metasedimentary subprovinces in the Superior province. It is bounded on the south by the Wawa volcanoplutonic subprovince, and on the north by the Wabigoon subprovince. It consists of schist derived from turbiditic sedimentary rocks and a complex suite of granitic intrusions and associated migmatite. In northeastern Minnesota, the subprovince displays a roughly symmetrical distribution of metasedimentary rocks on the north and south, that grade irregularly through zones of schist-rich 86 �migmatite, to a central axial zone composed largely of polyphase granitoid migmatite and younger granite. Metamorphic grade mimics this symmetry, with generally higher grade rocks in the central axis and lower grade near the bounding volcanoplutonic subprovinces. The accretionary prism model of Williams (1990) implies sediment deposition by submarine fans and abyssal turbidites. Timing of deposition is constrained by youngest detrital zircons at about 2698 Ma, and intrusion ages of about 2688 Ma (Davis and others, 1990). The migmatitic and plutonic rocks in the axial zone are known collectively in Minnesota as the Vermilion Granitic Complex (Southwick and Sims, 1980). The migmatite is divided into schist-rich and granite-rich components, depending on the ratio of paleosome to neosome. These can be equated generally with the terms metatexite (low degree of partial melting) and diatexite (nearly complete fusion), respectively, of Sawyer, 2008. Field relationships within the complex (Southwick, 1991) indicate that earliest granitoid phases are leucogranite, granodiorite, and trondhjemite, which make up the leucosome of a broad area of migmatite. The migmatite is interlayered at all scales with paleosomes of biotite schist and amphibolite, and is cut by dikes, sills, and irregular masses of two mica leucogranite with accessory garnet, and biotite granite that contains minor magnetite. The latter forms a large intrusion known as the Lac La Croix granite, and apophosial intrusions that produce aeromagnetic anomalies highlighting broad fold structures. The earlier migmatite was derived from partial melting of a metasedimentary protolith. Southwick (1991) suggested that the younger Lac La Croix-type granite is a further distillation of granitic liquid from partial melting of the older migmatite and metasedimentary rocks. The apophosial intrusions of Lac La Croix granite generally decrease in thickness and continuity westward, suggesting that the western portion of the complex may represent the roof zone of a batholith cored by massive granite. DEFORMATION AND METAMORPHIC HISTORY Both terranes have undergone a complex deformation history. As described by Day (1990), three main phases of deformation are recognized. D1 produced tight to isoclinal, northeasttrending folds plunging to the southwest and locally overturned to the southeast. This event may locally have produced recumbent folds over a broad region (Bauer, 1985; Poulsen and others, 1980). It occurred shortly after deposition and involved burial to produce metamorphic conditions of moderate pressure and temperature (Valli and others, 2004). D 2 deformation was synchronous with peak regional metamorphism to upper greenschist facies in the Wabigoon subprovince and amphibolite facies in the adjacent Quetico subprovince. This is considered lowpressure, moderate temperature conditions by Valli and others (2004); however there is a progressive increase in metamorphic conditions across the Quetico subprovince from 0.2 GPa and 500 C on the west near International Falls, to 0.5-0.6 GPa and 780C in the east. D2 deformation produced small-scale F2 folds, and an axial-planar cleavage and schistosity that strikes to the east-northeast. Folds and other asymmetric features indicate that D 2 deformation was a dextral transpressional event, which probably initiated movement along the major faults. D3 deformation is recorded in the youngest movement along major fault zones. Vertical offset during D3 or D4 events, and subsequent differential erosion produced the contrasting metamorphic grades across the Rainy Lake-Seine River Fault. Combining structural, metamorphic, and published geochronologic data, Valli and others (2004) infer ages of tectonometamorphic events in the Quetico subprovince as follows: D1 Moderate pressure and temperature metamorphic event shortly after deposition of turbidites in the range of 2698-2690 Ma 87 �D2-3 Biotite-sillimanite-garnet peak metamorphism (0.6 GPa 700C) during transpression which began ~2689 Ma D4 Low pressure, moderate temperature metamorphism in the range of 2671-2667 Ma According to Fralick and others (2006), clastic deposition in the northern Quetico began about 2698 Ma during D1 and continued to <2696 Ma, supporting the interpretation of a transition from accretionary wedge to foreland basin deposition. Subsequent deformation during D 1 was associated with accretion of these sediments onto the active Wabigoon arc. Deformation during D2 and D3 may represent docking of the Wawa arc from the south. FIELD TRIP STOP DESCRIPTIONS This field guide is based on mapping by Hemstad and others (2000, 2001, 2002) and Day (1990), and an earlier guide produced for the Institute on Lake Superior Geology (Ojakangas and others, 1982). Some of the stops described below were also visited during the 1982 meeting and are described in more detail in that guide book—for reference, prior stop numbers are shown in [brackets]. Because of road layout, the order of stops is not necessarily geochronologic. Stops 1-6 and 9 lie within the Wabigoon subprovince (Fig. 6.1); Stops 7, 8, 10, and 11 are within the Quetico subprovince to the south (Figs. 6.1 and 6.2). All UTM coordinates are given in NAD 83, zone 15. Figure 6.1—Geologic map of the Rainy Lake area, modified from Day, 1990, showing locations of field trip stops 6-1 through 6-9. 88 �STOP 6-1 Archean monzonite cut by Paleoproterozoic diabase dikes Location: 0.65 miles east of US Hwy 53 on MN Hwy 11; ~500 feet north of Hwy 11 (UTM 471,295E / 5,383,386N). Description: Pink, coarse-grained, porphyritic hornblende-quartz monzonite. Rock displays a pronounced trachytoid foliation that trends N70E. Unit contains irregular autoliths of monzodioritic to dioritic composition, and rare xenoliths of schistose metasedimentary rocks, which form the country rock into which the intrusion was emplaced. The long axes of inclusions are typically parallel to foliation, implying that foliation is largely magmatic; however, tectonic and metamorphic overprint is also likely. Note that some of the clasts in conglomerate of the Seine Group at STOP 6-6 have textures similar to this intrusion. Several fine-grained diabase dikes, having varied thicknesses and a northerly trend, cut the monzodiorite. STOP 6-2 Archean metagraywacke and slate Location: Just east of village of Rainier; 3 miles east of US Hwy 53 on MN Hwy 11; ~400 feet N of Hwy 11 (UTM 474,888E / 5,384,188N) Description: This outcrop exposes fine-grained plagioclase-biotite schist of graywacke and mudstone protolith, metamorphosed to upper greenschist facies. Bedding is locally visible and parallels cleavage and schistosity at N68E, dipping 75N. Stratigraphic younging is northward, based on graded bedding and cross-cutting relationships of basal turbidite beds. This is consistent with the outcrop position on the north side of an east-trending, westward plunging anticline that is overturned slightly to the south. Recessed aligned depressions on the outcrop surface generally parallel the foliation and may represent boudinaged carbonate-rich horizons or zones of concretions. The rock is cut by a number of quartz veins emplaced along small-scale faults. NE-trending veins show sinistral offset, NW-trending ones are dextrally offset. Late silicification ―veinlets‖ trending N25W create a ribbed relief on the outcrop surface. The temporal relationship between metagraywacke exposed here and similar but more highly metamorphosed schist in the Quetico subprovince to the south is unresolved. STOP 6-3 Schistose feldspathic-lithic quartzite of the Seine Group [1982 stop #2] Location: Approximately 2.5 miles east of Rainier on MN Hwy 11 to County Road 109; turn south on 109 and follow it south and east for about 1.2 miles. Ridge of outcrop trends east from here. (UTM 478,990E / 5,382,630N) Description: This outcrop ridge extends more or less parallel to bedding and foliation. The rock is medium-grained, light gray to tan. Framework grains consist of quartz and feldspar and are angular to subrounded and slightly flattened in the regional foliation. Matrix consists of sericite, chlorite, and quartz. Beds range in thickness from 5 cm to 1 meter, planar lamination and trough cross stratification are common. This outcrop lies within an irregular belt of quartzite and conglomerate trending to the east-northeast into adjacent Canada. It appears to be fault-bounded on the north by the Tilson Bay Fault, and stratigraphic younging is consistently southward. Though not in evidence here, this unit and that of STOP 6-6, are inferred to unconformably overlie metasedimentary and metavolcanic strata of the Wabigoon subprovince. STOP 6-4 89 �Metavolcanic and meta-igneous rocks [1982 stop #3] Location: Return on County Road 109 to MN Hwy 11, proceed east 1.0 mi. to outcrop on north side of highway (UTM 479,745E / 5,383,009N) Description: At least two rock types are exposed here; a schistose green to gray chloritic tuff containing iron-carbonate, and a coarser grained meta-igneous rock composed of what may have been pyroxenes, now pseudomorphed by amphibole. Both rock types display S 2 schistosity. Though strongly deformed, this exposure is typical of the local greenstone. STOP 6-5 Tectonite in the Rainy Lake-Seine River fault zone Location: Continue east on Mn Hwy 11 for 2 mi., crossing Tilson Creek to outcrops and boat landing on north side of highway (UTM 482,770E / 5,382,850N) Description: These outcrops lie within a broad zone of shearing associated with the Rainy LakeSeine River fault that separates Wabigoon and Quetico subprovinces. The zone continues eastward to Little American Island, where Minnesota‘s only gold mine (to date) operated from 1893 to about 1898—see Hemstad and Jirsa, Field Trip 4, this volume for more details. The main part of the fault is represented topographically by the northeast-trending swamp visible just to the south. Outcrops consist of amphibolitic and chloritic schist, inferred to be of basaltic protolith, and minor chert-magnetite iron-formation. Both carry a strong, vertically dipping, N50E-trending cleavage and abundant detached quartz veins in the subparallel shear fabric. The rocks contain minor amounts of pyrite. Some exploration drilling was done near here in the late 1980s (Newmont), to investigate the possibility of more significant mineralization, presumably at junctions of the regional schistosity (S 2) and local shear structures. STOP 6-6 Conglomerate and quartzite of the Seine Group Location: Continue east on MN Hwy 11 for 1.2 mi. to Gold Shores Road (County Rd 138), turn NE on 138 and follow it around (one-way) for about 1 mile to roadcuts (UTM 485,876E / 5,383,470N) Description: This large area of outcrop includes polymictic, clast-supported conglomerate, underlain to the north and locally interbedded with feldspathic lithic quartzite similar to that at STOP 6-3. The conglomerate contains rounded pebbles, cobbles, and boulders of felsic plutonic rocks, mafic to felsic volcanic rocks, chert, biotite schist, and iron-formation. Plutonic clasts are subspherical to elliptical, whereas volcanic clasts tend to be moderately elongated in the regional lineation. The texture of some plutonic clasts is similar to monzonite exposed at STOP 6-1. The schistose matrix is composed of fine- to medium-grained biotite- or chlorite-rich lithic arenite that contains angular to subrounded granules of volcanic rock fragments, intrusive rock fragments, plagioclase, quartz, and chert. Bedding is crude to massive. Subtle, fining-upward to coarsening-upward sequences are common in the conglomerate. Note that some volcanic clasts display pronounced concentric zonation, manifest by color and surface relief. It is unclear whether this represents original mineralogic zonation or metamorphically modified weathering. Similar observations have been made of clasts in other ―Timiskaming-type‖ or successor-basin conglomerates near Virginia, Minnesota (Jirsa, 2000) and in the Ogishkemuncie conglomerate in the Boundary Waters Canoe Area (Jirsa and Starns, 2008). 90 �STOP 6-7 Voyageurs National Park Visitor Center Location: Return to MN Hwy 11 and continue east for 0.8 mi. to south-trending road to Black Bay visitor center and boat landing (UTM 488,050E / 5,381,191N) Description: Visitor center displays and restrooms should be open. Several roadcuts along this trail and lakeshore outcrops near the visitor center expose biotite schist of the northern-most Quetico subprovince. This will be the subject of STOP 6-8. STOP 6-8 Quarry and gravel pit in Quetico metasedimentary rocks Location: Return to MN Hwy 11 and turn west, follow Hwy 11 ~0.8 miles to Gold Shores Road and walk south to large quarry and pit (UTM 484,630E / 5,382,325N) Description: Outcrops of fine- to medium-grained, light- to dark-gray schist derived from rhythmically bedded, texturally graded graywacke and shale. The rock is composed dominantly of plagioclase, quartz, biotite, and muscovite; and minor garnet, sillimanite, and hornblende in beds of appropriate bulk composition. Individual beds are typically 5–20 centimeters thick; some sandy beds are as thick as two meters. Outcrop displays folded discontinuous layers of boudinaged concretions, sandy beds, or quartz veins. En echelon gash quartz veins occur locally. STOP 6-9 Wabigoon subprovince metabasalt and iron-formation [1982 stop #10] Location: Continue west on MN Hwy 11 for about 9 mi. to MN Hwy 332 near the east edge of International Falls. Drive south and east on Hwy 332 for 2.5 mi. to large road cuts (UTM 472,350E / 5,380,800N) Description: Schistose rocks of basaltic and iron-formation protolith are exposed on both sides of the highway. Rock is strongly foliated amphibolitic schist, with lenses and layers of tightly folded magnetite-, chert-, garnet- and sulfide-bearing iron-formation. The strong foliation nearly obliterates bedding and other features. Fabric asymmetry indicates much of the fabric is a product of dextral transpression attributed in part to proximity to the Rainy Lake-Seine River Fault just to the south (as at STOP 6-5). STOP 6-10 Quetico subprovince—granite-rich migmatite of the Vermilion Granitic Complex [1982 stop #15] Location: Continue south on Hwy 332 for about 1.7 mi. to US Hwy 53. Follow US Hwy 53 south and east for 19.5 miles to large road cut (UTM 493,730E / 5,361,730N) Note: this is 5.4 miles east of Ray at jct. Hwys 53 and 217, and 1.6 miles west of hwy 122 entrance to Kabetogama landing. Description: This complex and photogenic outcrop is typical of granite-rich migmatite in the Vermilion Granitic Complex (Fig. 6.2). Overall, the unit mapped by Southwick and Ojakangas (1979) as granite-rich migmatite contains between 5% and 25% paleosomatic inclusions. In this area where granite is dominant, paleosome blocks and slabs up to several meters in size are rafted in neosome. Some paleosome blocks have sharp boundaries characteristic of agmatic migmatite, some have distinct flow fabric of schlieren migmatite, and some have diffuse boundaries representing partial assimilation typical of nebulitic migmatite (vernacular of Mehnert, 1968). The neosome is coarse-grained, light gray to pink, muscovite-biotite granite of the Lac La Croix-type with weak foliation. The paleosome includes biotite schist, amphibolite, tonalite, and early migmatite displaying variable degrees of deformation. The early migmatite 91 �has paleosomes of amphibolite and biotite schist, and neosome of tonalite or granodiorite. Much of the amphibolite has a relict porphyritic texture pseudomorphed by biotite, resembling lamprophyric intrusions exposed in adjacent greenstone terranes. Figure 6.2—Geologic map of the International Falls area, modified from Southwick and Ojakangas, 1979 (original map scale 1:250,000), showing locations of field trip stops 6-9 to 6-11. STOP 6-11 Quetico subprovince—schist-rich leucogranite migmatite of the Vermilion Granitic Complex Location: Continue south on US Hwy 53 for about 18.4 miles to road cuts; just southeast of Ash Lake (UTM 507101E / 5,339,900N) Description: These road cuts expose shallowly dipping schist-rich migmatite of the Vermilion Granitic Complex. According to the mapping convention of Southwick and Ojakangas (1979), the map unit is defined by a leucogranitic neosome content of 10% and 15%. The paleosome consists largely of biotite schist that displays strong metamorphic foliation (S 2?) that was deformed by D3 in broad open folds. No petrography has been conducted from samples of this exposure, so details of neosome composition and metamorphic mineral content are unknown. Nevertheless, it is apparent the schist is cut by a variety of leucogranitic intrusions that are strongly boudinaged and crumpled in planes of S 2 foliation and refolded. 92 �REFERENCES Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, 13:657660. Czeck, D., and Fralick, P., 2002, Structure and sedimentology of the Seine conglomerate, Mine Centre area, Ontario; Field Trip 3 in Institute on Lake Superior Geology Proceedings v. 48, pt. 2., p. 37-67. Corcoran, P.L., and Mueller, W.U., 2007, Time-transgressive Archean unconformities underlying molasse basin-fill successions of dissected oceanic arcs, Superior Province, Canada: Journal of Geology 115:655-674. Corfu, F., and Davis, D.W., 1992, A U-Pb geochronological framework for the western Superior Province, Ontario: in Geology of Ontario, Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M., eds., Ontario Geological Survey Special Vol. 4, Part 2, p. 1335-1346. Davis, D.W., Pezzutto, F., and Ojakangas, R.W., 1990, The age and provenance of metasedimentary rocks in the Quetico subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province: Earth and Planetary Science Letters 99: 195205. Davis, D.W., Poulsen, K.H., and Kamo, S.L., 1989, New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada: Journal of Geology 97:379-398. Day, Warren, C., 1990, Bedrock geologic map of the Rainy Lake area, northern Minnesota: U.S. Geological Survey Miscellaneous Investigation Series I-1927, scale 1:50,000. Devaney, J.R. and Williams, H.R., 1989. Evolution of an Archean subprovince boundary: A sedimentological and structural study of part of the Wabigoon-Quetico boundary in northern Ontario: Canadian Journal of Earth Sciences, 26:1013-1026. Fralick, P., Purdon, R.H., and Davis, D.W., 2006, Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentalogical, geochemical, and geochronological evidence: Can. J. Earth Sci. 43:1055-1070. Hemstad, C.B., Ojakangas, R.W., and Southwick, D.L., 2000, Bedrock geologic map of the Island View quadrangle, Koochiching County, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-105, scale 1:24,000. Hemstad, C.B., Ojakangas, R.W., Southwick, D.L., and Nemitz, M., 2001, Bedrock geologic map of Cranberry Bay quadrangle, Koochiching and St. Louis Counties, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-110, scale 1:24,000. Hemstad, C.B., Southwick, D.L., and Ojakangas, R.W., 2002, Bedrock geologic map of Voyageurs National Park and vicinity: Minnesota Geological Survey Miscellaneous Map M-125, scale 1:50,000. Jirsa, M.A., 2000, The Midway sequence: a Timiskaming-type, pull-apart basin deposit in the western Wawa subprovince, Minnesota: Can. J. Earth Sci. 37:1-15. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Lake fire area, parts of the Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie Lake 7.5 minute quadrangles, northeastern Minnesota. Minnesota Geological Survey Open-File Report OF08-05, scale 1:24,000. Ojakangas, R.W., Day, W.C., and Southwick, D.L., 1982, Archean geology of the International FallsKabetogama area, Minnesota: Field Trip 2 in Institute on Lake Superior Geology Proceedings 28:89-139. 93 �Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J., 2006, Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies: Can. J. Earth Sci., 43: 1085-1117. Poulsen, K.H., Borradaile, G.J., and Kehlenbeck, M.M., 1980, An inverted Archean succession at Rainy Lake, Ontario: Canadian Journal of Earth Sciences 17:1358-1369. Sawyer, E.W., 2008, Atlas of migmatites: Canadian Mineralogist, Special Pub. 9, NRC Research Press, Ottawa, Ontario, Canada, 371p. Southwick, D.L., 1991, On the genesis of Archean granite through two-stage melting of the Quetico accretionary prism at a transpressional plate boundary: Geological Soc. of America Bulletin 103:1385-1394. Southwick, D.L., and Ojakangas, R.W., 1979, Geologic map of Minnesota, International Falls sheet: Minnesota Geological Survey, scale 1:250,000. Southwick, D.L., and Sims, P.K., 1980, The Vermilion Granitic Complex—A new name for old rocks in northern Minnesota: U.S. Geological Survey Professional Paper 1124A, p. A1-A11. Valli, F., Guillot, S., and Hattori, K.H., 2004, Source and tectono-metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada: Precambrian Research, 132:155-177. Williams, H.R., 1990, Subprovince accretion tectonics in the south-central Superior Province: Can. J. of Earth Sci. 27:571-581. 94 �Field Trip 7 MINERAL DEPOSITS OF THE MINE CENTRE – RAINY RIVER AREA, ONTARIO Peter Hinz Mines Group Ontario Ministry of Northern Development, Mines and Forestry 435 James Street South, Suite B002 Thunder Bay, ON P7E 6S7 Chris R. White Numax Resources, Inc., Ely, Minnesota & Paul B. Albers Numax Resources, Inc., Prior Lake, Minnesota Delio Tortosa Geological Consultant Q-Gold Resources Ltd., Fort Frances, Ontario Photo of the Golden Star Mine, Mine Centre area, circa. 1898 from the Report of the Bureau of Mines, Volume VII, First Part, 1898. 95 �FOREWORD This trip will examine sites containing a variety of mineral deposits in the Mine Centre Area. Historically gold was the primary focus of mineral exploration, development and extraction. However in recent years sub-economic deposits of iron, titanium, copper, nickel and zinc have been identified and are being evaluated by junior exploration companies. The trip will visit four mineral deposit types in the Mine Centre area: lode gold; magmatic copper-nickel, PGEs; magmatic iron-titanium; and pyroclastic-hosted diamonds. REGIONAL SETTING Introduction and Tectonic Setting (taken from Czeck and Fralick, ILSG 2002) The central portion of the Superior province is characterized by alternating subprovinces of metavolcanic-plutonic and metasedimentary natures (Fig. 1). Figure 1. The Superior Province. From Card and Ciesielski (1986). One popular tectonic interpretation for the central portion of the Superior Province is of repeated island arc, microcontinent collisions. The collisions are evidenced by rocks that can be interpreted as arc sequence subprovinces (metavolcanics) and their corresponding accretionary prism subprovinces (metasediments) (Langford and Morin, 1976; Hoffman, 1989; Percival and 96 �97 Figure 2. Simplified geological map of the Mine Centre – Rainy Lake area with field trip stops. �Williams, 1989; Card, 1990; Hoffman, 1990). In general, the ages in the greenstone belts are similar along strike, but differ systematically across strike (Hoffman, 1989). This is consistent with the island-arc accretion model. A history of southward accretion has been proposed to explain the juxtaposition of Superior Province terranes (Langford and Morin, 1976; Percival and Williams, 1989; Card, 1990). The Seine Conglomerate, located in the Rainy Lake region of the western Superior Province, was deposited along the boundary between the Wabigoon metavolcanic/plutonic subprovince and the Quetico metasedimentary subprovince (Fig. 2). The structural observations along the Wabigoon–Quetico boundary are consistent with an oblique island-arc microplate collision circa 2.7 Ga. In this part of the Superior Province, it seems likely that first the Quetico acted as a subduction prism during accretion of the Wawa to Wabigoon. Then, it was effectively shifted from the subduction prism setting to a back-arc setting, as subduction shifted (Percival and Williams, 1989). In the Rainy Lake region, a series of lithostratigraphic terranes were assembled together along structurally controlled, stratigraphically discordant boundaries during the collisions. The boundary between the Wabigoon and Quetico Subprovinces in this region is divided into three primary blocks by dextral wrench faults (Poulsen, 1986). Each of the small terranes and subterranes may have undergone a somewhat unique history of formation and deformation. The Quetico Fault forms the northern boundary, separating the granite-greenstone terrain of Wabigoon Subprovince from the Coutchiching Group argillites and the Seine Conglomerate. The Seine River-Rainy Lake Fault forms the southern boundary of these sedimentary sequences with the Quetico turbiditic metasediments [sic] to the south. The wedge-shaped area lying between the two major fault systems is itself dissected by splays off the major east-west [sic] faults, which isolate the lithic units, destroying stratigraphic integrity. This problem has resulted in historical speculation on the lateral equivalency of the Seine and Coutchiching sediments (Merritt, 1934) and the Coutchiching and Quetico (Lawson, 1913). STRUCTURAL GEOLOGY (taken from Poulsen, Geological Report 266) The 2 most prominent structural features in the study area are the Quetico and the Rainy Lake– Seine River faults. Both truncate distinctive lithologic units which merge at a low angle with the faults in a manner that suggests a dextral sense of displacement (Map 2525) [sic]. The Quetico Fault, up to 1 km wide, strikes east and contains heterogeneous rock types possessing an intense steep foliation that parallels the fault. Mylonite is common and pseudotachylite occurs locally. The mylonite itself is commonly folded into minor folds of dextral asymmetry. Felsic plutonic rocks are the most probable protoliths of mylonites, and protomylonites containing centimetresized feldspar porphyroclasts are similar to porphyritic quartz monzonites that are common phases of the Algoman intrusions. This observation, plus the marked truncation of the Ottertail Lake pluton, indicate clearly that the fault postdates the emplacement of the late-stage Algoman granitoid suite. The rocks of the Rainy Lake–Seine River fault zone contrast lithologically with those of the Quetico Fault. The sharp contact with the Quetico metasedimentary rocks is marked by a zone as little as 50 m wide, composed of chlorite schist and phyllonite. Some of these schists have been 98 �mapped historically as mafic volcanic rocks, but detailed examination reveals that they are commonly heterolithic with enclaves of conglomerate, iron formation and carbonaterich schist. The nature of this zone is well-illustrated by the changes that occur in metagabbro along the fault near the Scott Islands. Outcrops of gabbro to the north of the fault contain local chloritic shear zones that parallel the trace of the main fault. The width and abundance of these zones increase southward to the main schistose zone, which marks the fault. Small folds superimposed on the steep schistosity and sigmoidal quartz veins in the vicinity of the fault near Neil Point, Minnesota, 3 km southwest of Red Pine Island, indicate a right-hand sense of displacement. Figure 3. Schematic diagram illustrating structural features of Rainy Lake Wrench Zone. Short solid arrows identify downward facing units. From Poulsen (1986). Structure The rocks of the Mine Centre–Fort Frances area contain evidence of progressive deformation involving folds, ductile shear zones and faults. While some of the above features likely formed contemporaneously, there is evidence of a continued transition from ductile to brittle deformation. The attitudes of most structures appear to be dominated by incremental shortening about a subhorizontal axis oriented west-northwest. This imparts a dominant northeasterly trending structural ―grain‖ to the rocks of the area. Folds, Cleavage And Lineation Variations in the distribution, attitudes and facing of mappable lithologic units attest to the existence of large folds within the study area. One of these, the Rice Bay Dome, is not a simple structure but can be related to the development of 3 distinctive fold sets (Poulsen, Borradaile and Kehlenbeck 1980). New lithologic mapping of the magnetic ultramafic unit exposed in this area has resulted in further definition of these folds. Early folds (F1) were recumbent and involved substantial inversion of the stratigraphic succession. An early foliation (S1), recognized locally 99 �by extreme flattening of pillow lavas, is probably related to D1. Refolding about axes (F2) that are confined to an east-northeast-striking axial surface (S2) produced a complex interference structure (Figure 8). This superposition results in F2 folds that face structurally downward onto the dome. D2 structures are common and small F2 folds are locally coaxial with pronounced lineations (L2) which result from crystallographic and dimensional orientation of metamorphic minerals. Cleavage (S2) that is axial planar to F2 folds is generally well developed. A third fold set (F3) is accompanied by the development of kink bands and a crenulation cleavage (S3) that strikes northwest. D3 minor structures are particularly abundant in the Bear Passage area. Away from the Rice Bay Dome, similar sets of minor structures can be recognized. Rarely, 3 discrete cleavages have been observed in a single outcrop. While direct temporal correlation of such cleavages with similar structures in the Rice Bay Dome is unwarranted, the persistence of east-northeast-and northwesterly striking sets throughout the area suggests a genetic relationship to a west-northwest- oriented axis of shortening. These cleavages are commonly axial planar to small folds but their direct relationship to big folds is more ambiguous. In some cases, they appear to transect large structures defined by younging reversals, implying the possible existence of early fold sets such as those mapped at Rice Bay. Minor fold axes and lineations have variable attitudes, but shallow plunges on east-northeast and west-southwest trends are common. Nearvertical stretching lineations are well-developed west of the Rice Bay Dome. Shear Zones And Faults The attitude of minor fold axes and cleavage are clearly controlled by proximity to the Quetico and Rainy Lake–Seine River faults. The sigmoidal pattern of cleavage orientation suggests that these faults involve a zone of ductile deformation in which rotation of early-formed structures has taken place. Deflection of marker units indicates right-hand components of displacement for both faults so that the intervening terrane can be considered to be a dextral wrench zone (Figure 3). The orientations and senses of mesoscopic ductile shear zones across the area support this interpretation. Three common orientations exist: 2 sets of right-hand shear zones parallel to each of the major faults can be distinguished from a northwesterly striking, left-hand conjugate set (see Figure 3). The interpreted direction of regional shortening (see Figure 3) is consistent with that indicated by the folds. 100 �Mineral Deposit Types Poulsen (2000a) proposed the mineral deposit classification shown in Table 1, deposit Type 5, Diamond-bearing ultramafic pyroclastic rocks has been added by the author. The presence of diamonds in the ultramafic pyroclastic rocks had not been identified at the time of Poulsen‘s field work. Table 1. Mineral deposits classification, Mine Centre–Fort Frances area Type 1 Stratabound Mineralization Hosted by Felsic to Mafic Metavolcanic Rocks A. Sphalerite-galena-chalcopyrite associated with siliceous volcanic rocks B. Sphalerite-chalcopyrite associated with intermediate to mafic amygdaloidal volcanic flows, tuffs and breccias C. Sphalerite-chalcopyrite associated with iron formation D. Lean iron formation (mainly chert-magnetite and massive pyrite-pyrrhotite, minor chalcopyrite common) Type 2 Mineralization Hosted by Layered Gabbroic Intrusions A. Chalcopyrite associated with gabbro and leucogabbro near the base of sills (Northrock) B. Disseminated chalcopyrite associated with siliceous phases of intrusions C. Ilmenite-magnetite-apatite-rutile lenses in the upper portions of the intrusions (Barber Lake property) (Stop 3) Type 3 Vein Mineralization A. Quartz-gold-sulphide veins in shear zones and cleavage-parallel dilatant zones (Foley Mine, McKenzie-Gray; Stop 1) B. Quartz-molybdenite-pyrite veins and disseminations in unmetamorphosed granitoid rocks Type 4 Disseminated chalcopyrite-pyrrhotite mineralization hosted by ultramafic metavolcanic rocks (Northrock) (Stop 4) Type 5 Diamond-bearing ultramafic pyroclastics (Grassy Portage Ultramafic Pyroclastic) (Stop 5) 101 �FIELD TRIP STOPS (Stops are located by UTM co-ordinates based on NAD 83, UTM Zone 15) Depart from the Fort Frances Memorial Sports Centre and drive east on Highway 11 for approximately 65 km to the hamlet of Mine Centre. Continue an additional 1.3 km and turn right (south) onto the Shoal Lake Road. Drive south along Shoal Lake Road for approximately 8.5 km to the entrance of the Foley Mine property, turn right onto the access road. (0526320E 5395406N) STOP 1– Vein Mineralization - Lode Gold Q-Gold Resources Limited (Foley Mine and McKenzie-Gray Prospect) Delio Tortosa This portion of the trip will be led by Delio Tortosa, Consulting Geologist, Q-Gold Resources Ltd. Q-Gold Resources Ltd. has mineral rights over a large portion of the historical Mine Centre Mining Camp (Figure 4). Past-producing gold mines and numerous mineral prospects occur in quartz and quartz-carbonate fissure veins oriented north-northwest and dipping steeply. The predominant host rock is the Bad Vermilion Felsic Intrusion that ranges in composition from trondhjemite to tonalite and granodiorite. The intrusion has a sigmoidal shape that is characteristic of the rock units and major faults that occupy the area between the Quetico Fault to the north and the Rainy River – Seine River Fault to the south. Typically, the mineralized veins are laminated and occur within shear zones with a left-hand sense of movement. The quartz contains pyrite, chalcopyrite, arsenopyrite, galena and sphalerite. Free gold is commonly associated with the sulphide-rich zones and displays a strong ‘nugget’ effect along the vein systems. STOP 1a: Foley Mine Area The Foley Mine is the best-developed mine in the area with 2.5 kilom of drifting on seven levels to a depth of 260 m. Initial development and mining took place in the 1890’s, and the mine was further developed to greater depth in the 1920’s, with extensive drifting but little gold production. A total of 5,267 ounces of gold was produced between 1893 and 1934. There are two main veins related to the Foley Mine on surface: the Bonanza Vein, which is currently overburden-covered and where the Foley Shaft is situated, and the nearby Jumbo Vein (Figure 5). The Jumbo Vein is well-exposed west and south of the Foley Mine over a distance of 200 m with a north-northwest strike and steep dip. The wall rocks consist of sheared trondhjemite with the shear fabric displaying a left-hand sense of movement. The Bonanza Vein appears to be a splay off the Jumbo Vein. All the vein systems in the area appear to be interconnected either along strike and/or at depth. 102 �Chip sampling by Q-Gold Resources along the Jumbo Vein resulted in a weighted average of 5 g/t Au, 15 g/t Ag, and 0.33% Zn. Two 25-ton bulk samples taken by Nipigon Gold Resources in 1992 returned 0.5 oz/t Au and 0.05 oz/t Au – reflecting the ‘nugget effect’ for gold in these vein systems. STOP 1b: McKenzie Gray Prospect A substantial amount of mineral exploration and bulk sampling was carried out on the McKenzie Gray Prospect between 1984 and 1995. In 1992, Nipigon Gold Resources Ltd. completed 15 diamond drill holes and estimated a mineral resource of 63,000 tons grading 0.17 oz/ton Au, 1.13 oz/ton Ag, 0.25% Cu, and 3.05% Zn (not compliant with NI 43-101). In 1984 the Mine Centre Joint Venture Group processed a 28-ton bulk sample through a local mill resulting in an average grade of 7.2 g/t Au, 112.1 g/t Ag, 10.18% Zn, and 0.11% Pb. A 1995 report by Nipigon Gold Resources, reported that a 204-ton bulk sample from the open pit processed through their on-site test mill returned an average grade of 0.17 oz/ton Au. In 2009 Q-Gold Resources Ltd. completed a diamond-drilling program consisting of 12 NQ holes spaced on sections 15 m apart to evaluate the northwestern extension of the McKenzie Gray vein system. The drill results indicated that the vein ranges in width from less than 1 m up to 7 m wide with weighted assay values of 17 g/t Au, 21 g/t Ag, and 2% Zn. The weighted average grade of all mineralized intersections was 4.7 g/t Au, 30.8 g/t Ag, and 2.9% Zn. The McKenzie Gray Prospect can be subdivided into two major vein systems: the McKenzie Gray Vein and the East Vein (Figure 6). The McKenzie Gray Vein is thought to be a crack-seal textured vein that crosscuts the East Vein, an extension-type vein (Glidden, 1990). The McKenzie Grey Vein contains much of the Au-Ag-Zn-Cu-Pb mineralization, while the East Vein is noted for its high silver content (possibly argentite). The McKenzie Gray Vein has a strike of 310° to 315°, dipping 75° southwest to vertically. The vein has an anastomosing character, splitting into two veins and then joining into a wider single vein along strike and down dip. The vein consists of reddish quartz that is bounded by and contains strongly foliated/sheared and altered sections of trondhjemite with the foliation generally parallel to the vein direction. The McKenzie Gray vein is mineralized with varying concentrations of sulphides that carry significant amount of gold and silver. The most prevalent minerals are: sphalerite, pyrite, chalcopyrite, galena, and a gray metallic mineral (possibly argentite). Gold occurs as 10-micron grains within the sphalerite and as 10-40 micron grains within the quartz (Larouche, 1995). The East Vein is situated immediately east and on the footwall side of the McKenzie Gray vein. It averages between 3 and 5 m wide and has an irregular shape trending sub-parallel to the McKenzie Gray vein and dipping 60° to 80° southwest. The vein is composed of white to gray quartz and is weakly mineralized, primarily with pyrite and minor amounts of chalcopyrite, and a gray metallic mineral (possibly argentite). The quartz vein contains numerous inclusions of altered trondhjemite. One of the wider sections of the East Vein intersected by the Q-Gold drilling program consisted of a 6.7 m-wide zone containing 54 g/t Ag, 0.2 g/t Au, and 0.33% Zn. The weighted average grade of all intersections of the East Vein was 35 g/t Ag. 103 �Figure 4: Regional Geology, Gold Mines and Prospects (Internal Company Report, Q-Gold Resources Ltd.) 104 �Figure 5: Foley Mine area shafts and veins (Internal Company Report, Q-Gold Resources Ltd.) 105 �Figure 6: McKenzie Gray Prospect, Detailed Geology (Internal Company Report, Q-Gold Resources Ltd.) 106 �STOP 2 – Historical Stop – Arastra (Optional) Peter Hinz Return to Highway 11 turn left and drive approximately 7.1 km west, pull off to the north side of the road. (0522191E 5400245N) If conditions are favourable walk around the perimeter of the wetland to the low concrete structure at the edge of the open water. The Mine Centre arastra (a.k.a. arrastra) is a historical oddity in northwestern Ontario. An arastra, also known as a drag-stone mill or ‗chili mill‘ is a pre-industrial form of a crusher used in South and Central America for crushing vein material. Local residents have told the author that this is one of three known in the local Mine Centre area, it is believed these are the only known arastras in existence in Ontario. A small pile of quartz-carbonate vein material adjacent to the arastra could have come from either the Olive Mine located 2.5 km to the northwest or the Stellar Mine located 1.7 km, both of which host gold in quartz-carbonate veins. Figure 7. An arastra from the U.S. midwest, circa. 1890’s. Image from http://www.calisphere.universityofcalifornia.edu/ Figure 8. The author beside the Mine Centre arastra. 107 �STOP 3 – Magmatic Iron-Titanium Mineralization – Numax Resources Property Paul Albers & Chris White Continue west on Highway 11 for approximately 3.5 km to the Barber Lake Road,, turn south and drive 15.4 km (0509484E 5390837N). There is a gate located at approximately 10.6 km. This portion of the trip will be led by Chris White and Paul Albers, Numax Resources Ltd. Introduction Numax Resources, Inc. (Numax) holds a 100% interest in the Mine Centre Fe-Ti-V Property. The property comprises 45 contiguous mineral claims totaling 5,188 ha in the Kenora Mining Division and is situated along Seine Bay of Rainy Lake and Bad Vermilion Lake (Fig. 1). The Mine Centre Property has been the focus of Numax‘s exploration efforts dating back to the summer of 2004. The property was previously explored by Hunter in 1911, Goodwin from 1917-1918, Butler Brothers in 1943, and Stratmat Limited from 1956-1958. While relatively inactive at the Mine Centre Property for numerous decades, Numax commenced field work in 2004 that included grid line cutting, two ground geophysical surveys (magnetic and electromagnetic), extensive trench excavation, channel sampling of trenches, field sampling, diamond drilling, and prospecting. During the fall of 2009, geologic mapping (Scale 1:5000) was conducted across a majority of the property. Along with this, ten trenches were mapped in detail (Scale 1:400). Figure 9: Location of the Numax Resources, Inc., Mine Centre Property, modified from Poulsen, 2000. 108 �Local Geologic Setting The igneous stratigraphy of the Seine Bay/Bad Vermilion Lake intrusion features interlayered anorthosite, gabbro, melagabbro, pyroxenite, semi-massive oxide, and massive oxide. The intrusion has been rotated such that layers are now oriented sub-vertically in the west, and dipping at 30-60° in the east. Layers generally strike easterly in the west, and northeasterly in the east, with a bend in the intrusion near the Snake Lake area. Graded modal layering of silicate minerals indicates that layers top to the north and northwest, respectively. Zones of massive and semi-massive oxide demonstrate the geologic expression of, and correlate with magnetic anomalies discovered during Ontario Geological Survey airborne geophysical surveys (2009, 1980), and a ground magnetometric survey conducted by Numax (Simoneau, 2008). These massive and semi-massive oxide zones are present as discrete, continuous to semicontinuous layers of magnetite, ilmenite, and titaniferous magnetite, covering a strike length of at least 10 km, and likely extend in both to the east and west. The Seine Bay/Bad Vermilion Lake intrusion is uniquely situated between two major fault zones (the Quetico Fault to the north and the Rainy Lake-Seine River Fault to the south; Fig. 1). Though the gabbros and anorthosites of the intrusion appear to be largely undeformed, numerous discrete (metre-scale) shear zones exist within the intrusion, typically trending in a general easterly direction. It is quite possible that deformation within the intrusion has been largely accommodated by zones of massive oxide as the contacts between massive oxides and gabbroic rocks locally exhibit an obliquely trending (to the contact) mineral lineation emanating from the massive oxide into the gabbro, which dominantly features Fe-Ti-oxide smearing into the gabbro. Additionally, several large shear zones have been identified within the intrusion; a fault displaces barren anorthositic rocks in contact with oxide- and sulfide-bearing gabbroic rocks. In the area of Numax‘s Mine Centre Property, the Seine Bay/Bad Vermilion Lake intrusion is sandwiched between metamorphosed felsic intrusive rocks to the north, and mafic meta-volcanic rocks to the south, which largely underlie the northern shore of Seine Bay. However, mafic intrusive rocks of the Seine Bay/Bad Vermilion Lake are again present to the south of Seine Bay (Poulsen, 2000). The exact natures of both the upper and lower contacts of the intrusion are not completely understood at this time. The felsic intrusive rocks to the north display a distinct equigranular texture with spherical quartz eyes. Occurrences of quartz eyes (1-3 mm in size), can be observed locally in the mafic intrusive rocks of the Seine Bay/Bad Vermilion Lake intrusion, proximal to the upper contact. Additionally, interlayers of felsic and mafic intrusive rocks can be found locally near the upper contact. The mafic meta-volcanic rocks exposed on the northern shore of Seine Bay could be a very large inclusion block within the intrusion, or more likely could be related to the regional faulting in the area. The contact between mafic metavolcanic rocks and mafic intrusive rocks appears intrusive, as it is extremely heterogeneous and features pegmatitic gabbro intermixed with mafic metavolcanic rocks. Closest to the mafic meta-volcanic rocks, apophyses of gabbroic melt migrate downward into the mafic volcanic rocks; while closer to the gabbro above, blocks (1-3 m) of mafic metavolcanic rocks occur as inclusions within the gabbro. 109 �In the eastern part of the property a shear zone displaces pegmatitic gabbroic rocks adjacent to anorthositic rocks. This shear zone is within 10-20 m of the lower contact with mafic metavolcanic rocks. Quite possibly, this shear zone has displaced the far western extent of the intrusion eastward, causing the intrusion to become ―stacked,‖ and displacing a portion of the mafic meta-volcanic footwall to its current position within the intrusion. Description of Map Units The lithologic units present in Numax‘s Mine Centre Property are remarkably correlative in outcrop and exposed trenches, generally along an easterly strike. Figure 2 displays a simplified geologic map of the central Seine Bay/ Bad Vermilion Lake intrusion as it relates to Numax‘s Property. The lithologic units present on the property include: Felsic volcanic undivided (FVu) – Felsic volcanic rocks (undivided), typically interpreted as inclusions or groups of inclusions within the Seine Bay/Bad Vermilion Lake intrusion, typically very fine-grained, fresh and weathered surfaces white to light pinkish – beige, locally contains fine- to medium-grained disseminated chalcopyrite up to 1% found in both the Heat-Sink Trench and the Trenches on Line 28. Massive and Semi-massive Oxide (MOX) – Massive (90-100% Fe-Ti oxide) and Semi-massive (30-90% Fe-Ti oxide) mineralization of iron – titanium oxide, typically hosted by medium- to coarse-grained pyroxenite, fresh surfaces are metallic black to dark purple, weathered surfaces are rusty orange to brown, magnetic strength varies from moderate to very strong, typically contains trace to 2% disseminated and massive sulfide (1-2 cm thick) consisting of pyrrhotite + chalcopyrite + pyrite, sulfides are typically fine- to very fine-grained, oxides are typically medium- to coarse-grained. Mafic Intrusive, oxide-bearing (MIox) – Oxide-bearing (5-30% Fe-Ti oxide) mafic intrusive rocks, typically consisting of homogeneous, medium- to coarse-grained gabbro, melagabbro, pyroxenite, and rare leucogabbro, fresh surfaces are medium grey, and weathered surfaces are medium grey to brown, magnetic strength typically moderate to strong, contains lenses and layers of massive oxide that are <1 m thick, clinopyroxene is locally ophitic, but is typically intergranular, locally contains rare to trace disseminated sulfide mineralization, alteration dominantly includes moderate chlorite after pyroxene. Mafic Intrusive (MI) – Non-oxide-bearing (<5% Fe-Ti oxide) mafic intrusive rocks, typically consisting of homogeneous, coarse- to medium-grained anorthosite and leucogabbro, and locally gabbro, fresh and weathered surfaces are light grey, locally displays modal layering between plagioclase and clinopyroxene, typically consisting of layers <1m thick that top to the north-northwest. Mafic and Felsic Intrusive undivided (MFIu) – Undivided mafic and local felsic intrusive rocks, assumed from Poulsen, 2000. 110 �Figure 10: Simplified geologic map of the central Seine Bay/Bad Vermilion Lake intrusion, modified from Albers and White, 2010. 111 �Description of Map Units, continued Felsic Intrusive (FI) – Felsic intrusive rocks dominantly consisting of granodiorite, quartz diorite, tonalite, and trondhjemite, typically medium- to coarse-grained, these rocks typically display the effects of contact metamorphism largely demonstrated by the texture of quartz, forms the hanging wall to the Seine Bay/Bad Vermilion Lake intrusion in the map area. Mafic Volcanic undivided (MVu) – Undivided mafic volcanic rocks, typically consisting of homogeneous, massive, fine- to very fine-grained basalt, typically displays contact metamorphism to hornfels, locally plagioclase-phyric, locally displays pillows, local sulfide mineralization, occurs both as inclusions within the Seine Bay/Bad Vermilion Lake intrusion, and as an extensive ―finger‖ occurring across the length of the intrusion in the map area, an extremely heterogeneous intrusive contact is observed in the Discovery Zone and Discovery Zone North Trench between the basalt ―finger‖ and the Seine Bay/Bad Vermilion Lake intrusion in which prominent sulfide and oxide mineralization is displayed. Massive and Semi-massive Oxide assumed (MOXa) – Massive and Semi-massive oxide, assumed from Ontario Geological Survey (2009) airborne geophysical survey (approximate). Field Trip Stop The field trip consists of one stop, which will traverse a complex of trenches excavated on section line 28. The trenches include Beaver Dam, Buck, and Doe trenches. Figure 3 displays a geologic map of these trenches with accompanying Fe2O3 and TiO2 assay data profiles, as well as a profile of the ground magnetometric geophysical survey data (Simoneau, 2008). Ten distinct lithologies, which display correlative stratigraphy through the complex of trenches and outcrops on section line 28, and with adjacent trenches and outcrops at least 600 m to the east and west, are displayed. Detailed lithologic descriptions that correspond to Figure 3 are documented below (from north to south). A) Melagabbro: Medium- to fine-grained, weakly to non-magnetic, weak chlorite alteration, locally verges on pyroxenite; silicate modal mineralogy is as follows: 68-88% clinopyroxene, 515% plagioclase, and 5-15% olivine, trace disseminated and veinlets of very fine-grained chalcopyrite + pyrrhotite (<0.5%), <2% iron oxide, olivine seems to occur as clusters with plagioclase and weathers to rusty brown creating a pock-marked appearance, weathered surface is medium slate grey, fresh surface dark slate grey, cut by a dike of felsic to intermediate composition which is very fine-grained, non-magnetic, and contains tiny phenocrysts of white alkali feldspar, clear to white quartz, and 1-2 mm needles of hornblende, weathered surface is white, fresh surface is medium blue/grey, upper contact buried beneath overburden/swamp, lower contact gradational, at least 33 m thick. 112 �Figure 11: Geologic map (Scale 1:400) of the Trenches on Line 28 with associated Fe2O3 and TiO2 profiles from channel samples, and walking magnetometric geophysical survey profile. 113 �B) Melagabbro (layered): Similar in composition to the melagabbro above (A), but contains less olivine and is medium- to coarse-grained, displays prominent modal layering between plagioclase and pyroxene, layers are typically <1 m thick and in many instances thinner layers (<10 cm thick) stack on top of thicker layers, plagioclase-rich tops are typically 1-4 cm thick, layers are very consistent with sharp contacts and local fault off-sets (<10 cm), layers dip subvertically and strike at ~245° with a topping to the northwest, lower contact is gradational over ~3 m and displays a zone with thin fractures or shears (1 mm to 2 cm thick) filled with chlorite and skirted by plagioclase-rich gabbro that abruptly dissipates back to melagabbro, fractures are irregularly spaced (1 cm to 0.5 m apart) with a subvertical dip and strike of ~240°, ~43 m thick. C) Oxide Pyroxenite: Fine-grained, weakly to non-magnetic, strong chlorite alteration, ~5% iron oxide which is clumped together with very fine-grained plagioclase, trace very fine-grained chalcopyrite + pyrrhotite, weathered and fresh surfaces are pale green and purplish, lower contact abrupt, ~8 m thick. D) Oxide Melagabbro to Oxide Pyroxenite: Medium-grained, weakly to moderately magnetic, strong chlorite alteration, <3% very fine-grained granular olivine, 5-15% iron oxide, trace finegrained disseminated chalcopyrite + pyrrhotite, local patchy veining or fracture fillings of chlorite <0.5 m thick and discontinuous with pinching and swelling, local very distinct change in weathering: broken-up and sheared with a strong foliation dipping sub-vertically and a general southwest trend (too magnetic to obtain strike), near the bottom of this unit is a thin subvertically orientated band of fibrous-fanning amphibole (2 mm to 1 cm thick) generally striking southwest (too magnetic to obtain strike), lower contact is sharp to gradational over 3-4 m, ~25 m thick. E) Massive Oxide: Medium-to fine-grained, strongly magnetic, strong chlorite alteration, hosted by pyroxenite, rare fine-grained disseminated chalcopyrite, vague localized modal layering displaying as thin bands (1 mm to 3 cm thick) of chlorite (very soft with blue-green color) semicontinuous and flat-lying, sub-vertical generally trending southwest (too magnetic to obtain strike), spaced 10-20 cm apart commonly occurring as pairs with a thinner layer (2-4 cm thick) overlying a thicker (10-20 cm thick) layer, 4-9 m thick, lower contact is sharp with a 0.5 m thick lens or dike locally cross-cutting the contact between this massive oxide and the gabbro below, dike contains 1% fine-grained disseminated chalcopyrite and stringers of chlorite, weathered and fresh surfaces are white (felsic volcanic?), ~4-10 m thick. F) Oxide Gabbro: Medium-grained, weakly to non-magnetic, strong chlorite alteration, ~5% iron oxide, rare fine-grained disseminated chalcopyrite + pyrrhotite, thin sinuous and continuous laminations of <1 mm thick plagioclase and chlorite occur in within 0.5 m of upper contact, contains local layers (<10 cm thick) of massive oxide oriented normal to strike of upper contact, lower contact is abrupt over ~1 m, with lineated iron oxide emanating obliquely to contact (~30°) into oxide gabbro from massive oxide below showing a somewhat boudinage appearance, dipping sub-vertically (too magnetic to obtain strike), 3-25 m thick. G) Gabbro (lineated/sheared/folded): Fine- to medium- to coarse-grained, locally weakly magnetic, strong chlorite alteration, pyroxene and iron oxide are typically lineated and stretched 114 �in direction of lineation (too magnetic to obtain trend), local pervasive folding with up to nine folds observed in a single structure, typically folds are rimmed with chlorite (2 mm to 2 cm thick) and dominantly composed of quartz and plagioclase with local iron oxide, locally strongly foliated dipping sub-vertically and striking southwest (too magnetic to get strike), but foliation is normal to mineral (iron oxide) lineation at contact above, numerous layers (10 cm to 1.5 m thick) of massive oxide dipping sub-vertically with strike normal to upper contact, contacts between massive oxide and gabbro are sharp and irregular, massive oxide is hosted by pyroxenite with local olivine and displays sub-vertically oriented laminations (1-2 mm thick) of chlorite striking normal to contacts, locally grades to oxide gabbro toward lower contact, lower contact sharp, up to 45 m thick. H) Massive Oxide: Medium- to fine-grained, strongly magnetic, strong chlorite alteration, hosted by pyroxenite, weathered surface is brown/rusty-orange to dark-deep-reddish/purple, fresh surface is metallic dark-purple/black, local silver-white staining generally as small (several cm) sinuous patches, local discontinuous sinuous bands of massive pyrrhotite + pyrite + chalcopyrite (<2 cm thick), local lenses that form a discontinuous layer of strongly chloritized sheared gabbro (1 m thick x <3 m long), lower contact consists of a 1-2 m cliff face plunging beneath the overburden, at least 7-10 m thick. I) Felsic Volcanic: Strongly metamorphosed and altered, massive appearance, weathered surface is pinkish/orange to beige with intermixed patchy blue-green, fresh surface is generally pale blue-green-grey, mafic phase is strongly altered to chlorite, local pale-yellow epidote alteration, local thin (<1 mm thick) folded bands that show a somewhat preferred orientation with strike (~270°) of contact, local foliation and shearing toward top of zone, rare fine-grained disseminated pyrite and possibly chalcopyrite, could be the same as dikes above, lower contact buried beneath overburden and standing water, at least 37 m thick. J) Dikes and/or Felsic Volcanic Inclusions: Very fine-grained, non-magnetic, locally contains up to 1% fine-grained disseminated chalcopyrite, local phenocrysts (1-2 mm in size) of white alkali feldspar, clear to white quartz, and needles of hornblende, local irregular-shaped clasts of beige to light reddish/orange quartz (1-15 cm long), weathered surface is white to light blue-grey, fresh surface is medium to light blue-grey, contacts are generally sharp, 1-4 m thick. Iron-titanium-oxide geochemical data collected from continuous channel sampling through the trenches display very distinct profiles that correlate remarkably well with the geology observed in the trenches (Fig. 3). Restricted (<1.5 m) zones or layers of massive oxide within gabbroic rocks show spikes in Fe2O3 and TiO2, while felsic to intermediate dikes display corresponding decreases in Fe2O3 and TiO2 values. The profiles are most interesting through units C, D, E, and F. Here the profiles display a steady increase from north to south through the pyroxenite and oxide pyroxenite, hitting a peak of roughly 60% Fe 2O3 and 25% TiO2 in unit E (massive oxide). The profile of the ground magnetometric geophysical survey (Simoneau, 2008) along section line 28 is also quite correlative with the observed geology (Fig. 3). The survey profile displays spikes of >80,000 gammas in zones of massive oxide, and corresponding decreases in unmineralized gabbro and felsic rocks to <60,000 gammas. As discussed above and presented in Figure 3, the geology, geochemistry, and geophysics of the trenches on line 28 are remarkably correlative. The igneous stratigraphy of the Seine Bay/Bad 115 �Vermilion Lake intrusion throughout the Numax Mine Centre property also correlates remarkably well, with both geologic mapping, and airborne geophysical surveys (Ontario Geological Survey, 2009), showing continuous to semi-continuous layers of massive oxide through the property for a strike length of at least 10 km, and likely up to 15 km. An extensive drilling program focused on confirming and defining this enormous Fe-Ti prospect is both warranted and forthcoming. Return to Hwy 11 turn left and drive west for 24.4 km, turn left (south) on the bush road and drive 2.25 km to the Y-intersection. Take the right branch and continue for 1.95 km to the Beaverpond Zone (0495043E 5394021N) STOP 4 – Magmatic Copper-Nickel-PGE Mineralization – MetalCORP Limited – North Rock Property Peter Hinz NOTE: This portion of the field trip guide is taken from the MetalCORP website www.metalcorp.ca. MetalCORP Limited‘s 100%-owned North Rock Property is located approximately 25 km east of Fort Frances and is easily accessed via Highway 11 and then 4 km via a gravel road. The property consists of 370 claims or 5,920 ha (14,800 acres). Figure 12. Property outline and generalized geology, North Rock property, MetalCORP Resources Ltd. From MetalCORP website www.metalcorp.ca. 116 �The Property is underlain by the 20 km long Grassy Portage layered mafic intrusion and hosts four known zones of magmatic copper-nickel sulphide mineralization; the Beaver Pond, the Main South, the East and the West zones all of which occur at or near the base of the intrusion, along its western contact. GENERAL GEOLOGY: The property is underlain by mafic and ultramafic metavolcanic rocks, which are in contact with the Grassy Portage gabbroic intrusion (Figure 20). The ultramafic rocks are tuffs and lapilli tuffs that are generally actinolite-rich and intensely magnetic. Metabasaltic flows overlie these to the south, and pillow shapes indicate southward facing. The contact of the pillow lavas with the Grassy Portage intrusion is sharp, and is occupied by a 20 m thick unit of coarse-grained, hornblende-rich melagabbro. The melagabbro grades south-eastward to medium- to coarsegrained gabbro composed of subequal proportions of hornblende and plagioclase. Layers of anorthosite composed of andesine intrude the gabbro at the southern margin of the property, where they are spatially related to masses of magnetite-actinolite amphibolite. Numerous mafic to intermediate dikes cut the gabbroic rocks. Copper-Nickel Mineralization The most significant of the zones is the Beaver Pond Zone, which was discovered by Noranda in 1958 and subsequently explored from underground via a 90 metre deep shaft and one drift on the 70 m level. Resource estimates for the Beaver Pond Zone, reported by Bergman (1973) range (depending on the cut-off grade) from 1,020,458 tons grading 1.17% Cu to 265,230 tons grading 2.08% Cu. The resource estimate was calculated to a depth 90 m and the deposit appears to remain open to depth. One drill hole intersected 2.21% Cu over 36.5 feet at a vertical depth of 175 m. (Note that these historical resource estimates are not current and should not be relied upon as they were not prepared in compliance with National Instrument 43-101.) Other significant drill intersections reported by Noranda from within the Beaver Pond Zone include: 4.3% Cu over 8.1 m, 4.1% Cu over 7.8 m, 3.4% Cu over 8.0 m, 1.5% Cu over 21.3 m, and 1.0% Cu over 55.5 m. (These Noranda assays have not been verified and as such should not be relied upon). There is no record of any further mineral exploration work having been conducted on or near the Beaver Pond Zone, or on any of the formerly leased claims in the area, since the underground program was concluded in 1973. Very little historical data exist regarding the abundance and distribution of nickel, cobalt, platinum group metals (PGM) or gold in the magmatic sulphide deposits on the North Rock Property. However, recent grab sampling of a 10,000 ton surface stockpile of material recovered from underground development of the Beaver Pond Zone yielded assays of up to 8.9% Cu, 0.8% Ni, 0.05% Co, 1.6 g/t Pt, 0.7 g/t Pd and 0.7 g/t Au. Although grab sample assays are typically not representative of the overall grade, these results do confirm the presence of significant nickel, cobalt, PGM and gold values associated with the copper mineralization. 117 �Figure 13. Mineralized zones and assays, North Rock property, MetalCORP Limited. From MetalCORP website www.metalcorp.ca. The three other known sulphide zones are less-explored and occur over a 1.5 km strike length along the base of the intrusion extending northeast from Beaver Pond Zone. These areas, as well as several other sulphide occurrences higher up in the intrusion, are all considered highly prospective for hosting additional magmatic nickel-copper (+/- PGE's, Co) sulphide mineralization. MetalCORP conducted surface exploration, an airborne magnetometer/electromagnetic survey and three phases of drilling on the Property from 2005 to the end of 2007. The first phase of drilling was carried out in 2005 and consisted of 14 diamond drill holes totaling 3,906 m. Phase 1 drilling was designed to test the depth extension of the known copper mineralization at the Beaver Pond Zone. All drill holes intersected widespread, varied amounts of net-textured copper-nickel sulphide mineralization similar to that reported by previous drilling by Noranda. Hole 4 appears to have intersected the core of the zone returning 1.5% copper over 13.7 m. The drilling also intersected a hanging wall Platinum Group Metal (PGM) zone that can be traced for 500 m along strike and to a depth of 200 m. Assay values as high as 3.7 g/t Pt + 7.1 g/t Pd over 0.8 m were reported. The Company completed an airborne magnetic/EM geophysical survey over the property which detected several anomalies with a 1 km long anomaly coincident with the East zone which is 1.2 km northeast and along strike of the Beaver Pond zone. Mapping and sampling confirmed the 118 �presence of PGM mineralization at the East zone and a Phase 2 drilling program was set up to test the mineralization. The Company drilled 21 holes totaling approximately 4,000 m with several holes intersecting PGM mineralization. Hole 20 intersected 3.7 m (2.5 m true width) from 151.9 to 155.6 m grading 1.2 g/t Pt, 0.1 g/t Pd, 0.1 g/t Au, 0.6% Cu and 0.2% Ni (see March 6 and April 19, 2006 news releases) www.metalcorp.ca. In June 2007 the Company announced a Phase 3 diamond drilling program of 25 holes totaling approximately 8,000 m to test several airborne EM conductors over a 20 km strike length on the entire Property. Drilling in the Belacoma area, approximately 4.0 km northeast of the Beaver Pond Zone, intersected platinum and palladium mineralization as well with the following values: 3.5 g/t Pd over 2.8 m, 1.2 g/t Pt and 1.0 g/t Pd over 1.0 m, 2.9 g/t Pd over 0.9 m and 1.6 g/t Pd over 0.9 m. A number of other airborne EM conductors were tested, with the majority explained by pyrrhotite veins and/or sulphide iron formation. The sulphide iron formation in the Nickel Lake area was found to be zinc-rich, with the best assay from hole #57 of 0.5% Zn over 19.6 m. Return to Hwy 11, turn left and drive for 2.7 km, pull off to the right into the small bush lane. Walk through the bush 120 m to the northwest to the outcrops on the south side of the beaver pond (497137E/5396262N) STOP 5 – Diamonds in the Grassy Portage Ultramafic Pyroclastic – MetalCorp – GUP Property The Grassy Portage Ultramafic Pyroclastic (GUP) is located approximately 33 km east-northeast of Fort Frances. The property straddles highways 11 and 502 and extends out into Grassy Portage Bay, Rainy Lake (Figure 14). Portions of the property are accessible by road or boat. The property is held by MetalCORP Limited under option from Messrs. R. Cousineau, L. Cousineau and K. Desjardins, all of Fort Frances. The general geology of the area is described by Poulsen (2000): The rocks of the Rainy Lake area are part of the Archean Superior Province and form a fault-bounded wedge between 2 subprovinces, the Wabigoon granitegreenstone terrane to the north and the Quetico metasedimentary terrane to the south. The Quetico and Rainy Lake-Seine River faults define this wedge, which is interpreted to be a dextral wrench zone and which displays distinct stratigraphic, structural and metamorphic relationships. Poulsen (2000) also described the GUP: A unique ultramafic unit occurs in the Redgut Bay-Grassy Portage Bay area. Rocks from this unit have distinctive textures as well as a magnesian chemical composition; samples plot as basaltic komatiites. Locally, the rock consists of angular to subrounded fine-grained clasts, up to 5 cm in diameter, set in a fine-grained chlorite-tremolite matrix. The rocks are moderately to strongly magnetic and, where visible fragments are not present, they are fine-grained magnetic tremolite schists. As a whole, this ultramafic unit strongly resembles the "ashrock" unit (Joliffe 1955) that overlies the iron ore horizon of the Steep Rock Group north of the Quetico Fault at Atikokan. 119 �The author collected 15 samples from various parts of the property during the 2000 field season; all samples plotted at the boundary between basaltic komatiite (BK) and picritic (ultramafic) komatiite (PK) on a Jensen Cation plot (Jensen 1976) (Figure 15). A plot of total alkalis (Na2O + K2O) versus silica (SiO2) (Le Maitre 1989) shows that the majority of samples plot within the komatiite or meimechite field (see Figure 16). The differentiation between the two rock types is based upon titanium oxide content, meimechite having greater than 1% TiO2, and komatiite, less than 1% TiO2. Meimechite is defined as, "…the extrusive equivalent of kimberlite" (Mitchell, 1985). Schaefer and Morton (1991) conducted research on both the GUP and the Dismal Ashrock of the Steep Rock Group. Their work indicated that "GUP is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. Both pyroclastic units contain cored and composite lapilli, evidence of explosive volcanism. Locally, some of the lapilli fragments are highly vesicular (up to 30% by volume), greater than reported for any other komatiite." A variation in clast composition was noted at various locations in the GUP. Cored clasts up to 10 cm across were noted on the shoreline (495819E/5395232N) and islands (499902E/5394822N) of Grassy Portage Bay. At the Beaver Pond Occurrence (497137E/5396262N) clast-dominant GUP is seen to be in contact with a fine-grained matrix-dominant GUP. The contact between the two units suggests that the younging direction is roughly to the east. This is based on what is interpreted, by the author, to be scouring of the fine-grained tuffs during passage of a pyroclastic debris flow. The majority of clasts appear to be ultramafic in composition with <2% of exotic origin. At all locations the clasts range from subangular to subrounded, with a maximum dimension of 15 cm. Rocks of the GUP contain only trace amounts of sulphide minerals (pyrite +/- pyrrhotite). Some sections contain appreciable amounts of euhedral magnetite, visible to the naked eye. A bronzecoloured mica was observed on a small island in Grassy Portage Bay and identified as phlogopite in the field; X-ray diffraction analysis confirmed the presence of phlogopite/biotite. Talc was also noted in some sections. Shaefer and Morton (1991) and Poulsen (2000) concurred that the GUP has undergone amphibolite-grade metamorphism. Only late-stage, weak carbonatization was noted by the author. Schaefer and Morton (1991) commented on the structure of the area: "GUP outcrops form an arcuate fold interference pattern, are strongly deformed …". However, outcrops visited by the author display very little deformation, the only structural fabric was a weak foliation observed at the Beaver Pond occurrence. This apparent lack of deformation is supported by observed clast morphology, which ranges from subangular to well-rounded. The property and surrounding area has undergone extensive exploration at various times since the late 1800s for copper, zinc, nickel, PGM, gold and iron. Prior to 2000, no record of diamond exploration is documented in the area. Under an agreement with the property owners, INCO Ltd. conducted a sampling program on the property in 2003, with samples being analysed by Lakefield Research (L. Cousineau, Prospector, personal communication, 2003). One sample collected by INCO was reportedly identified as a hypabyssal kimberlite. 120 �Figure 14. Geology of the Grassy Portage ultramafic pyroclastic (modified from Schaefer and Morton 1991). Rectangles represent sample areas. (From Lichtblau et al. 2004). 121 �BK – Basaltic komatiite CA – Calc-alkaline andesites CB – Calc-alkaline basalts CD – Calc-alkaline dacites CR – Calk-alkaline rhyolites HFT – High iron tholeiites HMT – High – magnesium tholeiites PK – Picritic komatiite TA – Tholeiitic andesites TD – Tholeiitic dacites TR – Tholeiitic rhyolites Figure 15. Jensen cation plot (Jensen 1976) for samples collected from the Grassy Portage ultramafic pyroclastic. Triangles represent sample plots, this study. Figure 16. Alkali versus silica plot (LeMaitre 1989) for samples collected from the Grassy Portage ultramafic pyroclastic. Diamonds represent sample plots, this study. 122 �As part of its ongoing exploration program MetalCORP announced in February 2008 the discovery of micro-diamonds on the property in the Grassy Ultramafic Pyroclastic. The Company took a 100 kg sample and sent it for processing by Kennecott Canada Exploration Inc. ("Kennecott") in its Mineral Processing Laboratory, located in Thunder Bay, Ontario. A total of six diamonds were recovered with one sitting on the 0.212 mm sieve, three were on the 0.106 mm sieve and two were less than the 0.106 mm sieve size. In December 2008 Kennecott and MetalCORP signed an option agreement to explore for diamonds on the property but in September 2009 MetalCORP was advised by Kennecott that it had completed processing a 1,200 kg sample taken during its summer exploration program and, based on the results, concluded that the diamond potential is very low. Accordingly, Kennecott terminated its program and its option and returned the property to MetalCORP. Figure 17. SEM images of four of the six diamonds recovered by MetalCORP from the Grassy Portage Ultramafic Pyroclastic. 123 �REFERENCES Albers, P.B., and White, C.R. 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario. Unpublished report prepared for Numax Resources, Inc. Bernatchez, R. A. 2009, A Report on the Fe-Ti-V and Cu-Ni-PGM and other Mineral Potential for the Mine Centre Property. Unpublished report prepared for Numax Resources, Inc. Bernatchez, R.A., 2008, The 4 Intrusive cycles of the Seine Bay/Bad Vermilion Lake Intrusion, Unpublished schematic drawing prepared for Numax Resources Inc. Card, K. D., 1990. A review of the Superior Province of the Canadian Shield, a product of Archean accretion. Precambrian Research 48, 99-156. Davis, D.W., 1990. Geological study of the Winnipeg River - Wabigoon subprovince boundary. Report on Energy Mines and Resources Research Agreement 45, 21 p. Davis, D. W., Poulsen, K. H., Kamo, S. L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, pp 379-398. Devaney, J.R., Williams, H.R., 1989. Evolution of an Archean subprovince boundary: a sedimentological and structural study of part of the Wabigoon-Quetico boundary in northern Ontario. Canadian Journal of Earth Sciences v26, pp 1013-1026. Fralick, P., Davis, D., 1999. The Seine-Coutchiching problem revisited: sedimentology, geochronology and geochemistry of sedimentary units in the Rainy Lake and Sioux Lookout Areas. In: Harrap, R. M., Helmstaedt, H. (Eds.), 1999 Western Superior Transect Fifth Annual Workshop 70, pp. 66-75. Glidden, D.J., 1990 Report on the summer 1990 Exploration Program on the McKenzie Gray Property, Mine Centre Area, Kenora District, Ontario , for Nipigon Gold Resources Ltd. Hoffman, P.F., 1989. Precambrian geology and tectonic history of North America. In: Bally, A.W., Palmer, A.R. (Eds), The geology of North America; an overview. The geology of North America A, pp.447-512. Hoffman, P.F., 1990. On accretion of granite-greenstone terrane. In: Robert, F., Sheahan, P.A., Green, S.B. (Eds), Greenstone gold and crustal evolution; NUNA conference volume, pp.3245. Jensen, L.S. 1976. A new cation plot for classifying subalkalic volcanic rocks; Ontario Department of Mines, Miscellaneous Paper 66, 22p. Jolliffe, A.W., 1955. Geology and iron ores of Steep Rock Lake; Economic Geology, Volume 50, No. 4, pp 373-398. Kennedy, M. C., 1984. The Quetico Fault in the Superior Province of the southern Canadian Shield. MSc. thesis, Lakehead University. Langford, F. F., Morin, J. A., 1976. The development of the Superior Province of northwestern Ontario by merging island arcs. American Journal of Science 276, 1023-1034. Larouche, C. 1995. Separation of finely disseminated gold from zinc-copper ore; Report for Nipigon Gold Resources Limited. Lawson, A. C., 1913. The Archaean geology of Rainy Lake re-studied. Memoir - Geological Survey of Canada 40, pp 1-115. 124 �Le Maitre, R.W., ed. 1989. A classification of igneous rocks and glossary of terms; Blackwell, Oxford, 193p. Lichtblau, A., Hinz, P., Ravnaas, C., Storey, C.C., Kosloski, L. and Raoul, A. 2004. Report of Activities 2003, Resident Geologist Program, Red Lake Regional Resident Geologist Report: Red Lake and Kenora Districts; Ontario Geological Survey, Open File Report 6127, 104p. Merritt, P.I., 1934. Seine-Coutchiching problem. Geol. Soc. Am. Bull. 45, pp 333-374. Mitchell, R.S. 1985. Dictionary of rocks; Van Nostrand Reinhold Company, New York, 222p. Ontario Geological Survey, 2009, Ontario airborne geophysical surveys, Magnetic and Electromagnetic Surveys, Grid and Profile Data (ASCII and Geosoft Formats) and Vector Data, Mine Centre Area; Ontario Geological Survey. Geophysical Dataset 1061a ISBN 9781-4435-0854-4 [DVD] ISBN 978-1-4435-0855-1 [ZIP FILE]. Percival, J. A., Williams, H. R., 1989. Late Archean Quetico accretionary complex, Superior province, Canada. Geology 17, pp 23-25. Poulsen, K. H., 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), Tectonic evolution of greenstone belts Technical Report 86-10, pp. 177-179. Poulsen, K. H., 2000b. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121p. Poulsen, H. K., 2000, Precambrian geology and mineral occurrences, Mine Centre – Fort Frances area; Ontario Geological Survey, Map 2525, scale 1:50,000. Poulsen, K. H., Borradaile, G. J., Kehlenbeck, M. M., 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, pp 1358-1369. Schaefer, S.J. and Morton, P. 1991. Two komatiitic pyroclastic units, Superior Province, northwestern Ontario: their geology, petrography, and correlation; Canadian Journal of Earth Sciences, v.28, pp 1455-1470. Severson, M.J., and Hauck, S.A., 1990, Geology, Geochemistry, and Stratigraphy of a portion of the Partridge River Intrusion, NRRI/GMIN-TR-89-11, 236 p. Simoneua, P., 2008, Magnetometric (Walking Mag) and Electromagnetic VLF survey on Mine Centre Property. Unpublished report prepared for Numax Resources, Inc. Stone, D., Hallé, J., and Murphy, R. 1997a, Precambrian geology, Mine Centre area; Ontario Geological Survey, Preliminary Map P. 3372, scale 1:50,000. Stone, D., Hallé, J., and Murphy, R. 1997b, Precambrian geology, Mine Centre area; Ontario Geological Survey, Preliminary Map P. 3373, scale 1:50,000. Tabor, J. R., Hudleston, P. J., 1991. Deformation at an Archean subprovince boundary, northern Minnesota. Canadian Journal of Earth Sciences v28, pp 292-307. Wilcox, R. E., Harding, T. P., Seely, D. R., 1973. Basin wrench tectonics. The American Association of Petroleum Geologists Bulletin 57, 74-96. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980a. Mine Centre Area (Eastern Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2202, scale 1:15 840. Wood, J., Dekker, J., Jansen, J. G., Keay, J. P., Panagapko, D., 1980b. Mine Centre Area (Western Half), District of Rainy River. Ontario Geological Survey Preliminary Map P. 2201, scale 1:15 840. 125 �Field Trip 8 GEOLOGY AND ENVIRONMENTAL ISSUES OF THE STEEP ROCK MINE Andrew Conly Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Rob Purdon Ontario Ministry of Natural Resources, Thunder Bay, ON P7E 6S7 Canada Lindsay Moore Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada View from north rim of the Hogarth Pit during the late 1970’s. Courtesy J. Baechler) 126 �Introduction Flooding since 1979 of the decommissioned Steep Rock iron mines open pits north of Atikokan, Ontario has lead to the formation of the joint Hogarth-Roberts‘, Errington and Caland pit lakes (Fig. 1). Currently, the two largest pit lakes, Hogarth and Caland, have maximum water depths of 200 m, with waters levels expected to rise another 90 m, reaching the overflow elevation of 390 m (the original lake water level elevation). Overflow has been estimated to occur as soon as 2030 (Van Crook, 2005), but other estimates indicate overflow will take place around 2060 or later (Jackson, per. comm., 2007; Mikkeleson, unpublished data). Consequently, the overflow of the pit lakes is predicted to flow into the West Arm and possibly into the Seine River system, which flows into the International Boundary Waters of Rainy Lake and Lake of the Woods. The main issue with the waters from the pit lakes is their elevated sulphate contents (up to 1500 mg/L SO42-), which have been shown to be acutely to chronically toxic for aquatic life (McNaughton; 2001; Goold, 2008; Godwin, 2009). In addition to water quality issues, it is anticipate that Crown, municipal and private lands and roads, a section of Provincial Highway 622 and portions of the Ontario Power Generation Atikokan Generating Station site will undergo varying degrees of flooding. Consequently, Steep Rock is currently one of the top three hazardous environmental sites in Ontario (Laderoute, pers. comm., 2008). Figure 1. Map showing the locations of pit lakes at the former Steep Rock iron mine site (image from Google Earth). 127 �In 1988 the Steep Rock mine site was returned to the Province of Ontario where the Ministry of Natural Resources (MNR) has managed the risks to public safety and the environment in accordance with the 1991 MNR Steep Rock Management Plan. However, this plan did not anticipate the water quality issues, which were identified by researchers from Lakehead University, nor did it anticipate a recent proposal to use pit lakes for tailings disposal. Remediation work conducted on the site has been sporadic. However, in 2000 Ministry of Northern Development Mines and Forests (MNDMF) spent $650,000 to remove industrial buildings under the Province‘s Abandoned Mine Hazard Abatement Program. Furthermore, all PCB materials were removed from the site by 2008 for treatment at approved waste disposal facilities, as well as the removal of some of the buried metal waste. Owing to the environmental concerns of the Steep Rock site, in 2008 the MNR received approval to commence the development of a long-term management strategy that will address issues related to surface and ground water quality and hazardous lands. The Steep Rock Mine Rehabilitation Project, a multi-year effort to develop a sustainable long-term management plan for the site has been initiated. This project is governed by a Project Steering Committee with director-level participation from: Ministry of Natural Resources (MNR), Ministry of Northern Development Mines and Forests (MNDMF), Ministry of Energy and Infrastructure (MEI), Ministry of Municipal Affairs and Housing (MMAH), Ministry of the Environment (MOE), Ministry of Transportation (MTO), and Cabinet Office. As part of the Steep Rock Mine Rehabilitation Project, the potential for contaminant release and/or discharge of impaired water from the pit lakes will be examined and remedial strategies developed to ensure that the downstream waters of the Seine River and Rainy Lake remain protected. History Preceding mining, Steep Rock Lake, host of the Steep Rock Mine, was a large, 2050 ha, Mshaped lake located north of the town of Atikokan in northwestern Ontario (Fig. 2). Although iron was discovered as early as 1891, several engineering challenges owing to the nature of the ore body, being located beneath the lake, prevented its extraction. However, following the increasing demands for iron ore during World War II, the Canadian Government declared the development of the Steep Rock Iron Mine as a project ―essential to the war effort‖ under ―The War Measures Act‖. To mine the ore, it was necessary to divert the Seine River that flowed through Steep Rock Lake around the middle and eastern arms of the Steep Rock Lake (Fig. 3). This large river, draining a watershed of approximately 5000 square km, was diverted north around the mine through Finlayson Lake to the northern end of the West Arm of Steep Rock Lake that ultimately retained the flow of the river. This diversion required the construction of two dams that served to separate the West Arm from the rest of Steep Rock Lake. Following the construction of these two dams, work to expose the ore body began. This required draining of the middle and east arms of Steep Rock Lake and the removal of approximately 15 million m3 of lake-bottom silt and varved clay that were ultimately pumped into the West Arm. 128 �Figure 2. Steep Rock Lake prior to mining and diversions (courtesy of the Ministry of Natural Resources). In 1944, the first ore was mined from Errington Pit with subsequent ore bodies having been developed in the Hogarth and Roberts’ pits ca. 1950. During the removal the lake bottom sediment of the latter two ore bodies, a massive release of approximately 4 million m3 of colloidal silt and clay into the Seine River system and ultimately into Rainy Lake (located 145 km downstream) prompted a second diversion of the Seine River system. This diversion required the transformation of the West Arm of Steep Rock Lake into a closedsystem settling basin (Fig. 4) where the once 60 m deep lake now has an average depth of 3 m owing to the deposition of nearly 90 million m 3 of silt and clay sediment. In 1955, a second major dredging project was underway to access the ore body located along the east arm that was still under 30 m of water and 120 m of fine glacial silt and clay. Marmion Lake, now divided in half through the construction of several new dams, served as catchment of the approximately 120 million m 3 of dredged sediment from the dredging project. The newly accessible deposit became the Caland Pit, which started production in 1960. The Steep Rock ore was predominantly hematite, and at 63% iron, could be shipped directly to steel mills and loaded into blast furnaces following the reduction of the material into 6-inch chunks and smaller. A more highly processed, finely crushed ore rolled into marble-sized balls and fired to produce transportable pellets later replaced the previously used ―direct shipping‖ of the ore. No chemical processing was used on the Steep Rock mine site. 129 �Figure 3. Seine River flow following first Steep Rock diversion (1940‘s). Dash line shows the original flow pattern through Steep Rock Lake (courtesy Ministry of Natural Resources). Figure 4. Seine River flow following the second Steep Rock diversion (1950‘s). Dash line shows the flow pattern of the 1940 diversion (courtesy Ministry of Natural Resources). 130 �Figure 5. Location of open pits (1955-1979; courtesy Ministry of Natural Resources). Open pit mining continued until 1979 when both Steep Rock (operating the Errington, Roberts‘, and Hogarth Pits) and Caland reached the bottom of their conical pits and could no longer reach the deeper ores (Fig.5). Following the termination of mining, Steep Rock and Caland had removed 53 million tons and 35 million tons of ore, respectively. Following the closure of the Caland open pit, Caland Ore returned their leased lands to Steep Rock Iron mines. By 1985, Steep Rock Mines determined that no further development was expected, informing the Ministry of Natural Resources (MNR) of their intention to return their mining lands to the Province. At the time of surrender there were no specific regulations regarding mine closures and hence Steep Rock Iron mines were under no legal obligations regarding site remediation. However, Steep Rock Iron mines paid nearly a half million dollars to the MNR to defray any current and immediate future costs associated with mine site management as well as ownership and responsibility of two PCB sites (currently under Control Orders issued by the Ministry of the Environment). Over the years, obsolete mining equipment was either removed or buried with hundreds of thousands of dollars spent on removing structures and materials that the Province did not want. On April 1, 1988, Steep Rock Iron mines and the MNR signed the Surrender Agreement at which point the Province of Ontario regained ownership and management of the Steep Rock site. Following the surrender, the Steep Rock site was largely left alone, allowing the pits to slowly fill with water where maximum water depths currently exceed 200 m. Between 1988 and 2009, the Caland pit hosted the Snow Lake Fish Farm and today the site is largely utilized as an unofficial recreation facility for the residents of Atikokan. 131 �Geology The Steep Rock area is in northwestern Ontario within the Superior province, on the southern border of the Wabigoon Subprovince adjacent to the Quetico Subprovince (Kusky and Hudleston, 1999; Shklanka, 1972; Card, 1990; Stone et al., 1992). The rocks in the area are structurally repeatedly folded and faulted. Three major metamorphic events resulted in three major periods of deformation. These coincide with the major orogenic events known as Kenorian, Hudsonian, and Grenville (Shklanka, 1972). The geology of the Steep Rock area is comprised of the Marmion Gneiss Complex, the Steep Rock Group, the Dismal Ash Formation and the Witch Bay Formation (Kusky and Hudleston, 1999; Fig. 5). Figure 5. Geological cross-section of Errington Pit (after Kusky and Hudleston, 1999). The Marmion Complex is part of a regional belt of arc-type plutons and volcanics in the central Wabigoon Province (Kusky and Hudleston, 1999). It consists of several different units with ages between 3001 Ma and 2928 Ma (Stone et al., 1992). These units to consist of mafic tonalitic gneiss, a more leucocratic tonalite containing amphibolitized remnants of mafic rocks, and several units of felsic and intermediate tuffaceous rock, all intruded by granodiorite and gabbro dykes (Kusky and Hudleston 1999; Stone et al., 1992). Overlying the Marmion Gneiss Complex is the Steep Rock Group, consisting of the Wagita Formation, the Mosher Carbonate Formation and the Jolliffe Ore Zone. The Wagita Formation consists of metamorphosed siliciclastic rocks ranging from argillaceous rocks to conglomerate (Shklanka, 1972; Wilks and Nisbet, 1988; Stone et al., 1992; Kusky and Hudleston, 1999). It is 0 to 150 m thick and not well exposed on the Steep Rock property (Kusky and Hudleston, 1999). The Mosher Formation of the Steep Rock Group overlies the Wagita Formation and is up to 500 m in thickness. It was deposited at ca. 3.0 Ga on the rifted arc margin of the Marmion Complex (Kusky and Hudleston, 1999). The formation and consists of primarily of dolostone and 132 �limestone, which is often brecciated, and in some localities is characterized by geologically significant and well developed, giant stromatolite mounds (Wilks and Nisbet, 1988; Kusky and Hudleston, 1999). The Jolliffe Ore Zone is 100 to 400 m thick and overlies the Mosher Formation and is composed of (in ascending stratigraphic order): the Paint Rock Member, Geothite Member and Pyrite Member (Kusky and Hudleston, 1999; Jolliffe, 1966). The Manganiferous Paint Rock Member has large blocks of carbonate rock, while the brecciated Geothite Member resembles a banded iron formation (Tanton, 1926, 1941a, 1941b, 1946; Bartley, 1939; Smith, 1942; Wilks and Nisbet, 1988). The Dismal Ashrock Formation unconformably overlies Jolliffe Ore Zone and consists of mafic to ultramafic volcanic and pyroclastic rocks, which range in thickness from 100 to 400 m (Schaefer and Morton, 1991; Kusky and Hudleston, 1999). The Dismal Ashrock Formation occurs as a series blocks and lenses of komatiitic volcanic breccia, mafic pillow basalt, komatiitic volcaniclastic rock, carbonate, tonalitic gneiss, and goethite-hematite-rich rock surrounded by a matrix of fragmental komatiitic volcaniclastic rock (Kusky and Hudleston, 1999). Overlying the Dismal Ash Formation is the Witch Bay Formation, which consists of a complex assemblage of metamorphosed mafic volcanic and volcanoclastic rocks that are approximately 2932 Ma in age (Kusky and Hudleston, 1999). The Witch Bay Formation displays physical characteristics of island-arc deposits while being geochemically similar to mid-ocean ridge basalts (Kusky and Hudleston, 1999; Tomlinson et al., 1995). These rocks are correlative in terms of age and lithology to meta-volcanic rocks in the Finlayson belt (Kusky and Hudleston, 1999). Previous Environmental Studies Lakehead University undergraduate and graduate students have been studying the two pit lakes since 1998, and in cooperation with the Ministry of Natural Resources from 2002 to 2009. Work by Lakehead University has largely involved seasonal sampling for water quality assessment of Hogarth and Caland pit lakes, and it was through these studies that the water quality issues were first identified. McNaughton (2001) conducted a limnological study of the two pit lakes to determine the potential of developing an aquaculture facility on Hogarth similar to the one on Caland. Vancook (2005) used GIS mapping to predict the path and timing of overflow of Caland into Hogarth and subsequent outflow into the Seine River system. Through the integration of mineralogy, geochemistry and stable isotope geochemistry, MacDonald (2005) determined that the high sulphate levels originated largely from inflowing ground waters rather than contaminated surface waters. Cockerton (2007) explored the influence of water-rock interactions in the two pits and found that the main cause of the difference in water chemistry between the two pits is the apparent higher pyrite content of the Jolliffe ore zone below Hogarth. Goold (2008) determined that the toxicity of Hogarth is due largely to the elevated levels of dissolved sulphate and less so to heavy metals which are present in very low concentrations in both lakes. Perusse (2009) conducted the first groundwater survey, investigating water quality issues of two tailings impoundments. Godwin (2009) used experimental water-rock interaction experiments coupled with toxicity identification and evaluation studies in order to predict water chemistry and toxicity at the time of outflow. Current research continues with a complete hydrodynamic 133 �model being developed by L. Mikkelsen and a bioremediation investigation into the use of permeable reactive barriers to promote sulfate reduction being conducted by S. Shankie. Water Quality – Pit Lakes Caland exhibits classic meromictic conditions with a dichotomy in water quality versus depth forming a mixolimnion of oligotrophic water for a depth of 20 m and a sulphate-saline, anoxic monimolimnion for the remaining 180 m. Sulphate levels in the monimolimnion reached up to 450 mg/L. Freshwater inflow has created the thinning mixolimnion. Hogarth has uniform water quality throughout its 200 m depth except for a very shallow, recently forming, freshwater surface layer. Sulphate levels in Hogarth often exceed 1500 mg/L. The entire Hogarth water column is sulphate-saline and aerobic. McNaughton (2001) determined that water from Hogarth was acutely toxic and there were no apparent signs of aquatic life in the lake; however, a recent study (Goold, 2008) found that it now appears to be chronically toxic. Snails and minnows were observed in shallow littoral zones of the lake during the summer of 2006 (Godwin, 2009). In 2004, Hogarth merged with Roberts‘ pit at the south end and appears to have a very similar chemical profile to Hogarth. The concentration of dissolved ions in Caland is lower than that of Hogarth particularly with respect to major cations (Ca2+, Mg2+, Na2+, K+) and dissolved sulfate. All of these parameters increase with depth in each lake. Goold (2008) found that chronic toxicological affects produced by water from the Hogarth pit were likely caused by the elevated levels of dissolved ions especially sulfate (SO42-). As a consequence, Hogarth has higher concentrations of dissolved ions compared to Caland, and subsequently higher conductivity, hardness, and TDS (total dissolved solids) levels. Likewise, these parameters all increase with depth in both pit lakes. The pH of the two lakes is circum neutral to slightly basic. The pH in Caland is on average 8.05 at 2 m and decreases with depth to an average of 7.50 near bottom, whereas Hogarth has an average pH of 7.80 at 2 m and decreases with depth to 7.10 near bottom. The alkalinity is greater is Caland (130-173 mg/L CaCO3) than in Hogarth (94-126 mg/L CaCO3) and values increase with depth in both lakes. As a consequence of the Snow Lake Fish Farm on Caland, levels of certain organic constituents are greater throughout the water column in Caland as compared to Hogarth (Godwin, 2009). Caland water has higher dissolved organic carbon (DOC) concentrations than Hogarth (Caland: 2.9-3.2 mg/L; Hogarth: 1.0-1.3mg/L). Curiously the maximum DOC contents in Caland occur at 18 m whereas, at the same depth, Hogarth displays the minimum value. However, the average total nitrogen (Caland: 0.42-0.50 mg/L; Hogarth 0.40-0.75 mg/L) and total phosphorus (Caland: 0.032-0.037 mg/L; Hogarth: 0.029-0.037 mg/L) values from 2005-2008 do not differ greatly between the two pit lakes. The isotopic composition of dissolved sulphate is depleted relative to pyrite from the submerged ore zone and surface waste rocks ( 34S = -1.5 to 10‰) and other pit wall rocks ( 34S = -1.6 to 4.9‰), consistent with oxidation of ore zone pyrite and subsequent fractionation due to precipitation of saturated sulphate minerals from the water column. The oxygen isotopic composition of dissolved sulphate indicates that Hogarth sulphate is produced by anaerobic oxidation of pyrite, whereas the oxygen isotope composition of Caland sulphate is consistent with aerobic oxidation of pyrite (Fig. 6a). 134 �The near neutral pH of both pit lakes is the result of interactions with carbonate wall rocks. The carbon isotopic composition of DIC from Hogarth is similar to that isotopic composition of the carbonate wall rocks ( 13C = -4.0 to 0.6‰; Fig. 6b). However, DIC in Caland pit water is more depleted and suggests either isotopic exchange with isotopically depleted DOC generated by fish farming activities or atmosphere exchange. Figure 6. a) The 34S and d18O isotopic composition of dissolved sulphate, and b) DOC composition of Hogarth and Caland pit lakes (from Conly et al., 2008). 13 CDIC and Water Quality – Ground Water Figure 7 shows the average composition of major anions and cations, Fe, Mn, total metals and pH for all ground water monitors and selected surface waters. Shallow ground waters contained within pyrite-quartz and goethite-hematite-quartz tailings/waste rock are sulphate-dominant, whereas the one regional ground water is chlorine-dominant. Interactions with pyritic waste rock produces ground waters with elevated sulfate (>3000 mg/L), low pH (<6) and high Fe (>95 mg/L). Fe-oxide + quartz tailings produce pH-neutral ground waters with moderate to high concentrations of sulphate (200-1000 mg/L) and are Fe-depleted (<1 mg/L). The difference in pH, sulphate and Fe levels reflects the following: i) the relatively insoluble behaviour of goethite and hematite in pH-neutral waters; ii) possible occurrence of soluble Fe-sulphates; and, iii) the relative abundance of pyrite and the rate at which it is oxidized. Surface waters parallel the observed trends for ground waters. However, for both sites anion and cation abundances are typically lower, and likely reflect dilution from surface run-off and precipitation. 135 �Figure 7. Schoeller diagrams, with pH (inset), showing the average compositions for ground water and surface water from sites 1 (A) and 2 (B). Abbreviations: Alk – alkalinity; AGW – Atikokan ground water (from Conly et al., 2009). 136 �Figure 8 shows the variation in carbon and sulphur isotopes for Steep Rock ground water and surface water. 34S ratios for ground water from both sites are within the reported range for the two pit lakes, and indicate that the dissolved sulphate is produced from the oxidation of pyrite. 13 C, and is within the range reported for the two pit lakes, and reflects exchange with atmospheric CO 2 and carbonate 13 dissolution. C of ground water from Fe-oxide tailings is both more depleted and variable. Such values are interpreted to reflect isotopic exchange with CO2 released from buried organic-rich (bog) soils. Ground waters that are both highly depleted 13C and enriched 34 S reflect methanogenesis and bacterial sulfate reduction, arising from interaction with organic-rich (bog) soils from below. Figure 8. Variation in 13C and 34S of ground water and surface water (from Conly et al., 2009). Data for other waters (excluding regional water; unpublished) from Conly et al. (2008). 137 �Field Trip Stops Stop Number 1 Location 2 Caland Lookout 3 Red, Green and Moore‘s Ponds 4 Errington Pit 5 6 B-2 Shaft Building Roberts Pit 7 ―Party Point‖ Hardy Dam Details Largest earthen dam owned by the Province of Ontario Originally constructed in 1944 Holds back Rawn Reservoir, former flow into Steep Rock Lake directed into Atikokan River through a series of tunnels to the southwest Highway 622 will be under approximately 5 m water. Triangulation Island Snow Lake Fish Farms (now defunct) Caland Flats (flooding) Water quality, sulphate, TDS, meromictic conditions Rock slope stability AMD causing low pH pH buffered by carbonate host rocks elevated sulphate, high TDS pyritic waste rock piles slope stability First open pit mined Pit floor approximately 180m below current water level ―Green Pond shows evolution of water quality ―Yellow Creek‖ Fe pptn on creek bed PCB Storage site Forced entry, public safety Pit lake 180 m deep Water quality Slope stability Narrows Dam, inundation scenarios Slope stability Water quality, sources of impacts 138 UTM Coordinates Zone 15, nad’83 606391E 5403980N 603135E 5408000N 600055E 5405046N 600133E 5405282N 599167E 5405329N 599206E 5405782N 600118E 5407376N �Stop Number 8 Location 9 A-2 Shaft Remnants 10 B-1 Shaft/Vault 11 Narrows Dam 12 West Arm Dam SR Pelletizing Plant Details Soil impacts (As, metals) Relict structures Closure activities Underground openings Public safety Unauthorized salvage operations in underground workings Unknown infrastructure Overlook of pit, ―Road to Nowhere‖ Water Quality Slope stability Original diversion Flooding/inundation Pyrite rich material Mobilisation of sediments in West Arm Repairs 2007 Changes to Steep Rock Lake Settling Basin Ongoing risk 139 UTM Coordinates Zone 15, nad’83 600365E 5408468N 599630E 5408685N 598614E 5408105N 598931E 5407490N 597854E 5402450N �Figure 9. LIDAR ortho map showing field trip stops and other points of interest. 140 �References Bartley, M.W., 1939. Iron deposits of the Steeprock Lake area. Ontario Department of Mines, Annual Report, v. 48, p 419-421. Card, K.D., 1990. A review of the Superior Province of the Canadian Shield. Geoscience Canada, v. 13, p. 5-13. Cocketon, S., 2007. An experimental water-rock interaction study into the origin of the chemical characteristics of the Hogarth and Caland pit lakes, Steep Rock Lake, Atikokan. H.B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 46 p. Conly, A.G., Lee, P.F., Goold, A. and Godwin, A., 2008. Geochemistry and stable isotope composition of Hogarth and Caland Pit lakes, Steep Rock iron mine, northwestern Ontario, Canada. In Rapantove N. and Hrkal, Z. (eds), Mine Water and the Environment Proceedings v. 10, p. 555-558. Godwin, A., 2009. Geochemical and toxicological investigation of the Hogarth and Caland Pit Lakes, former Steep Rock iron mine site. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 105 p. Goold, A.R., 2008. Water quality and toxicity investigations of two pit lakes at the former Steep Rock iron mines, near Atikokan, Ontario. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 90 p. Hayes, J.M., Kaplan, I.R., and Wedeking, K.W., 1983. Precambrian organic geochemistry, preservation of the record. In Earth‘s earliest biosphere, its origin and evolution. Edited by J.W. Schopf. Princeton University Press, Princeton, N.J. p. 93-132. Joliffe, A.W., 1966. Stratigraphy of the Steeprock Group, Steep Rock Lake, Ontario. GAC Special Paper No. 3, p. 75-96. Kusky, T.M. and Hudleston, P.J. 1999. Growth and demise of an Archean carbonate platform, Steep Rock Lake, Ontario, Canada. Canadian Journal of Earth Science. v. 36, p. 565-584. MacDonald, J.C., 2005. An integrated mineralogy, geochemistry, and stable isotope investigation into potential sulphate sources of the Hogarth Pit Lake, Steep Rock Iron Mine, Atikokan. B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 52 p. McNaughton, K.A., 2001. The limnology of two proximal pit lakes after twenty years of intense flooding. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 78 p. Perusse, C., 2009. Hydrogeological and geochemical assessment of two tailings impoundments at the former Steep Rock iron mine Atikokan, Ontario. H.B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 39 p. Schaefer, S.J., and Morton, P., 1991. Two komatiitic pyroclastic units, Superior Province, northwestern Ontario: their geology, petrology, and correlation. Canadian Journal of Earth Science, v. 28, p. 1455-1470. Shklanka, R., 1972. Geology of the Steep Rock Lake area, District of Rainy River. Part 1. Geology of Steep Rock Lake. Ontario Department of Mines and Northern Affairs, Geological Report 93, p. 1-80. 141 �Smith, F.G., 1942. Notes on the iron ores of Steeprock Lake, Ontario. University of Toronto Studies, Contributions to Canadian Mineralogy, v. 47, p. 71-75. Stone, D., Kamineni, D.C., and Jackson, M.C., 1992. Precambrian geology of the Atikokan area, northwestern Ontario. Geological Survey of Canada, v. 405, 106 p. Tanton, T.L., 1926. Mineral deposits of Steeprock Lake map-area, Canada. In Summary Report 1925, Part C. Geological Survey of Canada, p. 1–11. Tanton, T.L., 1941a. Origin of the hematite deposits at Steeprock Lake, Ontario. Transactions of the Royal Society of Canada, Section 4: Geological Sciences, v. 35, p. 131–141. Tanton, T.L., 1941b. Areas in the vicinity of Steeprock Lake, Rainy River District, Ontario. Geological Survey of Canada, Paper, p. 41-13. Tanton, T.L. 1946. The iron ore at Steeprock Lake. Transactions of the Royal Society of Canada, Section 4: Geological Sciences, v. 40, p. 103-110. Tomlinson, K.Y., Hall, R.P., Hughes, D.J., and Thurston, P.C., 1995. The Archean Steeprock greenstone belt, NW Ontario: geochemistry of platformal komatiitic volcanics and overlying tholeiitic lavas — evidence for secular and tectonic separation. Precambrian 95, Abstracts, p. 273. Vancook, M., 2005. The limnology and remediation of two proximal pit lakes in Northwestern Ontario. M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 81 p. Wilks, M.E. and Nisbet, E.G., 1988. Stratigraphy of the Steep Rock Group, northwestern Ontario: a major Archaean unconformity and Archaean stromatolites. Canadian Journal of Earth Sciences, v. 25, p. 370-391. 142 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2010 Description An account of the resource Institute on Lake Superior Geology. International Falls, Minnesota. May 19-22, 2010. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2010 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/73d8517270988c319ee40c8061c3d887.pdf 5ed6d41d45acb966d9a218591d343814 PDF Text Text 57TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ASHLAND, WISCONSIN MAY 18-21, 2011 PROCEEDINGS VOLUME 57 PART 1 – PROGRAMS AND ABSTRACTS ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 57TH ANNUAL MEETING MAY 18-21, 2010 ASHLAND, WISCONSIN HOSTED BY: NORTHLAND COLLEGE TOM FITZ Chair Proceedings Volume 57 Part 1 – Program and Abstracts Edited by: Tom Fitz, Allison Mills, Kristi Wilson, Cassandra Bodette, and Drew Cramer Cover Photo: Intrusive breccia in the Mellen Intrusive Complex, Mellen, WI �57TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 57: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1: IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH - TYPE INTRUSION OF THE DULUTH COMPLEX TRIP 2: MIDCONTINENT MICROCOSM TRIP 3: GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS TRIP 4: GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE TRIP 5: BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM TRIP 6: GEOLOGY OF COPPER FALLS STATE PARK TRIP 7: GEOLOGY OF THE MONTREAL RIVER MONOCLINE TRIP 8: THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA TRIP 9: GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX Reference to material in Part 1 should follow the example below: LaMaskin, T.A., 2011, Testing models of late Paleoproterozoic Penokean orogenesis in the Great Lakes Region, U.S.A. using sedimentary provenance: planning the investigation [abstract]: Institute on Lake Superior Geology Proceedings, 57th Annual Meeting, Ashland, WI, v. 57, part 1, p. 53-54. Published by the 57th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ILSG 2011 ii Program and Abstracts �TABLE OF CONTENTS PROCEEDINGS VOLUME 57 PART 1— PROGRAM AND ABSTRACTS Previous Institutes on Lake Superior Geology, 1955-2010........................... iv Sam Goldich and the Goldich Medal............................................................. vi Past Goldich Medalists and the 2010 Goldich Medal Recipient ................. viii Goldich Medal Committee........................................................................... viii Citation for 2010 Goldich Medal Recipient .................................................. ix ILSG Student Research Fund ........................................................................ xi Student Paper Awards ................................................................................... xii Eisenbrey Student Travel Awards ............................................................... xiii Report of the Chairs of the 56th Annual Meeting ......................................... xv 2011 Board of Directors ............................................................................. xvii 2011 Session Chairs .................................................................................... xvii 2011 Student Paper Awards Committee ..................................................... xvii 2011 Banquet Speaker ............................................................................... xviii Program ........................................................................................................ xix Schedule of Events ....................................................................................... xx Schedule of Talks and Posters .................................................................... xxi Abstracts .................................................................................................. xxviii Author Index ................................................................................................. 97 ILSG 2011 iii Program and Abstracts �PREVIOUS INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-2010 ILSG YEAR PLACE CHAIRS 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz and C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims and R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley and E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray ILSG 2011 iv Program and Abstracts �28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. La Berge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey and R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage and W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller, Jr. and M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst and R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin and P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey, Jr. and B.A. Brown 48 2002 Kenora, Ontario P. Hinz and R.C. Beard 49 2003 Iron Mountain, Michigan L.G. Woodruff and W.F. Cannon 50 2004 Duluth, Minnesota S.A. Hauck and M. Severson 51 2005 Nipigon, Ontario P. Hollings and M.C. Smyk 52 2006 Sault Ste. Marie, Ontario R.P. Sage and A.C. Wilson 53 2007 Lutsen, Minnesota L.G. Woodruff and J.D. Miller, Jr. 54 2008 Marquette, Michigan T.J. Bornhorst and J.S. Klasner 55 2009 Ely, Minnesota J.D. Miller, Jr., G.J. Hudak, D.M. Peterson 56 2010 International Falls, Minnesota P. Hollings, P. Hinz, M. Smyk, M. Jirsa, and T. Boerboom ILSG 2011 v Program and Abstracts �SAM GOLDICH AND THE GOLDICH MEDAL Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam‘s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970‘s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for ―outstanding contributions to the geology of the Lake Superior region.‖ Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 ILSG 2011 vi Program and Abstracts �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL ILSG 2011 vii Program and Abstracts �PAST GOLDICH MEDALISTS 1979 1980 1981 1982 1983 1984 Samuel S. Goldich not awarded Carl E. Dutton, Jr. Ralph W. Marsden Burton Boyum Richard W. Ojakangas 1995 Gene La Berge 1996 David L. Southwick 1997 Ronald P. Sage 1998 Zell Peterman 1999 Tsu-Ming Han 2000 John C. Green 1985 1986 1987 1988 1989 1990 1991 Paul K. Sims G.B. Morey Henry H. Halls Walter S. White Jorma Kalliokoski Kenneth C. Card William Hinze 2001 2002 2003 2004 2005 2006 2007 1992 William F. Cannon 1993 Donald W. Davis 1994 Cedric Iverson John S. Klasner Ernest K. Lehmann Klaus J. Schulz Paul Weiblen Mark Smyk Michael G. Mudrey Joseph Mancuso 2008 Theodore J. Bornhorst 2009 L. Gordon Medaris, Jr. 2010 William D. Addison and Gregory R. Brumpton 2011 GOLDICH MEDAL RECIPIENT Dean M. Rossell Kennecott Exploration Company Salt Lake City, Utah GOLDICH MEDAL COMMITTEE Serving for the meeting years shown in parentheses Al MacTavish, Industry member - Chair (2008-2011) Mary Louise Hill, Academic member (2009-2012) Laurel Woodruff, Government member (2010-2013) ILSG 2011 viii Program and Abstracts �CITATION FOR 2011 GOLDICH MEDAL RECIPIENTS Dean M. Rossell It is my privilege and great pleasure to introduce Dean Rossell as this year's Goldich Medal recipient. Dean has been an ILSG contributor and stalwart attendee of its meetings and field trips since his student days many years ago. Dean grew up in Minneapolis, and graduated from the University of Minnesota-Duluth in 1979, studying under some of the early recipients of the Goldich Medal (Ralph Marsden, Dick Ojakangas, and John Green). Immediately after graduation he got a temporary job in Nevada, and that summer, by tracing boulders up dry washes, was the co-discoverer of what soon thereafter became the Trout Creek barite mine. But he promptly returned to the Lake Superior region, graduating from Michigan Technical University in 1983, studying under the aegis of another Goldich medalist, Ted Bornhorst. His M.Sc. thesis was entitled "Alteration of the Deer Lake Peridotite in the Vicinity of the Ropes Mine, Marquette County, Michigan." This grew out of his summer student work at that old gold mine, which Callahan Mining Corp. was putting back into production after nearly a century of abandonment. After graduating from MTU, he spent the next seven years as an exploration geologist for Resource Exploration, Inc. out of Marquette, working for Bill Bodwell. His wife Karen also worked in the Resource Exploration office in those days as a draftsperson. Dean himself never spent much time in the office, though. His work for clients during this period included: re-logging all drill core and re-interpreting the Keweenawan Western Syncline sediment-hosted chalcocite deposit, Michigan; mapping, sampling and drilling VMS base metal and shear zone-hosted gold targets in the Archean Ramsay-Lake Gogebic greenstone belt, Michigan; initiating and carrying out field exploration and drilling on numerous grassroots-developed gold targets in the Archean greenstones and felsic intrusives of the Virginia Horn, Minnesota; and drilling for platinum group elements near the Reserve Mining property in the Duluth Complex, Minnesota. In short, he came up the old fashioned way, doing the field work in numerous exploration programs, mapping, sampling and drilling rocks around the Lake Superior region. Beginning in 1991 Dean worked as a contract geologist for Kennecott Exploration Company out of Crystal Falls, Michigan, initially doing mapping and field sampling in the search for Proterozoic sedex base metal deposits. It was during this time that he alertly found a nickelcopper sulfide-bearing boulder in a logging roadcut that led to a complete change in direction of that program. A few months later, he was the first to discover nickel-copper mineralization in the BIC layered mafic intrusion, and he strongly advocated an investigation of the Yellow Dog peridotite. In 1995 Dean gained permanent status with Kennecott and carried on with his tireless and imaginative efforts to explore for base metals in Keweenawan Rift-related terranes on both sides of the border. His investigations eventually led to the discovery of the Eagle nickelcopper-PGE deposit at the Yellow Dog peridotite in 2002, the result of a nearly singlehanded effort on his part. This very high grade Michigan deposit has now received its permits for underground mining, and promises to be an important contributor to the economy of the Upper Peninsula. ILSG 2011 ix Program and Abstracts �In the early days of 21st Century exploration for nickel-copper in the Black Sturgeon district of Ontario, he acquired and tested one of the known ultramafic intrusions (Kitto), and tested others in elsewhere in Ontario that he is still not authorized to talk about. He carried his ideas into Minnesota, leading Kennecott to search for mineralization in buried ultramafic intrusions in the Animikie Basin. Again with the imagination and persistence that are his hallmarks, this work eventually led to discovery of the Tamarack deposit in Carlton County, Minnesota. Several years of drilling turned up only anomalous Ni-Cu values at Tamarack, but Dean's careful persistence eventually paid off with Kennecott's announcement in June 2008 of a long drill intercept of very high grade nickel-copper. This deposit is currently being intensively evaluated by Kennecott, and Tamarack has the promise of being a larger version of the Eagle deposit in Michigan. Dean and his family are now based in Salt Lake City, but he still doesn't spend much time in the office. That's not where the rocks are. He has been to nearly every part of the world where one can see the right rocks. In fact, his title in Kennecott's Project Generation Group is "Global Nickel Specialist", and he's now internationally regarded as an expert on exploration for ultramafic-hosted Ni-Cu-PGE deposits. Despite the competitive secrecy that surrounds such efforts, Dean has shared his expertise and knowledge with others in the geologic community when able. As is the case with most company geologists, Dean has had to be restrained in publishing his work, but has made the scientific aspects of it known at ILSG meetings: Duluth, 2004 (Eagle deposit presentation and abstract), Nipigon 2005 (Eagle deposit poster session). He also published on the Eagle deposit at the AIME-SEG meeting in Salt Lake City in 2005 (presentation and abstract), and prepared and led the field trip to the BIC intrusion at the Marquette ILSG in 2008. He has also contributed more indirectly, getting Kennecott to provide access and thesis research funds for graduate students studying the geology of the Eagle and Tamarack deposits, and he has been an important supporter of student involvement in the ILSG. Although his base is now in Utah, he still spends most of his time in the Lake Superior region, something not likely to change anytime soon. Dean Rossell is a standout in the ranks of exploration geologists: a real minefinder, and one who gives back to the community of his peers (especially the ILSG community), as well. Please join me in congratulating Dean as the 2011 recipient of the Goldich Medal of the Institute on Lake Superior Geology. Doug Duskin April, 2011 ILSG 2011 x Program and Abstracts �ILSG STUDENT RESEARCH FUND The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute‘s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. The applications are due to the ILSG Secretary by August 31st of each year Awards will be made by October 1st of each year. Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. Details of the application process can be found on the ILSG web site. The proposal will need to be signed by researcher‘s supervisor. In 2010 the ILSG Board of Governors awarded one $500 award from the Student Research Fund. The winner is listed below, along with a description of his research project. Scott Scheiner (Kent State University) – Source regions of sediments in the Animikie Basin Scott is working with Dan Holm (Kent State University) and Terry Boerboom (Minnesota Geological Survey) studying the geochemistry and clast mineralogy the greywacke and turbidite sequences of the Animikie basin to determine the source of the sediments. ILSG 2011 xi Program and Abstracts �STUDENT PAPER AWARDS Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: The contribution must be demonstrably the work of the student. The student must present the contribution in-person. The Student Paper and Poster Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper and Poster Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute. Student papers are noted on the Program. STUDENT PAPER AWARDS The student paper committee comprised of Graham Wilson (Chair, Turnstone Geological Services Ltd.) Jim Miller (University of Minnesota – Duluth) and Anthony Pace (Ontario Geological Survey) yet again had a difficult job of selecting among the 16 student oral and poster presentations. The following students were granted awards of $200 each, in recognition of their excellence of research and presentation: Andrew Ryan Petrographic and geochemical analysis of a Nipigon diabase sill Ryan, Andrew J. and Zieg, Michael J., Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA ILSG 2011 xii Program and Abstracts �Steve Hoaglund U-Pb zircon geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large, multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift Hoaglund, S.A.a*, Miller, J.D.a, Crowley, J.L.b, and Schmitz, M.D.b a Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN, b Department of Geosciences, Boise State University, Boise, ID Victoria Stinson Structural control at Hammond Reef gold deposit north of Atikokan, Ontario Stinson, Victoria R. and Hill, Mary Louise, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario EISENBREY STUDENT TRAVEL AWARDS The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name "Eisenbrey" was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. "Ned" Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The annual Chair in consultation with the Secretary-Treasurer determines the number of awards and value. Recipients will be announced at the annual banquet. The student travel award application is available on the ILSG website. The following general criteria will be considered by the annual Chair, who is responsible for the selection: The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. Students who are the senior author on either an oral or poster paper will be given favored consideration. It is desirable for two or more students to jointly request travel assistance. In general, priority will be given to those in the Institute region who are farthest away from the meeting location. Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. ILSG 2011 xiii Program and Abstracts �The following 28 students were awarded travel grants to help defray the cost of meeting attendance. Award amounts varied by travel arrangement and distance from $75-$300. We think this is a record number of annual awards, thanks largely to generous contributions from corporate and individual sponsors. Robert Cundari—Lakehead University, Thunder Bay, ON Adam Fage—Lakehead University, Thunder Bay, ON Maura Kolb—Lakehead University, Thunder Bay, ON Seamus Magnus—Lakehead University, Thunder Bay, ON Sean O’Hare—Lakehead University, Thunder Bay, ON Raya Puchalski—Lakehead University, Thunder Bay, ON Robert Scott—Lakehead University, Thunder Bay, ON Steve Siemieniuk—Lakehead University, Thunder Bay, ON Victoria Stinson*—Lakehead University, Thunder Bay, ON James Blount—Lawrence University, Appleton, WI Rachel Carver—Lawrence University, Appleton, WI Suzanne Craddock—Lawrence University, Appleton, WI Sarah Ehlinger—Lawrence University, Appleton, WI Meaghan Gallagher—Lawrence University, Appleton, WI Megan Luedtke—Lawrence University, Appleton, WI Sarah Shay—Lawrence University, Appleton, WI Amanda Van Lankvelt*—Lawrence University, Appleton, WI Levi Markwood—Slippery Rock University, PA Andrew Ryan*—Slippery Rock University, PA Dan Cervin—University of Minnesota, Duluth, MN Avery Cota—University of Minnesota, Duluth, MN Dan Foley—University of Minnesota, Duluth, MN Shelby Frost**—University of Minnesota, Duluth, MN Steve Hoaglund*—University of Minnesota, Duluth, MN Cabin Ross—University of Minnesota, Duluth, MN Stephanie Theriault—University of Minnesota, Duluth, MN Michael Totenhagen—University of Minnesota, Duluth, MN Natalie Pietrzak—University of Western Ontario, London, ON *Also recipient of 2010 STUDENT PAPER AWARD **Also recipient of 2009 STUDENT RESEARCH GRANT ILSG 2011 xiv Program and Abstracts �REPORT OF THE CHAIRS OF THE 56TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY INTERNATIONAL FALLS, MINNESOTA The 56th Annual Institute on Lake Superior Geology on May 19-22, 2010 was a cross-border meeting with an international team of co-chairs. The meeting in International Falls was chaired by Mark Jirsa, Terry Boerboom, Mark Smyk, Peter Hinz and Pete Hollings. Despite this the meeting was a great success with a total of 171 registrants, including 30 students. The two-day technical session held at the Holiday Inn on Thursday and Friday included 23 talks and 17 posters presentations, including 11 oral and 5 poster presentations by students. This year‘s Goldich Medal recipients were Bill Addison and Greg Brumpton whose tireless work in the Thunder Bay area led to the first recognition of the ejecta layer from the Sudbury impact. Bill and Greg were presented their medals at the annual banquet by another Goldich Medalist, Bill Cannon from the USGS. Fittingly, the evening banquet talk was presented by Dr. Bevan M. French of the Smithsonian Institution who discussed his work on meteor impacts. The meeting offered eight field trips that highlighted various aspects of the geology and ore deposits of two countries. Pre-meeting field trips included Mineral deposits of the Rainy River area led by Wally Rayner and C.J. Baker (Rainy River Resources), and Craig Ravnaas (Ontario Geological Survey), a one-day trip on Geology of the Archean succession at Atikokan led by Phil Fralick (Lakehead University) and Denver Stone (OGS) and Structural geology along the Quetico fault led by Howard Poulsen (Consultant) and Dyanna Czeck (UW-Milwaukee). On Friday afternoon following the completion of the technical session, two field trips were offered. The first took participants on a boat trip through the Archean geology of Voyageurs National Park and Little America gold mine led by Chris Hemstad (Boreal Explorations) while Mark Jirsa (MGS) led a group to the Ash River neutrino detector laboratory and the Archean Vermilion Granitic Complex. Post-meeting trips included a Transect through the Quetico-Wabigoon subprovince boundary led by Mark Jirsa (Minnesota Geological Survey) and Chris Hemstad (Boreal Exploration), Mineral deposits of the Mine Centre – Rainy Lake area led by Peter Hinz (OGS) and last, but not least, Geology and environmental issues of the Steep Rock mine led by Andrew Conly (Lakehead University) and Rob Purdon (Ontario Ministry of Natural Resources). The Institute‘s Board of Directors met on May 20, 2010. The meeting was called to order by Chair Peter Hinz. The meeting was attended by George Hudak, Ted Bornhorst, Jim Miller, Mark A. Jirsa - Treasurer (2011), Pete Hollings, Terry Boerboom and Mark Smyk. Secretary Hollings took the minutes of the Board meeting that are as follows: 1. Accepted report of the Chairs for the 55th ILSG, Ely, Minnesota; as printed in the Proceeding Volume (Miller), and minutes of last Board meeting, May 7, 2009 (Hollings) 2. Received, discussed, and accepted 2009-2010 ILSG Financial Summary (Jirsa). ILSG 2011 xv Program and Abstracts �3. Received, discussed, and accepted 2009-2010 report of the Secretary (Hollings). 4. Approved Hollings to continue as Institute Secretary (this was later presented to the membership and approved) 5. Approved Peter Hinz as on-going ILSG Board member 6. Discussed application process for ILSG Student Research Awards. Hollings to revise the application to be a two-page form, not including references, figures or tables. Application dates to remain unchanged 7. Approved Ashland, Wisconsin at the site for the 57th annual ILSG meeting. The meeting will be hosted by Tom Fitz with help from Jim Miller, Laurel Woodruff and Bill Cannon. 8. Discussed and approved replacing Terry Boerboom as the ―member from government‖ on Goldich Committee (end of term 2010) with Laurel Woodruff 9. It was agreed that the Institute would adopt the following policy ―The Institute will not distribute email advertisements for ‗for-profit‘ organizations or individuals. The institute will post links for such organizations on the Institute web site and will include a logo and brief notice in the annual proceedings in return for a donation‖. Hollings to prepare a new page on the web site highlighting institute sponsors and donors. The co-chairs would like to thank all the sponsors for their support of the meeting, all those who helped us organize and run the meeting, the Backus Community Center who put together a magnificent banquet on very short notice using community volunteers, and to all presenters and participants without whom the meeting could not have happened. We never cease to be amazed by the patience, enthusiasm, and passion of the membership of the ILSG. Respectfully submitted, Pete Hollings, Mark Smyk, Peter Hinz, Terry Boerboom and Mark Jirsa Co-Chairs, 56th Institute on Lake Superior Geology ILSG 2011 xvi Program and Abstracts �2011 BOARD OF DIRECTORS Board appointment continues through the close of the meeting year indicated in parentheses, or until a successor is selected. Tom Fitz, Chair (2011-2014) Northland College Ashland, Wisconsin Peter Hinz, (2010-2013) Ministry of Northern Development, Mines and Forestry Thunder Bay, Ontario George Hudak, (2009-2012) NRRI, Duluth Ted Bornhorst, (2008-2011) Michigan Technological University Houghton, Michigan Mark A. Jirsa, Treasurer (2008-2011) Minnesota Geological Survey St. Paul, Minnesota Peter Hollings, Secretary (2010-2013) Lakehead University Thunder Bay, Ontario 2011 SESSION CHAIRS Marcia Bjørnerud, Lawrence University Bill Cannon, U.S. Geological Survey Todd LaMaskin, Wisconsin Geological Survey Mark Jirsa, Minnesota Geological Survey George Hudak, Natural Resources Research Institute Jim Miller, University of Minnesota – Duluth Peter Hollings, Lakehead University Mark Smyk, Ontario Geological Survey 2011 STUDENT PAPER AWARDS COMMITTEE Elizabeth Gordon, University of Wisconsin – Parkside (Chair) Laurel Woodruff, U.S. Geological Survey James Shannon, MMG – Minerals and Metals Group ILSG 2011 xvii Program and Abstracts �Thursday Evening Awards Banquet with Keynote Address by: Dr. Huifang Xu (University of Wisconsin – Madison) Banded Iron Formations: Unique Products from the Interactions between Komatiites and Seawater of the Early Earth ILSG 2011 xviii Program and Abstracts �PROGRAM ILSG 2011 xix Program and Abstracts �SCHEDULE OF EVENTS TUESDAY, MAY 17 4:00 p.m. – 9:00 p.m.: Registration 4:00 p.m. – 9:00 p.m.: Cash bar is open WEDNESDAY, MAY 18 (ALL TRIPS DEPART AND RETURN TO THE AMERICINN) 7:30 a.m. – 6:00 p.m. Field Trip 1 – Duluth Layered Series trip (Rendezvous in Duluth at 9:00) 8:00 a.m. – 6:00 p.m. Field Trip 2 – Midcontinent Microcosm Field Trip 3 – Bayfield Peninsula 4:00 p.m. – 10:00 p.m.: Registration continues 7:00 p.m. – 10:00 p.m.: Poster Session, Ice-breaker social at AmericInn THURSDAY, MAY 19 8:00 a.m. – 12:00 p.m.: 8:10 a.m. – 11:40 a.m.: 11:40 a.m. – 1:15 p.m.: 1:15 p.m. – 4:40 p.m.: 6:00 p.m. – 7:00 p.m.: 7:00 p.m. – 9:30 p.m.: Registration Continues Technical Session I Lunch break. Lunch on your own. Technical Session II Social mixer and cash bar Banquet, awards ceremony, and keynote address FRIDAY, MAY 20 (ALL TRIPS DEPART AND RETURN TO THE AMERICINN) 8:00 a.m. – 11:40 a.m.: Technical Session III 11:40 a.m. – 1:15 p.m.: Lunch break. Lunch on your own. 1:15 p.m. – 2:40 p.m.: Technical Session IV 3:00 p.m. – 6:00 p.m.: Field Trip 4 – Ashland Lakefront Site Field Trip 5 – Bad River Watershed Culverts Field Trip 6 – Copper Falls Hike 6:30 p.m. – 9:00 p.m.: Barbecue at Northland College. Shuttle vans to/from AmericInn provided. SATURDAY, MAY 21 (ALL TRIPS DEPART AND RETURN TO THE AMERICINN) 8:00 a.m. – 5:00 p.m.: Field trip 7 – Montreal Monocline Field Trip 8 – Unconformity at Denham, MN Field Trip 9 – Mellen Complex End of meeting. ILSG 2011 xx Program and Abstracts �SCHEDULE OF TALKS AND POSTER SESSIONS THURSDAY, MAY 19 Technical Session I: Regional geology and geophysics, Archean stratigraphy and structure. Session Co-chairs: Marcia Bjørnerud and Bill Cannon 8:10 Introductory comments 8:20 Bruce A. Brown and Peter Schoephoester Preserving Wisconsin’s mining history: Building a GIS-based mined-lands inventory as a resource for planning, environmental management, future mineral exploration, and historical research 8:40 Val W. Chandler and Richard S. Lively Application of the horizontal-to-vertical spectral ratio (H/V) passive seismic method in Minnesota 9:00 Mark A. Jirsa, Terrence J. Boerboom, Val W. Chandler, John H. Mossler, Anthony C. Runkel, and Dale R. Setterholm Highlights of the new Bedrock Geologic Map of Minnesota 9:20 John M. Esch Bedrock topography, glacial drift thickness and bedrock outcrop maps, Upper Peninsula of Michigan 9:40 Coffee Break and Poster Session 10:00 H. Paul Gilbert Stratigraphic investigation of the Neoarchean Bird River Belt, Southeast Manitoba, Canada 10:20 Paul Geller*, Phil Fralick, Mary Louise Hill, and Patricia Gillies A review of the Western Superior Lithoprobe Line 3 10:40 Brittney Swoffer* and Mary Louise Hill A deformation history of the Ivanhoe Lake Fault 11:00 Robert J. Scott*, Maura J. Kolb, and Mary Louise Hill Comparison of microscale and regional scale structural control on gold mineralization in the North Caribou greenstone belt, Superior Province, northwestern Ontario 11:20 Victoria Stinson* and Mary Louise Hill Microstructural control on gold mineralization in greenschist to amphibolite facies metamorphism, Greenstone, Ontario ILSG 2011 xxi Program and Abstracts �11:40 a.m. – 1:15 p.m.: Lunch break. Lunch on your own. Technical Session II: Paleoproterozoic geology and mineral deposits Session Co-chairs: Todd LaMaskin and Mark Jirsa 1:15 Introductory Comments 1:20 Amanda Van Lankvelt*, David Schneider , Keiko Hattori, and John Biczok Neoarchean magmatism in the NW Superior Craton: Granitoids of the North Caribou Terrane 1:40 Michael A. DeVasto*, Dyanna M. Czeck, and Prajukti Bhattacharyya Quantifying deformation fabrics and chemistry across small granitic ductile shear zones from Mountain, Wisconsin 2:00 Thomas W. Buchholz, Al Falster, and William B. Simmons Compositional trends in chevkinite group minerals from the Wausau Complex, Marathon County, WI. 2:20 Todd A. LaMaskin Testing models of late Paleoproterozoic Penokean orogenesis in the Great Lakes Region, U.S.A. using sedimentary provenance: planning the investigation 2:40 Coffee Break and Poster Session 3:00 Michael R. Easton, and Larry M. Heaman Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization 3:20 Phillip Larson, John Swenson, and Marsha Patelke The Biwabik Iron Formation: geochemical and textural evidence for deposition of iron-formation in a Paleoproterozoic epeiric sea 3:40 Steph Theriault*, Jim Miller, Mike Berndt, and Ed Ripley The mineralogy, spatial distribution, and isotope geochemistry of sulfide minerals in the Biwabik Iron Formation 4:00 Michael Totenhagen*, Penny Morton, and Phil Larson Characterization of gangue minerals in lower cherty ores of the Biwabik Iron Formation at United Taconite LLC 4:20 Thomas Waggoner, Doug Duskin, Musa Karakus, and John Gartner Pyroclastic Magnetite Bombs in the Hemlock Formation, Iron County, Michigan 6:00 – 7:00 ILSG 2011 Poster Session during Social Mixer xxii Program and Abstracts �FRIDAY, MAY 20 Technical Session III: Paleoproterozoic to Mesoproterozoic geology and mineral deposits. Session Co-chairs: George Hudak and Jim Miller 8:10 Introductory comments 8:20 Breanne Beh* and Phil Fralick Depositional Processes Operating on the Paleoproterozoic Gowganda Ice Margin: an example from the Marquette, Michigan area 8:40 Philip Fralick and Kamil Zaniewski Sedimentology of a wet, pre-vegetation floodplain assemblage: the Mesoproterozoic Outan Island Formation, Sibley Group, Ontario, Canada 9:00 Carl Christian*, Peter Hollings, and Mark Smyk Chemistry and petrology of Midcontinent Rift-related intrusive rocks of the Sibley Peninsula, Ontario 9:20 Robert Cundari*, Peter Hollings, and Mark Smyk Compilation and reevaluation of the geochemistry of Midcontinent Rift-related intrusive rocks around Thunder Bay, Ontario 9:40 Coffee Break and Poster Session 10:00 Mark Smyk, Peter Hollings, and Robert Cundari The Pillar Lake volcanics: new insights into an enigmatic Mesoproterozoic volcanic suite near Armstrong, Ontario 10:20 Dayton N. Ryan*, James D. Miller, and Jeffery D. Vervoort The geochemical evolution of the Sonju Lake intrusion: an assimilation-fractional crystallization model 10:40 Dan Cervin*, Penny Morton, Jim Miller, and Richard Patelke Characterization of precious metal occurrences in the NorthMet deposit of the Partridge River Intrusion, Duluth Complex, Minnesota, USA 11:00 Daniel J. Foley*, and James D. Miller Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan 11:20 Erik R. Tharalson* and Thomas Monecke Geology and mineralization of the Serpentine Cu-Ni deposit, Duluth Complex Minnesota 11:40 – 1:15: Lunch break. Lunch on your own. ILSG 2011 xxiii Program and Abstracts �Technical Session IV: Midcontinent Rift mineral deposits Session Co-chairs: Peter Hollings and Mark Smyk 1:15 Introductory comments 1:20 Brian D. Goldner and James D. Miller Petrology of the Ni-Cu-PGE-mineralized Tamarack Intrusion, Aitkin and Carlton Counties, Minnesota 1:40 Allan D. MacTavish, Geoffrey J. Heggie, V. Roland Goodgame, Justin R. Johnson, Anthony E. Beswick, William E. Stone, and Keith P. Watkins Platinum-Palladium-Copper-Nickel Mineralization at the Thunder Bay North Deposit, Ontario 2:00 Dean Peterson, Phil Larson, Gabriel Sweet, and Jack Gibbons The Mineral Exploration Trifecta 2:20 Presentation of student awards * Indicates eligibility for Student Paper Award. If presenter is not the first author, then the name of the presenter is underlined. POSTER PRESENTATIONS Regional Geology and Geophysics 1) Val W. Chandler, and Richard S. Lively Upgrade of the gravity and rock properties databases at the Minnesota Geological Survey 2) Mark A. Jirsa, Terrence J. Boerboom, and Val W. Chandler Bedrock Geologic Map of Minnesota—Precambrian Geology Archean and Paleoproterozoic Geology 3) Jakob Wartman*, Ron Morton, George Hudak, and Cory Hercun Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, Northwestern Ontario 4) Ryan Rague and Steven Losh Fluid flow events in the Biwabik Iron Formation, Minnesota ILSG 2011 xxiv Program and Abstracts �5) Brittany Saylor*, Jonathan Stencil, Michael DeVasto, and Prajukti Bhattacharyya Whole rock geochemical analyses of sheared granitic rocks from Mountain, Wisconsin 6) Jolene T. Traut* and Dyanna M. Czeck Using cleavage refraction and microstructural evidence to determine the relative rheology of naturally deformed quartzites and phyllites from Baraboo, Wisconsin Midcontinent Rift Rocks 7) Terrence J. Boerboom and John C. Green Bedrock geologic maps of the Grand Marais and Kadunce River Quadrangles, North Shore of Minnesota 8) Eric J. Carlson, Terrence J. Boerboom, Corey J. Holton, Kyle W. Kubitza, Lucy Mulvey, and Eric Scheurer Mesoproterozoic bedrock in the Kadunce River Quadrangle, NE Minnesota— Precambrian Research Center capstone 9) Katherine Cummings* and Marcia Bjørnerud Possible eukaryotic macrofossils in the 1.1 Ga Copper Harbor Formation, Michigan 10) Evgeniy V. Kulakov, A.V. Smirnov, and J.F. Diehl Paleomagnetism and Paleointensity as recorded by 1.08 GA Lake Shore Traps (Keweenaw Peninsula, Upper Michigan) 11) Elisa J. Piispa, Aleksey V. Smirnov, and Lauri J. Pesonen Paleomagnetism of Midcontinent Rift rocks from the northern shore of Lake Superior (Ontario, Canada): Preliminary results Midcontinent Rift Mineral Deposits 12) Laetitia Amisse* and Sarah J. Barnes Role of Te-As-Bi-Sb (TABS) in the distribution of PGE and formation of PGM: Application to the Duluth Complex 13) Matthias Queffurus* and Sarah J. Barnes The use of S/Se ratios in magmatic Ni-Cu-PGE sulfide deposits and its implications for exploration: Example of the Duluth Complex, Minnesota, USA 14) Cabin Ross*, George Hudak, Ron Morton, Tom Quigley, and Bob Mahin Preliminary stratigraphy and physical volcanology associated with the Paleoproterozoic Backforty VMS deposit, Menominee County, Michigan ILSG 2011 xxv Program and Abstracts �15) Jillian E. Votava and Theodore J. Bornhorst Towards a geochemical characterization of native copper ores of the Keweenaw Peninsula, Michigan Quaternary Geology and Geological Education 16) Maverick Deschamp* and Jennifer McGuire The uptake of lead by Brassica rapa; implications for hunting in agricultural fields 17) Corrie T. Floyd*, Kent M. Syverson, and Christina M. Hupy Using LiDar data and ArcGIS to evaluate subtle glacial landforms associated with the early Chippewa and Emerald Phase ice-margin positions, Barron County, Wisconsin 18) George Hudak, Stephen Monson Geerts, Larry Zanko, April Severson, Allison Severson, and Bryan Bandli The Minnesota Taconite Workers Health Study: Environmental study of airborne particulates - 2011 update 19) William Bajjali, Cole Holstrom, and Dan Fuller Comparing sources of high salinity of two Urban Streams and their effects on Superior Bay in City of Superior 20) Stephen Mattox Using Credit-by-Exam to Connect Advanced High School Geology Courses to University Geology Programs: Lessons Learned from a State-wide Pilot Study in Michigan Precambrian Research Center Field Camp 21) Ryan Birkmeier, Tyler Boley, Brittnee Brannan, Ryan Doucette, Mark Jirsa, and Aubrey Lee Geologic mapping of Neoarchean rocks near Ogishkemuncie Lake, by students of the Precambrian Research Center’s 2010 field camp 22) Jim Miller, Ben Brooker, Max Hadley, Levi Markwood, Jeff Olson, and Alex Tomlinson 2010 Precambrian Field Camp Mapping in the Jack Lake Area, Cook County, Northeastern Minnesota ILSG 2011 xxvi Program and Abstracts �23) Amy L. Radakovich, Charlie T. Parent, , Molly E. Partridge, Andrew D. Ritts, Rita Pierce, and George J. Hudak Reconnaissance Bedrock Geological map of the northern part of Sudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota 24) Alexandra M. Vallowe, Ernest J. Thalhamer, Damon L. Rhoades, and Dean M. Peterson Surface and subsurface geologic maps of the Soudan Underground Mine State Park, St. Louis County, Northeastern Minnesota * Indicates eligibility for Student Paper Award ILSG 2011 xxvii Program and Abstracts �ILSG 2011 xxviii Program and Abstracts �ABSTRACTS ILSG 2011 xxix Program and Abstracts �ILSG 2011 xxx Program and Abstracts �Role of Te-As-Bi-Sb (TABS) in the distribution of PGE and formation of PGM: Application to the Duluth Complex Laetitia Amisse and Sarah J. Barnes Sciences de la Terre, Université du Québec à Chicoutimi, 555 Bld. de l’Université, Saguenay, Québec, Canada G7H 2B1laetitia.amisse1@uqac.ca and SarahJane_Barnes@uqac.ca Platinum-group elements (PGE) are commonly present in minerals such as pyrrhotite, pentlandite and chalcopyrite (BMS) crystallized from sulfide liquid. However, recent studies of deposits have shown that while much of the Re, Os, Ir, Ru, Rh are in solid solution in BMS, Pt and in some cases Pd are absent of these phases. The general observation shows that Pt and Pd are present in platinum-group minerals (PGM) with elements like tellurium, arsenic, bismuth and antimony (TABS). In some cases the PGM are closely associated with BMS but there are some examples where they are not. This raises the question of what role these elements play in collecting PGE. More and more authors think that TABS have an effect on the behavior of Pt and Pd. Some authors (Dare et al., 2010) suggest that PGE may crystallize as PGM directly from the sulfide liquid. Others authors (Hanley, 2007; Holwell and McDonald, 2010) suggest that TABS are derived from country rocks and formed an immiscible TABS liquid where collected PGE. However, one difficulty in testing these hypotheses is that TABS especially Bi and Te are present at very low concentrations in crustal rocks and are volatiles, which makes them difficult to determine. Some authors (Hall and Pelchat, 1997) introduce a more sensitive routine method for determining TABS based on hydride generation and inductively coupled plasma-mass spectrometry (HG-ICP-MS). This study will consist of two parts. The first will consist developing the HG-ICP-MS at laboratory of ―Université du Québec à Chicoutimi‖ to obtain better detections limits. This second part will investigate whether TABS have redistributed or collected PGE at the Dunka Road deposit of the Duluth Complex. REFERENCES Dare, S.A.S., Barnes, S.J., and Prichard, H.M., 2010a, The distribution of platinum group elements (PGE) and other chalcophile elements among sulfides from the Creighton Ni– Cu–PGE sulfide deposit, Sudbury, Canada, and the origin of palladium in pentlandite: Mineralium Deposita, doi 10.1007/s00126-010-0295-6. Hall, G.E.M., and Pelchat, J.C., 1997, Determination of As, Bi, Sb, Se, and Te in Fifty Five Reference materials by Hydride Generation ICP-MS: Geostandards Newsletter, v. 21, p.85–91. Hanley, J.J., 2007, The role of arsenic-rich melts and mineral phases in the development of high-grade Pt–Pd mineralization within komatiite-associated magmatic Ni–Cu sulfide horizons at Dundonald Beach South, Abitibi subprovince, Ontario, Canada: Economic Geology, v.102, p. 305–317. Holwell, D.A., and McDonald, I., 2010, A review of the behavior of platinum group elements within natural magmatic sulfide ore systems: the importance of semimetals in governing partitioning behavior: Platinum Metals Review, v. 54, p. 26–36. ILSG 2011 1 Program and Abstracts �Comparing sources of high salinity of two urban streams and their effects on Superior Bay in City of Superior William Bajjali, Cole Holstrom, and Dan Fuller University of Wisconsin-Superior, Department of Natural Sciences, Superior, WI A study of water quality was conducted for Newton and Faxon Creeks that run through an urban area in the city of Superior and discharge into Hog Island Inlet (HII) and Superior Bay, respectively. Faxon Creek was monitored for comparison purposes and recorded salinity concentrations much higher than that of Newton Creek. The chloride concentration was greater than that of Newton Creek and much higher than the secondary maximum contaminant level established by the Environmental Protection Agency. Water salinity of the creeks is attributed mainly to the chloride and sodium concentrations. Both Newton and Faxon Creeks revealed different chemical makeups. Faxon Creek reveals that all ion concentrations in its water are higher than in Newton Creek with the exception of sulfates and nitrates. The average chloride:sulfate ratio is 13.5 times higher in Faxon Creek than in Newton Creek. Nitrate concentrations in Newton Creek are almost 4 times the natural abundance of nitrate in natural water and the phosphorous is 3 to 4 times the recommended concentration value set by EPA. A geochemical model reveals that the water of HII is the result of a mixing ratio of about 90 % of Lake Superior water and 10 % of Newton Creek water. The source of water contaminations in Newton Creek originate mainly from the discharge of treated wastewater from Murphy Oil with minimum contributions from surface runoff. The source of contamination in Faxon Creek is mainly due to urban surface runoff that is influenced by road deicing practices. ILSG 2011 2 Program and Abstracts �Depositional processes operating on the Paleoproterozoic Gowganda ice margin: an example from the Marquette, Michigan area Breanne Beh and Philip Fralick Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, philip.fralick@lakeheadu.ca Glacial sediments of the Huronian Supergroup outcrop along the north shore of Lake Huron and were likely deposited on what is thought to have been a divergent continental margin (Fralick and Miall 1981; 1989). The glaciogenic Enchantment Lake Formation, part of the Chocolay Group of the Marquette Range Supergroup, is chronostratigraphically correlated to the Huronian Supergroup based on U-Pb age determination on detrital zircon, 2317 6 Ma, and diagenetic xenotime, 2133 11 Ma (Vallini et al. 2005). As the Gowganda is the thickest and most commonly preserved of the three Huronian glacial events it is reasonable to assume it correlates to the Enchantment Lake Formation. Two metamorphosed stratigraphic sections were logged on the north side of the Dead River Basin located near Marquette, Michigan. According to the mapping discussed in a United States Geological Survey professional paper (Puffett 1974), the sections are identified as the Enchantment Lake Formation of the Marquette Range Supergroup. Puffett‘s (1974) map shows that the Enchantment Lake is underlain by Archean rocks to the north, indicating the sections young to the south. The logged sediments were grouped into ten lithofacies: 1) Shale, 2) Siltstone, 3) Siltstone with coarse grains, 4) Diamictite, 5) Fine sandstone, 6) Hummocky cross-stratified sandstone, 7) Coarse sandstone, 8) Granule to small pebble conglomerate, 9) Pebble conglomerate and 10) Boulder lense. These lithofacies are probably a subaqueous glacial outwash fan. The presence of an iceberg dumped boulder mound (Fig 1A) as well as hummocky cross-stratification (Fig 1B) indicates that deposition on the fan was occurring in an open water setting. Siltstone is the most common sediment throughout the logged section. It makes up the matrix of the diamictite as well as the conglomerates. In some locations, it is also interbedded with fine- and coarse-grained sandstone. The fact that the siltstone occurs in conjunction with the majority of the other lithofacies indicates that it is likely the background sediment deposited from suspension settling of subaqueous outwash material. Resedimentation events such as small slumps and debris flows are thought to account for the incorporation of coarse-grained lithofacies in the predominantly siltstone succession. In particular, these resedimentation events probably produced the siltstone with coarse grains lithofacies as these layers represent a mixing of two separate grain-size populations in a manner not compatible with traction current deposition (Fig 1C). Throughout the section, stringers of coarse-grains as well as small lag deposits indicate that current reworking of the sediments has occurred by removing the finer grains (Fig 1D). Currents created by the glacial outwash would have been incapable of creating this winnowing effect because they were sediment ladened. The erosive currents were likely geostrophic currents generated in the open water environment during storm events. ILSG 2011 3 Program and Abstracts �A B C D Figure 1. A) A cobble from the boulder mound compressing underlying sediments. B) Hummocky cross-stratification. C) Siltstone with coarse grains indicated by the arrow. D) A small lag deposit of granule to small pebble conglomerate. REFERENCES Fralick, P.W., and Miall, A.D. 1981, Grant 84: Sedimentology of the Matinenda Formation, in Pye, E.G., ed., Geoscience Research Grant Program, Summary of Research 1980 – 1981: Ontario Geological Survey, Miscellaneous Paper 98, p. 80-89. Fralick, P.W., and Miall, A.D., 1989, Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake area, Ontario, Canada: Sedimentary Geology, v. 63, p. 127-153. Puffett, W.P., 1974, Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S. Geological Survey, Professional Paper 788, 53 p. Vallini, D.A., Cannon, W.F., and Shulz, K.J. 2005, New age data for the Chocolay Group, Marquette Range Supergroup: implications for the Paleoproterozoic evolution of the Lake Superior and Lake Huron regions: Institute on Lake Superior Geology 51st Annual Meeting, Proceedings and Abstracts, v. 1, p. 64. ILSG 2011 4 Program and Abstracts �Geologic mapping of Neoarchean rocks near Ogishkemuncie Lake, by students of the Precambrian Research Center’s 2010 field camp Ryan Birkmeier1, Tyler Boley1, Brittnee Brannan1, Ryan Doucette1, Mark Jirsa2, and Aubrey Lee1 1 2010 Field Camp Students, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 2 Minnesota Geological Survey (MGS), University of Minnesota, 2642 University Avenue W., St. Paul, Minnesota 55114 This presentation shows the results of mapping Neoarchean bedrock in the Ogishkemuncie Lake area (Fig. 1) by 5 students of the Precambrian Research Center field camp. Figure 1. Simplified geology of the Cavity Lake fire area (modified from Jirsa and Starns, 2008), showing location of 2010 mapping near Ogishkemuncie Lake. The Precambrian Research Center—a branch of the University of Minnesota, Duluth— conducted its 4th season of field camp in 2010. The final phase of training is a ―Capstone Project‖ that provides students an opportunity to create new bedrock geologic maps in areas of poorly understood geology. The students were charged with mapping the area to help unravel the complexity and construct a sequence of depositional and deformation events ILSG 2011 5 Program and Abstracts �Based on prior mapping, Ogishkemuncie Lake lies along a complex unconformity that is marked locally by paleosaprolite and dissected by faults. The unconformity is overlain by the Ogishkemuncie Lake sequence, consisting of conglomerate and other strata containing sedimentary structures indicating deposition by alluvial fan, fluvial, and fluvial-lacustrine (or marine) processes, and syn- and post-depositional faulting. The unconformity is underlain by the Paulson and Jasper Lake sequences composed of metabasalt, metagabbro, trachyandesite and trachyandesitic volcanic conglomerate. An offset at the south end of the lake between the trachyandesite and volcanic conglomerate units, combined with a shear fabric containing sigma-shaped clasts indicates a component of right-lateral displacement along an anastamosing fault zone in and adjacent to Ogishkemuncie Lake. Carbonate alteration and multiple cycles of quartz veining are pervasive along the fault zone. The exposure of the trachyandesite unit in the southwest and northeast of the lake are evidence of a large synform. The Ogishkemuncie conglomerate contains clasts derived from the Paulson Lake and Jasper Lake sequences, together with abundant clasts of Saganaga Tonalite, recently dated at ~2690 Ma (Driese and others, 2011). The clast-supported, moderately-sorted deposits in the Ogishkemuncie sequence are indicative of braided stream deposition. The sequence also contains lenses of matrix-supported debris flow deposits indicative of alluvial fans. The conglomeratic strata are locally interlayered with units of graded mudstone and sandstone that may represent deposition in standing water in a fluvial-lacustrine system. These findings are consistent with the evolution of a pull-apart basin in which braided streams flowing along the basin axis were temporally dammed by transgressing alluvial fans (Fig. 2). Figure 2. Model representing deposition of the Ogishkemuncie Lake sequence prior to regional deformation and metamorphism (from Driese and others, 2011); north is toward upper right of figure. REFERENCES Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, in press. Jirsa, M.A., and Starns, E, Geologic map of the Cavity Lake Fire Area: MGS Open-File Report 08-05. ILSG 2011 6 Program and Abstracts �Bedrock geologic maps of the Grand Marias and Kadunce River Quadrangles, North Shore of Lake Superior, Minnesota Terrence J. Boerboom1 and John C. Green2 1 Minnesota Geological Survey, boerb001@umn.edu 2 University of Minnesota-Duluth, jgreen@d.umn.edu The Minnesota Geological Survey has continued ongoing mapping of the bedrock geology of 7.5‘ quadrangles adjacent to the shore of Lake Superior as part of the USGS STATEMAP program, resulting to date in eighteen published 1:24,000 scale maps from Duluth to beyond Grand Marais, in addition to 10 quadrangles already published under the former USGS COGEOMAP program. The Grand Marais and Kadunce River quadrangles (Boerboom and Green, 2010; Boerboom and Green, 2011) are the most recent of these geologic maps (figures 1 and 2). All maps in this series are available as plotter printed maps, or as PDF and GIS shapefiles at the MGS website (www.mngs.umn.edu). Outcrop mapping was augmented by over 100 sets of high-quality water well cutting samples, collected at 10 foot intervals by McKeever Well Drilling of Little Marais, Minnesota. These provided a crucial glimpse of the volcanic stratigraphy in the third dimension as well as information where the bedrock is poorly exposed. The area of this map lies in the central portion of the northeast limb of the 7-10 km thick North Shore Volcanic Group (NSVG), which include abundant felsic volcanic rocks (e.g. the Devil Track, Maple Hill, Kimball Creek, Devil‘s Kettle, Trout Lake, and unnamed units of rhyolite, and the Kadunce River icelandite) as well as andesitic to mafic lava flows (e.g. the Red Cliff basalts and the Marr Island Lavas). In addition there are several thin hypabyssal diabase dikes and sills emplaced into the volcanic rocks, and near the northern margin of the maps the volcanic rocks are intruded by largely felsic rocks related to the Duluth Complex (e.g. Pine Mountain granophyre). This mapping has refined the volcanic stratigraphy of the NSVG in this area, working down-section towards the base of the northeast limb of the NSVG. This work has also shown that the lavas in and near Grand Marais have been pervasively intruded by diabase dikes, likely along fault planes, that now separate much of the volcanic rocks into isolated blocks which are disheveled and rotated to varying degrees. Minor amounts of hybrid felsic magmas, likely related to emplacement of the diabase, are also present. In keeping with prior work, the NSVG is subdivided into informal lithostratigraphic packages separated by major compositional changes, by intrusions or faults across which correlation is tenuous, or where thick flows or flow sequences form mappable units. The informal lithostratigraphic packages shown on these maps include the Grand Marais felsites (rhyolite and icelandite), the Croftville lavas (which includes the Pincushion Mountain trachybasalt of Green, 2002), the Devil Track and Maple Hill rhyolites, the Red Cliff basalts, the Kimball Creek rhyolite, the Kadunce icelandite, the Marr Island lavas, and the Brule River lavas. The Brule River lavas contain a substantial proportion of felsic volcanic rocks including the Devil‘s Kettle rhyolite (1,097.7 ±1.7 Ma: Davis and Green, 1997), and the newly named Trout Lake rhyolite. The Devil Track and Kimball Creek rhyolites are interpreted as rheoignimbrites with large aspect ratios, i.e. long strike length compared to flow thickness (Green and Fitz, 1993). The aphyric Devil Track rhyolite flow, the largest felsic flow in the NSVG, is approximately 250 meters thick and extends at least 40 km west and inland from the mouth of Devil Track River (and an unknown distance to the east, beneath Lake Superior). This flow is inferred to have been either a hot superliquidus lava flow or possibly a thick, hot rheoignimbrite that flowed and underwent complete recrystallization after deposition (Green and Fitz, 1993). The Kimball Creek rhyolite is estimated to be the second largest felsite flow in the North Shore Volcanic Group, with an aspect ratio similar to the Devil Track rhyolite. It exhibits fiamme textures and tightly folded flow layering which are restricted to the top and bottom of the unit, but the majority of the exposures are fine-grained uniform rhyolite that do not contain recognizable flow structures (Green and Fitz, 1993). ILSG 2011 7 Program and Abstracts �Intrusive rocks in these map areas consist of thin dikes and subconformable sills most likely related in timing to the Beaver Bay Complex, and deeper-seated granophyric rocks that are part of the Pine Mountain granophyre (1,095.3 ±3.8 Ma; Vervoort and others, 2007). Although this long-term mapping project is on hold for the 2011 field season, the Minnesota Geological Survey has plans to resume mapping in 2012, working east up the shore towards the Canadian border. Figure 1 (left). Location map showing the extent of the Keweenawan Midcontinent Rift System in Minnesota, and the locations of the Grand Marais and Kadunce River quadrangles (dark gray boxes). Figure 2 (right). Simplified geologic map of the Grand Marais and Kadunce River quadrangles, showing the names of the major lithostratigraphic units discussed in the text REFERENCES Boerboom, T.J., and Green, J.C., 2010, Bedrock geology of the Grand Marais quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-189, scale 1:24,000. Boerboom, T.J., and Green, J.C., 2011, Bedrock geology of the Kadunce River quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-190, scale 1:24,000. Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Science, Volume 34, No. 4, April 1997, p. 476-488. Green, J.C., and Fitz, T.J. III, 1993, Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Green, J.C., 2002, Volcanic and sedimentary rocks of the Keweenawan Supergroup in northeastern Minnesota, in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, p. 94-102. Vervoort, J.D., Wirth, K., and Kennedy, B., 2007, The magmatic evolution of the Midcontinent rift. New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research, vol. 157, no. 1-4, p. 235-268. ILSG 2011 8 Program and Abstracts �Preserving Wisconsin’s mining history: Building a GIS-based mined-lands inventory as a resource for planning, environmental management, future mineral exploration, and historical research Bruce A. Brown and Peter Schoephoester Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 A comprehensive statewide inventory of metallic and nonmetallic mining sites has never been compiled for Wisconsin. A GIS-based inventory, including current and historic sites would be useful to planners, developers, and environmental regulators as a guide to potential hazards such as improperly abandoned shafts and drill holes, areas of potential subsidence, sites that may require additional reclamation, and possible sources of surface and groundwater contamination. Access to documents and maps of historical lead-zinc and iron mining areas would also be of interest to historians. A complete inventory of metallic and nonmetallic mining sites could provide a guide to undeveloped resources for the future. Many of the pieces necessary to assemble a statewide inventory already exist or are in various stages of preparation. The WGNHS recently completed scanning, editing, and georeferencing more than 1400 section maps from the Wisconsin Mineral Development Atlas. This data set contains maps of hundreds of underground zinc mines and crevice lead mines dating from the 1820s to 1978. More than 28,000 exploration drilling logs were recently scanned and added to this data set. A Mines, Pits, and Quarries database (MPQ) currently under construction will contain locations, geologic information, and engineering test data for current and recently active aggregate and industrial mineral mines. It will also incorporate the historic WGNHS Road Materials Survey, which provides a record of aggregate mining and exploration activity back to the early 1900s. Historic nonmetallic and metallic sites compiled by the U.S. Geological survey in the Mineral Resource Data System (MRDS) and by the former U.S. Bureau of Mines in the Mineral Availability System and Mineral Industry Location System (MAS/MILS) have been integrated into the MPQ GIS. WGNHS is also in the process of constructing a locational database for the mineral exploration and engineering cores stored at the sample repository in Mt. Horeb, WI. Several important historical data sets still need to be inventoried and evaluated. Examples that will require significant work are the extensive collection of documents and paper maps related to Iron mining and exploration in the Gogebic Range, Florence District, Baraboo District, and the Mayville Ironstone mines. This material has significant local and statewide historical value, and contains locations of long forgotten potential hazards a well as valuable geologic data. We are beginning to sort and evaluate this material to prioritize documents that need to be scanned or digitized to include in the statewide inventory. The age and fragile condition of many of these documents requires that they be handled carefully in the scanning process. The WGNHS does not have staff and facilities for archiving, and after essential data is captured, many original documents are being transferred to the UW Archives or appropriate regional archive centers of the State Historical Society where they can be preserved while still available for research. ILSG 2011 9 Program and Abstracts �Compositional trends in chevkinite group minerals from the Wausau Complex, Marathon County, WI. Thomas W. Buchholz 1, A. U. Falster 2, and W. B. Simmons2 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2 Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, Louisiana 70148 The Wausau Syenite Complex (WSC) is composed of four plutons, from the oldest (1565 Ma +3-5 )(Van Wyck, 1994), most alkalic Stettin Pluton, through the younger and progressively less alkalic and more silicic Wausau and Rib Mountain Plutons, to the youngest (1505.9 ± 2.7 Ma) (Dewane & Van Schmus, 2007), most silicic Nine Mile Pluton. Over the last several years chevkinite group minerals have been identified from three sites within the Wausau Complex. Chevkinite group minerals are complex titanosilicates, most of which contain Light Rare Earth Elements (LREE, La to Eu), Ca and Fe as essential components. Extensive substitution often allows the incorporation of significant Nb, Ta, Th, HREE and Zr, among other elements. Recently chevkinite group minerals (probably either chevkinite-(Ce) or perrierite-(Ce) have been identified from three sites within the Wausau Complex. The first occurrence noted is at a site just north of the Aspirus Hospital in Wausau, where phases of the Wausau Pluton near a contact with a large quartzite xenolith were exposed by quarrying to cut back a hillside. Small pegmatoidal veins and clots are exposed in the approximate center of the cut and are largely composed of feldspars, quartz, an unidentified amphibole, zircon and locally abundant grains of a usually more-or-less altered chevkinite-group mineral. Accessory minerals include apatite, monazite and allanite-(Ce). Both mafic and more felsic phases of the veins carry chevkinite-group minerals. The grains generally present a mottled, altered appearance that is probably the result of post-emplacement hydrothermal alteration. These altered grains range in color from dark yellow through reddish hues to dark glassy black. Highly altered grains are chalky white in color. Careful examination of heavy mineral separates nonetheless resulted in finding unaltered material suitable for analysis. The second occurrence is in a long-abandoned and overgrown quarry or borrow pit located near the SE corner of CTH O and U in the Stettin Pluton. The small, poorly exposed pegmatites at this site include sparse, small glassy brown prismatic crystals of a chevkinitegroup mineral associated with quartz, microcline, albite, biotite, aegerine, ilmenite, probable rhabdophane-(Ce), unusually heavy-rare-earth-element enriched xenotime-(Y), an unidentified amphibole, fayalite, an euxenite-like Y-Nb oxide phase, and magnetite. The third occurrence is in heavy mineral separates prepared from a fine-grained, miarolitic phase of a locally megacrystic mafic dike cutting Nine Mile granite in the Ladick quarry. Clearly younger than the host granite, the mafic dike may be similar in age to a dike of similar material in the Koss quarry, classified as meladiorite, with a K-Ar age of 1307.2 +/- 41 Ma (Cordua, 2004). Associated minerals include amphiboles, biotite, chlorite, fluorapatite, and very rare columbite- and monazite-group species. Most samples examined are metamict, so no XRD determination could be made as to whether the chevkinite-group minerals are chevkinite-(Ce) or perrierite-(Ce), two chemically similar members. However, the samples were analyzed by electron microprobe (EMP), and when CaO vs. FeO is plotted as suggested by MacDonald & Belkin (2002) the compositions ILSG 2011 10 Program and Abstracts �from all three sites clearly fall into the chevkinite field, providing strong evidence that the mineral in question in all three cases is probably chevkinite. EMP analysis also shows that chevinikite-group minerals from each site exhibit a distinct chemical composition based on the rations of Nb + Ta versus Ti + Fetotal (Fig. 1). The most Nb + Ta-depleted samples are from the ―Ladick meladiorite‖. Samples most enriched in Nb + Ta are from the ―Aspirus felsic syenite phase‖, whereas those from the ―Aspirus melasyenite phase‖ and the ―Stettin syenite‖ exhibit intermediate Nb + Ta contents. While the range in chemical variation is relatively small, it likely reflects the abundance of these elements in the respective melts, thus the degree of fractionation. A chondrite-normalized plot of LREE contents shows overall relatively small variations in the LREE; all sites track rather closely with the exception of Eu. The observed greater Eu anomaly for the Stettin and Ladick samples is likely due to greater substitution of Eu2+ in plagioclase. Hence, the ―Ladick meladiorite‖ dike, though probably significantly younger and not part of the WSC, is chemically similar and thus may have originated from a similar source as the intrusive centers of the WSC. REFERENCES Cordua, William S., 2004, Enigmatic 1300 – 1400 Ma Mafic Pluton from the Koss Pit, Marathon County, WI [abstract]: Institute on Lake Superior Geology Proceedings, 50th Annual Meeting, Duluth, MN, v. 50, part 1, Program and Abstracts, p. 48-49 Dewane, T. J., Van Schmus, W. R., 2007, U-Pb geochronology of the Wolf River batholith, north-central Wisconsin: Evidence for successive magmatism between 1484 Ma and 1468 Ma: Precambrian Research, v. 157, p. 215-234. Macdonald, R., Belkin, H.R., 2002, Compositional variation in minerals of the chevkinite group: Mineralogical Magazine, v. 66, p. 1073-1098. Van Wyck, N., 1994, The Wolf River A-type magmatic event in Wisconsin: U/Pb and Sm/Nd constraints on timing and petrogenesis. [abstract]: Institute on Lake Superior Geology, 40th Annual Meeting, Houghton, MI, v. 40, part 1, Program and Abstracts, p. 81-82. ILSG 2011 11 Program and Abstracts �Geochemistry and petrology of Midcontinent Rift-related intrusive rocks of the Sibley Peninsula, Ontario Christian Carl1, Peter Hollings1, and Mark Smyk2 1 Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 2 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada The 300 km2 Sibley Peninsula projects south into Lake Superior approximately 30 km east of Thunder Bay, Ontario. The geology of the peninsula consists of Southern Province assemblages, including intrusive rocks of the ~ 1.1Ga Midcontinent Rift. Due to their location within Sleeping Giant Provincial Park, the Sleeping Giant Sill (SGS) and the numerous dikes on the Sibley Peninsula have not been geochemically sampled nor classified. With permission from Ontario Parks, this study was undertaken to classify the Sibley Peninsula dikes and the SGS based on their geochemistry and to determine relative ages of the SGS and dikes of various orientations. Figure 1: Map of sample locations (Hollings et al., 2010). Thirty-four dike samples and five SGS samples were collected and analyzed for whole rock geochemistry at the Geoscience Laboratories (Fig. 1). Dikes with three different orientations were sampled: 1) numerous ~070˚-striking dikes, which represent the majority of ILSG 2011 12 Program and Abstracts �the dikes on the peninsula, 2) two 110˚-striking dikes and 3) two 029˚-striking dikes. The ~070˚-striking dikes were classified as Pigeon River Dikes based on Mg# versus TiO2 and Gd/Ybn versus La/Smn discrimination diagrams (Fig. 2). The 110˚-striking dikes plotted near or within fields defined for ultramafic intrusions of the MCR (Hollings et al., 2007). However, due to their orientation, they are unlikely to be feeders to any of the ultramafic assemblages found in the Nipigon Embayment, northeast of the study area. The 029˚striking dikes also plotted in the field defined for mafic / ultramafic sills and intrusions on the Mg# versus TiO2 plot and in the field defined for Nipigon Sills on the Gd/Ybn versus La/Smn diagram (Fig. 2). Although these dikes are geochemically similar to Osler volcanic rocks of the neighbouring Black Bay Peninsula, they were just 18 and 20 cm wide, respectively, and tapered eastward, making them unlikely candidates as feeders to Osler volcanic rocks. Discrimination diagrams show that the SGS is of Logan affinity and now represents the easternmost recognized Logan Sill (Fig. 2). The thickness of the SGS was determined to be 194 m, making this the thickest known Logan Sill. (A) (B) Figure 2. (A) Plot of Gd/Ybn versus La/Smn. Fields from Hollings et al (2007). Normalizing values of Sun and McDonough (1989). (B) Plot of Mg# versus TiO2. Fields from Hollings et al. (2007). Cross cutting relationships indicate that the ~070˚-striking dikes are younger than both the SGS and the 029˚-striking dikes. The SGS and the 029˚-striking dikes had lower silica and higher magnesium weight percentages than the ~070˚-striking dikes, which may suggest that more primitive magmas were locally emplaced earlier. The ~070˚-striking dikes displayed the largest negative niobium anomalies which could indicate an increase in contamination of magma sources over time, since these dikes were the youngest intrusions observed in this study. REFERENCES Hollings, P., Hart, T., Richardson, A., and MacDonald, C., 2007. Geochemistry of the Midproterozoic intrusive rocks of the Nipigon Embayment, Northwestern Ontario. Canadian Journal of Earth Sciences, 44: 1087-1110. Hollings, P., Smyk, M.C. and Carl., C. 2010. Geochemistry of Midcontinent Rift–Related Dikes on the Sibley Peninsula, Thunder Bay: A Preliminary Report; Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.10-1 to 10-3. Sun, S. and McDonough, W., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Magmatism in the Ocean Basins. Geological Society, Special Publication No. 42, pp. 313–345. ILSG 2011 13 Program and Abstracts �Mesoproterozoic bedrock in the Kadunce River Quadrangle, NE Minnesota—Precambrian Research Center capstone Eric J. Carlson, Terrence J, Boerboom, Corey J. Holton, Kyle W. Kubitza, Lucy Mulvey, and Eric Scheurer Precambrian Research Center, University of Minnesota-Duluth, Duluth, MN 55812 The Precambrian Research Center (PRC) field camp culminates in several ‗capstone‘ mapping projects located in different geological terranes in northeastern Minnesota. One of the 2010 PRC capstone projects was focused on mapping an area within the Midcontinent rift system, mainly in lava flows that are part of the North Shore Volcanic Group, intruded by thin diabase dikes and sills likely related to the Beaver Bay Complex. The mapping crew consisted of 4 students (Carlson, Holton, Kubitza, and Scheurer), one teaching assistant (Mulvey) and an instructor (Boerboom). After a day of orientation the students/TA/instructor were broken into three pairs who mapped independently, and the pairings were switched on a daily basis. Since this capstone had the luxury of staying indoors, all outcrop data (outcrop areas, structure, lithology, etc.) was entered into a GIS database file each evening. This capstone project was chosen to augment a concurrent mapping project by the Minnesota Geological Survey in the Kadunce River 7.5‘ quadrangle, which is located on the shore of Lake Superior just east of Grand Marais (figure 1). The information and geologic interpretations obtained by this capstone project were incorporated into a geologic map published by the Minnesota Geological Survey (Boerboom and Green, 2011). Most of the exposures within this capstone project area are located either on the shore of Lake Superior, or in the many small streams which are incised into the adjacent highlands. Bedrock is well exposed in these streams, particularly on the slopes that face Lake Superior, affording an excellent opportunity to view flow features within the strata of the lava flows. Outside of the stream valleys outcrops are restricted to scattered exposures in topographically high areas, which are typically overgrown clearcuts of various generations. All of the students had a turn locating not only the sporadic outcrops, but also ourselves, in the highland bush country. The map area is dominated by felsic lava flows, including the Devil Track and Maple Hill rhyolites, the Kimball Creek rhyolite, and the Kadunce icelandite. In addition to these felsic flows, the map area included the entire thickness of the Red Cliff basalt flows, and the uppermost basalt flow of the Marr Island lavas, a thick series dominated by volcanic rocks of andesitic composition that also includes some thinner flows of tholeiitic basalt. The stratigraphy of the volcanic pile is more or less perpendicular to the trend of the stream valleys, which enabled us to correlate map units from one stream to the next. One of the challenges was the seemingly simple task of differentiating the Kadunce icelandite from the overlying Kimball Creek rhyolite, since both have very similar macroscopic textures with nearly identical phenocryst assemblages and only subtle variations in the textures of the matrix. As mappers we learned to identify volcanic flow features such as amygdaloidal flow tops, pipe vesicles, etc., and subsequently identified several flow contacts within the individual units. During the map compilation stage we were supplied aeromagnetic data that was used to infer the extension of small diabase dikes exposed in the river channels; however we were slightly overzealous in using the magnetic data and did end ILSG 2011 14 Program and Abstracts �up accidentally incorporating some areas of known volcanic exposures into the diabase dike units. Data collected during field work, including outcrop locations, lithology, structure, flow contacts, etc., was digitized using Arcmap software, which was also used for construction of the final geologic map. Figure 1 (left). Location map showing the extent of the Keweenawan Midcontinent Rift System in Minnesota, and the locations of the capstone mapping project (Kadunce River quadrangle). REFERENCE Boerboom, T.J., and Green, J.C., 2011, Bedrock geology of the Kadunce River quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series Map M-190, scale 1:24,000. ILSG 2011 15 Program and Abstracts �Characterization of precious metal occurrences in the NorthMet deposit of the Partridge River Intrusion, Duluth Complex, Minnesota, USA Dan Cervin1, Penny Morton1, Jim Miller1, and Richard Patelke2 1 Department of Geological Science, University of Minnesota Duluth, Duluth, MN, 55812 2 PolyMet Mining, PO Box 475, County Road 666: Hoyt Lakes, MN 55750 The NorthMet deposit is a Cu-Ni-PGE magmatic sulfide ore body located along the northwestern margin of the Partridge River Intrusion (PRI), which is part of the 1.1 Ga Duluth Complex. PolyMet Mining Company is currently seeking a permit to develop an open pit mine at the site, which is about 7 miles south the town of Babbitt, MN. During pilot-plant test runs by PolyMet, approximately 75% of the total mass of precious metals (mostly Pd, Pt, Au) known to exist from assay data were recovered; total sulfide recovery was 90%. In a sulfide flotation beneficiation process, it is assumed that precious metals are contained within sulfide minerals as small (micron-sized) platinum group minerals (PGM), as Au-Ag minerals and alloys, or in solid solution. The 75% recovery implies that many precious metal mineral and alloy phases (PMM) may not be hosted by sulfide minerals. Building on a summary report of PGM occurrences in the Duluth Complex by Severson and Hauck (2003), this study seeks to more completely characterize the mineralogical and textural occurrences of PMM in the NorthMet ore feed and concentrates. This information is not only of importance to the beneficiation of NorthMet ores, but also to the understanding of the metallogenesis of PGE-Au in magmatic sulfide deposits. The energy dispersive spectrometer-equipped scanning electron microscope at University Minnesota Duluth was used in backscatter electron composition mode (BEC) to conduct detailed compositional scanning of polished thin sections to locate PMM. As the 75% precious metal recovery would predict, NorthMet PMM primarily occur in association with sulfide minerals. Of the 347 PMM investigated in this study, 234 (67%) were hosted by sulfide minerals (mostly chalcopyrite and pentlandite), either as inclusions or at sulfide grain boundaries (Fig. 1). The remaining 33% (133) of PMM were found in a variety of primary silicates, secondary silicates, and apatite (Fig. 1). Forty-four percent of sulfide-hosted PMM are located at sulfide grain boundaries, 56% occur as inclusions in sulfide. However, 3D studies by Godel et al. (2010) indicate that most PGM observed as inclusions in sulfides in 2D studies are in fact boundary occurrences. Boundary PMM are susceptible to isolation from a sulfide host by silicate alteration minerals, these isolated PMM are presumably unrecoverable by sulfide flotation beneficiation processes. The preliminary conclusions of this study are: 1) The lower recoveries of precious metals relative to base metals in NorthMet ores is largely due to PMM boundary occurrences being lost to tailings because they are isolated in gangue minerals or are broken off of sulfide grains during comminution. 2) The occurrence of PMM in secondary silicates (chlorite, amphibole, sericite) that appear to be replacing sulfide minerals implies that PGM-Au-Ag can be isolated from a sulfide host by post-magmatic deuteric fluids causing sulfide dissolution. 3) PMM that appear to be hosted in primary silicates occur in very close association with fine-grained, interstitial sulfide minerals (generally <0.5 mm distant). Many PMM that appear to be hosted in primary silicates in SEM BEC images most likely have a sulfide association that is not always apparent in 2D SEM imagery. A preliminary spot check of primary silicate PMM occurrences with transmitted and reflected light microscopy indicates ILSG 2011 16 Program and Abstracts �that sulfides, which are not visible in SEM images, are visible as opaque minerals below the surface of the polished thin sections. Additional petrographic study is needed to confirm whether PMM actually occur in primary silicates without associated sulfide or secondary minerals, for if this is the case, this would have profound implications for the metallogenesis of PGE. Figure 1: Frequency of PMM occurring in various types of sulfide and silicate minerals in NorthMet ores; cp=chalcopyrite, pn=pentlandite, cb=cubanite, tn=talnakhite, bn=bornite; High Plag= An >60, Low Plag=An <60, cpx=clinopyroxene, and opx=orthopyroxene. REFERENCES Godel, B., Barnes, Sara J., Barnes, Stephen J., Maier, W.D., 2010, Platinum Ore in Three Dimensions: Insights from High Resolution X-ray Computed Tomography; Geology, v. 38, no. 12, pp.1127-1130. Severson, M., Hauck, S., 2003, Platinum Group Elements and Platinum Group Minerals in the Duluth Complex, Natural Resources Research Institute Technical Report NRRI/TR-2003/37,University Minnesota Duluth, 312 p. ILSG 2011 17 Program and Abstracts �Application of the horizontal-to-vertical spectral ratio (H/V) passive seismic method in Minnesota Val W. Chandler and Richard S. Lively Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 chand004@umn.edu Determining the thickness of unconsolidated materials over bedrock is a common and sometimes difficult geologic problem. It is especially difficult in areas of Minnesota where glacial deposits may be over 200 meters thick and few drill holes penetrate to bedrock. Test drilling in such deep sediment can be prohibitively expensive, seismic soundings can also be slow and costly, and most electrical resistivity methods do not penetrate deeply enough. Recent technological advances in seismometers has opened a window for passive seismic data acquisition, also known as the horizontal-to-vertical -spectral ratio method (HVSR or H/V). This method relies solely on ambient seismic noise, and shows great promise for rapidly and inexpensively estimating the thickness of unconsolidated deposits over bedrock. The H/V method uses continuous, background vibrations in the subsurface to estimate the fundamental resonant frequency of a layer of unconsolidated materials. Data collection and processing was developed (largely in Europe) to delineate areas of high-risk for earthquake damage (SESAME, 2004). If the acoustic impedance (density*seismic velocity) at the overburden-bedrock contact differs by a factor of at least 2, the thickness (Z) of the unconsolidated materials can be estimated by the relationship: Z=af0b Where f0 is the fundamental resonant frequency (for shear waves) of the unconsolidated sequence, and a and b are parameters that are empirically determined for a given region. Although the theory behind the H/V method is still developing, it appears that the behavior of shear waves (transversely vibrating body waves) and Rayleigh waves (backwardrolling surface waves) near the fundamental resonant frequency of the overburden layer are the principal components. At this frequency, the horizontal component of shear waves tend to maximize and the vertical component of Rayleigh waves tend to minimize. When the average horizontal spectrum of seismic noise is divided by the corresponding vertical spectrum, a pronounced peak results at the resonant frequency. This resonant frequency is used in the above equation to compute an overburden thickness. In Minnesota, passive seismic data have been acquired at over 180 sites in northeastern, east-central, and southeastern parts of the state, with encouraging results. Through analysis of data from 34 sites with bedrock control from drill holes, we have obtained parameters for the above equation (a=130.96, b=-1.3304) specific to Minnesota. Estimates from the data of depth to bedrock have an error of 15-20% over depths ranging from a few meters to greater than 150 meters. We have found that the H/V method must be used with appropriate caution. For example, some relatively poor H/V results are obtained in parts of southwestern Minnesota, where thick regolith and unusually dense tills may complicate the simple, sharply contrasting 2-layer model that is preferred for this kind of analysis. Although, the H/V method cannot match seismic refraction profiling for overall accuracy and amount of derived information ILSG 2011 18 Program and Abstracts �per line, the advantage of passive seismic is seen in rapid data collection, low field deployment costs, and ease of data analysis. For example, at a bedrock depth of around 500 feet, H/V depth estimates are obtained within 20 minutes, and are accomplished by a single individual and possibly a vehicle. In comparison, only one or two seismic refraction soundings per summer day would be possible at these depths, and this usually with a crew of 3-4 individuals, at least two large vehicles, and an energy source. Finally, a 3 component, broadband seismograph that is designed for H/V surveying is small, light weight and costs a fraction of that required for a conventional multi-channel seismic profiling system. All things considered the H/V method should prove to be a powerful tool for geologic and depth to bedrock studies, and geo-engineering investigations in the Lake Superior region. ACKNOWLEDGEMENTS Professor Justin Revenaugh at the University of Minnesota, and the IRIS consortium generously supported the pilot stages of this study in various ways and information on equipment and techniques from John Lane of the U.S. Geological Survey is greatly appreciated. This study was supported by the Minnesota Legislature through the County Geologic Atlas Program of the Legislative and Citizens Commission on Minnesota Resources. Additional support was through the State Special Appropriation of the Minnesota Geological Survey. REFERENCES SESAME (Seismic Effects assessment using Ambient Excitations), 2004, Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations. Measurements, processing and interpretation, WP12 European commission - Research general directorate project no. EVG1-CT-2000-0026 SESAME, report D23.12, 62 pp. ILSG 2011 19 Program and Abstracts �Upgrade of the gravity and rock properties databases at the Minnesota Geological Survey Val W. Chandler and Richard S. Lively Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 chand004@umn.edu Gravity and rock property data have been a significant resource for geological investigations in Minnesota. Used in conjunction with the upgraded, aeromagnetic database, the gravity and rock property data at the Minnesota Geological Survey (MGS) have been foundational to mapping of Precambrian bedrock geology. During 2009-2011 the MGS has upgraded these databases, with significant additions of new and preexisting data that have become available over the last decade as well as improving the location information for better usage in a digital environment. Much of the upgrade of the gravity database involved relocating stations with subsequent recalculation of the Bouguer Anomaly. Many of the older stations had location errors of 200 meters or more made when the data was originally digitized from the field maps. Field numbers and other logistical information were lost during this early digitization. To remedy this, the original gravity location data were imported into an ArcGis environment. The points were compared with locations shown on the original field maps, which were usually located at spot elevations. The point locations were moved to match what was shown on the field maps. The original station numbers were also restored and, if available, other information was added, including observers, date of acquisition, and the meter that was used. If the field materials were missing, the stations were at least moved to their most likely position along roads and trails. The re-computed Bouguer anomaly values equate to differences of up to 0.15 milligals for most of the pre-existing stations, although differences up to 0.25 milligal error exist for some of the oldest stations. In addition 2,000 new gravity stations were added to the database. Of these, 878 stations were acquired at spacings of 100250 meters in 1979 along seismic reflection lines in west-central Minnesota. The remaining stations were acquired in 2003-2010 as part of various Statemap and County Atlas mapping in northeastern and southwestern Minnesota. Many of these latter stations lie within large coverage gaps in the earlier data. The rock properties database has been expanded from 4161 to over 8800 entries. The new database includes determinations for over 5125 densities, 6890 magnetic susceptibilities, and 980 measurements of Natural Remnant Magnetization (NRM). Most entries are based on individual measurements from hand or core samples. The new density data also include 739 determinations that were based on seismic refraction velocities. These produce density estimates for materials that were underrepresented in the previous release, including glacial deposits, Cretaceous rocks, Paleozoic rocks, and Keweenawan sandstones. In addition, results from cuttings and core samples acquired by previous investigators resulted in several thousand magnetic susceptibility and several hundred density measurements being added to the database. Finally, the location of many rock property sample sites have been improved through GIS overlays with detailed topographic bases and precisely mapped locations of outcrop. ILSG 2011 20 Program and Abstracts �The upgraded gravity and rock property databases, which are web-accessible, represent a significant improvement over the previous available versions. They should serve the needs of the MGS and the broader geological community for many years to come. ACKNOWLEDGEMENTS University of Minnesota students Jack Barland and Roger Morrison provided valuable assistance thought much of the data compilation. This study was supported by the Minnesota Legislature through the Minerals Coordinating Committee and the Minnesota Department of Natural Resources. ILSG 2011 21 Program and Abstracts �Possible eukaryotic macrofossils in the 1.1 Ga Copper Harbor Formation, Michigan Katherine Cummings and Marcia Bjørnerud Geology Department, Lawrence University, 711 E Boldt Way, Appleton WI 54911 bjornerm@lawrence.edu Unusual and possibly biogenic cm-scale features have been found on the bedding plane of a siltstone layer within the ca. 1.1 Ga Copper Harbor Formation, the lowermost of the rift-filling sedimentary units of the Midcontinent Rift. The Copper Harbor Formation includes cobble- to boulder-conglomerates, sandstones, siltstones and mudstones, representing alluvial fan, braided stream, and ephemeral lake environments within the axial valley of the Midcontinent Rift (Elmore 1984). Calcified cabbage-sized stromatolites occur at several horizons in the upper part of the Copper Harbor formation; some of these grew on top of a substrate of mud, while others encrusted cobbles and boulders (Elmore, 1094; Planavsky and Bjørnerud, 2006). The newly discovered features were found in a siltstone layer between two stromatolitic horizons at Horseshoe Harbor, near Copper Harbor, Keweenaw County, Michigan. In plan view, the enigmatic features are circular to slightly elliptical with a diameter of 0.3-1.0 cm (Fig.1). Many have a transecting lenticular element or septum that cuts across them in a ‗theta‘ geometry. These septa lie at random orientations in the ca. 25 x 30 cm bedding plane slab in which more than 50 of the features were found. In cross section, the features are funnel-shaped, narrowing downward to a blunt tip. In several of the features, the funnel terminates in a mm-scale crystal of calcite (Fig 2). Finer calcite cements the fine-grained material in the interior of the features and also defines the edges of the funnel shapes. Petrographic and XRD analyses indicate that the rest of the interior material is silt-sized quartz and feldspar together with minor clay (halloysite) and iron oxides, the same minerals found in the surrounding rock. SEM imaging of the material inside and outside the features similarly revealed no obvious differences in grain size or texture. Some of the funnel-shaped elements have a crude layering defined by iron oxides, but it is not easily traceable from the enclosing rock. The layering could either be primary or a product of post-depositional pressure dissolution. These features do not resemble any common sedimentary or diagenetic structures. They are unlikely to be dewatering structures since there is no evidence of ‗soft-sediment‘ distortion of the surrounding layering. Their consistent and unusual shapes make it seem unlikely that they are concretions or similar structures. A National Academy of Sciences workshop (Knoll, 1999) proposed the following criteria for identifying microbial fossils in Precambrian rocks: 1) Provenance [the source and geologic context of the parent rock must be firmly established]; 2) Age [the parent rock must be have a reasonably well constrained age]; 3) Indigenousness [the features must be embedded in the rock matrix]; 4) Syngenicity [the features must have been formed at the same time as the parent rock]; and 5) Biogenicity – the features must be definitively biological. The features from the Copper Harbor Formation meet the first four of these criteria; the last, of course, is the most difficult to establish unambiguously. ILSG 2011 22 Program and Abstracts �Fig. 1: The siltstone slab with unusual bedding plane features. Lower right: Close-up of one feature, highlighting the transecting element. Fig. 2: Thin section (cross-polarized light) of one of the features in cross section, showing calcite crystal at terminal end of the funnel (arrow). If they are biogenic, it seems most likely that the features represent either body or trace fossils of a eukaryotic organism (Andrew Knoll, personal communication, 2010). They are far too large to have been formed by single prokaryotes, and their symmetry and consistent geometry would seem improbable for colonies of individuals. Eukaryotic organisms were diverse and well-established by Mesoproterozoic time in the marine realm, but there are fewer reports of eukaryotic terrestrial biota (Knoll et al. 2006). Fedonkin and Yochelson (2002) described fossils of colonial eukaryotes on bedding surfaces of freshwater siltstones within the 1.5 Ga Appekunny Fm. of the Belt Series in Montana. In a study of the 1.1-1.2 Ga Torridonian sequence of Scotland, similar both in age and depositional setting to the Copper Harbor Formation, Prave (2002) argued for a diverse terrestrial fluvial and lacustrine ecosystem, based on extensive and well-preserved microbial crusts and evidence for unusually cohesive sandy sediment. The Copper Harbor stromatolites indicate that in spite of the high-energy sedimentary environment, there was an active lacustrine ecosystem within the valley of the Midcontinent Rift at ca. 1.1 Ga, perhaps seeded via wind transport of dormant reproductive structures (Planavsky and Bjørnerud, 2002). The newly discovered features described here may indicate an even greater degree of biodiversity than previously recognized. The Midcontinent Rift is already renowned for its unparalleled volcanic effusion rates and worldclass copper deposits; perhaps it should also be re-examined for clues to the early colonization of the continents by Life. REFERENCES Elmore, R., 1984. The Copper Harbor Conglomerate: A late Precambrian fining-upward alluvial fan sequence in northern Michigan. Geol. Soc. America Bulletin 95, 610-617. Fedonkin, M., & Yochelson, E., 2002. Middle Proterozoic 1.5 Ga Horodyskia moniliformis: Oldest Known Tissue-Grade Colonial Eucaryote. Smithsonian Contrib. Paleobiology 94. Knoll, A., ed., 1999. Size Limits of Very Small Organisms: Proceedings of a Workshop. Washington, D.C: National Academy Press. 164 pp. Knoll, A.., Javaux, E., Hewitt, D., and Cohen, P., 2006. Eukaryotic organisms in Proterozoic oceans. Philosophical Transaction of the Royal Society 361, 1023-1038. Planavsky, N., and Bjørnerud, M., 2004. Blowing in the Wind: The Copper Harbor stromatolites revisited. Proceedings of the Institute on Lake Superior Geology 50, 137-138. Prave, A., 2002. Life on land in the Proterozoic: Evidence from the Torridonian rocks of northwest Scotland. Geology 30, 811-814. ILSG 2011 23 Program and Abstracts �Compilation and reevaluation of the geochemistry of Midcontinent Rift related intrusive rocks around Thunder Bay, Ontario Robert Cundari1, Peter Hollings1, and Mark Smyk2 1 Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 2 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada Recent geochemical, geochronological, and geophysical data have provided further insight into the relationship of Midcontinent Rift (MCR)-related intrusive units south of Thunder Bay (e.g. Hollings et al., 2010). These data have shown that the intrusive relationships are more complicated than previously thought and further work is needed in order to understand these relationships and put them in context within the MCR. The initial phase of this study will: 1) identify the issues surrounding the intrusive units within the study area and how they may potentially be resolved; and 2) present preliminary findings of a geochemical compilation for MCR rocks in Canada, with particular focus on indices of fertility for Ni-Cu-PGE deposits. Mapping focused around the Arrow River has revealed an extension of the recently recognized Devon basalts (Cundari, 2010). Outcrops on the western bank of the Arrow River exhibited an amygdaloidal texture and a cracked ―elephant skin‖ flow-top texture similar to Devon basalt outcrops observed ~4 km to the west, suggesting that the unit may be significantly more extensive than previously recognized. A La/Sm cn ratio of 2.98 and a Gd/Yb cn ratio of 3.13 are well within the range (La/Sm cn = 2.6 to 3.1; Gd/Yb cn = 3.0 to 3.4) for the unit (Cundari, 2010), supporting this interpretation. Digital elevation models will also be generated in order to help correlate laterally extensive volcanic units and sills. Additional geochemistry and reinterpretation of geological maps suggest that a broadly northeast-trending dyke swarm, hitherto collectively termed Pigeon River dykes, may in fact comprise two distinct trends: one trending northeast and the other trending eastnortheast. Dating by Heaman et al. (2007) revealed a considerable range of ages for Pigeon River dykes (i.e. east-northeast-trending 1078±3 Ma Arrow River dyke and the northeasttrending 1141±20 Ma Rita Bolduc dyke), suggesting that it may be possible to subdivide the Pigeon River swarm. Although preliminary data suggest that there are no significant geochemical variations between the two orientations, closer examination of the geochemistry, as well as additional detailed field work, is planned to further investigate this question. A detailed compilation of existing data sets, comprising ~2400 data points of spatially referenced samples with whole rock geochemical analyses (including ~800 samples with fire assay PGE data), has been undertaken. This database represents all major MCR units in Canada and is currently being analyzed in order to investigate geochemical variations in these rocks. Only preliminary interpretations of these data sets have been undertaken (e.g. Hollings et al., 2007a, 2010). Detailed study of these data sets will be conducted, particularly in regard to PGE mineralization. Preliminary observations suggest that Logan sills display broadly higher Cu/Pd ratios than those of the Nipigon sills, the Osler volcanic rocks and the ultramafic intrusions of the Nipigon Embayment (Figure 1). Samples from a detailed traverse through the Osler Group stratigraphy undertaken by Hollings et al. (2007b) have been ILSG 2011 24 Program and Abstracts �reanalyzed for PGE in order to look for detailed variation in one of the few MCR-related sequences in Canada where the stratigraphy is well-understood. This study will allow for investigations in PGE distribution over a ~3 million-year interval and the implications for possible Cu-Ni-PGE mineralization. Figure 1: Age versus Cu/Pd graph. Median age dates were taken from Heaman et al. (2007). REFERENCES Cundari, R. 2010. The Geology and Geochemistry of the Devon Volcanics, South of Thunder Bay, Ontario. Unpublished honours thesis. Lakehead University, 62p. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences 44, p.1055–1086. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences 44, p.1087–1110. Hollings, P., Fralick, P., Cousens, B., 2007b. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario. Canadian Journal of Earth Sciences 44, p.389–412. Hollings, P., Smyk, M., Heaman, L.M., and Halls, H., 2010. The geochemistry, geochronology, and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, v.183, Issue 3, p.553- 571. ILSG 2011 25 Program and Abstracts �The geochemical evolution of the Sonju Lake intrusion: an assimilationfractional crystallization model Dayton, R.N.1*, Miller, J.D.1, and Vervoort, J.D.2 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 USA School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164, USA 2 The Sonju Lake Intrusion, located within the Beaver Bay Complex near Finland, MN, is the most completely differentiated intrusion related to the Midcontinent Rift System (Stevenson, 1974; Wieblen, 1982). The Sonju Lake intrusion exhibits a cumulate stratigraphy consistent with closed system differentiation of tholeiitic magma by fractional crystallization (Stevenson, 1974; Miller et al., 1993; Miller and Ripley, 1996). The Finland granite, composed of micrographic-textured ferromonzonite to leucogranite, forms the hanging-wall to the Sonju Lake intrusion. Field relationships from outcrop and drill core through the Sonju Lake intrusion and the overlying Finland granite show a cyclic to irregularly gradational contact. This relationship, the smooth compositional variations across the felsic-mafic contact, and the parallel zonation of the two subunits of the Finland granite with the strike of the mafic cumulates of the Sonju Lake intrusion are consistent with the Finland granite being a late stage felsic differentiate of the Sonju Lake intrusion (Miller and Green, 2002; Miller and Ripley, 1996). However, geophysical modeling of gravity and aeromagnetic data implies a volume of granophyre that approaches that of the Sonju Lake intrusion and therefore greatly exceeds the volume of felsic material that could be accounted for by differentiation of a mafic body of that size (Miller et al., 1990). Miller and Ripley (1996) suggested that the Finland granite was emplaced first and acted as a density barrier to the upward movement of the mafic Sonju Lake magma. Since underplating of the mafic magma would be expected to cause melting in the lower portions of the Finland granite, the lithologically and chemically gradational contact between the two bodies might instead represent a mixing zone between the mafic magma of the upper Sonju and the partially melted base of the Finland granite. The major and minor element geochemistry of the Finland granite resembles an extreme differentiate of the Sonju Lake intrusion and precludes using lithogeochemical data to evaluate the extent of mixing between mafic and felsic magmas. However, because the granite has an isotopic signature that is distinct from the mafic rocks of the Sonju (Vervoort et al., 2007, and this study), a radiogenic isotope study has a better chance of assessing the extent of assimilation of the partially melted Finland granite by the Sonju Lake intrusion and the effect of this assimilation on the compositional evolution of the well differentiated Sonju Lake intrusion. The Finland granite and Sonju Lake intrusion have distinct and identifiable isotopic compositions of Nd. Samples from the leucogranite range from -3 to -3.6 εNdat 1096 Ma with an average of -3.4 εNd for five samples (Vervoort and Green,1997, Vervoort et al., 2007, this study). Samples from the Sonju Lake intrusion, below the cumulus arrival of apatite, range from 1.47 to -1.62 εNd. The rocks above and including the uppermost cumulate layer of the Sonju Lake intrusion consistently exhibit decreasing values of εNd with increasing stratigraphic height closer to the Finland granite. If the consistent isotopic signature of the leucogranite(-3 to -3.6 εNd) is taken as the initial εNd of the Finland granite as a whole, it is clear that the underlying quartz ferromonzodiorite, with εNd compositions of 1.7 to -2, contains a more radiogenic Nd component attributable to the Sonju Lake intrusion. ILSG 2011 26 Program and Abstracts �The quartz ferromonzodiorite may be a late-stage differentiate of the Sonju Lake intrusion that assimilated non-radiogenic Nd from the Finland granite. An assimilation-fractional crystallization model is presented to evaluate Nd isotopes in the upper portions of the Sonju Lake intrusion and overlying Finland granite (Fig. 1).Anomalously negative εNd values for samples from the SLI-1 core that profiles the felsic-mafic contact west of the main exposure area are enigmatic. Figure1. Concentration of Nd vs. εNd(1096) showing AFC model curves and simple mixing curve for identical end members. Ticks on mixing curve are 10% increments. REFERENCES Miller, J.D., Jr, Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: Geochemical and geophysical evidence of petrogenetic relationships. 36th Annual Institute on Lake Superior Geology, p. 66–68. Miller, J.D., Jr., Green, J.C., Chandler, V.W., and Boerboom, T.J., 1993, Geologic map of the Finland and Doyle Lake quadrangles, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series M-72, 1:24,000 scale. Miller, J.D., Jr., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G. (ed.):Layered Intrusions: Amsterdam, Elsevier, p. 257-301. Miller, J.D., Jr., Green, J.C., 2002. Geology of the Beaver Bay Complex and related hypabyssal intrusions. in Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, pp. 106-143 Stevenson, R.J., 1974. A mafic layered intrusion of Keweenawan age near Finland, Minnesota. M.S. Thesis, University of Minnesota Duluth, 160 pp. Vervoort, J.D. , Wirth, K., Kennedy, B., Sandland, T. , and Harpp, K.S., 2007. The magmatic evolution of the Midcontinent rift: new geochronologic and geochemical evidence from felsic magmatism, Precambrian Research157, p. 235– 268 Vervoort, J.D., Green, J.C., 1997. Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Ndisotope evidence for melting of Archean crust. Canadian Journal of Earth Science. 34, p. 521–535. Weiblen P.W., 1982, Keweenawan intrusive igneous rocks. Geological Society of America Memoir 156, p. 57-82 ILSG 2011 27 Program and Abstracts �The uptake of lead by Brassica rapa; implications for hunting in agricultural fields Maverick Deschamp and Jennifer McGuire Department of Geology, University of Saint Thomas, 2115 Summit Ave, St Paul, MN 55105 According to the Center for Disease Control and Prevention, about 1 in 22 children in America have high levels of lead in their blood. Research has shown that the long-term effects of elevated concentrations of lead in the blood of children can be severe including learning disabilities, decreased growth, hyperactivity, impaired heading, and brain damage. In 1987 Minnesota banned the use of lead shot for hunting waterfowl because it concluded that the use of lead shot was killing waterfowl and poisoning their predators. However, lead is used in other forms of hunting and thus may also be significant in the environment suggesting it should also be removed from use. To test this hypothesis, I grew Brassica rapa, a fast growing wild mustard plant, exposed to varying amounts of lead in its soil. The contamination was lead shot that would be commonly used in hunting and recreational shooting. In total, 10 plants were grown to maturity with lead ranging from 0-20 grams per pot. The plants and the soil from which they were grown, with remaining lead shot removed, were then digested using EPA method 3050B, a rigorous hydrochloric and nitric acid digestion that accesses all environmentally available elements, and analyzed using an ICPAES. The data shows that the plants and soil increased in total lead corresponding with the amount of lead per pot. This result shows that with increased lead present, the plants and soil become enriched in lead and potentially pose a greater risk to consumers. ILSG 2011 28 Program and Abstracts �Quantifying deformation fabrics and chemistry across small granitic ductile shear zones from Mountain, Wisconsin Michael A. DeVasto1, Dyanna M. Czeck1, Prajukti Bhattacharyya2 1 Dept. of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201. Dept. of Geography and Geology, University of Wisconsin-Whitewater, Upham 119, Whitewater, WI 53190 2 INTRODUCTION Deformation processes in ductile shear zones can be profoundly affected by the presence of fluids (Wawrzyniec et al., 1999). Fluid-rock interaction also has potential for causing volumetric changes in shear zones, which are important to consider during kinematic analyses (Ramsay and Graham 1970). The purpose of our study is to develop a way to systematically and quantitatively describe textural and chemical changes across small-scale ductile shear zones, which may be indicative of the presence of fluids during deformation. The small (cm scale) shear zones in deformed granite of the Mountain Shear Zone (MSZ) in Mountain, Wisconsin are ideal for this study for a variety of reasons. 1) Sims et al. (1989) conducted a detailed study of the MSZ, allowing for a more detailed structural analysis. 2) Granite is an excellent rock to study microtextures due to its isotropy and simple mineralogy and chemistry. 3) The scale of these shear zones is ideal for a high resolution sampling method in order to achieve a complete geochemical dataset through the strain gradient. GEOCHEMISTRY Tracking geochemical changes across the strain gradient can act as an indicator of fluid-rock interactions during deformation and volume loss (Newman and Mitra, 1992; Yonkee et al., 2003). Bialek (1999) summarizes the inconsistency in results between many studies, which indicate that ductile shear zones may or may not have significant fluid input and volume change. These inconsistencies may be due to lack of resolution in the chemistry data, which typically are only collected in the protolith and shear zone. Therefore, , we plotted individual elemental chemistry, determined through x-ray fluorescence, with respect to distance from the shear zone (Yonkee et al. 2003). This method provides more information than the commonly used isocon method, which lacks spatial relevance and may misrepresent elements with small changes in concentration. Our granitic shear zones have a variety of geochemical gradients. Preliminary results in one shear zone show relatively no major element chemical changes. Preliminary results in another shear zone from the same outcrop show enrichments in Fe2O3, MgO, Al2O3 and K2O with depletion in SiO2 and Na2O. CaO, P2O5, and TiO2 concentrations vary along the transect, but show no obvious trend with respect to the shear zone. In comparison, the isocon method indicates minor depletions/enrichments in SiO2, Fe2O3, MgO, and Al2O3, whereas other analyzed elements remained ―immobile‖. A GIS TECHNIQUE TO QUANTIFY FABRIC FORMATION Microstructures may be indicative of fluid interaction and localized volume loss, offering a different, yet largely descriptive, approach to studying fluid-rock interaction during ILSG 2011 29 Program and Abstracts �deformation. To completely convey the importance of microstructures there needs to be a method to quantify various features. Li et al. (2008) developed a semi-automatic digitizing model within ArcGIS to improve the ease of tracing crystal grain boundaries. We have expanded the use of this model by applying it to deformed polymineralic rocks. Because this model works within a GIS, we can build a spatial database of microstructural information, including mineralogy and deformation fabrics, which can be queried for statistical and geospatial information. The average nearest neighbor tool is a good example of what even a simple spatial analysis can do. Running this tool yields a plethora of information, most notably the nearest neighbor index (figure 1), which will indicate whether the distribution of grains is statistically either clustered or dispersed. Our data show that quartz and plagioclase are both significantly dispersed outside of the shear zone, yet abruptly change to a ―less-dispersed‖ pattern inside the shear zone. The significance of this spatial Figure 2: ArcGIS graphical output for distribution still needs to be determined. This the calculated Nearest Neighbor Index. study will help to establish a quantitative A) corresponds to inside the shear zone. method for thin section fabric analysis. B) corresponds to samples outside of Moreover, insight from this information will the shear zone. allow us to find a potential link between textural and chemical analyses in deformed rocks. REFERENCES Bialek, D. 1999. Chemical changes associated with deformation of granites under greenschist facies conditions: the example of the Zawidów Granodiorite (SE Lusatian Granodiorite Complex, Poland). Tectonophysics 303, 251-261. Li, Y., Onasch, C. M. and Guo, Y. 2008. GIS-based detection of grain boundaries. Journal of Structural Geology 30, 431-443. Newman, J. and Mitra, G. 1993. Lateral variations in mylonite zone thickness as influences by fluidrock interactions, Linville Falls fault, North Carolina. Journal of Structural Geology 15, 849863. Ramsay, J. G. and Graham, R. H. 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences 7, 786-813. Sims, P. K., Klasner, J. S. and Peterman, Z. E. 1990. The Mountain Shear Zone, Northeastern Wisconsin –A Discrete Ductile Deformation Zone Within the Early Proterozoic Penokean Orogen. Geologic Survey Bulletin, Precambrian Geology of Lake Superior Region, A1-A15. Wawrzyniec, T., Selverstone, J. and Axen, G. J. 1999. Correlations between fluid composition and deep-seated structural style in the footwall of the Simplon low-angle normal fault, Switzerland. Geology 27, 715718. Yonkee, W. A., Parry, W. T. and Bruhn, R. L. 2003. Relations between progressive deformation and fluid-rock interaction during shear-zone growth in a basement-cored thrust sheet, Sevier orogenic belt, Utah. American Journal of Science 303, 1-59. ILSG 2011 30 Program and Abstracts �Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization R.M. Easton1 and L.M. Heaman2 1 Precambrian Geoscience Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 mike.easton@ontario.ca 2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 larry.heaman@ualberta.ca Between 1956 and 1996, 12 mines in the Elliot Lake camp produced 138,500 tonnes of uranium metal at an average grade of 763 ppm U (900 ppm U3O8). The uranium ore was hosted in quartz pebble conglomerate beds within the lower part of the Matinenda Fm near the base of the Paleoproterozoic Huronian Supergroup. The Matinenda Fm is composed almost entirely of trough and planar crossstratified, greenish to buff, coarse-grained sandstones deposited in a braded fluvial system (Fralick 2003). The ore-conglomerates occur mainly in NW-oriented valleys (Roscoe 1969), with current direction from the NW (McDowell 1957; Pienaar 1963). Numerous workers (see Roscoe 1969 for list) have suggested a paleoplacer origin for the mineralized conglomerates, however the location and distance of the source region is problematic. McDowell (1957) suggested a source 225 to 400 km to the W-NW of Elliot Lake (roughly the Wawa greenstone belt), but previous detrital zircon studies of the Matinenda Fm near Sault Ste. Marie and Espanola (Rainbird and Davis 2006, Easton and Heaman 2008) indicate a dominant zircon population ranging from 2650 and 2670 Ma, not the >2700 Ma zircons expected for a Wawa greenstone belt source. Due to increased exploration interest in the camp, in 2009 the Ontario Geological Survey began compilation mapping in the camp (Easton 2009, 2010). As part of this study, 4 Matinenda Fm sandstones were collected to better understand its provenance. U-Pb isotopic analyses were carried out using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the University of Alberta. The samples were collected from a 180 m thick section of the Matinenda Fm exposed along a hydro transmission line west of Pecors Lake on the south limb of the Quirke Lake syncline. Two samples are from the Ryan Member, one 5 m below and the other 7 m above the mineralized Main Conglomerate Bed (09RME-0344 and 09RME-0343, respectively). A third sample (09RME-0342) is from the lower part of the Stinson Member, roughly 75 m above the Main Conglomerate Bed. The fourth sample (09RME-0129) is from the uppermost Stinson Member, roughly 10 m below the unexposed contact with the overlying McKim Formation. The Ryan Member samples adjacent to the Main Conglomerate Bed (09RME-0344 and 09RME-0343) give near identical weighted average 207Pb/206Pb ages of 2648.9±4.3 (MSWD=2.1, n=100) and 2648.5±6.3 (MSWD=2.2, n=118), respectively. The youngest concordant grains are 2620 and 2634, and the oldest grains are 2776 and 2684 Ma, respectively. Sample 09RME-0344 contained 4 concordant grains between 2702 and 2776 Ma. Sample 09RME-0343 contained a significantly higher proportion of discordant grains, perhaps due to metamict effects related to the underlying uranium mineralization. Sample 09RME-0342, located roughly 85 m above the base of the formation, has a weighted average 207Pb/206Pb age of 2638.5±5.2 (MSWD=0.55, n=72), but contains younger grains (2546 Ma) and 3 Mesoarchean grains which give an upper intercept age of 2836±31Ma. The stratigraphically highest sample, 09RME-0129, has peaks at 2657 and, with a minor peak at 2708, suggesting a slightly more diverse provenance. The grains between 2640 to 2710 Ma give a weighted average 207Pb/206Pb of 2667.4±4.0 (MSWD=8.9, n=79). The youngest concordant grain is 2580 Ma and there are 4 concordant grains between 2706 and 2760 Ma. These results are consistent with the recently obtained 2687±2 Ma age of felsic metavolcanic rocks at Elliot Lake (Easton 2010), and with the ages of Archean granitic rocks NE of Elliot Lake that indicate emplacement between 2620 and 2670 Ma (J. Ayer, Ontario Geological Survey, personal communication 2011), all consistent with local sourcing. The age and geochemical data are consistent with nearby Archean granites being an important source component, as suggested by Roscoe (1969). The ages are younger than those typical of the Abitibi or Wawa greenstone belts and contrast with those from other sandstone units in the Huronian Spg which show a more diverse provenance consisting of mixed ILSG 2011 31 Program and Abstracts �<2700 and >2700 populations and a variety of Mesoarchean grains (Rainbird and Davis 2006; Easton and Heaman 2008). Geochemistry of the sandstones (Table 1) confirms the observations of Fralick (2003). The presence of monazite is reflected in LREE enrichment and the presence of negative Eu anomalies throughout the Matinenda Fm. This contrasts with the lack of Eu anomalies in units immediately overlying the Matinenda Fm. The increase in SiO2 and decrease in Al2O3 and K2O in the better sorted upper Stinson Member is consistent with the observed decreased clay mineral content in this unit, however, Th and total REE content do not decrease, indicating that heavy mineral content was unchanged. The low K2O and high CIA value for sample 09RME-0129 may indicate that this sample underwent minor metasomatism compared to sample 09RME-0341, located only 5 m higher in the section. The Huronian metavolcanic rocks in the area lack any diagnostic trace element signature (such as high Cr) that could be detected in the sandstones. In summary, local sourcing of the Matinenda Fm means that the character of the adjacent Archean basement needs to be accounted for in exploration. Table 1. Summary of geochemical data for Matinenda Formation samples along the study transect. All samples are sandstones, except for sample 09RME-0318 which is an uraniferous quartz-pebble conglomerate bed. Geochronology samples are indicated in bold. Ratios listed are those used by Fralick (2003). Data from GeoLabs, Ontario Geological Survey, Sudbury. Abbreviations: CIA = chemical index of alteration. Sample 09RME- SiO2 wt.% 0341 5.79 5.11 0129 - 4.17 0342 - 8.67 0343 0318 7.16 - 0344 9.88 Al2O3 wt.% K2O wt.% Zr ppm 9 .50 9 .02 8 .28 7 2.87 8 .13 7 2.04 2 .42 3 .37 9 .04 1 .64 5 .32 1 .28 1 2 0 1 6 3 6 7 2 70 6 7 Th ppm 7 9 8 4 3 6 1 112 9 7 Total REE (ppm) 4 89 6 71 4 9 9 6 > 759 1 1 La/ Sm Gd/ Yb 1 .7 2 .8 2 .3 6 3.8 1 3.4 9 0.3 9 2.1 9 2.9 9 1.8 1 4.9 1 2.4 1 6 Al/ K Zr/ Hf 3 .8 1 5 3 .2 8 1 1 .9 1 9 2 3 15 6 09 0 3 .2 2 41 8 8 8 6 5 6 3 7 4 6 1 3 1 5 2 3 .3 6 9 25 7 1 .9 2 3 7 3 10 .8 5 .2 3 1 CIA 1 20 .0 4 Zr/ TO2 3 .8 6 .5 Ce/ Th 6 REFERENCES Easton, R.M. 2009. Compilation mapping, Pecors-Whiskey Lake area, Superior and Southern provinces; in Summary of Field Work and Other Activities, 2009, Ontario Geological Survey, Open File Report 6240, p.10-1 to 10-21. Easton, R.M. 2010. Compilation mapping, Pecors-Whiskey Lake area, Superior and Southern provinces; in Summary of Field Work and Other Activities, 2010, Ontario Geological Survey, Open File Report 6260, p.8-1 to 8-12. Easton, R.M. and Heaman, L.M. 2008. Detrital zircon geochronology of Huronian Supergroup sandstones located within the Vernon structure, north of Espanola, Ontario; 54th Institute on Lake Superior Geology, Proceedings, v.54, pt.1, p.21-22. Fralick, P.W. 2003. Geochemistry of clastic sedimentary rocks: ratio techniques; in Geochemistry of sediments and sedimentary rocks: Evolutionary considerations to mineral deposit-forming environments, D.R. Lentz, editor; Geological Association of Canada, GeoText 4, p.8-103. Pienaar, P.J. 1963. Stratigraphy, petrology, and genesis of the Elliot Group, Blind River, Ontario, including the uraniferous conglomerate; Geological Survey of Canada, Bulletin 83, 140p. Rainbird, R.H. and Davis, W.J. 2006. Detrital zircon geochronology of the western Huronian Basin; Institute on Lake Superior Geology, Proceedings, 52, pt.1, Programs and Abstracts, 55-56. Roscoe, S.M. 1969. Huronian rocks and uraniferous conglomerates in the Canadian Shield; Geological Survey of Canada, Paper 68-40, 205p. ILSG 2011 32 Program and Abstracts �Bedrock topography, glacial drift thickness and bedrock outcrop maps, Upper Peninsula of Michigan John M. Esch, Michigan Dept. of Environmental Quality, Office of Geological Survey, P.O. 30256, Lansing, MI 48909, eschj@michigan.gov Few studies exist concerning the thickness of the glacial drift, the underlying bedrock surface topography or the distribution of outcrops in Michigan. Newly assembled bedrock topography, glacial-drift thickness and bedrock outcrop maps were constructed using traditional depth to bedrock data from water, oil and gas, and mineral wells. Due to the sparse well control over much of the Upper Peninsula, nontraditional bedrock elevation sources such as bedrock shorelines and outcrops, SSURGO county soil data, waterfalls, quarries, aerial photos and General Land Office surveyor‘s plats were also used (Esch, 2011). In the Upper Peninsula, the bedrock surface consists of high relief resistant bedrock highlands and cuestas as well as lowlands and deep bedrock valleys. The bedrock surface in places has well defined drainage networks with tributaries as well as long linear valleys. The bedrock surface has been sculpted by hundreds of millions of years of preglacial erosion and by paleo-river channels cut into it during the numerous glacial ice advances over the last 2.5 million years. There may also be local relationships between of certain bedrock valleys and scarps with jointing and deeper bedrock structures. The Upper Peninsula is characterized by two distinct bedrock surface regions. In the western Upper Peninsula, Precambrian crystalline bedrock forms large, high relief bedrock uplands with bedrock elevations generally greater that 1000 feet above mean sea level (amsl). Archean Granite and Gneiss generally form bedrock highlands. In fact Mount Arvon consisting of Archean Granite and Gneiss in eastern Baraga County has the highest land surface and bedrock elevation in the state at 1979 feet amsl. The Portage Lake Volcanics and the Copper Harbor Conglomerate form a prominent bedrock and land surface ridge along their extent. This contrasts with the eastern Upper Peninsula where bedrock consists of less resistant Paleozoic sedimentary rocks with elevations generally less than 1000 feet amsl and lower relief. Cuestas and bedrock valleys are the dominant bedrock features in the eastern Upper Peninsula. Bedrock valleys are most numerous in eastern Chippewa County. The Niagaran cuesta and Cambro-Ordovician Sandstone cuestas are especially prominent bedrock features. Locally, the Stonington Formation/Big Hill Dolomite and Black River Formation form lower relief and less extensive cuestas. The Mackinaw Breccia forms localized bedrock highlands north of the Mackinaw Straits. Broad bedrock lowlands generally occur over the Jacobsville Sandstone except at the northern part of Sugar Island in Chippewa County. One of the lowest bedrock locations in the Upper Peninsula at 200 feet amsl occurs in a deep bedrock valley at Rudyard in Chippewa County. This bedrock valley runs from Brimley on Lake Superior past Rudyard to St. Martin Bay on Lake Michigan. It has some of the thickest drift in the Upper Peninsula at 435 feet thick. Parts of the Waiska and the Pine Rivers overlay this bedrock valley. Another deep bedrock valley underlies the Sturgeon River valley in Baraga and Houghton Counties near Pelkie. The bottom of the bedrock valley is around 200 feet amsl and drift is 400 feet over it. It is unclear if there is any relationship between the deep bedrock valleys in the eastern Upper Peninsula and the deep north-south troughs on the floor of the east side of Lake Superior which are at a much lower elevation. ILSG 2011 33 Program and Abstracts �Drift thickness ranges from nonexistent at bedrock outcrops to 450 feet. It averages 40 feet thick but can change thickness abruptly over short distances. Drift is locally thicker overlying bedrock valleys, broad bedrock lowlands, along the back cuesta lowlands and over end moraines. This contrasts with the Lower Peninsula where drift averages 270 feet thick and is up to 1,320 feet thick which is the thickest glacial drift on land in North America. The land surface topography in the Upper Peninsula generally mimics the underlying bedrock topography. Modern drainage often overlies broad bedrock lowlands, bedrock valleys and back cuesta lowlands. In eastern Menominee, western Delta and southwestern Marquette counties the bedrock is a surprisingly smooth downward sloping surface to the southeast. The bedrock formations are Trenton, Black River and Prairie du Chien. In parts of this area the land surface mimics this southeast sloping surface. These maps will assist geologists in understanding glacial lobe dynamics, in seismic data processing for oil and gas exploration, bedrock geology mapping, in aggregate and mineral exploration. These maps should be especially helpful for ground-water exploration in the highly variable drift and generally limited aquifer areas over crystalline bedrock in the western Upper Peninsula and for finding potential glacial aquifers in bedrock valleys. Figure 1: Upper Peninsula of Michigan bedrock topography and major bedrock valleys REFERENCES Esch, John, M., 2011, Michigan bedrock topography, glacial drift thickness and bedrock outcrop maps, Geological Society of America Abstracts with Programs, Vol. 43, No. 1, p. 56 ILSG 2011 34 Program and Abstracts �Using LiDAR and ArcGIS to evaluate subtle glacial landforms associated with the Early Chippewa and Emerald phase ice-margin positions, Barron County, Wisconsin Corrie T. Floyd1, Kent M. Syverson1, and Christina M. Hupy2 1 Department of Geology, 2Department of Geography and Anthropology, University Wisconsin-Eau Claire, Eau Claire, WI 54702-4004, floydct@uwec.edu Glacial landforms in Barron County, Wisconsin, are a result of at least four glacial advances during the Pleistocene Epoch (2.58 to 0.012 Ma) (Johnson, 1986; Syverson and others, 2011). During the early Chippewa and Emerald Phases (Late Wisconsin Glaciation, 31-17 ka), till of the Pokegama Creek and Poskin Members of the Copper Falls Fm. were deposited over till of the River Falls Fm. (Illinoian Glaciation, >130 ka). These tills are all reddish-brown, sandy, and lithologically indistinguishable. Johnson (1986) was unable to map the location of the Emerald Phase ice margin using glacial geomorphology because these till surfaces do not display obvious glacial landforms in the field. Johnson used Late Wisconsin lake sediment in the northwest-flowing Fourmile Creek valley as evidence for the Emerald Phase ice-margin position (Fig. 1). In addition, Fourmile Creek makes a 90degree bend, and Johnson (1986) attributed this to flow along the Emerald Phase ice margin. LiDAR data obtained from the Barron County Land Information Office was used to evaluate the early Chippewa and Emerald Phase ice-margin positions proposed by Johnson (1986). High-resolution terrain models have been generated using the LiDAR data. The point spacing of the data is about 3 ft, and Figure 1. Elevation model of the study area and the compared to ground truth points, the root proposed ice-margin positions of the early Chippewa and Emerald Phases. Johnson (1986) mapped the mean square error (RMSE) of the survey is Emerald ice margin based on the sharp bend in 0.29 ft. Even with LiDAR‘s high resolution, Fourmile Creek downstream (NW) from a broad, flat primary glacial landforms are lacking on till valley filled with Late Wisconsin lake sediment (crosssurfaces in the 90 sq km study area. hatched area). Each ―x‖ marks meltwater features discovered by LiDAR terrain analysis. The river valley However, ArcMap‘s 3D analyst sub-parallel to the proposed Emerald Phase ice margin tools and ArcScene‘s 3D visualization might have been incised along a recessional icecapabilities reveal three distinct fluvial marginal position. channels incised in the Poskin and Pokegama Creek till surfaces (Fig. 1). Longitudinal and cross-sectional profiles were generated to study channel morphology. These meltwater channels are sinuous, 900 to 1000 ft long, and reach maximum depths of 20 ft. They are different from modern channels because they cut across drainage divides, are sub-parallel to the land contour in some places, and typically appear abruptly on the landscape with little area for water catchment. Channel MC-1 (Figs. 1, 2) is an example of a lateral meltwater channel formed during deglaciation from the early Chippewa Phase ice maximum. Overall, these channels are evidence for a younger, lessmodified landscape impacted by the Late Wisconsin Glaciation as compared to the River ILSG 2011 35 Program and Abstracts �Falls Fm. till surface from the Illinoian Glaciation which lacks fresh glaciofluvial and glacial landforms. LiDAR data did not reveal a sharp landform-assemblage difference between the Late Wisconsin and Illinoian till surfaces. Figure 2. (A) 3D view of lateral meltwater channel MC-1 generated with ArcScene 10. Note: Vertical elevation is exaggerated 4x and the arrows represent water-flow direction. (B) A 2D map view of MC-1 created in ArcGIS 10. Near the midpoint of the channel, modern drainage is toward the northeast. This valley must have been filled with ice to force the water flow to the east during deglaciation. REFERENCES Johnson, M.D., 1986, Pleistocene Geology of Barron County, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 55, 42 p. Syverson, K.M., Clayton, L., Attig, J.W., and Mickelson, D.M., eds., 2011, Lexicon of Pleistocene Stratigraphic Units of Wisconsin: Wisconsin Geological and Natural History Survey Technical Report 1, 180 p. ILSG 2011 36 Program and Abstracts �Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan Daniel J. Foley1 and James D. Miller1 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 The Bovine Igneous Complex (BIC), located 8 km southeast of the town of L‘Anse, Michigan, is a small basin-shaped mafic/ultramafic intrusion emplaced in the southwestern part of the Baraga Basin. Although age dating of the intrusion has so far been unsuccessful, the BIC intrusion was very likely emplaced during the early magmatic stage of Midcontinent Rift development, given its similarities to other early stage intrusions, such as Tamarack (MN) and Eagle (MI). Investigated by Kennecott as a possible Cu-Ni-PGE prospect, the intrusion has undergone extensive exploration drilling since 1995. This work has shown the intrusion to be weakly to moderately mineralized with Cu-Ni-PGE-enriched sulfides. Metal tenors provided by initial drilling averaged less than .5% Cu and Ni, and less than 350 ppb Pt and Pd (Rossell, 2008). For this study, which is part of Dan Foley‘s MS thesis, two drill cores that profile the BIC (08BIC044 and BIC0101) were investigated for their petrographic attributes, cryptic mineral compositions, and whole rock geochemistry. A detailed (1:6,000) re-mapping of the BIC was also conducted for this study. Preliminary field and petrographic studies by Rossell (2008) interpreted the intrusion to be a simple three unit system composed of a basal wehrlite/melagabbro, overlain by a clinopyroxenite/gabbro, and finally an oxide gabbro. Field mapping, core logging, and petrography conducted for this study have found that the lithostratigraphy of the BIC is a somewhat more complicated. The stratigraphy can be subdivided into three main zones – a lower ultramafic zone, an upper ultramafic zone, and a gabbro zone, each of which can be further subdivided by cumulate mineralogy. As profiled in core 08BIC044 (Fig. 1), the lower ultramafic zone is in sharp contact with a footwall of granitic gneiss at about 670m. A medium fine-grained feldspathic wehrlite (Ol cumulate with intercumulus Cpx and Pl) occurs at the basal contact and gradually coarsens up section and becomes less feldspathic. At about 525m, augite abruptly increases in mode and becomes granular to create a feldspathic olivine pyroxenite (Cpx+Ol cumulate with intercumulus Pl). The contact with the base of the upper ultramafic zone, at about 500m, is marked by the abrupt reappearance of feldspathic wehrlite that is vari-textured and contains abundant inclusions of chert and carbonate. Several fine-grained mafic dikes cut the lower 70 meters of this heterogeneous wehrlite. Above the uppermost dike, a more homogeneous, medium-grained feldspathic wehrlite (Ol cumulate with intercumulus Cpx and Pl) persists up to about 205m, at which point cumulus augite reappears and the modal rock type becomes a feldspathic olivine clinopyroxenite (Cpx+Ol cumulate with intercumulus Pl). At about 75m, an abrupt increase in the Fe-Ti oxide mode to about 10% and a loss of olivine generates a feldspathic oxide clinopyroxenite (Cpx+Ox±Ol cumulate with intercumulus Pl). Soon thereafter (~ 60m), plagioclase becomes abundant (>50%) and lath-shaped to create an oxide gabbro (Pl+Cpx+Ox cumulate). Apatite becomes a cumulus phase at about 50m to create an uppermost cumulate of Pl+Cpx+Ox+Ap. Outcrops of apatitic oxide gabbro, at presumably higher stratigraphic levels than seen in drill core, contain patches of interstitial granophyre. Assuming upward-directed crystallization, this igneous stratigraphy implies a cumulus paragenesis of: Ol Cpx+Ol // Ol Cpx+OlCpx+Ox±Ol Pl+Cpx+OxPl+Cpx+Ox+Ap. The cumulus regression evident at the lower and upper ultramafic zone contact and the heterogeneous nature of the basal upper ultramafic zone strongly implies that this contact demarks two major magma emplacement events. Further evidence of two episodes of magma emplacement come from mineral chemical data on olivine and augite. Cryptic variations of Fo and En components through core 08BIC044 ILSG 2011 37 Program and Abstracts �(Fig. 1) show trends that are consistent with two major episodes of emplacement followed by fractional crystallization. The base of each ultramafic zone is characterized by decreased En content of postcumulus augite which is consistent with chilling of a parental magma. Fo content of olivine in the lower ultramafic zone remains elevated, which is consistent with chilling of primocrystic olivine. Olivine at the base of the upper ultramafic zone shows a decrease in Fo suggesting reequilibration of a new magma pulse with the resident magma. As both the lower and upper ultramafic zone wehrlites transition into olivine clinopyroxenites, both Fo and En decrease, which is consistent with progressive iron enrichment due to fractional crystallization. Interestingly, the upper ultramafic zone and overlying gabbro zone progress to more evolved cumulates, but the cryptic variation is more muted than in the lower ultramafic zone. Noting that the upper ultramafic cumulates are more adcumulate (i.e. contain less postcumulus minerals) than the lower ultramafic zone cumulates, the more subdued cryptic variation of the upper cumulates may be due to a lower trapped liquid shift. A suite of 27 samples have been submitted for lithogeochemical and assay analyses, but the results were not available at the time of this writing. We hope to report on the geochemical data at the meeting. The whole rock geochemistry will be used to evaluate whether the two magma pulses involved similar parental magma composition. Analyses of wehrlite from the base of the lower ultramafic zone and mafic dikes from the base of the upper ultramafic zone will be evaluated as potential candidates for chilled parent magma compositions. The geochemical data will also be used to evaluate the history of sulfide saturation and metallogenesis during the crystallization of the BIC magma(s). Figure 1. Lithostratigraphy and cryptic variation of Fo in olivine and En in augite in DDH 08BIC044. Unit abbreviations are fWERfeldspathic wehrlite, fOCPfeldspathic olivine clinopyroxenite, Db-diabase, fOxCP – feldspathic oxide clinopyroxenite, OxGB – oxide gabbro, AOxGB – apatitic oxide gabbro ILSG 2011 38 REFERENCES Rossell, D., 2008. Geology of the Keweenawan BIC Intrusion. 54th Annual Institute on Lake Superior Geology, Part II: Field Trip Guidebook, pp. 181-193. Program and Abstracts �Sedimentology of a wet, pre-vegetation floodplain assemblage: the Mesoproterozoic, Outan Island Formation, Sibley Group, Ontario, Canada Philip Fralick1 and Kamil Zaniewski2 1 Department of Geology, Lakehead University, P7B 5E1, Thunder Bay, Ontario, Canada philip.fralick@lakeheadu.ca 2 Department of Geography, Lakehead University, P7b 5E1, Thunder Bay, Ontario, Canada Kamil.Zaniewski.lakeheadu.ca Descriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style. This is probably in part due to the scarcity of well-developed floodplain successions in fluvial assemblages of these ages. Those studies that do exist are concerned with depositional process on semi-arid and arid floodplains, which can be modeled by comparison with the recent. This investigation adds to the knowledge base on wet, pre-vegetation floodplain deposits and processes with a description of a continuous succession of drill-core through an extensive, 1,400 Ma, delta-top channel-floodplain assemblage. This succession forms the upper portion of the Outan Island Formation in the central portion of the one kilometer thick Sibley Group, present in northwestern Ontario, Canada. The sub-aerial floodplain deposits are dominated by flaser to lenticular bedded, fine-grained sands to shales, with abundant small shale rip-up clasts. Iron oxide-rich, millimeter-thick shale laminae denote exposure surfaces. Soft-sediment deformation of these units is ubiquitous with loading and injection features the most prominent. Where soft sediment deformation is minor the layering appears lensy, with the ripple laminated, fine-grained sandstones able to maintain their appearance. However, significant sections of the core suffer more severe deformation with the layering developing a wispy appearance. Thicker medium-grained sandstones, representing crevasse splays, commonly have protocols (very immature soils) developed in their upper portions. Geochemistry indicates downward movement of meteoric water through the upper portions of the crevasse splays. Well-laminated sediments with wave ripples and only rarely containing rip-up clasts and soft-sediment deformation were deposited in floodplain ponds. These sub-aqueous units are commonly orange or green rather than the ubiquitous red of the sub-aerial deposits. Reduction spots centered on specks of carbon, which is probably of organic origin, indicate that life did exist on the sub-aerial surface, but the highly oxidizing nature of the sedimentary environment led to little preservation of this material other than the prolific, transported carbon specks. These deposits differ from post-vegetation floodplain sediments in having: 1) better preservation of layering without rootlet bioturbation; 2) dominance of rippled sand on the floodplain, probably due to lack of vegetation induced baffling; 3) large-scale generation of small, intraformational clasts, possibly as a result of less rootlet binding; 4) desiccation crack fills consisting of soil peds and locally derived intraclasts, probably delivered due to erosion of the floodplain surface during rainfall events; 5) ubiquitous soft-sediment deformation features in sub-aerial deposits; and, 6) welllaminated, oxidized sediment that accumulated in floodplain ponds. ILSG 2011 39 Program and Abstracts �Figure 1. Diagram showing the types of layering developed in various sub-environments of the depositional system. A- Main Channel; B and C- Levee; E and F- Proximal to Distal Crevasse Splay; H- Wave reworked Crevasse Splay; D and G- Floodplain; I, J and KFloodplain Pond. ILSG 2011 40 Program and Abstracts �A review of the western Superior lithoprobe line 3 Paul Geller, Phil, Fralick, Mary Louise Hill, and Patricia Gillies Lakehead University Department of Geology 955 Oliver Road, P7B 5E1, Thunder Bay, Ontario, Canada. pgeller@lakeheadu.ca The Lithoprobe project was a Canada-wide project that had a multi-disciplinary approach to mapping the deep crust. Line 3 of the Western Superior transect crosses from the Eastern Wabigoon Subprovince into the Quetico Subprovince. The transect passes through two terranes of the Eastern Wabigoon Subprovince: the Onaman-Tashota Terrane and the Beardmore-Geraldton Belt. The Onaman-Tashota Terrane is a patchwork of igneous intrusive and extrusive rocks. The Beardmore-Geraldton Belt is a collection of interlayered metasedimentary and metaigneous units. The Quetico Subprovince is metasedimentary, primarily composed of metaturbidites intruded by felsic igneous rocks and migmatized in places. The seismic section does not follow a straight-line but instead has an east-west jog to the otherwise north-south trend (Figure 1). This, as well as the related areas of low data confidence, has important ramifications for the interpretation. Gravity data was used in collaboration with surface geology to add an extra degree of robustness to the investigation of structures at depth. A structure is present, dipping at approximately 12° to the south at the boundary between the Quetico Subprovince and the Wabigoon Subprovince. Structures inferred to be the Paint Lake Fault and the Jellicoe River Fault appear below the surface expression of these features, with dips of approximately 86°. Other features that seem to appear in the section must be dismissed at present as they occur in areas of low confidence. Figure 1. A 3D representation of the seismic line from surface to approximately 48 km depth, with associated geology and interpreted geology. ILSG 2011 41 Program and Abstracts �Stratigraphic investigation of the Neoarchean Bird River Belt, SE Manitoba, Canada H. Paul Gilbert Manitoba Geological Survey 360-1395 Ellice Ave. Winnipeg Manitoba Canada R3G 3P2 The Neoarchean Bird River Belt (BRB) is located in the Bird River Subprovince within the southwestern Superior Province, between flanking older cratonic blocks — the 3.0-2.87 Ga North Caribou Superterrane to the north and the 3.4-2.8 Ga Winnipeg River Subprovince to the south (Percival et al., 2006). The BRB consists of diverse volcanic and sedimentary rocks in two structural panels (north and south), separated by the relatively younger turbiditic Booster Lake Formation that extends through the centre of the belt. The calcalkaline north panel rocks (ca. 2733 Ma) are akin to modern subduction-related rocks at active continental margins, whereas the tholeiitic sequence in the south panel (ca. 2725 Ma) documents incipient rifting in an extensional tectonic regime (Fig. 1). The arc–type rocks are flanked to both north and south by mid-ocean-ridge basalt (MORB)–type sequences that are interpreted as relatively older than the arc-type rocks, and may be associated with early arc rifting in a back-arc setting (Gilbert et al., 2008). The north and south panel rocks, although distinguished by different composition and age, have broadly similar internal stratigraphies. The south panel contains a lower felsic volcanic sequence overlain by voluminous pillowed mafic flows and an upper, diverse volcano-sedimentary assemblage. The north panel is also interpreted to consist of a lower felsic volcanic sequence – Peterson Creek Formation (PCF), overlain by varied sedimentary and volcanic rocks – the Diverse Arc assemblage (DAA), which includes a pillowed amygdaloidal andesite member at the base. The PCF is a felsic volcanic complex composed mainly of volcanic fragmental rocks and subordinate rhyolite flows. A spherulitic zone within massive flows that extends laterally within the PCF for over 20 km represents a stratigraphic break where rapid chilling was associated with alteration and penecontemporaneous devitrification (Fig. 2). Compositionally distinctive, intrusive (and possibly extrusive) sanukitoid-type rocks (Fig. 1) are interpreted as late- or post-DAA in age. Whereas these rocks intrude the DAA as well as Booster Lake Formation rocks, sporadic clasts of apparent sanukitoid type within some DAA conglomeratic deposits suggest the deposition of DAA rocks and Booster Lake Formation turbidites may have been, in part, contemporaneous. Detrital zircon U-Pb data that indicate maximum depositional ages for the DAA (2706 ±23 Ma) and Booster Lake Formation (2712 ±17 Ma) are consistent with this model (Gilbert, 2008). Overlap of sanukitoid-type magmatism with late volcanism is also supported by the occurrence of synvolcanic sanukitoid massive to fragmental volcanic rocks with an inferred igneous age between 2715 Ma and 2723 Ma in the contiguous Rice Lake belt, north of the BRB (Anderson, 2008). ILSG 2011 42 Program and Abstracts �Figure 1. Th/Ta vs Yb plot of mafic to intermediate volcanic rocks in the Bird River Belt north and south panels, and late sanukitoid-type intrusive rocks. OA = Oceanic arc; ACM = Active continental margin; WPVZ = Within-plate volcanic zone. Figure 2. Chondrite-normalized REE plot of massive and spherulitic rhyolite (PCF). REFERENCES Anderson, S. D. 2008. Geology of the Rice Lake area, Rice Lake greenstone belt, southeastern Manitoba (parts of NTS 52L13, 52M4); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Report GR2008-1, 97 p. Gilbert, H.P., 2008: Stratigraphic investigations in the Bird River greenstone belt, Manitoba (part of NTS 52L5, 6); in Report of Activities 2008, Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, p.121-138. Gilbert, H.P., Davis, D.W., Duguet, M., Kremer, P.D., Mealin, C.A. and MacDonald, J. 2008: Geology of the Bird River Belt, southeastern Manitoba (parts of NTS 52L5, 6); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Map MAP2008-1, scale 1:50 000 (plus notes and appendix). Percival, J.A., McNicoll, V. and Bailes, A.H. 2006: Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and >2.98 Ga continent margin sequences and its implications for Archean terrane accretion, western Superior Province, Canada; Canadian Journal of Earth Sciences, v. 43, p. 895–927. ILSG 2011 43 Program and Abstracts �Petrology of the Ni-Cu-PGE-mineralized Tamarack Intrusion, Aitkin and Carlton Counties, Minnesota Brian D. Goldner*1 and James D. Miller2 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 *Present Address: Rio Tinto Exploration, 224 N 2200 W Salt Lake City, UT, 84116 gold0334@umn.edu 2 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 mille066@umn.edu The Tamarack intrusion is an unexposed mineralized ultramafic intrusion located near the town of Tamarack, about 50 miles west of Duluth, Minnesota. Rio Tinto Exploration (previously Kennecott Exploration) has been conducting exploration drilling of the Tamarack intrusion for Cu-Ni-PGE sulfide deposits since 2001. The intrusion was emplaced into black slates of the Paleoproterozoic Animikie Basin during the early magmatic stage of the 1.1Ga Midcontinent Rift and a new U-Pb baddeleyite age reported here yields an age of 1105.6 ± 1.3 Ma. Drilling and geophysical data indicate that the Tamarack intrusion has a tadpole-like shape that is about 13 km long and is between 1 and 4 km wide (Fig. 1). The narrow tail area of the intrusion, which is the site of greatest exploration drilling, is composed of exclusively of ultramafic rock types. The wider ―body‖ area at the southeastern end of the intrusion is composed of a wider variety of rock types ranging from lherzolite to granophyric gabbronorite. Core logging, petrographic observations, mineral chemical analyses, lithogeochemical analyses, and XRF scanning of drill core were employed on four drill cores from the Tamarack intrusion to evaluate its emplacement and crystallization history. Core logging and petrography in three cores from the tail area shows that this part of the intrusion can be subdivided into two texturally and modally distinct units of olivine cumulates – a lower Feldspathic Lherzolite Unit composed of coarse-grained olivine mesocumulates with poikilitic pyroxenes and plagioclase, and an upper Lherzolite Unit composed of medium-grained olivine mesocumulates to adcumulates. The contact between the two lherzolite units was investigated all three cores. In one, the contact occurs across a meter-thick zone of intense alteration; in another, it shows the two lherzolite lithologies irregularly interlayered; and in a third, the feldspathic lherzolite is contains gabbroic inclusions of unknown origin. The one 268-m-long drill core investigated from the body area ends in an olivine mesocumulate similar to the upper Lherzolite Unit of the tail area. At 225m, the lherzolite gives way up section to a 10m-thick interval of intergranular olivine websterite (Cpx+Opx+Ol cumulate) which in turn gives way to an intergranular lherzolite (Ol+Cpx+Opx cumulate). At 210m, plagioclase abruptly increase in mode (~50%) and changes from poikilitic to granular resulting in a gabbronorite. Granular augite and orthopyroxene persist into the gabbronorite, oxide increase to about 5 vol. %, but olivine disappears. This gabbronorite is interpreted to a Pl+Cpx+Opx ±Ox cumulate. The upper 50 meters of the core (below a 59m-thick cap of till) contains interstitial to irregular segregations of granophyre and shows the reappearance of olivine. Disseminated Ni-Cu-PGE sulfide mineralization is present throughout most of the lithologies studied here, but is particularly abundant in the tail area at the basal contact of the ILSG 2011 44 Program and Abstracts �Feldspathic Lherzolite unit and in a zone straddling the contact between the two lherzolite units. Whereas mineral chemical and whole rock data within the feldspathic lherzolite and lherzolite units found in the tail of the intrusion show little cryptic variation, the chemical attributes of lithologies in the body areas show evidence extreme differentiation. Olivine composition ranges from Fo84 in the lowermost lherzolite and becomes progressively evolved to Fo10 in the uppermost granophyric gabbronorite. Other mineral and whole rock data both show smooth gradations from the lherzolite to the gabbronorite which are consistent with these lithologies having formed from one parental magma by fractional crystallization in a closed system. The main petrologic conclusions of this study are: 1) The two lherzolitic units in the tail area formed from a similar high-Mg olivine tholeiitic parent magma. The composition of this parent magma is estimated from subtracting 30% Fo89 olivine phenocryst composition from a chilled margin found at the basal contact of the Feldspathic Lherzolite unit. The resulting composition is comparable to other picritic compositions found at the base of MCR-related Mamainse Point Volcanics. 2) The emplacement of the lower feldspathic lherzolite preceded that of the upper lherzolite in the tail area. The differences in texture and modal mineralogy between the two lherzolite units are attributed to the more rapid cooling of the earlier feldspathic lherzolite creating an orthocumulate in contrast to the more adcumulate upper lherzolite. 3) The sulfide mineralization straddling the lherzolite contact in the tail area is attributed to country rock assimilation and S contamination of the leading edge of the lherzolite parent magmas during the two main episodes of emplacement. The sulfide mineralization in upper part of the Feldspathic Lherzolite is thought to be related to downward infiltration of sulfide liquid from the overlying Lherzolite unit magma upon its emplacement into the semi-molten core of the Feldspathic Lherzolite. 4) Finally, the well differentiated lithologic sequence comprising the body area is interpreted to have resulted from closed-system fractional crystallization of the second magma pulse that created the upper Lherzolite unit in the tail area (Fig. 1). Tai l Body till Gn Lh fLh Figure 1. Possible correlation of lithologies between the tail and body of the Tamarack intrusion. fLh – feldspathic lherzolite unit; Lh – lherzolite unit; Gn – gabbronorite unit ILSG 2011 45 Program and Abstracts �The Minnesota Taconite Workers Health Study: Environmental study of airborne particulates - 2011 update George Hudak1, Stephen Monson Geerts1, Larry Zanko1, April Severson1, Allison Severson1, Bryan Bandli2 1 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811 2 Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Since 2008, the Natural Resources Research Institute (NRRI) has been conducting a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) by characterizing airborne particulate matter within taconite operations, in communities surrounding taconite operations on the MIR, in population centers in other regions of northeastern Minnesota, as well as particulate matter deposited in lake sediments (Figure 1). NRRI‘s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort investigating long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore). The UM School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling is performed within taconite operations, MIR communities, and nonMIR communities by NRRI scientists during both winter and summer seasons. Sampling at taconite operations takes place at four locations: 1) secondary crushers; 2) magnetic ILSG 2011 46 Program and Abstracts �separators/concentrators, agglomerators/ ball drums, and the kiln/pellet discharge area. Sampling within MIR communities takes place on the rooftops of public buildings, whereas sampling in non-MIR communities occurs on rooftops or in remote locations so that background air quality can be evaluated. Airborne particulate are collected using a microorifice uniform deposit impactor (MOUDI) (Marple et al., 1991), which enables sizefractionated particulate collection, and a total suspended particulate filter (TSP). Particulate samples are evaluated via gravimetric analysis and subsequently subjected to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health‘s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization‘s Method 10312 Ambient air – Determination of Asbestos Fibers – Direct-Transfer TEM Method (ISO 10312, 1995). Over the past year, the NRRI has completed particulate sampling within MIR taconite operations, MIR communities, and non- MIR communities. This includes 14 sampling events at taconite operations and 67 sampling events at locations within communities in northeastern Minnesota, as summarized in Table 1. As well, continued analysis of lake sediment samples from ―North of Snort Lake‖ indicates collection of sediment dating back to ~1840, which pre-dates iron mining on the MIR. At Silver Lake, logging activities in the early part of the 20th century disrupted the sediments; however, we believe we will have good post-1915 lake sediment data, which will mark the period where the transition from natural ore to taconite mining took place. Continued analysis, interpretation and reporting will take place in 2011. Taconite Facility United Taconite In-Plant Sampling Events Sampling Events Taconite Facility 2 events while active Keetac Hibbing Taconite Sampling Events 1 event while active, 1 event while inactive 1 event while active, 3 events while active 3 events while active 1event while active, Northshore 1 event while inactive Minntac 1 event while active Minorca Community Sampling Events Community Sampling Location Sampling Events and Number per Season Silver Bay High School 11 Events (4 Winter / 7 Summer) Virginia Court House 9 Events (4 Winter / 5 Summer) Hibbing High School 9 Events (4 Winter / 5 Summer) Keewatin Elementary School 6 Events (3 Winter / 3 Summer) Babbitt Municipal Building 15 Events (7 Winter / 8 Summer) Duluth NRRI Rooftop 10 Events (4 Winter / 6 Summer) Ely Fernberg Site 7 Events (4 Winter / 3 Summer) Table 1. Summary of in-plant and community sampling events REFERENCES ISO 10312, 1995, Ambient air – determination of asbestos fibers – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A microorifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-2006-02, 10 p. ILSG 2011 47 Program and Abstracts �Bedrock geologic map of Minnesota—Precambrian geology Mark A. Jirsa, Terrence J. Boerboom, and Val W. Chandler Minnesota Geological Survey (MGS), 2642 University Ave., St. Paul, MN, (www.mngs.umn.edu) Digital products associated with the new bedrock geologic map of Minnesota (MGS State Map Series S-21) allow removal of Cretaceous, Paleozoic, and some parts of Mesoproterozoic strata to reveal an interpretation of the underlying Precambrian bedrock (simplified on Figure 1). Corresponding GIS data tables permit attribute searches based on age, terrane, and lithologic and nomenclatural subdivisions; which allow users to create attribute-based thematic maps. Companion data containing an outcrop database and all known geochronologic analyses linked to sample locations were published in 2010 as MGS Open-File Report OF10-02. Collectively, these products provide a new interpretation of Minnesota‘s Precambrian terranes, summarized here. Much of the Precambrian geology on S-21 is depicted in the context of major orogenic, rifting, and meteorite impact events. The understanding of these events in the Lake Superior region has evolved considerably in recent years by the works of many authors. The Archean rocks are subdivided approximately by their apparent temporal relationship to three major periods of deformation; the first (D1) at about 2695 Ma, which may be equated with the Shebandowanian orogeny in adjacent Ontario; the second (D2) at about 2680 Ma occurred during the Minnesotan orogeny and produced regional transpressive fabrics, folds, and metamorphism to greenschist-amphibolite grade (orogenic nomenclature via Percival and others, 2006). The former may represent collision of the Wawa subprovince with the composite Superior superterrane to the north. The latter can be attributed to oblique collision of the Minnesota River Valley subprovince with the Superior superterrane along a north-dipping suture known as the Great Lakes Tectonic Zone. A D3 event is manifest in the Quetico subprovince as broad folds involving D2 fabrics, and elsewhere by faulting. Structures bounding major components of the Superior Province are locally inferred to be thrust faults that initially formed during terrane assembly. Although little is known in detail about their vergence and offset history, the interpretation here builds on seismic surveys in Minnesota, and extrapolation from results of the Western Superior Lithoprobe and NATMAP Transects in adjacent areas of Canada (Percival and Helmstaedt, 2006). The Proterozoic rocks are the products of 4 orogenic and rifting events. The Paleoproterozoic rocks contain evidence for a cycle of rifting and continental collision during the Geon 18 Penokean orogeny. An ejecta blanket resulting from the 1850 Ma Sudbury meteorite impact forms a definitive time-line within the Animikie Group at the stratigraphic top of Gunflint and Biwabik Iron Formations. Crust in the southern half of the state was deformed, metamorphosed, and plutonized to varying degrees during Geon 17 Yavapai orogenesis. The Yavapai Province in extreme southeastern Minnesota contains older continental crust deformed during Geon 17 and intruded by granitic rocks. Although deposition of strata assigned to the Animikie Basin began during the late stages of Penokean orogenesis, it is likely that sedimentation continued during the Yavapai event. The effects of later orogenic events in Minnesota are unclear. The Geon 16 Mazatzal orogeny is well delineated in adjacent Wisconsin; however, only scant evidence of metamorphic overprint is documented in the McGrath Gneiss in Minnesota, and alteration of the Sioux Quartzite is assigned to a Geon 14 hydrothermal event. Mesoproterozoic rocks result from Geon 11 continental rifting, producing volcanic and sedimentary rocks of the Keweenawan Supergroup and plutonic rocks of the Midcontinent Rift Intrusive Supersuite. Mafic dike swarms of inferred Paleoproterozoic and Mesoproterozoic age represent significant episodes of crustal extension. ILSG 2011 48 Program and Abstracts �Figure 1. Terrane map of Precambrian bedrock showing subprovinces of the Archean Superior Province (caps), significant components of Paleoproterozoic bedrock, the Mesoproterozoic Midcontinent rift, and mafic dikes (gray) of Paleoproterozoic and Mesoproterozoic age. REFERENCES Percival, J.A., and Helmstaedt, H., 2006, The Western Superior Lithoprobe and NATMAP transects: Introduction and summary: Can. Jour. Earth Sci. 43:743-747 and articles therein. Percival, J.A., Sandborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J., 2006, Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies: Can. Jour. Earth Sci. 43:1085-1117. ILSG 2011 49 Program and Abstracts �Highlights of the new bedrock geologic map of Minnesota Mark A. Jirsa, Terrence J. Boerboom, Val W. Chandler, John H. Mossler, Anthony C. Runkel, and Dale R. Setterholm Minnesota Geological Survey (MGS), 2642 University Ave., St. Paul, MN, (www.mngs.umn.edu) The latest bedrock geologic map of Minnesota—MGS State Map Series S-21*—is a new construct that incorporates existing geologic maps where prior mappers had adequate ground control, and new interpretations based on drill hole, geophysical, and unpublished data where they did not. The interpretation differs significantly from previous maps to reflect the new data and accommodate scale. It portrays our current geologic understanding of the temporal and geographic distribution of units within major Precambrian terranes (Jirsa and others, this volume) and of Phanerozoic strata. The previous state-wide compilation of bedrock geology, S-20, was created in 2000, modified in 2003, and produced at a scale of 1:1,000,000. Although S-20 was a suitable image for its time, the new map shows considerably more detail at twice the scale; it incorporates recent detailed mapping and previously unpublished data; and its interpretations are reconciled with reprocessed, highresolution geophysical data. Digital files associated with this map allow removal of Cretaceous, Paleozoic, and some parts of Mesoproterozoic strata to reveal an interpretation of the underlying Precambrian bedrock. Corresponding GIS data tables permit attribute searches based on age, terrane, and lithologic and nomenclatural subdivisions. Ancillary files, published in 2010 as MGS Open-File Report OF10-02*, include new state-wide maps of outcrops, bedrock topography, depth to bedrock, and geochronologic data. Major areas of improvement include the depiction of Precambrian rocks beneath Phanerozoic strata, and the subdivision of Archean units in a broad area of subdued magnetic character north of the Great Lakes Tectonic Zone that was formerly referred to as ―the Quiet Zone.‖ Mafic dike swarms are the remnants of significant crustal extension events during the Paleoproterozoic and Mesoproterozoic. Despite this significance, and the prominent magnetic fabric created by dike anomalies, this map is the first attempt at state-wide depiction. Improvements in Phanerozoic geology include a much more accurate western erosional edge of Paleozoic bedrock, subdivision of Cambrian strata into two units, and the use of modern stratigraphic nomenclature that is more broadly correlative with adjacent jurisdictions. The Phanerozoic geology was compiled from mapping that required contouring of bedrock surfaces using test hole and water well data. Much of the Cretaceous depiction, for example, is taken from grid-subtraction of contoured elevations of top and bottom of units. The new map provides a jumping-off point for future scientific and exploration endeavors. It should be noted that the geologic depiction is based solely on geophysical maps and scant drill hole information in many areas. In addition, the creation of S-21 did not involve logging of all drill core and MGS well-cuttings sets, targeted geophysical modeling, and test drilling, as would be conducted for larger-scale mapping. The lack of ground-truth in much of central and western Minnesota is a call to new drilling. Recent studies in other regions have shown that both temporal and tectonic considerations are needed to identify greenstone belts with enhanced base and precious metal prospectivity. In light of this, the dearth of high-precision geochronologic and lithogeochemical data for Precambrian supracrustal rocks in Minnesota clearly needs to be addressed. Work on the topography of major bounding surfaces, and the depiction of geology beneath them, is on-going. *MGS Open-File Report OF10-02 and State Map Series S-21 were prepared and published with the support of the Minnesota Legislature, as administered by the Minerals Coordinating Committee. ILSG 2011 50 Program and Abstracts �Paleomagnetism and Paleointensity as recorded by 1.08 GA Lake Shore Traps (Keweenaw Peninsula, Upper Michigan) Evgeniy V. Kulakov1, A.V. Smirnov1, J.F Diehl1 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University,1400, Townsend Drive, Houghton, Michigan, 49931, USA. Here we report new results of paleomagnetic and paleointensity investigations carried out on samples collected from a sequence of lava flows (12 lava flows) interbedded within the Copper Harbor Conglomerate on the Keweenaw Peninsula of Michigan. This sequence, known as the Lake Shore Traps (LST), represents the latest stage of volcanic activity associated with North American Mid-Continent Rift (MCR). Our sampling localities were on Silver Island (10 lava flows) situated along the western shore of Keweenaw Peninsula north of Eagle Harbor, MI and on the mainland (two lava flows) directly east of Silver Island. The Silver Island lava flows are equivalent to the upper lava flows of the Middle LST at the tip of the Peninsula while the mainland lava flow are equivalent to lower lava flows of the middle LST (Diehl and Haig, 1994; Diehl et al., 2009). At the tip of the Peninsula, the upper and lower sections of the middle LST are separated by a 27 m thick conglomerate layer. The LST flows exposed along the western coast of Keweenaw Peninsula and at its northern tip are characterized by different strike and dip angles. That allows us to use the paleomagnetic directional data to test the conclusion by Hnat et al. (2006) that the curvature of MCR in the Keweenaw Peninsula, where structural trends vary from NE/SW to E/W to NW/SE, is a primary rift structure (as opposed to a secondary feature caused by oroclinal bending of the rift related rocks). Characteristic remanent magnetizations (ChRM) were isolated by thermal or alternating field demagnetization, or by combination of both techniques. ChRM was interpreted as primary magnetization, acquired during the original cooling of these lavas. Mean paleomagnetic directions were combined with the results from the previous study (Diehl and Haig, 1994) of the Lower and Upper Middle LST, sampled at northern tip of Keweenaw Peninsula. The group means (N=2) for Upper and Lower Middle LST from the western sites coincide at 95 per cent confidence with those reported for same flows exposed on the northern tip of the peninsula. This provides further support to the conclusion that the curvature of MCR is primary in its origin. We also performed determinations of the strength of Earth magnetic field (paleointensity) recorded by the Lake Shore Traps. The paleointensity database for the Precambrian is very limited and needs to be significantly extended. Determinations of absolute paleointensity were conducted using Thellier method (Thellier and Thellier, 195?) on 107 samples from nine lower lava flows and ten upper flows. 43 per cent (46 samples) of determinations passed our reliability criteria and have been accepted. Paleointensity (36 μT) recorded by the oldest lower flows is relatively high and comparable with the modern field strength. Paleointensity decreases to ~13.2 μT toward in the younger lower flows. All upper flows are characterized by low values of paleofield with mean of ~13.1 μT. Our paleointensity values are less than 50% the values reported by Pesonen and Halls (1983). We suggest that their values are biased because of the use of low-temperature demagnetization data for calculating paleointensity. ILSG 2011 51 Program and Abstracts �REFERNECES Diehl, J.F., T.D. Haig, 1994, A paleomagnetic study of the lava flows within the Copper Harbor Conglomerate, Michigan: new results and implications, Canadian Journal Sciences, 31, 369-380. Diehl, J. F., A.J. Durant, C. Schepke, 2009, Paleomagnetism of Silver Island, Keweenaw Peninsula, Michigan: Additional Support (?) for the Primary Curvature of the MCR, Eos Trans. AGU, 90(22), Jt. Assem. Suppl., Abstract GP11E-01. Hnat, J.S., B.A. van der Pluijm, R. van der Voo, 2006, Primary curvature of the MidContinent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA), Tectonophysics, 425 Pesonen, L. J., H.C. Halls, 1983, Geomagnetic field intensity and reversal asymmetry in late Precambrian Keweenawan rocks. Geophysical Journal. Royal Astronomical Society, 73, 241-270. Thellier, E., and O. Thellier, 1959, Sur l‘intensite´ du champ magne´tique terrestre dans le passe´ historique et ge´ologique: Annales Ge´ophysique, v. 15, p. 285–378. ILSG 2011 52 Program and Abstracts �Testing models of late Paleoproterozoic Penokean orogenesis in the Great Lakes Region, U.S.A. using sedimentary provenance: planning the investigation Todd A. LaMaskin Department of Environmental Science, University of Wisconsin -Extension & Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705, lamaskin@wisc.edu Aspects of the overall plate-tectonic setting and regional controls on crustal deformation during late Paleoproterozoic time in the Lake Superior region of the U.S. midcontinent remain debated. Synthesis of the geology and tectonic history of the region suggests that both marginal-basin regime and Cordilleran-style plate-margin tectonic processes were well established in late Paleoproterozoic time; however, the driving forces that produced crustal deformation and coeval subsidence are not fully understood. During Paleoproterozoic time, the southern periphery of Laurentia was a convergent margin that experienced episodic southward continental growth through accretion of both arc and micro-continent lithosphere (Fig. 1). Tectonic "events" during the Penokean orogenic interval (ca. 1.9–1.75 Ga) represent the initiation of this long-lived stage of accretionary southward growth of the North American craton. The Penokean Orogenic Province (POP; Fig. 1) of the Lake Superior region provides an excellent testing ground for evaluating the style and timing of lithospheric accretion in the Proterozoic growth of Laurentia. The POP contains Archean and Paleoproterozoic rocks that represent two distinct magmatic arcs and their associated sedimentary basins (Marshfield and Pembine-Wasau terranes, respectively), as well as two temporally distinct, spatially overlapping sedimentary successions: (1) turbidite iron-formation basins of the Menominee Group, and (2) a collisionrelated foreland fold-thrust complex (Baraga-Animikie Groups). The original composition of weathered source rocks is a dominant control on the makeup of terrigenous sediments, therefore, geographic and stratigraphic variations in sediment provenance in both successions can provide important constraints on the tectonic evolution of the region. Investigation of sedimentary successions in the POP is planned using U-Pb and Hf detrital zircon analysis, mudrock trace-element and Sm-Nd geochemistry, and (U-Th)/He thermochronology. The sampling program is designed to test specific predictions of proposed and new models for the tectonic setting of individual basins, and for the tectonic evolution of the region as a whole. ILSG 2011 53 Program and Abstracts �Figure 1. Map showing major Precambrian crustal provinces in the United States prior to accretion of the Mazatzal Province and formation of Mesoproterozoic igneous and tectonic provinces. ILSG 2011 54 Program and Abstracts �The Biwabik Iron Formation: geochemical and textural evidence for deposition of iron-formation in a Paleoproterozoic epeiric sea Phillip Larson1, John Swenson2, and Marsha Patelke3 1 Duluth Metals Limited, 306 W Superior Street #610, Duluth MN 55802, USA 2 Department of Geological Sciences, 1114 Kirby Drive, University of Minnesota Duluth, Duluth MN 55812, USA 3 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth MN 55811, USA The Paleoproterozoic Biwabik Iron Formation of northeastern Minnesota, USA, consists of interbedded ‗cherty‘ granular iron-formation (GIF) and ‗slaty‘ banded ironformation (BIF) lithofacies. Advances in geochronology have demonstrated that the 1.85 Ga Biwabik IF is the chronostratigraphic correlative of other Paleoproterozoic iron-formations preserved along the margins of the 2.7 Ga Superior craton, including the Gunflint, Ironwood, Vulcan, Negaunee, Temiscamie, Sokomon, and Kipalu iron formations. The Biwabik and Gunflint Ifs are essentially undeformed flat-lying platform sequences, at present extending several 100 km inboard of the craton margin. However, in contrast the majority of the circum-Superior iron-formations are preserved as deformed rocks in orogenic belts at the craton margin. In common, all these iron-formations, ranging in thickness from 10s to several 100 m, are sandwiched within clastic sequences. In the case of the Biwabik IF, chemically precipitated Fe occurs in both the footwall and hangingwall clastic sequences, indicating that the trigger for iron-formation accumulation was not the commencement of Fe precipitation, but rather the cessation of clastic input into the basin. While the origin of GIF is contested (shallow-water oolite vs. reworked BIF), the association with high energy environments is clear. The preponderance of GIF in the Biwabik IF, as well as the occurrence of Mn-rich bioherms interbedded with GIF, suggests a predominantly shallow-water depositional environment. Mineralogy of the Biwabik IF is dominated by Fe-oxides, silica, and a variety of Febearing silicate and carbonate species. Lithofacies are characterized by distinct mineralogical assemblages, suggesting a strong control imposed on mineralogy by the depositional environment. Textural evidence indicates essentially all mineral species present are the product of diagenetic reactions, chiefly silica addition, recrystallization of primary precipitates into oxide, carbonate, and silicate species, and conversion of ferric Fe-bearing precipitates to ferrous Fe-bearing minerals. Magnetite strongly correlates with carbonatebearing iron-formation, and magnetite enrichment suggests significant ferrous Fe mobility during late diagenesis. Wrapping of bedding around early chert concretions in BIF indicates significant volume and mass loss subsequent to deposition. Stylolites are a common feature in both silicate-carbonate BIF and GIF; they too indicate significant mass loss during early diagenesis, likely from carbonate-rich iron-formation precursor sediments. A significant fraction of the precipitates deposited on the seafloor seem to have remobilized soon after deposition. This evidence indicates that iron-formation accumulation is strongly controlled by physical and chemical processes in the benthic zone, and is largely independent of the processes controlling Fe precipitation in the water column. The most abundant elements in iron-formation – Fe, Si, Mg, Mn, Ca, and P – were deposited as chemical precipitates. In contrast, Al, Ti, and to a lesser extent K, were deposited as terrigenous sediment. In the Biwabik IF, Al and Ti strongly correlate, suggesting ILSG 2011 55 Program and Abstracts �they are ultimately sourced from a long-lived, homogeneous reservoir. We suggest this reservoir was ultimately either windblown or suspended clay and silt. Al2O3 concentration in the Biwabik IF averages 0.44%. Concentrations are lowest in GIF, and highest in BIF, ranging from 0.10% in the coarsest GIF to 1.84% in the Intermediate Slate. Assuming PAAS-like compositions, these translate into terrigenous input contributions of 0.4% to 9.5%, averaging 2.5%. In comparison, the insoluble residue of modern Bahamian periplatform carbonates is 1-2% (Boardman et al. 1986). We infer that the bulk of Biwabik IF sedimentation therefore occurred in an environment similarly isolated from terrigenous input as the modern Bahamas platform. These lines of evidence suggest that the Biwabik IF and its correlative ironformations were deposited in a single continental-scale depositional system. The bulk of the iron-formation accumulated in shallow-water depositional environments in conjunction with carbonate, likely at very low rates. Indeed, a lack of any significant terrigenous input may have been the key factor responsible for accumulation of significant thicknesses of ironformation. The clear textural, geochemical, and mineralogical evidence for co-precipitation of ferric iron and carbonate suggests a depositional environment essentially analogous to Paleozoic epeiric sea carbonates, if not modern platform carbonates. We suggest that deposition of the Biwabik IF and its correlative iron-formations occurred in an epeiric sea (and adjacent continental margins) that nearly, if not completely, inundated the Superior craton through much of Biwabik IF time. Iron-formation a was a ubiquitous feature of the Paleoproterozoic sea, and accumulation on the Superior craton only ceased when renewed orogenic activity along the craton margin (e.g. Penokean orogeny) significantly increased the flux of terrigenous sediment onto the craton, with consequent dilution of iron-formation accumulation. REFERENCES Boardman, M.R., Neumann, A.C., Baker, P.A., Dulin, L.A., Kenter, R.J., Hunter, G.E., and Kiefer, K.B. 1986. Banktop responses to Quaternary fluctuations in sea level recorded in periplatform sediments. Geology 14: 28-31. ILSG 2011 56 Program and Abstracts �Platinum-Palladium-Copper-Nickel mineralization at the Thunder Bay North Deposit, Ontario A.D. MacTavish1, G.J. Heggie1, V.R. Goodgame2, J.R. Johnson1, A.E. Beswick3, W.E. Stone4, and K.P. Watkins5 1 Magma Metals (Canada) Limited, 1004 Alloy Drive, Thunder Bay, Ontario, Canada P7B 6V1 2 Excelsior Mining Corp., Suite 1240, 1140 West Pender Street, Vancouver, British Columbia, Canada V6E 4G1 3 Consulting Geoscientist, 11 Little Panache Road, Whitefish, Ontario, Canada P0M 3E0 4 Magma Metals Limited, Suite 2700, Brookfield Place, 161 Bay Street, Toronto, Ontario, Canada M5J 2S1 5 Magma Metals Limited, Level 3, 18 Richardson Street, West Perth, Western Australia WA6005 The drill discovery of the Thunder Bay North (TBN) Pt-Pd-Cu-Ni deposit by Magma Metals (Canada) Limited (Magma) in late 2006 was the result of the initial 2001 discovery of glacially transported mineralized peridotite boulders on the west shore of Current Lake. The TBN deposit is located ~50 km northeast of the city of Thunder Bay, Ontario. Since December 2006 Magma has drilled 630 holes (120,000 m) that have defined 9.5 million tonnes (Mt) indicated at 2.3 gpt Pt Equivalent (Pt-Eq) and 0.27 Mt inferred at 2.8 gpt Pt-Eq. Conceptually, 90% of the defined deposit can be mined by an open pit. Recoverable metals include Pt, Pd, Cu, Ni, Ag, Au, Rh and Co. The TBN deposit occurs within the 1120±23 Ma Current Lake Intrusive Complex (‗CLIC‘) which crosscuts Archean rocks of the Quetico Sub-province. The CLIC is one of three mafic-ultramafic intrusive complexes which are part of a network of magma conduits associated with the Keweenawan age Mid-Continent Rift. The conduits comprise narrow, nearly flat-lying, sinuous tubes and small tabular bodies that are classified as chonoliths, rather than sills or dykes. The emplacement of these conduits is controlled by pre-existing sub-horizontal and sub-vertical structures. Local conduit morphology is controlled by the rheology of the enclosing country rocks. The Archean country rocks consist of variably deformed fine wackes and siltstones metamorphosed to lower amphibolite-grade, and granitoid rocks typified by foliated granodiorite and tonalite. Conduits occurring within the metasedimentary rocks form tabular bodies, whereas those within granitoid rocks form tubes. The CLIC is undeformed, only weakly altered, and has been preserved in almost the same orientation as it formed. The earliest CLIC intrusive phases are a complex series of variably inclusion-rich, hybridized gabbro to diorite rocks that exhibit a wide variety of textures. The hybrids intrude along preexisting sub-horizontal and sub-vertical structures, are hematized, and contain carbonate ocellae. The mineralized mafic-ultramafic rocks are emplaced through these earlier hybrid rocks, and comprise two or more pulses of sulphide-bearing peridotite and olivine melagabbro. The mafic-ultramafic rocks exhibit chemical differentiation, but lack layering, and all internal contacts are gradational. The upper portions of the conduits contain diffuse zones of varitextured (taxitic) gabbro. Serpentinization of olivine is observed near the conduit margins and is attributed to late magmatic (deuteric) processes, rather than postmagmatic hydrothermal activity and metamorphism. The host rocks immediately adjacent to ILSG 2011 57 Program and Abstracts �the CLIC are hematized for 5 to 10m. Metasedimentary hanging-wall rocks exhibit strong in-situ brecciation, carbonatization, epidotization and sericitization, and contain up to 5% finely disseminated pyrite. This brecciation and alteration is less prominent in metagranitoid hanging-wall rocks. Mineralization within the TBN deposit is dominated by thick zones of disseminated pyrrhotite, chalcopyrite, pentlandite, and minor cubanite. Disseminated mineralization commonly fills the entire conduit within the Current Lake and Bridge zones of the deposit. Small, lenticular, very high-grade zones of net-textured and massive sulphides occur within troughs and depressions at the base of the conduit within the western Beaver Lake Zone. Most of the mineralization within the central and eastern Beaver Lake Zone is disseminated to blebby and is also confined to basal troughs and depressions. In addition to the basal mineralization, the ‗Cloud Zone‘ forms a diffuse zone containing <2% very finely disseminated chalcopyrite that occupies the upper third of the conduit in the northern Beaver Lake area. Sub-horizontal and sub-vertical sulphide veins and veinlets, up to 3 cm thick, are common within the Current Lake and Bridge zones, but rare within the Beaver Lake Zone. Composite sulphide blebs, up to 2 cm in diameter, are common throughout the wellmineralized portions of the deposit. Angular to sub-angular, fractured or broken sulphide fragments, up to 3 cm in length, are minor, but observed throughout the deposit. There is a strong positive correlation of sulphide abundance to base and precious metal grades. A very strong positive correlation of Pt, Pd, Cu and Ni indicates a pristine magmatic system with little re-distribution of metals. The Pt:Pd ratio value is 1.07; the Ni:Cu ratio value is approximately 0.5:1.0; the PGE and base metals tenor is relatively consistent throughout most of the deposit; and the chalcophile metal pattern is fractionated. The magmatic conduit system presently defined within the TBN area is comparable in size to other sulphide-rich magma conduits elsewhere. The CLIC shares a number of common features with the conduits observed at Noril‘sk-Talnakh (Russia): 1) the mineralized intrusive rocks form chonoliths; 2) both systems exhibit sill-like marginal mafic rocks; 3) taxitic (varitextured) phases are observed in both; 4) both systems contain both basal and internal mineralization; 5) both magmatic systems were dynamic, 6) both are closely associated with large igneous provinces, and 7) both show evidence of marked wall rock alteration. The CLIC is part of a much larger magmatic system which formed from a deep sourced magma, focused magma flux, and considerable wall-rock assimilation. The sulphides within the TBN deposit are strongly fractionated (PGE and Cu-rich), which reflects entrained sulphides being transported to their present location from a crystallizing and fractionating Ni-rich sulphide trap located deeper within the system. This scenario strongly suggests there may be one or more Ni-rich, massive sulphide deposits located magmatically upstream from the TBN Deposit. Magma continues to explore along strike from the defined TBN deposit and has identified low-grade mineralization at 1040 m depth, 2 km to the southeast, within the downplunge extension of the CLIC. Exploration also continues on the two other conduit systems identified within The Thunder Bay North land package. The most advanced work is at the Steepledge Lake Intrusive Complex (SLIC), located 3 km west of the CLIC, where drilling has intersected rocks and disseminated Pt-Pd-Cu-Ni mineralization similar to that present within the CLIC. ILSG 2011 58 Program and Abstracts �Using Credit-by-Exam to connect advanced high school geology courses to university geology programs: Lessons learned from a state-wide pilot study in Michigan Stephen Mattox1 and Sandra Rutherford2 1 Department of Geology, Grand Valley State University, Allendale, MI 49401-9403, mattoxs@gvsu.edu 2 Geography and Geology Department, Eastern Michigan University, Ypsilanti, Michigan 48197, srutherf@emich.edu To address the national shortage of geologists and to diversify the geoscience workforce we are constructing a seamless path from rigorous high school geology classes taught by well-trained teachers to geoscience programs at state universities and colleges. This program is modeled on an existing high school – university collaboration that has added a significant number of students to the career pipeline. We have obtained Memorandums of Understanding (MOU) from school districts with high populations of diverse students that have administrators and teachers ready to join the program. Discussions and visits with department chairs and faculty at numerous state universities and colleges resulted in a second set of Memorandums of Understanding to award college credit to students that successfully pass a rigorous credit by exam. Although geologists have vigorously lobbied for an AP course and exam in geology, the College Board has resisted because they perceive that demand will be low. The lack of an AP course has made it difficult for those high schools nationwide that offer advanced (i.e., college-level content) geology courses to obtain appropriate recognition for their students‘ accomplishments. Mattox designed a test, with input from my GVSU peers, that includes 60 multiple choice, 10 essay questions, a map test with skills and landforms, and a rock and mineral exam (essentially what we use in our physical geology introductory course). We give the exam over 5-6 hours on two different days. About 70-80% of students from a local high school pass. With the same exam at a different high school no students passed, although several were close. This exam is prepared, reviewed, and administered by geology department faculty. The authors received a NSF planning grant titled ―Collaborative Project: Collaborations for Building Michigan Geology Talent‖ and significant accomplishments include: Mattox visited eight state universities and colleges and discussed credit-by-exam and the high school advanced geology exam with geology department faculty and university administrators. A template MOU was drafted for high schools and for universities. To date, seven signed MOUs to award credit-by-exam and be involved with the high school advanced geology exam have been received from EMU, GVSU, Lake Superior State University, MTU, Wayne State University, Western Michigan University, and Hope College (a private college). Geology faculty at two other universities (Central Michigan University and University of Michigan-Dearborn) are guiding MOUs through administrative review at their institutions. Michigan State University is interested but currently focused on curriculum restructuring of their geology program. Mattox is currently in dialogue with ILSG 2011 59 Program and Abstracts �faculty at the University of Michigan and Northern Michigan University to establish site visits and visits with administrators. By the end date of this planning grant at least 7 of 10 state universities with B.S. geology degree programs will have signed agreements to award credits to students that pass an approved high school advanced geology exam. Recruiting targeted high schools with underrepresented groups throughout Michigan has been initiated in our planning grant. Rutherford received MOUs from school districts, including the Detroit school district with 30 high schools and diverse student populations. Administrators and teachers are interested in adding the geology course. If funding continues, we plan to support existing highly trained teachers that are ready to offer the course. Teachers that need additional training will be supported to earn a M.S. in geoscience education from EMU. Our proposal also includes funds for teaching materials. The MOU asks the high school to return the materials they were provided and releases the obligation to pay for the teachers‘ Masters if the high school does not teach the advanced geology class. Additional insights from this project include: We have developed a poster to be displayed in high school classrooms where the advanced geology course and exam are being offered. Administrators were surprising receptive to and supportive of a rigorous high school geology credit by exam course. Numerous teachers are excited at the prospect of earning a M.S. degree to be better prepared to teach the geology credit by exam course. Many university faculty were unaware that credit by exam was already available at their university. This mechanism made establishing a high school credit by exam easier. After reviewing the existing exam most faculty immediately embraced the idea of credits that pass the exam. We have demonstrated the feasibility of establishing a statewide network of universities to award college credit for passing a credit by exam during a high school geology course. This material is based upon work supported by the National Science Foundation under OEDG Grant No. NSF 08-605 1006652. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. ILSG 2011 60 Program and Abstracts �2010 Precambrian field camp mapping in the Jack Lake area, Cook County, Northeastern Minnesota Jim Miller, Ben Brooker, Max Hadley, Levi Markwood, Jeff Olson, and Alex Tomlinson Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As part of the 2010 Precambrian field camp, a crew of five students and an instructor (Miller) conducted five days of field mapping bedrock geology in the Jack Lake area of northeast Minnesota. This area is composed of intrusive and volcanic rocks formed during the 1.1 Ga Midcontinent Rift (MCR) in northeastern Minnesota. This year‘s mapping project expanded on capstone mapping projects conducted in 2007 (Frost et al., 2007) and 2009 (Blakely et al., 2009). These mapping projects have focused on an as yet unnamed, well differentiated layered mafic intrusion that forms the western part of the Brule Lake –Hovland gabbro of Miller et al. (2001). A 1:12,000-scale bedrock geologic map was generated of a 4 mile by 1 mile area that profiles the igneous stratigraphy of the intrusion. A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/capstone. Prior to the detailed mapping conducted by the PRC field camp students, the general geology of this area was poorly known. Township-scale outcrop mapping by Grout et al. (1959) showed the area to be dominantly composed of gabbroic to felsic intrusive rocks, mafic volcanic rocks and minor volcanogenic sedimentary rocks. They noted that the gabbroic rocks are well layered and portions are particularly rich in Fe-Ti oxide. Reconnaissance mapping by Davidson (1972) designated the mafic intrusion as an olivine gabbro unit that he interpreted as an early intrusion relative to granophyric and anorthositic rocks in the area (Davidson, 1977; Davidson and Burnell, 1977). The regional geologic map of northeastern Minnesota, which focused on the geology of the Duluth Complex (Miller et al., 2001), incorporated Davidson‘s (1972) mapping of the area, but his olivine gabbroic unit is now interpreted to be a relatively late intrusion that is roughly equivalent to the Beaver Bay Complex to the south. 2010 capstone mapping was conducted along a north-south-trending chain of lakes and rivers that included Kelly Lake, Jack Lake, Weird Lake, Mama Bear Lake, and the Temperance River. Exposures along the shorelines of these waterways provided a nearly complete profile across the moderately south-dipping internal structure of the unnamed mafic intrusion and the overlying Eagle Mountain granophyre. The igneous stratigraphy of the approximately 2 km-thick intrusion generally defines a progressively differentiated sequence. However, it is not clear whether this differentiation took place entirely by in situ fractional crystallization. The lowest unit exposed in the north of the map area, along the Temperance River, is a complex mixture of medium-grained, augite troctolite to olivine gabbro and fine-grained, intergranular gabbroic rocks of uncertain origin – diabase intrusions or hornfels basalt inclusions? Overlying this mixed rock unit, the small Mama Bear Lake area, is an augite troctolite (Pl+Ol cumulate) unit that is ophitic to subophitic, poorly to moderately foliated, and medium- to coarsegrained. The augite troctolite grades into an interval that contains abundant hornfels inclusions of basalt and volcanogenic sandstone. The rock hosting the inclusions ranges from augite troctolite to olivine gabbro. This inclusion-rich unit, which is exposed at the north end of Weird Lake, is overlain by a very thick sequence of foliated, intergranular oxide gabbro to olivine oxide gabbro (Pl+Cpx+Ox±Ol cumulate). This unit locally contains cm- to m-thick layers rich in Fe-Ti oxides and occasional inclusions of gabbroic anorthosite. The lower section of this oxide gabbro unit is intruded by a complex network of intermingled diabase, ferromonzondiorite, and quartz monzonite that mostly appear to be sill-like. These mixed intrusion also occur higher in the sequence where they appear to be more dike-like. In the northern part of Jack Lake, the oxide gabbro abruptly grades into an apatite oxide gabbro with the appearance of 3-5% apatite prisms (Pl+Cpx+Ox+Ap cumulate). The apatite ILSG 2011 61 Program and Abstracts �oxide gabbro unit is homogeneous, medium-grained, and well foliated. The oxide gabbro and the apatite oxide gabbro units are each about 800 m thick and comprise about 80% of the intrusion stratigraphy. At the south end of Jack Lake, the apatite oxide gabbro grades into a prismatic ferromonzodiorite, which grades into a ferromonzonite, and which in turn grades into micrographic leucogranite of the Eagle Mountain granophyre. The transition from apatite oxide gabbro to leucogranite occurs over a 200 m-thick interval and appears to represent a zone of mixing and assimilation between the younger mafic intrusion and the older granophyre body. The Jack Lake sequence generally correlates with the igneous stratigraphy mapped to the east during 2007 and 2009 capstone projects. The main correlation horizon that links the Jack Lake sequence with areas to the east is the interval rich in hornfels inclusions of basalt and volcanogenic sandstone. This interval can be traced for over six miles from the north end of Weird Lake, to the north end of Vern Lake, and along the north shore of Homer Lake. This volcanic screen was actually identified by Grout et al. (1959), but curiously is not shown on Davidson‘s maps (Davidson, 1977; Davidson and Burnell, 1972). Below the hornfels-rich horizon, the troctolite-dominant lithologies of the Temperance River /Mama Bear Lake area generally correlate with what Frost et al. (2007) and Blakely et al. (2009) termed the Axe Lake sequence to the east. The oxide gabbros and apatite oxide gabbros of the Jack Lake area generally correlate with what is called the Homer Lake sequence to the east. One notable difference is that the 800m-thick apatite oxide gabbro unit in the Jack Lake area dramatically thins by an order of magnitude to the east. Plans are to return to the area for the 2011 capstone and continue mapping to the west. In addition, Ben Brooker will be conducting more mapping throughout the area next summer, as well as petrographic and mineral chemical studies, to better understand the emplacement and crystallization history of this intrusion for his MS thesis at UMD. A major objective of Ben‘s study is to evaluate the genetic relationship between the lower troctolitic cumulates and the upper oxide gabbroic cumulates. Although these rock types could represent the progressive differentiation of a mafic magma, their separation by a screen of volcanic and interflow sedimentary rocks suggests that they may represent two discreet intrusions formed by different parental magmas. Ben also promises to name the intrusion. REFERENCES Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M., Jr., 1977, Cherokee Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-30, 1:24,000. Davidson, D.M., Jr., and Burnell, J.R., 1977, Brule Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-29, 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000. Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 ILSG 2011 62 Program and Abstracts �The Mineral Exploration Trifecta Dean Peterson, Phil Larson, Gabriel Sweet, and Jack Gibbons Duluth Metals Limited, 306 West Superior Street, Suite 610, Duluth, MN 55802 Mineral exploration is the research activity of the mining industry. At its foundation are ideas, visions, or intuitive geologic thoughts that are evaluated through repeated phases of data gathering (geological, geochemical, and geophysical studies), interpretation, and drilling. Mineral deposit models have arisen as mental paradigms that act as guidebooks to help geologists process information. Effective mineral deposit models increase discovery rates and decrease exploration and mining costs. However, it is becoming increasingly apparent that analog deposit targeting models are decreasing in their effectiveness. Adherence to these decreasingly effective analog deposit targeting models has created a misalignment between future societal mineral resource needs and the ability of the mineral exploration industry to meet those needs. Over the last several decades, companies have focused on brownfields exploration around existing mines with a trend toward lower grade open-pit resources at the expense of greenfields exploration for the next generation of high-quality deposits. In addition, we have moved into an energy-constrained and environmentally-concerned world and global society‘s demands that the mining industry reduce its physical, social and environmental footprint are translating into legislation. Duluth Metals recognizes these challenges and has implemented a three-part fundamental philosophical shift in our mineral exploration programs to support the discovery of new, high-quality ore deposits. We at Duluth Metals have coined this shift the ―Mineral Exploration Trifecta‖, which utilizes (1) the mineral system approach, (2) innovative technology, and (3) trained people. Mineral System Approach A mineral system approach to exploration attempts to understand ore deposits as expressions of multi-scale systems encompassing the Earth that focus mass and energy transfer to ultimately form ore deposits. This approach to mineral exploration leads to systematic, scale-dependent targeting models and allows for the recognition of the largestscale footprints of ore-forming systems. Of particular importance to us at Duluth Metals is the recognition of the physical magmatic processes in the Duluth Complex intrusions we are exploring. We view all these processes from the perspective of the initial conditions of the intrusion in general (and the target area in detail) and attempt to understand the chemical processes that are intimately paired with these physical processes. We ask each other simple questions like: (1) Where did magmas in intrusion X behave in a manner that was conducive for the concentration of ore minerals? (2) Where are their changes in the geometry of intrusions Y, i.e., areas of flow expansion, and areas of flow focusing? Technology The quantity and quality of data available to exploration geologists increases greatly with each passing year. Great innovations in geophysical techniques, geochemical analyses, and geospatial data management occur seemingly every month and a sound mineral exploration program must create and utilize these data in inventive ways into their ore ILSG 2011 63 Program and Abstracts �deposit targeting models. Duluth Metals is fully engaged in the innovations being developed for the minerals exploration industry and have: (1) acquired state-of-the-art airborne geophysical data over a 160,000 acre survey area and have seemingly innumerable 2D and 3D datasets of magnetics, gravity, and conductivity; (2) set in motion a large geochemical survey that will utilize QEMSCAN & MLA technologies as well as in-the-field geochemical analyses via a hand-held XRF; and (3) have built an impressive computer system with stateof-the-art software (ArcMap GIS, Oasis Montaj, Encom PA) to integrate all of these data into a single exploration platform. People Duluth Metals recognizes that the science of mineral exploration is more than just applied economic geology. It is a distinct discipline in its own right that embraces aspects from many other fields including; probability theory, risk-management, economics, and decision science. Most importantly, mineral exploration depends on well trained people who know geology and know how to think big, utilize technology, and protect the environment. Duluth Metals is a proud sponsor of the Precambrian Research Center and has other research ties to the University of Minnesota to help answer focused earth system questions (e.g. AMS to understand magmatic flow directions ILSG 2011 64 Program and Abstracts �Paleomagnetism of Midcontinent Rift rocks from the northern shore of Lake Superior (Ontario, Canada): Preliminary results Elisa Piispa1, Aleksey Smirnov1, Lauri Pesonen2 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 630 Dow, ESE Building, 1400 Townsend Drive, Houghton, MI 49931, USA 2 Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland We report the preliminary results of our paleomagnetic and rock magnetic investigations of a suite of extrusive and intrusive rocks representing the Mesoproterozoic Midcontinent Rift system along the northern shore of Lake Superior in Ontario, Canada. Specifically we investigated: (1) The abundant ~1144 Ma lamprophyre dykes near Marathon. These dykes, coeval with the Abitibi diabase dyke swarm, possibly represent the first magmatic stage of the Midcontinent Rift; (2) The ~1100 Ma copper-nickel-PGE-mineralized layered intrusion named Crystal Lake Gabbro South of Thunder Bay; (3) The ~1115 Ma Logan Sills predominating in the Southern Thunder Bay; (4) At least three different dyke sets: the NE –trending Pigeon River dykes with two potential magmatic episodes at ~1079 and ~1141 Ma, the ~1109 Ma NW-trending Cloud River dykes, and the ~1109 Ma Mount Mollie dyke; (5) The rocks of ~1108 Ma Coldwell Complex which represents the largest alkaline intrusion related to the Midcontinent Rift; and (6) Newly discovered Devon Township basaltic outcrops NE of Thunder Bay with an unknown age. A preliminary set of 220 independently oriented samples of these rocks were subjected to a detailed paleomagnetic study using both alternating field and thermal demagnetization, baked contact tests, and the measurements of magnetic hysteresis and other rock magnetic properties. In combination with the recently published high precision ages, we plan to use these paleomagnetic data to test the models of the apparent geomagnetic reversal asymmetry during the Precambrian as well as to further refine both the Apparent Polar Wander Path between 1144 and 1079 Ma and the evolution of the Midcontinent rifting sequence. ILSG 2011 65 Program and Abstracts �The use of S/Se ratios in magmatic Ni-Cu-PGE sulfide deposits and its implications for exploration: Example of the Duluth Complex, Minnesota, USA Matthias Queffurus and Sarah J. Barnes Université du Québec à Chicoutimi, DSA, matthias.queffurus@uqac.ca S/Se ratios have been used in the study of magmatic Ni-Cu-PGE sulfide deposits as petrogenetic indicator of ore forming processes (e.g., Eckstrand et al., 1989). Despite the utility of S/Se ratios, no study has been dedicated specifically to the characteristics of the processes that could modify these ratios. We have compiled S, Se, δ34S and metals data from more than fifty magmatic Ni-Cu and PGE deposits and added some new Se data for the country rocks from the Duluth Complex with the aims of: a) characterizing the deposits, b) better constrain the application of S/Se and, c) considering a wider range of processes than have been discussed in the past. Se concentrations were determined by TCF-INAA technique (Thiol Cotton Fiber - Instrumental Neutron Activation Analysis), developed by Savard et al. (2006). Our results indicate that some processes, other than those mentioned in the past, i.e. the assimilation of country rocks and the remobilization of S post-cumulate, could modify the S/Se ratios and these include: a) changes in silicate to sulfide liquid ratio, R-factor (Thériault and Barnes, 1998); b) the segregation of the sulfide liquid (Barnes et al., 2009); c) a fractionation between MSS and ISS. These processes influence the concentrations of S and Se in mineralized rocks, i.e. S/Se ratio, and may overprint the original source of S and the interpretation of the genesis of the deposit. At the Duluth Complex, where country rocks assimilation is believed to play a key role in the genesis of the mineralization, the preliminary results of our work show that most of the Virginia Formation samples have S/Se ratios lower than the ore. Only the samples close to the contact of a particularly S-rich layer, known as the Bedded Pyrrhotite Unit (BPU) and the xenoliths, have S/Se ratios higher to than sulfidebearing igneous rocks. The concentration of S, high S/Se ratio and sulfur isotopic values of rocks samples from the BPU strongly indicate that this layer would be the main source of S assimilated by the mafic magma. REFERENCES Eckstrand, O.R., Grinenko, L.N., Krouse, H.R., Paktunc, A.D., Schwann. P.L., and Scoates, R.F., 1989, Preliminary data on sulphur isotopes and Se/S ratios, and the source of sulphur in magmatic sulphur in magmatic sulphides from the Fox River Sill, Molson Dykes and Thompson nickel deposits, northern Manitoba. In: Current Research, Part C. Geological Survey of Canada v. 89-1C, p. 235-242. Barnes, S.-J., Savard, D., Bédard, L.P., and Maier, W.D., 2009, Selenium and sulfur concentrations in the Bushveld Complex of South Africa and implications for formation of the platinum-group element deposits. Mineralium Deposita, v. 44, p. 647-663. Savard, D., Bédard, L.P., and Barnes, S.-J., 2006, TCF selenium preconcentration in geological materials for determination at sub-μg.g-1 with INAA (Se/TCF-INAA): Talanta v. 70, p. 566-571. Thériault, R.M., and Barnes, S.-J., 1998, Compositional variations in Cu–Ni–PGE sulfides of the Dunka Road deposit, Duluth Complex, Minnesota: the importance of combined assimilation and magmatic processes: Canadian Mineralogist v. 36, p. 869-886. ILSG 2011 66 Program and Abstracts �Reconnaissance Bedrock Geological map of the northern part of Sudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota Amy L. Radakovich, Charlie T. Parent, Molly E. Partridge, Andrew D. Ritts, Rita Pierce, and George J. Hudak Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811 The ―capstone‖ project for the Precambrian Research Center (PRC) summer field camp encompasses detailed field mapping and map production in small groups with faculty from the field camp. During the fifth and sixth weeks of the 2010 field camp, five PRC field camp students, under the direction of PRC Faculty member George Hudak, mapped Neoarchean rocks in the northeastern part of Soudan Underground Mine State Park and the northwestern part of Minnesota‘s newest state park, Lake Vermilion State Park (Radakovich et al., 2010). This capstone mapping project sought to: 1) identify the lithologies and stratigraphy of the Neoarchean supracrustal strata in this area; 2) define and characterize the nature of the contact between various units of the Neoarchean supracrustal strata and intrusive rocks; 3) obtain a better understanding of geological structures and their orientations within the area; and 4) to make a comprehensive bedrock geological map for future use by the new Lake Vermilion State Park. Prior to mapping, detailed 1:5000 scale laminated field mapping sheets were produced. One side of each field mapping sheet consisted of a georeferenced air photo, and the other side of the mapping sheet consisted of a corresponding georeferenced topographic map. Mapping was completed by means of lakeshore mapping from canoes, as well as numerous traverses through the bush. Following each day of field mapping, students and faculty transferred their field data to a master map, enabling the generalized geology of the region to be established by the middle of the fifth week of field camp. During the sixth week of field camp, students produced a digital version of the field map utilizing a variety of software (ArcView, AutoCad, Surfer, Adobe Illustrator). Neoarchean supracrustal rocks consist of a NE/SW striking, NW facing homoclinal sequence comprising three distinctive rock sequences. Up-section, these sequences are: (1) the Central Basalt Sequence of the Lower Member of the Ely Greenstone Formation, which is composed of medium green to dark green, aphyric to sparsely plagioclase-phyric, sparsely amygdaloidal sheet flow- and pillowed facies basalt lava flows which are locally strongly quartz-epidote altered; (2) the Soudan Member of the Ely Greenstone Formation, an interbedded sequence comprising: a) laminated to medium-bedded, planar-bedded, locally chaotically folded, dark gray to red-brown, Algoma-type oxide facies iron formation; b) medium- to dark green, aphyric to locally sparsely plagioclase-phyric, massive to moderately amygdaloidal andesite to basalt sheet flow facies lava flows; and c) localized gray to gray green, quartz ± plagioclase-phyric rhyodacite tuffs and polymict lapilli tuffs which are generally less than 10m in stratigraphic thickness. (3) the Gafvert Lake Member of the Lake Vermilion Formation, which is dominantly composed of intermediate to felsic volcaniclastic rocks comprising: a) a basal unit composed of massive, light gray, quartz- and plagioclase-phyric polymict dacite to ILSG 2011 67 Program and Abstracts �rhyodacite tuff; b) interbedded light gray, massive, quartz- and plagioclase-phyric dacite to rhyodacite tuff and pumiceous lapilli tuff; c) epiclastic deposits comprising light gray to brownish-gray, polymict volcaniclastistic conglomerates and sandstones derived primarily from felsic rocks; and d) white to gray, thinly-bedded to massive, fine- to locally coarse-grained dacite to rhyodacite tuff breccias which locally exhibit graded bedding. Chemical sedimentary rocks composed of: a) light gray to black, laminated to very thickly bedded chert deposits; and b) light gray to dark gray, laminated to medium-bedded Algoma-type iron formation, occur in two distinctive stratigraphic horizons up to 300 meters thick in the central and northeastern parts of the field area. Intrusions into the supracrustal assemblages include feldspar ± quartz porphyritic dacite to rhyodacite dikes and sills, medium-grained granodiorite dikes, and medium-grained gabbro sills. An east-west striking shear zone in the northeastern part of the field area is composed of highly foliated basalt or chlorite schist. Our analysis leads us to interpret the following sequence of events for the development of the bedrock geology visible today in this part of the Vermilion District: 1) deposition of submarine basalt lava flows, comprising both sheet flows and pillowed facies, in a relatively deep water (>500 meters water depth) setting (Peterson and Patelke, 2003; Hudak et al., 2007); 2) deposition of Algoma-type iron formations of the Soudan Member of the Ely Greenstone Formation as a result of hydrothermal activity during volcanically quiescent periods, (Peterson, 2003) and deposition of interbedded mafic and felsic volcanic and volcaniclastic strata associated with intermittent volcanism; 3) the onset of voluminous explosive dacitic to rhyodacitic felsic volcanism, alternating with periods of volcanic quiescence characterized by continued chemical sedimentation and localized clastic sedimentation; 4) synvolcanic to post-volcanic intrusion of mafic to felsic sills and dikes; and 5) structural deformation, probably associated with regional D2 deformation (Peterson and Patelke, 2003), that formed the sheared mafic rocks and chlorite schists that occur in the northeastern part of the field area. REFERENCES Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota. Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Programs and Abstracts, p. 42-43 Peter, J. M., 2003, Ancient iron formations: their genesis and use in the exploration for stratiform base metal sulphide deposits, with examples from the Bathurst Mining Camp: in Lentz, D. R., ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral Deposit-Forming Environments: Geological Association of Canada, GeoText 4, p. 145-176. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-2003/29, 88 p. Radakovich, A. L., Parent, C. T., Partridge, M. E., Ritts, A. D., Pierce, R., and Hudak, G. J., 2010, Reconnaissance Bedrock Geological Map of the northern part of Soudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota: Precambrian Research Center Map Series, PRC/MAP-2010/04, 1:5000 scale. ILSG 2011 68 Program and Abstracts �Fluid flow events in the Biwabik Iron Formation, Minnesota Ryan Rague and Steven Losh Dept. of Chemistry and Geology, FH 241, Minnesota State University, Mankato MN 56001 Fluids have interacted with banded iron formation rocks of the Paleoproterozoic (~1.85 Ga) Biwabik Fm, exposed in the Mesabi range, during diagenesis and dissolution/oxidation. Burial diagenesis likely took place shortly after deposition at temperatures of around 150° – 200° C based on mineral assemblages and published oxygen isotope thermometry (Perry et al., Econ. Geol. 68, 1110-1125), whereas dissolution of silicates and carbonates and oxidation of iron to hematite (forming ‗natural ore‘) has long been interpreted as a much younger supergene phenomenon associated with intense chemical weathering, perhaps during the Cretaceous. However, fluid inclusion and bulk geochemical analysis (specifically rare earths) of veins and whole-rock samples, respectively, from the Hibbtac, Thunderbird, Thunderbird South, Fayal, and LTV#6 pits, shows that oxidation may have taken place over a range of burial depths and temperatures, hence over a protracted period of time. Quartz vein samples from a variety of paragenetic settings (mainly from unoxidized low-angle faults, high-angle veins (+/- calcite +/- minnesotaite) without oxidation, and veins in high-angle faults (+/- Fe oxides) associated with oxidation) show considerable overlap of fluid inclusion homogenization temperatures (mean = 154° +/- 74° C, n= 467) and salinity (9.5 +/- 5.3 wt % NaCl equivalent, n=166) in the sample set. These temperatures are at the low end of those inferred for diagenesis; addition of a temperature correction due to fluid pressure, estimated at ~ 50° C corresponding to ~ 3-4 km burial depth, improves the correlation. Notably, fault breccia samples associated with oxidized (red hematitic) iron formation in the Hibbtac and Thunderbird South pits show elevated temperature and salinity, not consistent with supergene alteration. Rare earth element analysis of samples of unoxidized and oxidized iron formation and veins from nearby faults indicate that fluids that precipitated quartz in the fault zones had locally interacted with a large amount of iron formation, acquiring its rare earth element ‗fingerprint‘ in terms of positive europium, thulium, and/or cerium anomalies (relative to NASC). In the Fayal pit, ‗natural ore‘ and associated quartz / calcite + hematite fault breccia are both characterized by strong cerium anomalies consistent with near-surface oxidation. In the Hibbtac pit, both oxidized iron formation and quartz + hematite fault breccia associated with oxidized ore show no cerium anomaly. Instead, both rock types show pronounced Tm anomalies (as does unoxidized iron formation at that location), indicating local control on the chemistry of fluids associated with oxidation and absence of large volumes of externallyderived supergene fluids during oxidation of iron formation adjacent to the sampled fault. Thus an early oxidation event, separate from late supergene ‗natural ore‘ formation, may have tapped a mixture of diagenetic and oxidizing meteoric waters during uplift shortly after burial. ILSG 2011 69 Program and Abstracts �Preliminary stratigraphy and physical volcanology associated with the Paleoproterozoic Back Forty VMS deposit, Menominee County, Michigan Cabin Ross1, George Hudak2, Ron Morton1, Tom Quigley3, and Bob Mahin3 1 Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812 2 Precambrian Research Center, NRRI, University of Minnesota Duluth, Duluth, MN 55811 3 Aquila Resources Inc., E807 Gerue Street, Stephenson, MI 49887 Aquila Resources‘ Back Forty volcanogenic massive sulfide (VMS) deposit is the most recent VMS discovery within the Wisconsin Magmatic Terrane. The deposit is located approximately 200 meters east of the Menominee River, approximately 25 kilometers west of Stephenson in Michigan‘s Upper Peninsula. This research, which represents the first detailed volcano logical and hydrothermal alteration studies of the Back Forty VMS deposit, aims to: 1) establish the physical, chemical, and spatial attributes of the lithologies associated with the Back Forty deposit; 2) establish the stratigraphy in the vicinity of the Back Forty deposit and its relationship with VMS mineralization; 3) establish the physical, chemical, and spatial attributes of metamorphosed hydrothermal alteration mineral assemblages associated with the Back Forty deposit; 4) evaluate the spatial relationships of the alteration mineral assemblages to the Back Forty deposit mineralization; 5) identify chemical variations associated with alteration mineral phases at the Back Forty deposit, and evaluate the spatial relationships of these mineral compositional variations with the Back Forty mineralization; and 6) develop a coherent geological model of the volcanic setting and mineralizing environment associated with the genesis of the Back Forty VMS deposit. Our study involved a multi-faceted approach including geological mapping, diamond drill core logging, petrographic, lithogeochemical, and mineral chemistry analyses. Geological mapping of the Back Forty deposit and surrounding area was completed at a 1:100 or 1:500 scale. Information mapped at each outcrop included descriptions of lithology, alteration minerals present, alteration intensity, mineralization, structural features and measurements, and other unique features (i.e. weathering patterns, magnetism, etc.). Regional mapping was limited to outcrop sampling with reference to previous work by Greenberg and Brown (1984). The drill cores investigated were selected based on location within mineralized zones, and previously defined stratigraphy and alteration types (Quigley et al., 2008). Twenty-three diamond drill core were selected throughout the mineralized zone with an additional 22 cores evaluated for regional lithology and alteration zone characteristics (a total of approximately 10,000 meters of drill core). 170 samples were selected for petrographic analysis based on lithological distribution, alteration mineral representation, variable spatial relationships to massive sulfides zones (i.e. above, below, or away from), and volcanic textures. Seventy-seven samples were submitted to Activation Laboratories Ltd. (ACTLABS, Ancaster, Ontario) for whole rock lithogeochemical major and trace element analysis. Preliminary stratigraphy of the Back Forty deposit includes four main tuff units: A) massive, coarse-grained, quartz-phyric, rhyolitic lapilli tuff with 7-10%, <1-5mm euhedral to subhedral quartz crystals and angular quartz crystal fragments in a coarse, inequigranular quartz-rich matrix containing possible pumice and glass fragments; B) massive, quartzfeldspar-phyric, rhyolitic-rhyodacitic tuff with 5-7%, <1-5mm, subhedral quartz crystals and angular quartz crystal fragments and 1-3%, <1-2mm subhedral feldspar crystals and feldspar ILSG 2011 70 Program and Abstracts �crystal fragments in a variably coarse- to fine-grained, quartz-rich matrix; C) massive, finegrained, quartz-phyric, rhyolitic-rhyodacitic tuff breccia with 5-7%, <1-4mm quartz crystal fragments and quartz shards in a fine-grained, quartz-rich matrix that contains fragments of unit A; D) massive, coarse-grained, aphyric, rhyolitic-rhyodacitic tuff to tuff breccia containing quartz-phyric tuff fragments in an aphyric, coarse-grained, quartz-rich matrix. Units A-C are interbedded with laminated ash and tuffaceous sedimentary rocks. Compositional classification (Winchester and Floyd, 1977) and tectonic affinity (Pearce at al., 1984) based on immobile trace elements indicate that host rocks for the Back Forty mineralization are arc-associated FII-type rhyodacites and rhyolites. Alteration mineral assemblages and alteration zoning are still under review. Initial studies indicate the presence of four primary alteration minerals of various abundances: sericite, Mg-chlorite, iron-rich chlorite and quartz. Mineralization at the Back Forty is generally bounded by laminated tuffaceous sedimentary rocks within the massive tuff units and could indicate conformable massive sulfide zones. However, remnant textures in massive sulfide zones suggest replacement-style mineralization (Doyle and Allen, 2003). To date, drilling at the Back Forty deposit has outlined a combined NI 43-101 compliant measured and indicated resource (open pit and underground development potential) of 18.1 million tonnes grading 0.19% copper, 2.48% zinc, 1.63 grams/tonne gold, and 20.04 grams/tonne silver (Aquila Resources‘ Press Release, October 15, 2010; www.aquilaresources.com). Preliminary stratigraphic interpretations place tuff units A-D from oldest to youngest, respectively. Further analysis is needed to more completely define these relationships. Detailed physical volcanological environment is under investigation. Thick sequences of felsic volcanic rocks may be consistent with a caldera setting; however, more widespread regional stratigraphic studies will be needed to further test this interpretation. REFERENCES Doyle, M.G. and Allen, R.L., 2003, Subsea-floor replacement in volcanic-hosted massive sulfide deposits: Ore Geology Reviews, 23, 183-222. Greenberg, J.K. and Brown, B.A., 1984, Bedrock Geology of Wisconsin, Northeast Sheet: Wisconsin Geological and Natural History Survey Map 84-2, Scale 1:250,000. Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, 25, 956-983. Quigley, T., Mahin, B., and Aquila Field Office Geologic Staff, 2008, Back Forty Geology and Mineralization: 54th Annual Institute on Lake Superior Geology, Field Trip #3. Winchester, J.A. and Floyd, P.A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, 20, 325-343. ILSG 2011 71 Program and Abstracts �Whole rock geochemical analyses of sheared granitic rocks from Mountain, Wisconsin Brittany J. Saylor, Jonathan C. Stencil, Michael A. DeVasto, and Prajukti Bhattacharyya University of Wisconsin – Whitewater, 800 W. Main St,. Whitewater, WI 53190 Ductile deformation deep within the earth‘s crust and mantle are localized within narrow bands called shear zones. The purpose of our research is to gain a better understanding of how chemistry and mineral alignment patterns change across shear zones. We conducted our research on samples collected from around the Mountain Shear Zone exposed near the town of Mountain, Oconto County, Northern Wisconsin (Figure 1a-b: Sims et al. (1989a, b, 1991)) concluded that rocks in the study area belong to the eastern part of the Pembine-Wausau Terrane, which was accreted to the Superior craton during the Penokean Orogeny approximately 1.8 billion years ago. The study region was affected by ductile shearing about 25-35 million years after the main continent-arc collision event of the Penokean Orogeny (Sims et al., 1990, 1991). Most of the shear zones of comparable age in Wisconsin are buried under glacial deposits, so the exposed Mountain Shear Zone provides an excellent opportunity to study rock deformation processes that have taken place deep within the earth. Here, we present our preliminary observations on whole rock geochemistry, variation of magnetic susceptibility values along and across deformed samples collected from the study area. In order to see whether whole rock chemistry changes with shear deformation we collected both deformed and undeformed granitic rock samples from the study area (Figure 1b), and recorded their locations using a hand-held GPS unit. We analyzed the whole rock geochemistry of selected samples using X-Ray Fluorescence (XRF) at the UW Milwaukee Campus to see how the whole rock chemistry differs in deformed and undeformed rocks. We also used the Magnetic Susceptibility Meter SM 30 to measure how the magnetic susceptibility values change within the same deformed samples. The major minerals in the studied samples are quartz, potassium feldspar, plagioclase, biotite, and hornblende, with normative magnetite. Based on the QAP diagram, the analyzed samples can be classified as grandodiorite. The studied samples can be classified as peraluminous S-type granite common in orogenic belts. We used samples M0513-O, M0513-Q, and M1003 as examples of undeformed rocks, and M1002-1 and M1002-A was taken from different parts of a sample showing a shear zone. The results of the chemical analysis show that the chemical compositions of even undeformed samples are different from each other. One explanation for this might be that there was more than one episode of minor granitic intrusion after the shearing event. Observations of rock texture in sample M1002-AB seems to support this hypothesis. The magnetic susceptibility values per unit area of rock surface changes with distance in deformed samples, but seems to show no correlation with mineral alignment patterns. This variation could be due to higher concentration of magnetic minerals like magnetite in some parts of the analyzed samples as indicated by normative mineralogy. We need to conduct detailed geochemical analyses including REE and trace element analyses to see if we can identify different generations of granite in the study area. We also ILSG 2011 72 Program and Abstracts �need to conduct detailed mineralogical and textural analyses using an optical microscope to study the effects of shearing on our samples. Figures Figure 1a Figure 1b Figure 1. (a). A simplified regional geologic map modified after Sims, (1990) showing the study area outlined in red. (b). Detailed geologic map of the area around Mountain Shear Zone, modified after Sims, (1989a). REFERENCES Sims, P.K. 1989a. Geologic map of Proterozoic rocks near Mountain, Oconto County, Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series map I-1903, scale 1:24,000 Sims, P.K., Van Schmus, W.R.., Schulz, K.J., and Peterman, Z.E., 1989b, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen. Canadian Journal of Earth Sciences, 26, 2145-2158. Sims, P.K., Klasner, J.S., and Peterman, Z.E. 1990. The Mountain shear zone, northeastern Wisconsin – a discrete ductile deformation zone within the early Proterozoic Penokean Orogen. U.S. Geological Survey bulletin,1904, Contributions to Precambrian geology of Lake Superior region Sims, P.K., Klasner, J.S., Day, W.C., and Peterman, Z.E. 1991. Mountain shear zone, Oconto County, Wisconsin – A post-Penokean discrete ductile deformation zone. Field trip guide, 37th Annual Meeting of Institute on Lake Superior Geology, Eau Claire, Wisconsin. ILSG 2011 73 Program and Abstracts �Comparison of microscale and regional scale structural control on gold mineralization in the North Caribou Greenstone Belt, Superior Province, Northwestern Ontario Robert Scott1, Maura Kolb1, and Mary Louise Hill1 1 Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada; rjscott@lakeheadu.ca The North Caribou greenstone belt is bounded on the north and east by the North Caribou-Totagan Lake shear zone that divides the North Caribou terrane from the Island Lake domain. The greenstone belt is host to the gold deposit at Musselwhite Mine, where gold is found in banded iron formation metamorphosed to the lower amphibolite facies. Microstructural analysis shows that gold is commonly found in the strain shadows of garnet where strain is lower. Strain shadows at the regional scale may also provide an area of lower strain that serves as a preferential location for gold mineralization. Understanding regional structural control on gold mineralization will provide an important tool in the exploration for gold. On the microscopic scale, less competent matrix minerals (including quartz, biotite and grunerite) deform in a ductile manner while more competent minerals (including garnet and arsenopyrite) are more rigid and deform in a brittle manner. The competency contrast between garnet and the matrix creates strain shadows around the garnet; these areas of low strain are favourable locations for gold mineralization. Similarly, the lithological competency contrast between rigid granitic plutons and the more ductile metasedimentary and metavolcanic greenstone belt creates areas of low strain on the regional scale. The North Caribou batholith shows microstructural evidence of solid state deformation by dislocation creep along its eastern margin, indicating that it was crystallized and solid before being deformed by the North Caribou-Totogan Lake shear zone system. Although the batholith is more competent and therefore strain is localized in the more ductile greenstone belt, the edge of the batholith is affected by the deformation. At Libert Lake, a splay off the main North Caribou-Totagan Lake shear zone appears to anastomose around the Southern pluton. In this area there is evidence for recovery through grain boundary area reduction (GBAR) in addition to evidence for dislocation creep. GBAR suggests a significant decrease in strain to allow for recovery at the elevated temperatures of metamorphism. There is a common spatial relationship between gold deposits and strain shadows on the regional scale, some of which are major gold camps including Hemlo, Red Lake and Musselwhite Mine, where the gold deposits are situated in greenstone belts proximal to several granitic plutons. The low strain (strain shadow) areas are produced by competency contrast between plutons and greenstone belts and in these cases host significant gold mineralization. Regional structural control of gold mineralization is similar in geometry to the microstructural control, and lithological competency contrast may be a useful tool for predicting the occurrence of other economic orogenic gold deposits. ILSG 2011 74 Program and Abstracts �The Pillar Lake Volcanics: new insights into an enigmatic mesoproterozoic volcanic suite near Armstrong, Ontario Mark Smyk1, Peter Hollings2, and Robert Cundari2 1 Ontario Geological Survey, Ministry of Northern Development, Mines and Forestry, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada 2 Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The Pillar Lake volcanic suite was first recognized and mapped by Macdonald (2004) and is the subject of an ongoing study by Magee (in progress). Initial observations suggested that it largely consisted of a sequence of flat-lying pillowed and massive flows and autoclastic and hyaloclastic breccias. The volcanic rocks unconformably overlie the Badwater gabbro (~1599 Ma) and Badwater syenite (~1590 Ma) and are capped by Keweenawan Inspiration diabase sills (1159 + 33 Ma), yielding an apparent thickness of 20 to 40 m (Hart and Macdonald 2007; Heaman et al. 2007). Titanite in an andesitic unit yielded a 207Pb/206Pb age of 1129.0 + 4.6 Ma; no zircon nor baddeleyite was recovered in the sample (Heaman et al. 2007). Reinvestigation of these volcanic rocks in 2010 was prompted by a new exposure south of Armstrong that had been created during ballast quarry development. The quarry face exposes a ~15 m section of thin (0.5 to 2 m), flat-lying, variably altered, columnar-jointed, basaltic andesite flows, capped by a diabase sill. Individual flows may persist over the 130 m length of the exposure while others bifurcate and terminate as thin tendrils in flow breccia. Autobrecciated zones are rubbly weathering and occupy the spaces between thin, pinching flows. Thin flow top breccias separate flows. Massive flows contain zones of pipe amygdules at their bases and tops. The morphology and disposition of these flows is suggestive of an intercalated, subaerial pahoehoe and a‘a flow succession. Ropy flow tops were noted at a nearby location for the first time during this investigation. Ellipsoidal features seen in some sections that were initially considered to be pillow forms may now be reinterpreted as pahoehoe toes. These volcanic rocks are variably altered along fractures, flow contacts and within brecciated zones. This hydrothermal alteration is typically manifested as a beige to pink discolouration of the dark grey-black flows resulting from the destruction of primary ferromagnesian minerals and the introduction of alkali feldspar, sericite and quartz. Void spaces along joints and fractures and in vesicles has been occupied by large (< 3 cm), black, euhedral actinolite crystals. Small acicular asbestiform crystals of edenite were also identified by x-ray diffraction analysis. Alteration is characterized by increases in Al 2O3, K2O, Na2O, and SiO2 and decreases in CaO, Fe2O3, MgO, P2O5 and TiO2. The trace element geochemistry of the sill that caps the quarry face is identical to that of the Inspiration sill identified by Hart and Macdonald (2007) and we propose that this sill is part of that intrusive suite. Samples of the volcanic rocks display REE enrichment and negative Nb anomalies comparable to the range of samples of the Pillar Lake volcanic suite reported by Magee (personal communication, 2011). Samples of the Inspiration sills are geochemically indistinguishable from the Pillar Lake volcanic rocks and to least-altered pillowed samples with the lowest LOI values. The similarity between these two enigmatic ILSG 2011 75 Program and Abstracts �suites suggests that they may be derived from the same source. MacDonald and Tremblay (2005) distinguished the Inspiration sill, which exclusively overlies the Pillar Lake rocks, on the basis of its normal polarity and distinct geochemistry. Primary clinopyroxene in the Inspiration sill was commonly replaced by actinolite (Schandl, 2004). MacDonald et al. (2005) also noted a remarkable lack of chilled margins on the Inspiration sill and suggested that it was older than the Nipigon sills. Later work by Heaman et al. (2007) determined an age of 1159±33 Ma for the Inspiration sill, which is consistent with the early period of normal polarity. We propose that the Pillar Lake basalts may in fact be may be coeval with the Inspiration sill which, in turn, may represent either a subvolcanic intrusion or possibly a massive, ponded flow / lava lake. This may account for the extensive alteration in the Pillar Lake basalts with the sill acting as an impermeable cap to hydrothermal fluids, which were concentrated in the underlying volcanic flows. The similarity of the Pillar Lake and Inspiration sill magmatism to other magmas of the MCR, including the Nipigon sills and Osler volcanic rocks (Hollings et al., 2007), suggests that these rocks may represent a very early extrusive to subvolcanic phase of rift activity. The location of these rocks close to the ~1599 Ma Badwater gabbro, the ~1590 Ma Badwater syenite and the ~1540 Ma English Bay anorogenic granite, which have been interpreted as evidence for a long-lived crustal weakness (Hollings et al. 2004), may offer an explanation for how these magmas were erupted so early in the rift history. REFERENCES Hart, T.R., MacDonald, C.A., 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44, 1021–1040. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences 44, 1055–1086. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences 44, 1087–1110. Hollings, P., Fralick, P., Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario. Canadian Journal of Earth Sciences 41, 1329–1338. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. MacDonald, C.A. and Tremblay, E. 2005. Lake Nipigon Region Geoscience Initiative, Bedrock Mapping Project: Geology of the northwest Nipigon Embayment; unpublished poster, Ontario Geological Survey. MacDonald, C.A., Tremblay, E. and Easton, R.M. 2005. Precambrian geology of the west-central map area, Nipigon Embayment, northwestern Ontario, Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Open File Report 6164, 49p. Magee, A. (in progress). Geology and geochemistry of the Pillar Lake volcanic sequence, northwestern Ontario; unpublished M.Sc. thesis, Carleton University, Ottawa ON. Schandl, E.S. 2004. Petrographic data from the northwest Nipigon Embayment, Lake Nipigon Region Geoscience Initiative (LNRGI); Ontario Geological Survey, Miscellaneous ReleaseData 141. ILSG 2011 76 Program and Abstracts �Microstructural control on gold mineralization in greenschist to amphibolite facies metamorphism, Greenstone, Ontario Victoria R. Stinson and Mary Louise Hill Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 Canada; vrstinso@lakeheadu.ca In Greenstone, Ontario, north of Lake Superior in the Superior Province, gold mineralization is associated with regional shear zones within the Beardmore-Geraldton and Onaman-Toshota greenstone belts. Metamorphic grade increases from greenschist facies near Jellicoe, Ontario to amphibolite facies in Longlac, Ontario. Gold is hosted within various lithologies, including iron formation, mafic volcanics, and granodiorite, typically in association with ductile and brittle-ductile structures such as deformed quartz veins. On the microscopic scale, gold is found along quartz-quartz and quartz-muscovite grain boundaries, as inclusions in porphyroblasts of pyrite and albite, and in pressure shadows around competent porphyroblasts. Gold is present in association with similar microstructures regardless of lithology and metamorphic grade. Mafic volcanics at the Prodigy Gold Incorporated Hercules and Castlewood properties north of Jellicoe have been metamorphosed to the greenschist facies with stable metamorphic mineral assemblages that include chlorite and albite. Gabbro at the Prodigy Gold Inc. Milestone property in Longlac has been metamorphosed to amphibolite facies with a stable metamorphic mineral assemblage of hornblende and plagioclase. Within shear zones at Milestone, epidote and chlorite replace hornblende indicating retrograde greenschist facies metamorphism during deformation after peak amphibolite facies metamorphism. All samples with gold mineralization from both Jellicoe and Longlac have microstructures characteristic of ductile deformation at temperatures of greenschist facies metamorphism or higher including quartz grains with irregular grain boundaries, undulose extinction in quartz, and subgrains in quartz. In samples from Longlac with gold mineralization, plagioclase has undulose extinction consistent with ductile deformation at temperatures of amphibolite facies metamorphism or higher. In all samples, gold is found along subgrain boundaries in quartz and along grain boundaries, commonly within muscovite. Pyrite commonly has strain fringes of quartz that have been rotated, indicating non-coaxial strain. Inclusions of gold are within porphyroblasts of pyrite and albite, and also within pressure shadows around the porphyroblasts. Gold is present in similar microstructures regardless of metamorphic grade. ILSG 2011 77 Program and Abstracts �A deformation history of the Ivanhoe Lake Fault Brittney Swoffer and Mary Louise Hill Lakehead University, Department of Geology, 955 Oliver Road, P7B 5E1, Thunder Bay, Ontario, Canada. bswoffer@lakeheadu.ca Ivanhoe Lake is located 220km east of Wawa, Ontario, Canada. The Ivanhoe Lake Fault runs northeast to southwest and is the boundary between the high grade metamorphism of the Kapuskasing Structural Zone and the low grade metamorphism of the Abitibi Subprovince. This difference in grade of metamorphism is what makes the Ivanhoe Lake Fault so interesting to study. Through analysis of the microstructures seen in thin section a long deformation history was revealed. Ductile deformation characteristic of deep burial as well as deformation at or near the brittle-ductile transition zone were evident. This affects a wide area that extends to either side of the fault. This was followed by brittle fracturing at a shallower depth of burial. The brittle deformation is confined to a narrow zone found along Ivanhoe Lake. Examples of ductile deformation within the thin sections include: undulatory extinction in quartz, subgrains in quartz and feldspars, deformation twins in plagioclase and microcline, as well as irregular grain boundaries for all grains. The brittle-ductile transition zone is documented by the existence of pseudotachylites. These glassy rocks are characteristic of large faults. The brittle history is documented by quartz or calcite filled fractures found at both the outcrop and microscopic scale. These brittle structures commonly cross cut the ductile features. This cross-cutting relationship suggests that the fault has had a long history which began with ductile deformation and progressed into brittle deformation. ILSG 2011 78 Program and Abstracts �Geology and mineralization of the Serpentine Cu-Ni deposit, Duluth Complex Minnesota Erik R. Tharalson and Thomas Monecke Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA, etharals@mines.edu The Serpentine Cu-Ni deposit is located approximately 7km southeast of Babbitt, MN and is hosted within the South Kawishiwi intrusion, in the 1.1 Ga Duluth Complex (Fig. 1). The footwall rocks to the deposit are comprised of metagreywacke and banded iron ores of the Paleoproterozoic Virginia and Biwabik Iron Formations. Figure 1: Generalized geologic map showing the locations of deposits within the Duluth Complex (modified from Severson and Hauck, 2008). The South Kawishiwi intrusion has been studied in detail and an igneous stratigraphy has been established by Severson (1994). However, the identified igneous stratigraphy is not recognizable in the area of the Serpentine deposit, possibly due to the occurrence of a local magmatic feeder intruding the stratigraphy or even the occurrence of separate intrusion. The Serpentine deposit is located immediately adjacent to the Grano fault (Fig. 1), which is believed to have been instrumental in controlling the location of a feeder for the Bathtub intrusion (Severson & Hauck, 2008) and the Local Boy massive sulfide deposit (Severson, 1994). Drill core logging in the Serpentine area has identified a heterogeneous mix of troctolitic rocks with minor norites. Igneous textures are characterized by subhedral to euhedral plagioclase with intergranular olivine, augite, and oxides. Plagioclase is locally enclosed in subophitic to ophitic augite. The Serpentine deposit was discovered in 1958 by AMAX following up an EM survey (Kulas, 1979). The deposit consists of two resources: (1) a disseminated sulfide resource yielding 430 million tons of 0.41% Cu and 0.14% Ni, and (2) a semi-massive sulfide resource of 7 million tons of 0.88% Cu and 0.30% Ni (Kulas, 1979). Disseminated mineralization occurs as thick intervals (>50 feet) of interstitial sulfides making up <5% by volume. Pyrrhotite is the dominant sulfide, but chalcopyrite and cubanite locally occur in ILSG 2011 79 Program and Abstracts �equal amounts. The semi-massive and massive sulfides are pyrrhotite-rich with up to 7% copper sulfides and minor pentlandite. The massive sulfide intervals display sharp and gradational contacts and can be up to 19 feet thick. They often display variable textures; including rounded pyrrhotite with interstitial copper sulfides and pegmatitic pyrrhotite with wormy, crosscutting copper sulfides. Most of the semi-massive to massive sulfide occurs within the footwall Virginia Formation or at the basal contact of the Virginia Formation with the South Kawishiwi intrusion. Sulfur isotope values for the Virginia Formation were analyzed by Arcuri and others (1998) and found to range from 8.1 to 29.1 per mil, with most values above 15 per mil. New sulfur isotope values for massive sulfides ranging from 13.1 to 16.3 per mil are consistent with the Virginia Formation representing the principal sulfur source. Understanding the deposit geology and reasons for the accumulation of massive sulfide at Serpentine will help to better focus exploration in developing this deposit. Continued research will establish whether the deposit occurs in proximity to a local magmatic feeder. REFERENCES Arcuri, T., Ripley, E.M., and Hauck, S.A., 1998, Sulfur and oxygen isotope studies of the interaction between pelitic xenoliths and basaltic magma at the Babbitt and Serpentine Cu-Ni deposits, Duluth Complex, Minnesota. Kulas, J.E., 1979, Serpentine reserve; MINNAMAX Project Babbitt, Minnesota: Internal AMAX Company Report, 17p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, Technical Report NRRI/TR-93/34, 210 p., 15 pls. Severson, M.J., and Hauck, S.A., 2008, Finish logging of Duluth Complex drill core (and a reinterpretation of the geology at the Mesaba (Babbitt) deposit): Natural Resources Research Institute, Technical Report NRRI/TR-2008/17, 68 p., 94 pls. ILSG 2011 80 Program and Abstracts �The mineralogy, spatial distribution, and isotope geochemistry of sulfide minerals in the Biwabik Iron Formation Steph Theriault1, Jim Miller1, Mike Berndt2, and Ed Ripley3 1 Department of Geological Sciences, University of Minnesota-Duluth, Duluth, MN 55812 2 Division of Lands and Minerals, MN Department of Natural Resources, St. Paul, MN 55155 3 Department of Geological Sciences, Indiana University, Bloomington, IN 47405 Sulfate cycling in the St. Louis River Watershed (SLRW) requires knowledge of the distribution of sulfide minerals undergoing oxidation in the region, including those from the Biwabik Iron Formation (BIF). The BIF occurs along the Mesabi Range in northeastern Minnesota and continues to be extensively mined for taconite. Associated tailings basins and waste rock piles containing BIF and the overlying Virginia Formation are located along the northern portions of the SLRW. Although the BIF is not considered a sulfide-bearing formation, it contains measurable amounts of sulfide minerals, primarily in the form of pyrite and pyrrhotite (Gundersen and Schwartz, 1962). Three notable previous studies have been conducted on the geochemistry of primary sulfide minerals in the Animikie Basin sediments. The sulfur isotope (δ34S) values cited in each of the studies suggest that primary sulfide minerals of the Animikie Group sediments are the result of bacterial reduction of late Paleoproterozoic seawater sulfate (Poulton et al., 2010; Johnston et al., 2006; Carrigan and Cameron, 1991). However, these studies principally focused on primary sulfide minerals which formed during or soon after the deposition of the BIF. This study, which is being conducted in conjunction with the Minnesota Department of Natural Resources sulfate cycling study of the SLRW, seeks to delineate the mineralogy, distribution, and sulfur isotope geochemistry of sulfide minerals in the BIF. This includes minerals formed by secondary processes, such as the thermal metamorphism of the 1.1 Ga Duluth Complex and the formation of natural ores. These data can be used to better define the major source components of sulfate to the SLRW and to help determine how sulfate, once released, behaves in the environment. Samples containing visible sulfide minerals were collected from five well-spaced drill cores across the Mesabi Range (Fig. 1). Each sulfide-bearing stratigraphic subunit of the BIF (upper slaty, upper cherty, lower slaty, intermediate slate, lower cherty) was sampled in each drill core. Commonly multiple samples were collected when various sulfide mineral habits were encountered in a given unit. In all, 123 samples were collected; 63 were prepared for sulfur isotope analysis and 26 were made into polished thin sections for petrographic study of mineral paragenesis and mineral identification. Results were compared to sulfur isotope data for dissolved sulfate in streams leading from the mining operations. Preliminary results show a complex relationship between geographic location, stratigraphic position, and type of sulfide occurrence. A large range of δ34S values were obtained from samples collected for this study, spanning from +80.37‰ to -36.11‰ (see Fig. 1). Also, four different sulfide mineral habits were noted in the BIF: euhedral cubes, euhedral framboids or spheroids, anhedral ―blebs,‖ and veins. Primary sulfides, in this study, appear mainly as anhedral ―blebs‖ but also occurred as euhedral cubes. They are characterized by a limited range of slightly heavy δ 34S values, from +2‰ to +13‰, similar to those cited in the studies mentioned above. In this study, for example, the primary sulfide isotope values were observed throughout the intermediate slate layer and in the eastern-most, metamorphosed portion of the Mesabi Range. Metamorphism, including the transition from pyrite to pyrrhotite, does not appear to have affected sulfur isotopic composition of the sulfide minerals. However, it is possible that the δ34S values were homogenized during metamorphism and now artificially appear primary. Sulfide minerals formed through secondary processes occur in veins or as euhedral framboids, spheroids, or cubes. They correspond to the more extreme δ34S values, which presumably formed through processes such as weathering and hydrothermal alteration. ILSG 2011 81 Program and Abstracts �In conclusion, both primary and secondary processes are evident in the formation of sulfide minerals throughout the BIF, with secondary sulfide values spanning a broader range than the limited, +2‰ to +13‰, values associated with primary sulfides. Sulfur isotope values for sulfates in stream water samples collected close to mining operations have a relatively narrow range of +4‰ to +9‰, as well. This suggests that primary sulfides within the BIF may contribute most of the sulfate found in the streams. The veins and other noted sparse occurrences sampled in this study may also contribute to the sulfate in area streams, but it is possible their contribution is negligible compared to that of the primary sulfides or there is a balance where the average values from secondary sulfides are, coincidentally, isotopically similar to that of the primary sulfides. However, because sampling was conducted only for visible sulfide minerals in this study, it is possible this correlation is only coincidental. Additional sulfur isotope analyses in the region will help to further delineate all possible sources of sulfate to the SLRW. Figure 1. Distribution of sulfur isotope (δ34S) values at each core location across the Mesabi Range in northeastern Minnesota. Values are reported in per mil (‰) from sulfide mineral occurrences taken throughout the Biwabik Iron Formation and lower most portions of the Virginia Formation. REFERENCES Carrigan, W.J. and Cameron, E.M., 1991, Petrological and stable isotope studies of carbonate and sulfide minerals from the Gunflint Formation, Ontario: evidence for the origin of early Proterozoic iron-formation, Precambrian Research, v. 52, p. 347-380. Gunderson, J.N. and Schwartz, G.M., 1962, The Geology of the Metamorphosed Biwabik Iron-Formation, Eastern Mesabi District, Minnesota. Johnston, D.T., Poulton, S.W., Fralick, P.W., Wing, B.A., Canfield, D.E., and Farquhar, J., 2006, Evolution of the oceanic sulfur cycle at the end of the Paleoproterozoic, Geochimica et Cosmochimica Acta, v. 70, p. 5723-5739. Poulton, S.W., Fralick, P.W., and Canfield, D.E., 2010, Spatial variability in oceanic redox structure 1.8 billion years ago, Nature Geoscience, v.3, p. 486-490. ILSG 2011 82 Program and Abstracts �Characterization of gangue minerals in lower cherty ores of the Biwabik Iron Formation at United Taconite LLC Michael Totenhagen1, Penny Morton2, and Phil Larson3 1 M.S. Candidate Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall 1114 Kirby Drive, Duluth, MN 55812 2 Associate Dean Swenson College of Science and Engineering, Associate Professor Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall 1114 Kirby Drive, Duluth, MN 55812 3 Senior Geologist Duluth Metals, 306 West Superior Street, Suite 60, Duluth, MN 55802 Gangue minerals in lower cherty ores of the Biwabik Iron Formation at United Taconite's Thunderbird mine in Eveleth Minnesota cause an observed decrease in plant water quality during processing. Cliffs Natural Resources (CNR) thinks this problem may be related to a particular gangue mineral staying suspended in plant water. CNR has identified this mineral as a Fe-rich talc occurring in the LC4 ore unit, a cherty thick to thin bedded ore as defined by United Taconite. This project focuses on the gangue minerals in lower cherty ore at United Taconite‘s Thunderbird mine through petrography, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and microprobe analysis, to describe both the mineral abundance and composition in diamond drill core. Fe-rich talc, minnesotaite, stilpnomelane and Fe-Al rich talc display the most intense variations in abundance, occurrence and composition in comparison with carbonates, oxides and quartz. The total abundance of these minerals varies between 10-70%, with the highest abundance occurring in the LC4 and LC2 ore units. There is no apparent spatial variation along strike (Fig 1. A). However, the LC4 and LC2 ore units contain the majority of Fe-Al rich talc and the LC5 ore unit contains a high concentration of Mg-rich stilpnomelane (Fig 1. B). Some of the Fe-rich talc contains elevated amounts of Al as shown by both SEM and microprobe analysis (Figure 1. A & B). This suggests that there might be an unknown mineral (Fe-Al rich talc) occurring with Fe-rich talc, minnesotaite and stilpnomelane. This Fe-Al rich talc appears to be an early stage mineral as it is commonly overprinted by minnesotaite and stilpnomelane but overprints early quartz and carbonate. ILSG 2011 83 Program and Abstracts �A. B. Figure 1. A. Compositional variation by drillhole of talc-minnesotaite, Al rich talc, and stilpnomelane in the LC4 ore unit. B. Stratigraphic compositional variation in drill hole 1081001. These results are consistent with CNR observations of stratigraphic variation in Ferich talc, however we have identified a Fe-Al-rich talc which has not been previously reported. Both of these minerals might be responsible for the decrease in water quality during processing of some LC4 ore. Further research is needed to follow the character of these minerals through the concentrating process to confirm if the observed variation in abundance, composition, and distribution are responsible for variable results during processing. ILSG 2011 84 Program and Abstracts �Using cleavage refraction and microstructural evidence to determine the relative rheology of naturally deformed quartzites and phyllites from Baraboo, Wisconsin Jolene T. Traut and Dyanna M. Czeck Department of Geosciences, University of Wisconsin–Milwaukee, P.O. Box 413, Milwaukee, WI 53201 Understanding the rheology of rocks is a first-order question in tectonic studies where we aim to understand how rocks flow in response to stress. Rock rheology is largely studied through experimental work, but cannot be easily applied to natural deformation because of the long time-scales involved and the complications derived from rocks containing multiple minerals, all with different strengths. The Baraboo syncline near Baraboo, WI is an ideal test case to compare natural deformation to rheologic experimental results conducted on quartz because the main lithology at Baraboo is a quartzite derived from a clean quartz arenite. Within the Baraboo syncline, the variation in lithology (quartzite and phyllite) results in a difference in how these rocks responded to deformation and metamorphism during the regional contraction from the Mazatzal orogeny. We conducted a detailed analysis of the cleavage refraction through a graded contact between the quartzite and phyllite layers in several areas around the syncline (Figure 1) and combined those results with mineralogy determined by X-ray diffraction (XRD). The angle between bedding and cleavage varies between approximately 89° in the most competent quartzite layers (approximately 99% quartz and 1% pyrophyllite) and 15° in the least competent phyllite layers (approximately 82% quartz and 13% pyrophyllite). Due to the increase in pyrophyllite, cleavage readily forms in the phyllite causing a smaller cleavage/bedding angle than what can be seen in the more competent quartzite. Where possible, we determined effective viscosity ratios using the Treagus (1999) cleavage refraction method. Preliminary cleavage/bedding angle data suggest that the effective viscosity ratios range between 1.0 – 3.7, which are values comparable to other studies. Microstructural evidence indicates that varying degrees of recrystallization of quartz occurred during deformation, and it is possible that the amount of smaller, recrystallized grains respond differently than the original depositional grains. By combining point counting methods and the total quartz abundance determined through XRD, we will be able to extract whether these recrystallized grains impact the overall rheology. ILSG 2011 85 Program and Abstracts �Figure 1: Field photos of cleavage refraction in a graded contact between quartzite and phyllite located in the Williams Quarry near Baraboo, WI on the north limb of the syncline. The black lines represent the orientation of bedding, and the white lines represent the orientation of cleavage as it refracts through the two lithologies. REFERENCES Treagus, S., 1999. Are viscosity ratios of rocks measurable from cleavage refraction? Journal of Structural Geology, 21, 895-901. ILSG 2011 86 Program and Abstracts �Surface and subsurface geologic maps of the Soudan Underground Mine State Park, St. Louis County, Northeastern Minnesota Alexandra M. Vallowe1, Ernest J. Thalhamer2, Damon L. Rhoades3, and Dean M. Peterson4 University of Minnesota Duluth, Precambrian Research Center 1 Virginia Polytechnic Institute and State University 4044 Derring Hall (0420) Blacksburg, VA 24061, USA 2 SUNY College: University at Buffalo, Buffalo, NY 14260-1608 3 University of Central Missouri, Warrensburg, MO 64093 4 University of Minnesota Duluth, University of Minnesota Duluth, Duluth, MN 55812 Three Precambrian Research Center field camp students, under the guidance of Dean M. Peterson, completed an underground and surface mapping project at the Soudan Mine, northeastern Minnesota. The purpose of the mapping was to provide the Soudan Underground Mine State Park an updated geological map of the surface and subsurface geology of the Soudan Mine for use in geologic tours and scientific research. In addition, the underground geologic map of the 27th level has been, and continues to be used extensively in numerous geobiology studies. These maps are the result of five days field mapping by the authors in August 2010. The surface geology (Map 1) was mapped for several hours in the afternoon over a period of five days. The authors worked as individuals to cover the mapping area. The authors recorded notes and observations, collected samples, took structural measurements (bedding orientations, lineation, foliation), and plotted data on a 1:5,000 scale field map. Location was determined using a handheld GPS unit, and structural measurements were taken using a Brunton Pocket Transit. Surface outcrops were exposed by peeling back moss layers and sweeping up remaining soils with whisk brooms. Open pits and other ―manmade‖ exposures were also available within the mapping area, but with limited access. The surface bedrock map displays sheared rocks composed of chlorite (5c) and sericiteschists (5s), intrusive rocks such as granite (Gr) and quartz feldspar porphyry (Qfp), and the Upper Sequence rocks which include oxide facies iron formation (4a), lapilli tuff (2e), and greywacke (3a). The surface map also shows two major shear zones which are truncated by the Soudan fault to the east, as well as a minor shear zone. Our assumptions and interpretations of the surface field data were influenced by previously existing map data. Information from Peterson and Patelke‘s 2003 surface map, as well as historical drill core data (circa 1954) from the Oliver Mining Division of US Steel Corporation was incorporated into building the final map. The geology of the 27th level west drift was mapped by the authors over a period of five nights (5:00 PM – Midnight). The authors worked as a team to locate contacts, collect samples, take structural measurements (cleavage, lineation, fault orientations, etc.), record notes and observations, and plot important features on a field map. Geologic information from horizontal drill holes were used to interpret the geology adjacent to the drift. Locations in the mine were determined by measuring tape. Structural data were measured using a Brunton Pocket Transit; there was no deviation of local magnetic field observed. In contrast to the surface mapping, the drift provided complete outcrop exposure with rare exceptions where the drift walls and ceilings were obscured by pipes and other man-made structures. ILSG 2011 87 Program and Abstracts �The drift map (Map 2) includes rock units that correlate to units on Map 1, however, there are two below ground units, gabbro (Gb) and fragmented schist (5f), that do not show a surface expression. Map 2 includes the geology of the 27th level east drift which was previously mapped by Peterson and Patelke (2003). Map 2 also includes annotations by the authors which indicate the locations of significant drift features that may be relevant for mine employees, visitors, and scientists. REFERENCES Oliver Iron Mining Company., 1953-1954. Soudan Mine Area Worksheet. Peterson, D. M. and Patelke, R.L., 2003.Bedrock Geologic Map of the Soudan Mine Area, St. Louis County, Northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29. Scale 1:5,000. Peterson, D.M. and Patelke, R.L., 2003. Geologic Map of the 27th Level East Drift, Soudan Mine. Peterson, D.M. and Patelke, R.L., 2003. National Underground Science and Engineering Laboratory (NUSEL) Geological Site Investigation for the Soudan Mine, Northeastern Minnesota. Economic Geology Group, Natural Resources Research Institute, University of Minnesota Duluth. ILSG 2011 88 Program and Abstracts �Neoarchean magmatism in the NW Superior Craton: Granitoids of the North Caribou Terrane Amanda Van Lankvelt1, David Schneider1, Keiko Hattori1, and John Biczok2 1 Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5 Canada Goldcorp Canada Ltd., Musselwhite Mine, P.O. Box 7500, Thunder Bay, ON, P7B 6S8 Canada 2 The North Caribou Terrane (NCT), located in northwestern Ontario and eastern Manitoba, is the core of the Archean Superior Province. Like many Archean terranes, the NCT is characterized by central greenstone belts of strongly deformed metavolcanics and metasediments surrounded by variably deformed granitoids. We are investigating these external granitoids that comprise the larger batholiths surrounding the North Caribou Greenstone belt in the NCT in an attempt to understand the magmatic history and processes involved in batholith formation. Mapping and petrologic work identified two distinct groups of granitoids in the region: voluminous, massive to gneissic tonalite, trondhjemite, granodiorite (TTG) batholiths and massive pegmatitic to aplitic K-feldspar-rich granites, which intrude the margins of the greenstone belt and are also ubiquitous throughout the TTG suites. These rocks cross-cut all rock types and occur mostly as convoluted, brittle-ductile dykes (Figure 1). There are two types of pegmatitic rocks: an undated suite consisting primarily of K-feldspar and quartz with minor oxides, including magnetite, and S-type intrusions, containing muscovite, biotite, garnet, and less K-feldspar. Notably, zircon and titanite are absent from pegmatites. The TTG batholiths have a wider variety of compositions, ranging from tonalite to granite (sensu stricto). Important accessory minerals include titanite, zircon, epidote, and hornblende. Epidote tends to occur in cross-cutting quartz veins, which suggests a late- to post-magmatic metasomatic event. This thermal episode was accompanied by moderatetemperature strain as evidenced by undulose extinction and bulging recrystallization in quartz and planar alignment of biotite and amphibole. Migmatitic textures in discrete parts of the batholiths indicate that partial melting of, or melt segregation within, the TTG batholiths could be the source for the K-feldspar-rich pegmatites. We performed Al-in-hornblende geobarometry on several batholith samples spanning granitic, granodioritic and tonalitic compositions; emplacement depths are variable across the batholith, ranging from pressures of 3.0±0.5 kbar (~13 km) to the southeast (tonalite) and 6.5±0.5 kbar (~20 km, granodiorite) to the north of the belt. To complement our geobarometric data, we also carried out U-Pb geochronology on igneous titanites and zircons using LA-ICP-MS methods to determine crystallization ages. The oldest zircons have a ca. 3.0 Ga inherited core, but the majority of grains have well-defined concordant ages of ca. 2850 Ma or ca. 2730 Ma. The former age signature is more prominent in the north of the belt, in contrast to the ca. 2730 Ma age that is more localized in the south-centre margin of the belt. In one sample, zircons have 2850 Ma cores and 2730 Ma rims. Titanite ages are similar to those from zircon. These new U-Pb ages are in accord with the few previously reported ages from the NCT (e.g. Breaks et al. 2001, Klipfel 2002, Wyman et al. 2010). The original core of the Superior Province, recorded in the inherited zircons in this study and detrital and volcanic zircons from elsewhere in the NCT (Percival 2007, Lin et al. 2006) had formed by 3.0 Ga. This crust was subsequently deformed and intruded by, or incorporated into, 2850 Ma granitoids, which form the large batholiths that comprise much of ILSG 2011 89 Program and Abstracts �the NCT. At this stage, the granitoids of tonalitic composition were emplaced at midcrustal levels. Later magmatism, at 2730 Ma, was more evolved with granodioritic to granitic composition. This stage of magmatic activity took place at shallower, mid- to uppercrustal depths. The most recent magmatic activity (granitic pegmatites) was post-deformation, occurred near the brittle-ductile transition, and has a highly evolved composition. The discrete magmatic events recognized in the NCT shows a long history of magmatism, over 300 m.y., and re-working of existing crust in the interior of the craton. Figure 1: Pegmatite intruding strongly lineated amphibolite (Amp). Notice brittle and ductile structures (cracks parallel to lineation and pinched-out amphibolite rafts), suggesting pegmatites were intruded near the brittle-ductile transition. REFERENCES Breaks, F., Osmani, L., DeKemp, E. 2001. Geology of the North Caribou Lake Area, Northwestern Ontario, Ontario Geological Survey, Open File Report 6023. 80 pp. Klipfel, P. 2002. Musselwhite U-Pb zircon and Ar-Ar dates: synthesis and interpretation. Internal report. 23 pp. Lin, S., Corkery, M., Bailes, A., and Davis, D. 2006. Geological evolution of the northwestern Superior Province: Clues from geology, kinematics, and geochronology in the Gods Lake Narrows area, Oxford-Stull terrane, Manitoba. Canadian Journal of Earth Sciences. v. 43. pp. 749-765. Percival, J. 2007. Geology and Metallogeny of the Superior Province, Canada. in Goodfellow, w., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, pp. 903-928. Wyman, D.A., Hollings, P., Biczok, J. 2011. Crustal evolution in a cratonic nucleus: Granitoids and felsic volcanic rocks of the North Caribou Terrane, Superior Province Canada. Lithos. in press. ILSG 2011 90 Program and Abstracts �Towards a geochemical characterization of native copper ores of the Keweenaw Peninsula, Michigan Jillian E. Votava and Theodore J. Bornhorst A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931 The Keweenaw Peninsula is well known for its world-class native copper deposits. These deposits have been subject to many geologic studies yet the native copper ores are poorly characterized in terms of bulk geochemical composition. This study represents a first step towards a geochemical characterization of the native copper ore. The samples for this study came from the Caledonia native copper mine near Mass, MI since there is underground access. The Caledonia is part of the southern extension of the main native copper district where native copper is hosted in several different mineralized basalt lava flow tops, collectively known as the Evergreen Series. At Caledonia the native copper is hosted by two brecciated flow tops and is accompanied by secondary minerals filling amygdules and spaces between fragments. The secondary minerals include: quartz, feldspar, pumpellyite, chlorite, calcite, and epidote. The Caledonia mine has yielded ~3.1 million kg of refined copper with an average grade of 1.24%. Native copper mineralization is typical of that found elsewhere in the district. For this study, 16 underground grab samples were taken at thirteen locations and 1 background sample of massive, visually unaltered basalt from the surface rock pile. Underground mineralized samples were focused on areas with high and variable degrees of alteration. The goal was to obtain a sufficient number of samples with different amounts of copper that bracketed the average copper in the ore. Thereby, the median abundance of constituents in the suite of mineralized samples will be used to represent the geochemical composition of the native ore. Representative subsets of each grab sample were prepared and analyzed for 64 elements by Actlabs using 4-Acid digestion with ICP or ICP-MS finish, INAA, and Hg by cold vapor FIMS. The Caledonia background sample was compared to fresh tholeiitic basalt (BCR-1) to insure Caledonia background values were reasonable and if not, BCR-1 was used for the background value. Ratios of enrichment for each element were calculated using the median of the mineralized samples. Using these ratios each element was placed into one of 6 categories: enriched, depleted, potentially enriched or depleted, same as background, and indeterminate. Ratios greater than 1.0 were considered to be enriched while ratios less than 1.0 were considered depleted. Ratios around 1.0 were generally considered same as the background (no change). Elements with moderate variance or with some (less than half) of the samples less than the detection limit were placed into the appropriate potentially depleted or potentially enriched category. Those with high variance or with more than half the samples below the lower detection limit were considered indeterminate. The native copper ore is enriched in Cu, Ag, Hg, As, Te, Sr, Ca, Cd, and Se while depleted in Ba, Rb, Na, Nb, K, and V. Each of these elements was plotted against copper and the highest correlations are with Hg and Ag (ρ= 0.597 and 0.635, respectively). Strikingly, when rank correlations between all of these are calculated, Hg and Ag had a rank correlation coefficient of 0.966 and a log-log plot illustrates this high correlation (Fig. 1). Heretofore, the native copper ores were well known for the association of Ag and As with Cu. The abundance of calcite as an alteration mineral is consistent with the enrichment of Ca and Sr. The enrichment of Cd, Te, and Se and potential enrichment of Au is previously ILSG 2011 91 Program and Abstracts �not documented. The strong enrichment of Hg is also previously not documented although geochemical studies of stamp sands suggested this enrichment (Kerfoot, personal communication). That Hg so strongly correlates with Ag suggests a synchronous timing of deposition. The combination of Hg, Se, Te, and Au are often found associated with gold deposits elsewhere and their genetic connection to the native copper deposits is unknown at this time. This study highlights the value in characterizing the geochemical composition of the native copper ores. Additional sampling is needed to validate the results presented here since they only represent one mine. Table 1: Chemical characterization of native copper ore from Caledonia Mine, MI. Values represent amount of element at the median Cu level (1.6%). Categories described in text. Composition of Native Copper Ore with 1.6% Cu at Caledonia Mine Enriched Depleted Ag 9.68 ppm 65 ppb H g As 4.5 ppm Te 0.4 ppm Sr 837 ppm Ca 12.8 % 0.9 ppm C d Se 0.7 ppm Potentially Enriched Pb 25 ppm 1 ppb A u Ba Rb 14 ppm 2.70 ppm Na Nb K V 0.1 % 0.6 ppm 0.38 % 120 ppm Potentially Depleted Zn 52 ppm Cr 140 ppm Same as Background Al 7.79 % Be 0.6 ppm Same as Background Nd 18 ppm P 0.045 % Ce Dy Er Eu Fe 36 ppm 4 ppm 2.3 ppm 1.57 ppm 7.35 % Pr Sb Sc Sm Tb Ge Gd Hf Ho 0.3 ppm 4.6 ppm 1.9 ppm 0.8 ppm La Lu Mn 18 ppm 0.3 ppm 1040 ppm Indeterminate Co Li Re Sn 4.5 ppm 0.2 ppm 21 ppm 4.4 ppm 0.7ppm Mg Ti Tl In Ir W Ta S Mo Bi Th Tm U Y 1.5 ppm 0.3 ppm 0.3 ppm 23 ppm Ga Ni Br Cs Yb Zr 2 ppm 65 ppm 10000 Hg (ppb) 1000 Figure 3: Plot of Ag versus Hg and the apparent relationship between silver and mercury in the samples from Caledonia Mine. 100 10 1 0.1 0.01 1 100 10000 Ag (ppm) ILSG 2011 92 Program and Abstracts �Pyroclastic Magnetite Bombs in the Hemlock Formation Iron County, Michigan Thomas Waggoner1*, Doug Duskin2, Musa Karakus3 and John Gartner4 1 141 Chippewa Negaunee, MI 49866 thomaswaggonergeo@hotmail.com 2 210 Union St. Camden, SC 29020 3 Cliffs NR 550 E. Division Ishpeming, MI 49849 4 PMR 723 Riverside Plaza Iron River, MI 48935 The Mt. Hemlock Stratovolcano has a footprint of at least 2000 square miles and reaches a height of 30,000 feet without inclusion of the two Kiernan Sills. Recent age dating (Schneider, 2002) places a single rhyolite unit age at 1874+-7 Ma. The volcanic pile includes massive/vesicular/pillowed basalts, agglomerates/hyaloclastites, rhyolite flows/tuffs, basic volcanoclastics, ash beds, slates and at least two bands of carbonate chert iron formation. Limited outcrop exposure and drilling has prevented better definition of the detail lithology so each new drill hole offers additional useful information. Basalt outcrops on the west side of Mt. Hemlock (Johnson, 1975; Dann, 1978; Foose, 1981) indicate a high magnetite content and samples from the southwest flank show the iron content doubles in the top 6,000 feet of the Hemlock volcanics. The increase in iron oxide content is associated with two banded iron formations of limited areal extent that formed on the inclined flank of the volcano. In 2007 Prime Meridian Resources drill tested a 2,000 nT total field magnetic anomaly over outcrops that showed traces of chalcopyrite. Located in the NE ¼ NE 1/4 Sec. 32 T. 43N. R. 31 W., inclined NQ diamond core hole KS-102-2 was drilled on an angle to the east designed to intercept the volcanics and test the metal content. The hole penetrated thin cherty zones alternating with thin amygdaloidal basalts near the top followed by a tuff unit, amygdaloidal basalt and ending in pyroclastics consisting of rounded and flattened vesicular bombs of basalt and magnetite resting in fine volcanoclastic sediments. SEM-EDS, XRF and XRD were employed to identify the rock constituents. Iron values ranged from 8 to 25%, mostly as magnetite, while titanium oxide values ranged from 2-6.5%. Both metals are present in unusually high amounts. The TiO2/FeO ratio plotted against FeO illustrates the Hemlock is quite different from other basalts. Even though titanium is elevated, it does not match the larger increase in iron oxide. Basaltic units of the Hemlock Formation have already been reported to contain high percentages of titanium free magnetite (Johnson, 1974, Dann, 1978). Magnetite in some bombs account for up to 80% of the groundmass. The magnetite is confined to the ground mass and does not fill the vesicles indicating primary crystallization. The drill hole is located between the basal pyroxenite and the lower gabbro zone of the differentiated West Kiernan sill. Some metasomatism of the proximal Hemlock occurred during emplacement of the sill. Both the Hemlock and West Kiernan sills have undergone sodium and potassium metasomatism. Original minerals including pyroxene, calcium feldspars and ilmenite were replaced by epidote, albite, titanite, Fe-chlorite, biotite, quartz, potassium minerals, calcite, ilmenite, chalcopyrite, pyrite and calcium REE fluorocarbonate + barite in biotite. Many of these minerals can be found as amygdaloidal fillings. The sodium, silica and potassium alteration has converted the pyroxenes, feldspar and ilmenite to albite, chlorite, biotite and titanite leaving the original magnetite and apatite ILSG 2011 93 Program and Abstracts �unaltered. The subhedral and euhedral magnetite quickly crystallized from the magma and migrated toward the vesicle openings that formed quickly by rapid de-gassing. The fine magnetite crystals range from 3-50 microns in size and are generally devoid of any inclusions, including titanium minerals. It is postulated the gravity separation took place in a quiescent magma chamber where iron oxide rich segregations pooled. The subsequent violent eruption of a gas laden magma allowed for creation of vesicular magnetite pyroclastic bombs that varied in size from pebbles up to more than 8 inches that quickly solidified and deposited in water on the slope of the volcano in a random pattern. High magnetite tholeiitic basalts and magnetite rich bombs are very rare in extrusive volcanic rocks of any age. One example is the high grade magnetite bombs preserved in the 2 Ma El Laco deposit located in Chile (Henriquez, 1998) where they are part of magnetite flows. We can speculate that near cessation of active volcanism an iron oxide rich magma was tapped by sea water and formed a large hydrothermal plume that vented distal to the central vent on the sea bottom instantly forming fine crystals of magnetite or specularite depending on piping, distance or chemistry at the instant of entry into a reduced temperature/pressure gradient. The Fence River iron formation on the east flank of the Amasa uplift exhibits both magnetite and specularite lithologies. Drilling, mapping and analytical examination have shown that parts of the Hemlock Formation originated from unusually rich iron oxide magma that resulted in the creation of vesicular magnetite rich pyroclastics. REFERENCES Dann, J.C., 1978, Major-Element Variation within the Emperor Igneous Complex and the Hemlock and Badwater Volcanic Formations, MTU MS Thesis, 198 pages. Foose, M.P., 1978, Geologic Map of the Ned Lake Quadrangle, Iron and Baraga Counties, Michigan, USGS Map I-1284, Scale: 1:62,500. Henriquez, F., Nastrom, J.O., 1998, Magnetite Bombs at El Laco Volcano, Chile, GFF V. 120 p. 269-271. Johnson, D.J., 1975, Petrology of a Portion of the Hemlock Formation, Iron County, Michigan. MTU MS Thesis, 55p. Schneider, D.A., et al, 2002, Age of Volcanic Rocks and Syndepositional Iron Formation, Marquette Range Supergroup; Implications for the Tectonic Setting of Paleoproterozoic Iron Formations of the Lake Superior Region, Canadian Journal of Earth Sciences, V. 39, p. 999-1012. ILSG 2011 94 Program and Abstracts �Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, Northwestern Ontario Jakob Wartman1, Ron Morton1, George Hudak2 and Cory Hercun3 1 Dept. of Geol. Sciences, Univ. of Minnesota Duluth, Duluth, Minnesota 55812 2 Precambrian Research Center / NRRI, Duluth, Minnesota 55811 3 Rainy River Resources Ltd., P.O. Box 3, Emo, Ontario P0W 1E0 The Rainy River Gold Project (RRGP)is located 75km northwest of Fort Frances, Ontario, within the Rainy River greenstone belt. This advanced stage exploration project has a NI43-101 compliant gold resource of 3.42 Moz indicated and 3.17Moz inferred (from Rainy River Resources Press Release, February 2011) represented by low grade (<2g/t), lowmoderate grade (2-10g/t), and high grade (>10g/t) gold mineralization. The nature of the gold mineralization in this deposit has been the subject of controversy, and several competing models have been proposed to explain its genesis. Initial exploration in 1967 suggested that the deposit wasa shear zone-hosted resource. However, recently completed exploration drilling has now defined large, diffuse zones of gold mineralization in dacitic volcanic and volcaniclastic rocks, suggesting, in part, a syn-genetic genesis for the gold mineralization. While previous studies have examined structural regimes and timing of gold mineralization, this research focuses on the physical volcanology and hydrothermal alteration associated with the deposit. Field mapping is difficult due to a paucity of outcrop, and geological correlations are complicated by polyphase deformation, hydrothermal alteration, and both regional, and locally contact, metamorphism. This study included comprehensive fieldwork involving mapping of all available outcrops at a 1:24,000 scale and exploration drill core logging along three sections totaling ~7,400 m of core. Fieldwork, supplemented with 210 thin sections and 73 geochemical samples, has enabled distinction of stratigraphy, volcanic facies, and hydrothermal alteration assemblages. Drill core is locally intensely altered and deformed, resulting in many of the units having a false pyroclastic appearance. Despite this, strata associated with the RRGP contain some well-preserved primary textures. These primary textures indicate that the volcanic facies in the deposit include coherent dacitic flows and associated syn-volcanic intrusions with autoclastic breccias, hyaloclastites, peperites, and syn- to post-depositional resedimented volcaniclastic deposits. The coherent dacitic flows are massive, range in thickness up to 150 m, and in lateral extent for 2500 meters. Coherent dacite flows grade into a heterogeneous facies, characterized by pods and lobes of coherent dacite, enveloped by autoclastic breccia and hyaloclastite. Flows are interspersed with strongly altered volcaniclastic sediments that are locally punctuated by peperites. Volcanic facies reconstruction indicates the presence of lobe-hyaloclastite dome/flow complex fed, and locally intruded by, synvolcanic dacite hypabyssal intrusions. An apparent feeding fissure is centered to the west of the main area of mineralization. Hydrothermal alteration is widespread throughout the deposit and is marked by silicification, chloritization, sericitization, and carbonitization (as well as minor epidote and local biotite). Quartz, sericite and chlorite are ubiquitous in the deposit. Grant‘s (1986) isocon method and Large et al.‘s (2001) box plot where applied to the rocks at the RRGP and quantify the chemical alteration resulting from the hydrothermal alteration. Alteration ILSG 2011 95 Program and Abstracts �assemblages are dominantly stratabound, and related to original rock permeability, with flow tops, autoclastic breccias, and volcaniclastic sediments being most strongly altered. Shearzones also preserve stronger alteration intensities. Gold mineralization appears to have occurred within a synvolcanic, low-sulfidation (Simmons et al., 2005) epithermal system. Elevated gold values are strongly correlated with highly permeable units and increased alteration intensity, suggesting enhanced mineralization in areas that experienced higher water: rock ratios. Post-volcanic remobilization of the gold appears to have occurred, as the highest gold values in the deposit are spatially related to shear-zones and associated quartz-carbonate-epidote veins. REFERENCES Grant, J. A., 1986, The Isocon diagram—A simple solution to Gresen‘s equation for metasomatic alteration: Economic Geology, v. 81, p. 1976–1982. Large, R.R., Gemmell, J.B., Paulick, H., and Huston, D., 2001, The alteration box plot: A simple approach to understanding the relationship between alteration mineralogy and lithogeochemistry associated with VHMS deposits: Economic Geology, v. 96, p. 957– 972. Simmons, S.F., White, N.C., and John, D.A., 2005, Geological characteristics of epithermal precious and base metal deposits: Economic Geology, 100th Anniversary Volume, p. 485–522. ILSG 2011 96 Program and Abstracts �AUTHOR INDEX PROCEEDINGS VOLUME 57 PART 1— PROGRAM AND ABSTRACTS Amisse, L. and Barnes, S.J .................................................................................................................... 1 Bajjali W., Holstrom C., and Fuller, D. ...............................................................................2 Beh, B. and Fralick, P ..........................................................................................................3 Birkmeier, R., Boley, T., Brannan, B., Doucette, R., Jirsa, M., and Lee, A. .......................5 Boerboom, T.J. and Green, J.C. ...........................................................................................7 Brown B.A. and Schoephoester, P. ......................................................................................9 Buchholz, T.W., Falster A., and Simmons, W. B. .............................................................10 Carl, C., Hollings, P., and Smyk, M. .................................................................................12 Carlson E.J., Boerboom T.J., Holton C.J., Kubitza, K.W., Mulvey, L., and Scheurer, E. ................................................14 Cervin, D., Morton, P., Miller J., and Patelke, R. ..............................................................16 Chandler, V.W. and Lively, R.S. .......................................................................................18 Chandler, V.W. and Lively, R.S. .......................................................................................20 Cummings, K. and Bjørnerud, M.......................................................................................22 Cundari, R., Hollings, P., and Smyk, M. ...........................................................................24 Dayton, R.N., Miller, J.D., and Vervoort, J.D. ..................................................................26 Deschamp, M. and McGuire, J. .........................................................................................28 DeVasto, M.A., Czeck D.M., Bhattacharyya, P. ...............................................................29 Easton, R.M. and Heaman, L.M. .......................................................................................31 Esch, J.M............................................................................................................................33 Floyd, C.T., Syverson, K.M., and Hupy, C.M. ..................................................................35 Foley, D.J., and Miller, J. D. ..............................................................................................37 Fralick, P. and Zaniewski, K. .............................................................................................39 Geller, P., Fralick, P., Hill, M.L., and Gillies, P. ...............................................................41 Gilbert, H.P. .......................................................................................................................42 Goldner B.D. and Miller J.D..............................................................................................44 Hudak, G., Monson Geerts, S., Zanko, L., Severson A., Severson, A., and Bandli, B. ....46 Jirsa, M.A., Boerboom, T.J., and Chandler, V.W. .............................................................48 Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Mossler, J.H., Runkel, A.C., and Setterholm., D.R......................................50 Kulakov, E.V., Smirnov, A.V., and Diehl, J.F. .................................................................51 LaMaskin, T.A. ..................................................................................................................53 Larson, Phillip, Swenson, J., and Patelke, M. ....................................................................55 MacTavish, A.D., Heggie, G.J., Goodgame, V.R., Johnson, J.R., Beswick, A.E., Stone, W.E., and Watkins, K.P. ...................57 Mattox, S. ...........................................................................................................................59 Miller, J., Brooker, B., Hadley, M., Markwood, L., Olson, J., and Tomlinson, A. ...........61 Peterson, D., Larson, P., Sweet, G., and Gibbons, J. .........................................................63 Piispa, E.J., Smirnov, A.V., Pesonen, L.J. .........................................................................65 ILSG 2011 97 Program and Abstracts �Queffurus, M. and Barnes, S.J. ..........................................................................................66 Radakovich, A.L., Parent, C.T., Partridge, M.E., Ritts, A.D., Pierce, R., & Hudak G.J. ..67 Rague, R. and Losh, S. .......................................................................................................69 Ross, C., Hudak, G., Morton, R., Quigley, T., Mahin, B. .................................................70 Saylor, B.J., Stencil J.C., DeVasto, M.A., and Bhattacharyya, P. .....................................72 Scott, R. J., Kolb, M.J., and Hill, M.L. ..............................................................................74 Smyk, M., Hollings, P., and Cundari, R. ...........................................................................75 Stinson, V. and Hill, M.L. ..................................................................................................77 Swoffer, B., and Hill, M.L. ................................................................................................78 Tharalson, E.R., and Monecke, T. ....................................................................................79 Theriault, S., Miller, J., Berndt, M., and Ripley, E. ...........................................................81 Totenhagen, M., Morton, P., and Larson, P. ......................................................................83 Traut, J.T. and Czeck, D.M. ...............................................................................................85 Vallowe, A.M., Thalhamer, E.J., Rhoades, D. L., and Peterson, D.M. .............................87 Van Lankvelt, A., Schneider, D., Hattori, K., and Biczok, J. ............................................89 Votava, J.E. and Bornhorst, T.J .........................................................................................91 Waggoner, T., Duskin, D., Karakus, M., and Gartner, J....................................................93 Wartman, J., Morton, R., Hudak, G., and Hercun, C. ........................................................95 ILSG 2011 98 Program and Abstracts � https://digitalcollections.lakeheadu.ca/files/original/3d0c78b21ab22e0ec00084aad9293a15.pdf 09d9f20af1d143b5a95f8a7098f16c65 PDF Text Text 57TH ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY ASHLAND, WISCONSIN MAY 18-21, 2011 PROCEEDINGS VOLUME 57 PART 2 – FIELD TRIP GUIDEBOOK ��INSTITUTE ON LAKE SUPERIOR GEOLOGY 57THANNUAL MEETING MAY 18-21, 2010 ASHLAND, WISCONSIN HOSTED BY: NORTHLAND COLLEGE TOM FITZ Chair ProceedingsVolume 57 Part 2 – Field Trip Guidebook Edited by: Tom Fitz, Allison Mills, Kristi Wilson, Cassandra Bodette, and Drew Cramer Cover Photo: Copper Falls at Copper Falls State Park near Mellen, Wisconsin – T.Fitz �57TH INSTITUTE ON LAKE SUPERIOR GEOLOGY CONTENTS OF PROCEEDINGS VOLUME 57: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD TRIP GUIDEBOOK TRIP 1:IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH - TYPE INTRUSION OF THE DULUTH COMPLEX TRIP 2:MIDCONTINENT MICROCOSM TRIP 3:GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS TRIP 4:GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE TRIP 5:BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM TRIP 6:GEOLOGY OF COPPER FALLS STATE PARK TRIP 7:GEOLOGY OF THE MONTREAL RIVER MONOCLINE TRIP 8:THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA TRIP 9:GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX Reference to material in Part 2 should follow the example below: Bjørnerud, M., and Cannon, W.F., 2011, Midcontinent microcosm: Geology of the Atkins Lake–Marengo Falls area: Institute on Lake Superior Geology, 57th Annual Meeting, Ashland, WI, v. 57, part 2, p. 31-46. Published by the 57th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca ILSG website: http://www.lakesuperiorgeology.org ISSN 1042-9964 ILSG 2011 ii Field Trip Guidebook �TABLE OF CONTENTS PROCEEDINGS VOLUME 57 PART 2: FIELD TRIPS TRIP 1:IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH – TYPE INTRUSION OF THE DULUTH COMPLEX LEADER: JIM MILLER 1 TRIP 2:MIDCONTINENT MICROCOSM: GEOLOGY OF THE ATKINS LAKE – MARENGO FALLS AREA LEADERS: MARCIABJØRNERUD&WILLIAM F. CANNON 31 TRIP 3:GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS LEADERS: DICK OJAKANGAS, DREW CRAMER, & TOM FITZ 49 TRIP 4:GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE LEADER: JAMES R. DUNN 79 TRIP 5:BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM LEADERS: MICHELE WHEELER & CASSANDRA BODETTE 87 TRIP 6:GEOLOGY OF COPPER FALLS STATE PARK LEADERS: ALLISON MILLS, DREW CRAMER, & TOM FITZ 97 TRIP 7:GEOLOGY OF THE MONTREAL RIVER MONOCLINE LEADER: WILLIAM F. CANNON 111 TRIP 8:THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA LEADER: TERRY BOERBOOM 127 TRIP 9:GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX LEADER: TOM FITZ 163 ILSG 2011 iii Field Trip Guidebook �Locations of ILSG 2011field trips. Numbers on the map correspond to field trip numbers. The lead editor, Tom Fitz, would like to thank the field trip leaders, the reviewers, and the dedicated team of editors who have contributed their time and effort to creating this field trip guidebook. Their efforts are greatly appreciated by everyone who will use this book. We hope the guidebook will help foster understanding and appreciation of the remarkable geology the Lake Superior region. ILSG 2011 iv Field Trip Guidebook �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 1 IGNEOUS STRATIGRAPHY OF THE LAYERED SERIES AT DULUTH – TYPE INTRUSION OF THE DULUTH COMPLEX Rocks of the Duluth Layered Series -J. Miller ILSG 2011 1 Field Trip 1 �ILSG 2011 2 Field Trip 1 �Field Trip 1 Igneous Stratigraphy of the Layered Series at Duluth – Type Intrusion of the Duluth Complex James D. Miller Department of Geological Sciences and Precambrian Research Center University of Minnesota Duluth This trip guide is modified from a guidebook entitled Field Guide to the Geology and Mineralization of Mafic Layered Intrusions of the Duluth and Beaver Bay Complexes, Northeastern Minnesota (Miller and Severson, 2009). The guidebook was developed for a field trip associated with the Precambrian Research Center Professional Workshop on Field, Petrographic and Mineralization Characteristics of Mafic Layered Intrusions held October 4-10, 2009 GEOLOGY AND PGE MINERALIZATION OF THE LAYERED SERIES AT DULUTH The well-exposed gabbroic rocks forming the escarpment above the city of Duluth have long been recognized as the type section of the Duluth Complex. While early surveys recognized the presence of two distinct rock types in the Duluth area (Winchell, 1899), Grout was the first to interpret the layered gabbros as a product of convection and magma differentiation (Grout, 1918a-e). Taylor (1964) produced the first detailed-scale (1:24,000) geologic map of the complex in the Duluth area and defined the main distinctions between the layered and anorthositic series. Moreover, based on field, petrographic, and very limited geochemical data, Taylor recognized basic similarities between the layered series at Duluth and the Skaergaard intrusion, which Wager and Deer (1939) had established as the classic example of a mafic intrusion formed by closed-system fractional crystallization of a tholeiitic magma. More recently, detailed mapping in the Duluth area has delineated much more about the structure and cumulate stratigraphy of the layered series (Miller and Green, 2008a & b; Green and Miller, 2008). The petrology of the layered series was summarized by Miller and Ripley (1996) and Miller and Severson (2002). The Layered Series at Duluth (DLS) is a well-differentiated, 3- to 4.5-km-thick, eastdipping sheet-like mafic layered intrusion that is the southernmost intrusion of the Duluth Complex. Exposure is very good along the 600'-high escarpment above Lake Superior and the St. Louis River estuary, but is spotty inland. Aeromagnetic data show that the DLS has a north-south strike length of about 60 kilometers long and eventually pinches out into the Boulder Lake intrusion. The hanging wall of the DLS consists of olivine gabbroic and troctolitic anorthosite of the anorthositic series, which according to U-Pb dating was emplaced just prior to DLS intrusion at 1099 Ma (Paces and Miller, 1993). The footwall of ILSG 2011 3 Field Trip 1 �the DLS at its southern end is reversed polarity lavas of the Ely's Peak basalts (Fig. 1-1). Layering and foliation in the DLS, which dip 20º-40º to the east, and modeling of aeromagnetic and gravity data indicate that the basal contact is not conformable with shallow-dipping (~15°) Keweenawan basalt flows in the footwall, but instead dips at a greater than 35° angle (Miller and Green, 2008b). Aeromagnetic data further imply that the basalt is cut out by the DLS to the north, whereupon the footwall becomes graywacke and slates of the Thomson Formation. Figure 1-1. Geology of the Duluth Complex at Duluth showing the field trip stops. ILSG 2011 4 Field Trip 1 �Figure 1-2. Igneous stratigraphy and cryptic variation of olivine, pyroxene and plagioclase composition through the Layered Series at Duluth. mg# values of olivine and pyroxene and An content of plagioclase are based on multiple microprobe analyses (error bars indicate one standard deviation). From Miller and Severson, 2002, Figure 6.8.Note that the cumulate codes for augite are A/a rather than the C/c (clinopyroxene) used in the text. The DLS is divided into five major zones based on dominant rock type (Fig. 1-1). The basal contact zone is composed of coarse-grained, taxitic olivine gabbro and augite troctolite (Stop 1). The lowermost cumulate sequence is the troctolite zone (Stops 1, 2A & 2B). It is 1 to 2 kilometers thick, consists mostly of homogeneous foliated troctolitic (PO) cumulates, and locally displays modal and textural layering, especially in its lower third. The cyclic zone (Stops 3, 4, & 5) forms the medial section of the DLS and is characterized by cyclical variations in cumulus mineralogy between troctolitic and gabbroic (PCFO) cumulates (Fig. 1-4). The persistent occurrence of gabbroic cumulates defines the gabbro zone. Gabbroic cumulates, in turn, grade upward into unlaminated (noncumulate) apatitic ILSG 2011 5 Field Trip 1 �Figure 1-3. Geology of the Cyclic Zone of the DLS showing field stops 1-3 to 1-5. Note that the cumulate codes for augite are A/a rather than the C/c (clinopyroxene) used in the text. ILSG 2011 6 Field Trip 1 �Figure 1-4. Cryptic variation of Fo and En in outcrop samples from the cyclic zone taken along two profiles north and south of Interstate 35 (Fig. 1-3). Note that the cumulate codes for augite are A/a rather than the C/c (clinopyroxene) used in the text. ILSG 2011 7 Field Trip 1 �quartz DLS ferromonzodiorite, which composes most of the upper contact zone (Stop 6). This quartz ferromonzodiorite complexly mixes with a fine-grained biotitic ilmenite ferrodiorite, which ultimately forms a "chilled" contact with anorthositic series rocks (Stop 7). A body of melanogranophyre that irregularly cuts through the anorthositic series probably represents the uppermost differentiate of the DLS (Stop 8). This igneous stratigraphy is complimented by cryptic layering of cumulus mineral compositions (Fig. 1-2) and together these imply the generally formed by bottom-up fractional crystallization of a moderately evolved, olivine tholeiitic parent magma. Although the DLS is overall, a well-differentiated intrusion, the repeated progression from troctolitic to gabbroic cumulates in the cyclic zone indicates that it did not fractionally crystallize as a closed system. The cyclic zone consists of at least five macrocycles (each 50200 m thick) within which troctolitic cumulates grade upward to gabbroic cumulates (Figs. 13, 1-4). Macrocycle boundaries are marked by the abrupt regression in the cumulus mineralogy from gabbroic back to troctolitic cumulates. The gabbroic parts of the macrocycles commonly contain inclusions of anorthositic series rocks and the very uppermost parts of the macrocycles locally have discontinuous layers of fine-grained gabbroic adcumulate (microgabbro). This cyclicity in phase layering does not correspond to a complimentary cryptic variation in mineral chemistry (Fig. 1-4). Based on these characteristics, Miller and Ripley (1996) suggested that the macrocyclic phase layering is predominantly related to devolatilization and decompression that attended magma venting events from a shallow (< 5 km) chamber, with magma recharge possibly having played a secondary role. Geochemical data were acquired from 83 hand samples that profile the general stratigraphy of the DLS. The sampling focused on specific horizons, particularly in the cyclic zone, that contain visible sulfide mineralization or appear to represent perturbations to the magmatic system (Fig. 1-5, 1-6). Most of the samples have Pt+Pd concentrations below 15 ppb; 14 samples have concentrations between 15 and 150 ppb and five samples have concentrations >150 ppb. Whole rock S and Cu concentrations (Fig. 1-5) imply that the DLS magma 1) was consistently sulfide-undersaturated during troctolite-zone crystallization; 2) achieved intermittent saturation during crystallization of the cyclic zone; and 3) was fairly consistently saturated as the gabbro zone accumulated. In contrast to the singular large increase in Cu abundance and Cu/Pd ratio observed in the Sonju Lake intrusion (See Day 3 description), the DLS shows erratic variability in these parameters (Fig. 1-5). Such variability probably reflects the openness of the DLS system to magmatic recharge. Nevertheless, a two to three order-of-magnitude range in the Cu/Pd ratio through the cyclic zone indicates some level of efficiency in the ability of sulfide melt to extract PGE from silicate magma (e.g., Maier and others, 1996). Six hand samples that contain the most enriched Pt+Pd and sulfide concentrations come from the upper portions of macrocycles I and II (Figs. 1-5 & 1-6) and the two highest values are associated with gabbro-microgabbro interfaces. The model of magma venting to explain the phase layering of the cyclic zone (Miller and Ripley, 1996) also can explain the elevated concentrations of sulfide and PGE at cyclic zone boundaries of cumulus regression. Experimental data at low pressures (1-2 kb) suggest a positive correlation between pressure and sulfide solubility in hydrous tholeiitic magmas (Carroll and Rutherford, 1985). If valid, this implies that magma decompression due to venting from shallow differentiated magma chambers may trigger sulfide saturation (or oversaturation in a saturated magma). ILSG 2011 8 Field Trip 1 �Devolatilization resulting from venting of a volatile-rich magma would also cause an abrupt increase in fO2 , thereby having a compounding effect on reducing sulfur solubility (Poulson and Ohmoto, 1990). The attractiveness of a magma venting as a trigger for sulfide saturation in terms of PGE enrichment is that it would promote chamber-wide sulfide segregation if the system was well mixed, or a rain of sulfide out of the roof zone if the magma chamber was compositionally zoned. Both situations would promote high R factors, though the second scenario would required enough overproduction of sulfide so as to survive the descent to the cumulate floor. Figure 1-5. Stratigraphic variation in sulfur, copper, copper/palladium, and palladium + platinum through the Layered Series at Duluth (Fig. 8.11, RI-58). ILSG 2011 9 Field Trip 1 �Figure 1-6. Geology of the cyclic zone of the DLS showing locations and concentration ranges of Pt+Pd from outcrop samples. (Modified from Fig. 8-12, Severson et al., 2002). ILSG 2011 10 Field Trip 1 �FIELD STOP DESCRIPTIONS STOP 1-1: Basal Contact Zone and Lower Troctolite Zone, Duluth Layered Series and Ely’s Peak Basalts Location: Railroad Grade of the former Duluth, Winnipeg, and Northern Railway, Bardon Peak, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Secs. 33 SE & 34 SW; approx. NAD 83 UTM – Start: 557740E, 5170300N; End: 558460E, 5170040N). Duration: 90 min. Description: This stop traverses about 400 meters of stratigraphic section of the lower part of the Layered Series at Duluth (DLS). The exposures will be accessed by walking the railroad grade from the western side of Ely‘s Peak. Along the way, several extensive exposures of Ely‘s Peak basalts (including a tunnel) will be encountered. The Ely‘s Peak basalts form the footwall of the Duluth Complex and show the effects of intense thermal metamorphism by the gabbro. The Ely‘s Peak basalts are comprised of about 1.5 kilometers of gently (10-15 ) dipping, tholeiitic basaltic lavas and represent the earliest volcanic eruptions into the Midcontinent Rift. Starting at the basal contact, which trends ~N-S in a valley between Ely‘s and Bardon‘s peaks, a variety of rock types belonging to the basal contact zone and the lower part of the troctolitic zone of the DLS are exposed in roadcuts and outcrops along the abandoned railroad grade as it arcs around Bardon Peak. Several macrocyclic layers grading in mode and texture are apparent in the lower part of the troctolitic zone and are thought to represent major recharge and differentiation/cooling episodes associated with the early integration stage of DLS emplacement. The exposures on the north side of the tracks are schematically shown in Figure 1-7 and areas of particular interest are flagged along the route and described below. A. Basal Contact The traverse starts at the irregular contact between strongly hornfelsed mafic volcanics of the Ely‘s Peak basalts and taxitic olivine gabbro and augite troctolite of the basal contact zone (Flag A). Granophyre dikes cut both rock types, but display lobate contacts with the coarse gabbro suggesting two-liquid mixing. Grout (1918e) cited this mixing of granophyre and gabbro as evidence of silicate immiscibility. An alternative explanation is that the granophyre represents anatectic melts from the footwall that were not completely assimilated into the mafic magma. Contacts between the hornfels basalt and the gabbro are commonly characterized by coarse augite prisms oriented perpendicular to the contact. Roadcuts on the downslope side of the railroad grade (Flag A‘) show a small, irregular intrusion of coarse gabbro into hornfels basalt. B. Basal Contact Zone This exposure of taxitic olivine gabbro to augite troctolite with its variable textures (medium-grained to pegmatitic, subophitic to ophitic, nonfoliated to poorly foliated) and variable modal compositions are characteristic of the basal contact zone. In contrast to other layered series intrusions along the northwestern margin of the Duluth Complex that locally contain abundant Cu-Ni(-PGE) sulfide (see Day 2 field stops), the basal contact zone of the DLS is devoid of significant sulfide mineralization. ILSG 2011 11 Field Trip 1 �ILSG 2011 12 Field Trip 1 Photo 1-1. Strong sheet jointing in feldspathic dunite formed from intense serpentine veining. Area 1G’ Photo 1-2. Trough layering at Area 1-I. Note thickening of leucocratic layers toward the axis of the trough. Figure 1-7. Outcrop exposed along the north side of the Duluth, Winnipeg and Northern railroad grade showing variability in rock type across the basal contact zone (BCZ) and several macrocycles in the lower troctolitic zone (TZ1-3). �C. Lower Macrocyclic Unit of the Troctolite Zone Exposed here is a modally layered, medium-grained, moderately foliated, ophitic augite troctolite. This foliated POcf cumulate marks the base of the troctolitic zone and the first of three macrocyclic layers exposed along this traverse. At the east end of this exposure, the rock grades to a medium coarse-grained, augite troctolite to olivine gabbro. D. Upper Part of Lower Macrocycle This exposure of medium coarse- to very coarse-grained, non-foliated to moderately foliated, ophitic to subophitic olivine oxide gabbro characterizes the upper part of the lower macrocycle (TZ1, Fig. 1-7). The exposure at D‘ is coarser grained and generally nonfoliated. E. Contact between Lower and Medial Macrocycles Down from the grade is a sharp contact between coarse-grained non-foliated olivine oxide gabbro in a lower exposure (similar to outcrop D‘) and a coarse-grained feldspathic, biotitic oxide peridotite above. Following this contact upslope to the west, the peridotite gives way to a medium-grained melatroctolite overlying coarse olivine oxide gabbro. This abrupt transition from coarse olivine gabbro to medium melatroctolite is interpreted to mark a major recharge event into the DLS magma chamber. The origin of the coarse-grained oxide peridotite is unclear (see Stop 1-2A for a discussion). The oxide peridotite commonly contains trace amounts of sulfide (note local Cu staining) and has higher PGE concentrations (10-30 ppb) than surrounding rock types. F. Middle Section of Medial Macrocycle Medium- to medium coarse-grained, moderately foliated, locally layered augite troctolite is exposed in small outcrops on either side of trail leading north of tracks. These exposures occur in the midsection of the second macrocycle (TZ2, Fig. 1-7). G. Contact between Medial and Upper Macrocycles Exposures nearest the north side of the railroad grade are of massive, biotitic coarsegrained oxide peridotite which form the upper part of the medial macrocycle (as at E). Following the flagging to an outcrop ledge, an abrupt contact is observed between the oxide peridotite and medium fine-grained, well foliated, feldspathic dunite to melatroctolite. The orientation of the contact between the feldspathic dunite and the oxide peridotite is locally irregular. It is unclear if this irregularity is due to intrusion of the peridotite into the dunite or due to scouring of the dunite into the peridotite. Evidence for both processes are found elsewhere in this area. Here and in exposures along this ledge, the feldspathic dunite/melatroctolite display a very pronounce sheet jointing formed by intense serpentine veining (Photo 1-1). Following the flagging over the top of this ledge leads to another 5meter-high ledge outcrop that displays lenticular interlayering of troctolite and melatroctolite. Lenses of troctolite consistently thicken to the west suggesting that they may denote channel or trough layering. A similar type of lenticular (channel?) layering is more completely displayed at I, which is approximately at the same stratigraphic level. Also evident at the eastern end of this upper exposure is steeply inclined serpentine veining that refracts through the different layers. ILSG 2011 13 Field Trip 1 �H. Lower Part of Upper Macrocycle. At the beginning of a very deep cut is well layered melatroctolite grading up from the feldspathic dunite observed at G and forming the lower part of the uppermost macrocycle (TZ3, Fig. 1-7). The melatroctolite grades upward into a medium-grained, well foliated and intermittently layered, ophitic augite troctolite. At two locations (flagged as H‘), the troctolite is cut by irregular steeply oriented bodies of oxide olivine gabbro pegmatite. Some have suggested that this pegmatite and the oxide peridotite are related late magmatic features. I. Layered Troctolite of the Upper Macrocycle At the highest part of the cut is an exceptional exposure of lenticular layered melatroctolite and leucotroctolite. Similar to the upper exposure at G, trough/channel structures are seemingly implied by the bowing of the layering and the consistent thickening of leucocratic layers in the axis of the trough. Several trough structures have been recognized in the Troctolitic Zone in the Bardon Peak area. The axis of the troughs tend to trend easterly (i.e., in the down dip direction of regional layering). Stop 1-2A: lower troctolite zone, Duluth Layered Series, Duluth Complex (OPTIONAL) Location: Skyline Parkway near Bardon Peak, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec. 34, SW of NW; NAD83 UTM (Area E) 558540E, 5170280N). Duration: (60 min.) Description: This stop examines rocks comprising the lower part of the Duluth Layered Series in roadcuts and barren knob exposures at the south end of Skyline Parkway. This outcrop area is approximately 500 m above the moderately dipping (~45°) basal contact of the Duluth Complex against shallow-dipping (<15°) basalt flows, which form Ely‘s Peak (rocky hill to the west). This area is situated in the lower part of the troctolitic zone, but upsection of the upper exposures observed in the traverse at Stop 1-1. As seen in that section, this part of the Troctolite Zone continues macrocyclic layering of melatroctolite grading upward to coarser augite troctolite. Because the layering is similar to that seen below, it is unlikely that we will be able to visit this exposure. It is included here for those who which to return at a later date. Figure 1-8 shows the geology of the area with eight areas of interest (A-H) noted and generally described below. The PO to OP cumulate rocks exposed at this stop include ophitic olivine gabbro, augite troctolite, troctolite, melatroctolite, and feldspathic dunite. In general, the more olivine-rich melatroctolites and dunites tend to be finer grained than the more augite-rich troctolites and olivine gabbros. These rocks display a variety of types and scales of layering. Macrocyclic layering of rock types is evident on a meter to decimeter scale—typically finer grained troctolite, melatroctolite, and feldspathic dunite upward to from coarser grained augite troctolite to olivine gabbro. Cryptic differences in olivine composition are evident with melatroctolite to feldspathic dunite being more Mg-rich (Fo62-65) compared to augite troctolite-olivine gabbro (Fo48-60). Contacts between macrocycles are abruptly (<10 cm) to narrowly gradational (<1 m) (areas C, D, and G, Fig. 1-8). Within macrocycles, various type of finer-scale layering is common. Subtle grain-size layering locally occurs in the mediumto coarse-grained augite troctolite to olivine gabbro intervals (area E). Centimeter-scale isomodal layering and decimeter-scale graded modal layering is very common in the more melatroctolitic rocks and locally has very rhythmic alternations (area F). Trough layering is locally implied by variable orientations of modal layering (areas A and C) and the tendency ILSG 2011 14 Field Trip 1 �for the more melatroctolitic layers to pinch out along strike. However, some of what appears to be pinch out may be due to faulting along ENE-trending structures through the area (Fig. 1-8). Very abrupt reorientation and steepening of lamination in Area G suggests that some faulting may be contemporaneous with crystallization. Deeply weathered, coarse-grained, biotitic oxide dunite to peridotite, similar to that seen at Stop 1-1 (areas E & G), occurs in two locations here (areas B and G, Fig. 1-8). Petrographic studies show the rock to be composed of 50-90% granular olivine, 3-20% Fe-Ti oxide (mostly ilmenite), 2-25% ophitic augite, 1-4% biotite, and 0-20% interstitial plagioclase. Primary minerals are commonly altered to chlorite, serpentine, talc, and actinolite. Olivine compositions in these bodies are Fo50-59 and therefore are similar to the augite troctolite and olivine gabbro cumulates in the area. The limited aerial extent of these oxide ultramafic bodies suggest that many may occur as small pipes or dike-like bodies. However, as seen at Stop 1-1, they also occur as stratabound, sub-conformable lenses at olivine gabbro-melatroctolite contacts. Figure 1-8: Outcrop geology of the Bardon Peak area denoting areas of interest at Stop 1-2A. Dashed lines are inferred faults. Narrow lines are topographic contours portraying 50 foot intervals. Ross (1985) suggested that these small oxide plugs and lenses are metasomatic replacement bodies formed by volatile fluxing out of the footwall basalts. Severson (1995; Severson and Hauck, 1990) found similar oxide ultramafic (OUI) bodies along the western and northwestern margin of the Duluth Complex to be spatially related to iron-formation inclusion and suggested that partial melting and assimilation of these inclusions may have generated the bodies. Another possible explanation is that they formed by mobilization of ILSG 2011 15 Field Trip 1 �interstitial volatile-rich magma extracted from lower cumulates in the basal contact zone which became ponded beneath fine-grained melatroctolite/dunite layers at the base of macrocyles. More study, especially isotopic data, are clearly needed to resolve the origin of these bodies. STOP 1-2B: troctolite zone, Duluth Layered Series and Overview of Duluth Complex. Location: Skyline Parkway, Bardon Peak, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec.34 SW; approx. NAD 83 UTM - 5170450N, 558740E). Duration: 15 min. Description: This brief stop, which corresponds to Area H in Figure 1-8, provides a panoramic view of the breadth of the Duluth Complex at Duluth. A 5 kilometer-thick stratigraphic section of the layered series and the overlying anorthositic series are exposed along the 200 meter high, and 15 kilometer long escarpment above the St. Louis River estuary. The top of the Duluth Complex on the rise just above downtown Duluth which is marked by a cluster of radio towers and the Enger Tower landmark. This is where our trip will end today (Stop 9). As exemplified by the layering of the troctolitic rocks exposed at this overlook, the general internal structure of the DLS dips 20-25 to the east, thus the northeast trending escarpment cuts an acute angle across the intrusion. The anorthosite series overlying the DLS can be recognized by its higher topographic expression. STOP 1-3: Cyclic Zone, Duluth Layered Series Location: Spirit Mountain Ski Resort, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec 23 NW; approx. NAD 83 UTM - 5174250N, 559952E). Duration: 30 min. Description: About 100 m east of the north side of the ski lodge, near the top of the northernmost chairlift is series of outcrop ledges that display over a 10-meter-thick section about 16 successive meter-scale mesocycles (Fig. 1-9) that mimic the decameter-scale macrocycles that characterize the medial Cyclic Zone of the DLS (Fig. 1-3). Each mesocycle is composed of a medium fine grained troctolite at its sharp base with the cycle below. About 1/3 of the way up, the troctolite develops small clots (~5mm) of subpoikilitic augite and lesser oxide, which then grades on a cm-scale to a medium-grained, intergranular oxide olivine gabbro (Photos 1-3 and 1-4). All rocks display well developed foliation of plagioclase (in troctolite) and plagioclase and augite (in oxide olivine gabbro). Although the thickness of each mesocycle gradually increases from 30 cm to 100 cm upsection, the proportion of gabbro to troctolite comprising each cycle remains generally constant at about 2:1 (Fig. 1-9). The mesocycles fundamentally show a rapid progression from a two-phase (PO) to a four-phase (PCOF) cumulate. The mesocyclic boundaries represent even more abrupt regressions from four-phase back to two-phase cumulates. ILSG 2011 16 Field Trip 1 �4 5 Photo 1-4. Close-up of contact relationships between oxide olivine gabbro (OG) and troctolite 6 (Tr). 1-3 1-4 Photo 1-3. Mesocyclic phase layering at Stop 1-3. Cycles 4 and 5 and the upper part of Cycle 6 are visible in this photo. Microprobe analyses of olivine and augite in samples collected across mesocycles 1 through 4 and 10 through 13 (Fig. 1-9) show subtle but systematic variations in mg# (MgO/(MgO+FeO), mole%). Olivine (and less consistently augite) show a 2-3% increase in mg# in the troctolite intervals compared to the gabbro intervals. The mg# increase can be recognized on the scale of a single thin section at the sharp mesocyclic boundaries. The maximum mg# typically is found in the central part of the troctolite intervals then gradually decreases as subophitic augite clots appear. The mg# stays remarkably constant through the gabbro intervals. The average mg# of each mesocyclic zone is also remarkably constant across the multiple cycles investigated (Fig. 1-9). ILSG 2011 17 Field Trip 1 �Figure 1-9. Stratigraphic variations in mesocycle thickness, volume % of gabbro composing each mesocycle, and mg# (MgO/(MgO+FeO), mole%) of olivine and augite through the mesocycles exposed at Stop 1-3. White parts of the mesocycles indicate troctolite intervals (PO cumulate), yellow indicates subophitic augite troctolite (POcf cumulate), and orange indicate olivine oxide gabbro intervals (PCFO cumulate). Figure after Miller and Stifter (2010). This may at first glance seem to argue for recharge of more primitive magma as the cause for the phase layering. However, the shift in mg# in olivine may also reflect a shift in the Fe-Mg equilibrium between melt and olivine when augite becomes a cumulus phase. Trace element data, particularly REE, also show a significant difference between the troctolite and gabbro layers, but this can be attributed to differing partition coefficients for the main cumulate phases (Miller and Stifter, 2010). Collectively, the field, petrographic, mineral chemistry, and lithochemical attributes of the mesocyclic layering observed here does not unambiguously favor either a magma recharge or decompression venting model for its origin. More data and evaluation are needed to definitely determine the origin of the mesocyclic and larger macrocyclic layering of the Cyclic Zone of the Duluth Layered Series. Stop 1-4: Cyclic Zone, Duluth Layered Series Location: North of I-35, west of Boundary Ave., slope north of Northern Engine & Supply Co. and Spirit Mtn. Red Roof Motel, Duluth. West Duluth 7.5' quadrangle (T49N, R15W, Sec 15 SE, NAD 83 UTM - 5175100N, 559350E (area E)) Duration: 60 min. Description: This stop examines the first macro-scale transition from troctolite cumulates to olivine gabbro cumulates that defines the base of the Cyclic Zone (Fig. 1-3). ILSG 2011 18 Field Trip 1 �From behind the Northern Engine and Supply Co., head north and uphill to outcrops of medium-grained ophitic augite troctolite (A, Fig. 1-10). This POcf cumulate contains about 7-10% interstitial Fe-Ti oxide and augite, with the latter commonly forming 2-4 cm oikocrysts. Progressing about 70 m to the east over intermittent outcrop ledge, the augite troctolite gradually becomes coarser grained and more enriched in augite and oxide (B, Fig. 1-10). Figure 1-10: Outcrop geology and cryptic variation along south facing slope at Stop 1-4. Igneous foliation and layering dip at 15-25° to the east. Exposure corresponds to the top of macrocyclic unit I marking the base of the cyclic zone (Fig. 1-3). Troctolitic rocks 120 m east of the powerline behind the motel indicate the presence of an unexposed cumulus reversal. Crossing a 35-m gap in exposure, the next outcrop is of a medium- to very coarse grained, subophitic to ophitic olivine gabbro (PcOf; C, Fig. 1-10). In one area of the exposure, this rock type is in sharp, discordant, but unchilled contact with a coarse-grained, moderately laminated gabbroic anorthosite that is probably an inclusion of the anorthositic series. Beyond an 8-m gap is a 10-m-high outcrop knob that grades from a medium-grained, subophitic augite troctolite with 1/2-cm high density augite oikocrysts near the base (D, Fig. 1-10) to a texturally layered, medium- to fine-grained, intergranular oxide olivine gabbro at the top (E. Fig. 1-10). Textural layering on top of knob is defined by 2- to 20-cm-thick, laterally discontinuous layers of fine grained, moderately laminated, intergranular oxide olivine gabbro (microgabbro) that intermittently occurs within medium-grained, well-laminated, intergranular oxide olivine gabbro. Contacts between the medium gabbro and the microgabbro range from sharp to gradational on a scale of centimeters. In some areas, the textural layering drapes over football-sized blocks of a coarser grained, subophitic olivine gabbro, similar to the rock type seen just to the west (down section). Gossan of sulfidebearing areas are locally present. If time allows, we will follow flagging tape about 200 meters from the powerline (Fig. 1-10) to the parking lot behind the Red Roof motel. Exposed in an 80m-long roadcut and pavement exposure is medium-grained, well foliated, locally layered augite troctolite, thus implying that a cumulus reversal was crossed between site E and here. This rock is interpreted to form the lower part of the second macrocycle unit (Fig. 1-3). Exposure of this macrocycle boundary to the south shows that the texturally layered oxide olivine cumulate ILSG 2011 19 Field Trip 1 �interval (E, Fig. 1-10) is only locally present and appears to be lenticular in form (Fig. 1-3). In some exposures, the boundary between macrocyles I and II is marked by a coarse-grained ophitic olivine gabbro with anorthositic series inclusions (as seen at C, Fig. 1-10) in sharp contact with troctolite. With the exception of the medium-grained, subophitic augite troctolite at D, cryptic variations through this sequence show a general decrease in An, Fo, and En (Fig. 1-10). Plagioclase, olivine, and augite compositions in the troctolite above the layered gabbro define a regression to more primitive values. Mineral compositions are also more primitive in the microgabbro layers compared to the enclosing medium-grained gabbro. Across the cumulus reversal from oxide olivine gabbro to troctolite, Fo, En and An increase slightly. Again a straightforward interpretation of this sequence is that it represents progressive crystallization differentiation leading to multiple saturation in plagioclase, olivine, augite, and ilmenite, followed by recharge of a more primitive magma causing a regression to saturation in just plagioclase and olivine. The occurrence of anorthosite inclusions and the microgabbro layers suggest however that any such recharge event may have been preceded by an eruption event from the chamber. Miller and Ripley (1996) interpreted the microgabbro lenses to have formed by decompression quenching of magma due its reaching water saturation in the roof zone (see Stops 1-5 and 1-7 for further discussion). Decompression may also have triggered sulfide saturation which caused local stratiform enrichment of PGE at this and other cumulus regression. Although some enrichment in PGE above background is noted from samples taken from site E, more pronounced sulfide and PGE enrichment are evident at this macrocycle interface to the south and at the interface between the second and third macrocycles (Figs. 1-5, 1-6). Stop 1-5: Cyclic Zone, Duluth Layered Series Location: Roadcut on Skyline Parkway at Thompson Hill Rest Area. West Duluth 7.5' quadrangle (T49N, R15W, Sec 14, center, NAD 83 UTM - 5175350N, 560700E) Duration: 90 min (including lunch). Description: In the layered sequence of gabbroic rocks exposed along this 200-mlong roadcut (Fig. 1-11), a reversal in cumulus paragenesis can be seen that is characteristic of higher levels in the cyclic zone. The west end of the roadcut begins with a coarse-grained, moderately laminated, subophitic olivine gabbro, which locally is intergranular and elsewhere is leucocratic (sample A, Fig. 1-11). Augite in this rock is marginally cumulus but becomes definitely so quickly upsection where it consistently has an anhedral granular to subprismatic habit. This coarse-grained, intermittently layered (locally graded), olivine-poor (<5%) oxide gabbro (sample B) classifies as a PCFO cumulate. Beyond a poorly exposed interval, this gabbro includes a 2-m-thick interval wherein minor olivine becomes subpoikilitic and concentrated in layers (sample C). Another 3 m above this, the coarse gabbro passes into a medium-grained, well-laminated, subpoikilitic olivine-bearing oxide gabbro (samples D and D') which displays layering of olivine oikocryst concentration and elsewhere isomodal layers rich in Fe-Ti oxide and pyroxene. The very strong foliation and subhedral to euhedral habit of cumulus phases (plagioclase, pyroxene, and ilmenite) impart an adcumulate texture to this rock. Over a poorly exposed interval about 15 m long is an altered, coarse-grained, ophitic gabbroic anorthosite (sample E) that is texturally and mineralogically identical to rocks in the anorthositic series. At the beginning of the next well-exposed section of roadcut, several similar gabbroic anorthosite inclusions are found in ILSG 2011 20 Field Trip 1 �a coarse-grained, subophitic to intergranular olivine gabbro (sample F), which gradually grades upward into a more consistently subophitic texture over the remainder of the roadcut (sample G). This rock type closely resembles that at the west end of the roadcut and indicates a downgrading in the cumulus status of pyroxene (and oxide?) and a reemergence of cumulus olivine. Figure 1-11: Geology and cryptic variation along Skyline Parkway roadcut near Thompson Hill rest area; Stop 1-5. View is to the north. Dip of lamination and layering is exaggerated; averages about 20° to the east. Contacts between units are gradational over thicknesses of 10 cm to 1 m. The cryptic variation of En and Fo across the cumulus regression exposed in this section (Fig. 1-11) is the reverse of that expect from magma recharge. Although there is a general decrease in these parameters below the cumulus regression as would be expected due to progressive differentiation, a sharp reversal takes place at samples D and D' – the most differentiated samples based on phase mineralogy. The increase in Fo and En in these samples may reflect their adcumulate nature (i.e., low trapped-liquid component) or perhaps reflect a shift in the equilibrium compositions of mafic silicates due to the cumulus crystallization of Fe-Ti oxide. Nevertheless, the expected increase in Fo and En above the recharge horizon is not observed in either the pyroxene or the olivine. Indeed, the lowest Fo olivine in the section occurs above the cumulus regression. Again, an alternative explanation to magma recharge is that the textural and compositional variations across this interval reflect decompression of the chamber due to eruption to the surface. Decompression of a volatile-enriched magma would cause supercooling and multiple saturation of the magma and thereby explain the abrupt decreases in grain size and cumulus phase changes without much compositional variation. Magma expulsion through the roof of the layered series would also explain the occurrence of a gabbroic anorthosite inclusion and elsewhere throughout the cyclic zone. The hydrothermally altered nature of the gabbroic anorthosite is consistent with a volatile-rich environment in the anorthositic series cupola of ILSG 2011 21 Field Trip 1 �the DLS magma chamber. The cumulus reversal to ophitic olivine gabbro without an increase in mg# could be explained by repressurization of a devolatized magma. STOP 1-6: Gabbro Zone, Duluth Layered Series and Anorthositic Series Location: Roadcuts on Skyline Parkway above Oneota Cemetery. Duluth Heights 7.5' quadrangle (T49N, R15W, Sec 1, SE, approx. NAD 83 UTM - 5177970N, 562995E) Duration: 30 min. Description: In roadcuts along a 0.5-km section of Skyline Parkway, rocks forming the contact between the layered series and the anorthositic series may be examined. Exposed in low, deeply weathered outcrops at the west end of the section is a poorly to well foliated, intergranular, apatitic olivine ferrodiorite cumulate (PCFOAp ). It is composed mostly of plagioclase (zoned An38-50, avg. An44), augite (avg En33Fs27Wo40), and bladed ilmenite. In addition, as much as 10% olivine (Fo30), 5% orthopyroxene (En45), 5% apatite, 3% biotite, and 5% granophyric mesostasis is present. Generally medium-grained, it locally varies in grain size from fine to coarse. Medium- to fine-grained variants have well-developed foliation, whereas coarser grained ferrodiorite tends to be poorly foliated to nonfoliated. Coarser ferrodiorite tends to be more granophyric, as well. By being locally well-laminated, olivine-bearing, and lacking ferromonzodioritic component, the ferrogabbro here is not typical of the upper contact zone of the layered series; relationships more characteristic of the upper contact zone will be seen at the next stop. Rather, this rock is more typical of gabbroic cumulates of the gabbro zone. About 35 meters east of the ferrogabbro, a prominent roadcut displays several common variants of anorthositic rocks that compose the anorthositic series. The western end of the roadcut is composed of a poorly to moderately laminated, poikilitic olivine gabbroic anorthosite (OGA) that contains olivine (Fo42-46) oikocrysts up to 6 cm across, as well as interstitial augite and oxide. Progressively eastward, this rock type grades into a subpoikilitic to granular olivine gabbroic anorthosite. At the high point in the roadcut, this granular OGA contains an inclusion of olivine (Fo60)-bearing anorthosite (Photo 1-5). Whereas the enclosing OGA contain about 80-85% plagioclase (avg. An62), this inclusion contains more than 95% plagioclase (unzoned An65). The contact between OGA and the anorthosite inclusion is sharp with some concordant alignment of plagioclase in OGA. Photo 1-5. Olivine-bearing anorthosite inclusion in subpoikilitic olivine gabbroic anorthosite as Stop 1-6. Hammer at contact. ILSG 2011 22 Field Trip 1 �The inclusion relationships and the outcrop-scale variability in internal structure and lithology (mode, texture, olivine habit, and lamination development) of the anorthositic rocks here are ubiquitous features of the anorthositic series. These complexities led Taylor (1964) to characterize the anorthositic series as an igneous breccia. He and later Miller and Weiblen (1990) concluded that these rocks formed by repeated intrusions of plagioclase crystal mush. This stop is at the western end of a westward projection of the layered series-anorthositic series contact (Fig. 1-2). A possible explanation for the irregular trace of the contact is that it reflects the original shape of the DLS roof. This would seem to be a difficult shape to maintain if the anorthositic series rocks were hot and lighter than the DLS magma. Rather, it seems more likely that the westward projection of the anorthositic series represents a large inclusion that detached from the roof zone and settled down to the cumulate floor of the DLS at the time of gabbro zone crystallization. Evidence for this is suggested by a truncated cryptic variation in the DLS leading up to this contact compared to the thicker DLS section to the north and the atypical character of the ferrodiorite in contact with the anorthositic series here (compare with next stop). Stop 1-7: DLS ―Chill‖, Granophyre, and Anorthositic Series Location : Skyline Parkway, Duluth Heights 7.5' quadrangle (T50N, R14W, Sec 32, SE, NAD 83 UTM - 5179675N, 564880E). Duration: 30 min. Description: Exposed at the northeast end of this roadcut is a fine-grained mafic rock with intermingled granophyre that together cut coarse-grained olivine gabbroic anorthosite of the anorthositic series. Since early mapping by Grout in the Duluth area in 1912, this rather innocuous exposure has been a keystone outcrop in interpreting the intrusive history of the Duluth Complex. This fine-grained mafic rock can be traced up over the ridge to the west where it merges into the upper contact zone of Layered Series. Grout (1918a) and later Taylor (1964) saw this exposure as evidence that the anorthositic series was considerable older and cold when the layered series was intruded. This paradigm was accepted by all subsequent workers on the Duluth Complex up through the 1980‘s. It came therefore as a shock when high resolution U-Pb age dating (Paces and Miller, 1993) showed that the anorthositic series and the layered series were virtually identical in age (within 0.5 Ma relative to the 22 Ma span of Midcontinent Rift magmatic activity). This precipitated a major paradigm shift in the perception of the intrusive relationships between these two series here and throughout the Duluth Complex. A closer look at this DLS ―chill‖ reveals that it is not a thermal quench of the Layered Series at all. This rock type is found at the contact with the anorthositic series throughout most this area and has a remarkably homogeneous composition with an mg# of about 37 (Table 1-1). In thin section, it is a subprismatic biotitic oxide ferrodiorite. Applying its composition to the MELTS routine indicates that it should be in equilibrium with augite, ilmenite, and plagioclase with compositions comparable to gabbroic cumulates found in the cyclic zone and gabbro zones of the DLS. In sum, this rock is much too evolved to have produced the troctolitic rocks of the layered series. Rather than this being a thermal quench, Miller and Ripley (1996) have interpreted this rock to represent decompression quenching of an evolved, water-saturated DLS magma during venting at a time when the cyclic zone was crystallizing. Whereas decompression of a water-undersaturated magma should result in superheating and a suspension of crystallization (or at least a significant change in phase ILSG 2011 23 Field Trip 1 �equilibrium), decompression under water-saturated conditions should cause supercooling and quenching. This model is fits nicely with the explanation for the cyclic zone with which this composition is apparently comagmatic. Photo 1-6. Lobate contacts between fine-grained oxide ferrodiorite (fd) and medium-grained melanogranophyre (mg), which is in sharp contact with coarsegrained gabbroic anorthosite (ga) at Stop 1-7. fd mg gaga The lobate contacts between the irregular masses of medium-grained granophyre and the fine-grained ferrodiorite host (Photo 1-6) gives the appearance of two magma mixing. The ferrodiorite is not of a composition that would indicate that these two liquid formed by immiscibility. An alternative explanation is that these felsic magmas were derived from anatectic melting of various inclusion carried into the DLS chamber. Because of their high silica content and low density, these felsic melts did not readily mix or assimilate with mafic melt, but rather rose to the roof zone where they ponded beneath the anorthositic series cupola. During magma venting from the chamber, the felsic melts became entrained and irregularly mixed with the mafic magmas. While decompression under water-saturated conditions caused rapid crystallization of the mafic magma, the felsic melt became irregularly entrapped in the quenched mafic host and cooled more slowly to a mediumgrained texture. ILSG 2011 24 Field Trip 1 �Table 1-1. Compositions of six fine-grained ferrodiorite samples collected at the DLS-AS contact. Stop 1-8: Ferrodiorite/Melanogranophyre Composite Intrusion in Anorthositic Series Location : Roadcuts on Skyline Parkway on the south and north sides of Twin Ponds swimming area. Duluth 7.5' quadrangle (T50N, R14W, Sec 21, E central; NAD 83 UTM 5180870N, 566980E). Duration: 30 min. Description: Outcrops and roadcuts north and south of Twin Ponds expose contact relationships between very altered anorthositic series rocks and an irregular composite body of intermediate to felsic rocks. Exposed in the roadcut south of the T-junction is an altered, coarse-grained, granophyric gabbroic anorthosite. All ophitic pyroxene has been replaced by fibrous amphibole, and a granophyric mesostasis locally composes 5-15% of the rock. The altered and granophyre-enriched character of anorthositic rocks is common over most of the upper part of the anorthositic series in the Duluth area (Fig. 1-1). ILSG 2011 25 Field Trip 1 �About 15 m east of the T-junction, an abrupt but unchilled contact is exposed between the gabbroic anorthosite and a medium-grained, apatitic olivine ferromonzodiorite. As seen in thin section, all the olivine and most of the clinopyroxene has been replaced by amphibole, chlorite, and iron oxide, and plagioclase is commonly mantled by dusty Kfeldspar which becomes intergrown with quartz to form a granophyric mesostasis. Locally the ferromonzodiorite is moderately laminated, and in one area it displays a streaky modal layering that is moderately inclined to the southeast. About 75 m east of the junction, a very complex contact between the ferromonzodiorite and a plagioclase-porphyritic quartz ferromonzodiorite is encountered which appears to be a hybrid mixture of gabbroic anorthosite and ferromonzodiorite. Over a 25 m interval, this hybrid rock grades into variably altered and granophyric gabbroic anorthosite. With the exception of small irregular bodies of quartz monzodiorite about 140 m from the junction, the remainder of the roadcut is altered and granophyric gabbroic anorthosite. On the north side of the Twin Ponds, a roadcut exposes a pink, mafic quartz monzonite (or melanogranophyre) cut by a 2-m-wide tholeiitic diabase dike. The melanogranophyre is composed of about 50% subhedral plagioclase, 5-10% skeletal prisms of Fe-pyroxene and amphibole, and 5% granular Fe-oxide in a felsic matrix of micrographically intergrown dusty K-feldspar and quartz. This rock type makes up 80% of the irregular composite body label as melanogranophyre in Figure 1-1. Uphill (north) from the roadcut in intermittent pavement outcrops, the melanogranophyre can be observed to grade into more mafic compositions (quartz ferromonzodiorite to ferrodiorite) as sharp but unchilled contacts with gabbroic anorthosite are approached. The alteration, contact relationships, and variety of rock types observed here indicate that evolved magma and hydrothermal fluids emanating from the underlying DLS passed through the anorthositic series over a protracted period of time and at a variety of scales. The very altered and granophyre-rich character of anorthositic rocks observed here and over much of the upper part of the anorthositic series suggests that late-stage melts and hydrothermal fluids fluxed through the intercumulus pore spaces of the (partially molten?) anorthositic series over a large area. In addition to this widespread percolation, the composite melanogranophyre body appears to represent a discrete conduit through which DLS-originating magmas passed and partially crystallized on their way to higher level intrusions or surface eruptions. The crude zonal distribution of composition more ferrodioritic at the margins and more quartz monzonitic in the interior—is consistent with this conduit being open during crystallization of at least the upper third of the DLS. Moreover, the hybridization between the composite body and the gabbroic anorthosite host suggest that the anorthositic series was partially molten at least in the vicinity of this conduit. Stop 1-9: Upper Contact of Duluth Complex and North Shore Volcanic Group. Location : Radio Tower Hill. East of corner on gravel road corresponding to 5th Ave and 9th Street. Duluth 7.5' quadrangle (T50N, R14W, Sec 28, NE; NAD 83 UTM 5182260N, 567650E). Duration: 45 min. Description: The east side of Radio Tower Hill overlooking downtown Duluth approximates the slope of the upper contact of the Duluth Complex with the overlying lava flows of the North Shore Volcanic Group, which now underlie all the lower ground to the northeast along the shore. As observed at the southeastern crest of the hill, the rock exposed ILSG 2011 26 Field Trip 1 �over most of this slope is a medium- to fine-grained, plagioclase-porphyritic (5-15%), subophitic to ophitic olivine gabbro that contains many decimeter- to meter-sized basaltic hornfels inclusions. In thin section, the gabbro typically displays intense hydrothermal alteration resulting in sericitic breakdown of plagioclase and uralitic, chloritic, and oxide replacement of pyroxene and olivine, though its primary subophitic to ophitic texture is typically preserved. The basaltic inclusions are locally intensely recrystallized and the gabbro shows little to only a weak chill around the inclusions. Less commonly, an inclusion of gabbroic anorthosite is present in the gabbro. To the west (downsection), basaltic inclusions become less common and plagioclase phenocrysts become more abundant, but in a nonsystematic way. The contact with the main suite of gabbroic anorthosite is not well exposed but seems to be abrupt and may actually finger into the anorthositic series as dikelike apophyses (Fig. 1-12). Figure 1-12: Outcrop geology of Radio Tower hill area, showing the starting and ending locations of Stop 1-9. Open circles indicate the location of radio towers. Along a telephone line down the slope to the northeast, the gabbro gradually becomes finer grained and less porphyritic. At the base of the slope, the gabbro abruptly grades into a pink, medium-grained, intergranular to micrographic hornblende quartz monzonite (Fig. 112). The quartz monzonite is composed of ~15% amphibole and ferroaugite, 5% granular Fe-oxide, 40% plagioclase, 30% dusty K-feldspar, and 10% graphic to granular quartz. Abundant miarolitic cavities indicate a shallow depth of crystallization. The contact relationship between the gabbro and the quartz monzonite is best seen in a roadcut along the driveway to 415 W. Skyline Parkway. Here, the two phases, along with some basaltic inclusions, occur in a complex mixture over a 15-m-wide zone. The melanogranophyre ILSG 2011 27 Field Trip 1 �probably formed by partial melting of intermediate to felsic lavas of the overlying North Shore Volcanic Group which form the hanging wall of the Duluth Complex . A strongly recrystallized porphyritic felsic (icelandite) volcanic rock can be observed in low roadcuts about 40 m northeast of Skyline Parkway on W. 8th St. A possible interpretation of the porphyritic gabbro is that it represents the upper border phase of the anorthositic series that formed by flow differentiation of plagioclase crystals away from the contact with overlying volcanic rocks. An alternative explanation is that this body represents a discrete intrusion, perhaps comagmatic with the layered series, which was emplaced at the anorthositic series-volcanic contact. Except for its highly altered state, its primary mineral compositions and texture are similar to the basal contact zone rocks of the layered series. REFERENCES Carroll, M.R., and Rutherford, M.J., 1985, Sulfide and sulfate saturation in hydrous silicate melts: Journal of Geophysical Research, v. 90C, p. 601-612.Grout, 1918a-c Green, J.C., and Miller, J.D., 2008, Bedrock geology of the Duluth quadrangle, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-182, scale 1:24,000. Grout, F.F., 1918a, Internal structures of igneous rocks; their significance and origin with special reference to the Duluth Gabbro. Journal of Geology 26, 439-458. Grout, F.F., 1918b, Two-phase convection in igneous magmas. Journal of Geology 26, p. 481-499. Grout, F.F., 1918c, A type of igneous differentiation. Journal of Geology 26, 626-58. Grout, F.F., 1918d, The lopolith, an igneous form exemplified by the Duluth gabbro. American Journal of Science, series 4, vol. 46, p.516-522. Grout, F.F., 1918e, The pegmatites of the Duluth Gabbro. Economic Geology, v. 13, p.185197. Maier, W.D., Barnes, S.J., De Klerk, W.J., Teigler, B., Mitchell, A.A., 1996, Cu/Pd and Cu/Pt of silicate rocks in the Bushveld Complex: implications for platinum-group element exploration: Economic Geology, v. 91, p. 1151-1158. Miller, J.D., and Green, J.C., 2008a, Bedrock geology of the Duluth Heights and eastern portion of the Adolph quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-181, scale 1:24,000. Miller, J.D., and Green, J.C., 2008b, Bedrock geology of the West Duluth and eastern portion of the Esko quadrangles, St. Louis County, Minnesota. Minnesota Geological Survey Miscellaneous Map M-183, scale 1:24,000. Miller, J.D., and Ripley, E.M., 1996, Layered intrusions of the Duluth Complex, Minnesota, USA. in Cawthorne, R.G., ed., Layered Intrusions: Amsterdam, Elsevier Science, p. 257-301. Miller, J.D., and Severson, M.J., 2002, Chapter 6: Geology of the Duluth Complex, in Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, p. 106-143. ILSG 2011 28 Field Trip 1 �Miller, J.D., and Stifter, E, 2010, Cyclical phase layering in the Duluth Complex at DuluthEvidence for periodic magma venting from a shallow staging chamber?. Annual Institute on Lake Superior Geology, Proceedings Volume 56, Part 1- Program and Abstracts, International Falls, MN. p. 44-45. Miller, J.D., and Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: Examples of rocks formed from plagioclase crystal mush. Journal of Petrology 31, p. 295-339. Paces, J.B., and Miller, J.D., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochonological insights to physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no.B8, p. 13,99714,013. Poulson, S.R. and Ohmoto, H., 1990, An evaluation of the solubility of sulfide sulfur in silicate melts from experimental data and natural samples: Chemical Geology, v. 85, p. 57-75. Ross, B.A., 1985. A petrologic study of the Bardon Peak peridotite, Duluth Complex. Unpublished M.S. thesis, University of Minnesota, Minneapolis, 140 p. Severson, M.J., 1995, Geology of the southern portion of the Duluth Complex: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report NRRI/TR 95/26, 185 p. (with plates). Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236 p. (with plates). Severson, M.J., Miller, J.D., Peterson, D.M., Green, J.C., and Hauck, S.A., 2002, Mineral potential of the Duluth Complex and related intrusions, in Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey. Report of Investigations 58, p. 164-200. Taylor, R. B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Minnesota Geological Survey Bulletin 44, 63 p. Wager, L.R., and Deer, W.A., 1939, Geological investigations in East Greenland, Part III. The petrology of the Skaergaard intrusion, Kangerdlugssuaq, East Greenland. Meddr. Gronland. 105, 352 p. Winchell, N.H., 1899, The Geology of Minnesota. Geological and Natural History Survey of Minnesota, Final Report v. 4, 354p. w/100 plate ILSG 2011 29 Field Trip 1 �ILSG 2011 30 Field Trip 1 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 2 MIDCONTINENT MICROCOSM: GEOLOGY OF THE ATKINS LAKE –MARENGO FALLS AREA Photomicrograph of ultracataclasite from the north side of the Marengo River below Marengo Falls. The protolith is probably basalt of the Siemens Creek Volcanic series. The field of view is about 2 mm. – M. Bjørnerud ILSG 2011 31 Field Trip 2 �ILSG 2011 32 Field Trip 2 �Field Trip 2 Midcontinent Microcosm: Geology of the Atkins Lake –Marengo Falls area Marcia Bjørnerud1 and William F. Cannon2 1 2 Lawrence University, 115 South Drew Street,Appleton, WI U.S. Geological Survey, 12201 Sunrise Valley Drive,Reston, VA ___________________________________________________________________________ OVERVIEW OF FIELD TRIP STOPS 1. Paleoproterozoic Ironwood Iron-formation, metadiabase sills, and breccias near Atkins Lake 2. Mesoproterozoic Mellen-type granite near Atkins Lake 3. Mesoproterozoic Siemens Creek Volcanics contact metamorphosed to pyroxene hornfels and granophyre 4. Penokean shear zone in Paleoproterozoic Palms Formation, Marengo River 5. Paleoproterozoic Bad River Dolomite: Stromatolites, Penokean deformation, contact metamorphism, Grandview Quarry, Marengo River 6. Marengo Falls, South side: Neoarchean Puritan Batholith; fault breccias, cataclasites and pseudotachylyte along the Atkins Lake-Marenisco Fault 7. Marengo Falls, North side: Ultracataclasite along the Atkins Lake-Marenisco Fault INTRODUCTION Archean and Proterozoic rocks exposed over about 16km2 between Atkins Lake and Coffee Lake in southeastern Bayfield County (Fig. 1) chronicle almost all of the major Precambrian geologic events in the history of the southern Superior Craton. The oldest rocks are part of a locally gneissic quartz monzonite complex, the Puritan Batholith, with an igneous Rb-Sr age of 2710+140 Ma (Sims et al., 1977). At the regional scale, this complex is part of one of the youngest Archean granite-greenstone belts in the Superior Province, and it intrudes greenstones of the Neoarchean Ramsay Formation. In the Atkins Lake – Marengo River area, the Puritan Batholith is nonconformably overlain by the Paleoproterozoic (ca. 2200 Ma) Bad River Dolomite. The Bad River Dolomite is in turn separated by an unconformity from rocks of the ca. 1875 Ma Menominee Group (Palms Formation and Ironwood Iron-formation), which locally contain mafic volcanic rocks and diabase sills (Cannon et al., 2008). These Paleoproterozoic rocks provide insight into climate and biogeochemical cycles during the transition to an oxidizing atmosphere (Bekker et al., 2006) and have deformational fabrics (folds, strong cleavage, local mylonite zones) that record the ca. 1850 Ma Penokean Orogeny. The youngest rocks in the area are Mesoproterozoic ILSG 2011 33 Field Trip 2 �basaltic lava flows (Siemens Creek Volcanics, ca. 1110 Ma) and a layered mafic complex (the Mineral Lake Intrusion, also ca. 1100 Ma), both related to the Mid-continent Rift. All of the stratified units show static contact metamorphic textures near their contacts with the Mineral Lake Intrusion. Thus the area constitutes a microcosm of the regional bedrock geology, and the cross-cutting relationships among the units provide clear constraints on the relative timing of different phases of deformation and magmatism (Cannon etal., 2008, Bjørnerud, 2010a). The rocks in the Atkins Lake-Marengo Falls area are part of a regional structure called the Montreal River Monocline (Fig. 2), a northward-dipping sequence of Archean through Mesoproterozoic rocks that exposes a 35-km thick section of the crust as it existed at the time of the closure of the Midcontinent Rift (see also chapter 7 of this guidebook). The monocline extends almost 100 km along strike, from Gogebic County in northern Michigan to Bayfield County, Wisconsin (Schmidt, 1976). The regional northerly tilt of the strata is thought to have been caused by south-directed post-rifting reverse displacement on a system of lithosphere-scale, north-dipping listric faults, including the Atkins Lake-Marinesco and Pelton Creek Faults, which may have formed initially as normal faults during the development of the rift (Cannon et al., 1993). Because the rift-filling lavas and sediments formed a much thicker stratigraphic section to the north of these normal faults, the subsequent reactivation of the normal faults as reverse faults caused the Mesoproterozoic riftrelated rocks to be thrust over older, Archean and Paleoproterozoic rocks (in contrast to the usual older-over-younger stratigraphic pattern caused by reverse faulting in a simple ‗layer cake‘ sedimentary sequence). The Atkins Lake-Marenisco fault cuts higher in the crustal section on its western terminus, where Paleoproterozoic rocks form the footwall, than in the east, where it transects Archean crust (Cannon et al., 2008). Apart from the major faults bounding the Montreal River Monocline on the south, there is no evidence for significant stratigraphic disruption or repetition within the exposed section in north-central Wisconsin. Therefore, the steeply dipping strata of the monocline provide a complete transect of the crust at ca. 1000 Ma. The listric shape of the Atkins Lake-Marenisco fault seems well constrained by the outcrop patterns in the hanging wall (north side): the northward-shallowing, northerly dips of Proterozoic strata, and the fanning pattern of southward dips seen in (originally vertical) Midcontinent rift-related dikes intruded into Archean rocks (Fig. 2) (Cannon et al., 1993, 2008). Also, in the eastern half of the monocline, where the fault cuts through Archean basement rocks, there is a well-defined biotite Rb-Sr age front as one approaches the fault from the north. The Archean rocks north of this line preserve Archean and Paleoproteorozoic (Penokean) ages, showing that they have remained below the 270° C Rb-Sr blocking temperature for biotite since at least early Proterozoic time. South of the front, the Archean rocks yield anomalous Mesoproterozoic Rb-Sr ages (ca. 1040-1060 Ma), indicating that they cooled through the blocking temperature at the time of closure of the Midcontinent rift and were brought up from mid-crustal depths by rotational displacement along the curved surface of the Atkins Lake-Marenisco fault (Cannon et al., 1993; Holm et al., 2007). The Rb-Sr age front thus represents an exhumed Mesoproterozoic isotherm. These regional relationships indicate that the Atkins Lake-Marenisco fault was a major structure that accumulated many kilometers of reverse displacement in Mesoproterozoic time (Aldrich, 1929; Cannon et al., 1993, 2008). ILSG 2011 34 Field Trip 2 �ILSG 2011 35 Field Trip 2 �Figure 2. Schematic cross sections through the Montreal River Monocline in northern Michigan and Wisconsin (Modified from Cannon et al., 1993). Sections (a) and (b) depict eastern and western parts of the monocline, respectively, as indicated in the inset. The major reverse faults originated as riftbounding normal faults, creating a thicker stratigraphic section to the north. MCR =Midcontinent rift; Paleoprot = Paleoproterozoic metasedimentary rocks; PCFZ = Pelton Creek Fault Zone (inferred). Dashed lines indicate the positions of isotherms prior to closure of the rift. Thin lines in Archean rocks in upper section represent orientations of Midcontinent rift-age mafic dikes. Filled star shows location of Marengo Falls (Stops 6, 7); open star shows inferred depth of formation of pseudotachylytes at Stop 6. Stop 1. Paleoproterozoic Ironwood Iron-formation, metadiabase sills, and breccias (Text modified from Klasner and LaBerge, ILSG Field Guide 1996) Numerous exposures of the Ironwood Iron-formation and the Palms Formation, both part of the Menominee Group, occur in a 4,000 m long northeast-trending zone from Atkins Lake to the Marengo River (Fig. 1), where the Early Proterozoic units are truncated by the Atkins Lake-Marenisco fault. The Palms Formation consists of an upper massive quartzite member and a lower argillaceous quartzite member, similar to exposures elsewhere on the Gogebic Range. (Due to time limitations, we will not visit exposures of the Palms Formation at this stop.) The Ironwood Iron-formation consists of magnetic wavy-bedded and laminated cherty iron-formation and argillite, some of which is also magnetic. In places, mafic igneous bodies—either sills or flows—occur in contact with iron-formation. ILSG 2011 36 Field Trip 2 �A group of closely spaced outcrops along a ridge east of Atkins Lake (N46.27904, W 91.02187) shows typical metamorphosed Ironwood Iron-formation, unusual intraformational breccias, and gabbro sills (Fig. 3). These outcrops are a good representation of the Ironwood in the western extremity of the Gogebic Range, and of the igneous and tectonic overprints that are more pronounced here than in the central part of the range. At this stop, enigmatic breccia units are spatially associated with the upper and lower contacts of a concordant mafic body in the Ironwood Iron-formation (Fehrer and Flood, 1995). These breccias contain angular to sub-rounded clastsof banded siliceous sedimentary rock,ranging from cm to several m in length, within a matrix of a fine-grained, massive mafic igneous rock. There is a marked gradation in grain size in the mafic igneous material, from basalt-like at the contacts with the iron-formation, where the breccia occurs, to diabasic and gabbroic away from the contacts (Fig. 3). Clasts within the breccia are mainly siliceous argillite;a minority is magnetic. Puzzlingly, the population of clasts appears to be more siliceous than the rock in the immediately adjacent iron-formation. Most clasts, regardless of size and composition, are elongate and tabular. Bent or folded clasts are not uncommon. The argillite clasts consist of alternating layers of recrystallized quartz, fine-grained chlorite, biotite or actinolite, garnet, and variable amounts of magnetite. The fine-grained mafic matrix contains conspicuous vesicles, especially around clasts. The matrix is composed of medium-grained amphiboles and plagioclase laths, probably a relict ophitic texture. Plagioclase phenocrysts are present locally. Limited geochemical analysis indicates a tholeiitic composition for the matrix. It should be noted that the mineralogy and, to some extent, the textures of these rocks have likely been modified by Mesoproterozoic contact and regional metamorphism. The origin of the breccias is problematic. Formation by debris flows, faulting or impact ejecta can be ruled out because the matrix is igneous and not fragmental. Outcropscale relationships are arguably consistent with either intrusive or extrusive magmatic activity. The presence of breccia units at both the upper and lower margins of the igneous body seems most simply interpreted as evidence that they formed during intrusion of a sill. The presence of angular, and locally deformed, blocks of country rock within mafic materialcould be evidence for intrusion into semi-consolidated sediments at shallow depth -or could indicate that the clasts were hot enough to deform ductilely in a mobile magma that was shearing against the walls of more rigid host rock. Alternatively, the breccia and mafic rocks could represent an extrusive basaltic flow that incorporated ‗rip-up‘ clasts from the sediments over which it flowed. Sometime later, this stratigraphic horizon may have become the site for the concordant intrusion of a mafic sill that separated the clast-bearing basalt flow into two parts. Features consistent with this origin include: 1) perfectly concordant contacts with thinly laminated sediments at several outcrops, and the lack of any observed crosscutting contacts; and 2) the marked textural contrast between the fine-grained breccia matrix and presumably intrusive coarse-grained, inclusion-free metadiabase and gabbro. Whether the breccias are the result of extrusive or shallow intrusive igneous activity, they do indicate that mafic magmatism occurred during or soon after iron-formation deposition in the western part of the Gogebic Range. Field relationships at this site are similar to those in the eastern part of the Gogebic Range, where the Emperor Volcanic Complex is interlayered with the Ironwood Ironformation (Klasner et al., 1998). In both areas, platformal sediments, including the Bad River Dolomite, the Palms Formation, and the Sunday Lake Quartzite in the east, are similar to ILSG 2011 37 Field Trip 2 �equivalent rocks in the central part of the Gogebic Range. However, evidence for igneous activityduring the time of deposition of the Ironwood Iron-formation is much scarcer in the central part of the range, limited to a few minor ash beds (R. G. Schmidt, personal communication, 1995). The larger volume of igneous rock on either end of the Gogebic Range indicates a far more volcanically active environment in these regions. Figure 3. Detailed map (Klasner and LaBerge, unpublished) of the area east of Atkins Lake showing outcrops and geologic relationships among iron-formation, breccia units, diabase and gabbro. Descriptions of the numbered localities are provided below. Locality 1. This outcrop consists of layered magnetic, greenish, actinolitic iron-formation with metadiabase, and a breccia zone with clasts of iron-formation in a fine-grained metadiabase matrix. Bedding orientations at this outcrop are consistent with orientations of bedding elsewhere in the area, indicating that the outcrop is in place and not a detached raft of iron-formation within the igneous rock.Walk northeast about 60 m to the top of a hill. Locality 2. At this location, large thin slabs of argillite lie in various orientations in a matrix of locally vesicular metadiabase. Some of the slabs are contorted or folded, and one is rolled into a ball with a central layered portion surrounded by a concentric zone of smaller ILSG 2011 38 Field Trip 2 �clasts in the metadiabase matrix. Slabs range in length from less than 1 cm to more than 1 m.Walk east along the crest of the hill about 90 m to a small NNW-trending valley. When in the valley, follow the edge of the hill toward the SSW for about 60 m. Locality 3. Strongly magnetic, wavy-bedded, granular cherty iron-formation. Here lenticular chert beds are up to 15 cm thick. This bedding style is typical of wavy-bedded iron-formation farther east along the Gogebic Range.Walk north-northeast along edge of hill about 25 m. Locality 4. Thin-bedded, non-magnetic chert-argillite or argillaceous ironformation.Some cherty layers appear boudinaged. The argillite unit is about 50 m thick and grades into a more argillaceous unit with less cherty layers to the north.Walk northeast about 55 m. Locality 5. The top of the argillaceous iron-formation layer isin sharp contact here with the overlying breccia unit, in which argillite clasts aresurrounded by metabasalt matrix. Clasts in the breccia tend to become largerhigher in the section (northward from the contact). Remnants of what appears to be a thin slab of iron-formation lie along the top of the breccia unit both here and at locality 6.Walk east-southeast about 10 m. Locality 6.Exposed here is the southeast continuation of the thin slab of iron-formation that lies along the contact between the breccia on the southwest and massive metagabbro on the northeast.Walk northeast about 50 m. Locality 7.Massive metadiabase/metagabbro.Walk northwest about 35 m. Locality 8. Contorted clasts, mainly argillitic, in a vesicular metabasaltmatrix. Stop 2. Mesoproterozoic (?) Mellen-type granite A small undated granite body is exposed in several outcrops east of Atkins Lake, in the open woods south of Forest Service Road 379 (N46.27739, W91.02542). The granite is pink and white, with a medium- to coarse-grained equigranular to porphyritictexture. Principal minerals are potassium feldspar, plagioclase, biotite, and quartz. The granite intrudes Paleoproterozoic metasediments of the Ironwood Iron-formation, and the lack of metamorphic effects and or deformational fabric within the granite suggests that it is Mesoproterozoic in age. This granite is very similar in appearance to the well-known Mellen granite, which is a phase of the Mellen Intrusive Complex, a major magmatic unit of the Midcontinent Rift exposed over a large area more than 20 km to the northeast of this site. In addition to the Mellen granite, the Mellen Intrusive Complex is made up of two large, layered gabbroic bodies -- the Potato River and the Mineral Lake intrusions -- and several related igneous bodies, including the peridotitic Rearing Pond intrusion, and a group of gabbro and granophyre sills. A granophyre within the Mineral Lake intrusion has yielded a U-Pb zircon age of 1102.0 ± 2.8 Ma (Zartman et al., 1997). An indistinguishable U-Pb zircon age (within error) of 1100.9 ± 2.8 Ma has been determined for the Mellen granite, which intrudes and brecciates gabbro of the Mineral Lake intrusion (Zartman et al., 1997). ILSG 2011 39 Field Trip 2 �Stop 3. Mesoproterozoic Siemens Creek Volcanics contact metamorphosed to pyroxene hornfels and granophyre(text from Cannon et al., ILSG Field Guide 1996) The Siemens Creek Volcanics, the lowermost formation within the Powder Mill Group, is the oldest volcanic unit in the Midcontinent Rift in northern Wisconsin. It consists of a sequence of flood basalts erupted as relatively thin flows. These Keweenawan rocks are thrust southward over Paleoproterozic rocks (such as those seen at Stops 1, 4, and 5) along the Atkins Lake-Marenisco fault (Figs. 1 and 2). The northward tilt caused by the faulting has exposed the basal contact of the Mineral Lake intrusion together with a structurally lower sequence of intruded and metamorphosed volcanic rocks. At this stop (N46.2805, W91.0268) an unusually highly metamorphosed section of the Siemens Creek Volcanics is exposed on an isolated hill about 100 m north of Forest Service Road 210 (Club Lake Road), just north of Atkins Lake. The Atkins Lake-Marenisco fault lies under the low ground between the road and the hill. No indications of faulting have been found in the outcrops we will examine, but other low outcrops just south of the road (on private land) contain zones of markedly sheared basalt. The Siemens Creek Volcanics at this locality are intensely recrystallized to pyroxene hornfels. Most original textures are obliterated. Locally, textures possibly representing relict amygdular flow tops can be seen, but these occur discontinuously, making it difficult to recognize distinct lava flows. Small bodies of pink, granophyric granitic rock are common and appear to represent segregations of magma generated by partial melting of the basalt during high-grade metamorphism. These intensely metamorphosed and partly melted rocks can be traced for about 10 km along strike, beyond which they grade both east and west into less metamorphosed equivalents. This unit is up to about 1 km thick and is bounded on the north by the western extension of the Mineral Lake intrusion and related intrusive rocks. Although it is tempting to ascribe the intense metamorphism at this stop to contact effects of the Mineral Lake intrusion, the grade and geographic extent of the metamorphism seen here are much greater than those in the border zone of the gabbro in other areas. Additional conductive heating during the time that these rocks resided deep in the crust, at the base of the thick sequence of lavas and sediments that filled the Midcontinent Rift, may have contributed to the unusually high metamorphic grade. Stop 4. Penokean shear zone in Paleoproterozoic Palms Formation, Marengo River On the south bank of the Marengo River immediately downstream from the private bridge off Forest Road 198 (N46.28965, W90.99405), laminated argillite of the lower part of the Palms Formation is exposed in several subdued outcrops. The rockshere have been intensely sheared within a NE-trending zone that is at least 60 m wide. This shear zone, presumed to be of Penokean age, is essentially bedding parallel and now strikes50° NE and dips 50°NW. It separates a thick metadiabase sill in the footwall from the upper Palms Formation and thinner metadiabase sills in the hanging wall. At this site, bedding in the Palms is tightly to isoclinally folded and transposed into a mylonitic foliation. The folds are commonly dismembered, with limbs so strongly attenuated that only relict fold hinges can be seen. These small folds typically plunge about 50 degrees to the northwest and their axes lie within the plane of the mylonitic foliation. Some of the relict fold hinges apparently acted like rigid porphyroclasts during shearing and display sigma-type tails that indicate dextral shear (in the present orientation). ILSG 2011 40 Field Trip 2 �We interpret this fault to have formed as a bedding-parallel thrust during the Penokean Orogeny. The current northwestward dip of the Paleoproterozoic strata and the fault are a result of regional tilting that occurred during inversion of the Midcontinent Rift at about 1060 Ma. If the strata are restored to their attitude prior to this rotation, they are nearly flat lying and the fault likewise is a nearly flat surface across which the upper plate was thrust northward. Thus, the shear zone seen here is, to our knowledge, the best exposed example of a Penokean thrust fault in this part of the Lake Superior region. Stop 5. Paleoproterozoic Bad River Dolomite: Stromatolites, Penokean deformation, contact metamorphism, Grandview Quarry, Marengo River Please note that Stops 5-7 are on private land owned by the S.C. Johnson family, who have been generous in making these outcrops accessible to geologists for educational purposes. Steep slopes and loose rocks at these sites pose hazards, and field trip participants are responsible for their own safety. The oldest Paleoproterozoic unit in the western Gogebic Range, the Bad River Dolomite of the Chocolay Group, is exposed here in an abandoned quarry that was developed on a natural glaciated bedrock outcrop (N46.28509, W90.98608). In this area, the Bad River Dolomite lies nonconformably and discontinuously on top of the Neoarchean Puritan Batholith (Stop 6), but in the eastern Gogebic range it overlies the Sunday Quartzite, interpreted as a shallow marine tidally influenced unit. At this site, the Bad River Dolomite strikes approximately N50°E and dips 50°-60° NW, concordant here with the overlying Palms Quartzite (Stop 4), but regional relationships indicate that the contact with the Palms is disconformable (Cannon et al., 2008). The Bad River Dolomite is a fine-grained chert-bearing dolostone in which massive, meter-scale strata give way up-section to laminated dolostone with horizons of silicified stromatolites. The stromatolites are well exposed in cross section on the glacially scoured surface at the rim of the quarry and are typically 20-30 cm in diameter. Along the Marengo River, the Bad River Dolomite experienced metamorphism as a result of the emplacement of the nearby Mineral Lake intrusion, and radiating bundles of pale green tremolitic amphibole can be seen in and around chert nodules and within stromatolitic layers, where silica and dolomite are interbedded at the centimeter scale. These relationships record the classic calcsilicate reaction: Dolomite + SiO2 + H2O Tremolite + Calcite + CO2, which occurs either 1) in the presence of very CO2-rich fluids under low pressure conditions (ca. 1-2 kbar) or 2) over a wider range of fluid compositions at higher pressures (>3 kbar) (Spear, 1993). The isotropically radiating clusters of tremolite indicate nucleation-limited growth and static metamorphism (i.e., under low deviatoric stress). In places the tremolite is very fibrous, even asbestoform. Fine mineralogical specimens can be found in the waste rock piles in the old quarry. The quarrying also exposed several meter-scale Keweenawan dikes that cut discordantly across the tilted beds. In a detailed geochemical and isotopic study of carbonate rocks from the Chocolay Group across the Lake Superior region, Bekker et al., (2006) found that the Bad River Dolomite (where it is less metamorphosed) has a 13C signature distinct from that of other Chocolay Group carbonates (e.g., the Kona Dolomite in the Marquette Range and the Gordon Lake Formation in Ontario). They concluded that the Bad River is slightly younger than these units, deposited after the early Proterozoic (Gowganda) glaciations but just before the ILSG 2011 41 Field Trip 2 �global biogeochemical upheaval recorded by the large positive carbon isotope excursion known as the Lomagundi event. Stop 6. Marengo Falls, S side: Neoarchean Puritan Batholith; fault breccias, cataclasites and pseudotachylyte along the Atkins Lake-Marenisco Fault The deeply incised valley of the Marengo River in southeastern Bayfield County, Wisconsin (sections 14 and 15, T44N, R5W) provides a 0.5 km transect through the Atkins Lake-Marenisco fault zone and is one of the few places within the Midcontinent Rift where rocks along a major rift-closing fault are well exposed (Fig. 4). At the falls of the Marengo River (N46.286, W90.964), the Puritan Batholith is juxtaposed across the Atkins LakeMarenisco Fault with the Ironwood Iron-formation and Paleoproterozoic diabase sills, and also with both basaltic lava flows (Siemens Creek Volcanics) and a layered mafic complex (the Mineral Lake Intrusion) of the Midcontinent Rift. Based on regional outcrop relationships, discussed in the Introduction, the vertical throw on the Atkins Lake-Marenisco Fault is probably on the order of 15 km (Fig.4). The river flows northward over Archean rocks until it intersects the fault zone, marked by a 15 m high waterfall near the southern boundary of section 14 (Fig. 4). At that point, the river bends sharply to the west-northwest, subparallel to the fault zone, and cuts through rocks in the core of the zone. Along this transect, the Puritan Quartz Monzonite can be traced from an undeformed condition above the falls to an ultracataclastic state a few hundred meters downstream. Rocks exposed between these two locations preserve intermediate textures that record the temporal evolution of the fault zone (Bjørnerud 2010a). The nearly undeformed Puritan Quartz Monzoniteis a medium- to coarse-grained (0.5-2.0 cm), locally pegmatitic granitoid with consertal igneous texture in thin section. It is comparatively poor in mafic minerals and those present (mainly biotite, some hornblende) are largely altered to chlorite. The feldspars, principally albitic plagioclase and microcline, are commonly sericitized, and this becomes more pronounced near the fault zone. Away from the fault, feldspars show little evidence of internal deformation, although in places where a weak gneissic fabric is developed, a fine-grained (0.1 mm) polygonalized texture indicates partial annealing following ductile deformation, presumably during Neoarchean time (Schmidt, 1976). The earliest fault-related microstructures, found in rocks immediately downstream from Marengo Falls, are quasi-ductilely deformed albite and microcline grains with kinked or bent twin planes and patchy extinction. In some samples, very small (ca. 0.05 mm) subgrains have developed along the most deformed parts of these crystals. These textures are overprinted by more brittle features. In the steep cliffs on the south side of the river, the quartz monzonite has been broken into heterogeneous breccias and cataclasites. Theserocks lack any prominent planar fabric, slickenlines or other indications of slip sense; they are simply pervasively fragmented. ILSG 2011 42 Field Trip 2 �Figure 4.Geologic map of the Atkins Lake-Marenisco Fault zone in the Marengo Falls area, Bayfield County, Wisconsin (modified from Cannon et al., 2008).Star marks location of Marengo Falls (Stops 6 & 7). The location of Stop 5 is just west of the area marked Xb. The non-parallelism of section lines reflects the difficulty early surveyors had in mapping areas with extensive outcrops of magnetite-bearing iron-formation. In a series of outcrops of the Puritan quartz monzonite on the south side of the river, extending about 200 m immediately west from an abandoned hydroelectric station, breccias and cataclasites are complexly interlaced with very dark veins, 0.2 to 6 cm thick, of what appears to be devitrified (mainly chloritized) pseudotachylyte (Bjørnerud, 2007, 2010a). Several types of textural evidence support this interpretation. First, the chlorite in this material is texturally isotropic, occurring as uniformly spaced radial bundles 0.1-0.2 mm in diameter. This suggests that it nucleated in a homogeneous and isotropic medium – i.e., glass or on primary microlitic spherules in glass sometime after the deformation had ceased. Second, this chloritic material typically includes relatively large (0.5-2 mm), angular clasts from the protolith (including quasi-ductilely deformed feldspar grains), while finer clasts are conspicuously absent. The absence of fines is consistent with preferential melting of smaller rock fragments, a phenomenon documented in natural and experimental pseudotachylytes (e.g., Tsutsumi, 1999). Third, the contacts between some of the chloritic material and the host rock are distinctly scalloped, a partial melting texture observed in pseudotachylyte injection veins (Swanson, 2006). Fourth, in some of the thicker veins, a disproportionate number of the surviving clasts are embayed grains of titanite, a refractory accessory mineral in the protolith that could have withstood temperatures sufficient to melt the major minerals (quartz and alkali feldspar). The irregular, cuspate edges of these crystals are again suggestive of partial assimilation by a melt phase. Finally, some of the chloritic material ILSG 2011 43 Field Trip 2 �preserves ‗pseudomorphs‘ of apparent flow textures similar to those observed in younger, still-glassy pseudotachylytes. Collectively, these observations support the interpretation that much of the black chloritic material in the fault zone originated as pseudotachylyte. The spatial relationships among the various types of fragmented rock and the pseudotachylytes shed light on the dynamic processes in the fault zone over time. In thin sections of some pseudotachylyte-bearing samples, a continuous gradient in grain size can be observed from cataclasite to ultracataclasite to pseudotachylyte, suggesting that the textures formed in a single slip event. More commonly, though, fine-grained cataclasites or microbreccias transect and/or include scattered pieces of the pseudotachylytic material, indicating that these granulated rocks are generally younger than the pseudotachylytes. One group of samples shows two distinct generations of pseudotachylyte, which in turn are both incorporated as clasts in a cross-cutting breccia. Some of these microbreccias appear to have dilational and intrusive (fracture Mode I), rather than shearing (Mode II), cross-cutting relationships with the pseudotachylyte and the surrounding rock. As such, they are not cataclasites in the strict sense of having been produced only by grinding and frictional wear. They occur as dike-like fingers that ‗explosively‘ disaggregate the pseudotachylyte and make inroads into fractures in the host rock. These microbreccia veins are typically matrix- rather than clast-supported and contain parallel bands of uniform grain size, commonly fining inward from wall to interior, possibly a result of dynamic sorting of fluidized granular material. Such textures may record thermal pressurization of fluids during a seismic event (Otsuki et al., 2003; Bjørnerud, 2010b). Stop 7.Marengo Falls, N side: Ultracataclasite along the Atkins Lake-Marenisco Fault The most extreme comminution of grains has occurred in rocks exposed in prominent bluffs on the north side of the river. In outcrop, these ultracataclasites are dark gray, massive, and transected by anastomosing veins of chlorite, quartz and zeolites. In thin section, one can discern very fine (<0.1 mm) angular feldspar grains in a still finer matrix of chlorite with a weak preferred orientation. These rocks are so modified by deformation and mineralization that their protolith is unclear; they are likely derived from the basaltic lava flows of the Siemens Creek Formation, which crops out to the north of the river below the falls(Fig. 4). In the very first geologic description of these rocks, C.E. Wright (1879), who mapped the Penokee Gap under the direction of T.C. Chamberlin in 1876, noted their unusual character, giving them the non-genetic designation ―chloro-silicious rocks‖ (Fig. 5). These fault rocks are overprinted by multiple generations of vein networks, with a wide array of orientations, indicating a protracted period of fracture-hosted fluid flow that continued after the main period of faulting and cataclasis had ceased. Many of these veins have a fibrous texture, indicating a ‗crack-seal‘ mechanism of formation, and some appear to record almost isotropic dilation of the fault rock. Vein minerals include chlorite, zeolites, quartz, and calcite. These same minerals are common as vein fillings and slickenfibers in Midcontinent Rift basalts in northern Wisconsin (Garbowicz and Bjørnerud, 2003) and in the Upper Peninsula of Michigan, where they occur together with hydrothermal native copper deposits known to have formed between about 1060 and 1050 Ma (Bornhorst et al., 1988). Summary: The AtkinsLake-Marenisco fault zone of north-central Wisconsin provides a rare window into faulting processes along the flanks of the Midcontinent Rift during its inversion at ca. 1060 Ma. The presence of quasi-ductilely deformed feldspars and cm-thick ILSG 2011 44 Field Trip 2 �pseudotachylyte fault veins indicate that faulting began at mid-crustal levels and that the fault slipped in one or more large earthquakes. Cross-cutting cataclasites and matrix-supported microbreccias record progressive comminution and intermittent fluidization of rocks in the fault zone. Elevated fluid pressures almost certainly played a role throughout the life of the fault. Reverse slip on the steeply dipping AtkinsLake-Marensico fault and other rift bounding faults coincided in time with widespread hydrothermal mineralization, including native copper deposition, in rocks of the Midcontinent Rift. The Atkins Lake- Marenisco fault is a thick-skinned lithosphere-scale structure, and its characteristics support the hypothesis (Cannon, 1994) that large deviatoric stresses were transmitted far inland during the Grenville Orogeny. Figure 5. The first geologic map of the Marengo Falls area, by C.E. Wright (1879), based on field work done in the summer of 1876. The ultracataclasites along the Atkins LakeMarenisco fault zone are mapped as ―Chloro-silicious rocks‖; the Puritan Quartz Monzonite as ―Granite‖ and Siemens Creek Volcanics as ―Greenstone‖. Only the strike of the Ironwood Iron-formation (―Magnetic schist‖) is inaccurate (shown as NW rather than NE). ILSG 2011 45 Field Trip 2 �REFERENCES Aldrich, H. R., 1929. The geology of the Gogebic iron range of Wisconsin. Wisconsin Geological and Natural History Survey Bulletin 71. Bekker, A., Karhu, J. and Kaufman, A., 2006. Carbon isotope record for onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America. Precambrian Research 148, 145-180. Bjørnerud, M., 2007.Evidence for paleoseismic events during closure of the Midcontinent Rift, Atkins Lake-Marenisco fault zone, northern Wisconsin. Proceedings of the Institute on Lake Superior Geology 53, 6-7. Bjørnerud, M., 2010a. Evidence for Grenville-age seismicity and thick-skinned deformation in northern Wisconsin, USA. Journal of Geology 188,45-58. Bjørnerud, M., 2010b. Rethinking conditions necessary for pseudotachylyte formation: Observations from the Otago schists, S Island, New Zealand. Tectonophysics 490, 6980. Bornhorst, T., Paces, J., Grant, N., Obradovich, J., and Huber, N. K., 1988. Age of native copper mineralization, Keweenaw Peninsula, MI. Economic Geology 83, 619-625. Cannon, W.F., 1994. Closing of the Midcontinent Rift: A far-field effect of Grenvillian compression. Geology 22, 155-158. Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30‘x60‘ quadrangles, Wisconsin and Michigan: U.S. Geological Survey, Miscellaneous Investigation Series Map I-2556, scale 1:100,000. Cannon, W.F., LaBerge, G., Klasner, J., and Schulz, K., 2008. The Gogebic Iron Range – A sample of the northern margin of the Penokean fold and thrust belt. U.S. Geological Survey Professional Paper 1730. Cannon, W.F., Peterman, Z., and Sims, P. K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline – a 35-km thick cross section of the mid-continent rift. Tectonics 12, 728-745. Feher, L. and Flood, T., 1995. Vesicles and breccia due to injection of mafic magma into partially lithified sediments of the early Proterozoic Ironwood Iron Fm., Gogebic Range, NW Wisconsin. Proceedings of the Institute on Lake Superior Geology, 41, 1315. Garbowicz, A. and Bjørnerud, M., 2003. Paleostress inferences from slip vectors in the eastern part of the Wisconsin segment of the Midcontinent rift, Proceedings of the Institute on Lake Superior Geology 49, 18. Holm, D., Schneider, D., Rose, S., Mancuso, C., McKenzie, M., Foland, K. and Hodges, K., 2007, Proterozoic metamorphism cooling in the southern Lake Superior Region of North America and its bearing on crustal evolution. Precambrian Research 157, 106126. Klasner, J.S., LaBerge, G.L., and Cannon W.F., 1998, Geologic map of the eastern Gogebic iron range, Gogebic County, Michigan: U.S. Geological Survey Geologic Investigations Series map I-2606, scale 1: 24,000. Klasner, J, and LaBerge, Gene L., 1996, Lake Namekagon and Penokee Gap areas, west Gogebic Range, Wisconsin. Proceedings of the Institute on Lake Superior Geology, Part 2: Guidebooks to Field Trips, Field Trip 5, p. 83-108. ILSG 2011 46 Field Trip 2 �Otsuki, K., Monzawa, N., and Nagase, T., 2003. Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms. Journal of Geophysical Research108 B4, doi: 1029/2001JB001711. Schmidt, R., 1976. Geology of the Precambrian W (lower Precambrian) rocks in western Gogebic County Michigan. U.S. Geological Survey Bulletin 1407. Sims, P. K., Peterman, S., and Prinz, W., 1977. Geology and Rb-Sr age of Precambrian W Puritan quartz monzonite, northern Michigan. U.S. Geological Survey Journal of Research 31, 185-192. Spear, F., 1993, Metamorphic phase equilibria and pressure-temperature-time paths: Monograph 1, Mineralogical Society of America, Washington, D.C. Swanson, M., 2006. Pseudotachylyte-bearing strike-slip faults in mylonitic host rocks, Fort Foster brittle zone, Kittery, Maine. In Abercrombie, R., McGarr, A., Kanamori, H. and DiToro, G., eds. Earthquakes: Radiated Energy and the Physics of Faulting. Geophysical Monograph 170. Washington D.C., American Geophysical Union, 166179. Tsutsumi, A., 1999. Size distribution of clasts in experimentally produced pseudotachylytes. Journal of Structural Geology 21, 305-312. Wright, C.E., 1879, The Huronian Series west of Penokee Gap, Chap. IV in T.C. Chamberlin, ed., Geology of Wisconsin, Survey of 1873-1879, Volume III. Madison WI: Wisconsin Commissioners of Public Printing. Zartman, R.E., Nicholson, S. W., Cannon, W .F., and Morey , G.B., 1997. U-Th-Pb zircon ages of some Keweenawan Supergrouprocks from the south shore of Lake Superior. Canadian Journal of Earth Sciences 34, 549 -561. ILSG 2011 47 Field Trip 2 �ILSG 2011 48 Field Trip 2 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 3 GEOLOGY OF THE BAYFIELD PENINSULA: KEWEENAWAN BAYFIELD GROUP AND PLEISTOCENE DEPOSITS Bayfield Peninsula – K.Wilson ILSG 2011 49 Field Trip 3 �ILSG 2011 50 Field Trip 3 �Field Trip 3 Geology of the Bayfield Peninsula: Keweenawan Bayfield Group and Pleistocene deposits Richard Ojakangas1, Drew Cramer2, and Tom Fitz2 1 University of Minnesota – Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 E-mail: rojakang@d.umn.edu 2 Northland College, 1411 Ellis Avenue, Ashland, Wisconsin 54806 E-mail: tfitz@northland.edu THE KEWEENAWAN BAYFIELD GROUP Introduction In the Lake Superior region of the Midcontinent Rift System (MRS), as much as 20,000 m of dominantly basaltic sub-aerial flood lavas are overlain by 10,000 m of sedimentary rocks (Figs. 1, 2) (Behrendt et al, 1988; Cannon et al, 1989). The exposed volcanic rocks range in age from 1109-1086 Ma; the greatest bulk was extruded during a magnetically reversed interval around 1108-1105 Ma, and during a magnetically normal interval around 1097-1094 Ma (Green, 1995). The overlying, unmetamorphosed, sedimentary rocks, comprise the Oronto Group and the younger Bayfield Group (Fig. 3). Although we will not see the Oronto Group rocks on this field trip, a short description is provided here to set a sedimentological and petrographic background for the Bayfield Group rocks that we will see. Cessation of volcanism was followed by the deposition of the Oronto Group, which includes the Copper Harbor Conglomerate, the Nonesuch Formation, and the Freda Sandstone (Daniels, 1982). On the Keweenaw Peninsula of Michigan, the Copper Harbor Conglomerate, which is as thick as 1340 m, contains basaltic flows that represent some of the last volcanism within the basin. The Lakeshore Traps in the Copper Harbor have been dated at 1087.2 +/-1.6 Ma (Davis and Paces, 1990), and an intrusive on Michipicoten Island in eastern Lake Superior at has been dated at 1086 Ma (Palmer and Davis, 1987). The Copper Harbor is largely a coarse, clast-supported conglomerate; the clasts are dominantly volcanic and were derived from a variety of volcanic rocks. Sandstones, which are also largely composed of volcanic detritus, are common, especially in the upper half of the formation; generally, they are lithic arenites. Siltstones and shales are less abundant, at least in the exposed portion of the formation, but may be more abundant at depth nearer the axis of the basin. Deposition was on alluvial fans and associated braided alluvial plains; rare stromatolite horizons formed in standing bodies of water during pauses in sedimentation. Aprobable correlative unit is the subsurface Solor Church Formation of southeastern ILSG 2011 51 Field Trip 3 �Figure 1. Generalized geologic map of the Midcontinent Rift System in the Lake Superior region showing the study area. From Adamson, 1977, modified from Dickas, 1986. Figure 2. Aeromagnetic map of Northern Wisconsin. From Nicholson et al., 2006. ILSG 2011 52 Field Trip 3 �Figure 3. Stratigraphic correlation chart for Keweenawan Supergroup rocks in Minnesota, Wisconsin, and Michigan. From Morey and Van Schmus, 1988. ILSG 2011 53 Field Trip 3 �Minnesota. The Nonesuch Formation, which is as thick as 200 m, is noted for its copper content (mainly chalcocite) and the presence of petroleum seeps in the now-closed White Pine Copper Mine. The Nonesuch is largely a fine-grained sequence of gray to black siltstone with minor thin shales and carbonates as well as local, graded sandstone beds and conglomerates. The fine sediment was deposited in a restricted lacustrine basin,where organic material could accumulate as a result of a reducing chemical environment (Elmore et al, 1988; Suszek, 1995). The Freda Sandstone, as thick as 1525 m, is comprised dominantly of red to buff sandstone with intercalated red siltstone and shale; the sandstones are more mature than those lower in the group. The environment of deposition fits a braided stream model (Fig. 4). Figure 4. Generalized reconstruction of what the Midcontinent Rift System may have looked like after cessation of volcanism about 1086 Ma. Note the streams entering the rift from the adjacent highlands and the presence of lakes (white). From Ojakangas and Matsch, 1982. The overlying Bayfield Group, 2100 m thick, is comprised from the base upwards of the Orienta Sandstone, the Devils Island Sandstone, and the Chequamegon Sandstone. All three units are best exposed on the Bayfield Peninsula and the Apostle Islands. Correlative units are found in Minnesota, Michigan, and Ontario (Ojakangas and Morey, 1982). The ILSG 2011 54 Field Trip 3 �contact between the subhorizontal Bayfield Group and the steeply dipping underlying Oronto Group is not exposed, but field and geophysical data can be interpreted as indicating an unconformable relationship (Morey and Ojakangas, 1982). Figure 5 shows the distribution of the three formations in the Bayfield Group. Figure 5. Distribution and attitudes of the three Bayfield Group formations. From Adamson, 1997. Orienta Sandstone The Orienta, and its equivalent several miles to the west in Minnesota (the Fond du Lac Formation), are comprised of red feldspatho-lithic arenite, red siltstone, and shale units with fining-upward sequences that are typical of deposition in meandering river systems;large-scale trough cross-beds are common andparting lineation and current ripple marks are also present. Devils Island Sandstone The white,buff, and orange Devils Island Sandstone is about 100 m thick. It is a quartz arenite, as is its equivalent unit,the Hinckley Sandstone, to the west in Minnesota, (Tryhorn and Ojakangas, 1972). Symmetrical ripples suggest a lacustrine environment as the final site of deposition, but eolian processes must have been an important factor in both the textural and the mineralogical maturation of the sand; creating a quartz arenite from felspatho-lithic sands (Fig. 6). The discovery of adhesian ripples and other eolian features by Johnson et al (2001) in the Hinckley support this suggestion, although the latter works also suggest the presence of a braided stream environment. More recently, Galston and Havholm (2008) have identified sedimentary features that they interpret as eolian dunes, adhesian ripples and wind ripples that were deposited on a floodplain with braided streams. An ILSG 2011 55 Field Trip 3 �equivalent to the east in Michigan may be an upper portion of the Jacobsville Sandstone (Kalliokoski,1982) as exposed on the Middle Branch of the Ontonogan River on Michigan Highway 28. Quite likely, the maturation of the sand on the vegetation-less surface of that time was due to eolian processes, which are the most effective agents of such maturation. Figure 6. Compositional triangle showing the relative abundances of quartz, feldspar, and lithics (rock fragments) in the formations of the Oronto and Bayfield Groups. The Chequamegon Formation is outlined with the dashed line. Note the increasing compositional maturity upwards from the base of the Oronto Group through the Bayfield Group. From Adamson, 1977, after Ojakangas, 1986. Chequamegon Sandstone This feldspatho-lithic unit resembles the Orienta Sandstone, but overlies the Devils Island. The similarity and stratigraphic relationship has long been questioned, largely because the formations are subhorizontal and exposures are scattered. However, Adamson (1997), on the basis of both field and well data, concluded that the Chequamegon does indeed overlie the Devils Island. Another question involving the Bayfield Group is whether the unfossiliferous units could be nonmarine Cambrian deposits (e.g., Hamblin, 1958; Ostrom, 1967; personal communication, A.C. Runkel).Paleocurrent data from several sources show that the dominant currents moved toward the north or northeast, toward the basin axis ILSG 2011 56 Field Trip 3 �in both sedimentary groups (Fig. 7).Whereas intrabasinal volcanogenic detritus is dominant in the Oronto Group, extrabasinal granitic detritus dominates in the younger Bayfield Group. Figure 7. Generalized paleocurrent trends in the Bayfield Group of Wisconsin, the Fond du Lac Formation and the Hinckley Sandstone of Minnesota, and the Jacobsville Sandstone of Michigan and Ontario. The stippled areas in southeastern Minnesota and southwestern Wisconsin are subsurface occurrences. The dashed line represents the axis of the Lake Superior Syncline. From Ojakangas and Morey, 1982. Beginning in 1869, the Chequamegon was extensively quarried for ―brownstone‖ on the Bayfield Peninsula as well as on several of the Apostle Islands. There were also quarries in the Orienta and in the equivalent Fond du Lac Formation near Duluth; in total, there were 12 quarries. The stone was shipped to Chicago, Milwaukee, Detroit, Toledo, Cincinnati, St. Paul and Kansas City, as well as used locally in lighthouse construction and in buildings such Washburn‘s main street bank building (now a museum) and Northland College‘s Wheeler Hall. ILSG 2011 57 Field Trip 3 �Apostle Islands Because of both time and uncertain weather, it was decided that a boat trip to the Apostle Islands was not possible as an ILSG field trip. However, we have included a short summary of the islands. (Take an excursion boat trip on your own some day from Bayfield.) Why the name? Presumably, the early French explorers concluded there were 12 islands, and named them after the 12 apostles of the Bible. Actually, there are 22 islands, 21 of which are in the Apostle Islands National Lakeshore, established in 1970, that also includes part of the Bayfield Peninsula north of the Red Cliff Indian Reservation (Fig. 8). The bedrock on 19 of the islands is the Chequamegon Sandstone;not so surprisingly, the bedrock on Devils Island, the northernmost island, is the Devils Island Sandstone, and Sand Island, the north-westernmost island, consists of the Orienta Sandstone. Well-exposed sedimentary structures are common. Glacial deposits of the Miller Creek Formation blanket most of the islands, but red to buff bedrock is visible in cliffs, sea caves, sea stacks, arches and elsewhere along the shorelines where erosion has exposed the bedrock. Several excellent beaches and a few tombolos are present. Locally, there are sand dunes. The islands look quite pristine, but they were extensively logged for white pine, hemlock, yellow birch and maple between 1850 and 1970. Brownstone quarries were established on Basswood Island in 1869, and later on Hermit and Stockton Islands. There are six lighthouses on the islands, built between 1857 and 1891. Figure 8. The Apostle Islands. From Nuhfer, 2004. ILSG 2011 58 Field Trip 3 �PLEISTOCENE GEOLOGY Introduction The Bayfield Peninsula has been repeatedly glaciated over the last 2.4 million years, but the most recent glaciations have wiped out evidence of previous glaciations, leaving a record of only the last 30,000 years. Glaciers scoured down to bedrock, then deposited many tens of meters of till and outwash as they melted away. Glacial lakes intermittently occupied the margins of the ice sheets where deposits of lacustrine sediment accumulated, much of which was reworked by minor glacial advances. These sediments have been divided into two units: the sandy Copper Falls Formation deposited as till and outwash before 11,500 BP, and the younger, finer-grained Miller Creek Formation consisting of lacustrine and glacially reworked lacustrine sediments deposited between 11,500 and 9,500 years BP (Fig. 9) (Clayton, 1984). F igure 9. Glaciations in Wisconsin. From Syverson and Colgan, 2004. ILSG 2011 59 Field Trip 3 �Glaciation The Bayfield Peninsula was most recently glaciated by the Superior Lobe and the Chippewa Sub-lobe of the Laurentide Ice Sheet, during the Wisconsin Glaciation from 25,000 BP to 9,500 BP. Deposits from several glacial advances have been identified in northern Wisconsin, the youngest of which is referred to as the Lake View Advance (Fig. 10). The ice-margin positions have been determined by a combination of lithostratigraphic evidence, ice-margin landforms, and inferred connections in areas where no evidence remains (Clayton, 1984). Figure 10. Furthest extent and flow direction of the Lake View Advance, between 10,000 BP and 9500 BP. From Clayton, 1984. Surficial Units Copper Falls Formation The Copper Falls Formation lies above pre-Pleistocene rock types and is generally composed of sandy till derived from Keweenawan sandstones in the Superior basin (Fig. 11). The Copper Falls Formation is at the surface in large areas of northern Wisconsin, and is prevalent at elevations higher than 1000 feet where it has not been covered up by younger deposits of lacustrine and reworked lacustrine sediments. The type section for the Copper Falls Formation is a river cutback near Copper Falls. Although the Copper Falls Formation is primarily composed of sandy till, it contains other sediment types including common proglacial stream deposits. These proglacial stream deposits are often interbedded with lithologically similar till. The formation on average is 40-80% sand, 15-50% silt, and 2-20% clay. The Copper Falls is slightly calcareous and the clay component is mostly composed of illite and smectite (Clayton, 1984). ILSG 2011 60 Field Trip 3 �Figure 11. The stratigraphic relationship between the Keweenawan rocks, the Copper Falls Formation and the Miller Creek formation. From Clayton, 1984. Miller Creek Formation The Miller Creek Formation, overlying the Copper Falls Formation, was deposited between 11,500 and 9,500 BP, and is composed of offshore clayey sediment and clayey till derived from fine-grained offshore sediment. The Miller Creek Formation has two subunits on the western half of the Bayfield Peninsula: the Hanson Creek Member and the overlying the Douglas Member. It is the majority of the subsurface material between 1000 feet and lake level (602 feet). The Hanson Creek Member is generally unlaminated and is 45-70% clay, 20-45% silt, 3-20% sand, trace pebbles, cobbles, and boulders. The silt and clay particles are typically 10% carbonates. The Douglas Member generally is 45-85% clay, 1040% silt, 3-20% sand, trace pebbles, cobbles, and boulders. Where it overlies sand it may contain up to 60% sand. It is usually dull reddish brown and calcareous. East of the Bayfield Peninsula the Miller Creek Formation is not divided into named members. It is 30-65% silt, 10-60% clay, 5-30% sand, trace pebbles, cobbles, and boulders. It is usually reddish brown and where it overlies sand may be more sandy (Clayton, 1984). The presence of wellpreserved laminations in some parts of the Miller Creek Formation, and their absence in other places, is indicative of the variable depositional environments and post-depositional histories of the sediments. Some parts are undisturbed lake sediments, while other parts were reworked, mixed with coarser sediment, and deformed by glacial action by later ice advances (Clayton, 1985; Shumway and Iverson, 2008). FIELD TRIP STOPS Head west out of Ashland on U.S. Highway 2. Just outside of town, we are driving to a young beach ridge built up at the mouth of Fish Creek (Fig. 12). This formed as the result of very rapid sedimentation from Fish Creek at the shallow southwest end of Chequamegon Bay. Three miles west of Ashland, past the junction of U.S. Highway 2 and Wisconsin Highway 13, is the Northern Great Lakes Visitor Center, which has excellent displays of natural and cultural history, including some that are geologic. Between the junction with Wisconsin Highway 13 and County Road E in Ino, a distance of 12 miles, we are driving on ILSG 2011 61 Field Trip 3 �the Pleistocene Miller Creek Formation. The glacial topography has been greatly modified by Glacial Lake Duluth. From just west of Ino, traveling 20 miles to Brule, , we are driving on the older Pleistocene Copper Falls Formation. Figure 12. Map of field trip stops. Stop 1: Brule River Outlet of Glacial Lake Duluth In Brule, turn south on Wisconsin Highway 27 and proceed for 3 miles to of Winneboujou. Winneboujou is located at the bottom of a steep-walled outlet channel, presently 100 feet deep, about one-mile-wide, that was cut when Glacial Lake Duluth drained via the Bois-Brule River outlet into the St. Croix River, which enters the Mississippi River (Figs. 13, 14). Figure 13. Generalized map showing the BoisBrule outlet that carried waters southward from Glacial lake Duluth to the Sta. Croix River and hence to the Mississippi River. The heavy line near the bottom represents the southernmost advance of the Late Wisconsin glaciers. From Dott and Attig, 2004. ILSG 2011 62 Field Trip 3 �When Glacial Lake Duluthbreached this outlet, at an elevation of about 1100 feet, the Bois-Brule flowed southward, cutting this deep and wide channel. The present underfit Brule River now flows northward into Lake Superior, with its headwater 30 miles to the south. Native Americans, early explorers, and fur traders came up the Mississippi and St. Croix Rivers and then traversed a two-mile portage from the St. Croix River into the present Brule River. This divide is located a few miles north of Solon Springs. Figure 14. Landscape image of the Bois-Brule outlet of Glacial Lake Duluth just south of the village of Brule. Note the Glacial Lake Duluth plain. From Dott and Attig, 2004. ILSG 2011 63 Field Trip 3 �Between Brule and Maple, a distance of 8 miles, we are again driving on the younger Miller Creek Formation. At Maple, we descend over several low, sandy beach ridges (elevations about 1080 feet) onto lake-modified glacial topography, the bed of Glacial Lake Duluth. Three miles west of Maple is the village of Poplar, located on this lake bed. Proceed about 5 miles and turn right on Country Road U. Follow County Road U a short distance to the Amnicon State Falls Park entrance on the left. Stop 2A: Amnicon Falls State Park: Orienta Sandstone, Keweenawan Volcanics, and the Douglas Fault. The Douglas Fault has thrust Middle Keweenawan basalt (Chengwatana Volcanics) northward over the Late Keweenawan Orienta Sandstone (Fig. 15). A waterfall exists as the Amnicon River flows over the basalt into the gorge cut into the much softer, subhorizontal red Orienta Sandstone. The basalt flows dip at 30-40 to the southeast whereas the Orienta dips at about 5 . These relationships are easily seen after crossing the bridge and observing the east bank from the west side of the river. Note that the Orienta has been dragged into a near-vertical attitude adjacent to the few-meter-thick gouge and breccia zone between the fault and the Orienta. Prior to a collapse of the side of the gorge several years ago, at the base of the short stairway on the east bank, steep slickensides were visible on the sandstone beneath the breccia. Figure 15. Sketch of the Douglas Fault as exposed at the base of the main falls in Amnicon Falls State Park. The arrows show the relative motion along this fault. From Dickas, 1989, adapted after Thwaites, 1912. ILSG 2011 64 Field Trip 3 �You can walk on the Orienta Sandstone a short distance downstream from the bridge on the northeastern bank. Note trough cross-bedding and parting (current) lineation.The Douglas Fault defines the northwestern edge of a major feature of the Midcontinent Rift, the St. Croix Horst. (see Fig. 2). It has been traced by geophysical methods from the Twin Cities through northwestern Wisconsin and into Lake Superior, and may have 3000 m of displacement (Craddock, 1972). There is now some question about whether it connects with the Isle Royale Fault beneath Lake Superior. The southeastern side of the horst is marked by the Lake Owen-Keweenawan Fault system, also reverse faults. It is generally assumed that these faults were originally normal faults during the formation of the rift, but became reverse faults later due to compressional forces perhaps caused by the collision of the Grenville micro-continent with eastern North America. A paleomagnetic study by Watts (1981) estimated the age of the compressive event at 950-1040 Ma. Stop 2B: (Optional) Quarry in Keweenawan Volcanics Exit the park and return to U.S.Highway 2. Turn right (west) and drive a short distance to an entrance road into a quarry operated by Northwoods Paving of Ashland. Drive a short distance behind the crushed rock piles to basalt exposures on the quarry wall. Nearby is gabbro of the Amnicon Intrusion (Nicholson et al, 2006) that has altered and deformed these flows. Return to County Road U on which the entrance to Amnicon Falls State Park is located, and proceed north for 5 miles to Wisconsin Highway 13. Proceed eastward for about 35 miles to Port Wing. Note that we are driving this entire distance on the flat bed of Glacial Lake Duluth (the Lake Superior Lowland), and that this flat area (the Douglas Member of the Miller Creek Formation) is deeply dissected by numerous rivers and smaller streams, all flowing northward into Lake Superior (Fig. 16).Locally some of the rivers have cut down to the underlying Pleistocene Copper Falls Formation and to the Bayfield Group sandstones. Figure 16. Lake–modified glacial topography intersected by incised stream valleys. U.S. Geological Survey Poplar NE Quadrangle (7.5-minute, topographic, 10-ft contour interval. T. 48 N.. R. 12 W. From Clayton, 1984. ILSG 2011 65 Field Trip 3 �Stop 3: Orienta Sandstone at Twin Falls Park in Port Wing Upon entering Port Wing, pull off to the right into Twin Falls Park. Follow the path a few hundred feet to an observation deck above the falls. Note that the Flag River has cut a deep gorge through the glacial beds and into the Orienta Sandstone. Stop 4: Devils Island Sandstone at Siskiwit Falls, Cornucopia In Cornucopia, turn right (south) on Superior Avenue and continue for one block. Turn left (east) on County Road C and continue 0.4 miles to Siskiwit Falls Road. Turn left and continue for one block. Park on shoulder just before the bridge over the Siskiwit River. The falls, really a rapids, formed by water passing over a series of sandstone beds,is just upstream from the bridge. This, unfortunately, is the only easily accessible exposure of the quartz arenite of the Devils Island Sandstone. At low water, a total thickness of about 6 meters can be seen. Look for current ripple marks and cross-bedding. If the water is high, the only observation possible may be a close look at the composition of loose pieces of the formation under a handlens—99% fine-grained quartz. Good rounding of grains may be visible in coarser-grained pieces. Transect Across the Bayfield Peninsula Stops 5 through 10 constitute a transect across the peninsula, from the highest elevations in the middle of the peninsula down to the lakeshore on the east side (Fig. 17). All of the stops are on public land, with the exception of Stop 7, which is on private land. Locations are given in public land survey grid, and UTM coordinates (Zone 15T), datum NAD 1927. The transect crosses the transition zone between sandy Copper Falls sediments and clay-rich Miller Creek sediments on both sides of the Peninsula (Fig. 18). From Siskiwit Falls, go east on Siskiwit Falls Road, then turn right (south) on Stage Road, which becomes Star Route Road. Take Star Route road southeast for approximately three miles uphill to where the road levels off and turns east. After passing over the top of the hill, go left on Settlement Dump Road. Stop 5 is at the end of this road. The road has a steep grade and loose sand, so it may not be passible at all times of the year. Figure 17. Map showing locations of field trip stops on the Bayfield Peninsula. ILSG 2011 66 Field Trip 3 �Figure 18. Surficial geologic map showing field trip stops 4-10. Miller Creek Formation: gl—lake-modified glacial topography (teal on map), gw—wave-planed topography (green), gh—valley sides(darkest green), b—shoreline sediment (orange); Copper Falls Formation: suc—uncollapsed proglacial stream sediment (Valhalla surface) (reddish orange), sc— collapsed proglacial stream sediment (red), sg—hummocky stream sediment overlain by silty material (light green). Modified from Clayton, 1985. Stop 5: Glacial Outwash and the Glacial Lake Duluth Beach Location: Gravel pit at the north end of Settlement Dump Road; UTM coordinates: 0649710 mE, 5185261 mN; T50N, R5W, Sec 18, south-central. (Fig. 19) Stop 5A: Glacial Outwash Above the Lake Duluth Beach Travel up Settlement Dump Road watching for exposures of sediment; the best exposures are in a gravel pit at the top of the hill (UTM coordinates listed above). This is glacial outwash of the Copper Falls Formation deposited by meltwater rivers flowing out of the Superior Glacial Lobe to the west and the Chippewa Lobe to the east. Here in the central part of the Bayfield Peninsula, at elevations above 1100 feet, outwash of the Copper Falls Formation is gravelly sand with few cobbles and boulders. There is a wide variety of clasts types, which is typical of the Copper Falls Formation. The hilltop to the north of the pit is the highest point on the Bayfield Peninsula, with an elevation of 1426 feet – which is 824 feet above the present surface of Lake Superior and well above all the glacial lake beaches. The sandy soils and relatively high topographic relief in the area make for excessively well drained soils. These soils support a sparse forest consisting of only drought-tolerant trees such as jack pine, red pine, and scrubby red oak. Be careful of glass and debris in this gravel pit. ILSG 2011 67 Field Trip 3 �Stop 5A Stop 5B Figure 19. Topographic map showing location of Stop 5 area (U.S.G.S., 1964a). Go back down Settlement Dump Road and go right (west) on Star Route Road. In 0.25 miles stop on the flat stretch where the road turns from west to northwest. You are now on a beach of Glacial Lake Duluth – stop 5B. Stop 5B: Glacial Lake Duluth Beach Terrace Location: On Star Route Road; UTM coordinates: 0647828 mE, 5180050 mN; T50N, R5W, Sec. 19, north-central (Fig. 19). There is a distinct terrace at an elevation of 1100 feet throughout this part of the southern Lake Superior region, and is especially well defined here on the Bayfield Peninsula. This beach terrace was created 9500 BP when Lake Duluth was at its highest level (Fig. 20). The terrace at 1100 feet is a distinct step in the landscape and an important transition zone from very sandy sediments on and above the beach level to finer sediments on the slopes below. The sandy outwash seen in stop 5A underlies all areas above the beach – deep deposits of gravelly sand with no silt or clay. The beach terrace goes all the way around this hill, indicating that at one time it was an island within Glacial Lake Duluth. Continue northwest on Star Route Road back toward Siskiwit Falls. In 0.7 miles, turn left (west) on Mountain Road and travel two miles to County Road C. Turn south on County Road C and go 3.5 miles to North Boundary Road. Turn left (east) on North Boundary Road East, which is also labeled FR 439, and head toward the Bayfield County Sportsman‘s Club. Stop 7 is one mile east on this road. ILSG 2011 68 Field Trip 3 �Figure 20. Cross section of a beach terrace. Letter a represents the lake level, b is the outer edge of the wave-built terrace, and c is the base of the wave-built terrace From Clayton, 1984. Stop 6: Pitted Outwash Plain Location: North Boundary Road East; UTM coordinates: 0647828 mE, 5180050 mN; T50N, R6W, Sec 36, south-central (Fig. 21). Stop 6 67 Figure 21. Topographic map of Stop 6 area (U.S.G.S., 1964a). Areas above the 1100-foot beaches have a variety of landforms including outwash plains, eskers, outflow channels, and kettles of all sizes. These features were created during melting of the ice of the Airport Advance, the last glacial advance to cover the high interior of the Bayfield Peninsula, about 12,300 BP (Clayton, 1984). The large flat outwash plain, at an elevation of 1250-1280 feet, in the middle of the Peninsula is the Valhalla Surface. ILSG 2011 69 Field Trip 3 �Numerous large blocks of ice were left behind in some areas as the ice margin retreated, and subsequently the blocks were buried in thick deposits of sandy outwash. As the ice blocks melted, kettles formed in the otherwise flat outwash plain. These pits are especially abundant in Sections 30, 31, and 36 (T50N, R6W), some up to 180 feet deep and 3000 feet across with very steep slopes (Figure 22). Here at stop 6 is a relatively small pit, the southern-most kettle in a large kettle field. Just to the east is a road that goes north into the heart of the kettle field if time permits, we will walk the road north into the kettle field. The absence of surface water here is very revealing about the high permeability of the sands of the Copper Falls Formation. There are no rivers in this high interior part of the Bayfield Peninsula because all water infiltrates and is drained by groundwater. The presence of kettles nearly 200 feet deep that are dry on the bottom is evidence of the great depth of the water table. Return to County Road C and turn south. Travel 7.5 miles to Church Corner Road and turn right (south). In 0.5 miles stop at the crest of the hill. This is stop 8. Stop 7: Beach Terrace and Former Bay Location: Church Corner Road; UTM coordinates: 0653786 mE, 5173208 mN; T49N, R5W, Sec. 27, west-central (Fig. 22). This is private land so do not explore far off the road without seeking permission from land owners. Stop 7 Figure 22. Topographic map of Stop 7 area (U.S.G.S., 1964b). This stop is on the terrace of the highest Lake Duluth beach, the 1100-foot beach level visited on stop 5B. This terrace marks the highest and longest lived lake level of the numerous lake levels that existed in the Superior Basin during deglaciation, thus it is the topographically most distinct terrace and the only one which has significant sandy beach deposits (Farrand, 1969). Below this terrace the topography was planed by waves of at least ILSG 2011 70 Field Trip 3 �five short-lived lower lake levels, so lower beaches are less well defined. On the east side of the road, there is an exposure of pebbly sand typically present on this terrace. The sand of this beach is now used as a resource for road construction and other uses, and several sand pits exist along contours of this hill. The view to the north is of the Fourmile Creek valley and the Bayview ridge beyond. At the time that the Glacial Lake Duluth shoreline was here, about 9500 bp, this was a large bay closing to the west. The hydrology changes dramatically below 1100 feet, where the sand of the Copper Falls Formation is overlain by silt and clay of the Miller Creek Formation. There is typically a transition zone between these two areas, below which fine sediments and soils prevent much infiltration of precipitation. As a result, nearly all the rivers on the Bayfield Peninsula start at the edge of the fine sediments just below an elevation of 1000 feet. This stop provides an excellent view of the transition zone where the sediment changes from sand on the hill slopes to mud on the bottom of the valley. The transition is rapid on steep slopes such as this one, with clay-rich sediments of the Miller Creek Formation dominating below elevations of about 950 feet. At that elevation the hydrology and soils change, so there is a corresponding change in forest types and ecosystems, from the dry jack pine and red oak forests above to wet-tolerant species at lower elevations. Although the differences in the forest types are much less pronounced today than they were prior to the forests being cut over in the late 19th century, the differences are still apparent. At the time of first settlement, the ―Lake Superior clay plain‖ was occupied by spruce, fir, and cedar, whereas today it is primarily red maple, aspen, and birch. Travel 0.5 mile and turn left (east) on Maple Hill Road. Travel 1 mile to County Road C and turn right (east) toward Washburn. In 2 miles, turn left (north) on Big Rock Road. Travel 1.5 miles to Big Rock Park and stop at the parking lot just north of the bridge over Sioux River -- this is stop 9. Stop 8: Chequamegon Sandstone and Miller Creek Formation Location: The parking lot at Big Rock Park; UTM coordinates: 0658541 mE, 5174862 mN; T49N, R5W, Sec. 24, east-central (Fig. 23) Figure 23. Topographic map of Big Rock Park area at Stop 8 (U.S.G.S., 1964b). Stop 8 ILSG 2011 71 Field Trip 3 �Stop 8A: Chequamegon Sandstone Location: The Sioux River valley downstream of Big Rock Park; UTM coordinates: 0658541 mE, 5175068 mN (Fig. 23). From the parking lot hike north on the trail along the Sioux River for about ¼ mile to two long outcrops of Chequamegon Sandstone. The northern outcrop is located at 0658541 mE, 5175068 mN. Pleistocene deposits have been eroded away in the Sioux River valley to expose the Chequamegon Sandstone, the stratigraphically highest formation in the Bayfield Group. The Chequamegon Sandstone here is ared feldspathic sandstone that has enough clay on weathered surfaces to give it a chalky white appearance. It is poorly cemented and friable, so it is not very resistant to erosion. Although it is composed primarily of sand-sized clasts, there are sparse quartz pebbles and some mud chips; the chips are soft red mud, which easily erodes out of the sandstone, leaving deep holes in the surface of the outcrop. Some of the mud chips in this outcrop are up to 25cm in length. Bedding here is horizontal and has large cross beds 0.5 to 1m in height, with dip of cross strata indicating the flow was in a northerly direction, similar to paleocurrent indicators throughout the Bayfield Group. Some of the bedding is disturbed by various soft-sediment deformation features, including at least one excellent ball-and-pillow structure exposed in the southern outcrop. The sediment along the base of the cliff is sand derived from weathering of the friable sandstone immediately above. However, about 2m away from the cliff face the sediment is hard sandy mud. Most of the Sioux River valley is filled with this red mud – sediment typical of the Miller Creek Formation. The presence of Miller Creek mud deep within a bedrock gorge indicates that the gorge was carved into the sandstone prior to the deposition of the mud. Thus, it is likely that the Sioux River gorge was carved during a Pleistocene interglacial time, then filled with mud during deposition of the late Pleistocene Miller Creek Formation, then re-excavated to create the river valley present today. This situation is similar to that of Fish Creek west of Ashland, and Sand River on the northern side of the Bayfield Peninsula (Nuhfer, 2004). Hike back out the trail, past the parking lot, south along the road to the banks on the east side of Big Rock Road – this is stop 9B. Stop 8B: Fine mud of the Miller Creek Formation Location: East side of Big Rock Road, just south of the bridge over Sioux River. UTM coordinates: 0658570 mE, 5174755 mN (Fig. 23). The bank is composed of red mud of the Miller Creek Formation. The Bayfield County Highway Department has attempted to stabilize the bank, with moderate success; the clay of the Miller Creek Formation is notorious for its instability, especially where oversteepened by erosion or excavation. The mud here in the Sioux River valley is especially fine grained for the region, consisting of approximately 60% clay, 35% silt, and 5% sand, with few, if any, pebbles. Although no laminations have been seen in this exposure, there are excellent examples of horizontal, undeformed, thin laminations in the mud upstream along west side of the Fourmile Creek valley (just downstream of the Hwy C bridge). ILSG 2011 72 Field Trip 3 �Drive south on Big Rock Road, back to Hwy C. Go left (east) on C into Washburn. At the intersection of C and Hwy 13, go straight across the intersection (south) onto South 8th Avenue West. In about 0.5 mile enter Thompson‘s West End Park. The artesian well is on the right (west) as you first enter the park – stop 10. Stop 9: Flowing Artesian Well Location: Thompson‘s West End Park in Washburn; UTM coordinates: 0660194 mE, 5170054 mN; T48N, R4W, Sec. 6, SE1/4 (Fig. 24). Stop 10 Stop 9 Figure 24. Topographic map of Washburn and stops 9 and 10 (U.S.G.S., 1964b). The fine-grained sediments of the Miller Creek Formation create a confining layer over more-permeable sediments of the Copper Falls Formation. The very permeable sands of the Copper Falls Formation exposed at the surface in the middle of the Bayfield Peninsula – areas above the 1100-foot-elevation beaches – serve as the recharge area for this confined aquifer. This situation creates high enough artesian pressure that the piezometric surface is above the land surface and flowing wells can exist. This hydrogeologic condition exists all around Chequamegon Bay, and there are numerous flowing artesian wells in the area. There are also natural springs, although none near here that have high flow rates. The artesian wells are popular sources of drinking water, and often there is a line of people waiting to fill their bottles. This is the historic Sprague Well: drilled in 1903, it was the first artesian well in Bayfield County; it is 119 feet deep and currently produces 43 gallons per minute (gpm). ILSG 2011 73 Field Trip 3 �The water has a constant temperature of 45o F and total dissolved solids concentration of 104 ppm. The interpretive sign at the well says that it originally produced 224 gpm from a fourinch pipe and was ―so free of minerals that it was piped directly into the sawmill boilers.‖ Return to Hwy 13 and turn left (east). Go east on Hwy 13 to downtown Washburn, then go right (southeast) on Central Avenue toward Lake Superior. Just before the town dock there is a parking lot on the left (north) and a small beach – stop 10. Stop 10: View of Chequamegon Bay Location: Washburn beach, near the town dock; UTM coordinates: 0661718 mE, 5170422 mN; T49N, R5W, Sec. 24, NE ¼ (Figs.24, 25) Long Island Washburn Oak Point Figure 25. Satellite image of Chequamegon Bay (image from UW SSEC and Wisconsin View). From the beach you can look out over Chequamegon Bay, which has served as a safe harbor to vessels on Lake Superior for centuries because it is so well protected from the wind ILSG 2011 74 Field Trip 3 �and waves of the open lake. This protection is provided by the Apostle Islands and Bayfield Peninsula to the north, and Long Island to the northeast (Figure 25). Long Island is a large barrier spit that has been created by deposition of sediment carried by longshore transport from the southeast. The shore of Lake Superior southeast of the bay has high bluffs of Miller Creek Formation which supply large volumes of sediment to longshore currents. The spit has grown since the lake level stabilized at its current level about 2000 years ago. Smaller spits first grew in the area where Oak Point is located today, and when the White River captured the Bad River 800 bp, the sediment supply increased, eventually leading to the development of the spit in its current position (Meeker, unpublished data). The power of the waves continues to deliver sediment and reshape the spit today. In historic times, it had been a series of unconnected barrier islands, thus its name Long Island (Bona, 1990; Collie, 1901). But deposition and erosion has caused it to alternate between being a barrier spit connect to the mainland, to being a barrier island. The last time it changed dramatically was in November 1975 when a large storm, the same storm that sank the Edmond Fitzgerald, caused deposition of enough sand to connect the sand bars into the large barrier spit present today. REFERENCES Adamson. Kent F., 1997, Petrology, Stratigraphy, and sedimentation of the Middle Proterozoic Bayfield Group, Northwestern Wisconsin, Unpublished M.S. Thesis, University of Minnesota Duluth, 203 p. Behrendt, J.C., Green. A.G., Cannon, W.F., Hutchinson, D.R., Lee, M.W., Milkereit, B., Agena, W.F., and Spencer, C., 1988, Crustal structure of the Midcontinent rift system: Results from GLIMPCE deep seismic reflection profiles: Geology v. 16, p. 81-85. Bona, L.J 1990, Geomorphology and Holocene geologic history of Long Island, Apostle Islands National Lakeshore, Wisconsin [abs.]: Geological Society of America, Abstracts with Programs, v. 22, no. 5, p. 4. Cannon, W.F., et al, 1989, The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305-332 Clayton, L., 1984, Pleistocene geology of the Superior Region, Wisconsin, Informational Circular 46, Wisconsin Geological and Natural History Survey, Madison, Wisconsin, p. 40. Clayton, L., 1985, Pleistocene geology of the Superior Region, Wisconsin: Wisconsin Geological and Natural History Survey, scale 1:250 000, 1 sheet. Clayton, L., Attig, J.W., Mickleson, D.M., Johnson, M.D., and Syverson, K.M., 2006, Glaciation of Wisconsin, Wisconsin Geological and Natural History Survey, Madison, Wisconsin, Educational Series 36, Third Edition. Collie, G.L., 1901, Wisconsin shore of Lake Superior: Geological Society of America Bulletin, v. 12, p. 197 – 216. Craddock, C., 1972, Regional geologic setting, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, p. 281291. ILSG 2011 75 Field Trip 3 �Daniels, P.A., 1982, Upper Precambrian sedimentary rocks: Oronto Group, MichiganWisconsin,in Wold, R.J. and Hinze, W.J., Geology and Tectonics ofthe Lake Superior Basin, Geological Society of America Memoir 156, p. 107-133. Davis, D.P and Paces, J.B., 1990, Time resolution of geological events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift System: Earth and Planetary Science letters, v. 97, p. 54-64. Dickas, A.B., 1986, Seismological analysis of arrested stage development of the Midcontinent Rift: Geoscience Wisconsin, v. 11, p. 45-562. Dickas, A.B, 1989, Lake Superior Basin segment of the Midcontinent Rift System, 28th International Congress Field Trip Guidebook T344, 62p. Dott, R.H. Jr., and Attig, J.H., 2004, Roadside Geology of Wisconsin, Mountain Press Publishing company, Missoula, MT, 345 p. Elmore, R.D., Milavec, G.J., Imbus, S.W., and Engel, M.H., 1989, The Precambrian Nonesuch Formation of the North American Mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition: Precambrian Research, v. 43, p. 191-213. Farrand, W.R., 1969, The Quaternary history of Lake Superior: Proceedings of the 12th Annual Conference on Great Lakes Research, p. 181-197. Galston, Lynn, and Havholm, K.G., 2008, Re-evaluation of the Proterozoic Devils Island Sandstone, Keweenawan Rift, Northern Wisconsin (Abs.), North-Central Section 42nd Annual Meeting 24-25 April, 2008, Geological Society of America Abstracts with Programs, v. 40, No. 5, p. 76. Green, J.C., 1995, Volcanic rocks of the Midcontinent rift system: A review, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Basement Tectonics 10, Kluwer Academic Publishers, Dordrecht/Boston/London, p. 65-67. Hamblin, W. K. 1958, Cambrian sandstones of northern Michigan, Michigan Geological Survey Publication 57, 149 p. Johnson. A.D., Beaster, K.F., Kohn,J.D., and Havholm, K.G., 2001, Braided-stream/eolian environment of Proterozoic Hinckley Sandstone, Keweenawan rift, East-Central Minnesota (Abs.), Geological society of America Annual Meeting, Nov. 5-8. 2001. Kalliokoski, J., 1982, Jacobsville Sandstone, in Wold, R.J. and Hinze, W.J., eds., Geology and Tectonics of the Lake Superior Basin. Geological Society of America Memoir 156, p. 147-155. Meeker, J., unpublished data: Northland College, Ashland, WI. Morey, G.B. and Ojakangas, R.W., 1982, Keweenawan sedimentary rocks of eastern Minnesota and northwestern Wisconsin, in Wold, R.J., and Hinze, W.J., eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America Memoir 156, p. 135-146. Morey, G.B. and Van Schmus, W.R., 1988, Correlation of Precambrian rocks of the Lake Superior Region, United States, in Harrrison, Jack and Peterman, Zell, eds., U. S. Geological Survey Professional Paper 1241-F, 31p. Nicholson, S. W., Cannon, W.F., Woodruff, L. G., and Dicken, C.L., 2006, Bedrock Geologic map of the Port Wing, Solon Springs, and part of the Duluth and Sandstone 30‘ X 60‘ quadrangles, Wisconsin and Minnesota.: U.S. Geological Survey, Scientific Investigations Map 2869. ILSG 2011 76 Field Trip 3 �Nuhfer, E.B., 2004, A Guidebook to the Geology of Lake Superior‘s Apostle Islands National Lakeshore and nearby areas of the Bayfield Peninsula of Wisconsin, with poetry by Mary P. Dalles: Eastern National, Fort Washington, PA, 141 p. Ojakangas, R.W., 1986, Reservoir characteristics of the Keweenawan Supergroup, Lake Superior Region: Geoscience Wisconsin, v. 11, p. 25-31. Ojakangas, R. W. and Matsch, C. L., 1982, Minnesota‘s Geology: University of Minnesota Press, Minneapolis, Minnesota, 255 p. Ojakangas, R.W., and Morey, G.B., 1982, Keweenawan sedimentary rocks of the Lake Superior Region: a summary, in Wold, R.J. and Hinze, W.J., eds., Geology and Tectonics of the Lake Superior Basin, Geological society of America Memoir 156, p. 157-164. Ostrom, M.E., 1967, Paleozoic stratigraphic nomenclature of Wisconsin: Wisconsin Geological and natural History Survey Information circular 8. Chart and txt. Palmer, H.C. and Davis, D.W., 1987, Paleomagnetism and U-Pb geochronology of volcanic rocks from Michipicoten Island, Lake Superior, Canada: Precise correlation of the Keweenawan polar wander track: Precambrian Research. v. 37, p. 157-171. Shumway, JR., and Iverson, NR., 2008, Magnetic fabrics of the Douglas Till of the Superior lobe: exploring bed-deformation kinematics, Quaternary Science Reviews, Elsevier, p. 107-119. Suszek, Thomas, 1995, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, Western Lake Superior Region: Midcontinent Rift System, in Ojakangas, R.W., Dickas, A.B.,, and Green, J.C., eds., Basement Tectonics 10: Kluwer Academic Publishers, Dordrecht/Boston/London., p. 25-28. Syverson, K.M. and Colgan, P.M., 2004, The Quaternary of Wisconsin: a review of Stratigraphy and glaciation history, in: Ehlers, J. & Gibbard, P.L. (eds.), Quaternary Glaciations -- Extent and Chronology, Part II: North America, Amsterdam, Elsevier Publishing, 295-311. Thwaites, F.T., 1912, Sandstones of the Wisconsin coast of Lake Superior: Wisconsin Geological and Natural History Survey Bulletin 25, 117 p. Tryhorn, A.D., and Ojakangas, R.W., 1972, Sedimentation and petrology of the Upper Precambrian Hinckley Sandstone of north-central Minnesota: in Sims, P.K, and Morey, G.B., eds., Geology of Minnesota: A Centennial Volume, Minnesota Geological Survey, p. 431-435. U.S. Geological Survey, 1964a, Cornucopia quadrangle, 7.5-minute series topographic map: scale 1:24,000. U.S. Geological Survey, 1964b, Washburn quadrangle, 7.5-minute series topographic map: scale 1:24,000. Watts D.R., 1981. Paleomagnetism of the Fond du Lac Formation and the Eileen and Middle River sections with implications for Keweenawan tectonics and the and the Grenville problem: Canadian Journal of Earth Sciences v. 18, p. 829-841. ILSG 2011 77 Field Trip 3 �ILSG 2011 78 Field Trip 3 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 4 GEOLOGY AND REMEDIATION AT THE ASHLAND/NORTHERN STATES POWER SITE Ashland Lakefront circa 1900 ILSG 2011 79 Field Trip 4 �ILSG 2011 80 Field Trip 4 �Field Trip 4 Ashland/Northern States Power Company Lakefront Superfund Site James R. Dunn1 1 Hydrogeologist, Wisconsin Department of Natural Resources E-mail: james.dunn@wisconsin.gov As with many cities of the late 1800‘s, Ashland Wisconsin‘s lights and heat were supplied by manufactured gas. From 1887 into the 1940‘s a manufactured gas plant (MGP) operated under various owners, making gas and producing wastes. The Ashland MGP produced mainly carbureted water gas from feed stocks of coal and oil. MGP wastes included oil tars (coal tar) and wastewater as well as boiler ash, retort brick, and purifier box wastes. The City of Ashland sits on the shores of Lake Superior‘s Chequamegon Bay. The MGP was located less than a block from the historic shoreline transected by a ravine that discharged to the bay. Over time the ravine was filled with wastes from the MGP and the lakebed area was filled mainly with wood waste from a series of saw mills that operated up to 1930. Regional geology in the Ashland area consists of unconsolidated glacial deposits overlying Precambrian aged sedimentary bedrock. Unconsolidated deposits consist of the Miller Creek Formation, which overlies the Copper Falls Formation. Precambrian-aged sandstone of the Oronto Group, is the uppermost bedrock unit in the Ashland area. Thickness of the sandstone unit has not been determined. The Oronto sandstone is most likely underlain by Precambrian basalt. Surficial soils in the Ashland area are underlain by red clay and silt deposits of the Miller Creek Formation, the predominant surficial lithologic unit in the Ashland area. The Miller Creek Formation is a fine-grained clayey silt to silty clay. It consists of lacustrine deposits and glacial till deposited during the last major advance of glacial ice. The thickness of the Miller Creek Formation in the Ashland area ranges from approximately 15 to 50 feet based on local well logs. The Miller Creek Formation encountered at the Site consists of clays and silts and range in thickness from 5.5 to 40 feet. The thinnest portion of the Miller Creek Formation is found at the base of the bluff where the former lake shoreline was located. This thin zone likely resulted from wave erosion of the Miller Creek Formation, forming the ―bench‖ shaped bluff and former lake shoreline. Data from soil borings and downhole geophysics confirm that the Miller Creek Formation thickens to the north of the bluff. Sand and gravel layers interbedded with silty clay lenses have been encountered near the contact of the Miller Creek Formation and the underlying Copper Falls Formation. The Copper Falls Formation consists of interbedded glacial clays, sands, and gravels. In addition to unconsolidated glacial deposits, fill soil units are present beneath the upper bluff area and Kreher Park. Fill soil encountered in the upper bluff area was placed in a former ravine that dissected the Miller Creek Formation in the vicinity of the former MGP ILSG 2011 81 Field Trip 4 �facility. Historic documents indicate that the ravine was filled by 1909. Kreher Park consists of fill material that was used to fill the former lakebed. Historic documents indicate that the former lakebed was filled beginning in the late 1800‘s, and continued to be altered by filling activities until the 1950‘s. The ravine fill unit consists of silty clay fill material mixed with solid and liquid MGP wastes, ash, cinders, slag, and fragments of concrete and brick, glass bottles, scrap steel, and wire. Fill material at Kreher Park consists predominantly of silty clay soil mixed with rocks, bricks, concrete, wood, and miscellaneous debris. A buried clay berm is also located along the shoreline on the northeast side of the Site near the former wastewater treatment plant WWTP. This fill unit ranges in thickness from 3 to 7 feet. A wood waste layer consisting of wood planks was encountered beneath the fill soil. Native soil underlying the fill unit includes a beach sand layer (predating filling of the lakebed) that ranges in thickness from 0 to 5.5 feet at Kreher Park. The Miller Creek Formation was encountered below the fill and beach sand and ranges in thickness from 7 feet at the MW-7 well nest to 40 feet at the MW-26 well nest. Offshore geology of the Chequamegon Bay inlet area consists of a discontinuous layer of submerged wood chips and debris of varying size on the lake bottom underlain by variably fine to medium grained sediments. The sediments are underlain by silts and clays of the Miller Creek Formation. The Copper Falls Formation was not encountered during investigation of the offshore sediments. Consequently, the thickness of the Miller Creek Formation below the bay is unknown. Regional hydrogeologic units correspond to regional geologic units. The three aquifers that occur in the Lake Superior Basin in the vicinity of Ashland include the following: • The Pleistocene sand and gravel aquifer (Copper Falls Formation); • The Precambrian sandstone aquifer (Oronto Formation); and • The Precambrian basalt aquifer. The Copper Falls aquifer occurs between 25 to 55 feet below ground surface in the Ashland area. Based on deep piezeometer boring information, the Copper Falls aquifer is over 150 feet thick Sandy till units within the aquifer yield low volumes of water (5 to 10 gallons per minute gpm), while sand and gravel lenses can yield up to 100 gpm. The Copper Falls aquifer is confined by the overlying Miller Creek Formation near the Chequamegon Bay shoreline where the Miller Creek Formation has a higher plasticity and clay content. The low permeability Miller Creek Formation functions as an aquitard for the underlying Copper Falls aquifer in this area. Based on a series of investigations of the site, MGP wastes including free product oils and tars are present within the filled ravine, Miller Creek Formation, Copper Falls Formation, Kreher Park (filled lakebed) fills, and approximately 17 acres of Chequamegon Bay. The contaminants within the sediments manifest themselves as slicks during windy conditions. A pilot test, pump and treat system has operated in the Copper Falls Formation since 2001. In 10 years of operation over 11,000 gallons of oil/tar free product has been collected and over 2 million gallons of contaminated groundwater treated. ILSG 2011 82 Field Trip 4 �In consultation with the Wisconsin Department of Natural Resources, the USEPA has approved the Record of Decision for the remediation of contaminated soil, sediment and groundwater at the Ashland/NSP Superfund site. EPA‘s final cleanup plan includes: Removing soil from the most contaminated areas of Kreher Park and the Upper Bluff/Filled Ravine that overlooks the park area, treating the soil on-site and reusing it after treatment. Using barriers to contain and stop the movement of contaminants in ground water, treating the ground water in-place, and adding wells to extract and treat ground water. Digging up wood waste and contaminated sediment near and along the shore of Chequamegon Bay and dredging contaminated sediment in the bay, covering excavated and dredged sections nearshore and offshore with six inches of clean material, and treating contaminated sediment after removal. The estimated cost for this cleanup plan is between $84 million and $98 million. Photo 1: Product slick on Chequamegon Bay, Ashland, Wisconsin.Photo by Ed Monroe ILSG 2011 83 Field Trip 4 �Photo 2: Product slick on Chequamegon Bay, Ashland, Wisconsin.Photo by Ed Monroe REFERENCES URS, 2007, Remedial Investigation Report – Ashland/Northern States Power Lakefront Superfund Site USEPA, 2010, Record of Decision, Ashland/Northern States Power Lakefront Superfund Site ILSG 2011 84 Field Trip 4 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 5 BAD RIVER WATERSHED CULVERT RESTORATION PROGRAM Hagger Road west culvert – before replacement. -T. Fitz ILSG 2011 85 Field Trip 5 �ILSG 2011 86 Field Trip 5 �Field Trip 5 Bad River Watershed Culvert Restoration Program Michele Wheeler1 and Cassandra Bodette2 1 Bad River Watershed Association P.O. Box 875 Ashland WI 54806 2 Northland College 1411 Ellis Ave, Ashland WI 54806 INTRODUCTION The Bad River watershed (BRW), a sparsely populated, largely forested area of northern Wisconsin, covers 700,000 acres in the Lake Superior basin. It is one of the largest watersheds in the Great Lakes basin. The BRW encompasses parts of Ashland, Bayfield and Iron Counties, draining approximately 1,094 square miles. With 1,140 miles of streams, the BRW supports one of the most diverse fish assemblages in the Lake Superior basin. Watershed Geology The BRW is divided into three primary drainage and topographic zones: the upper basin, transition zone and clay plain (Figure 1). Figure 1. General watershed zones in the Bad River system The upper basin is characterized by sandy glacial till with frequent rock outcroppings and a poorly developed drainage network with no valleys (Fitzpatrick, 2005). The lower portion of the watershed is known as the Lake Superior clay plain. This area is dominated by ILSG 2011 87 Field Trip 5 �clay till soils deposited across the landscape by glaciers and glacial Lake Duluth. Streams are often found in entrenched, alluvial valleys in this zone. The middle portion of the watershed that separates these two zones is known as the transition zone. This area is characterized by shallow sandy soils, as well as mixed sand and clay soils. The topography is characterized by steep unstable stream banks, entrenched valleys with steep ravines, with many springs and seeps (Fitzpatrick, 2005). Culvert Program Description Background When culverts are employed to cross streams with roads, fish habitat may be significantly compromised in at least two ways. First, culverts can indirectly reduce habitat quality by accelerating sedimentation. Sediment can come from roadside ditches that empty directly into stream channels, in stream erosion associated with high water velocities, and bank erosion caused by misaligned crossings. In addition to these regular, smaller scale inputs, major crossing failures deliver large volumes of sediment into waterways. As a result of sediment loading from ―blow outs,‖ stream channels can gradually fill in, further affecting fish habitat. As the stream substrate becomes covered with sand and fine sediment, both food and water quality can be diminished, and spawning gravels can be covered. The result can affect the growth and survival of fish. In addition, the Bad River watershed is the largest producer of sediment into Lake Superior from Wisconsin waters (Figure 2), largely as the result of non-point sources like failed crossings. Figure 2. Mouth of the Bad River, and a destroyed culvert after a blowout event. Second, culverts may block the ability of fish to move up and downstream. In this case, juvenile fish may not be able to reach food or find cover from predators in critical rearing habitats. Adult fish may not be able to reach spawning habitats. Fish may not be able ILSG 2011 88 Field Trip 5 �to retreat to tributary streams during floods or find relief in cold water during the heat of summer. Consistent with the mission of the United States Fish and Wildlife Service (USFWS) and the Bad River Watershed Association (BRWA) to maintain the integrity of the Bad River Watershed, the culvert project was initiated to build community support and capacity to address problems at road stream crossings. The Culvert Program includes inventory, prioritization, coordination, restoration and monitoring elements in the overall goals to restore naturally functioning stream channels in the Bad River watershed. Inventory Inventory protocols were developed to identify sites that are likely fish barriers and/or significant contributors of sediment passage problems. Protocols included recording dimensions of the culvert, measurements from the stream channel, and conditions at the crossing that reflect how the stream and culvert interact. Partners throughout the region combined efforts to conduct surveys with local citizen volunteers, Ashland and Iron counties, US Forest Service, Northland College and the Bad River tribe contributing to survey efforts. To date, 996 of the 1,154 crossings in the watershed have been inventoried. Prioritization Site specific criteria have been developed for both fish passage and sedimentation concerns. Existing fisheries criteria include a drop greater than 6 inches and in-pipe velocity greater than 1 foot/second. Existing sedimentation criteria include sites with steep, unvegetated and eroding embankments. Erosion of stream banks above and below crossings is use to estimate alignment issues. Town road crew knowledge of eroding or regularly washing out sites was also considered as a sedimentation criteria. Results are summarized into this Needs Assessment that outlines problem sites by sub basin. The Needs Assessment shows that blockages to fish migration and sedimentation problems at road stream crossing are widespread and identifies sites with the most significant problems. There are 164 crossings on the priority list that were identified using these criteria, 69 of which are on trout streams. Strategic Plan The primary purpose of this document is to provide local road and fish managers with information that can be used as a planning tool for crossing replacements. Restoration In the past 5 years, about 10 culvert replacement projects in the watershed have been designed and funded by fishery interests to ensure proper fish passage and to reduce excess sediment inputs into the system. Monitoring Assumptions have been made about the benefits of these projects, based on past experience, design standards and scientific literature: More fish pass through the culverts after replacement than before and this is a positive thing for fishery populations and fish communities. Erosion from sites has decreased immediately and in the long-term. ILSG 2011 89 Field Trip 5 �Sediment from upstream head-cuts (created when new culverts are placed into channel beds rather than perched above them) move effectively through the culvert and are carried downstream, improving upstream and within-culvert habitat. Mobilized sediment from culvert replacement is not negatively impacting downstream habitat in gentle-sloped channels or coastal wetlands, such as the Bad River Slough, even though the influence of excess sediment load to downstream habitat quality is not known. The BRWA and USFWS led an effort to test these assumptions by initiating a monitoring program of culvert replacements in the Bad River watershed. Companion fish, habitat, and channel stability protocols were being developed as a result of a coordinated monitoring workshop held in December 2008. The purpose of the workshop was to identify monitoring protocols that can be used to evaluate how well restoration work in the BRW is achieving program goals. At the workshop, participants (listed in Appendix A) developed specific recommendations for Culvert Program monitoring in the Bad River watershed. These recommendations included objectives, critical questions to be answered, parameters, and metrics for both physical and biological aspects of a monitoring program for the Culvert Program. Specific protocols followed are provided in Appendix B. Monitoring Objectives and Questions Objective 1: Reconnect artificially fragmented stream channels. Question: Have we restored fish passage? Objective 2: Determine changes in species assemblages associated with culvert replacements. Question: Are there differences in species richness between upstream and downstream reaches of culverts prior to restoration? Question: Are there differences between treatment and references reaches prior to restoration? Question: Are treatment reaches more similar to reference reaches post restoration? Objective 3: Restore or improve instream habitat & prevent accelerated erosion and sedimentation Questions: Has the channel morphology, slope, and sediment characteristics improved or restored (relative to reference reach), upstream, downstream and within the culvert and does this result in quality habitat? This question relates to the following sub-questions: Is there deposition of fine sediment and bar formation? Is the composition of streambed sediment appropriate for biotic health? Is there habitat complexity in the channel geomorphic features? (riffles/pools) Can bedload move appropriately through the culvert? Is the stream aggrading/degrading or widening/narrowing? Is the channel morphology similar to a preferred reference reach? Are the banks eroding? ILSG 2011 90 Field Trip 5 �Education Although many culverts that pose fisheries and sedimentation problems have been replaced in the Bad River watershed by natural resource professionals, road managers from towns, counties and state agencies are involved in a far greater number of replacements each year. Investment by road managers in fish friendly culvert installation techniques is essential to adequately restore connectivity and reduce sedimentation in the watershed. Therefore, the BRWA has invested in education and outreach activities to transportation managers to encourage consideration of stream channel integrity as much as road maintenance at road stream crossings. Thanks to BRWA and partner sponsored events, elected officials and/or road crew members from all 15 townships in the watershed have attended fish friendly culvert installation trainings. BRWA staff have prepared township specific summaries of culvert inventory data, and have worked with townships to strategically plan culvert replacements. Outreach sheets summarizing problems at existing culverts, restoration solutions and monitoring results have been prepared and distributed to all streamside landowners and town officials. Regular press releases and articles in local newsletters inform the general public of restoration efforts to conserve fish and their habitats, building public support for future restoration projects. FIELD TRIP STOPS We will be visiting three to four sites within the transition zone of the Bad River Watershed. At each stop, we will describe conditions at the crossing prior to restoration and compare the 3 different treatment techniques. Stop 1: Baffled culverton a Marengo Tributary Location: Hager Road West - Latitude 46.398877 N Long. 90.931833 W (Fig. 3) Description: The shallow, fast-moving water and drop at the culvert exit prevented fish from moving upstream. If a new pipe was set sufficiently low enough to eliminate the drop, the stream would have headcut substantially. Therefore the new culvert was installed at a 3.75% slope (based on natural channel conditions) with baffles throughout the pipe to prevent a velocity barrier (Figs. 4 and 5). !( 2011 Troutmere Cr. Restoration Site Transition Zone Control Site 2010 Hager Rd Restoration sites Transistion Zone streams Perennial Streams Intermittent Streams !( Troutmere Creek Control site Hager Road West ILSG 2011 Hager Road East 91 Figure 3. Map showing locations of Hagar Road and Troutmere Creek sites. Field Trip 5 �Figure 4. Hagar Road west culvert; the photo on the left shows the old culvert before replacement, the photo on the right shows the new culvert. Before restoration Figure 5. Longitudinal profile of the unnamed tributary of the Marengo River showing the stream profile before and after replacement of the Hagar Road west culvert. ILSG 2011 92 Field Trip 5 �Stop 2: Culvert Replacement on a Marengo Tributary Location: Hager Road East -Latitude 46.399013 N Longitude 90.925979 W Description: The culvert was old and needed to be replaced (Figure 6). There was no sand, gravel, or rock within the culvert to mimic a natural stream bottom. The shallow, fast moving water and plunge at the culvert exit prevented smaller fish from moving upstream. The old culvert was replaced with a new, larger culvert set into the stream bottom to simulate a natural channel. Larger culverts prevent erosion by allowing water to pass through during high flow events. Larger culverts also slow the water down, making it easier for fish to swim up through the pipe. Figure 6. The Hagar Road east culvert before replacement (top photo), and after replacement (bottom photo). ILSG 2011 93 Field Trip 5 �Stop 3: Raising the grade at Troutmere Creek Location: County C - Latitude 46.406291 N Longitude 90.911559 W Description: The pipe was in pretty good shape at this crossing (Figure 7). The 10inch drop at the end of the culvert prevented many fish from accessing the 2 miles of stream above. Instead of replacing the culvert, the stream channel will be built up by placing large boulders in the stream below the culvert. This will cause sediment to fill in around the boulders, raising the stream bed and eliminating the 10-inch drop at the outlet of the culvert. It is anticipated that the boulders and sediment will form a terrace-like stream bottom similar to a staircase. Figure 7. Culvert at Troutmere Creek. Stop 4 (Time allowing): Control for Hager Road sites Location: Midway (County Line) Road; Lat. 46.406302 N Long. 90.916723 W Description: This site is a control site for transition zone restoration sites. Discussion Evaluation of the monitoring results calls for clarification and refinement of what we intend to achieve with our Culvert Program. Overall, monitoring set out to characterize site scale and broad scale population response to culvert replacement restoration. Movement protocols served to establish if passage was restored and if habitat was affected (site scale), and fish community metrics served to provide insights to population response (catchment scale). For the later to be effective, a better understanding of current fisheries habitat, distribution, status and limiting factors is required. In addition, a desired outcome, or fish population target, that is agreed upon among agencies is paramount to success. At the outset of the program, natural resource managers recognized that these building blocks of effective management are currently lacking for the watershed. Monitoring protocols intended to improve understanding of population response to restoration given these constraints. Recommendations for the monitoring program are provided for watershed ILSG 2011 94 Field Trip 5 �scale context in evaluating restoration projects (catchment scale) and specifically for future culvert restoration projects (site scale). Recommendation for monitoring at catchment level Increase examination of habitat-fisheries linkages Improve catchment level assessments as context for restoration benefits Continue fisheries monitoring to examine efficacy of various treatment sites Monitoring protocols were developed with the intention of considering subwatershed scale effects of culvert replacements. Evaluation of monitoring program data has shown a good assessment of reach scale effects, however only a small proportion of the watershed has been sampled. At present, the status of fisheries habitat, fish distribution and status is not well known. In order to consider culvert restoration projects in a landscape scale context, additional data is needed. Existing modeling efforts may help provide this context. Some work is underway through U.S. Fish and Wildlife Service (USFWS) initiatives to provide landscape level analysis on these topics. The USFWS Great Lakes Basin Fish Habitat Partnership is in the process of conducting habitat condition assessments for several species. Fish habitat condition can be estimated and changes in habitat conditions from restoration and protection actions could be tracked. The USFWS is also currently involved in classifying catchments in the Lake Superior basin that maintain self-sustaining brook trout populations. Researchers have identified subwatershed metrics found useful in identifying changes in brook trout status.By compiling data from state and federal partners, this effort will describe the distribution, status, and stressors of brook trout throughout the region, including in the Bad River watershed. This landscape scale modeling work will include data inputs to ground truth predicted brook trout status in areas where status is unknown. Upon review of monitoring program results, local managers recommend focusing local monitoring on efficacy of different structure types, in different zones of the watershed. In particular, the steep erosive channels in the transition zone area of the watershed raise question about longer term channel adjustments following culvert restoration. In particular, continued monitoring of sites with different culvert techniques (in pipe baffles, grade control below, or a larger pipe embedded into the stream bottom) will provide insights into the longer term efficacy of these techniques. This is a research oriented approach to evaluate culvert replacements in the different zones and different types of culvert designs. This program has demonstrated that collaboration among groups is essential to implementing responsible watershed restoration programs over the long term. This collaboration assists all partners in achieving their respective objectives, in a way that maximizes watershed integrity. ACKNOWLEDGEMENTS The BRWA‘s Culvert Project has been part of an integrated effort in concept and structure from its inception. We sincerely thank all partners who have contributed to this project, including: ILSG 2011 95 Field Trip 5 �Bad River Band of LakeSuperior Chippewa Indians, Natural Resource division, the Nature Conservancy, Sigurd Olson Environmental Institute, BRWA volunteer culvert inspectors, Ashland County Land and Water Conservation, Iron County Land and Water Conservation, Bayfield County Land and Water Conservation, U.S. Forest Service, Wisconsin Department of Natural Resources, U.S. Fish and Wildlife Service, U.S. Geologic Survey, Trout Unlimited, local townships, Natural Resources Conservation Service, and landowners adjacent to culvert sites. Funding and resources for the BRWA Culvert Project have been provided by: Wisconsin Environmental Education Board, US Fish and Wildlife Service, National Fish and Wildlife Foundation, Pri-Ru-Ta, ABDI Land Conservation Department, Wisconsin Department of Natural Resources, the Great Lakes Aquatic Habitat Network and Fund, Wisconsin Coastal Management Program, Great Lakes Basin Fish Habitat Partnership, Army Corps of Engineers Section 154 funds, and U.S. Forest Service Resource Advisory Committee REFERENCES Bad River Watershed Association (BRWA). 2007. Culvert Program Needs Assessment. Ashland, WI. 174 p. Bad River Watershed Association (BRWA). 2008. Culvert Program Strategic Plan. Ashland, WI. 24 p. Fitzpatrick, F. 2005. Project Update - Investigation of Erosion, Sedimentation, Channel Migration, and Streamflow Trends for the Bad River, Wisconsin. U.S.D.I. Geological Survey and Bad River Tribe of Lake Superior Chippewa Natural Resources Department, project update/no publication number. ILSG 2011 96 Field Trip 5 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 6 GEOLOGY HIKE AT COPPER FALLS STATE PARK Devils Gate at Copper Falls State Park -T. Fitz ILSG 2011 97 Field Trip 6 �ILSG 2011 98 Field Trip 6 �Field Trip 6 Geology hike at Copper Falls State Park Allison Mills, Drew Cramer, and Tom Fitz Northland College, 1411 Ellis Avenue, Ashland, WI 54806 ___________________________________________________________________________ COPPER FALLS GEOLOGIC SETTING Introduction Copper Falls State Park, near Mellen, Wisconsin, is 2,676-acre park with hiking trails, scenic waterfalls, excellent bedrock exposures, and beautiful surficial geologic features (Figure 1). The Bad River flows from south to north though the park, and has cut through glacial sediments, erodingdeeply into bedrockto form the Copper Falls gorge. After a 12foot drop at Copper Falls,the Bad River travels about twomilesthrough the gorge. Downstream, theTyler Forks River plunges 30 feet over volcanic ledges to join the Bad River before flowing throughthe narrow passage of Devil‘s Gate. The gorge has excellent exposures of Keweenawan volcanic and sedimentary rocks overlain by Pleistocene glacial deposits. This guidebook explains the geology along the1.7- mile Doughboys‘ Nature Trail that takes hikers around the top of the gorge (WDNR, 2009). Copper Falls State Park was established in 1929, aided by the Civilian Conservations Corps and the Works Progress Administration. Several buildings at the park are from this era and contain some nice gabbro and slate building stones. While Copper Falls State Park was preserved for its natural beauty, people once sought native copper at the site. Though no significant deposits were discovered, mine shafts can still be found within the park. In the early 1900s, the Bad River was diverted to prevent flooding in the mine shafts; the original channel is visible along the trail at stop 8 (WDNR, 2009). Bedrock Geology The bedrock exposed in the park is part of the 1100 Ma Keweenawan Midcontinent Rift (MCR) sequence. The MCR was created when the Laurentian craton began to pull apart, resulting in huge outpourings of flood basalts withinmajor rift valleys. Many thousands of feet of volcanic rock accumulated in the rift‘s central graben, but the tectonic activity stopped before an ocean basin could form. Volcanic activity was most voluminous between 1109-1094 Ma, and, with waning extension and volcanism, the rift filled with sediments. The MCR then underwent tectonic compression, and the central graben was partially inverted. As a result rocks along the southern arm of the rift now dip steeply to the north.One volcanic and three sedimentary bedrock units related to the MCR can be seen in Copper Falls State Park: the upper part of the Kallander Creek Volcanics, and the Copper Harbor Conglomerate, the Nonesuch Formation, and the conglomeritic facies of the Freda Sandstone,all part of the Oronto Group (Figure 2). ILSG 2011 99 Field Trip 6 �Figure 1. Location of Copper Falls State Park. These rock units are situated on the southern flankof the Montreal monocline –a large structure created when the rocks of the central graben of the MCR were tipped north during compression of the rift about 1060 Ma ago(Figure 3). Erosion of the monocline has exposed more than 35 km of crustal rocks. Layeringin the monocline strikes northeast-southwest and dips steeply north (Cannon et al., 1996; Cannon et al., 1993b; and Chapter 7 of this guidebook volume). The Kallander Creek Volcanics consist of mostly basalt, with some andesite and rhyolite—the lowermost flows are flood basalts, while the upper sections are dominated by andesites and rhyolites—erupted from a large central volcano that was probably located in the region of Copper Falls Park (Cannon et al., 1993a). This central volcano acted as a topographic high during the early phase of volcanism of the MCR at the time, and somewhat after, the Kallander Creek eruptions. Overlying bedrock units pinch-out in the Mellen area and thicken to the east away from this area. The intrusive Mellen Complex is interpreted to have been emplaced within the volcano late in its magmatic activity. The time of igneous activity is known from U-Pb zircon ages—rhyolite in the middle of the Kallander Creek Volcanics has an age of 1107 +/- 2 Ma, and granite in the Mellen Complex has an age of 1102 +/- 2 Ma (Cannon et al., 1993a). Thus, the central volcano was probably active from about 1108 Ma to 1102 Ma ago (Cannon et al., 1993a). ILSG 2011 100 Field Trip 6 �Magnetic Polarity Formation Age (Keweenawan Rocks) MellenIntrusions (1102 Ma) Kallander Creek Volcanics (~1108 - 1099Ma) Siemens Creek Volcanics Bessemer Formation Tyler Ironwood Palms Bad River Dolomite Puritan Batholith (2750 Ma) Ramsey Formation Bergland Group Powder Mill Group Portage Lake Marquette Range Supergroup Freda Nonesuch Copper Harbor Pleistocene Oronto Group Copper Falls / Miller Creek Normal Mesoproterozoic Reversed Normal Reversed Paleoproterozoic Neoarchean Figure 2. Stratigraphic table of formations in the Copper Falls area. The Kallander Creek through Copper Falls Formations are visible in Copper Falls Park. Note that the table shows rocks by age, not position – the Mellen Intrusions were mostly emplaced between the Siemens Creek and Kallander Creek Volcanics (after Green, 1982; Nicholson et al., 1997). The eastward thickening of the units above the Kallander Creek is referred to as ―The Wedge‖ on interpretive signs within the park. In fact, the Portage Lake Volcanics, just above the Kallander Creek Volcanics and below the Oronto Group, pinches out east of Copper Falls State Park and is not visible on this field trip. Therefore, going up-section, the sedimentary units above the Kallander Creek arethe Copper Harbor Conglomerate, the Nonesuch Formation, and the Freda Sandstone, all of which are much thicker to the east in Michigan (Figure 2). The shale and siltstone of the Nonesuch is overlain by interbedded red and brown conglomerate and sandstone of the Freda. These units record the waning of volcanic activity in the MCR and the subsequent filling of the rift with sediments. Rocks of the Mellen Complex are located nearby, butare not exposed in the area of the Copper Falls gorge. The Mellen Complex consistsprimarilyof gabbro and granite intrusions that were emplaced along the base of the MCR volcanic pile. The southern part of Copper Falls park is underlain by granite of the Mellen Complex, and a small gabbro quarry ILSG 2011 101 Field Trip 6 �can be seen on the west side of State Highway 169 south of the park, and along State Highway 13 north of Mellen (seeChapter 9 of this guidebook volume). F E D C B F B A G Figure 3. Bedrock geologic map of the Copper Falls area (from Cannon et al., 1996). Formations are: A, Yk) Kallander Creek andesite and basalt; B) Kallander Creek rhyolite; C) Portage Lake Volcanics, D) Copper Harbor Conglomerate; E) Nonesuch Formation; F) Freda Sandstone; G) Mellen Granite; Ymgp) Granophyre of Potato River Intrusion in Mellen Complex. The inset box shows the area of this field trip. Surficial Geology Bedrock exposed in the park is overlain by Pleistocene glacial deposits of the Copper Falls and Miller Creek Formations that record the retreat of the last of the Wisconsin glacial lobes prior to 9,500 BP (Clayton, 1984). The Copper Falls Formation contains sandy till and outwash deposited by the Chippewa Lobe prior to 11,500 BP, whereas the younger Miller Creek Formation is comprised of clay-rich deposits associated with Glacial Lake Duluth that were deposited between 11,500 and 9500 BP (Figure 3). Areas occupied by these two different deposits are often separated by a ribbon of sand – part of the Miller Creek Formation – at an elevation of 1100 feet that marks the highest beach of Glacial Lake Duluth. Deposits of sandy till of the Copper Falls Formation cover the bedrock in the southern part of the park and can be seen in the banks of the Bad River along the field trip route. Here the Copper Falls Formation is a mix of sand and silt with some gravel and boulders but has considerable local variability. The topography in the vicinity of the park is primarily controlled by the form of bedrock hills that have little or no glacial sediment on top, separated by intervening valleys where the Copper Falls Formation is tens of feet thick (Figure 4). Fine-grained sediments of Miller Creek Formation form large banks just downriver of the field trip route. ILSG 2011 102 Field Trip 6 �Soils that developed atop these sediments on topographic highs, such as in the landscape of the park, are moderately well-drained. These soils support a mixed forest with abundant white pine, sugar maple, and hemlock. In spite of the permeable sediments in the region, topographic lows are poorly drained and have a high water table because of the deranged drainage pattern. In these wetter areas, soils support spruce, fir, and cedar forests. There are also numerous kettle lakes and bogs that have thick accumulations of peat. Downstream (north) of the gorge the bedrock is covered by clay-rich sediment of the Miller Creek Formation. It is the distinctive red clay of the Lake Superior ―clay plain‖ in this region. The red clay is hard and stable when dry, yet soft and unstable when wet, which results in the erosion of very steep banks in the meanders of the Bad River immediately north of the gorge and Doughboys‘ Nature Trail. Figure 3. Surficial geologic map of the Copper Falls area (from Clayton, 1984). The area of this field trip is shown in the inset box. Areas represented in green are the Copper Falls Formation; those in blue-green are Miller Creek Formation. The strip of orange represents a beach deposit from Glacial Lake Duluth. Hydrology The Bad River starts at Caroline Lake, southeast of the park and the town of Mellen. The land in the headwaters of the Bad River is poorly drained and has numerous bogs. The water flowing from this headwater region is rich in tannins and organic acids derived from organic material in the peaty bogs. This gives the Bad River its characteristic brown color and is the source of the name ―Copper Falls‖. ILSG 2011 103 Field Trip 6 �From Caroline Lake the river flows southwest through a large area of poor drainage, then turns north and goes through the bedrock gorge of the Penokee Gap. The Bad River then flows through the City of Mellen, continuing north into the park. Within the park, the river meanders across a floodplain in a relatively flat area probably held in place by the nick point defined by the hard basalt at the head of the Copper Falls Gorge. The river then plunges over the ledge at Copper Falls and flows through the deep gorge. The presence of the gorge is likely due to nonresistant rock, which may be due to a combination of poor cementing in sedimentary rocks and closely spaced fractures in the felsite. Also, the rock may be highly fractured due to faulting. Figure 4. Topographic map of the Copper Falls area. From U.S.G.S. maps of the Mellen quadrangle (1967) and the High Bridge quadrangle (1984). FIELD TRIP STOPS This field trip guide is for a walking tour of the Copper Falls State Park Doughboys‘ Nature Trail starting and ending at the parking lot of the concessions building near the river. The stops are listed in order going around the Copper Falls trail loop in a clockwise direction, although the hike can be done in either direction (Figure 5). Helpful interpretive signs mark many of the significant geologic features along the trail and will be mentioned here. Also, bring along binoculars; some of the outcrops are inaccessible by foot. This area is represented on the Mellen and Highbridge topographic quadrangle maps(U.S.G.S.), on the Pleistocene map of Clayton (1984), and the bedrock geologic maps of Cannon et al. (1996) and Sims (1992). All locations are given in UTM coordinates (Zone 15 T), NAD 1927.Maps and brochures are also available at the park; some of which are accessible on-line through the Wisconsin DNR website. ILSG 2011 104 Field Trip 6 �Figure 5. Field trip stops 1-9 along the Doughboys‘ Nature Trail. Map courtesy of the Wisconsin Department of Natural Resources. Stop 1. Bridge over the Bad River Location: UTM coordinates: 681444 mE, 5137895 mN The bridge is located past the concessions building and marks the trailhead for the Doughboys‘ Nature Trail. Upstream of the bridge, thereis a large bank of silty sand and some gravel of the Copper Falls Formation. Boulders eroded out of the slope make up the channel and banks of the river. There is a widevariety of lithologies in these boulders, reflecting the diverse bedrock geology of the region. Rock exposed under the bridge is reddish gray basalt of the Kallander Creek Volcanics. The rock is slightly amygdaloidal and extensively fractured. The amygdules and fractures are filled with calcite stained red with hematite. During the Keweenawan eruptions, these flows were deeply buried by younger lava flows and sedimentary rocks, deep enough to undergo zeolite facies metamorphism. Oxidation of ferromagnesian minerals created abundant hematite that gives the rocks their dark reddishtint. No lava flow contacts are ILSG 2011 105 Field Trip 6 �visible in this part of the section, butthe lava flows seen from the bridge are tipped to near vertical, dipping steeply to the north. The bridge, built in the 1930s by the CCC, sits on a foundation constructed with Mellen gabbro. During mining activities in the early 1900s, the Ruggles Mining Company blasted the basalt exposed belowthe bridge in order to remove a small bedrock ridge to prevent flooding in the mine shafts. This allowed the river to flow directly east before turning north into the gorge; prior to blasting, the river flowed south around a big meander bend, visible at Stop 8. Stop 2. Copper Falls Overlook Location: UTM coordinates: 681510 mE,5138011 mN Along bend in trail with good view of the falls; history-focused interpretive sign nearby. In 1902, Copper Falls was reportedly nearly 30-feet high, but several large bedrock pieces have broken off, and today the falls is about 12-feet high. During spring floods, the water can rise high enough to flow over the basalt island separating the channels. The rock of the falls is basalt of the Kallander Creek Volcanics, similar to that seen at Stop 1. Look carefully to see if you can see any lava flow contacts or interflow sedimentary units. The interpretive sign here says that the gorge formed because of a fault—what are your thoughts? Stop 3. Brownstone Falls Overlook Location: UTM coordinates: 681682 mE, 5138203 mN Along trail in bend that offers view of Brownstone Falls; interpretive sign nearby. Brownstone Falls marks the confluence of Tyler Forks River and Bad River and reveals a contact between the mafic and felsic rocks of the Kallander Creek Volcanics.Felsic rocks are visible along the west side of the gorge, just left of the falls, apparent by theirreddish color and closely spaced fractures. This quartz- and plagioclase-phyric unit is informally named the Sheep Farm rhyolite and is the uppermost unit of the Kallander Creek Volcanics. It is at least 300 m thick and is likely the product of a single eruption. It has been dated at 1099 Ma, and lies just above the horizon of the principle magnetic reversal of the Keweenawan from reversed to normal polarity (Zartman et al., 1996; Cannon, chapter 7 of this guidebook volume). Don‘t climb over the fence, but examine the rocks alongside the path, and see what you can find. Along the trail, watch for some rare Northern Wisconsin vegetation: cutoversurviving white pines, eastern hemlock, and ravine-hugging cedars. The gorge, with its steep sides, has offered protection for the hemlock and cedar from over-browsing by deer.The cedars also show evidence of soil creep by the way their trunks curve down slope. Stop 4. Devils GateOverlook Location: UTM coordinates: 681506 mE,5138285mN Overlookalong the trail above Devils Gate Despite the name, this part of the field trip is really quite tame and the river‘s gradient has decreased. From this vantage, several formations of the Oronto Group can be seen across the river. The beds strike about 24 degreesnortheast-southwestand dipnorthwest at 86 degrees. The Copper Harbor Conglomerate, characterized by mainly red and brown conglomerate, sandstone, and siltstone, is straight across the gorge;there is some crude ILSG 2011 106 Field Trip 6 �bedding apparent. These rocks represent a prograding alluvial fan complex in the MCR (Daniels, 1982). The Nonesuch Formation—up-section and to the left—is characterized by mostly gray to black shale, siltstone, and mudstone. These fine-grained sediments were derived from local sources and deposited in alternating lacustrine and fluvial environments (Cannon et al., 1993a). The conglomeritic facies of the Freda Sandstone, the farthest outcrop visible to the left, is characterized by red and brown interbedded sandstone and conglomerate with thick bedding. The clasts of this facies are approximately equal proportions of immature, subangular to well-rounded, and poorly sortedpebbles offelsic and mafic rocks (Cannon et al., 1996). These clasts indicate that volcanoes were still serving as the main source of sediment in the rift. Stop 5. View of Devils Gate from Bridge Location: UTM coordinates: 681389 mE, 5138371mN Looking upstream into Devil‘s Gatefrom the bridge is another view of the Oronto Group members. The name ‗Devil‘s Gate‘ may come from the wicked turbulence and power of the Bad River in this constriction, especially notable during floods. This stop is unique as three units of the Oronto Group are visible. The different units here are relatively thin as they pinch-out towards Mellen;the thinning is especially apparent when these same formations are compared at different locations, particularly as they thicken northeastwards into the Michigan Upper Peninsula. The varying slopes indicate different rock resistance: the Nonesuch Formation has clearly eroded back, while the conglomerates and sandstones of the Copper Harbor and Freda have formed more resistant outcrops protruding into the river. The Copper Harbor Conglomerate is the farthest upstream, while the Freda Sandstone is closest to the bridge. From the next overlook along the trail, there is a good close-up view of the siltstones and sandstones of the Freda Sandstone. Stop 6. Overlook of Tyler Forks River Location: UTM coordinates: 681742 mE, 5138243mN Follow the wooden bridge path down to the farthest overlook, which is almost directly over Brownstone Falls. From this vantage, north of the confluence, we are looking upstream into the Bad River gorge with Tyler Forks River on the left, just to the right, Brownstone Falls plunges over a ledge of the Kallander Creek Volcanics.Felsic rock is visible on the right; a close-up of the outcrops seen at Stop 3. To the left is Tyler Forks Cascades—a series of short turbulent fallstumbling over felsite. A better view of the cascades can be seen at the overlook a short distance back along the wooden walkway. Stop 7. Brownstone Falls: a Different Perspective Location: UTM coordinates: 681723 mE, 5138192 mN Past the bridge, along the trail, is another good view of Brownstone Falls from the east side of the confluence. At this stop, there isanother view of Brownstone Falls:to the right, the Tyler Forks River flowsin from the northeast, joins the Bad River below the falls, andcontinues northwest through Devil‘s Gate. Across the gorge, looking towards Brownstone Falls, an overflow channel can be seen carved into the rock;this abandoned channel funnels water into the Bad ILSG 2011 107 Field Trip 6 �River during floods. More mafic and felsic rocks of the Kallander Creek Volcanics are visible—do you see any evidence of a contact? Stop 8. Old Bad River meander Location: UTM coordinates: 681477 mE, 5137861 mN Along trail, just before the first bridge; old channel is to the left. In the early 1900s, the Ruggles Mining Company put in several shafts near the current concessions building, prospecting for copper. To prevent flooding, workers blasted a new channel so the Bad River could be redirected. Today, the old meander bend is still visible, though trees and other vegetation have successively reclaimed the channel. As for the mining, although some of these same formations are rich in copper father east, this endeavor proved unsuccessful. Stop 9. Flagstones in the path at the concession building Locally quarried slate flagstones in the sidewalk near the concessions buildings are from the Paleoproterozoic Tyler Formation. They show good examples of thin bedding, graded beds, concretions, and other sedimentary structures. Additionally,more slate and gabbro were used in the bridges, buildings, and along the trails, particularly on the steps. Around the corner from the first waypoint, on the west side of the concessions building, is a large chimney constructed of anorthositic gabbro of the Mellen Complex. Stop 9a. Location: UTM coordinates: 681383 mE, 5137847 mN Along the paved path, near the southwest corner of the concessions building kitchen. This particular location, along the south wall of the concessionsbuilding, has several flagstones that show thinly interbedded mudstone and sandstone. Look for interesting sedimentary structures in other flagstones.The angle of slaty cleavage varies from being parallel to bedding to being perpendicular to bedding, with many angles in between. Stop 9b. Location UTM coordinates: 681330 mE, 5137846 mN Along the paved path, closer to the parking lot. Thislocation, closer to the parking lot, has some good examples of graded bedding. All of this slate comes from the Paleoproterozoic Tyler Formation. The Tyler Formation crops out sporadically south and east of the park,and is considerablydown-section of the Keweenawan rocks found in Copper Falls State Park. ACKNOWLEDGEMENTS Thanks to Laurel Woodruff for her astute editing. ILSG 2011 108 Field Trip 6 �REFERENCES Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30- x60-minute quadrangles, Wisconsin and Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map I-2556, scale 1:100,000. Cannon, W.F., Nicholson, S.W., Zartman, R.E., , Peterman, Z.E., and Davis, D.W., 1993a, Kallander Creek Volcanics—a remnant of a Keweenawan central volcano centered near Mellen, Wisconsin [abs.] Institute on Lake Superior Geology, 39 th Annual Meeting, Proceedings and Abstracts, p. 20-21. Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993b, Crustal-scale thrusting and the origin of the Montreal River monocline—a 35-km-thick cross section of the midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Clayton, L., 1984, Pleistocene geology of the Superior Region, Wisconsin: Wisconsin Geological and Natural History Survey, Information Circular no. 46, 40 p., map scale: 1:250,000. Daniels, P.A., Jr., 1982, Upper Precambrian sedimentary rocks: Oronto Group, MichiganWisconsin, inWold, R.J., and Hinze, W.J., eds., Geology andtectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 107-134 Green, J.C., 1982, Geology of Keweenawan extrusive rocks,inWold, R.J., and Hinze, W.J., eds., Geology andtectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 57-82. Nicholson, S.W., Shirey, S.B., Schulz, K.J., Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: Implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520. Sims, P.K., 1992, Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin and northern Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map I-2185, scale 1:500,000. U.S. Geological Survey, 1984, High Bridge, Wisconsin quadrangle, 7.5-minute topographic map: photorevised 1980, scale 1:24,000, 1 sheet. U.S.Geological Survey, 1967, Mellen, Wisconsin quadrangle, 7.5-minute topographic map: photorevised 1980, scale 1:24,000, 1 sheet. Wisconsin Department of Natural Resources, 2009, Copper Falls State Park, http://dnr.wi.gov/Org/land/parks/specific/copperfalls/ (January 2011). Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1996, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. ILSG 2011 109 Field Trip 6 �ILSG 2011 110 Field Trip 6 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 7 GEOLOGY OF THE MONTREAL RIVER MONOCLINE Steeply dipping Freda Sandstone at the mouth of the Montreal River -W.F. Cannon ILSG 2011 111 Field Trip 7 �ILSG 2011 112 Field Trip 7 �Field Trip 7 Geology of the Montreal River Monocline William F. Cannon U.S Geological Survey MS 954 Reston, Virginia 20192 Note: This fieldtrip was originally run for the 42nd Annual Institute on Lake Superior Geology in 1996. At that time, poor road conditions including deep snow on some back roads, ―vehicular traction issues‖, and high water in streams prevented a complete examination of all intended localities. New developments in the past 15 years and hopes for better field conditions in 2011 lead us to rerun the trip for the 57th Annual Meeting. The Field Guide is based largely on the original publication in 1996 (Cannon, 1996) with a few modifications based on new exposures and recent studies. INTRODUCTION South of Lake Superior, near the Wisconsin-Michigan border, a remarkable monoclinal sequence of rocks exposes, at the present land surface, what had been the upper 35 km of the crust (Figure 1) prior to uplift and monoclonal tilting during inversion of the Midcontinent rift. This section is seen in a sequence of steeply to vertically dipping and north-facing volcanic and sedimentary units of Paleoproterozoic and Mesoproterozoic age and underlying Neoarchean volcanic rocks. Figure 1. Cross section of the Montreal River monocline showing the stratigraphic position of field trip stops and approximate burial depths. ILSG 2011 113 Field Trip 7 �This structure, named the Montreal River monocline by Cannon and others (1993), resulted from southward-directed thrusting on a northward-dipping listric thrust of crustal scale, the Atkings Lake-Marenisco fault (Figure 2). The age of thrusting is known to be about 1040-1060 Ma, the date of reset biotite Rb-Sr ages in Archean rocks that were upthrust and cooled on the upper plate above the fault (Cannon and others, 1993). Figure 2. Schematic cross section of the Montreal River monocline showing the inferred crustal-scale structures responsible for its formation. Figure 2 shows the regional structural context of the Montreal River monocline. The section extends approximately from the Bayfield Peninsula on the northwest to Mercer, Wisconsin on the southeast, a distance of roughly 100 km. To the northwest, a prominent basement high, White‘s Ridge, limited the depositional extent of much of the Keweenawan volcanic section and remained a positive topographic feature until the close of the extensional phase of the rift. As more widespread thermal subsidence supplanted extension, the ridge was buried beneath the Oronto and Bayfield Groups, which remain nearly flatlying. Southeastward from White‘s Ridge to the present outcrop belt of the Keweenawan volcanics, the volcanic section becomes progressively thicker. This geometry suggests that the deepest parts of the rift in this area may actually have been to the south of the preserved volcanic rocks, in an area since uplifted and deeply eroded. The present steeply northdipping attitude of the rocks of the Montreal River monocline is believed to be the result of crustal-scale ramping on the Atkins Lake-Marenisco fault. The Atkins Lake-Marenisco fault is interpreted to penetrate the entire crust and to have thrust rift units and their Paleoproterozoic and Archean basement southward. The listric geometry of the fault surface caused the upper plate to tilt northward, after which erosion has exposed the present crustalscale cross section. The involvement of Archean rocks in the monoclonal structure is shown by the southerly dip of originally vertical diabase dikes and by Mesoproterozoic Rb-Sr ages of biotite, which mark the time of uplift and cooling. The 270o isotherm, the Rb-Sr blocking temperature of biotite, was located by determining the Rb-Sr age of biotite in Arcehan rocks. Rocks north of the isotherm (originally above the isotherm) remained cooler than 270o throughout Keweenawan burial and retained ages reflecting Paleoproterozoic and Archean conditions. Rocks between the isotherm and the Atkins Lake-Marenisco fault were heated above 270o by Keweenawan burial and the Rb-Sr ages were reset. In this area biotite ages ILSG 2011 114 Field Trip 7 �range from 1040-1060 Ma and date the time of uplift, erosion, and cooling caused by thrusting along the Atkins Lake-Marenisco fault. South of the fault Mesoproterozic burial was not deep and biotite ages continue to reflect Paleoproterozoic thermal events. The trip begins at the mouth of the Montreal River (Figure 3) where extensive exposures of vertically dipping Freda Sandstone, the youngest unit exposed in the monocline, form bluffs along the Lake Superior shore. Based on projections of seismic data from Lake Superior these strata may have been buried by as much as 5 km of overlying clastic rocks now preserved beneath the lake. The trip progresses southward for about 25 km through successively older and steeply dipping to vertical strata of the Mesoproterozoic Keweenawan Supergroup, the Paleoproterozoic Marquette Range Supergroup, and into Neoarchean metavolcanic rocks. The Paleoproterozoic and Neoarchean rocks seen at stops 8 and 9 have been exhumed from lower crustal depths, but lack the expected high metamorphic grade and deformation typical of rocks from such depths. The generally low-grade metamorphism and lack of Mesoproterozoic penetrative deformation attest to a short residence time at these depths. Rapid subsidence during rifting and rapid uplift during rift inversion brought these rocks to great depth and returned them to shallower depths at a rate that did not allow conductive heating to achieve typical deep crustal temperatures. ILSG 2011 115 Field Trip 7 �Figure 3. Geologic map of the field trip area showing location of stops. Geology generalized from Cannon and others (1996). Stop 1: Freda Sandstone at the mouth of the Montreal River (46.566, -90.4169)From the parking area at the top of bluff overlooking Lake Superior walk down the trail to exposures along the lakeshore. The Freda Sandstone is the youngest unit exposed in the Montreal River monocline. The rocks here, representing the middle to upper parts of the Freda, may have been buried by as much as 3-5 km under younger parts of the Freda and younger sediments of the Bayfield Group. The sandstone is well exposed in high continuous bluffs along the Lake Superior shoreline. It is mostly a well bedded and commonly crossbedded feldspathic sandstone. Dips are essentially vertical. Volcanic rock fragments are a major constituent, indicating that riftderived volcanic rocks composed much of the source area. Muscovite is also a common detrital mineral in many layers indicating that an Archean basement terrane was also exposed that contributed a substantial part of the detritus. Stop 2: Middle part of Oronto Group along Parker Creek (46.5241, -90.4199) These exposures are best reached by walking west from Highway 122 to Parker Creek. The coordinates are approximately at the contact between the Copper Harbor Conglomerate and Nonesuch Formation. These exposures are on private land. The top of the Copper Harbor Conglomerate, the Nonesuch Formation, and the lower part of the Freda Sandstone can be seen along the incised bed of Parker Creek. The Parker Creek section is the most complete exposure of the Nonesuch and adjacent units in the Lake Superior region. Beds dip from 75°N to vertical. Detailed stratigraphy of the Parker Creek section was described by Suszek (1991) on whose work the following generalized description is based. The upper part of the Copper Harbor Conglomerate consists of massive to crudely bedded clast-supported conglomerate with cross-bedded, pebbly sandstone interbeds. Clasts consist mostly of basalt and rhyolite with lesser granite and gabbro, all probably derived from slightly older Keweenawan rocks. Banded iron-formation, jasper, quartzite, and chert clasts are a minor component but are widespread, indicating that pre-Keweenawan basement was also exposed to the south within the drainage basin. The Nonesuch Formation is about 130 m thick and comprised of mostly gray to pale brown siltstone, sandstone, and minor shale. It is generally thinly bedded to laminated, but thicker massive beds also occur. Small-scale trough crossbedding is common. There is an overall coarsening-upward trend to the formation, although many individual units display fining-upward textures. The fine-grained rocks of the Nonesuch Formation lie on coarse conglomerate of the Copper Harbor. A transitional bed, one meter thick, contains reddishbrown, trough-crossbedded sandstone at the base and grades upward into gray-brown calcareous siltstone. The upper contact of the Nonesuch with the Freda Sandstone is gradational over about 20 m, in which gray siltstone and shale are interbedded with reddishbrown, medium to coarse sandstone. ILSG 2011 116 Field Trip 7 �Stop 3: Top of the Porcupine Volcanics at SaxonFalls (46.5380, -90.3751) Drive to the end of the road to Saxon Falls dam. The exposures are along the river bed below the dam. Basalt flows and interflow conglomerate and sandstone of the uppermost part of the Porcupine Volcanics are exposed along the Montreal River below the dam at Saxon Falls flowage. The uppermost exposed unit is thin-bedded red sandstone and siltstone, which is underlain by a massive basalt flow about 100 m thick. This flow is underlain by about 30 m of interbedded conglomerate and sandstone. The lowest exposed unit is another basalt flow with an amygdular, partly brecciated top. About the upper half meter of the flow shows typical rubbly flow-top breccia, and the upper three meters are coarsely amygdular. The most abundant amygdule-filling mineral is thomsonite, which occurs in masses of coarse radiating blades in the larger amygdules. The predominance of this low temperature zeolite indicates that rocks at this level of burial were only weakly heated before uplift. Stop 4: Rhyolite at top of Kallander Creek Volcanics (46.5108, -90.3265) This exposure is on private land on an active farm. Permission for access should be requested from the land owner. The uppermost unit of the Kallander Creek Volcanics is a thick, mostly massive, quartz- and plagioclase-phyric rhyolite. We informally designate this unit the Sheep Farm rhyolite and this stop is our informal type locality. The unit is at least 300 m thick and appears to be the product of a single eruptive event. It can be traced from this locality westward for about 30 km to Copper Falls State Park near Mellen, where it is the resistant rock that holds up Brownstone Falls on the Tyler Forks River. At that locality, we have located by field measurement the horizon of the principal magnetic reversal recorded in the Keweenawan section. The reversal lies a few flows beneath the rhyolite. The rhyolite has been dated at 1099 Ma (Zartman and others, 1996), so the age of the magnetic reversal must also be approximately 1099 Ma. At this stop, the exposures reveal the basal part of the flow, which contains numerous blocks of rhyolite up to about 20 cm across. These blocks have a slightly different texture and phenocryst content than the host rhyolite. Stop 5: Icelandite in Kallander Creek Volcanics (46.4936, -90.3292) The exposures to be examined are in a gravel pit to the northeast of the intersection of Lakehead Road and US Highway 2 The upper member of the Kallander Creek Volcanics contains abundant rocks of intermediate composition in addition to basalt and rhyolite. We have proposed that the upper part of the Kallander Creek Volcanics represents the partly eroded remains of a broad central volcano, and that the Mellen Complex is the crystallized remnants of a magma chamber that was intruded into the volcanic edifice late in its eruptive history (Cannon and others, 1993). ILSG 2011 117 Field Trip 7 �At this locality our data show that the rock knob in the gravel pit is a fine-grained reddishbrown icelandite (SiO2=64-66 wt%; Ti02=1.26 wt%; CeN/YbN=4.5) that contains small plagioclase phenocrysts. The rock has a strong primary foliation, perhaps indicating that it is the basal zone of a flow, and weathers into platy fragments parallel to the nearly vertical foliation. Small outcrops nearby show a coarser, more massive icelandite. The icelandite is overlain by a basalt (Si02=50.24wt%; Ti02=3.78 wt%; CeN/YbN=3.7) that is exposed in low outcrops along the power-line north of the gravel pit. Stop 6: Upson Lake area—Siemens Creek Volcanics, Bessemer Quartzite, and Tyler Formation (46.3867, -90.4442) Drive road along the south shore of Upson Lake to the small parking area at the west end of the lake. Walk north and east along logging road about 300 meters and then north to exposures on higher part of the south-facing slope. At this locality the base of the Keweenawan section is exposed in contact with the underlying Tyler Formation. At the top of the prominent bluff north of Upson Lake the lowermost basalt flows of the Siemens Creek Volcanics are exposed. They are pillowed and brecciated as a result of eruption into a shallow lake in which the Bessemer Quartzite had been deposited. The flows have a distinctive chemical composition marked by low A1203 (812 wt%), and high Cr, Ni, and MgO. In addition, rare earth element (REE) patterns are quite steep (CeN/YbN = about 15) and heavy REEs have low abundances. These chemical characteristics indicate that the magmas formed by a small degree of partial melting of a deep, garnet-bearing enriched mantle source (postulated to be a mantle plume), and that the magma erupted directly to the surface without undergoing significant crystal fractionation. Flows of this chemical character can be recognized in the field by the presence of small phenocrysts of clinopyroxene. These clinopyroxene-phyric flows have been traced from near Bessemer, Michigan, to just west of this locality. They apparently represent a unique magma composition that was erupted only at the very outset of volcanism. Identical flows have been recognized at the base of the North Shore Volcanics in Minnesota, both at Ely's Peak and at Pigeon Point and Grand Portage (Green, 1977). An interflow sedimentary bed is exposed between the first and second flows. The sediments are, in part, an algal laminite and are now substantially metamorphosed so that original calcareous layers are a fine-grained mat of wollastonite. The metamorphism was caused, at least in part, by emplacement of the Potato River Gabbro about 1 km up-section. The Siemens Creek Volcanics are underlain by the Bessemer Quartzite, the oldest unit in the Keweenawan Supergroup. Scattered exposures of white to gray laminated quartzite are on the slope below the basalt exposures. Passing down-section, the Bessemer contains basal beds of coarse conglomerate composed largely of Paleoproterozoic metasedimentary clasts, including iron-formation. Small garnet porphyroblasts are common in the matrix. A small exposure near the base of the slope shows an angular unconformity between basal conglomerate of the Bessemer and thin-bedded argillite of the Tyler Formation. This is the only exposure known to us where the unconformable relationship between the Keweenawan Supergroup and the Marquette Range Supergroup can be seen. Stop 7: Tyler Formation (46.3500, -90. 5051) Exposure is a roadcut on the north side of High 77. ILSG 2011 118 Field Trip 7 �This roadcut exposes typical thin-bedded graywacke and argillite of the Paleoproterozoic Tyler Formation. Although dips are nearly vertical, the lack of penetrative fabric indicates little structural effect of the Penokean orogeny on the Tyler in this area. The highest grade metamorphic mineral is biotite, which is common throughout the matrix. Stops 8 and 9: Mt. Whittlesey area The prominent ridge of Mt. Whittlesey (Figure 4) is held up by the Ironwood Ironformation. An unusually great thickness of iron-formation here resulted from structural repetition by thrust faults and folds produced during the Penokean orogeny. Mt. Whittlesey lies at the easternmost extent of the Penokean (Paleoproterozoic) foreland fold and thrust belt. Here, a series of thrusts repeats the Paleoproterozoic section, having detached it from Archean rocks along a basal decollement. The thrusts all rise in the section to the east along a series of lateral ramps. Displacement along the basal decollement diminishes eastward as splays from the decollement successively rise in the section. East of Stop 9 the Paleoproterozoic section lies unconformably on Archean rocks. ILSG 2011 119 Field Trip 7 �Figure 4. Geologic map of the Mt. Whittlesey area showing the locations of Stops 8, and 9. Stop 8: Ironwood Iron-formation in Mt. Whittlesey (46.30226, -90. 61633) Vehicle access is via a poorly maintained road that extends eastward from County Road MM. Some steep sections of road are susceptible to washouts and may be impassable at times. From the Berkshire mine ruins walk eastward on an ATV trail up the slope to large bedrock exposures. Excellent exposures of the Ironwood Iron-formation occur on the north face of the ridge west of the summit, and along a railroad grade near the Berkshire mine ruins. At the former locality, a large area was cleaned of overburden in the 1920s in anticipation of open pit mining. Only a small pit was developed near the base of cleared area; the remainder provides an excellent, glacially polished exposure in which many details of sedimentary ILSG 2011 120 Field Trip 7 �features are extraordinarily well displayed. At the Berkshire mine ruins an abandoned railroad grade provides a continuous exposure of several hundred meters of section and a tight fold is exposed in the cut. The iron-formation contains several lithologic variations. The classic five member internal stratigraphy defined to the east (Hotchkiss, 1919; Huber, 1959) consists of alternating members of even-bedded, carbonate iron-formation (slaty iron-formation) and irregularly-bedded hematitic jasper. This stratigraphy becomes more complex and irregular westward along the iron range; we have not been able to apply it west of Mt. Whittlesey. The iron-formation exposed here characteristically has many lithologic variations on a small scale so that individual exposures may show a complete range of lithologies interbedded on a scale of meters or less. The following detailed description of the Ironwood exposed on the northwest flank of Mt. Whittlesey was prepared by Philip Fralick as part of an informal guidebook for the Workshop on Geology, Mineralogy, and Genesis of Precambrian Iron-Formations held at the University of Minnesota-Duluth Precambrian Research Center in October 2010. It is based largely on a detailed study of these exposures reported in Pufahl and Fralick (2004). Scattered outcrops in the vicinity of Mt. Whittlesey define three upward-coarsening, asymmetric cycles composed of a basal unit of slaty iron-formation that is gradational into an upper unit of trough cross-stratified cherty iron-formation. The middle cycle provides an unparalleled opportunity to investigate cycle sedimentology as it is completely stripped of overburden to appraise the Fe resources of this area. Like the cycles above and below, it consists of ~14 m of slaty iron-formation gradationally overlain by ~16 m of cherty ironformation, which is capped by a ~1 m thick unit of convolute-bedded, slaty iron-formation. Pufahl and Fralick (2004) conducted a detailed description of a 40x44 m glacially polished exposure through 1.5 cycles combined with a reconnaissance study of poorly exposed strata above and below the logged section. This area provides an excellent opportunity to investigate the depositional controls governing the accumulation of ironformation facies. Their research was augmented with geochemical analysis of iron-formation facies in order to elucidate the chemical controls on iron-formation precipitation. This approach has yielded important insights into the relative roles tectonics, fluctuating hydraulic regime, and changing water chemistry played in governing the distribution of iron-formation facies. ILSG 2011 121 Field Trip 7 �Figure 5. Schematic map of vertically dipping asymmetric cycles developed in Superior-type iron-formation of the study area. The slaty unit forming the lower portion of cycle 1 overlies a slumped unit of convolutedly-bedded chemical mudstones and is gradational into a succession of cherty grainstone lenses. These coarse-grained beds are sharply overlain by another slumped chemical mudstone unit. The base of a second coarsening upward cycle (cycle 2) forms the upper portion of the outcrop. ILSG 2011 122 Field Trip 7 �Grainstone lenses become progressively thicker upwards in cycles with the largest at cycle tops, where they are sharply overlain by a unit of slumped chemical mudstone. Cycles formed through progradation when offshore-directed storm currents transported progressively greater quantities of chert sand intraclasts that were formed in nearshore settings, outboard into middle and distal shelf environments (Fig. 5). Abrupt subsidence events, probably resulting from normal faulting associated with extensional tectonism, repeatedly terminated shallow-water chert grainstone accumulation and may have also generated the slumped units at cycle boundaries. The episodic storm currents are also interpreted to have transported biologically oxygenated waters over the shallow-water, inner shelf outboard along the seafloor into otherwise anoxic bottom waters of the strongly stratified distal shelf (Fig. 6). The consequence of such transport and mixing was rapid deposition of chemical mud, mainly as precipitated Fe-oxide. In many cases, the resultant decrease of Fe2+ in the water column, together with pelagic inorganic precipitation of SiO2 and rainout of terrigenous clays, resulted in submillimeter- to millimetre-thick, chemically graded laminae. The concomitant decreasing Fe2+/Mn2+ ratio also led to increasing Mncompound precipitation and enrichment in the upper portions of some chemically graded laminae (Fig. 7). The outcrop is mostly two-dimensional and glacially polished so that sedimentary features are well displayed. It is sub-parallel to the trend of the paleoshoreline but oblique enough so that paleocurrent trends can be ascertained. Figure 6. Physical depositional model for an asymmetric iron-formation cycle. Rapid transgression was followed by formation of a shoaling upwards, prograding cycle. Offshore flow during or immediately after major storm events built shelf sand sheets from chert and Fe-oxide intraclasts which progressively migrated into more distal environments as water depth decreased. ILSG 2011 123 Field Trip 7 �Figure 7. (1) Chemical depositional model for an asymmetric iron-formation cycle. Low free oxygen levels allowed Fe and Mn to remain dissolved in a water mass with near-neutral pH. The main O2 production took place near-shore where light penetration to the seabed allowed cyanobacteria to flourish. (2) Storm-generated offshore flows, which were responsible for generating hummocky cross-stratification in more shore-proximal locations, transported the oxygenated coastal waters to the mid and outer shelf where they mixed with Fe- and Mn-bearing offshore waters, with resultant chemical precipitation. Storm-wave mixing of the stratified water body may have also played a role in this process, although an O2-bearing surface layer is not necessary. The succession represented by Column A reflects the formation of thick Fe-oxide laminae overlying grainstone lenses. The succession illustrated in Column B would have developed in a more distal shelf position resulting in thinner and finer grained sand lenses and thinner chemical layers than in Column A. The assemblage which developed furthest offshore is shown in Column C. ILSG 2011 124 Field Trip 7 �Figure 8. Progressive formation of a millimetre-scale chemical mud laminae. Chemical grading is interpreted to be the result of changes in water chemistry driven by the precipitation process. (A) Offshore currents generated during storm events delivered nearshore oxygenated water to deeper portions of the shelf, where it mixed with normal (Fe 2+-, Mn2+- and PO43--bearing) ocean water. (B) Precipitation proceeded relatively rapidly as the abundant Fe2+ and O2 combined to probably form FeOOH‘s. (C) As Fe content decreased in seawater precipitation rate slowed and ambient clastic and SiO2 deposition gained in importance. (D) As the Fe/Mn ratio in the aqueous phase decreases, Mn2+ more effectively combined with oxygen. A progressively decreasing Fe2+/PO43- ratio results in more scavenging of PO43- by the FeOOH‘s. Carbon is delivered to the sediment by the rainout of phytoplankton from the surface ocean. (E) Diagenesis and low-grade metamorphism resulted in the mineral assemblage present today. ILSG 2011 125 Field Trip 7 �Stop 9A: Palms Formation and Ironwood Iron-formation along Ballou Creek, east end of Mt. Whittlesey (46.31083, -90.57756) The exposures are in a long road cut along the west side of Lake Road. Recent road improvement along Lake Road has produced a set of roadcuts along the west side the valley of Ballou Creek that provide a cross section of most of the Palms Formation and part of the lower Ironwood Iron-formation. Starting at the south, at the lower part of the section, a nearly continuous exposure, about 75 m long, of laminated argillite and sandstone illustrates the lower part of the Palms Formation. Proceeding north, up-section, along the outcrop many typical sedimentary structures of the Palms are well exposed. These features have been interpreted to indicate sedimentation on a tidal flat or tidally influenced lagoon. Farther north, a covered interval about 75 m wide is probably underlain by the upper part of the laminated lower Palms Formation and quartzite of the upper Palms Formation. Next, to the north, is a 30 m exposure of the upper part of the Palms Formation quartzite and the Ironwood Iron-formation. The uppermost quartzite is coarse-grained and cross-bedded. The transition to the Ironwood Iron-formation takes place in a 0.5 m interval containing reworked fragments of the Palms surrounded by magnetite. The overlying Ironwood Ironformation varies from even-bedded magnetite-rich material to irregularly bedded jasper at a scale of a meter or less. Stop 9B. Contact of Paleoproterozoic strata and Archean metavolcanic rocks on the southeast flank of Mt. Whittlesey (46.30834, -90.58632) Walk west from Lake Road about 600 meters along the base of the ridge. Low outcrops, near the base of a prominent ridge supported by the Palms Formation, reveal the basal contact of Paleoproterozoic strata with underlying icelandite breccia of Neoarchean age. The breccia has a penetrative structural fabric, best expressed by the prominent elongation of clasts. Long axes plunge moderately southward, but must have plunged moderately to steeply northward prior to tilting of the rocks into the monoclinal structure. The basal Paleoproterozoic unit is a chert breccia that locally is the basal unit of the Palms Formation. We have interpreted this breccia to be residual concentration of chert produced by dissolution of the Bad River Dolomite, a cherty carbonate rock that is preserved sporadically beneath the breccias. The chert fragments were reworked during the transgression of the Palms Formation over a long-lived erosion surface that separates the Bad River and Palms. The upper meter or two of the icelandite breccia is strongly sheared parallel to the contact with the chert breccia, and the Archean fabric typically is completely overprinted by this secondary fabric. Shearing is weak to absent in the chert breccia. To our knowledge, these outcrops are the only ones in the western part of the Gogebic Range where a basal detachment between the Paleoproterozoic strata and Archean basement rocks can be directly observed. We believe the Paleoproterozoic strata were thrust northward along a nearly flatlying basal contact during the Penokean orogeny. Because of later tilting of the Montreal River monocline, the present apparent displacement reflects a down-to-the-north normal fault. This fault can be traced westward, and about 1 km to the west of these exposures it passes up- ILSG 2011 126 Field Trip 7 �section so that the Bad River-Palms-Ironwood sequence of the upper plate is juxtaposed over Ironwood Iron-formation in the lower plate, producing the unusually wide outctrop belt of Ironwood along higher parts of Mt. Whittlesey. Just east of this stop, the basal thrust appears to pass upward into the Paleoproterozoic strata. Exposures just east of Ballou Creek show that the base of the Paleoproterozoic rocks lies unconformably on Archean volcanic rocks without an intervening shear zone. The Archean rocks seen at Stop 9B are the structurally lowest parts of the monocline that are seen on the field trip. But the trace of the Atkins LakeMarerinsco fault, the master thrust that formed the Montreal River Monodline, lies about 8 km farther to the south. Thus, all of the stratigraphic section to the north that is seen on the trip, plus an additional 8 km of Archean rocks to the south, form the upper plate of the Atkins Lake-Marenisco fault, a coherent structural unit that was uplifted and tilted late in the Mesoproterozoic history of the region. REFERENCES Cannon, W.F., 1996, Geology of the Montreal River monocline: a traverse through 25 km of the crust: Proceedings of the Institute on Lake Superior Geology, v. 42, part 3, Fieldtrip guidebook, p. 49-63. Cannon, W.F., Woodruff. L.G, Nicholson, S. W., and Hedgeman, C.A., 1996, Bedrock geologic map of the Ashland and northern part of the Ironwood 30‘x60‘ quadrangles, Michigan and Wisconsin: U.S Geological Survey Miscellaneous Investigations map I-2566, scale 1:100,000. Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline - a 35-km-thick cross section of the Midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Green, J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region, in Baragar, W. R. A., and others, (eds.), Volcanic regimes in Canada: Geological Association of Canada Special Paper 16, p. 407-422. Hotchkiss, W.O., 1919, Geology of the Gogebic Range and its relation to recent mining developments: Engineering and Mining Journal, v. 108, p. 443-452, 501-507, 537541, 577-582. Huber, N.K., 1959, Some aspects of the origin of the Ironwood Iron-formation of Michigan and Wisconsin: Economic Geology, v.54, p.82-118. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic range, Lake Superior region, USA: Sedimentology, v. 51, p. 791-808. Suzek, T.J., 1991, Petrology and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent rift system: Duluth, Minn., University of Minnesota, Duluth, unpublished M.S. thesis, 198 p. Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1996, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. ILSG 2011 127 Field Trip 7 �ILSG 2011 128 Field Trip 7 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 8 THE ARCHEAN/PALEOPROTEROZOIC UNCONFORMITY NEAR DENHAM, MINNESOTA Pillowed basalt flow near Denham, Minnesota - T. Boerboom Field Tri ILSG 2011 129 Field Trip 8 �ILSG 2011 130 Field Trip 8 �Field Trip 8 Transect from Archean basement to the Animikie basin, East-central Minnesota Terry Boerboom Minnesota Geological Survey boerb001@umn.edu INTRODUCTION This field trip runs parallel to the western margin of the Mesoproterozoic Midcontinent Rift System, where the continental crust is dominated by the geon 18 (1,800 – 1,900 Ma) Penokean Orogen and is intruded by rocks of Yavapai age (1,800 – 1,772 Ma; geon 17). Together, these orogenic components form a major curvilinear orogenic belt that spans an area in Minnesota from the Mesabi Iron Range to central Minnesota, with outlying intrusions as far south as the Iowa border (Figure 1). The first part of the trip will start on Archean basement rocks (the McGrath Gneiss dome) which are non-conformably overlain by the Paleoproterozoic Denham Formation, which in turn is overlain by the eastern extension of the overlying Little Falls Formation. The Denham Formation consists of basal coarse clastics interlayered with pillow basalts that were erupted into a shallow-water environment, overlain by a thick unit of dolomitic marble, in turn overlain by a thick sequence of turbidites correlative with the Little Falls Formation. The Denham Formation and the lower part of the Little Falls Formation have been metamorphosed to staurolite-zone amphibolite facies. The remainder of the trip will examine outcrops of turbiditic sediments of progressively lower metamorphic grade, ending in the low-grade Thomson Formation near Carlton, Minnesota. The stops will examine the structural attributes of the rocks and the gradual decrease in metamorphic grade from south to north, highlighting the difficulty of defining a boundary between the Animikie basin to the north and the older rocks of the Penokean orogen to the south. One of the stops will be in an area traditionally mapped as part of the older rocks south of/underneath the Thomson Formation, but which are now considered to be part of the Thomson Formation. We will also discuss the different generations of deformation evident in the rocks, and the likelihood that the later deformation may be related to the geon 17 (1,700 – 1,800 Ma) Yavapai Orogeny. The reader is greatly encouraged to make use of the numerous references listed, which provide detailed studies that tie together the various components of the Penokean Orogen as well as the effects of the Yavapai and Mazatzal orogens in Minnesota, Wisconsin, Michigan, and Ontario. A special volume of Precambrian Research (Volume 157 , August 2007) contains the latest interpretations of the Paleoproterozoic history of the Lake Superior region. ILSG 2011 131 Field Trip 8 �Figure 1. Simplified geologic map of southern two-thirds of Minnesota showing extent of eastcentral Minnesota batholith and other intrusions of known and inferred Yavapai age in the Minnesota ILSG 2011 132 Field Trip 8 �River Valley subprovince; also extent of Yavapai-interval intrusive rocks in central Minnesota (stippled pattern). TECTONIC ELEMENTS OF EAST-CENTRAL MINNESOTA Early workers (e.g. Marsden, 1972, Morey and others, 1981) correlated components of the Mesabi Iron Range (Pokegama, Biwabik, and Virginia Formations) with those of the Cuyuna North and South Ranges (Mahnomen, Trommald, Rabbit Lake Formations), and considered the Thomson Formation (equivalent to the Virginia Formation) to include the higher metamorphic grades south of Thomson Dam, e.g.stops 8-4 and 8-5 of this trip. They considered the Denham Formation to be part of an older sequence termed the Mille Lacs Group, which also included such units as the Trout Lake dolomite north of the Cuyuna Range, the Little Falls Formation, the Glen Township Formation, and other mafic volcanic and high-grade metasedimentary units. Subsequently Southwick and others (1988) reinterpreted the entire Penokean Orogen in terms of a plate-tectonic model, with the aid of high-resolution aeromagnetic data that was unavailable to earlier workers. They considered the Cuyuna Range and everything to the south (Figure 1) to be part of an older series of folded and thrust-faulted panels that increased in metamorphic grade (hence greater degrees of exhumation), from north to south, culminating in the southern-most plutonic terrane near St. Cloud. In this model, northwarddirected thrust-stacking created uplift along the southern margin of the Animikie basin, and the basin developed to the north of the uplifted crust in response to flexural loading. All of the components were considered to be of Penokean age. The next step in refining our understanding of the Penokean Orogen in Minnesota was and is the acquisition of age dates. The first significant studies began in the 1990‘s, through multiple geochronological studies initiated by Daniel Holm at Kent State University, and later by others. Ar-Ar ages define widespread ca. 1,760 Ma metamorphism throughout the Penokean Orogen (e.g. Holm and others, 1993; Holm andLux, 1996; Holm and others, 1998). Extensive dating of various components of the east-central Minnesota batholith (Holm and others, 2005) show that nearly all of the intrusions in the heart of the ‗Penokean Orogen‘ internal zone are actually between 1,772 – 1,800 Ma (Yavapai), and only the Bradbury Creek granodiorite (1,858 – 1,877 Ma) is Penokean in age. More recently, in-situ U-Pb geochronological studies of hydrothermal xenotime and metamorphic monazite have further documented metamorphic and hydrothermal events related to both the Penokean and Yavapai orogens (e.g. Schneider and others, 2004; Vallini and others, 2007; Holm andothers, 2007). Detrital zircon studies (e.g. Wirth and others, 2006) have provided ages for source materials for many of the units. Ongoing U-Pb zircon dating of thin ash layers and interbedded volcanic rocks within the lower parts of the Animikie basin and the Baraga basin in Michigan (e.g. Schneider and others, 2002; Fralick and others, 2002; Addison and others, 2005) and the important recognition of the Sudbury ejecta layer (e.g. Addison and others, 2005; Jirsa and others, 2008; Cannon and others, 2010) has allowed for confident correlation of the major depositional basins in the region and allowed for a more thorough interpretation of the Paleoproterozoic evolution of the Lake Superior region. For example, Schulz and Cannon (2007), using newly available dates for igneous and metamorphic events in combination with the Penokean-wide Sudbury impact marker layer, have compiled a robust interpretation of the geologic history of the Penokean and Yavapai ILSG 2011 133 Field Trip 8 �events in the Lake Superior region. In their synopsis, the Penokean orogeny began ca. 1,880 Ma via docking of the Pembine Wausau terrane to the southern margin of the Superior craton, resulting in subduction jumping to the south and the development of back-arc basins in both the Superior craton edge and the arc. Extension and subsidence of the back arc basins produced volcanic grabens within a broad, shallow sea into which the major iron formations (i.e. Mesabi-Gunflint, Ironwood, and Negaunee) were formed. The iron formations at the south edge of the basin (e.g. Marquette and Gogebic ranges) are coeval with volcanic rocks whereas deposition at the north edge of the basin (e.g. Mesabi and Gunflint ranges) was distal to the volcanic activity. At around 1,850 Ma (a time for which the Sudbury impact layer provides a basin-wide marker horizon; Figure 2), a fragment of Archean crust (Marshfield terrane in Wisconsin) arrived at the subduction zone and converged with the Superior craton, causing subsidence in front of the collision zone, and forming a broad foreland basin. Some parts of the basin saw uplift and possible subaerial exposure of the iron formation for a time, prior to inundation and burial by a transgressive sediment sequence (Animikie and Baraga basins). Sedimentation into the southern part of the Animikie foreland basin began at 1,850 Ma and ended by 1,833 Ma, whereas in the northern part of the basin sedimentation did not start until ca. 1,835 Ma and continued until at least 1,770 Ma. Tectonic thickening produced medium-grade metamorphism of the sediments, which continued at the south edge of the basin until ca. 1,830 Ma when the deformed sediments and accreted arc terranes were intruded by post-tectonic granitic rocks (Van Schmus, 1976), marking the end of Penokean orogenesis. Younger tectonic and thermal events that overprint the Penokean orogen, previously ascribed to the Penokean orogeny, are now known from metamorphic and thermochronologic studies to be Yavapai-interval events that post-date the Penokean by tens of millions of years (e.g. Holm and others, 2007; Tohver and others, 2007). Renewed Yavapai-interval activity that peaked in mid-geon 17 produced doming of the Archean crust (e.g. McGrath Gneiss dome in Minnesota and other gneiss domes in Michigan – the ‗gneiss dome corridor), accompanied by mid-crustal level emplacement of the east-central Minnesota batholith, and widespread resetting of metamorphic ages in the rocks of east-central Minnesota. Detrital zircons as young as 1,770 Ma (Heaman and Easton, 2005) in the northern reaches of the Animikie basin (Rove Formation in Ontario) show that deposition continued into the Yavapai-interval accretion event, at least in the upper portions of the basin. Deformation at the southern margin of the Animikie basin may have begun in the late stages of Penokean orogenesis, but likely continued during geon 17 (Scheiner, Boerboom, and Holm, 2011). ILSG 2011 134 Field Trip 8 �Figure 2. Correlation diagram of Paleoproterozoic strata across Minnesota, Wisconsin, and Michigan, showing the 1,850 Ma Sudbury impact layer timeline. From Cannon and others, 2010. Figure 3. General geologic map of eastcentral Minnesota showing the major subdivisions of the Penokean Orogen. Red box outlines field trip traverse. ILSG 2011 135 Field Trip 8 �LITHOLOGIC UNITS Animikie Basin Foredeep The Animikie basin is conceptualized as a foreland basin that developed in front (north) of the fold and thrust belt (everything south of the Animikie basin in Figures 1 and 3). It includes the main depositional basin as well as two outliers that probably represent erosional remnants of a larger, formerly continuous area. Strata of the Animikie basin unconformably overlie both Archean crust and deformed rocks of the Penokean Orogen fold and thrust belt (e.g. the Cuyuna North Range). The strata at the north edge of the Animikie basin are essentially undeformed, with a gentle south dip, whereas at the southern margin they are folded into a series of generally upright and open, east-west trending folds (stop 89). Slaty cleavage associated with folding first appears about 15 km south of the Mesabi Iron Range, and increases in strength southward in conjunction with progressively tighter folds (Southwick and others, 1998). A narrow belt along the southern edge of the basin contains an early bedding-parallel metamorphic foliation that is folded into upright folds which are apparently coaxial with folds related to the single deformation to the north. The Animikie Basin is composed of a thin lower unit of quartzite and siltstone (e.g. the Pokegama quartzite on the Mesabi Iron Range; Figures 1, 2, 3, 5) overlain by the Biwabik and Gunflint Iron Formations, and a thick upper sequence of turbidites and shale (the Virginia-Thomson Formations and related outliers, and the Rove Formation). The Pokegama quartzite onlaps ca. 2,700 Ma Archean granite-greenstone terrane to the north, and contain only locally-derived 2.9 – 2.6 Ga detrital zircons eroded from adjacent Neoarchean crust (Wirth and others, 2006). In Ontario, the 1,850 Ma Sudbury ejecta layer (Addison and others, 2005) lies above an 1,878±1.3 Ma volcanic ash layer near the top of the Gunflint Formation (Fralick and others, 2002) and below an 1,836±5 Ma ash layer near the base of the Rove Formation (Addison and others, 2005). In addition an ash layer near the base of the Virginia Formation on the Mesabi Iron Range in Minnesota yields a U-Pb zircon age of 1832±3 Ma (Addison and others, 2005). The nearly 40 m.y. hiatus at the north side of the basin is ca. 15 m.y. greater in duration than the same hiatus associated with the same unconformity at the south side of the basin in Michigan, which was more proximal to the Penokean tectonic front. (Shulz and Cannon, 2009). The position of the Mille Lacs Group and Cuyuna Range strata in Minnesota relative to deposition of the Animikie basin fill is still unresolved, but geophysical evidence implies that the Cuyuna North Range Group continues beneath, and is unconformably overlain by, strata of the Animikie Group (Figures 3 and 5). Detrital zircon studies to date from turbiditic sediments at the southern edge of the Animikie basin in the Thomson Formation (e.g. stop 8-9 of this trip) yield ages mostly in the 2.05 – 1.80 Ga range (Wirth and others, 2006). They report similar ages for detrital zircons from the Rove Formation in northeastern Minnesota and from the Tyler Formation in northern Wisconsin. Heaman and Easton (2005), obtained detrital zircon ages as young as 1,777 Ma from near the top of the Rove Formation in Ontario, approximately 400 m above the base (Maric and Fralick, 2005). This age overlaps with the age of the east-central Minnesota batholith in Minnesota (Holm and others, 2005), and indicates that sedimentation, at least in the uppermost strata of the Rove portion of the Animikie basin, was synchronous with Yavapai magmatic activity to the south. Using the interpretation that the Penokean orogeny ended at ca. 1,830 (Schulz and Cannon, 2009), these data imply that much of the deposition into the basin may have occurred during Yavapai time. ILSG 2011 136 Field Trip 8 �Thomson Formation The Thomson Formation is correlative with the Virginia Formation, but is situated at the southeastern margin of the Animikie Basin (Figures 1, 3, and 5). As reported by Morey (1978), the type locality of the Thomson Formation (stop 8-9) is composed of 34-46 percent graywacke, 27-39% siltstone, and 27% slate. The Thomson Formation has been metamorphosed to the greenschist facies, and is cut by a series of northeast-trending diabase dikes that are presumed to be associated with the Midcontinent rift system. The bulk of the Thomson Formation is deformed into a series of open, east-trending folds with axes that dip vertical to steeply south and plunge shallowly to the east and west. However, Holst (1984) recognized a ‗southern structural terrane‘ which contains an early, near bedding-parallel, S1 foliation that is inferred to be parallel to poorly exposed recumbent isoclinal folds, and the entire area is proposed to be on the upper limb of a large recumbent fold. This early fabric is refolded along open and upright folds having axial traces that plunge gently both east and west, which has crenulated the S1 cleavage and produced an S2 cleavage. The second generation of folds has the same style and orientation as those to the north, where only one generation of folding (regional D2) has produced a single slaty cleavage. The boundary between the once- and twice-deformed rocks has been interpreted by Holst (1984) as the edge of a thrust nappe, south of which the rocks contain two deformations and north of which the rocks were only affected by the second deformation. However, recent mapping (e.g. Boerboom, 2009) across this transition shows that there seems to be a progressive decrease in metamorphic grade from south to north, and the transition between twicedeformed and once-deformed rocks (e.g. Figure 3) is not easily demarcated. The twicedeformed ‗southern structural terrane‘ includes rocks interpreted to be part of the Penokean fold and thrust belt as well as the southern edge of the Thomson Formation/Animikie basin, without a clear-cut structural front to separate the two entities. An alternate explanation, based on magnetic and petrofabric studies, proposes that the early subhorizontal S1 cleavage in the southern zone formed at the same time as the single subvertical cleavage in the northern zone (Sun and others, 1995 and references therein). Their paleomagnetic studies showed that the natural remnant magnetism of the Thomson Formation record the field during deposition and compaction, and was not reset during deformation. This field is recorded by ferromagnetic minerals (e.g. hematite). Anisotropy of magnetic susceptibility, recorded primarily by the preferred orientation of metamorphic chlorite, records minima perpendicular to the single upright cleavage in the northern zone (i.e. subhorizontal), whereas it is normal to the early S1 cleavage in the southern zone (i.e. subvertical: Figure 4). They invoke a model, based on studies of other foreland orogens, involving simple shear on subhorizontal planes (south) transitioning into pure shear with subhorizontal shortening (north). Late-stage regional shortening produced the second deformation, which shows up preferentially in the southern zone due to favorable orientation there of the S 1 cleavage perpendicular to compression, whereas in the northern zone the later deformation is not apparent because the preexisting fabric was already oriented subparallel to the later compressional event. This model may be a means to explain the transitional nature observed between the northern and southern (once- and twice-deformed) panels of the Thomson Formation (e.g. between stops 8-8 and 8-9 of this trip), where the strength of S2 cleavage in the southern zone seems to progressively weaken to the north. However, the samples for the paleomagnetic ILSG 2011 137 Field Trip 8 �study were confined to the immediate area of the transition, and do not include the highergrade metamorphic facies to the south. The promising use of future paleomagnetic studies should incorporate samples from the higher-grade terrane to the south. Figure 4. Alternative model for explaining the contrasting deformational history of the northern vs. southern zones of the Thomson Formation. From Sun and others, 1995. On prior geologic interpretations, the southern margin of the Thomson Formation was considered to be the point at which the large area of featureless aeromagnetic signature associated with the Animikie basin ends, and where characteristically ‗busy‘ signatures associated with the Mille Lacs Group begins. However, based on remapping and relogging of drill cores and cuttings, the southern margin of the Thomson Formation is now considered to include an east-west belt marked by moderately high gravity in combination with an eastwest belt of discontinuous linear aeromagnetic anomalies (Figure 5). This linear zone wraps southwest into apparently tightly-folded strata between rocks of the Cuyuna South Range and the Mille Lacs Group, and the argument could be made that the entire Cuyuna South Range is the continuation of this southern-most belt now mapped as Thomson Formation. The discontinuous lenses shown as ‗sulfidic horizons‘ in the base of the Thomson Formation on Figure 3 are characterized by large cubic pyrite porphyroblasts. Most of these zones are confined to the very base of the Thomson, but some lenses occur as far as 4 km north of the ILSG 2011 138 Field Trip 8 �base, well within classic Thomson Formation. In addition to the sulfidic horizons, this lower unit includes generally thin units of chert, and minor amounts of mafic hypabyssal intrusions (Boerboom, 2009). These sulfidic horizons may be similar to those in the base of the Baraga basin in Michigan, e.g. the Bijiki Iron Formation. Figure 8-5. First-vertical derivative aeromagnetic anomaly map superimposed on secondverticalderivative gravity. Note the moderate gravity high that along the southern margin of the Animikie basin that extends southwest into rocks currently mapped as Cuyuna South Range. Compare to Figure 3. Fold and Thrust Belt The fold and thrust belt (Figure 3) shows decreasing metamorphic grade and structural complexity (i.e. a decreasing depth of tectonic burial) from south to north. It contains a complex assemblage of thrust-stacked, folded and faulted metasedimentary and metavolcanic rocks and associated hypabyssal mafic sills, which overall are of low to moderate metamorphic grade except for the southern margin which have been metamorphosed to the amphibolite facies. The medial zone includes the South range of the Cuyuna district and the Mille Lacs Group and associated rocks (Figure 1, 3 and 5), which encompass a broad area that extends from the Moose Lake area south of the Animikie basin to as far southwest as Stearns County. Iron-formations in the medial zone are closely associated with mafic volcanic rocks and euxinic shale. The external zone, which forms the ILSG 2011 139 Field Trip 8 �northwest-most panel and includes the Cuyuna district North range, is composed of folded and weakly metamorphosed strata including the Mahnomen, Trommald, and Rabbit Lake Formations, but also includes a substantial proportion of mafic volcanic rocks. Ironformations (mainly the Trommald) are associated with fine-grained argillaceous rocks as well as mafic volcanic rocks. Aeromagnetic data (Figure 5) and geophysical models clearly indicate that tightly folded strata of the Cuyuna North range district continue to the east beneath unconformably overlying strata of the Animikie group, a relationship that has been verified by geophysical models (Carlson, 1985). Also clearly visible on Figure 5 is the truncation of magnetic anomalies associates with the Mille Lacs Group and Cuyuna South Range by the overthrust Little Falls Formation, which is magnetically quiet. Denham and Little Falls Formations The Denham Formation (Morey, 1978) is composed of a heterogeneous mixture of interbedded pelitic to carbonate-cemented, arenitic arkosic sedimentary rocks, calc-alkaline pillow basalt, dolostone, and fragmental volcanic rocks. This sequence nonconformably overlies the Archean McGrath Gneiss, and has been subjected to amphibolite-facies metamorphism. Drill holes (cores and cuttings) from north and east of the Denham locality show that the Denham is transitional upward into graywacke now inferred to be the eastern extension of the Little Falls Formation (Figure 1, 3, and 5), with the transition marked by a 1m thick zone of graphitic argillite. Aeromagnetic and widely scattered outcrop data indicate that the Denham Formation extends westward along the north edge of the McGrath Gneiss to Mille Lacs Lake, and the Little Falls Formation continues west and south a considerable distance past Mille Lacs Lake. Old drilling records and corroborating aeromagnetic anomalies also indicate that infolded remnants of the Denham Formation are present within the McGrath Gneiss (Figures 3 and 5). Stops 8-1 through 8-3 of this field trip will cover the transition from the McGrath Gneiss to the base of the Little Falls Formation. The age of the Denham Formation is constrained by the underlying McGrath Gneiss (2,557 15 Ma; Figure 6; Holm and others, 2005). The Denham Formation and overlying Little Falls Formation are interpreted as a panel thrust northwest over the Mille Lacs Group, which is cut by a granite stock (Mille Lacs granite; Figure 3) that yields a U-Pb zircon age of 2,009 Ma (Holm and others, 2005). Volcanic rocks in the Denham Formation have provided a Sm-Nd isochron age of 2197 39 Ma (Beck, 1988). Thus, the age of the Denham Formation is poorly constrained to somewhere between 2,550 to possibly as young as 2,009 Ma, depending upon the relationship to the Mille Lacs Group. Monazite U-Th-Pb singlespot electron microprobe analyses and single-spot ion microprobe 207Pb/206Pb analyses of samples from the Little Falls Formation yield mainly 1,741 – 1,776 Ma Yavapai-interval ages, and a sample from stop 8-3 of this trip also yielded an age of 1,844 7 Ma in addition to a 1,788 4 age, thus this location records both Penokean and Yavapai metamorphic events (Schneider and others, 2004; Holm and others, 2007). ILSG 2011 140 Field Trip 8 �Figure 6. Locations of various types of age date determinations in east-central Minnesota. Base map is same as in figure 8-3. Rocks of the Denham and Little Falls Formations have undergone regional amphibolite-grade metamorphism and at least two periods of deformation attributed to Penokean and Yavapai Orogenic events. The first deformation event was synchronous with metamorphism to the garnet zone of the amphibolite facies (Holm, 1986). It produced an early S1 foliation that is typically bedding-parallel, and a locally strong, shallowly plunging, stretching lineation (Figure 7a). S1 foliation and S0 bedding were subsequently folded along steeply dipping axes (Figure 7b), concurrent with or followed by ca. 1,760 Ma peak metamorphism that produced staurolite. In the Denham valley (stop 8-2), the stratigraphic sequence dips variably north, having local F2 folds with overturned limbs. North of the valley in the overlying Little Falls Formation (stop 8-3) bedding and S1 in metagraywacke are nearly horizontal, and are deformed into open F2 folds with local crenulation features. Farther north the bedding dips mostly south, defining a broad, regional-scale, upright F2 syncline. ILSG 2011 141 Field Trip 8 �Figure 7. Equal area, lower hemisphere projections of structural elements of the Denham Formation. A. Measured lineations; B. Poles to measured foliations. Open diamonds represent the main, early foliation, which has been refolded by a second deformation that produced only a weak north-dipping foliation (crosses). Despite deformation and metamorphism, the stratigraphy of the Denham Formation forms a coherent package that is shown schematically in Figures 8. Omitting the prefix 'meta' for clarity, the base of the Denham Formation consists of interbedded siltstone and cross-stratified pebble conglomerate. This is overlain by coarse-grained and locally conglomeratic arkose that apparently pinches out laterally. The primary clastic grains in the dolomitic arkose are well preserved owing to the abundance of dolomite in the matrix, which accommodated most of the deformation. The arkose is interbedded with amygdaloidal basalt flows that contain well-preserved primary macroscopic flow features. The flows grade stratigraphically upward (northward) from massive bases to pillowed interiors to fragmental upper portions. The volcanic flows are thickest at the eastern outcrop limit, where at least four flows of nearly 1000 feet total thickness were recognized, and thin westward to two flows of 300 feet total thickness. An upper sequence of mafic fragmental volcanic rocks nearly merges to the east with the pillow basalts (see Figure 10 under stop 8-2), possibly implying that later stages of volcanic activity were more explosive in nature and that both volcanic horizons emanated from a common center to the east, which may presently be buried beneath the Mesoproterozoic Fond du Lac Formation. Arkosic and pelitic strata (now staurolite-garnet-mica schist) between the flows and fragmental volcanic sequence pinch out to the east where the flow package thickens, and are not present in drill holes to the north and east of the Denham valley. The northern-most outcrops in the valley consist of very pure dolomite, now marble, which contains ptygmatically folded and strongly lineated quartz veins. Drill cores show that the dolomitic marble is at least 500 feet thick, and is overlain by graywacke (e.g. stop 8-3) that is exposed discontinuously to the north for some distance. A thin layer of graphitic argillite marks the contact between the dolomite and overlying graywacke. ILSG 2011 142 Field Trip 8 �Figure 8. Schematic stratigraphic section of the Denham Formation, showing relative location of field trip stops. Field and petrographic observations imply that clastic detritus in the Denham Formation was derived in large part from a weathering residuum on the subjacent McGrath Gneiss. Near the contact with the Denham Formation, the McGrath grades abruptly from granite gneiss containing quartz, orthoclase, plagioclase, and biotite to strongly foliated, quartz- and sericite-rich schist that contains orthoclase, but no plagioclase. The arkosic parts of the Denham Formation similarly lack plagioclase, and are composed of quartz and orthoclase grains, together with small clasts of granitic gneiss. Studies of saprolite developed beneath Cretaceous sedimentary rocks on Precambrian crystalline rocks in southwestern Minnesota may provide an analog (Setterholm and others, 1989). These studies demonstrate that plagioclase and mafic minerals such as biotite and hornblende are the first minerals to alter to clay during the weathering process, and that orthoclase and quartz are the most resistant to weathering. The basal Cretaceous strata locally consist of reworked saprolite, including beds of cross-stratified sandstone and nearly pure kaolinitic shale. Exposures of basal Cretaceous sedimentary rocks locally contain detrital orthoclase and quartz derived by slight reworking of weathered granite grus, set in a carbonate matrix. Similar processes apparently occurred in the Paleoproterozoic by erosion and reworking of weathered McGrath Gneiss into beds of calcareous arkose and kaolinitic shale. These were subsequently metamorphosed to produce recrystallized dolomitic arkose and staurolite-garnet-sericite ILSG 2011 143 Field Trip 8 �schist. Slight weathering of orthoclase may have liberated potassium for the inferred conversion of kaolinite to sericite during metamorphism. The Denham Formation is interpreted to represent a rift-margin assemblage deposited during the Paleoproterozoic, similar to and perhaps temporally equivalent to the Chocolay Group in Michigan. In this setting, the McGrath Gneiss was part of the continental margin that was weathered and eroded to provide detritus to an evolving rift basin undergoing active, shallow water volcanism. Interbedded arkose and dolomite higher in the stratigraphic section represent foundering of the shelf and deepening water, possibly by subsidence of localized grabens. The paucity of clastic material in the upper, dolomite-dominated part of the sequence indicates that deposition of coarse detritus was restricted to the shallow, nearshore environment adjacent to the McGrath Gneiss. The stratigraphically upward sedimentological gradation of dolomite to graywacke (aka Little Falls Formation) indicates further deepening water and associated turbidite deposition. The deformation of the Denham Formation is inferred to be the product of basin closure during the Penokean Orogeny, later influenced by deformation related to Yavapai orogenic activity which produced doming of the underlying McGrath Gneiss. McGrath Gneiss Dome (gneiss dome/plutonic terrane) The Neoarchean McGrath Gneiss occurs within the internal zone of the Penokean orogen as discussed prior (Southwick and others, 1988). The McGrath Gneiss is exposed in several localities between Denham and Mille Lacs Lake to the west. Although this unit locally contains layered gneissic structure, it is best characterized as a metamorphically foliated porphyritic granite. The 2557 Ma McGrath Gneiss is only slightly younger than the undeformed 2600 Ma Sacred Heart Granite of the Minnesota River Valley in southwestern Minnesota (Doe and Delevaux, 1991). Holm and Lux (1996) interpreted the basement-cover contact of the McGrath gneiss dome to be a detachment fault because of an apparent 50 m.y. cooling age difference between basement and cover rocks. However, more recent detailed Ar ion laser data show that the basement and cover rocks have essentially the same lower temperature thermal history (Figure 9; McKenzie, 2004). Also, as discussed under stop 8-1, field and petrographic evidence (Boerboom and others, 2001) indicates that the McGrath Gneiss had a saprolite developed on it at the time of deposition of the overlying Denham Formation. Together, these data indicate that the boundary may simply be a domed nonconformable contact. Further evidence of this lies in the inlier of Denham Formation that is infolded into the top of the McGrath Gneiss dome (e.g. Figure 3). The McGrath Gneiss dome and adjacent Denham Formation represent the culmination of the north to south increase in metamorphic grade, from greenschist facies at the base of the Animikie basin to upper amphibolite facies. This southward increase in metamorphic grade is considered to reflect an increase in depth of exposure from the foreland basin at the north to the deeply eroded crust of the Penokean orogen at the south, related to geon 18 accretion. Plutonic activity in east-central Minnesota between 1,800 – 1,772 Ma, likely due to Yavapai-interval convergence (Holm and others, 2005) was more or less concurrent with formation of the McGrath Gneiss dome, and with widespread regional ca. 1,750 Ma regional metamorphism. ILSG 2011 144 Field Trip 8 �Figure 9. Spot age data (in Ma) of muscovite grains from Ar ion laser data (McKenzie, 2004). A. McGrath gneiss basement near stop 8-1; B. metapelite cover rocks at stop 8-3. Malmo Structural Discontinuity (MSD) The internal zone of the Penokean Orogen (the gneiss dome – plutonic terrane and the Denham-Little Falls Formations) are separated from the Penokean fold and thrust medial zone (e.g. Mille Lacs Group) by the Malmo structural discontinuity (originally termed the Malmo thrust), inferred to be a major, south-dipping fault (Wunderman and Young, 1987). The western end of the Malmo structural discontinuity (MSD: figure 1, 3, and 5) separates relatively low metamorphic grade sedimentary and volcanic rocks to the north from relatively high-grade metamorphosed granitic and sedimentary rocks and Yavapai intrusive igneous rocks to the south. The western portion of the MSD is clearly marked by the sharp truncation of linear aeromagnetic anomalies associated with thin iron-formations in the Cuyuna south range and Mille Lacs Group, indicating that the ‗internal‘ zone containing the Little Falls panel was thrust west-northwest over lower-grade rocks of the medial zone. Also, recent metamorphic geochronology indicates that the ages of metamorphism differ across the MSD (geons 17 and 18 to the north and dominantly geon 17 to the south). These data all imply that the MSD is a post-Penokean geon 17 structure along which the gneiss dome – plutonic terrane (i.e. the Penokean orogen internal zone and the McGrath Gneiss dome) was uplifted and subsequently exhumed, most likely during or after emplacement of the Yavapai-interval east-central Minnesota batholith. East of Mille Lacs Lake, the location of the MSD is more obscure due to the lack of contrast in geophysical data. Historically the eastern end of the MSD has been placed at the northern margin of the McGrath Gneiss. However, more recent mapping (e.g. Boerboom, 2009) indicates that the MSD may lie a short distance north of the McGrath Gneiss (Figure 3). The graywacke unit (stop 8-3), which overlies the Denham Formation (stop 8-2), contains the same metamorphic and lithological attributes as the Little Falls Formation, and if this correlation is correct the MSD must lie north of the Denham and Little Falls Formations, an observation supported by outcrop and drill core lithologies. South of where the MSD is currently mapped the rocks are composed of only metagraywacke and related rocks with no interbedded volcanic rocks. North of where the MSD is mapped, the bedrock types are a ILSG 2011 145 Field Trip 8 �diverse assemblage of carbonate, iron formation, graphitic schist and phyllite, and mafic intrusive and volcanic rocks. West-northwest-directed thrusting may have also caused more displacement and hence a greater degree of exhumation to the west, whereas to the east the MSD fades into more of a strike-slip regime, with less vertical displacement and hence less contrast in metamorphic grade. FIELD TRIP STOPS The general geologic setting and locations of stops are shown on Figure 1. A detailed location map is included with each stop description, with a base made at scale from a portion of the appropriate 7.5‘ quadrangle map. The UTM coordinates are reported in NAD83, zone 15. The regional setting of the various rock units is given in the introduction, and only facts pertinent to each specific stop are given below. --From the parking area near Carlton go west on Highway 210 through Carlton then south on I-35 to the Willow River exit (no. 205) – a distance of approximately 34 miles. Go west to Willow River, turn north on Highway 61 for 3 miles to intersection with State Road 52, go west on 52 for approximately 6.2 miles (road changes from pavement to gravel). The road makes a right angle turn to the north towards the town of Denham, but instead turn south and walk or drive on small dirt path to gate. Proceed past gate into open pasture until road crosses a gentle rise and west end of outcrop (stop 8-2a). Outcrop is 0.65 miles south of 52. ILSG 2011 146 Field Trip 8 �Road map showing stop locations Stop 8-1: McGrath Gneiss and contact with overlying Denham Formation Location: T.45N., R.21W., Sec. 36 Denham 7.5’ quadrangle UTM 503979 easting; 5132367 northing HIGHLIGHTS: ARCHEAN MCGRATH GNEISS AND METAMORPHOSED SAPROLITE (AMG), CONGLOMERATE AT BASE OF PALEOPROTEROZOIC DENHAM FORMATION (PPDS). Note:Stops 8-1, 8-2, and 8-3 are shown in apparent stratigraphic context on Figure 8. These stops will examine in order the north-facing transition from the Neoarchean McGrath Gneiss (stop 8-1), to the overlying Paleoproterozoic Denham Formation (stop 8-2), and then to graywacke of the Little Falls Formation that stratigraphically overlies the Denham Formation (stop 8-3). ****Stops 8-1 and 8-2 are on PRIVATE PROPERTY. Please procure permission from landowner before entering**** At this locality, the northernmost outcrops of the McGrath Gneiss (metamorphosed porphyritic granite) grade abruptly from the typical mineral assemblage of quartz, orthoclase, plagioclase, and biotite to strongly foliated, quartz- and sericite-rich schist that contains coarse relict orthoclase, but no plagioclase. The latter assemblage, present just below the base of the Denham Formation, may be the product of metamorphism of partially weathered granite, in which the plagioclase feldspar had weathered to clay, but quartz and potassium feldspar remained fresh, analogous to the weathering patterns noted in younger saprolites that underlie Cretaceous rocks in Minnesota. A sample of McGrath Gneiss from 20 miles southeast of this locality yielded a U-Pb zircon age of 2557±15 Ma. (Holm and others, 2005). Detrital zircons from the metasedimentary rocks just above the McGrath Gneiss contact (see next paragraph) yield ages as young as 2,072±17 Ma and as old as 3,447±17 Ma, with spikes around 2,625, 2,900, and 3,300 Ma. Detrital zircons from stratigraphically higher dolomitic arkose (stop 8-2) yield ages as young as 2,173±11 Ma and as old as 3,505 ±8 Ma, with groups around 2,600, 2,800, and 3,360 Ma (Wirth and others, 2006). Just north of the McGrath gneiss is a thin layer of very poorly exposed biotite schist of sedimentary protolith which contains irregular lenses of coarse conglomerate with abundant pebbles of quartz and pink orthoclase, but no plagioclase (unit Ppds onFigure 10). Similarly, ILSG 2011 147 Field Trip 8 �the clastic component of the arkosic beds stratigraphically higher in the Denham Formation (stop 8-2b) is comprised almost entirely of quartz and orthoclase feldspar with essentially no plagioclase feldspar. Together, these observations are consistent with the interpretation that the McGrath gneiss was weathered to the point that plagioclase and mafic minerals had altered to clay minerals, and this weathered residuum acted as a source of detritus for the overlying Denham Formation. Figure 10. Bedrock geologic map of the Denham Formation type locality, showing stop locations. Black circles are locations of exploratory drill holes and gray areas are mapped outcrops. McGrath Gneiss-Amg; Denham Formation-Ppds (metasiltstone), Ppda (metadolomitic arkose), Ppdb (metabasalt), Ppdg (metapelite), Ppdv (meta-fragmental mafic volcanic rocks), Ppdm (dolomitic marble); Little Falls Formation equivalent metagraywacke – Ppgs. --Return to vehicles. Drive back east 0.75 miles on State road 52 to a small gated trail to the south. Walk south down trail for approximately 0.5 miles following edge of ravine to south-most outcrop on west side of ravine (see inset map). This stop will traverse from south to north over outcrops located along west edge of ravine. There are also outcrops on the east side of the ravine that show parts of the stratigraphy not exposed on the western valley traverse. **Private Property ** ILSG 2011 148 Field Trip 8 �Stop 8-2: Denham Formation Location: T.45N., R.21W., Sec. 25, SE 1/4 Denham 7.5’ quadrangle UTM Start: 505406 easting; 5132575 northing UTM end: 505208 easting, 5133330 northing HIGHLIGHTS: INTERBEDDED BIOTITE SCHIST(SILTSTONE), PILLOW BASALT, DOLOMITIC ARKOSE, AND DOLOMITIC MARBLE. From bottom to top, the Denham Formation consists of fine-grained metamorphosed siltstone (Ppds), dolomite-cemented arkosic arenite with rare cobbles of McGrath gneiss (Ppda), pillowed basalt flows (amphibolite; Ppdb), shale (staurolite-garnet-mica schist; Ppdg), interbedded dolomitic arkose and dolostone (Ppda), fragmental basaltic volcanic rocks (amphibolite; Ppdv), and at the top of the section, pure dolostone (marble; Ppdm). See Figure 8 (repeated below) for stratigraphic section of the Denham Formation. A. (South most). Gray biotite schist interpreted as metamorphosed siltstone (unit Ppds), overlain by brownish-tan weathered dolomitic arkose (unit Ppda). B. Pillow basalt flows (unit Ppdb). Classic basalt flow sequences, with massive bases that grade upward (north) into amygdaloidal pillow basalt, which in turn grades to coarsely fragmental or blocky flow tops. The fragmental and amygdaloidal character of the flows indicates eruption into a shallow-water environment. Two distinct flows are present in the valley, but in hills to the east where the unit thickens, as many as four flow sequences were identified. Strong stretching lineation evident in fragmental portions of flow is accentuated by differential weathering between clasts and matrix. Basalt geochemistry shows slight calc-alkaline tendencies (Southwick and others, 2001). Outcrop gap is inferred to represent garnet-staurolite-mica schist (unit Ppdg) that is exposed both east and west of this gap. C. Dolomitic arkose (unit Ppda). Weathered reddish-tan color due to abundant dolomite in matrix. Coarse sand-sized detrital grains of orthoclase and quartz, and rare cobbles identical to the McGrath Gneiss. The original clastic grains in the arkosic rocks are well preserved despite metamorphism, largely because strain has been partitioned into the comparatively more ductile dolomitic matrix. As discussed at the McGrath gneiss stop (8-1), the arkosic rocks contain almost no plagioclase, presumably due to sediment derivation from a weathered McGrath Gneiss source. ILSG 2011 149 Field Trip 8 �Figure 11. Photomicrograph of dolomitic arkose from Stop 8-2C. D. Fragmental volcanic rocks (unit Ppdv). Dark green amphibolitic clasts and scattered clasts of white cherty material are strongly flattened and lineated. In map view, this fragmental unit nearly merges to the east with the underlying pillow basalt, possibly indicating a common volcanic source located to the east beneath the Keweenawan Fond du Lac Formation. Holm and others (1993) obtained Ar/Ar plateau ages of 1,756 ±16 Ma on a hornblende separate, and 1754 ±13 Ma on a biotite separate from this amphibolitic unit. E. Dolomitic marble (unit Ppdm). Tan weathered, dark gray fresh. In drill holes (cores and cuttings) to the east marble changes from gray to white with depth, with intercepts of massive dolomite as much as 500 feet. ILSG 2011 150 Field Trip 8 �Figure 7 repeated. Stratigraphic section of the Denham Formation, showing relative locations of field trip stops. Age of McGrath gneiss from nearby outcrop, and age ranges of detrital zircons shown in boxes. ---Return to vehicles. Drive east approximately 0.75 miles on State road 52 to a “T” intersection with gravel road to the north. Go north on gravel road 0.65 miles to junction of the Soo Line Trail – a former railroad grade now utilized as an ATV trail. Park at trail junction and walk northeast on trail about 500 feet to outcrops and low road cuts. Stop 8-3: Metagraywacke and argillite Location: T.45N., R.20W., Sec. 19, SE 1/4 Denham 7.5’ quadrangle UTM: 506343 easting; 5134441 northing HIGHLIGHTS: GRADED BEDS, METAMORPHIC ASSEMBLAGE ILSG 2011 151 Field Trip 8 �Railroad cuts and flat outcrops of metagraywacke with pelitic beds. Pelitic units are micaceous and contain small garnet and staurolite crystals, although the latter are more abundant further to the northeast. Abundant 1 mm diameter garnets are synkinematic with respect to S1, and have a well-developed internal schistosity defined by quartz and ilmenite inclusions (Figure 12). At this locality, inclusion-rich cores are surrounded by haloes of inclusion-free garnet, and staurolite has overgrown both the schistosity and the crenulation cleavage. Garnet rim analyses give final equilibration temperatures of 520-590 degrees C and a pressure of ca. 6 kbar (Holm and Selverstone, 1990). Chemically homogeneous monazite grains at this locality give a prominent metamorphic age domain of ca. 1788 4 Ma and a less prominent age domain of 1840 Ma (McKenzie, 2004). In the series of outcrops to the northeast of this stop, bedding and S 1 cleavage are gently folded along a series of shallow east-plunging, northeast-striking F2 fold axes which are oriented similar to those at the last stop but are more open and upright. A series of 10-30 cm-thick graded beds and scoured crossbedding visible in the flat outcrop north of the trail give a sense of younging to the north. Figure 12. Photomicrograph of rolled garnet from stop 83. G – garnet; S – staurolite; B- biotite, matrix is quartz and plagioclase. Drill cores from approximately 1 mile southeast of this stop (locations shown on Figure 10) show that this metagraywacke conformably overlies the marble to the south (Stop 8-2). The transition between the graywacke and marble is marked by a thin (1 meter) interval of carbonaceous argillite. ---Return to vehicles. Take State road 52 back east to Highway 61. Turn left (north) on Highway 61 and go approximately 7.5 miles to junction with Highway 27 west at stop light in Moose Lake. Turn left (west) on Highway 27 a short distance to the Soo Line ATV trail; just before trail turn left into parking lot. Walk north along Soo Line trail for approximately 0.2 mile to outcrops along trail. ILSG 2011 152 Field Trip 8 �Stop 8-4: Schist at Moose Lake Location: T.46N., R.19W., Sec. 20, NE 1/4 Moose Lake 7.5’ quadrangle UTM start: 517815 easting; 5144635 northing UTM end: 518205 easting, 5145203 northing HIGHLIGHTS: GARNET-GRADE METAGRAYWACKE AND PELITIC SCHIST; FOLDED BEDDING AND S1 FOLIATION This pelitic schist is part of the same belt of rocks as the last stop, but here is north of the staurolite isograd (McSwiggen, 1987). A well-developed, bedding-parallel, moderately south-dipping S1 foliation is folded by steeply to moderately south-dipping F2 folds which are accompanied by an axial-planar crenulation cleavage (Figure 13). Internal inclusion trails in small (0.5 mm diameter) garnets show as much as 160 degrees of rotation. These garnets give final equilibration temperatures of 440-500 degrees C. The age of metamorphism here is ca. 1830 Ma based on monazite ages obtained from a similar grade rock south of this locality. Figure 13. Photomicrograph of crenulated micaceous phyllitic schist from this stop. Fine-grained micas define S1 fabric which is parallel to bedding; both are folded by D2 deformation. An east-northeast trending diabase dike of presumed Mesoproterozoic age may be visible towards the north end of this set of outcrops, but the exposure has become quite overgrown. The dike is well exposed farther north along this trail but this trip will not go there. ILSG 2011 153 Field Trip 8 �---Return to vehicles and drive back east to Highway 61. Turn left (north) on Highway 61 for approximately 4.5 miles to Barnum, turn right (west) on County Highway 6 for a short distance and park at the Willard Munger trail parking lot. Stop 8-5: Phyllitic schist at Barnum Location: T.46N., R.19W., Sec. 1, NW ¼ and Sec 2, NE ¼. Barnum 7.5’ quadrangle UTM: 523180 easting; 5149900 northing HIGHLIGHTS: BIOTITE-GRADE METAGRAYWACKE AND PELITIC SCHIST; FOLDED BEDDING AND S1 FOLIATION. Fine-grained, phyllitic, silvery micaceous schist metamorphosed to biotite isograd; this location is north of the garnet isograd (McSwiggen, 1987). Contains a strong subhorizontal to south-dipping, approximately bedding parallel foliation that is weakly folded along small east-plunging (5-10 degrees) fold axes. A weak S2 cleavage parallels the fold axes; in thin section a strong S2 crenulation cleavage is obvious (Figure 14). Micaceous minerals are dominantly chlorite and sericite with minor biotite. Figure 14. Photomicrograph of crenulated micaceous phyllitic schist from this stop. Earlier S1 foliation is strongly overprinted and transposed by S2 fabric; S1 is refracted through a sandy bed, in upper right part of photograph. --Return to Highway 61. Turn left (north) on Highway 61 for 6 miles to Mahtowa, continue northeast for another 0.5 miles. Veer left (north) on Brandt Road for 0.8 miles and turn left(west) onto Boundary Road/Township Road 85 after the first curve and proceed to second driveway on the ILSG 2011 154 Field Trip 8 �left (3260 Boundary Road). **Private property permission must be obtained before entering** From farmhouse walk south on trail across pasture to powerline, then east across low ground to knob of white quartz. Stop 8-6: Base of Thomson Formation, mafic intrusion Location: T.47N., R.18W., Sec. 4, NW ¼ and Sec 5, NW ¼. Barnum 7.5‘ quadrangle UTM start: 528370 easting; 515950 northing UTM end: 527540 easting, 5159462 northing Highlights: Chert, pyrite porphyroblasts, metagabbro. This stop is just south of what was ‗traditionally‘ considered to be the south edge of the Animikie basin, but is now considered to be the lowermost part of it, based on remapping and analogues to Michigan. There are a variety of rock types here including recrystallized chert, metagabbro/diorite, and pyritic slate. We will first walk east along the powerline to point ‗A‘ on the above figure, then will go back west to examine outcrops of phyllitic slate (B), and then to outcrops of sheared, porphyritic metagabbro (C). On the index map above the three black dots are the locations of core holes drilled in the 1980‘s by Rocky Mountain Energy. Summaries of those drill core logs are listed at the end of this stop. Substop A: This is a small knob composed mainly of recrystallized black chert that is flooded by white to pinkish-colored, pre-cleavage, quartz veins. A smaller outcrop to the west contains abundant small cubes of oxidized pyrite and is stained yellow and red. Petrographically this chert is composed of fine-grained saccharoidal quartz, minor muscovite, and (where black) abundant submicroscopic dusty opaque inclusions that are not identifiable by optical methods. Scattered rhombic shapes imply that there were dolomite porphyroblasts, which are now replaced by fine-grained quartz. McSwiggen (1987) identified thin intervals of dark gray chert in the drill cores drilled adjacent to this outcrop as being phosphatic. Other drill cores obtained along this lower, basal-most unit of the Animikie basin further west also intersect thin to thick beds of chert interbedded with pyritic slate. Substop B. Between substops A and C are some small outcrops of dark gray, finegrained phyllitic slate/schist in which bedding and S1 cleavage are kinked by a later S2 fabric. Scattered 2-3 cm square pits represent weathered-out cubes of pyrite. Also visible are small dark green clots that may be retrograde-altered porphyroblasts formed during an earlier prograde metamorphic event. The dark retrograded porphyroblasts and the pyrite cubes are both found elsewhere at this same stratigraphic horizon, for example near Kalevala township and near the Tamarack intrusion to the west (Figure 3). The retrograded ILSG 2011 155 Field Trip 8 �porphyroblasts (possibly after garnet or andalusite) contain an earlier, rotated S1 metamorphic foliation; pressure shadows are also evident (see Figure 15). Figure 15. A). Photomicrograph of drill core samples showing rotated, retrograde-altered porphyroblasts similar to those present at this stop. Bedding-parallel S1 foliation is highlighted by black lines and S2 fabric by white lines). B.) Close-up of rotated porphyroblasts. C.) Photograph of drill core from approximately 15 km west of this stop from the same stratigraphic horizon, showing characteristic pyrite porphyroblasts. Substop C: Green, variably shear-foliated metagabbro metamorphosed under greenschist-facies conditions. In less deformed lozenges 5-10% white plagioclase phenocrysts up to 3cm in size are preserved, but most of the phenocrysts are broken and strung out in the foliation plane and appear as flaser-like grains. Schist is composed of sericite, quartz, carbonate, chlorite, sphene/leucoxene, and minor sulfides, and is locally overprinted by rhombic carbonate and rarely, coarse-grained muscovite porphyroblasts. The foliation dip varies from 25 degrees east at the north end of the unit to 60 degree east at the south end; this is at a high angle to the regional fabric and is likely the result of cleavage refraction through this more competent unit. In thin section the main (presumably S1) fabric is weakly folded by a later D2 crenulation cleavage. Summary of drill core logs; locations of core holes shown on location map above: Drill Core MLCH-9 (vertical hole) 39-53 – Thinly laminated graphitic argillite. 53-430 – Strongly sheared/mylonitic, porphyritic metagabbroic rock. Flattened plagioclase phenocrysts give the rock a flaser-gneiss like appearance. Below 395 is generally less deformed. 430-550 – Thinly laminated tightly folded graphite-rich interbedded slate and metagraywacke with pyrite cubes, ankerite clots, and some bedding-parallel pyritic layers. Last 5 feet is strongly graphitic. ILSG 2011 156 Field Trip 8 �Drill Core MLCH-11 (0, -60) 9-36 – Dark gray sooty graphitic phyllitic argillite, variably pyritic, with pyrite cubes and local thin layers semi-massive sulfides. 36-42 – Fine-grained silicic graywacke with white siliceous lapilli-like fragments. 42-45 – Dark grayish-black chert, similar to nearby outcrop. 45-83 – Meta-argillite and graywacke, variably sulfidic and graphitic, minor chert. 83-137 – Variably shear-foliated metagabbroic rock. 137-242 – Fine-grained thinly laminated argillite with carbonate spots, locally graphitic, local layers rich in disseminated sulfides. 242-354 – Variably shear-foliated, porphyritic metagabbroic rock. Drill Core ML-1 (azimuth and inclination unknown; core incomplete) 9-105 – Weakly crenulated phyllitic slate, argillite, and graywacke, locally graphitic with minor stratiform sulfides. ---Return to Highway 61. Turn left (north) on Highway 61 for 6.6 miles to Gillogly Road. Turn right (south) on Gillogly Road and proceed 0.64 miles around curve to small outcrop just north of the crest of the hill, on the east edge of the road.--Stop 8-7: Weakly D2 – crenulated slate and graywacke Location: T.48N., R.17W., Sec. 21, NW 1/4 Iverson 7.5’ quadrangle UTM Start: 537374 easting; 5164270 northing HIGHLIGHTS: S1 FOLIATION AT A HIGH ANGLE TO BEDDING, BOTH AFFECTED BY D2 FOLDING. This stop is near the northern edge of the twice-deformed portion of southern edge of the Animikie basin. Graded beds strike roughly N60W, 60°N, with younging directions upright to the northeast. Bedding can easily be traced by following layers of carbonate concretions. This is one of the rare places where S1 foliation (developed mainly in the upper pelitic portions of the graded beds) is observed at a high angle to bedding. F2 fold axes plunge approximately 15° to N85E. This is one of the key outcrops used by Holst (1984; Figure 16) to conclude that the entire area of twice-deformed metasedimentary rocks is on the upper limb of a large, north-directed thrust nappe. The following sketch is from Holst (1984), showing the relationships between bedding (S0), early S1 cleavage, and later S2 cleavage. ILSG 2011 157 Field Trip 8 �Figure 16. Sketch showing relationships between bedding (S0), early S1 cleavage, and later S2 cleavage for stop 8-7. From Holst, 1984). --Go back to Highway 61 and continue north approximately 3 miles to the junction with Highway 210. Turn right (east) on Highway 210 and go approximately 2 miles into town, then take a right on County Highway 3/1 for 0.6 miles then stay left (east) on County Highway 1 Follow Highway 1 for 1.3 miles then turn left (east) on small road and park at Munger Trail adjacent to Highway 1. Walk north approximately 400 feet on the Munger trail to rock cuts on both sides of trail. --- Stop 8-8: Slate and graywacke with minimal S1 foliation Location: T.48N., R.17W., Sec. 21, NW 1/4 Cloquet 7.5’ quadrangle UTM Start: 545770 easting; 5166742 northing HIGHLIGHTS: VERY WEAK S2 FOLIATION, KEWEENAWAN DIABASE DIKE This stop is the northern-most location within the southern, twice-deformed terrane, and at this point the S1 fabric which is obvious to the south is nearly absent. On the index map above, the dashed line is the break between one deformation (north of the line) and two deformations (south of the line), as mapped by Clark (1985). Here the dominant cleavage is subvertical and parallel to gentle east-west trending F2 fold hinges; the earlier S1 cleavage is very weak and difficult to see. Thin sections show a very weak bedding-parallel foliation cross-cut by the stronger S2 cleavage (Figure 17). Bedding varies from thinly bedded slategraywacke sequences to thicker sandy graywacke beds up to 3 meters thick. Due to the nature of the outcrop the bedding can be hard to observe and a good way to see it is by ILSG 2011 158 Field Trip 8 �observing patterns of cleavage refraction through pelitic and sandy beds, and in a few places graded beds can be seen which show upright younging directions. Carbonate concretions are weakly flattened in the slaty cleavage plane. This stop and the last are both within classic Animikie basin-type sediments, i.e. low-grade graywacke-slate sequences, but unlike the bulk of the Animikie basin to the north, these have been twice-deformed, in the same manner as the phyllites and schists to the south which show progressively higher grades of metamorphism from north to south, as seen on earlier trip stops. The metamorphic grade and style of deformation here is virtually identical to that of the Thomson Formation at the type locality at Thomson Dam, which will be the last stop of this trip. Figure 17. Photomicrograph of slate from stop 8-8 showing very weak early S1 foliation (highlighted by black lines; parallel to bedding which is not visible in this photo), cut by S2 cleavage that is parallel to D2 fold axes (highlighted by white lines). Diabase Dike: Towards the north end of this outcrop the graywacke is cut by a near vertical, 6mwide diabase dike that is oriented approximately N45E. The dike has strongly chilled margins and in the center is fine-medium grained. Petrographic examination shows the chilled margin to be devitrified glass with small spherulitic or variolitic structures; the dike interior is composed of plagioclase, sub-prismatic augite, opaque Fe-Ti oxides, and minor olivine. Approximately 30% of the rock in the dike interior is turbid, opaque-dusted mesostasis that is likely devitrified glass which contains abundant small prismatic pyroxene grains. This dike is part of the Carlton dike swarm; see stop 8-10 for further description. ILSG 2011 159 Field Trip 8 �Stop 8-9: Slate and graywacke of Thomson Formation; Keweenawan diabase dikes Location: T.48N., R.16W., Sec. 5, SW 1/4 Cloquet 7.5’ quadrangle UTM Start: 546612 easting; 5168119 northing HIGHLIGHTS: VARIETY OF SEDIMENTARY STRUCTURES, FOLDS, DIABASE DIKES This is the type locality for the Thomson Formation, which is southern equivalent of the Virginia Formation along the Mesabi Iron Range. The Virginia Formation overlies the Biwabik Iron Formation, and collectively the Virginia/Thomson Formations cover a broad area (the Animikie basin) that extends south from the Mesabi Iron Range to a few miles south of this locality (Figure 1). Deformation of the Animikie strata increases to the south, in closer proximity to the Penokean fold and thrust belt. Here, near the south edge of the Animikie basin, the strata are folded into a series of gently east- and west-plunging, open symmetric to locally asymmetric, slightly overturned folds with near-vertical axial-planar cleavage. Several northeast-trending diabase dikes of Mesoproterozoic age cut the Thomson Formation. These dikes are part of the Carleton County dike swarm, which is one of a number of dike swarms that flank the Midcontinent rift system. The dikes here were emplaced along northeast-trending joints in the graywacke, and they exhibit well-developed subhorizontal columnar cooling joints and have chilled margins. The approximately 2 meter wide dike just below the parking area is reversely polarized, and like most of the dikes in the Carlton County swarm has a composition of high Fe-Ti continental tholeiite (Green and others, 1987). Overall, dikes of the Carlton swarm range from centimeters to greater than 60 meters in width. Narrow contact metamorphic albite-epidote hornfels are found adjacent to the contacts of only the thicker dikes. Green and others (1987) have summarized the geologic, geochemical, and paleomagnetic data on the dikes, and has demonstrated that these swarms are most likely Mesoproterozoic in age. ---Return to your vehicles in the parking lot to end the trip.--- ILSG 2011 160 Field Trip 8 �REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Minneapolis, University of Minnesota Ph.D. dissertation, 273 p. Boerboom, T.J., 2001, Bedrock geologic map and sections, pl. 2 of Boerboom, T.J., project manager, Geologic Atlas of Pine County, Minnesota: Minnesota Geological Survey County Atlas C-13, pt. A., 7 pls., scale 1:100,000. Boerboom, T.J., 2009, Plate 2 – Bedrock Geology, pl. 2 of Setterholm, D.R., project manager, Geologic Atlas of Carlton County, Minnesota, Minnesota Geological Survey County Geologic Atlas Series C-19, pt. A, 7 pls, scale 1:100,000. Boerboom, T.J., Runkel, A.C., and Chandler, V.W., 2001, Bedrock geology of Pine County, in Boerboom, T.J., project manager, Contributions to the geology of Pine County, Minnesota: Minnesota Geological Survey Report of Investigations 60, p. 1-18. Boerboom, T.J., Southwick, D.L., and Severson, M.J., 1999, Bedrock geology of the Mille Lacs Lake 30 x 60 minute quadrangle, east-central Minnesota: Minnesota Geological Survey Miscellaneous Map M-100, scale 1:100,000. Cannon, W.E., Schulz, K.J., Horton, J.W. Jr., and Kring, D.A., 2010, The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA: GSA Bulletin v. 122, no. 1/2, p. 50-75. Carlson, K.E., 1985, A combined analysis of gravity and magnetic anomalies in east-central Minnesota: Minneapolis, Minn., University of Minnesota, M.S. Thesis, 138 p. Clark, R.C., 1985, The structural geology of the Thomson Formation: Cloquet and Esko quadrangles, east-central Minnesota: M.Sc. Thesis, University of Minnesota-Duluth, 114 p. Doe, B.R., and Delevaux, M.H., 1991, Lead-isotope investigations in the Minnesota River Valley-- late-tectonic and posttectonic granites, in Morey, G.B., and Hanson, G.N., eds., Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper, v. 182, p. 105-112. Fralick, P., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, V. 39, p. 1085-1091. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G., Jr., Myers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: Evidence for evolution of the Middle Proterozoic midcontinent rift system of North America: in Halls, H.C., and Fahrig, W.F., Mafic Dyke Swarms, Geol. Assoc. Can. Spec. Pap. 34, pp. 289302. Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area, Ontario: constraints from U-Pb zircon and baddeleyite dating (Abs.): Institute on Lake Superior Geology 51, Part 1 – Program and Abstracts, p. 24-25. ILSG 2011 161 Field Trip 8 �Holm, D.K., 1986, A structural investigation and tectonic interpretation of the Penokean Orogeny: east-central Minnesota: M.S. Thesis, University of Minnesota – Duluth, 105 p. Holm, D.K., Holst, T.B., and Lux, D.R., 1993, Postcollisional cooling of the Penokean orogen in east-central Minnesota: Canadian Journal of Earth Sciences v. 30, p. 913-917. Holm, D.K., and Selverstone, J., 1990, Rapid growth and strain rates inferred from synkinematic garnets, Penokean orogeny, Minnesota: Geology, v. 18, p. 166-169. Holm, D.K., and Lux, D., 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, Minnesota: Geology, v. 24, p. 343-346. Holm, D.K., Darrah, K., and Lux, D., 1998, Evidence for widespread ~1760 Ma metamorphism and rapid crustal stabilization of the Early Proterozoic Penokean Orogen, Minnesota: American Journal of Science v. 298, p. 60-81. Holm, D., Holst, T., and Ellis, M.A., 1988, Oblique subduction, footwall deformation and imbrication: a model for the Penokean orogeny, Minnesota: Geological Society of America Bulletin, v. 100, p. 1811-1818. Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, D., 2005, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis; Geological Society of America Bulletin, March/April, v. 117, no. 3/4, p. 259-275 Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., Mckenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution: Precambrian Research v. 157, p. 106-126. Holst, T.B., 1984, Evidence for nappe development during the Early Proterozoic Penokean Orogeny, Minnesota: Geology, v. 12, p. 135-138. Jirsa, M.A., Weiblen, P.W., Vislova, T., and McSwiggen, P.L., 2008, Sudbury impactite layer near Gunflint Lake, NE Minnesota: Institute on Lake Superior Geology Volume 54, Part 1 – Programs and abstracts, p. 42-43. Maric, M., and Fralick, P., 2005, Sedimentology of the Rove and Virginia Formations and their tectonic significance: Institute on Lake Superior Geology 51, Part 1 – Program and abstracts, p. 41-42. Marsden, R.W., 1972, Cuyuna district, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 227-239. McKenzie, M.A., 2004, Age pattern and nature of Late Paleoproterozoic metamorphism of the Penokean crust, east-central Minnesota: M.S. thesis, Kent State University, 105 p. McSwiggen, P.L., 1987, Geology and geophysics of the Denham-Mahtowa area, east-central Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-63, scale 1:48,000. Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for eastcentral Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p. Morey, G.B., Olsen, B.M., and Southwick, D.L., 1981, Geologic map of Minnesota, eastcentral Minnesota, bedrock geology: Minnesota Geological Survey, scale 1:250,000. Scheiner, S.W., Boerboom, T.J., and Holm, D.K., 2011, Reinterpretation of Paleoproterozoic sedimentation and deformation in east-central Minnesota: Geological Society of ILSG 2011 162 Field Trip 8 �America Abstracts with Programs, Vol. 43, No. 1, p. 167; Northeastern (46th Annual) and North-Central (45th Annual) Joint Meeting (20–22 March 2011). Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1460, 96 p. Schneider, D.A., Holm, D.K., O‘Boyle, C., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA, in Whitney, D.L., Teyssier, C, and Siddoway, C.S., eds., Gneiss domes in orogeny: Geological Society of America Special Paper 380, p. 339-357. Schulz, K.A., and Cannon, W.E., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, V. 157, p. 4-25. Setterholm, D.R., Morey, G.B., Boerboom, T.J., and Lamons, R.C., 1989, Minnesota kaolin clay deposits—A subsurface study in selected areas of southwestern and east-central Minnesota: Minnesota Geological Survey, Information Circular 27, 99 p Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean Orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. Southwick, D.L., Boerboom, T.J., and McSwiggen, P.L., 2001, Geochemical characterization of Paleoproterozoic volcanic and hypabyssal igneous rocks, east-central Minnesota: Minnesota Geological Survey Report of Investigations 57, 33 p. Sun, WeiWei, Hudleston, P.J., and Jackson, M., 2005, Magnetic and petrofabric studies in the multiply deformed Thomson Formation, east-central Minnesota: Tectonophysics, vol. 249, iss. 1-2, p. 109-124. Tohver, E., Holm, D.K., van der Pluijm, B.A., Essene, E.J., and Cambray, F.W., 2007, Late Paleoproterozoic (geon 18 and 17) reactivation of the Neoarchean Great Lakes Tectonic Zone, northern Michigan, USA: Evidence from kinematic analysis, thermobarometry, and 40Ar/39Ar geochronology: Precambrian Research vol. 157, Nos. 1-4, p. 144-148. Vallini, D.A., Cannon, W.F., Schulz, K.J., and McNaughton, N.J., 2007, Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: Evidence for a widespread 1786 Ma overprint based on xenotime geochronology: Precambrian Research V. 157, p. 169-187. Van Schmus, W.R., 1976, Early and middle Proterozoic history of the Great Lakes area, North America: Philos. Trans. R. Soc. Lond., Ser A 280 (1298), p. 605-628. Wirth, K.R., Vervoort, J., Craddock, J.P., Davidson, C., Finley-Blasi, L., Kerber, L., Lundquist, R., Vorhies, S., and Walker, E., 2006, Source rock ages and patterns of sedimentation in the Lake Superior region: results of preliminary U-Pb detrital zircon studies: Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, Part 1 – Programs and abstracts, p. 69-71. Wunderman, R.L., and Young, C.T., 1987, Evidence for widespread basement decollement structures and related crustal asymmetry associated with the western limb of the Midcontinent Rift [abs.]: Institute on Lake Superior Geology, 33rd Annual Meeting, Wawa, Ontario, Proceedings and Abstracts, v. 33, pt. 1, p. 85-86. ILSG 2011 163 Field Trip 8 �ILSG 2011 164 Field Trip 8 �57TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY FIELD TRIP 9 GRANITIC, GABBROIC, AND ULTRAMAFIC ROCKS OF THE KEWEENAWAN MELLEN INTRUSIVE COMPLEX Intrusive breccianear Mellen, Wisconsin – T. Fitz ILSG 2011 165 Field Trip 9 �ILSG 2011 166 Field Trip 9 �Field Trip 9 Granitic, gabbroic, and ultramafic rocks of the Mellen Intrusive Complex in northern Wisconsin Tom Fitz Northland College 1411 Ellis Avenue Ashland, Wisconsin 54806 tfitz@northland.edu INTRODUCTION The Mellen Intrusive Complex (MIC) is a group of Keweenawan Midcontinent Rift (MCR) intrusions underlying 380 km2 in northern Wisconsin (Fig. 1). The Complex was intruded as large sill-like bodies into the lower part of the Keweenawan volcanic sequence 1102 Ma ago (Zartman et al., 1997). The rocks of the MIC, along with rest of the rocks in the region, were tilted steeply north during the compression and folding that created the Lake Superior Syncline (Cannon et al., 1993a). Subsequent erosion has exposed a complete cross section of this magmatic system – from the feeder dikes that brought magma from depth, to the subvolcanic magma chambers, and the overlying volcanic sequence (Woodruff, 2005). The MIC is bi-modal in composition, meaning mafic and felsic rocks dominate and there is a relatively small volume of intermediate rocks. This is similar to the rest of the igneous rocks of the MCR, and is typical of continental rifts. Anorthositic gabbro and granite are the two most common lithologies in the MIC, but there is a wide variety of rock types. This variety of rocks, as well as the fact that they are mostly pristine and well exposed, makes the MIC an excellent showcase of igneous petrology. The goal of this trip is to see this variety of rocks. The Mellen Complex is one of four major intrusive complexes of the MCR – these are: the Duluth Complex in northeastern Minnesota; the Nippigon sills near Lake Nippigon in Ontario; the Coldwell Complex on the north shore of Lake Superior in Ontario; and the Mellen Complex in Wisconsin (Miller, 2007). Another MCR layered mafic intrusion, the relatively small Clam Lake Intrusion, occurs at depth in the Clam Lake area 25 km southwest of the MIC. It is not exposed at the surface but is known from its magnetic and gravity signatures, and from two drill cores (Mudrey et al., 2003). Midcontinent rift magmatism produced an estimated 1,000,000 km3 of igneous rock over a period spanning 29 Ma. (Klewin and Shirey, 1991; Miller, 2007). It occurred in three main pulses: an early episode from about 1115 to 1107 Ma; the main phase that produced a huge volume of volcanic and plutonic rocks from about 1102 to 1092 Ma; and a third, less prodigious phase, from 1092 to 1086 Ma (Paces and Miller, 1993; Miller, 2007). Magnetic ILSG 2011 167 Field Trip 9 �polarity reversals occurred during rifting, so the remnant magnetism of the rocks has been important in deciphering the history of the MCR and making correlations between rocks in different parts of the rift: The early pulse of magmatism occurred during a time of reverse magnetic polarity, followed by a switch to normal polarity before the main phase of activity (Halls and Pesonen, 1982). Area of this field trip. Figure 1. Location map showing the Mellen Intrusive Complex in black, and the area of this field trip outlined in the white box. From the Bedrock Geologic Map of Wisconsin (Mudrey et al., 1982) The age of the MIC is known from U/Pb dating of zircon crystals from granite which yield an age of 1100.9 +/- 1.4 Ma, and from granophyre that give an age of 1102 +/- 2.8 Ma (Zartman et al, 1997). These dates, along with the contact relationships within the MIC, indicate the magma was intruded over a relatively short period of time very early in the main pulse of MCR magmatism. Its age is nearly the same as the Kallander Creek Volcanic rocks which it intruded, so the intrusions of the MIC probably represent crystallized magma chambers in the subvolcanic plumbing system of a large volcanic center located near what was is now Mellen, Wisconsin (Cannon, 1993b). INTRUSIONS OF THE MELLEN COMPLEX ILSG 2011 168 Field Trip 9 �The MIC consists of four intrusions– from west to east these are: the Mineral Lake Intrusion, the Rearing Pond Pluton,the Mellen Granite in the central region, and the Potato River Intrusions in the east (Fig. 2). The Mineral Lake and the Potato River Intrusions are each made up of two or more intrusions, whereas the Rearing Pond and Mellen Granite appear to be single intrusive bodies (Olmsted, 1979; Klewin, 1990; Seifert et al., 1992). Mellen Granite Rearing Pond Intrusion Mellen, Wisconsin 0 10 20 Miles N Figure 2. Map of the four main intrusions of the Mellen Intrusive Complex. The intrusions were emplaced into the lower parts of the volcanic sequence and likely fed magma upward to the surface where it was extruded as lava flows. In the eastern part of the Potato River intrusion, the sill-like gabbro bodies are nearly concordant and are within the lower part of the 2- to 4-km-thick Kallander Creek Volcanics(Klewin, 1990; Seifert et al., 1992).The intrusions are not exactly concordant however, and the level of emplacement gets progressively deeper toward the west (Fig. 3). The bottom of the MIC cuts through the Kallander Creek Volcanics and the underlying Siemens Creek Volcanics in the area of Mellen, Wisconsin. West of Mellen, the intrusions are in contact with the Paleoproterozoic Tyler Formation, and completely separate overlying volcanic rocks from the basement rocks on which they were deposited. In the area of Mineral Lake, the MIC crosses the Tyler and into the underlying Ironwood Formation. Then, finally, at the western end of the MIC the intrusions cutcross Archean rocks, so the entire Paleoproterozoic and lower Keweenawan volcanics are missing. In some areas, the intrusions occur at multiple levels and are separated by screens of volcanic rock preserved between parallel sill-like intrusive bodies. The upper intrusions must have been emplaced at relatively shallow levels since miarolitic cavities are quite common. The total thickness of the MIC varies from about 1 km to just over 5 km. ILSG 2011 169 Field Trip 9 �Strong igneous lamination defined by parallel alignment of plagioclase crystals is very apparent in many of the gabbroic rocks. Additionally, the large mafic bodies have largescale Figure 3. layering, with more mafic, olivine-rich gabbroic rocks near the base, which transition upward to a main section of anorthositic gabbro, which grades upward into intermediate to Geologic map of the western part of the Mellen Intrusive Complex. Field trip stops are show in white boxes. Formations are labeled on the map and rocks of the Mellen Complex have these symbols: Mineral Lake Intrusion (shown light green and dark green): Ymo: olivinebearing gabbroic rocks; Ymg: gabbro, anorthositic gabbro, gabbroic anorthosite; Ymf: ferrrodiorite; Ympg: granophyre. Rearing Pond Intrusion (shown in blue): Yrp: olivine gabbro; Yrs: serpentinized peridotite. Mellen granite ILSG 2011 (shown in medium Fre da San dsto ne 1 Gab bro, An orth osit e 0 1 2 3 4 5 M ile s Kal lan der Cre ek Vol cani cs 6 2 3 5 Puri tan Bat holi th Oli vin e Ga bbr o 4 Tyl er For mat ion 12 7 N 9 8 170 10 , 11 Kal lan der Cre ek Vol cani cs Con glo mer ate of Fre da For mat ion Field Trip 9 �felsic rocks near the top. The cumulate texture and broad compositional layering indicates the magmas were emplaced as crystal mushes that underwent crystal settling and fractional crystallization, and possibly some contamination, to create the variety of rocks present today (Seifert et al., 1992). However, well-defined small-scale compositional layering is not common. Some of the granitic intrusions have sharp chilled contacts against the gabbroic rocks, which means that not all of the felsic magma was created by fractionation within the preserved intrusions. Rocks of the Mellen Complex were extensively quarried for building stone starting in the early 20th century and some quarrying activity continues today. The black, coarsegrained gabbro, which has the trade name ―black granite‖, was widely used for decorative stone and monuments. It serves as structural stone in many historic buildings in the area, including numerous buildings in the town of Mellen and at Copper Falls State Park. In 1963, a monument made of Mellen ―black granite‖ was shipped to Arlington National Cemetery for the tombstone of the late President John F. Kennedy. Although no cutting and polishing operations remain in the area, several quarries still extract gabbro and granite for ballast rock and rip rap. Small abandoned quarries are common in the area and provide excellent bedrock exposures. The importance of quarrying in the local history is evident in the name of the Mellen school athletic teams – the ―Granite Diggers‖. Mineral Lake Intrusion The Mineral Lake Intrusion (MLI) consists of two intrusive bodies separated by a screen of Kallander Creek Volcanics (Seifert et al., 1992). The lower intrusion is 4.5 km thick and has a basal olivine gabbro, a thick central section of anorthositic gabbro, and an upper ferrodiorite zone. The composition of the plagioclase changes gradually upward in the intrusion from a modal value of An64 near the base to An23 near the top (Seifert et al., 1992). Cumulate texture is common and Seifert et al (1992) concluded that trapped intercumulus liquid constituted 25–35% of the volume of the accumulated crystal mush. The central portion of the intrusion consists of a large volume of coarse-grained anorthositic gabbro composed of plagioclase feldspar, augite, and minor amounts of olivine and magnetite. The igneous lamination defined by the alignment of large plagioclase crystals is distinct in most of this zone. The lamination dips about 70o northwest and is generally parallel to the regional structure, which indicates the crystals are aligned parallel to the floor of the intrusion, probably due to crystal settling. The central zone grades upward into gabbroic rocks with hornblende instead of olivine and augite, and eventually into diorite with small amounts of quartz and alkali feldspar. The diorite is iron rich and has a relatively high proportion of magnetite, a characteristic special enough to warrant the name ―ferrodiorite‖. It is a dark-colored rock with an appearance similar to gabbro, but close inspection reveals that its mineralogy is distinctly different than that of gabbroic rocks – it is composed of plagioclase feldspar, hornblende, magnetite, in some places biotite, and minor amounts of quartz and alkali feldspar. It does not contain olivine or augite as do most of the gabbroic rocks lower in the MLI. Based on REE data, Olmsted et al. (1986) concluded that the ferrodiorite is the product of fractional crystallization and that final crystallization took place at a temperature of 850 o– 750oC. Granophyre is abundant in the upper parts of the MLI, especially in the upper part of the higher of the two intrusions. The granophyre is brick red and usually fine to medium ILSG 2011 171 Field Trip 9 �grained for a granitic rock. It has spectacular micrographic intergrowths of quartz and microperthitic orthoclase feldspar that often penetrates the entire rock. Although it is usually visible only at the microscopic scale, it is this textural characteristic that distinguishes it from granite. Leighton (1954) found that the micrographic texture is well developed only in rocks in which the An content of the plagioclase is within the feldspar exsolution lamellae is within a restricted range. Klewin (1990) concluded that the roof-zone granophyres were residual liquids intruded along the upper margin of the MLI. Rearing Pond Intrusion The Rearing Pond Intrusion (RPI) is a relatively small intrusion mostly contained within the larger MLI. It is interpreted to have intruded as a single magma body into the MLI, but the nature of the contact between the two bodies is unknown because of poor exposure. The RPI is the most mafic intrusion of the MIC, being dominated by Mg-rich olivine gabbro and troctolite. Cumulate layers change upward from an olivine-rich peridotite layer near the base of the intrusion, through olivine gabbro in the middle, to gabbro near the top. The composition of plagioclase changes vertically as well, from An80 near the base to An60 near the top. This sequence is interpreted to have been created by fractional crystallization of olivine, followed by olivine, plagioclase, and clinopyroxene (Olmsted, 1979). Although the basal peridotitelayer is partially serpentinized, it locally has wellpreserved olivine crystals and igneous textures. Potato River Intrusion The Potato River Intrusion (PRI) is similar to the MLI in that it has a lower zone of olivine gabbro overlain by a thick section of anorthositic gabbro that has conspicuous igneous lamination, and granophyre near the top (Klewin, 1990; Tabet and Mangham, 1978). It differs from the MLI in that it has a larger volume of anorthosite. Mellen Granite The Mellen Granite (MG) occupies the central portion of the MIC and is in contact with the MLI on the west and the PRI on the east (Fig. 3). It is a medium-grained,grey to pink biotite granite. It includes xenoliths of gabbro and ferrodiorite that closely resemble those rock types elsewhere in the MIC and are probably locally derived. The lack of significant chilled margins in the contact zone indicates the granite was intruded while the mafic rocks were still relatively hot, and the presence of miarolitic cavities indicates the granite was emplaced at a depth of less than about 6 km. Together those facts attest to the rapid succession of emplacement of the MIC high in the Keweenawan volcanic sequence. The granite is interpreted to be a stock intruded near the end of the igneous activity that created the MIC and Kallander Creek Volcanics (Cannon et al., 1993b). In some areas on the east and west sides of the MG, the contact between the granite and the gabbroic country rocks contains a zone of extremely brecciated gabbro xenoliths (Sikkila, 2002). The gabbro country rocks were complexly intruded by the granite,creating intrusive breccia comprised of angular blocks that appear to have been shattered by the intrusion. Additionally, there are often xenoliths of several different types of gabbroic rocks and ferrodiorite mixed together in the breccia zone, suggesting that the brecciation was followed by mixing of the inclusions within the granite (Cervin, 2009; Goscinak, 2010). The granite in some breccia zones has highly variable texture showing a complex arrangement of ILSG 2011 172 Field Trip 9 �pegmatite, graphic granite, and hypidiomorphic granular granite. The combination of a diverse range mafic rock xenoliths, complex brecciation, and highly variable granitic textures makes this a very distinctive and intriguing intrusive breccia. FIELD TRIP STOPS The field trip starts near Sanborn, Wisconsin and goes through the Chequamegon National Forest east to Mellen, Wisconsin on gravel roads maintained by the U.S. Forest Service (Fig. 4). The roads are generally well maintained but may be impassible during mud season. The U.S.G.S. topographic quadrangle maps covering the area of this field trip are, from west to east: Marengo Lake, Mineral Lake, and Mellen, Wisconsin. All locations in this chapter are given in UTM coordinates (Zone 15 T) with NAD 1927, which is the datum for all the local topographic maps. This guide focuses on the western part of the MIC– specifically the MLI, the RPI, and the MG. It has one stop in the very western part of the PRI and a dike that penetrated up through the underlying Tyler Formation into the PRI. A detailed description of the eastern part of the Mellen Complex, with numerous stops in the PRI, can be found in the 1989 ILSG field guide chapter by Klewin, et al. In this chapter, rock names for the gabbroic rocks are based on the classification system of Miller et al., 2002, and the felsic and intermediate rocks are classified according to the IUGS classification system (Middlemost, 1991; Streckeisen, 1976). C County Rt. C Co. Lin e Rd. Ba ss La ke R oa d C FR 18 7 Stop 4 Stop 1 Rt. 13 C Stop 12 County Rt. C Stop 3 FR-189 Spring Brook Rd. Stops 9, 10, 11 Stop 2 Mellen FR-199 Quarry Road English Lake Stop 7 Rt. 13 Mineral Lake Stop 5 Stop 6 0 Stop 8 GG FR-187 1 2 3 4 5 Miles N Figure 4. Road map showing location of field trip stops. ILSG 2011 173 Field Trip 9 �Stop 1: Granophyre at Morgan Falls Location: 0659762 mE, 5134883 mN;T45N, R4W, Sec. 30, west-central. Park at the Forest Service parking lot for Morgan Falls and St. Peter‘s Dome located on the east side of Forest Service road FS-199(Fig.5). The UTM coordinates listed above are for the parking lot. From the parking lot, walk east on the trail to St. Peter‘s Dome for approximately 0.25 miles, then bear right (south) at the fork and take the trail to Morgan Falls. This area has good exposure of gabbro and granophyre in the upper part of the MLI. These intrusions were emplaced as large sills concordant with lava flows of the Kallander Creek Volcanic rocks, which are exposed on the hills across the valley to the north. Screens of Kallander Creek Volcanics are preserved between the sills but are not well exposed because they are relatively non-resistant in comparison to the hard intrusive rocks. In the area of this stop, volcanic rocks of the Kallander Creek served as the roof rocks, below which is a relatively thin roof zone of gabbro, below which is a large granophyre sill that is exposed at Morgan Falls and on St. Peter‘s Dome (Cannon et al., 1996). Stop 1 Parking Stop 1 Figure 9. Topographic map showing the area of Stop 1. Stop 1-A: Gabbro of the Mineral Lake Intrusions Location: Near Morgan Falls, on the west side of trail; 0660363 mE, 5134684 mN. The first outcrops encountered along the trail are just south of the bridge over Morgan Creek, on the west side of the trail. These outcrops are all in gabbro of the MLI. This is the uppermost intrusion of the MLI, and this outcrop is approximately 200 – 300 m below the roof of the intrusion. The rock is gabbro,or oxide gabbro, containing approximately 80% plagioclase feldspar, 15% oxide mineral, at least some of which is magnetite, 5% ILSG 2011 174 Field Trip 9 �intercumulus augite and hornblende, and a trace of quartz. This lithology is typical of this part of the MLI. Stop 1-B: Granophyre at Morgan Falls Location: At the end of the Morgan Falls trail; 0659762 mE, 5134884 mN. Scenic Morgan Falls is a 70-foot cascade in a narrow rock canyon where Morgan Creek falls over dark red granophyre. The joint pattern in the rock is distinct and exerts a strong control on the form of the rock face and the flow of the water over the falls. The rock at the falls is a dark red, medium-grained granophyre. It is composed almost entirely of quartz and microperthitic orthoclase feldspar crystals intergrown in a micrographic texture. The micrographic texture is not readily apparent in hand sample, especially in finer-grained samples, but is very distinctive in thin section (Fig. 6). The contact between the granophyre and the overlying gabbro is exposed in the sides of the valley to the west, but the slopes are extremely steep and treacherous so we will not be visiting the outcrop. The contact is sharp and the granophyre has a distinct chilled margin against the medium-grained gabbro, indicating the granophyre intruded into the gabbro after the gabbro had cooled. Figure 6. Photomicrograph of granophyre from Morgan Falls at Stop 1. The image was taken with plane transmitted light, and is shown here in black-and-white. The rock is composed primarily of alkali feldspar (dark grey) and quartz (light grey) intergrown in micrographic texture. The field of view is 2 mm. ILSG 2011 175 Field Trip 9 �Stop 2: Anorthositic Gabbro of the Mineral Lake Intrusion Location: South side of road FS-189; T45N, R4W, Sec. 30, west-central; 0666203 mE, 5133192 mN. There are several large outcrops in the woods immediately south of the road. The rock at this location is coarse-grained anorthositic gabbro with a strong parallel alignment of plagioclase feldspar defining a lamination that dips 70o northwest. This rock is very typical of the central zone of the MLI, and is the most common rock type in the intrusion overall. The rock is composed of 80–85% plagioclase, the rest being intercumulusferromagnesian silicate minerals, now biotite and hornblende. The southern part of the outcrop has some small-scale modal layering. Stop 3: Olivine Melagabbro and Peridotite of the Rearing Pond Intrusion Location: T45N, R4W, Sec. 26, SW1/4; start of hike: 066638 mE, 5133660 mN; hilltop destination: 666713 mE, 5134266 mN. This stop involves a 0.4-mile bushwhack north from FS-189 to a hilltop with exposures of olivine melagabbro and peridotite. From the starting location hike N10oW to the ridge, then N65oE to the end of the ridge where there are several large outcrops of olivine melagabbro. There is a concrete foundation and detritus from a work camp on the highest point. Stop 3 Start of hike to Stop 3 Stop 2 Figure 7. Topographic map of the area of Stops 2 and 3. ILSG 2011 176 Field Trip 9 �The rock exposed on the hilltop is olivine melagabbro composed of approximately 50% plagioclase feldspar, 25% olivine, 20% augite, and 5% oxide mineral. There is no lamination, which is typical of the RPI. Hike northwest, downhill a short distance to outcrops of peridotite. The rock is black, medium-grained, olivine-rich, partially serpentinized peridotite. The outcrop extends down the steep hill to the valley floor – hike carefully down this slope and then return to FR-189. There are few outcrops of the peridotite, but Klewin et al., 1987 describe another outcrop of this unit about 1 km northeast of this stop. Perhaps the best exposures are located in the ledges above the Brunsweiler River in T45N, R4W, Sec. 23 SW1/4; 0667514 mE, 5135895 mN. We will not be visiting these outcrops on this trip because of their remote location. At that location the rock is slightly coarser grained and less serpentinized. It is composed of approximately 50% olivine, 15% orthopyroxene, 10% opaque oxide mineral, and 25% serpentine(Fig 8). The large orthopyroxene crystals enclose euhedral olivine crystals. Some of the serpentine contains thin veins of parallel-aligned acicular crystals oriented perpendicular to the margins of the vein, which is typical of serpentinites. Figure 8. Photomicrographs of serpentinized peridotite of the Rearing Pond Intrusion. The upper image was taken with plane light, the lower image is the same view, but with crossed polars. The rock is composed primarily of olivine (highly birefringent), magnetite (opaque), and serpentine (fine-grained, low birefringence). Field of view is 2 mm. This rock is not from the outcrop at Stop 3, but is from an outcrop of the same unit at 0667514 mE, 5135895 mN. ILSG 2011 177 Field Trip 9 �Stop 4: Ferrodiorite in the Upper Part of the Mineral Lake Intrusion Location: T45N, R4W, Sec 25, north-central,0668365 mE; 5134955 mN The rock at this stop is within the thin ferrodiorite unit at the top of the MLI. The dark-colored ferrodiorite somewhat resembles gabbro, but it‘s mineral composition is distinctly different than that of the underlying gabbroic rocks. The rock is composed of plagioclase, hornblende, magnetite, and minor amounts of alkali feldspar and quartz. The proportion of alkali feldspar and quartz increases to the north, toward the top of the unit. Stop 5: Olivine Gabbro of the Mineral Lake Intrusion Location: T44N, R4W, Sec. 13, NW1/4; Park along the road near the entrance to the Mineral Lake boat ramp (0668420 mE, 5129159 mN). This stop includes one outcrop a short distance into the woods on the west side of Mineral Lake Road (Stop 4A), and several outcrops on along the ridge on the east side of the road (Stop 4B) (Fig. 9). Stop 5 Figure 9. Topographic map showing the area of Stop 5. Stop 5A: Coarse-Grained Gabbro Location: T44N, R4W, Sec. 13, NW 1/4; 0668392 mE, 5129146 mN. This outcrop is located in the woods on west side of Mineral Lake Road. The rock is coarse-grained gabbro composed of approximately 80% plagioclase feldspar, 20% intercumulus pyroxene that is now mostly actinolite, 5% oxide minerals, some of which is magnetic, some of which is not. It does not show laminations or compositional layering. ILSG 2011 178 Field Trip 9 �Stop 5B: Troctolite and olivine melagabbro Location: 0668468mE, 5129193 mN. The outcrops of this stop are located in the woods a short distance east of Mineral Lake Road. The low ridge trending northeast toward Potter Lake has numerous exposures of the olivine gabbro, troctolite, and melatroctolite of the basal zone of the MLI. The rock varies in grain size and proportion of olivine, with some of the finer-grained rocks containing up to 50% olivine. There is a trace of sulfide mineral present. The map of Cannon et al, 1996 shows a Cu-Ni zone here, and Bakheit (1981) studied the petrography of Cu-Ni mineralization in this area. It is likely the rockshere are compositionally layered, although it is not apparent in individual exposures because of the relatively small size of most outcrops. These rocks are within 200 m of the base of the MLI. The basal contact zone is not exposed in the area of Mineral Lake, but it is exposed in one place approximately 2.5 km east of here near English Lake. This field trip will not be visiting that basal contact exposure because of difficulties with access, but the site is described in the Klewin et al., 1989ILSG guidebook chapter (their Stop 7). The exposed contact has a chill zone where fine-grained gabbro of the MLI is in contact with contactmetamorphosed rocks of the Tyler Formation. Stop 6: Metamorphosed Iron Formation Below the Mineral Lake Intrusion Location: T44N, R4W, Sec 14, NE 1/4; 0667764 mE, 5128518 mN. This outcrop is located in the woods just south of County Road GG. Use caution while parking along the road because the shoulder is narrow; there a small pull-off on the shoulder on the north side of the road not far west of the outcrop that has adequate room for one car. This outcrop is in the upper part of the Ironwood Iron Formation where iron formation has been contact metamorphosed by the MLI. Although the contact is not exposed, olivine gabbro of the MLI is not far to the north. The rock here is black, dense, metamorphosed siliceous iron formation. It is composed of approximately 50% dark green to black olivine, 30% black amphibole, and 20% quartz. This is a relatively rare lithology – rocks containing both quartz and olivine are not common and are restricted to just a few geologic settings, one of which is where siliceous iron formation has undergone hightemperature metamorphism. The situation here, where the Ironwood Iron Formation was contact metamorphosed by the MLI, hadone of these special circumstances in which olivine crystals could grow in a silicic metamorphic rock. The fact that the olivine is in an iron-rich rock and occurs with quartz – which is not stable with Mg-rich olivine – suggests it is ironrich (fayalitic) olivine. Stop 7: Quarry in Coarse-Grained Anorthositic Gabbro of the Mineral Lake Intrusion Location: T44N, R3W, Sec. 4, NW 1/4; 0672101 mE, 5132430 mN. This abandoned quarry is in very coarse-grained anorthositic gabbro. The rock is composed of plagioclase feldspar, hornblende, some augite, and non-magnetic oxide minerals. The quarry dump contains large blocks of freshly broken gabbro displaying a wide range of textures, including some pegmatitic gabbro. Hornblende is more common than augite as the ferromagnesian mineral in these rocks, which is common in upper parts of the MLI. ILSG 2011 179 Field Trip 9 �Stop 8: Feeder Dike at Mellen Location: T44N, R2W, Sec. 6, NE 1/4; 0680368 mE, 5132423 mN; near the intersection of Fayette Avenue and Highway 77 in Mellen, Wisconsin. This outcrop is fine-grained olivine gabbro within a dike that cuts across the Tyler Formation. It lies immediately below rocks of the PRI and is likely a feeder dike approximately 100 – 200 m wide that carried magma upward into the base of the PRI. This rock is considerably finer grained than most of the gabbroic rocks of the MIC becausethe relatively small size of this intrusion caused it to cool quite rapidly. The dike creates a resistant ridge that trends south from here, down-section, into the area of less-resistant slate of the Tyler Formation. Stop 9: Anorthositic Gabbro of the Potato River Intrusion Location: East side of Wisconsin Highway 13, 0.5 miles north of Mellen, Wisconsin; T45N, R2W, Sec. 31, NE ¼; 0680249 mE, 5133847 mN. This is a large highway road cut outcrop of anorthositic gabbro in the western part of the PRI. It is composed of plagioclase feldspar with a strong parallel alignment that dips north, and minor amounts of augite and magnetite. At the south end of the outcrop there is a 5-cm-wide basalt dike that dips south, cutting across the lamination. Further north along the outcrop, the rock surface is not freshly broken gabbro, but instead is joint and minor fault surfaces covered with epidote and calcite. In a few areas where freshly broken gabbro is exposed, there are tabular zones approximately 2 cm wide of very fine-grained light-colored rock interpreted to be gabbroic mylonite created along minor faults. The next outcrop to the north along the highway is similar in that its surface is mostly fine-grained epidote. The area apparently has numerous small-scale faults, along which the rock broke when it was blasted during highway construction, so what is exposed on the surface is mostly epidote. Stop 10: Intrusive Breccia of the Mellen Granite – South Outcrop Location: T45N, R2W, Sec. 30, SW ¼; 0679689 mE, 5134810 mN. This is a large highway road cut on the west side of Wisconsin State Highway 13, 1.3 miles north of Mellen. The rocks on the upper slopes of this outcrop are unstable so climbing would be hazardous, and if knocked out of place, the blocks would pose a significant hazard to anyone below. Thus, do not climb the outcrop, examine it from ground level. This outcrop is in the contact zone of the MG; the main body of the MG lies to the west, and the PRI lies to the east. Grey granite of the MG has intruded and brecciated mafic and intermediate rocks of the MIC, creating an intrusive breccia that is volumetrically about half inclusions and half granite. The granite is grey, medium-grained hypidiomorphic granite typical of the MG. The xenoliths are composed of anorthositic gabbro, biotite-rich gabbro, ferrodiorite, minor troctolite, and probably others as well. The ferrodiorite inclusions tend to be finer grained than other inclusions. The biotite-rich gabbro, informally called the ―biotite cluster gabbro‖, tends to be the coarsest grained of the inclusions. Cervin‘s (2009) study of this outcrop and that of Stop 11 included whole-rock chemical analyses of the MG and fivedifferent xenolithic rocks from the breccia zone (Table 1). The dark-colored mafic- and intermediate-rock xenoliths have been broken into a complex array of angular fragments, some of which are not completely fragmented but have thin intrusions of granite that nearly penetrate the xenolith (see photograph on the cover page ILSG 2011 180 Field Trip 9 �of this chapter). In some xenoliths, the granite is finer grained within the fractures than in the surrounding granite. This is interpreted to be the result of filter pressing of the magma – liquid was forced into the cracks but larger feldspar crystals in the crystal mush could not travel into the small nascent fractures. No rocks other than MIC lithologies are present as inclusions, so they are probably all locally derived. However, the inclusions constitute a wide variety of rock types. Some of this heterogeneity may be due to intrusion of the granite into a zone of the PRI that had a complex mixture of rock types in one area, but at least some of the compositional heterogeneity is likely the result of entraining and mixing of inclusions from different sources within the MIC. Table 1. Whole-rock chemical analyses of Mellen Granite and selected xenoliths in the intrusive breccias. Sample 8-10-13 8-10-11 8-10-16 8-10-1 8-10-14 8-10-15 Rock Mellen Ferro- Anortho Gabbro Hbl-rich Fn-grain Granite Diorite Gabbro Fe-Dior Troctolite SB NB SB NB SB SB SiO2 72.22 55.82 51.24 48.07 46.95 43.44 % 0.01 Al2O3 13.45 12.74 22.81 20.72 11.14 12.11 % 0.01 Fe2O3(T) 1.9 13.09 6.71 9.48 20.35 20.08 % 0.01 MnO 0.02 0.174 0.076 0.11 0.236 0.229 % 0.001 MgO 0.25 3.01 1.97 6.61 4.83 18.08 % 0.01 CaO 1.13 5.67 10.29 9.53 9.1 5.57 % 0.01 Na2O 3.47 3.29 4.3 2.76 3 1.64 % 0.01 Location Units Detection Limit Analyte K2O 4.9 2.02 0.62 0.62 0.75 0.21 % 0.01 TiO2 0.161 2.461 1.28 0.682 3.439 0.16 % 0.001 P2O5 0.03 0.47 0.25 0.06 0.54 0.03 % 0.01 LOI 0.52 0.7 0.39 0.94 -0.01 -0.99 % Total 98.04 99.44 99.94 99.58 100.3 100.6 Ba 2449 594 197 123 211 62 ppm 2 Sr 102 171 395 376 176 199 ppm 2 Y 77 72 20 6 61 2 ppm 1 % 0.01 Sc 1 25 10 9 35 6 ppm 1 Zr 626 518 117 36 321 35 ppm 2 Be 4 3 1 <1 3 <1 ppm 1 V 11 266 213 93 441 31 ppm 5 Locations: SB=South Breccia outcrop; NB=North Breccia outcrop Method: FUS-ICP Actlabs Report Date: 14/04/2009 Also reported in: Cervin, 2009 ILSG 2011 181 Field Trip 9 �Stop 11: Intrusive Breccia of the Mellen Granite – North Outcrop Location: T45N, R2W, Sec. 31, SW ¼; 0679517 mE, 5135384 mN. This is a low road-cut outcrop on the west side of Highway 13, 1.8 miles north of the town of Mellen. The rock at this outcrop is similar to that of Stop 9 – intrusive breccia with granite enclosing angular inclusions of a variety of mafic and intermediate lithologies of the MIC. The granite is different here though – it is pink granite with a wide variety of textures. Some is granitewith hypidiomorphic granular texture typical of the MG, but this grades into zones of graphic granite that in turn grade inward to pegmatitic zones. Some of the pegmatitic zones have concentric zoning around central miarolitic cavities that contain euhedral quartz crystals, others are filled with calcite. There is also zoning of the granite outward from inclusions (Fig. 10), suggesting the inclusions influenced the crystallization of the granite. The textures of this granite indicate complex crystallization processes in the granite of the breccia zone. Sikkila (2002) concluded there was significant potassium metasomatism here, which altered plagioclase to potassium feldspar and caused alteration of ferromagnesian minerals in both the granite and the xenoliths. The complex variation in textures and the potassium metasomatism indicate that water was important in petrologic processes the during and after crystallization of this rock. Figure 10. Intrusive breccia at ―the north breccia outcrop‖ of Stop 11. The rock has darkcolored, angular inclusions of ferrodiorite enclosed in pegmatitic granite. Quarter for scale. Goscinak‘ s (2010)detailed mapping of this outcrop revealed that the lithology of the inclusions defines three zones within the outcrop (Fig. 11). Ferrodiorite is the dominant rock type of inclusions in the central section, whereas ―biotite-cluster gabbro‖ occurs at the north and south ends of the outcrop. If, as the evidence suggests, the intrusive breccia was created by forceful injection of granite at a shallow level, then perhaps the magma exploded to the surface as a pyroclastic eruption. This hypothesis is consistent with the presence of large rhyolite units in the Kallander Creek Volcanics, so perhaps there is a connection between the intrusive breccias and the older rhyolites. ILSG 2011 182 Field Trip 9 �Figure 11. Zones of xenolith composition in the ―north breccia outcrop‖ of Stop 10. From Goscinak, 2010. Stop 12: Fault in Mellen Granite Location: T45N, R3W, Sec. 27, west-central; 0674116 mE, 5134740 mN. This quarry is owned and operated by Milestone Materials Corporation. Permission must be obtained from Milestone Materials before venturing into the quarry. This is an active quarry with significant hazards such as loose rock on quarry walls and in rock piles. Use great caution while in the quarry; do not climb on any rock faces, and do not even approach the tall faces on the south side of the quarry. Piles of freshly broken rock in the quarry allow detailed examination of the granite, which is typical pink granite of the MG. Inclusions of ferrodiorite are common, some of which have gradational contacts with the granite. The outstanding feature at this stop is the large fault surface on the east side of the quarry. The fault surface is green due to a neomineral coating of epidote on and within the fault zone. Slickenlines and fault mullions on the surface clearly indicate strike-slip motion. The fault is not a single surface but instead is a zone of nearly parallel surfaces that enclose lens-shaped bodies of largely undeformed granite. The fault zones are 1–2 cm wide and contain angular fragments of granite enclosed within the fine-grained epidote-rich cataclasite. The potassium feldspar in the granite along the fault is slightly darker red, evidently due to metasomatism. The timing of faulting and the amount of displacement on the fault are unknown, but since the rock is identical on both sides of the fault, the offset is not on the order of kilometers. Ferrodiorite is exposed along both sides of the access road into the quarry. There is an especially good outcrop of the ferrodiorite on the west side of the road: 0674139 mE, 5134942 mN. The contact between the ferrodiorite and the granite is not exposed here, but based on their relationships elsewhere, the granite is probably intrusive into the ferrodiorite. ACKNOWLEDGEMENTS Dan Cervin and Chris Goscinak studiedthe ―north breccia outcrop‖ of Stop 11 for their senior capstone research projects at Northland College. Their companionship during our exploration of that outcrop and many others within the MIC is greatly appreciated. Their ILSG 2011 183 Field Trip 9 �work has both answered and raised many questions about the origin of the intrusive breccias. I am grateful to Bill Cannon for his suggestions for field trip stops and for his time in the field showing me some especially interesting rocks in and around the MIC. REFERENCES Bakheit, A.K., 1981, Petrography of Cu-Ni mineralization in Mineral Lake area, Ashland County, Wisconsin: Madison, Wisconsin, University of Wisconsin—Madison, M.S. thesis, 104 p. Cannon, W.F., Woodruff, L.G., Nicholson, S.W., and Hedgman, C.A., 1996, Bedrock geologic map of the Ashland and the northern part of the Ironwood 30‘x60‘ quadrangles, Wisconsin and Michigan: U.S. Geological Survey, Miscellaneous Investigation Series Map I-2556, scale 1:100,000. Cannon, W.F., Nicholson, S.W., Zartman, R.E., Davis, D.W., 1993a, Kallander Creek Volcanics—a remnant of a Keweenawan central volcano centered near Mellen, Wisconsin [abs.], Institute on Lake Superior Geology, 39th Annual Meeting, Thunder Bay, Ontario, Proceedings, p. 20-21. Cannon, W.F., Peterman, S.E., and Sims, P.K., 1993b, Crustal-scale thickening and the origin of the Montreal River Monocline—a 35-km-thick cross section of the Midcontinent Rift in Northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. Cervin, D., 2009, Geology of igneous breccia in the border zone of the Mellen Granite near Mellen, Wisconsin: Northland College, Ashland, Wisconsin, senior thesis, 21 p. Goscinak, C., 2010, Lithologies and intrusive relationships of the igneous breccia of the Mellen Igneous Complex near Mellen, Wisconsin: Northland College, Ashland, Wisconsin, senior thesis, 10 p., 1 plate. Halls, H.C., and Pesonen, L.J., 1982, Paleomagnetism of Keweenawan rocks, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior Basin: Geological Society of America Memoir 156, p. 173-202. Klewin, K.W., 1990, Petrology of the Proterozoic Potato River Layered Intrusion, Northern Wisconsin, USA: Journal of Petrology, v. 31, p. 1115-1139. Klewin, K.W., and Shirley, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent rift system: Tectonophysics, v. 213, p. 33-40. Klewin, K.W., Olmstead, J.F., and Seifert, K.E., 1989, Rock types and relationships of the Mellen Igneous Complex: Institute on Lake Superior Geology, 35th Annual Meeting, Duluth, MN, Field Trip Guidebook, p. C1-C15. Leighton, M.W., 1954, Petrogenesis of a gabbro-granophyre complex in northern Wisconsin: Bulletin of the Geological Society of America, v. 65, p. 401-442. Middlemost, E.A.K., 1991, Towards a comprehensive classification of igneous rocks and magmas: Earth-Science Reviews, v. 31, no. 2, p. 73-87. Miller, J., 2007, The Midcontinent Rift in the Lake Superior Region: A 1.1 Ga large igneous province: Large Igneous Provinces, http://www.largeigneousprovinces.org/07nov.html Miller, J.D. Jr., Green, J.C., and Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex ILSG 2011 184 Field Trip 9 �and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations 58, 207 p. w/ CD-ROM. Mudrey, M.G., Brown, B.A., and Greenberg, J.K., 1982, Bedrock geologic map of Wisconsin: Wisconsin Geological and Natural History Survey, scale 1:1,000,000. Mudrey. M.G., Ervin, C.P., and Olmsted, J.F., 2003, Middle Keweenawan Basin evolution inferred from geophysical analysis of a strongly magnetic intrusion, Clam Lake, Wisconsin: Wisconsin Geological and Natural History Survey, Open-File Report 2003-2004, 15 p. Olmsted, J.F., 1979, Crystallization history and textures of the Rearing Pond gabbro, northwestern Wisconsin: American Mineralogist, v. 64, p. 844-855. Olmsted, J.F., Windom, K.E., and Seifert, K.E., 1986, Mineralogy and chemistry of the ferrodiorite of the Mineral Lake Intrusion, Mellen, Wisconsin [abs.]: Institute on Lake Superior Geology, 32nd Annual Meeting, Wisconsin Rapids, WI, Proceedings, p. 65. Paces, J.B., and Miller, J.D., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent rift system: Journal of Geophysical Research, v. 98, no. B8, p. 13,997-14,013. Seifert, K.E., Peterman, Z.E., and Theiben, S.E., 1992, Possible crustal contamination of Midcontinent Rift igneous rocks: examples from the Mineral Lake Intrusions, Wisconsin: Canadian Journal of Earth Sciences, v. 29, p. 1140-1153. Sikkila, K., 2002, Description of a pegmatite occurrence on the east margin of the Mellen Granite, State Highway 13, Ashland County, Wisconsin [abs.]: Institute on Lake Superior Geology, 48th Annual Meeting, Kenora, Ontario, v. 48, part 1, p. 48. Streckeisen, 1976, To each plutonic rock its proper name: Earth-Science Reviews, v. 12, no. 1, p. 1-33. Tabet, D.E., and Mangham, J.R., 1978, The geology of the eastern Mellen Intrusive Complex, Wisconsin: Geoscience Wisconsin, v. 3, p. 1-19. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007, The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism: Precambrian Research, v. 157, p. 235-268, doi: 10.1016/j.precamres.2007.02.019. Woodruff, L.G., 2005, Geology of the Midcontinent rift in western Lake Superior [abs.], Geological Society of America, Abstracts with Programs, North-Central Section 39th Annual Meeting. Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages from some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561. ILSG 2011 185 Field Trip 9 �Northland College Van #1 on the verge of meeting its deserved demise at a ―black granite‖ quarry near Mellen, Wisconsin. ILSG 2011 186 Field Trip 9 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2011 Description An account of the resource Institute on Lake Superior Geology. Ashland, Wisconsin. May 18-21, 2011. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2011 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/aee6ad921cb93985dfe1d1a5fddfcefc.pdf 40a74911abcb44b762a3664bbc18cf2d PDF Text Text 58th Annual Meeting Institute on Lake Superior Geology Thunder Bay, Ontario - May 16-20, 2012 Part 1 – Proceedings and Abstracts �58th Annual Meeting Institute on Lake Superior Geology May 16-20, 2012 Thunder Bay, Ontario HOSTED BY: Pete Hollings Chair Lakehead University Proceedings - Volume 58 Part 1 – Proceedings and Abstracts Edited by Pete Hollings (Lakehead University) Cover Photos: Top - Lac des Iles open pit, Middle - Kakabeka Falls in flood, notice light coloured tuff units on right, Bottom - the Sea Lion in 1985, Silver Islet (photos courtesy of Al MacTavish and Bill Addison). �Proceedings of the 58th ILSG Annual Meeting - Part 1 58th Institute on Lake Superior Geology Volume 58 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1 & 13: Sudbury Impactoclastic Debrisites at Thunder Bay Trip 2: Geology of the Sibley Peninsula Trip 3: Lac des Iles mine Trip 4: Shebandowan Mine Area Trip 5: Geology of the Thunder Bay area Trip 6: Thunder Bay Amethyst Mine Trip 7: building stone tour of Downtown Port Arthur, Thunder Bay Trip 8: Highway 527 Transect Trip 9: Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites Trip 10: Geoarchaeology of Thunder Bay Trip 11: Midcontinent rift intrusions Trip 12: Musselwhite mine Reference to material in Part 1 should follow the example below: Baumann, S., and Cummings, K., 2012. Bedrock Geology of Copper Falls State Park, Ashland County, Wisconsin. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 - Proceedings and Abstracts, v. 58, part 1, 1-2. Published by the 58th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 -i- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2012.............................................................. iii Sam Goldich and the Goldich Medal...................................................................................v Goldich Medal Guidelines................................................................................................ vii Goldich Medalists.............................................................................................................. ix 2012 Goldich Medal Recipient.......................................................................................... ix Goldich Medal Committee ............................................................................................... ix Citation for Goldich Medal Recipient..................................................................................x Eisenbrey Student Travel Awards..................................................................................... xii Student Paper Awards....................................................................................................... xii ILSG Student Research Fund........................................................................................... xiii Student Paper Awards Committee.................................................................................... xiii Session Chairs . ............................................................................................................... xiii Board of Directors............................................................................................................ xiv Local Committee.............................................................................................................. xiv Banquet Speaker.............................................................................................................. xiv Report of the Chair of the 57th Annual Meeting ..............................................................xv Sponsors.......................................................................................................................... xvii Program.......................................................................................................................... xviii Abstracts..............................................................................................................................1 Author index....................................................................................................................101 - ii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2012 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage - iii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 # Date Place Chairs 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste. Marie, Ontario A. Wilson & R.Sage 53 2007 Lutsen, Minnesota L. Woodruff & J. Miller 54 2008 Marquette, Michigan T. Bornhorst & J. Klasner 55 2009 Ely, Minnesota J. Miller, G. Hudak & D. Peterson 56 2010 International Falls, Minnesota M. Jirsa, P. Hollings, T. Boerboom, P. Hinz & M. Smyk 57 2011 Ashland, Wisconsin T. Fitz 58 2012 Thunder Bay, Ontario P. Hollings - iv - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 -v- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Institute on Lake Superior Geology Goldich Medal - vi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. - vii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. - viii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Goldich Medalists 1979 Samuel S. Goldich 1996 David L. Southwick 1980 not awarded 1997 Ronald P. Sage 1981 Carl E. Dutton, Jr. 1998 Zell Peterman 1982 Ralph W. Marsden 1999 Tsu-Ming Han 1983 Burton Boyum 2000 John C. Green 1984 Richard W. Ojakangas 2001 John S. Klasner 1985 Paul K. Sims 2002 Ernest K. Lehmann 1986 G.B. Morey 2003 Klaus J. Schulz 1987 Henry H. Halls 2004 Paul Weiblen 1988 Walter S. White 2005 Mark Smyk 1989 Jorma Kalliokoski 2006 Michael G. Mudrey 1990 Kenneth C. Card 2007 Joseph Mancuso 1991 William Hinze 2008 Theodore J. Bornhorst 1992 William F. Cannon 2009 L. Gordon Medaris, Jr. 1993 Donald W. Davis 2010 William D. Addison & Gregory R. Brumpton 1994 Cedric Iverson 2011 Dean M. Rossell 1995 Gene La Berge 2012 Goldich Medal Recipient Jim Miller University of Minnesota Duluth, Duluth, Minnesota Goldich Medal Committee Serving through the meeting year shown in parentheses Mary Louise Hill (2012) Lakehead University Laurel Woodruff (2013) United States Geological Survey Graham Wilson (2014) Turnstone Consulting Mary Louise Hill, as out-going senior member of Institute Board of Directors, is liaison between Goldich Medal Committee and the Board through the 2012 meeting. - ix - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Citation for Goldich Medal Recipient Dr. Jim Miller is indeed a worthy recipient of the 2011 Goldich Medal. Jim exemplifies the best of what this award represents on the basis of his contributions to our understanding of Lake Superior geology and his longstanding involvement with the Institute on Lake Superior Geology. Most of us know Jim for his work in the Midcontinent Rift in Minnesota over more than 25 years. The major emphasis of Jim’s research with the Minnesota Geological Survey and the University of Minnesota since 1985 has been field, petrologic, and metallogenic studies of the igneous rocks of this 1.1 billion-year-old rift. His work has focused on producing bedrock geologic maps of the Duluth Complex and related intrusive terranes of the Rift in northeastern Minnesota and on studying the petrology and crystallization history of various intrusions within it. Jim has authored or co-authored dozens of peer-reviewed reports, papers and maps and is regarded world-wide as an expert in the Duluth Complex and the Midcontinent Rift. He has 54 contributions in ILSG publications, including both abstracts and field trip guidebooks. Jim’s work is not purely academic, however, his research and mapping has led to the evaluation of the coppernickel-platinum group element potential of many Midcontinent Rift intrusions. One of the first collaborations that Jim and I worked on was a field trip for the International Geologic Correlation Program looking at mafic intrusions in Ontario and Minnesota, a trip that we reprised in 2002 and 2010 for the International Platinum Symposium. Jim has had a long-standing history with this symposium, presenting papers, workshops and field trips on the economic potential of the Lake Superior region. Jim is highly regarded by geologists in the mineral exploration industry and frequently takes the opportunity to present his research and ideas at industry-based symposia. From 1989 to 2005, Jim was the principal investigator for a series of six biennial grants from the Minnesota State Legislature’s Minerals Coordinating Committee. Specific projects included: • Shallow drilling project, central Duluth Complex; • Geologic mapping in the Duluth area; • Petrology and metallogenesis of the Duluth Complex at Duluth; • Geologic mapping in the Allen quadrangle; • Digital geologic mapping of the central Duluth Complex; • PGE potential of the Sonju Lake Intrusion; • Geology and mineral potential of the Duluth Complex; • Geology and PGE potential of the Greenwood Lake Intrusion and PGE evaluation of mafic intrusions outside the Duluth Complex. Jim’s research, as well as that of the many undergraduate and graduate students he has supervised, has contributed to the understanding of the general tectonic and magmatic history of the Midcontinent Rift and its mineral deposits. This has led to new ideas in rift development and metallogeny. Jim’s ideas and discoveries have led to industry interest and investment, including the drilling of a recently discovered PGE-bearing reef in the Sonju Lake intrusion. In addition to his research, Jim has also worked with the Minnesota Department of Natural Resources to produce geological displays for North Shore State Parks. He continues to be an advocate of public education and works with researchers in different disciplines such as health, public policy and the environment in making informed decisions involving geology and mineral development. -x- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Another major focus of Jim’s work involves educational outreach. Jim frequently speaks to school groups, teaches non-credit classes and leads field trips in an attempt to introduce Minnesota geology to non-geologists. He has been actively involved in planning and implementation of workshops for kindergarten to Grade 12 earth science teachers for many years. Jim has taught Compleat and Practical Scholar Courses, non-credit classes on Minnesota’s geology sponsored by the University of Minnesota’s General College. Course offerings have included: • Drifting Continents/Expanding Oceans: An Introduction to the Dynamic Earth (1994-98); • The Making of Laurentia – Minnesota’s Geologic History (1994-99); • What’s this Rock? An Introduction to the Geology of Minnesota’s North Shore (2003-05). Since 2006, Jim has taught non-credit, one-day summer field classes at the University of Minnesota’s College of Continuing Education’s Curiosity Camp entitled “A Geology Tour of the Twin Cities”. The Teacher Inquiry-based Minnesota Earth Science (TIMES) Project was a federally funded program sponsored by Hamline University that organized two week-long summer field courses for middle school earth science teachers in Minnesota. Jim served as guest lecturer and assisted on field trips for projects between 2000 and 2009 and was the lead instructor for the 2008 program. The Minnesota Mineral Education Workshop began in 1997. Jim served as an instructor, field trip leader and program coordinator for this annual three-day workshop for K-12 earth science teachers that offers short courses and field trips on the geology and mineral resources of Minnesota. He has also lectured on the geology of Minnesota for the national Elderhostel Program since 1986. Jim, now a professor at the University of Minnesota-Duluth, continues to enthusiastically teach geology and encourage students. He is a staunch advocate of economic geology and tirelessly lobbies administration, government and industry for their ongoing support of the geology program. Most recently, Jim has spearheaded the creation and administration of the Precambrian Research Center (PRC), which was established to satisfy an urgent, long-term need for geologists trained in the mapping and study of Precambrian rocks and their mineral deposits. As a director, Jim continues to canvass the mineral industry for sponsorships, thesis support and ideas on maintaining the relevance of the PRC. The PRC organizes professional workshops and is planning to take a major role in managing the popular Minnesota Minerals Education Workshop for teachers. Jim is a member of the Geological Society of America (GSA), Prospectors and Developers Association of Canada (PDAC), Society for Mining, Metallurgy, and Exploration (SME) and the Society of Economic Geologists (SEG). He served in 1995 as Co-chair of UNESCO-sponsored International Geologic Correlation Program (IGCP) Project 336 on the topic of mineral deposits associated intracontinental rift systems. From 2003 to the present, Jim has served as principal investigator for one-year grants from the U.S. Geological Survey’s EDMAP program, which go toward funding student field research. He is currently Vice Chair of Operations, Minnesota Center for Minerals Resource Education and was Field Trip Chairman for the 2011 Geological Society of America meeting in Minneapolis. Jim has co-chaired the annual meeting of the Institute on Lake Superior Geology three times and has remained a staunch supporter of the Institute for over 30 years (1979-present). His numerous contributions to our understanding of Lake Superior geology are well-regarded by colleagues, peers and clients alike. He is a vocal advocate of education in the earth sciences. He continues to be captivated by the geology and mineral potential of the Lake Superior region. He exemplifies the dedication and enthusiasm that are the hallmarks of previous Goldich Medal recipients. Jim is indeed a worthy addition to this list. Submitted by Mark Smyk, P.Geo. Ontario Geological Survey, Thunder Bay - xi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. Student Paper Awards Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the - xii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 next volume of the Institute. Student papers will be noted on the Program. ILSG Student Research Fund The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • Details of the application process can be found on the ILSG web site. • The proposal will need to be signed by the researcher’s supervisor. In 2011 the ILSG Board of Governors awarded two $500 awards from the Student Research Fund. The winners were Evgeniy Kulakov from the Department of Geological and Mining Engineering and Sciences, Michigan Technological University for his work on the “Paleomagnetism of the Geordie (Coubran) Lake Basalts” and Ernest Thalhamer from the University of Wisconsin - Milwaukee for his work on “Shear Zones and Strain in the Rainy Lake area”. Student Paper Awards Committee Ryan Weston - Magma Metals (Canada) Ltd. Gordon Medaris Jr. - University of Wisconsin – Madison Michael Easton - Ontario Geological Survey Session Chairs Mary Louise Hill - Lakehead University George Hudak - Precambrian Research Centre Mark Severson - Natural Resources Research Institute Mark Smyk - Ontario Geological Survey Ann Wilson - Ontario Geological Survey Laurel Woodruff - US Geological Survey - xiii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Mark Smyk - General Chair 2012 meeting (2015) Tom Fitz (2014) - Northland College, Ashland, Wisconsin Peter Hinz (2013) - Ontario Geological Survey George Hudak (2012) - NRRI, Duluth Pete Hollings - Secretary (2013) - Lakehead University, Thunder Bay, Ontario Mark A. Jirsa - Treasurer (2014) - Minnesota Geological Survey Local Committee Chair Pete Hollings - Lakehead University, Thunder Bay, Ontario Volume Editors Pete Hollings & Bill Addison - Lakehead University, Thunder Bay, Ontario Allan MacTavish - Magma Metals (Canada) Ltd., Thunder Bay, Ontario Organising Committee Bill Addison - Thunder Bay, Ontario Dorothy Campbell - Ontario Geological Survey, Thunder Bay, Ontario Peter Hinz - Ontario Geological Survey, Thunder Bay, Ontario Al MacTavish - Magma Metals (Canada) Ltd. Mark Smyk - Ontario Geological Survey, Thunder Bay, Ontario Banquet Speaker Dr. David Overstreet Human Adaptation to Late Pleistocene Landscapes - A View from Southeastern Wisconsin Study of mammoth bone piles in Kenosha County Wisconsin has provided evidence of human-mammoth interaction between 13,500-12,500 radiocarbon years before present. Chipped stone tools in association with two of the bone piles and micro-wear analyses support the thesis that the carcasses were butchered. Bone modification analyses also support this conclusion. Whether or not the mammoth remains represent active or moribund prey is open to debate and thus the results do not speak to the role of human predation in Late Pleistocene extinctions. - xiv - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Report of the Chair of the 57th Annual Meeting Ashland, Wisconsin The 57th ILSG was held in Ashland, Wisconsin on May 18-21, 2011. Hosted by Northland College and chaired by Tom Fitz, the meeting was attended by a total of 247 people, with most people also attending field trips. Seventy-one students attended, 30 of who received financial support through the Eisenbrey scholarship fund. The two-day technical session began on Thursday morning with oral presentations on regional geology and then proceeded in chronological order from Archean topics through Friday afternoon’s presentations on Quaternary geology. A total of 30 talks were given, 16 of which were presented by students. The poster sessions were also arranged chronologically from Archean through recent, with a total of 24 posters, 18 of which were presented by student authors. The 2011 Goldich Medal was awarded to Dean Rossell of Kennecott Exploration Company. Doug Duskin presented the award during the annual banquet and supplied numerous examples of how Dean’s research has expanded our understanding of ore deposits in the region. Huifang Xu of the University of Wisconsin – Madison gave the keynote address, talking about the origin of banded iron formations by interaction between komatiite and seawater. The meeting offered nine field trips that highlighted the geology of the southwestern Lake Superior region. Three pre-meeting trips were run on Wednesday, including Jim Miller’s trip “Igneous Stratigraphy of the Layered Series at Duluth – Type Intrusion of the Duluth Complex”; Marcia Bjørnerud and Bill Cannon’s “Midcontinent Microcosm: Geology of the Atkins Lake – Marengo Falls Area”; Dick Ojakangas, Drew Cramer, and Tom Fitz’s trip “Geology of the Bayfield Peninsula: Keweenawan Bayfield Group and Pleistocene Deposits”. Friday’s technical sessions and the presentation of student awards were followed by three late-afternoon field trips: “Geology and Remediation at the Ashland/Northern States Power Site” led by Jamie Dunn; “Bad River Watershed Culvert Restoration Program” led by Michele Wheeler and Cassandra Bodette; and “Geology of Copper Falls State Park” led by Allison Mills, Drew Cramer, and Tom Fitz. Three field trips were run on Saturday: “Geology of the Montreal River Monocline” led by Bill Cannon; “The Archean/Paleoproterozoic Unconformity near Denham, Minnesota” led by Terry Boerboom; and “Granitic, Gabbroic, and Ultramafic rocks of the Keweenawan Mellen Intrusive Complex” led by Tom Fitz. On Friday evening Northland College hosted a beer and barbeque social event for ILSG participants outside the college’s student center. A variety of door prizes were given away and a good time was had by all. The Institute’s Board of Directors met on May 19, 2011 to discuss business and preliminary plans for future Institutes. The meeting was attended by Ted Bornhorst, Peter Hinz, George Hudak, Mark Jirsa (Treasurer), Pete Hollings (Secretary), and Tom Fitz (2011 Chair). Secretary Hollings took the minutes of the meeting, that are as follows: 1. Accepted report of the Chairs for the 56th ILSG, International Falls, Minnesota; as printed in the Proceedings Volume (Hinz), and minutes of last Board meeting, May 20, 2010 (Hollings). 2. Received, discussed, and accepted 2010-2011 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2010-2011 report of the Secretary (Hollings). 4. Approved Jirsa to continue as Institute Treasurer until 2014 (this was later presented to the membership and approved). - xv - �Proceedings of the 58th ILSG Annual Meeting - Part 1 5. Approved Tom Fitz as on-going ILSG Board member. 6. Approved Thunder Bay, Ontario at the site for the 58th annual ILSG meeting. The meeting will be hosted by Pete Hollings with help from Mark Smyk and Peter Hinz. Ted Bornhorst offered to host the 59th Annual Meeting in Houghton. 7. Discussed and approved replacing Al MacTavish as the “member from industry” on Goldich Committee (end of term 2011) with Graham Wilson. 8. There was some discussion as to whether or not presentations from the meeting should be posted on the ILSG web site or included on a CD given to participants. 9. It was agreed that a copy of the DVD with all proceedings to date would be included with the volumes mailed out to our standing orders. The Chair would like to thank the student committee made up Northland College students Kristi Wilson, Allison Mills, Cassie Bodette and Drew Cramer for their hard work in organizing and running the meeting. Appreciation also goes to the field trip leaders and session chairs, to the sponsors of the meeting, to the ILSG Board and the many people who worked to make the 2011 Institute a successful, educational, and enjoyable meeting. Respectfully submitted, Tom Fitz Chair, 57th Institute on Lake Superior Geology - xvi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Sponsors The following organizations made generous contributions to the 58th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior region. - xvii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Program Wednesday May 16 7:30 a.m. Field Trip 3: Lac des Iles Pd mine Leader: John Corkery (North American Palladium Ltd.) 8:00 a.m. Field Trip 1: Sudbury Impactoclastic Debrisites at Thunder Bay Leaders: Bill Addison and Greg Brumpton 8:00 a.m. Field Trip 2: Geology of the Sibley Peninsula Leaders: Dr. Philip Fralick (Lakehead University) Mark Smyk & Riku Metsaranta (Ontario Geological Survey) 8:00 a.m. Field Trip 4: The Shebandowan greenstone belt Leaders: Alan Aubut (Sibley Basin Group Geological Consulting Services Ltd.) and Dorothy Campbell (Ontario Geological Survey) 6:00 p.m. Return of Trips 1-4 4:00 p.m. - 8.00 p.m. Registration (Barcelona Room, Travelodge Airlane) 7:00 p.m. - 9.00 p.m. Ice Breaker Social (Barcelona Room, Travelodge Airlane), Poster Setup (Tiberio Room, Travelodge Airlane) and Core Shack (Courtyard, Travelodge Airlane) Thursday May 17 7:30 a.m. - 4:00 p.m. Registration (Barcelona Room, Travelodge Airlane) 8:20a.m. - 8:30 a.m. Introductory Remarks - Peter Hollings, Chair Technical Session I NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Session Chairs: Ann Wilson (Ontario Geological Survey) and Mary Louise Hill (Lakehead University) 8:30 a.m. Gilbert, H. Stratigraphy and tectonic setting of Neoarchean arc volcanic rocks in the Bird River Belt, Manitoba, Canada 8:50 a.m. Chaffee, M.*, Miller, J., Hollings, P., Heggie, G., MacTavish, A., and Bandli, B. Petrographic and geochemical study of the hybrid rock unit associated with the Current Lake Intrusive Complex, Magma Metals’ Thunder Bay North Property 9:10 a.m. Brooker, B.* and Miller, J. Geology and petrology of a Mesoproterozoic layered mafic intrusion in portions of the Brule Lake and Cherokee Lake 7.5’ Quadrangles, northeastern Minnesota 9:30 a.m. Good, D., McLean, K., Epstein, R., Linnen, R., and Samson, I. Geology of the Layered Series, Coldwell Alkaline Complex, Ontario 9:50 a.m. Cundari, R.*, Hollings, P. and Smyk, M. Petrogenesis and crustal contamination of the Nipigon sills: a geochemical and spatial reevaluation 10:10 a.m. - 10:40 a.m. Coffee Break, Poster Session and Core Shack - xviii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 10:40 a.m. Rousell, D., Petrus, J., Easton, R., Tinkham, D. and Napoli, M. The tectonometamorphic, magmatic and mineralization history of the Wanapitei Complex, Grenville Front tectonic zone, Ontario 11:00 a.m. Berkley, J. New York’s Adirondack Mountains: Window to the Mesoproterozoic Grenville Orogen 11:20 a.m. Van Lankvelt, A.*, Schneider, D., Hattori, K. and Biczok, J. Anatomy of a Mesoarchean batholith 11:40 a.m. Kuzmich, B.*, Hollings, P. and Campbell, D. Geochemistry and petrology of the Dog Lake Granite Chain, Quetico Basin, Northwestern Ontario 12:00 p.m. - 1:30 p.m. Lunch Break, Poster Session and Core Shack (ILSG Board Meeting by invitation) Technical Session II Session Chair: Mark Severson (Natural Resources Research Institute) and Laurel Woodruf (USGS) 1:30 p.m. Horner, S.* and Fralick, P. Chert stromatolites in the basal Gunflint Formation, Kakabeka Falls: Primary precipitation or silicification? 1:50 p.m. Phillips, B., Dean, F. and Zaniewski, K A catastrophic breach and inter-lake flow through the Marks Moraine via Brule Creek and Cedar Creek, Thunder Bay Region, Ontario. A small but significant detail of the eastern outflow history of Lake Agassiz 2:10 p.m. Beh, B.* and Fralick, P. Depositional processes operating on the Paleoproterozoic Gowganda ice margin: an example from the Espanola area, Ontario 2:30 p.m Kerkermeier, L.* and Fralick, P. Ferromanganese precipitates in lacustrine environments of Northwestern Ontario and Nova Scotia: Effects on arsenic concentration 2:50 p.m. - 3:20 p.m. Coffee Break, Poster Session and Core Shack 3:20 p.m. Frederiksen, A., Deniset, I., Bollman, T., Van der Lee, S. and Darbyshire, F. Erosion of Archean lithosphere by subduction and rifting: a tomographic image of central North America 3:40 p.m. Pesonen, L. and Veikkolainen, T. Supercontinents during the Proterozoic – A paleomagnetic view with Keweenawan data (Lake Superior) as an example 4:00 p.m. Swanson-Hysell, N., Burgess, S., Maloof, A., and Bowring, S. Temporal context of the Mamainse Point succession: a record of magmatic activity and fast plate motion across the “latent stage” of Midcontinent Rift development 4:20 p.m. Easton, R.M. The source of the Elliot Lake uranium ores: The neodymium isotope story 4:40 p.m. Medaris, G., Driese, S., Boerboom, T. and Jirsa, M. Granitic saprolites in the Lake Superior region: K-metasomatism, intensity vs. magnitude of weathering, and estimates of atmospheric pCO2 - xix - �Proceedings of the 58th ILSG Annual Meeting - Part 1 7:00 p.m Annual Banquet and Award Presentation (Barcelona Room, Travelodge Airlane) Announcement of 59th Annual Meeting Location 2012 Goldich Award Presentation to Jim Miller 2012 Banquet Address - Dr. D. Overstreet Meeting participants not registered for the banquet are welcome to attend the address Friday May 18 9:00 a.m. - 12:00 p.m. Registration Technical Session III Session Chairs: George Hudak (Precambrian Research Centre) and Mark Smyk (OGS) 8:30 a.m. Stinson, V.* and Hill, M.L. Regional to microstructural control of gold mineralization along the Quetico-Wabigoon subprovince boundary 8:50 a.m. Thalhamer, E.* and Czeck, D. Analyzing ductile shear zone network geometries in the Grassy Portage Sill, Rainy Lake Region, Northwestern Ontario, Canada 9:10 a.m. Heggie, G., MacTavish, A., Johnson, J., Weston, R. and Ma, L. Structural control on the emplacement of the TBN-Igneous Complex 9:30 a.m. Bailes, A., Galley, A., Paradis, S.and Taylor, B. Using large synvolcanic alteration zones to explore for VMS deposits at Snow Lake, Manitoba, Canada 9:50 a.m. Scott, G., Karakus, M., Shinkle, D. and Mitchell, A. Exploration history, mineralogy and genesis of the Black Thor chromite deposit, Ring of Fire Intrusion, Northwestern Ontario 10:10 a.m. - 10:40 a.m. Coffee Break, Poster Session and Core Shack 10:40 a.m. Bornhorst, T. and Williams, W. The Copperwood sediment-hosted stratiform copper deposit, Upper Peninsula, Michigan 11:00 a.m. Blaske, A., Braun, G., Leier-Engelhardt, P., Mottl, R., Anderson, D. and Bornhorst, T Glacial geology of the Copperwood Project, Gogebic County, Michigan, and environmental and engineering implications for site development 11:20 a.m. Waggoner, T and Karakus, M. The Little Commonwealth exploration - an example of a SEDEX Deposit 11:40 a.m. Mikkelsen, L. and Conly, A. Predictive water level and preliminary hydrodynamic models of Caland and Hogarth pit lakes, Atikokan, Ontario 12:00 p.m Presentation of Best Student Paper Award and Eisenbrey Awards 1:00 p.m. Field Trip 5: Geology of the City of Thunder Bay Leader: Mark Smyk (Ontario Geological Survey) 1:00 p.m. Field Trip 6: Panorama Amethyst Mine Leader: Steve Kissin (Lakehead University) - xx - �Proceedings of the 58th ILSG Annual Meeting - Part 1 1:00 p.m. Field Trip 7: Port Arthur building stone walking tour Leader: Peter Hinz (Ontario Geological Survey) 6:00 p.m. Return of Trips 5-7 7:00 p.m BBQ (Whitewater Golf Club - bus service provided leaving from the Travelodge Airlane beginning at 6.30 p.m.) Poster Presentations Baumann, S. and Cummings, K. Bedrock Geology of Copper Falls State Park, Ashland County, Wisconsin Boerboom, T. Bedrock geologic map of Morrison County, Central Minnesota Bollmann, T.*, Van der Lee, S., Frederiksen, A., Lou, X. Preliminary results of P- and S-wave delay times in the Superior Region from transportable array and SPREE stations Buchholz, T., Falster, A. and Simmons, W. Accessory minerals of a roadside pegmatite, Orr, Minnesota Craddock, S.* and Craddock, J. Strain variations in carbonates across the Proterozoic Grenville Orogen Cundari, R.*, Hollings P., Scott, J. and Campbell, D. Geology and geochemistry of the Coubran Lake Basalts, a Midcontinent Rift-related sequence within the central Coldwell Complex, Marathon, Ontario Gaspar, B*, and Hill, M.L. The black line faults of the Red Lake gold mines, Ontario Gasparotto, M.*, and Hill, M.L. A microstructural study of the Otjikoto Deposit, Namibia: Inferences from gold mineralization in the Superior Province Goscinak, C.* and Hansen, V. Quartz Fabric analysis of the Kawishiwi Shear Zone, NE Minnesota Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C., and Hudak, G. Preliminary bedrock geology map of the Eastern part of Lake Vermilion State Park, St. Louis County, NE Minnesota Hudak, G., Monson Geerts, S., Zanko, L., Severson, A., Severson, A. and Bandli, B. The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulates - 2012 Update Jirsa, M. Bedrock geologic map of the Crane Lake and Brule Narrows 30’X60’ quadrangles, Quetico subprovince, northern Minnesota Jirsa, M., Baggetto, L., Eliason-Johnson, G., Hansen, D., Hoxsie, E., Kilpatrick, K. Reconnaissance geologic mapping of Neoarchean rocks in the central Boundary Waters Canoe Area Wilderness by students of the Precambrian Research Center’s 2011 field camp. - xxi - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Johnson, T.*, Shannon, J., Boerboom, T., Wendlandt, R. Geochemistry of mafic dikes from the Carlton Dike Swarm in Minnesota Kern, A.*, Kulakov, E., Smirnov, A. and Diehl, J. Paleomagnetism of the alkaline Coldwell Complex: New Results, new Insights. Koroscil, J.*, Hill, M.L. and Fralick, P. Thrust faulting in the Gunflint Formation north of Lake Superior Kulakov, E.*, Smirnov, A. and Diehl, J. Paleomagnetism of the Geordie Lake and Silver Mountain basalts Lee, A.*, Albers, P., Miller, J., Severson, M. and Deen, T. Bedrock Geologic map of the Seine Bay/Bad Vermilion Lake intrusion, Mine Centre, Ontario McCormick, K. and Paterson, C. Mafic intrusions along the southern boundary of the Superior Craton, SD: What is their potential for NiCu-PGE mineralization? Miller, J., Brooker, B., Asp, K., Leu, A., Parisi, A. and Sletten, D. 2011 Precambrian Field Camp Mapping in the Sawbill Lake Area, Cook County, Northeastern Minnesota Pietrzak-Renaud, N. A Metallogenic Model and Geometallurgical Treatment of the Basal Iron Ores within the Negaunee Iron Formation, Tilden Mine, Michigan Piispa, E.*, Smirnov, A., Pesonen, and Diehl, J. Paleomagnetic, rock magnetic and geochemical data from the Keweenawan dykes and baked rocks from the Little Mountain and Sugar Loaf areas, Michigan, USA Roe, C.*, and Bjørnerud, M. The ca. 1650 Ma Freedom Formation, Baraboo Range, Wisconsin: A late iron formation in the Lake Superior Region Santaguida, F., Lappalainen, M., Jones, S., Voipo, T., Siikaluoma, J. and Ylinen, J. Geologic Setting of the Kevitsa Ni-Cu-PGE Deposit, Central Lapland Greenstone Belt, Finland Selagi, J.*, and Hill, M.L. Microstructural analysis of porcellaneous nepheline syenite, Coldwell Complex, Ontario Shahabi Far, M.*, Samson, I., Gagnon, J., Linnen, R., Good, D. Textural and compositional variation in apatite and plagioclase in the Marathon PGE-Cu deposit, Northwestern Ontario; implications for fluid-rock interaction Taylor-Hollings, J. Quartz quarry sites in northwestern Ontario: A new ‘prospect’ for geoarchaeologists investigating lithic raw material sources Van der Lee, S., Frederiksen, A., Bollmann, T. and Spree Team. SPREE: Field experiment to study deep structure of the Midcontinent Rift Vaughan, A.*, Swanson Hysell, N. and Feinberg, J. Paleomagnetic data in stratigraphic context from 1.1 Ga Osler Group basalt flows on Simpson Island, Ontario: Evidence for rapid plate motion of Laurentia in the late Mesoproterozoic - xxii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Wilson, G. and McCausland, P. The curious meteorite harvest of the Lake Superior Region, I. Overview. Wilson, G. and Kissin, S. The curious meteorite harvest of the Lake Superior Region, II. Gems in the Rough NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Saturday May 19 6:00 a.m. Field Trip 12: Musselwhite gold mine (participants should check in at the Wasaya Airlines counter at Thunder Bay Airport by 6 AM for a 7 AM departure) Leaders: John Biczok (Musselwhite Mine) 8:00 a.m. Field Trip 8: Highway 527 Transect Leaders: Mark Smyk (OGS) and Philip Fralick (Lakehead University) 8:00 a.m. Field Trip 9: Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mines Leader: Mark Puumala (OGS) 8:00 a.m. Field Trip 10: Geoarchaeology of Thunder Bay Leaders: Scott Hamilton (Lakehead University) 8:00 a.m. Field Trip 11: Midcontinent rift intrusions Leaders: Rob Cundari & Pete Hollings (Lakehead University) and Mark Smyk (Ontario Geological Survey) 8:00 a.m. Field Trip 13: Sudbury Impactoclastic Debrisites at Thunder Bay Leader: Greg Brumpton 6.00 p.m. Return of Trip 8 to Armstrong, overnight there 6.00 p.m. Return of Trips 9-11 & 13 7.45 p.m. Return of Trip 12 to Thunder Bay Airport Sunday May 20 8:00 a.m. Field Trip 8: Highway 527 Transect Leaders: Mark Smyk (OGS) and Philip Fralick (Lakehead University) 6.00 p.m. Return of Trip 8 - xxiii - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Using large synvolcanic alteration zones to explore for VMS deposits at Snow Lake, Manitoba, Canada BAILES, Alan, Bailes Geoscience, 6 Park Grove Drive, Winnipeg, MB, R2J3L6, Canada, GALLEY, Alan, Exploration Research Director, CMIC, 2-250-155 Queens Street, Ottawa ON, Canada, PARADIS, Suzanne, GSC, P.O. Box 6000, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada and TAYLOR, Bruce, GSC, 601 Booth Street, 7th Floor, Room 702, Ottawa, ON K1A 0E8, Canada Due to its rich mineral endowment, excellent outcrop exposure and large domains of hydrothermally altered rocks, the juvenile 1.89 Ga Snow Lake arc volcanic assemblage is an important natural laboratory for applying modern volcanic massive sulphide theories to ancient deposits that are well exposed in cross section. For 25 years, a series of multidisciplinary investigations have examined regionally metamorphosed products of subseafloor hydrothermal events at Snow Lake (Galley, 1993; Galley et al, 1993; Bailes and Galley 1996, 2007; Galley at al., 2002). This presentation will summarize some of the important findings of these studies and place the VMS mineralization, including the recently discovered 30 million tonne Au-rich Lalor Lake VMS deposit, into a stratigraphic-hydrothermal model. Alteration zones at Snow Lake are characterized by extensive zones with anomalous 1.82 Ga metamorphic mineral assemblages, including porphyroblasts of garnet, staurolite, amphibole, biotite, gahnite and/or kyanite, produced from altered rocks created during pre-metamorphic, 1.89 Ga, synvolcanic, hydrothermal, fluid-rock interaction. Three separate episodes of hydrothermal alteration (Fig. 1) are recognized that span evolution of the host volcanic rocks from a primitive (Anderson sequence) to mature arc (Chisel sequence) geotectonic setting. Figure 1. Schematic cross section of large-scale zones of alteration affecting rocks of the Snow Lake arc assemblage. The alteration zones comprise those associated with the Anderson sequence VMS deposits (Anderson alteration zone), those associated with a formational pyrrhotite-pyrite zone (Foot-Mud alteration zone) and those associated with the Chisel-Lalor VMS deposits (Chisel-Lalor alteration zone). The alteration pipe west of Chisel Lake VMS deposit (C) is the footwall pipe for the Lalor Lake VMS deposit (not shown). -1- �Proceedings of the 58th ILSG Annual Meeting - Part 1 The geological, geochemical, mineralogical and isotopic attributes of the zones indicate that they include VMSrelated and VMS-unrelated, and were produced at high- and low- temperatures and in seafloor and sub seafloor (intra-stratal) environments. Semi-conformable alteration zones at Snow Lake are up to 20 km in strike length and 0.8 km wide. Their large exploration ‘footprint’ compared to associated ‘pipe-like’ alteration zones and VMS deposits means that such zones provide a useful target to ‘vector-in’ exploration to VMS depositional settings within volcanic belts. The VMS-related semi-conformable alteration zones at Snow Lake display diagnostic variations in intensity and style of alteration along strike towards VMS deposits, are stratigraphically underlain by altered portions of synvolcanic intrusions, are cut by discordant zones of more intensely altered rocks, and can be demonstrated to have formed by interaction with high temperature(>350°C) hydrothermal fluids. References Bailes, A.H, and Galley, A.G., 1996. Setting of Paleoproterozoic volcanic-associated massive sulphide deposits, Snow Lake, Manitoba, in Bonham-Carter G.F., Galley A.G., Hall, G.E.M. (eds.) EXTECH I: A multidisciplinary approach to massive sulphide research in the Rusty Lake and Snow Lake greenstone belts, Manitoba, Geological Survey of Canada, Bulletin 426, p. 105-138. Bailes, A.H., and Galley, A.G., 2007. Geology of the Chisel–Anderson lakes area, Snow Lake, Manitoba (NTS areas 63K16SW and west half of 63J13SE); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, MAP Geoscientific map 2007-1, 1 colour map with accompanying notes, scale 1:20 000. Galley, A.G., 1993. Charactersitics of semiconformable alteration zones associated with volcanogenic massive sulphide districts, Journal of Geochemical Exploration, v. 48, p. 175-200. Galley, A.G., Bailes, A.H., and Kitzsler, G., 1993- Geological setting and hydrothermal evolution of the Chisel Lake and North Chisel Zn-Pb-Ag-Au massive sulphide deposit, Snow Lake, Manitoba, Exploration Mining Geology, v. 2, p. 271295. Galley, A.G., Bailes, A.H., Hannington, M., Holk, G., Katsube, J., Paquette, F., Paradis, S., Santaguida, F., and Taylor, B, compiled by B. Hillary, 2002. Database for CAMIRO Project 94E07: interrelationships between subvolcanic intrusions, large-scale alteration zones and VMS deposits, Ontario, Manitoba, Quebec and Sweden, Geological Survey of Canada Open File 4431, 1-CD-ROM. -2- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock Geology of Copper Falls State Park, Ashland County, Wisconsin BAUMANN, Steven D.J. and CUMMINGS, Katy E., Midwest Institute of Geosciences and Engineering, Geology Section, Chicago, Illinois, email: steveb@migeweb.org The geology of Copper Falls State Park in Wisconsin has been a point of geologic and economic interest since the 19th century. The Park contains both Precambrian sedimentary rocks that have been tilted vertical and Precambrian igneous intrusions. The sedimentary rocks are comprised of the Oronto Group and are about 1060 Ma to about 1088 Ma. Beneath the sedimentary rocks are the basalt and andesite flows of the Powder Mill Group, which includes the local “Sheep Farm Rhyolite”. The Powder Mill Group ranges in age from about 1100 Ma to 1109 Ma old. The Powder Mill Group is intruded by the 1102 Ma Mellen Intrusive Complex which is a mix of granites, ferrodiorites, gabbros, and granoporphyries. In June of 2011, the authors mapped the Park at a scale of 1:14,400 and an area of about 6 mi2. The result was a bedrock geology map with new information on the structure and geologic history of the Park. If you tour the Park you will see yellow signs depicting a fault between the Powder Mill and Oronto Groups. The Powder Mill Group does show signs of breccia within, but this is more likely the cause of “reworked” igneous flows. Plus there is no evidence for faulting anywhere else in the Park. If faulting had occurred, it most likely would have affected the softer Oronto Group sediments. There is evidence on top of the Sheep Farm Rhyolite of a thin “weathered” zone. The field evidence suggests that the Powder Mill-Oronto Group contact is erosional and not faulted. There are four distinct alternating flows of basalt and andesite of the Powder Mill Group within the Park. The individual flows were not named but contain an andesite at the base and basalt at the top and are part of the Kallandar Creek deposits. These four main flows show internal signs of brecciation and minor sedimentary deposits. There are likely more flows within the four larger flows, but their lateral extent is difficult to determine with the existing exposures in the Park. The Oronto Group shows some internal variations previously unnoted. The Freda Sandstone exhibits an intertonguing relationship between sandstone and shale, with two large sandstone tongues. The Sandstone facies also exhibits sharp glauconitic contacts with the finer shale facies at the Horseshoe Outcrop (Fig. 1A). The two basal Oronto Formations, the lower Copper Harbor Conglomerate and the younger Nonesuch Shale show a complex intertonguing relationship at Devil’s Gate, where they are excellently exposed (Fig. 1B). The two basal formations are also at their thinnest in the park and pinch out near the west center of the Park. This is due to their deposition on significant paleotopography of the Powder Mill Group. The overall structure of the geologic units in the Park is vertical bedding from Brownstone Falls to Devil’s Gate. As you head north the beds begin to deviate from the vertical and dip about 80° to the northwest and the finer facies of the Freda Sandstone dominates. The Powder Mill and Oronto Groups were deposited nearly flat and deformed into their near vertical position by the Mellen Creek Intrusions to the south and southeast. Like the Copper Harbor Conglomerate and Nonesuch Shale, the Sheep Farm Rhyolite pinches out within the Park (Fig. 1C). -3- �Proceedings of the 58th ILSG Annual Meeting - Part 1 A C B Figure 1. A) Contact between the sand and shale facies at the Horseshoe Outcrop. B) Inter tonguing beds within the Nonesuch Shale at Devil’s Gate. C) The Sheep Farm Rhyolite on the east cliffs near Devil’s Gate References Baumann, S.D.J. and Cummings, K.E., 2011. Bedrock Geologic Map of Copper Falls State Park, Ashland County, Wisconsin. Midwest Institute of Geosciences and Engineering, Map M-082011-1A. Cannon, W.F., 1997. Bedrock Geologic Map of the Ashland and Northern Part of the Ironwood 30’ X 60’ Quadrangle, Wisconsin and Michigan. United States Geologic Survey Miscellaneous Information Series, Map I-2566. Fitz, T. (Editor), 2011. 57th Institute on Lake Superior Geology’s Annual Meeting May 18-21, Ashland Wisconsin. Institute on Lake Superior Geology’s 57th Meeting, Part 2. Ostrom, M.E., 1973. Guidebook to the Precambrian Geology of Northeastern and North Central Wisconsin. 19th Institute on Lake Superior Geology’s Annual Meeting. -4- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Depositional Processes Operating on the Paleoproterozoic Gowganda Ice Margin: An example from the Espanola Area, Ontario BEH, Breanne and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, bbeh@lakeheadu.ca Glacial sediments of the Huronian Supergroup outcrop along the north shore of Lake Huron and were likely deposited on what is thought to have been a divergent continental margin (Fralick and Miall, 1981; 1989). The rocks of the Gowganda Formation record one of three glacial events preserved in the Supergroup and are therefore of interest for developing further glaciomarine models (Puffett, 1974) and furthering understanding of this early stage in Earth’s history. Data has been collected in five main study areas in an attempt to cover as much of the ancient continental margin as possible. The Espanola study area consisted of the most extensive and well exposed outcrops of the Gowganda Formation, with more than a kilometer of nearly continuous exposure, therefore it will be used as an example to show how the depositional history can be interpreted. Two metamorphosed stratigraphic sections were logged on McGregor Bay and one on Iroquois Bay, which are located near Espanola, Ontario. The logged sediments were grouped into six lithofacies associations: 1) Planar Cross-Stratified Sandstone Lithofacies Association, 2) Diamictite Lithofacies Association, 3) Interlayered Siltstone and Fine-Grained Sandstone Lithofacies Association, 4) Slump Lithofacies Association, 5) Fine- to Coarse-Grained Sandstone Lithofacies Association and 6) Quartz-Rich Sandstone Lithofacies Association. These lithofacies associations (LA) likely represent a sequence of depositional environments on a shallow continental shelf. Initially, the shelf was dominated by what were likely large-scale, low-angle sandwaves (Fig. 1A) interbedded with successions of wavy bedding and possible hummocky cross-stratification indicating an Figure 1. A) Large-scale, low-angle planar cross-stratified sandstone. B) A dropstone compressing underlying laminations. C) A mud-block conglomerate of slumped prodelta deposits. D) Herringbone cross-stratified sandstone. -5- �Proceedings of the 58th ILSG Annual Meeting - Part 1 open-water setting with tidal and storm processes reworking the sediments. The shelf then gradually evolved into an environment dominated by diamicite layers. The diamictite layers have dropstones as well as evidence of current activity indicating outsized clasts were likely being introduced into the environment as ice-rafted debris (Fig. 1B). Resedimentation events in the form of debris flows are thought to account for conglomeratic layers that are common in the Diamictite LA. The Interlayered Siltstone and Fine-Grained Sandstone LA, along with the Slump LA (Fig. 1C), closely resemble deposition from suspension in a prodelta setting where large slump events are common. The gradual transition into a more sandstone dominated LA, with an abundance of current related sedimentary structures, is indicative of the shallowing and coarsening upwards succession common to deltaic deposits. A final transition into the Quartz-Rich Sandstone LA indicates a return to a sandy, current-dominated open continental shelf environment with abundant tidally generated sedimentary structures such as herringbone cross-stratification (Fig. 1D). The southernmost sections contain only mass-flow diamictite with no massive diamictite. Thus, the likelihood of an ice shelf in this area is remote indicating the Gowganda Formation does not represent snowball Earth conditions. References Fralick, P.W., and Miall, A.D., 1981. Grant 84: Sedimentology of the Matinenda Formation, in Pye, E.G., ed., Geoscience Research Grant Program, Summary of Research 1980 – 1981: Ontario Geological Survey, Miscellaneous Paper 98: 80-89. Fralick, P.W., and Miall, A.D., 1989. Sedimentology of the Lower Huronian Supergroup (Early Proterozoic), Elliot Lake area, Ontario, Canada: Sedimentary Geology, 63: 127-153. Puffet, W.P., 1974. Geology of the Negaunee quadrangle, Marquette County, Michigan: U.S. Geological Survey, Professional Paper 788: 53 p. -6- �Proceedings of the 58th ILSG Annual Meeting - Part 1 New York’s Adirondack Mountains: Window to the Mesoproterozoic Grenville Orogen Berkley, Jack, Department of Geosciences, Houghton Hall, SUNY Fredonia, Fredonia, NY 14063 USA The Adirondack Mountains occupy most of the northeastern lobe of New York State, with associated Grenville metamorphic rock exposures and structures extending north through the Laurentian Lowlands and into Ontario, Canada (Fig. 1). Isotopic data give whole-rock ages of roughly 1.3 to 0.9 Ga corresponding to the late Mesoproterozoic, thus are generally coeval with the Midcontinent Rift system exposed prominently in the Lake Superior region. Orogenic evolution of the Grenville Orogeny was complex, and arose from at least two major continent-continent collisions producing the eastern (including the Adirondacks) and southern chains (including Texas and Mexican exposures) during the assembly of the supercontinent Rodinia. The Adirondack region reveals evidence for Himalayan-magnitude mountains that were part of a nearly continuous chain stretching from present-day Scandinavia through North America, Africa, Australia and Antarctica. Major Grenville exposures in North America include the (1) Grenville Province (Labrador-Quebec-Ontario), which includes the AdirondackNY-Laurentian Lowland region, (2) inliers within the Green, Taconic, Blue Ridge Appalachian chain, (3) the Llano County, Texas granite-gneiss/schist terrain, and (4) the Oaxacan Complex, southern Mexico. Figure 1. Generalized geology of the Adirondacks with ages of principal units. ANTanorthosite; HSRG-Hyde School & Rockport granites; HWK-Hawkeye granite; LMG-Lyon Mtn. granite; MCG-mangerite, charnockite, granite; RDAG-Rossie diorite & Antwerp diorite; RMTG-Royal Mtn. tonalite & Granodiorite; CCZ-CarthageColton mylonite shear zone; HL-Highlands; LL-Lowlands. After McLelland, 2008 The overall timeframe of Grenville deformation, metamorphism and plutonism began with the Elzevirian Orogeny (from 1240-1220 Ma), followed by the Shawinigan event (1190-1140 Ma), the Ottawan (1090-1020 Ma), and the 1010-980 Ma Rigolet Orogeny (Rivers, 2008). Grenville tectonic history – as exemplified in the Adirondacks – consisted of major convergent folding events (Shawinigan and Ottawan) with intermittent relatively quiescent periods featuring rifting episodes and erosion-deposition of sedimentary units in rift basins and shelf areas. These quiet spans also spawned the extensive intrusion into countryrock of AMCG (anorthositemangerite-charnockite-granite) plutonic suites, well-represented in the Adirondacks. Evidence gleaned from extensive modern studies, including recent data from TIMS and SHRIMP single-grain isotopic determinations (e.g., McLelland, 2008) shows that subduction with formation of island arcs long preceded collision events, with magmatic arc intrusive and volcanic rocks eventually riding over continental margins on overlying NW-vergent thrust sheets. The closing of the Iapetus seaway during the Neoproterozoic-Paleozoic Appalachian orogenies some 700 million years later presents parallels to Grenville tectonic mechanisms. -7- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Exposed areas in the Grenville orogen consist of three litho-structural “belts”, the Gneiss Belt, Metasedimentary Belt, and the Granulite Terraine. Relatively narrow shear zones separate each belt. In the Adirondacks the Metasedimentary Belt occupies most of the Laurentian Lowlands terrain consisting of marble, calcsilicates, rare gypsum-anhydrite layers (mostly in mine excavations) along with various gneiss and amphibolite terrains. The Carthage-Colton shear zone separates the Metasedimentary Belt from the Granulite Terrain, which makes up the bulk of geology within the Adirondack State Park. As its name implies, most rocks in the Granulite Terrain are assigned to the granulite metamorphic facies, while Metasedimentary Belt assemblages predominantly plot in the amphibolite facies. Greenschist and lower P-T assemblages are absent in the Adirondacks. Various models have been advanced seeking to establish a genetic and regional kinematic connection between the Grenville orogen and the Midcontinent rift system (for a synopsis, see Hauser, 1996). Early ideas postulated dextral strike-slip movement along the Grenville front causing opening of the Midcontinent rift (e.g., Anderson and Burke, 1983), however, these models were fraught with problems. Principal among these is that isotopic dates along the Canadian Grenville front are consistently younger than Keweenawan terrains, also shown by cross-cutting truncation of the eastern rift arm by the Grenville front near Detroit. Current thinking posits a primary role of an uprising mantle plume in producing the midcontinent rift at around 1.10 Ga (e.g., Nicholson and Shirey, 1990), but with some interplay with the Grenville belt. For example, creation of horst ridges during later convergent rift inversion may have been influenced by Grenville shortening during early Ottawan times, around 1.08 Ga (McLelland, 2008). Also, strike-slip offsets along the southern extent of the western rift arm may be influenced in some manner by the southern (Texas-Mexico) Grenville front (Hauser, 1996). References Anderson, S. and Burke, K., 1983. A Wilson Cycle approach to some Proterozoic problems in eastern North America, in Medaris, L.G. et al. eds., Geological Society of America Memoir, 161: 75-93. Hauser, E., 1996, Midcontinent rifting in a Grenville embrace, in van der Pluijm, B. and Catacosinos, P., eds., Geological Society of America Special Paper, 308: 67-75. McLelland, J., 2008, Geologic setting and characteristics of Adirondack anorthosite and related mangerite-charnockitegranite (AMCG suite), Field Trip Guidebook, New York State Geological Association, 80th meeting: 1-18. Nicholson, S. and Shirey, S., 1990, Midcontinent rift volcanism in the Lake Superior region: Sr, Nd and Pb isotopic evidence for a mantle plume origin. Journal of Geophysical Research, 95: 10, 851-10,868. Rivers, T., 2008, Assembly and preservation of lower, mid, and upper orogenic crust in the Grenville Province – Implications for the evolution of large, hot, long-duration orogens. Precambrian Research, 167: 237-259. -8- �Proceedings of the 58th ILSG Annual Meeting - Part 1 Glacial Geology of the Copperwood Project, Gogebic County, Michigan, and Environmental and Engineering Implications for Site Development BLASKE, Allan R., 401 S. Washington Square, Lansing, Michigan 48933, allan.blaske@aecom.com, BRAUN, Gary, AECOM, 11425 W. Lake Park Dr., Milwaukee, Wisconsin 53224, LEIER-ENGELHARDT, Paula, AECOM, 1035 Kepler Drive, Green Bay, Wisconsin 54311, MOTTL, Robert, AECOM, 1035 Kepler Drive, Green Bay, Wisconsin 54311, ANDERSON, Dave, Orvana Resources US Corp., 10199 Lake Road, Ironwood, MI 49938 and BORNHORST, Theodore J., A. E. Seaman Mineral Museum, Michigan Technological University, Houghton, MI 49931. The Copperwood Project is located in the northwestern portion of Gogebic County, Michigan, between the Black River and the Presque Isle River, on the west side of the Porcupine Mountains. In 2008, Orvana Minerals Corporation began development of a stratiform copper deposit at the base of the Nonesuch Shale in the Western Syncline. Data on the glacial overburden were collected through rotosonic soil borings, monitoring well installations, test pits, grain size analyses, and hydrogeological testing methods. AECOM collected data necessary for an Environmental Impact Assessment pursuant to Michigan’s Non-Ferrous Metallic Mining Regulations (Part 632) funded by Orvana Minerals Corporation. The broader context of the data provided here is provided in the publically available Part 632 Mine Permit application for the Copperwood Project. Research regarding the glacial deposits in the western Upper Peninsula of Michigan is generally scattered and not especially detailed. Most available information is on the regional scale, rather than a local scale. Farrand and Bell (1982) mapped the area west of the Porcupine Mountains as “lacustrine clay and silt.” Hack (1965) described the Ontonagon Plain (east of the Porcupine Mountains) to be underlain by reddish-brown glacial lake sediments and till. He described three units within the glacial deposits – lower, intermediate, and upper units. Thin lacustrine deposits related to Glacial Lake Duluth are described as a veneer of strongly-laminated clay, silt and sand of variable thickness. More recently, Petersen (1985 and 1986) described the glacial deposits of the area as thin drift over bedrock, generally less than 30 feet thick, composed of calcareous red till of fine sand, silt, and clay with a small percentage of angular clasts. Detailed investigation at the Copperwood site indicated the unconsolidated materials consist primarily of a reddish-brown glacial till. The glacial section most closely matches that described by Hack (1965). The overburden material was interpreted as subglacial till based on the following evidence: • Uniform composition and consistently found to be massive, matrix-supported diamicton with little evidence of interbedded sorted sediments; • Little structure and no observed stratification, lamination or fining upward or downward trends; • The material is widespread and consistent over several square miles of the project area; • Grain-size distribution curves are typical of those associated with subglacial tills; and • The matrix is uniform based on USDA grain size distribution plots. The average composition of the cohesive (silt/clay) samples encountered at the site was found to be approximately 50% silt, 20% clay, and 30% sand. The gravel/cobble portion of the material was approximately 10 to 20% and consisted primarily of dark reddish-brown sandstone (Freda Sandstone). The fine fraction (<200 sieve size) were determined to be predominantly quartz, with very minor amounts of clay minerals. The matrix also contains carbonate (fizzes when exposed to dilute hydrochloric acid). Layers of coarser (non-cohesive) material were observed throughout the site in various borings. These soil types generally consisted of fine to medium-grained sand with varying amounts of silt and clay. Granular deposits are thin and isolated, with none greater than 10 feet thick. These deposits are not laterally extensive, and for the most part cannot be correlated between adjacent borings. They are interpreted to be lacustrine, intra-till sands, or subglacial meltwater deposits. These deposits are typically stratigraphically between the upper and lower till -9- �Proceedings of the 58th ILSG Annual Meeting - Part 1 units, but some are located at the base of the overburden, lying directly on the top of the bedrock surface. Thin, surficial, lacustrine deposits related to Lake Duluth are present across the site, as wave-cut benches and beach deposits. The beach deposits are generally less than 10 feet thick and less than 200 feet wide. These beach deposits consist of sand and gravel deposits adjacent to the wave-cut bluffs that parallel topography. The unconsolidated overburden at Copperwood is interpreted to be the result of least two advances of the Ontonagon lobe. The first advance moved south out of the Lake Superior basin, and deposited the lower till unit, as well as the granular deposits on the bedrock surface. The ice then retreated, depositing the lacustrine/subglacial meltwater deposits which are located between the upper and lower till units, where present. The youngest glacial advance occurred as the ice moved out of the western Lake Superior basin and overrode previous deposits, and influenced the orientation of surface drainage features. Glacial Lake Duluth occupied the area after final retreat of the Ontonagon lobe. This lake had an elevation as high as approximately 1,200 feet msl. Drainage of this lake continued as ice continued to retreat. Several Lake Duluth shorelines are present across the site between elevation 650 feet msl and 1,125 feet msl. The Lake Nipissing shoreline (elevation 615 feet msl) is not present at the site and likely has been destroyed by erosion along the current Lake Superior shoreline. The characteristics of the glacial material have significant implications for site development. The very low hydraulic conductivity of the till limits surface water infiltration producing streams that are runoff-dominated and flashy. Streams are present in deeply incised, closely-spaced parallel valleys, impacting space for site development. Abundant wetlands are present across the site, due to the poor infiltration, and are not hydraulically connected to groundwater. The very low hydraulic conductivity also leads to very slow movement of groundwater. Hydraulic testing and migration modeling suggests that a tailings disposal facility can be constructed in the clay till soil thus using a native material as natural liner. Orvana Minerals Corporation generously allowed publication of this abstract. References Farrand, W. R. and Bell, D. L., 1982, Quaternary Geology of Northern Michigan, Michigan DNR, Geologic Survey Division. Hack, J. T., 1965, Postglacial Drainage Evolution and Stream Geometry in the Ontonagon Area, Michigan, USGS Professional Paper 504-B. Peterson, W. L., 1985, Surficial Geologic Map of the Iron River 1 degree x 2 degree Quardangle, Michigan and Wisconsin, USGS Miscellaneous Investigations Series, Map I-1360-C. Peterson, W. L., 1986, Late Wisconsinan Glacial History of Northeastern Wisconsin and Western Upper Michigan, USGS Bulletin 1652. - 10 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock Geologic map of Morrison County, Central Minnesota BOERBOOM, Terrence J., Minnesota Geological Survey, boerb001@umn.edu Morrison County, located in the center of Minnesota, has recently been remapped with help from the USGS STATEMAP mapping program, and eventually it will become integrated into the MGS County Atlas Program. Although the basic geologic framework was well established prior to this mapping effort, much has been learned by logging of all available drill cores, integration of old exploration data, and outcrop examinations. Scattered bedrock outcrops are present throughout the southern two-thirds of the county, and more than 200 drill holes targeted at iron ore were placed in the northern part of the county prior to the 1960’s. Although the core from many of these was not preserved, drill logs, including iron assay data, locations, and some cross-sections, are available for all. The only iron mine to come out of this effort was the tiny Gorman mine (U of M 1954 Mining directory reports operation from 1951-1952, with 116,000 tons shipped). More recently, about 100 drill holes were placed into other parts of the county; these are a mixture of diamond and mineral exploration, scientific research, and other borings. About 50 of these have useful core or cuttings available and the rest have descriptions deemed reliable for use. In total 85 cores and cuttings were logged. In areas between drill holes and bedrock outcrops, the bedrock geology is inferred, with varied degrees of confidence, from geophysical data. Morrison County is located along the southern Margin of the Superior Province – Wawa subprovince, and also straddles the Penokean Orogen (Fig. 1). Archean bedrock is inferred to be primarily felsic gneisses and metamorphosed supracrustal rocks intruded by felsic to mafic plutons. The rest of the county is composed of Paleoproterozoic bedrock that includes, from north to south, basal quartzite and turbidites of the Animikie basin, the Cuyuna North and South Ranges (external and medial zones of the fold-and-thrust belt, Fig. 1), the Little Falls Formation, and the east-central Minnesota batholith (both within the internal zone; Fig. 1). The external and medial zones of the Penokean fold-and-thrust belt are composed of phyllitic muddy to quartzose sedimentary rocks interbedded with mafic flows and sills and thin iron-formations, all metamorphosed under lower greenschist-facies conditions. In contrast, the Little Falls Formation, within the internal zone, is composed only of turbidites metamorphosed under amphibolite-facies conditions, and ultramafic to felsic intrusive plugs and plutons. The break between low-grade and high-grade rocks, the Malmo structural discontinuity (MSD), is modeled as a southdipping thrust fault. The MSD is clearly recognizable on aeromagnetic data, where sharp linear magnetic trends associated with iron-formations to the north are truncated by low, uniform aeromagnetic patterns to the south. South of the MSD, the metamorphic grade of the Little Falls Formation increases in proximity to the ECMB, from biotite at the northwest, through a narrow band of garnet, into a wide zone of sillimanite-garnetstaurolite to the southeast. Cordierite occurs around the margins of what are inferred to be gneiss domes. Adjacent to mafic intrusions staurolite has been thermally metamorphosed to an   - 11 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 assemblage of hercynite and corundum. The ECMB batholith is composed of several separate intrusions that range from tonalite to granite. Some of these include the approximately 1,800 Ma Hillman tonalite, the Pierz granodiorite (1796±19 Ma), the Foley granite (1779±4 Ma non-magnetic phase and 1774±1 Ma moderately magnetic phase), the Freedhem granodiorite (ages of 1775±3 to 1776±7 Ma; intruded by 1773±2 granite dike) and unnamed granites, one of which is 1766±8 Ma and another that is 1784±8 Ma (all U-Pb zircon ages reported in Holm et al., 2005). Small dominantly mafic to ultramafic, but also felsic, plug-like intrusions are abundant. Fourteen of these have either been drilled or are exposed, and the rest are mapped on the basis of small, sharp aeromagnetic anomalies. Those drilled or exposed consist predominantly of porphyritic gabbronorite to norite, and others include ultramafic olivine-pyroxene hornblendite, hornblendite, hornblende gabbro, and diorite to tonalite. The gabbronorite contains euhedral phenocrysts of augite, variably altered olivine, and rare plagioclase in a fine-grained, weakly trachytoid groundmass composed of lath-shaped plagioclase, intergranular bronzite hypersthene and varied proportions of augite, and lesser poikilitic biotite/phlogopite, interstitial alkali feldspar, and apatite. One of these bodies gives an Ar-Ar plateau age of 1791±8 Ma (Jirsa et al., 2006). Only two of the intrusions sampled are ultramafic; these are coarse-grained, phlogopitic pyroxene-olivine hornblendite with 3040% poikilitic, pargasitic Mg-hornblende, 30-35% variably altered olivine, 15-20% augite, 2-10% phlogopite, 1-15% secondary actinolitic amphibole, 3-6% secondary oxides, and minor chlorite, chromite, and apatite. These intrusions contain high Cr, Ni, and MgO. A second distinct type of hornblendite is dark green, medium-grained, and composed mainly of deep green equant hornblende. This unit forms plugs, is observed as a 3m-wide dike that cuts the Little Falls Formation, and as local phases in the gabbronorite intrusions. A third distinct type of hornblende-rich intrusion is composed medium- to coarse-grained, strongly prismatic-foliated (magmatic), weakly plagioclase-phyric, hornblende gabbro. This intrusion is substantially larger than the other plug-like bodies and yields an Ar-Ar plateau age of 1770±6 Ma (Jirsa et al., 2006). Thin sills of dioritic rocks within the Little Falls Formation wallrock adjacent to the mafic rocks, as well as irregular zones within the mafic and ultramafic intrusions, are identical to small, discrete dioritic plugs. These felsic members contain strongly zoned plagioclase phenocrysts, and petrographic textures indicate that they are likely the product of contamination by melting of the adjacent Little Falls Formation. Geochronological studies show that supracrustal rocks in the Penokean fold and thrust belt were metamorphosed as recently as 1,760 – 1,770 Ma, which compares with emplacement ages of 1,800 – 1,776 Ma for the ECMB, indicating that the metamorphism may be attributed to widespread crustal thickening and heating related to pluton emplacement. These metamorphic and pluton emplacement ages correspond to the Yavapai Orogeny (e.g. Holm and others, 2007, and references therein). Intrusions within the ECMB show mafic/felsic magma mingling textures, and the ECMB is likely related to the small outboard plugs but emplaced at a deeper crustal level in the heart of the orogenic belt where there was greater crustal thickening and wholesale crustal melting. References Holm, D.K., Van Schmus, R.V., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and Schneider, D., 2005, Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis, Geological Society of America Bulletin, v. 117, p. 259-275. Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution: Precambrian Research, v. 157, nos. 1-4, p. 106-126. Jirsa, M.A., Miller, J.D., Jr., Severson, M.J., and Chandler, V.W., with contributions by Boerboom, T.J., Lively, R.S., Keatts, M.J., and Holm, D.K., 2006, Final Report on the geology, geochemistry, geophysical attributes, and platinum group element potential of mafic to ultramafic intrusions in Minnesota, excluding the Duluth Complex: Project Summary: Minnesota Geological Survey Open File Report OF-06-3, 49 p. - 12 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Preliminary results of P- and S-wave delay times in the Superior Region from transportable array and SPREE stations BOLLMANN, Trevor, VAN DER LEE, Suzan, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208, FREDERIKSEN, Andrew, Department of Geological Sciences, University of Manitoba, 125 Dysart Rd., Winnipeg, MB R3T 2N2 and LOU, Xiaoting, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208 The Superior Province is cut by a Proterozoic structure named the Midcontinent Rift (MCR). It is one of the most striking gravity anomalies in North America on account of its length and height in mgals. The anomaly is caused by large masses of mafic/ultra mafic rock deposited along the rift axis around 1.1. Ga. To investigate the deep structure of this region we measured delay times of teleseismic waves recorded at 242 seismic stations from the Earthscope Transportable Array (TA) and the Superior Province Rifting Earthscope Experiment (SPREE). To date, 6144 P-wave and 3013 S-wave delay times were measured from 33 teleseismic earthquakes. Using a new, interactive automated delay time assessment interface, we could measure absolute delay times, relative to global seismic velocity model iasp91. Interesting features found by mapping the average delay times are: a region of negative delays around Lake Nipigon and regions of positive delays in central Minnesota into Wisconsin, and in south-central Iowa (P-wave only). These delay times are most affected by the structures in the upper mantle due to the fact that the lithospheric portion of the delay time is less than 5 percent of the whole delay time. Since deeper structures cause larger variations with distance and azimuth it is not a surprise that the delay times vary between individual events. Looking at delay time maps of individual events show patterns of planar wave arrivals. These patterns will be investigated to discern their origins and if they are related to their azimuth or distance. More measurements will be added as new TA data is downloaded from the IRIS Data Management Center and as SPREE servicing runs are completed in the next 1.5 years. References Kennett, B.L.N., E.R Engdahl. 1991. Travel times for global earthquake location and phase identification, Geophysical Journal International, v. 105, p. 429–465 Lou, X., S. Lloyd, S. Van der Lee. 2012. A Python/Matplotlib Tool for Measuring Teleseismic Body Wave Arrival Times. (In Preparation) Bedle, H., and S. van der Lee. 2006. Fossil flat-slab subduction beneath the Illinois basin, USA, Technophysics, v. 424(1-2): p. 53-68 - 13 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The Copperwood Sediment-Hosted Stratiform Copper Deposit, Upper Peninsula, Michigan BORNHORST, Theodore J., A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 and WILLIAMS, William C., Orvana Minerals Corporation, 181 University Avenue, Suite 1901, Toronto, Ontario M5H 3M7 The Copperwood copper deposit is located in Gogebic County, Upper Peninsula, Michigan within the Porcupine Mountains sediment-hosted copper district. It was discovered in 1956 by United States Metal Refining Company. In 2008, Orvana Minerals Corporation began exploration and development work at Copperwood. The company is planning first production in 2014, which would be the first Upper Peninsula copper mine since 1996. The copper mineralization occurs as very fine-grained chalcocite hosted by siltstones and shales of the lowermost Nonesuch Formation, which is underlain by red-bed sandstones, conglomerates, and minor siltstones of the Copper Harbor Formation. The mineralized beds are stratigraphically equivalent to the lower copper mineralized units at the now closed White Pine Mine about 30 kilometers to the northeast. These rocks are part of a thick section of clastic sedimentary rocks that comprise the upper fill of the Mesoproterozoic (1.1 to 1.0 Ga) Midcontinent rift. The copper deposit is a tabular body averaging 2.5m thick, lacks significant basal undulations, and dips 8° to 12° northward on the southwest limb of the Western Syncline, an open fold with a shallow plunge to the northwest. Copper mineralization occurs as 5 to 50 micron disseminated grains of chalcocite and as concentrations along laminations. The deposit is relatively undeformed with only one shallow, north-dipping reverse fault recognized; it’s displacement is as much as 7m. Gangue minerals in the ore body are quartz, clinochore, muscovite/illite, plagioclase, K-feldspar, calcite, and hematite. Reported Canadian National Instrument compliant Proven and Probable Reserves, fully diluted, are 27.42 million metric tonnes averaging 1.41 % Cu and 3.6 ppm Ag for contained metal of 852 million pounds of copper and over 3.2 million ounces of silver. A model for sedimentation and mineralization was based on the logging of core from nearly 200 drill holes at Copperwood and over another 100 drill holes throughout the Western Syncline. The host rocks were likely deposited under mostly anoxic conditions along the margins of a lake. In these conditions, reduced organic carbon-bearing muds with very fine-grained, syngnetic pyrite were deposited in the copper-bearing sequence. Red intervals within the otherwise black to gray sequence suggest periodic oxic conditions. During diagenesis and incipient lithification, copper-bearing, saline basinal fluids passed vertically through the sequence and formed chalcocite after pyrite; copper was likely leached from the underlying red-bed paleoaquifer. The stratigraphic sequence was folded during the Grenvillian compression of the rift strata long after lithification. In addition to the formation of the Western Syncline and recognized reverse faulting, calcite veinlets were emplaced at Copperwood. The Copperwood deposit has the characteristics of a reduced-facies, sediment-hosted stratiform copper deposit, or Kupferscheifer-type. Although the mineralized sequence is stratigraphically equivalent to that of the White Pine deposit, it lacks the structural complexity and hydrothermal overprint characteristic of that deposit. The simple mineralogy and geochemistry, single copper-mineralizing event, and minimal deformation distinguishes the Copperwood deposit from other more complex, reduced-facies sediment-hosted stratiform copper deposits throughout the world. Orvana Minerals Corporation has granted permission to publish this abstract. The results and interpretations herein are the responsibility of the authors. Certain statements constitute forward-looking statements or forwardlooking information within the meaning of applicable securities laws. Readers are cautioned not to put undue reliance on forward-looking statements. - 14 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geology and petrology of a Mesoproterozoic layered mafic intrusion in portions of the Brule Lake and Cherokee Lake 7.5’ Quadrangles, northeastern Minnesota BROOKER, Ben, and MILLER, Jim, Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812, USA The Duluth Complex and related Mesoproterozoic intrusions of northeastern Minnesota comprise one of the largest igneous complexes on Earth (Miller and Severson, 2002). These intrusions, and the comagmatic volcanics into which they were emplaced, are part of the 1.1 Ga Midcontinent Rift (MCR). Many areas of northeastern Minnesota have sufficient bedrock exposure to be mapped at a detailed scale (e.g., 1:24,000). Some areas, however, are underlain by the Duluth Complex and related MCR rocks, particularly those contained within the Boundary Water Canoe Area Wilderness, have been only cursorily investigated or mapped at a reconnaissancescale. One such area of incomplete mapping, which has been a particular curiosity since the acquisition of high resolution aeromagnetic data over northeastern Minnesota (Chandler, 1983), is located in the Sawbill Lake area of Cook County. Part of what is generalized as the Brule Lake-Hovland gabbro on the M-119 map of NE Minnesota (Miller et al., 2001) is characterized by a strongly banded, tightly curved anomaly pattern. Reconnaissance mapping in the anomaly area by Grout et al. (1959) and Davidson (1977; Davidson and Burnell, 1977) indicates a predominance of well foliated and locally layered, oxide-rich olivine gabbro. Recently, several areas of the gabbro were mapped in detail by PRC field camp students (Frost et al., 2007; Blakely et al., 2009; Brooker et al., 2010; Asp et al., 2011). For the first author’s MS thesis project, he is focused on integrating this previous mapping with his own bedrock mapping to establish the geology of a previously unrecognized, well differentiated mafic layered intrusion, which we have named the Sawbill Lake intrusion (SLI). The detailed mapping of the SLI indicates that it is composed of troctolitic cumulates in its lower section, oxide gabbro cumulates in its medial section, and apatite ferrogabbro cumulates to ferromonzonite in its upper section. The footwall of the SLI is composed of a mix of intermediate to gabbroic rocks, anorthositic gabbros, and Pl-porphyritic diabase. The hanging wall is composed of leucogranites of the Eagle Mountain granophyre. The gradational nature of the contact between SLI ferromonzonite and Eagle Mtn granophyre implies that the SLI underplated and partially melted the granophyre body, as found elsewhere throughout the Duluth Complex (Miller and Severson, 2002). Petrographic analysis of samples collected along multiple profiles through the intrusion confirmed the cumulate stratigraphy of the SLI implied from field mapping, namely POaPCFaPCFOA. The majority of samples collected were concentrated along three main profile lines in different areas of the intrusion. One set of profiles was in the west end, one in the central, and the other in the eastern end of the SLI. One of the more striking features within the intrusion is the presence of an interval rich in subconformable blocks of basaltic and sedimentary hornfels, which were first recognized by Grout et al. (1959). The interval is located immediately above the transition between troctolite and oxide gabbro cumulates. Although the cumulate stratigraphy of the SLI shows a typical progression that could be related to progressive differentiation of a tholeiitic mafic magma, the presence of a persistent volcanic screen at the major transition in cumulate mineralogy suggests that this intrusion probably did not form by uninterrupted closed system fractional crystallization. Possible explanations for these relationships are 1) the hornfels interval separates two distinct intrusions, 2) the onset of oxides becoming cumulus caused the liquid in the magma chamber to become less dense and allowed volcanic rocks in the hanging wall to fall to the cumulate floor of the chamber, or 3) the hornfels blocks mark a significant venting event that depressurized the magma chamber and triggered the saturation (cumulus arrival) of pyroxene and oxide. To determine which of these scenarios is most likely, EDS analyses of olivines and pyroxenes were collected from the samples profiling the intrusion with the intention of evaluating whether the abrupt mineralogical change marked by the hornfels interval also corresponds to a compositional break. The mineral chemical data collected from over 68 samples does not unequivocally support any one theory for the emplacement and crystallization history of the SLI, but is does seem to favor the venting model best. Perhaps - 15 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the most significant result of the mineral chemical data is that it shows a smooth cryptic variation throughout the entirety of the intrusion in both the olivine and pyroxene. The lack of any significant chemical break at the hornfels interval would seem to indicate that the SLI fractionally crystallized in situ from one major pulse of magma, thus supporting the density inversion and venting models. The problem with the density inversion model is that one would expect that if hornfels blocks fell to the crystallized floor due to the cumulus crystallization of oxide, one would expect to find inclusions of the volcanic rocks throughout the upper oxide gabbroic part of the intrusion. This is not the case. Alternatively, the singular occurrence of volcanic hornfels at horizon marked by the abrupt change in pyroxene habit from subophitic-ophitic below to granular above, but no significant mineral chemical change, seems more consistent with a significant venting model. While decompression attending a venting event could have an profound affect on the phase equilibrium of the system to where augite and oxide may become cumulus, it should have a minimal effect on the chemistry of the magma system. The presence of high density subophitic-ophitic pyroxenes below the screen indicate that the magma was very close to becoming saturated in augite such that venting could have triggered the cumulus arrival of augite and oxide. References Asp, K., Leu, A., Parisi, A., Sletten, D., Brooker, B., Miller, 2011, Bedrock geology of the Sawbill Lake area: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2011-04, 1: 12,000. Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock geology map of Homer Lake and adjacent areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Brooker, B.P.,Hadley, M.L., Markwood, L.W., Olson, J., Tomlinson, A.P., and Miller, J.D.,2010, Bedrock geologic map of the Jack Lake and Weird Lake areas, Cook County, northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/Map-2010-05, 1: 12,000. Chandler, Val W, 1983, Aeromagnetic map of Minnesota, Cook and Lake counties: Minnesota Geological Survey, Aeromagnetic Map Series, Map A-1, scale 1:250,000 Davidson, D.M., 1977, Reconnaissance geologic map of the Cherokee Lake quadrangle, Cook County, Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-30, scale 1:24,000 Davidson, D.M., Jr. and Burnell, J.R., Jr., 1977, Reconnaissance geologic map of the Brule Lake quadrangle, Cook County, Minnesota: Minnesota Geologic Survey Miscellaneous Map M-29, scale 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000 Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The geology of Cook County, Minnesota: Minnesota Geological Survey Bulletin 39, 163 p. Miller, J.D., and Severson, M.J. 2002. Geology of the Duluth Complex. In Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota, Minnesota Geological Survey Report of Investigations 58, p. 106-143. Miller, J.D., and Green, J.C., 2002, Geology of the Beaver Bay Complex and related hypabyssal intrusions. In Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota, Minnesota Geological Survey Report of Investigations 58, p. 144-163. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000, 2 sheets. - 16 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Accessory minerals of a roadside pegmatite, Orr, Minnesota Buchholz, Thomas, 11140 12th Street North, Wisconsin Rapids, Wisconsin 54494, Falster, Alexander, and Simmons, William, Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, Louisiana 70148 While returning from ILSG 2010, the first author took some samples from a pegmatite exposed on the west side of US 53 just north of Orr, MN. Interesting results from heavy mineral separates prepared from the samples encouraged a return visit in August 2011, in which samples were taken from discrete zones of the pegmatite. Heavy mineral separates were prepared at the University of New Orleans, mounted in epoxy, and examined under SEM/EDS. Selected grains were analyzed with the electron microprobe. Rock units in the vicinity of the pegmatite were mapped by Jirsa (2011) as Neoarchean biotite schist-rich migmatite intruded by Lac La Croix granite. The pegmatite is zoned, roughly horizontal, and with multiple alternating zones of aplite and pegmatite. The footwall is granititic with scattered pegmatite stringers, but the hanging wall is not well exposed; rafts or screens of biotite-rich schist in the uppermost pegmatite exposure suggest close proximity to the top of the dike. Zones are not continuous, but pinch and swell laterally. No discrete massive quartz core such as frequently observed in pegmatites is present, but pegmatite zones do have distinctive mineralogy, exhibiting varying degrees of fractionation; hence a more highly fractionated, core zone can be distinguished. Aplite zones throughout the dike are generally lacking in HFSE-bearing mineralization, though zircons are present in all phases. Lower pegmatite zones adjacent and near the lower contact carry Mn and Zn-bearing ilmenites, along with Th-rich allanite. Nb- rich titanite in these zones appears to accommodate Nb by charge balance using Al+Fe3+<-->Nb5++Ta5+. Magnetite is common in all pegmatite zones, and carries significant Zn and some Mn, reflecting pegmatite enrichment in these elements. The third pegmatite zone is perhaps the most highly fractionated, and is thusly considered to correspond to the core zone of the pegmatite. Mn-rich almandine garnet, magnetite, Th-rich monazite, possible chevkinite(Ce), and columbite-Fe have been noted. Sparse zircons exhibit significant Hf enrichment, ranging from about 2.1 wt% to about 5.3 wt% HfO2. Sparse columbite-Fe is somewhat enriched in Ta, with up to almost 16 wt% Ta2O5. Notably, detectable Sc is present, ranging from 0.021 to 0.031 Sc2O3. Some grains of columbite-(Fe) were found embedded in magnetite grains, and detectable Nb (in the range of 0.0X to 0.00X wt%) was found in the adjacent magnetite. Since Nb cannot be reasonably incorporated in the magnetite structure it is likely present as minute grains of Nb-bearing minerals, most likely columbite-(Fe). Heavy mineral separates from the biotite-rich metamorphic rock rafts in the upper portion of the pegmatite exposure carry abundant titanite and sparse zircons. The titanites carry Nb just barely above detection limit and exhibit variable composition suggesting alteration, likely the result of metasomatism via pegmatite fluids that introduced the Nb. Since it is generally accepted that pegmatites form from a single injection of relatively volatile-rich, fractionated melt, the interlayered aplite and pegmatite suggest fluctuating conditions of crystallization, from nucleation-suppressing volatile-rich conditions facilitating coarse pegmatite formation, to nucleation-enhancing less volatile-rich conditions facilitating aplite formation. The low abundance of volatile-rich phases, with the exception of biotite in pegmatite layers, suggests a relatively “dry” melt; pegmatite phase formation proceeded until volatile levels decreased to the point that mica formation and volatile-induced nucleation-inhibition ceased, and aplite formation proceeded until buildup of volatiles again inhibited nucleation and allowed pegmatite formation to resume. It is interesting to note that Grout (1926) noted abundant magnetite in pegmatites of the Vermillion Batholith, now the Lac La Croix granite. Evidently few analyses of the magnetite were made, but the one analysis included - 17 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 by Grout (magnetite in border phases of the Vermillion Batholith: 0.26 % Mn, p. 74) notes significant Mn contents. There is no evidence that Zn was looked for by these researchers, but it is possible that Zn-rich magnetites, noted in our study, may be more common in these pegmatites than has been recognized. References Jirsa, Mark A., 2011. Bedrock Geology of the Crane Land and Brule Narrows 30’ X 60’ quadrangles, Northern Minnesota, Miscellaneous Map Series Map M-192, University of Minnesota, Minnesota Geological Survey Grout, Frank F., 1926. The Geology and Magnetite Deposits of Northern St. Louis County, Minnesota, Bulletin No. 21, Minnesota Geological Survey, 220 pages. - 18 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Petrographic and geochemical study of the hybrid rock unit associated with the Current Lake Intrusive Complex, Magma Metals’ Thunder Bay North Property CHAFFEE, Matthew, MILLER, James, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812, HOLLINGS, Peter, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, HEGGIE, Geoff, MACTAVISH, Allan, Magma Metals Ltd, 1004 Alloy Drive, Thunder Bay, ON P7B 6A5 Canada, BANDLI, Bryan, Department of Geological Sciences, University of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812 The Current Lake Intrusive Complex (CLIC) is one of several recently discovered ultramafic to mafic intrusions associated with the Midcontinent Rift (MCR) that host Ni-Cu-PGE deposits (Heggie, 2005; Ware et al., 2008; Rossell, 2008; Ding et al., 2010; Goldner, 2011; Foley, 2011). Largely unexposed in the Current Lake area about 50 kilometers northeast of Thunder Bay, Ontario, the CLIC was discovered when glacially transported ultramafic boulders containing disseminated sulfides were located along the shores of Current Lake (MacTavish and Smyk, 2010; Goodgame et al., 2010). Subsequent geophysical surveys and drilling have outlined a 3.6 kilometer long, tubular ultramafic chonolith that is composed mostly of wehrlite to dunite and is intruded into granitic and metasedimentary rocks of the Quetico subprovince. Over the course of drilling, it was discovered that an intensely altered and contaminated heterogeneous rock unit, termed the hybrid rock unit (HRU), commonly occurred at the margins of the CLIC. The focus of this study is to characterize the mineralogical, textural and geochemical attributes of the HRU in order to understand the origin of its heterogeneity and address the question of its petrogenetic relationship to the CLIC. Ten cores profiling two representative sections across the CLIC were re-logged and sampled. Six cores profile the narrow (30-50m) Current Lake zone where the CLIC intrudes granitic country rocks northwest of the Quetico fault. Four cores profile the wider (up to 600m) Beaver Lake zone southeast of the Quetico fault where the country rock is metasedimentary schists and minor granite. A total of 139 petrographic thin sections that are representative of the various textures and lithologies were studied for this project. 52 samples were submitted to Acme Laboratories in Vancouver, British Columbia for trace element and PGE fire assay analysis. XRF analyses of the ground pulps were conducted in the UMD Research Instrumentation Lab for major element chemistry. Having only recently completed the geochemical analyses, we will focus here mostly on the results of the petrographic study. Initial core logging conducted by Magma Metals distinguished two types of hybrid lithologies, which they termed red hybrid and grey hybrid by their obvious coloration differences. Typically, the red hybrid occurs on the margins of the CLIC or as narrow offshoots into the country rock. The grey hybrid typically occurs sandwiched between the marginal red hybrid and mineralized wehrlite/dunite. In places, the red hybrid can be missing and the grey forms the CLIC margin in direct contact with the country rock. Like the red, the grey can also occur as isolated offshoots into the country rock. Core logging and petrography of what has been identified as the red hybrid show it to be an intensely altered and hematized, texturally variable (prismatic to subophitic), fine to medium-grained, quartz-bearing gabbro to quartz diorite, with the dominant lithology being quartz leucogabbro with 2-5% sulfide (mostly iron sulfide). The rock is fine-grained at its contact with country rock, but is medium-fine grained near its abrupt contact with the grey hybrid. Occurrences of this rock type in the Current Lake and Beaver Lake zones are generally similar, with the exception that the Beaver Lake zone contains large, chlorite-mantled quartzite and granitic xenoliths and has more abundant interstitial quartz. A significant discovery is that some of what was logged as grey hybrid because of its color is mineralogically and texturally a quartz leucogabbro that lacks hematization. To steer clear of using secondary coloration as a primary criteria for classifying rock types, we refer to this lithology as the quartz leucogabbro unit (QLG) Petrographic investigations of rock types that have been logged as the grey hybrid are mineralogically and - 19 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 texturally distinct from the QLG unit and instead bear more resemblance to the CLIC ultramafic rocks. The average “grey hybrid” rock type is a moderately to intensely altered, fine to medium-fine grained, intergranular to subpoikilitic feldspathic wehrlite to melagabbro with 3-5% Ni-Cu sulfide. As such, we have renamed the grey hybrid, the feldspathic wehrlite (FW) unit. However, significant changes in mode and texture and alteration occur across the 1-3 meter wide FW unit from its outer contact with country rock or the QLG unit inward to the mineralized werhlite/dunite. Where the FW unit is adjacent to the QLG unit, the contact occurs over several centimeters width and is marked by 1) an abrupt decrease in grain size, 2) a rapid decrease in plagioclase mode and its change from lath shaped to subpoikilitic, and 3) a rapid increase in clinopyroxene mode and its change in habit from subophitic to anhedral granular. Progressing inward from the altered, fine-grained, intergranular melagabbro at the QLG-FW contact, several gradual changes occur within the FW unit: alteration decreases, grain size increases to medium-grained, subpoikilitic plagioclase mode decreases, clinopyroxene mode decreases and its texture changes from anhedral granular to subpoikilitic, and granular olivine quickly appears and become the dominant phase. In one Current Lake zone core where the FW unit is in direct contact with the granitic country rock, the rock grades over 1-3 meter wide interval from a very fine grained (chilled) altered gabbro at the contact to a subpoikilitic feldspathic wehrlite. The modal and textural changes across the FW unit are interpreted as indicating that this unit is a rapidly cooled marginal phase of the mineralized ultramafic intrusion. The contact relationships between the QLG and mineralized ultramafic intrusion are best explained by a twostage model of emplacement. The QLG unit was emplaced first, incorporated abundant country rock xenoliths, became strongly contaminated, and extensively hydrothermally altered. This precursor intrusion “set the table” for the main intrusive event by establishing a conduit system and preheating and devolitalizing the country rock. The emplacement of the main CLIC ultramafic magma generally followed the same path as the QLG unit probably by reaming out and inflating the semi-molten core of the precursor intrusion. The incorporation of silica, sulfides, and volatiles residing in the QLG precursor intrusion during the dynamic emplacement of the main chonolithic ultramafic intrusion resulted in high tenor sulfide mineralization. Preliminary evaluation of lithogeochemical data indicate that despite its intense felsic contamination and hydrothermal alteration, the trace element characterisitic of the QLG unit are consistent with its having similar parent magma as the ultramafic rocks of the CLIC. These results also indicate that the granitic and metasedimentary rocks of the Quetico subprovince are a reasonable source of contamination. Further geochemical modeling is underway in order to confirm the apparent genetic relationship between the QLG and the mineralized ultramafic CLIC. References Ding, X., Li, C., and Ripley, E.M., 2010, The Eagle and East Eagle sulfide ore-bearing mafic-ultramafic intrusions in the Midcontinent Rift System, upper Michigan: geochronology and petrologic evolution. G3-Geochemistry, Geophysics and Geosytems, Volume 11, Number 3, 22p. Foley, D.J., 2011. Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan. M.S. thesis. University of Minnesota Duluth, Duluth, MN. Goldner, B.D., 2011. Igneous petrology of the Ni-Cu-PGE mineralized Tamarack intrusion, Aitkin and Carlton Counties, Minnesota. M.S. thesis. University of Minnesota Duluth, Duluth, MN. Heggie, G., 2005, Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, Northwestern Ontario. M.Sc. thesis, Lakehead University, Thunder Bay, ON MacTavish, A. and Smyk, M.C., 2010, Thunder Bay North Project, Magma Metals Limited. In Miller, J.D., Smyk, M.C. and Hollings, P.N. (eds.).Cu-Ni-PGE deposits in mafic intrusions of the Lake Superior region: A field trip for the 11th International Platinum Symposium; Ontario Geological Survey, Open File Report 6254, 166p. Rossell, D., 2008. Geology of the Keweenawan BIC intrusion: Institute of Lake Superior Geology. 54th Annual Meeting, Marquette, MI, Proceedings and Abstracts, part 2. p. 181-199. - 20 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Strain Variations in Carbonates across the Proterozoic Grenville Orogen Craddock, Suzanne D., Geology Department, Lawrence University, Appleton, WI, 54911, Craddock, John P., Geology Department, Macalester College, St. Paul, MN 55105 The Grenville orogeny involved the accretion of numerous terranes, likely with reversals of slab polarity, adding a width of ~800 km of new crust to Laurentia as part of the supercontinent Rodinia (Easton, 1991; Carr et al., 2000). Top-to-the-northwest thrusting is the common mechanism for accretion but there is a component of post-orogenic collapse (Rivers, 2012) documented by southeast trending kinematic indicators (Carlson et al., 1990). We report the results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Taconic allocthon. Forty three carbonates were collected resulting in 57 strain analyses on mechanically twinned calcite with the following distribution: Parry Sound domain (1), Bancroft domain (20), Harvey-Cardiff domain (1), Belmont domain (4), Marzinaw domain (1), Frontenac domain (1), Adirondack Lowlands (5), Adirondack Highlands (8), and the Taconic allochthon (2). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone and a roadcut near Ft. Ann, NY. From northwest to southeast, the twinning strain fabric is dominantly a layer-parallel shortening fabric oriented N-S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW-SE) across the Composite Arc Belt (Central Metasedimentary Belt) to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW-NE). The easternmost Adirondack Highlands preserve a vertical shortening strain in calcite veins that are cross-cut by veins found in the overlying Taconic allochthon; the Ordovician limestones and veins preserve a layer-parallel shortening strain oriented at 330°. Marbles from the Bancroft shear zone contain calcite grains with two sets of twin lamellae (e1 and e2). The better-developed e1 sets (n=406) record a LPS fabric oriented NW-SE whereas the younger e2 lamellae (n=146) preserve a marginparallel (SW-NE) LPS fabric. Both the e1 and e2 strains record an overprint strain (NEV) that records vertical shortening, perhaps related to the collapse of the Ottawan orogenic lid. References Carlson, K.A., van der Pluijm, B.A. and S. Hamner, 1990, Marble mylonites of the Bancroft shear zone: evidence for extension in the Canadian Grenville: Geol. Soc. Am. Bulletin 102, p. 174-181. Carr, S.D., Easton, R.M., Jamieson, R.A. and N.G. Culshaw, 2000, Geologic transect across New York and Ontario: Canadian J. Earth Sciences 37, p. 193-216. Easton, R.M., 1991, The Grenville Province and the Proterozoic history of central and southern Ontario: Geology of Ontario, Ontario Geologic Survey Special volume 4, part 2, Chapter 19, p. 715-906. Rivers, T., 2012, Upper-crustal orogenic lid and mid-crustal core complexes: signature of a collapsed orogenic plateau in the hinterland of the Grenville Province: Can. J. Earth Sc. 49, p. 1-42. - 21 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Petrogenesis and crustal contamination of the Nipigon sills: a geochemical and spatial re-evaluation CUNDARI, Robert, HOLLINGS, Peter, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, and SMYK, Mark, Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada A compilation and re-evaluation of 2397 publically available, spatially referenced samples with associated whole-rock geochemistry has yielded previously unrecognized variation within the Midcontinent Rift-related Nipigon sills of the Nipigon Embayment. Nipigon diabase sills represent the most volumetrically significant Midcontinent Rift-related unit in Canada covering an area in excess of 20 000 km2 (Sutcliffe, 1991). 796 Nipigon sill samples have been investigated using Th/Yb ratios as a means of evaluating crustal contamination. This investigation revealed three distinct Nipigon sill types comparable to the three distinct suites of sills suggested by Hollings et al. (2007) based on radiogenic isotope data. This discrimination supports the isotopic signatures for each group as higher Th/Ybpm values are consistent with a more negative eNd(t=1000Ma) values. Ranges for the three Nipigon sill types are summarized in Table 1. Table 1. Summary of geochemical data for the Nipigon sills   Th/Ybpm   Nb/Ybpm   Nipigon  I   Nipigon  II   Nipigon  III   0.75  –  1.65   1.2  –  1.8   1.5  –  2.2   1.97  –  3.4   3.4  –  5.0   Nb/Nb*   0.425  –  0.65   0.35  –  0.55   Gd/Ybpm   1.0  –  1.9   1.0  –  1.9   La/Smpm   εNd(t=1000Ma)   1.2  –  1.8   1.60  –  2.0   -­‐0.5  to    -­‐1.5   1.5  to  3.0   5.0  –  6.5   0.3  –  0.5   2.2  –  2.6   1.0  –  1.9   >  5.0   Populations of more-contaminated samples (Nipigon types II and III) dominantly lie within two areas (Fig. 1); a north-trending, linear group in the southwestern Nipigon Embayment and an arcuate array in northern and eastern Lake Nipigon. Nipigon sill types II and III display higher Th/Ybpm values, lower Nb/Nb* values and more negative eNd(t=1000Ma) values when compared to Nipigon sill type I. These populations appear to be proximal to major structures such as the Black Sturgeon fault, southwest of Lake Nipigon, (Fig. 1). Type I Nipigon sills are located peripherally to Nipigon sill types II and III and display lower Th/Ybpm values, higher Nb/Nb* values and less negative eNd(t=1000Ma). Type I Nipigon sills do not appear to be related to any known major structures. Preliminary analyses show that Archean rocks of the Quetico Subprovince appear to control the Th/Ybpm values, whereas Sibley Group sedimentary rocks have a stronger control on Nb/Nb*. Type II and III Nipigon sills display higher Th/Ybpm values than type I sills, consistent with the model in which type II and III sills have assimilated metasedimentary rocks of the Quetico Subprovince. The higher Nb/Nb* values of type I Nipigon sills likely reflects a greater degree of interaction with the rocks of the Sibley Group suggesting shallower level contamination (as the Sibley is not present at depth within the Nipigon Embayment). The more crustalcontaminated nature of type II and III sills in conjunction with their proximity to major structures, suggest that magmas feeding these sills ascended through structures that provide more interaction with Quetico rocks at greater depths. This is supported by type II and III samples showing more negative eNd(t=1000Ma) values than - 22 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 type I samples, a signature that infers interaction with older continental crust. The source magma feeding type I Nipigon sills possibly exploited the same pathways yet had less interaction with continental crust at depth as the system was armoured by previously ascended magmas. As type I melts were laterally emplaced, they were contaminated by Sibley Group sedimentary rocks as displayed by elevated Nb/Nb* signatures. Figure 1. Map of the Nipigon Embayment showing the distribution of Nipigon sill types I (yellow circles), II (green squares) and III (red triangles). Major faults after Hart and MacDonald (2007). Figure 2. A) Nb/Ybpm vs. Th/Ybpm plot; B) eNd(t=1000Ma) vs. Th/ Ybpm Nipigon sill types I (yellow circles), II (green squares) and III (red triangles). Radiogenic isotope data from Hollings et al. (2007), Normalizing values from Sun and McDonough (1989). References Hart, T.R., and MacDonald, C.A., 2007. Proterozoic and Archean Geology of the Nipigon Embayement: implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44: 1021-1040. Hollings, P., Richardson, A., Creaser, R. and Franklin, J., 2007. Radiogenic isotope characteristics of the mid-Proterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario. Canadian Journal of Earth Science 44: 1111-1129. Sun, S.S., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins. Geological Society, Special Publication No. 42, 313345. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior area. In Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M. Stott. Ontario Geological Survey, Special Vol. 4, Part 1, pp. 405–484. - 23 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geology and Geochemistry of the Coubran Lake Basalts, a Midcontinent Riftrelated sequence within the central Coldwell Complex, Marathon, Ontario CUNDARI, Robert, HOLLINGS, Peter, Department of Geology, Lakehead University 955 Oliver Road Thunder Bay, Ontario, Canada, P7B 5E1, SCOTT, John and CAMPBELL, Dorothy, Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada The Coubran Lake Basalts occur as part of a large, preserved roof pendant of volcanic rocks in the center of the ~28 km diameter Mesoproterozoic Midcontinent Rift-related, alkalic Coldwell intrusive complex near Marathon, on the northern shore of Lake Superior. In one location, basalt is exposed in a 3 to 5 m wide by~350 m long stripped area. Thin (~2 m), amygdaloidal flows appear to be draped over the side of a hill and exhibit ropy flow tops, suggesting subaerial emplacement (Figs. 1 A and B). The volcanic pile is estimated to comprise five or six individual flows at this locality. The basalts are locally hornfelsed, altered (Fig. 1C) and are intruded by syenite dykes (Fig. 1D). They likely predate the main phase of syenitic magmatism that produced the central part of the Coldwell intrusive complex. Figure 1. Photos showing characteristic features of the Coubran Lake Basalts. Major element chemistry for all groups displays very little variation, with SiO2 values ranging from 49.33 to 51.98 wt%, TiO2 values ranging from 0.78 to 0.99 wt%, and MgO ranging from 4.77 to 7.34 wt%. However, three groups (types A, B, and C) can be distinguished based on their incompatible element abundances. Basalt type A lies towards the base of the unit, forming the lowermost 10% of the exposed sequence and displays strong LREE enrichment (La/Smn = 7.44 to 9.32) with HREE ratios comparable to that of types B and C (Gd/Ybn = 2.48 to 2.52). Basalt type B dominates the succession and displays light rare-earth element (LREE) enrichment (La/ - 24 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Smn = 2.45 to 4.77) and heavy rare-earth element (HREE) fractionation (Gd/Ybn = 1.96 to 2.39). Basalt type C comprises only three samples sporadically located through the sequence. It is characterised by LREE enrichment (La/Smn = 1.99 to 2.94) and HREE fractionation (Gd/Ybn = 1.97 to 2.10). It is distinguishable from basalt type A and B based on the absence of a negative niobium anomaly and lower abundances of large-ion lithophile elements (LILE). These samples appear to mark the tops of geochemically distinct flows. Midcontinent Rift-related volcanic rocks have been classified and correlated as five distinct groups (basalt types I through V) based on their major element, trace element and Nd isotopic analyses (Nicholson et al., 1997). Basalt type II represents a reversely polarized group of volcanic rocks deposited during an early phase of magmatism (>1105 Ma). They are characterized by LREE enrichment, HREE fractionation, as well as a negative niobium anomaly (Fig. 2). Primitive mantle-normalized diagrams show the Coubran Lake Basalts to be similar to basalt type II, which includes the upper Siemens Creek Volcanics, the central suite of the Osler Group and the recently recognized Devon Volcanics (Cundari, 2010). Furthermore, the Coubran Lake Basalts may be genetically related to the Two Duck Lake Gabbro of the Coldwell Complex based on preliminary evaluation of trace element abundances. Further correlative work, including radiogenic isotope analysis and paleomagnetism, is ongoing in order to place the Coubran Lake Basalts in context with the Coldwell Complex and other Midcontinent Riftrelated volcanic units. Figure 2. Primitive mantle normalized plots show distinctions between the four Coubran Lake Basalt types. Basalt type II data from Nicholson et al. (1997). Normalizing values from Sun and McDonough (1989). References Cundari, R., 2010. Geology and Geochemistry of the Devon Volcanics. South of Thunder Bay, Ontario. Unpublished HBSc thesis, Lakehead University. 68p. Nicholson, S.W., Shirey, S., Schulz, K., and Green, J., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Sciences 34: 504-520. Sun, S.S., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins. Geological Society, Special Publication No. 42, 313345. - 25 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The source of the Elliot Lake uranium ores: The neodymium isotope story EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada, mike.easton@ontario.ca Uranium ore in the Elliot Lake camp is hosted in quartz pebble conglomerate beds within the lower part of the Matinenda Fm near the base of the Paleoproterozoic Huronian Supergroup. The Matinenda Fm is composed mainly of trough and planar cross-stratified, greenish to buff, coarse-grained sandstones deposited in a braded fluvial system (Fralick, 2003). Many workers (see Roscoe, 1969 for list) have suggested a paleoplacer origin for the mineralized conglomerates, but whether the source region was local (cf. Roscoe, 1969) or distal (cf. McDowell, 1969) has remained problematic. Based on a detrital zircon provenance study through the Matinenda Fm, Easton and Heaman (2011) suggested that the Matinenda Fm was locally sourced, especially the basal, mineralized, Ryan Member. This study reports neodymium isotopic data obtained from the Elliot Lake area, including the detrital zircon samples used by Easton and Heaman (2011). This data provides additional insights into the source rocks of the Matinenda Fm. Neodymium data are presented in Figure 1 and were obtained from the Isotope Geochemistry and Geochronology Research Centre at Carleton University. Archean felsic volcanic and granodiorite samples from the Whiskey Lake greenstone belt (samples -0108, -0180, -0202), all of which have been dated by U-Pb TIMS on zircon, have positive eNdT values close to the depleted mantle evolution curve. Paleoproterozoic rocks of the Thessalon Fm (sample -0042), and the East Bull Lake intrusive suite (sample -0335 and data from Prevec (1993)) have ΕNdT values ranging from 2.58 to -2.28, suggesting derivation from a primary magma originating from a depleted mantle source, which locally was affected by minor amounts of crustal contamination.   Figure 1. Epsilon neodymium versus age plot for Archean and Paleoproterozoic samples from the Elliot Lake area. Archean rocks indicated by circles, Paleoproterozoic mafic rocks by filled squares, Matinenda Fm samples by open squares. Numbers refer to sample numbers mentioned in the text. - 26 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 In contrast, the Matinenda Fm. sandstone samples all have negative eNdT, ranging from -0.52 to -9.21. Samples with the highest negative eNdT (-0129, -0344) are also enriched in Th, most likely due to the presence of monazite. The magnitude of the negative eNdT values in the Matinenda Fm indicates a negligible contribution from the volcanic and intrusive rocks of the Whiskey Lake greenstone belt, all of which have positive eNdT. The Matinenda Fm data can be explained if the sandstones contain a significant component derived from a suite of radiogenic granites located 30 to 40 km north and northwest of Elliot Lake which have eNdT of -6.19 (sample -1519). These radiogenic granites typically contain 8 to 33 ppm U and 30 to 50 ppm Th (Easton, 2010), thus they are also a potential source of uranium. In contrast, uranium contents of other Archean felsic intrusions in the Elliot Lake area are 1 to 4 ppm (Easton, 2010). It may be no coincidence that the most negative of the Matinenda Fm samples (-0344) was collected only a few metres below the mineralized Main Conglomerate Bed. In conclusion, the neodymium data, as well as the detrital zircon and geochemical data reported by Easton and Heaman (2011), are all consistent with a local source region that included radiogenic granites found north and northwest of Elliot Lake. The absence of similar radiogenic granites north of the Huronian Supergroup between Elliot Lake and Sault Ste. Marie may explain why the Matinenda Fm west of Elliot Lake contains no significant uranium occurrences. References Easton, R.M. 2010. Compilation mapping, Pecors-Whiskey Lake area, Superior and Southern provinces; in Summary of Field Work and Other Activities, 2010, Ontario Geological Survey, Open File Report 6260, p.8-1 to 8-12. Easton, R.M. and Heaman, L.M. 2011. Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization; 57th Institute on Lake Superior Geology, Proceedings, v.57, pt.1, p.31-32. Fralick, P.W. 2003. Geochemistry of clastic sedimentary rocks: ratio techniques; in Geochemistry of sediments and sedimentary rocks: Evolutionary considerations to mineral deposit-forming environments, D.R. Lentz, editor; Geological Association of Canada, GeoText 4, p.8-103. McDowell, J.P. 1957. The sedimentary petrology of the Mississagi quartzite in the Blind River area; Ontario Department of Mines, Geological Circular 6, 31p. Prevec, S.A. 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanism near Sudbury, Ontario; unpublished PhD thesis, University of Alberta, Edmonton, Alberta, 223p. Roscoe, S.M. 1969. Huronian rocks and uraniferous conglomerates in the Canadian Shield; Geological Survey of Canada, Paper 68-40, 205p. - 27 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Erosion of Archean lithosphere by subduction and rifting: a tomographic image of central North America FREDERIKSEN, Andrew, DENISET, Ian, Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, BOLLMANN, Trevor, VAN DER LEE, Suzan, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208-2150 and DARBYSHIRE, Fiona, Université du Québec à Montréal, Centre GEOTOP, CP 8888, succ. Centre-Ville, Montréal, Québec, H3C 3P8 The core of the North American continent was assembled by Precambrian accretionary processes, which are generally believed to also be responsible for the formation of the underlying continental lithosphere. Signatures of lithospheric accretion are preserved in the lithosphere in a form detectable by observations of seismic velocity and fabric. These signatures, however, are subject to later modification by later tectonic processes, such as orogeny, rifting, and mantle plume impingement. We examine the lithosphere of the region surrounding the southwest portion of the Archean Superior Province using a large teleseismic data set from Canadian and American sources. The western Superior is associated with high seismic P velocity and a strong, consistent eastwest pattern of shear-wave splitting. The western edge of the Western Superior velocity and splitting anomalies lies ca. 200 km east of the crustal contact with the Proterozoic Trans-Hudson Orogen, perhaps indicating that Trans-Hudson orogenic processes eroded the Superior lithosphere. To the southwest, the Superior lithosphere is disrupted by a low-velocity lithospheric channel ca. 250 km wide striking northwest through Minnesota and the Dakotas, which is also associated with a sharp weakening of anisotropy. The origin of this feature is enigmatic, as it does not correspond to any known crustal feature, though it appears to be truncated by the Mid-Continent Rift and so may represent a failed rift arm at the lithospheric level. The rift itself is only partially resolved, but shows indications of low velocities; resolution of the Mid-Continent Rift will improve over the next few years due to a combination of the eastward migration of the Earthscope Transportable Array and new data from the temporary Superior Province Rifting Earthscope Experiment (SPREE). - 28 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The black line faults of the Red Lake gold mines, Ontario GASPAR, Brandon and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Black line faults are a common feature in the Red Lake gold mines. Characterized by their distinctive black color, they are planar structures that cross-cut and displace various lithologies, dykes, and gold-bearing veins. They may have anomalous traces of gold but are usually barren. Although black line faults along the Red Lake mine trend vary in size, geometry and amount of displacement, they all share the same general composition of mostly tourmaline, chlorite, and interstitial quartz. They have previously been interpreted as minor faults associated with brittle late-stage deformation; however thin-section analysis indicates that some are overprinted by ductile deformation. Rather than representing a discrete late-stage event, they are better characterized as compressive shear fractures that formed during progressive high-temperature deformation that was dominantly ductile. Black line faults are significant in that they represent a time of brittle-ductile deformation and the presence of complex hydrothermal fluids, typically related to orogenic gold mineralization. - 29 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 A microstructural study of the Otjikoto Deposit, Namibia: Inferences from gold mineralization in the Superior Province GASPAROTTO, Marc and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Shear-zone-hosted gold deposits in the Superior province of northwestern Ontario are characterized by heterogeneous deformation in high-temperature brittle-ductile shear zones. Since microstructure is critical to identifying this type of deformation, microstructural analysis may provide a vector to gold mineralization in orogenic gold deposits where scarce outcrop inhibits structural mapping. The Otjikoto project in Namibia is an example of such an orogenic gold deposit. It is situated in the province of Otjozondjupa, approximately 300 km north of the capital city of Windhoek. A 20-metre-thick layer of calcrete covers the exploration area, making the mapping of structural features difficult. Research for this Lakehead University Honours thesis was carried out through the microstructural analysis of thin sections (obtained from Auryx Gold) that were collected from drill core. The microstructures produced by deformation exhibited in the minerals of this deposit provide information about the relative timing of metamorphism, deformation, quartz veins and gold mineralization of the Otjikoto project. The metapelitic Okonguarri Formation, host to the Otjikoto gold mineralization, contains evidence for peak metamorphism at the temperatures and pressures of the sillimanite zone of the amphibolite facies. Grain size reduction of feldspar indicates that mylonitization also occurred at amphibolite facies temperatures. The formation also exhibits brittle deformation, specifically in the competent amphibole and garnet porphyroblasts, which occurs during synchronous metamorphism and ductile deformation. The mineralized quartz veins are synorogenic as evidenced by both brittle and ductile deformation under at least greenschist facies metamorphism. In particular, boudins of coarse-grained quartz (interpreted to be boundinaged quartz veins) indicate the mutual overprinting of brittle and ductile deformation which is characteristic of a shear-zone-hosted gold deposit. Based on evidence for the synchronous metamorphism, brittle deformation and ductile deformation, and the synorogenic mineralized quartz veins, a shear-zone-hosted gold deposit model should be considered for the Otjikoto project. - 30 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Stratigraphy and tectonic setting of Neoarchean arc volcanic rocks in the Bird River Belt, Manitoba, Canada. GILBERT, H.P., Manitoba Geological Survey (360-1395 Ellice Ave., Winnipeg, Manitoba R3G 3P2 The Neoarchean Bird River Belt (BRB) in the western Superior Province is currently the focus of geological and geochemical investigations that have led to a revised interpretation of its tectonic setting and geological history. A collaborative mapping project initiated in 2005 by the Manitoba Geological Survey is currently focused on detailed studies of economically important mafic-ultramafic intrusions and rare-element–bearing pegmatites. The BRB is part of a continental arc, juxtaposed against the north margin of the Winnipeg River Subprovince, which extends east for over 150 km, as far as Separation Lake in Ontario. The stratigraphy of this ca. 2.75 to 2.70 Ga belt documents a history of arc magmatism and associated sedimentation, as well as subsequent deformation that resulted from the convergence and ultimate collision of flanking older cratonic terranes to the north and south: North Caribou Terrane (NCT) and Winnipeg River Subprovince (WRS), respectively. BRB arc activity in the Neoarchean occurred during an interval of widespread magmatism in the central part of the NCT and along its north and south margins (Percival et al., 2006). In many cases the Neoarchean arc assemblages are contiguous with older, Mesoarchean assemblages, but the latter appear to be absent in the BRB. The oldest known rocks in the BRB are mid-ocean-ridge basalt (MORB)-like rocks flanking the arc volcanics along both north and south margins of the belt, and thought to represent back-arc and ocean-floor settings respectively. The northern MORB type is notable for the presence of mafic-ultramafic intrusions (2745 Ma Bird River Sill, Wang, 1993; Mayville intrusive complex), thought to be genetically related to their host rocks, and associated with Cu-Ni-Cr-PGE ore deposits (Mealin, 2006). North (2735-2731 Ma) and south (2725 Ma) panels of arc-type rocks in the BRB (Gilbert et al., 2008) are stratigraphically distinctive but they are probably part of an evolving arc volcanic succession that was associated with subduction of oceanic crust at a continental margin to the north. Sm-Nd isotopic data and crustal residence ages of the arc volcanic rocks indicate the source magma was influenced by older ca. 3.0 Ga continental crust. Crustal assimilation is also indicated by the fact that 70% of the mapped area of BRB arc volcanics consists of felsic to intermediate rock types. Remnants of the cratonic block north of the BRB may be represented by Mesoarchean (2.85 Ga) granitoid rocks within the multiphase Maskwa Lake Batholith, which also contains 2725 Ma granitoid intrusion(s) of synvolcanic age. Geochemical discrimination of incompatible elements and element ratios in the arc type rocks shows a systematic trend from least evolved compositions in the north panel, towards progressively more evolved compositions in the lower, main and upper parts of the south panel, consistent with a change from arc to arcrift (extensional) tectonic settings (Gilbert et al., 2008). The upper part of the north panel is a lithologically diverse, mainly volcaniclastic to epiclastic succession (Diverse arc assemblage, DAA), possibly representing an extensional basin. A youngest detrital zircon date of 2706 ±23 Ma for a turbidite deposit provides a rough estimate for the maximum depositional age of the DAA. Turbidite deposits (Booster Lake Formation, BLF) in the central part of the BRB are thought to represent an elongate rift basin that extends east between the north and south panels of arc rocks. The turbidites are interpreted as analogous with 2713-2704 Ma orogenic sedimentary rocks and related paragneiss in the English River Subprovince, commonly interpreted as a fore-arc basin that extends for 800 km along the south margin of the Uchi domain (Hrabi and Cruden, 2006). A 2712 ±17 Ma youngest detrital zircon date provides an approximate depositional age for the BLF. Fluvial-alluvial deposits of slightly younger age (2697 ±18 Ma) in the eastern part of the BRB structurally overlie the margin of the arc volcanic rocks. The coarse siliciclastic rocks are interpreted as synorogenic, more proximal equivalents of the BLF turbidites. Contacts between the various tectonic components (arc, back-arc and orogenic sedimentary rocks) appear to be invariably faulted. - 31 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Synvolcanic granitoid and gabbroic plutons (2723 Ma) intrude the south panel arc rocks, but similar plutons are absent in the north panel that contains, instead, relatively younger sanukitoid rocks. The light-rare-earthelement–enriched and strongly fractionated sanukitoid suite is represented by (1) clasts in DAA conglomerate, (2) dikes and sills in north panel arc volcanic rocks, and (3) minor intrusions in BLF turbidites. The sanukitoid magmatism thus spanned syn- and post-volcanic intervals. Terminal collision of the NCT and WRS resulted in regional folding, faulting and locally tectonic intercalation between BLF turbidites and arc volcanic rocks. Posttectonic (ca. 2660-2646 Ma) granitoid plutons of probable anatectic origin, and rare-element-bearing pegmatites (e.g. 2640 ±7 Ma Tanco pegmatite; Baadsgaard and Černý, 1993) are among the last manifestations of the sequence of events - initiated ca. 3.0 Ga - that were associated with the convergence and collision of Mesoarchean cratonic blocks in the western Superior Province. References Baadsgaard, H. and Černý, P. 1993: Geochronological studies in the Winnipeg River pegmatite populations, southeastern Manitoba; Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Program with Abstracts, v. 18, p. A5. Gilbert, H.P., Davis, D.W., Duguet, M., Kremer, P.D., Mealin, C.A. and MacDonald, J. 2008: Geology of the Bird River Belt, southeastern Manitoba (parts of NTS 52L5, 6); Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, Geoscientific Map MAP2008-1, scale 1:50 000 (plus notes and appendix). Hrabi, R.B. and Cruden, A.R 2006: Structure of the Archean English River subprovince: implications for the tectonic evolution of the western Superior Province, Canada; Canadian Journal of Earth Sciences, v. 43, p. 947-966. Mealin, C.A. 2008: Geology, geochemistry and Cr-Ni-Cu-PGE mineralization of the Bird River Sill: evidence for a multiple intrusion model; M.Sc. thesis, University of Waterloo, 155 p. + 1 folded colour map. Percival, J.A., Sanborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H. and White, D.J. 2006: Tectonic evolution of the western Superior Province from NATMAP and LITHOPROBE studies; Canadian Journal of Earth Sciences, v. 43, p. 1085–1117. Wang, X. 1993: U-Pb zircon geochronology study of the Bird River greenstone belt, southeastern Manitoba; M.Sc. thesis, University of Windsor, Windsor, Ontario, 96 p. - 32 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geology of the Layered Series, Coldwell Alkaline Complex, Ontario GOOD, David, McLEAN, Katrina, EPSTEIN, Rachel, Stillwater Canada Inc, 11 Sydenham St., Dundas Ontario, Canada, L9H 2T5, LINNEN, Robert, Department of Earth Sciences, Western University, London, Ontario, Canada, N6A 3K7 and SAMSON, Iain, Department of Earth and Environmental Sciences, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada, N9B 3P4 The Eastern Gabbro forms the outer margin of the Coldwell Alkaline Complex, a large lopolith emplaced during the Mid Continent Rift Event at 1108 Ma (Heaman and Machado (1992) The Eastern Gabbro is up to 1500 meters thick and, as described by Shaw (1997), formed by multiple intrusions of evolved basaltic magma with a subalkaline parentage into a partial ring dike structure that cut the Archean country rock.     Figure 2. Ternary diagram for CIPW Norm abundances demonstrates the linear modal trends within the layered series. FH-08-05 Fly Hill samples contain a higher proportion of pyroxene and less plagioclase than the typical oxide augite melatroctolites. Figure 1. Eastern margin of the Coldwell Complex showing the Eastern Gabbro, Marathon Deposit, and Syenitic Rocks. Geochemical sampling locations are indicated by red symbols. The Eastern Gabbro is shown to be a composite intrusion made up of at least three distinctive components or gabbroic series including the early Fine Grained Series, the Layered Series and the Marathon Series. This study presents the results of detailed petrography of the Layered Series at five sites covering 24 of the 31 km strike length of the Eastern Gabbro (Fig. 1). The Layered Series makes up the majority of the Eastern Gabbro and is compositionally, geochemically - 33 - �Proceedings of the 58th ILSG Annual Meeting - Part 1   Figure 3. (a) Modal layering in the Layered Series shows cross bedding, gradational variation of plagioclase and clinopyroxene and strong contrast between cyclical layers. (b) Polished thin section of a plagioclase free interval within the oxide augite melatroctolite unit. and texturally similar for all sample locations. The Layered Series is dominated by massive to modally layered olivine gabbro (Fig. 3a), but also includes relatively thick units of weakly layered oxide augite melatroctolite (Fig. 3b). Contacts between these units are typically gradational. The oxide augite melatroctolite unit at the Fly Hill location contains iron rich clinopyroxene. CIPW Normative abundance of minerals are plotted on ternary diagrams (Fig. 2) to highlight the sequence of layered rocks that trend from gabbroic anorthosite to olivine melagabbro. The Mg number calculated for olivine gabbro displays a narrow range of values from 35 to 65 and is similar to the results of Shaw (1997). Samples of oxide augite melatroctolite at Fly Hill located near the top of the Layered Series represent the most fractionated rocks with an Mg number of 10 and samples from the Three Finger Lake area represent the most primitive gabbro with Mg numbers up to 65. References Heaman, L.M., Machado, N., 1992. Timing and origin of Midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contrib. Mineral. Petrol., 110: 289-303. Shaw C.S.J., 1997, The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke, Lithos, v. 40, p.243-259. - 34 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Quartz Fabric Analysis of the Kawishiwi Shear Zone, NE Minnesota GOSCINAK, Christopher and HANSEN, Vicki, Department of Geological Sciences, University of Minnesota Duluth, 1049 University Drive, Duluth MN 55812 Structural fabrics within the Vermilion district, NE Minnesota, including metamorphic foliation and mineral lineation are well established and can be summarized as northeast striking, near vertical foliation containing oblique to vertical lineations (Sims, 1972, 1976; Hudleston 1976; Hudleston et al., 1988; Shultz-Ela & Hudleston, 1991; Bauer & Bidwell, 1990; Goodman, 2008; Karberg, 2009; Johnson, 2009; Erikson, 2010). The oblique to vertical lineations are present throughout the Vermilion district, yet areas of subhorizontal lineation orientations are also present locally. Two different interpretations emerge from previous studies: 1) dextral transpression associated with terrane accretion (e.g., Hudleston, 1976; Hudleston et al., 1988) and 2) lineation-parallel shearing consisting of regional dip-slip shearing and later, more focused, strike-slip shearing (e.g., Goodman, 2008; Karberg, 2009; Johnson, 2009; Erikson, 2010). The Kawishwi Shear Zone (KSZ), one of several Vermilion district shear zones, shows structural relationship particularly well. Foliation consistently strikes east-northeast and dips vertically. Lineations are broadly down-dip with discrete areas of subhorizontal. Goodman (2008) performed a structural and kinematic analysis of the KSZ and interpreted lineation-parallel shearing with dip-slip shearing followed by strike-slip shearing. However, previous studies do not specifically constrain flow within L-S tectonites relative to lineation. This study aims to characterize the kinematic pattern of flow through use of quartz fabric analysis of c- and a-axis petrofabrics. The data reveal the dominant slip planes and direction of flow during deformation, and also provide deformation temperature and strain geometry information. Oriented samples from the KSZ were analyzed. Sample KS7J contains a vertical lineation and quartz petrofabric data indicate flow nearly parallel to the lineation, thus dominantly dip-slip displacement. Quartz microstructures are consistent with greenschist deformation. Sample KS6UI, collected from a localized zone with strike-parallel lineation, displays Quartz petrofabric data indicative of slip along the prism <a> plane with flow parallel to lineation. However, quartz microstructures are inconsistent with this higher temperature slip plane; suggesting another mechanism to activate slip. One such mechanism, orthogonal reactivation, accounts for high temperature slip at lower temperatures due to shearing in two orthogonal directions (Oliver, 1996). In this model, initial shearing produces a crystallographic preferred orientation within quartz. Subsequent orthogonal shearing exploits weaknesses within the aligned quartz lattice and triggers slip along high temperature slip planes. Quartz petrofabrics, in particular a-axis data, support this model. References Bauer, R. L and Bidwell, M. E., 1990. Contrasts in the response to dextral transpression across the Quetico-Wawa subprovince boundary in northeastern Minnesota. Canadian Journal of Earth Sciences, 27, 1521-1535. Erickson, E., 2010. Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean. Canadian Journal of Earth Sciences, 47, 1463-1479. Goodman, S., 2008. Structural and Kinematic Analysis of the Kawishiwi Shear Zone, Superior Province. M.S. Thesis, University of Minnesota Duluth, MN. Hudleston, P. J., 1976. Early deformational history of Archean rocks in the Vermilion district, northeastern Minnesota. Canadian Journal of Earth Science vol. 13, 579-592 Hudleston, P.J., Schultz-Ela, D., Southwick, D. L., 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences, vol 25, 1060-1068. Johnson, Thomas K., 2009, Structural, Kinematics, and Hydrothermal Fluid Investigation of the Gold-Bearing Murray Shear Zone, Northeastern Minnesota, M.S. Thesis, University of Minnesota Duluth, MN. Karberg, S M., 2009. Structural and Kinematic Analysis of the Mud Creek Shear Zone, Northeastern Minnesota. M.S. Thesis, University of Minnesota Duluth, MN. - 35 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Oliver, Douglas, 1996. Structural, Kinematic and Thermochronometric Studies of the Teslin Suture Zone, South-Central Yukon Territory. PhD dissertation, Southern Methodist University, TX. Schultz-Ela, D.D., Hudelston, P.J., 1991. Strain in an Archean greenstone belt of Minnesota. Tectonophysics, 190, 223-268. Sims, P. K., 1972. Vermilion district and adjacent areas. In Geology of Minnesota: a centennial volume, P. K. Sims & G. B. Morey editors. Minnesota Geological Survey 49-62 Sims, P.K., 1976. Early Precambrian tectonic-igneous evolution in the Vermillion district, northeastern Minnesota. Geol. Soc. Am. Bull. 87, 379-389. - 36 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Structural control on the emplacement of the TBN-Igneous Complex. HEGGIE, Geoff, MACTAVISH, Allan, JOHNSON, Justin, WESTON, Ryan, and MA, Leon, Magma Metals (Canada) Ltd. 1004 Alloy Drive, Thunder Bay, Ontario, 1gheggie@magmametals.ca The Lake Superior area was the focus of extensional and compressional deformation during the development of the Mesoproterozoic Midcontinent Rift (MCR) rift sequence. Age determination carried out around the MCR identify a rift evolution sequence spanning ~60 m.y. and subdivided into 4 stages (Heaman et al., 2007). It is within Stage 2 (1115-1105 Ma) that a series of mafic-ultramafic intrusions prospective for Ni-Cu-PGE were emplaced proximal to the main rift axes (e.g. Seagull, Kitto, Disraeli, Hele, Thunder Bay North, Tamarack and Eagle Intrusions). Pre-existing Archean structures are postulated to be a strong influence on the early rift architecture and are implicated to be a primary control on the distribution of these Stage 2 intrusions (Hollings et al., 2007; 2010). The Thunder Bay North (TBN)-igneous complex was emplaced during Stage 2 of the rift event. The complex comprises a series of contemporaneous layered intrusions, dykes, sills and chonoliths (Fig. 1). Magma Metals Ltd. has been actively exploring the TBN-complex since 2006 with the discovery of Pt-Pd-Cu-Ni mineralization hosted within the complex. To date the TBN-Complex hosts an indicated resource of 9.8 Mt at 2.34 g/t Pt-Eq for 741 000 contained Pt-Eq ounces and an inferred resource of 0.53 Mt at 2.87 g/t for 49 000 contained Pt-Eq ounces (MMW, Feb. 23, 2012). During the delineation of the mineral resource a number of lines of evidence were observed within the TBN-complex to support the hypothesis that pre-existing Archean structures controlled the spatial emplacement of intrusions, the morphology of igneous bodies, and consequently the distribution of mineralization. Structural controls on the TBN-complex can be divided into two groups: 1) macro-controls which are evident in the morphology of the intrusion, but not apparent in drill core, and 2) micro-controls which are readily seen Figure 1. Plan map overview of the TBN-Complex as defined by diamond drilling and airborne magnetics. Outline of the igneous complex shown in orange, mineral resource in red. - 37 -   �Proceedings of the 58th ILSG Annual Meeting - Part 1 in drill core. Macro-structural controls (Fig. 2) include the east-west trending Quetico fault which defines the boundary between Archean metasedimentary rocks to the south and anatectic granites to the north. This structure provides a zone of weakness for the intrusive complex to develop along. Exfoliation and horizontal joints in the Archean rocks provide vertical discontinuities for the lateral development of sills. Regional conjugate joint sets and faults in the texturally homogneous anatectic granite provide a network of rheological discontinuities that were exploited during the development of chonoliths. Of the micro-structural controls, fault zones and zones of decreased rock quality are identifiable in the host rocks. The development of invasive magma fingering into these incompetent structural breccias are strong evidence for the influence of pre-existing structure controlling the emplacement, progressive development and final morphology of the intrusions. The progressive development and morphology of the complex influence magma flow dynamics, which in turn effect particle settling and the distribution of fluid load, specifically the distribution of immiscible sulfides and crystal accumulation.   Figure 2. Idealized schematic cross-section showing differing structures controlling the morphology of intrusions. References Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magnetism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Hollings, P., Smyk, M., Heaman, L.M., Halls, H., 2010. The geochemistry, geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada: Precambrian Research, v. 183, p. 553-571. Hollings, P., Fralick, P., Cousens, B., 2007. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario: Canadian Journal of Earth Sciences, v. 44, p. 389-412. Magma Metals Ltd., Feb. 23, 2012. News release, Magma Metals increases mineral resources at TBN to 790 00 PlatinumEquivalent ounces. SEDAR, 5p. - 38 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Preliminary Bedrock Geology Map of the Eastern part of Lake Vermilion State Park, St. Louis County, NE Minnesota HEIM, Nick, SCOTT, Heather, KILDUFF, Rob, RAHTZ, Christine, VIAL, Andrew, YOUNG, Spencer, MAHR, Chris, and HUDAK, George, Precambrian Research Center, Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN, 55811 Each year, students from the Precambrian Research Center (PRC) geology field camp complete “capstone” projects that encompass approximately one week of detailed field mapping followed by one week of mapmaking and map publishing. During the fifth and sixth weeks of the 2011 field camp, seven PRC field camp students, under the direction of PRC Faculty member George Hudak, mapped Neoarchean rocks in the eastern half of Minnesota’s newest state park, Lake Vermilion State Park (Heim et al., 2011). This capstone mapping project sought to: 1) identify the lithologies and stratigraphy of the Neoarchean supracrustal strata in this area; 2) define and characterize the nature of the contact between various units of the Neoarchean supracrustal strata and intrusive rocks; 3) obtain a better understanding of geological structures and their orientations within the area; and 4) to complete field mapping which will result in a comprehensive bedrock geological map for future use by employees of, and visitors to, Lake Vermilion State Park. Prior to mapping, detailed 1:5000 scale laminated field mapping sheets were produced. One side of each field mapping sheet consisted of a georeferenced air photo, and the other side of the mapping sheet consisted of a corresponding georeferenced topographic map. Mapping was completed by means of lakeshore mapping from canoes, as well as numerous traverses through the bush. Following each day of field mapping, students and faculty transferred their field data to a master map, enabling the generalized geology of the region to be established by the middle of the fifth week of field camp. During the sixth week of field camp, students produced a digital version of the field map utilizing a variety of software (ArcView, AutoCad, Surfer, Adobe Illustrator). In the area mapped, the Neoarchean supracrustal rocks consist of a NE/SW striking, NW facing homoclinal sequence comprising several distinctive volcanic and sedimentary sequences, synvolcanic and post-volcanic intrusive rocks, and sheared rocks adjacent to post-volcanic deformation zones. Up-section, these sequences are: (1) the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation, which is composed of; a) light bluish green to green, aphyric to sparsely plagioclase-phyric, highly amygdaloidal sheet flow - and pillowed facies andesite and basalt; b) medium-bedded to massive andesite to basalt tuffs and lapilli tuffs; c) grayish-green, aphyric to quartz-phyric coherent rhyolite lava flows and associated flow breccias; and d) green to tan, massive polymict breccias comprising up to 20cm diameter clasts of pumice, scoria, and/or amygdaloidal basalt/andesite; (2) the Central Basalt Sequence of the Lower Member of the Ely Greenstone Formation, which is composed of medium green to dark green, aphyric to sparsely plagioclase-phyric, sparsely amygdaloidal sheet flow - and pillowed facies basalt lava flows which are locally strongly quartz-epidote altered; (3) the Soudan Member of the Ely Greenstone Formation, an interbedded sequence comprising: a) laminated to medium-bedded, planar-bedded, locally chaotically folded, dark gray to red-brown, Algoma-type oxide facies iron formation; b) medium- to dark green, aphyric to locally sparsely plagioclase-phyric, massive to moderately amygdaloidal andesite to basalt sheet flow facies lava flows; c) gray to gray green, massive quartz ± plagioclase-phyric rhyodacite tuff; and d) light gray, massive polymict lapilli tuffs comprising up to 5% quartzand plagioclase-phyric coherent rhyodacite lapilli and 10-15% dark green chlorite-rich lapilli interpreted to be chloritized pumice; (4) the Gafvert Lake Member of the Lake Vermilion Formation, which is composed of; a) a basal unit - 39 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 comprising light gray, massive, polymict volcaniclastic conglomerates and sandstones; b) light gray, massive, quartz- and plagioclase-phyric polymict dacite to rhyodacite tuff and lapilli tuff containing up to 20% quartz- and plagioclase phyric coherent dacite and rhyodacite lapilli and up to 7% locally quartz- and plagioclase-phyric pumice lapilli; c) light gray, massive, quartz- and plagioclase-phyric tuff and lapilli tuff containing up to 15% subangular quartz- and plagioclase-phyric pumice lapilli; d) gray, thinly-bedded to massive tuff breccias with poorly developed graded bedding; e) red to dark gray, laminated to medium bedded oxide facies iron formation; and f) light gray to black laminated to very thinly bedded chert. Numerous sills and dikes intrude the supracrustal assemblage in the area. Intrusive bodies which appear to be synvolcanic include a) grayish green to black, medium-grained, locally ophitic gabbro; b) gray to gray-green, medium-grained diorite; and c) light gray, massive, quartz- and plagioclase-phyric diorite and quartz-diorite characterized by euhedral blue-gray quartz phenocrysts up to 1cm in diameter. Post-volcanic intrusive rocks identified include: a) pink to green-gray medium-grained hornblende diorite; b) light gray to pale green gray, quartz- and plagioclase-phyric diorite to quartz diorite sills and dikes; and c) light gray, fine- to medium-grained feldspar-phyric diorite. Locally, sericite-dominant schists and phyllites, chlorite-dominant schists and phyllites, and chlorite-rich sheared mafic volcanic rocks occur along discrete ductile to brittle-ductile structures. Our analysis leads us to interpret the following sequence of events for the development of the bedrock geology visible today in this part of the Vermilion District: 1) deposition of highly amygdaloidal sheet- and pillowed facies andesite and basalt lava flows, felsic lava flows, primary mafic tuffs and lapilli tuffs, and associated resedimented polymict volcaniclastic rocks in a shallow submarine setting (Hudak et al., 2007); 2) deposition of submarine basalt lava flows, comprising both sheet flows and pillowed facies, in a relatively deep water (>500 meters water depth) setting (Hudak et al., 2007); 3) deposition of Algoma-type iron formations of the Soudan Member of the Ely Greenstone Formation as a result of hydrothermal activity during volcanically quiescent periods and deposition of interbedded mafic and felsic volcanic and volcaniclastic strata associated with intermittent volcanism; 4) the onset of voluminous explosive dacitic to rhyodacitic felsic volcanism, alternating with periods of volcanic quiescence characterized by continued chemical sedimentation and localized clastic sedimentation; 5) synvolcanic to post-volcanic intrusion of mafic to felsic sills and dikes; and 4) structural deformation, probably associated with regional D2 deformation (Peterson and Patelke, 2003) that formed the sheared mafic rocks and chlorite schists that occur in the northeastern part of the field area. References Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C, and Hudak, G., 2011, Preliminary Bedrock Geology Map of the Eastern Part of Lake Vermilion State Park, St. Louis County, NE Minnesota: Precambrian Research Center Map Series, PRC/MAP-2011/01, 1:5000 scale. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota. Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Programs and Abstracts, p. 42-43 Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological Site Investigation for the Soudan Mine, NE Minnesota: Natural Resources Research Institute, Technical Report NRRI/ TR-2003/29, 88 p. - 40 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Chert Stromatolites in the Basal Gunflint Formation, Kakabeka Falls: Primary Precipitation or Silicification? HORNER, Simon and FRALICK, Philip, Department of Geology, Lakehead University, Thunder Bay, On, Canada, P7B 5E1, philip.fralick@lakeheadu.ca Modern-day stromatolites primarily form in carbonate-dominated environments through binding particulate matter to the sticky extracellular polysaccharides secreted by the cyanobacteria. Evidence of the operation of this process in constructing stromatolites extends back into the Phanerozoic. However, Precambrian stromatolites do not contain abundant silt- to sand-sized particles characteristic of this process. Instead they are primarily composed of material that has precipitated from seawater. This is commonly a carbonate phase, though chert stromatolites are common in some Proterozoic and Archean sequences. In recent years controversy has developed in the literature as to whether the chert stromatolites in the Gunflint Formation and similar rock units represent silicification of carbonate or are primary silica precipitates (see: Knoll and Simonson, 1981; Simonson, 1985; Knoll, 1985; Siever, 1992; Maliva et al., 2005). Some of this work utilized outcroppings of the Gunflint Formation that are highly diagenically altered, and thus ambiguous concerning primary mineralogy. Roadwork on Highway 588 near Kakabeka Falls exposed an excellent example of chert microbialites and stromatolites entombed in ankeritic grainstone with a sharp boundary between the two, providing an opportunity to add to the debate on the possible primary origin of the silica in chert stromatolites. The 1.878±1 Ga Gunflint Formation is a chemical sediment dominated shelf succession deposited on Archean basement. It records a number of transgressive-regressive cycles. Hofmann (1969) and Franklin et al. (1982) described stromatolites occurring at two different stratigraphic levels; at the base of the Gunflint growing on Archean units and the basal conglomerate and secondly, on a hard-ground in the middle of the Formation developed during regression. The basal biogenic sediment at Kakabeka Falls directly overlies Archean granodiorite and consists of a lower one meter thick, continuous chert microbialite. The millimeter-scale, crinkly layering is sub-horizontal, with organic carbon only giving the unit a dark colouration in limited places. A large stromatolitic head develops off the upper microbialite layers. It is the size of a beach ball, with a slightly narrower base. There appears to be a sharp contact between the microbialite-stromatolite and carbonate grainstone, which overlies the microbialite and abuts against the stromatolite. The layering in the stromatolite does not build on top of the lower grainstone laminae, indicating that the stromatolite was fully grown before the grainstone entered the environment. Samples of all lithologies were investigated with a petrographic microscope, a field emission scanning electron microscope with an energy dispersive spectrometer, inductively coupled plasma atomic emission spectrometry and mass spectrometry. The diagenetic history is more complex than it appears to the naked eye. The stromatolite contains abundant microquartz, megaquartz, chalcedony, and organic matter. The silica is rather clean and well developed with little to no carbonate inclusions or ghosts. Carbonate present in the stromatolite is seen replacing chalcedony fans. Distinct growth zones can be seen in the outer portions of the carbonate. Rarely, quartz crystals present in the stromatolite show carbonate overprinting, replacing them to varying degrees. Carbonate is visible replacing megaquartz grains and overprinting microquartz as very fine grained specular carbonate. Growth zoning is present in most of the carbonate grains. The carbonate grainstone unit overlying the stromatolite has been highly silicified. The quartz is texturally similar to the quartz in the stromatolite; microquartz and megaquartz are abundant with minor amounts of chalcedony. The megaquartz is seen in the intergranular spaces as space-filling cement. Higher magnification shows extinct carbonate grains that have been silicified with small amounts of carbonate remaining behind. However, further from the contact with the stromatolite the carbonate grainstone unit is silicified to a much lower degree. Some of the carbonate grains have experienced some silicification, - 41 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 but the rock is mainly composed of carbonate with some organic material. Silica in the form of microquartz is forming within the carbonate grains. A closer observation reveals that the quartz developing in the carbonate grains is itself being over-printed by carbonate growth. The carbonate grains have growth zoning in their outer portions. Rare earth element plots for the ankerite and chert are generally similar, though this probably reflects both inheriting the seawater pattern. The most striking difference between the two lithologies is the Ba/Sr ratio. The stromatolitic cherts have one consistent ratio, whereas the carbonates and known replacement cherts from the area have a different consistent ratio. All of the above is consistent with the microbial and stromatolitic cherts being a primary precipitate, but it is not conclusive. If the silica is primary new models will need to be developed to explain how Precambrian cherty stromatolites form. References Franklin, J.M., et al 1982. Proterozoic geology of the northern Lake Superior area. Geological Association of CanadaMineralogical Association of Canada Field Trip Guidebook, Trip 4. Hofman, H.J. 1969. Stromatolites from the Proterozoic Animikie and Sibley Groups, Ontario: Geological Survey of Canada, Paper 68-69. Knoll, A.H., 1985. Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats: Philosophical Transactions of the Royal Society of London, v. B311, p. 111-122. Knoll, A.H., and Simonson, B., 1981. Early Proterozoic microfossils and penecontemperaneous quartz cementation in the Sokoman Iron Formation, Canada: Science, v. 211, p. 478-480. Maliva, R.G., Knoll, A.H., and Simonson B.M., 2005. Secular change in the Precambrian silica cycle: Insights from chert petrography. Geological Society of America, v.117; no.7/8; p. 835-845 Siever, R. 1992. The silica cycle in the Precambrian. Geochimica et Cosmochimica Acta Vol. 56, p. 3265-3272. Simonson, B.M., 1985. Sedimentology of cherts in the Early Proterozoic Wishart Formation, Quebec-New-foundland, Canada: Sedimentology, v. 32, p. 23-40. - 42 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulates - 2012 Update HUDAK, George, MONSON GEERTS, Stephen, ZANKO, Larry, SEVERSON, April, SEVERSON, Allison, Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN, 55811, and BANDLI, Bryan, Department of Geological Sciences, University of Minnesota Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Since 2008, the Natural Resources Research Institute (NRRI) has been conducting a detailed characterization of mineral dust in northeastern Minnesota. The purpose of this research is to evaluate the effects of past and present emissions from taconite mining and processing on air quality throughout the Mesabi Iron Range (MIR) by characterizing airborne particulate matter within taconite operations, in communities surrounding taconite operations on the MIR, in population centers in other regions of northeastern Minnesota, as well as particulate matter deposited in lake sediments (Fig. 1). NRRI’s sampling and characterization work represents the community/environmental component of the Minnesota Taconite Workers Health Study, a broad University of Minnesota (UM) research effort investigating long-standing questions regarding the impact of dust derived from mining and processing of taconite (iron ore). The UMN School of Public Health (SPH), with whom NRRI is collaborating, is responsible for the human health- and exposure-related components of that effort, which include: 1) an occupational exposure assessment; 2) a mortality study; 3) a cancer incidence study; and 4) a respiratory health survey of taconite workers and spouses. Figure 1. Locations of taconite processing plants on the Mesabi Iron Range being sampled during this study (after Oreskovich and Patelke, 2006) Air sampling is performed within taconite operations, MIR communities, and non-MIR communities by NRRI scientists during both winter and summer seasons. Sampling at taconite operations takes place at four locations: 1) secondary crushers; 2) magnetic separators/concentrators, agglomerators/ ball drums, and the kiln/ pellet discharge area. Sampling within MIR communities takes place on the rooftops of public buildings, whereas sampling in non-MIR communities occurs on rooftops or in remote locations so that background air quality can be evaluated. Airborne particles are collected using 1) a micro orifice uniform deposit impactor (MOUDI) (Marple et al., 1991), which enables size-fractionated particulate matter collection, and 2) a total suspended particulate filter (TSP). Particulate matter is evaluated via gravimetric analysis and subsequently subjected - 43 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 to comprehensive particulate matter characterization that includes: 1) scanning electron microscopy (SEM) imaging; 2) energy dispersive spectroscopy (EDS); 3) electron backscattered diffraction (EBSD); 4) proton induced x-ray emission (PIXE); 5) the Minnesota Department of Health’s 852 Method Transmission Electron Microscopy (TEM) Analysis for Mineral Fibers in Air; and 6) the International Standard Organization’s Method 10312 Ambient air – Determination of Asbestos Fibers – Direct-Transfer TEM Method (ISO 10312, 1995). Over the past year, the NRRI has completed particulate matter sampling within MIR taconite operations, MIR communities, and non- MIR communities. This includes 14 sampling events at taconite operations and 79 sampling events at locations within communities in northeastern Minnesota (73) and Minneapolis (6), as summarized in Table 1. Continued analysis of lake sediment samples from “North of Snort Lake” indicates collection of sediment dating back to ~1840, which pre-dates iron mining on the MIR. At Silver Lake, logging activities in the early part of the 20th century disrupted the sediments; however, we believe we will have good post-1915 lake sediment data, which will mark the period where the transition from natural ore to taconite mining took place. Continued analysis, interpretation and reporting will take place in 2012. Table 1. Summary of in-plant and community sampling events Taconite Facility United Taconite In-Plant Sampling Events Sampling Events Taconite Facility 2 events while active Keetac Hibbing Taconite   Sampling Events 1 event while active, 1 event while inactive 1 event while inactive, 3 events while active 3 events while active 1event while active, Northshore 1 event while inactive Minntac 1 event while active Minorca Community Sampling Events Community Sampling Location Sampling Events and Number per Season Keewatin Elementary School 7 Events (3 Winter / 4 Summer) Hibbing High School 9 Events (4 Winter / 5 Summer) Virginia City Hall 10 Events (5 Winter / 5 Summer) Babbitt Municipal Building 16 Events (7 Winter / 9 Summer) Silver Bay High School 13 Events (4 Winter / 9 Summer) Ely Fernberg Site 6 Events (3 Winter / 3 Summer) Duluth NRRI Rooftop 12 Events (7 Winter / 5 Summer) UMN Mech. Eng. Rooftop 6 Events (3 Winter / 3 Summer) References ISO 10312, 1995, Ambient air – determination of asbestos fibers – direct transfer transmission electron microscopy method, 51p. Marple, V. A., Rubow, K. L., and Behm, S. M., 1991, A micro orifice uniform deposit impactor (MOUDI): description, calibration, and use: Aerosol Science and Technology, v. 14, p. 434-446. Oreskovich, J. A., and Patelke, M. M., 2006, Historical use of taconite byproducts as construction aggregate materials in Minnesota: A Progress Report: Natural Resources Research Institute Report of Investigation NRRI-RI-2006-02, 10 p. - 44 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock geologic map of the Crane Lake and Brule Narrows 30’X60’ quadrangles, Quetico subprovince, northern Minnesota JIRSA, Mark A., Minnesota Geological Survey (jirsa001@umn.edu) The Crane Lake-Brule Narrows map portrays a complex history of sediment deposition, multiple intrusive events, and several periods of deformation and metamorphism. It lies within the Quetico subprovince of the Archean Superior Province (Fig. 1, small inset). The bedrock consists of biotite-plagioclase schist, granitoid and minor mafic intrusions, and complex migmatite containing multiple paleosomatic and neosomatic components. The schist was derived from graywacke and pelitic sediments deposited ~2695 Ma in an accretionary prism during early stages of collision between the Wawa subprovince island arc to the south, and the evolving Superior superterrane to the north. Later stages of this D1 collisional event produced tilting, broad nappe structures, and Figure 1. Location and regional geologic setting of Crane Lake-Brule Narrows map (MGS miscellaneous map M-192). Adjacent 1:100,000-scale maps recently published by MGS are numbered: Bigfork (M-176); Vermilion Lake (M-141); ElyBasswood Lake (M-148). thrust imbrication of volcanoplutonic rocks in subprovinces north and south of the Quetico subprovince, and recumbent folding within the Quetico. An early suite of leucogranite, granodiorite, trondhjemite, and tonalite is interlayered on all scales with biotite schist. This early suite has compositional and approximate temporal similarities with the 2690 Ma Saganaga Tonalite. A second deformation event (D2) occurred at about 2680 Ma, and produced regional penetrative fabrics, folds, and prograde metamorphism to greenschist and amphibolite facies. A third deformation event (D3) is manifest in the Quetico subprovince as broad, east- and west-plunging folds of D2 fabrics (synforms, antiforms), and by faulting. At least part of this deformation was synchronous with migmatization and emplacement of 2-mica leucogranite and a slightly younger, typically red, variably magnetic biotite granite known as the Lac La Croix granite. A U-Pb zircon age of the latter in adjacent Canada - 45 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 is approximately 2666 Ma, which is consistent with published ages for metamorphism inferred to be related to D3. The metamorphic grade is more or less symmetrical along the axis of the Quetico subprovince, with greenschist grade at the margins and middle to upper amphibolite facies near the axis. Metamorphism was generally syntectonic with D2 and D3; and a contact metamorphic overprint occurs locally adjacent to the Lac La Croix Granite. Bedrock in the western part of the map area is cut by Paleoproterozoic diabasic dikes that constitute the easternmost extent of the ~2076 Ma, Kenora-Kabetogama dike swarm. The geologic interpretation is based on archived and newly acquired outcrop and geophysical data augmented with geophysical and digital land surface topographic maps. Nowhere in Minnesota is bedrock so pervasively exposed, and a strong correlation is apparent between topography and the structure and composition of bedrock (Fig. 2). For example, large areas of granitic bedrock in the south and east are topographically higher than areas of migmatitic and schist-rich rocks in the north and west. Differential weathering produced curvilinear topographic relief that crudely depicts foliation trends and reveals complex folding. Prominent fractures and faults in all rock types are manifest in linear topographic lows. The aeromagnetic data similarly reflect foliation, folding, and fracture-controlled oxidation. Ground magnetic susceptibility data acquired from lithologically diverse outcrops indicate that granitic components typically produce higher signatures. The new image created by combining these data sources mimics at the map scale the complex magmatic, migmatitic, deformation, metamorphic, and alteration features observed at the outcrop scale. Mapping was supported by the U.S. Geological Survey STATEMAP element of the National Cooperative Geologic Mapping Program. Figure 2. Image of digital land surface topography in the Crane Lake-Brule Narrows area. Highest elevations, international border, and lake names are white. - 46 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Reconnaissance geologic mapping of Neoarchean rocks in the central Boundary Waters Canoe Area Wilderness by students of the Precambrian Research Center’s 2011 field camp. JIRSA, Mark, Minnesota Geological Survey, University of Minnesota, 2642 University Avenue W., St. Paul, Minnesota 55114; jirsa001@umn.edu, BAGGETTO, Louis, ELIASON-JOHNSON, Garret, HANSEN, Darren, HOXSIE, Erin, and KILPATRICK, Kayla, 2011 Field Camp Students, Precambrian Research Center, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811 The Precambrian Research Center (PRC)—a branch of the University of Minnesota, Duluth—conducted its 5th season of field camp in 2011. The final phase of field training is a “Capstone Project” that provides students an opportunity to create new geologic maps in areas of poorly understood geology. In contrast to detailed mapping that is typical of Capstone exercises, the students in this group were charged with reconnaissancestyle mapping in parts of four 1:24,000-scale quadrangles along a 44 mile canoe route in the Boundary Waters Canoe Area Wilderness (BWCAW). The route traversed ~22 lakes (Fig. 1), and a great variety of primarily Archean metavolcanic, metasedimentary, and intrusive rocks. Students mapped shoreline outcrops by canoe, and traced pertinent contacts during short inland traverses and at portages. Much of the area hasn’t been mapped in significant detail since J.W. Gruner’s work 1927-1938 (published in 1941). Although that work was remarkably detailed for its time, base maps were rudimentary and thus, surface control was uncertain. This project was intended to help place Gruner’s work into an accurate geographic context, verify or refute his observations, understand the nature of contacts, and apply modern geological nomenclature and interpretations. The regional bedrock is part of the Neoarchean Wawa subprovince of Superior Province (Fig. 2). Two main suites occur; pillowed volcanic and volcaniclastic rocks of the Newton Lake Formation that are cut by Figure 1. Map of central BWCAW showing route of 2011 geologic mapping (dark dashed line). Some of the larger lakes along the route are labeled. - 47 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 2. Generalized geologic map of northeastern Minnesota showing map area and geologic entities mentioned in text. felsic intrusions including the Basswood Lake granodiorite, and sedimentary rocks of the Knife Lake Group that locally contain fragments derived from the Newton Lake and Basswood Lake rocks, and thus are inferred to be younger. The geology is parceled into lozenges of strata having internally consistent stratigraphic relationships, separated by anastomosing shear and fault zones. Offset magnitude on those faults is largely unknown, and correlation of geologic units from one lozenge to another is not always possible. Significant discoveries from the reconnaissance mapping include: 1) The presence of iron-formation interlayered with volcaniclastic strata near Moose Lake in a sequence of rocks mapped regionally as the Newton Lake Formation. 2) The Basswood Lake granodiorite is compositionally and texturally identical with parts of the 2690 Ma Saganaga Tonalite, and wall-rock relationships are similar, suggesting temporal equivalence. 3) Identification of an unconformity and paleosaprolite between metagabbroic rocks likely equivalent to the Newton Lake Formation, and sedimentary strata of the Knife Lake Group. 4) Fault-bounded lozenges appear to represent slightly different crustal levels of exposure. For example, the lozenge exposed at Knife Lake was likely uplifted to expose the older, weathered and eroded metagabbroic floor of the Knife Lake Group depocenter. Geologic maps of all capstone projects can be downloaded or viewed at the PRC website: http://www.d.umn. edu/prc. - 48 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geochemistry of mafic dikes from the Carlton Dike Swarm in Minnesota JOHNSON, Teresa, Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401, SHANNON, James, MMG, 390 Union Boulevard, Suite 200, Lakewood, CO 80228, BOERBOOM, Terrence, Minnesota Geological Survey, 2642 University Avenue W., St. Paul, MN 55114, and WENDLANDT, Richard, Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 The Carlton Dike Swarm (CDS) is a reversely-polarized diabase dike swarm associated with the earlyreversal period of the ~1.1 Ga Midcontinent Rift System. The mafic dikes occur west of Duluth centered on Carlton County, MN and are generally oriented rift parallel. Previous characterization of the CDS has been completed by field mapping, petrography, geochemistry and aeromagnetics. Continued characterization of the CDS is being completed to delineate possible subgroups based on structural orientation, mineralogy, texture, geochemistry and geochronology. The four dike subgroups under consideration are (1) the standard CDS diabase dikes described by Green et al. (1987) and Reichhoff (1987), (2) a Ti-enriched set that may be related to the reversely polarized Esko intrusion, (3) an ultramafic-like set described by Boerboom (2009) and (4) the NWtrending orthogonal set. In addition two normally-polarized dikes in Carlton County are included to contrast temporal differences. Detailed characterization studies of rift-related dike swarm intrusions may elucidate temporal-spatial relationships to extrusive and intrusive rocks of the Midcontinent Rift including mineralized, magmatic Cu-Ni chonoliths and Duluth-type intrusions. Distinct petrographic differences in olivine modes, oxide compositions/modes, phenocryst phases/modes and quench textures are initially being used to delineate subgroups. Based on these criteria, NW-trending dikes have similar petrographic attributes to the standard CDS. The variations of ilmenite and magnetite exsolution and oxidation characteristics are being investigated further to use as a distinguishing factor. Quench textures of chain olivine, Ti-rich augite, plagioclase and ilmenite in the chilled margins of Esko-like dikes are similar to the mineralogy and quench textures found in the upper part of the Esko intrusion. The standard CDS dikes, NW-trending and normally-polarized dikes have chilled margins ranging from glassy to aphanitic. Further comparisons of the chilled margins among subgroups are in progress using whole rock geochemistry and electron microprobe analyses. Considering the difficulties in age determination for mafic rocks, QEMSCAN® is being used to identify possible uranium enriched zirconium minerals. Representative thin sections from each of the proposed subgroups are analyzed at 4 µm intervals with BSE imaging (Fig. 1). Points with a designated brightness (BSE95-150) are further analyzed with EDS to determine the mineralogy. The minerals identified with this technique are zircon, zirconolite, baddeleyite and monazite. For age determination, samples with zirconium minerals greater than 20 µm will be candidates for bulk analysis with ID-TIMS, and samples with grain sizes smaller than 20 µm will be considered for in-situ analysis with SIMS methods designed by Chamberlain et al. (2010). The final selection of grains for in-situ analysis is enhanced by additional high resolution BSE and CL imaging to characterize mineral associations, compositional zoning, intergrowths and overgrowths. - 49 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 1. BSE image of NW-trending dike; white circles show locations of zirconium minerals References Boerboom, T.J., 2009, C-19 Geologic Atlas of Carlton County, Minnesota [Part A]: Minnesota Geological Survey, County Atlas Series. Chamberlain, K.R., Schmitt, A.K., Swapp, S.M., Harrison, T.M., Swoboda-Colberg, N., Bleeker, W., Peterson, T.D., Jefferson, C.W., and Khudoley, A.K., 2010, In situ U–Pb SIMS (IN-SIMS) micro-baddeleyite dating of mafic rocks: Method with examples: Precambrian Research, v. 183, no. 3, p. 379–387. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey Jr., M.G., Meyers, P.E., Pesonen, L.J., and Wilband, J.T., 1987, Keweenawan dykes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America, in Mafic Dyke Swarms, Special Paper 34, Geological Association of Canada, p. 289–302. Reichhoff, J.A., 1987, Two Keweenawan Basaltic Dike Swarms in the Duluth Area, Minnesota: University of Minnesota, Duluth, 206 p. - 50 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Ferromanganese Precipitates in Lacustrine Environments of Northwestern Ontario and Nova Scotia: Effects on Arsenic Concentration KERKERMEIER, Leah and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada Freshwater lakes in Northwestern Ontario and Nova Scotia have iron and manganese precipitates growing at the sediment-water interface. The precipitates appear as disc shaped nodules with ringed structures composed of alternating iron- and manganese-rich laminations surrounding a nucleus of a pebble or cobble (Fig. 1). On the surface of some of the structures, micro-columnar stomatolites are formed (Fig. 2). The nodules are often found covered with a thick bacterial mat. Precipitates collected from Lake Shebandowan and Sowden Lake in northwestern Ontario and Lake Charlotte in Nova Scotia have been analyzed. The nodules have revealed extreme naturally concentrated arsenic with values as great as 3500ppm.   Figure 1. A ferromanganese nodule collected from Sowden Lake, Northwestern Ontario Figure 2. Micro-columnar stromatolites found on the surface of a ferromanganese nodule To understand such an arsenic concentration, natural environments able to create reduction/oxidation boundaries are being examined. The precipitates are composed primarily of the redox sensitive metals iron and manganese allowing for speculation that arsenic concentration has a relationship with the redox potential of an environment. In a reduced environment, such as some groundwaters, iron and manganese can be transported in solution until reaching an oxidized environment, such as lake water. It is proposed that arsenic will behave similar to phosphorous in a lake environment due to its molecular similarities and valance state, therefore also having an attraction towards metals such as iron (Takamatsu et al., 1985). Two possible natural redox environments can occur in a lacustrine environment allowing for precipitation of redox sensitive metals. When reduced groundwater comes into contact with oxidized lake water by means of a spring or diffuse flow on a lake bottom, redox sensitive elements can oxidize and precipitate out of solution coating a nucleation point such as a pebble. This is hypothesized to be occurring in the study area in Lake Charlotte and areas of Lake Shebandowan. The concentration of arsenic, by means of this redox method, can be high due to the ability of groundwater to leach a large area and transport ions in solution for long distances, concentrating the dissolved material in the nodules. - 51 -   �Proceedings of the 58th ILSG Annual Meeting - Part 1 The second postulated redox environment involves upwelling of anoxic, deeper lake water carrying redox-sensitive elements to the oxic water in the shallows (Dean et al. 1981). In conjunction with this process, microorganisms concentrate dissolved manganese and iron and upon death are buried in the lake bottom sediment. The anoxic environment in the substrate allows for the reduction of iron and manganese, releasing the soluble reduced forms of these metals into sediment pore waters. These ions then diffuse upward into the lake water, until contact with photosynthetic microorganisms enables re-oxidization and precipitation onto a nucleus, often a pebble (Dean et al. 1981). Continued accretion of iron and manganese will form a nodule. This redox process probably creates ferromanganese precipitates that do not accumulate substantial amounts of arsenic due to lack of a large reservoir that can be leached. It is hypothesized that areas of Shebandowan and Sowden Lake fall into this category. References Dean, W.E., Moore W.S., Nealson, K.H. 1981. Manganese cycles and the origin of manganese nodules, Oneid Lake, New York, USA. Chemical Geology, 34: 53-64. Stevens, L.B., Investigation of ferromanganese nodule precipitation and arsenic uptake in modern biochemical sediments: Lake Charlotte, Nova Scotia. Unpub. B.Sc. thesis, Lakehead University, 70pp. Takamatsu,T., Kaswashima, M., and Koyama, M., 1985. The role of Mn2+- Rich Hydrous Manganese Oxide in the Accumulation of Arsenic in Lake sediments. Water Resource. 19 (8): 1029-1032. - 52 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetism of the Alkaline Coldwell Complex: New Results, New Insights. Kern, Ashley, Kulakov, E.V., Smirnov, A.V., and Diehl, J.F, Department of Geological and Mining Engineering and Sciences, Michigan Technological University,1400, Townsend Drive, Houghton, Michigan, 49931, USA. The alkaline Coldwell Complex is a ≈1.1 Ga intrusive complex which is thought to have been emplaced during three different episodes of magmatism. Previous paleomagnetic work by Lewchuk and Symonds (1990) has shown that rocks representing magmatic episodes I and III exhibit reversed (R) magnetizations with mean directions that are significantly different. The rocks representing episode II were characterized by normal (N) magnetizations which are symmetrical with respect to the R magnetizations. Lewchuk and Symonds (1990) interpreted these results as indicating the reversal asymmetry seen in other Keweenawan age rocks is not the effect of a long-standing non-dipole field but the effect of plate motion. To further investigate the reversal asymmetry question in Keweenawan-aged rocks, we have begun a new paleomagnetic study of the alkaline Coldwell Complex at the Earth Magnetism Laboratory of Michigan Technological University. We are also using the micro-baddeleyite method to refine the ages of the various episodes of Coldwell Complex magnetism, but are only reporting our preliminary paleomagnetic results at this meeting. Six to ten samples were collected from 32 sites along the Trans-Canadian HWY 17 (previously sampled and studied by Lewchuk and Symons (1990)). In addition, we sampled 11 sites north of Marathon and near Middleton. Characteristic remanent magnetizations (ChRM), in our study, have been isolated by thermal and alternating field demagnetization techniques. Mean paleomagnetic directions have been categorized according to the existing model of the three episodes of magmatism. The mean paleomagnetic directions for episodes I and III have been found to be statistically similar at 95 percent confidence, making these episodes indistinguishable with paleomagnetism unlike the Lewchuk and Symons’ study. We will discuss two possible scenarios of the geomagnetic field behavior during the formation of the Coldwell Complex: 1. Multiple reversals RaNaR, as recorded consecutively by episodes I, II and III; 2. Single reversal from R to N polarity, where episodes I and III were emplaced at the same time (single episode) or in pulses during a time interval insufficient to result in any difference between the primary ChRM directions. The normal polarity paleomagnetic direction of episode II appears to be antipodal to both reversed components as well as to the mean direction calculated from all reversely magnetized sites. The reversal test (McFadden, 1990) is positive in classification C for both RaNaR reversals and positive in the same classification for the RaN case. References. McFadden P. L., M. W. McElhinny, 1990. Classification of the reversal test in palaeomagnetism. Geophysical Journal International, 103, 3, pp 725–729. Lewchuk M.T., D.T.A. Symons, 1990. Paleomagnetism of the Late Precambrian Coldwell Complex, Ontario, Canada. Tectonophysics, 184, pp 73-86. - 53 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Thrust faulting in the Gunflint Formation north of Lake Superior KOROSCIL, Jesse, HILL, Mary Louise and FRALICK, Philip, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1 New exposure of the Gunflint-Rove contact at the Terry Fox monument north of Thunder Bay reveals several thrust faults that cut through the Sudbury impact layer and into the overlying Rove Formation. This discovery has implications for the timing of thrust-fault deformation here and elsewhere in the area north of Lake Superior. Previously, thrust-fault deformation in the Gunflint Formation was interpreted to be associated with the Penokean orogeny (Hill and Smyk, 2005). However, new observations of thrust faults cutting the impact layer and overlying Rove Formation mean that this deformation is too young to be Penokean. The thrust faults exposed in this outcrop are mainly small discrete bedding plane faults with few kinematic indicators observed in the hanging wall or footwall. The faults are most visible where they cut across bedding; once identified they can be traced along the outcrop. Thrust faults within the impact layer are identified by slickenlines on fault plane surfaces as well as possible duplication of strata, resulting in localized thickening of the impact layer. The faults can be traced along strike until they are either covered by overburden or cut by a prominent normal fault which displaces all units in the hanging wall down to the south several meters. The timing of thrust-fault deformation can be constrained based on cross-cutting relationships and published ages of the 1878 Ma Gunflint Formation (Fralick et al., 2002), 1850 Ma Sudbury impact event (Krogh et al., 1984) and the 1832 Ma Rove Formation (Addison et al., 2005). References Addison, W.D, Brunpton, G.R., Valinni, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event: Geology, v. 33, p. 193-196. Fralick, P.W., Davis, D.W. and Kissin, S.A., 2002, The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Canadian Journal of Earth Sciences, v. 38, p. 1085-1091. Hill, M.L. and Smyk, M.C., 2005, Penokean fold-and-thrust deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario [abstract]: in Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, v.51, part 1, p.26. Krogh, T.E., Davis, D.W. and Corfu, F., 1984, Precise U-Pb zircon and baddeleyite ages for the Sudbury area: in E.G. Pye et al. (eds.), The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey Special Volume 1, p. 431-446. - 54 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetism of the Geordie Lake and Silver Mountain basalts Kulakov, Evgeniy V., Smirnov, A.V. and Diehl, J.F., Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400, Townsend Drive, Houghton, Michigan, 49931, USA. A paleomagnetic study of 17 basaltic lava flows exposed in the northern part of the Coldwell Complex, near the Geordie Lake (GL) (Ontario, Canada), and 13 lava flows that crop out on Silver Mountain (SM) (Baraga County, Northern Michigan) has been carried out in order to investigate the paleomagnetic directions of these rocks and to determine their relationship to other rocks of the North American Midcontinent Rift (MCR). The Geordie Lake basalts consist of a series of flat-lying lava flows that overlay gabbros of the northern part of the Coldwell Complex and are assumed to be similar in age (~1108 Ma) to these gabbros. The lava flows at Silver Mountain, a 300 foot-high glacial polished dome-shaped hill, are part of the Siemens Creek volcanics, the lowermost formation of the Powder Mill Group. This sequence of lava flows dip 15 degrees to north-west. Silver Mountain is surrounded by Cambrian and younger sediments as well as glacial deposits. Six to ten samples were collected from each lava flow at both locations. Well-defined characteristic remanent magnetizations (ChRM) were isolated using thermal demagnetization techniques. Both the GL and the SM CHRMs define a reversed (R) polarity magnetization. Mean directions for each flow as well as group means were calculated using standard Fisher statistics. Group mean paleomagnetic directions of GL (D=109.1°, I=-63.1°, α95=3.81°, K=89, N=17) and SM (D=107.9°, I=-60.4°, α95=2.43°, K=232, N=13) are statistically indistinguishable after the correction of SM directions for structural tilt. In addition, both the SM and GL mean directions are statistically similar to the published data for reversely magnetized Lower North Shore Volcanics (LNSV) (e.g. Books, 1968; Halls and Pesonen 1982) dated at 1107±1.9 Ma using the U-Pb method (Davis and Green, 1997). Paleomagnetic poles calculated from GL and SM paleomagnetic direction are located at lat=42.9°N, long=206.5°E (α95=3.6°) and lat=39.8°N, long=202.8°E (α95=3.4°), respectively. These poles fall directly on the eastern arm of the North American Precambrian apparent polar wonder path (APWP) (Logan Loop (Robertson and Fahrig, 1971)) and are similar to that calculated for 1107 Ma reversely magnetized LNSV (lat=41.5°N, long=202.7°E) at the 95 percent confidence level. Therefore, we conclude that rocks of the Geordie Lake and Silver Mountain represent the earliest stage of rift related volcanic activity. Similarity of paleomagnetic directions and paleomagnetic poles for these sequences suggests no clockwise rotation of Midcontinent Microplate after 1107 Ma, as proposed by Hauser (1996). References. Books, K., 1968.Magnetization of the lowermost Keweenawan lava flows in the Lake Superior area. USGS Processional Paper. 600-D, D248–D254. Campbell R.E., 1952 Geophysical Investigation of the Silver Mountain Are – Houghton County, Michigan, MS Thesis, Michigan college of Mining and Technology. Davis, D.W., Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of. Earth Sciences. 34 (4), 476–488. Halls, H.C., and Pesonen, L.J. 1982. Paleomagnetism of Keweenawan rocks. In Geology and tectonics of the Lake Superior Basin. Geological Society of America, Memoir 156, pp. 173 -201. Hauser E.C, 1996, Midcontinent rifting in a Grenville embrace, Geological Society of America, Special paper, 308, pp 67-75 Davis, D.W., Green, J., and Manson, M. 1995. Geochronology of the 1.1 Ga North American Mid-Continent Rift [abstract). Proceedings of the 41st Conference of the Institute on Lake Superior Geology, Marathon, Ont., May 1995, Part I, Program and Abstracts, pp. 9-10. Robertson, W.A., W.F. Fahrig, 1971. The great Logan paleomagnetic loop-the polar wandering path from the Canadian Shield rocks during the Neohelikian Era. Canadian Journal of Earth Sciences, 8, 1355-1372. - 55 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geochemistry and Petrology of the Dog Lake Granite Chain, Quetico Basin, Northwestern Ontario KUZMICH, Ben, HOLLINGS, Pete, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, CAMPBELL, Dorothy, and Scott, John, Resident Geologist Program, Ontario Geological Survey, Thunder Bay, Ontario P7E 6S7 The Dog Lake Granite Chain is composed of six ovoid magnetite-bearing granitic intrusions within the Quetico Basin, Northwestern Ontario (Fig. 1). From east- to west the intrusions have been termed the Penasen Lake, White Lily, Barnum Lake, Trout Lake, Silver Falls, and Shabaqua intrusion. These granites have been mapped previously mapped as syn- to post-tectonic, massive to foliated granite/granodiorites (Ontario Geological Survey, 2006). Of the intrusions, only the Barnum Lake intrusion has been studied in detail and was classified as a homogenous quartz-monzonite with lesser amounts of syenite and gabbroic phases (Steinert, 1975; Kelhenbeck, 1977). The Dog Lake Granite Chain displays a linear trend, which parallels the tectonic boundary between the Wawa-Abitibi terrane to the south, and the Quetico Basin to the north. Figure 1. Aeromagnetic map of the Dog Lake Granite Chain (modified from Ontario Geological Survey, 1999) Petrologic and geochemical data has been used to classify the granites as both I- and S-type. The I-type granites can be classified into three broad groups a microcline-phyric monzonite/quartz-monzonite, syenite/quartzsyenite, and a monzodiorite. These granites are massive, silica poor, largely metaluminous, and characterized by a hornblende+ magnetite+ sphene +/-pyroxene assemblage. The microcline-phyric monzonite/quartz-monzonite and syenite/quartz-syenite groups are characterized by a positive εNd (+1.44 to +2.11). The I-type granites have - 56 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 been recognized within the Trout Lake, Barnum Lake, White Lily, and Penasen Lake intrusions. The S-type granites are typical of the Quetico Basin, and have a largely peraluminous affinity, variable εNd signatures (-1.44 to +1.09), and are characterized by a muscovite+ biotite+/- garnet assemblage. The S-type granites have been sampled within the Silver Falls, Trout Lake, and White Lily intrusions. The recognition of I-type granites within the Quetico Basin, which is predominantly composed of S-type granites requires a different model for the formation of this relatively rare rock type. It is suggested that the formation of I-type granites within the Quetico Basin involves the partial melting of the mantle wedge beneath the Wawa-Abitibi island arc. The mafic melt would have underplated the Archean lithosphere possibly near the contact between the Wawa-Abitibi terrane and the Quetico Basin. The mafic melts would then have evolved into granitic melts through fractionation, thus giving rise to the I-type granites. Small volumes of these melts were then emplaced within the Quetico Basin, possibly along weaknesses associated with lithosphere scale structures at the boundary between the Wawa-Abitibi terrane and Quetico Basin. The majority of the underplated melts may also have aided in the production of large S-type granites, which are typical of the Quetico Basin. These S-type melts formed from the melting of sedimentary rocks, and may have interacted with the I-type granites, producing variations in isotopic and geochemical signatures as seen within the S-type granites of this study. Although magnetite-bearing granitic intrusions within the Quetico Basin are not unique, the identification and classification of I-type granites has not been widely documented. The regional implication of I-type granites within the Quetico Basin is both profound and complex. It is suggested that other magnetite-bearing metaluminous granites within the Basin (e.g., the Vermillion complex, northern Minnesota) may have formed through a similar processes as the Dog Lake Granite Chain. References Kehlenbeck, M.M. 1977. The Bamum Lake pluton, Thunder Bay, Ontario; Canadian Journal of Earth Sciences, v.14, p.21572167. Ontario Geological Survey 1999. Single master gravity and aeromagnetic data for Ontario, GeoSoft® format; Ontario Geological Survey, Geophysical Data Set 1036. Ontario Geological Survey 2006. 1:250 000 Scale Bedrock Geology of Ontario; Ontario Geological Survey, Miscellaneous Release---Data 126-revised. Steinert, G. 1975. Structure and petrology of the Barnum Lake intrusive; unpublished BSc thesis, Lakehead University, Thunder Bay, Ontario, 67p. - 57 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Bedrock Geologic map of the Seine Bay/Bad Vermilion Lake intrusion, Mine Centre, Ontario LEE, Aubrey, UMD Department of Geological Sciences, Duluth, MN, ALBERS, Paul, Freeport-McMoRan Copper & Gold, Oro Valley, Tucson, Arizona, MILLER, Jim, UMD Department of Geological Sciences, Duluth, MN, SEVERSON, Mark, Natural Resources Research Institute, UMD, Duluth, MN, DEEN, Tobin, University of Minnesota Department of Earth Sciences, Minneapolis, MN The Seine Bay-Bad Vermilion Lake Intrusion (SB/BV) is a layered plutonic sequence of gabbroic rocks exposed along the shores of Seine Bay (Rainy Lake) and Bad Vermilion Lake in Northwest Ontario, Canada. It is approximately 14 km long and 1-3 km wide, stretching along the northern shores of Seine Bay in the southwest and Bad Vermilion Lake in the northeast with the town of Mine Centre marking its northeast tip. The SB/BV lithologically consists of a layered mafic intrusive package of anorthositic to gabbroic to pyroxenitic rocks with layers/lenses of semi-massive and massive oxides that have been significantly altered and possibly metamorphosed. The intrusion occurs between a hanging wall of felsic intrusive rocks at its northern contact and a footwall of mafic volcanic rocks at its southern contact. The nature of these contacts and the relative magmatic history of each unit are currently poorly known. Although gold has historically been the primary focus of mineral exploration in the Mine Centre area, deposits of iron, titanium, copper, nickel and zinc have been identified in recent years by various junior exploration companies (Hinz et al., 2010). Lode gold, magmatic Cu-Ni-PGE, magmatic Fe-Ti-V, and pyroclastichosted diamond deposits in the Mine Centre area are currently being evaluated for economic viability. Numax Resources, Inc. (Numax) holds 45 contiguous mineral claims totaling 5,188 ha in the Kenora Mining Division, covering the length of the SB/BV. While the Mine Centre property was somewhat inactive for decades, Numax’s 2004 field season saw a jump in activity including prospecting, diamond drilling, trenching, channel sampling,   Figure 1. Bedrock Geologic Map of the Seine Bay/Bad Vermilion Lake Intrusion, Numax Resources, Inc., Mine Centre, Ontario, Canada. Compiled and finalized by A. Lee, this map was generated with data collected by A. Lee during the summer of 2011 and by P. Albers and C. White during the summer of 2009. (Lee et al., 2011) - 58 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 field sampling, and two ground geophysical surveys (magnetic and electromagnetic). In 2009 Numax contracted Paul Albers and Chris White to conduct geologic bedrock mapping at 1:5000 scale and detailed trench mapping at 1:400 scale across the southwest portion of the property (Albers and White, 2010). In the summer of 2011, Aubrey Lee with assistance from Tobin Deen was recruited to map the northeast portion of Numax’s property at 1:5000 scale. A total of 83 hand-samples were collected during Lee’s mapping and have subsequently been geochemically assayed and cut into thin-sections. This field work and the sample suite constitute the basis for Aubrey Lee’s MS thesis at UMD. Recent geologic mapping of the SB/BV has illustrated that massive, semi-massive and disseminated oxide layers containing significant Fe-Ti mineralization exist within the intrusion (Lee et al., 2011). These layers are generally persistent along the strike of the intrusion, but may anastomose and bifurcate, as indicated by bedrock mapping, magnetic surveys, and drill core. The layers are typically hosted by pyroxenite and contain the oxides magnetite and ilmenite. Titaniferous magnetite may also be present. Oxide-rich layers range in width from a few centimeters to over 20 meters and number dozens. They dip sub-vertically and likely extend along with the host intrusion to a considerable depth. Magnetic surveys of the Mine Centre area reveal three distinct magnetic highs extending the length of the intrusion and merging in the northeast where the exposure of the intrusion narrows along Bad Vermilion Lake. It is also unknown how many massive Fe-Ti oxide-rich layers exist within the intrusion. Drill core from three holes drilled by Numax establish the lateral continuity of the layers along the strike of the intrusion. The principal objectives of this study are to characterize the lithologic, petrographic, and geochemical attributes of the SB/BV through analyses of hand samples, channel samples, and drill core from three widely spaced traverses across its stratigraphy. The research goals are three-fold: 1) to establish the lithologic and chemical stratigraphy of the intrusion so as to delineate the history of emplacement and crystallization within the magma chamber; 2) to fully characterize the mineralogic, textural, and chemical attributes of the Fe-Ti oxiderich layers with the goal of documenting their composition and understanding their origin; and 3) to assess the history of sulfide saturation in the intrusion so as to evaluate the potential for precious metal mineralization. More specifically, the research will address the following questions: 1) Recognizing that the formation is a gabbroic layered intrusion that has been significantly altered, deformed, and possibly metamorphosed, what are the original igneous rock types that comprise the intrusion?; 2) How many massive Fe-Ti oxide-rich layers exist within the intrusion, how do they compare in mineralogy, texture and chemistry, what is the nature of lateral and vertical variations of the layers, and which layers contain the highest, most homogeneous Fe, Ti, V, and P content?; 3) Was the cyclically layered gabbroic sequence formed in a closed system or was it affected by multiple magma pulses into the chamber, and what mechanism acted to concentrate multiple layers of massive oxides?; 4) What was the parental magma composition of the intrusion?; 5) What is the nature of the contact between the mafic intrusion and the felsic rocks at its top and the basaltic rocks at its base and what do these relationships reveal about the magmatic history of these rock units?; and 6) What is the potential for PGE reef-style mineralization? References Albers, P.B., and White, C.R. 2010. Report on the Geology and Mineral Potential of the Seine Bay/Bad Vermilion Lake Intrusion, Mine Centre Property, Mine Centre, Ontario. Unpublished report prepared for Numax Resources, Inc. Hinz, P., White, C.R., Albers, P.B., and Tortosa, D., 2010, Mineral Deposits of the Mine Centre – Rainy River Area, Ontario: Institute on Lake Superior Geology, 56th Annual Meeting, International Falls, MN, v. 56, part 2, p. 95-125. Lee, A., Albers, P., White, C., and Deen, T., 2011, Bedrock Geologic Map of the Seine Bay/Bad Vermilion Lake Intrusion, Numax Resources Inc., Mine Centre, Ontario, Canada. - 59 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Granitic saprolites in the Lake Superior region: K-metasomatism, intensity vs. magnitude of weathering, and estimates of atmospheric pCO2 MEDARIS, Gordon Jr, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, medaris@geology.wisc.edu, DRIESE, Steven, Department of Geology, Baylor University, Waco, TX 76798, Steven_Driese@baylor.edu, BOERBOOM, Terry, Minnesota Geological Survey, St. Paul, MN 55114, boerb001@umn.edu and JIRSA, Mark Minnesota Geological Survey, St. Paul, MN 55114, jirsa001@umn. edu. Paleosols provide insights into past climates, atmospheric compositions, and the role of terrestrial biomass in weathering processes. A geochemical comparison of nine granitic paleosols in the Lake Superior region (Table 1), ranging in age from 2690 to 100 Ma, has been undertaken to evaluate weathering conditions in the region over time. Table 1. Characteristics of selected granitic saprolites in the Lake Superior region Overlying Locality Age, Ma Formation Saganaga 2690 Ogishkemuncie Pronto 2450 Matinenda Ville Marie 2450 Lorrain McGrath ~2200 Denham Baraboo 1700 Baraboo Sibley 1500 Pass Lake Monticello 500 Mt. Simon EC MN 100 Cenomanian SW MN 100 Cenomanian Saprolite Saprolite max pCO2 paleo- 1 Protolith Depth, m PIA MW tonalite granite granite gneiss granite granite tonalite granite gneiss 70.5 97.7 99.6 93.0 96.5 90.3 94.9 97.9 98.8 <16 6.2 8.6 n.d. 4.3 2.3 2.1 >36 >22 <47.4 13.4 7.6 n.d. 7.9 5.6 2.2 >34.7 >33.8 2 3 K <2.2 2.0 1.1 n.d. 0.7 1.0 0.3 -2.3 -2.1 × PAL4 latitude <137 11 4.5 n.d. 6.4 12 2.5 6.8 6.8 25º N 40º N 40º N 10º N 50º N 25º N 10º S 40º N 40º N PIA, Plagioclase Index of Alteration: 100 × (Al2O3 – K2O)/(Al2O3 + CaO* + Na2O – K2O) Magnitude of Weathering: integrated loss of SiO2+MgO+CaO+Na2O+K2O (moles/cm2) 3 integrated change in K2O (moles/cm2); for Precambrian & Cambrian saprolites, relative to estimated weathered material (see text) 4 partial pressure of CO2 times pre-industrial level, calculated for 100,000 years of weathering n.d., not determined 1 2 In A-CN-K diagrams (Fig. 1), the compositions of Precambrian and Cambrian saprolites are displaced markedly from a “normal” weathering trend towards the K apex (except for the Barron saprolite), indicating a substantial degree of K-metasomatism. Such metasomatism precludes use of the Chemical Index of Alteration (CIA) as an indicator of the intensity of weathering. Instead, the Plagioclase Index of Alteration (PIA), which is a measure of plagioclase removal, may be used and yields values >90 for all but the Saganaga saprolite (Table 1). The % change of oxides was calculated for each saprolite relative to its protolith, taking Al2O3 as the immobile constituent. For metasomatised saprolites, K2O loss was taken to be 75% of Na2O loss, by analogy with the Cretaceous saprolites and modern day weathering profiles. The magnitude of weathering (MW) is a measure of mass removal and is defined as the loss of SiO2, MgO, CaO, Na2O, and K2O, integrated over the depth of weathering. Although most of the saprolites have comparably high intensities of weathering (PIA >90), the Cretaceous saprolites have appreciably larger magnitudes of weathering compared to the Precambrian and Cambrian saprolites, i.e. MW values of 22 to 36 moles/cm2 vs. 2 to 9 moles/cm2 (Table 1). The Saganaga - 60 - �Proceedings of the 58th ILSG Annual Meeting - Part 1   Figure 1. Compositions of protoliths and saprolites in terms of molar Al2O3, CaO*, Na2O, and K2O. saprolite is an exception to this pattern, most likely due to uncertainty in the depth of weathering. The mass of K2O added to the weathered Precambrian and Cambrian saprolites is 0.3 to 2.0 moles/cm2, an amount which is comparable to the mass of K2O removed by weathering from the Cretaceous saprolites, 2.1 to 2.3 moles/cm2 (Table 1). It is apparent that under high (= modern) atmospheric pO2, Fe is retained in most Cretaceous saprolites, whereas in Neoarchean and Proterozoic profiles Fe is generally lost, consistent with previous interpretations of lower pO2; the Cambrian Monticello saprolite shows a pattern for Fe loss with depth more like the Precambrian profiles. Following Sheldon’s method (2006, Precam. Res., v. 147, 148-155), pCO2 has been calculated for the Precambrian and Cambrian saprolites (Fig. 2, Table 1). Among the parameters involved, the MW and duration of weathering are most critical. The uncertainty in duration of weathering may preclude direct comparison of pCO2 among the individual saprolites, although the results in Figure 2 indicate a generally higher level of atmospheric pCO2 associated with the Precambrian and Cambrian saprolites compared to the pre-industrial level. Note that the depth of weathering and pCO2 estimate for the Saganaga saprolite are uncertain. Despite elevated concentrations of atmospheric CO2 and lowto mid-paleolatitudes, the Precambrian saprolites have significantly lower MW values than do the Cretaceous saprolites, perhaps reflecting enhanced weathering in Cretaceous time in response to organic compounds contributed by vascular land plants. References Boerboom 2011 ILSG Proc 57/2: 127-163; Driese & Medaris 2008 J Sed Res 78: 443-457; Driese et al. 2007 J Geol 115: 387-406; Driese et al 2011 Precam Res 189:1-17; Nedachi et al 2005 Chem Geol 214: 21-44; Rainbird et al 1990 J Geol 98: 801-822; Rogala et al 2007 Can J Earth Sci 44: 1131-1149; Setterholm et al 1989 Minn Geol Surv Inf Circ 99 pp - 61 -   Figure 2. pCO2 vs. duration of weathering �Proceedings of the 58th ILSG Annual Meeting - Part 1 Mafic intrusions along the southern boundary of the Superior Craton, SD: What is their potential for Ni-Cu-PGE mineralization? McCORMICK, Kelli A. and PATERSON, Colin J., Dept of Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 E. St. Joseph St., Rapid City, SD 57701, USA A significant volume of mafic intrusive rocks is present along the southern margin of the Superior Craton, mainly within the Superior Boundary Zone, in southeastern South Dakota, northeastern Nebraska, and northwestern Iowa (McCormick, 2010). This region of the North America Craton experienced a number of orogenies in the Proterozoic, including the Trans-Hudson, Penokean, Yavapai, and Mazatzal. Most certainly at least three of these have significantly affected the southern margin of the Superior craton in this tri-state region. These intrusions of interest are typically 700 to 900 ft deep. Existing samples of these rocks are limited to the upper few feet of each body with the exception of a fairly well-studied 2890 Ma layered mafic-ultramafic intrusion in northwestern Iowa (Windom et al., 1993), a 1733 Ma metagabbro present just south of the Spirit Lake Tectonic Zone (Van Schmus et al., 2007) in Union County, South Dakota, and an undated mafic intrusion in Clay County, South Dakota near the town of Wakonda. The location of these mafic intrusions along a craton margin, the potentially elevated heat flow over an extended period of time due to the active orogenesis, and the subsequently strong possibility of a mantle contribution to these mafic bodies, suggests the potential for existence of an economic Ni-Cu-PGE deposit within the region. Our objective is to characterize and understand the regional tectonic and metallogenic framework of the southern margin of the Superior Craton in South Dakota and contiguous states in a number of integrated projects focusing on petrologic, geochemical, isotopic, and geochronological studies. We hope to initiate the research through the study of the two 1000 ft cores of the Wakonda intrusion drilled by WMC exploration in 2003/2004. References McCormick, K.A., 2010, Precambrian basement terrane of South Dakota: South Dakota Geological Survey Bulletin 41, 35 p. Van Schmus, W.R., Schneider, D.A., Holm, D.K., Dodson, S., and Nelson, B.K., 2007, New insights into the southern margin of the Archean-Proterozoic boundary in the north-central United States based on U-Pb, Sm-Nd, and Ar-Ar geochronology, in Holm, D.K., Schneider,D., and Chandler, V.W., eds., Proterozoic tectonic and crustal evolution of the Upper Great Lakes region, North America: Precambrian Research, 157, issues 1-4, 80-105. Windom, K.E., Van Schmus, W.R., Seifert, K.E., Wallin, E.T., and Anderson, R.R., 1993, Arch-ean and Proterozoic tectonomagmatic activity along the southern margin of the Superior Province in northwestern Iowa, United States: Canadian Jour. Earth Sci., 30, 1275-1285. - 62 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Predictive water level and preliminary hydrodynamic models of Caland and Hogarth pit lakes, Atikokan, Ontario. MIKKELSEN, Larissa1 and CONLY, Andrew, Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada. 1present address, True Grit Consulting Ltd. 1127 Barton Street Thunder Bay, Ontario P7B 5N3 Canada Recent limnology studies of the pit lakes on the former Steep Rock Iron Mines property (McNaughton, 2001; Gould, 2008) have shown that Caland and Hogarth pit lakes are meromictic and holomictic, respectively, and that, in general, measured concentrations of sulphate within the lakes are at or above Ontario Drinking Water Standards (ODWS) or Environmental Protection Agency (EPA) Secondary Maximum Contaminant Level (SMCL). Pit lake water quality can be influenced by hydrodynamics. For example, unexpected overturn of a meromictic pit lake can transport dissolved metal laden waters that are toxic to aquatic life to the surface, or where overturn in a holomictic pit lake transports dissolved oxygen down to submerged tailings producing acid mine waters where sulphide minerals exist. Changes in hydrodynamic behavior of Caland or Hogarth will influence whether the water released from the super pit lake will be of good quality or have elevated sulphate concentrations, potentially impacting downstream drinking water sources. Predictions of future water quality in the pit lakes are important to assess the type of rehabilitation efforts, if any, that may be required at the former Steep Rock Iron Mines property with respect to out flowing water quality (Conly et al., 2008; Godwin et al., 2010). This research develops a predictive analytical model of pit lake water level over time that is used to define water volume inputs for the hydrodynamic models. Preliminary hydrodynamic models of Caland and Hogarth pit lakes are designed to assess if the model can accurately simulate current conditions as well as future limnology, as flooding continues and Caland and Hogarth join forming a super pit lake before out flowing into the West Arm Reservoir. Future water quality is assessed by proxy using a linear relationship between water salinity and sulphate concentrations in the Steep Rock pit lakes and data deficiencies with respect to the models are discussed. The future water levels in the pit lakes are modeled using a void filling approach. The volume of Caland, Hogarth and South Roberts open pits and Fairweather Lake are calculated based on ultimate pit limits and contours based on 1986 aerial photography. The surface area of the pit lakes between the contours is calculated using linear interpolation at a 1 m interval. A water balance for each pit defines the volume of water entering the pits as the sum of precipitation landing on the lakes plus runoff and groundwater minus evaporation from the pit lake surfaces. The surface area of the pit lake and watershed area are variable depending on the accumulated volume in the pit lakes. The model predicts that water from Caland will flow into Hogarth in 2057 and the super pit lake will flow in the West Arm in 2077 or 45 years and 65 years from present day. Compared to the Regional Engineering model for the Steep Rock pit lakes (MNR, 1986), which has proven to be accurate, this study prediction is the same as the Regional Model except this study predicts it will take 8 years longer to completely fill. The methods used to construct the two different models are similar, but this study benefits from a longer data set for meteorological parameters and that the remaining volume capacity of Hogarth pit lake when the two lakes join is taken into account in subsequent water level predictions. This study was the first hydrodynamic modeling attempted for the Steep Rock pit lakes. The hydrodynamics of Caland and Hogarth are modeled using Dynamic Reservoir Simulation Model (DYRESM) from the Centre for Water Research at the University of Western Australia (www.cwr.uwa.edu.ca). DYRESM simulations of current conditions accurately portray the observed limnological characteristics of Caland and Hogarth pit lakes (McNaughton, 2001), including: i) that Caland is meromictic and has a lower salinity relative to Hogarth; and, ii) that Hogarth develops a temporary meromix. Simulations of the two pits joining indicate that Caland’s freshwater lens is maintained, but is thinner, and Hogarth pit lake develops a permanent meromix. Simulations of when the pit lakes outflow into the West Arm indicate that Caland maintains its upper freshwater lens and that a fresh water lens is seen in Hogarth is quickly lost when the pit lakes join. In most cases, variations in simulation parameters including additional inflows, alteration of the inflow salinities, and the use of a slower rebound rate to define the - 63 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 DYRESM water balance only produced minor changes in the simulation. Based on the linear trend between salinity and sulphate concentrations, the DYRESM salinity profiles suggest that: i) future outflow waters from Caland into Hogarth will have sulphate concentrations ranging from 0 mg/L to 100 mg/L; and, ii) waters that outflow from Hogarth will be toxic with sulphate concentrations ranging from 1700 mg/L to 1900 mg/L. In general, the sulphate concentrations in Caland are below maximum acceptable limit of the ODWS and EPA SMCL, while those in Hogarth exceeded these standards. Simulation results for when the pit lakes outflow suggest that the waters will be toxic. However, results may be influenced by the initial profile used to execute the model due to the short simulation period. Further work will address this issue by lengthening the simulation period and by using different initial profiles of the pit lake water column to assess their influence on the pit lake hydrodynamics. Other limitations to the model that should be addressed in future work include: the ill-defined inflow volumes and chemistry overtime which must be assumed for the length of the simulation period; the availability of information pertaining to site hydrogeology was limited; and, the light extinction coefficient should be measured because the presence of iron oxides influences light absorption in water. References Godwin, Amy Lynn, Lee, Peter Ferguson, Conly, Andrew George, Goold, Andrea R. (2010): Predicting toxicity of future combined pit lakes at the former Steeprock Iron Mine near Atikokan, Ontario. – In: Wolkersdorfer, Ch. & Freund, A.: Mine Water & Innovative Thinking, 343 – 347; Sydney, Nova Scotia (CBU Press), 343 – 347. Conly, A. G., Lee, P. F., Godwin, A., Cockerton, S. 2008. Experimental Constraints for the Source of Sulfate Toxicity and Predictive Water Quality for the Hogarth and Caland Pit Lakes, Steep Rock Iron Mine, Northwestern Ontario, Canada. – In: Rapantova, N. & Hrkal, Z.: Mine Water and the Environment. – Paper #139; Ostrava (VSB – Technical University of Ostrava). (http://www.imwa.info/docs/imwa_2008/IMWA2008 _139_ Conly.pdf Accessed December 8, 2011) Gould, A.R. 2008. Water quality and toxicity investigations of two pit lakes at the former Steep Rock iron mines, near Atikokan, Ontario. Unpublished MSc thesis, Lakehead University, Thunder Bay, ON, Canada, 122 p. McNaughton, K.A. 2001. The limnology of two proximate pit lakes after twenty years of flooding. Unpublished MSc. thesis, Lakehead University, Thunder Bay, ON, Canada, p. 85. MNR (Ministry of Natural Resources). 1986. Report on the Surrender of Mining Claims By Steep Rock Inc., Steep Rock Lake Atikokan District., Ontario. 146. - 64 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 2011 Precambrian Field Camp Mapping in the Sawbill Lake Area, Cook County, Northeastern Minnesota MILLER, Jim, BROOKER, Ben, ASP, Kris, LEU, Adam, PARISI, Andrew, and SLETTEN, Dan, Precambrian Research Center, University of Minnesota Duluth, Duluth, MN 55812 As part of the 2011 Precambrian field camp, a crew of four students, a teaching assistant (Brooker) and an instructor (Miller) conducted five days of field mapping bedrock geology in the Sawbill Lake area of northeast Minnesota. This area is composed of intrusive and volcanic rocks formed during the 1.1 Ga Midcontinent Rift (MCR) in northeastern Minnesota. This year’s mapping project expanded on three previous capstone mapping projects conducted to the east (Frost et al., 2007; Blakely et al., 2009; Brooker et al., 2010). These mapping projects have focused on a previously unnamed, well differentiated layered mafic intrusion that forms the western part of the Brule Lake –Hovland gabbro of Miller et al. (2001). This intrusion is now called the Sawbill Lake intrusion (SbLI) since the intrusion is centered and well exposed in and around the large lake. A 1:12,000-scale bedrock geologic map (Asp et al., 2011) generated of a 3 mile by 3 mile area profiles the igneous stratigraphy of the lower two-thirds of the intrusion and the footwall rocks. A pdf version of this and other Precambrian field camp capstone geologic maps can be downloaded from the PRC website: www.d.umn.edu/prc/fieldcamp/ capstone. Prior to the detailed mapping conducted by the PRC field camp students, the general geology of this intrusion was poorly known. Township-scale outcrop mapping by Grout et al. (1959) showed the area to be dominantly composed of gabbroic to felsic intrusive rocks, mafic volcanic rocks, and minor volcanogenic sedimentary rocks. They noted that the gabbroic rocks are well layered and portions are particularly rich in Fe-Ti oxide. Reconnaissance mapping by Davidson (1972) designated the mafic intrusion as an olivine gabbro unit that he interpreted as an early intrusion relative to granophyric and anorthositic rocks in the area (Davidson, 1977; Davidson and Burnell, 1977). The regional geologic map of northeastern Minnesota, which focused on the geology of the Duluth Complex (Miller et al., 2001), incorporated Davidson’s (1972) mapping of the area, but his olivine gabbroic unit is now interpreted to be a relatively late intrusion that is roughly equivalent to the Beaver Bay Complex to the south. 2011 capstone mapping was conducted in the northern half of Sawbill Lake, and reached to the west into Handle and Java lakes and to the north into Ada Lake. Exposures along the shorelines of these waterways provided a nearly complete profile of the lower two thirds of the SbLI as well as a footwall composed of ferrodiorite of unknown affiliation. The internal structure of the SbLI defines a southeast plunging synform with foliation and intermittent layering dipping moderately to the south at the east end and moderately east at the south end. The igneous stratigraphy of the approximately 1.5 to 3 km-thick intrusion generally defines a progressively differentiated sequence. Unlike the complex mixture of medium-grained, augite troctolite to olivine gabbro and fine-grained, intergranular gabbroic rocks observed at the basal contact during last summer’s mapping two miles to the east (Brooker et al., 2010), the basal contact exposed in the Ada Lake area shows well foliated and intermittently layered troctolite and melatroctolite in sharp contact with a well foliated ferrodiorite. This contact is also exposed along the western shoreline of Sawbill Lake, but here melatroctolite is rare. The troctolite grade upward into an medium-grained, moderately foliated augite troctolite (Pl+Ol cumulate) unit which contains 2-3 oikocrysts of ophitic to subophitic augite. Exposed in Ada Creek and through the north central part of Sawbull Lake is the gabbroic unit rich in hornfels inclusions of basalt and volcanogenic sandstone that has been traced over six miles between Sawbill and Homer lakes. This inclusion-rich unit, which is exposed at the north end of Weird Lake, is overlain by a very thick sequence of foliated, intergranular oxide gabbro to olivine oxide gabbro (Pl+Cpx+Ox±Ol cumulate) exposed along the shorelines of northeastern Sawbill Lake. The cm- to m-thick layers in Fe-Ti oxides found in this unit to east are not very abundant in the Sawbill Lake area. Some occurrences of intermingled intrusions of diabase, ferromonzondiorite, and quartz monzonite found to the east were observed in the Sawbill Lake however. that mostly appear to be sill-like. The cumulus arrival of apatite which characterizes - 65 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the upper part of the SbLI was not observed in this map area, but was found to the south in the Burnt and Smoke Lake area (see Brooker and Miller, this volume). Following the 2010 capstone project, Ben Brooker chose to develop an MS thesis project at UMD that would integrated the previous mapping by Grout, Davidson, and the PRC field camp projects and conduct additional mapping to create a complete geological picture of the Sawbill Lake Intrusion. He has conducted petrographic studies of over 200 thin sections to characterize the mineralogic and textural attributes of the main rock types. To document the cryptic variation throught the SbLI, he has conducted mineral chemical analyses of olivine and pyroxene from a suite of samples collected along several profiles across the intrusion. As reported elsewhere in this volume (Brooker and Miller, 2012), the main topical objective of this study is to evaluate the genetic relationship between the lower troctolitic cumulates and the upper oxide gabbroic cumulates. Although these rock types could represent the progressive differentiation of a mafic magma, their separation by a screen of volcanic and interflow sedimentary rocks suggests that they may represent two discreet intrusions formed by different parental magmas. References Asp, K., Leu, A., Parisi, A., Sletten, D., Brooker, B., and Miller, J., 2011, Bedrock Geology of the Sawbill Lake area. Precambrian Research Center, PRC/MAP-2011-04, 1: 12,000. Blakely, S., Brown, A., Foley, D., Rowland, A., Stifter, E., and Miller, J., 2009, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2009-01, 1: 12,000. Brooker, B., Hadley, M., Markwood, L., Olson, J., Tomlinson, A., and Miller, J., 2010, Bedrock geology map of the Jack Lake and Weird Lake areas, Cook County, northeastern Minnesota. Precambrian Research Center, PRC/MAP-2010-05, 1: 12,000. Davidson, D.M., Jr., 1972. Eastern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota - A Centennial Volume. Minnesota Geological Survey, p. 354-360 Davidson, D.M., Jr., 1977, Cherokee Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-30, 1:24,000. Davidson, D.M., Jr., and Burnell, J.R., 1977, Brule Lake Quadrangle, Cook County, Minnesota: Minnesota Geological Survey, Miscellaneous Map Series, M-29, 1:24,000. Frost, S.J., Juda, N.A., and Miller, J., 2007, Bedrock Geology Map of Homer Lake and Adjacent Areas; Cook County, Northeastern Minnesota: University of Minnesota Duluth, Precambrian Research Center, PRC/MAP-2007-02, 1: 12,000. Grout, F.F., Sharp, R.P., and Schwartz, G.M., 1959, The Geology of Cook County Minnesota: Minnesota Geological Survey Bulletin 39, 163p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.E., 2001, Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Miscellaneous Map Series, M-119, scale 1:200,000 - 66 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Supercontinents during the Proterozoic – A paleomagnetic view with Keweenawan data (Lake Superior) as an example. PESONEN, Lauri J. and VEIKKOLAINEN, Toni, Division of Geophysics & Astronomy, University of Helsinki, FI-00014 Helsinki, Finland The importance of supercontinents in understanding the geological evolution of the Earth has been recently discussed in several articles (e.g., Pesonen et al., 2012, and references therein). Geological processes linked to the existence of supercontinents include concepts such as large igneous provinces (LIPs), mantle superplume events, low latitude glaciations (“Snowball Earth”) and the high obliquity-theory of the rotation axis, carbon isotope excursions, fragmentation of continental dyke swarms, truncations of tectonic belts and major rifts, matching of conjugate orogenic belts, episodic nature of distributions of magmatic activities, discoveries of Precambrian ophiolites and the concept of true polar wander. Three supercontinent assemblies (Pangaea 350-165 Ma, Gondwanaland 550-400 Ma, Rodinia ~ 1050-750 Ma) have existed during or since the Neoproterozoic. The two younger assemblies have been constructed by sea floor magnetic data (Pangaea), and by palaeomagnetic and biostratigraphic results supported by geology (Gondwanaland). A current debate concerns the relative positions of the continents in supercontinent Rodinia, and the timing of its assembly and breakup. The consequences of supercontinents on geological evolution of the Earth have led some people to look also for pre-Rodinian supercontinents. In this paper, we use the updated palaeomagnetic database combined with new geological information, to define the positions of the continents during Paleoproterozoic (2.5 - 1.5 Ga) and Mesoproterozoic eras (1.5 - 1.04 Ga). We do the reconstructions relying mainly on isotopically (U-Pb) dated high quality palaeopoles since this method provides strict information of ancient latitudes of the cratons throughout the Proterozoic time. The new database is both geographically and temporally more abundant than any previous database and contains 2813 observations. Before putting the continents to a supercontinent assembly, we have tested the validity of the geocentric axial dipole model (GAD) of the Paleo-Mesoproterozoic geomagnetic field using four methods. The tests yield support to the GAD-model, but do not rule out a ca. 5% non-dipole (octupole) field. During the Proterozoic, the continents lie predominantly in low to intermediate latitudes. The sedimentological indicators of palaeoclimate are generally consistent with the palaeomagnetic latitudes, with the exception of the Early Proterozoic, when low latitude glaciations took place on several continents. The Proterozoic continental configurations are also in agreement with current geological models of the evolution of the continents. Three large continental landmasses existed during the Proterozoic. The oldest one is the Neoarchaean Kenorland. The protracted break up of Kenorland during the 2.45-2.10 Ga interval is manifested by mafic dykes and sedimentary rift-basins on many continents. The two Meso- to Neoproterozoic supercontinents are Columbia (known also as Nuna or Hudsonland) and Rodinia, respectively, which were predominantly in moderate to low paleolatitudes during their life span, although during the Paleoproterozoic some parts of Columbia, notably India (Dharwar craton) and Australia (Yilgarn craton), occupied polar latitudes. The pre-Columbia orogenies were due to a complex set of collisions, rotations and transform or strike slip faultings that caused the orogenic belts to appear obliquely. The final amalgamation of Columbia did not happen until ca. 1.53 Ga. Columbia broke up at ca. 1.18 Ga during several rifting episodes, followed by a short period of independent drift of most continents. The youngest assembly is the Meso-Neoproterozoic supercontinent Rodinia, which was formed by continentcontinent collisions during ~ 1.10-1.00 Ga and which involved most of the continents. A new model for its assembly and configuration is presented, which suggest that multiple Grenvillian age collisions took place during 1.10-1.00 Ga (Fig. 1). In building up the Rodinia model, the 1.2-1.0 Ga apparent polar wander paths from Laurentia (Keweenawan Lake Superior data), Kaapvaal (Umkondo Igneos Province), Yilgarn (intrusive), dyke data from Baltica and North China, as well data from Siberian sediments are strikingly similar containg a large - 67 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 1. (a) Reconstruction of continents at 1.04 Ga showing the Rodinia configuration. Data available from Laurentia (L), Baltica (B), Amazonia (Am), Australia (A), Congo/SãoFrancisco (C-Sf), Siberia (S), India (I), North China (NC) and Kalahari (K). The Grenvillian age orogenic belts are shown in red. (b) Geologically made reconstruction of LaurentiaBaltica-Amazonia (= the 0.9 Ga SAMBA-assembly by Johansson, 2009). loop (“the Logan Loop”). The similarity suggests that the various cratons experienced very rapid drift during 1.15-1.10 Ga just before docking to Rodinia, or the whole Earth suffered a true polar wander, or “non-dipole” episode during that time. References Pesonen, L.J., Mertanen, S., Veikkolainen, T., 2012. Paleo-Mesoproterozoic Supercontinents - A Paleomagnetic View. GEOPHYSICA (in press). Johansson, Å., 2009. Baltica, Amazonia and the SAMBA connection-1000 million years of neighbourhood during the Proterozoic? Prec. Res., 175, 221-234. - 68 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 A catastrophic breach and inter-lake flow through the Marks Moraine via Brule Creek and Cedar Creek, Thunder Bay Region, Ontario: A small but significant detail of the eastern outflow history of Lake Agassiz. PHILLIPS, Brian, Department of Geography, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada, DEAN, Frederick, Research Technician, 3482 Rosslyn Road, Rosslyn, Ontario P7K 0P8, Canada, and ZANIEWSKI, Kamil, Department of Geography, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The search for the eastern outlets of glacial Lake Agassiz began with John Elson’s pioneering aerial reconnaissance in Northwestern Ontario (Elson, 1957). His air photo interpretation revealed six possible outlets: Brule Creek, Dog River, Kaiashk River, Kaministiquia River, Pitikigushi River and Pillar Lake. Field studies followed in the summer of 1961 (Zoltai, 1963). Stephen Zoltai was tasked with ground truthing Elson’s work, a project that would lead to the publishing of “The Surficial Geology of Thunder Bay”, Map S265 (1965a, Zoltai, 1965b). Zoltai concluded that two outlets, the Brule Creek spillway and the Kaministiquia River spillway (and the associated Dog River tributary) were not related to glacial Lake Agassiz. He concluded that “the Kaministiquia –Dog River spillway was functioning to carry away meltwater from the area of the Kaiashk Moraine, while the Brule spillway follows closely the outer margin of the Marks Moraine” (Zoltai, 1963 p.110). The remaining spillways would become the well known “Nipigon outlets” (Zoltai, 1965a,b, 1967; Elson, 1967; Teller and Thorleifson, 1983). Interest in the two all-but-forgotten spillways waned until Thorleifson (1996) brought about their revival in his review of Lake Agassiz history. Work by Minning at al (1994) began to reveal that Lake Agassiz levels were still high south and west of Lake Nipigon and the eastern “Nipigon outlets” could not have been functioning to draw down early Lake Agassiz levels. This lead Thorleifson (1996) to propose that the Matawin-Shebandowan outlet (into the Kaministiquia River spillway) and the Savanne River outlet (into the Dog River spillway) were active. Research in the Fort Frances area by Bajc et al (2000) further supported high Lake Agassiz levels along the Ontario/Minnesota border, prompting Teller et al to publish “Alternative Routing of Lake Agassiz Overflow During the Younger Dryas: New Dates, Paleotopography and a Re-evaluation” (2005). This lead to renewed interest in Glacial Lake Kaministiquia, the Kaministiquia spillway and the chronology of glacial and deglacial events in the Thunder Bay area (Loope, 2006; Loope et al, 2006). In 2009, Lowell et al published “Radiocarbon Deglaciation Chronology of the Thunder Bay, Ontario area and Implications for Ice Retreat Patterns”, in which a key point made was that ice at the Brule Moraine blocked the Matawin-Shebandowan outlet. The low point between the Arctic and Great Lakes watersheds was determined as between Squeers and Watershed lakes, and since the Brule Moraine was not breached here, they concluded that no overflow had taken place (Lowell et al 2009). However, the conclusion was based on digital elevation maps, and is considered in error by the present authors. Squeers and Watershed lakes lie at the top of Squeers Creek at 484m (1588’) ASL, while the Squeers Creek spillway, mapped by Mollard and Mollard (1980), runs at the bottom of the drainage line from 472m to 453m (1550’ to 1486’) ASL. Bajc (2000) established the level of Glacial Lake Agassiz in this very area, based on the South Burchell Lake delta grading to 472m (1550’) ASL. At 472m (1550’) there is a 2km (6562’) breach in the Brule Moraine, leading to Sawmill Bay on Lake Shebandowan. Mollard and Mollard (1980) mapped the deepest incision of the Squeers Creek spillway to be over 500m (1640’) wide. This breach area was used before and after the Steep Rock readvance (Dean and Phillips, 2011) and flow was through Lake Shebandowan, down the Shebandowan River and into Glacial Lake Kaministiquia, also a part of the Dog-Kaministiquia spillway. Burwasser (1981) determined three distinct levels of Glacial Lake Kaministiquia: 1450’ (442m), 1500’ (457m) and 1550’ (472m) ASL. Earlier, Zoltai (1963) had noted the highest level as 1540’ (469m) ASL. With the sill between Kashabowie and Lac de Milles Lac lakes close to this high level, it must be concluded that the 1540-1550’ (468-472m) level was during the peak of the Marquette readvance - 69 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 when ice was holding up Lake Kaministiquia and before breaching the Marks Moraine and ultimately flowing south into a pro-glacial lake. Phillips and Fralick (1994), determined from a perched delta north of Thunder Bay, that a post Marquette level of Glacial Lake Kaministiquia was 1500’ (457m) ASL, and Bajc (2000) noted 1510’ (460m) ASL as a Marquette ice held level of Glacial Lake Kaministiquia. This would have been when red clays stopped flowing west out of Glacial Lake Kaministiquia through Lac de Milles Lac to be deposited in front of the Hartmann Moraine (Zoltai, 1965b; Teller and Thorleifson, 1983). Loope (2006) suggested that Glacial Lake Kaministiquia could have drained catastrophically following retreat of Marquette ice. This could have been when the level first dropped to 1500’ (457m) ASL (Phillips and Fralick, 1994). This level would have been the start of the Brule Creek spillway (Elson, 1957), running out of the Brule Creek valley, and along the Marks Moraine and the valley of Cedar Creek. There, based on the characteristics of a feature named in this paper as the “Cedar Creek Delta”, the authors present evidence of a catastrophic breaching of the Marks Moraine, waters of Glacial Lake Kaministiquia flowing south into “Glacial Lake Cedar Creek”, initially at the 442m (1450’) level. Subsequently, Glacial Lake Kaministiquia lowered to the 442m (1450’) level (Burwasser, 1981), before finally breaching the Marks Moraine along the Kaministiquia River valley at Kakabeka (Phillips and Fralick, 1994; Loope, 2006) and draining away into the Upper and Lower Beaver Bay levels of Lake Superior (Phillips and Fralick, 1994). References Bacj, A.F., 2000. Glacial History and Regional Till Sampling in the Archean Greenstone Belt, Institute on Great Lakes Geology, Proceedings of the 46th Annual Meeting, Thunder Bay, Ontario, 46, 32pp. Bacj, A.F., Schwert, D.P., Warner, B.G. and Williams, N.E., 2000. Canadian Journal of Earth Sciences, 37: 1335-1353. Burwasser, G.J., 1981. Ontario Geological Survey, Miscellaneous Paper 94, 10pp. With Map P2203, Scale 1:50,000. Dean, J.F. and Phillips, B.A.M., 2011. Pre and Early Agassiz Outlets to the Western Superior Basin, in Miller, J.D., et al., Eds. Archean to Anthropocene: Field Guides to the Mid-Continent of North America, Geological Society of America Field Guide 24, p.317-349, doi:10.1130/2011.0024(15). Elson, J.A., 1957. Lake Agassiz and the Mankato-Valders Problem, Science, 126, 3281, 999-1002. Elson, J.A., 1967. Geology of Glacial Lake Agassiz, in Mayer-Oakes, W.J. Ed. Life, Land and Water, Winnipeg, University of Manitoba Press, 37-95. Loope, H.M., 2006. Deglacial Chronology and Glacial Stratigraphy of the Western Thunder Bay Lowland, Northwestern Ontario, Canada, MSc. Thesis, University of Toledo, Ohio. Loope, H.M., Fisher, T.G., Lowell, T.V. and Hajdas, I., 2006. Geological Society of America Annual Meetings, Philadephia, Proceedings and Program, 38 (7) 72. Lowell, T.V., Fisher, T.G., Hajdas, I., Glover, K., Loope, H., and Henry, T. 2009. Quat. Science Reviews 28, 1597–1607. Minning, W.G, Cowan, W.R., Sharpe, D.R. and Warman, T.A., 1994, Geological Survey of Canada Memoir 436, 74p. Mollard, D.G. and Mollard, J.D., 1980. Lac de Mille Lacs Area (NTS 52B NE) Northern Ontario Engineering Geology Terrain Study, 56, 28p. with Database Map, Lac de Mille Lacs Area, Districts of Rainy River and Thunder Bay, Ontario Geological Survey, Map 5074, Scale 1:100000. Phillips, B.A.M. and Fralick, P.W., 1994. Journal of Great Lakes Research 20 (2), 390-406. Teller, J.T. and Thorleifson, L.H., 1983. The Lake Agassiz-Lake Superior Connection in Teller, J.T. and Clayton, C., Eds., Glacial Lake Agassiz, Geological Association of Canada Special Paper 26, 261-290. Teller, J.T., Boyd, M., Yang, Z., Kor, P.S.G. and Faro, A.M., 2005. Alternative Routing of Lake Agassiz during the Younger Dryas; New Dates, Paleotopography and Re-evaluation, Quaternary Science Reviews, 24, 1890-1905. Thorleifson, L.H., 1996. Review of Lake Agassiz History, in Teller, J.T., et al.,Eds., GAC Field Trip Guide Book for GAC/ MAC of Canada Joint Annual Meeting, Winnipeg, 55-84. Zoltai, S.C., 1963. Glacial Features of the Canadian Lakehead Area, The Canadian Geographer, 7, 3, 101-115. Zoltai, S.C., 1965a. Glacial Features of the Quetico-Nipigon Area, Canadian Journal of Earth Sciences, 2, 247-269. Zoltai, S.C. 1965b. Thunder Bay - Surficial Geology, 1:506,880, Map S265, Ontario Department of Lands and Forests. Zoltai, S.C., 1967. Eastern Outlets of Lake Agassiz, in Mayer-Oakes, W.J., Ed., Life, Land and Water, Winnipeg, University of Manitoba Press, 107-120. - 70 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 A metallogenic model and geometallurgical treatment of the Basal Iron Ores within the Negaunee Iron Formation, Tilden Mine, Michigan PIETRZAK-RENAUD, Natalie, Earth Sciences Department, Western University, 1151 Richmond Street, London, Ontario, N6A 3K7, Canada The complex paragenesis of the Tilden ores can be tied to the regional tectonic framework. Variable mineral chemistry and textures of basal Negaunee iron ores mined in the Main Tilden Pit have led to metallurgical difficulties. Core-logging and detailed petrography supported by microprobe investigations, identify three upward fining lithofacies within the Main Pit Carbonate and overlying Martite ore domains: 1) Basal Clastics; 2) Medial BIF; and 3) Granular Iron Formation. Growth fault related subsidence controlled deposition of the Basal Clastics, comprised of detrital quartz dispersed in a matrix of chlorite, cemented by initial ferri-hydrite, chert and Mg-siderite. Subsequent starvation of any clastic input led to cyclic iron-silica precipitation throughout the deposition of Medial BIF. Increasing wave action accompanied marine transgression caused deposition of granular rip-up clasts to form the Upper Granular Iron Formation. These domains are cross-cut by numerous chloritized feeder dykes and are capped by a greenstone Pillar, indicating overlapping mafic magmatism. Low grade regional metamorphism attending the 1850 Ma Penokean arc-continent collision led to magnetite growth, carbonate grain coarsening and chlorite crystallization. Metamorphic fluids facilitated martite replacement of magnetite and deformation led to developing local platy specularite schists. A late retrograde hydrothermal overprint post-dates peak thermal conditions that accompanied the development of the 1750 Ma Republic Metamorphic Node. This is expressed by high-Fe chlorite, Fe-dolomite/ankerite, zoned Mn-rich siderite with associated trace Cu-Fe sulphide and REE bearing fluro-apatite and monazite, diagnostic of an “IOCG” type signature. The development of a metallogenic model accounting for the multistage paragenetic sequence has shown that textures and mineral chemistries reflect a complex evolution. Bulk chemical analyses do not always reliably predict concentrate chemistries. Textural variations and mineral speciation of the different lithofacies can affect grinding and liberation and must be taken into account. Therefore, ore treatment processes are more effective when the genesis of the ore deposit is fully understood. This complex paragenetic history accounts for the unpredictable geometallurgical response of the basal Negaunee iron ores. Treatment difficulties relate to: 1) variable silica-iron separation that occurs due to liberation from detrital quartz versus massive mosaic and granular textured chert; 2) the bulk iron content is expressed not only in iron oxide but in iron carbonate and iron chlorite; and 3) the intense late IOCG overprint proximal to the Southern Shear Zone. - 71 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetic, rock magnetic and geochemical data from the Keweenawan dykes and baked rocks from the Little Mountain and Sugar Loaf areas, Michigan, USA PIISPA, Elisa, SMIRNOV, Aleksey, Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, 630 DOW ESE Building, Houghton, MI 499311295, USA, PESONEN, Lauri, Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, PO Box 64, FIN-00014 Helsinki, Finland, and DIEHL, Jimmy, Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, 630 DOW ESE Building, Houghton, MI 49931-1295, USA Rock magnetic, paleomagnetic and geochemical data from the Keweenawan diabase dykes from the Little Mountain (Baraga County) and Sugar Loaf (Marquette County) areas are presented. The study areas are located in the Upper Peninsula of Michigan where the dykes intrude metasedimentary and metavolcanic rocks of Animikean age, forming part of the Marquette Range Supergroup. This supergroup unconformably overlies a dominantly granitic basement of Archean age. At least three Keweenawan dyke swarms crop out in the area. Most of the Middle Keweenawan units have dual polarities and there is a tendency of the R polarity units to be slightly older according to radioisotope dating, paleomagnetic studies, and stratigraphic evidence. Previous paleomagnetic studies have shown that the E-W trending Baraga and Marquette dykes are of reversed polarity, with minor but significant differences in their paleomagnetic and physical properties. Our study reveals the presence of normal polarity dykes in the study area. Based on cross-cutting relationships, they are younger than the R-polarity dykes. The top-hill exposure of the Little Mountain, where the cross-cutting of the dykes takes place, is complex: the rocks have been hit by lightning and the dykes and their baked contact rocks are of mixed polarity. In contrast, the down-hill sites, where the N and R dykes are more distant from each other, they reveal either N- or R-polarities (the NE-SW and the E-W trending dyke respectively). The R-polarity and N-polarity dykes have slightly different petrophysical, rock magnetic and paleomagnetic properties. Geochemical signatures of the two dykes are also different: typically the R-polarity dykes have higher Fe- and lower Al- and Mg-values than their N-polarity counterparts. In addition, the R-polarity dykes of the Baraga County are paleomagnetically, petrophysically and geochemically slightly different than the R-polarity Marquette dykes. The same is not true for N polarity dykes from the two areas. The differences between the R polarity dykes may reflect a small difference in age (Baraga dykes being older). The new paleomagnetic and geochemical data together with cross-cutting relationships also hint that an even older N-polarity Keweenawan dyke event is present in the Sugar Loaf area. These older dykes may represent the earliest stage of the intrusive activity associated with the Mid-Continental Rift. - 72 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The ca. 1650 Ma Freedom Formation, Baraboo Range, Wisconsin: A late iron formation in the Lake Superior Region ROE, Carly and BJØRNERUD, Marcia, Geology Department, Lawrence University 711 E Boldt Way, Appleton WI 54911, USA, bjornerm@lawrence.edu Iron formation deposition in the Lake Superior region is typically considered to have ceased by about 1850 Ma, with the end of deposition of Animikie Group iron formations including the Gunflint, Biwabik, and Ironwood Formations in western Ontario, northern Minnesota, Wisconsin, and the Upper Peninsula of Michigan. However, a little-known iron formation occurs within the latest Paleoproterozoic (ca. 1650 Ma) Freedom Formation, part of the Baraboo sequence of southern Wisconsin. The Freedom Formation does not occur in outcrop, but was the target of iron mining in the early 1900s, and drill cores taken more than a century ago were recently acquired by the core repository of the Wisconsin Geological and Natural History Survey (WGNHS). The Freedom Formation, which is at least 300 m thick, lies conformably above the Seeley Slate, which in turn overlies the well-known Baraboo Quartzite (Dalziel and Dott, 1970). According to a 1904 WGNHS report by Samuel Weidman, the Freedom Formation includes interbedded ferruginous slates, ferruginous cherts, and iron-rich dolostones, and is made up almost entirely of quartz, dolomite, hematite, magnetite, kaolinite and chlorite. The lower half of the formation consists mainly of thinly bedded slates and cherts and lesser dolomite, as well as localized stratiform bodies of iron ore (mainly hematite) with up to 65% Fe. The upper half of the formation is predominantly dolomitic. We studied core samples originally drilled for the Cahoon Iron Mine (43.4470 N, 89.7565 W), on the south limb of the Baraboo syncline about 3 km northwest of Devils Lake. The cores had been stored for at least 80 years in an abandoned mine building before they were obtained by the WGNHS, and no related documents other than depth tags were preserved. As a result, we are not sure of the precise stratigraphic position of the cores we examined, but the lithologies present – primarily ferruginous slate and chert – are consistent with Weidman’s (1904) descriptions of the lower half of the Freedom Formation. Most of the rock in the cores shows clastic textures and mm-cm scale layers of quartz, magnetite, and lesser carbonate. In places, the magnetite is altered to hematite. The layers are somewhat irregular or lensoid, commonly varying in thickness even across the width of the 1.5 cm cores. In thin section, the magnetite is seen to occur in mm-scale clumps or granules that are subspherical but lack any obvious concentric banding that would indicate they formed as ooids. These textures are similar to those observed in classic Lake Superior granular iron formations (GIFs). Unlike banded iron formations (BIFs) sensu stricto, which represent laminated chemical muds, GIFs originated largely as well-sorted sands, most likely as a result of intrabasinal erosion and redeposition of pre-existing iron formation, and are younger than most BIFs (Simonson, 2003; Bekker et al., 2010). XRF analysis of trace metals in the Freedom Formation show Fe-normalized concentrations comparable to those in the Ironwood Iron Formation, a GIF-type deposit. In contrast, the chemistry, mineralogy and textures of the Freedom Formation do not resemble Phanerozoic ironstones, which tend to be highly oolitic, with chamosite, glauconite and/or pyrite as major mineral constituents (van Houten and Bhattacharyya, 1982). The geochemical factors leading to deposition of the Freedom Formation are not clear. The Baraboo Quartzite and Seeley Slate are both supermature sedimentary units deposited in a shallow marine environment adjacent to a rhyolitic/granitic landmass that had undergone intense and sustained tropical weathering (Medaris et al., 2003). The contact between the Seeley Slate and Freedom Formation seems to be conformable and gradational, suggesting that the Freedom Formation was deposited under similar conditions. While Phanerozoic ironstones are also thought to have accumulated in tropical settings with intense chemical weathering, they typically occur at the base of transgressive sequences and represent protracted periods of sea level lowstand (van - 73 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Houten and Bhattacharyya, 1982). This does not appear to have been the case for the Freedom Formation, which is underlain by more than 1000 m of uninterrupted marine sediments. The Freedom Formation therefore seems to be more closely related in genesis to other (earlier) Paleoproterozic GIFs than to Phanerozoic ironstones. If this is the case, its existence challenges models that point to an end for iron formation deposition at ca. 1850 Ma as a result of deep ocean euxinia (Canfield, 1998), the Sudbury impact (Slack and Cannon, 2009) or cessation of komatiitic volcanism (Wang et al., 2009). Instead, we suggest with Planavsky et al. (2011) that ferruginous ocean conditions and GIF deposition may have persisted well after early Paleoproterozoic time. Remaining questions about age of the Baraboo Sequence and the timing of its deformation are relevant to understanding the significance of the Freedom Formation. The youngest detrital zircons in the Baraboo Quartzite indicate that it can be no older than 1710 Ma (Medaris et al., 2003). There are, however, no direct constraints on the ages of the overlying units nor on the timing of the Baraboo folding event, although indirect arguments suggest that it occurred at ca. 1630 Ma (Holm et al., 1998). 40Ar/39Ar analyses of muscovite in breccias within the Baraboo quartzite consistently yield ages in the range of 1470-1450 Ma (Medaris et al., 2003), and it is possible that the deformational event could have occurred as late as this time (van Lankvelt and Bjørnerud, 2010). If so, the Freedom Formation could be as young as Mesoproterozoic and may indicate that Paleoproterozoic-type ocean chemistry prevailed much longer than previously thought. References Bekker, A., et al., 2010. Economic Geology, v. 105, p. 467-508. Canfield, D., 1998. Nature, v. 396, p. 450-453. Dalziel, I., and Dott, R. H. Jr., 1970, WGNHS Information Circular no. 14. Holm, D., Schneider, D., and Coath, C., 1998, Geology, v. 26, p. 907-910. Medaris, L. G. Jr. et al., 2003, Journal of Geology, v. 111, p. 243-257. Planavsky, N., et al., 2011, Nature, v. 477, p. 448-451. Simonson, B., 2003. Geological Society of America Special Paper 30, p. 231-241. Slack, J., & Cannon, W., 2009, Geology, v. 37, p. 1011-1014. van Houten, F. and Bhattacharyya, D., 1982, Ann. Rev. Earth Plan. Sci., v. 10, p. 441-457. van Lankvelt and Bjørnerud, 2010, ILSG Proceedings, v. 56, p. 67-68. Wang, Y., et al., 2009. Nature Geoscience, v. 2, p. 781-784. Weidman, S., 1904, WGNHS Bulletin XIII, Economic Series 8. - 74 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The tectonometamorphic, magmatic and mineralization history of the Wanapitei Complex, Grenville Front tectonic zone, Ontario ROUSELL, D.H., PETRUS, J.A., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6 Canada drousell@laurentian.ca, japetrus@gmail.com, EASTON, R.M., Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 mike.easton@ontaro. ca, TINKHAM, D.K., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6 Canada, dtinkham@laurentian.ca and NAPOLI, M.G.,Vale, Copper Cliff, ON mars.napoli@vale.com This presentation summarizes the results of new fieldwork and U-Pb geochronology from the Wanapitei Complex (WC), located 13 km ESE of Sudbury (Fig. 1). The WC is elliptical in plan view (6 x 2.5 km) and is located 400 m SE of the Grenville Front (Fig. 1). The WC is surrounded by a variety of gneissic rocks of the Grenville Front tectonic zone, including orthogneiss, paragneiss, amphibolite, migmatite, mylonite and breccia. All marginal contacts of the WC are sheared and no chilling is present near the margins. All new U-Pb data reported herein were obtained from the laser-ablation -ICP-MS facility at Laurentian University. The WC is divided into three segments each separated by a covered interval: a large NW and smaller SE and NE segments (Fig. 2). The WC was intruded twice by felsic dikes and twice by mafic dikes, subsequently underwent local folding and shearing, and finally was cut by late NWtrending pegmatite dikes (Napoli, 2003). The latter, dated regionally at 979±3 Ma (Easton et al., 1999), strike parallel to a major joint set and cut all rocks of the WC as well as a hornblende-plagioclase gneiss body in the SE segment. Within the SE segment a deformed, sill-like, hornblendeplagioclase gneiss body (4.4 x 0.36 km) trends NE and is in   sheared contact with the WC (Fig. 2). A cluster of concordant Figure 1. Regional setting of the Wanapitei Complex zircons from it yields a Concordia age (Ludwig 1998) of 2377±4 Ma (site 2, Fig. 2), similar to the age of the Creighton granite (Smith, 2002). This age represents a minimum age for emplacement of the body. The WC in the SE segment consists of olivine norite, norite and hornblende gabbro. Olivine and plagioclase occur as phenocrysts enclosed by hypersthene and augite, suggesting that the former two minerals are cumulates, subsequently enclosed in post-cumulus pyroxene. The olivine norite is also characterized by coronas around olivine and hypersthene, resulting from the reactions (1) olivine + plagioclase a bronzite + pargasite-spinel; (2) olivine + pyroxene a bronzite; and (3) pyroxene + plagioclase a pargasite-spinel. Other mafic rocks include norite and gabbro with varied amounts of secondary hornblende. Discordant zircon from a hornblende norite yielded an upper intercept age of 1747+12/-6 Ma (Prevec, 1993). In the NE segment of the WC, toward the margin of the complex, gabbro grades into recystallized gabbro and then into a breccia containing clasts of garnetiferous gabbro and infolded boudins of country rock. The bulk of the NW section of the WC consists of gabbro which was intruded by porphyritic quartz monzonite emplaced as large dikes and anastomosing vein systems which form an intrusion breccia. A sample of one of these dikes yielded a single population of concordant zircons with a Concordia age of 1707±4 Ma (site 1, Fig. 2), slightly younger than the age of 1747+12/-6 Ma from the same unit reported in Easton et al. (1999). To the NE, the breccia passes into garnet metagabbro, which has a Concordia age of 1735±3Ma (site 4, Fig. 2). Finegrained mafic dikes (68°/90°), typically 1 m wide with non-chilled contacts, are widely distributed in the WC. Locally garnetiferous mafic dikes with chilled margins cut injection breccia in the WC, and a sample yielded 2 - 75 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Figure 2. Geological map of the Wanapitei Complex showing the location of new U-Pb sample locations (sites 1 to 4). Map modified from Rousell and Trevisiol (1988).   clusters of concordant zircons, with Concordia ages of 1694±7 Ma and 1640±10 Ma, respectively (site 3, Fig. 2). Discordant zircons from the same sample define a Pb loss line trending toward 1000 Ma. Mineral exploration in the WC began in the 1940s, and is ongoing. The SE and NE segments of the WC contain 17 occurrences of sulphide mineralization (pyrrhotite, chalcopyrite, pentlandite, pyrite) all hosted by norite. The sulphide minerals are interstitial to the silicates, are inter-cumulus, and of primary magmatic origin. Although not all rocks of the WC are deformed, all are metamorphosed to upper amphibolite facies, with the possible exception of the olivine norite (do the coronas reflect subsolidus reactions or metamorphic overprinting?). Based on new and previous geochronology, the WC likely crystallized deep in the crust from multiple magma injections between 1747 and 1707 Ma during Killarney Belt magmatism. It was subsequently emplaced into its current position, in the solid state, along shear zones near the end of the Grenville Orogeny. During this emplacement, it was dismembered into three segments, with later independent tilting and/or rotation, and may be the remnant of a much larger intrusion. References (abbreviated titles) Easton, R.M., Davidson, A., and Murphy, E.I. 1999. Guidebook A2, GAC Sudbury’99, 52 p. Ludwig, K.R., 1998. On the treatment of concordant uranium-lead ages; GCA, v.62, p.665-676. Napoli, M.G. 2003. Geology of the Wanapitei Complex; MSc, Laurentian University, Sudbury. Prevec, S.A. 1993. Early Proterozoic mafic intrusions; PhD thesis, Univ Alberta, Edmonton, 223p. Rousell, D.H. & Trevisiol, D.D. 1988. Wanapitei complex; Mineralium Deposita; v.23, p.138-149. Smith, M.D. 2002. The timing and petrogenesis of the Creighton pluton; MSc, Univ Alberta, 123p. - 76 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Geologic setting of the Kevitsa Ni-Cu-PGE Deposit, Central Lapland Greenstone Belt, Finland SANTAGUIDA, F., LAPPALAINEN, M., JONES, S., VOIPIO, T., SIIKALUOMA, J. and YLINEN, J. First Quantum Minerals Limited. 1 Kaikutie, Sodankylä, Finland The Kevitsa Ni-Cu-Pt-Pd Mine is a new operation in northern Finland commencing full production in 2012. The current measured and indicated mineral resource (NI 43-101 compliant) for the deposit is 240Mt @ 0.30% Ni, 0.41% Cu, 0.21 gpt Pt, 0.15 gpt Pd, 0.11 gpt Au using a 0.1% Ni cutoff value. Additionally, an inferred resource of 34.7 Mt exists with comparable grade. The deposit is located within the Central Lapland Greenstone Belt; a Paleo-Proterozoic volcano-sedimentary sequence. Ultramafic intrusive and volcanic rocks are widespread and occur at several stratigraphic levels within the greenstone belt sequence (Hanski et al., 2001). The host ultra-mafic intrusion at Kevitsa has been dated at 2058 ± 4 Ma and is considered in general to be 2050 Ma in age based on dates from other cross-cutting dykes (Mutanen and Huhma, 2001). Mineralization consists largely of disseminated sulphides defined as Regular Ore, False Ore, Ni-PGE Ore and Contact Ore. Sulphide minerals are dominantly pyrrhotite, pentlandite, and chalcopyrite; although pyrite is also locally present. The Regular Ore constitutes approximately 95% of the resource. The deposit is hosted within a thick (> 1km) peridotite rock unit segregated as olivine pyroxenite, websterite and plagioclase-bearing pyroxenite (Figure 1). A ubiquitous amphibole overprint has replaced the primary minerals, affecting mainly clino-pyroxene, making rock identification locally impossible in hand specimen and in drill core. A crude internal layering is evident within the peridotite highlighted by horizons of sulphide mineralization. Individual sulphide lenses are 10s to 100s meters in thickness typically associated with the olivine pyroxenite layers, suggesting emplacement of magmas as pulses. Olivine-rich, “dunite”, xenoliths occur locally and are aligned within discrete zones discordant to the internal layering. In places, the dunite is seen as irregularshaped masses intercalated with the peridotite groundmass. The Ni-PGE Ore is spatially associated with these dunite zones. Dunitic rocks also occur below the peridotite unit, but are poorly defined. The peridotite unit is overlain by gabbro which forms a comparatively thin lithologic unit (typically < 200m) in the SW portion of the intrusion. The dunite, peridotite and gabbro geochemically occur as a fractionation series. A large, discrete, serpentinitized dunite body exists within the central portion of the intrusion that is geochemically distinct from the main Kevitsa body and is un-mineralized. Presently, this body is not well understood, but is considered to cut the Kevitsa peridotite unit. The immediate country rocks surrounding the Kevitsa Intrusion consist of mafic volcanic flows and tuffs interlayered with clastic metasediments and sulphide-rich carbonaceous schists. Continuing exploration to depth has improved the 3D geologic model of the deposit and of the intrusion. Both 2D- and 3D-seismic surveys have been utilized to delineate regional- and mine-scale structures as well as to define the basal Kevitsa Intrusion contact in un-drilled areas. Several new research initiatives have commenced recently to aid the exploration effort and in general will better position the Kevitsa deposit within the current Ni-Cu-PGE genetic models. - 77 - �Proceedings of the 58th ILSG Annual Meeting - Part 1     Figure 1. Bedrock geology of the Kevitsa Intrusion area defined by surface mapping and diamond drilling. The surface projection of the complete Ni-Cu-PGE orebody is shown by the dotted line. The geologic cross-section is drawn along the dashed line (drill section 12150N); note: the cross section is an expanded scale from the map. References Hanski, H., Huhma, H., Rastas, P., and Kamenetsky, V., 2001. The Paleoproterozoic komatiite-picrite association of Finnish Lapland. Journal of Petrology, 42: 855-876. Mutanen, T. and Huhma, H., 2001. U-Pb geochronology of the Koitelainen, Akanvaara and Keivitsa layered intrusions and related rocks. Geological Survey of Finland, Special Paper 33: 229-246. - 78 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Exploration history, mmineralogy and genesis of the Black Thor chromite deposit, Ring of Fire Intrusion, Northwestern Ontario Scott, G.W., Karakus, M., Shinkle, D. A. and Mitchell, A. R., Cliffs Natural Resources Inc., Thunder Bay, Ontario The McFaulds Lake Black Thor chromite deposit, an emerging world class chromite deposit, is located in northwestern Ontario approximately 550 km north of the City of Thunder Bay in the Ring of Fire greenstone belt. The initial discovery of the deposit was made by Freewest Resources Inc. in 2008 when a diamond drill hole intersected approximately 100 m of massive chromitite. Subsequent geophysical surveys and drilling have outlined a deposit amenable to open pit mining 400 m deep with a strike length of approximately 3 km, occurring in vertically dipping, sheet-like zones with mineable widths between 50 and 100 m. Chromite mineralization has been confirmed beyond the open pit zone to a depth of approximately 700 m and remains largely open downdip. Cliffs Natural Resources (CNR) acquired Freewest in 2010, and is currently proceeding with a definitive feasibility study and environmental assessment for the project. The chromite mineralization is hosted by a multi-phase layered ultramafic intrusion (Ring of Fire Intrusion or RFI) consisting of: peridotite, olivine cumulates, pyroxenite and gabbro occurring in three discrete fault bounded domains: the Southwest Domain, the Central Domain and the Northeast Domain. All domains feature some degree of layering and repetition of massive chromitite horizons. These features may be attributed a combination of primary magmatic and structural origins. Magma mixing, sulphide removal and sialic crustal contamination are possibilities with respect to the saturation of the magma in chromium. The volume of chromite mineralization in the deposits does not balance with the magma volume of the delineated extent of the RFI. Emplacement in a magma conduit has been proposed as a possible physical mechanism for the concentration of chromite mineralization of the magnitude represented by the deposits. Samples from drill holes in the Northeast Domain and from the Southwest Domain were studied at Cliffs Technology Laboratories to determine the bulk chemical composition, chemical composition of chromite, typical chromite textures, free magnetite content and gangue mineralogy. The Cr2O3 content of chromite grains increases from 45% in disseminated mineralization to over 52% in massive chromitite. Similarly, Cr/Fe ratio of chromite grains increases from 1.45 in disseminated mineralization to 1.96 in massive chromitite. Chromite mineral content ranges from as low as 7.8% in disseminated to more than 88% in massive chromitite. Semi-massive mineralization contains 35-63% chromite. The gangue minerals in disseminated mineralization are primarily sjoegrenite, lizardite and to a lesser extent antigorite, whereas clinochlore was the primary gangue mineral in massive chromitite. Disseminated mineralization zones were also characterized by the presence of magnetite and absence of talc-carbonate veins. Several distinct types of chromite textures are recognized. They are (#1) un-deformed and unaltered chromite, (#2) un-deformed chromite with slight surface alteration, (#3) deformed and fractured chromite but with preserved grain outline, (#4) intense cataclastic textured chromite, (#5) completely silicate altered chromite, (#6) chromite with many gangue or sulfide inclusions, and (#7) very fine precipitated chromite with very high chromia content. Types #1-#5 are the most common textures and can occur within the same drill core. The mean grain size of chromite grains in the massive type is slightly larger (200-220 µm) than in semi-massive (~160-167 µm) and in disseminated samples (180-190 µm). - 79 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Microstructural analysis of porcellaneous nepheline syenite, Coldwell Complez, Ontario SELAGI, Josh and HILL, Mary Louise, Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON Canada P7B 5E1 The Coldwell Complex is a large Proterozoic alkaline igneous complex on the north shore of Lake Superior west of Marathon, Ontario. From east to west across the complex, three distinct magmatic centres form an overlapping sequence; Center 2 is characterized by biotite-bearing alkaline gabbro and nepheline syenite (Mitchell and Platt, 1982). Within Center 2, a fine-grained porcelaineous variety of nepheline syenite is recognized as being chemically equivalent to the more typical coarse-grained nepheline syenite. This porcelaineous nepheline syenite outcrops in a narrow arcuate zone along the western shore of Redsucker Cove. Microstructural evidence for dislocation creep indicates that the porcelaineous syenite is a mylonite formed by localized solid-state deformation. As the Coldwell Complex has not been subjected to regional metamorphism, the high temperature required for ductile solid-state deformation in the porcelaineous syenite must be related to the magmatic activity. Localized high-temperature ductile strain within this arcuate zone could have led to catastrophic brittle failure along ring faults culminating in caldera collapse. This discovery may have implications for understanding the processes associated with caldera collapse in large alkaline igneous complexes. Reference Mitchell, R.H. and Platt, R.G., 1982, Mineralogy and petrology of nepheline syenites from the Coldwell Alkaline Complex, Ontario, Canada: Journal of Petrology, v. 23, p. 186-214. - 80 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Textural and compositional variation in apatite and plagioclase in the Marathon PGE-Cu deposit, Northwestern Ontario: Implications for fluid-rock interaction SHAHABI FAR, Maryam, SAMSON, Iain, GAGNON, Joel, Department of Earth and Environmental Sciences, University of Windsor, LINNEN, Robert, Department of Earth Sciences, Western University, and GOOD, David, Stillwater Canada Inc The Marathon PGE-Cu deposit is hosted within the Two Duck Lake gabbro (TDLG) of the Mesoproterozoic (1108 ± 1 Ma) Coldwell alkaline complex. Three zones of mineralization with different textural, mineralogical and geochemical characteristics have been identified: Footwall Zone, Main Zone, and W-Horizon. The relative roles of magmas and volatiles in Cu and PGE enrichment in this deposit remain a point of debate. The TDLG is coarse-grained to pegmatitic, and comprises plagioclase with variable amounts of interstitial ophitic clinopyroxene, olivine, and magnetite. Apatite, biotite and amphibole are common accessory minerals. Hydrothermal alteration is a common feature, especially in the Main and Footwall Zone, and is dominated by replacement of primary igneous minerals by amphibole, chlorite, biotite, and serpentine. Epidote and carbonate are also present as alteration minerals in some samples. Preliminary petrographic studies and SEM-CL imaging have revealed complex textures in both plagioclase and apatite, including evidence of dissolution and replacement, normal or reverse zoning, oscillatory zoning, and patchy zoning in plagioclase. Preliminary LA-ICP-MS analyses of various textural types of plagioclase and apatite have been conducted to evaluate any relationship between textural complexities and chemical variations. Magmatic plagioclase is characterized by a strong positive Eu anomaly, and has higher REE concentrations in the rims relative to the cores. Plagioclase with replacement rims occurs in the vicinity of quartz-feldspar granophyre patches, suggesting alteration by fluids exsolved from the granophyric melts. This is supported by the lower An content (An33-50) of the replacement plagioclase compared to the magmatic plagioclase that was replaced (An60-75). These rims also contain higher concentrations of Cu and Pb compared to magmatic plagioclase. Apatite occurs as euhedral to subhedral acicular and tabular prisms, and as anhedral crystals hosted by both altered and un-altered minerals. SEM-CL imaging shows that apatite crystals can exhibit growth (oscillatory) and replacement zoning, and EDS analyses indicate that the F and Cl contents are variable. In different rocks, apatite either predated (early apatite) or postdated plagioclase crystallization (late apatite). Apatite crystals from mineralized Main and Footwall Zone samples generally have high Cl contents, whereas apatite from the W-Horizon has low Cl contents, and generally have fluorapatite end-member compositions. Apatite crystals from the Footwall Zone (the lowest stratigraphic zone) have the highest Cl/F values, whereas those from the W-Horizon (the highest stratigraphic zone) show the lowest Cl/F. In addition, zoning, re-crystallization textures, and the presence of primary fluid inclusions are more common in apatite crystals from Main and Footwall Zone samples compared to the W-Horizon. Apatite consistently has a strong negative Eu anomaly, and early apatite has higher REE concentrations in the rims compared to the cores. The magnitude of the negative Eu anomaly, however, increases from core to rim. Late apatite has a REE abundance between that of the core and rim of early apatite. This suggests that late apatite probably crystallized from a new influx of magma rather than from continued crystallization of a resident, more fractionated magma. Replacement rims of apatite crystals show lower ∑REE, higher Cl, and higher transition metal (e.g., Ti, Fe, Cu) contents compared to early apatite. The similar REE core to rim evolution of the apatite and plagioclase suggests that early apatite and plagioclase coprecipitated; similarly, the sequestration of Eu by plagioclase resulted in the decrease in Eu* in apatite from core to rim. The observed spatial changes in Cl/F in apatite can be explained by compositionally distinct magma pulses, responsible for each mineralized zone, as most apatite crystals are euhedral to subhedral, formed early in the paragenetic sequence, and are magmatic. Intracontinental basaltic melts, however, (cf. Bushveld, and Stillwater complexes) are reported to be poor in Cl. Thus, apatite crystallizing from such a melt is expected to - 81 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 be fluorapatite. Much of the chalcopyrite in the Main Zone is reported to replace pyrrhotite and is intergrown with hydrous silicate minerals, which suggests that Cu was introduced into the system by fluids. This observation can be explained by a process in which fluids flux through the Footwall Zone, and transport Cu to the Main Zone during its upward migration. Therefore, a zone-refining process in which volatiles, derived from footwall country-rock dehydration, migrated up through the crystallizing gabbros is an attractive alternative by which Cl could be added to the system. The low Cl contents of apatite in the W-horizon can be explained if these fluids did not migrate upward into the W-Horizon, or the W-Horizon represents late-stage magma infiltration. Thus, the alteration of the primary igneous minerals, the re-crystallization textures, high Cl/F exhibited in apatite within the Footwall and Main Zones, and higher Cl and transition metal contents of the apatite replacement rims can be attributed to the interaction of a fluid phase at suprasolidus and/or subsolidus conditions. References Barrie, C.T. et al. 2002, Contact-type and magnetitite reef-type Pd-Cu mineralization in ferroan olivine gabbros of Coldwell Complex, Ontario, in Canadian Institute of Mining, Metallurgy and Petroleum, p. 321-337. Boudreau, A.E. & Hoatson, D.M., 2004, Halogen variations in the Paleoproterozoic Layered mafic-ultramafic intrusions of east kimberley, western Australia: Implications for platinum group element mineralization, Economic Geology, 99(5), p. 1015-1026. Boudreau, A.E. & Kruger, F.J., 1990, Variation in the composition of apatite through the Merensky cyclic unit in the western Bushveld Complex, Economic Geology, 85(4), p. 737-745. Boudreau, A.E. & McCallum, I.S., 1989, Investigations of the Stillwater Complex: Part V. Apatites as indicators of evolving fluid composition, Contributions to Mineralogy and Petrology, 102(2), p. 138-153. Boudreau, A.E., Mathez, E.A. & McCallum, I.S., 1986, Halogen geochemistry of the Stillwater and Bushveld Complexes: evidence for transport of the platinum-group elements by Cl-rich fluids, Journal of Petrology, 27(4), p. 967-986. Dahl, R., Watkinson, D.H. & Taylor, R.P., 2001, Geology of the Two Duck Lake intrusion and the Marathon Cu-PGE deposit, Coldwell Complex, Northern Ontario, Exploration and Mining Geology, 10(1-2), p. 51-65. Ginibre, C., Worner, G. & Kronz, A., 2007, Crystal zoning as an archive for magma evolution, Elements, 3(4), p. 261-266. Good, D.J. & Crocket, J.H., 1994a, Genesis of the Marathon Cu-platinum-group element deposit, Port Coldwell alkalic complex, Ontario; a Midcontinent rift-related magmatic sulfide deposit, Economic Geology, 89(1), p. 131-149. Good, D.J., 2010, Abstracts, 11th International Platinum Symposium, Applying multistage dissolution upgrading and 3d-GIS to exploration at the Marathon Cu-PGE deposit, Canada. p. 21-24. Heaman, L.M. & Machado, N., 1992, Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex, Contributions to Mineralogy and Petrology, 110(2), p. 289-303. Ruthart, R.G., Linnen, R.L., Samson, I.M. & Good, D.J., 2010, Abstracts, 11th International Platinum Symposium, Characterization of high-PGE low-Sulphur mineralization at the Marathon PGE-Cu deposit, Ontario. Samson, I.M., Fryer, B.J. & Gagnon, J.E., 2008, The Marathon Cu-PGE deposit, Ontario: Insights from sulphide chemistry and textures, Geochimica et Cosmochimica Acta Supplement, 72, p. 820. Watkinson, D.H. & Ohnenstetter, D., 1992, Hydrothermal origin of platinum group mineralization in the Two Duck Lake intrusion, Coldwell Complex, Northwestern Ontario, Canadian Mineralogist, 30, p. 121-136. - 82 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Regional to microstructural control of gold mineralization along the QueticoWabigoon subprovince boundary STINSON, Victoria R., and HILL, Mary Louise, Dept. of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1 Canada Gold occurrences in Greenstone, Ontario are located along major regional shear zones, along the Quetico and Wabigoon subprovince boundary within the Superior province. On the regional scale, gold mineralization occurs within anastomosing ductile to brittle-ductile shear zones in pressure shadows around competent plutons or in ductilely deformed fractures within the plutons. In outcrop and drill core gold mineralization is observed in areas of heterogeneous deformation, especially at rheological or lithological contacts, such as along an amphibolite-granodiorite contact within the Klotz Lake shear zone east of Longlac, Ontario. Microstructure mimics the regional structure, as gold is found in fractures in competent minerals as well as in pressure shadows around the competent minerals, such as the gold found in fractured garnet west of Caramat, Ontario. The regional shear zones in Greenstone are transcurrent, ductile to brittle-ductile shear zones that typically trend east-west, parallel to the Quetico-Wabigoon subprovince boundary. Deviations from this eastwest trend occur where shear zones anastomose around older, competent granitoid plutons, like the Elmhirst meta-granodiorite pluton north of Jellicoe. Areas of highest strain along the pluton margins exhibit evidence of dislocation creep, such as quartz ribbons and grain size reduction, and host sporadic gold mineralization. No gold mineralization is present in the interiors of the plutons where strain fabric is minimal or absent. The highest gold mineralization is present immediately adjacent of the zones of highest strain and within the competent plutons, usually in ductiley-deformed fractures or along rheological contrasts. Significant gold mineralization is also present in pressure-shadows around the deformed plutons, where complex folding is typically located. Pressure shadows that host gold are present on all scales, including the microscopic scale. Gold is commonly found in pressure shadows around garnet, plagioclase, hornblende, and pyrite throughout the QueticoWabigoon subprovince boundary. Hetereogeneous deformation on the microscopic scale hosts gold mineralization within fractures of competent minerals, such as pyrite and garnet, while the less competent matrix minerals, like quartz, deform more ductilely. Gold is typically located at the grain boundary between competent and less competent minerals. Fractured meta-granitic plutons adjacent to shear zones that host gold are analogues to gold found in fractured garnet within a schistose fabric, although on much different scales. Exploration for gold mineralization along the Quetico-Wabigoon subprovince boundary should continue west of Lake Nipigon and east of Longlac and focus on heterogeneous deformation within shear zones on all scales. Heterogeneous deformation continues north into the Quetico and south into the Wabigoon and so should the exploration for new gold occurrences. - 83 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Temporal context of the Mamainse Point succession: a record of magmatic activity and fast plate motion across the “latent stage” of Midcontinent Rift development SWANSON-HYSELL, Nicholas L., Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, 291 Shepherd Labs, 100 Union Street SE, Minneapolis, MN 55455, USA, BURGESS, Seth D., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA, MALOOF, Adam C., Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA and BOWRING, Samuel A., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA A popular model for the development of the Midcontinent Rift (MCR) proposes that there were four stages of magmatism: Early Stage (>1109-1106 Ma), Latent Stage (1106-1100 Ma), Main Stage (1100-1094 Ma) and Late Stage (1094-1086 Ma) (Miller and Vervoort, 1996; Vervoort et al., 2007). U-Pb dates of both extrusive (Davis and Green, 1997) and intrusive (Paces and Miller, 1993; Vervoort et al., 2007) sequences in the western part of the Lake Superior basin largely fall within either the early or main magmatic stages–leading to the interpretation that the period between them was a latent stage characterized by minimal eruptive activity. The early stage of MCR volcanism is characterized by magnetizations of reversed polarity while the main stage of MCR volcanism is characterized by magnetizations of normal polarity. In the Powder Mill Group and the North Shore Volcanic Group (NSVG), U-Pb dates on extrusive felsic units (Davis and Green, 1997, Zartman et al., 1997) reveal condensed stratigraphy during the time period of the latent magmatic stage accompanied by the switch from reversed to normal magnetizations. In contrast, the succession in the eastern Lake Superior basin exposed at Mamainse Point has been interpreted to represent a much more continuous record spanning nearly the entire duration of rift volcanism (Klewin and Berg, 1990). This interpretation of more continuous eruption in the eastern Lake Superior basin has stemmed, in part, from the presence of multiple geomagnetic reversals in the Mamainse Point succession (R-NR-N; first identified by Palmer, 1970) that have not been recognized in other extrusive successions in the rift. These reversals suggest that eruptive activity continued locally, albeit with some shorter duration lulls marked by conglomeratic units, while there was the hiatus of the latent magmatic stage elsewhere in the rift. Due to the large change in paleomagnetic inclination throughout the development of the MCR (Davis and Green, 1997, Swanson-Hysell et al., 2009), paleomagnetic data can be used to provide rough chronostratigraphic constraints through comparison of undated paleomagnetic poles to those with age control. Applying this approach with the paleomagnetic data set we have developed at Mamainse Point, we find that the lowermost flows at Mamainse Point correlate to those from the lower northeast sequence of the NSVG (the early magmatic stage with a U-Pb crystallization age of 1107.9±1.8 Ma determined from the intercept of a linear regression with the concordia curve; Davis and Green, 1997) and that the upper normal flows correlate with the upper southwest sequence of the NSVG (the main magmatic stage with concordia intercept U-Pb crystallization ages of 1096.6±1.7 Ma and 1098.4±1.9 Ma; Davis and Green, 1997). This result implies that the intervening stratigraphy at Mamainse Point spans the latent magmatic stage during which there were at least three reversals of the geomagnetic field. A U-Pb crystallization age of 1096.2±1.9 Ma for a felsic unit within the lower reversed zone at Mamainse Point was presented in an ILSG abstract and tentatively interpreted to be extrusive (Davis et al., 1995)—although the unit was mapped as a felsic intrusion by Giblin (1969). An extrusive interpretation implies that all the geomagnetic reversals within the succession occurred during the main magmatic stage—in conflict with the consistently normal magnetostratigraphy of the upper sequence of the NSVG and the Portage Lake Volcanics as well as the comparative analysis of paleomagnetic poles. New field observations document that this fine-grained unit with quartz phenocrysts has a cross-cutting relationship with the basalt flows such that the felsic unit both overlies and underlies the pahoehoe flow top of a single basalt flow. These observations indicate the unit is - 84 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 intrusive with its age providing a minimum rather than absolute age constraint on the lower reversed polarity zone at Mamainse Point. During a field season when fortuitously low lake levels led to excellent exposure of the stratigraphy in Flour Bay, we collected a crystal-rich tuff in close stratigraphic proximity to the base of the “Great Conglomerate” within the upper reversed polarity zone. We will present a new chemical abrasion TIMS U-Pb date from euhedral prismatic zircons from this tuff that anchors the Mamainse Point stratigraphy in absolute time and demonstrates that the record does indeed span much of the latent magmatic stage. These results strengthen the temporal framework of the Mamainse Point succession and allow for refined stratigraphic correlations across the rift. Furthermore, these results along with an expanded paleomagnetic data set support the interpretation that significant plate motion of North America was ongoing between the early and main magmatic stages that led to the decrease in the paleomagnetic inclination. References Davis, D., Green, J., and Manson, M., 1995. Geochronology of the 1.1 Ga North American Mid-Continent rift. Institute on Lake Superior Geology Proceedings, 41: 9-10. Davis, D. and Green, J., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Science, 34: 476–488. Giblin, P. E., 1969. Kincaid Township. Preliminary Geological Map, Ontario Department of Mines, 553. Klewin, K.W. and Berg, J.H., 1990. Geochemistry of the Mamainse Point volcanics, Ontario, and implications for the Keweenawan paleomagnetic record. Canadian Journal of Earth Sciences, 27: 1194-1199. Miller, J.D., Jr., and Vervoort, J.D., 1996. The latent magmatic stage of the Midcontinent rift: A period of magmatic underplating and melting of the lower crust. Institute on Lake Superior Geology Proceedings, 42: 33-35. Paces, J.B. and Miller, J.D., 1993. Precise and U-Pb ages of Duluth Complex and related mafic intrusions northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift system. Journal of Geophysical Research, 98: 13,997-14,013. Palmer, H.C., 1970. Paleomagnetism and correlation of some Middle Keweenawan rocks, Lake Superior. Canadian Journal of Earth Sciences, 7: 1410-1436. Swanson-Hysell, N.L., Maloof, A.C., Weiss, B.P., Evans, D.A.D., 2009. No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts. Nature Geoscience, 2: 713–717. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., and Harpp, K.S., 2007. The magmatic evolution of the Midcontinent Rift: New geochronologic and geochemical evidence from felsic magmatism. Precambrian Research, 157: 235-268. Zartman, R.E., Nicholson, S.W., Cannon, W.F. and Morey, G.B., 1997. U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior, 34: 549-561. - 85 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Quartz quarry sites in northwestern Ontario: A new ‘prospect’ for geoarchaeologists investigating lithic raw material sources TAYLOR-HOLLINGS, Jill, Department of Anthropology, 13-15 HM Tory Building, University of Alberta, Edmonton, AB, T6G 2H4 Flintknapping is the process of creating stone tools by utilizing percussion and pressure flaking techniques on siliceous minerals. Preservation of lithic materials in the archaeological record provides some of the best evidence for reconstructing past human activities. In the boreal forest of northwestern Ontario, Aboriginal people began flintknapping about 9500 years ago, as evident from dated contexts of stone tools, cores, and resultant flaking debris (debitage). Despite this lengthy time frame, there is limited knowledge of archaeological sites in general and even less information about primary, secondary, or tertiary sources that Aboriginal people accessed to obtain useable siliceous material for stone tool manufacturing. Archaeological quarries are typically large sites with dense clusters of tools, debitage, and diverse processing areas. However, few quarries sites have been found in the Canadian boreal forest. Over the last seven years, rare opportunities enabled eight brief archaeological surveys to take place on the Bloodvein River in Ontario’s Woodland Caribou Provincial Park (now enlarged into a Signature Site); these projects are part of ongoing collaborations with Ontario Parks as well as Pikangikum, Lac Seul, and Little Grand Rapids First Nations in their traditional territories (e.g., Taylor-Hollings, 2006). The Ontario portion of this Canadian Heritage river is about 106 km long and all projects required fly-in transportation to this remote area. These surveys enabled data collection for the author’s PhD research as well as park and community-based land use planning. The first survey indicated that the most common lithic artifacts in that area consisted of various forms of quartz: rock crystal, white, yellow, and smoky. However, it was unknown where this quartz was sourced. As Bakken (2011:124) suggests, “quartz is a complicated issue in regional raw material studies, and one that remains to be resolved”. Thus, while looking for archaeological sites, it was decided to look for quartz sources, such as vein quarries or pebbles and cobbles, since the latter forms are known from northern Minnesota sites (Bakken, 2011). As part of the larger Canadian Shield, the Bloodvein River lies within the North Caribou Terrane, a collage of ~3-2.8 billion years old diverse lithotectonic assemblages within the northern Archean Superior Province (Thurston et al., 1991; Corfu and Stone, 1998). This part of the terrane was formerly known as the Berens River subprovince as defined by Corfu and Stone (1998). Geological mapping (Rickaby, 1923; Corfu and Stone, 1998) and research has been minimal in the Bloodvein River area. Published data is available for only one sample of a biotite tonalite from along that river system in Ontario (Corfu and Stone, 1998:2985). This is likely explained by the Bloodvein River being part of the Woodland Caribou Signature Site, where mineral prospecting has not been allowed since at least the 1980s, although possibly as early the 1940s when it first became a protected area (OMNR, 2004). Underlying most of the Bloodvein River in Ontario is a biotite granite suite consisting of a massive granodiorite to granite unit generally containing tonalite inclusions (Corfu and Stone, 1998). Within these types of bedrock outcrops along the river, there are a large number of quartz veins available for quarrying stone tool materials. However, sources of the most common quartz artifacts were previously unknown until this study. Although few archaeologists have looked for quartz vein quarrying previously in the boreal forest, the Woodland Caribou Signature Site presented an opportune locale to look for these sites where there was minimal mineral sampling known or evident. Due to the extensive amount of prospecting in northwestern Ontario since the last century, particularly in the nearby Red Lake area, it is important to consider carefully the evidence for precontact quartz quarrying as opposed to quartz sampled for mineral exploration. Three types of evidence for Aboriginal quarrying activities were found along the Bloodvein River during these surveys: (1) quartz veins - 86 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 located adjacent to other types of archaeological sites typically with quartz artifacts (n=9); (2) quarried quartz veins with artifacts found beside or in them (n=5); and (3) mined quartz veins with evidence of non-recent quarrying (n=7). All of the places considered to be the result of ancient quarrying activities were weathered and had lichen developed on them, sediments deposited and/or soil development, which had been disturbed to reveal the quartz vein. No pebble or cobble based quartz sources were found, but they may be present in the tills found in the Lac Seul moraine to the east of the park. Since locally derived quartz is evidently the most commonly found raw material in the Bloodvein River area, Aboriginal people from this region were selecting reliable, nearby quartz vein sources more often than typically higher quality exotic types from afar. There is a specific Ojibwe word for white quartz, wiininwaabik (fat rock), whereas most other minerals and rocks are generically known as asin. Perhaps this distinction stems from the longstanding importance of this material in lithic technology within Anishinaabeg territories. In addition, the Bloodvein River examples indicate that geoarchaeologists do need to search for these types of small-scale quartz vein quarry sites in the boreal forest where quartz artifacts are typically prevalent. References Bakken, K.E., 2011. Lithic raw material use patterns in Minnesota. Unpublished Ph.D. dissertation, Department of Anthropology, University of Minnesota. Corfu, F., and Stone D., 1998. The significance of titanite and apatite U-Pb ages: Constraints for the post-magmatic thermal-hydrothermal evolution of a batholithic complex, Berens River area, northwestern Superior Province, Canada. Geochimica et Cosmochimica Acta 62(17):2979-2995. Ontario Ministry of Natural Resources (OMNR), 2004. Woodland Caribou background information. Queens Printer, Ontario. Rickaby, H.C., 1923. Bloodvein River to Twelfth Base Line, 1922. 32nd Annual Report, Part II:49-59, Ontario Department of Mines. Clarkson W. James, Printer to the King’s Most Excellent Majesty, Toronto. Taylor-Hollings, 2006. Stage two archaeological research at Knox and Peisk Lakes in the Woodland Caribou Signature Site, northwestern Ontario. Research report submitted to the WCSS Park Superintendent, Ontario Ministry of Culture, Pikangikum First Nation, and research partners. Thurston, P.C., Osmani, I.A., and Stone, D., 1991. Northwestern Superior Province: review and terrane analysis; in Thurston, P., Williams, H., Sutcliffe, R., Stott, G. (Eds.), Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, 80-142. - 87 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Analyzing ductile shear zone network geometries in the Grassy Portage Sill, Rainy Lake region, Northwestern Ontario, Canada THALHAMER, Ernest, and CZECK, Dyanna M., Department of Geosciences, University of WisconsinMilwaukee, P.O. Box 413, Lapham Hall 366, Milwaukee, WI 53201 The Grassy Portage Sill (GPS) is a ~2.7 Ga metagabbroic sill located in the Rainy Lake region of northwestern Ontario (Fig. 1). The Rainy Lake region is located in the Superior Province between the metavolcanic Wabigoon subprovince to the north and the metasedimentary Quetico subprovince to the south (Card and Ciesielski, 1986). Two regional faults bound the region and intersect to the east, forming a wedge which defines the Rainy Lake zone (Poulsen, 1986). This area was regionally deformed due to dextral transpression resulting from the Kenoran Orogeny (~2.7 Ga) (Poulsen et al., 1980; Davis et al., 1989; Czeck and Hudleston, 2003; Druguet et al., 2008). The GPS is approximately 20 km long and 1-2 km wide, and has undergone heterogeneous strain along its length. This strain variation is a function of the competence contrast between the GPS, the gneissic Rice Bay Dome to the west, and the metavolcanic and metasedimentary units between the two (Druguet et al., 2008; Carreras et al., 2010). The GPS has a higher competence than the adjacent metavolcanic and metasedimentary units, but all have a lower competence than the Rice Bay Dome. Figure 1. Geologic map of the Grassy Portage Sill and surrounding region with regional foliations indicated by dashed lines. Modified from Druguet et al. (2008) from mapping by Poulsen (2000). Within the GPS, anastomosing ductile shear zone networks accommodated the bulk of deformation within the largely competent sill (Fig. 2; Carreras et al., 2010), although a penetrative foliation also formed locally. The network geometries vary along the length of the sill, apparently related to strain variations. At nearly all locations, steeply dipping dextral and sinistral sets of shear zones formed, presumably simultaneously as evidenced by mutually offsetting cross-cutting relationships where present. Conjugate shear zone sets initiated at a near perpendicular orientation, and through progressive deformation have rotated towards one another. At the least strained sites, the gabbro has a pervasive foliation, but few, if any, shear zones. The varying responses of the GPS to deformation may be explained by lithological heterogeneity. Where grain size is coarse, the rock is likely stronger and deformation is localized into discrete shear zones. Where grain size is fine, the rock is generally weaker and pervasive foliation forms. Regionally the bulk shortening direction is oriented NW-SE, due to the N-S oriented shortening accompanied by dextral transpression. Estimates of bulk shortening direction based on shear zone geometries within the GPS indicate wide variation along the length of the unit, but outcrops can be grouped into three general clusters. In - 88 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the north, bulk shortening is predominantly oriented NE-SW. In the central part, shortening is oriented between N-S and NW-SE. In the south, the shortening direction is oriented NW-SE. Overall the shortening direction appears to “wrap” around the GPS, which is similar to the trend of the foliation variation within surrounding units. The variations seen at the unit scale can be attributed to a regional scale “strain shadow” imparted by the highly competent Rice Bay gneissic dome. Figure 2. Photograph of the anastomosing shear zone network within metagabbro at a highly strained outcrop. Shear zones have been traced, and are defined by narrow zones of intense foliation surrounding undeformed or less deformed lozenges. They define an anastomosing network pattern. References Card, K.D., Ciesielski, A., 1986. DNAG subdivisions of the superior province of the Canadian shield. Geoscience Canada 13, 5-13. Carreras, J., Czeck, D.M., Druguet, E., Hudleston, P.J., 2010. Structure and development of an anastomosing network of ductile shear zones. Journal of Structural Geology 32, 656-666. Czeck, D.M., Hudleston, P.J., 2003. Testing models for obliquely plunging lineations in transpression: a natural example and theoretical discussion. Journal of Structural Geology 25, 959-982. Davis, D.W., Poulsen, K.H., Kamo, S.L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398. Druguet, E., Czeck, D.M., Carreras, J., Castaño, L.M., 2008. Emplacement and deformation features of syntectonic leucocratic veins from Rainy Lake zone (Western Superior Province, Canada). Precambrian Res. 163, 384-400. Poulsen, K.H., Borradaile, G.J., Kehlenbeck, M.M., 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, 1358-1369. Poulsen, K.H., 1986. Rainy LakeWrench Zone: an example of an Archean Subprovince boundary in Northwestern Ontario. In: deWit,M.J., Ashwal, L.D. (Eds.), Tectonic Evolution of Greenstone Belts, Technical Report 86-10, pp. 177–179. Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. OGS Report 266. - 89 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 SPREE: Field experiment to study deep structure of the Midcontinent Rift VAN DER LEE, Suzan, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208, FREDERIKSEN, Andrew, Department of Geological Sciences, University of Manitoba, 125 Dysart Rd., Winnipeg, MB R3T 2N2, BOLLMANN, Trevor, Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208 and SPREE Team By about 1 Ga North America’s midcontinent region completed a formation process broadly similar to the current American west, with convergence along the Grenville Orogeny and extension along the contemporaneous Mid-continent Rift (MR). Now buried under platform sediments, more than a half million cubic km of dense igneous rock was deposited in the MR. These rocks generate an elongated 60+ mgal Bouguer gravity anomaly and a correlated magnetic anomaly, which cuts curiously through major geologic units such as the Superior and Yavapai Provinces. Although the MR is the most striking surface geophysical anomaly in the midcontinent, no evidence has yet been found for current geologic activity or correlated anomalies in the mantle lithosphere. The installation this year of the first swath of Earthscope-USArray stations east of the Mississippi River allows us to shed light on this enigmatic anomaly. Specifically, our seismic field experiment SPREE (Superior Province Rifting Earthscope Experiment) aims to uncover important details such as the depth and lateral extent of crust and mantle structures related to rifting, rift cessation/inversion, or post-rift stabilization. In addition, SPREE can answer questions about the longevity and healing ability of Proterozoic continental lithosphere. SPREE is a collaboration between Canadian and US universities. Eighty-three Flexible-Array (FA) seismic stations were installed between April and June this year on and near the MR. The array configuration includes an extension of the US-based Transportable Array (TA) into Ontario north of Lake Superior as well as three lines of 10-km spaced stations along and across the MR west and east of the northern Mississippi. SPREE recorded the M2.5 western Minnesota earthquake (April 29), the M7.3 Fox Islands earthquake (June 24), the M5.8 Virginia earthquake, and an unusually high number of moderate earthquakes in the relatively stable continental interior, from eastern Ontario to Oklahoma. We aim to use the recorded earthquakes and ground motion noise to detect microseismicity as well as to construct a multi-scale, three-dimensional image of the seismic velocity and discontinuity structure of the study region’s lithosphere and underlying mantle. These investigations will reveal the deep structure of the mid-continent rift. Moreover, the central location of SPREE with respect to North America’s seismically active plate boundaries contributes optimally to the seismic imaging of the sub-continental mantle elsewhere in stable North America. - 90 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Anatomy of a Mesoarchean Batholith VAN LANKVELT, A., SCHNEIDER, D.A., HATTORI, K., Science de la Terre/ Earth Sciences, Université d’Ottawa/ University of Ottawa, 140 Louis-Pasteur Ottawa, ON K1N 6N5 Canada and BICZOK, J. Goldcorp Canada Ltd., Musselwhite Mine, P.O. Box 7500 Thunder Bay, ON P7B 6S8 Canada The North Caribou greenstone belt (NCGB) lies in the North Caribou Terrane (NCT) of the Superior Province at the northeastern boundary of the North Caribou Core and the Island Lake Domain (Stott et al. 2010). The NCGB hosts the Musselwhite gold mine, a banded-iron-formation-hosted orogenic lode gold deposit, and structural controls on the deposit have been linked to intrusions along the margins of the NCGB (Stott and Biczok 2010). The most prominent of these intrusions is the North Caribou Batholith (NCB), which forms the southern and western boundaries of the west-central half of the NCGB. The contact of the NCB with the greenstone belt is marked by a crescent of low magnetic susceptibility relative to the centre of the NCB, which has been interpreted by Stott and Biczok (2010) as a “crescent pluton,” like those found elsewhere in the Superior Province. These crescent plutons intrude along the contact of greenstone belts and older gneissic terranes and impose a strain aureole on the less-competent greenstone belts, causing them to form arcuate shapes. Since the margin of the NCB conforms to the structural and geometric constraints of crescent plutons, this area has been dubbed the North Caribou Pluton (NCP). This interpretation prompted an investigation of the NCP to determine if this pluton is a discrete, younger intrusion. New U-Pb ages from zircon and titanite, along with whole rock and zircon geochemistry, and amphibole and plagioclase thermobarometry illuminate the evolution and relationship between the NCP and the rest of the NCB. Rocks in the NCB range from tonalites to granites and have a wide variety of textures and compositions. Textures throughout the NCB include enclaves of isotropic, equigranular, weakly-deformed granitoids, strongly foliated and lineated gniesses, and migmatites. Some of the migmatites are partially-melted amphibolites, which occur near the northern margin of the intrusive complex, but there are also granitoids with schlieren and other textures suggesting either partial melting, melt segregation, or magma mixing. Late K-feldspar-rich pegmatites occur around the margins of the NCB, in both the NCP and the central batholith. U-Pb geochronology on zircon and titanite was conducted using LA-ICP-MS methods at the University of New Brunswick. Single zircon ages range from 3132-2693 Ma, with a majority of rock ages falling between 2870 Ma and 2830 Ma. These ages are consistent with those reported by previous workers in the NCB and elsewhere in the NCT (e.g. Biczok et al., 2012). Ages from titanite are similar to the younger zircon populations. In the central NCB, titanite ages from 2798-2791 Ma are found in rocks with zircon ages of 2834-2833 Ma. This 30 m.y. difference could represent cooling through the different closure temperatures of titanite and zircon (500 and 900 ºC, respectively). The titanite in the NCP, however, record ages between 2766 Ma and 2748 Ma, with a ~100 m.y. difference between the zircon and titanite ages. As the titanites from these rocks are roughly aligned with the foliation, and the ages match zircon ages in younger intrusions surrounding the NCGB, these ages likely reflect later tectonism. To further assess the relationship between the rocks in the NCP and those in the central NCB, depth and temperature of emplacement was estimated using the plagioclase-amphibole thermobarometer and the Ti-inzircon thermometer. Titanium concentrations in zircon were measured using LA-ICP-MS, and calculated temperatures range from 830-760 ºC in the NCP and 790-750ºC in the central NCB. Plagioclase-amphibole temperatures and pressures were calculated following Holland and Blundy (1994) and Anderson and Smith (1995). Only five samples from the NCB have the appropriate mineralogy for the thermobarometer, only one of which was in the NCP. The temperature/ pressure conditions for the NCP sample are 600 ± 20 ºC and 6.3 ± 0.2 kbar (~22 km). Since this sample also contained isotopically disturbed titanite, it is possible that the plagioclase and amphibole compositions record this later event, rather than the emplacement conditions. The PT conditions in the central part of the batholith range from 640-620 ºC and 7.5-6.7 kbar (~26-23 km). - 91 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 These data provide some evidence supporting the crescent pluton model of the NCP. The shape of the NCGB, the magnetic signature of the NCP, and the structural data from previous workers are similar to the observations of crescent plutons in other greenstone belts. The model requires an intrusive contact between the crescent plutons and the supracrustal rocks, and the partially-melted amphibolites at the contact between the NCP and the NCGB clearly demonstrate that this contact is intrusive. Major, minor, and trace element geochemistry from the NCP is less variable than that from the rest of the NCB, despite a wide range of SiO2 concentrations, and REE patterns in zircon in the NCP have distinct, shallower slopes than those in the NCB. This suggests that the rocks in the NCP are more similar to each other than those in the rest of the NCB. U-Pb ages from titanite in the NCP are also younger than those in the central NCB. There are, however, new data that do not support a late crescent pluton. Zircon U-Pb ages in the NCP range from 2870-2858 Ma, which is slightly older than the ages from the centre of the NCB, which are 2852-2833 Ma. There is also no difference between the textures in the pluton and the batholith, as both contain gneisses and isotropic granitoids. Temperatures and pressures are also indistinguishable across the contact between the NCP and the central NCB. The older zircon ages in the NCP suggest either that the NCP represents a chill-margin on the NCB or a discrete earlier intrusion. Since the NCB has a map area of ~2800 km2 and does not exhibit cumulate textures, it is unlikely the entire complex was emplaced as a single melt batch. It is more likely that the primary conduit for the magma was near the centre of the NCB, and that several intrusions emanating from this area were emplaced at roughly the same depth over ~40 m.y.. This model predicts older granitoids on the margins of the intrusive complex, which have been shouldered aside radially by continued magmatism at the centre of the complex. This is consistent with the aeromagnetic data and textures observed in the NCB, as well as the geochronology, geochemistry and thermobarometry. Since this study did not investigate fabrics and structures related to the intrusions, we cannot comment on whether the kinematics of this model are consistent with these data. Further studies of the NCB, including fabric analysis, could constrain the dynamics of the intrusions and improve the proposed model. References Anderson, J.L., Smith, D.R. 1995. The effects of temperature and fO2 on the Al-in-hornblende barometer. American Mineralogist, 80: 549-559. Biczok, J., Hollings, P., Klipfel, P., Heaman, L., Maas, R., Hamilton, M., Kamo, S., Friedman, R. 2012. Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite mine. Precambrian Research, 192-195: 209-230. Holland, T., Blundy, J. 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology, 116: 433-447. Stott, G.M., Biczok, J. 2010. North Caribou Greenstone Belt: Gold and Its Possible Relation to the North Caribou Pluton Emplacement—A Belt-wide Contact-Strain Aureole? Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260: 22-1-22-12. Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M., Goutier, J. 2010. A Revised Terrane Subdivision of the Superior Province. Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260: 201-20-2. - 92 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Paleomagnetic data in stratigraphic context from 1.1 Ga Osler Group basalt flows on Simpson Island, Ontario: Evidence for rapid plate motion of Laurentia in the late Mesoproterozoic VAUGHAN, Angus A., Department of Geology, Carleton College, 1 North College St., Northfield, MN 55057, SWANSON-HYSELL, Nicholas L.and FEINBERG, Joshua M., Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, 291 Shepherd Labs, 100 Union Street SE, Minneapolis, MN 55455 The Osler Group represents the extrusive component of the early magmatic stage of the Midcontinent Rift in the northern Lake Superior region. The Osler Group overlies the epicontinental sediments of the Mesoproterozoic Sibley Group with angular unconformity. The lowest 100 m of stratigraphy of Osler Group contains rift-related sandstones and conglomerates (Hollings et al., 2007). In some locales, these basal sedimentary units are overlain by a quartz-feldspar porphyritic rhyolite for which a U-Pb zircon age of 1107.5 +4/-2 Ma has been reported (Davis and Sutcliffe, 1985). This age was obtained from an outcrop of rhyolite porphyry on Black Bay Peninsula, about 40 km to the west of Simpson Island. This unit was tentatively interpreted as extrusive (Davis and Sutcliffe, 1985; Lightfoot et al., 1991; Hollings et al., 2007), and the age has been interpreted as constraining the time at which Osler Group volcanism commenced. However, observations made during this study of a quartz-feldspar porphyry unit on Simpson Island, mapped as equivalent to the unit from which the age was obtained (Giguere, 1975), provide additional evidence for Giguere’s inference that this unit is actually intrusive. A thin veneer of basalt, ranging in thickness from several cm down to 1-2 mm, is variably present overlying the porphyry. The basalt veneer displays pahoehoe flowbanding, which is unlikely to have developed if the flow was originally this thin, implying that the felsic unit intruded into the basalt, cutting into an originally thicker flow. We also observed protrusions of porphyry surrounded by host basalt, providing further support for an intrusive relationship. This evidence suggests that the 1107.5 Ma age is a minimum age for the eruption of the first Osler basalt flows, rather than an absolute age for that point in the Osler Group stratigraphy. Above the felsic porphyry is a succession of relatively continuous tholeiitic basalt flows, with minor siltstone and conglomerate interbeds. The flows range in thickness from 5 cm to about 40 m thick; individual flows are recognized by a transition from massive basalt at the base of flows (occasionally with pipe vesicles in the basal 10-20 cm) to vesicular basalt towards the top, and by occasionally well-exposed pahoehoe flow tops. A rhyolite, interpreted as extrusive by Davis and Sutcliffe (1985), occurs near the top of the Osler Group basalt flows at Agate Point (stratigraphically higher than the highest flow on Simpson Island). Davis and Green (1997) obtained a U-Pb zircon age from the Agate Point Rhyolite of 1105 ± 2 Ma, which because of the extrusive nature of this rhyolite, is a robust age for that point in the Osler Group stratigraphy. It is worth noting that this age is statistically indistinguishable from the rhyolite porphyry discussed above. Here we report the first paleomagnetic investigation of the lower portion of the Osler Group stratigraphy, which builds on the work of Halls (1974) in the Nipigon Strait region. Oriented paleomagnetic cores were collected from 45 basalt flows in 5 stratigraphic sections along the eastern and southern shores of Simpson Island, spanning about 2500 m of Osler stratigraphy. Alternating field demagnetization, thermal demagnetization and low-temperature cycling experiments show that the majority of the remanent magnetization in Osler flows is held by magnetite and records a thermal remanent magnetization acquired at the time of eruption. Paleomagnetic data from the Osler Group suggest rapid plate motion during the early stages of the Midcontinent Rift. A preliminary paleomagnetic pole (λp = 220.6, φp = 44.2, A95 = 5.0) was calculated for 8 measured flows towards the bottom of the Osler Group and another preliminary pole (λp = 197.9, φp = 35.2, A95 = 6.3) was calculated for 9 flows towards the top of the reversely magnetized Osler flows. These two poles imply ~20° of latitudinal motion during the time the Osler Group was erupted. Significant latitudinal change during the lower reversed polarity zone of the Midcontinent Rift supports similar data from correlative stratigraphy at Mamainse Point, Ontario and represents the second place in the rift where such motion has been documented in - 93 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 the lower reversed polarity zone (Swanson-Hysell et al., 2009). These data add to the database of paleomagnetic poles from Keweenawan volcanics that together imply rapid plate velocities (>20 cm/yr) for Laurentia during the development of the Midcontinent Rift. References Davis, D. W., and Green, J. C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, p. 476-488. Davis, D. W., and Sutcliffe, R. H., 1985, U-Pb Ages from the Nipigon Plate and Northern Lake-Superior: Geological Society of America Bulletin, v. 96, p. 1572-1579. Giguere, J. F., 1975, Geology of St. Ignace Island and adjacent islands, District of Thunder Bay: Canada (CAN), Ontario Geological Survey, Toronto, ON, Canada (CAN), p. 35. Halls, H. C., 1974, Paleomagnetic Reversal in Osler Volcanic Group, Northern Lake Superior: Canadian Journal of Earth Sciences, v. 11, p. 1200-1207. Hollings, P., Fralick, P., and Cousens, B., 2007, Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario: Canadian Journal of Earth Sciences, v. 44, p. 389-412. Lightfoot, P. C., Sutcliffe, R. H., and Doherty, W., 1991, Crustal Contamination Identified in Keweenawan Osler Group Tholeiites, Ontario - a Trace-Element Perspective: Journal of Geology, v. 99, p. 739-760. Swanson-Hysell, N. L., Maloof, A. C., Weiss, B. P., and Evans, D. A. D., 2009, No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts: Nature Geoscience, v. 2, p. 713-717. - 94 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The Little Commonwealth exploration - An example of a SEDEX deposit WAGGONER, Thomas, 141Chippewa Negaunee, MI 49866, thomaswaggonergeo@hotmail.com and KARAKUS, Musa, Cliffs NR 550 E. Division, Ishpeming, MI 49849 The Little Commonwealth Exploration (LCE) is located in Florence Co., Wisconsin. The LCE represents a subaqueous synsedimentary exhalative deposit where silica, iron and manganese rich minerals are deposited on an elevated mound that formed a barrier separating sand (quartzite) to the west from mud (slate) to the east. The entire deposit outcrops in a small area encompassing less than 80 acres. The surface geology is primarily Michigamme slates with minor quartzites, conglomerates and isolated hydrothermal banded iron formation deposits. The quartzite can be characterized as a vitreous white ortho-quartzite except at the LCE where banded magnetite chert and Mn-chlorite, Mn-stilpnomelane, Mn-garnet chert breccias were deposited. The garnet, chlorite, stilpnomelane and ilmenite are all enriched in manganese characteristic of mineralization associated with SEDEX/VMS deposits. The rock with abundant spessartine garnet has been called coticule. Beginning with the deposition of the quartzite unit within the Michigamme formation a restricted area was subjected to hydrothermal mineralization. Some minerals are replacement of existing sediments (i.e. Dunkel Exploration located one mile to the northwest) while others are subaqueous exhalites like the one found at the LCE. Hot fluids carried Fe, Si, Al, Mn, Mg, P, B and REE. Upon reaching the surface and experiencing a significant drop in the pressure and temperature some of the minerals immediately crystallized and were deposited directly on the slope of the mound. Specularite appears to be later as the crystal laths display a random orientation usually enclosing magnetite or ilmenite. Late addition of sulfur, Fe++, copper and arsenic is evidenced by the presence of small quantities of sulfides. The major minerals present are magnetite, chert, specularite, Mn-chlorite, MnStilpnomelane, Mn-garnet and apatite. Minor minerals include Mn-ilmenite, monazite, tourmaline, pyrite, chalcopyrite and arsenopyrite. The only carbonate identified was a few grains of kutnohorite and dolomite. The manganese oxide content in the garnet is quite variable (Table 1) while the manganese in the chlorite and stilpnomelane is about more constant at about 1.8%. Ilmenite exhibits MnO values of ~10%. Table 1. Garnet Composition Oxide SiO2 Al2O3 FeO MnO MgO CaO FC-103 @80' 35.00 18.62 31.41 7.38 0.47 3.62 FC-104 Breccia Breccia @ 250' 2A E. Sft 2B E. Sft 35.09 35.37 36.12 18.5 18.95 19.14 28.39 26.53 27.38 12.91 13.87 20.23 0 0 0.69 1.93 2.31 2.53   The rocks appear to have experienced low grade greenschist alteration as evidenced by replacement of the garnet by either Mn-chlorite or Mn-stilpnomelane. Some of the magnetite is believed to be the result of immediate crystallization on exit from the vent and has undergone some oxide overgrowth resulting in crystals that are +100 microns in size. The rare earth elements reside in monazite which yields a high LREE pattern and a very low HREE suite in a chondrite normalized plot (Fig. 1). A slight negative Eu value would support the presence of hydrothermal fluids but with abundant sea water dilution. The age of the LCE deposit is constrained between 1836 and 1833 Ma based on a tuff age in the basal Rove formation and the local intrusive of the Tobin Lake granite indicating the magma chamber that produced the iron oxide rich Hemlock volcanics and associated with the northward subduction zone was still viable near the end - 95 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 LITTLE COMMONWEALTH MARTITE CHONDRITE NORMALIZED REE LOG SCALE: SAMPLE/CHONDRITE 100.00 10.00 TOTAL REE 121.48 1.00 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu RARE EARTH ELEMENTS Figure 1. Chondrite normalized REE plot for Little Commonwealth martite of the Penokean Orogeny. When the Wisconsin Magmatic terrain moved northeast along a series of NW-SE major faults it resulted in three parallel allochthons that were tilted and rotated exposing repeat sections of Michigamme strata. There are abundant indications within the three fault blocks to indicate the SEDEX mineralization is not an isolated occurrence. A number of iron oxide rich showings have been noted in the Michigamme formation particularly from the quartzite unit upward. Replacement of existing sediments is indicated in a few of the shows like the Dunkel located one mile northwest of the LCE. The LCE is an excellent example of a synsedimentary exhalative deposit with both SEDEX and IOCG features situated adjacent to an active hydrothermal vent (not located) that produced iron oxides-chert-Mn silicates. A series of much larger and more active vents can easily be envisioned as a source for Lake Superior Type iron deposits found in the Animikie Basin. References Dutton, C.E., 1971, Geology of the Florence Area, Wisconsin and Michigan, USGS Prof. Paper 633, 54p. Johnson, R.W., 1958, Geology of the Little Commonwealth Area, Florence County, Wisconsin, USGS OFR 58-53, 98p. Schneider, D.A., et al, 2002, Age of Volcanic Rocks and Syndepositional Iron Formations, Marquette Range Supergroup: Implications of the Tectonic Setting of Paleoproterozoic Iron Formations of the Lake Superior Region, Can. J. Earth Sci. v. 39, p 199-1012. - 96 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The curious meteorite harvest of the Lake Superior region I - Overview WILSON, Graham C., Turnstone Geological Services Limited, P.O. Box 1000, Campbellford, ON K0L 1L0 and McCAUSLAND, Phil J.A., Department of Earth Sciences, University of Western Ontario, London, ON N6A 3K7 Introduction The historic trove of discrete meteorite falls and finds in the region surrounding Lake Superior is 52 (the official total, on 10 April 2012, as recounted in the on-line database of the Meteoritical Bulletin, http://www. lpi.usra.edu/meteor/metbull.php?), plus five more in Manitoba awaiting official recognition. Current official tallies for large countries include: U.S.A. (1,633); Australia (646); Chile (206); India (136, plus 4 insufficiently documented); Argentina (74); and Canada (60, plus 13 “in progress” of certification). Our chosen region for this short survey includes the four Superior-bounding jurisdictions plus Manitoba (with 3 finds in and near Whiteshell Park, which is closer to Superior than the populous south of Ontario). For the three states the documented haul of meteorites corresponds to a cumulative mean density of 32 in 645,304 km2, or 1 meteorite recovered per 20,166 km2. For the two provinces the total is 25 in 1,724,192 km2, or 1 meteorite recovered per 68,968 km2 (in this instance, the cosmic odds appear 3.4 times better in the U.S.!). The two sub-regions have respective populations of 20.891 and 14.291 million, and mean population density of the three states is 3.9 times that of the two provinces (data from National Geographic Society, 2011). The table breaks down the 57 meteorites into their most basic families (see Norton and Chitwood, 2008, for a readable taxonomy of meteorites; Grady, 2000, may be the last global meteorite catalogue in print). Irons and the rarer stony-irons (the two regional examples are both the olivine-metal mixtures known as pallasites) are quite distinct from Earth rocks. The stony meteorites, both the more primitive, undifferentiated chondrites and the rare, igneous achondrites, vary in their properties but can be much harder to identify, especially when weathered. There are 29 irons, two stony-irons and just 26 stones, including 25 ordinary chondrites (43.9% of the total). The latter (Met.Bull., 10 April 2012) are 87.4% of all authenticated meteorites, irons just 2.5% (but 51% of the regional meteorite haul), stony-irons just 0.6% (the other 9.5% are a legion of achondrites and rare chondrites). Note also the rarity of achondrite recoveries: none in the region, depending on the classification of Mount Morris (Wisconsin), nor in the whole of Canada; and of the reduced enstatite chondrites (just one in Ontario – Blithfield – which being near the Ottawa valley lies far from Superior). Table 1. Meteorites of the Lake Superior region   State / province Ontario Manitoba Minnesota Wisconsin Michigan Irons 8 4 4 7 6 Stonyirons 1 1 0 0 0 Achondrites Chondrites 0 0 0 0 0 7 4 4 7 4 All meteorites 16 9 8 14 10 Impact structures 5 2 0 2 1 The volume of research on the meteorites varies greatly, as will be seen: although all the meteorites are in Grady (2000) and/or the Met.Bull. database, no other specific records were found* for eight of the 32 U.S. meteorites, while 21 of these were described by 1 to 10 citations each, and just three by 20-plus articles. A tabular review of Canadian meteorites is available at http://www.turnstone.ca/canamet4.pdf (update of April 2012, cf. Wilson and McCausland, 2012). * The more exact rendering of “found” is “accrued to the MINLIB database by 13 April 2012”: the above link further explains the methodology and serendipity involved. Note that the MINLIB coverage of Canadian meteorites will be more complete than for those recovered south of the border. - 97 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Ontario The significant meteorite haul in Ontario (n=16; seven irons, one stony-iron, six ordinary chondrites and an enstatite chondrite, plus a 2.7-kg iron named in, and for, Toronto but probably found in Quebec) has been won largely along the populous corridor extending from Ottawa W.S.W. towards Detroit. There are 11 finds and five falls, most recently the Grimsby (2009) fall (Brown et al., 2011). The sole meteorite from northern Ontario is the Osseo iron, recovered in the Temiskaming region. The Manitouwabing iron and Southampton pallasite (Kissin et al., 2012) were recovered east of Georgian Bay and along the eastern shore of Lake Huron, respectively. The only meteoritic calling-cards west of Sudbury, in fact, seem to be the Slate Islands impact structure and the debris layer from the 1850 Ma Sudbury impact, preserved around Thunder Bay (e.g., Cannon and Addison, 2007). References Brown, P., McCausland, P.J.A., Fries, M., Silber, E., Edwards, W.N., Wong, D.K., Weryk,R.J., Fries, J. and Krzeminski, Z. (2011). The fall of the Grimsby meteorite I: Fireball dynamics and orbit from radar, video, and infrasound records. Meteoritics & Planetary Science 46, 339 363. Cannon, W.F. and Addison, W.D. (2007) The Sudbury impact layer in the Lake Superior iron ranges: a time line from the heavens. Abs. 53rd Annual Meeting, Institute on Lake Superior Geology, vol.53 part 1, 89pp., 20 21, Lutsen, MN. Grady, M.M. (2000) Catalogue of Meteorites. Natural History Museum, London / Cambridge University Press, 5th edition, 690pp. plus CD ROM. Kissin, S.A., Macrae, N.D. and Keays, R.R. (2012) Southampton: Canada’s third pallasite. Can.J.Earth Sci., in press. National Geographic Society (2011) Atlas of the World. National Geographic Society, Washington, DC, 9th edition, 137+153pp. Norton, O.R. and Chitwood, L.A. (2008) Field Guide to Meteors and Meteorites. Springer Verlag London Limited, 287pp. Wilson, G.C. and McCausland, P.J.A. (2012) Canadian meteorites: a brief review. Abs. Geol.Assoc. Canada / Mineral. Assoc. Canada, St. John’s, Newfoundland / manuscript submitted to Can.J.Earth Sci. - 98 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The curious meteorite harvest of the Lake Superior region II - Gems in the rough WILSON, Graham C., Turnstone Geological Services Limited, P.O. Box 1000, Campbellford, ON K0L 1L0 and KISSIN, Stephen A., Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5B From Ontario, we continue a counterclockwise tour of the meteorites closest to Lake Superior … Manitoba Three irons and two chondrites await certification, for an unofficial total of nine finds. The five “candidates” include an H4 chondrite plucked from a road by a grader operator and recognized for its unusual nature, and the informally-named Pinawa and Lone Island Lake irons. These last, plus Bernic Lake, were all retrieved from the southeast corner of the province by a single person, between 2002 and 2005. Minnesota Minnesota has two small (<10 g) meteorites of muddied provenance (like Toronto), an iron and a stone. To be a Minnesota meteorite is to be doomed to obscurity, except for three interesting irons. Redressing the balance of the tiny, misplaced finds is the 22.39-kg Turtle River IIIAB iron, largest find in the state. Best-studied is Arlington, one of the rare IIE irons that may strictly be non-magmatic, arising as melt pools on the surface of their parent body (Wasson and Wang, 1986). The Anoka iron has also been a test case in iron meteorite classification. Minnesota is home to one fall and seven finds. Wisconsin The Trenton iron, a 505-kg find collected in 1858 and subsequent years, remains the largest meteorite known in the five jurisdictions. At least 29 articles refer to it, 1961 onwards, making it, after Allegan, perhaps the bestresearched meteorite in the Upper Great Lakes States. Five falls and nine finds are recorded. The Hammond iron (24 kg) was ploughed up in a cornfield in St. Croix County (a synonym) in 1884. George Kunz etched a piece and noted troilite (FeS) nodules and a Widmanstatten pattern (Fisher, 1887). There is an intriguing possibility of kinship between the Pine River iron with silicate inclusions, and Mount Morris (Wisconsin), two murkilydocumented finds within 15 km of one another. The latter has been interpreted to be the remains of a large silicate inclusion in Pine River, and itself possesses the mineralogy of a winonaite, a rare achondrite (Bevan and Grady, 1988). The most recent meteorite fall in the Superior region was the Mifflin L5 chondrite on the evening of 14 April 2010. Michigan The closest known meteorite visitor to Lake Superior is Iron River, a IVA iron find in 1889, ~75 km south of the head of Keweenaw Bay. The best-researched meteorite in the Upper Great Lake States is probably Allegan (H5 chondrite fall of 1899) with 48 references on file, 1903 onwards (in 1903 just three meteorites were known in the state – the next 109 years have added but 7 more: six finds and four falls are known). Eight are described by Del Chamberlain (1968). The Rose City H5 chondrite is another interesting Michigan meteorite, a highly shocked, black chondrite (23 references known, third-most-studied local meteorite). As an impact melt breccia, with recrystallized olivines, melt veins and masses of metal (Rubin, 1995), it has been compared to the Portales Valley H6 stone (a 1998 fall in New Mexico), and has been exhibited at the Smithsonian and in the Arthur Ross Hall of Meteorites at the A.M.N.H. in New York City. Conclusions - 99 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 The modest meteorite trove of the region is easy to attribute to low population density, predominant lakes and forests, and large distances. Persons in the region are often transient or seasonal (e.g., hunters, loggers, cottagers) cf. farmers. The factors all limit the chances of tracking a fireball and making a recovery, thus finds are predominant (a regional total of 15 falls and 42 finds, cf. India, with at least 10 witnessed falls recovered in 2001-2008 alone: http://www.turnstone.ca/indmet.htm). In general, the best prospects for recovery of a fall are a well-tracked fireball crossing populous areas to land on farmland. It is surely not a coincidence that 9 of the 10 recovered falls in the three states are in Wisconsin and Michigan. At least five of the 32 US meteorites were ploughed up, and a further three or more found in fields. A Wisconsin farmer is more apt to find a meteorite than a logger in Minnesota or the U.P. of Michigan! The best prospects for finds may be discoveries in a zone of glacial stillstand. The three recent iron finds in southeast Manitoba may lie in such a concentration (Hildebrand et al., 2006). The predominance of iron finds in the region may speak to their unique nature, since they are relatively easy for the general public to recognize as something out of the ordinary. References Bevan, A.W.R. and Grady, M.M., 1988. Mount Morris (Wisconsin): a fragment of the IAB iron Pine River? Meteoritics 23, 349-352. Del Chamberlain,V., 1968. Meteorites of Michigan. Michigan Geol.Surv. Bull. 5, 20pp. Fisher, D., 1887). Description of an iron meteorite from St. Croix Co., Wisconsin. Amer.J.Sci. 134, 381-383. Hildebrand, A.R., Beech, M., Kissin, S.A. and Quade, G.B., 2006. A possible meteorite lag deposit after continental glaciation in southeastern Manitoba. Meteoritics & Planetary Science 41, A77. Rubin, A.E., 1995. Fractionation of refractory siderophile elements in metal from the Rose City meteorite. Meteoritics 30, 412 417. Wasson, J.T. and Wang, J., 1986. A nonmagmatic origin of group IIE iron meteorites. Geochim. Cosmochim. Acta 50, 725 732. - 100 - �Proceedings of the 58th ILSG Annual Meeting - Part 1 Author Index Albers, P.  Anderson, D.  Asp, K.  Baggetto, L.  Bailes, A.  Bandli, B.  Baumann, S.  Beh, B.  Berkley, J.  Biczok, J.  Bjønerud, M.  Blaske, A.  Boerboom, T.  Bollmann, T.  Bornhorst, T.  Bowring, S.  Braun, G.  Brooker, B.  Buchholz, T.  Burgess, S.  Campbell, D.  Chaffee, M.  Conly, A.  Craddock, J.  Craddock, S.  Cummings, K.  Cundari, R.  Czeck, D.  Darbyshire, F.  Dean, F.  Deen, T.  Deniset, I.  Diehl, J.  Driese, S.  Easton, R.  Eliason-Johnson, G.  Epstein, R.  Falster, A.,  Feinberg, J.  Fralick, P.  Frederiksen, A.  Gagnon, J.  Galley, A.  Gaspar, B.  Gasparotto, M.  Gilbert, H.  Good, D.  Goscinak, C.  58 9 65 47 1 43 3 5 7 91 73 9 11, 49, 60 13, 28, 90 9, 14 84 9 15, 65 17 84 24, 56 19 63 21 21 3 22, 24 88 28 69 58 28 53, 55, 72 60 26, 75 47 33 17 93 5, 41, 51, 54 13, 28, 90 81 1 29 30 31 33, 81 35 Hansen, D.  Hansen, V.  Hattori, K.  Heggie, G.  Heim, N.  Hill, M.  Hollings, P.  Horner, S.  Hoxsie, E.  Hudak, G.  Jirsa, M.  Johnson, J.  Johnson, T.  Jones, S.  Karakus, M.  Kerkemeier, L.  Kern, A.  Kilduff, R.  Kilpatrick, K.  Kissin, S.  Koroscil, J.  Kulakov, E.  Kuzmich, B.  Lappalainen, M.  Lee, A.  Leier-Engelhardt, P.  Leu, A.  Linnen, R.  Lou, X.  MacTavish, A.  Mahr, C.  Ma, L.  Maloof, A.  McCausland, P.  McCorkmick, K.  McLean, K.  Medaris, G.  Mikkelsen, L.  Miller, J.  Mitchell, A.  Monson Geerts, S.  Mottl, R.  Napoli, M.  Paradis, S.  Parisi, A.  Paterson, C.  Pesonen, L.  Petrus, J.  - 101 - 47 35 91 19, 37 39 29, 30, 54, 80, 83 19, 22, 24, 56 41 47 39, 43 45, 47, 60 37 49 77 79, 95 51 53 39 47 99 54 53, 55 56 77 58 9 65 33, 81 13 19, 37 39 37 84 97 62 33 60 63 15, 19, 58, 65 79 43 9 75 1 65 62 67, 72 75 �Proceedings of the 58th ILSG Annual Meeting - Part 1 Phillips, B.  Pietrzak-Renaud, N.  Piispa, E.  Rahtz, C.  Roe, C.  Rousell, D.  Samson, I.  Santaguida, F.  Schneider, D.  Scott, G.  Scott, H.  Scott, J.  Selagi, J.  Severson, Allison  Severson, April  Severson, M.  Shahabi Far, M.  Shannon, J.  Shinkle, D.  Siikaluoma, J.  Simmons, W.  Sletten, D.  Smirnov, A.  Smyk, M.  Stinson, V.  Swanson-Hysell, N.  Taylor, B.  Taylor-Hollings, J.  Thalhamer, E.  Tinkham, D.  Van der Lee, S.  Van Lankvelt, A.  Vaughan, A.  Veikkolainen, T.  Vial, A.  Voipo, T.  Waggoner, T.  Wendlandt, R.  Weston, R.  Williams, W.  Wilson, G.  Ylinen, J.  Young, S.  Zaniewski, K.  Zanko, L.  69 71 72 39 73 75 33, 81 77 91 79 39 24, 56 80 43 43 58 81 49 79 77 17 65 53, 55, 72 22 83 84, 93 1 86 88 75 13, 28, 90 91 93 67 39 77 95 49 37 14 97, 99 77 39 69 43 - 102 - � https://digitalcollections.lakeheadu.ca/files/original/48334c7f548fa7b9dc6dbb56d0f14bba.pdf 89a9b157235d7048264752ca3185ccee PDF Text Text 58th Annual Meeting Institute on Lake Superior Geology Thunder Bay, Ontario - May 16-20, 2012 Part 2 – Field Trip Guidebook �Sponsors The following organizations made generous contributions to the 58th Annual Meeting. We thank them for their commitment to the Institute on Lake Superior Geology. For the past 50 years this organization has thrived as a result of the interest of individuals, corporations, universities and government agencies. The dedication to an exchange of scientific ideas and a passion for field trips has enabled the Institute to provide one of its primary objectives – to promote better understanding of the geology of the Lake Superior Region. �58th Annual Meeting Institute on Lake Superior Geology May 16-20, 2012 Thunder Bay, Ontario HOSTED BY: Pete Hollings Chair Lakehead University Proceedings - Volume 58 Part 2 – Field Trip Guidebook Edited by Pete Hollings, Al MacTavish and Bill Addison Cover photos: Top - Neoarchean conglomerate in the Max Lake area, Hwy 527, Wabigoon Subprovince, Middle - Silver Islet Mine, Lake Superior, Right - Inspiration diabase sills, Chimney Lake near Armstrong (all photos courtesy of Mark Smyk). �58th Institute on Lake Superior Geology Volume 58 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1 & 13: Sudbury Impactoclastic Debrisites at Thunder Bay Trip 2: Geology of the Sibley Peninsula Trip 3: Lac des Iles mine Trip 4: Shebandowan Mine Area Trip 5: Geology of the Thunder Bay area Trip 6: Thunder Bay Amethyst Mine Trip 7: building stone tour of Downtown Port Arthur, Thunder Bay Trip 8: Highway 527 Transect Trip 9: Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites Trip 10: Geoarchaeology of Thunder Bay Trip 11: Midcontinent rift intrusions Trip 12: Musselwhite mine Reference to material in Part 2 should follow the example below: Addison, W., and Brumpton, G., 2012. Field trips 1 & 13 - Sudbury impactoclastic debrisites at Thunder Bay. In; Hollings, P., MacTavish, A. and Addison, W. (Eds.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 2 - Field trip guidebook, v.58, part 2, 2-26. Published by the 58th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table of Contents Introduction, safety considerations and acknowledgements................................................1 Field trips 1 & 13 - Sudbury Impactoclastic Debrisites at Thunder Bay . ..........................2 Field trip 2 - Geology of the Sibley Peninsula...................................................................27 Field trip 3 - Lac des Iles Mine . .......................................................................................56 Field trip 4 - Shebandowan Mine Area ............................................................................67 Field trip 5 - Guide to the Thunder Bay area ....................................................................74 Field trip 6 - Thunder Bay Amethyst Mine . .....................................................................82 Field trip 7 - Building stone tour of downtown Port Arthur, Thunder Bay, Ontario ........93 Field trip 8 - A geologic transect across the Western Superior Province and Nipigon Embayment, Thunder Bay to Armstrong, Ontario . ................................................101 Field trip 9 - Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites ..............................................................................................................136 Field trip 10 - Geoarchaeology of the Thunder Bay area ..............................................150 Field trip 11 - Midcontinent Rift-Related Mafic Intrusions around Thunder Bay...........189 Field trip 12 - The Musselwhite Gold Deposit................................................................208 -i- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada This volume is intended to serve not only as a guide for 58th ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently we have included UTM coordinates in the NAD 83 datum for the majority of stops, as well as instructions on how to reach them. For some of the stops on private land we have witheld the UTM coordinates to respect the privacy of the property owner. As some of the stops are on private and staked land, please be sure to obtain the land owners’ permission before entering their land. For upto-date information on land ownership please contact the Thunder Bay Resident Geologists’ Office (807 475 1331). Sample collection is prohibited at some stops on private land or in Provincial Parks. Many of the fieldtrips will be visiting stops along either major highways or busy logging roads. Please take care when crossing or parking along these roads. For those field trips that are visiting active mine sites personal protective equipment will be required. Please notify the field trip leaders if you have any medical conditions that may be of concern during the trip. Each trip leader is equipped with a first aid kit and satellite/ cell phone, so please notify them of any incident. We would like to thank all authors who contributed to this field guide and also all those who provided comments and assisted with the running of the field trips themselves. We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties. We are particularly grateful to the Musselwhite and Lac des Iles mines for running field trips on their properties. Figure 1. Map showing the general locations of field trips for the 2012 meeting. -1- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trips 1 & 13 - Sudbury Impactoclastic Debrisites at Thunder Bay Bill (W.D.) Addison and Greg (G.R.) Brumpton Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Abstract Eight outcrops of chaotic debrisite containing ejecta from the 1850 Ma Sudbury impact event have been identified in and near the city of Thunder Bay, Ontario, 650 km west of the center of the Sudbury crater. Ejecta features include devitrified vesicular impact glass, spherules, accretionary lapilli, microtektites and tektites, and shocked quartz grains containing relict planar features including planar deformation features. The original volume of ejecta has been significantly reduced by carbonate replacement and recrystallization, so that today ejecta only make up ~ 20 % of the debrisite volume. Major debrisite components include ripped up clasts of carbonate grainstones, stromatolites and chert of the 1878 Ma Gunflint Formation. These Gunflint boulder to coarse sand-sized clasts commonly fine upward, in marked contrast to the chaotic nature of the remainder of the debrisite. Seven of the eight sites have had the upper portion of the impact layer removed by glaciation. The eighth site shows a complete stratigraphic section from the Gunflint Formation, up through the ejecta bearing layer, and into the overlying 1832 Ma Rove Formation. The sequence of events deduced from these outcrops is as follows. 1. Mafic volcanic ash was deposited and reworked in a carbonate dominated, near-shore environment that supported microbial mat growth and stromatolites. 2. These areas were then subaerially exposed. 3. Upon impact, earthquakes fractured some stromatolites as well as the underlying Gunflint Formation chert and carbonate. 4. Impact-generated density currents (base surges) stripped the area of loose sediment and incorporated ripped up Gunflint chert-carbonate breccia clasts, before being deposited as a chaotic variable layer. 5. In an ensuing period of subaerial exposure lasting < 18 m.y., blocky, meteoric calcite cements formed in this material while weathering and erosive reworking modified the deposits. 6. The Rove Sea then transgressed the area depositing the overlying Rove Formation carbonaceous shale. Introduction In 2005, Addison et al. documented an ejecta layer formed by an 1850 Ma (Krogh et al., 1984) impact event in cores drilled north of Lake Superior in Ontario and Minnesota. Features in this Sudbury impact layer (SIL) included planar shock features, notably planar deformation features (PDF) in quartz grains; accretionary lapilli; devitrified microtektites and tektites; and devitrified vesicular impact glass (DVIG). The ejecta were linked to the 1850 Ma Sudbury impact by their presence between an 1878 Ma Gunflint Formation tuff (Fralick et al., 2002) approximately 105 metres below the ejecta and tuffs variously dated at 1827 ± 8 Ma, 1832 ± 3 Ma and 1836 ± 5 Ma from the Rove and Virginia Formations 5 to 6 metres above the ejecta (Addison et al., 2005). Sudbury is the only known terrestrial impact from this time interval (Earth Impact Database, 2012) and an oceanic impact would not have produced the quartz- and feldspar-rich, cratonsourced ejecta. The identification of the SIL has led to the discovery of about 30 additional ejecta-bearing drill core and outcrop sites (Fig. 1) in the Lake Superior region (Cannon and Addison, 2007; Pufahl et al., 2007; Jirsa et al., 2008; Cannon et al., 2010). Eight of them are in the northern Gunflint Formation outcrop area in and near Thunder Bay, Ontario (Fig. 1). Locations are supplied for all sites except one in a private yard which is omitted to protect the owner’s privacy. These new sites are 660-680 km from Sudbury. Using the crater radius of 130 km as determined by Spray et al. (2004), these new outcrops are 5.1-5.2r (crater radii) from the Sudbury crater center. The approximate boundary between proximal and distal ejecta is usually given as 5 crater radii (French, 1998) but this boundary is a transition zone, not a sharp line. The ejecta features seen on this field trip are consistently proximal, not distal. This field trip will examine outcrops for macroscopic ejecta and non-ejecta features (Table 1) and relate -2- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. Approximate locations of some Sudbury impact layer (SIL) localities in the Lake Superior region. Concentric lines represent multiples of the final Sudbury impact crater radius of ~130 km as determined by Spray et al. (2004). them to the dynamics of the Sudbury impact, the second largest and fourth oldest impact known on Earth (Earth Impact Database, Jan. 10, 2012). A large impact results in a sequence of events at the impact site which subsequently played out in the Thunder Bay area. The Earth Impacts Effects Program (impact.ese. ic.ac.uk/ImpactEffects/) allows an estimate of the time of delivery and magnitude of events from the impact by inputting: 1) distance from impact (660 km); 2) projectile diameter; 3) projectile density; 4) projectile velocity; 5) impact angle and; 6) target rock type (crystalline – granitic at Sudbury). A velocity of 25 km/s with an impactor diameter of 23 km, along with the other variables noted above, produces a final radius of 134 km, very close to the actual value of Spray et al. (2004). It is interesting that the SIL thickness predicted by the model is not matched by reality at Thunder Bay. For instance, the maximum ejecta thickness seen is ~4 m at Hillcrest Park, where it was once thicker because the exposure top is erosively truncated. The only complete stratigraphic exposure of the SIL is 3.2 m. Likewise, complete SIL in drill cores BP99-2 and PR98-1 from ~35 km south of Thunder Bay do not Table 1. Major effects of a Sudbury-sized impact at Thunder Bay, 660 km from its epicenter, as predicted by the Earth Impacts Effects Program (impact.ese.ic.ac.uk/ImpactEffects/). The field trip outcrops will show the effects of earthquakes and some of the types of ejecta features generated at the impact site. Ambiguous evidence of air blast will be seen at one site. Effects from the fireball have not been identified so far. -3- �Proceedings of the 58th ILSG Annual Meeting - Part 2 exceed 0.8 m in thickness. All of these values are well short of the model’s predicted 12 m thickness which raises questions either about the model or about the SIL’s post-depositional history or both. Terminology and Features Like other branches of geology, the geology of large extraterrestrial impacts has its own rapidly evolving vocabulary which is not widely known in the larger geological community. Therefore terms and specialized impact features are best defined or described and illustrated before seeing and discussing the outcrops on this field trip. Ejecta Ejecta is a collective name for anything thrown out of the crater during the impact. It includes target rock breccia clasts of all sizes from µm- to km-scale. It includes melt which cooled to form glass clasts of various shapes and sizes, most of them now devitrified. It also includes dust and glassy spherules which condensed from rock vapour ejected high into Earth’s atmosphere and even above it. Ballistic Ejecta Curtain Many ejecta components initially travel outward as a curtain on a ballistic trajectory, most of it landing at about 2r from the crater center (French, 1998). There, this massive amount of material lands, severely abrading the landscape and incorporating the abraded material into an outward-rolling debris flow. Debrisite Shanmugam (2006) argues that tsunamite should not be used to describe tsunami deposits because it describes a process and does not deal with clast sizes like conventional sedimentary terminology e.g., sandstone, claystone, etc. He proposes the term “debrite” for tsunami deposits because of their wide variety of clast sizes. By inference, impact related deposits should not be called impactites. We choose debrisite as the best descriptor for these deposits which result from four sets of related energetic events: 1) the impact; 2) impact-induced earthquakes; 3) ejecta traveling in ballistic trajectories and; 4) ground-hugging density currents (called base surges in earlier literature). Thus, the SIL is composed of debris with clasts in the µm to metre size range and of variable origins. Even though all debrisite sites reported here contain ejecta, debrisite is not synonymous with ejecta because ejecta features only comprise about 20 % of debrisite while localized areas of some outcrops seemingly lack ejecta. Ground-hugging Density Currents (Base surges) Much of the early impact literature applied volcanic terminology to impact generated deposits. In fact, a number of the SIL deposits south of Lake Superior were identified by searching the literature for pyroclastic deposits, then checking to see if ejecta features were present (W. F. Cannon, personal communication, 2008). Base surge is one such borrowed term but we use both base surges and the newer impact literature term ground-hugging density currents interchangeably. Matrix (not ejecta) The largest debrisite component by volume is carbonate matrix (Figs. 2A-D), with calcite > dolomite > ankerite. Carbonate has partially replaced most ejecta features (Figs. 3A-D, F) and likely obliterated many more of them. It has also infilled vesicles (Figs. 3A-D). Pervasive carbonate recrystallization, with crystals up to 5 mm maximum dimension, has further destroyed ejecta features (Figs. 3A & F, 2C). There are areas in the matrix comprising angular submillimetre to millimetre carbonate clasts composed of crystals ≤ 20 µm in size (Fig. 2). These clasts may represent Gunflint carbonate ground up in the turbulent events leading to deposition of the SIL but, if so, most show at least minor recrystallization planes. Silica also replaces ejecta features (Fig. 3E). Silica is most visible as anastomosing chert at submillimetrescales in most thin sections as well as centimetre-scale bands in outcrops. Such silica is usually microcrystalline and is clearly a post-depositional feature. Today, the matrix comprises ~80 % of the debrisite volume leaving ejecta features at ~20 %. At the time of deposition the ratio of ejecta to matrix must have been a significant but unknown amount higher, judging from the volume of partially carbonate-replaced ejecta remnants. Stromatolites (not ejecta) Seven of the eight identified outcrop sites show: 1) debrisite lying directly on stromatolites (Fig. 4A) or microbialite mats and/or; 2) broken subcentimetre to decimetre-size stromatolite clasts within the debrisite. The stromatolite or microbialite clasts attest to violent events breaking them up and mixing them into the -4- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. Splashform devitrified vesicular glass (DVIG) clasts in carbonate matrix. A – GTP site; plane polarized light (pp). B – GTP site; crossed polarizers (xp). C – Private yard site (pp). D – Atypical abundance of DVIG fragments, splashform or otherwise. Hwy 588 (pp). debrisite. Subrectangular Blocks of Upper Gunflint ChertCarbonate (not ejecta) Prior to highway reconstruction in 2011 the original Terry Fox exposure showed the upper 0.5 m of Gunflint chert-carbonate bedrock heavily fractured with the subrectangular blocks slightly separated from each other but still basically in situ (Fig. 5B). Subrectangular blocks of Upper Gunflint chertcarbonate, commonly exceeding 0.5 m maximum dimension (Fig. 4B), are found at or near the base of most debrisites (Table 1). They, along with fractured chert clasts of all sizes, show upward fining within the chaotic debrisite. The chert-carbonate blocks usually have sharply angled corners, except at the Private Yard (Fig. 4B), GTP and BB sites where some subrounded blocks exist among angular blocks. None of the blocks show weathering rinds. Anastomosing Silica and Agate with Mini-stalactites (not ejecta) Hillcrest Park, with its 3.5-4 m thick debrisite, shows extensive post-depositional anastomosing chert deposits, some of which include banded agate in localized zones. These chert deposits flow around debrisite clasts but never cut through them. In places, the chert and agate has been deposited in debrisite voids. Near the top of the Hillcrest Park deposit, two vugs contain silica and agate stalactites 1-3 cm long (Fig. 4E). The Banning Bluff and Baseball Central sites and the DVIG-rich, recessively weathering layer at the Terry Fox site also show anastomosing chert but on a smaller scale than Hillcrest Park. Micro-scale anastomosing chert is seen in thin sections from all sites. The agate layer at the top of the TF site shows digitate projections from both the base and top of the layer which may have been miniature stalactites and -5- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3. Varying degrees of vesicle deformation in devitrified vesicular impact glass (DVIG) clasts and spherules. A – undeformed vesicles infilled by calcite. Note recrystallized carbonate at top center and right; GTP Site; plane polarized light (pp). B – ovoid vesicles aligned subvertically and infilled with calcite. There is no evidence of lateral compression of the clast, so presumably the vesicles were deformed prior to deposition; GA site (pp). C – calcite-infilled, deformed spherules in cluster; Private Yard (pp). D – fibrous spherule rim, partially destroyed by carbonate replacement; GA site; crossed polarizers (xp). E – collapsed and partially collapsed spherules. Presumably the spherules were hollow prior to collapse. Hwy 588 (xp). F – fractured spherule rim bits and devitrified whole spherules. Note recrystallized carbonate at upper left and lower right; Hwy 588 site (pp). -6- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 4. Various features of SIL sites. A – Gunflint Formation stromatolites exposed on a glacially truncated surface. While it is not recognizable in the photo, debrisite lies over stromatolites at upper right of the photo. Private Yard site. B – Angular to slightly subangular clast-supported Gunflint Formation breccia with a finer DVIG-rich and calcite-rich matrix, all of which lies directly on Gunflint stromatolites. The angular clasts suggest a short travel distance from their point of origin. C – DVIG clasts within a recrystallized calcite matrix. The silicate devitrification product supports growth of a black lichen, whereas calcite prevents lichen growth. The vesicles are calcite infilled. Private Yard site. D – Orange weathered accretionary lapilli in a recrystallized carbonate matrix. Hillcrest Park. E – Stalactites hanging from top of a vug with agate flowstone deposited on bottom of vug, an indicator of postdepositional subaerial exposure. Hillcrest Park. F – Ocean transgression sequence beginning with an iron-rich alteration profile at the bottom of the photo which marks the top of the debrisite. Above it are rip-ups composed of mudstones or clasts of the iron-rich alteration profile embedded in a carbonate matrix. The boundary between these two units marks a disconformity. The rip-up zone grades into siltstones of the lower Rove Formation at the top of the photo. Original Terry Fox site. Scale is graduated in centimetres. -7- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. A – Earthquake fractured black Gunflint chert. The fractures are thought to have opened during passage of the dilational phase of the earthquake wave, allowing very fine-grained ankeritic sand (light gray) to fill the cracks preventing them from closing back up. Highway 588 site. B – Rectangular to subrectangular earthquake fractured uppermost Gunflint chert-carbonate clasts which delaminated along bedding planes (between dotted lines). These blocks are still more or less in situ, the base surge having failed to rip them up. The more blocky material on top of it is the SIL debrisite. This old Terry Fox site, was removed by highway reconstruction in 2011. stalagmites at one point but, if so, silica deposition continued until they were all encased in a solid agate mass. Unshocked Quartz and Feldspar Grains (some may be ejecta, some are not) Both angular and subrounded detrital quartz and feldspar grains are found in the debrisite at all eight sites, with some thin sections showing as many as 1020 grains per slide. The grains range in size from 40 µm to 800 µm, too small to be seen on this field trip. The angular grains tend to be at the small end of this size range, are shard-like and are likely ejecta. Subrounded grains are probably detrital sand picked up by the base surges flowing across the landscape. Neither grain type is seen in Gunflint Formation rocks. Planar Features in Quartz Grains (ejecta) A few quartz grains show planar features, some of them PDFs as defined by French (1998 and references therein). Nearly all of the planar features have been found within accretionary lapilli at the Hwy 588 site and Hillcrest Park (Figs. 6A-4D). Up to three intersecting sets of PDF are seen in quartz grains, which are typically 50-100 µm in size. A single quartz grain from the Hwy 588 site has planar fractures (Fig. 6D) with their characteristic wide spacing and thick lines (French, 1998). A quartz grain from the Terry Fox upper iron-rich alteration zone shows similar features. The dark lines of PDFs are isotropic quartz glass formed when the high pressure shock waves instantaneously destroy the quartz crystal structure without melting it. They are diagnostic of extremely high pressure shock waves, only obtained in nature by impacts, but they are microscopic and, thus, not seen on this field trip. Spherules (ejecta) Most spherules seen near Thunder Bay are frozen melt droplets ejected during the impact. Spherule sizes range from 50 µm to 1 mm. Except for very rare single spherules, all spherules are clustered, with cluster sizes ranging from 1 mm to 5 cm maximum dimension (Figs. 3E, 5F). Extrapolating from the two dimensions seen in thin section clusters to three dimensions, spherule numbers probably ranged from a few tens of spherules to perhaps 1,000 per cluster. Some spherule clusters contain quartz grains within them. Spherule clusters are the dominant ejecta feature by volume in these debrisites. Despite that, identifiable spherule clusters will not be seen on the field trip. Spherules show prominent rims (Figs. 3D-F). Many rims are round and complete; however, other rims are variously deformed, ranging from slightly ovoid, through ovoid, to totally collapsed where all that remains are two flat rim layers squeezed together -8- �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 6. Planar features in quartz grains within accretionary lapilli. A – two PDF sets; Hillcrest Park; plane polarized light (pp). B – single PDF set; Hwy 588 site; crossed polarizers (xp). C – two PDF sets, with the less distinct set along the right side of the grain; Hillcrest Park (xp). D – planar fractures; Hwy 588 (pp). (Fig. 3E), suggesting that these spherules were hollow. In some cases, a portion of the rim has fractured but remains attached to an otherwise almost intact spherule. Fractured rim pieces are also seen scattered amongst intact spherules, or they are randomly oriented in a cluster, presumably at the site of a former spherule (Fig. 3F). Sometimes the spherules in an entire cluster are collapsed, but in other cases, only a few spherules within a cluster are collapsed. Clusters showing spherules with little or no deformation generally show spherules with 1-3 contact points with adjacent spherules indicating that they have experienced little post depositional compaction (Simonson, 2009). replaced spherules are rimless or else the rims were destroyed during devitrification. Spherule rims range from amorphous features composed of unidentified clay minerals (Fig. 3F) to crystalline silica (Fig. 3E) and calcite rims, and crystalline rims of as yet unidentified minerals. Rims also vary in thickness. Some of the smallest clay- Carbonate replacement of spherules is pervasive. Judging from the remnants, we estimate that >50 % of the spherules have been replaced by recrystallized carbonate, most commonly calcite and less so, dolomite and rarely ankerite. The volume of material Spherules generally show one of two general core types. The first is featureless and composed of as yet unidentified clays. The second core type is crystalline, most commonly calcite, and less commonly silica. The cores are usually centered within the spherule but in some cases they are off-center. The outer boundary of the cores is typically smooth but some cores show botryoidal ingrowths from the core edge towards the center. In other cases carbonate replacement has produced uneven core boundaries. -9- �Proceedings of the 58th ILSG Annual Meeting - Part 2 replaced by carbonate is probably significantly higher than this because portions of thin sections are pure recrystallized carbonate, offering no clue as to what the original material was in those areas. Perhaps the most notable feature of the spherules and spherule clusters is their extensive morphological and compositional variability. Devitrified Glass (ejecta) There are three categories of devitrified glass: 1) devitrified vesicular impact glass (DVIG) clasts and, 2) rare microtektites (<1 mm in size) and tektites (>1 mm in size) and 3) spherules. It will be seen best at the Private Yard site (Table 1.1). DVIG clasts are usually irregularly shaped (Fig. 4C) but splashform (streamlined) shapes are also present (Figs. 2A-C). They range in size from 1-2 mm up to 5 cm. Vesicles in DVIG are usually infilled with carbonate, most commonly calcite. Vesicle shapes range from round (Fig. 3A) to ovoid (Figs. 3B-C). If most vesicles in a clast are ovoid, they show a preferred orientation along their long axes (Figs. 3B, C). Vesicle size, whether within a single clast or between clasts, is also variable (Figs. 3A-C). There are few positively identifiable microtektites and tektites in these deposits but carbonate-replaced microtektite and tektite shapes are more numerous. However, the Gunflint Formation has iron-rich chloritic granules that have many shapes in common with splashform microtektites and are the same size (average 0.8 mm). The two can only be distinguished if some remnant of their internal structure has not been replaced by carbonate. Microtektites and tektites show a blue-gray platy or granular fabric under crossed polarizers, whereas chloritic granules have a blotchy black appearance in plane polarized light. Thus, if a microtektite shape is totally carbonate replaced, there is no way of visually determining whether it was a microtektite or a Gunflint chlorite granule. Accretionary Lapilli (have ejecta and non-ejecta components) Accretionary lapilli consist of fine clasts of accreted target rock, usually quartz, some of which show shock induced planar deformation features (PDFs) and feldspar. The accretionary lapilli form in the base surge from the impact and as such incorporate non-target dust-sized particles picked up by the ground-hugging base surges. Impact-generated base surge dynamics are poorly understood and super-computers are not yet powerful enough to model these complex flows (N. Artemevia, personal communication, 2008). Accretionary lapilli and armored lapilli are found in outcrop only at Hillcrest Park and Hwy 588, and then only within localized areas of the larger exposure at each site. They range in size from 2-13 mm maximum dimension at Hillcrest Park and 5-25 mm maximum dimension at the Hwy 588 site. Lapilli show rounded to subrounded shapes (Figs. 7A-D). Lapilli fragments are present but rare, so they have undergone little breakage. The lapilli range from fairly uniformly gray accreted grains to ones with alternating dark gray, thick bands with thinner bands of very fine black amorphous material (Fig. 7B). The alternating dark gray and black laminations may be repeated up to two times in larger lapilli. The black laminations appear in ~ 35 % of lapilli. By volume, the most common feature in lapilli is 10-50 µm carbonate crystals. Larger carbonate clasts, up to 2.5 mm maximum dimension, form the cores of the few lapilli in which cores are visible. Lapilli show carbonate recrystallization, but where this has occurred, it has enlarged the original crystals only marginally. In contrast, large recrystallized carbonate crystals up to 5 mm in size are abundant in the debrisite outside the lapilli abutting their outer margins, so some unknown factor has inhibited large scale carbonate recrystallization within lapilli. All lapilli also contain 30-250 µm grains of quartz and feldspar, which are found scattered in the gray, coarser-grained areas of the lapilli. Quartz and feldspar grains comprise about 5% of the total lapilli grain population (carbonate grain clusters being the majority). Approximately 1% of the quartz grains exhibit PDFs, making them very difficult to find. The features in the Hillcrest Park lapilli are more poorly defined than those at Hwy 588. The Hillcrest Park lapilli thin sections were prepared from weathered rock, whereas the Hwy 588 lapilli thin sections were prepared from unweathered rock. Geologic Setting The Sudbury impact occurred during a period of tectonic activity along the southern margin of the Superior craton. Prior to the Sudbury event, two interpretations for the area’s geologic setting exist. Kissin and Fralick (1994), Hemming et al. (1995), Van Wyck and Johnson (1997) and Pufahl et al. (2004) see it - 10 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 7. Accretionary lapilli. A – ‘stack of cards’ type accretionary lapilli; Hwy 588 site. Terminology after Schumacher and Schmincke (1991, 1995). B – armored, banded accretionary lapillus. Black band is an extremely fine-grained unidentified black substance. The nucleus is a clast of fine-grained, angular, fractured carbonate; Hwy 588 site. C – accreted material resembling an accretionary lapillus but < 2mm in diametre. Yancey and Guillemette (2008) have called such structures sublapilli, a term which we adopt. Hwy 588 site. D – Accretionary and armored lapilli draped unconformably over a stromatolite, composed of silicified carbonate, which was abraded to its present configuration likely by a base surge immediately preceding the deposition of the lapilli; Hwy 588 site, polished surface. The gray component in all lapilli photos is primarily fine-grained, angular, fractured carbonate clasts whose individual crystals are usually < 10 µm maximum dimension. These clasts are typically < 50 µm in size but they may be as large as 500 µm. Quartz and feldspar grains are a minor component among the carbonate clasts within the lapilli. The black lapilli on the polished surface in D resemble those in A, B and C when seen in thin section. Lapilli from Hillcrest Park are not shown because they are heavily weathered and their features are less distinct. as a backarc basin formed on this margin as extension, possibly subduction roll-back had caused an area of the continental crust to subside and be flooded. An alternative explanation is summarized by Schneider et al. (2002) and Schulz and Cannon (2007) involving successive island arc collisions and the development of a foreland basin which subsided to receive the Gunflint Formation sediments on its northern margin. Pufahl et al. (2010) described the backarc basin evolving into a foreland basin. Chemical sediments (chert, iron oxides and iron carbonates) and volcaniclastics of the Gunflint Formation were deposited onto Archean basement rocks (Gill, 1926; Tanton, 1931; Moorehouse and Goodwin, 1960) in a nearshore marine setting and organized into fining- and coarsening-upwards successions (Fralick and Barrett, 1995) on this open, wave and tide dominated environment (Ojakangas, 1983; Fralick, 1988; Pufahl et al., 2000; Pufahl and Fralick, 2004). Shegelski (1982) interpreted - 11 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 8. A – general stratigraphy of the Gunflint and Rove Formations showing location of the Sudbury Impact Layer (SIL) and the locations of dated zircon. B – more detailed stratigraphy for 10 m above and below SIL. C – composite cartoon of debrisite features from all eight SIL sites. No site shows all features. stromatolites and carbonate at the top of the Gunflint Formation in the Thunder Bay area as a carbonate-rich lagoon environment marking the end of the Gunflint Formation. A depositional hiatus exists between the top of the 1878 Ma (Fralick et al., 2002) Gunflint Formation and the overlying 1832 Ma (Addison et al., 2005) basal Rove Formation, probably caused by the 1860-1835 Ma (Sims et al., 1989) Penokean Orogeny - 12 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 to the south, which resulted in crustal up-warping and withdrawal of the sea (Johnston et al., 2006; Cannon and Schulz, 2009). Alteration of this subaerial Gunflint surface, including development of meteoric calcite cement, silicification, and agate/pyrite veins and vugs, occurred during this time interval (Tanton, 1931; Fralick and Burton, 2008). The SIL lies on this stromatolitic, silicified carbonate surface at the top of the Gunflint Formation (Figs. 8, 9). The SIL is overlain by carbonaceous black shale and grainstone of the Rove Formation which records the end of the Penokean Orogeny and the beginning of crustal relaxation and flooding of the area. The Rove sediment was likely eroded from the Trans-Hudson Orogen to the northwest (Maric and Fralick, 2005; Johnston et al., 2006). Today, the Gunflint and Rove Formations lie on a homocline dipping southeast towards Lake Superior at an average of 5° (Gill, 1926). These rocks remain unmetamorphosed except for localized zones adjacent to diabase sills and dikes (Tanton, 1931). The Debrisite Outcrops Figure 9. Cartoon stratigraphic column as seen at the pre 2010 Terry Fox Lookout rock cut, Hwy 11-17. This is the only complete outcrop exposure extending from the Gunflint Formation, through the Sudbury impact layer and up into the Rove Formation in the Thunder Bay area. Disconformities exist at the Gunflint Formation-sheared debrisite contact and the alteration profile-dolomite contact. The outcrops located since 2005 (Fig. 10) contain ejecta features similar to those seen in the drill cores described by Addison et al. (2005; Fig. 1). However, there are significant variations in the particular ejecta features present from outcrop to outcrop (Table 2) and on a decimetre- to metre-scale within a single outcrop. Only the Terry Fox site (TF) displays a complete Figure 10. Debrisite containing Sudbury impact event ejecta in and near The City of Thunder Bay. Note: one site in a private citizen’s yard is not shown to protect their privacy. Sites 2-6 are either on private property or in city parks and, as such, are “No Hammer” and “No Collecting” zones. - 13 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 GTP abandoned railway rock cut Garden Avenue Quarry area Private yard, Thunder Bay Highway 11-17 at Terry Fox Lookout Hillcrest Park, Thunder Bay yes no yes yes no no no yes yes yes ?? yes yes no no no no ?? no yes yes yes no yes yes yes no no no no no no yes yes yes yes yes yes yes yes no no no no yes no no yes no no no yes no yes yes yes yes yes yes yes yes yes ?? yes yes ?? no ?? no no no no no no no ?? no yes yes no yes yes yes no no no yes no yes ?? no no ?? ?? no yes no yes yes no ?? yes no no yes yes yes yes yes yes yes yes no yes no ?? no no no yes no no no yes yes yes no yes 0.4 0.4 0.5 0.6 ~2 ~2 2.7 ~4 Site Feature Stromatolites or microbialite mats in situ below debrisite base Ripped up stromatolite or microbialite clasts in debrisite Subrectangular Gunflint blocks, >0.5 m maximum dimension in debrisite Fractured black chert or chertcarbonate more or less in situ below ejecta base Angular chert clasts and shards, sub-cm to 3 dm max. dimension in debrisite Post-depositional anastomosing chert in debrisite Alteration profile (possible paleosol) below base of debrisite Devitrified vesicular glass with carbonate in-filled vesicles Accretionary lapilli Microtektites in debrisite thin sections Gunflint Formation iron granules PDF in quartz or feldspar grains/shards Isotropic quartz containing crystallites Subrounded to angular quartz & feldspar grains in debrisite matrix Top of debrisite deposit visible Bottom of debrisite deposit visible Approximate debrisite thickness (m) - 14 - Banning St. bluff below Waverly Towers Baseball Central, Central Ave. Highway 588 ditches Table 2. Each debrisite site has a unique combination of ejecta and non-ejecta features that are summarized in Table1. Please note that “no” has two meanings: 1. the feature may not be present at all at that site and; 2. we have not found the feature but it may be present. For instance, more thin sections might show PDFs where none are currently found. “??” means that the evidence for this feature is weak and ambiguous. Again, more thin sections might resolve the ambiguity. �Proceedings of the 58th ILSG Annual Meeting - Part 2 stratigraphic column extending from the Upper Gunflint Formation, through the debrisite and up into the Rove Formation (Fig. 9). The other seven sites (Fig. 10) are all erosively truncated. Most of these sites are also briefly described in Jirsa et al. (2011). All of the ejecta-bearing debrisites, except the TF site, are seen primarily in plan view and range in area from as little as 10 m2 at the Highway 588 (Hwy 588) site to over 1000 m2 at Baseball Central and Garden Avenue. Most sites also show some portion of themselves in cross-section. Preserved debrisite thickness ranges from 0.4 m at the Hwy 588 and Grand Trunk Pacific Railway (GTP) sites to 3.5-4 m at Hillcrest Park (Table 2). Weathered accretionary lapilli are present and are confined to a localized area comprising <5 % of the total exposure face. The patch of 3-13 mm diameter lapilli (Fig. 7D) is located 1.5-2.5 m above the base of the exposure. Planar features and PDFs are present in quartz grains within accretionary lapilli (Figs. 4A, 4C). Planar features have not been found in quartz grains outside accretionary lapilli within the debrisite matrix. Scattered angular Gunflint chert and chert-carbonate rip-ups range in size from 0.5 m maximum dimension near the base of the deposit to 1-2 mm near the deposit top. One disintegrating heavily weathered round granitic boulder 33 cm across lies at the base of the debrisite. The SIL shows four major components: 1) a matrix of carbonates, commonly dolomite and calcite and least commonly ankerite; 2) Gunflint Formation clasts in the submillimetre to metre size range; 3) ejecta and; 4) minor components of uncertain origin such as subrounded quartz and feldspar grains. The debrisites are chaotic, showing large variations in the percentage of the various components both in surface and crosssectional exposures. Gunflint Formation clasts show upward fining when seen in cross-section thicknesses >1 m, whereas there is little evidence of upward fining in the other components. Carbonate-replaced microtektites may be present based upon size and shape of some features. However, carbonate-replaced Gunflint Formation chlorite granules have sizes and shapes similar to microtektites making it impossible to distinguish between the two if they are totally carbonate replaced as described previously. So far, only one confirmed microtektite has been observed at Hillcrest Park compared to tens of carbonate-replaced microtektite or granule shapes. N.B. The only sites on public land are Highway 588 and Terry Fox on Highway 11-17. Please respect “no hammering” and “no collecting” at all other sites. Spherules appear in clusters in which the spherules are frequently deformed or crushed. Many apparent spherule clusters are heavily altered by carbonate replacement making it difficult to determine whether the feature is carbonate-replaced DVIG or whether they are really spherules. Most ejecta features at Hillcrest Park are poorly preserved because of a combination of carbonate or silica replacement and weathering. Stop 1. Hillcrest Park UTM coordinates: NAD83; 16U 0334728E / 5366952N Hillcrest Park has debrisite exposures on a dip slope with a true thickness of 3.5-4 m, the thickest of all exposures. However, it was once thicker because it lacks a carbonate cap topped by shale, which marks the transition from the debrisite to the Rove Formation seen at the Terry Fox site and in drill cores (Addison et al., 2005). An intermittent, erosively truncated, 5-15 cm thick microbialite layer lying on Upper Gunflint chert-carbonate lies beneath the debrisite. The Hillcrest Park lane cliff face shows four chaotic, undulating, largely ungraded lenses with one lens displaying a prominent U-shaped channel. The lenses become thinner upwards. Each shows a heterogeneous mix of features and a chaotic patchiness at decimetreto metre-scales. Irregularly shaped DVIG clasts up to 2 cm maximum dimension are a common debrisite feature. Some DVIG vesicles are ovoid or totally flattened. This is the best site to view post depositional anastomosing black chert and light gray and black banded agate in the debrisite. The agate usually appears to have been deposited in vugs. Centimetre-sized silica stalactites occur in two vugs (Fig. 4E) near the top of the debrisite exposure. Stop 2. Private Yard (no UTM coordinates to protect owner’s privacy) A bedrock exposure, about 5 m by 15 m, in a private yard in Thunder Bay contains a spectacular debrisite exposure composed mainly of Gunflint chert-carbonate breccia (Fig. 4B) and ejecta, primarily DVIG, which is surrounded and partially replaced by blocky calcite - 15 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 cement (Figs. 4C, 3C, 2C). The debrisite remnant preserved here is 0-0.5 m thick and unconformably overlies stromatolites and chloritic grainstone of the uppermost Gunflint Formation (Fig. 4A). An iron-rich alteration zone exists approximately 30 cm below the erosive contact between the debrisite and the Gunflint bedrock. DVIG clasts are up to 2 cm across. Vesicles range from round to ovoid to nearly flat. Angular quartz and feldspar grains, chert shards, and chloritic granules are also present. Quartz grains with PDFs have not been found here. Stop 3. Banning Street Bluff (BB) UTM coordinates: NAD83; 16U 0335129E / 5367236N A bluff at the north end of Banning Street shows both SIL and Upper Gunflint Formation clast-supported breccia composed of cobble to boulder-size clasts as large as 3-4 m maximum dimension, separated in places by pyritic and carbonaceous black shale similar to the Rove Formation.. There are both subrectangular Gunflint Formation chert-carbonate blocks and manysided, nearly equidimensional chert blocks. As with Gunflint breccia and clasts at other sites, none show weathering rinds. Stromatolite clasts rest upside down and on their sides in the debrisite. The only ejectabearing debrisite lies at the base of the breccia pile, the inverse of the sequence at the GTP site. Other non-ejecta features include millimetre-scale, sharply angular chert fragments. One chert fragment contains chloritic granules similar in shape and size to microtektites. Three ejecta-bearing 26 mm by 45 mm thin sections showed only one subrounded quartz grain. Postdepositional anastomosing chert is present but it is not nearly as common as at Hillcrest Park. There is meager evidence of ejecta in the BB debrisite with DVIG clasts and both crushed and uncrushed spherule clusters being the most obvious ejecta features. A 250 µm clast showed one set of enigmatic planar features in a quartz crystal within it, plus a second crystal with two sets of possible relict PDFs. Microtektites were not observed in the BB thin sections. We interpret this site to be a slide deposit which occurred after the SIL layer was lithified and after transgression by the Rove Sea. Stop 4. Highway 11-17, Terry Fox Lookout (TF) UTM coordinates: NAD83; 16U 340112E / 5372511N The Terry Fox site today was created by Hwy 1117 reconstruction in 2011 (Fig. 11). The old rock face, now removed for fill, was about 40 years old and its weathered surface showed a number of faint features brought out by the weathering (Fig. 12). Had we not had the benefit of the weathered surface we probably would not have been able to interpret the new rock face (Fig. 11). The following description is based on the now-removed outcrop. Given another 3-5 decades, the new face will probably resemble the old one. This is the only outcrop showing a complete ~ 3 m cross-section of the ejecta-bearing debrisite layer extending from Gunflint chert-carbonate up into the basal Rove Formation, which is overlain in turn by a diabase sill (Figs. 9, 10). An iron-rich alteration profile, heavily replaced by secondary pyrite, lies ~ 1 m below the base of the debrisite and a few metres northeast of the main outcrop. The basal SIL is a recessively weathering, locally sheared, clastic layer about 0.5 m thick containing crushed spherule clusters, some of which are aligned subvertically instead of in the usual subhorizontal position. Several sets of subhorizontal slickensides, whose striae are aligned at a 140º azimuth, are found at various levels within this layer. Postdepositional anastomosing chert has replaced much of this basal sheared layer, obliterating considerable structural detail. Non-ejecta features include centimetre to millimetre-sized angular chert clasts and angular, subrounded to round Gunflint Formation iron carbonate clasts plus two rounded crystalline rocks with prominent alteration rinds. The presence of clasts with weathering rinds reinforces the idea that Gunflint clasts lacking such rinds were freshly fractured by impactgenerated earthquakes before being incorporated into the debrisite. The main body of the 2.2 m thick debrisite lies in sharp contact over the basal sheared clastic unit. It is so heavily replaced by recrystallized dolomite that any possible ejecta features are only seen as vaguely outlined shapes on weathered surfaces (Fig. 13) or in thin section. Almost all detail, including any vesicles in possible DVIG shaped clasts, has been destroyed. Tektites and microtektites may be present, based upon shape and rare faint devitrification textures. A single, polycrystalline, rounded quartz grain shows faint planar features. Both angular and rounded millimetre- - 16 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Ocean transgression sequence Pyritic (iron-rich) alteration profile Recessively weathering spherule-rich layer Ejecta-bearing SIL debrisite (2.5 m) SIL basal horizontally sheared zone Earthquake shattered Gunflint chert ccarbonate Figure 11. The current Terry Fox outcrop is a nearly featureless gray carbonate face. Given several decades of weathering, faint features within the debrisite should begin to appear as in Figure 12. White vertical scale is 1 m. Rusty area is weathered rock. scale chert clasts are also present, but not common. A 5-20 cm thick undulating, dark brown, recessively weathering, spherule-cluster-rich layer appears as a groove across the cliff face at the top of the dolomitereplaced debrisite. This mass of spherule clusters is much more concentrated than seen at any other location or than is suggested by faint shapes in the main dolomite-replaced layer immediately beneath it. These concentrated clusters seem to be the residuum of a thicker layer. Plentiful thin anastomosing post depositional chert strands weave through this spherulerich material but on a much finer scale than at Hillcrest Park. Red-brown agate 3-8 cm thick lies on top of the spherule-rich layer. Laterally discontinuous vertical digitate projections extend down from the top and project up from the base of this agate layer. They are similar in shape and size to the agate stalactites in vugs at Hillcrest Park (Fig. 4E), except that in this case the spaces between the projections were subsequently infilled by more agate. The red-brown colour is similar to that of the iron-rich alteration profile overlying it but it is a less saturated hue. An iron-rich alteration profile on top of the spherulerich layer, consisting of hematite has been largely replaced by secondary pyrite. Prominent deformed spherule clusters are locally present. The total thickness of all these ejecta-bearing layers is 3 m. The top of this iron-rich layer marks a return to carbonate deposition. The basal 10-15 cm of this 80100 cm thick carbonate zone is unstratified and shows dark, angular, commonly rectangular, millimetrecentimetre-size rip-up mudstone clasts and probable Gunflint Formations clasts (Fig. 4F). This is followed by millimetre- to centimetre-scale layered carbonate strata topped by a zone with a few poorly defined, - 17 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Diabase Sill Rove shale Ocean transgression sequence Iron-rich alteration profile & recessively weathering spherule layer Ejecta-bearing base surge debrisite (2.2 m) Earthquake shattered Gunflint bedrock-sheared debrisite contact zone Unshattered Gunflint chert carbonate Figure 12. The weathered Terry Fox outcrop on Highway 11-17, Dec. 26, 2010. The weathered surface brought out faint features that had been carbonate replaced, notably devitrified vesicular impact glass (DVIG) shapes and tektites. Pick is 0.9 m tall. laterally discontinuous beds containing centimetrescale, angular carbonate clasts. The carbonate then makes an abrupt transition to 1015 cm of gray siltstone and is overtopped by 10-15 cm of black, rusty weathering shale characteristic of the Rove Formation. The black shale is interrupted by 5 cm of chert before returning to 0.9-1.2 m of black, rusty weathering shale which is overlain in turn by a diabase sill more than 8 m thick. The shale is less friable than typical lower Rove shale, probably the result of low grade metamorphism induced by the overlying sill. Stop 5. Grand Trunk Pacific Railway Rock Cut (GTP) UTM coordinates: NAD83; 16U 0326399E / 5363836N A cut through an outcrop knob on the abandoned GTP right-of-way, approximately 0.5 km east of Mapleward Road and north of Highway 11-17, shows about 4 m composed of Upper Gunflint Formation clast- - 18 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 chloritic granules are abundant at this site but single granules are rare within the secondary carbonate cement. Stop 6. Highway 588 (Hwy 588) UTM coordinates: NAD83; 16U 0307539E / 5357977N Figure 13. Weathered carbonate-rich debrisite surface of pre-2011 Terry Fox rock cut. The rounder light features are likely microtektites or DVIG, however, they are sufficiently indistinct that this is not certain. Many of the more angular features are probably large recrystallized carbonate (likely dolomite) crystals. The reddish tinge is due to a light iron staining. supported, boulder-sized breccia lacking visible ejecta features topped by up to 0.4 m of DVIG-rich debrisite similar to that seen at the private yard. The Gunflint breccia blocks range from well rounded to angular. The largest clast is a rectangular calcite-cemented slab of grainstone 0.4 m thick by 5 m long containing an upside down stromatolite indicating that the slab is overturned. The small amount of matrix between blocks appears similar to the blocks but finer grained. The dip of the Gunflint beds increases westward, suggesting a nearby fault beneath overburden. The east side of the knob had exposures of fractured but in situ, sharp-cornered Gunflint chert-carbonate. The fractures remain closed and did not show any infilling of the ankeritic grainstone seen at Hwy 588. This exposure has now been destroyed by ATV traffic. The overlying ejecta-bearing debrisite is dominated by irregularly shaped and splashform DVIG clasts (Figs. 2A &B). Vesicle shape ranges from round (Fig. 3A) to ovoid (Fig. 3B) to crushed (Figs. 3E &F) depending on the clast examined. All vesicles but the crushed ones are infilled with calcite. A small number of deformed spherule clusters are present. A single 1.0 mm accreted sublapillus (pellet) composed of quartz and feldspar grains, similar to accretionary lapilli, is present but accretionary lapilli (>2 mm diametre) are absent. Rounded to angular sub millimetre quartz grains are present. Gunflint Formation clasts containing blotchy black When first observed in 2000, the Hwy 588 outcrop was a bedrock exposure in the ditch on the northwest side of the highway, 2.4 km southwest of the hamlet of Stanley. It was a glacially polished and striated surface showing erosively truncated stromatolites up to 0.5 m diametre, some of which were surrounded by accretionary lapilli 3-25 mm in diametre (Fig. 7D). Ankeritic grainstone and chloritic grainstone surrounded other stromatolites. This exposure was subsequently blasted to deepen the ditch and the blasted rock now lines the ditch slopes, giving a highly fragmented cross-section and plan view of the exposure. Since then we have exposed bedrock in the ditch about 50 m southwest of the first exposure. It shows a glacially striated surface of exposed stromatolites and shattered, but in situ black chert with an ankeritic grainstone filling in the cracks. The chert is assumed to have fractured during the compressional stage of impact-triggered earthquake waves with the fractures then opening during the dilational wave phase. Fine granular material then fell into the openings, preventing them from closing and subsequently the material was lithified (Fig. 5A). Thin sections prepared from the blasted material show a variety of ejecta features, the most obvious being accretionary lapilli (Figs. 7A-D) which have yielded quartz and feldspar grains showing planar deformation features (PDFs) and planar fractures (Figs. 6B &D). Planar features have not been found in larger subrounded and angular quartz and feldspar grains contained within the debrisite generally as opposed to within accretionary lapilli. This is the only site in which DVIG is not the most obvious ejecta feature within the debrisite. In fact, no DVIG has been observed, however carbonate and silica replaced clusters of spherules are present (Figs. 3E &F). Non-ejecta features include subrounded to round chert grains in carbonate cement, subcentimetre stromatolite fragments and mudstone and shale ripups. Chloritic, blotchy, black Gunflint Formation granules, similar in shape and size to microtektites, are present within the carbonate cement. Carbonatereplaced microtektite shapes are present but since - 19 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 they lack residual internal structure, it is impossible to determine if they were microtektites or carbonatereplaced Gunflint chlorite granules. Optional stops - Garden Avenue Quarry (GA) and Baseball Central (BC). These two outcrops will not be visited on the trip because their features are similar to the outcrops we will visit. However, should you wish to visit them their locations follow. Garden Avenue Quarry, accessed off Hwy 11-17: UTM Coordinates: NAD83; 16U 032695E / 5363209N. eroded lower Rove Formation plus the underlying SIL, leaving topographic depressions, most of which were subsequently post-glacially infilled by lakes, swamps or sediments. Where erosional surfaces have reached the Upper Gunflint Formation chert and chert-carbonate, these more resistant layers have provided a floor to further erosion which persists today. Thus, the few remaining ejecta-bearing debrisite outcrops are small erosional remnants of a once extensive SIL debrisite sheet exposed along the Rove-Gunflint contact. Fractured Gunflint Formation Bedrock and Gunflint Rip-ups These eight small outcrop areas appear anomalous given the large area over which debrisite must have been originally deposited. Initially, we anticipated finding the SIL along the entire exposed length of the contact between the Gunflint and Rove Formations. Not so. In Ontario the Upper Member of the Gunflint Formation is a widespread “very complex unit” with “beds of ferruginous carbonate and chert” (Moorehouse and Goodwin, 1960). The chert ranges from chalcedony to microcrystalline quartz. We hypothesize that this surficial or near-surficial Gunflint chert-carbonate was fractured by the powerful earthquakes which arrived in the study area approximately two minutes after impact (Marcus et al., 2000, Earth Impact Effects Program: http://impact.ese.ic.ac.uk/ImpactEffects/), providing much of the sharply angular chert breccia subsequently embedded in the overlying debrisite (Fig. 8B). However, other sub centimetre chert clasts embedded in the debrisite are quite rounded, yet their appearance is indistinguishable from Gunflint chert. This suggests that such clasts are Gunflint chert and that they underwent extended travel and abrasion within a ground-hugging density current. These detrital clasts may have been produced by conventional erosional processes but this type of detrital material has never been noted elsewhere in the Upper Gunflint Formation. The Gunflint Formation outcrops sporadically from Thunder Bay to the Slate Islands in Lake Superior 165 km east of Thunder Bay (Sage, 1991). Thus, it is possible for small chert clasts to have traveled as much as 150 km in the violent base surge environment, perhaps producing the rounded clasts from ripped up earthquake-fractured chert. The basal 9.8 m of the Rove Formation consists of siltstone and friable shale interspersed with forty-six, 2-15 cm thick, poorly lithified, greenish-gray tuff beds (Maric, 2006). Just a single year of weathering has reduced the tuff beds in exposed drill cores to flaky mush. Two further multi-layered tuffaceous zones occur in the 60 m above this basal zone. Thus, wherever the lower Rove Formation and ejecta-bearing debrisite were exposed, pre-glacial weathering, glacial gouging and post-glacial weathering have removed the easily The Hwy 588 (Fig. 5A) and Terry Fox (Fig. 5B) sites are alone in showing fractured but still in situ Gunflint Formation chert bedrock with ankeritic grainstone deposited in the cracks between the blocks. The depth of this widespread fracturing at the Hwy 588 site is unknown because no visible vertical section is present but, judging from blasted bedrock lining the ditch banks, fractures are estimated to have penetrated as much as 0.5 m based on what is seen in cross-section at the Terry Fox site. The GTP site also shows in situ Baseball Central, accessed off Central Avenue: UTM Coordinates: NAD83; 16U 332333E / 5364300N. Interpretation and Discussion Do All Eight Sites Contain Sudbury Ejecta? Only the Hillcrest Park and Hwy 588 outcrops contain shocked quartz with PDF sets indicating that their debrisites are of impact origin, which leaves the origin of the debrisites at the other six sites open to question. However, a variety of features (Table 1) indicate that all sites share a common origin, namely their common stratigraphic position, their chaotic nature, upward fining Gunflint Formation clasts, DVIG clasts, spherules, and iron-rich alteration profiles, to name the more obvious ones. Therefore, all eight outcrops are attributed to the Sudbury impact event. So Extensive a Deposit, So Few Outcrops - 20 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 fractures outlining rectangular blocks of Gunflint chert-carbonate, but the fractures remain closed and are not infilled. The fractures at these Gunflint sites are smaller in scale than those in Archean granite attributed to Sudbury event earthquakes at Silver Lake, Michigan (3.8r) by Cannon and Schulz (2008). There, Paleoproterozoic sediments were injected into the Archean granite fractures. The shattering of the Hwy 588 chert may well be similar to events at Silver Lake where it is suggested that the dilational phase of seismic waves opened fractures, allowing emplacement of overlying soft sediments into the openings (Cannon and Schulz, 2008). Bedrock fractures in the Barton Creek dolomite at Albion Island, Belize are similarly ascribed to seismic fracturing during the Chicxulub event by Ocampo et al. (1996). There is no evidence of in situ bedrock fracturing at other sites. However, there is ample evidence of fractured Gunflint carbonate and chert-carbonate and stromatolites in the form of rectangular blocks and clasts within the debrisite, at other sites. These blocks suggest that near-surface Gunflint Formation chertcarbonate and carbonate was seismically fractured and delaminated along bedding planes and that some of this fractured material was ripped up by base surges and incorporated into the debrisite. Similarly deposited angular to sub angular clasts are reported from the Chicxulub event in Belize (Kenkmann and Schönian, 2006). The lack of alteration rinds on either rounded or angular Gunflint breccia fragments at any site suggests that they were not derived from pre-impact features such as weathered talus. However, alteration rinds typical of weathered surfaces are present on a rounded granite boulder and on an unidentified crystalline cobble at the base of the debrisite at Hillcrest Park and on two crystalline cobbles in the basal shear zone at the Terry Fox site. The presence of weathering rinds on these non-Gunflint clasts supports the view that Gunflint clasts, all of which lack weathering rinds, were derived from freshly earthquake shattered Gunflint bedrock and subsequently ripped up and incorporated into the debrisite by the base surge. Deposits of Gunflint breccia and lapillistone are found between the Gunflint and Rove Formations at Gunflint Lake, Minnesota, 760 km (5.8r) from Sudbury (Jirsa et al., 2008, 2011). The areal extent of the 7 m thick Gunflint Lake debrisite is much greater than comparable Gunflint breccia zones at the BB and GTP sites at Thunder Bay. It is also nearly twice as thick as any Thunder Bay site. This contradicts the general westward thinning of the SIL (Addison et al., 2005). If the Gunflint Lake deposits were emplaced by base surges, and the base surges had lost sufficient energy by the time they reached the Gunflint Lake area, the entrained debris could have piled up into thick ‘ramparts’ as described for end-of-flow Martian base surge deposits (Kenkmann and Schönian, 2006; Osinski, 2006; Mouginis-Mark and Garbeil, 2007). Basal Shear Zone The slickensides seen at the base of the Terry Fox debrisite are similar to “highly chaotic shear planes often connected with polished and striated surfaces…” described for Chicxulub debris by Kenkmann and Schönian (2006) and Wigforss-Lange et al. (2007), and to “a thin basal shear zone” seen in the Stac Fada Member debrisite in Scotland (Amor et al., 2008). The slickenside striae are aligned at an azimuth of 140º, which supports the idea that the slickensides are related to drag shearing during deposition of the overlying fast moving base surge arriving from Sudbury, which lies at an azimuth of 108°. However, a fault just north of the new entrance to the Terry Fox Welcome Centre may also have produced these locally present slickensides. Peritidal or Subaerial Depositional Environment? It seems intuitive, with the chaotic SIL resting directly on microbialite mats and stromatolites, that it was deposited in peritidal lagoon environments that were subsequently reworked by tsunamis. As observations accumulated we were forced to reexamine this idea. Was the Area Subaerial Prior to Debrisite Deposition? The uppermost 30 m of the Gunflint Formation shows an upward-shoaling succession from thin fine grainstone layers in chemical and clastic mudstone to dominantly grainstone layers to thicker and coarser ripple-laminated and cross-stratified grainstones. These are overlain by the chert-carbonate, stromatolitic limestone, and grainstones, which are erosively terminated by the overlying debrisite. This falling stage sequence suggests that the area was nearly emergent prior to emplacement of the SIL, but it does not show that the area was subaerial at the time of SIL deposition. Moorehouse and Goodwin (1960) noted that the uppermost Gunflint Formation is composed of a thin, calcite-rich unit which they designated the Limestone - 21 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Member. This is the material that directly underlies the debrisite in many locations. It is composed of ironchlorite grainstone and iron-chlorite-rich layers in stromatolites that are cemented by blocky calcite. This calcitic cement is probably meteoric in origin (Fralick and Burton, 2008). This cement shows 100x vanadium and 10x uranium enrichment relative to lower Gunflint Formation background levels, indicative of a redox front in a subaerial environment (Fralick and Burton, 2008). Was the Area Subaerial During SIL Deposition? This is really a question of whether the SIL was deposited by base surges, by tsunamis, or by some combination of the two. Despite the top of the Gunflint Formation being a gently sloping, recently subaerial shallow lagoon environment with an ocean towards the south, the evidence for these being tsunami-emplaced deposits is weak. We see no evidence of tsunami lithologic couplets created by wave runup and backwash (Nishimura and Miyaji, 1995; Scheffers and Kelletat, 2004; Fujino et al., 2006). Nor is there any evidence of sand or other particle injection into the Gunflint substrate, a feature created by very high dynamic pressures of large tsunamis (Le Roux and Vargas, 2005). On the other hand, like the K/P boundary, these deposits are “sedimentologically complex, differing in architecture and composition from place to place” (Smit et al., 1996). However, in the SIL the complexity is very localized compared to the K/P deposits and it lacks the multi-unit stratigraphy described by Smit et al (1996). Thus, these SIL debrisites are different from the K/P deposits which are ascribed to large tsunamis even though both share some common ejecta. The heterogeneous distribution of devitrified glass clasts also argues against tsunami deposition of these deposits. These clasts range from tektites (once solid, non vesicular glass) with a density of about 2200 kg/ m3 to highly vesicular clasts, some of which may have been able to float on water. With this range in density, some clast sorting based on density would be expected during tsunami deposition, with the highly vesicular clasts being preferentially laid down towards the top of the deposit and with a higher proportion of the least vesicular, denser clasts deposited towards the bottom of the debrisite. This should be especially notable in the waning stages of tsunami wave recession. We observe no evidence of this. Tsunamis cannot be totally ruled out for the deposits described here. If such tsunamis were weak and reworked only the topmost freshly deposited debrisite, the record of such could have been erased during postdepositional subaerial exposure (discussed below). But, had a tsunami reworked the top of base surge deposited debrisite, water would presumably have settled into the hot, non-reworked debrisite and we would expect to see fluid or vapour escape pipes. We have seen none, suggesting that even weak tsunamis never reached these deposits. The absence of key tsunami features casts doubt on the SIL debrisite being deposited or reworked by tsunamis. Base surge deposit features are present. The TF site and Hillcrest Park are the only sites with sufficient thickness to see the structures typical of base surge deposits and the TF site is so heavily overprinted by dolomite recrystallization that structures within it are barely visible. U-shaped channels and massive bedding are among the typical volcanic base surge features (Hattson and Alvarez, 1973; Fisher and Schmincke, 1984; Gencalioğlu-Kuşcu et al., 2007; Branney and Brown, 2011). Volcanic base surge beds show distinct upward fining (Fisher and Schmincke, 1984; Dellino et al., 2004). Upward fining in these SIL beds is either absent or, at best, ambiguous, with the notable exception of upward fining of Gunflint clasts in these otherwise chaotic features. Like tsunami deposition, key features of base surge deposits are obscured or missing because of the small scale of the outcrops relative to the scale of the SIL, or the glacial truncation of most outcrops, or, in the case of the TF site, the massive carbonate recrystallization which obscures so much detail. The question of whether these debrisites were deposited by tsunamis or base surges cannot, at this stage, be unequivocally answered, but the evidence obtained to date supports base surge deposition of the SIL at Thunder Bay. Future work comparing features of the Thunder Bay sites to sites elsewhere in the Lake Superior region may help to answer the question more definitively. Did the Area Remain Subaerial After Debrisite Deposition? There is 5-6 m of sediment between the top of the 1850 Ma (Krogh et al., 1984) Sudbury impact event debrisite and dated zircons from three sites in the Rove Formation and one site in the correlative Virginia Formation with an age of 1832 Ma (Addison et al., 2005). This low sedimentation rate for an approximately 18 m.y. period suggests a depositional hiatus. The disconformity at - 22 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 the debrisite-Rove Formation contact seen at the TF site (Fig. 8F) also supports this view. In addition, the silica stalactites at Hillcrest Park (Fig. 8E) could only have been produced in a subaerial environment. There is a sequence of three lithofacies at the top of the TF debrisite that offers important support for a prolonged period of subaerial exposure: 1) The lowermost of the three lithofacies is the recessively weathering, spherule and DVIG-rich layer. It suggests that a significant but unknown thickness of the carbonate component of the debrisite was leached away, leaving behind a winnowed spherule-DVIG residuum. 2) The 2-8 cm thick layer of agate and chert with upward and downward pointing projections within it, which may have been stalactites and stalagmites before the void was totally infilled by agate and chert, seems to be the product of leaching of overlying material and redeposition at this lower level. 3) The 5-30 cm thick iron-rich alteration profile at the top of the TF site may be a paleosol. It also contains spherules, DVIG, and microtektites. This paleosol hypothesis will remain so until further work tests the idea, but it is in the zone where a paleosol would be expected and the concept is consistent with the other interpretations. There was a period of subaerial exposure after deposition of the SIL but its duration is unknown. The great mystery is how any unconsolidated debrisite survived such a long period of subaerial exposure. How Did Any Debrisite Survive? The reasons for debrisite survival are unknown but the Rove Formation suggests a possible mechanism for debrisite preservation. Its lower 9.8 m contains 46 tuff layers, which decrease in frequency from seven layers per metre at the base of the deposit to zero within that thickness (Maric, 2006). The combination of tuffs found in the Gunflint Formation below the SIL, combined with the 46 tightly spaced tuffs immediately above the SIL in the Rove Formation, suggests tuff deposition may have been ongoing during the depositional hiatus. If so, the tuffs may have borne the brunt of weathering during the period of subaerial exposure rather than the debrisite. The tuffs may also have provided silica leachate which was subsequently deposited as the anastomosing chert and agate throughout the middle to upper SIL seen at Hillcrest Park and at BB and BC sites, thus helping to preserve it. Sumary and Conclusions Eight SIL outcrops containing ejecta from the 1850 Ma Sudbury impact event have been identified in and near the city of Thunder Bay, Ontario, north of Lake Superior. The SIL was likely deposited by base surges on a subaerially exposed carbonate succession forming the top of the Gunflint Formation. The primary debrisite component by volume is recrystallized carbonate in which Gunflint chert and chert-carbonate breccia and ejecta are embedded. Today, ejecta are a minor component of the total debrisite volume, however, at the time of deposition, it was undoubtedly greater because carbonate replacement has destroyed many features, while recrystallization of carbonate further obscured features. Ejecta features include shocked quartz grains with relict planar features including PDFs and planar fractures, unshocked quartz and feldspar grains, spherules, DVIG clasts, rare microtektites and tektites and accretionary lapilli. Seven of the eight sites have had some portion of the Sudbury impact layer (SIL) removed by glaciation and subsequent weathering. The eighth site near Terry Fox Lookout shows a complete stratigraphic section from the Gunflint Formation, through the SIL and up into the overlying Rove Formation. Disconformities appear at both the base and top of the SIL. The study area has had a complex history, summarized as follows. 1. The upper Gunflint Formation shows an upward fining sequence ending with mafic volcanic ash being deposited and reworked into a carbonatedominated, nearshore environment supporting microbialite mat growth and stromatolites. 2. Regression of the Gunflint Sea was completed at some unknown time prior to the 1850 Ma Sudbury impact event. Prior to deposition of the SIL, blocky, meteoric calcite cements formed beneath the subaerial surface. 3. Approximately two minutes after the impact, violent earthquakes fractured and delaminated lithified portions of the Upper Gunflint Formation, as evidenced by still in situ fractured rock at the Terry Fox, Hwy 588 and GTP sites. 4. The earthquakes were followed by ground-hugging density currents (base surges) which stripped all unlithified material down to bedrock and ripped up, ground up and entrained some portion of the earthquake-fractured upper Gunflint Formation rock. The base surges then contained the following mixture of features: 1) clasts of fractured carbonate in the fine sand to fine gravel size range; 2) ripped up clasts of Gunflint fractured chert, chertcarbonate and stromatolites; 3) ejecta consisting of - 23 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 DVIG, spherules, accretionary lapilli, tektites and microtektites, and quartz and feldspar grains and shards, some of which show planar features and PDFs; and 4) small clasts of uncertain origin. 5. The sharply angular nature of most Gunflint chert and chert-carbonate clasts indicates a relatively short travel distance. Slightly rounded chert-carbonate clasts are less common and probably traveled only slightly further from their source than the angular ones. None of these clasts show weathering rinds. Some submillimetre- and millimetre-scale chert clasts are well rounded and could have traveled as much as 165 km from the furthest east Gunflint Formation known today at Slate Islands. 6. Accretionary lapilli formed within the base surges. Some accretionary lapilli passed through zones with varying water vapour concentrations, allowing them to accumulate alternating coarser-grained layers and finer-grained layers. Armored lapilli are also present. 7. The debrisites deposited by these base surges show chaotic patchiness with significant changes in clast sizes and composition over metre and even centimetre distances within the deposits. The one exception to this chaos is an upward fining of Gunflint clasts within the otherwise chaotic debrisite. 8. The SIL was subaerially exposed after deposition as evidenced by anastomosing chert and agate within the debrisite, centimetre-scale agate stalactites in debrisite vugs and the tri-level lithofacies of relict winnowed spherule clusters, agate, and iron-rich alteration profile at the top of the TF site. Silica and carbonate replacement and recrystallization probably began during this period of subaerial exposure. An unknown quantity of debrisite was removed during this period of subaerial exposure. 9. Tuffs deposited on top of the debrisite may have provided sufficient protection to allow survival of some of the debrisite. They could also have provided leachate which led to the extensive deposition of anastomosing chert in the SIL seen at both megascopic and microscopic levels. 10. The Rove Sea then transgressed over the area, first depositing about one metre of carbonate and siltstone before the lower Rove Formation organicrich mud began accumulating, intercalated with numerous volcanic ash layers. 11. Compaction and carbonate replacement during diagenesis probably continued destroying features in the debrisite. Silica replacement did the same to a lesser extent. Carbonate recrystallization further destroyed or obscured features. Acknowledgements The following have graciously assisted us and educated us: Bill Cannon, Don Davis, Phil Fralick, Mary Louise Hill, Pete Hollings, Mark Jirsa, Steve Kissin, Paul Knauth, Jon North, Dick Ojakangas, Rick Ruhanen, Klaus Schulz, Mark Smyk, Daniela Vallini, Paul Weiblen, and Laurel Woodruff. Our heartfelt thanks to all of the above and to Lakehead University for giving us access to its labs and instruments. Large portions of this guide have been imported from: Addison et al. (2010). 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Range Supergroup: implications for tectonic setting of the Paleoproterozoic iron formations of the Lake Superior region: Canadian Journal of Earth Science, v. 39, p. 999-1012. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4-25 Schumacher, R., and Schmincke, H.-U., 1991, Internal structure and occurrence of accretionary lapilli – a case study at Lacher See volcano: Bulletin of Volcanology, v. 53, p. 612-634. Schumacher, R., and Schmincke, H.-U., 1995, Models for the origin of accretionary lapilli: Bulletin of Volcanology, v. 56, p. 626-639. Shanmugam, G., 2006, The tsunamite problem: Journal of Sedimentary Research, v. 76(5), p. 718-730. Shegelski, R.J., 1982, The Gunflint Formation in the Thunder Bay area, in Franklin, J.M., ed., Field Trip Guidebook 4: Geological Association of Canada, p. 14-31. Simms, M.J., 2003, Uniquely extensive seismite from the latest Triassic of the United Kingdom: Evidence for bolide impact?: Geology v. 31, p. 557-560. Osinski, G., 2006, Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars: Meteoritics & Planetary Science, v. 41, p. 1571-1586. Simonson, B.M., Sumner, D.Y., Beukes, N.J., Johnson, S., and Gutzmer, J., 2009, Correlating multiple Neoarchean-Paleoproterozoic impact spherule layers between South Africa and Western Australia, Precambrian Research, v. 169, p. 100-111. Pufahl, P.K., Fralick, P.W., 2000, Depositional environments of the Paleoproterozoic Gunflint Formation in Fralick, P.W. (ed.), Institute on Lake Superior Geology, Proceedings, v. 46, Part 2 – Field Trip Guidebook, 45 pp. Smit, J., et al., 1996, Coarse-grained, clastic sandstone complex at the K/T boundary around the Gulf of Mexico: Deposition by tsunami waves induced by the Chicxulub impact?: Geological Society of America Special Paper 307, p. 151-182. Pufahl, P.K., and Fralick, P.W., 2004, Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA: Sedimentology, v. 51, p.791-808. Spray, J.G., Butler, H.R., and Thompson, L.M., 2004, Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling: Meteoritics & Planetary Science, v. 39, p. 287-301. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007, Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan: Geology, v. 35, p. 827–830. Pufahl, P.K., Hiatt, E.E., and Kyser, T.K., 2010, Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition?: Geology, v. 38, p. 659-662. Sage, R.P., 1991, Slate Islands: Ontario Ministry of Northern Development and Mines, Ontario Geological Survey Report 264: xi + 111 p., 1 map. Scheffers, A., and Kelletat, D., 2004, Bimodal tsunami deposits – a neglected feature in paleo-tsunami research in Schernewski, G., and Dolch, T., eds., Geographie der Meere und Küsten, Coastline Reports 1: p. 67-75. Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A., 2002, Age of volcanic rocks and syndepositional iron formations, Marquette Tanton, T. L., 1931, Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario: Geological Survey of Canada Memoir 167, 222 p. Van Wyck, N., and Johnson, C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin, Geological Society of America Bulletin, v. 109, p. 799-808. Wigforss-Lange, J., Vajda, V., and Ocampo, A., 2007, Trace element concentrations in the Mexico-Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous-Paleogene boundary: Meteoritics & Planetary Science., v. 42, p. 1871-1882. Yancey, T. E., and Guillemette, R. N., 2008, Carbonate accretionary lapilli in distal deposits of the Chicxulub impact event: GSA Bulletin, v. 120, p. 1105-1118. - 26 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 2 - Geology of the Sibley Peninsula Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Riku Metsaranta Ontario Geological Survey, Precambrian Geoscience Section, Sudbury, Ontario, Canada Introduction The Sibley Peninsula extends into Lake Superior, approximately 25 km east of the City of Thunder Bay (Fig. 1). The peninsula, approximately 52 km long and 10 km wide, separates Thunder Bay (the bay of Lake Superior, not the city) on the west from Black Bay on the east. It can be divided into two physiographic units, based largely on bedrock geology. Highlands or tablelands underlain by Mesoproterozoic Midcontinent Rift-related mafic sills intruding the Rove Formation and/or Sibley Group sandstones dominate the area west of Highway 587, rising as much as 380 m above Lake Superior at the Sleeping Giant. East of the highway, flat-lying Sibley Group siltstones and calcareous sedimentary rocks result in a relatively subdued topography. This field trip will examine a number of Paleoand Mesoproterozoic sedimentary and igneous units from the base of the Sibley Peninsula to its tip (c.f., Franklin et al., 1982; Fralick et al., 2000). Most of the stops are road-cuts so caution must be exercised. Do not stand on the paved portion of the road and be aware of vehicular traffic at all times. Sample collecting is not allowed without a collecting permit in Sleeping Giant Provincial Park. Archean basement below the Sibley Peninsula consists of metavolcanic and intrusive rocks of the Sibley Peninsula Figure 1. Regional geology east of Thunder Bay including the Sibley Peninsula south of Pass Lake. - 27 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 Wawa-Abitibi Subprovince. These are nonconformably overlain by the chemical-clastic sedimentary units of the 1878+-1 Ma Gunflint Formation (Fralick et al., 2002). The Animikie Basin, in which the Gunflint Formation was deposited, developed due to backarc spreading (Fralick et al., 2002) on the southern margin of Superior Province and forms a southwardthickening wedge sedimented on the shelf during transgressive-regressive-transgressive cycles (Fralick and Barrett, 1995; Pufahl, 1996; Pufahl and Fralick, 2000, 2004). The 1850 Ma (Krogh et al., 1984) Sudbury ejecta layer occurs near, or in places at, the top of the Gunflint Formation. These units will not be seen on this trip, as they form the subcrop. The 1835 Ma Rove Formation (Addison et al., 2005) disconformably overlies the Gunflint Formation. It consists of a lower siltstone dominated unit meters to 10 meters thick. This is overlain by approximately 100 meters of black shale representing a starved succession. The upper eight hundred meters of the formation is dominated by turbiditic, progradational parasequences outbuilding from distal deltaic bars to the north-northwest represented by lenticular to flaser bedded sandstones at the top of the preserved succession (Maric and Fralick, 2005; Maric, 2006). Sediment was probably derived from the Trans-Hudson Orogeny that was underway to the northwest. The Sibley Group (Fig. 2) lies disconformably on the Rove Formation, with the directly underlying shales showing the effects of Mesoproterozoic weathering. The age of the Sibley Group is poorly constrained. The best estimate is obtained from its polar wander position, which is the same as the 1450 to 1400 Ma Belt Supergroup (Elston et al., 1993, 2002; Evans et al., 2000). The Belt, which was deposited on the western side of North America, also records the same climatic fluctuations as the Sibley (Rogala et al., 2007). The Sibley basin originally developed as a down-sag accumulating conglomerates and sandstones filling topographic lows and then expanding to cover the area with sheet sandstones. The main sediment source was from the northwest, with Paleoproterozoic zircons dominating this population (Rogala et al., 2007). This indicates a probable source from the eroding TransHudson highlands. Lacustrine conditions developed throughout the area, with the lake becoming more saline with time. As the lake shrank, strand-line stromatolitic dolostone was deposited with a sub-aerial weathered upper surface and terra rosa (soil) development in places. Next a period of basin instability occurs as it down-tilts to the north, the basement collapses into   Figure 2. Stratigraphy of the Mesoproterozoic Sibley Group (From Rogala et al., 2007). a half-graben and within the basin, sub-aerial massflows are generated (Rogala, 2003; Rogala et al., 2005, 2007). Sediment feed is now from the southwest, comprising considerable Mesoproterozoic zircons. This indicates that the Penokean area was not a barrier to sediment transport from the south. The above is the highest stratigraphic unit we will see on this trip. The descriptions below include overlying units. The general geology of the Sibley Peninsula was most recently summarized by Carl (2011): - 28 - Gunflint Formation Gunflint Formation sedimentary rocks make up a chemical-clastic assemblage whose upper portion was deposited 1878Ma (Fralick et al., 2002). This formation crops out close to the northern limits of the Sibley peninsula near Pass Lake, Ontario. At this site, rare folding is present in Gunflint sedimentary layers. This �Proceedings of the 58th ILSG Annual Meeting - Part 2 folding is thought to be related to fold-andthrust belt deformation caused by Penokean compression (Hill and Smyk, 2005). Despite the presence of these compression-related folds, most Animikie Group sedimentary rocks in Ontario are undeformed (Sutcliffe, 1991). Animikie Group sedimentary rocks in Ontario have been classified as having a sub-greenschist metamorphic grade (Easton, 2000) and are frequently considered to be unmetamorphosed for convenience of interpreting depositional environments. northeastern shores of the Sibley Peninsula due to the southeastern dip direction of these rocks. On the southern tip of the Sibley peninsula, south of Perry Bay and Sawyer Bay, the Rove Formation is well represented. Here, sedimentary layers, consisting mostly of black shales, are present beneath the Sleeping Giant landform at elevations up to 369m. The occurrence of Rove Formation sedimentary rocks at such high elevations, when the south-easterly dip of this unit should cause it to be beneath the surface on the peninsula’s southern tip, can be explained by the Silver Islet fault described by William Logan as a transverse dislocation that lets down the succeeding formation by several hundred feet (Logan, 1847). This fault displaced Animikie Group sedimentary rocks as well as other assemblages and played a key role in the genesis of ore at the historic Silver Islet mine site (Horton, 1989). Eventually, the long-lived Animikie Basin closed and deposition of Rove Formation sedimentary rocks ceased. This resulted in a gap in the rock record on the Sibley Peninsula and allowed for erosion of Animikie Group sedimentary rocks. Rove Formation The Gunflint Formation was once thought to transition conformably into the overlying Rove Formation; however, a disconformity has been recognized between these two assemblages (Schulz and Cannon, 2007). The idea that the Gunflint and Rove Formations are discontinuous first became apparent when it was proposed that the Sudbury impact which occurred 1850Ma was a subaerial occurrence (Addison et al., 2005) in the Thunder Bay area (P. Fralick, pers. comm., 2011). This suggests there was a period during which no deposition was occurring that was coeval with the Sudbury impact (Schulz and Cannon, 2007). Using a volcanic ash layer near the base of the Rove Formation, Addison et al. (2005) determined an age of 1836Ma for basal Rove shales. This indicates deposition in the Animikie basin had resumed, and water was again present in this basin at 1836Ma. In the vicinity of the Sibley Peninsula, the Rove Formation has a shallow southeasterly dip (Horton, 1989) and a thickness greater than ~610m based on a diamond drill hole at Sibley Bay (Geul, 1973). Due to erosion, only the lower part of the Rove Formation can be found on the southern portion of the Sibley Peninsula. This basal portion of the Rove Formation consists mostly of black shales with interbeds of siltstone (Maric and Fralick, 2005). Sibley Group Sedimentary Rocks Large-scale subsidence following doming related to the intrusion of the approximately 1540Ma Mesoproterozoic English Bay Complex (Hollings et al., 2004) created an ovoid depression known as the Sibley Basin (Rogala, 2003). Located to the north of the Sibley Peninsula, the English Bay Complex is a granite-rhyolite assemblage (Hollings et al., 2004) with an age of 1537Ma (Davis and Sutcliffe, 1985). The formation of the Sibley Basin post dates igneous activity of the English Bay Complex, with deposition in this basin probably beginning slightly before 1500 Ma (Rogala, 2003). Sediments deposited in the Sibley Basin comprise the Sibley Group, a relatively flat, unmetamorphosed assemblage divided into five distinct formations (Franklin et al., 1980). The lowest three formations of the Sibley Group are found in abundance throughout Sleeping Giant Provincial Park and make up the majority of the surficial geology of the Sibley Peninsula. Rove Formation shales sporadically outcrop on the western shoreline of the peninsula from Sawyer Bay northwards and are never more than a few metres above the 183m elevation of Lake Superior. Also on the peninsula’s western shoreline, well-preserved Rove shale concretions can be seen next to the Kabeyun Trail between Clavet Bay and Hoorigan Bay. Similar Rove Formation sedimentary rocks are lacking on the Pass Lake Formation The oldest Sibley Group formation is the Pass Lake Formation which itself consists of two members: the Loon Lake Member and the Fork Bay Member (Cheadle, 1986). The Loon - 29 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Lake member is the lowermost assemblage of the Sibley Group and consists of conglomerate lenses that were deposited in depressions caused by erosion of the underlying rock (Franklin et al., 1980). This unit can be seen on the Sibley Peninsula directly adjacent to Pass Lake where it is in contact with the overlying sandstones of the Fork Bay Member. Cliff faces adjacent to Pass Lake are dominated by Fork Bay Member plane bedded sandstones which are also commonly seen in cliff faces close to the western shores of the Sibley Peninsula. In general, Fork Bay Member sedimentary rocks comprise sandstones which can be massive and well sorted, massive and poorly sorted, silty, laminated or rippled (Rogala et al., 2005) These sandstones are thought to have been deposited in a shallow, quiet, lacustrine environment (Franklin et al., 1980). evaporites has been interpreted to represent a clastic sabkha environment (Fralick et al., 2000). The dolomitic mudstones are at times overlain by sporadic stromatolitic chert-carbonate lithofacies which are indicative of shallow water, near-shore environments. These laterally discontinuous stromatolitic facies are part of the Middlebrun Bay Member and may represent migrating shorelines of partially restricted bays (Rogala, 2003). Above the Middlebrun Bay Member is the Fire Hill Member which is the uppermost member of the Rossport Formation. The Fire Hill Member can be extremely difficult to distinguish from the Channel Island Member (Rogala et al., 2005). For this reason, the previously characterized member assemblages (Cheadle, 1986) are now described as lithofacies associations (Rogala et al., 2005). The uppermost of these associations are primarily composed of a variety of conglomerates, sandstones, siltstones and mudstones. These uppermost sedimentary rocks were deposited onto a mudflat, with coarser, unsorted sediments and mud-chip conglomerates representing debris flows and slumping events (Rogala, 2003). Siltstones can be found at the top of the Rossport Formation which grade into the overlying Kama Hill Formation. Rossport Formation Fork Bay Member sandstones are overlain by the Rossport Formation, which consists of three members and ten facies associations. The Channel Island Member is the lowermost assemblage of the Rossport Formation and consists of dolomitic mudstones, which gradually increase in abundance as the Pass Lake Formation transitions into the Rossport Formation (Rogala, 2003). Many of the ten facies associations of the Rossport Formation contain siltstones and carbonates which provide clues to the types of environments that once existed when many rocks of the Sibley Peninsula formed. A unique cyclic siltstone-dolomite lithofacies association is present in the Rossport Formation, and has been interpreted to represent a shallow offshore environment where muds were deposited during wet periods and dolomite precipitated during periods of drought (Fralick et al., 2000). This suggests that the lake(s) in which sands of the Pass Lake Formation were deposited became progressively more saline and ultimately formed playa lakes (Rogala et al., 2005). This interpretation appears to be consistent with unpublished paleomagnetic results of G. Borradaile, which suggest at the time of sediment deposition, the Sibley Basin was near the Earth’s equator where arid conditions would have dominated (Fralick et al., 2000). Directly above the cyclic siltstone-dolomite layers, dolomitic mudstones containing mud cracks and gypsum nodules are present. The occurrence of these Kama Hill Formation The Kama Hill Formation is divided into four lithofacies associations. The first three consist of fine-grained sandstones and siltstones that are horizontally laminated, mud-cracked and rippled (Rogala et al., 2005). Horizontally laminated mudstones often cap these finegrained sandstone and siltstone units. These four lithofacies associations are thought to represent a floodplain system which periodically contained ponds. Thicker units of fine-grained sedimentary rocks which sometimes contain wave ripples, likely indicate the presence of long-lasting ponds (Rogala, 2003). Flooding events which covered the floodplains probably increased in occurrence until the formation of the subaqueous Outan Island Formation (Rogala et al., 2005). Outan Island and Nipigon Bay Formations Although not present on the Sibley Peninsula, the Outan Island and Nipigon Bay Formations represent important final stages in the depositional history of the Sibley Basin. The Outan Island Formation consists of mudstone, laminated - 30 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 streams), pebble to cobble conglomerate (ephemeral braided streams), trough crossstratified sandstone (ephemeral braided streams), massive cobble conglomerate (transgressive lag, reworking of braided stream deposits during lacustrine transgression), green sandstonesiltstone (wave and storm influenced fluvial dominated deltas), planar cross-stratified sandstone (nearshore migration of large sandwaves), and thinning upward sandstone (beach and storm remobilized nearshore sandstone sheets). sandstone/mudstone, siltstone, sandstone and conglomerate lithofacies associations (Rogala, 2003). The lower part of the Outan Island Formation has been interpreted as representing a deltaic environment, whereas its upper part represents a fluvial environment (Rogala, 2005). As described by Rogala (2003), the Nipigon Bay Formation is the uppermost formation in the Sibley Group that consists of a crossstratified sandstone lithofacies association and a horizontally laminated sandstone lithofacies association. These associations are thought to represent an ancient aeolian environment. This interpretation is supported by the high degree of sediment sorting seen in sandstones, as well as the presence of large-scale dune topography. The Nipigon Bay Formation was likely subjected to a semiarid to arid climatic regime and probably resembled a modern day desert. 3. The mixed siliciclastic-carbonate unit disconformably to conformably overlies the lower clastic unit and consists of the following lithofacies associations: red siltstone (nonsaline lake), red siltstone-dolostone (perennial saline lake, distal from clastic sources) and red siltstone-dolomitic sandstone (perennial saline lake, proximal to clastic sources). The Sibley Group has a minimum depositional age of 1339 Ma based on an Rb-Sr isochron constructed by Franklin (1978). This age is presently the youngest depositional age determined for the Sibley Group, with final deposition in the Sibley Basin probably occurring shortly after this date. The final formation of Sibley Group sedimentary rocks signaled the beginning of a roughly 200 million year hiatus in rock formation on the Sibley Peninsula. An expanded description of the lower Sibley Group was presented by Metsaranta (2006) in his M.Sc. thesis on the sedimentology, geochemistry and paleohydrology of these rock units: Based on the analysis of lithofacies associations and stratigraphy, the following conclusions can be made: 4. The upper clastic unit sharply overlies the mixed siliciclastic carbonate unit and consists of the sheet sandstone lithofacies association (ephemeral playa lake (?) or perennial lake with increased clastic supply with respect to underlying units), and the black chert-carbonate lithofacies association (shoreline). Subaerial exposure features are present at the top of the black-chert-carbonate lithofacies association and include the intraformational conglomerate lithofacies association (subaerial debris flows, intrusive and/or extrusive sedimentary breccias, terra rossa style soils, dissolution collapse breccias). 5. The mixed siliciclastic-carbonate-evaporite unit overlies the subaerial exposure surface at the top of the upper clastic unit. It consists of the massive dolostone (saline lake), the red siltstonesulfate (wet evaporate-rich mudflats around lake margins) and the fine-grained sandstone (dry, evaporate poor mud and sand flats around lake margins) lithofacies associations. 1. The portions of the Sibley Group studied (lithostratigraphic Pass Lake and Rossport Formations) contain a variety of distinct lithofacies associations. These lithofacies associations can be divided into 4 informally defined allostratigraphic units which roughly correspond to existing lithostratigraphic subdivisions. Based on the stable isotope, Sr isotope and trace element data, the following conclusions can be made: 2. The lower clastic unit forms the base of the Sibley Group and contains the following lithofacies associations representing distinct depositional settings: boulder conglomeratesandstone-calcrete (proximal ephemeral braided 1. Overall, the geochemical data supports a non-marine origin for the Pass Lake and Rossport Formations. 2. Low Sr isotope ratios from calcrete in the - 31 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 lower clastic unit suggest atmospheric deposition and weathering of Gunflint Formation carbonate bedrock was the primary source of cations for pedogenic carbonate rather than weathering of local silicate sources. Relatively 13C-rich calcrete carbon isotopic composition suggests little organic contributions to soil CO2. REE geochemistry suggests calcretes precipitated from oxidizing non-marine water. activity of the MCR and reached thicknesses in excess of 20km (Cannon et al., 1989). These basalts are generally tholeiitic in composition and are thought to be plume-sourced (Klewin and Shirey, 1992). Large mafic igneous bodies associated with the MCR are predominantly attributed to the upwelling of a mantle plume beneath the North American Continent (Burke and Dewey, 1973; Hollings et al., 2010a), with the most primitive magmas associated with the MCR being emplaced early in the rift’s history (Hart and MacDonald, 2007). This upwelling plume was responsible for the formation of numerous volcanic and intrusive units. The Logan and Nipigon Sills, Osler volcanic rocks, and what some have speculated to be Pigeon River dikes (Sutcliffe, 1991) dominate the MCR exposures proximal to the Sibley Peninsula. 3. S, Sr, REE and Y data for the mixed siliciclastic carbonate unit support a lacustrine origin for these rocks. Variations in S isotopic composition may be related to changes in the composition of sulfides weathering to supply sulfate to the system. MREE enriched PAAS normalized REE patterns for dolostone samples differ from those found in other carbonate lithofacies and this probably relates to more reducing conditions in lake waters relative to surface waters supplying the lake. Stratigraphic variations in C and O for this unit were created by evaporation and/or residence time effects. Logan Igneous Suite 4. Slightly enriched δ13C and δ18O values in stromatolitic units in both the upper siliciclastic unit and mixed siliciclastic-carbon-evaporite unit reflect a generally more arid evaporitic environment as compared to the mixedsiliciclastic unit. Shifts toward lighter δ13C in pedogenic carbonates from these units probably reflect a contribution of dissolved organic carbon. REE data for these units is consistent with a nonmarine, oxidizing depositional setting. Carl (2011) continued to describe the igneous units on Sibley Peninsula thus: Midcontinent Rift The Logan Sills and Nipigon Sills are part of what is now known as the Logan Igneous Suite (LIS; Hollings et al., 2007a). The LIS contains various diabase sill formations located north of Lake Superior (Hollings et al., 2007a). Using geochemical data, sills to the north of Thunder Bay have been classified as Nipigon Sills, with sills south of Thunder Bay referred to as Logan Sills (Hollings et al., 2007a). The diabase sill exposures on the Sibley Peninsula occur on the peninsula’s southern tip and make up the Sleeping Giant and Thunder Mountain landforms. For convenience, this sill will henceforth be referred to as the Sleeping Giant Sill (SGS). The SGS is located due east of Thunder Bay and has not been the subject of geochemical analysis in the past. Logan Sills At approximately 1.15 Ga, early stage mafic magmatism of the Midcontinent Rift (MCR) began (Heaman et al., 2007). Centered around Lake Superior, the MCR contains over one million cubic kilometres of mafic volcanic and plutonic rock (Klewin and Shirey, 1992). These rocks formed as the result of a major continental rifting episode (Shirey et al., 1994) that spanned at least 60 million years (Heaman et al., 2007). During this time, rifting of the Superior craton nearly resulted in the splitting of the North American continent and the formation of an ocean. This rifting event is mostly represented by flood basalts, which dominated the igneous Logan sills, such as those seen on Mt. McKay, commonly appear as mesas of the Nor’Wester Mountains found immediately south of Thunder Bay. These sills concordantly intrude sedimentary layers of the Rove Formation and have a gentle southwest dip (Sutcliffe, 1991). The Logan Sills have been classified as quartz-tholeiitic diabase that often contains labradorite, augite, pigeonite and iron-titanium oxides (Sutcliffe, 1991). Logan Sills are characterized by high TiO2 and Gd/Ybn when compared to Pigeon River dikes and Nipigon Sills (Hollings et al., 2010a). In hand sample, Logan Sills contain medium to coarse grains that are dark grey and ophitic-textured. Heaman et - 32 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 magmatic activity. This dike swarm is located on the northern shore of Lake Superior from Grand Portage, Minnesota, to the Black Bay Peninsula in Ontario (Osmani, 1991). Pigeon River Dikes can either cross-cut Logan Sills or terminate against these sills. Geochemical data suggest the Pigeon River Dikes could not have acted as feeders to the Logan Sills (Hollings et al., 2010a). Ages of Pigeon River Dikes range from 1078Ma for a dike near Arrow River in Devon Township to 1141Ma for a dike in Crooks Township (Heaman et al., 2007). This span of over 60 million years is appreciably longer than the duration of formation for most plume-derived large igneous provinces (Hollings et al., 2010a). Pigeon River Dikes have an olivine-tholeiitic composition (Sutcliffe, 1991) and can be distinguished from Logan Sills based on their low TiO2 and Gd/Ybn values (Hollings et al., 2010a). These values are broadly similar to the Nipigon Sills on Gd/Ybn versus La/Smn and Mg# versus TiO2 diagrams (Hollings et al, 2010a). Pigeon River Dikes generally have northeasterly strikes and very steep dips to the south (Osmani, 1991). Pigeon River Dikes have been cross-cut by the geochemically distinct Cloud River Dikes located south of Thunder Bay (Smyk and Hollings, 2007). On the Sibley Peninsula, numerous dikes having northeasterly strikes are present (Tanton, 1924). Dikes of the Sibley Peninsula have been shown to have both normal and reversed polarities suggesting a range of ages for these dikes (Pesonen and Halls, 1983). al. (2007) determined an age of approximately 1115 Ma for the sill that caps Mt. McKay, which presently serves as the only reliable dating of a Logan Sill (Heaman et al., 2007). Hollings et al. (2010a) reported small Nb anomalies and fractionated REE patterns in samples from Logan Sills. These characteristics along with elevated TiO2 values and low Mg# values are the best traits for distinguishing Logan Sills from other nearby sill suites (Smyk and Hollings, 2009). Nipigon Sills Located in the Nipigon Embayment (Sutcliffe, 1991), the Nipigon Sills represent a northern component of the MCR that may have formed in a failed arm of the rift (Richardson and Hollings, 2005). These sills may be up to 200m thick and intrude all other rocks in the area (Heaman et al., 2007). The Nipigon Sills are diabase comprised of medium-to-coarse-grained, lath-shaped, euhedral crystals of ophitic-textured plagioclase with abundant pyroxene as well as trace olivine and magnetite (Hart et al., 2005). The sills of the Nipigon Embayment were sometimes broadly referred to as Nipigon Sills and have recently been subdivided into five distinct sill suites based on geochemical analysis (Hollings et al, 2007a). These suites are the mafic Nipigon, Inspiration and McIntyre Sills and the ultramafic to mafic Jackfish and Shillabear Sills (Hollings et al., 2007a). The SGS is a mafic sill, and therefore only the mafic Nipigon, Inspiration, and McIntyre sills will be considered here. The Nipigon and Inspiration sills both have low La/Smn ratios with the Inspiration sills having elevated Gd/Ybn ratios compared to the Nipigon sills (Hollings et al., 2007a). The McIntyre sills are distinguished by their low La/Smn ratios and intermediate Gd/Ybn ratios compared to other sill suites (Hollings et al., 2007a). These suites can also be differentiated by plotting Mg# versus TiO2 with the McIntyre sills having TiO2 values elevated similarly to the TiO2 values recorded for Logan Sills (Fig. 2.3) near Thunder Bay (Hollings et al., 2007a). Sills of the Nipigon Embayment range in age from roughly 1106 Ma to perhaps as much as 1159 Ma and are considered to be amongst the oldest magmatic expressions of the MCR (Heaman et al., 2007). Osler Group The Osler Group consists primarily of mafic rocks found along the north shore of Lake Superior, representative of basaltic flows that occurred approximately 1108-1105 Ma (Hollings et al., 2007b). These basaltic flows reached a thickness of approximately 3 km and can be frequently found in outcrop to the northeast of the Sibley Peninsula. Osler Group basalts have chondrite-normalized La/Smn ratios that range from 1.5 to 3.9 and chondrite-normalized Gd/Ybn ratios ranging from 1.5 to 3.7 (Hollings et al., 2007b). Osler Group volcanic rocks are abundant on the Black Bay Peninsula, located east of the Sibley Peninsula. No volcanic rocks of any kind have been observed on the Sibley Peninsula. Pigeon River Dikes The Pigeon River Dikes occur as a northeast trending swarm that formed as a result of MCR - 33 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Road log for field tripFOR (NAD83, UTM Zone 16) ROAD LOG FIELD TRIP (NAD83, UTM Zone 16) STOP NAME Blende Creek Area: Gunflint Fm chert-carbonate Gunflint Fm chertcarbonate Rove Fm concretions Watson Site Kettle Brohm Site Pass Lake (Animikie / Sibley disconformity Pass Lake (basal conglomerate) Rossport Fm. Siltstones Rossport Fm. Lacustrine Sheet Sandstones Rossport Fm., Lacustrine Channel Sandstones Rossport Fm., Lacustrine Channel Sandstones STOP NO. LANDMARK (0 Km) DISTANCE (km) Junction Highways 1117 and 587 0.0 1A EASTING 0.6 5384392 369112 1.4 2.1 5383837 369581 3.5 5382426 370841 4.8 5.2 5382460 5381279 5380897 371737 371241 371236 4A 6.0 5380509 371853 4B 6.5 5380537 372294 5 7.7 5380075 373001 6 8.8 5378948 372877 7A 9.9 5377943 372842 10.1 5377728 372868 5376859 373095 1B CNR Trestle 2 3 optional optional Watson Site (turn-off) n.a. 7B Pass Lake Crossroad Rossport Fm., Subaerial Siltstone-Caliche NORTHING 8 3.8 10.6 10.9 Jakobsen Road 11.2 - 34 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Northwest-striking dykes in Rossport Fm. East-northeaststriking dyke in Rossport Fm 9A 9B 10 Sleeping Giant Provincial Park boundary Kay Lake Portage Drive Joe Creek trail Joeboy Lake Thunder Bay Lookout turnoff eastnortheaststriking dyke eastnortheaststriking dyke east-striking dyke Sifting Lake trailhead northeaststriking dyke Lake Marie Louise (north end) northeaststriking dyke Lake Marie Louise campground Plantain Lake trailhead Silver Islet loop junction Sawyer Bay trailhead eastnortheaststriking dyke Silver Islet General Store - 35 - 14.2 14.3 5374104 5374052 372312 372320 21.8 5368080 371382 22.6 5367443 370954 23.8 5366398 370595 23.9 5366287 370537 5365748 370280 5357977 366961 5355569 364889 14.4 16.9 17.2 17.8 19.5 21.1 24.2 24.6 29.8 34.1 34.2 34.8 35.7 37.3 37.5 38.5 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Rove Fm and east-northeaststriking- diabase dyke east-northeasttrending dyke in Rove Fm. / Silver Islet view 11 12 Sibley Creek (bridge) Loop Tjunction Middlebrun Bay trailhead Loop Tjunction 39.0 5354955 365488 39.9 5355427 366232 40.0 40.3 40.7 42.1   Figure 3. Stop locations, northern Sibley Peninsula; north to left of image from Google Earth Stop Descriptions Stops 1A,B: Blende Creek area (Gunflint Fm.) UTM coordinates: NAD83; 16U A - 0369112E / 5384392N, B - 0369581E / 5383837N Along Highway 587, rock cuts display thinly bedded, generally flat-lying sedimentary rocks of the Gunflint Formation. The outcrops we have driven past are composed of ankerite and siderite grainstones (medium-grained sand sized iron carbonates) referred to as granular iron formation (GIF). These are common in the Thunder Bay region, dominating the near-shore of the Animikie Basin. The iron carbonate grains were produced by wave erosion of carbonate precipitates and represent storm deposits in the near-shore. The iron may have precipitated as a carbonate in this shore-proximal zone due to photosynthesizing bacteria removing CO2 from the water and thus increasing the pH and driving the carbonate phase into supersaturation. The outcrop we are looking at has these carbonate grainstones weathering orangey-brown alternating with white chert layers (Fig. 4). In places the chert can be seen - 36 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   and faulted Gunflint strata on Highway 588, Stop 1B. Figure 4. Folded replacing the carbonate but other layers appear to be primary chert. In the older literature an outcrop such as this would be ascribed to deeper water due to less evidence of current activity. However, because of its shore proximal location it probably formed in a quieter water location near the strand-line, i.e., a sheltered lagoonal area behind on offshore bar. These exposures are somewhat unique in that the rocks are folded; elsewhere, they are undeformed. The hinge zones, where the majority of stress is focused, are commonly fractured (Fig. 4). These fractures may be occupied by quartz-calcite veins following the vertical axial plane. The outcrop to the west hosts numerous veins and vein breccias that strike between 40° and 45° and dip almost vertically to the southeast. These breccias contain sparry calcite, drusy quartz and also altered shale fragments, suggesting that these Rove Formation rocks likely occurred above this section during vein emplacement. A thin, northwest-dipping diabase dyke intrudes the Gunflint rocks at this location and is, in turn, cut by these veins. Recent examination of the Gunflint Formation near Pass Lake has led to the recognition of structures typical of Penokean (circa 1875 to 1835 Ma) fold-andthrust belt deformation (Hill and Smyk 2005). Discrete bedding-plane faults with locally developed gouge and breccia can be traced laterally into horizontal, hangingwall ramps with associated fault-bend folding. Foldand-thrust belt deformation is caused by regional compression. Previous workers had ascribed the folds to syn-sedimentary slumping and Keweenawan diabase sill emplacement and thought that they were attributable to local, rather than regional-scale, deformation. Displacement in fold-and-thrust belts tends to be localized along discrete bedding planes and not easily recognized. This may account for the perceived lack or absence of structures elsewhere in the Gunflint Formation (Hill and Smyk, 2005). Penokean structures on the northern side of Lake Superior represent the northward migration of thrust faults into the foreland (passive margin Archean basement + Gunflint Formation) caused by hinterland collision to the south. Stop 2: “Devil’s Flower Pots” (Rove Formation concretions) UTM coordinates: NAD83; 16U 0370841E / 5382426N Just north of Highway 587, a quarry face exposure of black, fissile Rove Formation shale displays lenticular and elliptical concretions, flattened along bedding planes (Fig. 5). These structures form during diagenesis, following initial compaction and dewatering of the sediments. They represent a concentration of a cementing agent (e.g., silica, calcite) focused during the migration of fluid through the sediments. They often are nucleated around a piece of organic material or other foreign object, which creates a perturbation - 37 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. Lenticular concretion in Rove Formation shale,  old quarry face, north of Highway 587, Stop 2. Concretion is almost 1 m in diameter. in fluid flow with a distinct chemistry. Because the cementing agent in this case is more resistant to weathering, these concretions stand out of the soft shale and may commonly completely detach form their host rock. Groundwater and surficial water flow through the shale has led to the dissolution and subsequent precipitation of a variety of low-temperature minerals (e.g. carbonates, sulphates, hydroxides) that occur as white and yellow encrustations on the bedrock surface. One of the more unusual of these secondary minerals is yellow magnesium aluminocopiaptite ((Mg,Al) (Fe,Al)4(SO4)6(OH)2.20H2O; Resident Geologist’s Files, Thunder Bay). Figure 6. 1835 Ma Rove shale, note bleached zone overlying oxidized zone, overlain by the approximately 1450 Ma basal sandstones and conglomerates of the Loon Lake Member,   Pass Lake Formation, Sibley Group, Stop 3. Stop 3: Watson Road section (Pass Lake and Rove formations) - private property; permission is required to access UTM coordinates: NAD83; 16U 0371737E / 5382460N A private access road extending up the mesa provides an excellent 150 m long section exposing the disconformity between the Rove Formation and the overlying basal conglomerate and sandstones of the Pass Lake Formation. The Rove shales immediately below the contact were subject to Mesoproterozoic weathering (Fig. 6).   Figure 7. Disrupted zone in the Rove shales underlying the Sibley Group, Stop 3. The origin of this structure is enigmatic, but may have been caused by fluid escape - 38 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 8. Close-up of conglomerate shale contact, Stop 3. Note the strong oxidation of both units. Geochemical investigations have outlined an oxidized zone below the contact grading to a more reduced zone with abundant chlorite a few tens of centimeters lower in the section. In one area what may be a dewatering or degassing structure strongly deforms the shale (Fig. 7). Very immature, iron oxide-rich conglomerates and sandstones of the Loon Lake Member, Pass Lake Figure 9. Typical Loon Lake Member coarse-grained sediments at Stop 3. From the bottom to the top of the photograph: 1) matrix-supported conglomerate, probably a high-density mass-flow; 2) a one clast-thick, pebble-cobble lag developed at the top of this conglomerate through erosion. This may represent either an Aeolian deflation lag or one developed by water erosion of the fine-grained fraction. 3) a boulder-cobble, matrix-supported, mass-flow conglomerate. This was probably a very high-viscosity flow as the larger clasts were suspended near the top of the flow. 5) an upper flow regime parallel-laminated sandstone probably deposited by sheet-flood on the alluvial fans surface; 6) a clast-supported fluvial conglomerate. Formation, overlie the Rove (Fig, 8). The conglomerate and sandstone layers are laterally discontinuous (Fig. 9), with some conglomerates in clast-support (fluvial deposits) and some in matrix-support (sub-aerial debris-flow deposits). Successions such as this in the Sibley are typical of arid to semi-arid alluvial fans (Cheadle 1986), though this would have been a very small one. The abundant hematite probably denotes a deep water table. Clasts are locally derived from the erosion of underlying units. This is sharply overlain by mature, well-sorted, medium-grained sandstones of the Fork Bay Member, Pass Lake Formation (Fig. 10). Detrital zircon geochronology and paleocurrents (Cheadle 1986; Rogala et al. 2007) indicate that the major source of this sediment was the Trans-Hudson highlands. The travel distance accounts for its maturity compared to the locally derived underlying conglomerates. The sandstone was deposited as sheet flows into the shallow nearshore of a lacustrine system that had flooded the area (Cheadle 1986; Rogala 2003; Metsaranta 2006; Rogala et al. 2007). These sandstone layers are laterally continuous, massive to parallellaminated, in places with trough cross-stratified or rippled tops (Fig. 11). Rare, odd features are present both in cross-sectional and bedding plane views in this outcrop (Figs. 12, 13). These may be dewatering pipes.     Figure 10. Sharp contact between the hematite-rich conglomerates of the Loon Lake Member and the wellsorted, buff sandstones of the Fork Bay Member, Pass Lake Formation, Sibley Group, Stop 3. - 39 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 11. Medium- to coarse-grained, well-sorted sandstone bed of the Fork Bay Member, Stop 3. The majority of the bed is upper flow regime parallel laminated, with a reworked, cross-stratified top. Stops (Optional): Kettle and Brohm Archaeological Site UTM coordinates: NAD83; 16U 0371241E / 5381279N and 371236E / 5380897N respectively Local archaeological sites are closely tied to paleo-shorelines, especially those associated with Lake Minong. When flooded to Minong levels (Fig. 14), Sibley Peninsula becomes a virtual island, connected to the mainland near Pass Lake with a series of baymouth bars, forming a spit between the sandstone cliffs (Geddes et al., 1987). The location of these sites may relate to the importance of this paleogeography in constricting the movement of caribou and other animals from the peninsula to the mainland. Although there is no organic preservation to confirm that these were caribou ambush and processing sites, it remains a compelling theory.   Figure 12. Bedding-plane view of odd concentric layering, Stop 3. The pattern is caused by erosion of the top of domed layers. These may be water escape structures with the basal portions of sand volcanoes preserved, or not.   Figure 13. Cross-section through a domal structure in a Fork Bay sandstone, Stop 3. - 40 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure  14. Reconstruction of shoreline near Pass Lake during Minong time (Hinshelwood 1990) A historic plaque on a roadside pull-off describes the archaeological discovery: In 1950, archeological investigations in this area uncovered a site which had been used as a workshop camp by a group of the earliest known people in this part of the Upper Great Lakes basin. Called Aqua-Plano Indians because they migrated from the western plains to fossil beaches of glacial and post-glacial lakes in - 41 - this region, they appeared about 9,000 years ago following the retreat of glaciers and the northward movement of plants and animals. They developed a distinctive tradition based primarily on large game hunting using weapons and specialized tools made of taconite, a stone that was obtained locally. Their way of life, which was closely related to the environment, disappeared as the climate grew warmer. �Proceedings of the 58th ILSG Annual Meeting - Part 2 Pass Lake section (http://www.ontarioplaques.com/Plaques_ STU/Plaque_ThunderBay17.html) This Quaternary geology of this area was also described by Geddes et al. (1987; Fig. 15 and 16): [Figure 15] shows the arrangement of bars which separated Pass Lake from open water about 9000 years B.P. The strait between Black Bay and Thunder Bay appears to have followed a subglacial channel scoured into the rock floor, and which forms a narrow trough along the length of Pass Lake. Geomorphological evidence of the environmental conditions at the time of habitation will be seen in gravel pit sections of the baymouth bars. Cryoturbated layers show a marked vertical alignment of platy shale fragments, suggesting that the bar surfaces were exposed to intensely cold conditions as they accumulated. The grounding of small icebergs, as evidences by the reorientation of bar features around depressions is also indicated [“Kettle” Stop]. Stop 4A: Disconformity between Pass Lake and Rove Fm.’s (UTM 371853E / 5380509N) UTM coordinates: NAD83; 16U 0371853E / 5380509N The rock cut adjacent to the railway at Pass Lake provides an excellent exposure of several types of flatlying sedimentary rocks and their contact relationships. This cliff is the type section for the Pass Lake Formation of the Sibley Group. Exposure is almost continuous for about 3.2 km along the tracks and gives a stratigraphic thickness of 50 m. The oldest rocks in the section, fine-grained, fissile shales of the Rove Formation (Animikie Group), are exposed at the northwestern end of the cliff exposure. Originally black, they were oxidized in pre-Sibley times, resulting in their purple and green colour. These rocks originated as 1835 Ma muds. The Animikie Group rocks are disconformably overlain by coarser-grained sedimentary rocks of the Sibley Group, similar to what we observed at the Figure  15. Baymouth bar complex in the vicinity of Pass Lake and Brohm archaeological site (Geddes et al. 1987). “Kettle” field trip stop corresponds to pond shown near the highway - 42 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 previous outcrop. Immediately above the AnimikieSibley disconformity, lenses of conglomerate comprise the base of the Pass Lake Formation (Fig. 17). This basal conglomerate contains angular to rounded pebbles, cobbles and boulders in a sandy matrix. These clasts are derived predominantly from erosion of Gunflint Formation chert and taconite; there are also clasts of quartz veins and Archean granite. (Gunflint rocks are exposed a few kilometres to the northwest on Highway 587). This conglomerate attains a thickness of approximately 3 m at the southeast end of the exposure, where it is sharply overlain by massive to parallel laminated, buff-coloured Pass Lake sandstone (quartz arenite). The constituent sand grains consist mainly of quartz, with minor chert and feldspar, with calcite cement lower in the cliff and silica cement higher up. The sandstones were deposited in the nearshore of the lacustrine system. Figure 16. Paleo-Indian sites, paleogeography and field trip stop locations (Hinshelwood, 2004) - 43 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 than those observed earlier. This opens the possibility that the conglomerates at this location were reworked by wave activity during initial lacustrine flooding. While examining the lithofacies in the field we will discuss the merits of each interpretation. The sandstone beds again represent sheet-floods forming sand-flats in the shallow lake. The thinning- and fining-upward sequence of sandstone beds is a classic example of a transgressive succession showing decreased sand supply through time as the shoreline moves further away from the area. Stop 5: Rossport Formation Siltstones UTM coordinates: NAD83; 16U 0373001E / 5380075N The dip of the strata to the south, probably developed due to block rotation during Mid-Continental rifting, allows us to observe higher levels of the Sibley Group as we drive down this stretch of the highway.   Figure 17. Disconformity between weathered Rove shales and Pass Lake basal conglomerate, Stop 4A Stop 4B: Pass Lake Formation A cliff on the far side of the railroad tracks contains the type section of the Pass Lake Formation. The basal conglomerate thins and thickens laterally, pinching down to pebbly sandstone in places. Clasts are generally surrounded and dominated by local Gunflint Formation lithologies. The matrix is poorly sorted. The conglomerates are overlain by a thinning upward sequence of sandstone beds (Fig. 18) capped by siltstones on the top of the cliff. Individual beds are reasonably laterally continuous though sometimes lense out. They are dominated by upper flow regime parallel lamination with occasional ripples and smallscale dunes on their tops. Both alluvial fan-braided fluvial and shallow lacustrine (Cheadle, 1986; Franklin et al., 1980 respectively) depositional environments have been proposed. The bedding organization of the conglomerates exposed here is somewhat different Figure 18. Thinning-upward sequence in Pass Lake sandstones, Stop 4B - 44 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 The fining- and thinning-upward trend of the sandstones in the last section culminates in the massive siltstones we see at this stop. Not much can be said about these structureless red siltstones. They appear to have formed from rainout sediment in the offshore portion of the lacustrine system. STOP 6: Rossport Formation Lacustrine Sheet Sandstones UTM coordinates: NAD83; 16U 0372877E / 5378948N Here we have another example of the offshore red siltstones, but with two sandstone layers in them (Fig. 19). The lower layer is actually composed of two amalgamated layers. These sheet sandstones probably represent large flood (storm) events during which the flow conditions were intense enough to transport the sand into the further offshore areas of the lake. Rare sedimentary structures, consisting of hummocky cross-stratification (Fig. 20), in the otherwise massive sandstones indicate storm generated currents deposited the sands. The presence of washed-out dunes at other locations indicates flow velocities were transitional from lower to upper flow regime, as opposed to the near shore sand sheets we looked at two stops back, which were deposited during upper flow regime conditions. The tops of the sand sheets were reworked by waves forming ripples.   Figure 20. Hummocky cross-stratification in a sheet sandstone, Stop 6. This type of layering is produced by the interaction of storm waves and an offshore flowing current carrying sand. These offshore currents, geostrophic flows, are produced by the storm surge draining away from land during the waning of the storm. Stop 7A (west side of road): Rossport Formation Lacustrine Channel Sandstones UTM coordinates: NAD83; 16U 0372842E / 5377943N This outcrop consists of somewhat chaotic layering (Fig. 21). It appears to have several steeply dipping faults running through it. On the southern (down-road) side of the outcrop the sandstone layers abruptly abut against red siltstone. Sandstone layers near the top of the outcrop appear to lever downwards. There are also two lithologies present here that we have not seen so far. The most evident of these is purple shale. It has an interesting mineralogy compared to the red siltstone. The red siltstone has abundant potassium-rich micas and clays. The purple shale does not contain these but instead has potassium feldspar (SEM-EDX and XRD analyses). This implies an alteration where potassium enrichment drove the standard hydrolysis weathering reaction backwards. The other new rock type is dolostone. Beds of it resemble the sandstone, but it is easily distinguished on fresh surfaces. Try to figure out what is causing the deformation of the layering. Stop 7B (east side of road): Rossport Fm., Lacustrine Channel Sandstones Figure 19. Sandstone sheets deposited in the off-shore during storm events, Stop 6. Fairweather deposition produced the siltstone between the sandstone sheets. UTM coordinates: NAD83; 16U 0372868E / 5377728N The layers strike across the road, meaning this section should be similar to the one we just looked at. - 45 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 21. Chaotic layering in an outcrop stratigraphically overlying and sheet sandstones and massive siltstones, Stop 7b. It is not. Before reading further look at the outcrop and see if you can figure out what is going on. The outcrop consists of three units. There are beds of sandstone at the top of the outcrop. Underlying these is a chaotic, poorly-sorted, jumble of siltstone, shale and dolomite clasts with a pebbly silt matrix. Underlying this is a thick layer of dolostone that has a very irregular upper surface (Fig. 22). In places the intraformational conglomerate extends down to ground level. Now that you possibly have a better idea what the lithologies are can you figure out what happened here? The answer: You are looking at a karst surface on the dolostone. It was buried by a sub-aerial massflow deposit, possibly triggered by a change in the basin slope at this time, from down to the southeast to down to the northwest. The sand-sheets that also come in at this stratigraphic interval are flowing from the southeast (Cheadle, 1986; Rogala, 2007). There is also the possibility that the chaotic unit represents a collapse breccia. Figure 22. Sub-aerial mass-flow deposits overlying a very irregular karst surface eroded into the dolostone at the bottom of the photo, Stop 7b. There is also a possibility that the intraformational conglomerate represents a collapse breccia. - 46 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 8: Rossport Formation., Subaerial SiltstoneCaliche STOPS 9A,B: Northwest-striking dykes in Rossport Fm. UTM coordinates: NAD83; 16U 0373095E / 5376859N UTM coordinates: NAD83; 16U 0372312E / 5374104N and 0372320E / 5374052N This outcrop represents the highest level of the Sibley Group that we will see on the peninsula. Again we have an outcrop of massive siltstone. However, the internal structuring here is very different than those previously examined (Fig. 23). This would have been more evident a few years ago when the outcrops were buried by small, centimeter and less, chunks weathered out from them. In the interim people, including PWF, discovered that this material can be used to make decorative garden paths and it has disappeared. If you have worked in the southwestern badlands you may have seen modern examples of this type of material coating the ground. The small chunks are soil peds that are baked in the sun. They commonly have clay coatings called cutans on their sides. Further evidence that what we are looking at represents arid to semi-arid soil is the presence of the light green layers. These are rich in dolomite and represent dolocrete layers which form in soils in semi-arid environments. They are created where evaporation is greater than precipitation and there is a net upward movement of water due to capillary action. Evaporation causes the soil fluid to be super-saturated and precipitate carbonate. We can see the peds and dolocrete layers in these exposures (Fig. 23), denoting the large, probably shallow, lake had gone from this area for good. Figure 23. Soils developed in the semi-arid environment of the Rossport Formation. The upper light greenish grey unit is a carbonate-rich horizon with some dolocrete. The red unit consists of soil peds, the small fragments it is disintegrating into. Two narrow, parallel, northwest-striking diabase dykes intrude Rossport Formation siltstones at these two locations. Sampling by Hollings et al. (2009) identified profound geochemical differences between northeast- and northwest-striking dykes on Sibley Peninsula. These dykes plot within, or very near the fields defined for mafic/ultramafic sills and intrusions on both Gd/Ybcn versus La/Smcn and Mg# versus TiO2 diagrams (Fig. 24). These fields are derived from mafic/   Figure 24. Gd/Ybcn versus La/Smcn plot and TiO2 versus Mg# plot for diabase dykes at Stops 9A,B and 10, as well as other mafic and ultramafic intrusions on the Sibley Peninsula and the northern Midcontinent Rift (Carl, 2011; Hollings et al., 2007a,b) - 47 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 ultramafic intrusions located in the Nipigon embayment northeast of the Sibley Peninsula (cf. Hollings et al., 2007a,b). The northwest strike directions make them spatially unsuitable candidates for feeders of any of the mafic/ultramafic intrusions located near Lake Nipigon, northeast of the Sibley Peninsula (Carl, 2011). REE patterns for these dikes do not display large negative niobium anomalies compared to the REE patterns of east-northeast-striking dikes. The small negative niobium anomalies suggest a more primitive source for these northwest-striking dikes compared to the eastnortheast striking dikes of the Sibley Peninsula. The SiO2 weight percentages of these dykes is 48.7 and 49.4, respectively. TiO2 and MgO weight percentages for these two dikes are roughly 3.35 and 3.65 and 7.74 and 7.19, respectively. No cross-cutting relationships were noted at the Highway 587 sample sites, and therefore a relative age relationship of these dikes to the east-northeast striking dikes (e.g., Stop 10) is unknown (Carl, 2011). Stop 10: East-northeast-striking dyke in Rossport Formation UTM coordinates: NAD83; 16U 0373382E / 5368080N At this location, a steeply dipping, east-northeaststriking diabase dyke crosscuts Rossport Formation siltstones (Fig. 25). There appear to be some localized contact metamorphic effects, including some hornfels and reduction of the iron in these hematite-rich sedimentary rocks. The geochemistry (Gd/Ybcn versus La/Smcn, TiO2 versus Mg#; Fig. 24) of east-northeaststriking dykes along Highway 587 falls into the field previously defined by Nipigon sills (Carl, 2011; Hollings et al., 2007a,b). They may be correlative with Pigeon River dykes of similar orientation south of Thunder Bay. STOP 11: Rove Formation and east-northeaststriking diabase dyke (UTM 5354955N, 365488E) UTM coordinates: NAD83; 16U 0365488E / 5354955N The unweathered Rove Formation can be seen here (Fig. 26). The dominance of siltstone raises the possibility that this is the lower portion of the Rove Formation. In fact the numerous siltstone layers imply that we may be looking at the lowest ten metres of the Rove Formation, as during initial transgression the more shore proximal location led to a more silt-rich interval. The fine parallel layering and lack of wave formed structures is interesting as it appears that no shallow water, coarser-grained lithofacies were deposited in the Rove. This may denote very rapid transgression or an arid climate limiting sediment delivery to the basin. Or Figure 25 . East-northeast-striking diabase dyke crosscutting Rossport Formation siltstones, Stop 10, south of Joeboy Lake. - 48 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 26. A grey siltstone dominated succession of the Rove Formation, Stop 11. This outcrop also contains thin dark shales and thicker, clay-rich sandstones. The Rove in this area either represents the basal siltstone-rich area or a portion of the turbiditic fan/ramp higher in the Formation. The diabase can be seen in the upper right of the photo. possibly, as paleocurrents indicate the Rove Formation represents foreland deposits associated with the TransHudson Orogeny, an early sediment starved phase was produced by development of a tectonic moat. This is common in other orogeny related black shale-turbidite sequences such as the Martinsburg Formation, which was deposited in the Taconic foreland. The other possibility is that this section is considerably higher in the Rove Formation. The presence of sandstones indicates that this may be a portion of the submarine Figure 27. Geology of the southern Sibley Peninsula (Tanton, 1924, 1931) showing stop locations. - 49 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 fan/ramp system that overlies the 100 meters of basal shales and siltstones. STOP 12: East-northeast-trending dyke in Rove Formation / Silver Islet view UTM coordinates: NAD83; 16U 0366232E / 5355427N This stop affords us the opportunity to look across the waters of Lake Superior to the former site of the Silver Islet Mine, 1.8 km to the south. This tiny speck, barely rising above the waves, was once home to the most famous silver mine in the world (Figs. 28, 29). The epic story of its discovery, development and legacy was summarized by Mohide (1985): The early regulations of the government with regard to the area of mining locations were, compared to modern ideas, generous to a fault. A mining location was required to be two miles in front by five miles in width, comprising an area of 6,400 acres. In 1856 a mining company had obtained from the Crown sixteen of these locations fronting at intervals on the north and northeast shores of Lake Superior, comprising in all 99,498 acres. The conditions required the grantee “to commence and bona fide carry on mining operations within a period of eighteen months”, under penalty of forfeiture of the lands. But the company did not comply with the conditions, neither did the government exact the penalty. The grant reserved all mines of gold and silver and imposed a royalty varying from 2 to 10 per cent of the value of the ore extracted. In, as we may suppose, its anxiety to start things moving, the government abandoned the reservations of gold and silver, repealed the royalties, and forgave one-half of the purchase money of 80 cents per acre. The only offset on the part of the government was to levy a tax of two cents per acre on all lands granted previous to 1868. All restrictions having been removed and facing an annual tax of approximately $3,140 the company apparently deemed it advisable to examine their lands for possible deposits of mineral. Their interest was probably quickened by the discovery of silver made by the McKellar brothers and others on the north shore of Lake Superior. The task of examination the company committed to Thomas MacFarlane, a well-known geologist and civil engineer. MacFarlane and his party set out in the spring of 1868 and cruised the locations one by one. On the Jarvis location they found a vein of silver, upon which considerable work was afterwards done and a quantity of silver recovered. Eventually the party arrived at the Woods location. He determined to make a complete study of the location and set his assistant, Gerald C. Brown, to survey the shore-line. While engaged in planting pickets on the islands in Lake Superior fronting the location, Brown landed on the tiny rock about the size of a ballroom to which Macfarlane afterwards gave the name of Silver Islet and here he noticed a vein carrying galena. Macfarlane thereupon visited the spot himself and put three of his men to work. On the north shore of the islet there was a vein having a width of 20 feet, which on the south divided into two branches, each seven to eight feet wide. On the 10th of July the first metallic silver was noticed by John Morgan, one of the exploring party, at the water’s edge on the east or hanging side of the west branch of the vein, in the form of small nuggets. A single blast was sufficient to detach all the vein rock carrying ore above the surface of the water, but the ore was traced some distance out into the lake, where instead of scattered nuggets of native silver, large patches of veinstone rich in galena were visible, intermixed with small particles and large nuggets of silver. The thickness of the rich part of the vein varied from a few inches to two feet and by working in the icy water with crowbars some rich pieces of ore were broken off. On the 15th of July three packages of the best specimens were shipped from Fort William, Thunder Bay, altogether 1,336 lbs. of ore having been obtained. This shipment was carefully weighed and sampled in the following December. Assays by Professor Chapman of Toronto, Dr. Hayes of Boston and Macfarlane himself, gave an average of 2,087 ounces Troy per long ton. Next year explorations were resumed on the rock, but winds and waves, together with the extreme coldness of the water, proved great hindrances. Nevertheless, by working with tongs and longhandled shovels in two to four feet of water, the party was able to raise and ship 46 half-barrels of good ore, weighing 9,455 Ibs. valued by Macfarlane on the basis of his assays at $6,751. Such results indicated a mine and amply justified further development. A shaft house and sleeping - 50 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 and dining rooms for the men were erected and strong barriers of two-inch plank built to protect them from the furious gales and sweeping winds of Lake Superior. Inflowing water slowed up work in the shaft and the contracted area of the mine severely hampered operations. Weather conditions, however, had their compensations for when the winter set in, the frozen surface of the lake provided solid footing for the men who managed to raise some nine tons more of the ore. The total quantity of ore recovered up to this time was 28,073 lbs. which realized after being smelted, the sum of $23,115. The mining company in March, 1870 sold not only the Woods tract, but all of its other locations, now 18 in number, to a new group, the price realized being $225,000. proved to be too large, for the amount realized was 5137,022 less than the value originally placed on the shipment. Frue had been succeeded as superintendent by Richard Trethewey, who decided to divert the vertical shaft to an incline following the line of the diorite dike, associated with the very rich ore of the earlier workings had been found. He carried it down to a further depth of 414 feet and drifts started at the bottom showed the vein to present highly promising conditions. Some very rich ore was encountered in a south drift. The hope was that the chimney of ore would again be tapped at the junction of the diorite and the slates, but although fugitive bunches of ore were met with, the mine failed to respond to expectations. Evidently the end was approaching. Minerals which frequently accompanied the silver, such as [zinc] blende, galena, pyrite, cobalt and nickel were found, but the silver was absent. Notwithstanding financial and mining difficulties, the work was continued until February, 1884. It was still intended to carry on, but a cargo of coal in charge of a drunken ship’s captain had failed to arrive before the close of navigation and no course was open but to allow the mine to fill with water and cease operations. The new company formed the Ontario Mineral Lands Company and selected Captain William B. Frue as manager, who at once set to work in September of that year. Frue was a man of remarkable quality and is worthy of some words of mention. Not only was he a skilled and experienced mine superintendent, but he seems to have been of heroic calibre. During the six years beginning with 1870 production of silver from the mine amounted to 1,561,882 fine ounces, but output was lessening year by year, due to unfavourable changes in the vein as the lower workings were reached. The company got into financial difficulties and facing a heavy deficit, sold all its holdings to a new concern known as the Silver Islet Consolidated Mining and Lands Company, with a capitalization of 51,000,000. The total production of the mine in ounces cannot be precisely stated, the records being incomplete, but the entire value is given at $3,500,000. At the average price of silver during the 16-year period of operations say $1.15 per ounce, this would represent a total output of about 3,044,000 fine ounces. To this may be added 16,769 ounces obtained during 1921 and 1922 by the Islet Exploration Company which removed a quantity of rich ore from the roof of the mine. The main production was from two very rich bonanzas, one of which was completely worked out in 1874, yielding over two million dollars. In shape this mass of ore resembled an irregular pear and consisted of arborescent silver. The second bonanza was found on the third level in 1878. It was remarkable for its width (5 feet solid across the breast) and for the occurrence of two previously unknown compounds [mixtures] of silver, huntilite and animikite. This deposit was phenomenal in its structure, the middle of the fourth level being sunk literally through solid silver, the metal projecting boldly from the four walls of the winze. In the breast it stood out in great arborescent masses in the shape of hooks The new company met with wonderful success, far exceeding their expectations. The first three levels were explored and much silver recovered from them. The mine, which had been allowed to fill up as far as the third level, was dewatered and work was carried on from the fourth to the tenth level. The results are thus described in the company’s report for 1878: “Silver of unparallelled riches was found in the winzes, in the drifts and in the stopes and rich stamp-mill rock abounded in all workings, the vein north of the shaft being particularly productive”. The year 1878 closed with an output of silver estimated at 724,632 ounces, of which 551,111 ounces was obtained from “packing ore” (i.e. one rich enough to be put up and shipped to the smelter in small packages) and 170,521 ounces from stamp mill concentrates. This estimate, however, - 51 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 28. Silver Islet ca. 1900 (top; www.canadiangeographic.ca) and today. Figure 29. Aerial view of present-day Silver Islet. Site plan of old mine from Barr (1988). Note that the original island consisted only of the small outcrop at the lower end of the present island. - 52 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 30. Stop locations, southern part of Sibley Peninsula. Image from Google Earth. and spikes and in gnarled, drawn out and twisted bunches. The width of the deposit was over 10 feet and including the accompanying stamp rock, it yielded about 800,000 ounces of silver. Probably nowhere, or at any rate nowhere in Canada, had mining been carried on under conditions so difficult as at Silver Islet, the area of which before enlargement by protective works, was no larger than a good-sized ballroom. Wind and water conspired to prevent an invasion of the tiny spot. 159p. Burke, K., and Dewey, J., 1973. Plume generated triple junctions: Key indicators in applying plate tectonics to old rocks. Journal of Geology, 81: 406-433. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., Behrendt, J.C., et al. 1989. The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling. Tectonics, 8: 305–332 Carl, C.F.J. 2011. Geochemistry and petrology of intrusive rocks of the Sibley Peninsula; unpublished HBSc thesis, Lakehead University, Thunder Bay, 77p. Silver Islet not only rose to fame as a prolific silver producer, having contributed half of all the silver in Canada in 1870’s. The Frue vanner, still in use in different forms today, was developed at Silver Islet in 1872. The use of steam-powered diamond drills and the Burleigh piston-type, compressed air-powered rock drill, was pioneered there. The discovery of Silver Islet led to the development and prosperity of Port Arthur (now Thunder Bay) and spurred the exploration and settlement of northwestern Ontario (cf. Barr 1988). References Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology,v.33, p.193-196 Barr, E. 1988. Silver Islet: Striking it rich in Lake Superior; Natural Heritage/Natural History Inc.,Toronto, ON, Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23: 527–542. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 15721579. Easton, R., 2000. Metamorphism of the Canadian shield, Ontario, Canada. II. Proterozoic metamorphic history. Canadian Mineralogist, 38: 319–344. Elston, D.P., Link, P.K., Winston, D. and Horodyski, R.J., 1993. Correlations of Middle and Late Proterozoic succession. In, ed. J.C. Reed et al., Precambrian: conterminous United States. The Geology of North America. Geological Society of America, Vol. C-2, 468-485. Elston, D.P., Enkin, R.J., Baker, J. and Kisilevsky, D.K., 2002. Tightening the Belt: paleomagneticstratigraphic constraints on deposition, correlation and deformation of the Middle Proterozoic (ca. 1.4 - 53 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Ga), Belt-Purcell Supergroup, United States and Canada. Geological Society of America Bulletin 114, 619-638. Evans, K.V., Aleinikoff, J.N., Obradovich, L.D. and Fuming, C.M., 2000. U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: evidence for rapid deposition of sedimentary strata. Canadian Journal of earth Sciences, 37, 1287-1300. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada. In, ed. A.G. Plint, Sedimentary Facies Analysis. International Association of Sedimentologists Special publication 22, 137-156. Fralick, P., Smyk, M., and Mailman, M. 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. Institute on Lake Superior Geology, 46th Annual Meeting, Thunder Bay, Ont., Proceedings, Vol. 46, part 2, Field Trip Guidebook, pp. 1–41. Fralick, P.W., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario: single zircon U-Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Sciences, v.39, no.7, p.1085-1091. Franklin, J.M. 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario. In Current research, part A. Geological Survey of Canada, Paper 78-1A, pp. 275–282. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17: 633–651. Franklin, J.M., McIlwaine, W.H., Shegelski, R.J., Mitchell, R.H. and Platt, R.G., 1982. Proterozoic geology of the northern Lake Superior area; Field Trip Guidebook, GAC-MAC Annual Meeting, Winnipeg, 71p. Geddes, R.S., Kristjansson, F.J. and Teller, J.T. 1987. Quaternary features and scenery along the north shore of Lake Superior; XIIth International Union for Quaternary Research, Excursion guide book C-12, 62p. Geul, J., 1973. Geology of Crooks Township, Jarvis and Prince Locations and Offshore Islands, District of Thunder Bay. Ontario Division of Mines, Geological Report 102, 46p. Hart, T.R. and MacDonald, C.A. 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, v.44, no.8, p.1021-1040. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, v.44, no.8, p.1055-1086. Hill, M-L. and Smyk, M.C. 2005. Penokean Fold-andthrust Deformation of the Paleoproterozoic Gunflint Formation near Thunder Bay, Ontario; 51st annual Institute on Lake Superior Geology, Program with abstracts, v.1, p.26. Hinshelwood, A. 1990. 1987 observations at the Brohm site (DdjE-1), Sibley Provincial Park, Conservation Archaeology North Central Region, Report 27, Ontario Ministry of Citizenship and Culture, Heritage Branch, Thunder Bay ON. Hinshelwood, A. 2004 Archaic Reoccupation of Late Palaeo-Indian Sites in Northwestern Ontario. In The Late Palaeo-Indian Great Lakes: Geological and Archaeological Investigations of Late Pleistocene and Early Holocene Environments. edited by Lawrence J. Jackson and Andrew Hinshelwood Mercury Series Archaeology Papers 165, Canadian Museum of Civilization, Gatineau. Hollings, P., Fralick, P. and Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario. Canadian Journal of Earth Sciences, 41, 1329-1338. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development; Canadian Journal of Earth Sciences, v.44, no.8, p.1087-1110. Hollings, P.N., Smyk, M.C. and Hart. T. 2007b. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes; 53rd Institute on Lake Superior Geology, Annual Meeting, Lutsen, Minnesota, May 2007, Proceedings Volume 53, Part 1, p.40-41. Hollings, P., Smyk, M., Halls, H. and Heaman, L. 2009. Mesoproterozoic Midcontinent Rift-related mafic intrusions in northwestern Ontario: Continuing geochemical, paleomagnetic, petrographic and geochronologic studies; in Institute on Lake Superior Geology 54th Annual Meeting, Proceedings and Abstracts, v.54, part 1, p.42-43. Hollings, P., Smyk, M., Halls, H. and Heaman, 2010a. L. in press. The geochemistry, paleomagnetism and geochronology of the dykes and sills associated with the Midcontinent Rift near Thunder Bay, Ontario, Canada; Precambrian Research, 2010a, doi:10.1016/j. precamres.2010.01.012. - 54 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Hollings, P., Smyk, M., and Carl, C. 2010b. Geochemistry of Midcontinent Rift–related dikes on the Sibley Peninsula, Thunder Bay: a preliminary report; in Summary of Field Work and Other Activities 2010, Ontario Geological Survey, Open File Report 6260, p.10-1 to 10-3. Horton, R., 1989. The Mining and Geologic History of the Silver Islet Mine, and a Conceptual Ore Genesis Model for the Deposit. Institute on Lake Superior Geology Proceedings, 35: 29-31. Klewin, K., and Shirey, S., 1992. The igneous petrology and magmatic evolution of the Midcontinent Rift system. Tectonophysics, 213: 33–40. Krogh, T.E., Davis, D.W. and Corfu, F., 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area, In, ed. Pye et al., The Geology and Ore Deposits of the Sudbury Structure. Ontario Geological Survey, Special Volume 1, 431-446. Gogebic range, Lake Superior region, USA. Sedimentology, 51, 791-808. Richardson, A., and Hollings, P., 2005. Geochemical Variation within the Mesoproterozoic Nipigon Diabase Sills. Institute on Lake Superior Geology 51st Annual Meeting, Proceedings Volume 51, Part 1 – Program and Abstracts, 52-53. Rogala, B., 2003. The Sibley Group: a lithostratigraphic, geochemical and paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, ON, 254 pp. Rogala, B., Fralick, P.W. and Metsaranta, R.T, 2005. Stratigraphy and sedimentology of the Mesopreterozoic Sibley Group and related igneous intrusions, northwestern Ontario. Lake Nipigon Region Geoscience Initiative, Ontario Geological Survey, Open File Report 6174, 128 pp. Kustra, C.R., McIlwaine, W.H., Fenwick, K.G. and Scott, J.F. (1977) Proterozoic rocks of the Thunder Bay area, northwestern Ontario; Field Trip Guidebook, 23rd Annual I.L.S.G. Meeting, Thunder Bay, 47p. Rogala, B., Fralick, P.W., Heaman, L and Metsaranta, R.T., 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario, Canada. Canadian Journal of Earth Sciences, 44, 1131-1149. Maric, M., 2006. Sedimentology of the Rove and Virginia Formations. Unpub. M.Sc. thesis, Lakehead University, Thunder Bay, ON. Schulz, K., and Cannon, W. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4–25. Maric, M. and Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, 51, 41-42. Shirey, S., Klewin, K., Berg, J., Carlson, R., 1994. Temporal changes in the sources of flood basalts: isotopic and trace element evidence from the 1100Ma old Keweenawan Mamainse Point Formation, Ontario, Canada. Geochimica et Cosmochimica Acta 58, 4475–4490. Metsaranta, R. 2006. Sedimentology and geochemistry of the Mesoproterozoic Pass Lake and Rossport Formations, Sibley Group; unpublished M.Sc. thesis, Lakehead University, Thunder Bay ON. Mohide, T.P. 1985. Silver; Ontario Ministry of Natural Resources, Mineral Policy Background Paper No. 20, 406p. Osmani. I.A. 1991. Proterozoic mafic dike swarms in the Superior Province of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, 661-681. Pesonen, L.J. and Halls, H.C. 1983. Geomagnetic field intensity and reversal asymmetry in late Precambrian Keweenawan rocks. Geophysical Journal of the Royal Astronomical Society, 73: 241-270. Pufahl, P.K., 1996. Stratigraphic architecture of a Paleoproterozioc iron formation depositional system: the Gunflint, Mesabi and Cuyuna iron ranges. Unpub. M.Sc. thesis, Lakehead University, 167 pp. Pufahl, P.K. and Fralick, P.W., 2000. Field trip 4 Depositional environments of the Paleoproterozoic Gunflint Formation. In, ed. P.W. Fralick, Institute on Lake Superior Geology, Proceedings Volume 46, Part 2: Field Trip Guide Book. Smyk, M.C. and Hollings, P.N. 2007. Midcontinent Riftrelated mafic intrusions north of the international border; 53rd Institute on Lake Superior Geology, Annual Meeting, Lutsen, Minnesota, May 2007, Proceedings Volume 53, Part 2, Field Trip Guidebook, p.53-80. Smyk, M. and Hollings, P., 2009. Project Unit 08-021. Mesoproterozoic Midcontinent Rift–Related Mafic Intrusions Near Thunder Bay: Update. Summary of Fieldwork and Other Activities 2009, Ontario Geological Survey, Open File Report 6240, p. 11-1 to 18-5. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior area; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 627658. Tanton, T.L. 1924. Thunder Cape, Lake Superior; Geological Survey of Canada, Publication 1902, scale 1:36 000. Tanton, T.L. 1931. Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario; Geological Survey of Canada, Memoir 167, 222p. Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on Palaeoproterozoic iron formation accumulation, - 55 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 3 - Lac des Iles Mine Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada John Corkery North American Palladium, Ltd., Thunder Bay, Ontario, Canada Regional Setting of the Lac des Iles Mine The Lac des Iles Mine area is underlain by mafic to ultramafic rocks of the Neoarchean Lac des Iles Intrusive Complex (LDI-IC). The LDI-IC is part of the Lac des Iles Suite, whose mafic intrusive rocks generally range in age between 2686 and 2699 Ma (c.f. Stone, 2010). These rocks have intruded a variety of metamorphosed granitoid and supracrustal greenstone belt rocks (ca. 2.9 to 2.7 Ga in age) of the Wabigoon Subprovince of the Superior Province (Fig. 1). The LDI-IC lies immediately north of the boundary between the volcano-plutonic Wabigoon and metasedimentary Quetico subprovinces. The LDI-IC is the largest of a series of mafic and ultramafic intrusions that occur along the Wabigoon-Quetico boundary and which collectively define a 30 km diameter circular pattern (Fig. 1). There are three broad, temporally diverse settings for Archean platinum group element (PGE) mineralization west of Lake Nipigon (c.f. Smyk et al., 2002): (1) Pre-tectonic mafic to ultramafic subvolcanic(?) intrusions intimately associated and coeval with greenstone belts of various ages in the Wabigoon Subprovince; (2) Mafic intrusive rocks occurring within syntectonic to post-tectonic, diorite-monzodiorite-monzonite suites with sanukitoid affinity within the Wabigoon and Quetico subprovinces (e.g., Shelby Lake Figure 1. Regional setting of the Lac des Iles complex and related ultramafic and mafic intrusions within the Wabigoon Subprovince (from Lavigne and Michaud, 2002). - 56 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. Geologic setting of the Lac des Iles complex and related ultramafic and mafic intrusions (from Lavigne and Michaud) batholith, Stone et al., 2003; Roaring River Complex, Schnieders et al., 2002). (3) Posttectonic, mafic to ultramafic intrusions related to late plutonism in the Wabigoon Subprovince, hosted by gneissic tonalite-granodiorite (e.g., Lac des Iles suite). Pre-tectonic, deformed mafic to ultramafic intrusions and/or coeval komatiitic metavolcanic rocks within greenstone belt assemblages may host coppernickel-PGE mineralization. This broad classification is equivalent to the “komatiitic-associated” and “intrusions comagmatic with volcanic rocks” settings for coppernickel-PGE ± chromium mineralization described by Fyon et al. (1992). Such deposits are characterized by remobilized, deformed and annealed, net-textured to massive sulfides. Examples include the past-producing Shebandowan Mine in the Shebandowan greenstone belt, west of Thunder Bay (8.64 million tonnes mined, grading 1.92% Ni, 0.98% Cu, 2.62 g/t Pt+Pd; Resident Geologist’s Files, Thunder Bay South District, Thunder Bay), and in the Obonga Lake belt, the Core Zone gabbro (2733±7 Ma; Tomlinson et al., 1999) and the Puddy Lake serpentinite (both may contain indications of such mineralization). Lavigne et al. (1991) reported metal contents from the Puddy Lake serpentinite up to 5.02% Cu, 2.1% Ni, 415 ppb Au, 1500 ppb Pt and 3750 ppb Pd; cobalt values of 0.07% were reported from drilling in the 1960s (Lavigne et al., 1992). PGE mineralization is also associated with rocks of the sanukitoid suite (ca. 2688 to 2690 Ma, Davis et al., 1990; Kamo, 2004; cf. Stern et al., 1989). The Shelby Lake batholith, which consists of hornblende leucogabbro (diorite) to monzodiorite and hornblende granodiorite to granite, contains disseminated sulfide zones in thin units of hornblende gabbro distributed along its northwestern margin (e.g., Turtle Hill and Stocker occurrences). Similar, somewhat larger sulfide occurrences have been described (Stone et al., 2003) at Wakinoo Lake, Towle Lake (e.g., Powder Hill, Stinger and Vande occurrences) and Legris Lake (e.g., Main and Poplar occurrences). Diamond drill holes in hornblende gabbro at Legris Lake contained 2.04 g/t Pd, 0.41 g/t Pt, 0.71 g/t Au, 0.42% Cu and 0.13% Ni over 9.95 m (News Release, Avalon Ventures Ltd. and Starcore Resources Ltd., November 10, 2000). The Roaring River complex (Stern and Hanson, 1991) consists of a variety of plutonic rocks including diorite, monzodiorite, monzonite, quartz monzodiorite and granodiorite, all of sanukitoid affinity; gabbroic and - 57 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 pyroxenitic mega-inclusions occur in these phases. Grab samples of the Mere showing contain up to 1249 ppm Ni, 3159 ppm Cu and 1.1 g/t Pt+Pd+Au and the Leigh (boulder) occurrence returned up to 2067 ppm Ni, 1920 ppm Cu and 1.23 g/t Pt+Pd+Au (Schnieders et al., 2002 and references therein). Disseminated to locally net-textured chalcopyrite, iron sulfides, pentlandite and magnetite typically characterize PGE- mineralized zones, which are commonly associated with intrusive contacts, polyphase intrusive breccia, and sheared and hydrothermally altered zones. Mafic to ultramafic intrusions of the Lac des Iles suite (ca. 2686 to 2699 Ma; Stone, 2010 and references therein; Davis, 2003; Kamo, 2004) and their associated copper-nickel-PGE mineralization at North American Figure 3. Geology of the Northern Ultramafic and Mine Block intrusions of the Lac des Iles Complex (from Lavigne and Michaud , 2002). - 58 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Palladium Ltd.’s Lac des Iles mine were most recently described by Stone et al. (2003). This suite includes the Buck Lake, Dog River, Taman Lake, Demars Lake, Bullseye and Tib Lake intrusions (see Fig. 2), as well as the Northern Ultramafic intrusion and Mine Block intrusion at Lac des Iles (Fig. 3). These leucogabbro and gabbronorite intrusions (± anorthosite, peridotite) range from 1 to 10 km in diameter and are considered to represent a continuum of the Quetico suite of mafic to ultra mafic intrusions (cf. MacTavish, 1999; Pettigrew et al., 2000). Michaud (1998), Lavigne and Michaud (2002), and Lavigne et al. (2005) provided recent descriptions of the deposits in the Mine Block intrusion (Fig. 4). Platinum group elements are associated with disseminated Cu-Ni-sulfide minerals in the matrix of magmatic breccia, in varitextured to pegmatitic gabbroic rocks (which together represent the Breccia Zone), and also in pyroxenite that is part of the HighGrade Zone. Platinum group elements also occur with sulfide-poor, varitextured to pegmatitic gabbro in the Roby and North Roby zones and are locally associated with strong silicate alteration (e.g., Roby Zone and portions of the High-Grade Zone; Lavigne et al., 2005). The Roby Zone is the product of multiple stages of intrusion, alteration and mineralization (Lavigne et al., 2005). Hinchey et al. (2005) put forward a schematic model illustrating a deposit model for the history of mineralization at the southern Roby Zone (Fig. 5). The textures of the Lac des Iles deposit are similar to those of contact-type PGE deposits, but there are fundamental differences between the two. The Lac des Iles deposit is not localized near the contact between the host intrusion and the country rocks and evidence of the assimilation of the host rocks is lacking. Instead, the mineralization at Lac des Iles has many features in common with layered intrusion-hosted deposits, in which pulses of primitive magma introduced the PGE. Unlike the quiescent magma chambers of most Figure 4. Geology of the Mine Block intrusion showing the main ore zones (from Lavigne and Michaud, 2002). - 59 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. Schematic mineralization model for the Lac des Iles Mine (Hinchey et al., 2005) layered deposits, the magmas at Lac des Iles were intruded energetically, forming breccias and magmamingling textures. Magmas formed by a high degree of partial melting in a depleted mantle source (Fig. 5, A1) became enriched in Cu, Pt, and Pd through fractional crystallization of olivine, chromite, and high- - 60 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 temperature PGM (Fig. 5, A2), segregated sulfide melt that had low Cu/Pd ratios along the conduit and the base of the magma chamber (Fig. 5, A3), and solidified as the early leucocratic gabbros. A second episode of partial melting in the mantle source produced another batch of fertile magma. As with the early magma, this magma was enriched in Cu, Pt, and Pd through fractional crystallization (Fig. 5, A2). This magma incorporated the earlier sulfide melt and intruded forcefully into the partially crystallized leucocratic rocks (Fig. 5, B1), causing brecciation and magma mingling, and solidified as fertile melanocratic gabbro. Aqueous fluids that separated from the melanocratic magma percolated through the cumulates, partially dissolving Pd and concentrating it in the High Grade ore zone adjacent to barren East Gabbro (Fig. 5, B2). History of Exploration and Mining The Lac des Iles area was initially mapped by Jolliffe (1934) and later by Pye (1968), Watkinson and Dunning (1979), Sutcliffe and Sweeny (1985, 1986), Sweeny and Edgar (1987), and Stone et al. (2003). The regional geology has been summarized by Stone (2010). Economic interest in the area was sparked by the ground-truthed aeromagnetic anomalies. Significant palladium mineralization was first discovered in the Roby Zone in 1963 by a prospecting syndicate. Various exploration programs were undertaken over the next 25 years by a number of companies, including Gunnex Ltd., Anaconda Ltd., Texas Gulf Sulphur Co. Inc., and Boston Bay Mines Ltd. In 1990, Madeleine Mines Ltd., a precursor to North American Palladium Limited (NAPL), developed the property. After intermittent production and continuing capital expenditures, commercial open pit production of the Roby Zone was achieved in December 1993. NAPL was formed through corporate reorganization. In 2000, an expansion program began and a new mill was commissioned in the second quarter of 2001 to achieve its rated 15,000 t per day throughput in August 2002. From 1999 to 2001, an extensive drilling campaign identified mineralization at depth, below the ultimate pit bottom. The drilling identified 2 zones with potential for underground mining: the Roby Underground Zone and the Offset Zone. On July 31, 2003, a positive pre-feasibility study for underground mining of the Roby Underground Zone (down-dip extension of the open pit Main Zone) was completed, and was followed by a feasibility study for underground mining in 2004. Development on the Roby Underground Zone started in 2004, with the ramp developed and the zone accessed in late 2005. Development muck was delivered to the concentrator in December 2005 and underground commercial production began in March 2006. The Offset Zone, discovered in 2000, was historically subdivided into the Offset High Grade Zone and the adjacent Roby Footwall Zone. The Offset Zone is the fault-offset, down-dip extension of the Roby Underground Zone that was mined below the Roby open pit until October 2008. A number of surface and underground drilling programs have targeted the Offset Zone since 2001. In 2008, a surface drilling program focused on exploring targets on the Mine Block Intrusion and on Table 1. Production figures for Lac des Illes (MD&A, 2011) Unit Ore Mined Waste Mined – Open Pit Mill Throughput Pd Head Grade Pd Recovery Pd Produced Pt Produced Au Produced Ni Produced Cu Produced Tonnes Tonnes Tonnes g/t % Oz Oz Oz Lbs Lbs 2011 2010 1,830,234 615,926 1,689,781 3.70 78.34 146,624 9,143 7,267 816,037 1,596,185 649,649 6.06 80.80 95,057 4,894 4,023 395,622 658,013 2009 Mine closed * Added; Ore Mined - Underground + Ore Mined - Open Pit - 61 - 2008 *3,676,418 6,964,501 3,722,732 2.49 75.30 212,046 16,311 15,921 2,503,902 4,623,278 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 2. Resource figures for Lac des Illes (MD&A, 2011) the Southeast Breccia Zone, situated adjacent to the southeastern corner of the open pit. The Cowboy Zone was discovered in June 2009 during infill drilling of the Offset Zone to support a prefeasibility study (news release, NAPL, June 25, 2009). It is located 30 to 50 m down-section to the west of the Offset Zone, extends for up to 250 m along strike and 350 m down-dip, and it remains open in all directions. Similar to the Offset Zone, the Cowboy Zone appears to consist of several mineralized subzones. Intersections include 5.10 g/t Pd over 4 m, 3.88 g/t Pd over 4 m, and 4.46 g/t Pd over 5 m. Open pit mining of the Roby Zone began in 1993. The open pit was operated by conventional truckandshovel mining, with low- and high-grade material stockpiled near the on-site concentrator. In May 2004, LDI collared a portal in the northwest wall of the pit and ramped down to access the Roby Underground Zone that continues down-dip from the Roby Zone hanging wall below the pit. LDI began processing development muck from the Roby Underground Zone in December 2005. The ramp was extended around the pit to the north and the new portal was opened in the east wall in 2006. The Roby Underground Zone reached commercial production at 2000 t per day in April 2006. Operations were suspended in October, 2008 due to the global economic downturn and depressed metal prices. Palladium production at Lac des Iles Mine resumed in April 2010 (news release, NAPL, April 14, 2010). NAPL expects to produce 140 000 ounces of palladium per year. Ore production from the Roby Underground zone is expected to increase to a target rate of 2600 t per day. A summary of ore mined is presented in Table 1. Since production began in 1993 at Lac des Iles Mine, almost 42 Mt of ore have been processed, and approximately 2.3 million ounces of palladium produced (see Table 1). A Mineral Resource Summary (December 31, 2008) is given in Table 2 (McCombe et al., 2009). Local and Property Geology Many of the following excerpts have been modified after McCombe et al. (2009). The mine lies in the southern portion of the Lac des Iles Intrusive Complex (LDI-IC) (see Figs. 2 & 3), in a roughly elliptical intrusive package measuring 3 km long by 1.5 km wide, termed the Mine Block Intrusive (MBI) (see Figs. 3 & 4). It hosts a number of PGE deposits and the most important of these is the Roby Zone with its three subzones: the North Roby Zone, the High Grade Zone, and the Breccia Zone. The MBI comprises rocks with a very wide range of textures and mafic and ultramafic compositions, ranging from anorthosite to clinopyroxenite, leucogabbronorite to melanonorite, and includes magnetite-rich gabbro. Textures include equigranular, fine- to coarsegrained, porphyritic and pegmatitic, varitextured - 62 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 units, and heterolithic gabbro breccias. These last three textural types are the most common host to PGE mineralization, including the Roby Zone. The MBI consists of two lithologically distinct domains. The oval-shaped domain immediately south of Lac des Iles is lithologically complex and contains widespread PGE mineralization, while the domain further to the south is dominated by massive, mediumgrained, PGE-barren gabbronorite (see Figs. 2 to 4). Extensive stripping has disclosed that the interior of the oval-shaped domain has an abundance of monolithic and heterolithic breccia with an average composition of gabbronorite. Within this area, individual lithological units are not laterally extensive and are chaotically distributed. The most laterally continuous unit is a massive, mediumgrained gabbro, referred to as East Gabbro (EGAB) (see Fig. 5). EGAB is adjacent to a varitextured gabbro “rim” to the west and more equigranular gabbronorite (GN) to the east. The varitextured rim is host to the Roby palladium deposit, where heterolithic gabbro breccia (HGABBX) commonly occurs as pipes and pods, and large blocks (~60 m) of varying composition. A pyroxenite unit (PYXT), at the contact between the EGAB and the HGABBX, is host to much of the High Grade Zone. The principal rock types in the Offset Zone area include the following: East Gabbro (EGAB) – is a well-known gabbro “marker unit” that is characteristically uniform and compositionally homogeneous. EGAB has very minor alteration, with local trace pyrite and epidote. It has no significant associated mineralization, and bounds the Roby Zone to the east. (i.e., hanging wall contact of the Roby Zone). Heterolithic Gabbro Breccia (HGABBX) – the principal host for the Roby Zone, consisting of a melanogabbro to gabbro matrix with variable clast composition, ranging from leucogabbro to pyroxenite. Clast percentage varies commonly from 15 to 60%. This unit comprises most of the economic ore grade material in the open pit and underground reserves. Varitextured Gabbro (VGAB) – the majority of rock types, excluding EGAB, have a varitextured counterpart. The VGAB varies from leucocratic to pyroxenitic, with grain sizes from fine to very coarse, to pegmatitic. The coarser-grained units form patches and “veinlets” within finer-grained counterparts. Gabbro (GAB) – the most common gabbros in the MBI are medium grained and equigranular, but range from fine to coarse grained and may locally be leucocratic to melanocratic. Magnetic Gabbro (MTGAB) – medium-grained, equigranular gabbro occurs within the MBI and contains black, fine-grained, interstitial magnetite (typically < 20%); magnetite content ranges from trace amounts to local, narrow layers of 60 to 95% magnetite. Pyroxenite (PYXT) – a steeply dipping, thin layer situated along the contact between the Heterolithic Gabbro and EGAB; it hosts the highest proportion of the High Grade Zone. This unit is responsible for much of the high PGE grades. Not all pyroxenites locally carry economic PGE grades. Gabbronorite (GN) – a 20 to 50 m thick, steeply dipping slab located along the northwestern contact of the EGAB; it is also a host unit of the High Grade Zone, although to a lesser degree than the PYXT. The gabbronorite appears to be a gradational extension of the pyroxenite to the northeast of the mine site. Gabbronorite Breccia (GNBX) – a palladiummineralized (Twilight Zone) heterolithic breccia, similar to the HGABBX but without pegmatitic phases or varitextured gabbro; it occurs as a roughly cylindrical pod, approximately 150 m in diameter, completely enclosed by the EGAB. Dikes – late, post-mineralization, mafic dikes vary from small, discrete bodies that occupy space within the modeled mineralized wire frames to large bodies that control the northern termination of the Offset Zone. A dike swarm approximately 30 m wide and trending approximately easterly was mapped at the southern extent of the Roby Zone. Two major faults have been interpreted to influence the Offset Zone. The Offset Fault structure displaces the High Grade Zone down and approximately 300 m to the west. This fault, easily picked out in diamonddrill core, is often marked by extensive fault gouge, fracturing, alteration of adjacent country rock, and infilling by mafic dikes. The B2 Fault has recently been recognized and interpreted from the underground Offset Zone diamond drilling. It lies approximately 20 to 40 m below and parallel to the westerly dipping Baker Fault and is marked by narrow intersections of fault gouge, fracturing and late mafic dikes. Mineralized Zones The Roby Zone is a bulk-mineable, PGE-enriched disseminated sulfide deposit with a minimum north to south length of 950 m, and a width of 815 m, including the Twilight Zone in the southwestern portion of the - 63 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 deposit. The Roby Zone consists of 3 distinct ore types: High Grade Ore (7.6% of volume), North Roby Ore (5.3% of volume), and Breccia Ore (87.1% of volume). The High Grade Ore is the primary ore type mined underground. High Grade Zone ore is hosted mainly within a 15 to 25 m thick unit of locally sheared pyroxenite/ melanogabbro. A host to high-grade PGE mineralization, it is located in the east-central portion of the Roby Zone, bounded by the barren EGAB hanging wall and HGABBX-hosted Breccia Ore to the west. The High Grade Zone is primarily confined to a 400 m long segment of the pyroxenite, although it does extend northward into the gabbronorite. The High Grade Zone, striking north-northwest to northnortheast, dips almost vertically near surface and flattens to nearly 45° at depth. Below the open pit, this zone is referred to as the Roby Underground Zone. The zone appears to be terminated down dip by a relatively shallow dipping fault, the Offset Fault. The Offset Zone, a higher grade zone similar to the High Grade Zone, is located below the Offset Fault structure, where it is displaced down and approximately 300 m to the west. The Offset Zone can be split into 3 horizons and has been divided into 3 subzones: the High Grade (HG) Subzone; the Mid (MID) Subzone; and the Footwall (FW) Subzone. High Grade Subzone mineralization is stratabound, along the contact between the EGAB and the mineralized HGABBX. Within the HGABBX, there is a high-grade core typically constrained to an easily recognized ultramafic unit, the pyroxenite. Width varies from 4 to 30 m, with an average of 15 m. Approximately 2% of the zone is occupied by late dikes (dilution). Less than 1% is occupied by shears and faults. The MID Subzone is proximal to the HG Zone, generally sharing a common boundary in the centre sections and then splitting away near the top and bottom areas. Palladium grades within the MID Subzone can approximate the high grades found within the HG Zone. Apparent widths can vary from 4 to 90 m, with an average of 15 m. Approximately 4% of the zone is occupied by late dikes (dilution). Less than 1% is occupied by shears and faults. The Footwall Subzone is a stand-alone band of higher grade mineralization that can be defined based on higher grade intersections within the Footwall varitextured gabbro mineralization. This subzone, located approximately 2 to 40 m from the MID Zone, was interpreted based on vertical continuity seen in the drill hole intersections. It is discontinuous and sinuous in plan and has less of a defined areal extent than the other zones. Apparent widths can vary from 4 to 20 m, with an average of 7 m. Approximately 1% of the zone is occupied by late dikes (dilution). Other mineralized zones present within the MBI, as shown in Figure 2-4, are described below: The Twilight Zone was removed with the mining of the open pit. The Baker Zone is located approximately 1 km northeast from the Roby and Twilight zones and contains similar rock types and textures. Gabbronorites/norites have been intruded by eastnortheast-trending, heterolithic melanogabbro breccia and lesser melanogabbro, leucogabbro breccia, varitextured gabbro and late pyroxenite dikes. Surface exploration has exposed the Baker Zone breccias and associated lithologies over a 150 by 55 m area. The heterolithic melanogabbro breccia hosts blebby to disseminated to narrow veinlets of sulphide with sporadic mineralization in the adjacent lithologies. The north-trending, shallowly westerly dipping Baker Fault appears to truncate the Baker Zone mineralization at depth. Extensive surface exploration by NAP occurred mainly from 1998 to 2001 and consisted of prospecting, stripping/trenching (including the main stripped area of approximately 200 by 120 m), channel sampling, geological mapping and ground induced polarization (IP) / resistivity surveys. Sixteen diamond-drill holes in 1998–99 tested the main portion of the Baker Zone over a 250 m strike length and to a maximum depth of 200 m. Subsequent exploration (trenching and diamond drilling) has tested possible strike extensions of the zone and the area below the Baker Fault. The Moore Zone is a low-grade, presently uneconomic, mineralized zone approximately 500 m south of the current Roby pit with similar lithologies and textures to other MBI breccias. The central area of interest is a small breccia pod measuring approximately 200 m long, varying from approximately 15 to 115 m wide, which occurs within the massive, medium-grained gabbronorite typical of the more southerly domain of the MBI. The main Moore Zone mineralization is located in the eastern portion of the breccia pod and appears to be structurally controlled (trending ~030°, dipping 70° east), ranging from 5 to 25 m thick. Prospecting, mapping, trenching, sampling and limited diamond drilling of the Moore Zone have indicated limited economic potential. - 64 - The Creek Zone is located approximately 2 km �Proceedings of the 58th ILSG Annual Meeting - Part 2 northeast of the Roby pit in the northeastern nose of the MBI, near the contact with the north LDI-IC. Surface trenching has exposed the main portion of the Creek Zone in an area 90 m long by 10 to 40 m wide. It is dominated by low-sulfide breccias that have intruded the varitextured gabbro rim of the MBI. The breccias consist of approximately 90% GBNR clasts and only approximately 10% MGAB matrix. Unlike the Roby Zone, mineralization is not dominantly hosted by the breccia matrix but seems to occur within the pegmatitic gabbronorite. Prospecting, mapping, trenching, sampling and limited diamond drilling of the Creek Zone have indicated limited mineralized potential. The platinum-group minerals at Lac des Iles Mine include the following (Lavigne and Michaud, 2001): Braggite (Pt,Pd)S Kotulskite Pd(Te,Bi)2 Isometrieite Pd11(Sb,Te)2As2 Merenskyite PdTe2 Moncheite PtTe2 Palladoarsenide Pd2As Sperrylite PtAs2 Stibiopalladinite Pd5Sb2 Stillwaterite Pd8As3 Mineralization Platinum group element and base metal mineralization at the Lac des Iles Mine appears to be dominantly stratabound along the contact between the EGAB and the mineralized HGABBX. Within the HGABBX, there is a high-grade core typically constrained to an easily recognized pyroxenite unit. Visible PGE mineralization is rare and its occurrence is difficult to predict. In general, economic PGE grades are anticipated within gabbroic to pyroxenitic rocks (in close proximity to the marker unit EGAB) that exhibit strong sausseritization of plagioclase feldspars, strong talcose alteration and association with either disseminated or blebby secondary sulfides. Higher PGE grades (mean – 7.89 g/t Pd, maximum – 55.95 g/t Pd) occur in those portions of the pyroxenite that are altered to an assemblage of amphibole (anthophylliteactinolite-hornblende)-talc-chlorite. The PGE tenor is not proportional to the sulfide content, and samples free of visible sulfide often contain more than 10 g/t Pd. The high-grade mineralization is located primarily within the western, highly altered portion of the pyroxenite, since much of the pyroxenite between the barren EGAB and the High Grade Zone is low grade. The higher grade “High Grade Ore” is not restricted to the pyroxenite as it commonly straddles the pyroxenite/ gabbro breccia contact to widths exceeding 250 m. The majority of platinum-group minerals occur either interstitially to sulfides as cumulus grains or are associated with sulfides at sulfide-silicate boundaries, occurring as discrete mineral inclusions within secondary silicates of altered rocks (Sweeny 1989; Lavigne and Michaud 2001). Palladium and platinum mineralization within the High Grade Zone consists primarily of fine-grained PGE sulfide, braggite and the telluride minerals merenskyite and kotulskite (Sweeny, 1989; Lavigne and Michaud, 2001). Vysotskite PdS Unnamed Ag4Pd3Te4 Unnamed Pd5As2 Melonite, gold, pentlandite Pd in solid solution Field Trip Stops Roby Zone open pit Baker Zone North VT Rim Trenches Exploration office and diamond drill core Directional drilling sites and Devico Unit Mill References Davis, D.W. 2003. U-Pb geochronology of rocks from the Lac des Iles area, northwest Ontario; Ontario Geological Survey, internal report, June 12, 2003. Davis, D.W., Pezzutto, F. and Ojakangas, R.W. 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: Implications for Archean sedimentation and tectonics in the Superior Province; Earth and Planetary Science Letters, v.99, p.195-205. Fyon, J.A., Breaks, F.W., Heather, K.B., Jackson, S.L., Muir, T.L., Stott, G.M. and Thurston, P.C. 1992. Metallogeny of metallic mineral deposits in the Superior Province of Ontario; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, pt.2, p.1091-1174. Hinchey, J.G., Hattori, K.H. and Lavigne, M.J. 2005. Geology, petrology, and controls on PGE mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada; Economic Geology, v.100, p.43-61. Jolliffe, F. 1934. Block Creek map area, Thunder Bay - 65 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 District, Ontario; Geological Survey of Canada Summary Report 1933, pt.D, p.7-15. Kamo, S. 2004. U-Pb geochronological investigations of rocks from the Lac des Iles area, northwestern Ontario, the Michipicoten Greenstone belt, Wawa, and the Tomiko Terrane, Mattawa, Ontario; Ontario Geological Survey, internal report, July 2004. Lavigne, M.J. and Michaud, M.J. 2001. Geology of North American Palladium Ltd.’s Roby Zone Deposit, Lac des Iles; Exploration and Mining Geology, v.10, Nos. 1 and 2, p.1-17. Lavigne, M.J. and Michaud, M.J. 2002. Geology of North American Palladium Ltd.’s Roby zone deposit, Lac des Iles; Exploration and Mining Geology, v.10, p.117. Lavigne, M.J., Michaud, M.J. and Rickard, J. 2005. Discovery and geology of the Lac des Iles palladium deposits; in Exploration for Platinum Group Element Deposits, Mineralogical Association of Canada, Short Course Series, v.35, Oulu, Finland, p.369-390. Lavigne, M.J., Scott, J.F. and Sarvas, P. 1991. Thunder Bay Resident Geologist’s District; in Report of Activities, 1990, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 152, p.107-126. Lavigne, M.J., Scott, J.F. and Sarvas, P. 1992. Thunder Bay Resident Geologist’s District; in Report of Activities, 1991, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 158, p.87-105. MacTavish, A.D. 1999. The mafic-ultramafic intrusions of the Atikokan–Quetico area, northwestern Ontario; Ontario Geological Survey, Open File Report 5997, 154p. Management’s Discussion and Analysis and Consolidated Financial Statements Fourth Quarter 2011 For the year ended December 31, 2011 p. 11 http:// www.napalladium.com/Theme/NAP/files/Q4%20 2011%20MDA%20and%20FS-Final%20Feb23.pdf McCombe, D.A., Blakley, I.T., Routledge, R.E. and Cox, J.J. 2009. Technical report on the Lac des Iles Mine, North American Palladium Ltd., internal report, Scott Wilson Roscoe Postle Associates Inc., 130p. Michaud, M.J. 1998. The geology, petrology, geochemistry and platinum group element-gold-copper-nickel ore assemblage of the Roby Zone, Lac des Iles mafic-ultramafic complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 183p. Pettigrew, N.T., Hattori, K.H. and Percival, J.A. 2000. Mafic-ultramafic intrusions of the central portion of the western Quetico subprovince, northwestern Ontario; in 2000 Western Superior Transect, 6th Workshop, Lithoprobe Report #77, University of British Columbia, p.104-110. Pye, E.G. 1968. Geology of the Lac des Iles area, District of Thunder Bay, Ontario Department of Mines, Geological Report 64, 47p. Schnieders, B.R., Scott, J.F., Smyk, M.C., Parker, D.P. and O’Brien, M.S. 2002. Report of Activities 2001, Resident Geologist Program, Thunder Bay South Regional Resident Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6081, 45p. Smyk, M.C., Mason, J.K., Schnieders, B.R. and Stott, G.M. 2002. A synopsis of Archean and Proterozoic platinum group element mineralization in the Thunder Bay District, Ontario; Extended Abstract Volume, 9th International Platinum Symposium, 25 July 2002, Billings, Montana, p.433-434. Stern, R.A. and Hanson, G.N. 1991. Archean highMg granodiorite: A derivative of light rare earth elementenriched monzodiorite of mantle origin; Journal of Petrology, v.32, pt.1, p.201-238. Stern, R.A., Hanson, G.N. and Shirey, S.B. 1989. Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province; Canadian Journal of Earth Sciences, v.26, p.1688-1712. Stone, D. 2010. Precambrian geology, central Wabigoon Subprovince area, northwestern Ontario; Ontario Geological Survey, Preliminary Map P.2229, scale 1:250 000. Stone, D., Lavigne, M.J., Schnieders, B.R., Scott, J. and Wagner, D. 2003. Regional geology of the Lac des Iles area; in Summary of Field Work and Other Activities 2003, Ontario Geological Survey, Open File Report 6120, p.15-1 to 15-25. Sutcliffe, R.H. and Sweeny, J.M. 1985. Geology of the Lac des Iles complex, District of Thunder Bay; in Summary of Field Work and Other Activities 1985, Ontario Geological Survey, Miscellaneous Paper 126, p.47-53. Sutcliffe, R.H. and Sweeny, J.M. 1986. Precambrian geology of the Lac des Iles complex, District of Thunder Bay, Ontario; Ontario Geological Survey, Preliminary Map P.3047, scale 1:15 840. Sweeny, J.M. and Edgar, A.D. 1987. The geochemistry, origin and economic potential of platinum-group element bearing rocks of the Lac des Iles complex, northwestern Ontario; in Geoscience Research Grant Program, Summary of Research, 1986-1987, Ontario Geological Survey, Miscellaneous Paper 136, p.140152. Tomlinson, K.Y., Davis, D.W., Percival, J.A., Hughes, D.J. and Thurston, P.C. 1999. Neoarchean supracrustal development in the central Wabigoon Subprovince: Nd isotope data and U/Pb geochronology; in Western Superior Transect Fifth Annual Workshop, LITHOPROBE Report 70, p.147-152. Watkinson, D.H. and Dunning, G.R. 1979.Geology and platinum-group mineralization, Lac des Iles complex, northwestern Ontario; Canadian Mineralogist, v.17, p.453-462. - 66 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 4 - Shebandowan Mine Area Alan Aubut, P.Geo. Sibley Basin Group Geological Consulting Services Ltd. Dorothy Campbell, P.Geo Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This field trip will focus on two main aspects of the geology of the Lake Shebandowan area: the presence of a suite of metasediment and metavolcanic rocks usually described as being “Timiskaming-type” that unconformably overlies older Archean, “Keewatintype” rocks; and the presence of numerous komatiitic ultramafic bodies within the underlying Keewatin metavolcanic rocks that in part are spatially associated with a Timiskaming-age pull-apart basin (Figs. 1 and 2). Many of the Archean terrains within the Canadian Shield are host to a neo-Archean sequence of shoshonitic/ alkali metavolcanic and fluvial metasedimentary rocks that occupy pull-apart basins typically spatially related to major sub-province transcurrent boundary faults such as the Kirkland Lake-Cadillac Fault and the Porcupine-Destor Fault. This suite of rocks is commonly referred to as “Timiskaming-type” after the Timiskaming Group found within the Abitibi Terrain. Examples of “Timiskaming-type” include the Oxford Lake Group in Manitoba, the Hauy Formation in the southern and northern parts of the Abitibi, the Opemisca Group near Chibougamau, the Seine Group of the Wabigoon Subprovince, and the Shebandowan Group (Card, 1990).   Figure 1. Shebandowan field trip stops - 67 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 2 – General Geology – Shebandowan area. From Pye and Fenwick (1965) Timiskaming-type rocks range in age from approximately 2700 Ma to about 2680 Ma (Card, 1990; Corkery et al., 2000). They unconformably overlie older metavolcanic sequences hereafter referred to as “Keewatin”. Timiskaming-type metavolcanic rocks usually are shoshonitic (high Al2O3 and K2O with TiO2 content < 1.3 wt.%) and are associated with fluvial meta-conglomerates and meta-sandstones. The Timiskaming-type rocks in the Lower Shebandowan Lake area consist of calc-alkaline metavolcanic rocks and alluvial-fluvial metaconglomerates and meta-sandstones. The metasedimentary rocks are immature, typically with cross-bedded arenites and conglomerates with some shale and ironstone. The metavolcanic rocks are calc-alkaline to alkali/shoshonitic subaerial volcanic rocks. They all display rapid facies changes and internal unconformities and typically only display late deformational and metamorphic events. have traditionally been considered intrusive bodies. While no spinifex textures, considered diagnostic of flow emplacement, have been found there are other features present that indicate they were deposited as flows. These include the fact that they are stratabound and commonly have ironstone or chert beds along one contact. In addition, generally accepted notions that ultramafics are intrusive does not take into consideration that the density of molten ultramafic rocks is too high to allow emplacement by density contrast and therefore must have relied more on processes such as over-pressure. This, combined with the fact these rocks are typically associated with extensional environments makes emplacement by intrusion, especially when they are conformable to local stratigraphy, extremely difficult to explain except by extrusive processes. The komatiitic bodies of the Shebandowan area - 68 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Road Log Field Trip Stop Descriptions Drive west from Thunder Bay on Highway 1117. At Shabaqua turn left onto Highway 11 and then drive 12.4 km to the Junction of Highway 11 and the Shebandowan Mine Road. Turn Left. Stop 1. Timiskaming-type metavolcanic debris flow with intercalated sediments. 0 km - Junction of Hwy 11 and Shebandowan Mine Road 0.7 km – bridge over Shebandowan River 1.9 km – Stop 1 3.1 km – Stop 2 5.7 km – Stop 3a 5.8 km – Stop 3b 5.9 km – Stop 3c 7.6 km – trail into Stop 4. Follow trail for 250 m, keeping to the left 15.9 km – gate barring entrance to Shebandowan mine site 16.4 km – junction with road to No. 1 shaft. 16.7 km – Stop 5. 17.1 km – Stop 6. Return to turn off to No. 1 Shaft. Reset trip meter. Turn Right (west). 1.7 km – Stop 7. 2.9 km – junction with Otto Lake road – keep to the right. 4.9 km – junction with road – keep to the right. 5.5 km – Stop 8. Return to Otto Lake road junction, reset trip meter and then turn right. 0.9 km – Stop 9. 1.2 km – Stop 10. Return to Gate house. Take the road to the right. Drive 325 metres and turn left and drive 120m to Stop 11. Return to Gate House and turn right and head east. 13.2 km – Junction with Duckworth Road – Turn Right. 17.6 – Junction – keep to the right. 18.5 – Junction – keep to the left. 19.0 – Junction with I Zone Road – turn right. 20.5 – Stop 12. UTM coordinates NAD83; 15U 0716408E / 5388223N The relatively undeformed metavolcanic debris flows at this stop consist of poorly sorted sub-angular to well-rounded fragments in a fine- to medium -grained tuffaceous matrix. A characteristic feature is the presence of hornblende phenocrysts in both the fragments and the matrix. Locally what appears to be graded bedding within the debris flows is present. One such locality is on the east side of the road where the debris flows are in contact with an intercalated greywacke unit. Here fragments within the debris flow fine to the south. On the west side of the road the same greywacke unit shows evidence of folding (fold axis plunging vertically) with graded bedding in the north limb indicating a synclinal structure. Compositionally the debris flows vary from basalt (<53% SiO2) to rhyolite (Brown, 1985). Shegelski (1980) has shown that they represent a typical calcalkaline volcanic suite with shoshonitic affinities. The presence of red pigmentation in these debris flows has led to much speculation as to their depositional environment. Pigmentation has resulted in clasts and matrix ranging from grey-green to red in colour. Locally grey-green clasts exhibit hematized rims while others are totally hematized. There are several possible processes that may have produced this red colouration: hydrothermal alteration after lithification; magmatic differentiation; deposition in an Archean oxygenated atmosphere resulting in red bed formation; or secondary oxidation during the Proterozoic or Phanerozoic. By plotting the ratios of ferric iron oxide (Fe2O3) to ferrous oxide (FeO) against SiO2 Shegelski (1980) ruled out magmatic differentiation. He then postulated that the red pigmentation was the product of red bed development. Due to the variability in red pigmentation doubt still remains as to whether it is a product of red bed development. In particular is the spectrum of pigmentation, including red fragments in a greygreen matrix, green fragments in a reddish matrix and fragments that are partially red and partially green. It’s due to this variability that has led others, such as Brown (1985), to believe the pigmentation is the product of varying degrees of hematization subsequent to deposition. - 69 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 2. Timiskaming-type conglomerate. UTM coordinates NAD 83; 15U 0715383E / 5387509N At this location two facies of the epiclastic suite of Timiskaming-type rocks are exposed. The dominant rock-type is poorly sorted, highly foliated metaconglomerate (Fig. 3). Note the heterolithic nature of the fragments, including minor Keewatin-type red jasper fragments. This particular outcrop is highly deformed with the clasts being stretched, forming a well-developed lineation plunging steeply to the southeast. Note the abundant iron carbonate alteration within the sandy matrix. In fault contact with the meta-conglomerate to the west are meta-mudstone and meta-siltstone. Here we have near vertical mineral lineations normal to rolls on the bedding planes. This unit is finely bedded with grading, although present, obscured by the deformation. sheared unconformity between the Timiskaming-type metasedimentary rocks and the underlying Keewatin metavolcanic rocks (15U 0712637E / 5387160N). Relatively pristine Keewatin felsic metavolcanic rocks are exposed to the west (15U 0712548E / 5387139N). Note the intense deformation of the meta-conglomerate on the north side of the road with deformation intensity increasing to the south. On the south side we have exposed highly foliated and carbonatized rocks that may or may not be Timiskaming-type metasedimentary rocks in contact with relatively undeformed, possibly Keewatin fragmental rocks. On the north side of the road (15U 0712742E / 5387227N) there are bands of conglomerate with a significant proportion of barren sulphide pebbles and cobbles. Also present are several felsic intrusive rocks that cut through at a low angle to the stratigraphy and which are also strongly carbonatized. Stop 4. Timiskaming-type Monzonite. Stop 3. Timiskaming-Keewatin contact. UTM coordinates NAD 83; 15U 0710946E / 5386868N UTM coordinates NAD 83; 15U 712653E / 5387161N Here we will examine what may possibly be the On the north side of the mine road is an overgrown logging road. Follow it, keeping to the left, for 230 metres. At this location, and several other outcrops to the east, we have exposed a small intrusion within the Timiskaming debris flow pile. This intrusion is interpreted to be a high level magma chamber that was parent to the volcanic system that produced the debris flows. Macroscopically the rock resembles the fragments found in the debris flows; both possess hornblende phenocrysts and have the same mottled green to red colourization. This pigmentation within an intrusive environment indicates that the reddish colourization was not the product of exposure to an oxygenated atmosphere but is more likely related to oxidizing deuteric fluids. The restriction of this alteration to the Timiskaming-type igneous rocks, both intrusive and extrusive, limits other possible interpretations. Stop 5. Deformed debris flows. UTM coordinates NAD 83; 15U 0702659E / 5385962N Figure 3. Timiskaming Conglomerate at Stop 2 This outcrop consists of strongly deformed hornblende-phyric Timiskaming-type debris flows.   Note the highly foliated nature reflecting its proximity to the Crayfish Creek Fault, approximately 100 metres to the north. Though strongly foliated note the many - 70 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 similarities with the undeformed debris flows seen at Stop 1. Stop 6. Shebandowan Mine Ultramafic Rocks. UTM coordinates NAD 83; 15U 0702841E / 5386253N This stop is beside the now capped production shaft used to extract the nickel-copper sulphide ore from the Shebandowan deposit. During its operation it produced 9.4 million tonnes grading 1.7% Ni, 0.9% Cu and 1.56 g/t total precious metals (Pt + Pd + Au). The outcrop consists of serpentinized peridotite (see photo below) with numerous narrow zones of talccarbonate schist that form an anastomosing network. It is this unit that is host to the nickel-copper sulphides. On the north side of the outcrop is a feldspar porphyry dike, an apophysis of the Shebandowan Lake Stock to the north, cutting across the peridotite. Stop 7. Ultramafic with iron formation. iron formation. The south outcrop is serpentinized peridotite and the iron formation is on the north side. There is another iron formation on the south side of this ultramafic unit (Fig. 4). Geological mapping by Morton (1982) found that tops in this area are to the south. Just to the west there are a series of outcrops of massive mafic flows with flow top breccias that confirm tops are to the south. Stop 8. Discovery Point. UTM coordinates NAD 83; 15U 0701334E / 5386459N The first sign of nickel mineralisation of what eventually became the Shebandowan Mine was made in 1913 by Jules Cross. At the time he was mapping for the Ontario Department of Mines and while doing mapping along the shoreline noted signs of nickel and copper mineralisation. The original showing can still be seen in the form of several pits right at the water’s edge at Discovery Point. Stop 8a. UTM coordinates NAD 83; 15U 0701100E / 5385538N This rock cut is through the contact between an ultramafic massive flow and an oxide-facies UTM coordinates NAD 83; 15U 0701350E / 5386512N Walk east on the road 75 metres. On the left is the Stop 7 IF Ultramafic Stop 9 Figure 4. Detailed geology of the Southwest Bay area (Morton, 1982).   - 71 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 concrete cap over the No. 1 Exploration shaft that was used for initial access to the Shebandowan ore body. debris flow. Stop 8b Here we have several examples of angular pieces of black chert ripped up off the chert horizon capping the ultramafic (exposed just to the north on the other side of the old logging road) and incorporated into the base of the felsic debris flow unit. UTM coordinates NAD 83; 15U 0701323E / 5386451N Return to vehicles. Walk 90 metres to the west until you see an old road on the left that works its way east down and along the hillside. Continue 120 metres to the bottom where there was once a boat landing at the waters edge. Work your way along the edge of the bay east for 50 metres to several old pits very close to the water. Here the mineralisation is at the north edge of the mine peridotite body where it is in contact with sheared and banded mafic metavolcanic rocks. Look for sheared peridotite with signs of nickel bloom (Fig. 5). UTM coordinates NAD 83; 15U 0698906E / 5385040N Stop 11. Pillowed Mafic Feldspar Phyric Flows. UTM coordinates NAD 83; 15U 0703244E / 5385343N Here we have a unit that probably can be used as a stratigraphic marker horizon as it has been found at several locations over a 5 kilometre strike-length. It consists of obvious pillows with well developed selvages but with feldspar phenocrysts, some up to several centimetres across. Note how the crystals are smaller as you approach the pillow margins. Tops are consistently to the south. Stop 12. Iron formation hosting felsic dike with gold-bearing quartz ladder veins. UTM coordinates NAD 83; 15U 0714705E / 5382490N At this stop we have Timiskaming oxide-facies iron formation intercalated with argillite intruded by a later felsic dike (Fig. 6). This dike is host to gold-bearing quartz ladder veins (Fig. 7). Fractures opened up in the dike due to the ductility contrast of the enclosing iron-rich argillites and the felsic dike. Later auriferous   Figure 5. Cu-Ni Mineralisation at Discovery Point (Stop8b). Stop 9. Contact between ultramafic flow and overlying felsic fragmental unit. UTM coordinates NAD 83; 15U 0699131E / 5385139N This ultramafic flow unit is a fine-grained peridotite with well-developed polyhedral jointing. It is cut by a felsic dike. Locally the contact with the overlying felsic debris flow is exposed and is characterised by a thin, strongly foliated black chert horizon capping the ultramafic, evidence that this unit was deposited as a flow. Stop 10. Fragments of Black Chert in the base of the Figure 6. Felsic dike cutting iron formation at Stop 12. - 72 -   �Proceedings of the 58th ILSG Annual Meeting - Part 2 References Aubut, A., Lavigne Jr., M.J., Scott, J. And Kita, J. 1990. Metallogeny, Stratigraphy and Structure of the Shebandowan Greenstone Belt; Field Trip 3 Guide Book, Mineral De[posits of Central Canada, CIM Thunder Bay Branch. Brown, H. 1985. A Structural and Stratigraphic Study of the Keewatin Type and Shebandowan Type Rocks West of Thunder Bay, Ontario; Unpublished MSc. Thesis, Lakehead University. Card, K.D. 1990. A Review of the Superior Province of the Canadian Shield, a product of Archean Accretion; Precambrian Research, Vol. 48, p. 99-156. Corkery, M.T., Cameron, H.D.M., Lin, S., Skulski, T., Whalen, J.B. and Stern, R.A. 2000. Geological Investigations in the Knee Lake belt (Parts of NTS 53L); in Report of Activities 2000, Manitoba Industry, Trade and Mines, Manitoba Geological Survey, p. 129-136.   Figure 7. Auriferous quartz ladder veins within the dike at Stop 12. fluids, likely carrying gold as thio complexes reacted with the iron oxides resulting in the formation of pyrite and precipitation of native gold. If one looks carefully at the exposed contact of the dike, where the iron formation has been eroded away and looking for pyrite concentrations you commonly will also find fine- to coarse-grained native gold. Figure 8 (from Aubut et al., 1990) shows the simplified geology of this location. Morton, P. 1982. Archean Volcanic Stratigraphy and Chemistry of Mafic and Ultramafic Rocks, Chromite, and the Shebandowan Ni-Cu Mine, Shebandowan, Northwestern Ontario; Unpublished PhD. Thesis, Carlton University. Shegelski, R.J. 1980. Archean Cratonization, emergence and Red Bed Development, Lake Shebandowan Area, Canada; Precambrian Research, Vol. 12, p. 331-347.. Pye, E.G and Fenwick, K.G. 1965. Atikokan-Lakehead sheet, geological compilation series, Kenora, Rainy River and Thunder Bay districts; Ontario Ministry of Northern Development and Mines, Ontario Geological Survey. Map 2065, Scale 1: 253,440. Figure 8. Simplified geology of Stop 12. From Aubut et al. (1990) - 73 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 5 - Guide to the Thunder Bay area Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Regional Geology Thunder Bay is situated on the northern margin of the Southern Province of the Canadian Shield. The Southern Province consists of Proterozoic rocks which unconformably overlie Archean basement rocks of the southern Superior Province (cf. Tanton, 1931; Pye, 1969). Archean basement rocks of the Wawa Subprovince are predominantly granitoid plutons and slivers of greenschist- to amphibolite-facies supracrustal (i.e. greenstone belt) rocks (Williams et al., 1991; Fig. 1). The Paleoproterozoic Animikie Group is represented locally by the Gunflint Formation and overlying Rove Formation. These dominantly sedimentary formations constitute a largely unmetamorphosed, undeformed, homoclinal succession which dips shallowly towards the center of the Mesoproterozoic Midcontinent Rift (MCR) to the southeast. The Gunflint Formation is a chemical-clastic assemblage which yielded a U-Pb age from reworked volcanic ash of 1878.3 ± 1.3 Ma (Fralick et al., 2002). These rocks grade upward into turbiditic sandstone and shales of the Rove Formation south of Thunder Bay. U-Pb zircon ages from ash beds in the basal Rove Formation yielded 1836+5 and 1832+3 Ma (Addison et al., 2005). A sandstone sample from the submarine fan portion of this succession yielded a youngest detrital zircon U-Pb age of approximately 1780 Ma (Heaman and Easton, 2006). The Sibley Group, exposed on the nearby Sibley Peninsula, has been subdivided into five formations; Figure 1. General geology of the Thunder Bay area, modified after Pye (1969) - 74 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 detailed descriptions of each formation have been reported previously (Franklin et al., 1980; Cheadle, 1986; Rogala, 2003; Rogala et al., 2005, 2007). The overall sedimentary environment indicates a fluctuating climatic scenario, in which the Sibley Group was deposited in a lacustrine system (Pass Lake Formation) that gradually evolved into a saline playa lake environment (Rossport Formation). As the climate progressively became drier, a sabkha-type environment developed (Kama Hill Formation). The Outan Island Formation represents the transition from subaqueous to subaerial conditions, and the Nipigon Bay Formation represents an aeolian environment (Rogala, 2003; Rogala et al., 2007). The depositional age for much of the Sibley Group is constrained between ~1340 and 1450 Ma. The northern margin of the Midcontinent Rift (MCR) is dominated by hypabyssal rocks of the Mesoproterozoic Midcontinent Rift Intrusive Supersuite (Miller et al. 2002), which intrude Paleoproterozoic Animikie Group and Mesoproterozoic Sibley Group sedimentary rocks and Archean basement (Fig. 2). Older hypabyssal rocks (1124 Ma Seagull intrusion, 1109-1113 Ma sills; Heaman et al., 2007) predominate in the Nipigon Embayment (cf. Hart and MacDonald, 2007). Volcanic and minor sedimentary rocks of the ca. 1108 to 1105 Ma Osler Group (Davis and Sutcliffe, 1985; Davis and Green, 1997) are exposed to the east on Black Bay Peninsula and on offshore islands in Lake Superior. Osler Group rocks are also intruded by mafic dykes and intrusive complexes (1095 Ma Moss Lake gabbro, Heaman et al., 2007; 1089 Ma St. Ignace Complex, Smyk et al., 2006) which represent the youngest local MCR magmatism. Ages in this part of the MCR range from ca. 1140 Ma (Heaman and Easton, 2007) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green, 1997). Hollings et al. (2007a) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite of Miller et al. (2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Starting about 11,000 years ago (Ka), Wisconsinan ice melted back from its position in central Minnesota and Wisconsin, and quickly exhumed the Thunder Bay region, forming recessional moraines during brief stillstand periods (Phillips, 2004; Phillips et al., 1994).   Figure 2. Schematic block diagram illustrating local stratigraphy (Pye 1969). - 75 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Giant can be seen across the waters of Thunder Bay (Fig. 3). The 240 m high cliffs facing Thunder Bay are the highest in Ontario. The Sleeping Giant is capped by a Logan diabase sill which has intruded Rove Formation shale and sandstone. Other prominent mesas and cuestas include Pie Island and Mount McKay and the other hills of the Nor’Wester range to the south. All of these hills consist of Rove Formation sedimentary rocks capped by Logan sills. Isle Royale (Michigan), visible on the distant horizon, consists of Keweenawan basalt flows (Portage Lake Volcanics) associated with the Midcontinent Rift. Raised beaches, which represent former lake levels of glacial Lake Minong, are visible within the city and extend westward up the Kaministiquia River valley. The most prominent of these in downtown Thunder Bay North, extending along Algoma Street, is associated with the Nipissing Great Lakes stage (ca. 5500 years ago), approximately 20 m above presentday Lake Superior. The large bell at this lookout rests upon the “Upper Limestone” member of the Gunflint Formation. This   The Lake Superior basin was occupied by Early Lake Minong, the shoreline of which is found close to the 1400 foot (427m) contour in the borderland area. About 10 Ka, ice re-advanced from north of Lake Nipigon, sweeping across the Superior Basin (Marquette Readvance). As that ice began to melt, glacial lakes were formed between the moraines and the retreating ice margins. As Superior ice melted, water levels lowered, forming a series of shoreline features down-slope and depositing thick lacustrine clays. Superior ice withdrew to the north of Lake Nipigon around 9.5 Ka, and for the first time since the Marquette Re-advance, the Superior basin was occupied by a single lake, Lake Minong. This lake level extended up the Kaministiquia embayment to Rosslyn, where a large delta structure was built. The Minong shoreline runs through the upper part of the city, being particularly evident in Boulevard Park where river mouth bars and terraces of the Current River are seen. The Minong shoreline in the city is strongly associated with Palaeo-Indian sites, the Cummins Site being the best-known. It is likely that as water levels fell, these early people moved down from the Arrow-Whitefish Lakes area into the Kaministiquia embayment. Little remains of the toolkit of these people other than a variety of knapped lithic tools made from taconitic chert that occurs in the local Gunflint Formation (cf. Hamilton, 1996). A number of field guides (e.g., Pye, 1969; Kustra et al., 1977; Franklin et al., 1982) have covered the Thunder Bay area, including those most recently during the 46th Institute on Lake Superior Geology (e.g. Pufahl et al., 2000; Phillips et al., 2000). Stop Descriptions Stop 1A: Hillcrest Park area UTM coordinates: NAD83; 16U 0334689E / 5366961N On a clear day, Sibley Peninsula and the Sleeping   Figure 3. Panoramic view (ca. 130 º) of Thunder Bay from Hillcrest Park (image from http://www.360cities.net/image/ hillcrest-park-thunder-bay#106.03,-0.78,54.2) Map above depicts field of view from this vantage point. - 76 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 unit is now interpreted as carbonatized ejecta related to the Sudbury impact event (ca. 1850 Ma) [see Field Trip guide 1, this volume]. Proceed down the concrete stairs, turn right and walk along the laneway. The ejecta here is characterized by debrisite breccia and accretionary lapilli (Fig. 4). Calcareous beach rock contains algal bioherms and basal cryptalgal laminites with fenestrae fabric which are overlain by coarse-grained, poorly sorted breccia. Stop 1B: Debrisite Breccia UTM coordinates: NAD83; 16U 334163E / 5366301N (n.b. Private Property, ask for permission to access) Another spectacular debrisite breccia (Fig. 5a) is exposed at the corner of Markland and Hill streets. This outcrop was mapped in detail by Shegelski (1982; Fig. 5b) and later described in the context of impactrelated brecciation by Addison et al. (2010): A bedrock exposure, ~5 m by 15 m, in a private yard…contains a spectacular exposure of Gunflint chert-carbonate breccia and ejecta, primarily devitrified vesicular impact glass, which are surrounded and partially replaced by blocky calcite cement. The debrisite remnant preserved here is 0-0.5 m thick and unconformably overlies stromatolites and chloritic grainstone of the uppermost Gunflint Formation. An iron-rich alteration zone exists ~30 cm below the erosive contact between the debrisite and the Gunflint bedrock. The most common ejecta feature is devitrified vesicular impact glass clasts up to 2 cm across. Vesicles range from round to ovoid to nearly flat. Angular quartz and feldspar grains, chert shards and chloritic granules are also present. Our route then takes us west along Highway 1117 toward the community of Kakabeka Falls. The route follows the Kaministiquia River valley, which is dominated by fluvio-lacustrine deposits related to proglacial lakes (Burwasser, 1977). Tills associated with Superior lobe ice predominate north of the valley, extending southward from the rolling hills of exposed Archean rocks. In contrast, the landscape south of the highway consists of flat plains underlain by flat-lying Gunflint and Rove Formation sedimentary rocks and Quaternary sediments, punctuated by diabase-topped cuestas. Just west of the junction of Highway 11-17 and the Highway 588 (Stanley) turn-off, glacio-fluvial gravels and sands of the Stanley delta formed where the Kaministiquia River entered into Lake Beaver Bay ca. 9.7 Ka (Phillips, 2004; Phillips et al., 2004). A well-formed bluff, representing a lower Beaver Bay phase (260 m / 853 feet) extends along the north side of the highway. The present-day river has deeply incised the delta. The town of Kakabeka Falls is built on the floor (at 277 m ASL) of an old distributary of the Kaministiquia River which cut through a higher terrace level. This terrace (~300 m / 984 feet) represents the highest level of the Stanley delta. The Crane archaeological site is found on the west side of the river, where the old river entered Lake Beaver Bay (Phillips et al., 1994, 2000; Phillips, 2004).     Figure 4. Accretionary lapilli in debrisite, Hillcrest Park (Stop 1A). Figure 5a. Debrisite breccia, corner of Markland and Hill streets, Stop 1B. - 77 - �Proceedings of the 58th ILSG Annual Meeting - Part 2  Figure 5b. Detailed map of the debrisite breccia outcrop at Stop 1B by Shegelski (1982). Stop 2: Kakabeka Falls Provincial Park Stop 2A: Junction of Highways 11-17 and 590 UTM coordinates: NAD83; 16U 0305176E / 5364668N Walk uphill on the west side of Highway-590. Highway excavation has revealed the sharp unconformity between Archean, gneissic, felsic plutonic rocks and the overlying Paleoproterozoic Gunflint Formation. Large, domical stromatolites occur on “highs” on the eroded Archean basement (Fig. 6). The associated sedimentary rocks, representing intertidal, foreshore sedimentation, are laminated cryptalgal cherts overlain by a wavy bedded grainstone-micrite facies. The latter is capped by chaotic, slumped, laminated chemical sedimentary rocks which are probably cryptalgal. A few metres further uphill, overlying the previous sequence is a brecciated and slumped pyritic black chert. Gunflint conglomerate (Kakabeka Member), exposed in the river gorge a few hundred metres to the north, forms the basal member of the Gunflint Formation in this area. A major, northeasttrending fault has resulted in a down-dropping of the block to the southeast. The Kakabeka gorge, ~600 m to the east, exposes rocks much higher up in the Gunflint stratigraphy. As noted by Pufahl et al. (2000) the first set of rapids above the highway bridge are formed by Archean granitoids. The slow-water area to the south is underlain by the Gunflint Formation. Kakabeka conglomerate patchily overlies the Archean basement. Silicified stromatolites are developed on the conglomerate or directly on the granitic basement. This is the location from which samples collected in the 1950’s yielded the first Gunflint cyanobacteria described in the literature. The samples, from the silicified stromatolites, were described by Tyler and Barghoorn (1954). Figure 6. Unconformity between Archean granitoids and Gunflint Formation, Stop 2A. - 78 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 2B: Kakabeka Falls UTM coordinates: NAD83; 16U 0305738E / 5364400N (n.b. Entry/parking fee is required in Kakabeka Falls Provincial Park. No sample collecting permitted) The park is dominated by a single, spectacular feature, Kakabeka Falls, which drops 39 m over sheer cliffs in Gunflint Formation sedimentary rocks. Kakabeka is an aboriginal word meaning “steep cliffs”. The age of the river gorge below the falls is still debated. If none of it existed prior to the glacial Lake Beaver Bay stage, then it is less than 9700 years old. The portage around the falls contains artifacts ranging from the Paleoindian to the historic (fur trade) periods. The falls owes its existence to the thin, lowermost chert-carbonate bed of the Gunflint Formation which forms a resistant cap rock to the softer underlying shales. Looking down the gorge, one can observe a lapilli-tuff member as a lighter grey unit near the base overlain by a thick sequence of black shales (Fig. 7). Note that shale is the predominant lithology in the Kaministiquia sections and this is, in fact, typical for the Gunflint Formation in general throughout the Thunder Bay region. As noted by Pufahl et al. (2000), this sequence represents the major volcaniclastic horizon present in the upper Gunflint Formation and is traceable to the south as the Biwabik Formation through the Mesabi Range. Basalts outcropping approximately 30 km to the southwest are probably correlative with this unit. The outcrop on the northern edge of the parking lot contains layers of banded chert-carbonate within black, fissile shale. The alternating, dark grey chert and brown siderite-ankerite layers display slump and soft-sediment deformation features. Microscopic examination of banded chert-carbonates reveals delicate lamination in the chert which resembles the “ribbon texture” of algal mats. The interlayered carbonate bands contain complex, microspherical structures which likely resulted by nucleation from a gel state. Local thick beds of carbonaceous siderite (2-3 wt% carbon) form carbonate iron formation; contemporaneous deposition of carbon and carbonate suggests biological activity during iron deposition (cf. Pufahl et al., 2000).   Figure 7. The gorge below Kakabeka Falls, developed in Gunflint Formation shales and tuffs (www.Audreyhansen.com) - 79 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 References no.8, p.1021-1040. Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology,v.33, p.193-196. Addison W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W. and Kissin. S.A. 2010. Debrisites from the Sudbury impact event in Ontario, north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits? Geological Society of America Special Papers, 2010, 465, p. 245-268. Burwasser, G. 1977. Quaternary geology of the City of Thunder Bay and vicinity; Ontario Geological Survey, Report 164, 70p. Cannon, W.F. and Addison, W.D. 2007. The Sudbury impact layer in the Lake Superior iron ranges: A time-line from the heavens; 53rd annual Institute on Lake Superior Geology, Lutsen, Minnesota, Proceedings volume with abstracts, v.1, p. 20-21. Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23, p.527–542. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon Plate and northern Lake Superior; Bulletin of the Geological Society of America, v. 96, p. 15721579. Fralick, P.W., Davis, D.W. and Kissin, S.A. 2002. The age of the Gunflint Formation, Ontario: single zircon U-Pb age determinations from reworked volcanic ash; Canadian Journal of Earth Sciences, v.39, no.7, p.1085-1091. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K. 1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, v.17, p.633–651. Franklin, J.M., McIlwaine, W.H., Shegelski, R.J., Mitchell, R.H. and Platt, R.G. 1982. Proterozoic geology of the northern Lake Superior area; Field Trip Guidebook, GAC-MAC Annual Meeting, Winnipeg, 71p. Hamilton, J. S. 1996. Pleistocene landscape features and Plano archaeological sites upon the Kaministiquia delta, Thunder Bay District; Lakehead University Monograph in Anthropology #1, 112p. Hart, T.R. and MacDonald, C.A. 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions; Canadian Journal of Earth Sciences, v.44, Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/ Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences, v.44, no.8, p.1055-1086. Hollings, P. and Smyk, M.C. 2008. Whatever happened to the Logan sills? Ongoing research into the geochemistry of Midcontinent Rift-related mafic intrusive rocks south of Thunder Bay: 54th Institute on Lake Superior Geology, Annual Meeting, Marquette, Michigan, May 2008, Proceedings Volume 54, Part 1, p.36-37. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A. 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development; Canadian Journal of Earth Sciences, v.44, no.8, p.1087-1110. Hollings, P.N., Smyk, M.C. and Hart. T. 2007b. Geochemistry of Midcontinent Rift-related mafic dykes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dykes; 53rd Institute on Lake Superior Geology, Annual Meeting, Lutsen, Minnesota, May 2007, Proceedings Volume 53, Part 1, p.40-41. Kustra, C.R., McIlwaine, W.H., Fenwick, K.G. and Scott, J.F. 1977. Proterozoic rocks of the Thunder Bay area, northwestern Ontario; Field Trip Guidebook, 23rd Annual I.L.S.G. Meeting, Thunder Bay, 47p. Miller, J.D., Green, J.C. and Severson, M.J. 2002. Terminology, nomenclature and classification of Keweenawan igneous rocks of northeastern Minnesota; in Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota; Minnesota Geological Survey, Report of Investigations Phillips, B. 2004. Of moraines, lake floors, deltas and shorelines: A brief summary of the deglaciation of the Kaministiquia embayment, Thunder Bay, Ontario; unpublished report, World Wide Website, http:// www.lakeheadu.ca/~geogwww/phillips/FOP%20 page_4.htm (accessed 2004). Phillips, B., Hill, C., Fralick, P. and Ross, B. 1994. Postglacial shorelines and Paleoindian migration along the northwestern shore of Lake Superior; Field Trip - 80 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Guidebook, 13th Biennial meeting of AMQUA, Minnepaolis, MN. Phillips, B., Stewart, J., Hamilton, S., Julig, P. and Ross, B. 2000. Geoarchaeology of the Thunder Bay area; 46th Institute on Lake Superior Geology, Thunder Bay, Ontario, Field Trip Guidebook, 40p. Pufahl, P., Fralick, P. and Scott, J.F. 2000. Geology of the Paleoproterozoic Gunflint Formation; in Institute on Lake Superior Geology, 46th Annual Meeting, Field Trip Guidebook. Pye, E.G. 1969. Geology and scenery, north shore of Lake Superior; Ontario Department of Mines, Geological Guidebook No.2, 148p. Rogala, B. 2003. The Sibley Group: a lithostratigraphic, geochemical, and paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 254 p. Rogala, B., Fralick, P.W., and Metsaranta, R. 2005. Stratigraphy and sedimentology of the Mesoproterozoic Sibley Group and related igneous intrusions, northwestern Ontario: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Open File Report 6174, 87 p. Rogala, B., Fralick, P.W., Heaman, L.M., and Metsaranta, R. 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario. Canadian Journal of Earth Sciences, v.44. Shegelski, R.J. 1982. The Gunflint Formation in the Thunder Bay area; in Franklin, J.M. ed, Field Trip Guidebook 4: Winnipeg, Manitoba; Geological Association of Canada, p.14-31. Smyk, M.C., Hollings P. and Heaman, L.M. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, p.61-62. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton, T.L. 1931. Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario; Geological Survey of Canada, Memoir 167, 222p. Tyler, S.A. and Barghoorn, E.S., 1954. Occurrence of structurally preserved plants in Precambrian rocks of the Canadian Shield; Science v. 199, p.606-608. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L. and Sage, R.P. 1991. Wawa Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, p.485-539. - 81 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 6 - Thunder Bay Amethyst Mine Stephen Kissin Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, Canada Introduction owing to the presence of Fe4+ , as originally shown by Cox (1977). Properties of Amethyst The proposed mechanism requires the coincidence of four geological conditions for the formation of amethyst: Amethyst, occurring in abundance in the Thunder Bay region, is a purple gemstone variety of quartz. It has been known for some time that an iron impurity in quartz is the underlying source of amethyst coloration (Holden, 1925). However, incorporation of iron of alone cannot account for the formation of amethyst, as many varieties of quartz contain trace amounts of iron, yet amethyst is relatively rare, and large deposits of amethyst are very rare. In a series of papers by Cohen and co-workers, culminating in a summary in Cohen (1989), a simultaneous sequence of reactions was proposed for the formation of amethyst. (1) (Al–O)- → (Al–O)° + e Ionizing radiation forms a hole center from oxidizing the substitutional Al-O bond. (2) Na+ + e- → Na° Electron from step 1 is trapped by an interstitial alkali metal ion. (3) Fe3+int → Fe4+int + e Induced ionizing radiation forms a trapped hole center via oxidizing the interstitial Fe3+. (4) (Al–O)°+e- → (Al–O)Trapped hole center is satiated as [AlO°] is reduced via gaining the electron from step 3. The presence of iron is positions interstitial with respect to the SiO4 framework was established by Adekeye and Cohen (1986), in noting its correlation with pervasive Brazil law twinning in colored sectors of amethyst crystals. Data on incorporation of the alkalis Na, K and Li and trivalent Al and Fe in quartz were reported by Deer et al. (1963), who further noted that the incorporation of Al3+ (and presumably Fe3+), is compensated by the incorporation of Na+ or Li+ into interstitial sites. The color of amethyst is produced by absorption of light in the visible region of the spectrum (1) The incorporation of Fe and Al, as well as Na or Li. This is not a limiting condition, as the small concentrations of these trace elements are readily available in hydrothermal solutions. (2) A source of ionizing radiation, either from U and Th or 40K in order to produce the defects in Fe and Al. (3) Deposition at generally rather shallow depth such that oxidizing conditions prevail and iron is in the form of Fe3+. (4) Deposition with a temperature range for the stability of Fe4+, the source of amethyst coloration. The mechanism proposed above is consistent with observed data and provides a logical mechanism for the formation of amethyst. However, Rossman (1994) noted that there are unestablished factors in the model such that its acceptance is tentative. Crystal forms expressed in amethyst are invariably simple, consisting only of combined positive {101 ̅1} and negative {011 ̅1} rhombohedra. The faces of one of the forms are generally largely and are designated as the major rhombohedron r, and the other form is designated as the minor rhombohedron z (Fig. 1). The only other form occasionally observed is the ditrigonal prism m (Frondel, 1962). Amethystine coloration is unevenly distributed in the crystal, generally with concentration in the major rhombohedral forms, in which Brazil law twins are also concentrated (Fig. 2; Frondel, 1962). The orientation of Brazil law twins in Figure 2 is typical of their occurrence in α-quartz; however, in amethyst the twins are polysynthetic with a typical width of 0.1 mm (McLaren and Pitkethly, 1982). The twin plane of the Brazil law is {101 ̅1}, which separates right-handed and left-handed orientations of quartz. McLaren and Pitkethly (1982) demonstrated that the composition plane of the Brazil law twin provides space for - 82 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. A typical amethyst crystal viewed perpendicular to the c-axis, illustrating the combination of positive {101 ̅1} and negative {011 ̅1} rhombohedra. - 83 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Garland (1994). Geology of the Thunder Bay amethyst mine Geologic Setting Figure 2. Etched basal α-quartz illustrating the typical occurrence of Brazil law twinning in which alternate bands contain left- and right-handed α-quartz (after Frondel, 1962). incorporation of Fe3+ and that iron is preferentially concentrated along this composition plane in amethyst. Amethyst Deposits in the Thunder Bay Area In this summary of the history of amethyst in the Thunder Bay area, Patterson (1985) reported that as early as 1642, Radisson described the use of “torquoise” as a gemstone by local indigenous peoples. However, it was not until 1862 that amethyst was commercially exploited in various mines associated with leadzinc and silver deposits. In the 1880s, amethyst was mined northeast of Thunder Bay in a place now called Amethyst Harbour. Interest in Thunder Bay amethyst declined around the turn of the century with the development of deposits of high quality, inexpensive amethyst from Brazil. In 1961, the construction of a road leading to a now abandoned fire tower revealed a very large amethyst deposit, which is now known as the Thunder Bay Amethyst Mine Panorama. In large vugs near the surface of the vein deposit, amethyst crystals of spectacular size were obtained. The development of the deposit with wide-spread sales and distribution of specimens revitalized interest in amethyst in the region (Sinkankas, 1976). The interest and activity in amethyst deposits in the Thunder Bay area led to the proclamation in 1975 designating amethyst as Ontario’s provincial gemstone (Patterson, 1985). A comprehensive report on amethyst deposits and mining activity in the Thunder Bay area was completed by The geological setting of the mine is complex, as an Archean and a Proterozoic record are preserved in the area. This record has been recently reviewed by Franklin et al. (1986) and will not be repeated in detail here. The Thunder Bay Amethyst Mine is hosted in the Archean Hilma Lake granite of McCrank et al. (1981). This pluton lies on the boundary of the Quetico Subprovince and the Shebandowan Subprovince, with typical greenstone lithologies on its southern margin and gneissic metasedimentary rocks on the northern margin. The Hilma Lake granite in the vicinity of the mine consists predominantly of monzonite, with compositional variation along the trend monzonite quartz monzonite - granite - granodiorite and pegmatite and pegmatitic textural variants (Jennings, 1985). Jennings’ study indicates that monzonite had been cut first by granodiorite, then by pegmatite, with some metasomatic alteration of early monzonite toward granodioritic composition. At the Greenwich Lake uranium occurrence, a vein-type occurrence located 10 krn to the northwest, Franklin (1978a) noted the presence of quartz monzonitic pegmatites containing 60 -100 ppm U in the form of uraninite. As these pegmatites are apparently comagmatic with the Hilma Lake granite, its uranium-rich character is likely a general feature. The Proterozoic rocks were deposited on the eroded Archean surface; however, the Animike Group (Gunflint and Rove formations) is missing in the vicinity of the amethyst mine. As indicated by Franklin et al. (1980), the Mesoproterozoic Sibley Group progressively onlaps Archean terrain in a northerly direction. The Sibley Group is presently absent in the vicinity of the Thunder Bay Amethyst Mine, although its presence as abundant fragments in mineralized breccias within the vein system indicates that these sediments were present as basement cover during the forming of the deposit. The significance of the Sibley Group is unclear in the face of contradictory evidence concerning its age and depositional setting. Franklin et al. (1980) noted that the Sibley Group is deposited at the location of a failed arm of an r-r-r triple junction, although they admitted to uncertainty as to the contemporaneity of sedimentation and rifting. Although some features of - 84 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 the Sibley Group are suggestive of a rift-filling deposit, the whole-rock Rb/Sr age of 1339 ± 33 Ma (Franklin, 1978b) is approximately 200 Ma prior to the main stage of rifting of the Midcontinent (Keweenawan) Rift (Van Schmus et al., 1982). Cheadle (1986), however, concluded on the basis of sedimentological studies that the Sibley Group was not deposited in a classical aulocogen, but represents a deposit on a sagging crust preceding rifting. The Sibley Group was more recently dated by U/Pb geochronology in zircons in a basal rhyolite unit at 1537 +10/-2 Ma (Davis and Sutcliffe, 1985). This timing makes a relationship with the Midcontinent Rift event unlikely, and Hollings et al. (2004) proposed that the Sibley Basin formed due to effects of a plume track that created an infracratonic basin. Other deposits located at or near the Sibley -Archean unconformity include the Dorion lead -zinc -barite veins (Fig. 3). The ore-depositing solution was considered to be a basinal, connate brine by Franklin and Mitchell (1977), an interpretation supported by the fluid-inclusion studies of Haynes (1988). As illustrated in Figure 3, there is a close spatial relationship between the lead-zinc -barite veins and the amethyst, and both are spatially related to the Sibley-Archean unconformity . Geological features of the mine The Thunder Bay Amethyst Mine is located within a first-order strike-slip fault, which strikes at 90-100º and dips steeply to the south. This fault is roughly parallel to one 2.1 km to the south, which strikes eastnortheasterly and has a vertical displacement of at least 125 m (Jennings, 1985), forming a major boundary to the Sibley Group’s depositional basin. The strike-slip fault hosting the amethyst deposit is offset by seven first-order strike-slip faults, five of which are illustrated in Figure 4, which strike 162 - 150° and dip vertically producing en echelon, pull-apart structures in the main fault. These structures are filled by breccias of granitic country rock and Sibley Group sedimentary rocks with large proportions of void space. The brecciated fault was subsequently mineralized by hydrothermal solutions. At least two periods of mineralization occurred, as an early generation of amethyst was clearly brecciated and subsequently coated by a second generation of amethyst. Figure 4 is an illustration of the state of the mine in 1987. At present, the main pit configuration is basically the same but has been deepened. In that year, an Figure 3. Local geology and location map of amethyst deposits and lead-zinc-barite deposits, showing the relationship of the former to the margin of the Sibley Group outcrop and the Hilma Lake granite. The location of the Thunder Bay Amethyst Mine is indicated by the star. Modified after Patterson (1985). extension of the vein system to the east was developed, offset to the north by a few metres by a strike-slip fault. Jennings (1985) subdivided the mineralization patterns into three basic types: (i) open fracture fillings, (ii) breccias with tectonic and collapse subtypes, and (iii) “honeycomb” veins. The strike directions of these veins are strongly clustered in two groups, one slightly west of north and parallel to the second stage of strike-slip faulting, and one easterly, parallel to the principal directions of - 85 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 4. Diagram of the main pit of the Thunder Bay Amethyst Mine. faulting. Open fracture fillings are common in the shallower zones of the deposit where low lithostatic pressure permitted the maintenance of open fissures following faulting. The veins are widest near the edges of collapsed breccias and at the intersections of oblique shears with the main fault zone. Fracture-fill mineralization occurred at the crystal-fluid interface as quartz crystals grew outward from the fracture walls. The crystals formed as parallel to radial growths with long crystal axes oriented perpendicular or subperpendicular to the growth surface. The crystal size invariably increases outward, and outward growth from opposite fractures resulted in an interlocking comb structure of euhedrally terminated crystals. This vein type may also contain vugs up to 2-3 m in diameter with large quartz crystals up to 10-15 cm in prism diameter. Tectonic breccias are here attributed to fault movement, as opposed to brecciation caused by collapse. Some breccia fragments are surrounded only by a later portion of the paragenetic sequence, suggesting that multiple fault motion during the mineralizing event has occurred. Breccia fragments of this type are invariably angular and may consist of fragments of earlier deposited vein material, which may have been thermally bleached. Collapse brecciation is not always differentiated from tectonic brecciation, and some collapse breccias have undergone subsequent tectonic brecciation and vice versa. Evidence of collapse brecciation is seen in the occurrence of Sibley Group lithologies not present in the mine area now, together with granite and diabase as breccia fragments. Sibley Group fragments are particularly abundant within channel- or pipe-like structures in which fluid transport and abrasion have produced subangular to subrounded fragments, which have undergone an appreciable degree of sorting. Collapse-breccia fragments are typically coated with successive layers of chalcedony, colorless quartz, and amethyst, producing a cockade structure. Vugs have developed in open space produced in the breccia in which crystals with prism diameters of up to 10 cm have grown. Honeycomb veins are the result of quartz crystallization that has occurred in all directions from small nuclei, usually chalcedony, hematite, or silicified granite fragments, rather than from a fracture wall. The amethyst and quartz are more massive than in the other types of veins, but the growth is chaotic. Mineralogy Amethyst and several varieties of quartz occur in the Thunder Bay Amethyst Mine, including colorless quartz; chalcedony; amethyst; the yellowish variety, citrine; and the greenish variety, prasiolite or “greened - 86 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 1. Paragenetic sequences observed in the veins of the Thunder Bay Amethyst Mine Stage Type Thickness (cm) Older sequence (observed as breccia fragments) 1 Chalcedony 0.1 -0.3 2 Colorless quartz 0.8-1.0 3 Chalcedony 0.1-0.2 4 Prasiolite 2.0-3.0 5 Prasiolite 2.0-7.0 Younger sequence (main vein deposits) 1 Chalcedony and hematite 0.1-0.3 2 Colorless to smoky quartz 0.5-1.0 2a Amethyst I (top of stage 2) 0.5 -0.8 3 Amethy st III - IV 2.0-3.0 4 Amethyst II - III 2.0- 12.0 5 Amethyst II -IV 2.0- 14.0 6 Black gem Up to 4.0 Notes: Variations observed include (i) late-stage greenish and yellowish-amethyst; (ii) late-stage smoky quartz; (iii) discontinuous hematitic and milky quartz capping to crystal terminations; and (iv) development of black gem in crystals, deposited in vugs. amethyst”. Smoky quartz has very limited development. The only variety of gemstone interest is amethyst, although the occurrence of the other varieties has aided in establishing the sequence of deposition. A grading system based on estimated intensity of coloration and clarity of specimens is in use at the mine, and this system has also aided in establishing the paragenetic sequence. Thus, the intensity of coloration may be from I (lightest) to IV (darkest) and clarity from a (clear) to f (opaque). Table 1 lists typical paragenetic sequences in an older sequence, which is present as breccia fragments in a younger sequence presently occupying the veins. The prasiolite in stages 4 and 5 of the older sequence appears to be thermally bleached amethyst on the basis of both its appearance and experimental evidence that heat-treated amethyst can be transformed to prasiolite (Lehmann and Bambauer, 1973). In the younger sequence, late-stage variations are noted, particularly as cappings to stage 5. A distinctive variety called black gem, a dark, brownish-black amethyst, is apparently characteristic of larger crystals grown in vugs in which iron-enriched, late-stage fluids were trapped. These frequently have final growth zone that contains abundant hematite inclusions, such that recent sales of such material has been called “Thunder Bay red”. It was this material, recovered in the early development of the deposit that led to the notorious statement by Sinkankas (1976, p. 204): “By far most of the amethyst is unsuited for lapidary purposes, with very little being free from flaws and hence useless for faceted gems or even baroques.” The compositions of specimens of amethyst by neutron activation analysis for selected trace elements (Table 2) revealed the presence of subequal concentrations of Fe and Al. As well, low concentrations of Ge were sought based on absorption spectra that indicated its presence. The low Ti concentrations are perhaps related to the spotty occurrence of rutile needles in the amethyst; needles occurring when concentrations are relatively greater. Other non-sulfide minerals. Barite is rare in the Table 2. Analyses of r-zones of amethyst for selected trace elements (in ppm; Kissin, 1997) Sample No. Fe Al Ge Ti DZS1* 217 447 0.5 n.d LZS2 102 393 0.5 0.01 BSS3 273 369 1.0 n.d. TPS4 368 249 0.5 n.d. ____________________ *DZS1 evidently contains solid inclusions, as high concentrations (in ppm) were noted; e.g. Ta 0.329, W 0.38, Eu 0.349, Sr 89.43, Zr 1.02, Nb 0.13, Ba 2985.26, La 3.85, Ce 0.35, U 0.387. All samples contain small, but detectable quantities of Co, Ni, Ga, Rb, Nb, Zr, Mo, Sn, Sb, Cs, La, Pr, Nd and U. - 87 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 veins at the Thunder Bay Amethyst Mine, although it is abundant in other amethyst mines of the district, where it follows the final stage of quartz deposition. It was not observed in the course of the present study, but has been noted in the mine. Calcite is fairly common in thin, monomineralic veins, but was not observed within the amethyst-bearing veins. The genetic link between the calcite veins and amethyst veins, if any, is unclear. Hematite is abundant as minute- (< 0.1 mm diam.) solid inclusions in stage 5 of amethyst deposition and occurs sporadically at other stages of deposition as well. Hematite occasionally occurs as a daughter mineral in fluid inclusions, particularly in stage 5 of crystallization. Rutile occurs as needles that transect the growth zones of the quartz in scattered locations within the mine. The orientations of the needles are apparently random; however, the possibility of crystallographically controlled growth directions has not been considered in detail. Native copper occurs in association with copper-iron sulfides. Sulfides. The common base-metal sulfides pyrite, chalcopyrite, galena, and sphalerite occur in small amounts throughout the vein succession and as veinlets and replacement bodies in altered granitic wall rock. Copper -iron sulfides, however, are predominant, and a sequence of the minerals cuprite-native copperchalcocite-covellite associated with hematite and pyrite was documented by McArthur et al. (1993). The copper -iron sulfides exhibit typical replacement textures (atoll structures, core-and-rim relationships) both in amethyst growth stages and in wall rock. The assemblages bornite + pyrite and chalcopyrite + pyrite and chalcopyrite + pyrite occur in wall rock only; however, spatial relations of wallrock sulfides to the veins do not reveal any pattern, owing in part to their scarcity. Malachite is present as a supergene product derived from these hypogene copper minerals. Wall-rock alteration mineralogy. Hematitization, chloritization, and kaolinitization are prominent in envelopes surrounding the veins within zones of brecciated granite; however, the alteration extends only a few centimetres into the granites outside of the zone of brecciation. Intense hematitization occurs fairly generally in altered rock nearest the amethyst veins. The strongly hematitized zone is generally only a few centimetres thick, but weaker hematitization is notable throughout the altered zone. Outward from the hematized zone is an irregular zone of highly chloritized rock ranging from a few to a few tens of centimetres thick. Sometimes associated with the chloritization is diffuse epidotization, which produced a pistachio green tint over zones up to a metre wide. Kaolinitization is widespread and pervasive through the breccia zone, imparting a chalky, white appearance to relict feldspars. Other clay minerals (e.g., montmorillonite and illite) may also be present; however, they have not been sought in a detailed examination. The pervasive kaolinitization has allowed weathering to penetrate into the brecciated zone, resulting in a soft and loosely aggregated matrix in which the near-surface exposures of the amethyst are contained. The nature of this matrix has enabled a good deal of the amethyst to be mined with a minimum of blasting. The hematite - chlorite - epidote alteration assemblages in the presence of ubiquitous quartz are characteristic of the propylitic alteration typical in many hydrothermal ore deposits. The kaolinite and other clay minerals are characteristic of the argillic alteration of hydrothermal ore deposits. The two alteration types are analogous at least in their relative timing to early peripheral propylitic alteration, which is overprinted by argillic alteration stemming from downward-infiltrating meteoric water. Genesis of the deposit The genesis of the deposits of the Thunder Bay Amethyst Mine were discussed in detail by McArthur et al. (1993). The conclusions of their study are given below; however, for details of the evidence, their paper should be consulted. Genetic speculations on the Thunder Bay Amethyst Mine are hampered at the outset by questions as to the timing of amethyst deposition, as discussed in the Introduction. The spatial and geochemical affinities of the amethyst deposits with the Dorion lead-zinc-barite veins and the relationships of both to the depositional margin of the Sibley Group sediments suggest that all three are interrelated. Franklin and Mitchell (1977) proposed that the lead -zinc -barite veins formed when, during diagenesis and settling of the Sibley Group sediments, metal-bearing brines were formed when expelled connate waters mobilized metals from the Sibley Group sediments and(or) weathered granitic basement rocks below the Archean-Proterozoic unconformity. The solutions thus formed would have hypothetically migrated through the basal Pass Lake Formation aquifer to escape at basin-marginal faults. Precipitation of sulfide, carried in chloride- and sulfate-bearing solution, occurred because of mixing of the relatively oxidized solution with H2S gas trapped at the Pass Lake Formation - 88 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 pinch-out. The amethyst deposits seem to be a variant of the lead-zinc-barite type of deposit in which the temperature was lower than that of the sulfide-rich lead-zinc-barite veins. The initially oxidizing to later reducing character of the solution is similar to that proposed for the lead-zinc- barite veins, but the relation to Pass Lake Formation pinch-outs is not present in most amethyst deposits. Rather, the amethyst deposits are generally hosted in granitic basement often with no Sibley Group sediments present. The amethyst deposits are richer in dissolved silica, having gained this component through the kaolinitization of feldspar during hydrothermal alteration of granitic country rock. As the amethyst deposits formed near the present or former unconformity with the Sibley Group, local reduction of the solution would have tended to occur as H2S was released during thermal breakdown of organic matter in the sediments. The quantity of sulfides precipitated would have been limited not only by the relatively small amount of H2S produced but also by the lower metal content of the solutions as compared with those depositing the Dorion lead-zinc-barite veins. The latter characteristic is inferred by a comparison of the results of this study with those of Haynes (1988) on the Dorion lead-zinc-barite veins. He found that fluid inclusions from these deposits are NaC1-CaC12 -H2O type on the basis of microthermometry and direct analysis of decrepitates. However, the fluid inclusions depositing sulfides are significantly more saline than those at the Thunder Bay Amethyst Mine in that they contain daughter salts. The more saline and higher temperature (105-203°C) fluid inclusions indicate that solutions that they represent would have had a better metal carrying capacity as chloride complexes. The similarity of the solution components to those at Thunder Bay Amethyst Mine lends support to the idea that the same event formed both types of deposits. The solutions depositing amethyst would have been cooler and less saline variants of those that formed the lead-zinc-barite veins. If the two types of deposit are genetically linked, both suffer from the problem of lack of knowledge of the timing of ore deposition. The maximum age of both is 1339 Ma, the whole rock Rb/Sr age of the Sibley Group (Franklin 1978b), as both types of veins cut Sibley Group rocks and contain breccia fragments of them. Franklin and Mitchell (1977) did not suggest a specific timing for formation of the Dorion lead-zinc-barite veins; however, their suggested mechanisms for creation of the deposit favor a timing soon after the deposition of the Sibley Group sediments. The expulsion of pore water would presumably occur during late diagenesis. However, as there is no evidence to suggest that the Sibley Group sediments have ever been deeply buried, the source of heat is a problem. If the timing of deposition were close to the formation of the Sibley depositional basin, it is possible that a thermal anomaly, perhaps augmented by seismic pumping, in the lower crust was responsible for both phenomena. Haynes (1988) suggested that the Dorion leadzinc-barite veins formed either in the environment of Keweenawan rifting or later, possibly in the Paleozoic. There is no geological evidence for activity in the Paleozoic in the western Lake Superior region, and the style of mineralization associated with Keweenawan events is different (silver deposits associated in part with Ni-Co arsenides; Franklin et al., 1986). Our preferred hypothesis is that the lead-zinc-barite veins and amethyst veins are associated with the timing of formation of and deposition in the Sibley basin. We, therefore, believe that these deposits are distinct from silver deposits in the Thunder Bay area and formed at a somewhat earlier time. The timing is, however, not at all certain, and some additional work is underway in an effort to resolve this remaining question. Summary Field and laboratory studies of the Thunder Bay Amethyst Mine reveal the following: (1) The vein system hosting amethyst deposits was formed by mineralization of an east-weststriking, steeply dipping strike-slip fault, opened into en echelon pull-apart structures by a series of later strike-slip faults, also dipping steeply and intersecting the first-formed fault at high angles. Much open space with brecciated and vuggy textures resulted. Breccia fragments include granitic host rock and Sibley Group sedimentary rocks, implying that the latter were present as a thin cover at the time of mineralization, although they are erosionally removed from the mine area at present. At least one early generation of amethyst is included as breccia fragments, indicating that fault movement continued during mineralization. (2) At least two phases of amethyst crystallization separated by a period of brecciation are present. The older sequence contains five stages of quartz growth, the latter two of which were originally amethyst, but were thermally bleached to prasiolite by the influx of hot solutions that deposited the younger sequence - 89 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 to the Dorion lead-zinc-barite veins. Both are believed to have been formed by solutions expelled and mobilized during diagenesis and compaction of the Sibley Group. The lead-zinc-barite veins formed in fractures at or near the margin of the Sibley depositional basin from solutions that were both hotter and more saline than those depositing amethyst. Amethyst-depositing solutions travelled longer distances in granitic basement, dissolving silica by alteration of feldspar. Although the amethyst-depositing solutions probably carried less metal as chloride complexes than did the solutions forming the lead-zinc-barite veins, less H2S at the site of deposition was probably the most significant factor causing a low sulfide content in the amethyst veins. of quartz. The younger sequence contains five and occasionally six stages of deposition, beginning with a stage of chalcedony and a stage of colorless quartz, followed by amethyst. Both sequences of deposition are traceable throughout the mine. (3) Sulfide minerals including pyrite, chalcopyrite, galena, and sphalerite accompany amethyst deposition as small mineral inclusions and occur, as well, as veinlets and replacement bodies in altered granitic wall rock. Copper and copper -iron sulfides are most abundant and, together with native copper and cuprite. Eh-pH relationships indicate that the solutions forming the deposit were initially rather oxidizing and weakly acidic. In the course of crystallization, the solution became more reducing and slightly more acidic. (4) Fluid-inclusion studies indicate that in the younger sequence of quartz deposition, homogenization temperatures range from 146.5 to 114.7°C (mean 132.1°C) as contrasted with 91.2-40.9°C (mean 68.4°C) for amethyst. Eutectic temperatures of frozen inclusions indicate that the solution was of the NaCl-CaC1-H2O system, with possible concentration of an additional halide salt component in late-stage fluids. Few inclusions contain daughter minerals, and those found are hematite and sphalerite in late-stage fluids. Final melting temperatures indicate a trend of decreasing salinity in later growth stages. (5) Oxygen isotopic determinations on quartz indicate a range of δ180 outside that of juvenile waters and end-member basinal brines. Progressive mixing of basinal brine with local meteoric water is suggested. (6) Sulfur isotopic analyses of pyrite yield δ34S of -0.4-0.6 ‰ and -1.4 ‰ in chalcopyrite. These volumes are consistent with derivation from H2S gas liberated by thermal action protection on organic material involving iron. The values are similar to those of the sulfur contained in sulfides in the Dorion lead-zinc-barite veins. (7) The presence Sibley breccia fragments cemented by quartz indicates that the veins cannot be older than 1339 Ma, the Rb/Sr age of the unit. However, a younger limit cannot be established at present. (8) On grounds of similarity in geological setting, proximity, composition of the ore-depositing solution, and sulfur isotopic composition, the amethyst veins are believed to be genetically related (9) The temperature conditions under which amethyst forms appear to have a high temperature limit; at the Thunder Bay Amethyst Mine this limit is no higher than approximately 115°C and may be as low as approximately 90°C. Temperatures as high as approximately 145°C but possibly as low as 115°C may be sufficient to thermally bleach earlier generations of amethyst in the influx of hot solutions. However, this theory of thermal bleaching has been recently criticized by Hebert and Rossman (2008), who attributed the development of greenish-grey to greenish quartz to the presence of H2O in the crystal. Our work (Klarner and Kissin, 201l) confirms the presence of water in IR absorption spectra; however, the water is largely contained in fluid inclusions, which are abundant and of secondary origin. Use of the highly focus beam of an FTIR microscope has shown that molecular water is of low and nearly identical concentration in both amethyst and “greened amethyst”. Work on this problem is continuing. Road Log Airlane Travelodge to Thunder Bay Amethyst Mine Leaving the Airline Travelodge, we will follow the portion of the Trans-Canada Highway 11-17 which is the Thunder Bay Expressway. The flat terrain is the remnant of the bottom of the Nipissing stage of ancestral Lake Superior, and proceeding northeasterly, we pass upward through strandlines of the receding Pleistocene lake. At 5.7 km is the intersection with Oliver Road, which leads to Lakehead University about 2 km to the east. Lakehead University itself is underlain by the Gunflint - 90 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Formation at or near the top of the unit. Shaly rocks near the top of the formation are exposed in the bed of the McIntyre River that flows through the campus; however, in recent years blocks of rock containing the Sudbury ejecta debrisite were excavated during construction of new student residences. These placed in various places around the campus as ornamentation or barriers to vehicular traffic. and steeply dipping Archean metavolcanic rocks was exposed on the left of the highway. Hopefully, new road construction will expose this unconformity once again. The hill is formed by the outcrop of the Mackenzie granite, an unmetamorphosed and undeformed, late Archean pluton. The highway continues on top the of granite, which contains occasional roof pendants of Archean metavolcanics. Continuing on at 6.7 km, outcrops of a Logan diabase sill are exposed on the left side of the expressway. These sills form the caps of the prominent mesas south of town and underlie the high ground in the northern section of Thunder Bay. Passing the junction of Red River Road (Highway 102) at 10.0 km, the expressway is on a level stretch marking the top of a Logan sill. After crossing the Mackenzie River at 35.7 km sparse outcrops of granite are replaced by poorly exposed Gunflint Formation until just past the junction with Highway 587 at 49.6 km. Here, well-bedded redstained carbonates of the Gunflint Formation crop out beside the highway. Passing onward to the East Loon Road at 55.2 km, turn left onto the road, then right on Bass Lake Road after 0.6 km. Continue to the turn off on the right to the private road to the mine. Proceeding along the mine road, it climbs steeply up from the Sibley basin onto the Archean Hilma Lake granite, ascending along a border fault surface. The expressway then passes downhill to the Current River at 15.9 km. In proceeding downhill outcrops of Logan sill diabase, Gunflint shale and Gunflint carbonate are successively exposed. The carbonate is ankeritic and is oxidized to yellowish orange. Climbing uphill from the Current River bridge, the expressway ends and the highway is again cutting into a diabase sill. A fault trends along the highway offsetting the sill on opposite sides of the highway. A few hundred metres further along the highway, the sill is dropped downward by a fault trending perpendicularly to the highway. Recent work has shown that this sill, known locally as the Terry Fox sill, is a Nipigon sill (Magnus and Kissin, 2010). Nipigon sills, which occur from here northeasterly to the Lake Nipigon area, are somewhat younger than Logan sills and can be distinguished on the basis of their trace element composition. Proceeding downhill and rounding a curve to the left, there is a high bluff on the left capped by a prominent diabase sill. The sill has intruded the top of the Gunflint Formation and the overlying Sudbury debrisite layer, which is capped by a thin remnant of Rove Formation shale. This is the only outcrop known in the Thunder Bay area that contains the complete debrisite layer. The east end of this outcrop is bounded by a fault that dropped down the section. Continuing onward, high ground on both sides of the highway are capped by sills; the sill on the right was extensively quarried for railway bed ballast and large stone for construction of the breakwall in the harbor. After the junction with Highway 527 at 20.1 km, the highway climbs the hill locally known as KOA hill. Prior to a widening of the highway about a decade ago, the angular unconformity between the Gunflint Formation At the top of the grade, there is a chance to view Lake Superior with Black Bay, the Black Bay Peninsula and the Sibley Peninsula, clear weather permitting. A few more kilometres brings the road to the mine. Amethyst Mine tour Note: Safety boots or shoes recommended. sandals or open-toed shoes. No The tour will pass through the operating mining area, which is not available to ordinary tourists. No collecting is allowed in this area. After visiting the mining area, there will be an opportunity to look for specimens in a designated collecting area. The charge for specimens is by weight. Hammering or chiseling is not permitted in the collecting area. Specimens are also for sale in the shop. References Adekeye, J.I.D., and Cohen, A.J., 1986, Correlation of Fe4+ optical anisotropy, Brazil twinning and channels in the basal plane of amethyst quartz, Applied Geochemistry, v. 1, p.153-160. Cheadle, B.A., 1986, Alluvial-playa sedimentation in the Lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Science, v. 23, p. 527-541. Cohen, A.J., 1989, New data on the cause of smoky and amethystine color in quartz. Mineralogical Record, v. 20, p. 365-367. - 91 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Cox, R.T., 1977, Optical absorption of the d4 ion Fe4+ in pleochroic amethyst quartz, Journal of Physics C: Solid State Physics, v. 10, p. 4631-4643. Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon Plate and northern Lake Superior. Geological Society of America Bulletin, v. 96, p. 1572-1579. Deer, W.A., Howie, R.A., and Zussman, J. 1963, RockForming Minerals,Vol. 4 Framework Silicates. John Wiley and Sons, Inc., New York, 435 p. Franklin, J.M. 1978a, Uranium mineralization in the Nipigon area,Thunder Bay District, Ontario. in Current Research, Part A. Geological Survey of Canada, Paper 78-lA, pp. 275-282. Report to Precious Purple Gemstones Ltd., Thunder Bay, Ont., 65 p. Kissin, S.A., 1997, Comprehensive research to colour enhance Canadian amethyst by heat treatment and irradiation. Final Report, Amsearch Colour Project, Northern Ontario Development Agreement SSC File #015SQ-2223440-2-9243, 36 p. and appendix. Klarner, J.M., and Kissin, S.A., 2011, Hydrothermal bleaching of amethyst at the Thunder Bay Amethyst Mine, Ontario. Geological Society of America Annual Meeting, Minneapolis, Paper No. 44-11. Lehmann, G., and Bambauer, H.U., 1973, Quartz crystals and their colors. Angewendtede Chemie International Edition, v. 12, p. 283-291. Franklin, J.M., 1978b, The Sibley Group, Ontario, in Rubidium strontrium isochron age studies report 2. Edited by R.K. Wanless and W.D. Loveridge. Geological Survey of Canada, Paper 77-14, p. 31-34. Magnus, S., and Kissin, S., 2010, Assimilation and petrogenesis in the Navilus and Terry Fox sills, Thunder Bay, Ontario; in Institute on Lake Superior Geology, Proceedings and Abstracts, v. 56, part 1, p. 36-37. Franklin, J.M., and Mitchell, R.H., 1977, Lead-zinc -barite veins of the Dorion area, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 14, p. 1963-1979. McArthur, J.R., Jennings, E.A., Kissin, S.A., and Sherlock, R.L., 1993, Stable-isotope, fluid-inclusion, and mineralogical studies relating to the genesis of amethyst, Thunder Bay Amethyst Mine, Ontario. Canadian Journal of Earth Science, v. 30, p. 19551969. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980, Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Science, v. 17, p. 633-651. Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D., 1986, Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 1576-1591. Frondel, C. 1962, The System of Mineralogy, 7th edition, Vol. III Silica Minerals. John Wiley & Sons, New York and London, 334 p. Garland, M.I., 1994, Amethyst in the Thunder Bay area. Ontario Geological Survey, Open-file Report 5891, 197 p. Haynes, F.M., 1988, Fluid-inclusion evidence of basinal brines in Archean basement, Thunder Bay Pb-Zn-Ba district, Ontario, Canada. Canadian Journal of Earth Sciences, v. 25, p. 1884-1894. Hebert, L.B., and Rossman, G.R., 2008, Greenish quartz from the Thunder Bay Amethyst Mine Panorama, Thunder Bay, Ontario, Canada. Canadian Mineralogist, v. 46, p. 111-124. McCrank, G.F.D., Misiura, J.D., and Brown, P.A., 1981, Plutonic rocks in Ontario. Geological Survey of Canada, Paper 80-23, 171 p. McLaren, A.C., and Pitkethly, D.R., 1982, The twinning microstructure and growth of amethyst quartz. Physics and Chemistry of Minerals, v. 8, p. 128-135. Patterson, G.C., 1985, Amethyst in the Thunder Bay area of Ontario, Canadian Gemologist, V. 6, p. 104-116. Rossman, G.R., 1994, Colored varieties of the silica minerals, in P.J. Heaney, C.T. Prewitt, and G.V. Gibbs, eds., Silica: Physical Behavior, Geochemistry and Materials Applications, Mineralogical Society of America, Reviews in Mineralogy, v. 29, p. 433-467. Sinkankas, J., 1976, Gemstones of North America, Vol, II. D. Van Nostrand Company, Inc., New York, 494 p. Van Schmus, W.R., Green, J.C., and Halls, H.C., 1982, Geochronology of Keweenawan rocks of the Lake Superior region: A summary, in R.J. Wold and W.H. Hinze, eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America, Memoir 156, p. 165-171. Holden, E.F., 1925, The cause of color in smoky quartz and amethyst, American Mineralogist, v. 10, p. 203-252. Hollings, P., Fralick, P., and Kissin, S., 2004, Geochemistry and geodynamic implications of the Mesoproterozoic English Bay graniterhyolite complex, northwestern Ontario. Canadian Journal of Earth Science, v. 41, p. 1329-1338. Jennings, E.A., 1985, Geology of the Thunder Bay Amethyst Mine and Precious Purple Gemstone claims. - 92 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 7 - Building stone tour of downtown Port Arthur, Thunder Bay, Ontario Peter Hinz Ring of Fire Secretariat, Ontario Ministry of Northern Development and Mines, 435 James Street South, Suite B332 Thunder Bay, Ontario, P7E 6S7, Canada Foreward This walking tour will examine a number of buildings in the downtown core of the Port Arthur portion of Thunder Bay. The buildings of interest are constructed from a range of stones, the majority of which were produced in northwestern Ontario. The walking tour will consider the geological and architectural features of each building. Introduction (taken from Hinz et al.,1994) The dimension and monument stone industry in northwestern Ontario has a long history and is linked to the development and prosperity of the region. One of the earliest commercial operations was located on Vert Island in Nipigon Bay of Lake Superior. The Mesoproterozoic Sibley Group yielded an attractive red sandstone which was extracted by the ChicagoVerte Island Sandstone Company. The stone was shipped to Chicago, Winnipeg, southern Ontario and other U.S. cities for construction uses. Development of some of the earliest quarries in the Marathon and Nipigon areas was directly related to the construction of the Canadian Pacific Railway in the late 1880’s. Syenites surrounding Marathon and sandstones south of Nipigon were used in the construction of railway trestles to span the Black, Pic, Little Pic, Steel and Nipigon rivers. Today these trestles show very little wear and are a testament to the long-standing durability of the stones. Although markets for dimension stone decreased in the early 1900’s, production continued at the Simpson Island sandstone quarry (1900-1910) and at the Bannerman and Horne quarry (1912-15) near Ignace. The next period of quarry development took place during the late 1920’s to early 1930’s. Five small scale quarries operated northwest of Marathon along the Canadian Pacific Railway. Black and brown granites were extracted and shipped to customers in Toronto, Buffalo, Chicago and Detroit. In 1932, the last of these quarries closed due to the loss of a market. In 1948, the Vermilion Pink Granite Company opened a quarry approximately 12 km southwest of the town of Vermilion Bay. This highly popular pink granite began production in 1954 and continued sporadically under various names until 1991 when the quarry, now named Granite Quarriers (GQI) Inc., closed. In 1981, Nelson Granite Limited of Sussex, New Brunswick began production of an identical granite from a quarry immediately south of the highway from the Granite Quarriers Inc. site. This quarry has operated year round since that time and is still in production. Currently, 2012, Nelson Granite Limited is the only stone producer operating in northwestern Ontario. Nelson Granite produces a range of colours including pink, yellow, green, brown and white granite from four quarries located north of Kenora and west of Vermilion Bay. Northwestern Ontario stone is shipped around the world for a range of uses including: building stone for interior and exterior uses; monumental stone; and landscape uses including pavers, benches and accent pieces. Detailed descriptions of the historic quarries, their operations, geology and geotechnical test results are provided in Hinz et al. (1994). Descriptions of current producers are available in the Kenora portion of the Report of Activities 2010, (Lichtblau et al., 2011). A list of building stone quarries in northwestern Ontario is given in Table 1. Geologic setting Sandstones of the Sibley Group The red and buff coloured sandstones utilized as building stone throughout Thunder Bay were sourced from quarries in the Nipigon to Rossport area. The sandstones represent lithologies hosted within the Pass Lake Formation of the Sibley Group, the following description is taken from Fralick et al. (2000). “Sedimentary rocks of the Sibley Group discontinuously outcrop on the north shore of - 93 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 1. Building stone quarries of northwestern Ontario. The asterisk (*) denotes stones which will be viewed at the various field trip stops. Sibley Group Sediments Quarry Cooke Point George Point La Grange Island Nipigon River * Quarry Island Ruby Lake Simpson Island * Vert Island * Wolf River Rock Type Marble Grey sandstone Red sandstone Variegated marble White sandstone Variegated marble Buff sandstone Red sandstone Buff sandstone Years of Operation 1931-1940 (?) Late 1800’s (?) 1882-1883 (?) 1883-1910 (?) Late 1880’s (?) 1996-1998 1904-1912 1881-1912 1913-15; 1921-31 Granitic Rocks Quarry Bulter Grey * Cold Spring C.P.R. Docker Township * Forgotten Lake GQI Quarriers Inc. Peninsula Granite Pine Green Red Deer Lake Redditt Rock Type Grey granite Black granite Black granite Pink granite Yellow granite Pink granite Brown granite Green granite Brown granite White granite Years of Operation 1892-1943, 1946-52, 1989 1931-38(?) 1880’s (?) 1981-present 1997-present 1948-1991 1880’s-1927(?) 1992-present 1996-present 2010-present Note: The usage of the term “granite” in the building stone industry is used regardless of lithology (eg. granite, syenite, gabbro) Lake Superior and around Lake Nipigon (Fig. 1). The flat-lying to gently dipping, clastic-carbonate succession occupies a broad oval area disconformably to unconformably overlying Mesoproterozoic, Paleoproterozoic and Neoarchean rocks. Sibley Group rocks are located in the Southern Province of northwestern Ontario. in thickness and consists of basal conglomerate and upward-thinning beds of quartz arenite. It was deposited in a shallow lacustrine environment (Franklin et al., 1980).” The fluvial and lacustrine strata of the Sibley Group are divided into three subhorizontal formations: 1) the lowermost is the Pass Lake Formation; 2) the Rossport Formation overlies it, followed by 3) the Kama Hill Formation. “This marble consists of contact metamorphosed, Mesoproterozoic, Rossport Formation (Sibley Group) dolostone and other, calcareous sedimentary rocks in the contact metamorphic aureole of Keweenawan diabase sills. It has previously been termed Nipigon River marble and was quarried from 1883 to ca. 1910 The Pass Lake Formation varies from 0 to 50 m Marble of the Sibley Group (taken from Fralick et al., 2000) - 94 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 1. Field trip stop locations: 1. Port Arthur Collegiate Institute; 2. Trinity United Church; 3. Masonic Temple; 4. Former Hymer’s Mens Wear; 5. Ontario Government Building. at a site on the eastern side of the Nipigon River, approximately 6 west of the Ruby Lake quarry (Hinz et al., 1994). A similar hornfelsed unit is described in more detail at Stop 2-6. Calcite, dolomite, epidote and opaque minerals were noted in thin section by Hinz et al. (1994) from the Nipigon River quarry.” Granites of the Wabigoon Subprovince (taken from Beakhouse et al., 1995) “The Wabigoon subprovince is a 150 kilometre wide volcanoplutonic domain that has an exposed strike extend of 700 kilometres, extending an unknown distance beneath Paleozoic strata at either end. The western Wabigoon region is characterized by interconnected, arcuate, metavolcanic ‘greenstone belts’ surrounding large elliptical batholiths. Granitoid rocks within the western Wabigoon region include large elliptical to multi-lobate batholiths that define the architecture of the greenstone belts as well as smaller stocks. Most of the large batholiths (Aulneau, Atikwa, Sabaskong) range compositionally from ultramafic to granitic but are predominantly tonalitic to granodioritic.” Both the Butler Grey and Vermilion Pink granites are sourced from intrusions within the Wabigoon subprovince. Storey (1986) described the geology of the Butler Grey quarry: “The rock is massive, light grey to white, biotite granite (approximately 5% biotite). There are local variations in grain size and resultant colour variations. There are a few minor patch pegmatites. A very weak foliation trends north-northwest.” Storey (1986) also described the Vermilion Pink granite: “The rock was classified as quartz monzonite by Mattison (1952) and granite by Pryslak (1976). A modal analysis from Mattison (1952) plots as granite in the Streckeisen (1976) classification.” Field trip stops Stops are located by UTM co-ordinates based on NAD 83, UTM Zone 16 The walking tour starts on the east side of the Port Arthur Collegiate Institute building. Park on Waverly Street on the south side of Waverley Park (0335224E 5367273N), walk to the east entrance of the Port Arthur Collegiate Institute. - 95 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 1. Port Arthur Collegiate Institute (aka. P.A.C.I.; 401 Red River Road) UTM coordinates: NAD83; 16U 0335208E / 5367358N This building, constructed in 1909 of Simpson Island buff sandstone (Pass Lake Formation), is an example of the Queen Anne style that was in common use from the 1880s to the 1910s. When the school board made the initial planning for the building, it was decided that it should be “erected for posterity, and not be of the ‘shack’ order”, so they chose the stately Queen Anne style. The original P.A.C.I. can be seen in Figure 2. Alterations in 1925 resulted in the addition of four more classrooms, and more renovations to the north and south in 1953 and 1962 created other rooms. These alterations used Indiana Limestone in an attempt to blend into the original building. The younger, middle Mississippian (335-340 Ma) aged limestone is easily distinguished from the much older Mesoproterozoic Sibley sandstones by the prolific presence of marine fossils such as crinoids, bryozoa and gastropods (Fig. 3). A gymnasium planned in 1964 and completed in 1974, provoked controversy as its design was incompatible with the rest of the school. Walk back to Waverley Street to view several residential houses which incorporate Simpson and Vert island sandstones (eg. 369, 349 and 332 Waverley Street) walking east approximately 190 metres along Waverley Street to the Algoma Street corner. Stop 2. Trinity United Church (30 Algoma Street South) UTM coordinates: NAD83; 16U 0335432E / 5367226N This building, completed in 1906, was formerly known as the Trinity Methodist Church, and became the Trinity United Church after the United Church of Canada was formed in 1925. Constructed of rough cut, Simpson Island buff sandstone (Pass Lake Formation), this structure is an example of the Late Gothic Revival style that was popular from the 1890s to the 1940s (Fig. 4). The unusual tower features very narrow windows (lancets), four buttresses, each capped with a pyramid shaped finial, and an extremely sharp hexagonal spire. The rest of the building also features very steeply pitched roofs, and arched windows in the Gothic style. Walk north and cross Red River Road, approximately 90 metres, then cross Algoma Street and walk east approximately 125 metres. Figure 2. The Port Arthur Collegiate Institute ca. 1909, from http://images.ourontario.ca/ - 96 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3. Left - Fossiliferous Indiana limestone, arrow points to a fan-like bryozoan. Right - Enlarged view of the fan-like bryozoan. Figure 4. Trinity United Church ca. 1930, from http://www.hpd.mcl.gov.on.ca/ - 97 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5. Masonic Temple, Red River Road, Nipigon River marble displaying mat-like stromatolites. Stop 3. Masonic Hall ( 262-270 Red River Road) UTM coordinates: NAD83; 16U 0335596E / 5367166N Built in 1910 and also known as the Shuniah Lodge, this stone, brick and concrete building replaced the old Masonic temple that was destroyed by fire in 1909. The first floor is made of cut Nipigon River marble (Rossport Formation) and the entrance features carved marble pilasters and decorative panels (Fig. 5). Originally there was a dome on the roof over the entrance, which has since been removed. The central portion of the building has a Mansard roof of French design. The building’s windows are decorated with alternating round and triangular pediments above them. Commercial space occupies the ground floor, while the lodge is located above. Continue east along Red River Road, crossing Court, St. Paul and Cumberland streets, approximately 300 metres. Cross Red River Road and proceed south for 60 metres to Lorne Street where you will see a red sandstone wall. Stop 4. Former Hymer’s Men’s Wear (17 Cumberland Street South, Lorne Street wall) UTM coordinates: NAD83; 16U 0335826E / 5366930N The north wall of this building is constructed of Vert Island red sandstone, the building was constructed circa. 1900. The stone is a brick red sandstone which is part of the Mesoproterozoic Sibley Group, Pass Lake Formation (Fig. 6a). Syneresis cracks are evident on one block. These cracks are caused by subaqueous Figure 6a (left). Red sandstone wall on the north side of the former Hymer’s Men’s Wear. 6b (right) Reductions band in Pass Lake formation sandstone. - 98 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 6c (left). Intraformational conglomerate, Pass Lake formation. 6d (right). Syneresis cracks in Pass Lake formation sandstone. shrinkage of sediments without dessication (Fig. 6d). Also visible in many blocks are intraformational conglomerate, buff coloured reduction spots and bands, ripples (Fig. 6b & c). Walk 35 metres east along Lorne Street to the Ontario Government Building. Stop 5. Ontario Government Building (189 Red River Road) UTM coordinates: NAD83; 16U 0335847E / 5366925N Opened in 1990, the Ontario Government Building was built to incorporate granite building stone from quarries in the Ignace and Vermilion Bay of northwestern Ontario, the stones included Butler Grey and Vermilion Pink (Fig. 7). The stones were used in the exterior and interior of the building and include polished, honed and flame finishes. The Ontario Government Building is described in the Ontario Architecture website (http:// www.ontarioarchitecture.com/postmodern.htm): “It is a classic Post Modern building in that it uses traditional architectural vocabulary in a new and impressive way. The front colonnade is a good example. The columns have neither bases nor capitals, but a decorative band level with the first floor lintels. There is an exaggerated cornice atop the architrave which has three horizontal bands. There is no ornament, not even fluting on the columns, and instead of marble, the columns are metal. Behind the colonnade, the building is a cutain wall of glass with an open concept foyer. Winding around the colonnade is a balustrade leading to other portions of the building and a landscaped front.” References Beakhouse, G.P., Blackburn, C.E., Breaks, F.W., Ayer, J.A., Stone, D. and Stott, G.M. 1995. Western Superior Province Fieldtrip Guidebook; Ontario Geological Survey, Open File Report 5924, 94 p. Fralick, P., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the mesoproterozoic Sibley Group (field trip guide): Institute on Lake Superior Geology Proceedings, 46th Annual Meeting, Thunder Bay, Ontario, v. 46, part 2, p. 5 Hinz, P., Landry, R.M. and Gerow, M.C. 1994. Dimension stone occurrences and deposits in northwestern Ontario; Ontario Geological Survey, Open File Report 5890, 191 p. Lichtblau, A.F., Ravnaas, C., Storey, C.C., Bongfeldt, J., McDonald, S., Lockwood, H.C., Bennett, N.A. and   Figure 7. Entrance of the Ontario Government Building with the flame-finished Butler Grey granite on the exterior and features of the Post Modern architecture. - 99 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Jeffries, T. 2011. Report of Activities 2010, Resident Geologist Program, Red Lake Regional Resident Geologist Report: Red Lake and Kenora Districts; Ontario Geological Survey, Open File Report 6261, 93p. Mattinson, C.R. 1952: A Study of Certain Canadian Building and Monumental Stones of Igneous Origin; Unpublished MSc Thesis, McGill University, Montreal, Quebec. Pryslak, A.P. 1976: Geology of the Bruin Lake-Edison Lake Area; District of Kenora; Ontario Division Mines, Geological Report 130, 61p. Storey, C.C. 1986. Building and Ornamental Stone Inventory in the Districts of Kenora and Rainy River; Ontario Geological Survey, Mineral Deposits Circular 27, 168p. Streckeisen, A. 1976: To Each Plutonic Rock Its Proper Name; Earth Science Reviews, Vol. 12, p. l-33. - 100 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 8 - A geologic transect across the Western Superior Province and Nipigon Embayment, Thunder Bay to Armstrong, Ontario Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Introduction This field trip along Highway 527 between Thunder Bay and Armstrong describes a 250 km long transect through Archean rocks of the western Superior Province of the Canadian Shield and the Mesoproterozoic Nipigon Embayment. This transect extends from the Wawa Subprovince near Thunder Bay, across the entire width of the Quetico Subprovince and into the Wabigoon Subprovince, south of Armstrong (Fig. 1). The Wabigoon and Wawa (ca. 3.0 to 2.7 Ga) are volcano-plutonic subprovinces, containing a number of greenstone belts. The intervening Quetico Subprovince (ca. 2.7 Ga) consists of metamorphosed clastic sedimentary rocks, their high-grade metamorphic equivalents and derived granitic rocks. Mesoproterozoic rocks. Bear in mind that this trip marks the first time that many of these stops have been visited and described as part of a formal field trip. Please exercise caution when stopping and viewing roadside exposures. Stott (2009) noted that the terminology of crustal subdivisions across the Archean Superior Province is slowly evolving such that regional lithologic subdivisions as subprovinces (Card and Ciesielski, 1986) are currently being reassessed in terms of terranes and adjacent, typically autochthonous domains (e.g., Percival and Helmstaedt, 2006; Stott et al., 2007). Subdivisions in the field trip area are shown in Table 1. The most recent geological synopsis in the transect area was provided by Hart and MacDonald (2007): These subprovinces are intruded or unconformably overlain in this area by a variety of Mesoproterozoic rocks of the Nipigon Embayment (Southern Province). These Mesoproterozoic rocks include Sibley Group (ca. 1.3 Ga) sedimentary rocks, the Badwater intrusive complex (ca. 1.6 Ga), and the English Bay felsic intrusive-volcanic complex (ca. 1.54 Ga). Voluminous mafic to ultramafic intrusive rocks related to the Mesoproterozoic Midcontinent Rift (ca. 1.1 Ga) predominate. Day One of the trip will highlight representative Archean lithologies while Day Two will focus on The Nipigon Embayment is underlain, from north to south, by Archean rocks of the English River, Wabigoon, and Quetico subprovinces (Fig. 2). Much of the Embayment is underlain by a series of east-trending 2950 to 2700 Ma greenstone belts separated by 3000 to 2690 Ma intrusive rocks of the central and eastern Wabigoon subprovince (e.g., Blackburn et al., 1991). Tomlinson et al. (2004) proposed a north– south subdivision of the Wabigoon subprovince into the Winnipeg River and Marmion terranes based on isotopic data. The boundary between Table 1. Geological subdivisions in the field trip area Existing Nomenclature Wabigoon Subprovince Proposed Nomenclature Winnipeg River Terrane (north) Marmion Terrane (south) Quetico Subprovince Quetico Basin Wawa Subprovince Wawa-Abitibi Terrane - 101 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. General geology of the field trip transect area, from Smyk and Franklin (2007) the Wabigoon and Quetico subprovinces is the Blackwater Fault to the east, south of Beardmore, and is located south of Peevy Lake to the southwest. The metasedimentary rocks of the Quetico subprovince are intruded by “I-type” tonalite to granodiorite and “S-type” muscovite and two-mica leucogranites (e.g., Williams, 1991; Breaks et al., 2003), with minor mafic to ultramafic and syenitic bodies (e.g., MacTavish, 1999; Pettigrew and Hattori, 2006). A minimum the terranes is obscured by later granites west of Lake Nipigon, but is correlated with the Humboldt Bay high-strain zone to the east of the lake. The southern margin of the Wabigoon subprovince consists of a series of metasedimentary and metavolcanic belts separated from the granite– greenstone terrane by the Paint Lake Fault east of Lake Nipigon and the Max Creek Fault west of the lake (e.g., Williams and Stott, 1991; MacDonald et al., 2005). The boundary between - 102 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. Hart and MacDonald’s (2007) generalized geology of the Nipigon Embayment (modified from Ontario Geological Survey 1993; MacDonald 2004; Hart and Magyarosi 2004; MacDonald et al. 2005; Hart 2005) and the Archean rocks of the Wabigoon and Quetico subprovinces surrounding and underlying the Embayment. intrusions of the Lac des Iles area (e.g., Sutcliffe, 1987; Tomlinson et al., 2002; Stone et al., 2003). The various intrusions of the Lac des Iles area… have been collectively referred to as the Lac des Iles suite by Stone et al. (2003) and include the Northern Ultramafic and Mine Block intrusions of the Lac des Iles Complex (e.g., Hinchey et al., 2005). Previous workers have suggested that the intrusions of the Lac des Iles suite may be part of a contemporaneous magmatic event (e.g., age of deposition for the metasedimentary rocks is constrained by ages of 2665 ± 2 and 2653+3/4 Ma on leucogranites immediately southwest of the Nipigon Embayment (Percival and Sullivan, 1988), as well as the 2688+6/-5 Ma Samuels Lake bodies (Pettigrew and Hattori, 2006), and the 2689 ± 2 Ma Black Pic monzodiorite (Zaleski et al., 1999). There are a number of late- to posttectonic mafic to ultramafic intrusions in the central Wabigoon subprovince, including the - 103 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stone et al., 2003; Pettigrew and Hattori, 2006). A number of intrusions identified as a result of mapping (e.g., Hart, 2000) and diamond drilling (e.g., MacDonald et al., 2005) may be part of the same magmatic event. diabase dyke was found that may correlate with the 1140 Ma Abitibi swarm (Ernst et al., 2006). Three Meso- to Paleoproterozoic [sic] lithologic units are located in the northwest portion of the Nipigon Embayment: the Badwater (Creek) intrusion, the Pillar Lake Volcanic rocks, and the English Bay Complex (Fig. 3). The mafic to felsic Badwater intrusion is unconformably overlain by flat-lying mafic pillowed volcanic rocks of the Pillar Lake volcanic unit (MacDonald, 2004). The English Bay volcanic–intrusive complex is located to the southeast, on the northwest shore of Lake Nipigon (e.g., Sutcliffe and Greenwood, A series of north-striking diabase dykes intrudes the Archean rocks of the Wabigoon and Quetico subprovinces. A paleomagnetic and geochemical study of the dykes located west of the Nipigon Embayment suggests that there are 2130– 2120 Ma reverse-magnetized and 2110– 2100 Ma normal-magnetized Marathon dykes (Ernst et al., 2006). A solitary northeast-trending Figure 3. Hart and MacDonald’s (2007) generalized geology of the Mesoproterozoic rocks of the Nipigon Embayment (modified from Ontario Geological Survey, 1993; MacDonald, 2004; Hart and Magyarosi, 2004; MacDonald et al., 2005; Hart 2005) - 104 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 1985a). All three units appear to be localized along major regional structures. Archean rocks (e.g., MacDonald, 2004; Hart, 2005). A lack of obvious textural, mineralogical, or geochemical variations within sill exposures hinders regional correlation of the sills and the development of a stratigraphic succession (e.g., Hollings et al., 2007). However, a recent airborne magnetic survey (Ontario Geological Survey, 2004), combined with new geochemistry and geochronology, has distinguished a number of distinctive sills with limited extents…, including the Inspiration sill (MacDonald 2004; Hollings et al., 2007a; Heaman et al., 2007) and the McIntyre sill (Richardson et al., 2005; Hollings et al., 2007; Heaman et al., 2007). Geological mapping suggests that the formation of the Nipigon Embayment was controlled by a series of north-, northwest- and northeast-trending faults that appear to correlate with prominent Archean basement structures (e.g., Hart, 2005; MacDonald et al,. 2005). Interaction between these faults formed an asymmetric basin or half-graben in the southwest portion of the Nipigon Embayment as originally defined by Coates (1972). Franklin et al. (1980) suggested that the faults defining parts of the Nipigon Embayment represented a failed arm of the Midcontinent Rift, and Sutcliffe (1987) and Lightfoot et al. (1991) proposed that the ultramafic intrusions and Nipigon diabase sills intruded along these faults. Alternatively, the faults may be the result of subsidence following an anorogenic thermal event as proposed by Fralick and Kissin (1995) and Hollings et al. (2004). Further work by Rogala et al. (2007) indicates that most fault activity related to halfgraben development was the result of broad subsidence that occurred more than 200 million years before the Midcontinent Rift, thus lending further credence to the interpretations of Fralick and Kissin (1995) and Hollings et al. (2004). Unconformably overlying the basement Archean and earlier Proterozoic rocks are the clastic and chemical sedimentary rocks of the Sibley Group. The Sibley Group is interpreted to have been deposited in a fluvial to shallowlacustrine environment with transitions to playa lake and sabkha environments followed by reflooding of the basin, recorded in the middle to upper stratigraphic parts of the sequence (e.g., Franklin et al., 1980; Cheadle, 1986; Rogala, 2003). The thickest accumulation of Sibley Group rocks in the western portion of the Nipigon Embayment is within a half-graben, defined by the faults in the area of the Black Sturgeon River, thinning toward the west (e.g., Coates, 1972). Rogala et al. (2007) interpret the initial period of fault activity as representing a change from broad subsidence to active basin formation in the Lake Nipigon area prior to 1339 ± 33 Ma, and prior to formation of the Midcontinent Rift. There are four sill-like mafic to ultramafic intrusions, with three (Disraeli, Seagull, and Hele) located to the south of Lake Nipigon and one (Kitto) located along the east side of the lake, emplaced into the Sibley Group and underlying Archean rocks (e.g., Sutcliffe, 1986, 1987; Hart and Magyarosi, 2004). Ultramafic-hosted PGE mineralization in the Seagull intrusion is located within discrete, laterally continuous zones, which a study by Heggie (2005), using wholerock geochemistry, isotope geochemistry, and mineral chemistry, suggests have been caused by sulphur saturation of the magma during initial stages of emplacement, with zones higher in the intrusion probably reflecting influxes of lessevolved magma. Other examples of fine-grained, massive mafic to ultramafic sills occur scattered through the Nipigon Embayment. The thickest are the Jackfish sill in the northwest corner of Lake Nipigon and the Shillabeer sill south of the lake (e.g., MacDonald, 2004; Hollings et al., 2007a). The current outline of the Nipigon Embayment is defined by a series of diabase sills estimated to cover an area in excess of 20 000 km2 (Sutcliffe, 1991). The shallow-dipping Nipigon diabase sills, ranging in thickness from <5 m to >180 m, intrude the mafic to ultramafic intrusions, Sibley Group, Pillar Volcanics, the English Bay Complex, and the underlying A synopsis of mineral deposits in the area was given by Smyk and Franklin (2007): A variety of metallic and non-metallic mineral deposit types occur within Archean and Proterozoic rocks in the area encompassing the Lake Nipigon Region Geoscience Initiative. Archean deposit types include: Algoma-type banded iron formation-hosted iron (e.g., Lake Nipigon iron range) ; volcanogenic massive sulphide copper-zinc (e.g., Onaman-Tashota belt); ultramafic intrusion-hosted chromium (e.g., Puddy-Chrome lakes); mafic to ultramafic - 105 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 2. Mapping of the Precambrian bedrock geology of the transect area by the Ontario Geological Survey Map Number P2984 ARM48C Scale 1:15 840 1:63 360 Year 1986 1939 Authors J.F. Scott, J.M. Seguin R.D. Macdonald 1:63 360 1971 M.E. Coates Eayrs Lake-Starnes Lake area 1:63 360 1969 L. Kaye Max Lake sheet 1:31 680 1967 E.G. Pye Southwest Portion of the Nipigon Embayment 1:100 000 2006 Hart, T.R. Lac des Iles Greenstone Belt 1:20 000 2001 Hart, T.R., MacDonald, C.A., Lepine, C.D. P3434 Heaven Lake Greenstone Belt 1:20 000 2001 P3560 Cheeseman-Black Sturgeon Lakes Area 1:50 000 2005 Kabitotikwia Lake Area 1:50 000 2005 Hart, T.R., MacDonald, C.A., Lepine, C.D. MacDonald, C.A., Tremblay, E., ter Meer, M. MacDonald, C.A., Tremblay, E., ter Meer, M. P3537 English Bay-Havoc Lake Area 1:50 000 2004 P3536 Waweig-Wabinosh Lakes Area 1:50 000 2004 Pashkokogan-Caribou lakes sheet 1:126 720 1974 M2235 M2172 M2136 P3580 P3435 P3559 P0962 Map Area MacGregor Township, west half Gorham Township and vicinity Disraeli Lake sheet intrusion-hosted copper-nickel-platinum group element (PGE) (e.g. Lac des Iles); and pegmatitehosted deposits of rare metals (Li, Ta, Be), uranium and molybdenum (e.g., Georgia Lake field; Black Sturgeon Lake; Anderson Lake, respectively). Mesothermal lode gold deposits are prominent in the Beardmore-Geraldton camp. MacDonald, C.A., ter Meer, M., Lepage, L., Préfontaine, S., Tremblay, E. MacDonald, C.A., ter Meer, M., Lepage, L., Préfontaine, S., Tremblay, E. Sage, R.P., Breaks, F.W., Stott, G., McWilliams, G., Bowen, R.P. Rift -related Osler Group volcanic and interflow sedimentary rocks. Native copper and Cusulphides occur in Mesoproterozoic Sibley Group sedimentary rocks, adjacent to ultramafic intrusions. These mafic to ultramafic intrusions, associated with Midcontinent Rift magmatism, host copper-nickel-PGE deposits (e.g. Seagull, Great Lakes Nickel). Silver-bearing veins occur in Paleoproterozoic Animikie Group sedimentary rocks in proximity to Midcontinent Rift-related Superior-type iron formation occurs in Paleoproterozoic Gunflint Formation. “Red-bed” copper occurs in Mesoproterozoic Midcontinent - 106 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 mafic intrusions (e.g., Silver Islet; Silver Mountain). Lead-zinc-barite veins, uraniumbearing veins and amethyst vein- and replacement -type deposits may be co-genetic and formed at or near the unconformity between Sibley Group basal sandstone and underlying Archean granitic basement (e.g., Dorion; Black Sturgeon Lake; McTavish Township). The hydrothermal systems that produced all of these veins were probably driven by heat associated with Midcontinent rifting. Many occur in structures related to riftbounding faults. Iron oxide-copper-gold deposits may occur near the English Bay intrusion. Embayment was undertaken as part of the Lake Nipigon Region Geoscience Initiative (Table 2). Stop descriptions Day One (Figs. 4 & 5) Stop 1-1: Pillowed Metavolcanic Rocks, Wawa Subprovince UTM coordinates: NAD83; 16U 0340852E / 5376037N Mapping of the Precambrian bedrock geology of the transect area by the Ontario Geological Survey (and its predecessors) ranges from detailed (e.g. 1:15 840) to reconnaissance-scale (1:250 000). Much of the detailed mapping focused on greenstone belts. Newer, comprehensive 1:50 000 mapping of the Nipigon This is a typical exposure of greenschist-facies, massive to pillowed, locally vesicular mafic metavolcanic rocks, foliated at 260/80. The unit is cut by small shear zones, mafic xenolith-bearing feldspar porphyry dykes and north-northeast-trending calcite veins. Similar veins to the northeast contain lead and zinc sulphides and may be related to a Mesoproterozoic   Figure 4. General geology of the transect area, showing the location of field trip stops along Highway 527 - 107 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 RoadLog Logfor for Field Field Trip Road TripStops Stops (N.B. Please exercise caution along highway and road right-of-ways.) (N.B. Please exercise caution along highway and road right-of-ways.) STOP NAME STOP NUMBER LANDMARK (0 Km) DISTANCE (km) NORTHING EASTING 5376037 340852 5378261 341252 5384413 344754 5389006 346350 5388319 346239 5391178 347109 DAY ONE Intersection of Hwy. 527 and Hwy. 11-17 Wawa – Pillowed Metavolcanic Rocks 1-1 2.8 Mt. Baldy Road Wawa Timiskaming Metasedimentary Rocks 2.9 1-2 5.1 Weigh Scales Compressor Station Road Wawa Penassen Lakes Stock 1-3 1-4A 1-4B 6.5 13.2 15.0 Beaverlodge Road (exit Highway 527 to access stops 1-4A, 1-4B) Beaverlodge Road, north spur, 1.3 km west off Hwy. 527 Beaverlodge Road, north spur, 1.0 km west off Hwy. 527 Kingfisher Lake Road Wawa – White Lily Lake Stock 5.2 12.4 Gibson's Road Magone Road Quetico Metasedimentary Rocks Wawa Feldspar-phyric granitoid 0.0 1-5 16.1 19.0 19.7 Escape Lake Road - 108 - 22.4 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Quetico Pegmatite Quetico Fault in Metasedimentary Rocks Quetico Fault in Granitoid Rocks 1-6 Barnum Road 25.3 Bush Road 100 m west off Hwy. 527 26.8 South Current River Monday Lake Road 27.9 29.6 31.1 5401948 348057 1-7B 31.5 5402246 348117 5405692 348152 5407714 345756 5416579 339226 5448888 328624 Shallownest Road 1-8 Quetico Migmatites 1-9 35.2 100 m east of Hwy. 527 38.4 Orr's Place Store Dorion Cut-Off Pace Lake Road 1-10 40.0 45.2 49.9 51.1 Mott Lake Road DeCourcey Lake Eaglehead Lake Road Pipeline Fensom Lake Road Mawn Lake Road Max Lake Road Camp 45 Road Wabigoon Conglomerate 346508 1-7A Quetico Pegmatitic Granite Quetico Cordieritebearing granitoids 5398003 1-11 56.6 64.1 72.5 74.5 80.8 82.2 84.2 87.5 88.1 Max Creek 88.3 Wabigoon – Tuff-breccia 1-12 88.6 5449317 328280 Wabigoon Pillowed Mafic Metavolcanic Rocks 1-13 88.8 5449467 328159 - 109 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Wabigoon Brecciated Metavolcanic Rocks 1-14 90.7 5450297 326447 Wabigoon – Banded Iron Formation 1-15 92.5 5451015 324893 5573375 356068 5564137 345619 LaChapelle Creek 93.7 Lac Des Iles Road 93.8 Poshkokogan Lake Road 103.6 Whistle Lake Road 104.9 DAY TWO Pillar Lake Volcanics Alarie Quarry Pillar Lake Volcanics – T-Junction, Mattice Lake Road Intersection, CNR Tracks and Hwy. 527, West end of Armstrong 0.0 Intersection, CNR Tracks and King St., East end of Armstrong turn-off to quarry 3.0 3.3 2-1 5.0 Intersection, CNR Tracks and Hwy. 527, West end of Armstrong MacKenzie Lake Inn Clearwater Lake Road Frontier Road Mattice Lake Road 0.0 2.5 6.2 7.8 10.0 Intersection of Mattice Lake Road with Highway 527 Bridge Badwater Creek turnoff Bridge 0.0 0.8 3.8 4.5 2-2 5.4 - 110 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Badwater Gabbro – Badwater Creek Road Badwater Gabbro - East of Badwater Creek Road Intersection of Mattice Lake Road with Badwater Creek Road 2-3A 2-3B Pillar Lake Volcanics Chimney Lake 2-4 Pillar Lake Volcanics – Hwy. 527 2-5 Nipigon Diabase Sill, Highway 527 2-6 Sibley Group Sedimentary Rocks and Nipigon Diabase Sills 2-7 0.0 0.6 5564306 347047 (outcrops 30 m and 60 m east of road) 0.8 5564142 347043 Intersection of Mattice Lake Road with Highway 527 0.0 outcrops 300 m east of highway) 2.4 5563345 349738 3.3 5562320 349460 Northern end of Waweig Lake 5.5 Bridge on Hwy. 527 at Gull River 0.0 Kabi River Bridge 20.2 24.2 5499194 342613 (outcrops 160 m southeast of highway) 26.8 5493814 341205 mineralizing event. Stop 1-2: Timiskaming Metasedimentary Rocks, Wawa Subprovince UTM coordinates: NAD83; 16U 0341252E / 5378261N These steeply dipping, clastic metasedimentary rocks are massive, thickly to thinly bedded, quartz-rich arkosic sandstones with abundant mudstone fragments. Thinly bedded turbidites display graded bedding with tops to the south, indicating that this sequence (at 270/85°) has been locally overturned. The northern Wawa Subprovince contains five major packages of sedimentary rock associations. There are: 1) cherts and iron formations deposited as inter-flow sediments in basaltic successions, 2) resedimented andesitic volcanic eruptive material forming areas of graded beds interlayered in calc-alkaline successions, 3) thick turbidite sequences similar to the Quetico turbidites deposited when the Quetico trench was full and sediment gravity flows gained access to the Wawa ocean floor to the south (Fralick et al., 2006), 4) strandline to deep shelf deposits formed at 2692 Ma (unpublished U-Pb zircon age on volcanic ash) during a period of shoshonitic volcanism related to initial collision between a Wawa island arc and the Quetico accretionary complex, and 5) 2686 Ma fluvial conglomerates and sandstones shed into pull-apart basins during transpression during the main orogeny. The succession we are examining is reasonably nondescript, but most closely resembles units present in the shelf succession of the 2692 Ma assemblage. At other locations, hummocky cross-stratification present closer to shore in this system attests to the operation of geostrophic flows draining storm surges away - 111 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 1-3: Subprovince Penassen Lakes Stock, Wawa UTM coordinates: NAD83; 16U 0344754E / 5384413N The Penassen Lakes stock is part of the Dog Lake granite chain, a linear series of at least six separate, possibly genetically related granitoid intrusions: Silver Falls, Trout Lake, Barnum Lake, Shabaqua, Penassen Lake and White Lily (Kuzmich et al., 2011; Fig. 7). They are emplaced in semi-pelitic to pelitic metasedimentary and gneissic rocks along the Quetico–Wawa subprovinces boundary north of Thunder Bay (Kehlenbeck, 1977). The intrusions form an approximately east-northeast-trending, evenly spaced chain spanning roughly 70 km, none of which have undergone geochronological analysis. The magnetic signature of these intrusions suggests that they may be distinct from the typical S-type granites found within the Quetico Subprovince. Detailed geochemical and petrographic studies of the granites by Kuzmich (in progress) and Kuzmich et al. (2011) will provide additional insights into the origins and petrogenesis, as well as the development of the Quetico Subprovince as a whole. The Penassen Lakes stock is a dark pink, massive, magnetic, medium-grained, quartz-monzodiorite to monzodiorite (Fig. 8). At this location, equigranular hornblende monzodiorite is cut by aplitic dykes and contains mafic xenoliths. Amethyst-bearing veins crosscut the stock at a locality approximately 600 m south along the highway.   Figure 5. General geology and field trip stops, Day One (Archean) from the coast. Below storm wave-base these flows deposited graded beds with the same appearance as slump induced turbidites. The lithologies present here are similar to these types of units, which are associated with hummocky cross-stratification and tidal flat deposits at other locations such as Finmark, west of Thunder Bay.   Figure 6. Overturned turbidites, Stop 1-2 - 112 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 7. Aeromagnetic image of the area north of Thunder Bay, with Dog Lake intrusions labeled (Kuzmich et al., 2011) Stop 1-4A: Metasedimentary Rocks, Quetico Subprovince UTM coordinates: NAD83; 16U 0346350E / 5389006N This site displays typical, thinly to thickly bedded Quetico metaturbidites (Fig. 9). Relict graded beds indicate southward younging and may have load casts at their bases. The fine-grained, pelitic tops appear to have porphyroblasts of what was tentatively identified as andalusite. Quartzo-feldspathic dykes host quartz veins in the necks of boudins. These dykes are locally cut by quartz-feldspar-muscovite pegmatite dykes, which may represent neosome generated by partial melting of the metasedimentary rocks. This corresponds to low- to medium-grade metamorphic assemblages outlined by Seemayer (1992) in this area along the southern margin of the Quetico Subprovince. Bear in mind that Wawa metavolcanic rocks have been noted north of this location, suggesting that the subprovincial boundary here may consist of intercalated panels of metavolcanic and metasedimentary rocks, intruded by late granitoids.  Figure 8. QAP diagram for samples of the Penassen Lakes stock (Kuzmich, in progress) As described by Seemayer (1992), these lowgrade metasedimentary rocks generally consist of quartz, plagioclase and biotite, with minor amounts of muscovite and chlorite. Porphyroblastic muscovite, - 113 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 medium-grained, magnetic, dark pink, alkali feldsparrich phase; and a medium- to coarse-grained, massive, magnetic, dark pink phase (Figs. 10 & 11). At this location, the intrusion consists of medium- to coarsegrained, K-feldspar-phyric, amphibole-bearing monzonite with a high magnetic susceptibility. Some slickensided surfaces were noted. The southern contact of the intrusion with metasedimentary country rocks is exposed approximately 700 m south along the highway in the vicinity of the Kingfisher Lake Road turn-off. STOP 1-6: Pegmatite, Quetico Subprovince UTM coordinates: NAD83; 16U 0346508E / 5398003N  Figure 9. Graded metaturbidites and pegmatite dykes, Stop Strongly peraluminous, muscovite-, cordierite- and garnet-bearing pegmatitic granite dikes, with local black tourmaline, were found to occur widely in the Quetico Subprovince along the Highway 527 from Walkinshaw Lake north to DeCourcey Lake (Breaks et al., 2003). Approximately 150 m west of Highway 527 and 40 m south of an old logging road, quartz-feldsparmuscovite pegmatite is exposed in a large whale-back outcrop. This locality, along with other pegmatites and related granitoid rocks, were described by Breaks et al. (2003): 1-4A Rare-element mineralization was discovered by the current survey within an extensive swarm of pegmatitic granite dikes at Onion Lake near Thunder Bay. The lens-shaped dikes of this swarm, as seen in the area near the junction of Highway 527 and the Barnett Lake road, occur as northeast-striking, whale-back glacial erosional andalusite, garnet and cordierite are more common near contacts with granitoids and are attributed to contact metamorphism. Kehlenbeck (1977) noted hornfelsic textures in these contact zones. Stop 1-4B: Feldspar-phyric granitoid, Wawa Subprovince UTM coordinates: NAD83; 16U 0346239E / 5388319N Just south of Stop 1-4A, there are exposures of an undeformed, medium-grained, feldspar-phyric granitoid with recessively weathered biotite and a moderate magnetic susceptibility. It may be a smaller, isolated intrusion related to one of the neighbouring (White Lily or Penassen Lakes) granitoid stocks. STOP 1-5: White Lily Lake Stock, Wawa Subprovince UTM coordinates: NAD83; 16U 0347109E / 5391178N Kuzmich (in progress) has subdivided the White Lily intrusion into two separate phases: a fine- to   Figure 10. QAP diagram for samples of the White Lily Intrusion (Kuzmich, in progress) - 114 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 11. TAS diagram showing samples from Penassen Lakes (diamonds) and White Lily stocks (circles), from Kuzmich (in progress). remnants that achieve a maximum size of 100 by 300 m. The internal units comprise: • muscovite-rich potassic pegmatite • quartz-rich patches with blocky K-feldspar, coarse muscovite books and sparse beryl • fine-to medium-grained, garnet-biotitemuscovite granite • • potassic pegmatite and enclosed quartz-rich patches. Dikes and foliation-concordant peraluminous granites and pegmatites were emplaced into Quetico Subprovince metasedimentary rocks during at least three intrusive episodes characterized by the following rock types: • garnet-biotite-muscovite pegmatitic leucogranite grey, garnet-biotite granite, fine- to medium-grained • cordierite and garnet-cordierite granite garnet and muscovite-garnet aplite • sheets of pegmatitic leucogranite and associated quartz-rich patches The quartz-rich patches locally contain pale green beryl up to 1 by 16 cm, as at locality 01-FWB-107 at Onion Lake (UTM 346199E, 5397916N, Zone 16). Black, tantalum-oxide minerals (ferrocolumbite: 27.31 weight % Ta2O5), up to 3 by 3 by 5 mm, were discovered at locality 01-JBS-52 (UTM 346512E, 5398007N, Zone 16) [STOP 1-6; Fig. 12] and apparently associated with local albitization of potassium feldspar megacrysts. Blocky potassium feldspar megacrysts up to 50 cm in diameter and muscovite books up to 10 cm in thickness were noted in the Stop 1-7A: Quetico Fault in Metasedimentary Rocks UTM coordinates: NAD83; 16U 0348057E / 5401948N As we near the Quetico Fault from the south, low-grade metasedimentary rocks give way to wellfoliated schists which retain evidence of a layered sedimentary protolith but which may display incipient anatexis (Seemayer, 1992). They are characterized by a dominant, subvertical west-striking foliation. North of - 115 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 12. Geology and rare metal mineral occurrences along Highway 527 (after Breaks et al., 2003) the Quetico Fault, migmatites predominate. This section along Highway 527 was described by Seemayer (1992): Mylonitic and cataclastic rocks of the subvertical Quetico Fault zone cut the migmatites of Block C [higher-grade subdivision]. The fault rocks outcrop for 2.5 km along Highway 527. Feldspars in mylonitized leucosome show a characteristic brick-red alteration colour which makes the fault rocks easy to identify. Recognizable stromatic migmatites in the fault zone show well-developed C-S fabric and abundant shear planes in the leucocratic layers. The fact that the migmatites have been sheared shows that final movement on the Quetico Fault post-dates - 116 - the migmatization and peak metamorphism, although the fault may have been initiated during an earlier period of transpression. Mackasey et al. (1974) attributed a dextral displacement of 100 km to the Quetico Fault. Detailed examination of the fault from Rainy Lake to Highway 527 by Kennedy (1984) found evidence to support the dextral sense of strike-slip motion. Kennedy found that brittle deformation followed the predominantly ductile deformation within the fault zone. Purdon (1989) concluded that motion along the Quetico Fault northeast of Thunder Bay was of a complex nature. An early dip-slip component inferred from subvertical stretching lineations on foliation surfaces was �Proceedings of the 58th ILSG Annual Meeting - Part 2 overprinted by slickenfibres resulting from dextral strike-slip motion. It is not surprising, then, that metasedimentary rocks showing incipient metamorphic differentiation are adjacent to wellsegregated, stromatic migmatites separated by the fault, although the initial bulk composition of the rocks on either side of the fault was likely very similar. at a site where a bulk sample of feldspar was taken by local company, Thunderbrick Ltd. in 1981 (Assessment Files, Thunder Bay Resident Geologist’s Office). Geochemical analyses and results of testing of its suitability for ceramics applications are not available. A pervasive foliation (~250/70) is locally kinkbanded. Granitic dykes and anastomosing, black patches of what may be pseudotachylite cross-cut the high-grade metamorphic rocks (Fig. 13). Foliated pegmatitic rocks consist of quartz, feldspar, muscovite, garnet and biotite. Feldspar crystals may reach up to 60 cm in size. Garnet euhedra occur as disseminated crystals or trains of crystals. Biotite bands are also evident. Stop 1-9: Migmatites, Quetico Subprovince UTM coordinates: NAD83; 16U 0345756E / 5407714N Stop 1-7B: Quetico Fault in Granitoid Rocks UTM coordinates: NAD83; 16U 0348117E / 5402246N North of the ravine, a large rock cut displays red, coarse-grained to pegmatitic, K-feldspar granitoids. These granitoid rocks host numerous chlorite- and epidote-coated and slickensided joint surfaces and are locally well-foliated. Locally developed, shallowly west-plunging lineations and patchy pseudotachylite were also noted. Stop 1-8: Pegmatitic Granite, Quetico Subprovince UTM coordinates: NAD83; 16U 0348152E / 5405692N Graphic-textured, pegmatitic granitoids are exposed As noted by Seemayer (1992), stromatic (with lesser schlieric and agmatitic) migmatites predominate north of the Quetico Fault. Leucosome consists of quartz, plagioclase and perthite or microcline. Melanosome consists of biotite, quartz and plagioclase. Porphyroblasts of garnet, cordierite and sillimanite and mineral assemblages devoid of muscovite, but including alkali feldspar, reflect regional highgrade metamorphic conditions. Their occurrence is suggestive of diatexis of pelitic protoliths (Seemayer, 1992). At this location, “lit-par-lit” migmatites, consisting of equal proportions of leucosome and melanosome display a gneissosity at 255/80° and schollen structure Figure 13. Narrow   zones of cataclasite and pseudotachylite associated with the Quetico Fault, Stop 1-7A - 117 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 14. Schollen and schlieric migmatite, Stop 1-9 muscovite, cleavelandite, quartz, brown black pyroxene and green fluorapatite). in large, rafted blocks. Quartzo-feldspathic, pegmatite neosome dykes cut the migmatites (Fig. 14). Garnet porphyroblasts up to 1 cm in diameter are also noted. Stop 1-10: Cordierite-bearing Granitoids, Quetico Subprovince STOP 1-11: Conglomerate, Wabigoon Subprovince UTM coordinates: NAD83; 16U 0328624E / 5448888N UTM coordinates: NAD83; 16U 039226E / 5416579N The 16 kilometer stretch of road that we have just Migmatitic rocks are intruded by cordierite-garnetmica pegmatites along this stretch of highway, where relict bands and schollen of biotitic schists are still evident. These pegmatitic rocks were described in detail by Breaks et al. (2003): Pegmatite sheets, at least 5 m thick, are evident on the Armstrong highway as at locality 01-FWB105 near Keelor Lake (UTM 339218E, 5416563N, Zone 16). These sheets consist of coarse-grained, garnet-muscovite-cordierite granite that contain 15 to 20% cordierite crystals pervasively altered to soft, dark green-black pseudomorphs (Fig. 15). The coarse [-grained] granite is gradational into muscovite-rich, miarolitic cavity-bearing, pegmatite patches (blocky potassium feldspar,   Figure 15. Cordierite crystals with incipient alteration along their margins, Stop 1-10 - 118 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure  16. Location of field trip stops 1-11 to 1-15 (Google earth image). Scale is 1.2 km long. traversed crosses a mainly metasedimentary terrain that is the western portion of the Beardmore-Geraldton area (Fig. 16). It is composed of three metasedimentary belts that are separated by metavolcanic belts. The northern metasedimentary belt is dominated by conglomerates and sandstones that were deposited by braided streams flowing from the Onaman-Tashota volcanic arc terrain to the north (Devaney, 1987; Fralick and Kronberg, 1997; Fralick et al., 1992; Fralick, 2003). The outcrop we will look at is part of this belt. The central metasedimentary belt to the east forms a northward younging, coarsening upward succession of ramp-fan turbidites (Barrett and Fralick, 1989) culminating in braid deltas interbedded with iron formation (Fralick and Pufahl, 2006). The same trend is present in the central metasedimentary belt here, though younging directions vary in the turbiditic portion due to isoclinal folding. The southern metasedimentary belt is composed of turbidites that are similar morphologically and geochemically to those in the central belt and the Quetico. This system served to deliver sediment to the Quetico trench as shown by these similarities and almost identical geochronology of their zircon populations (Fralick, 2003; Fralick et al., 2006). The main zircon population ranges from 2708 to 2698 Ma and represents erosion of synchronous calc-alkaline sub-areal volcanism occurring to the north. The clast composition reflects a source dominated by volcanic arc lithologies (mafic to felsic volcanics; diorite and other granitoids). Deep seismic profiling reveals that this succession was overthrust onto the volcanic arc rocks and was itself overthrust by the Quetico metasediments. This agrees with previous work that concluded the BeardmoreGeraldton belt represents a forearc basin between the Onaman-Tashota subareal volcanic arc to the north and the Quetico accretionary complex to the south (Barrett and Fralick, 1989; Eriksson et al., 1994, 1997). The outcrop we will examine forms a portion of the fluvial braided stream deposits that fed into the subaqueous portion of the basin to the south. Cobblepebble conglomerates represent longitudinal gravel bars with interbedded coarse- to medium-grained sandstones formed as waning flow sand sheets on the bar tails. Channels are represented by the thicker sandstones, commonly containing trough crossstratification, and some of the conglomeratic units. Small chute channels that cut into the bar tops during waning flow are represented by the sandstone lenses in the conglomerate. The rivers in this area were transitional to the south into sandy braided rivers, producing outcroppings of trough-cross-stratified, coarse- and medium-grained sandstone. This also formed in distributary mouth bars where the rivers flowed into the ocean to the south. The iron formations in the region are associated with this subaqueous, deltatop environment. In the marine basin to the south of the deltas turbidites accumulated in water that was shallow enough to allow storm reworking of the tops of these - 119 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 17. Conglomerate, Stop 1-11 layers into dunes and ripples. Changes in river mouth position allowed muddy successions a few meters thick to develop during sediment starved intervals. Stop 1-12: Tuff-breccia, Wabigoon Subprovince The sequence has been tectonised as evidenced by the flattening of the clasts in the conglomerate (Fig. 18). The mafic clasts have been affected the most and in places are reduced to ribbon-like shapes. In contrast, the granitoid clasts behaved as rigid bodies, tending to fracture rather than deform plastically. A deformed intermediate tuff-breccia occurs within the dominantly mafic metavolcanic sequence near the intersection of Highway 527 and Kingdon Lake Road. Flattened, buff-coloured pyroclasts (lapilli to bombs) have aspect ratios ranging from 2:1 to >10:1 and define a strong foliation (050/50). Both pyroclasts and matrix are feldspar-phyric. UTM coordinates: NAD83; 16U 0328280E / 5449317N Stop 1-13: Pillowed Mafic Metavolcanic Rocks, Wabigoon Subprovince UTM coordinates: NAD83; 16U0328159E / 5449467N Pillowed mafic flows are best-exposed on the eastern side of the highway (Fig. 20). They are intensely foliated (055/52°) at the southern end of the outcrop. Light green-weathering bun and mattress pillows are locally vesicular/amygdaloidal and have darker green selvages; small re-entrants were noted. Pillows have been flattened into ovoid shapes with aspect ratios ranging from 5:1 to 10:1, precluding unequivocal “tops” determination.   Figure 18. Deformed conglomerate, showing rotation of competent granitoid cobbles and flattening of less-competent clasts, Stop 1-11 - 120 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 19. Tuff-breccia, showing flattened pyroclasts, Stop 1-12 Stop 1-14: Brecciated Wabigoon Subprovince Metavolcanic Rocks, UTM coordinates: NAD83; 16U 0326447E / 5450297N perhaps both pyroclastic and autoclastic brecciation, is exposed on the eastern side of the highway. Flattened, buff (altered?) aphanitic fragments define a foliation at 050/55. An enigmatic volcanic unit, showing aspects of   Figure 20 Pillowed basalt flow, Stop 1-13 - 121 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 a focus of base metal exploration. Dome Exploration (Canada) Ltd. drilled this iron formation in 1972; no analytical results were reported (assessment files, Thunder Bay Resident Geologist’s Office, Thunder Bay). Day Two (Figs. 21 & 22) Stop 2-1: Pillar Lake Volcanic Rocks, Alarie Quarry UTM coordinates: NAD83; 16U 0356068E / 5573375N The Pillar Lake volcanic suite was first recognized and mapped by Macdonald (2004) and is the subject of an ongoing study by Magee (in progress). Initial observations suggested that it largely consisted of a sequence of flat-lying pillowed and massive flows and autoclastic and hyaloclastic breccias. The volcanic rocks unconformably overlie the Badwater gabbro (~1599 Ma) and Badwater syenite (~1590 Ma) and are capped by Keweenawan Inspiration diabase sills (1159 + 33 Ma), yielding an apparent thickness of 20 to 40 m (Hart and Macdonald, 2007; Heaman et al., 2007). Titanite in an andesitic unit yielded a 207Pb/206Pb age of 1129.0 + 4.6 Ma; no zircon nor baddeleyite was recovered in the sample (Heaman et al., 2007).   Figure 21. General geology and field trip stops, Day Two (Mesoproterozoic) Stop 1-15: Banded Iron Formation, Wabigoon Subprovince UTM coordinates: NAD83; 16U 0324893E / 5451015N A rusty-weathering sulphide-facies banded iron formation occurs within the volcanic succession. It is intercalated with tuffaceous and feldspathic, fragmental (pyroclastic?) rocks. The iron formation consists of pyrite, pyrrhotite + chalcopyrite and chert and has been Reinvestigation of these volcanic rocks in 2010 (Smyk et al., 2011) was prompted by a new exposure south of Armstrong that had been created during ballast quarry development (Fig. 23). The quarry face exposes a ~15 m section of thin (0.5 to 2 m), flat-lying, variably altered, columnar-jointed, basaltic andesite flows, capped by a diabase sill. Individual flows may persist over the 130 m length of the exposure while others bifurcate and terminate as thin tendrils in flow breccia (Fig. 24). Autobrecciated zones are rubbly weathering and occupy the spaces between thin, pinching flows. Thin flow top breccias separate flows. Massive flows contain zones of pipe amygdules at their bases and tops. The morphology and disposition of these flows is suggestive of an intercalated, subaerial pahoehoe and a’a flow succession. These volcanic rocks are variably altered along fractures, flow contacts and within brecciated zones. This hydrothermal alteration is typically manifested as a beige to pink discolouration of the dark greyblack flows resulting from the destruction of primary ferromagnesian minerals and the introduction of alkali feldspar, sericite and quartz. Void spaces along joints and fractures and in vesicles has been occupied by large (< 3 cm), black, eudhedral actinolite crystals. - 122 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure  22. Locations of stops 2-2 to 2-5 along Mattice Road and Highway 527 (Google earth image). Scale bar is 1452 m long.   Figure 23. Thin, amygdaloidal (a’a?) flows separated by interflow breccia, quarry face, Stop 2-1. The flow succession is overlain by an Inspiration diabase sill. - 123 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 24. Thin,  bifurcating flows separated by rubbly breccia, quarry face, Stop 2-1. Scale bar (left) is 1 m long Small acicular asbestiform crystals of edenite were also identified by x-ray diffraction analysis. Alteration is characterized by increases in Al2O3, K2O, Na2O, and SiO2 and decreases in CaO, Fe2O3, MgO, P2O5 and TiO2 (Fig. 25). The trace element geochemistry of the sill that caps the quarry face is identical to that of the Inspiration sill identified by Hart and Macdonald (2007), suggesting that this sill is part of that intrusive suite (Figs. 26 & 27). Samples of the volcanic rocks display REE enrichment and negative Nb anomalies comparable to the range of samples of the Pillar Lake volcanic suite reported by Magee (personal communication, 2011; Fig. 28). Samples of the Inspiration sills are geochemically indistinguishable from the Pillar Lake volcanic rocks and to least-altered pillowed samples with the lowest LOI values. The similarity between these two enigmatic suites suggests that they may be derived from the same source. MacDonald and Tremblay (2005) distinguished the Inspiration sill, which exclusively overlies the Pillar Lake rocks, on the basis of its normal polarity and distinct geochemistry. Primary clinopyroxene in the Inspiration sill was commonly replaced by actinolite Figure 25. Comparison of major element chemistry, unaltered and altered basaltic andesite, Stop 2-1 - 124 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 26. TAS plot for basaltic andesite (diamonds) and Inspiration diabase (circles), Stop 2-1 Figure 27. Plot of Mg# versus TiO2 for Pillar Lake volcanic rocks collected by Magee (in progress; squares) and at Stop 2-1 by Smyk et al. (2011; circles), compared to Inspiration sill and Osler Group volcanic rocks - 125 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 28. Chondrite-normalized REE plot for Pillar Lake volcanic rocks and Inspiration diabase (Smyk et al. 2011) (Schandl, 2004; Fig. 29). MacDonald et al. (2005) also noted a remarkable lack of chilled margins on the Inspiration sill and suggested that it was older than the Nipigon sills. Later work by Heaman et al. (2007) determined an age of 1159±33 Ma for the Inspiration sill, which is consistent with the early period of normal polarity. Smyk et al. (2011) proposed that the Pillar Lake basalts may, in fact, be coeval with the Inspiration sill which, in turn, may represent either a subvolcanic intrusion or possibly a massive, ponded flow / lava lake. This may account for the extensive alteration in the Pillar Lake basalts with the sill acting as an impermeable cap to hydrothermal fluids, which were concentrated in the underlying volcanic flows. The similarity of the Pillar Lake and Inspiration sill magmatism to other magmas of the Midcontinent Rift, including the Nipigon sills and Osler volcanic rocks (Hollings et al., 2007), suggests that these rocks may represent a very early extrusive to subvolcanic phase of rift activity. The location of these rocks close to the ~1599 Ma Badwater gabbro, the ~1590 Ma Badwater syenite and the ~1540 Ma English Bay anorogenic granite, which have been interpreted as evidence for a long-lived crustal weakness (Hollings et al., 2004), may offer an explanation for how these magmas were erupted so early in the rift history.  Figure 28.   Photomicrographs of basaltic andesite (left) and Inspiration diabase (right), crossed nicols, FOV = 4 mm in both images, Stop 2-1. - 126 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 2-2: Pillar Lake Volcanic Rocks, T-Junction, Mattice Lake Road indicate that they are likely Proterozoic. The first sample …, a mafic volcanic flow, was collected in 2003 to directly determine the age of these volcanic rocks. A total of about 30 colourless irregular zircon grains were recovered, but none of these have the physical characteristics readily attributed to volcanic zircon (generally too large). Two single zircon grains were analyzed but did not yield consistent results (Heaman and Easton, 2006) and were not considered to provide a good constraint on the time of volcanism. Subsequent mapping in the McLaurin Lake area revealed the presence of a flat-lying stratigraphic succession consisting of a lower, pillowed mafic volcanic flow, a thin sandstone horizon …and an overlying sequence of mafic and andesitic flows …, all overlain by an Inspiration diabase sill ... Because of the well-preserved stratigraphic relationships, this section was sampled in detail for geochronology, to at least bracket the age of volcanism, even if it were not possible to directly obtain an age from the volcanic rocks themselves. UTM coordinates: NAD83; 16U 0345619E / 5564137N Several low-lying outcrops and large boulders cluster around a T-junction on the Mattice Lake Road. First discovered by MacDonald (2004), these Pillar Lake volcanic rocks display features indicative of submarine extrusion. Supporting evidence includes autoclastic breccias (flow/pillow breccia), hyaloclastite and pillow forms (Fig. 29). MacDonald (2004) described these rocks in this vicinity: An apparently flat-lying succession of mafic metavolcanic rocks crops out near the north end of Pillar Lake. The unit extends over an approximately 20 to 25 km2 area and consists of pillowed flow breccia, hyaloclastite breccia, pillowed and massive flows. Examination of outcrops from several locations would suggest that the apparently flat-lying unit has a thickness between 20 and 40 m. Lower portions consist of 3 or 4 alternating beds of pillowed flows with accompanying flow top breccia and or pillow breccia and hyaloclastite overlain by a 1 to 2 m thick layer of massive flow and/or sill. Locally, this unit displays slightly flattened 10 cm to 1 m diameter pillows with interflow hyaloclastite. Concentric jointing and tortoise shell-like cracks are common. Alteration of this unit typically consists of weakly to moderately pervasive to moderate patchy hematite alteration and local weak to moderate patchy sericite alteration. Alteration intensity increases with proximity to the north tip of Pillar Lake and along a creek leading between Mundell and Pillar lakes. The flat-lying and relatively undeformed nature of these flows, combined with the well-preserved nature of relatively delicate primary features such as hyaloclastite may suggest that these rocks are Proterozoic rather than Archean. A sample of fine-grained grey andesite from McLaurin Lake was collected to establish the age of the Pillar Lake volcanics in this region. There was no zircon or baddeleyite recovered from this sample; however, a modest amount of rutile and titanite was recovered. The multigrain fractions of rutile (1) and titanite (2) both have low uranium contents (8.3 and 7.6 ppm) and contrasting Th/U (0.222 and 27.818, respectively). The rutile fraction is concordant with a 206Pb/238U age of 1106.4 ± 2.8 Ma. The titanite fraction is also within error of concordia with a slightly North of Pillar Lake, drilling in 2004 showed that the Pillar Lake volcanic rocks (unconformably?) overlie the 1599 Ma Badwater gabbro. The ongoing uncertainty regarding the absolute age of the Pillar Lake volcanic rocks was summarized by Heaman et al. (2007): When first discovered in 2003, there was some question as to whether these volcanic rocks were Archean or Proterozoic in age, although their flat-lying character and alteration patterns   Figure 29. Autoclastic pillow breccia and hyaloclastite, Stop 2-2 - 127 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 older 207Pb/206Pb age of 1129.0 ± 4.6 Ma. The titanite age of 1129 Ma is interpreted to be the best constraint on the age of the McLaurin Lake andesite. It could be a minimum age or it could closely mark the age of volcanism. The 1106 Ma rutile age is similar to the age of the McLaurin Lake diabase (reported previously) and could reflect thermal resetting during emplacement of the diabase (rutile has a much lower closure temperature to Pb diffusion of ~400°C). Corporation in 2004 and 2008 indicated that gabbroic rocks underlie Pillar Lake volcanic rocks from at least west of Pillar Lake, east to McLaurin Lake (Middleton, 2004; Middleton and Bennett, 2008). Stop 2-3A: Badwater Gabbro, Badwater Creek Road At this locality, the gabbro is typically coarsegrained and unaltered. Anorthositic bands, variations in cumulus and intercumulus minerals, and igneous foliation are suggestive of igneous layering (Fig. 31). Thin section petrography cited by Middleton and Bennett (2008) for a gabbro drilled 2.3 km east of this location listed a modal mineralogy as approximately. Plagioclase (labradorite/bytownite) 55% Clinopyroxene (augite?) 25% UTM coordinates: NAD83; 16U 0347047E / 5564306N Biotite 10% Olivine (partly relict) 3% Stop 2-3B: Badwater Gabbro, East of Badwater Creek Road Talc/sericite, minor iddingsite (after olivine) 2% Amphibole (secondary, actinolitic) 2% UTM coordinates: NAD83; 16U 0347043E / 5564142N The Badwater gabbro is best-exposed north of Pillar Lake, where it is overlain by Pillar Lake volcanic rocks. It was dated by Heaman et al. (2007) at 1598.7 ± 1.1 Ma. It was intruded by the Badwater syenite at 1590.1 ± 0.8 Ma (Fig. 30). Drilling by East West Resource Opaque (magnetite?) 2% (pyrrhotite?) 1 % Clay? /sericite (after plagioclase) trace Plagioclase forms mainly euhedral crystals up to about 4 mm long, with random orientations, partly   Figure 30. Badwater gabbro xenoliths in feldspar-phyric syenite, eastern shore of Pillar Lake - 128 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 31. Badwater gabbro, displaying igneous foliation and anorthositic lens/layer, Stop 2-3B. enclosing the mafic minerals. Pale brownish green clinopyroxene forms somewhat irregular, mainly subhedral crystals up to 4.5 mm long. Some crystals are partly altered, mainly around the rims, to minor secondary amphibole. Biotite forms ragged, irregular subhedral crystals mostly <2.5 mm in diameter, commonly wrapped around pyroxene, or interstitial to pyroxene and plagioclase. Olivine (or relict olivine) displays somewhat rounded or subhedral outlines up to almost 3 mm in diameter, commonly contained within pyroxene crystals or aggregates. The olivine is generally strongly fractured, with traces of minute secondary magnetite along the fractures. In places, the olivine is partly to locally completely replaced or pseudomorphed by very fine-grained talc/sericite or minor red-brown to greenish-brown iddingsite. Accessory opaque minerals appear to be mostly magnetite forming skeletal to irregular subhedral crystals up to 2 mm in diameter, interstitial to plagioclase, or associated with biotite and olivine or relict olivine. Sulfides (mostly pyrrhotite) form irregular subhedra up to 0.5 mm long and are also commonly interstitial to plagioclase and pyroxene, and are associated with biotite (Middleton and Bennett, 2008). STOP 2-4: Pillar Lake Volcanics, Chimney Lake UTM coordinates: NAD83; 16U 0349738E / 5563345N This outcrop on the northern shore of Chimney Lake is enigmatic (Fig. 32). It appears to be a volcanic breccia, but with variable clast compositions and what appears to be clastic matrix material. It could represent a mass-flow resulting from topography created by volcanism or fault movements related to magma recharge, though the lack of reaction rims makes a lahar improbable. Could it possibly be a “diatreme”? We will discuss its genesis in the field. A ~1m thick interflow sandstone unit described by Magee (in progress), approximately 1.5 km north of Chimney Lake consisted of basal sandstone, lithic arenite, and an upper quartz grain matrix breccia with locally derived basalt clasts. 50 detrital zircons from the interflow sandstone (Heaman et al., 2007) yielded a youngest concordant zircon of 1514 Ma. Dominant zircon populations fall between: 2700 to 2300 - 129 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 32. Fragmental unit, Pillar Lake Volcanics, Chimney Lake, Stop 2-4 Ma; 1950 to 1900 Ma; 1880 to 1780 Ma. The basal sandstone geochemical signature is andesitic (Magee, in progress). STOP 2-5: Pillar Lake Volcanic Rocks, Highway 527 UTM coordinates: NAD83; 16U 0349460E / 5562320N This road cut on the eastern side of Highway 527 provides a ~3 m section through flat-lying basalt flows with small (10 to 30 cm) ellipsoidal features, suggestive of bun pillows. However, Smyk et al. (2011) suggested that these features may be pahoehoe toes in cross-section. The suggestion that these were subaerial flows is supported by the discovery of ropy flow top in a dislodged boulder in the ditch at the base of the exposure (Fig. 33). STOP 2-6: Nipigon Diabase Sill, Highway 527 UTM coordinates: NAD83; 16U 0342613E / 5499194N An 85 m long, 12 m high road cut provides an exceptional exposure of a Nipigon diabase sill at this locality (Fig. 34). The diabase is massive, homogeneous, fine- to medium-grained and locally feldspar-phyric. Irregular joint faces have been infilled with coarse, drusy quartz-chlorite-calcite-pectolite(?) + malachite veins. An orthogonal set of shallowly south-dipping and steeply north-dipping joints suggests that the sill may be shallowly south-dipping. The glacially polished surface on top of the road cut displays scattered pink, feldspathic (granophyric?) patches and pectolite(?)coated joint surfaces. STOP 2-7: Sibley Group Sedimentary Rocks and Nipigon Diabase Sills UTM coordinates: NAD83; 16U 0341205E / 5493814N Sibley Group sedimentary rocks are typically preserved in this area of the Nipigon Embayment below diabase sills which protect them from erosion. At this locality, straddling an overgrown logging road, a 3 to 4 m section of white-weathering, thinly bedded Rossport Formation dolomites are sandwiched between two thin Nipigon diabase sills (Fig. 35). As a result, the calcareous sedimentary rocks have been contact metamorphosed, resulting in the formation of metamorphic calc-silicates. Where Sibley Group rocks rich in carbonates, such as the Middlebrun Bay or Channel Island Members of the Rossport Formation, are proximal to thick sills the mineralogy consists of sodium- and potassium-rich varieties of pargasite, tremolite, talc, magnesium-rich clinoclore and calcite. - 130 - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Figure 33. Cross-section through thin basalt flows, showing possible pahoehoe toes (left); ropy flow surface preserved in dislodged boulder (above); Stop 2-5.     Figure 34. Inspiration diabase sill, showing shallowly south-dipping joints, Stop 2-6 - 131 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Pargasite is common near the large sills, but tremolite, then talc, become important with distance away from the heat source (Rogala et al., 2005). This indicates temperatures in the 600°C dropping to 400°C range. The outcrop we are visiting appears to have initially been largely composed of dolomite. This combined with relict bedding and possible stromatolitic structures that are still evident indicates that this may represent the Middlebrun Bay Member. On the other (western) side of the road, a flat-lying outcrop displays well-developed polygonal jointing (aka “tortoise-shell” texture) that are characteristic of the upper chilled contacts of these sills (Fig. 36). Feldspathic alteration along these joints results in their raised appearance. Numerous, recessively weathering ovoid pits may be the remnants of fluid- and volatilerich pockets which migrated to the top of the cooling sill. Acknowledgements The authors wish to acknowledge the contributions of many people who provided suggestions and assistance in the development of this field trip and guidebook. John Scott, recently retired from the Resident Geologist Program, OGS, Thunder Bay, was an invaluable asset in suggesting sites and providing information. Assistance in the field during the scouting of field trip sites was provided by John Scott, Dorothy Campbell and Robert Cundari (RGP-OGS, Thunder Bay). The authors have benefited from field work and discussions in the field with Carole Ann MacDonald and Tom Hart (both formerly with Precambrian Geoscience Section, OGS), Dr. Peter Hollings (Lakehead University), Angelique Magee (Carleton University/Geological Survey of Canada) and Robert Middleton (formerly East West Resource Corporation). References Barrett, T.J. and Fralick, P.W., 1989. Turbidites and iron formations, Beardmore-Geraldton, Ontario: Application of a combined ramp-fan modelto Archean chemical and clastic sedimentation. Sedimentology, 36: 221-234. Blackburn, C.E., Johns, G.W., Ayer, J. and Davis, D.W. 1991. Wabigoon Subprovince; in Geology of Ontario; Ontario Geological Survey, Special Volume 4. pt.1, pp.302-381. Breaks, F.W., Selway, J.B. and Tindle, A.G. 2003. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6099, 179p.   Figure 35. Buff-weathering metadolostone, Stop 2-7 Card, K.D. and Ciesielski, A., 1986. DNAG #1. Subdivisions of the Superior Province of the Canadian Shield. Geoscience Canada 13, 5–13. Cheadle, B.A. 1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 23: 527–542. Coates, M.E. 1972. Geology of the Black Sturgeon River area, District of Thunder Bay. Ontario Department of Mines and Northern Affairs, Geoscience Report 98, 41 p.   Figure 36. “Tortoise-shell” texture developed in polygonal joints at the chilled top of an Inspiration diabase sill, Stop 2-7 Devaney, J.R., 1987. Sedimentology and stratigraphy of the northern and central metasedimentary belts in the Beardmore-Geraldton area of Ontario. Unpub. M.Sc. thesis, Lakehead University, 227 pp. Eriksson, K.A., Krapez, B. And Fralick, P.W., 1994. - 132 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Sedimentology of Archean greenstone belts: signatures of tectonic evolution. Earth Science Reviews, 37, 1-88. for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44, 1021–1040. Eriksson, K.A., Krapez, B. And Fralick, P.W., 1997. Sedimentological Aspects. In, ed. M.J. DeWit and L.D. Ashwal, Greenstone Belts. Clarendon Press, Oxford, 33-54. Hart, T.R., and Magyarosi, Z. 2004. Precambrian Geology of the Northern Black Sturgeon River and Disraeli Lake Area, Northwestern Ontario. Ontario Geological Survey, Open File Report 6138, 56 p. Ernst, R.E., Buchan, K.L., Heaman, L.M., Hart, T.R., and Morgan, J. 2006. Multidisciplinary study of N to NNE trending dykes in the region west of the Nipigon Embayment: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release Data, MRD 194. Heaman, L.M., and Easton, R.M. 2006. Preliminary U/ Pb geochronology results Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release — Data 191, 78 p. Fralick, P.W., 2003. Geochemistry of clastic sedimentary rocks: ratio techniques. In, ed. D.R, Lentz, Geochemistry of sediments and Sedimentary Rocks. Geological Association of Canada. Geotext 4, 85103. Fralick, P., and Kissin, S.A. 1995.Mid-Proterozoic basin development in central North America: Implications of Sibley Group volcanism and sedimentation. In Proceedings, 1995 International Geological Correlation Program, Project 336, Petrology and metallogeny of volcanic and intrusive rocks of the Midcontinental Rift System. pp. 51–52. Fralick, P.W. and Kronberg, B.I., 1997. Geochemical discrimination of clastic sedimentary rock sources. Sedimentary Geology, 113: 111-124. Fralick, P.W. and Pufahl, P., 2006. Iron formation in Neoarchean deltaic successions and the microbially mediated deposition of transgressive system tracts. Journal of Sedimentary Research, 76: 1-10. Fralick, P.W., Purdon, R.H. and Davis, D.W., 2006. Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentological, geochemical and geochronological evidence. Canadian Journal of Earth Science, 43. Fralick, P.W., Wu, J. and Williams, H.R., 1992. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield. Canadian Journal of Earth Sciences, 29: 2551-2557. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K.1980. Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, 17: 633–651. Hart, T.R. 2000. Precambrian geology, Garden Lake Area. Ontario Geological Survey, Open File Report 6037, 82 p. Hart, T.R. 2005. Precambrian geology of the southern Black Sturgeon River and Seagull Lake area, Nipigon Embayment, northwestern Ontario. Ontario Geological Survey, Open File Report 6165, 63 p. Hart, T.R., MacDonald, C.A., 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario. Canadian Journal of Earth Sciences 44, 1055–1086. Heggie, G.J. 2005. Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull intrusion, northwestern Ontario. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 156 p. Hinchey, J.G., Hattori, K.H., and Lavigne, M.J. 2005. Geology, Petrology, and Controls on PGE mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada. Economic Geology, 100: 43–61. Hollings, P., Fralick, P., Kissin, S., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay Granite-Rhyolite complex, northwestern Ontario. Canadian Journal of Earth Sciences 41, 1329–1338. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences 44, 1087–1110. Kaye, L. 1969. Eayrs Lake-Starnes Lake area; Ontario Deprtment of Mines, Geological Report 77, 29p. Kehlenbeck, M.M. 1977. The Barnum Lake pluton, Thunder Bay, Ontario; Canadian Journal of Earth Sciences, v.14, p.2157-2167. Kennedy, M.C. 1984. The Quetico Fault in the Superior Province of the southern Canadian Shield; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario. Kuzmich, B. (in progress). Geochemistry and petrology of the Dog Lake Granite Chain, Quetico Basin; unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario. Kuzmich, B., Hollings, P., Scott, J.F. and Campbell, D.A. 2011. Geochemistry and petrology of the Dog Lake granite chain, Quetico Subprovince, Thunder Bay: a preliminary report; in Summary of Field Work and Other Activities 2011, Ontario Geological Survey, - 133 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Open File Report 6270, p.8-1 to 8-8. Lightfoot, P.C., Sutcliffe, R.H., and Doherty, W. 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: a trace element perspective. Journal of Geology, 99: 739–760. MacDonald, C.A. 2004. Precambrian geology of the south Armstrong-Gull Bay area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6136, 42p. MacDonald, C.A. and Tremblay, E. 2005. Lake Nipigon Region Geoscience Initiative, Bedrock Mapping Project: Geology of the northwest Nipigon Embayment; unpublished poster, Ontario Geological Survey. MacDonald, C.A., Tremblay, E. and Easton, R.M. 2005. Precambrian geology of the west-central map area, Nipigon Embayment, northwestern Ontario, Lake Nipigon Region Geoscience Initiative; Ontario Geological Survey, Open File Report 6164, 49p. Mackasey, W.O., Blackburn, C.E. and Trowell, N.F. 1974. A regional approach to the Wabigoon-Quetico belts and its bearing on exploration in northwestern Ontario; Ontario Division of Mines, Miscellaneous Paper 58, 29p MacTavish, A.D. 1999. The mafic-ultramafic intrusions of the Atikokan-Quetico area, northwestern Ontario; Ontario Geological Survey, Open File Report 5997, 154p. Magee, A. (in progress). Geology and geochemistry of the Pillar Lake volcanic sequence, northwestern Ontario; unpublished M.Sc. thesis, Carleton University, Ottawa ON. Middleton, R.S. 2004. Diamond drilling on Red Granite property, Pillar Lake sheet, Armstrong, Ontario; unpublished assessment file, Thunder Bay North District, Thunder Bay, 58p. Middleton, R.S. and Bennett, N. 2008. Drill report, Armstrong (Red Granite) property, Pillar Lake area, Thunder Bay Mining Division, Ontario; unpublished assessment file, Thunder Bay North District, Thunder Bay, 125p. Ontario Geological Survey 1993. Bedrock geology, seamless coverage of the province of Ontario. Ontario Geological Survey, Data Set 6 Ontario Geological Survey 2004. Ontario airborne geophysical surveys, magnetic and gamma-ray spectrometer data, grid and vector data, ASCII format, Lake Nipigon Embayment Area. Ontario Geological Survey, Geophysical Data Set 1047a. Percival, J.A., Helmstaedt, H., 2006. The Western Superior Province Lithoprobe and NATMAP transects: introduction and summary. Can. J. Earth Sci. 43, 743–747. Percival, J.A. and Sullivan, R.W. 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario; in Radiogenic and isotopic studies: Report 2, Geological Survey of Canada, Paper 88-2, pp.97107. Pettigrew, N.T. and Hattori, K.H. 2006. The Quetico intrusions of the western Superior Province:Neoarchean examples of Alaskan/Ural-type mafic-ultramafic intrusions; Precambrian Research, v.149, pp.21-42. Purdon, R.H. 1989. The Quetico fault zone northeast of Thunder Bay, Ontario: kinematic indicators of dextral motion; unpublished H.B.Sc. thesis, Lakehead University, Thunder Bay, Ontario. Richardson, A., Hollings P., and Franklin, J. 2005. Geochemistry and radiogenic isotope characteristics of the sills of the Nipigon Embayment: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Open File Report 6175, 88 p. Rogala, B. 2003. The Sibley Group: a lithostratigraphic, geochemical, and paleomagnetic study. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 254 p. Rogala, B., Fralick, P.W., Metsaranta, R., 2005. Stratigraphy and sedimentology of the Mesoproterozoic Sibley Group and related igneous intrusions, northwestern Ontario: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Open File Report 6174, 128 pp. Rogala, B., Fralick, P.W., Heaman, L.M., and Metsaranta, R. 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario. Canadian Journal of Earth Sciences, 44, 1131-1149. Schandl, E.S. 2004. Petrographic data from the northwest Nipigon Embayment. Lake Nipigon Geoscience Initiative, Ontario Geological Survey, Miscellaneous Release of Data, MRD-141. Seemayer, B.E. 1992. Variation in metamorphic grade in metapelites in transects across the Quetico subprovince north of Thunder Bay, Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 163p. Schandl, E.S. 2004. Petrographic data from the northwest Nipigon Embayment, Lake Nipigon Region Geoscience Initiative (LNRGI); Ontario Geological Survey, Miscellaneous Release-Data 141. Smyk, M.C. and Franklin, J.M. 2007. A synopsis of mineral deposits in the Archean and Proterozoic rocks of the Lake Nipigon Region, Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, v.44, no.8, p.113. Smyk, M. Hollings, P. and Cundari, R. 2011. The Pillar Lake volcanics: new insights into an enigmatic Mesoproterozoic volcanic suite near Armstrong, Ontario; Institute on Lake Superior Geology Proceedings, Part 1, Program and Abstracts, v. 57, p.75-76, Ashland Wisconsin. - 134 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stone, D., Lavigne, M.J., Schnieders, B., Scott, J., and Wagner, D. 2003. Project Unit 95-014. Regional Geology of the Lac des Iles Area. Ontario Geological Survey, Open File Report 6120, pp. 15–1 to 15–25. Stott, G.M. 2009. Superior Province: The nature and evolution of the Archean continental Lithosphere; Precambrian Research 168 (2009) 1–3. Stott, G.M., Corkery, T., Leclair, A., Boily, M., Percival, J.A., 2007. A revised terrane map for the Superior Province as interpreted from aeromagnetic data. In: Institute on Lake Superior Geology Proceedings, 53rd Annual Meeting, Lutsen, MN 53-1, pp.74–75 (Abstract). Sutcliffe, R.H. 1986. Proterozoic rift related igneous rocks at Lake Nipigon, Ontario. Unpublished Ph.D. thesis, The University of Western Ontario, London, Ontario, 325 p. Sutcliffe, R.H. 1987. Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96: 201– 211. Sutcliffe, R.H. 1991. Proterozoic geology of the Lake Superior area. In Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M. Stott. Ontario Geological Survey, Special Vol. 4, Part 1, pp. 405–484. Sutcliffe, R.H., and Greenwood, R.C. 1985a. Geological series, Precambrian geology, Lake Nipigon area, Kelvin Island sheet, District of Thunder Bay. Ontario Geological Survey, Preliminary Map P.2838, scale 1: 50 000. Tomlinson, K.Y., Davis, D.W., Percival, J.A., Hughes, D.J., Thurston, P.C., 2002. Mafic to felsic magmatism and crustal recycling in the Obonga Lake greenstone belt, western Superior Province: evidence from geochemistry, Nd isotopes and U–Pb geochronology. Precambrian Res. 114/3-4, 295–325. Tomlinson, K.Y., Stott, G.M., Percival, J.A. and Stone, D. 2004. Basement terrance correlations and crustal recycling in the western Superior Province: Nd isotopic character of granitoid and felsic volcanic rocks in the Wabigoon Subprovince, N. Ontario; Precambrian Research, v.132, pp.245-274. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario; Ontario Geological Survey, Special Volume 4. pt.1, pp.383-403. Williams, H.R. and Stott, G.M. 1991. Subprovince accretion in the southern Superior Province; Geological Association of Canada-Mineralogical Association of Canada-Society of Economic Geologists, Field trip guidebook, 26p. Zaleski, E., Van Breemen, O., and Peterson, V.L. 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa–Quetico subprovince boundary, Superior Province, Ontario, constrained by U–Pb zircon dates of supracrustal and plutonic rocks. Canadian Journal of Earth Sciences, 36: 945–966. - 135 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 9 - Rehabilitation of the Past-Producing Shebandowan and North Coldstream Mine Sites Mark Puumala Ministry of Northern Development and Mines, 435 James St. S., Suite B002, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This trip will provide an overview of rehabilitation measures that have been implemented at two pastproducing mines that are located in the Shebandowan greenstone belt west of the City of Thunder Bay (Fig. 1). The Shebandowan Mine operated between 1971 and 1998, producing 9.29 million tonnes of ore grading 1.75% nickel, 0.88% copper, 0.063% cobalt, 0.0533 oz/ ton platinum group elements and 0.0575 oz/ton silver (Inco, 2001). The North Coldstream Mine operated between 1957 and 1967, producing approximately 2.5 million tonnes grading 1.97% copper, 0.012 ounces per ton gold and 0.22 ounces per ton silver (Golder Associates, 2002). Both mines produced significant quantities of acidgenerating tailings during their operational lives and provide a good illustration of historic and current mining waste management practices for base metal mines, and the technologies that are employed to prevent and mitigate adverse water quality impacts. Mine Rehabilitation Regulatory Framework Mineral exploration, mine development and mine rehabilitation in the Province of Ontario are regulated under the Mining Act. Part VII of the Mining Act deals principally with the rehabilitation of mines and mining lands and was proclaimed in 1991 (with the most recent significant amendments occurring on June 30, 2000). Under Part VII, proponents of all advanced exploration projects and operating mines are required to file a certified Closure Plan including financial assurance to indicate the method, schedule and cost of all rehabilitation to be conducted on the site once Figure 1. Location map illustrating Shebandowan and North Coldstream Mine sites relative to the City of Thunder Bay. - 136 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 closure commences. 1996: Mine Closure Plan accepted by MNDM. Closure Plans are not mandatory for historic mines that closed before 1991. However, all proponents of mining lands are responsible to ensure that any historic mine hazards on their property are progressively rehabilitated to prescribed standards. The minimum standards for mine rehabilitation are prescribed under Ontario Regulation 240/00 – Mine Development and Closure under Part VII of the Act. The Shebandowan Mine, which operated until 1998, is an example of a mine site that is being rehabilitated under a Mine Closure Plan, while the North Coldstream Mine, which closed before 1991, is currently undergoing progressive rehabilitation. 1998: Shebandowan Mine permanently ceased operation and work began to implement the Mine Closure Plan. Work completed to date includes flooding of tailings basin, waste rock relocation to tailings pond, infrastructure demolition and removal, capping of mine openings, closure of two landfills, and revegetation of disturbed areas. The Ministry of Northern Development and Mines (MNDM) is the government agency that is responsible for the administration and enforcement of the Mining Act. Shebandowan Mine Exploration and Development History The following summary of the historical exploration and development of the Shebandowan Mine property is based on information compiled from the files of the MNDM Mines and Minerals Division office in Thunder Bay. 1913: Nickel-copper ore found at Discovery Point by prospector Julian Cross. 1936: International Nickel Company (Inco) purchased property. 1936-1965: Various surface exploration programs were carried out. 1966-1967: No.1 development shaft completed, underground diamond drilling. 1968: Inco announced decision to develop mine and supporting facilities at Shebandowan. No.2 (production) shaft and mill commissioned. 1969-1974: Forest debris created by road and on-site construction removed, topsoil stockpiled for later usage and contouring, and revegetation began (hydro-seeding of grass with oats/rye, straw mulch; 15 000 seedlings / trees). 1973: Mine and mill complex officially opened June 28. 1986-1988, 1992-1995: Operations temporarily suspended due to economic conditions. 2001: Mine Closure Plan Amendment filed with MNDM. 2003: Option / joint-venture agreement signed between Inco (now Vale) and North American Palladium (NAP) to explore former mine property and environs. 2008: Underground exploration, ramp/decline advanced to collect bulk sample of ore from Shebandowan West deposit. This work was done under a separate advanced exploration Closure Plan filed by NAP. Operations were suspended due to the fall 2008 economic downturn. 2012: Vale continues to be responsible for the implementation of the Shebandowan Mine Closure Plan, while NAP is responsible for the Shebandowan West site. Denison Environmental has been contracted by Vale to carry out on-going site maintenance and rehabilitation activities. Deposit Geology The Shebandowan nickel-copper deposit is hosted in a serpentinized peridotite sill that forms part of a mafic metavolcanic rock-dominated sequence (Morin, 1973; Osmani 1997). The ore body is also located near the southern margins of the quartz diorite Shebandowan Lake Stock. The northwest-trending Crayfish Creek Fault (a regional-scale dextral transcurrent structure) is located immediately south of the ore body. Rocks to the south of the fault form a separate domain consisting largely of intercalated felsic, intermediate and mafic metavolcanic rocks (Osmani, 1997). The Shebandowan Mine ore body included three styles of mineralization: massive, breccia, and stringer ore (Osmani, 1997). Massive ore consisted of pyrrhotite, pentlandite, chalcopyrite, pyrite and magnetite. Breccia ore was comprised of pyrrhotite, chalcopyrite and pentlandite, and contains fragments of peridotite, mafic metavolcanics and granitic rock. Stringer ore occurred as stringers of chalcopyrite, pyrite and minor pyrrhotite and pentlandite in shear zones. The average width of the ore body was approximately 7.5 m (Inco, 2001) and was mined over a strike length - 137 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 of approximately 3.5 km to a maximum depth of approximately 1000 m. Field trip stops Stop 1: Discovery Point UTM coordinates NAD 83; 15U 0701365E / 5386525N Nickel-copper ore was first discovered 99 years ago along the shoreline of Lower Shebandowan Lake at Discovery Point. The initial underground exploration shaft was sunk at this location by Inco during the 1960s. The shaft location is now marked by a vented reinforced concrete cap and is located near the west end of the ore body (workings extend approximately 500 m further to the west). The underground mine workings are mostly located below the lake, and extend a further 3 km to the east. Rehabilitated vent raises are located on an island Photo 1. Reinforced concrete cap, Shaft No. 1 - Discovery Point   that can be seen from the outcrop near the shoreline immediately east of the shaft, while the No. 2 production shaft was located on a point behind the island. The mining method used at Shebandowan was cut and fill Figure 2. Satellite image of Shebandowan Mine site showing field trip stop locations. - 138 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3. Shebandowan Mine longitudinal section from Closure Plan (Inco, 2001). Figure 4. Plan view showing area of underground workings (Inco, 2001). - 139 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 stoping, with the mine workings being backfilled with a cemented mixture of 60% tailings and 40% alluvial sand (30:1 ratio of backfill to Portland cement). The minimum crown pillar thickness beneath the lake is 33 metres and an engineering study has indicated that there are not likely to be any long-term rock stability issues. have the potential to generate acid, a lined ore pad was also constructed adjacent to the pond in order to ensure that any contaminated runoff was not discharged to the natural environment. As per the requirements of the Closure Plan (North American Palladium, 2008), all ore was shipped off site for processing before activities at the site were suspended. Stop 2: Shebandowan West Prospect Stop 3: No. 2 Shaft Area UTM coordinates NAD 83; 15U 0700710E/ 5386900N UTM coordinates NAD 83; 15U 0702850E / 5386235N The Shebandowan West prospect consists of three shallow Ni-Cu mineralized zones (West, Road and D Zones) located immediately west and along strike with the Shebandowan Mine ore body. North American Palladium (NAP) http://www.napalladium.com/ English/projects/reserves-and-resources/default.aspx reports measured and indicated resources of 1,292,000 tonnes grading 0.91% Ni, 0.62% Cu, 1.09 g/t Pd, 0.34 g/t Pt, and 0.23 g/t Au. During 2008, a ramp was advanced to collect a bulk sample for metallurgical testing. After the project was suspended, the portal and vent raise were backfilled to prevent inadvertent access to the underground workings. These are considered to be temporary measures that would need to be upgraded and certified if the proponent were to decide to permanently close-out the site. The clearing at this stop was previously the location of the Shebandowan Mine headframe and hoist structures. These were demolished and removed from the site in 2001. Similar to the No.1 shaft, the No. 2 shaft was subsequently capped with reinforced concrete. The Mine Rehabilitation Code of Ontario requires that disturbed areas of mine sites be revegetated to stabilize surface soils, improve aesthetics, establish sustainable vegetation growth and support the To the west of the portal, NAP constructed an engineered containment pond to collect and retain water pumped from the underground workings. The pond is lined with high density polyethylene (HDPE) to prevent seepage. Because the ore is considered to   Photo 2. Ore pad and containment pond at Shebandowan West project site  Photo 3. View of No. 2 Shaft area. Concrete caps and pump house are visible in centre of photo. - 140 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 designated end use of the site. The No. 2 shaft area was contoured and seeded after the completion of demolition activities. Vegetation growth has been successful, with some native species (e.g., trees) already beginning to colonize the area. The last remaining original building on the site is the former process water pumphouse located adjacent to the lake. This pumping equipment now serves as a source of water that is used during periods of low precipitation to ensure that the tailings pond remains saturated. Stop 4: Mill Area UTM coordinates NAD 83; 15U 0703230E / 5385400N All buildings associated with the former mine/mill complex were demolished and removed from the site in 2003. Similar to the headframe area, the mine/mill complex area was seeded, and vegetation growth has been occurring. Revegetation efforts in this area have also included the planting of trees. An acid-generating waste rock pile was previously located in the low-lying area at the southwest corner of the clearing. This waste rock was relocated to the tailings pond, and is now located under water. Residual groundwater quality impacts continue to be monitored in groundwater monitoring wells that have been installed to the south of the former waste rock area. A storm water collection pond is located at the east end of the mill area clearing. Water collected in this pond contains elevated concentrations of metals  Photo 5. Storm water collection pond. Pump barge can be seen in centre of pond. Water is pumped to tailings pond at Dam No. 4. (most notably nickel) leached from soils in the mill area. Groundwater in the vicinity of the pond is also impacted by residual petroleum hydrocarbons from a fuel spill that occurred in its vicinity during mine operations. Water from the storm pond continues to be pumped to the tailings pond for treatment. This will continue until the water quality meets regulatory requirements for direct discharge to the environment. Groundwater quality in the area downgradient of the storm pond also continues to be monitored for acid rock drainage (ARD) and petroleum hydrocarbon impacts. Groundwater monitoring must continue until geochemical stability has been demonstrated and there are no longer any significant risks of adverse impact to downgradient receiving water bodies. Stop 5: Tailings Dam No. 4 Seepage Collection Pond UTM coordinates NAD 83; 707780E, 5384550N   Photo 4. View of rehabilitated mill area The Shebandowan Mine tailings were deposited in a 115 ha impoundment located approximately 1.5 km southeast of the mine/mill complex. The tailings contain a significant proportion of sulphide minerals and are considered to be acid-generating. Data presented in the 1996 Closure Plan (Inco, 1996) indicated that unoxidized tailings contain up to 13.2% sulphur and have NP/AP (neutralization/acid generation potential) ratios of approximately 0.16. Values less than 1 indicate that a material is acid generating. As a result, the tailings impoundment closure design includes a permanent water cover to prevent sulphide oxidation and acid production. The tailings area is contained in a basin - 141 - �Proceedings of the 58th ILSG Annual Meeting - Part 2  Photo 6. Shebandowan Mine tailings pond. Dam No. 4 is located to right hand side of pond. Splitter dyke can be seen at centre of photo crossing pond. This dyke limits wave development in pond. that is bounded by natural topography (bedrock outcrop areas) and six engineered dam structures. Although the tailings dams are designed to retain water, some seepage occurs. Seepage that collects in a pond at the toe of Dam No. 4 contains elevated concentrations of iron and nickel. As a result, this seepage is pumped back into the tailings pond for treatment. Groundwater monitoring is carried out downgradient of all of the tailings dams to monitor the groundwater quality impacts resulting from seepage. Although there are elevated concentrations of iron, manganese and sulphate in these seepage plumes, there is no evidence of acidic drainage (Wesa, 2009).  Photo 8. Tailings pond spillway Discharges from the tailings basin to the natural environment are regulated by the Ontario Ministry of the Environment under an Industrial Sewage Works approval. Since mine closure, the water quality in the tailings pond has improved to the point where no active treatment is required to meet the applicable effluent limits. However, monitoring will continue to be required until water quality meets the more stringent Provincial Water Quality Objectives. Stop 6: Tailings Pond Spillway UTM coordinates NAD 83; 0706315E / 5384850N Discharges from the tailings pond occur through a spillway located at the east end of the impoundment. The spillway is excavated through solid bedrock near the north abutment of Dam No. 2. Discharge from the tailings pond is intermittent, and only occurs following times of significant precipitation or snow melt. When discharge is occurring, water quality monitoring must be carried out to demonstrate that the water quality meets effluent limits. The receiving water body is Gold Creek, which is part of the Matawin River watershed, which ultimately flows to Lake Superior. North Coldstream Mine Exploration and Development History   Photo 7. Dam No. 4 seepage collection pond and pump house The following summary of the historical exploration and development of the North Coldstream Mine property is based on information compiled from the files of the MNDM Mines and Minerals Division office in Thunder Bay. - 142 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 corporate amalgamation. 1870s: Copper mineralization discovered. 1951-1957: Coldstream Copper Mines Ltd. carried out exploration and development program. 1957: Production commenced. 1958: Operations suspended. 1959: Company re-organized and name changed to North Coldstream Mines Ltd. 1960-1967: Mine produced approximately 2.5 million tons of ore grading 1.97% copper, 0.012 ounces per ton gold and 0.22 ounces per ton silver. 1968: Mill and associated infrastructure and surface rights sold to Nelson Machinery. 1971: North Coldstream Mines changes name to Coldstream Mines. 1976: Coldstream goes into receivership. 1991: Nelson Machinery placed into receivership. 1992-1995: MNDM ordered Nelson Machinery to submit a Closure Plan for mill site infrastructure, and Conwest to submit Closure Plan for tailings, mill yard. Subject to appeals, Mining Commissioner ruled that Nelson was responsible for rehabilitation of mill area, Conwest responsible for tailings and mine openings. 1996: Conwest acquired by Alberta Energy Company, re-named AEC West. Mineral rights later sold subject to an agreement that AEC West would retain responsibility for rehabilitation of tailings and mine workings. AEC West was subsequently re-named EnCana West and EWL Management (current corporate identity). 1998-2000: Tailings areas rehabilitated. 1977: International Mogul acquired mineral rights. 1982: Conwest becomes mineral rights owner through 2000: MNDM rehabilitated mill site Abandoned Mines Rehabilitation Fund. Figure 5. North Coldstream Mine field trip stop locations. - 143 - through �Proceedings of the 58th ILSG Annual Meeting - Part 2 2000-2002: Several mine openings to surface rehabilitated, fencing erected around open stopes. 2008-2012: Crown pillar stability investigations, additional tailings rehabilitation activities, work towards development of long-term monitoring program. Deposit Geology The North Coldstream deposit is located within an inferred s-folded metavolcanic and gabbroic rock sequence (Osmani, 1997). The gabbro intrudes along the contact between felsic and mafic metavolcanic rocks. The northeast-trending Burchell Lake Fault is located immediately west of the site. The North Coldstream Mine ore body is a 120 x 300 m silicified zone located at the contact between the gabbro and mafic metavolcanic rocks. Osmani (1997) has interpreted the mineralized zone as silicified gabbro. The ore zone consists of a high density network of chalcopyrite and pyrite veinlets, and massive and disseminated mineralization within a siliceous host rock that resembles chert. Field trip stops Stop 7: TMA-1 Tailings Area   Photo 10. TMA-1 tailings area in 1991. chalcopyrite and roughly similar quantities of pyrite. Tailings sampling carried out by CANMET in 1994 indicated that the average sulphur content was 5.7%, and that they are acid generating (Burns et al., 1999). The majority of TMA-1 is located over permeable soil (sand and gravel) with a deep water table. This hydrogeologic setting has resulted in the development of a significant ARD plume in the groundwater immediately below and downgradient of the tailings. This contaminant plume has low pH and contains extremely high concentrations of sulphate and metals (e.g., Fe, Cu, Co, Mn, and Ni). Tailings Management Area 1 (TMA-1) was used for the deposition of North Coldstream Mine tailings until 1962 (Burns et al., 1999) and was the largest of two tailings deposition areas that were used during mine operations. The ore contained approximately 2 to 8% Groundwater from TMA-1 migrates in a westerly direction toward the Wawiag River, which is known to be a location of groundwater discharge from the overburden aquifer. The overburden aquifer is located in a west-trending bedrock depression that controls the ARD plume flow direction. The core of the ARD plume sinks toward the bottom of the aquifer between TMA-1 and the river. Prior to reaching the river, the   Photo 11. Embankment on west side of TMA-1 in 1998. UTM coordinates NAD 83; 0678515E, 5386325N Photo 9. Aerial view of mine/mill complex and southwest end of TMA-1 tailings area in 1991 - 144   - �Proceedings of the 58th ILSG Annual Meeting - Part 2   Wawiag   R.   Groundwater   flow   TMA -­ 1   Figure 6. Approximate groundwater flow path from TMA-1. deepest portions of the plume become confined below a silt confining layer. As a result, the most severely impacted groundwater does not discharge to the river. Nevertheless, measurable water quality impacts attributable to TMA-1 do occur in the Wawiag River (i.e., elevated levels of Fe and Co), especially during periods of low flow. Deep ARD-impacted groundwater beneath the confining layer changes flow direction and migrates in a southwest direction through another overburden-filled bedrock depression that parallels the Wawiag River toward Burchell Lake. The ultimate location where the deep ARD plume is believed to discharge to Burchell Lake is approximately 1.5 km offshore at a depth of 40 to 70 m (Golder Associates, 2011). To date, no significant impacts to the lake have been documented as a result of the TMA-1 tailings plume. In 1998-1999, a vegetated low permeability cover with a capillary break was placed over the TMA-1 tailings in an effort to reduce groundwater quality impacts. This work has resulted in reduced concentrations of sulphate and metals in the tailings impact plume. However, portions of the plume remain acidic and it is expected to take decades for the acid rock drainage plume to be fully rehabilitated. Between 2007 and 2010, EWL Management carried out several environmental and geochemical investigations to better characterize site conditions. Some of the key findings of this work are listed below. • Shallow groundwater in the northern half of TMA1 has neutral pH and lower sulphate and metal concentrations than in the southern half. As a result, the northern portion of the TMA-1 cover appears to be functioning as expected. • September 2010 data for MW38B: pH = 7.4, sulphate = 1440 mg/L, Fe = 54 mg/L, Mn = 2.3 mg/L, Cu = <0.01 mg/L, Co = 0.0005 mg/L (Golder Associates 2010). • Acidic groundwater continues to be present in the southern half of TMA-1, indicating that this portion of the cover was not performing as expected. Photo 12. Vegetated TMA-1 cover as it appeared in the  summer of 2005. - 145 - • September 2010 data for MW35B: pH = 3.3, sulphate = 3390 mg/L, Fe = 1000 mg/L, Mn = 11 mg/L, Cu = 3.6 mg/L, Co = 4.7 mg/L (Golder �Proceedings of the 58th ILSG Annual Meeting - Part 2  Photo 13. Photograph taken in October 2011 during placement of cover over relocated tailings at TMA-1. Clay layer is visible to left, with overlying granular cover layer to right Associates 2010) • Significant aquifer recharge was occurring at the southeast end of TMA-1 immediately following major rainfall events. This is likely to have been responsible for the continued ARD generation in the south half of TMA-1. • Storm drainage in the eastern perimeter spillway was continuing to show signs of acid generation. • Two previously unidentified “orphan” tailings areas were found north and west of TMA-1. These were most likely related to mobilization from TMA-1 during historic storm events. • Two small tailings areas located to the south of TMA-1 were identified as ongoing sources of ARD that required additional rehabilitation. Photo 14. Reconstructed eastern perimeter spillway. East   side of tailings relocation area can be seen to left. During 2011, additional rehabilitation work was done on the site to address the on-going ARD issues. This work included the relocation of the orphan and southern tailings areas to TMA-1 and the reconstruction of the eastern perimeter spillway. The relocated tailings were placed over the eastern half of TMA-1 during the winter of 2011, and a low permeability clay cover was placed over them during the summer and fall. The eastern perimeter spillway was also reconstructed in 2011 to more effectively convey storm drainage around TMA-1. A key design element was the installation of a geosynthetic clay layer on the western side of the spillway to isolate drainage from the TMA-1 tailings. It is expected that this additional rehabilitation work will significantly improve storm drainage quality, reduce infiltration at the southeast end of TMA-1, and reduce contaminant loadings to the overburden aquifer. Stop 8: TMA-2 Tailings Area UTM coordinates NAD 83; 0679185E / 5386600N From 1962 to the end of the mine life in 1967, approximately 500,000 tonnes of North Coldstream Mine tailings were deposited in Halet Lake, which is now known as TMA-2 (Burns et al., 1999). Prior to 1998, a 3 ha tailings beach was located at the former tailings discharge location at the southwest end of the lake. These tailings were generating acid and contributing metal loadings to the tailings pond and downstream receiving water bodies. The TMA-2 outlet drains north to Background Lake, which subsequently drains toward the Wawiag River and Burchell Lake. During the summer of 1998, the majority of the TMA-2 tailings beach was relocated below water, with approximately 4,000 tonnes relocated to TMA-1 (Burns et al., 1999). The goal of the tailings relocation was to completely submerge the tailings and prevent further oxidation and acid generation. Suspended sediment, acidity and metals were released into the TMA-2 pond during tailings relocation. A temporary dam was constructed to prevent downstream discharge, and the pond was limed to reduce acidity and metal concentrations. Since completion of the tailings rehabilitation, water quality has stabilized, with neutral pH and low metal concentrations in the TMA-2 discharge (although copper and cobalt concentrations do slightly exceed Provincial Water Quality Objectives). Following relocation, some of the tailings along the west margin of TMA-2 remain less than 2 m below the water surface. Tailings in the central basin are much - 146 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 during 2008 due to the construction of a beaver dam on the outlet stream that regulated the water level and kept it well above the tailings. During the winter of 2012, EWL Management planned to construct an engineered water level control structure on the outlet stream in order to permanently maintain higher water levels in TMA-2. A long-term water quality monitoring program will be carried out to monitor the success of the tailings rehabilitation efforts at the North Coldstream Mine. This monitoring program will include the sampling of surface water and groundwater monitoring wells.  Photo 15. View of TMA-2 (Halet Lake) from former tailings beach area. farther below surface, ranging in depth from 7 to 12 m (Golder Associates, 2011). Because some tailings are relatively close to surface, it is important that the water elevation is maintained at a level that maintains permanent saturation. Prior to 2008, water levels occasionally dropped during dry years to expose some of the tailings. However, water levels rose substantially Stop 9: Mine/Mill Area UTM coordinates NAD 83; 0678120E / 5386040N All surface structures on the mine/mill site were demolished and removed in 2000. By this time, the buildings had deteriorated to the point where they had become significant safety hazards. This work was performed by the Ministry of Northern Development and Mines utilizing the Abandoned Mines Fund. Since the completion of the building demolition Figure 7. Map of North Coldstream mine/mill area showing locations of mine openings to surface. The locations of the former buildings are also shown. - 147 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 investigation and rehabilitation plan pillar areas. It is additional fencing program. is currently developing a final for all potentially unstable crown likely that this plan will include and a rock stability monitoring Stop 10: Burchell Lake “Ghost Town” UTM coordinates NAD 83; 0677550E / 5386090N  Photo 16. Glory Hole project, EWL Management and its predecessor companies have been working toward completing the rehabilitation of the mine openings to surface and the underground mine workings. By 2002, most of the shafts and raises had been permanently rehabilitated (Golder Associates, 2002). Certified reinforced concrete caps were constructed over the Nos. 3 and 4 shafts, and the 250 and 257 vent raises. The No. 2 shaft was backfilled to surface and its long-term stability has been certified. Areas with open stopes and/or unstable crown pillars have been fenced. These include areas near the No. 1 shaft, and above the 2-4-49 W and 2-449 E stopes. An open stope (known as the Glory Hole) is present in the fenced area around the 2-4-49 E stope. This is a general interest stop to view the remains of the former town site of Burchell Lake. Although a number of larger buildings were removed following mine closure, many abandoned houses remain. Mine management homes were located in a separate development located further to the south. These buildings continue to be used as seasonal cottages. EWL Management has carried out a crown pillar  Photo 18. Abandoned houses in former town site of Burchell Lake. References Burns, R.C., Orava, D.A., Zurowski, M. and Mellow, R.J., 1999. A case study of the rehabilitation of sulphide tailings at the Coldstream mine tailings management area no. 2: in Proceedings of Sudbury ’99 Mining and the Environment Conference, p. 301-308. Inco Limited Ontario Division, 2001. Shebandowan mine closure plan part I of II: unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 84p.   Photo 17. No. 4 Shaft and 250 Vent Raise are located below fenced vent pipes. Fencing was installed to protect against vandalism. Inco Limited Ontario Division, 1996. Shebandowan mine closure plan part I of II: unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 120p. - 148 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Golder Associates Limited, 2011. Coldstream mine site 2010 surface water and groundwater monitoring report; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 51p. Golder Associates Limited, 2002. Closure report, Coldstream mine openings, Burchell Lake area, Northwestern Ontario; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 32p. Morin, J.A., 1973. Geology of the Lower Shebandowan Lake area, District of Thunder Bay; Ontario Division of Mines, Geological Report 110, 45 p. North American Palladium Limited, 2008. Shebandowan West advanced exploration project closure plan; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 62p. Osmani, I.A., 1997. Geology and mineral potential Greenwater Lake area, west-central Shebandowan greenstone belt; Ontario Geological Survey, Geological Report 296, 135 p. Wesa Incorporated, 2009. Groundwater characterization assessment of potential impacts to existing water supply wells, Shebandowan mine; unpublished report, Ministry of Northern Development and Mines Thunder Bay Mines and Minerals Division office, 60p. - 149 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 10 - Geoarchaeology of the Thunder Bay area Brian Phillips Department of Geography (Emeritus), Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Scott Hamilton Department of Anthropology, Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Bill Ross Ross and Associates/Department of Anthropology, Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Pat Julig Department of Anthropology, Laurentian University, Sudbury, Ontario Joe Stewart Department of Anthropology (Emeritus), Lakehead University, 955 Oliver Road, Thunder Bay, P7B5E1 Objectives The field trip focuses on the deglaciation and lake level history of Thunder Bay and the immediately surrounding area (Fig. 1). In particular, we will examine evidence of Palaeo-Indian occupation along abandoned shorelines, river mouths and deltas of the Lake Minong stage of Lake Superior (circa 9.5 ka BP.). The trip includes a visit to the former mouth of the Current River at the north end of the city, where the Simmonds and McDaid sites are located. We then travel east along Highway 11/17 to view several PalaeoIndian sites currently undergoing salvage excavation along the path of highway development. The tour then returns to Thunder Bay to visit the Cummins site, and also the nearby Neebing R. sites. Next we visit the Rosslyn delta, on the Kaministquia River, the western extent of Lake Minong. From there we will travel west to Kakabeka Falls, where earlier Lake Beaver Bay features will be examined. Finally, the trip will 1 2 3 9 4 5 6 7 8 10 11 15 14 12 13 N 1 2 3 4 5 7 km Figure 1 Thunder Bay area orientation map. - 150 - Dog Lake Moraine Mackenzie Moraine Intola Moraine Marks Moraine Brule Moraine 6 Pass Lake site cluster 7 Mackenzie site cluster 8 Hodder Ave Cluster 9 Current River Cluster 10 McIntrye River Cluster 11 Cummins/Neebing Cluster 12 Breukelman-Evergreen Cluster 13 Breukelman Farm Cluster 14 Drezecky-Pawlick Sites 15 Crane Site Cache. �Proceedings of the 58th ILSG Annual Meeting - Part 2 Cummins Mackenzie Brohm Figure 2 The spatial relationship of probable Plano and Archaic sites with Lake Minong shorelines (dashed line) and exposures of tool stone deriving from Gunflint Formation (hatchured lines). After Hinshelwood (2004:234). mount the Marks moraine, providing a view that will place the day’s observations in context with pre and post Marquette ice marginal events and Palaeo-Indian presence. Introduction In the Thunder Bay area there is a strong, though not exclusive, relationship between Palaeo-Indian habitation, bedrock exposures of the favoured tool stone within the Gunflint Formation, and the abandoned shores of post-glacial lakes of the Superior basin (Fig. 2). Of particular importance are shores of Lake Minong, established about 9.5 ka B.P. This spatial relationship is perhaps illusory since it is clear that these people occupied a number of habitats, some far from ancient lakeshores, but in geomorphological mapping of these shorelines (often in the context of urban development), it is the lakeshore sites that have been most commonly found and reported. Evidence accumulated since MacNeish’s (1952) excavations at the Brohm Site (Pass Lake) on the Sibley Peninsula (Fig. 2) by archaeologists (Fox, 1975, 1980; Dawson, 1983b; Julig, 1984; Ross, 1997; Hinshelwood 2004) suggest that Palaeo-Indians migrated northeast from the Dakotas and Minnesota, and into the culde-sac formed between Lake Agassiz, the lakes of the Superior basin, and the retreating margin of the Laurentide ice sheet (Fig. 3). Lake Agassiz covered most of Manitoba and parts of Northwestern Ontario at its maximum extent, and contributed significantly to the complex hydrological sequence affecting the Lake Superior basin (Fig. 4). As such Lake Agassiz played an important role in the initial peopling of the greater part of northwestern Ontario since its spatial expanse shifted north over time with glacial retreat, and conditioning when land would have been available for northward human occupation (Fig. 5). While some fluted projectile points (i.e. early Palaeo-Indian Clovis; Fig. 6) have been encountered as far north as central and perhaps northern Minnesota, none have been found yet in northwestern Ontario. Perhaps Clovis technology disappeared before the ice had retreated from the region, or that insufficient research has been done in the rugged uplands of the cul-de-sac that was first deglaciated (and therefore the most likely zone where such finds might be made). - 151 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3 Proposed late Palaeo-Indian migration into the cul-de-sac formed between Lakes Agassiz and Minong and the Laurentide Ice Sheet (after Hamilton and Ross, 1997). In any case, the Thunder Bay area has yielded unfluted lanceolate projectile points, probably deriving from several late Palaeo-Indian cultures (often collectively referred to as Plano) (Fig. 6). These represent hunting groups who pursued game throughout the cul-de-sac at some time after the Marquette readvance (ca. 9,900 to 9,500 y BP) (Fig. 3). While a range of projectile point types have been recovered, Ross (1997) has proposed that these sites contribute to the “Interlakes Composite”- consisting of a series of inter-related local populations who jointly utilized the deglaciated landscape at some point after ca. 9,500 years ago. This material culture is characterized in part by paralleloblique flaked projectile points (Fig. 6a), and heavy use of siliceous stone deriving from the Gunflint Formation (i.e., Jasper Taconite, Gunflint Silica) that outcrops in the Thunder Bay area (Fig. 2). Other important raw materials include Knife Lake Siltstone, deriving from bedrock sources near the Minnesota/Ontario border, and a sparse array of non-local materials (including Hixton Silicified Sandstone from central Wisconsin and perhaps also Chalcedony from western North Dakota). Local Palaeo-Indian sites exhibit a strong preference for bedrock lithic sources, with only a minor percentage Figure 4 Repeated spills of water from Lake Agassiz into Lake Minong (Hamilton 1996:30 after Teller and Thorleifson 1983). - 152 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5 Northeastward retreat of the Laurentide glacier, with related northward shift of Lake Agassiz waters. A sparse array of early and middle Holocene archaeological sites suggest the time-transgressive northward migration of human populations (Hamilton nd). of the raw materials suggesting use of cobble and pebble sources. This forms a sharp contrast to other cultures in the region dating to the Middle and Late Holocene. This resulted in some repeatedly used archaeological sites where exposures of suitable stone coincide with the ancient beaches of Lake Minong. The famous Cummins Site represents one such quarry/ workshop that has yielded thousands of discarded flakes, blocks, preforms and other debris from tool/ preform fabrication, but with a very low relative frequency of formal or informal tools. Other sites, that likely served as short-term camps, seasonal aggregation places, hunting/ambush sites, observation points, and other functions also dot the landscape. While they also yield much discarded stone, a somewhat higher relative frequency of lost, broken or discarded tools are recovered, suggesting more generalized site functions. While the shorelines were not the only areas utilized on the early Holocene landscape, it is clear that they were important (likely used for seasonal ingathering), no doubt because of the spatial convergence of valued resources. Some of this site function variability is addressed at the various sites visited during the tour. Deglaciation History The broad details of the deglaciation of the Superior basin are shown in Figure 7. As ice of the Rainy River and Superior lobes withdrew from central Minnesota, a series of recessional moraines were left in place. The Vermilion moraine, trending northwest to southeast, across northeastern Minnesota, was followed by - 153 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 1999). Unfortunately these surface finds have not yet been subjected to absolute dating. A major event in the deglaciation history of the Lake Superior basin was the Marquette readvance (9.9 ka B.P.), during which the basin and its margins were briefly reoccupied by ice (Fig. 8b). Recessional moraines northeast of the Brule moraine were destroyed and more recent features (i.e., Marks Moraine, etc) formed in their place. This event also has considerable archaeological implications, since any evidence of occupation in the path of the Marquette readvance was likely buried or destroyed by the new ice cover. Hudson Bay ice pushed southwestward into the Lake Superior basin, to halt on its southern shore (Drexler, Farrand and Hughes, 1983). Only in a small portion of Whitefish Bay near Sault Ste. Marie, in Figure 6 Pettipas’ (2011) now-obsolete Lake Agassiz temporal sequence with proposed relationship to the PaleoIndian cultural historical sequence. He revised the lake sequence in light of new data published by Leverington and Teller (2003) and Fisher (2005, 2008). We include it here because it offers a sense of archaeological conventional wisdom regarding the cultural sequence. This will soon be updated in light of ongoing research conducted at the Mackenzie I Site that has yielded a very large collection of projectile points. This will allow development of a more regionally relevant typology. the Steep Rock moraine on the Canadian side of the border, and the Brule moraine (Fig. 7). Guided by these ice marginal positions, Lake Agassiz found an early eastern outlet through the Arrow/Whitefish lakes corridor (circa 11 ka B.P.) and, shortly after, through the Shebandowan lake corridor, ultimately using the Lake Nipigon spillways around 10.4 ka B.P. (Fig. 8a) to enter Early Lake Minong (Teller, 1985) which occupied the Superior basin. There is no reason to believe that warming climate and biological regeneration upon the uplands unaffected by the Marquette readvance would not have enabled immigration of animal herds and people into the Thunder Bay region at this time. Only tenuous evidence of such early occupation has been reported (Phillips, 1993: Ross, 1994), though recent work in the Arrow/Whitefish corridor has confirmed a number of sites that may represent an earlier Palaeo-Indian presence (McLeod and Phillips, pers. communication, Figure 6a A sketch and photo of lanceolate projectile point style from the Brohm Site. Interlakes Composite sites generally yield a very small assemblage of projectile points reminiscent of a range of late Palaeo-Indian (unfluted) styles. However the Brohm and Mackenzie I Sites have yielded a high percentage of points with lanceolate blade form, basal indentation through repeated flaking, and edge grinding along the lower lateral portions of the blade. Particularly notable is the pattern of oblique parallel flaking that seems to be an important feature of the knapping strategy that appears unrelated to the functional utility of the tools. - 154 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 7 Map of Quetico-Nipigon area showing moraines and direction of ice movement during various phases (from Zoltai, 1965a). the southeast corner of the basin, did Early Lake Minong remain an open lake (Farrand and Drexler, 1985). The Marquette lobe pushed westward against the steep Minnesota shore and, where less obstructed, flowed northwest up the Kaministiquia valley to the Marks moraine (Figs. 1, 7, 8). This prominent ridge, rising to over 470 m (1550’) in places, curves from Lappe through Mokomon and around the northwest of Kakabeka towards the Pigeon River (Zoltai, 1963, 1965a, 1965b; Burwasser, 1977, 1980; Burwasser and Ferguson, 1980). Contemporaneously, to the west of the Lake Superior basin, the Patricia ice lobe pushed towards the Rainy River district and halted to deposit the Dog Lake moraine. This runs northwest from Lappe and holds up Hazlewood, One Island, Hawkeye and Dog lakes. East of Lappe, where the two moraines meet, a line of glacial debris known as the Mackenzie interlobate moraine can be traced through Pearl and on to the Black Bay peninsula (Figs. 1, 8). A glacial lake, Lake Kaministiquia, was formed between the two ice margins (Teller, 1985) and, at Lappe, a huge sand and gravel delta was built by sediment pouring off the interlobate moraine. This scenario is still the subject of debate, however, and there is some field evidence that the Marks and Dog Lake moraines may be older features that were simply reoccupied by ice of the Marquette advance (Julig, McAndrews and Mahaney, 1990; Tickle, 1996; Noble, pers.comm.1999). Lake Agassiz, deprived of its eastern outlets by the advance of the ice, expanded in area and depth (the Emerson phase), and flooded catastrophically through the Clearwater and Athabasca valleys in Saskatchewan - 155 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 8 The retreat of the glaciers in the Superior Region between 10,400 and 9,500 years ago (from Philips, 1993:95). and Alberta into the Mackenzie River and the Arctic Ocean (Smith and Fisher, 1993). The Marquette readvance obliterated evidence of the earlier phase of Lake Superior’s shoreline history in most of the basin. Shoreline sites that prehistoric people might have occupied in the area of Thunder Bay before 9.9 ka B.P. may lie buried beneath Marquette deposits. In the four hundred years between 9.9 and 9.5 ka B.P., a period about which much more is known, ice withdrew from the Lake Superior basin (Figure 8c/8d). Then, again, Palaeo-Indian people followed game up the Interlakes corridor and into the Thunder Bay region, this time to settle, in part at least, on the shores of Lake Beaver Bay (Stuart, 1993) and Lake Minong (Phillips, 1988). Shoreline History At the peak of Wisconsinan glaciation, the northeastern margin of the Lake Superior basin was depressed by the weight of ice (isostatic depression) to a greater degree than the less heavily ice-loaded southwestern side. As a result, “rebound” (isostatic recovery) since that time has been greater on the north shore of the Lake Superior basin than on the south shore. Lake shorelines which had been originally horizontal became progressively tilted along an approximate northeast axis, such that a shoreline of the same chronological age increases in altitude from southwest to northeast along the western shore of Lake Superior. A theoretical archaeological site at Grand Marais, Minnesota, found at 184 metres, just above the present storm beach, will be of similar age to another theoretical site in Terrace Bay, Ontario, at the 300 metre contour (117 metres above the present lake), with both sites lying on the same tilted shoreline (Fig. 9). As the Marquette ice lobe wasted back, the eastern and western sides of the basin were exposed as separate entities (Fig. 8c). On the eastern side, a fairly stable Lake Minong, controlled by the height of the St. Mary’s River outlet at Sault Ste. Marie, extended northwards up the Ontario shore and westwards along the Michigan shore as they were exposed by melting ice (Farrand and Drexler, 1985). In the enclosed western end of the basin, a distinct succession of ever larger but progressively lower level lakes formed (Lakes Duluth, Highbridge, - 156 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 9 Shoreline diagram for the Pigeon River/Thunder Bay area (Farrand, 1960). Moquah, Washburn, Manitou, and Beaver Bay), each extending further northeast up the Minnesota - Ontario shore and east along the Wisconsin shore (Farrand and Drexler, 1985). Along the land-based margins of these water bodies were formed various coastal features, such as bluffs and beaches, by which the shorelines can be traced today. As ice vacated the basin, the eastern and western lakes were united and the single shoreline of Lake Minong was formed around the basin about 9.5 ka B.P. (Figure 8d). For the next 1,500 years, waterlevels in the Lake Superior basin declined as the St. Mary’s sill was eroded down to bedrock. A staircase of Post-Minong shorelines were formed, the last and lowest of which was Lake Houghton, about 8.0 ka B.P. By 8.0 ka B.P., due to isostatic uplift, the rising levels of Lake Huron had flooded into the St. Mary’s River and reversed the flow (Larsen, 1987). This backflooding led to slowly rising water-levels in the Lake Superior basin and culminated in the Nipissing lake stage around 5,000 years B.P. Known as the preNipissing transgression, this rise in water-level was imposed upon a still-tilting basin. On the north shore, east of Dorion, the rate of isostatic uplift remained more rapid than the rising waters of the pre-Nipissing period, with the result being that the shoreline marking the Nipissing maximum level lies at a lower altitude than all older shorelines, including that of Lake Houghton. On the south shore the pre-Nipissing transgression was more rapid than isostatic recovery, and wave action “inherited” the features of older shorelines, modifying and destroying portions of them, including pre-existing shoreline archaeological sites (Phillips, 1977). This causes a chronological discontinuity which is at its greatest near Duluth and decreases towards Dorion where the Houghton shoreline appears above the present waterlevel (Fig. 10). In Thunder Bay, evidence of Palaeo-Indian activities would normally have been traced as a continuum from the high Minong shoreline to the Houghton (which lies just below present water-level), but the pre-Nipissing transgression reoccupied the lower and later part of that record and cut a prominent bluff on which the General Hospital, the Court House, St. Joseph’s Hospital, and Lakehead University are built. The Nipissing shoreline can be traced up the valley towards Mapleward road and across towards Mount McKay. Thus, the later part of the Palaeo-Indian record, the important transition into the Archaic period, and a good portion of the - 157 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 10 The pre-Nipissing transgression and the resulting loss of potential archaeological sites along the shores of Lake Superior (Phillips, 1993:100). early Archaic is missing in Thunder Bay. Hinshelwood (2004) offers the observation that these shifting lake levels into the mid-Holocene suggests that some of the much older Plano lithic quarry and workshop sites might have been re-occupied during Archaic times. Several sites that are thought to be of Archaic age have also been encountered along the pre-Nipissing transgression strandline were it cuts through Lakehead University and adjacent properties. Local History Figure 11 represents an interpretation of the detailed history of the withdrawal of Superior (Marquette) ice from the Thunder Bay area, based on currently known information, though subject to revision. As ice wasted back from the Marks Moraine, shorelines on the south side of the moraine show that a body of water collected between the moraine and the ice front (Fig. 11a). This proglacial lake has been named Lake Cedar Creek (Jahnke, 1993), and there is tenuous evidence that it connected with high level lakes to the south, perhaps of Lake Duluth equivalence. As ice withdrew from the Kaministiquia embayment a small readvance to the Intola Moraine occurred (Fig. 11b). The higher levels of Lake Beaver Bay have been traced into this moraine (Stuart, 1993), and it is likely that Palaeo-Indian peoples entered the area at about this time, probably using the Marks moraine as a causeway. As ice withdrew further and water level declined to the lower levels of Lake Beaver Bay (Fig. 11c), the possibility of Palaeo-Indian occupation increases, and by the time Lake Minong was established in the Kaministiquia embayment, there is plenty of evidence to prove their presence (Fig. 11d). The shoreline diagram (Figure 9) shows that only as ice withdrew from the Thunder Bay region did the sequence of post-glacial lakes extend into the area. Beaver Bay shorelines can be found at Kakabeka, but only the lower levels extend eastwards through the city towards the Mackenzie River. A few notable sites are: 1) the Simmonds and McDaid sites at the mouth of the Current River, 2) the Hodder and Naomi Sites overlooking the Current River Mouth, 3) a site cluster around a possible Minong embayment near the present Mackenzie River, 4) the Biloski Site at the outlet of the McIntyre River into Minong, 5) Catherine, Neebing River and Cummins sites along the north side of the Kam embayment, 6) the Irene Site on a high bedrock-controlled upland overlooking the Minong shore, and 7) a collection of sites on the Rosslyn delta at the mouth of the Kaministiquia River. While many of these sites are associated with Lake Minong shores, some landscape associations are less simple and deserve more analysis (Hamilton, 1996). Some Palaeo-Indian sites unrelated to shorelines occur at High Falls on the Pigeon river, on Dog Lake (McLeod, 1982), at Harstone Hill, near the junction of the Whitefish and Kaministiquia rivers and near Kakabeka Falls where the Kaministiquia River might have been most logically crossed. The Crane site near Kakabeka, found in a vegetable garden, provides a - 158 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 11 An interpretation of post-Marquette history in the Thunder Bay area (Phillips et al., 1994). rich cache of beautifully crafted bifaces in a location apparently unrelated to any topographic feature, though on the surface of one of the higher Beaver Bay terraces. Because of their antiquity and the acidic nature of the Boreal forest soils, only the lithic materials remain to be found at these sites, though it is very likely that many other natural materials would have been used. Field Excursion Stops After leaving the hotel, we will travel up Edward/ Golf Links Rd. This route takes us across the lower flats of the Kaministquia River delta, with the bluff forming the Nipissing Transgression strand line occurring at Stop 1. Stop 1 Golf Lines Road-Thunder Bay Golf and Country Club. UTM coordinates: NAD83; 16U 0331712E / 5365013N While we will not leave the van, this location provides a view of the bluffs forming the strand associated with the Pre-Nipissing Transgression. To the left of Golf Links Road on the lower slopes of this bluff a taconite lithic scatter was encountered. Figure 23 provides a view of this beach feature, with several sites within the Lakehed University campus thought to be Archaic age. We will be returning along this route after decending down the Marks Moraine uplands, over the Upper Beaver Bay strand and the Lake Minong Strand to show the elevation contrast between these various beaches. - 159 - �Boulivard Proceedings of the 58th ILSG Annual Meeting - Part 2 A series of taconite lithic scatters are reported in this bedrock controlled upland area. They are likely Plano. Hinshelwood also assessed a taconite lithic quarry along the north side of the highway right of way overlooking the Current River. Naomi Hodder 260 m Current R. Hodder Ave. climbs a slope defined by ‘steps’ representing a series of Minong Lake phases. McDaid Simmonds 220m 300m Artificial Lake Figure 12 The Current River ‘mouth’ into Lake Minong, with the Naomi and Hodder Sites located on uplands well above glacial lakeshores. Note that Boulevard Lake is an artifacial headpond for the dam and old hydro generating station on the Current River located downstream. Stop 2 - Hillcrest Park Lookout. then reboard to cross Black Bay bridge and turn left on Centennial Park Rd to visit the McDaid site (Stop 4), adjacent to the roadway. UTM coordinates: NAD83; 16U 0334707E / 5366973N We follow a route east along Oliver Road, and then north up High Street for a brief visit to Hillcrest Park from where the city can be seen in context with local topography and Lake Superior. Stop 3 and 4 - Boulevard Park, the Bluffs and Centennial Park Rd. Figure 12 UTM coordinates: NAD83; 16U 0337219E / 5371002N We travel northeast on High St., over the St. Joseph/ Hillcrest island of Minong times, left on Balsam St, right on Hudson Ave and onto Arundel St. from which a left turn before Black Bay bridge will lead to the Bluffs Scenic Lookout (Stop 3). Walk down to Simmonds site. This site has been severely disturbed by park development and repeated cart track disturbance. We The Simmonds (DcJh-4) and McDaid (DcJh-16) Sites The present Current River runs in a channel incised into a gently lakeward dipping shelf of Gunflint formation that fronts a bounding rock wall, a structural feature, which now forms the ‘bluffs’ scenic lookout (Figs. 12, 13). In the Minong period, the river carried much water and sediment from inland proglacial lake flows and here evidence supports a major ‘bar building’ phase of coastal history, a series of river mouth spits or bars being formed on both sides of the river as water level generally declined. The highest Minong shoreline lay against the bounding rock wall at approximately the 252 m (827’) level, but between 240 and 236 m (787-774’) a series - 160 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 13 The changing geography and present characteristics of the Simmonds and McDaid sites (from Phillips, 1988:135). of sand bars formed parallel to the wall on the west bank of the river and a matching series of bars on the east side curve sharply into the then river mouth from a source on the same rock wall east of the point where it is cut by the river. It appears that the river entered Lake Minong along the rock wall, forming offshore and beach bars which extend south-eastwards across the shallow McVickers embayment to the southwest. Simultaneously, longshore transport from the east built bars partially across the river mouth at times, only to be later truncated by fluvial action. The Simmonds site on the west side, occurring on the parallel bars at about 236 m (774’) is matched in elevation and position by the McDaid site on the eastern curving bars (Figs. 13, 14). Neither site is a long-term habitation site but show evidence of activity typical of a river mouth camping and fishing site. Interestingly, the major bar building episode appears to have been just subsequent to the occupation of these two sites, two large curving bars being formed at 231 and 227 m (758-745’) on the east bank, the latter flat topped one largely a subaqueous feature that was probably contemporaneous with the supra-aqueous ridge form of the first. On the west bank a very long bar, now unfortunately truncated at its river mouth end, runs south west, in places broadening to over 100 m (328’) in width. In almost text-like manner, the mean grain size and sorting characteristics along its length confirm that it prograded out from the mouth across the McVickers embayment, probably mostly in subaqueous form. Some evidence of Palaeo-Indian activity has been found on the crests of these newer bars but no sites equivalent to those named. Stop 5 Naomi and Hodder sites. Figure 12. UTM coordinates: NAD83; 16U 0338765E / 5372052N Upon departure from the McDaid Site, the bus travels to Hodder Ave where it turns north to climb a long slope to its intersection with Highway 11/17 - 161 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 14 Geomorphic details of McDaid Site. (Fig. 12). Hodder Ave crosses a series of ancient beach strands marking various phases of Lake Minong. At the top of this slope where Hodder intersects the highway, considerable construction is underway. We may not be able to park near the intersection nor leave the van, depending upon construction activity at the time of our visit. The construction intercepted two archaeological sites located high on the bedrock-controlled uplands overlooking the Current River to the west and the waters of Lake Minong to the south. Both of these sites are far removed from modern or ancient water sources, and both are located on the north (leeward) side of the upland. Both the Naomi and Hodder Sites (Fig. 12) were salvage excavated as part of highway expansion by Western Heritage (an archaeological consulting firm based in western Canada). This position may have been calculated to offer protection from winds blowing off the glacial lake, and would have also provided a panoramic view of the extensive Current River valley to the north. If the landscape was comparatively open (taiga-steppe) then perhaps these sites might have offered a viewscape useful for observing game. These sites offer important ‘cautionary tales’ against undue emphasis on the apparent spatial association of PalaeoIndian sites and former Lake Minong shorelines. Clearly Plano settlement and land use was much more complex than first appearances would suggest. The tour continues east along Highway 11/17 for about 20 minutes to the location of another cluster of Palaeo-Indian sites associated with the former outlet of the Mackenzie River into Lake Minong (Fig. 15). While two sites are found near the river bank (one on each high bank overlooking the Mackenzie River gorge), several other sites are associated with beaches and sand spits found along a shallow former embayment (Fig. 15). Stop 6- Mackenzie Site Cluster (Figs. 1, 15) UTM coordinates: NAD83; 16U 0355847E / 5377436N Twinning of Highway 11/17 triggered extensive archaeological salvage excavations at several sites associated with high Minong Lake phases. Western Heritage is conducting ongoing salvage excavations. Thus, interpretation of these deposits remains preliminary and tentative. Again, because of active highway construction, it is not certain how close to this cluster of sites we will be permitted (safety and liability issues). - 162 - While the main site (Mackenzie I) is likely located �Proceedings of the 58th ILSG Annual Meeting - Part 2 Upland (Mackenzie Moraine) Stevens Former L. Minong and foreshore? Hydro line N L. Superior DEM courtesy of T. Sapic 1 km Figure 15 B/W version of a Digital Elevation Model of Mackenzie Site area, showing locations of Palaeo-Indian sites identified along highway construction corridor. on a reworked deltaic deposit where the Mackenzie River channel entered Lake Minong, several others have been discovered upon beach strand features within a shallow embayment of Lake Minong (Fig. 15). Archaeological inspection of where the hydro transmission lines cross the Mackenzie River in the 1970s led to the discovery of a Plano projectile point and two flakes in disturbed context. This site was named the Newton Site, and likely marks the southern extreme of the Mackenzie I Site (Fig. 16). The main part of the Mackenzie I Site remained undiscovered within the forest land to the north of the hydro transmission corridor until exploratory testing by Archaeological Services Inc. in anticipation of the construction of the new Mackenzie River bridge. Several sites in this locality are found on sandy sediment, and are associated with bedrock knob exposures that might have once formed rocky coastal headlands. Longshore sediment accumulation from a Mackenzie River source seems to be the most likely explanation for these sandy beach features. Enhanced archaeological deposition (consistent with mixed function encampment) also seems to be associated with these protected zones. This is dramatically evident with the distribution of material culture at the Mackenzie I site where artifact processing and spatial analysis is the furthest advanced. Also of note are several small and ephemeral springs that bisect the beach strandlines. The most prominent encampment is the Mackenzie I Site, located on sandy sediments accumulated north and northwest of a bedrock exposure along the west side of the Mackenzie River gorge (Fig. 17). More than 2,500 square metres of this site has been excavated, making it one of the largest Palaeo-Indian excavations ever conducted in Canada (Fig. 18). Many thousands of pieces of debitage have been recovered. However, in sharp contrast to most other excavated local Palaeo-Indian sites that are dominated by debitage, the Mackenzie I site has also yielded a wide variety of tools, including upwards of 200 complete or fragmentary projectile points. This diversity of artifact types, coupled with the discontinuous clustering of material culture suggests a repeatedly used aggregation site where diverse activities were undertaken. Because of the uncharacteristically rich recovery of diagnostic projectile points, this site will be an important type site for tool typologies useful for assessing Interlakes Composite in relation to other late Palaeo-Indian cultures throughout North America. The rich and diverse artifact recovery is enabling graduate student research addressing a wide range of topics (tool typology, lithic reduction strategies, sedimentology - 163 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Hyd ro l ine Newton Figure 16 Oblique air photo of new bridge over Mackenzie River, with Mackenzie I and II sites on each end. Early Lake Minong shorelines coincide with cleared area, with a main bank defined by white line. and geoarchaeology, artifact patterning, activity areas and site function, etc. While the Mackenzie River gorge is likely deeply downcut from its former configuration, the dense site distribution suggests repeated use of the embayment during Palaeo-Indian times. Notably, nothing except Palaeo-Indian diagnostic material has been encountered along the highway construction corridor, but reconnaissance by K.C.A. Dawson in the 1960s and 70s along the lower Mackenze River valley revealed a succession of Archaic and Woodland era sites at successively lower outlets into Lake Superior. Continued research in the Highway construction corridor has led to discovery, assessment and salvage excavation of several other sites, of particular note, the Woodpecker I and II and RLF Sites (Fig. 19). These sites are positioned upon the top of a sandy bank overlooking an extensive area of muskeg and beaver ponds. These wetlands are interpreted to be the former shoreline and shoals of Lake Minong, with the sandy/ fine gravel bank forming the beach strand line. The Woodpecker I and II sites were located by ASI during preliminary assessment of the highway corridor. These sites are initially interpreted to be localized lithic scatters. Subsequent forest clearing and mapping greatly improves interpretative resolution in their paleo-hydrological context (Figs. 20 and 21). The gently curving high bank illustrated in Figure 19 is interpreted to be the Lake Minong strandline, which forms a shallow embayment dotted with bedrock knob exposures that acted as rocky headlands along the former shore. These headlands broke the wave velocity along the shore, and allowed longshore accumulation of sediment on their leeward side. This resulted in a high raised bank with subtle berms and swales defining storm beach features. Upon one such berm is the small encampment/flaking station called the RLF site. The Woodpecker Sites are more complicated, with a bedrock dome enabling the development of a long spit or ridge of sandy sediment to accumulate to the west of the bedrock. This beach feature is bisected in at least three places by small streams that flow through underfit gullies across the beach strand. At issue is whether these streams are contemporaneous with Lake Minong water levels, or whether they are of mid to late Holocene derivation. Given the small drainage basin and rather small stream budget, coupled with the fact that no alluvial fan sediment accumulation is noted - 164 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 17 25 cm contour map of Mackenzie I Site showing bedrock with sandy/gravelly sediments between exposures. The extent of the block excavation is not fully documented here (see Fig. 18). Note the configuration of the contour lines suggests beach berms and back-beach swales. Well-sorted gravel deposits on the north side of main bedrock dome suggests former stream bed. Deep stratigraphic sections reveal deltaic sedimentation (well-sorted lens of fine pebbles, sands, and silts) overlaid by aeolian reworked fine sediment. in the muskeg lowland below the beach strand, it is suspected that they are of early Holocene derivation. That is, larger stream-flows drained downslope off the Mackenzie Moraine uplands to the north, and either accumulated in the swales behind the storm beach, or down and through the beach, with eroded sediment being carried away by wave action along the lakeward side of the beach strand line. When examining Figure 20 and 21, the most densely occupied portion of the Woodpecker I and II sites are associated with the banks - 165 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 ³ Bedrock Bedrock MACKENZIE RIVER 1 SITE Legend 7.5 http://gis.westernheritage.ca/aspnet_client/ESRI/WebADF/Print... Cores Drills Knifes Units Not Excavated Retouched Flakes Units Excavated Scrapers 0 Metres References Bifaces Map 15 15 Project No. 10-064-02 Date Nov 23, 2011 NAD83 UTM Z16N Scale 1:650 GIS LGK Map Figure 18 Preliminary plots of various tool types from the Mackenzie I excavations (Courtesy of Western Heritage). The plotted objects represent a very small fraction of the assemblage (formal and informal tools), while the many thousands of debitage, core fragments and other discarded debris is still being catalogued. The upper left image shows the extent of excavation in each of the two years of work at the site. When compared to Figure 17, it is evident that an important area for settlement was on the flat sandy surfaces to the immediate northwest of the main bedrock dome. This supports the speculation that these domes were important for breaking wave velocity and facilitating longshore sediment accumulation along their lee sides. We also speculate that these bedrock exposures where important considerations for settlement as they provided shelter from winds blowing off the lake. Such notions assume that the site was occupied while Lake Minong was at its high level. Bedrock Points (continued) Points Points (continued) 2 peices Lateral Frag Tip Frag Base Mid and Tip Tip and Base Base and Tip Midsection Base? Preform Complete Preform? Lateral Edge Tip Tip/Base Not_Points Grid_Total_Units 0 2 1 of 2 11-11-21 1:50 PM - 166 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 19 View northeast from existing highway along the construction zone. Note the ‘bank’ in the clearing (left side of frame) that likely is the former Lake Minong strandline. In the foreground the current highway cuts through this former beach feature. Note the location of the Woodpecker I, II, and RLF sites along the beach strand line. The former is located on the northwest side of a bedrock knob, while the RLF site is a small flaking station located on a berm (perhaps a high storm beach) overlooking the main strand line. of one such stream (Woodpecker I), or on the protected leeward side of the bedrock dome (Woodpecker II). Thus, occupation was densest along the protected leeward side of the bedrock dome and along the banks of the now dry stream bed. The RLF Site was discovered after a bulldozer cleared the dense coniferous forest along the north highway lane, revealing taconite flakes. Subsequent test-pitting and block excavation revealed a series of small and localized lithic clusters that are interpreted to represent short-term camps or lithic reduction stations. Topographic mapping (Figure 22) facilitated interpretation of the site locality to represent a raised storm beach berm that overlooks a back-beach swale. This high storm beach also overlooks a second swale (to the south and lakeward) and a second slightly lower storm beach berm located on the top brink of the bank forming the most prominent Lake Minong strandline (Figure 22). The RLF site is at least 50 metres north (inland) form the main strandline and at least 100 metres removed from the nearest stream bed that bisects the former beach zone. While yielding much less artifact material than the others in the area, such sites remain scientifically important because their relatively simple and brief depositional history render them much more archaeologically interpretable. Stop 7 McIntryre River outlet and the Biloski Site. Figure 23 UTM coordinates: NAD83; 16U 0332310E / 5367746N Upon completion of our examination of the Mackenzie River area, we return to Thunder Bay, travelling down the 11/17 Expressway past a series of sites that have been located along the former shores of Lake Minong (and also shores representing the pre-Nipissing Transgression (Fig. 23). In the interest of time, we will just stop along the highway. On the north side the low Minong bluff is seen, with a housing development on top. The Biloski site occupies this surface and a small sand bar at the bluff foot, where a small embayment and secondary river channel once existed. This and several other nearby sites are likely Plano age, but the proximity of the pre-Nipissing Transgression strand line suggests the close spatial association of Plano and Archaic sites, consistent with Hinshelwood’s (2004) observations. As the Expressway curves to the south, the area on the right holds a lengthy baymouth bar on the end of which lies the Catherine site, unseen from the road (Fig. 23). On the summit of Rabbit Mt. (275m, 902’), which rises behind the embayment, lies the Irene site (Fig. 23), a lookout site. - 167 - �- 168 - Stream Stream Beach strand? RLF Site Figure 20 The Woodpecker and RLF sites are located upon sandy and fine gravelly sediments that form part of the Lake Minong strandline. Note the position of the sites in proximity to small relict stream beds that drain across the strand and into the projected Lake Minong. Noteably, no alluvial fan is associated with the lower end of either of the stream beds associated with the Woodpecker Sites, suggesting that it is not a recent stream that eroded through the pre-existing sandy ridge. Continued excavation at the Woodpecker site locality suggests a much more extensive deposit on the north and west side of the bedrock dome illustated in Figure 21. This suggests encampment on flat sandy beach deposits that accumulated on the lee side of the proposed bedrock headland adjacent to a small spring flowing across the beach. The RLF site was discovered while archaeologists were walking the cleared north lane centre lane. Subsequent excavation revealed a small encampment/flaking station upon a raised berm overlooking the primary Lake Minong Beach strandline. While somewhat removed from the site, also note the small underfit stream bed located about 100 metres east of the site. Stream Newly defined extent of Woodpecker II site. Proceedings of the 58th ILSG Annual Meeting - Part 2 �- 169 - Muskeg formerly occupied by glacial meltwater Sparse Recovery Woodpecker I Trail Bedrock Woodpecker II Figure 21 25 cm contour map of the Woodpecker Sites located on a raised sandy ‘ridge’ formed north and west of a bedrock exposure. Low muskeg ground to the south of sandy ridge likely contained Lake Minong waters. Several abandoned or underfit stream beds bisect the sandy ridge, and suggest streams flowing into Lake Minong across this raised beach formed by longshore sediment accumulation on the lee side of a bedrock headland protruding into Lake Minong. Ongoing geoarchaeological and OSL dating research is assessing the viability of this interpretation. Archaeological materials were initially discovered along the banks of one of these relict streams, and also in disturbed context along the cart trail that bisects the site. Continued archaeological investigation reveals an extensive deposit on the northwest or lee side of the bedrock and along the east bank of the small abandoned stream bed. underfit Creek underfit Creek Proceedings of the 58th ILSG Annual Meeting - Part 2 �- 170 - Swale Area tested by Western Heritage Creek Figure 22 25 cm contour interval map of the RLF Site. This site was accidentially discovered by archaeologists while walking along the bulldozed north lane of the proposed highway. Lithic debitage exposed during bulldozing down trees led to shovel test reconnaissance, followed by Magnetic Gradiometer survey (Grey squares), and block excavation. Very localized dense clusters of debitage were encountered that suggests localized encampments or flaking stations. Much of the material appears to be located on the south flank of a raised berm (perhaps a sandy storm beach) about 50 metres north of the most prominent Lake Minong beach strand line. A small underfit stream bed is located ca. 100 metres east of the site. Locating such small and ephemeral (but highly interpretable) encampment zones is very difficult in forested conditions, particularly using conventional site prospecting techniques (5 metre interval shovel test pits). Western Heriage and Lakehead U. have been collaborating in experimentation of multi-proxy approaches to site discovery. Former L. Minong Main L. Minong Strandline RLF High Berm Swale Proceedings of the 58th ILSG Annual Meeting - Part 2 �Proceedings of the 58th ILSG Annual Meeting - Part 2 Biloski Irene McIntrye R. L. Minong (approx.) LU Black dots: aceramic (Plano or Con. Archaic) sites College reported along McIntrye beach strand lines R. pre-Nipissing Transgression (approx) 700 m Figure 23 Archaeological sites near the McIntyre River. Some of these sites have confirmed or probable Plano affiliation, specifically those near the L. Minong Strandline. While not yielding diagnostic artifacts, it has been proposed that the sites near the pre-Nipissing Transgression are of Arcahic affiliation. Also note the Irene Site, located on top of Rabbit Mountain, overlooking the Lake Minong shoreline. - 171 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Stop 8 - Mapleward Road, the Cummins Site. Figure 24 UTM coordinates: NAD83; 16U 0325670E / 5364042N The route turns west on Oliver Road and within 2 km rises up the distinct Minong bluff. A left turn on Mapleward Road sees a gentle descent to the Minong shore again, and the Cummins site. The Cummins Site (DCJi-1) The best published Palaeo-Indian site in the region is the Cummins site (Fig. 24), that is one of a number of Plano sites in the area where the Neebing River flowed into Lake Minong. Reported by a prominent local collector (Hugh Cummins) in 1962, the site was most intensively excavated as part of Dr. Pat Julig’s Ph.D. research. Initially considered a typical surface site, Julig et al. (1986) concluded that it is a rare stratified Palaeo-Indian site, under continued use over a long period of changing environmental conditions. The basic tool kit is reminiscent of Plains Plano culture (Dawson, 1983; Julig, 1984), and the recoveries imply diverse activities, albeit dominated by stone quarrying and knapping. This includes woodworking, fishing and beaver trapping in addition to the regular hunting of caribou and perhaps bison (McAndrews, 1982). Newman and Julig (1989) also attempted to extract and characterize blood residues from tools from this site, and propose a diverse diet. The Cummins Site was a major regional preform-making centre and the presence of exotic lithic components implies a broad geographical interaction with other groups in the region (Julig, 1984). The Cummins Site occurs across the surface of several large sand and gravel bars which trend northeastwards from the then Neebing river mouth across a broad southerly gentle dip slope of local Gunflint shales, overlain by a thin water washed silty till (Figs. 24 and 25). In these shales occurs jasper taconite, the major source of tool making material. Figure 25 shows the fenced area of the site and the morphological details (Julig et al., 1990). Figure 26 shows the paleogeographic reconstruction of events believed to have formed the area (Phillips, 1982). Longshore transport from the Neebing river mouth built a series of progressive bars across the shallow water rock shelf, recurving into a minor river valley which formed a sheltered embayment to the west of a rock island. A further bar was built along the front of existing ones eventually crossed the embayment in tombolo-like form to enclose a small lagoon, the Cummins Pond, after which lower and later shoreline features mimicked the established plan shape. A pollen core taken in the pond suggests this enclosure took place before 8.1 ka BP. (Julig et al., 1986). The plan shape and structure of the bars is not compatible with a declining water level margin, indeed the accumulation and building up of these large features on a gentle shelf slope is unlikely without transgressive wave action. Even so, such action across a gentle shore slope would not ordinarily build up a large supraaqueous bar without some initial encouragement to accumulate sediments along a line rather than disperse them in a sheet. The key to the existence of these bars in this location is underlying bedrock control. While Julig (1984) determined through resistivity and ground radar studies that variation in sediment character and bedrock irregularities could be traced, a more simple reconstruction of the bedrock profile is also possible. By surveying down the exposed bedrock dip-slope north of the site, the trend (see inset, Fig. 26) showed that beneath the site must lie a marked rock step, typical of many that occur in the present topography of these flat-lying shales, and often sharpened by wave action. Accumulation of sediments took place firstly against this rock step and subsequently over the top of it. It is possible that some till remained in the angle of the step. To the west of Mapleward Road an exposure of coarse sand and gravel with angular shale inclusions at the rear of the bar in contact with bedrock suggests overwash of the type that would be expected in this scenario. Once the linear feature was established, longshore sediment supply would extend its length and add to the lakeward face. However, overwash and building in elevation is characteristic of wave action either in an extreme storm event or a transgressive lake margin. Again, a ‘bar building’ period seems evident at Cummins. Julig (1984) found confirmatory evidence of habitation of these bars during the process of their accumulation, a buried layer of water worn taconite artefacts being associated with a period of overwash and construction. This suggests occupation of the site ...“when a lakeshore location offered advantages of longshore access, lagoon fishing and perhaps water transport” (Dawson, 1983b). A cremation burial site was recovered from a sand quarry wall on the beaches marked by the triangle in Figure 25. This was erroneously cited as being located on the rock island by Dawson (1983) since Bill Ross (pers. comm.) reports information from J.V. Wright that suggests that a cartographic error resulted in its - 172 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 fenced area DcJi-1 DcJi-11 DcJi-16 Harbourview Extension (Highway 11/17) 244 m Figure 24 The Neebing River cluster contains a series of probable Plano and Archaic sites associated with shoreline features of either the Lake Minong Strandline, or the Nipissing Transgression strand. Perhaps the most important of this group is the Cummins Site (DcJi-1), a registered National Historic Site, a portion of which is provincially owned (within fenced area). Much of the balance of this site is slowly being destroyed by urban development. J.V. Wright, K.C.A. Dawson and P. Julig have conducted excavations at the Cummins Site. Julig’s geoarchaeological examination, coupled with geomorphic mapping by B.A.M. Phillips and P. Fralick have enabled geomorphic interpretation. A Hinshelwood’s salvage excavations at DcJi-16 demonstrate Archaic reoccupation of this Plano site (2004). - 173 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 25 Cummins Site, with major paleo-lacustrine features (from Julig, McAndrews and Mahaney, 1990 based on Phillips, 1982). Black triangle marks the approximate location of the cremated human burial recovered by Wright in 1963 that was radiocarbon dated to 8480 ±390 (NMC-1216). misplacement, thereby miss-informing Dawson. This small sample was subjected to AMS radiocarbon dating, and the low collagen yield resulted in its complete consumption. The resultant date was 8,480 ±390 years BP (NMC-1216). Note that the very large sigma associated with this date renders it a not particularly precise measure of the antiquity of the Cummins occupation, but it remains the oldest dated human remains recovered so far in Ontario. Julig (1984) found artefacts both just below the aeolian sands that characteristically modified the topography of the shoreline features once the offshore shelf was exposed by lower lake levels (circa 8.0 ka BP) and in peat that accumulated in Cummins Pond. Indications are that long after the lake ceased to lap the beach face, the site remained used, at least until 7.5 ka BP (Julig et al., 1986). A well-developed, beach deposit is exposed at the Cummins site. Figure 27 shows a schematic representation of the sandy cliff face where the strandline deposits are visible. The following quotation describes the sequence. Low-angle, planar cross-stratified sands of - 174 - the foreshore dominate the exposure. Individual laminae dipping 3° to 12° lakeward (original swash-backwash surfaces) are arranged into packets which erosively truncate one another at very low angles. The planar cross-stratified sands are transitional both laterally and vertically into massive sands through a bioturbated zone. The bioturbated area represents a sparsely vegetated backshore environment while the massive sands were deposited as aeolian dunes. The foreshore sands are erosively truncated in the eastern portion of the cliff by magnetiterich sands. Intemal structures indicate that they are also foreshore deposits. The magnetite-rich foreshore laminae were formed during regression when storm wave activity reworked the lower portion of the beach, erosively truncating the older deposits. During the storm events sand was removed and stored in offshore bars. In intervening periods of fair weather, sand moved from the offshore bars back onto the beach and was winnowing by small waves, producing the magnetite-rich lag deposits filling scour truncations. The beach assemblage is overlain �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 26 The changing geography and present characteristics of the Cummins Site (from Phillips, 1988:133). by erosively based, trough cross stratified sands. These were formed after subaereal exposure of the area. Major rainfall events caused streams to flow off the adjacent rock knoll dissecting and reworking the upper layer of beach deposits. DcJi-16, located at the former outlet of the Neebing River into Lake Minong (Figs. 24 and 28). This stop on the side of the highway is approximately within the middle of this former site. In fact, the highway runs up the middle of the site, likely destroying most of the rivermouth feature upon which it was located. While the salvage excavation report has not been published, some of the relevant data is included in Hinshelwood’s 2004 publication. Phillips, Fralick and Ross, 1987. From INQUA Field Guide C-12, Eds. Geddes, Kristjansson and Teller. Stop 9. The Neebing River Site (DcJi-16). (Fig. 28) UTM coordinates: NAD83; 16U 0324874E / 5363477N In anticipation of highway development west from Thunder Bay, Andrew Hinshelwood conducted extensive excavations in the vicinity of the Neebing River south of the Cummins Site. One such site is He identifies and excavates a series of habitation areas on the terraces that successively built up at the river mouth. This site strongly resembles the site context noted at the McDaid Site, but with an important new observation. Figure 28 reports the recovery of several Archaic affiliated objects in much the same context as the Plano materials. He proposes that this locality (and - 175 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 27 Schematic representation of the cliff face at the Cummins Site (Phillips, Fralick and Ross 1987). others) were first occupied by Plano people when Lake Minong waters were nearby, but that it was reoccupied during Archaic times, perhaps because of its sandy riverbank sedimentary context coupled with nearby exposures of taconite. He also makes the point that Nipissing Transgression water levels had reflooded the Kaministiquia River delta to within about 2 km of DkJi-16, and that probable Archaic age sites are to be found to the south near the new outlet of the Neebing River into the Lake Superior Basin (Fig. 24). Stop 10 (several) - The Rosslyn Delta. Figure 23:A. UTM coordinates: NAD83; 16U 0317690E / 5360523N The route continues west up the Harbour-view Expressway extension, past the Neebing River site (DcJi-16), and then southwest to once more intercept the bluff defining the Lake Minong shoreline at the Highway weigh scales station. Turning left off the highway, once on the terrace, the road leads south to several fields in which lithic material has been found. Figure 28 Contour map of DcJi-16, at the former outlet of the Neebing River into Lake Minong. Note its similarity to the placement of the McDaid Site (Fig. 14). Also note the mixed Plano and Archaic objects recovered on this site that Hinshelwood (2004) interpreted as evidence of re-occuaption of this location. - 176 - �- 177 - H E F I C G A B B D Approx Minong Figure 29 Overview map of selected sites within the upper Kaministquia River Delta. Letters reference archaeological sites detailed in some of the following images. Dashed lines representing Strandlines are only approximate. 1.5 km Crane Strand line Strand line Proceedings of the 58th ILSG Annual Meeting - Part 2 �Proceedings of the 58th ILSG Annual Meeting - Part 2 These sites, the Dairy Farm, Breukelman Evergreen and Halow A. B and C, will be viewed (Fig. 30) and then the route will follow the north side of the Kaministiquia River towards Stanley. Enroute the upper and lower Drezecky sites will be seen (Fig. 32), as well as the Pawlick site which can be seen on the south side of the river. From Stanley the route will climb the terraces of the Stanley delta and return to Hwy 11-17. This stop provides a view north across the fields that contain a series of small lithic clusters associated with low sandy knolls around a shallow swale (Fig. 30). This swale coincides with the 750 foot contour line and suggests a shallow embayment of Lake Minong. The following extract of text and figures derives from S. Hamilton (2000). The paper addresses the problem of developing archaeological predictive models in light of environmental transformation, and as yet incompletely understood past land use practises. Among the many challenges is the issue of development of temporally sensitive palaeo-environmental reconstructions, and also models of land use relevant to the cultures under consideration. The apparent correlation between relict shorelines and Plano archaeological sites is well known in the Thunder Bay area where landscape evolution has been the subject of some study (Fig. 14). While many Plano shoreline archaeological sites are documented, the best known ones are lithic quarry and workshop sites such as the Cummins, Biloski, Simmonds, and McDaid Sites (Julig 1984, 1994; MacNeish 1952; Hinshelwood 1990; Phillips 1988, 1993). These large sites are well removed from modern shorelines, but are associated with the late Pleistocene shorelines of Glacial Lake Minong and Gunflint Formation bedrock exposures that are suitable for lithic tool production (Fig. 14; see Julig, McAndrews and Mahaney, 1990; Phillips 1988, 1993). However, these special-purpose sites do not characterize the full Palaeo-Indian settlement pattern. Rather, the dense recoveries from, and ready visibility of, these sites has resulted in their overrepresentation in the published archaeological literature. Archaeological reconnaissance upon agricultural fields within the Kaministiquia River delta has revealed a number of Plano or probable Plano sites in a wide range of landscape contexts (Hamilton, 1996) (Fig. 12). Many of these sites are found at or near Lake Minong shoreline elevations (i.e. 750 feet or 228.6 metres A.S.L.). When placed in a hypothetical Lake Minong environmental context, they are found associated with: 1) springs flowing into sheltered coves of Lake Minong [Figs. 29:A, 30] (Breukelman Evergreen); 2) on points of land that protruded out into Lake Minong to the northeast and southeast of the Breukelman Evergreen site cluster [Figs. 29:B, 30]; 3) upon low, well-drained sandy knolls surrounded by poorly drained floodplain/deltaic sediments [Figs. 29:C, 30] Halow C); 4) along deltaic backwater channels [Figure 29:D, Figure 31] DbJi-8, DbJi-7, DcJi-28, DcJi32); or 5) on sandy storm beaches developed upon relict deltas [Figs. 29:D, Figure 31] DcJi-30, DcJi-31). 6) raised Pleistocene terraces overlooking the present Kaministiquia River channel [Figs. 29:E, 30] Pawlick); 7) on high valley rims that offer panoramic views [Figs. 29:F, 32] Drezecky E); 8) along draws leading down to the Lake Minong shores [Figs. 29:G, 30] Halow A and B); 9) along upland streams well removed from late Pleistocene shorelines [Figs. 29:H, 33] DcJj-12, DcJj-13); and 10) upon well-drained upland knolls on what likely were formerly discontinuous permafrost uplands [Figs. 29:H, 33] Breukelman Field: lithic scatters A to E). Probable Plano lithic scatters have also been found upon isolated bedrock controlled knolls offering panoramic views of floodplains adjacent to the Lake Minong shoreline, [Figs. 29:I, 34] and at the top of the high bluff overlooking the gorge outlet of the Kaministquia River into the shoreline floodplain of Lake Minong [Fig. 32]. This range of landscape characteristics is certainly not exhaustive, but is sufficient to demonstrate the diverse microhabitats frequented by Plano people. These sites are consistent with other Lakehead Complex sites in terms of a very strong preference for Gunflint Formation lithic materials (Hamilton 1996). However, virtually - 178 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 30 Some archaeological sites identified at A, B, C and G in Figure 29. Sites DcJi-23 to 26 are small lithic clusters discovered on localized sandy knolls in the middle of the field. If the 750 foot contour is used as a proxy of Minong Shorelines, then they are found within a shallow embayment, while two other sites (DcJi-12 and Unnamed) are on points of land extending into the glacial lake. The sites in Halow A and B fields are small clusters or single finds oriented along draws or gullies draining towards Lake Minong, while DcJi-20 consists of localized taconite clusters on a point bar feature overlooking the Kam River. - 179 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 31 These sites referenced as D in Figure 29 are all located on localized sandy knolls within the cultivated field. Some are interpreted to be occuaptions upon a former storm beach (DcJi-29 to 30), while the others may reflect use of localized well-drained knolls within a former deltaic wetland at or immediately below the 750 foot contour. all of these sites are well removed from bedrock exposures, are significantly smaller than the quarry-workshop sites (such as the Cummins Site), and yield many fewer artifacts. When compared to the quarry/workshop sites, these small sites also yield a much higher relative frequency of tools, preforms and utilized flakes compared to debitage (Hamilton, 1996). This indicates that the small sites represent encampments, hunting stands and food procurement sites, rather than lithic extraction and reduction stations. Such observations are hardly surprising, but they do serve as cautionary tales regarding the dangers of predicting site distribution on the basis of our current incomplete heritage inventory. These examples are also important in that they demonstrate that modelling ancient human behaviour requires ongoing refinement, an understanding of human forager behaviour, and a well-developed sense of the nature and structure of the ancient landscape and its microhabitats.” Stop 11 Roadside stop to view Lower Beaver Bay strandlines Figure 37. UTM coordinates: NAD83; 16U 0309392E / 5361819N As we leave the Rosslyn delta (Minong, 9.5 ka B.P.) - 180 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 32 Location labelled F and G in Figure 29. Two small lithic scatters were encountered on the top brink of the gorge edge within Field Drezecky E. These are interpreted as game scouting sites given their commanding view across the lower gorge and Kam Delta to the south and east. Figure 33 Location labelled H in Figure 29. The sites within the Breukelman and Meyer fields are found in localized clusters on top of sandy well-drained knolls surrounded by now-dry drainage channels. These sites are far removed from paleoshoreline contexts and are interpreted as localized encampment zones within the interior region removed from glacial lake edges. - 181 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 area covered by the waters of a pre-Marquette lake (Early Lake Minong), now approximated by the 1400’ contour. On the basis of some long known sites, both in the interior and on the Minnesota northshore, Phillips and Hill (1995) proposed that a Palaeo-Indian routeway along the Minnesota shore turned inland just east of Judge C. Magney State Park, towards North and South Fowl lakes and the Whitefish Lake region. A number of more recently discovered sites in the Whitefish - Arrow Lake corridor supports the contention that Palaeo-Indian peoples were present in the area perhaps before and during the Marquette advance, and that the corridor continued to be used into the post-Marquette periods of Lake Beaver Bay and Lake Minong. Though conjectural, there is the possibility that after the retreat of Marquette ice had begun, the Marks moraine itself provided a high ground routeway from the Whitefish Lake area along the north side of the Kaministiquia valley and into the area to the north and east of Thunder Bay. Stop 12 - Kakabeka Falls UTM coordinates: NAD83; 16U 0305771E / 5364428N Figure 34 Location I in Figure 29. This set of small lithic clusters is located on the top brink of a localized upland facing north. It is interpreted as a hunting viewing location, whereby occupants could have watched the surrounding plains along the shores of Lake Minong (defined perhaps by the 750 foot contour line). and move up the present Kaministiquia valley, one is in effect travelling back in time. The Stanley delta was built into Lake Beaver Bay (9.7 ka B.P.), the first Superior lake to occupy the area recently vacated by Marquette ice retreating from the Marks moraine. Though perhaps less conspicuous, Palaeo-Indian sites of this period are also present. The larger step back in paleogeography is to consider the area that lies to the west of the maximum position of the Marks moraine, an area which remained unglaciated by the Marquette readvance and which was freed from ice around 11.0 ka B.P. Figure 35 shows the projected maximum position of the Marquette lobe and the large Just west of the junction of Hwy 11-17 and the Stanley turn-off, the huge structure of the Stanley delta is seen (Fig. 36). This delta was formed as the Kaministiquia River entered Lake Beaver Bay. A well formed bluff representing a lower Beaver Bay phase of 260m (853’), runs along the north side of the main highway, and the surface below it (250-240m; 820787’) is heavily exploited by the sand and gravel industry. The present Kaministiquia river has incised deeply into the delta, creating a terraced valley side. Kakabeka village is built on the floor (277m; 909’) of an old distributary of the Kaministiquia river which cut through a higher terrace still (Fig. 36). Remnants of this terrace surface at around 300m (984’) overlook the village. They represent the highest level of the Stanley delta, as it formed into Upper Lake Beaver Bay, and on the one on the west side of the Kaministiquia River, the Crane site was found. Kakabeka Falls is a major scenic attraction. It is formed as a result of a very resistant chert bed that caps the underlying, softer shales. It was just this sort of rock material that was desired for tool making. Today, a fairly spectacular gorge lies below the falls (Fig. 37). There has been some debate concerning the age of the gorge, since unless inherited and exhumed from a previous stage of river incision prior to the Marquette readvance, there is only post Beaver Bay time for it to develop its current grandeur. - 182 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 35 Plano and probable Plano sites found in the interior uplands around Arrow and Whitefish Lakes. Phillips and Hill (1995) proposed that these sites might predate or be contemporaneous with the Marquette Re-advance (Hamilton, 2000). Figure 36 Strandline and other features between Stanley Corners and Kakabeka Falls. - 183 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 An interesting abandoned falls and plunge pool on the west side of the gorge lies a short walk from the Park Information Centre, and represents a wide ‘horseshoe’ fall that would have been more spectacular than the present falls. Stop 13 (several) - The Marks Moraine. UTM coordinates: NAD83; 16U 0300280E / 5368976N The route will turn on Hwy 590 just west of Kakabeka, run across the High Beaver Bay terrace remnant (look for the cemetery on the right) and due west until turning north towards the Marks Moraine. As the dirt road rises up the outer face of the Marks moraine it crosses two distinct benches at 426m (1400’) and 442m (1450’). These shorelines of pro-glacial Lake Cedar Creek have been traced all along the outer slope of the Marks moraine (Jahnke, 1993) and represent still stands in a lake earlier than and isolated from the lakes of the Superior basin. The Marks Moraine. Stopping on the crest around 460m (1510’), the view south over the Whitefish valley and the rugged borderland country of the mesa-like NorWesters is spectacular. It is not hard to envisage ice pushing its way up to the Marks moraine, nor is it hard to imagine pro-glacial Lake Cedar Creek occupying the narrow strip between the moraine and the retreating ice margin. The moraine is not a simple structure. Although characterised by a till which contains diagnostic pieces of Sibley red sedimentary rocks, along much its length the Marks moraine joins and smothers isolated rock outcrops which probably greatly influenced the extent to which the ice pushed inland. The top of the moraine is remarkably flat in many places and is pockmarked with kettles. Spillway channels cross the feature in places suggesting that at one point Lake Kaministiquia to the north was a few metres higher than Lake Cedar Creek to the south. The moraine appears to have briefly existed as a narrow ridge between these two water bodies, and a possible post-Marquette routeway for animals and Palaeo-Indian people, from the unglaciated (Marquette) Arrow/Whitefish region and Interlakes corridor. Figure 37 Kakabeka Falls and the Kaministiquia River gorge. - 184 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 38 Cross section exposure of Conmee Pit (Tickle 1996). The Conmee Pit and spillway. Continuing north across the moraine and then turning east brings the excursion to a distinct channel cutting from north to south across the moraine surface. Here Conmee Township has several gravel pits. Apart from the huge boulder beds that suggest high discharges down this spillway at times, the pit reveals another key fact in understanding local deglaciation. Figure 38 shows a rare occasion when the western pit face was cleanly exposed. Tickle (1996) interpreted the sequence as consisting of two tills separated by fluvioglacial sediments. However, while one till was of diagnostic Sibley origin, the underlying one was of a northern provenance typical of the Patricia or Rainy River lobes. Elsewhere, at several places along the Marks moraine a thin Sibley till is plastered over fluvioglacial sediments. The observation suggests that the bulk of the Marks moraine may in fact be pre-Marquette in origin, with thin Marquette ice reoccupying the moraine. This has been the growing opinion of another local field worker (T. Noble, pers. communication, 1999) and looking at Figure 5, the possibility of the Marks moraine being the result of pushing the eastward part of the former Brule moraine is not unreasonable. Several exposures of contorted till structures also support this idea. Marks Moraine scenic lookout. Turning left out of the pit and again at the crossroad brings one to a point in the road from which a view east over the Kaministiquia valley, Thunder Bay city and the distant Sleeping Giant is obtained. The stop is useful only to impress viewers with scale of things. It is easy to imagine ice grinding past Mt. McKay and flowing up the valley, just as it is easy to imagine that a slight tilt of the present lake would bring the margin of the lake right up into the Kaministiquia and Whitefish valleys. Returning to the crossroad, the route will turn left down the face of the Marks moraine. The road is straight, save for a jog around a kettle hole, and, lower down, rock is exposed in the fields and along the stream beds. Turning right at Hwy 11-17 brings the excursion back to Kakabeka Falls. From here the route will take Oliver Road back to the University. The road climbs out of Kakabeka over the Upper Beaver Bay terrace remnant and then runs due east to Lakehead University. One landform of note on the way is a crag and tail feature, showing the flow of ice up the Kaministiquia valley. The ‘Old Barn’ restaurant lies on the tail and on the crag is the farmhouse, open for bed and breakfast (another time, eh!). Just east of Murillo but to the north of Oliver Road is the Murillo drumlin field, a series of long narrow forms parallel to the strike of the Gunflint shales, suggesting a good deal of bedrock control. References and Bibliography Arthurs, D. 1979 An Archaic site on the western Lake Superior shore. Report on file with the Historic Planning and Research Branch, Ontario Ministry of Culture and Communication, Toronto, 35 pp. Bjorck, S. 1985 Deglaciation chronology and revegetation in northwestern Ontario. Canadian Journal of Earth Science 22, 850-871. Burwasser, G.J. 1977 Quaternary Geology of the City of Thunder Bay and Vicinity. Ministry of Natural Resources, Ontario Geological Survey Report GR164. Burwasser, G.J. 1980 Quaternary geology of the Onion Lake and Sunshine area, District of Thunder Bay. Ministry of Natural Resources, Ontario Geological Survey Report MP94. Burwasser, G.J. and Ferguson, A. 1980 Quaternary geology of the Onion Lake and Sunshine area, District of Thunder Bay. Ministry of Natural Resources, Ontario Geological Survey Preliminary Map P.2203, Geological Series, 1:50,000. Clayton, L. 1983 Chronology of Lake Agassiz Drainage to Lake Superior: in Teller, J.T., and Clayton, Lee, eds., - 185 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Glacial Lake Agassiz, Geological Association of Canada Special Paper 26,291-307. River Delta, Thunder Bay District, Lakehead University Monographs in Archaeology, 1. Clayton, L. 1984 Pleistocene geology of the Superior region, Wisconsin, Information Circular No. 46, Wisconsin Geological and Natural History Survey, Madison, Wisconsin. Hamilton, S. 2000 Archaeological Predictive Modelling in the Boreal Forest: No Easy Answers Canadian Journal of Archaeology Vol. 24, p.p. 41-76. Clayton, L. and Moran, S. 1982 Chronology of Late Wisconsin glaciation in middle North America, Quaternary Science Reviews, vol. 1, pp. 55-82. Dawson, K.C.A. 1983a Prehistory of Northern Ontario. Thunder Bay Historical Museum Society. Dawson, K.C.A. 1983b Cummins Site: A Late PalaeoIndian (Plano) Site at Thunder Bay, Ontario. Ontario Archaeology 39,3-31. Drexler, C.W, Farrand, WR. and Hughes, J.D. 1983 Correlation of Glacial Lakes in the Superior Basin with Eastward Discharge Events from Lake Agassiz ill Teller, J.T., and Clayton, Lee, eds., Glacial Lake Agassiz, Geological Association of Canada Special Paper 26,309-329. Dudzik, M.J. 1993 The Palaeo-Indian tradition in northwestern Wisconsin, State Archaeology Regional Program, Siren, Wisconsin. Farrand, WR. 1960 Former shorelines in western and northern Lake Superior Basin, unpublished Ph.D. Thesis, University of Michigan, Ann Arbor Farrand, WR. and Drexler, C. W 1985 Late Wisconsin and Holocene history of the Lake Superior basin, In Karrow, P.F. and Calkin, P.E. eds., Quaternary Evolution of the Great Lakes, pp.17-32, Geological Association of Canada, Special Paper 30. Fisher, T. 2005 Strandline Analysis in the Southern Basin of Glacial Lake Agassiz, Minnesota and North and South Dakota. Geological Society of America Bulletin 117(11/12):1481-1496. Fisher, T. 2008 Chronological Status of Glacial Lake Agassiz. Geological Society of America, Abstracts with Programs 40(5):4. Fox, WA 1975 The Palaeo-Indian Lakehead Complex, in Nunn, P., ed. ,Canadian Archaeological Association, Collected Papers, Historic Sites Branch, Ontario Ministry of Natural Resources, Archaeological Research Report 6, 29-53. Fox, WT. 1980 The Lakehead Complex: New Insight Archaeological Research Report 13, Historical Planning and Research Branch, Ontario Ministry of Citizenship and Culture, Toronto, 127-151. Grootenboer, J. 1972 Former Shorelines in the Kaministikwia Plain and the Geomorphology of the Kakabeka Falls-Stanley Area. H. B.A. Dissertation, Department of Geography, Lakehead University, Thunder Bay, Ontario. Hamilton, J.S. 1996 Pleistocene Landscape Features and Plano Archaeological Sites upon the Kaministiquia Hamilton, S. nd A World Apart? Archaeology, Culture and History of Ontario’s Canadian Shield. In Before Ontario: New Insights from the Archaeology of the Eastern Great Lakes. edited by M. Munson and S. Jamieson. Submitted for review to McGill-Queens University Press. Hinshelwood, A 1990 1987 Observations at the Brohm Site (DdJe-1), Sibley Provincial Park, Conservation Archaeology North Central Region Report 27, Ontario Ministry of Citizenship and Culture, Heritage Branch, Thunder Bay, Ontario. Hinshelwood, A. 2004 Archaic Reoccupation of Late Palaeo-Indian Sites in Northwestern Ontario. In The Late Palaeo-Indian Great Lakes: Geological and Archaeological Investigations of Late Pleistocene and Early Holocene Environments. edited by Lawrence J. Jackson and Andrew Hinshelwood Mercury Series Archaeology Papers 165, Canadian Museum of Civilization, Gatineau. Huber, J.K. 1992 An overview of the vegetational history of the Arrowhead region, northeastern Minnesota in Field trip guidebook for the glacial geology of the Laurentian divide area, St. Louis and lake Counties, Minnesota, Minnesota Geological Survey, Field Trip Guidebook 18, pp. 55-64. Huber, J.K. and Hill, C.L. 1987 A pollen sequence associated with Paleo indian presence in northeastern Minnesota, Current Research in the Pleistocene, vol. 4, p. 89-91. Jahnke, R. 1993. Initial evidence of an early Post-Marquette pro-glacial lake on the flanks of the Marks Moraine, Thunder Bay, Ontario. HBSc. dissertation, Dept. of Geography, Lakehead University, Thunder Bay, Ontario. Johnson, L. L. and M. Stright (eds.) 1991 Paleoshorelines and prehistory: an investigation of method, CRC Press, Boca Raton, Florida. Julig, P. 1984 Cummins Palaeo-Indian Site and its Paleoenvironment, Thunder Bay, Canada. in Gramly, R.M., ed., New Experiments upon the Record of Eastern Palaeo-Indian Culture, Archaeology of Eastern North America 12,192-209. Julig, P.J., McAndrews, J.H., and Mahaney, WC. 1986 Geoarchaeological Investigations at the Cummins Palaeo-Indian Site, Thunder Bay, Ontario in Paleoenvironments: Geosciences, Current Research in the Pleistocene 3, 79-80. Julig, P.J., McAndrews, JH and Mahaney, WC. 1990 Geoarchaeology of the Cummins site on the beach of proglacial Lake Minong, Lake Superior basin, Canada. in N.P. Lasca and J. Donahue, eds., - 186 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Archaeological Geology of North America, pp. 2150. Geological Society of America, Centennial Special Volume 4, Ch.2. Julig, P.J. 1994 The Cummins Site Complex and PalaeoIndian occupations in the Northwestern Lake Superior Region, Ontario Archaeological Reports 2, Ontario Heritage Foundation, Toronto. Landmesser, C.W, T.C. Johnson, R.J. Wold 1982 Seismic reflection study of recessional moraines beneath Lake Superior and their relationship to regional deglaciation, Quaternary Research, vol. 17, p. 173190. Larsen, CE 1987. Geological History of Glacial Lake Algonquin and the Upper Great Lakes. U.S. Geological Survey Bulletin 1801, 36pp. Lehr, J.D. and H.C. Hobbs 1992 Glacial geology of the Laurentian divide area, SI. Louis and Lake Counties, Minnesota, Minnesota Geological Survey Field Trip Guidebook 18, prepared for the 39th Midwest Friends of the Pleistocene Field Trip. Leverett, F. 1929 Moraines and shorelines of the Lake Superior basin, U.S.G.S. Professional Paper 154-A Leverett, F. and F.W Sardeson 1932 Quaternary geology of Minnesota and parts of adjacent states United States Geological Survey· Professional Paper 161, U.S. Government Printing Office, Washington D.C. Levson, V.M. DE Kerr, and T.R. Giles 1993 Recognizing Late Pleistocene paleo shoreline levels from geomorphic and stratigraphic records of glaciofluvial delta, Current Research in the Pleistocene, vol. 10, p. 82-84 MacNeish, R.S. 1952 A Possible Early Site in the Thunder bay District, Ontario, National Museum of Canada Bulletin 120, 23-47. Matsch, C. L. and A F. Schneider 1986 Stratigraphy and correlation of the glacial deposits of the glacial lobe complex in Minnesota and northwest Wisconsin in Sibrava, B., D.Q. Bowen, and G.M. Richmond (eds.), Quaternary glaciations in the Northern Hemisphere: Quaternary Science Reviews, vol. 5, p. 59-64, McAndrews, J.H. 1982 Holocene environment of a fossil bison from Kenora, Ontario, Ontario Archaeology, 37, 41-51. McLeod, MP. 1982 A Re-Evaluation of the PalaeoIndian Perception of the Boreal Forest. Manitoba Archaeological Quarterly, 6, 4, 107-116. Mulholland, S. and E. Dahl 1988 Two late Palaeo-Indian projectile points associated with a Glacial Lake Duluth beach, Carlton County, Minnesota, The Minnesota Archaeologist, vol. 47, no. 2, p. 41-50. Newman, M. and Julig, P. 1989. The Identification of Protein Residues in Lithic Artifacts from a Stratified Boreal Forest Site. Canadian Journal of Archaeology 13, 119-132 Nordeng, S.C., A.P. Ruotsala, SA Nordeng 1987 Buried bog dates the time of the establishment of post-glacial Lakes Houghton and Nipissing in the western arm of the Lake Superior basin, North-central Section Geological Society of America Abstracts with Program, vol. 19, no. 4, p. 237. Pettipas, Leo 2011 An Environmental and Cultural History of the Central Lake Agassiz Region, with special reference to southwestern Manitoba, 12,000 - 7,000 BP. Manitoba Archaeological Journal Vol. 21, no. 1 and 2. Phillips, BA M. 1977 Shoreline Inheritance in Coastal Histories, Science 195,11-16. Phillips, BAM. 1982 Morphological Mapping and Palaeogeographic Reconstruction of Former Shorelines between Current River and Rosslyn, Thunder Bay, Ontario, Including Cummins Site DcJi-1. Report on file with Historical Planning and Research Branch, Ontario Ministry of Citizenship and Culture, Toronto, Ontario. Phillips, BAM. 1988 Palaeogeographic reconstruction of shoreline archaeological sites around Thunder Bay, Ontario. Geoarchaeology: An International Journal, 3, 2, 127-138. Phillips, BAM. 1993 A Time-Space Model for the Distribution of Shoreline Archaeological Sites in the Lake Superior Basin. Geoarchaeology: An International Journal, 8, 2, 87-107. Phillips, BAM. and W.A. Ross 1993 The Glacial Period and Early Peoples. In Thunder Bay: From Rivalry to Unity. pp. 2-15 Thunder Bay Historical Museum Society Phillips, BAM. and Hill, C. 1994 The Geology, Glacial and Shoreline History and Archaeological Potential of the Minnesota North Shore of Lake Superior: a background paper for Geomorphologial and Archaeological Studies of Individual State Parks on the North Shore. Report for Minnesota Department of Natural Resources, Division of Parks and Recreation, SI. Paul, MN. Phillips, BAM. and Fralick, P.W 1994. A Transgressive Event on Lake Minong, Northshore of Lake Superior - Possible Evidence of Lake Agassiz Inflow, circa 9.5 KA BP. Canadian Journal of Earth Science, v. 31, pp. 1638-1641. Phillips, BAM., Hill, C.L., Fralick, PW and WA. Ross 1994 Post-Glacial Shorelines and Palaeo-Indian Migration along the Northwestern Shore of Lake Superior, Guidebook for Fieldtrip E, 13th Biennial Meeting of AMQUA, Minneapolis, Mn. Phillips, BAM. and Hill, C.L. 1995 The deglaciation history and geomorphological character of apart of the region of the ‘Interlakes Composite’ - some new evidence. Symposium on ‘Archaeology, geomorphology and Paleoenvironmet: Paleo indian Occupations of the Western Great Lakes, 60th Annual Meeting of the Society for American Archaeology, Minneapolis, - 187 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 P.K. and G.B. Morey, eds. Geology of Minnesota: a Centennial volume, Minnesota Geological Survey, p. 561-578. MN. May 3-7. Ross, W 1980 A Palaeo-Indian Refined Biface from the Oliver Lake Area. Wanikan, 80, 3. Thunder Bay Chapter, Ontario Archaeological Society. Ross, W 1997 The Interlakes Composite: a re-definition of the initial settlement of the Agassiz-Minong Peninsula. Wisconsin Archaeologist, 76, 3-4. Sharp, R.P. 1953 Shorelines of the glacial Great Lakes in Cook County, Minnesota: American Journal of Science, 251, 109-139. Smith, D.G. and Fisher, T.G. 1993 Glacial Lake Agassiz: The northwestern ‘outlet and paleoflood, Geology 21, 9-12. Stewart, J.D., Ross, WA. and B.A.M. Phillips 1984 Investigations at the McDaid Site (DcJh-16), Thunder Bay, Report to the Ontario Heritage Foundation. Stewart, J.D., Ross, WA Faykes, A and A.S. Kissin 1989 Two Isolated Biface Finds from the Thunder Bay Area, Wanikan, 89, 3, 4-6, Thunder Bay Chapter, Ontario Archaeological Society. Stuart, A.J. 1993 Paleogeographical reconstruction of Lake Beaver Bay raised shorelines with correlation to possible Palaeo-Indian settlement, Thunder Bay region, Ontario. HBSc. dissertation, Dept of Geography, Lakehead University, Thunder Bay, Ontario. Taylor, F.B. 1895 The Nipissing beach on the north Superior shore, American Geologist, vol. 15, p. 304-314. Teller, J.T 1985 Glacial Lake Agassiz and its Influence on the Great Lakes. in. Karrow, P.F., and Calkin, P. E. eds., Quaternary Evolution of the Great Lakes, Geological Association of Canada Special Paper 30, 1-16. Wright, H.E. 1972b Quaternary history of Minnesota, in Sims, P.K. and G.B. Morey, eds. Geology of Minnesota: a Centennial volume, Minnesota Geological Survey, p. 515-547. Wright, H.E. Tunnel valleys, glacial surges and subglacial hydrology of the Superior lobe, Minnesota, Geological Society of America Memoir 136, p. 251-276. Wright, H.E., C.L. Matsch, and E.J. Cushing 1973 Superior and Des Moines Lobe, Geological Society of America Memoir 136, p. 153-185 Wright, H.E., W.A. Watts 1968 Glacial and vegetational history of northeastern Minnesota, Minnesota Geological Society Survey Special Publication 11 . Wright, J. V. 1963 An Archaeological Survey along the North Shore of Lake Superior Anthropology Papers, National Museum of Canada 3, Department of Northern Affairs and National Resources, Ottawa, 9 pp . Zoltai, S.C. 1963. Glacial features of the Canadian Lakehead, Canadian Geographer 7, 101-115 Zoltai, S.C. 1965a. Glacial features of the Quetico-Nipigon area, Ontario. Canadian Journal of Earth Science 2, 247-269. Zoltai, S.C 1965b. Thunder Bay - Surficial Geology, 1:506,880, Map S265, Ontario Department of Lands and Forests. Teller, J.T. 2004 Controls, History, Outbursts, and Impact of Large Late-Quaternary Lakes in North America. In The Quaternary Period in the United States: Developments in Quaternary Science, Vol. 1, edited by A. Gelespie, S. Porter and B. Atwater, pp. 45-61, Elsevier Amsterdam Teller, J.T. and Thorleifson, L.H. 1983. The Lake Agassiz -Lake Superior connection. In J.T. Teller and L. Clayton, eds., Glacial Lake Agassiz, pp. 261-290, Geological Association of Canada, Special Paper 26. Tickle, R. 1996. Depositional Systems developed during Deglacialion: Evidence from a portion of the Marks Moraine, Conmee Township, ON., HBA. dissertation, Dept. of Geography, Lakehead University, Thunder Bay, Ontario. Van, Hise, C.R. and C.K. Leith 1911 The Geology of the Lake Superior Region, United States Geological Survey, Washington. Winchell, N.H. 1901, Glacial Lakes of Minnesota, Geological Society of America Bulletin, vol. 12, p. 108-128. Wright, H.E., 1972a Physiography of Minnesota, in Sims, - 188 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 11 - Midcontinent Rift-Related Mafic Intrusions around Thunder Bay, Ontario Robert Cundari and Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This field trip covers an area that has been the focus of much recent research. The guidebook has benefited from geochronologic and geochemical studies conducted as part of the Lake Nipigon Region Geoscience Initiative (Heaman et al., 2007; Hollings et al., 2007a,b, 2010) as well as recent undergraduate theses (Puchalski, 2010; Cundari, 2010; Carl, 2011) and on-going Masters Theses (Cundari, in progress). These studies have elucidated the nature of magmatism in the northern part of the Midcontinent Rift (MCR) and have been used to augment and refine our previous understanding of these magmatic events. This trip focuses on a variety of mafic intrusions associated with the MCR in and around Thunder Bay. These intrusions encompass changes in the nature of early to mid-stage MCR magmatism over a span of ~20 million years. They include Nipigon sills, Logan sills, Pigeon River dykes and the Riverdale sill. Contacts with Paleoproterozoic Rove and Gunflint formations sedimentary rocks are well-exposed in this area and illustrate some of the mechanisms of dyke/sill emplacement, as well as magma-wallrock interactions. This guide builds upon those previously written and compiled by Franklin and Kustra (1972), Miller and Smyk (1995), Parker (2001), Miller et al. (2002) and Smyk and Hollings (2007). Bear in mind that when visiting exploration or private properties, permission must be granted by the property owner. Current ownership information can be obtained from the Resident Geologist’s Office, Ontario Geological Survey, in Thunder Bay. Please exercise caution along highway right-of-ways, near cliffs and along the lake shore. Regional Geology Situated within the Southern Province of the Canadian Shield, the field trip area is dominantly underlain by Paleoproterozoic Rove Formation clastic sedimentary rocks to the south of Thunder Bay and Gunflint Formation in and to the north of Thunder Bay (Animikie Group), both of which have been intruded by MCR-related mafic intrusions. Previous mapping has been conducted by Tanton (1936a,b), Geul (1970, 1973) and Smith and Sutcliffe (1989). Detailed mapping, geophysical surveys and diamond drilling undertaken by exploration companies have provided additional detail and much-needed information about sub-surface geology. Figure 1 shows the generalized geology of the Thunder Bay area. The field trip area is a rugged, upland area of diabasecapped mesas and ridges that occupies a 70 km by 30 km, northeast-trending topographic feature between Thunder Bay and the Minnesota border, termed the “Logan Basin” by North (2000). Logan Sills underlie and cap mesas that commonly rise 150 m above valleys consisting of deeply eroded,sub-horizontal, Rove Formation sedimentary rocks. Northwest of the Logan Basin, Archean granitoid rocks of the Superior Province form low, rolling hills. Southeast of the Logan Basin the topography is dominated by northeast-trending, linear ridges consisting of Pigeon River dykes. The area north of Thunder Bay displays less relief compared to the Logan Basin and has been described as peneplain by Tanton (1931). Mesoproterozoic diabase sills still provide the most dominant topographic features, as seen at the Silver Harbour Quarry (Stop 1), The Bluffs (Stop 2) and Mount McKay (Stop 3). Animikie Group Paleoproterozoic Gunflint and Rove sediments were deposited in the Animikie Basin, forming a southward-thickening wedge covering the southern margin of the Superior Province, which is truncated in east-central Minnesota and northern Wisconsin by Penokean magmatic terranes. Gunflint sedimentation - 189 - �Figure 1: Simplified geological map of the Thunder Bay area. Modified after Pye and Fenwick (1965) and Carter et al. (1973). Proceedings of the 58th ILSG Annual Meeting - Part 2 - 190 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 began approximately 2.1 Ga and ceased approximately 1.85 Ga, prior to, or during the Penokean orogeny. The nature of the sediment varies considerably, ranging from volcanic through clastic to chemical precipitates which form thick successions of iron formation. The Rove Formation is a turbidite-dominated shelf sequence, which overlies the Gunflint Formation (1878 +2 Ma; Fralick et al., 1998). It consists of a lower section of black, locally pyritic shales, which grades upwards into shales, interbedded with wackes. These clastic rocks were deposited by southeastward-moving turbidity currents, shed from the Archean craton to the north. The Rove has an approximate thickness of 500 to 600 m south and east of Thunder Bay, and thickens to the south. Rocks of the Rove Formation are flatlying or dip gently to the southeast. The shales are thinbedded, dark and fissile. Recent work by Amurawaiye (2001) and Maric and Fralick (2005) described a submarine ramp system in which the movement of coarse sediments into the deeper parts of the basin was mainly through the action of low- and high-density turbidity currents. Fair-weather and storm-generated currents dominated depositional activity at the edge of the basin. Amurawaiye (2001) concluded that approximately 70% of the Rove Formation locally consisted of organic shale. The lower 100 to 150 m of the Rove Formation and the correlative Virginia Formation in Minnesota, consist of alternating shale-siltstone and black, pyritiferous shale successions, probably reflecting fluctuations in sea level (Maric and Fralick, 2005). These successions, and especially the upper black shale, likely represent a major condensed interval deposited in water ~100 to 200 m deep. Lucente and Morey (1983) ascribed sedimentation of this interval to pelagic rainout of fine-grained sediment from dilute suspension or hemipelagic processes involving diffuse turbidity currents. The presence of abundant, submillimeter rip-up intraclasts also denotes the operation of sporadic bottom currents (Maric and Fralick, 2005). Tidal deposits present in correlative rocks to the south of Lake Superior confirms open connection to the ocean (Ojakangas et al., 2001). Above the upper, pure black shale interval, graded fine-grained sandstones are organized into a coarsening-upward succession approximately 100 m thick that is transitional into 400 m of medium-grained, sandstone-dominated, stacked parasequences (Maric and Fralick, 2005). This is overlain by lenticular to wavy bedded sandstones and shales with both wave and current ripples. The coarsening-upward to sandstone-dominated portion of the Virginia and Rove Formations has been interpreted as a submarine fan (Lucente and Morey, 1983; Maric and Fralick, 2005) with the uppermost ripple laminated succession representing progradation of distal distributary mouth bars of a delta (Maric and Fralick, 2005). A sandstone sample from the submarine fan portion of the succession yielded a youngest U-Pb detrital zircon age of approximately 1780 Ma (Heaman and Easton, 2006). The predominantly Paleoproterozoic zircon population and paleocurrents indicating sediment derivation from the north, strongly suggest the Trans-Hudson Orogen was the source of the detritus (Morey, 1973). Keweenawan Supergroup Mesoproterozoic intrusive, volcanic and minor sedimentary rocks associated with the MCR collectively constitute the Keweenawan Supergroup. On the northern margin of the MCR, Keweenawan rocks include a variety of intrusive rocks and Osler Group volcanic rocks, which represent some of the earliest magmatism in the MCR. As shown in Table 1, ages range from ca. 1140 Ma (Heaman et al., 2007) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green, 1997). A tabulated synopsis is provided below; bolded units occur within the field trip area. The majority of mafic and ultramafic rocks in the Lake Nipigon and northern Lake Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, interval between ca. 1115 and 1100 Ma (Heaman et al., 2007). Emplacement of alkalic intrusions, such as the 1108 Ma Coldwell Complex (Heaman and Machado, 1992), and filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake (1095±2 Ma) gabbros, and a Pigeon River dyke near Arrow River (1093±3 Ma; Heaman et al., 2007). Hypabyssal Mafic Rocks Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern remnants of the Midcontinent Rift, and have previously been referred to as the Logan sills (Stockwell et al., 1972), however, recent work suggests - 191 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 1: Geochronology data of MCR-related rocks in Northwestern Ontario Lithologic Unit St. Ignace Island Complex gabbro Arrow River Dyke Pigeon River Dyke Blake Gabbro Moss Lake Gabbro Crystal Lake Gabbro Mt. Mollie Dyke Cloud river Dyke Osler Osler Group rhyolite (central suite) Osler Group rhyolite (lower suite) St. Ignace Island Complex Rhyolite Coldwell Complex Logan Sills Nipigon Sills Ultramafic Intrusions Inspiration Sill Marathon lamprophyre dykes Locality / Age (Ma) St. Ignace Island / 1089.2 ±3.2 Reference(s) Smyk et al.(2006) Arrow River / 1078 ± 3 Rita Bolduc / 1141 ± 20 Blake Township / 1091.0 ± 4.5 Black Bay Peninsula / 1094.7 ± 3.1 Great Lakes Nickel / 1099.6 ± 1.2 1109.3±6.3 1109.2 ± 4.2 Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Hollings et al. (2010) Hollings et al. (2010) Agate Point / 1105±2 Davis and Green(1997) Black Bay Peninsula /1107.4 +4/-2 Davis and Sutcliffe (1985) St. Ignace Island / 1107.2 ± 2.4 Smyk et al.(2006) Coldwell Complex / 1108 ± 1 Mt. McKay / 1114.7 ± 1.1 Nipigon Embayment / 1114-1110 Nipigon Embayment / 1124-1113 Lake Nipigon / 1141 ± 20 McKellar Harbour / 1145 +15, -10 Heaman and Machado (1987) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Heaman et al. (2007) Queen et al. (1996) a geochemical difference between the sills to the north and south of the City of Thunder Bay (Hart, 2003; Hart et al., 2005). Hollings et al. (2007a) proposed that the term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite (Miller et al., 2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Logan sills generally consist of fine- to coarsegrained, ophitic to intergranular, quartz tholeiitic diabase/gabbro (Smith and Sutcliffe, 1987; Geul, 1970, 1973). Coarse-grained, intergranular gabbro, locally rich in granophyric mesostasis, is common in the interior of the thicker sills. The upper sections of the diabase sills are commonly plagioclase-porphyritic, containing as much as 60% phenocrysts. Chilled margin and bulk compositions are iron-rich, quartz-tholeiitic basalt. Compositional and textural variation in sills has been noted by North (2000) and Beskar (2001) in Blake Township, where vari-textured, “taxitic” gabbro has been described. Logan sills are recognized by their reversed magnetic polarity and generally take the form of columnarjointed, thick sheets and sills whose geometry is strongly controlled by the subhorizontal bedding of the country rock. They form conspicuous erosional remnants that create mesa and cuesta topography. From the international boundary area to Thunder Bay as many as six diabase sheets were emplaced nearly conformably into Animikie sedimentary rocks (Weiblen et al., 1972; Smith and Sutcliffe, 1987, 1989). Diamond drilling has also shown that stacked sills exist in the subsurface. For example, Dumont Nickel Inc. reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). North of the border, Smith and Sutcliffe (1989) reported sills up to 50 m thick, whereas in Minnesota, Jones (1984) studied four sills ranging from 50 to 160 m in thickness. Rare exposures of feeder dykes to sills and preserved sill terminations have been noted. The textural stratigraphy, which varies from a lower, ophitic zone to an upper pegmatitic zone, indicates that in most cases, the sills cooled as single units, probably over a period of 200 to 500 years (Smith and Sutcliffe, 1989). Chilled contact zones are developed against sedimentary country rocks; sedimentary xenoliths are rare. - 192 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Nipigon sills are commonly massive, medium- to coarse-grained, olivine-tholeiitic diabase/gabbros (Sutcliffe, 1989; Hart and MacDonald, 2007). Nipigon sills are dominantly present throughout the Lake Nipigon area but have also been recognized in the Thunder Bay area (Hollings et al., 2007b). Nipigon sills are characterized by a massive, subophitic to ophitic, plagioclase and clinopyroxene texture with trace to 3% olivine and 1-2% modal magnetite (Hart et al., 2005). Nipigon sills display a reverse magnetic polarity and generally form thick, columnar jointed sheets. Sills commonly intrude Sibley group sedimentary rocks but also can be found in contact with Archean rocks of the Quetico subprovince and the Marmion and Winnipeg River terranes. Sills often intrude earlier emplaced ultramafic units of the Nipigon Embayment as well as the 1129.0 ± 2.3 Ma Pillar lake Volcanic rocks and the 1129.0 ± 2.3 Ma English Bay Complex (Heaman et al., 2007) providing evidence for their emplacement during the second main phase of magmatism (Hart and MacDonald, 2007). The shallow dipping Nipigon diabase sills are estimated to cover an area in excess of 20 000 km2 (Sutcliffe 1991) ranging in thickness from <5m to >180m (Hart and Macdonald, 2007). Pigeon River dykes trend east-northeast to northeast and dip steeply to the southeast (Geul, 1970, 1973; Smith and Sutcliffe, 1989). Displacement and warping of the Rove Formation is evident along many of the dykes.Composite intrusions are noted in several dykes. Dyke widths average between 50 and 70 m, but may be as much as 150 m across in Ontario (Smith and Sutcliffe, 1987) and 500 m in Minnesota (Green et al., 1987). Forming northeast-trending, linear ridges, dykes can be traced semi-continuously for 15 km along strike. As noted by many workers, some dykes clearly crosscut Logan sills. However, Geul (1973) and Smith and Sutcliffe (1987) noted that others display somewhat ambiguous crosscutting relationships. In these latter cases, dykes may appear to merge with sills, suggesting that they were contemporaneous or that sills impeded the upward migration of the dykes. The presence of multiple sets of horizontal columnar jointing suggests the development of multiple or composite dykes. The dykes typically consist of ophitic diabase that may be plagioclase-porphyritic. A typical, nonporphyritic olivine diabase consists of approximately 60% plagioclase (zoned labradorite; An55-70), 20% augite + hypersthene, up to 15% olivine and up to 5% magnetite, ilmeno-magnetite and sulphides (Geul 1970, 1973). Average whole rock compositions of Pigeon River dikes are moderately evolved (Mg# = 52) olivine tholeiitic basalt. The Riverdale sill was first characterized by Hollings et al. (2007b) as being geochemically and petrographically distinct from the surrounding Logan sills and the Nipigon sills to the north. Puchalski (2010) described the geochemical and petrographical characteristics. The Riverdale sill lies within the southern city limits of Thunder Bay, close to the northern boundary of the Logan basin. The unit displays a sill morphology exposed over an area approximately 6 km long and 2 km wide with true thickness unknown as the upper contact is not exposed (Puchalski, 2010). Exposures within a quarry on West Riverdale Road (Stop 4) display a thickness of 10 m where detailed sampling and subsequent geochemistry and petrography analyses were completed. Paleomagnetic work performed by Hollings et al. (2010) confirmed the unit to display a reverse polarity. The following is summarized from Puchalski (2010). Rocks comprising the Riverdale sill are dominantly gabbronorites with lesser olivine gabbro present towards the centre of the intrusion. The gabbronorites are generally fine-grained and display no cumulate textures within any of the samples. Plagioclase typically occurs as subhedral laths with euhedral orthopyroxene and lesser clinopyroxene and olivine. Minor alteration is present in most samples as chlorite replacing pyroxene and sericite replacing plagioclase. The olivine gabbro samples lying toward the centre of the unit are petrographically similar to the gabbronorite samples except for a higher modal percentage of anhedral to euhedral olivine. Olivine grains are typically finegrained but may range to medium grained.Variable amounts of serpentine is found replacing olivine. A unit of mafic rock in Devon Township, south of Thunder Bay, was mapped by Tanton (1931) and was termed Rove Formation Basalts, but was subsequently mapped as a Logan diabase sill (Geul, 1970). Cundari (2010) described the detailed geological, petrographical and geochemical characteristics of the unit. The unit is exposed on a plateau 7 km long and 0.8 to 1.0 km wide. The unit is 4 to 6 m thick and is in apparent conformable contact with the underlying shales of the Paleoproterozoic Rove Formation, where a pronounced chilled margin consists of variolitic material up to 20 cm thick. The flow-top also exhibits a variolitic texture ~15 cm thick.The presence of ropy flow top and amygdules as well as quench textures, support a volcanic origin. - 193 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Major element chemistry reveals a tholeiitic, intermediate composition with samples plotting in the basaltic andesite to andesite fields as well as in the basaltic trachy-andesite to trachy-andesite fields on a TAS diagram. The unit typically has an intergranular texture consisting of randomly oriented plagioclase laths with interstitial chlorite, an alteration product of primary augite. Most samples contain minor serpentine (after olivine), opaque minerals, secondary quartz, oxides, pyrite and calcite. Amygdules are often present and are infilled with some combination of calcite, quartz, chlorite and pyrite. Lower flow contacts and flow-tops are typically glassy with abundant spherulites that sometimes coalesce into bands. This unit is definitively related to Midcontinent rift magmatism and is now referred to as the Devon Volcanics. Contact Metamorphism There is a remarkable range in the reported intensity and nature of contact metamorphic effects in Rove sedimentary rocks at diabase dyke and sill contacts, owing mainly to the subjectivity of the mapper and the exposures in question. Geul (1973) noted that sedimentary hornfelsic rocks are restricted to a narrow zone of baking between 2 to 10 cm wide at diabase dyke contacts. Metamorphosed siltstone displays two stages: first: slight recrystallization of biotite aggregates in an incipient hornfelsic texture; and second, a more complete recrystallization of biotite, surrounded by pale sericitic aggregates, set in a quartzo-feldspathic matrix. Conversely, Franklin (1970) suggested that contact effects existed up to 8 m from sill contacts and possibly up to 23 m.They were manifested as microporphyroblasts of mica and chlorite (a.k.a. “spotted alteration”), graphite destruction and the conversion of pyrite to pyrrhotite. Geul (1973) noted that minute particles (< 0.01 mm) of oxide and sulphide minerals are locally abundant in the contact zone. Rove Formation sedimentary rocks may be deformed along dyke contacts. As noted by Geul (1973) beds appear to dip toward the dykes or are “up-dragged” along dyke contacts. Deformed and fractured sedimentary rocks have been noted near sill terminations. Narrow, parallel tension gashes filled with quartzo-feldspathic leucosome/neosome occur in metatectic, deformed siltstones. Assimilation of country rock Pink granophyric features have been welldocumented in Logan sills to the west of Thunder Bay which are attributed to in-situ assimilation of granitic material (Blackadar, 1956). This theory was reevaluated by Magnus (2010) who studied assimilation features in the Navilus and Terry Fox sills. Although these outcrops will not be visited on this field trip, they provide a sound explanation for some of the assimilation and sill-top features observed throughout the trip (i.e., Stops 2 and 5). Two zones were reported to consistently appear around xenoliths present in the Navilus sill: a zone of quartzo-feldpathic intergrowths, or “granophyre”, adjacent to xenoliths, followed by a zone of pyroxene grains present on the interface between normal diabase magma and the granophyric zone (Magnus, 2010). Late-stage granophyric formations are also found interstitially between plagioclase and pyroxene grains with iron-oxides, likely representing immisicibility of a late-stage silica- and iron-rich liquid exsolved from the magma (Magnus, 2010). This premature exsolution of silica and iron-rich liquids from the magma are attributed to the introduction of silicate-rich xenoliths to the already silica-saturated, quartz-tholeiitic magma (Magnus, 2010). Magnus (2010) further concluded that geochemical variation between diabase with inclusions and normal diabase was caused by late-stage fractionation during crystallization as noted by variable depletion in high field-strength elements (HFSEs) with LIL enrichment attributed to in-situ assimilation. Discussion of Geochemistry As part of the Lake Nipigon Region Geoscience Initiative, whole rock analyses were performed on a number of Midcontinent Rift-related intrusions south of Thunder Bay as well as the Lake Nipigon region. Subsequent research conducted at Lakehead University provided additional sampling of laterrecognized units of interest (i.e., the Riverdale sill and the Devon Volcanics). Data from these studies, as well as data from previous mapping endeavors conducted by the Ontario Geological Survey, have been compiled in a database totaling 2400 spatially referenced points with whole-rock geochemical analyses. This database is currently being reevaluated by Cundari (in progress) to detect variability within units as well as put many of the obscure units in the context of the MCR. Current discrimination of intrusive units associated with the MCR is through trace element patterns (e.g., primitive mantle-normalized spider plots, as well as measures of heavy and light rare-earth - 194 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 present within the city of Thunder Bay and north around Lake Nipigon. This geographic distribution may be a result of either tapping different source regions at depth or the presence of a major compositional boundary (Hollings et al., 2007a). Geochemical and petrographic data show no evidence for fractionation within the Riverdale sill, with only slight variation present towards the lower margin and the centre of the sill. Samples within 1 m of the contact display negative niobium anomalies interpreted to be a result of crustal contamination at depth, with samples towards the centre characterized as olivine gabbros. This suggests that the sill is composed of two pulses of magma, with the more-contaminated first pulse intruded by a less-contaminated second pulse (represented by the olivine gabbro pushing the contaminated magma towards the outer margins of the sill). The Riverdale sill is geochemically distinct based on its heavy rare-earth element abundances when compared to the surrounding Logan sills. Displaying a high Gd/Ybn ratio denotes heavy rare earth element fractionation indicative of a deep-seated mantle melt sourced from below the garnet-spinel stability field (>100 km). As the Riverdale sill displays a Gd/Ybn ratio of 3.0 – 3.5, it was likely sourced from this region suggesting it is genetically related to the ultramafic intrusions of the Nipigon Embayment (Puchalski, 2010). Figure 2: Discrimination diagrams for mafic and ultramafic intrusions near Thunder Bay. Data are from Hollings et al. (2007a) and Puchalski (2010).Normalizing values from Sun and McDonough (1989). element abundances displayed by the plot of La/Smn (LREE) and Gd/Ybn (HREE; Fig. 2). Major element abundances, i.e. Mg# vs. TiO2, also show distinct populations between units (Fig. 2; Table 2). Nipigon and Logan sills show broadly similar morphological characteristics but can be distinguished from each other based on TiO2 abundances. The Logan sills present to the south of Thunder Bay within the Logan Basin, (e.g., Mt. McKay, Stop 5) display higher TiO2 content than the Nipigon sills (Stops 1 and 2) When the data from the dyke swarms are compared to the regional data set generated for the sills and intrusions of the Lake Nipigon embayment Hollings et al. (2007a) showed that Pigeon River dyke swarm closely resembles the sills of the Nipigon suite than the ultramafic intrusions or the Logan sills. In contrast, the Mt. Mollie swarm appears to be transitional between Nipigon sills and Inspiration sills. Additional isotopic and geochronological studies will be required in order to further investigate the relationships between these MCR-related intrusions. Rare-earth element geochemistry of the Devon Volcanics show the unit to be relatively enriched in both HREEs and LREEs, similar to the ultramafic sills of the Nipigon Embayment as well as the Riverdale Sill (Hollings et al., 2007a, 2010). A primitive mantlenormalized REE plot shows that the volcanic unit is characteristic of an Ocean-Island Basalt, but with a negative niobium anomaly, most likely the result of lower crustal contamination. This evidence is further supported by an εNd(t=1100Ma) of -3.48, which also suggests contamination of the unit by a lower crustal source. - 195 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Table 2: Geochemical analysis of select intrusive rocks from the Thunder Bay area. Data from: A) Hart and Magyarosi (2004), B) Hollings et al. (2011). 1 3 4 5 6 7 8 Nipigon sill Hornfelsed Rove Riverdale sill Riverdale sill Pigeon River dykes Pigeon River dykes Upper sill Silver Harbor quarry Squaw Bay road quarry Olivine gabbro Gabbronorite Whiskeyjack Whiskeyjack point N point S 03TRH201 03TRH202 DB-167 DB-12 RP-8QB RP-5Q DB-10 DB-11 Intrusive unit Description Sample 2 Logan sills Lower sill (upper contact) Source A A B B B B B B SiO2 48.36 48.86 48.32 76.56 46.4 48.64 53.14 50.62 TiO2 3.65 3.59 0.91 0.45 2.04 2.94 1.23 1.22 A12O3 13.74 14.46 15.45 10.48 8.16 9.89 14.62 14.31 FeOt 14.62 15.00 11.73 4.88 12.96 13.94 10.90 11.32 MnO 0.21 0.18 0.19 0.02 0.16 0.2 0.18 0.19 MgO 4.4 4.05 7.88 1.48 12.53 7.14 5.49 6.58 CaO 7.12 7.45 10.85 0.2 9.24 8.51 9.75 8.65 Na2O 2.81 3.43 2.11 2.11 1.51 3.09 2.64 2.85 K2 O 1.35 1.27 0.4 1.52 0.41 0.8 0.91 1.57 P2 O5 0.37 0.42 0.09 0.08 0.21 0.24 1.15 0.12 Volatiles 1.44 0.37 0.57 2.78 3.87 3.29 1.15 1.74 Total 99.7 100.76 99.81 101.09 98.93 100.24 101.38 100.45 mg# 23.13 21.26 40.18 23.28 49.16 33.87 33.50 36.76 Cr 30 41 112 190 >1300 266 68 11 Co 34 36 62 12 93 67 45 47 Ni 81 88 131 35 406 92 95 95 (ppm) The trace element characteristics of the volcanic unit suggest an origin in Keweenawan time as they are geochemically similar to units of the MCR (Hollings et al., 2007a) rather than Paleoproterozoic volcanic units of the Gunflint Formation. Field trip stops Stop 1: Silver Harbour Quarry UTM coordinates: NAD83; 16U 0354388E / 5374970N Location: Quarry adjacent to road cut at Silver Harbour boat launch. Silver Harbour Road off Lakeshore Drive. Description: The first stop on the trip offers an excellent exposure of a Nipigon sill. Material was quarried from this locality to create many of the breakwalls along this portion of the bay. Two localities are of interest here: the first being the actual quarry which exposes a typical Nipigon diabase sill; the second is a road cut at the northern end of the quarry which displays some enigmatic, late-stage features. The road cut can be reached by a trail along the northwestern side of the clearing. The features observed in Figures 4 and 5 appear to represent magma injected into the still-crystallizing sill. They are typically finer-grained than the surrounding medium- to coarse-grained, subophitic diabase. They appear as blobby masses or dykes with undulating contacts appearing to propagate up through the sill from depth. The amorphous form suggests that the surrounding sill material was not fully crystallized when these features were emplaced but must have been partially solid, as distinct contacts are preserved. These features may represent an episode of immiscible magma interaction. - 196 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip stops - Coordinates and Descriptions STOP NAME STOP NUMBER Description Silver Harbour Quarry sill 1 Quarry and adjacent road cut at Silver Harbour boat : launch The Bluffs 2 Flat-lying outcrop atop The Bluffs lookout; off Arundel St. 337266 E 5371048 N Baked Rove outcrop Optional Sandstone quarry 333765 E 5354546 N Pigeon River dykes (Chippewa park/Whiskeyjack point) 3 Two parallel, E-trending dykes cross-cutting Rove Formation sedimentary rocks; Whiskeyjack point, Lake Superior 336798 E 5355397 N Riverdale sill (Robin’s Donuts) Optional Easternmost exposure of Riverdale sill in Robin’s donuts parking lot; Highway 61 326487 E 5355715 N Riverdale sill Quarry 4 Riverdale sill exposed in contact with Rove sedimentary rocks; Candy Mountain Dr. 322410 E 5355212 N Mount McKay 5 Mount McKay lookout on top of lower sill; hike to upper sill via trail is optional 331126 E 5357384 N - 197 - NORTHING EASTING (NAD 83) 354388 E 5374970 N �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 3: Satellite image of the Thunder Bay area showing field trip stops. Figure 4: Photo of outcrop at Silver Harbour road cut showing immiscibility features. South side of road - 198 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 5: Photos of outcrop at Silver Harbour road cut showing immiscibility features. Left - dyke with offset on north side of road, Right - blob on south side of road. Samples SP-RC-016 and SP-RC-018 from the younger intrusion have higher silica contents as well as larger loss on ignition values (Table 3) suggesting that the late-stage material was contaminated in silica and hydrous minerals possibly from the surrounding sedimentary rocks of the Gunflint Formation. This is further supported by the elevated light rare-earth elements abundances of these samples compared to the host sill (Fig. 6). Alternatively, the later-stage material may represent a slightly more fractionated product of a typical Nipigon sill melt. This is supported by the pronounced negative europium and titanium anomalies (Fig. 6) representing plagioclase and magnetite fractionation, respectively. The most plausible scenario is that the material injected into the sill underwent both processes whereby the material has residency time in a shallow crustal magma chamber. Here it was able to fractionate plagioclase and magnetite as well as leach hydrous material including silica from the Gunflint Formation sedimentary rocks, resulting in the rareearth element enrichment, elevated silica content and loss on ignition. Stop 2: The Bluffs lookout UTM coordinates: NAD83; 16U 0337266E / 5371048N Location: The Bluffs lookout. Unmarked road off Arundel St. west of Lyon Blvd. W. Description: This stop provides a good vantage point for the city of Thunder Bay and Lake Superior, looking southeast. The pronounced topographic high Table 3: Major element chemistry for samples at Silver Harbour road cut. Data from Cundari (in progress) Sample SP-RC-016 SP-RC-017 SP-RC-018 Late-stage bleb Nipigon sill Late-stage dyke SiO2 TiO2 64.75 0.51 Al2O3 11.97 FeO 4.98 MnO 0.09 MgO 2.66 CaO 3.59 Na20 3.5 P205 0.11 LOI 5.6 Total 100.28 48.86 0.71 14.91 11.03 0.18 8.02 9.07 1.61 0.06 2.67 100.43 58.03 0.5 10.9 10.92 0.15 4.17 1.54 0.81 0.08 8.83 99.52 - 199 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 6: Primitive mantle-normalized trace element plots for samples from Silver Harbour quarry and road cut. Normalizing values from Sun and McDonough (1989). Data from Cundari (in progress). that forms the lookout is situated on a diabase sill of Nipigon affinity. The flat-lying outcrop to the east of the lookout/parking lot exposes the top a sill, commonly characterized by feldspar-phyric patches (Fig. 7). The likely source of the feldspar-phyric patches is earliercrystallized material or autoliths buoyantly rising to the top of the melt and crystallizing in situ. Stop (OPTIONAL): Hornfelsed Rove Formation Sandstone UTM coordinates: NAD83; 16U 0333765E / 5354546N Location: Clearing off Squaw Bay road. of hornfelsed sedimentary rocks resulting from Midcontinent Rift-related magmatism. In addition, a raised beach related to a higher stand (Minong or post-Minong stage?) of present-day Lake Superior is present towards the northeastern corner of the quarry (cf. Burwasser, 1977). A massive, ~4 m thick bed of sandstone displays a characteristic hornfelsed texture. From afar, this looks like a diabase sill but upon closer inspection can be identified as a thick sandstone bed that may have been metamorphosed by an overlying (and possibly underlying) intrusion (Fig. 8). The SiO2 content of this unit is 76.56 wt %. Description: This stop in Fort William First Nation provides excellent exposure of Rove Formation sandstone/wacke and one of the best examples Figure 7: Feldsapr-phyric patches present towards the top of a Nipigon sill at The Bluffs; Stop 2. - 200 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 8: Primitive mantle normalized trace element plots for hornfelsed Rove Formation sandstone as well as Gunflint formation sedimentary rocks. Data from Hollings et al. (2011). Normalizing values from Sun and McDonough (1989) Stop 3: Chippewa Park/Whiskeyjack Point UTM coordinates: NAD83; 16U 0336798E / 5355397N Location: Shoreline outcrops off Sandy Beach Rd. at Whiskeyjack Point Description: Along the shoreline of Lake Superior at Whiskeyjack Point, two east-northeast trending Pigeon River-style dykes crosscut Rove Formation shale. This stop also provides a panoramic view of Thunder Bay including the Sleeping Giant, which is capped by a Logan diabase sill (Carl, 2011), as is Pie Island and the mesas to the south along the shoreline. These dykes display a medium-grained, subophiticophitic texture. Jointing, measured here perpendicular to dyke trend, can be used as a proxy to define approximate trends in other dykes where contacts are not observed. Logan Basin. This contradicts the geochronology data as Pigeon River dykes have been dated at 1141 ± 20 Ma for the Rita Bolduc dyke (UTM 310563E 53247021N; NAD83) and 1078 ± 3 Ma (UTM 296694E 5324134N; NAD83) for the Arrow River dyke whereas Logan sills are dated at 1114.7 ± 1.1 Ma (Heaman et al., 2007). Further geochronological and paleomagnetic work is ongoing to resolve these issues. Stop (Optional): Riverdale sill at Robin’s Donuts, Highway 61 UTM coordinates: NAD83; 16U 0326487E / 5355715N Location: Outcrop in Robin’s Donuts parking lot, Highway 61. Geochemically, the Pigeon River dykes display broadly similar trace element patterns to those of the Nipigon sills suggesting they are genetically related, possibly representing the feeders to the Nipigon sills. However, the wide considerable distance between the two suggests that this is unlikely. An alternative explanation has been presented by Hollings et al (2010) who have suggested that the Pigeon River dykes tapped the same long-lived mantle reservoir as the Nipigon sills that were present throughout the Midcontinent rifting. Based on field relationships, Pigeon River dykes post-date Logan sills as several cross-cutting relationships have been documented throughout the Figure 9: Photo of a Pigeon River dyke outcrop at Whiskeyjack point; Stop 11-3. - 201 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 10: Primitive mantle-normalized trace element plots for Pigeon River dyes at Whiskeyjack Point, with Nipigon sill sample for comparison. Data from Hollings et al. (2011). Normalizing values from Sun and McDonough (1989) Description: This exposure of the Riverdale sill represents the easternmost expression of the unit as described by Puchalski (2010). This low-lying, moderately weathered outcrop of the Riverdale sill gabbronorite was the “discovery outcrop” for this unit (Smyk and Hollings, 2007). Stop 4: Riverdale sill in Quarry UTM coordinates: NAD83; 16U 0322410E / 5355212N Location: Quarry at east end of Candy Mountain Dr. Description: Sampling by Smyk and Hollings (2007) identified this as a Riverdale gabbronorite sill in Rove Formation shale, wacke and minor tuffaceous units (Fig. 11). Subsequent detailed petrographic and Figure 11: Photo of Riverdale sill quarry; Stop 4. - 202 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 12: Height vs. elemental abundance for Riverdale sill quarry samples. TiO2, SiO2 and MgO are in weight percent and Cr and Ni are ppm (Puchalski, 2010). geochemical analyses were carried out by Puchalski (2010). Samples were taken through stratigraphy at the quarry to investigate composition and contamination, as well as to test whether the sill had undergone differentiation. The following section provides a concise summary of those findings. The mafic intrusive rocks within the quarry are dominantly classified as gabbronorites with olivine gabbro present towards the centre of the sill. The gabbronorites are generally fine-grained with plagioclase occurring as subhedral laths. Orthopyroxene is present in greater abundance than clinopyroxene, occurring as anhedral to subhedral crystals. Varying degrees of alteration are manifested as sericitization of plagioclase and chloritization of pyroxene. The olivine gabbro is texturally similar to the gabbronorite, albeit with a higher modal percentage of fine-grained, anhedral to euhedral olivine. In most samples, olivine is replaced by serpentine, producing secondary quartz and calcite, as well as minor magnetite. Alteration is significantly greater in the narrow chilled margin at the contact. Pyrite occurs throughout the unit; minor chalcopyrite has also been noted. Sampling for whole rock major and trace element geochemistry was undertaken by Puchalski (2010) throughout the 10 m exposure at 1m intervals. Olivine gabbro samples display broadly similar trace element characteristics to those of the gabbronorite samples. Differences lie within the major element abundances; olivine gabbros are lower in SiO2 and elevated in MgO, Cr, Co, and Ni compared to the gabbronorite samples (Table 2). The sill does not display any evidence for differentiation as shown by the erratic trends of MgO, SiO2, TiO2, Cr and Ni through stratigraphy (Fig. 12). An olivine gabbro in the centre of the sill displays elevated MgO, Cr, and Ni values as well as a lower abundance of silica when compared to the surrounding samples. This is likely the result of a slightly more primitive magma intruding the centre of the sill. The lack of chilled margins between the olivine gabbro and the gabbronorite suggest that the sill had not fully crystallized when the second pulse intruded. A sample of a 60 cm wide north-trending diabase dyke which intrudes the sill near the western end of the quarry is geochemically comparable to the surrounding Logan sills. Contamination by the Rove shale is evident in samples taken from close to the contact (<1 m above the contact). These samples display higher SiO2 values as well as lower Nb/Nb* and Gd/Ybn values than the rest of the unit (Fig. 13). As the Rove shale displays significantly lower Nb/Nb* and Gd/Ybn values (Fig. 8) than that of the surrounding gabbronorite. The Rove shale is the likely source of this contamination signature. Two different pulses of magma are recognized within the Riverdale sill based on contamination signatures denoted by negative niobium anomalies. The lesscontaminated samples are typically found towards the core of the intrusion with rocks above and below displaying a greater degree of contamination (Fig. 14). Samples taken within 60 cm of a shale xenolith do not display a distinct negative niobium anomaly. This shows that the source of contamination responsible for the negative niobium anomaly is not the Rove shale but is likely a crustal component from depth. εNd(T=1100Ma) values of -1.6 to -1.9 for the Riverdale Sill are consistent with this model (Smyk et al., 2009). - 203 - Although the Riverdale sill is located near Logan �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 13: Height vs. elemental abundance (ppm) for Riverdale sill quarry samples. SiO2 is in weight percent (Puchalski, 2010). sills, it remains petrographically and geochemically distinct from them. Geochemical discrimination based on La/Smn (LREE) vs. Gd/Ybn (HREE) shows characteristics similar to those for the ultramafic units of the Nipigon Embayment (e.g., Disraeli, Kitto, Hele and Seagull), closely resembling the mafic to ultramafic Jackfish sill (Fig. 2). The Jackfish sill is finer-grained and displays a higher modal abundance of olivine than the Nipigon sills surrounding it (Hollings et al., 2007a). This suggests that the Riverdale sill may be genetically related to the ultramafic and mafic to ultramafic units of the Nipigon Embayment. This is consistent with the reversed polarity of the Riverdale sill (Hollings et al., 2010). Figure 14: Primitive mantle-normalized trace element plots for successive samples through stratigraphy at the Riverdale sill quarry (Puchalski 2010). Normalizing values from Sun and McDonough (1989) - 204 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 15: Primitive mantle normalized trace element plots for upper and lower sills at Mount McKay with Nipigon sill sample for comparison. Data from Hart and Magyarosi (2004) and Hollings et al. (2011). Normalizing values from Sun and McDonough (1989). Stop 5: Mount McKay outcrop (Fig. 16). UTM coordinates: NAD83; 16U 0331126E / 5357384N Location: Mount McKay scenic lookout, off Mission Rd. Description: This stop provides an exceptional view of the city of Thunder Bay as well as a great example of a stacked Logan sill sequence. The summit of Mount McKay at 482 m ASL is approximately 300 m higher than Lake Superior. Great exposures of Logan sills are abundant throughout the Logan basin south of Thunder Bay. From drill core, it has been reported that many sills are present at depth, as many at 14 as noted by Dumont Nickel Inc. who reported intersecting 14 gabbroic sills in a 705 m deep drill hole in central Pardee Township (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). It is inferred that many of the Logan sills are underlain by additional sills but exposures of this are rare. Mount McKay provides the best example of a stacked sill sequence in outcrop. The geochemistry of samples from the two sills is presented in Figure 15. The stop is centered on the lookout area, which represents the top of the lower sill at approximately 337 m ASL. Outcrop of the upper, ~60 m thick sill and adjacent, hornfelsed Rove wacke can be accessed by way of a hiking trail. If time permits, those interested in completing this hike to the upper sill may do so with extreme care. Feldspar-phyric patches similar to those observed at Stop 2 are present in an exposure of the fine-grained, upper, chilled contact of the lower sill found along a short trail to the west of the clearing next to a religious shrine. Polygonal jointing, characteristic of chilled contact zones, is well-developed in this References Amurawaiye, O. 2001. The Paleoproterozoic Rove Formation of northwestern Ontario: A turbidite-dominated shelf sequence; unpublished H.B.Sc thesis, Lakehead University, Thunder Bay, Ontario, 44p. Beskar, S. 2001. The Blake gabbro: A taxitic-textured gabbro sill south of Thunder Bay, Ontario; 47th Institute on Lake Superior Geology, Annual Meeting, Madison, Wisconsin, May 9-12, 2001, Proceedings Volume 47, Part 1, p.1 Blackadar, R.G. (1956). Differentiation and Assimilation in the Logan Sills, Lake Superior District, Ontario. American Journal of Science, vol. 254, p. 623-645. Burwasser, G.J. 1977: Quaternary geology of the City of Thunder Bay and vicinity, District of Thunder Figure 16: Photo showing polygonal jointing at the upper chill margin of the lower sill at Mount Mckay; Stop 5. - 205 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Bay; Ontario Geological Survey, Report 164, 70p. Accompanied by Map 2372, scale 1:50 000. Carl, C. 2011. Geochemistry and petrology of Midcontinent Rift-related intrusive rocks of the Sibley Peninsula, Ontario. Unpublished honours thesis. Lakehead University. Cundari, R. 2010. The geology and geochemistry of the Devon Volcanics, South of Thunder Bay, Ontario. Unpublished Lakehead University Honours Thesis. Davis, D.W. and Green, J.C. 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34, p.476-488. Davis, D.W. and Sutcliffe, R.H. 1985. 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Geology of Crooks Township, Jarvis and Prince Locations, and Offshore Islands, District of Thunder Bay; Ontario Department of Mines, Geological Report 102, 46 p. Green, J.C., Bornhorst, T.J., Chandler, V.W. et al. 1987. Keweenawan dikes of the Lake Superior region: evidence for evolution of the middle Proterozoic Midcontinent Rift of North America; in Mafic dike swarms, H.C. halls and W.F. Fahrig, eds., Geological Association of Canada, Special Paper 34, p.289-302. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; 49th Institute on Lake Superior Geology, Proceedings volume 49, Part 1, Programs and abstracts, p.21-22. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Hart, T.R., and MacDonald, C.A., 2007. Proterozoic and Archean Geology of the Nipigon Embayement: implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44: 1021-1040. Hart, T.R. and Magyarosi, Z. 2004. Northern Black Sturgeon River–Disraeli Lake area, Nipigon Embayment, northwestern Ontario: lithogeochemical, assay and compilation data; Ontario Geological Survey, Miscellaneous Release—Data 133. Heaman, L.M. and Easton, R.M. 2006. Preliminary U/ Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191, 79p. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., Macdonald, C.A. and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of earth Sciences 44: 1055-1086. Heaman, L.M. and Machado, N. 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: Evidence from the Coldwell complex; Contributions to Mineralogy and Petrology, v. 110, p. 289-303. Hollings, P., Hart, T., Richardson, A. And MacDonald, C.A., 2007a. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayement, northwestern Ontario. Canadian Journal of Earth Sciences v. 44: 1087-1110. Hollings, P.N., Smyk, M.C., Hart, T., 2007b. Geochemistry of Midcontinent Rift-related mafic dikes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dikes. 53rd Institute on Lake Superior Geology, Annual Meeting,Proceedings volume 53, Part 1. Lutsen, Minnesota, May 2007, pp. 40–41. Hollings, P., Smyk, M., Heaman, L.M., and Halls, H., 2010. The geochemistry, geochronology, and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research. Precambrian Research, v. 183, iss. 3, p.553-571. Hollings, P., Cundari, R., Pulchalski, R. and Smyk, M.C. 2011. Geochemistry of Midcontinent Rift- related mafic intrusions, Thunder Bay area; Ontario Geological Survey, Miscellaneous Release—Data 261 – Revised. Jones, N.W. 1984. Petrology of some Logan diabase sills, Cook County, Minnesota; Minnesota Geological Survey, Report of Investigations 29, 40p. Lucente, M.E. and Morey, G.B., 1983. Stratigraphy and sedimentology of the lower Proterozoic Verginia Formation, northern Minnesota. Minnesota Geological Survey Report of Investigations 28, 28 p. - 206 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Magnus, S. 2010. An investigation of the assimilation hypothesis in the Navilus Sill, Thunder Bay, Ontario. Unpublished Lakehead University Honours Thesis. intrusive rocks of the Thunder Bay area: in Summary of Field Work 1987, Ontario Geological Survey, Miscellaneous Paper 137, p. 248-255. Maric, M. And Fralick, P.W., 2005. Sedimentology of the Rove and Virginia Formations and their tectonic significance. Institute on Lake Superior Geology, v. 51, p. 41-42. Smith, A.R. and Sutcliffe, R.H. 1989. Precambrian geology of Keweenawan intrusive rocks in the Crystal LakePigeon River area: Ontario Geological Survey, Map P.3139, scale 1:50 000. Miller, J.D. and Smyk, M.C. 1995. Gabbroic intrusions of the International Boundary area; in Field trip guidebook for the geology and ore deposits of the Midcontinent Rift in the Lake Superior region; International Geological Correlation Program Project 336, Minnesota Geological Survey, Guidebook 20, p.171-181. Smyk, M.C., Hollings P. and Heaman, L.M. 2006.Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, 61-62. Miller, J.D., Smyk, M.C., Severson, M.J., Lavigne, M.J. and Middleton, R.S. 2002. PGE occurrences in mafic intrusions around western Lake Superior, USA and Canada; 9th International Platinum Symposium, Field Trip Guidebook, 135p. Smyk, M. and Hollings, P., 2007. Midcontinent rift-related mafic intrusion north of the international border. Proceedings of the Institute on Lake Superior Geology v. 53: 53-80. Morey, G.B. 1973. Stratigraphic framework of middle Proterozoic rocks in Minnesota. In, ed. G.M. Young, Huronian Stratigraphy and Sedimentation. Geological Association of Canada Special Paper 12, p. 211-249. North, J. 2000. Nature and distribution of Logan diabase sills and gabbro channels in the Keweenawan rift near Thunder Bay, Ontario: Brief comparison to Noril’sk; Abstract, 46th Institute on Lake Superior Geology, Annual Meeting, Thunder Bay, Ontario, Proceedings Volume 46, Part 1 (2000). Ojakangas, R.W., Morey, G.B. and Southwick, D.L., 2001. Palaeoproterozoic basin development and sedimentation in the Lake Superior region, North America; Sedimentary Geology, v. 141, p. 319-341. Parker, D.P. (ed.), Middleton, B., Schnieders, B.R., Smyk, M.C. and Scott, J.F. 2001. Intrusions of the Nipigon Basin; Superior PGE 2001, Canadian Institute of Mining and Metallurgy, Geological Society Field Conference, Thunder Bay, September 16-19, 2001, Field Trip Guidebook, 43p. Puchalski, R. 2010. The Petrology and Geochemistry of the Riverdale sill. Unpublished Lakehead University Honours Thesis. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: Evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, v.33, p.958-965. Rosatelli, M.P., 2002. Assessment report on the 2002 lithogeochemical rock sampling program, Pigeon River block. McVicar Minerals. Lrd. BHP Billiton World Exploration Inc., and Falconbridge Limited; Assessment Files, Thunder Bay South District, Thunder Bay, FN 2.24485, 36p. Smyk, M.C., Hollings, P., Heaman, L.M., 2006. Preliminary Investigations of the Petrology, Geochemistry and Geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario. Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, pp. 61–62. Stockwell, C.H., McGlynn, J.C., Emslie, R F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F. and Currie K L. 1972. Geology of the Canadian Shield, in Geology and Economic Minerals of Canada, edited by R.J.W. Douglas, Geological Survey of Canada, Economic Geology Report 1, 838 p. Sun, S.S., and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the ocean basins. Geological Society, Special Publication No.42, 313-345. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79. Tanton T.L., 1931. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63360. Tanton T.L., 1936a. Pigeon River area, Thunder Bay District. Geological Survey of Canada, Sheet 1, Map 354A, scale 1:63,360. Tanton T.L., 1936b. Pigeon River area, Thunder Bay District. Geological Survey of Canada, Sheet 2, Map 355A, scale 1:63,360. Weiblen, P.W., Mathez, E.A. and Morey, G.B. 1972. Logan intrusions; in Sims, P.K. and Morey, G.B. eds., Geology of Minnesota: A centennial volume; Minnesota Geological Survey, p.394-410. Smith, A.R. and Sutcliffe, R.H. 1987. Keweenawan - 207 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Field trip 12 - The Musselwhite Gold Deposit John L. Biczok Goldcorp Canada Ltd., Musselwhite Mine, PO Box 7500, Thunder Bay, ON P7B 6S8 Summary History of the Property The Musselwhite gold mine in northwestern Ontario began production in the spring of 1997 with quoted reserves of 1.8 million ounces of gold. Exploration efforts since that time have successfully replaced mined reserves in most years and added substantially to them in several years. By the end of 2011 Musselwhite had produced 3.34 million ounces with remaining proven and probable reserves of 2.28 million ounces and a measured+indicated resource of 146,000 ounces and an inferred resource of 917,000 ounces. Recorded exploration in the mine area began in 1962 when brothers Harold and Alan Musselwhite of Kenpat Mines Ltd. discovered the small Kenpat gold showing in quartz veins on the north side of Opapimiskan Lake as well as several showings in the iron formations on the south side. Over the next 5 years they conducted mapping, trenching and diamond drilling (12 holes totalling 773m). The Musselwhites re-staked the ground in 1973 and were subsequently financed by a syndicate of Dome Exploration, Canadian Nickel Co., Esso Minerals Canada Ltd. and Lacana Mining Corp. From 1976-1983 a major drilling program was undertaken in the West Anticline area followed by underground development and exploration in 1984. The West Anticline zone proved to be uneconomic and work soon shifted to the East Bay Synform area where the T-Antiform Zone was discovered by 1986. Exploration work carried on for another ten years and eventually the syndicate was reduced to two partners, Placer Dome (68%) and Kinross (32%). The T-Antiform ore zones had failed two early feasibility studies and failed to meet Placer Dome’s economic thresholds in a third study. However, the project proponents used a risk analysis study to convince the company’s board of directors that there was a very high probability of much more ore being found once the mine was in production and the go-ahead for construction was given in 1996 (Lewis, 1998). In 2002-3 the PQ Deeps ore zones were discovered followed by the Lynx Zone in 2010. After a series of corporate takeovers, Placer Dome’s interest became the property of Goldcorp Canada Ltd. in 2006 who subsequently bought out Kinross’ interest in 2007 and now hold 100%. Musselwhite is considered an orogenic gold deposit, hosted by tightly folded banded iron formation dated at ~ 2.98 Ga. It is located at the northern edge of the North Caribou Terrane (NCT), which forms the core of the North Caribou Superterrane (NCST) and the western Superior Province itself. The mine is adjacent to the ~2.86 Ga suture between the NCT and the Island Lake Domain to the north. Mineralization has been dated at 2.69 Ga, an age very close to that of gold occurrences elsewhere along the northern margin of the NCST. Unlike many orogenic gold deposits, but like a number of major BIF-hosted deposits, no major fault or shear zone that might have served as a pathway for mineralizing fluids has been found at Musselwhite. The current model for the formation of this deposit involves the development of mineralized high-strain zones along the steep limbs of the folded iron formation created during the flattening and folding event. This tectonic event was likely a result of the collision of the NCT with the Island Lake Domain 75km north of the mine, and/or the collision of the Northern Superior Superterrane ~200km to the north. Field trip participants will have the opportunity to observe weakly deformed, shallow-dipping BIF at several outcrops west of the mine, followed by steeply dipping exposures of the BIF which hosts most of the ore at depth. Underground stops will be dependent on the faces available at the time of the tour but we hope to visit well mineralized, highly strained portions of the orebodies. The following description of the geology is taken in part from Biczok (2007) and Biczok et al. (2012). Regional Geology The Musselwhite property covers a portion of the northwest-trending North Caribou Lake greenstone belt (NCGB) which is located in the northern margin of the Archean North Caribou Superterrane (NCST), adjacent to its internal boundary with the Island Lake Domain (Fig. 1). The NCST is a continental block consisting of ~3.0 Ga juvenile plutonic and minor volcanic rocks which underwent two periods of rifting - 208 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 1. Tectonic setting of the Musselwhite gold deposit. After Rayner and Stott (2005) and related deposition of arc sequences at 2.98-2.85 Ga and 2.85-2.71 Ga, followed by extensive reworking by continental arc magmatism at 2.75-2.70 Ga (Percival, 2007, and references therein). The North Caribou greenstone belt is one of the earlier arc sequences. Volcanic rocks of this belt have been dated at ~29822868Ma and, more specifically, those in the mine area at 2.98-2.97 Ga (Breaks et al., 2001; Biczok et al., 2012; unpublished Musselwhite data). The North Caribou greenstone belt (Fig. 2) has been mapped at various times by Satterly (1941), Emslie (1962), Andrews et al., (1981), and most recently a three year, multi-disciplinary effort in the mid1980’s by Breaks et al. (2001). These latter authors identified four dominantly volcanic rock suites in the Musselwhite mine area and these make up the McGruer Assemblage (Fig. 3): North Rim Metavolcanic Suite (NRU): Occurs in the northeast corner of Opapimiskan Lake and extends northwest from there over 60km along the northern margin of the greenstone belt. It consists largely of mafic and lesser ultramafic volcanic rocks. A minor felsic volcanic unit within this sequence was recently dated at 2868 Ma on Musselwhite’s behalf, confirming a previous age of 2870 Ma (Davis and Stott, 2001). South Rim Metavolcanic Suite (SRU): Occurs on the northwestern side of Opapimiskan Lake and extends north and northwest from there more than 50km along the southern margin of the NCGB. Regionally it is dominated by fine- to medium-grained, massive to pillowed basaltic flows with minor felsic and intermediate units and rare ultramafic units. Due to the paucity of felsic volcanic rocks in the SRU, only one age-date has been undertaken on rocks ascribed to the SRU by the OGS, that from an intermediate volcanic unit on the north shore of Opapimiskan Lake and based on only two zircons. These rocks were dated at 2982 Ma, however, it is not certain that they actually belong to the SRU, they are more likely part of the OMU. A number of authors have interpreted the SRU as being the folded repetition of the NRU on opposite sides of a major synform forming the axial core of the NCGB. Recent lithogeochemical work at the University of Ottawa suggests that the NRU and SRU formed in different tectonic settings and are not - 209 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 2. General geology of the North Caribou greenston belt with recent ages (after Breaks et al., 1987). Figure 3. Geologic map of the Musselwhite mine area - 210 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 equivalent (J. Duff, pers. comm.). Drilling by the Musselwhite exploration department over the past 10 years has identified a thick sequence of felsic volcanic flows, tuffs and volcaniclastic units beneath Opapimiskan Lake and at depth below the mafic volcanic rocks exposed on the northwest shore of the lake. This felsic pile varies from coarse pyroclastic rocks to very fine-grained, massive units (flows or welded ash tuffs?) to biotite-rich volcaniclastic units. These features, and the sheer volume of the felsic units. indicate that there was a major felsic volcanic edifice at this location which overlies the OpapimiskanMarkop suite and lies in the area mapped as part of the South Rim unit. However, the continuation of intraformational iron formations common in the upper Opapimiskan-Markop suite into the lower section of the felsic volcanic rocks of the felsic suite implies, at the very least, that this is largely a conformable contact. Alternatively, the Opapimiskan-Markop volcanism may actually include the felsic and mafic rocks on the north side of Opapimiskan Lake. U-Pb age dating of these felsic rocks is currently underway and may shed light on this issue. Opapimiskan-Markop Metavolcanic Suite (OMU): Occupies the central portion of the NCGB in the Opapimiskan Lake area and is dominated by mafic to ultramafic flows with intercalated clastic and chemical sedimentary units, including the banded iron formations which host the Musselwhite gold deposit (described in more detail in the following section). The lower portion of the volcanic pile is dominated by ultramafic to high-Mg basalts, iron formation and lesser siliciclastic sediments, a “primitive” sequence common in many greenstone belts of the NCS and interpreted to be products of plume-related rifting by Hollings and Kerrich (1999). The upper portion of the OMU, above the main BIF horizons, consist of predominantly tholeiitic basalts. As noted above, it is currently unclear which unit the felsic volcanic strata located above these tholeiitic basalts (Fig. 4) should be assigned to, the OMU or SRU. Further geochronological and lithogeochemical work is ongoing to answer this Figure 4. Generalized cross-section of the Musselwhite mine area and ore zones - 211 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 question. Forester Lake-Neawagank Metavolcanic Suite: Dominated by mafic and ultramafic volcanic units and occurs in the southeastern extremity of the belt. Granitoid gneiss and intrusions The greenstone belt is bounded to the north by the ~2.86 Ga Schade Lake granitic gneiss complex and various poorly-defined granitoid plutons within it. To the southwest is the ~2.85 Ga North Caribou Pluton and to the southeast is a poorly documented granitic batholith region assumed to also be ~2.85-2.86 Ga, but intruded by at least two 2.72 Ga plutons south of Musselwhite. These younger plutons are similar in age to those formed in a back arc position to the Confederation arc in the Uchi subprovince 200km to the south along the southern margin of the NCT. They are also similar to those found north of Musselwhite as far as the Hudson Bay Lowlands and potentially related to subduction of the Northern Superior Superterrane under the NCT. Further work is required to determine which of these suites the young plutons at Musselwhite belong to. Locally abundant S-type pegmatitic granite dykes occur throughout the area, particularly within areas underlain by metasedimentary rocks. These granites contain muscovite, garnet and tourmaline and are assumed to have formed by the partial melting of the metasedimentary rocks at depth. They have been dated at 2716 to 2669 Ma and are the only intrusive rocks known in the area which overlap the age of the mineralization (Biczok et al., 2012). Structural Geology Three deformation events have been recognized in the NCGB by previous workers (Hall and Rigg, 1986; Breaks et al., 2001). While the time between each event may be uncertain, there is good field evidence for these three discrete episodes of variably oriented strain. The earliest event, D1, is typically manifested only by tight to isoclinal folds in the iron formations, which are typically refolded by D2/F2. An excellent exposure of a large refolded F1 fold occurs in the West Anticline area (Fig. 5) and a classic basin and dome interference fold pattern occurs within the BIF on “Grunerite Island” in Opapimiskan Lake (Fig. 6). D2 is by far the dominant deformation event in the area and has produced a near vertical, moderate to strong planar north-trending foliation throughout the area with variably developed lineations, boudinage, and mesoscopic folds (Breaks et al, 2001). Syn-D2 shearing is commonly developed parallel to F2 and locally produces well-developed rootless folds in the Figure 5. F1 fold refolded by F2, West Anticline area. - 212 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 more highly strained margins of the iron formations. D3 is a relatively weak and localized event evidenced by minor warping and crenulation cleavages. Figure 6. Dome and basin interference fold pattern, Grunerite Island, Opapimiskan Lake. In the mine area, the strata have been folded into a broad antiform known as the West Anticline and the adjacent broad synform known as the East Bay Synform (Fig. 4). The West Anticline is an open fold with a relatively flat, undulating crest ~ 1 km across, featuring a series of smaller gentle folds that plunge to the north at <5° to ~30°. Rocks in this area are commonly only weakly deformed and preserve a variety of soft-sediment features in the iron formations including slumping (Fig. 7). In contrast, the East Bay Synform is bounded by limbs that dip steeply between ~70-90° and the keel is host to two 2nd order antiforms, the “T-Antiform” and the “W-Fold”. The folds plunge fairly consistently at ~12° to the north. This is a considerably higher strain setting than the West Anticline and primary structures are rarely preserved here. Late shears and faults are common in the volcanic rocks coring this synform and these are typically pervasively biotitized and laced with 20-40% thin calcite veinlets. Mineralized high-strain zones are predominantly developed in the upper margin of the Northern Iron Formation within the steepest portions of the fold limbs. The T-Antiform, the adjacent synformal keel (known as the “PQ Deeps”), and the east (PQ) limb of the East Bay synform host the bulk of the gold mineralization at Musselwhite. A major sub-vertical fault at ~3° to the fold axes has sliced the eastern limb (known as the PQ limb) into two pieces over a 700m interval and displaced the western portion ~1.3km to the south, forming a very large tubular sheath fold. This fault has fortuitously juxtaposed the ore zones over an interval of several hundred metres. Metamorphism Sedimentary rocks intercalated throughout the volcanic pile in the mine area commonly contain garnet +/- staurolite and the area is therefore considered to be of amphibolite grade. The greenschist-amphibolite isograd is thought to be located at least 5km from the mine to the north. Mine Stratigraphy Figure 7. Soft sediment slump features in BIF. Rocks hosting the Musselwhite gold deposit belong to the Opapimiskan-Markop Metavolcanic Suite (OMU) and have been subdivided into a detailed stratigraphy that is relatively consistent over the property, although major facies changes are locally observed along and - 213 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 across strike. This “mine stratigraphy” is depicted in Figure 8. folded areas including the Ranger, Red Wing and Thunderwolves. The lower portion of the OMU consists mainly of komatiitic basalts and ultramafic flows/intrusions, with local high-Mg andesite flows. This presence of andesite, which is commonly bleached and highly biotitized in the mine area, is somewhat unusual in such a maficultramafic sequence. Lithogeochemical analysis suggests that it formed by fractional crystallization of komatiite melt contaminated with either crustal TTG melts or felsic volcanic magma (Hollings and Kerrich, 1999). This sequence is overlain by two major banded iron formations (BIF) separated by 10-30m of maficultramafic volcanics and local high-Mg andesite. The Northern Iron Formation (NIF) sits ~20-30m above the SIF and is the main ore host at Musselwhite. This is a complexly layered horizon typically ~40m thick in total and consists of seven different facies thought to reflect varying proportions of clastic and chemical sedimentation combined with variations in the Redox conditions. Not every facies is always present across the drilled extent of the NIF, but where they are, the following stratigraphy is observed from the base to the top of the formation. The Southern Iron Formation (SIF) is the lowermost BIF and is a relatively monotonous sequence of thinly laminated magnetite and chert. There is generally little or no silicate, sulphide, or other facies within this horizon in the mine area. The SIF commonly occurs in two principal horizons, 5 to 20m thick, separated by 5-10m of basalt. The SIF hosts a number of small mineralized zones in tightly Unit 4H: This unit is a sulphidic iron formation, composed of 10-80% syngenetic pyrrhotite in a dark grey to black cherty argillite (Fig. 9). The continuity of the 4H is poor and where present its thickness varies from <10cm to as much as10m. Unit 4A: This unit is fairly ubiquitous throughout the mine area and is the most common basal unit of the NIF. It is composed of pale grey, weak-moderately magnetic chert, interlayered with 30-50%, diffuse bands of fine-grained, light yellow grunerite. Figure 8. Detailed stratigraphy of the Northern Iron Formation including presumed Redox conditions during formation. - 214 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Figure 9. 4H, massive pyrrhotite exhalite with chert fragments. Drill core sample 5.1cm (2”) wide. Unit 4B: Composed of thinly laminated to thickbanded (~1-2cm) chert-magnetite oxide-facies BIF, the 4B is typically 20-30m thick and forms ~3/4 of the NIF (and usually all of the SIF). It can be subdivided into two varieties: a lower, thick-banded, relatively pure chert-magnetite BIF (Fig. 10), and an upper, thinly laminated, more clastic-rich variety consisting of alternating intervals, typically <1-2cm thick, of thin, diffuse laminae of magnetic chert, and more homogeneous, medium-dark green, fine-grained amphibole-rich layers (Fig. 11). With increasing stratigraphic height these amphibole dominant bands become garnetiferous and may contain up to 25% pale red garnets <2mm in diameter. The amphibole+/-garnet layers are readily affected by hydrothermal alteration. Adjacent to mineralized zones and/or major quartz veins they are commonly altered to massive, finegrained black biotite, with an associated coarsening of the garnets. Figure 10. 4B, thick banded chert-magnetite BIF. Drill core sample 5.1cm (2”) wide. Figure 11. Laminated 4B (chert magnetite) below, clastic 4B with thin layers of green amphibole-garnet above. Drill core sample 5.1cm (2”) wide. Unit 4EA: Pristine 4EA is a silicate iron formation composed almost entirely of massive bands of garnet-grunerite with ~20-30% bands of pale grey, moderately magnetic chert <1-2cm thick (Fig. 12). The garnet-grunerite layers contain ~30-55% almandine garnets, 1-4mm across, in a fine-grained matrix of pale yellow grunerite and minor fine-grained disseminated magnetite. The 4EA forms the main orehost at Musselwhite and in mineralized zones it has undergone quartz flooding/veining, replacement of the original grunerite by green amphibole adjacent to the veins (hornblende or ferrotschermakite; Otto, 2002), significant coarsening of the garnets due to hydrothermal overgrowths, and pyrrhotite mineralization. There has been some debate over the years regarding the origin of the grunerite in the 4EA. Some have argued that it Figure 12. 4EA, garnet-grunerite silicate facies iron formation. Drill core sample 5.1cm (2”) wide. - 215 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 is the product of hydrothermal alteration of precursor magnetite + chert. While such alteration is locally observed on a small scale (mm to cm) in sheared, quartz veined portions of the 4B for example, it is difficult to envisage such a process producing the ~10m thick unit we see today that is continuous over many kilometers, always at the same stratigraphic heights, commonly has no evidence of hydrothermal alteration (such as quartz veins, calcic amphiboles, etc.), and displaying no gradation along strike into “fresh” magnetite+chert units. Perhaps most compelling is the nature of the BIF approximately 20km along strike to the north of Musselwhite in a greenschist grade region. Outcrops here are dominated by grunerite-chert layers with very little or no magnetite. What little magnetite is present occurs in very thin laminae, interbedded with grunerite, and delicately folded into complex patterns. It seems highly unlikely that the grunerite laminae here could have formed at the expense of the magnetite and still have preserved this delicate layering. The preferred explanation for the formation of the grunerite in the 4EA is that it is the product of metamorphism of the original iron-silica gels produced by seafloor hydrothermal vents. Unit 4F: The 4F is a garnet-biotite +/- staurolite schist with an average Fe2O3 content of ~25-30% and thus qualifies as an iron formation itself (Fig. 13). Typically it contains 30-55% subhedral garnets, 1-5mm across, in a fine-grained matrix of 60-70% biotite with lesser quartz, feldspar and magnetite. Intraformational 4F horizons within the basaltic pile commonly contain up to 30-40% anhedral, light yellow staurolite grains 1-2mm across. Unit 6: This is a thin, but semi-continuous unit, typically <1m thick, that occurs in the upper portion Figure 13. 4F, garnet-biotite-(Qtz-Fd)±staurolite schist; ferruginous metapelite. Drill core sample 5.1cm (2”) wide. Figure 14. 4E, garnetiferous amphibolite. Drill core sample 5.1cm (2”) wide. of the 4F sequence in the NIF. It is a light beige-grey, siliceous, fine-grained, equigranular, very homogeneous rock composed of 20-30% finely dispersed biotite and 70-80% quartz-feldspar. Lithogeochemical analyses indicate that this unit is very similar to the local felsic volcanic rocks but has relatively elevated levels of V, Mn and Ba. It is interpreted as a meta-sediment derived from a waterlain felsic ash tuff. Unit 4E: Where present, the 4E forms the uppermost unit of the NIF. It is generally a thin, <1m, fine-grained, massive, medium-dark green amphibolite containing 15-30% anhedral pale red garnets 2-4mm across (Fig. 14). It has little or no visible quartz or feldspar and averages 25% Fe2O3. The 4H, 4A, 4E and 4F all occur as discrete, “intraformational” horizons within the basaltic pile in addition to occurring within the NIF. These horizons are most commonly <2m thick but can swell to ten’s of metres within fold crests or keels. They are most abundant in the first 20-30m above the NIF and can be locally well mineralized. Overlying the NIF is a variable thickness of basalts ranging from <2m in the Esker fold area to 30-50m in the T-Antiform area. There are little or no komatiitic basalts or ultramafic units above the NIF. In the past, the various basalts in the mine area were designated as “BVol” (for “basic” or “basement” volcanics) or “2Vol” for the units near the NIF. Given the uncertainty of defining a “basement” in the mine area, these terms have been abandoned in recent years in favour of a generic “Unit 2” and its variants for all basalts and andesites. - 216 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Mineralization and Alteration Mineralization at Musselwhite is found largely within sub-vertical high-strain zones in the favourable iron formation units, primarily the silicate facies (4EA) and to a lesser extent the oxide facies (4B), where a number of the smaller ore zones are found (e.g., Jets, Ranger, Thunderwolves, lower portion of the PQ Deeps A-Block). Scattered small mineralized shears cut mafic volcanic rocks, however, to date these occurrences are of limited extent and uneconomic. Discrete mineralized shear zones up to 10m wide occur across a zone ~225m from the “Moose Zone” in the west through the T-Antiform and into the PQ Deeps in the east. Locally these shear zones coalesce into broad zones up to 40m wide (e.g. PQ Deeps C-Block). Individual mineralized shear zones tend to be quite persistent along strike, following the upper margin of the Northern Iron Formation and/or high strain zones within second order folds (e.g. the Jets Zone) for 2-3km. While the shear zones are primarily ductile, on both macroscopic and microscopic scales, there is evidence of associated brittle deformation that produced dilatant zones and allowed the infiltration of gold-bearing fluids. Gold mineralization within the 4EA is generally accompanied by substantial quartz veining or flooding, pyrrhotite formation, green amphibole (hornblendeferrotschermakite; Otto, 2002) replacement of the original grunerite-rich host (Fig. 15), a coarsening of the garnets, and local late-stage chlorite. The formation of pyrrhotite is thought to be a consequence of sulphidation reactions between the original gold-bearing bi-sulphide complexes and the iron-rich minerals of the BIF. The gold content is crudely proportional to the sulphur Figure 16. Reflected light photomicrograph of garnet cut by pyrrhotite-gold filled fracture. content of the mineralization in the ratio of 5 g/t Au for each 1% of sulphur. Gold occurs as free-milling native gold, most commonly in pyrrhotite-filled fractures in garnets (Fig. 16), with lesser amounts in the pyrrhotite, green amphibole, and rarely in quartz veins. The clastic 4B underlying mineralized 4EA is commonly highly altered itself. The original very fine-grained, green amphibole-rich laminae are replaced by massive finegrained biotite and 5-15% medium-grained secondary garnets; this alteration is especially common adjacent to quartz veins. In spite of the intensity of this alteration, it typically has nil to very low levels of gold or pyrrhotite. Surface Field Trip Stops Stop 1a: Trench #4, West anticline Lithology: This stripped outcrop on the west side of the exploration road exposes the Southern Iron Formation and the adjacent ultramafic rocks. The BIF here is dominated by chert with lesser beds of a light grey magnetite-amphibole-rich unit not seen outside the West Anticline area. Only along the margins of the outcrop does one see the chert-magnetite BIF which is predominant on the rest of the property. Figure 15. Photomicrograph of 4EA cut by quartz veins flanked by green amphibole replacement of grunerite (PPL). Comments: The change in the degree of strain is evident in this exposure as one crosses from the central, shallow-dipping area to the steeply-dipping, highly strained margins. The abundant small minor folds along the margins have an overall “Z” pattern with their axes plunging shallow to the (grid) north. On flat surfaces the tops of these folds appear as rootless or - 217 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 intrafolial folds. in these outcrops including: An unusual iron-rich chlorite schist, with a peculiar knobby or ovoid texture, appears to be locally interbedded with the cherty BIF as well as cross-cutting it. This unit averages ~40% Fe2O3, 15% Al2O3, 27% SiO2 and 6% MgO. It commonly contains intergrown magnetite and tourmaline grains and has been interpreted as an iron-rich metasediment that was locally injected as “dykes” through cracks in the more lithified BIF above. 1) The overall structural fabric is predominantly flattening rather than strike-slip movement or shearing. Stop 1b: Z-folded Chert-Magnetite BIF 3) The hanging wall basalts have a distinct pale pinkpurple color due to the pervasive fine-grained biotite alteration. Biotite alteration, bleaching of the amphiboles, and a strong foliation are typical of the basalts adjacent to the contact of the Northern Iron Formation and indicates that strain was partitioned (focused) at this contact. On the opposite (east) side of the road from the large stripped outcrop discussed above is a small exposure of 4B (well banded chert-magnetite BIF) that displays metre-scale Z-folds. Minor folds like these throughout the West Anticline can be related to their position on the series of undulating folds that make up the crest of the major antiform. Stop 2: Lakeshore Exposure of Gently Folded 4E/4EA Lithology: This small exposure near the shore of Opapimiskan Lake was only rediscovered in the Fall of 2011. What little of it was exposed at that time consisted of gently folded 4E and/or 4EA belonging to the Northern Iron Formation, plunging to the north. Comments: The outcrop was partially excavated in the Fall and will be power-washed in the spring prior to the field trip. It is expected to provide an excellent look at weakly deformed 4E / 4EA of the NIF, one of the few such exposures in the area. Stop 3: PQ Limb Section through the Northern Iron Formation Lithology: This is the only outcrop of the complete Northern Iron Formation found on the property and was created during overburden stripping operations related to the development of the PQ Shallows and the Ranger open pits. The exposures are part of the sub-vertical PQ limb, the eastern limb of the East Bay synform. The section begins with the eastern footwall which exposes deformed pillow basalts and passes through an almost complete section of the NIF, including a probable 4H, minor 4A, well-developed 4B, 4EA, 4F, Unit 6, and 4E. A small basaltic (“Bvol”) dyke is found within and roughly parallel to the trend of the 4EA. Comments: there is a wide range of features to note 2) There is a well developed zone of quartz veining and intense biotite alteration in the clastic 4B immediately below the 4EA. This type of alteration is typically barren and conversely biotite alteration such as this is relatively rare in well-mineralized 4EA. 4) There is a northeast trending series of small-scale folds and cleavages scattered throughout the exposures. These may be part of the D3 deformation event. References Andrews, A.J., Sharpe, D.R., and Janes, D.A. 1981. Preliminary Reconnaissance of the WeagamowNorth Caribou Lake Metavolcanic-Metasedimentary Belt, including the Opapimiskan Lake (Musselwhite) Gold Occurrence; in Summary of Field Work, 1981, Ontario Geological Survey, Miscellaneous Paper 100, p. 196-202. Biczok, J.L., 2007. 2006-2007 North Shore Project, Diamond Drilling Report. Assessment report filed with the Ontario Ministry of Northern Development and Mines on behalf of Goldcorp Canada Ltd. AFRI # 20000003409 Biczok, J.L., Hollings, P., Klipfel, P., Heaman, L., Maas, R., Hamilton, M., Kamo, S., and Friedman, R., 2012. Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite mine. Precambrian Research, 192-195 (2012), p. 209-230. Breaks, F.W., Osmani, I.A., and deKemp, E.A. 1987. Precambrian Geology of the OpapimiskanNeawagank Lakes Area, Western Part (Opapimiskan Project), Kenora district (Patricia Portion); Ontario Geological Survey, Map P.3080, Geological SeriesPreliminary Map, scale 1:31,680. geology 1986. Breaks, F.W., Osmani, I.A., and deKemp, E.A. 2001. Geology of the North Caribou Lake area, northwestern Ontario; Ontario Geological Survey, Open File report 6023, 80p. - 218 - �Proceedings of the 58th ILSG Annual Meeting - Part 2 Davis, D.W., and Stott, G.M., 2001. Geochronology of several greenstone belts in the Sachigo Subprovince, northwestern Ontario, #18 Project Unit 89-7. In: Summary of Field Work and Other Activities 2001, Ontario Geological Survey Open File Report 6070, 18-1 – 18-13. deKemp, E.A., 1987. Stratigraphy, provenance and geochronology of Archean supracrustal rocks of western Eyapamikama lake area, northwestern Ontario. Unpublished M.Sc. thesis, Carleton University, Ottawa, Ontario, 98p. Emslie, R.F. 1962. Wunnummin Lake (NTS 53A), Ontario; Geological Survey of Canada, Map l -1962, scale l inch to 4 miles or 1:253 440. Geology 1962. Hall, R.S. and Rigg, D.M., 1986. Geology of the West Anticline Zone, Musselwhite Prospect, Opapimiskan Lake, Ontario, Canada. In: Macdonald, A. J. (Ed.), Gold ’86: An International Symposium on the Geology of Gold Deposits, Proceedings Volume, 124-136. Hollings, P., and Kerrich, R., 1999. Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province. Precambrian Research, v. 93, p. 257-279. Lewis, T.D., 1990. Musselwhite: An exploration success. CIM Bulletin Vol. 91, No. 1017, pp. 51-55. Klipfel, P., 2002. Musselwhite U-Pb Zircon and Ar-Ar Dates, Synthesis and Interpretation. Internal Placer Dome Exploration research report. gold deposit Musselwhite Mine, Ontario, Canada. Unpublished M.Sc. thesis, Freiberg University of Mining and Technology. 86p. Percival, J.A., 2007. Geology and Metallogeny of the Superior Province, Canada, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods. Geol. Assoc. of Canada, Mineral Deposits Div., Spec Pub. No. 5, p. 903-928. Piroshco, D.W., Breaks, F.W., and Osmani, I.A., 1989. The Geology of Gold Prospects in the North Caribou Greenstone Belt, District of Kenora, Northwestern Ontario. Ontario Geological Survey, Open file Report 5698. Satterly, J. 1941. Geology of the Windigo-North Caribou Lakes Area, Kenora District (Patricia Portion); Ontario Department of Mines, Annual Report for 1939, v.48, Part 9, p. 1-32. Accompanied by Maps 48h and 49j. Stott, G., and Rayner, N., 2005. Discrimination of Archean domains in the “Sachigo Subprovince”, northwestern Ontario. Ontario Geological Survey, poster, Ontario Exploration and Geoscience Symposium, Toronto, Ontario, December 13-14. Thurston, P.C., Osmani, I.A., Stone, D., 1991. Northwestern Superior Province Review and Terrane Analysis: Geology of Ontario. Ontario Geological Survey, pp. 81–144 Otto, A., 2002. Ore forming processes in the BIF-hosted - 219 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2012 Description An account of the resource Institute on Lake Superior Geology. Thunder Bay, Ontario. May 16-20, 2012. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2012 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/f89df4b1f66e16d34fb5de82344c88e6.pdf c23729b90c272dc16d576f9f09f8c61b PDF Text Text �������������������� Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 1957 Description An account of the resource Institute on Lake Superior Geology. Michigan State University, East Lansing, Michigan. May 6-8, 1957. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 1957 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English